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Solid solubility of silver in the superconducting YBa₂Cu₃O compound Zhang, Chongmin 1991

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SOLID SOLUBILITY OF SILVER IN THE SUPERCONDUCTINGYBa2Cu30. COMPOUNDbyCHONGMIN ZHANGB.S.(Ceramics), Shanghai University of Science and Technology, China, 1982M.S.(Ceramics), Shanghai Institute of Ceramics, Chinese Academy of Science, 1985A THESIS IN PARTIAL FULFILMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIESDepartment of Metals and Materials EngineeringWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIADecember, 1991© CHONGMIN ZHANG, 1991In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature) Department of  IA JaiS 0.4J1^ater% et `; Eitple€ r 0 1 (jThe University of British ColumbiaVancouver, CanadaDate Dec 24, (99DE-6 (2/88)ABSTRACTThe solubility of silver in the YBa2Cu3Ox compound has been studied. Ceramic YBa2Cu3Oxpellets with added silver ranging from 0 to 10 wt% were prepared and characterized by X-raydiffraction (XRD), optical microscopy, scanning electron microscopy (SEM), energy dispersiveX-ray spectroscopy (EDX), wavelength dispersive X-ray spectroscopy (WDX), and secondary ionmass spectrometry (SIMS). The lattice parameters were precisely determined by a rigorous X-raydiffraction method and by using a USGS computer program which uses an iterative least squaresolution to calculate the lattice parameters and the standard deviations. The final precisions are±0m005A for lattice parameters a and b, and ±o.oca A for the lattice parameter c.Silver appears to be present within the YBa2Cu3Ox grains. More intragranular silver wasfound inside quenched samples than slowly cooled samples. In both cases the silver solubilityincreased with the nominal silver concentration, which reached a constant level as the nominalsilver concentration exceeded 2.5 wt%. The maximum silver solubilities are 0.41 and 0.37 wt%for samples quenched from 950°C and slowly cooled to room temperature, respectively. Theintragranular silver concentrations were found to be much lower than the nominal silver concen-trations added to the system.Along with the increasing intragranular silver concentrations, lattice parameters a and c, andunit cell volume of the orthorhombic YBa2Cu3Ox phase increased and appeared to reach constantvalues at higher silver concentrations. Lattice constant b remains relatively unchanged for allsilver concentrations. Structural chemistry analyses suggest that the dissolved silver ions maysubstitute copper ions at Cu(1) sites of the YBa2Cu3Ox unit cell. The maximum atomic fraction ofCu substituted by Ag is about 0.023 at room temperature, i.e. y-0.023 for YBa2Cu3_yAgy0x. Themaximum value of y is about 0.025 for samples quenched from 950°C.11111Normalized resistance was also found to increase with the silver concentration, whichattained a constant value at higher silver concentrations. Critical temperatures (Tc) are not sig-nificantly affected by the silver additions whereas the critical current density (Jc) is marginallyimproved for the sample with nominal silver concentration of 0.5 wt%.Table of ContentsABSTRACT^  iiTable of Contents  ivList of Figures  viList of Tables ^  viiiACKNOWLEDGMENT^  ix1 INTRODUCTION  11.1 The Breakthrough of Copper Oxide High Tc Superconductors ^ 11.2 Applications for High Temperature Superconductors ^  81.2.1 Josephson Junctions and SQUIDs ^  81.2.2 MRI Devices ^  91.2.3 Superconducting Magnets ^  101.2.4 Electric Power Applications  111.3 Difficulties of Practical Applications of Bulk YBa 2Cu3Ox Compounds ^ 111.3.1 Low Critical Current Density ^  111.3.2 Brittleness and Weakness  141.3.3 Oxygenation Behavior^  161.3.4 Weak Flux Pinning  191.4 Approaches to Overcome the Problems ^  201.5 Previous Work on Silver Addition to YBa 2Cu3Ox Compound^ 211.5.1 Improved Mechanical Properties by Silver Addition  211.5.2 Enhanced Critical Current Density by Silver Addition ^ 231.5.3 Improved Flux Melting Temperatures by Silver Addition  281.5.4 Other Studies on Silver Addition to YBa 2Cu3Ox ^  291.6 Objective of Present Study ^  31iv2 EXPERIMENTAL PROCEDURE^  332.1 Sample Preparation^  332.2 Scanning Electron Microscopy ^  362.3 Secondary Ion Mass Spectroscopy  382.4 Precise Determination of Lattice Parameters ^  392.5 Electrical Property Measurements^  442.5.1 Tc Measurement^  442.5.2 Normal State Resistance  452.5.3 Jc Measurement^  463 RESULTS^  473.1 X-ray Diffraction Phase Identification ^  473.2 Microstructure Studies ^  533.2.1 Optical Metallographic Examination ^  533.2.2 Electron Metallographic Examination  583.3 Secondary Ion Mass Spectroscopic Study^  633.4 Quantitative WDX Analysis ^  633.5 Precise Determination of Lattice Parameters ^  673.6 Electrical Property Measurements^  714 DISCUSSION ^  764.1 The Preferential Site of Silver Substitution ^  764.2 Impurity Phase and Its Effect on the Electrical Properties ^ 814.3 Previously Reported Results of Silver Addition to YBa2Cu30„ Compounds ^ 815 SUMMARY AND CONCLUSIONS^  846 REFERENCES^  85APPEDIX  91List of Figures1 The time evolution of the superconducting transition temperature^ 32 The perovskite structure for ABO 3 compounds ^  53 Crystal structure of YBa2Cu3Ox^  64 Nominal unit cells for the Bi- and Tl-containing superconductors ^ 75 Jc as a function of the misorientation angle ^  136 Y 1.50-BaO-CuO phase diagram at 950°C  147 Phase diagram along the join YBa2Cu30x-Y2BaCu05 ^  158 Transition temperature versus oxygen content of YBa 2Cu3Ox^  179 TGA trace for YBa2Cu3Ox heated and cooled under 1 atmosphere 0 2^  1810 Magnetic Properties of Type I & II superconductors ^  2011 EPMA Ag contents in YBa2Cu3 _xAgx06,3, of Gangopadhyay & Mason's ^ 2712 Flow chart of the processing steps for sample preparation ^ 3513 0-sin0 relationship of X-ray Diffraction ^  4014 Calibration curves for 28 measurements of X-ray diffraction ^ 4215 Methods to determine X-ray diffraction peak positions  4316 Schematic diagram of the sample and apparatus of T c measurements^ 4517 The geometry of the sample for Jc measurement ^  4618 Diffraction pattern of pure orthorhombic YBa2Cu3Ox ^  4819 Diffraction pattern of orthorhombic YBa2Cu30x+0.5%Ag  4820 Diffraction pattern of orthorhombic YBa 2Cu30x+1%Ag^  4921 Diffraction pattern of orthorhombic YBa 2Cu30x+1.5%Ag  4922 Diffraction pattern of orthorhombic YBa 2Cu30x+2.5%Ag ^ 5023 Diffraction pattern of pure tetragonal YBa 2Cu3Ox ^  50vivii24 Diffraction pattern of pure tetragonal YBa2Cu 30x+1.5%Ag^ 5125 Diffraction pattern of pure tetragonal YBa2Cu30x+10%Ag  5126 Optical micrographs of YBa2Cu30x+Ag samples^  5427 Electron micrographs of YBa 2Cu30x+Ag samples  5828 Silver precipitate in a YBa2Cu30„+5%Ag sample ^  6029 EDX spectrum from the grain boundary of a YBa 2Cu30x+1%Ag sample^ 6130 Positive SIMS spectra of YBa2Cu3Ox samples containing silver ^ 6231 Intragranular vs nominal silver contents ^  6632 Reproducibility of WDX data of intragranular silver contents ^ 6633 Lattice constant a of orthorhombic YBa2Cu3Ox vs silver content  6934 Lattice constant c of orthorhombic YBa2Cu3Ox vs silver content ^ 6935 Unit cell volume orthorhombic YBa 2Cu3Ox vs silver content  7036 Lattice constant b of orthorhombic YBa 2Cu3Ox vs silver content^ 7037 Resistance of YBa2Cu30x+Ag as a function of temprature  7238 Normalized resistance of YBa 2Cu30x+Ag vs silver content ^ 7439 Voltage as a function of electric current of YBa2Cu30„+1.0%Ag  7540 Voltage as a function of electric current of YBa 2Cu30x+10%Ag^ 7541 Analyzed silver content vs nominal silver content^  7642 Crystal structure of YBa2Cu3O7^  77List of Tables1 Critical Temperatures of RBa 2Cu30„ Group ^  42 Compounds and Critical Temperatures of (A0).M2Can_iCu„02,m.2Family^ 43 Summary of Jc Values of YBa2Cu30x+Ag Composites ^  264 Flux Metling Temperatures of YBa2Cu30x•Agy Samples  295 Intensities of the Strongest Peak of Orthorhombic YBa 2Cu3Ox ^  526 Intragranular Silver Concentrations Obtained from WDX 657 Lattice paramenters of orthorhombic YBa2Cu3Ox phase^  688 Electrical Properties of YBa2Cu30x+Ag Compounds  749 Bond length in YBa2Cu3O7^  8010 XRD Data of the Orthorhombic YBa2Cu3Ox Phase ^  9111 XRD Data of the Orthorhombic YBa2Cu3Ox Phase in 123+0.5%Ag Sample^ 9212 XRD Data of the Orthorhombic YBa2Cu3Ox Phase in 123+1.5%Ag Sample^ 9313 XRD Data of the Orthorhombic YBa2Cu3Ox Phase in 123+10%Ag Sample^ 94viiiACKNOWLEDGEMENTIt is my sincere pleasure to thank my research supervisor, Dr. Chaldader, for his encour-agement and guidance throughout the course of my work on this project. Thanks are also extendedto the faculty, staff and fellow graduate students in the Department of Metals and MaterialsEngineering, especially to Dr. Alina Kulpa and Mr. Glenn Roemer for their many valuable dis-cussions and help.The financial support from the department in the form of research and teaching assistant-ships is sincerely acknowledged.The Jc measurement provided by Mr. Reinhold Krahn in Department of Physics is gratefullyacknowledged.My special thanks are to my wife, Xiyi, for her understanding, support and patiencethroughout my graduate study, and for her sharing the happiness and hardships in the past years.ix11 INTRODUCTION1.1 The Breakthrough of Copper Oxide High T, SuperconductorsThe discovery of high-temperature superconductivity in B a-La-Cu-0 ceramic oxides witha critical temperature (Tc) at 30K by Nobel laureates Bednorz and Miiller fil in 1986 and thesubsequent confirmation and extension of this discovery by several groups in USA, China, andJapan triggered large scale research efforts worldwide. Superconductivity at temperature about90K, which breaks down the temperature barrier of liquid nitrogen (77K), was reported in March1987 in the Y-B a-Cu-0 system(21 . The general formula of these new high-temperature super-conducting compounds is RBa2Cu307.„, where R is a rare earth element which is most commonlyY and it is usually also referred to as 123 compound as it is known because of the stoichiometricratio of its cations. Achievement of superconductivity at such a high temperature has had twoimportant consequences. First, this discovery has acted as a catalyst in re-opening the interestof the scientific community and stimulated basic research across a broad spectrum of disciplinesand applied research. The interest arises out of the fact that the discovery, contrary to earlierpredictions by many theorists, demonstrates that superconductivity can occur not only at hightemperature but also in a wide range of oxides whose concentration of charge carriers is low.Second, and most importantly, this discovery has re-kindled the interest of industrial communityin the development and applications of high temperature superconducting materials. Previously,the widespread use of superconducting materials was limited by the necessity of using expensiveliquid helium (at $4 per liter) as the cryogen. With the development of new copper oxidesuperconducting materials which can be operated by cooling with inexpensive liquid nitrogen(at 250 per liter), the economic viability of superconductors' applications has dramaticallyimproved. Moreover, liquid nitrogen has an exceptional high latent heat of evaporation. In an2hour one watt of heat will evaporate 1.4 liters of liquid helium but only 0.016 liters of liquidnitrogen. This effect in large scale applications of superconductors is obvious and the cost tomaintain a cryogenic system has been reduced to 1/1,400, excluding the reduction in thermalinsulation, by using high temperature superconducting materials.Although several claims of superconductivity in the Y-Ba-Cu-O system at temperaturesnear 270K have been reported 131 , most measurements indicate that only a small portion of aspecimen exhibits superconductivity at the higher temperature. Furthermore, the apparenthigh-temperature superconductivity disappears over time. Two other notable oxide super-conductors: Bi2Sr2CaCu2O8 (Tc=110K) and T12Ba2Ca2Cu 30 1 ,3 (Tc=122K) were discoveredsubsequent to the discovery of YBa 2Cu 30x . Figure 1 shows the critical temperatures of historicsuperconductors versus their discovery year. This illustrates the dramatic progress triggered byBednorz and Miiller.There are two interesting features of these ceramic superconductors. First, wide chemicalsubstitutions for certain transition elements can be made without seriously degrading the criticaltemperature. Second, all of these ceramic superconductors have a layered structure derivedfrom the perovskite structure. Table 1 lists the critical temperatures of the RBa 2Cu3Ox group,where R is given. Another large group of ceramic superconductors is of the type(A0)mM2Ca„,,Cu.02,2, where the A cation can be Tl, Bi, Pb or a mixture of them. The valueof m is 1 or 2, and the M cation is Ba or Sr. The superconducting compounds in this familyand their critical temperatures are shown in Table 2. In these layered structure compounds,Cu-0 layers (also called basal planes) are the essential parts of the layer structure and the oxygenordering in the Cu-0 layers is crucial to the superconductivity of these ceramic superconductors.Unlike other transition elements in these ceramic superconductors, Cu can only be partially100 -2.'HP2 80=12a)eLEa) 60-I—c0Cl). _cEll 40 -H.20 -3TI-Ba-Ca-Cu-O120 -Bi-Sr-Ca-Cu-OY-Ba-Cu-ON2 77KLa-Sr-Cu-0Ne 27K^La-Ba-Cu-ONb 3GeNb3 SnHg^ He 4.2K01900I^.^I^,^1^,^1^.1920^1940^1960^1980^2000YearFigure 1 The time evolution of the superconducting transition temperature.Table 1 Critical Temperatures of RBa2Cu3Ox Group [51R T., (K)Eu 94Nd 92Gd 92Y 91Dy 91Tm 91Er 91Ho 91Sm 88Lu 88Yb 86Table 2 Compounds and Critical Temperatures of (A0),,,M 2Can_ iCui3O2,2 Family [51Compound 'I', (K)T1B a2CaCu207 90T1B a2Ca2Cu309 110T1B a2Ca3Cu40 11 122(T1,Bi)Sr2Cu05 50(T1,Bi)Sr2CaCu207 90(T1,Pb)Sr2CaCu2O7 90(T1,Pb)Sr2Ca2Cu309 122T12Ba2CuO6 90T12Ba2CaCu208 110T12Ba2Ca2Cu3010 122T12Ba2Ca3Cu40 12 119Bi2Sr2CuO6 12Bi2Sr2CaCu2O8 90Bi2Sr2Ca2Cu3O10 110Bi2Sr2Ca3Cu4O12 9045substituted by other elements otherwise the superconductivity would be destroyed. Due to thesecond feature described above these superconductors, including La-Sr-Cu-0 compounds, arealso known as high Tc cuprates.Perovskite structure (Figure 2), originated from the mineral perovskite (CaTiO 3), is oneof the typical structures for ABO 3 compounds. In this structure, large cations A and anions 0combine to form a close-packed cubic structure with the smaller, more highly charged B cationsin the octahedral interstices. The coordination numbers are 12 and 6 for A cations and B cations,respectively. The unit cell of YBa 2Cu301, as illustrated in Figure 3, contains three distortedperovskite building blocks, in which Y and Ba cations are at A sites, and Cu at B sites. For theorthorhombic phase of YBa2Cu30„, 05 sites are empty and 01 sites are filled orderly by oxygenwhereas in the tetragonal phase of YBa2Cu30„, 01 and 05 sites are either disorderly occupiedor empty. The structures of bismuth- and thallium-containing superconductors (Figure 4)consists of perovskite-like units containing one, two, or three Cu0 2 planes sandwiched betweenBi-0 or T1-0 bilayers. For the thallic compounds, these Cu0 2 planes can also be separated byT1-0 monolayers.Figure 2^The perovskite structure for ABO 3 compounds. Large open circles represent 0ions, the shaded circle A, and the small circles B ions.01Y0 Ba• Cu0 0b6Figure 3 Crystal structure of YBa2Cu30x.7(c)(b)1021 1122 1223(a)(0(e)(d)2021o TI or Bi• CaO Ba or Sr• Cu• 021222223Figure 4^Nominal unit cells for the bismuth- and thallium-containing superconductors.Only the 2021, 2122, and 2223 structures form in the Bi-Ca-Sr-Cu-O system,whereas all six structures form in the Tl-Ca-Ba-Cu-0 system.For the purposes of this study, only the YBa 2Cu30, compound was considered. Thereare several reasons for this. The YBa2Cu30„ compound has only one superconducting phaseand is the easiest to synthesize. This is contrasted to bismuth compounds which form multi-phase8components much more easily. Although one of the phases has a critical temperature of 110K,it is difficult to separate this phase from other phases which have lower critical temperatures.The YBa2Cu3Ox phase is safe to handle without special precaution because although bariumand copper compounds are generally toxic if ingested, they are not volatile. Thallium and manyof its compounds including T1203 are extremely toxic. Particularly, the toxin can readily passthrough unbroken skin. Also, YBa2Cu3Ox has been extensively investigated and such things asphase diagrams, crystal structures and physical properties are well documented. Moreover,among these ceramic superconductors, YBa 2Cu30„ has one of the highest flux melting tem-peratures under a given applied magnetic field. E611.2 Applications for High Temperature SuperconductorsThe possibility of superconductivity above the temperature of liquid nitrogen holds, ofcourse, tremendous promise for their future applications. The applications can be roughlygrouped into three categories: electronics, magnets and electric power applications.Though currents in electronic devices are small, due to restricted dimensions, currentdensities are similar to those in large machines. However, magnetic fields are generally small.On the other hand, thin films, as would be required for fabrication of a variety of electronicdevices, have already demonstrated their ability to meet the required current density.1.2.1 Josephson Junctions and SQUIDsA large number of superconducting devices are based on the properties of Josephsonjunctions. These junctions are formed by two superconducting films separated by a verythin insulating layer so that they are only weakly coupled. A simple Josephson junction isa switch which can be switched from the superconducting to the resistive state through the9electron tunneling effect which was discovered by Giaever in 1960.m The switch can beoperated either by the application of a voltage or by exceeding the critical junction current.The critical junction current is very strongly affected by small magnetic fields and thereforethe junction can sense very small changes of magnetic flux (on the order of 10 -19 Wb). SingleJosephson junctions can be used as (i) switches with a gate switching time of 10 12 second,(ii) SIS mixers to convert frequencies for millimeter-band receivers with extremely lownoise, (iii) detectors for electromagnetic radiation over a very broad range of frequencies,(iv) voltage standards, etc. A two-junction device, in which two Josephson junctions areconnected in parallel, is called a SQUID (superconducting quantum interference device)whose critical current of the junction is very sensitive to the magnetic flux that threads thejunction and, in fact, is modulated with the period of a flux quantum (90=2.07x10-15 Wb).SQUID devices can be used as voltmeters with sensitivities on the order of 10 45V, sensitivemagnetometers for biological as well as geophysical applications and instruments useful forphysicists to search for magnetic monopoles and gravity waves.1.2.2 MRI DevicesMagnetic resonance imaging (MRI) is used for noninvasive imaging of soft tissueswithout the need for exposure to radiation. MRI, which probes the nuclear magnetic res-onance (NMR) characteristics of the hydrogen nuclei in the body, can distinguish betweenthe various chemical environments in which the hydrogen atoms reside in various tissuesof the body. The key element in making this technique work is the large superconductingmagnet that provides both the bias magnetic field and the magnetic field gradient. MRI iscurrently the dominant market of superconducting materials and devices.101.2.3 Superconducting MagnetsAnother potential application of superconducting magnets is for the separation ofmagnetic materials from nonmagnetic matrices or even the separation of materials by theirdegree of magnetism, e.g. removal of impurities from food and industrial raw materials.The principle on which this method is based is that the force on a weakly magnetic particleis proportional to the product of its magnetic susceptibility and the gradient of the magneticfield. Superconducting magnets offer the possibility of high fields and high field gradientsand therefore could provide very efficient separation.The superconducting magnetic energy storage (SMES) system is another notableapplication of superconducting magnets. The concept is simple; the energy is stored in themagnetic field of a rather large, underground solenoid or toroid. For a persistent currentlevel I in the SMES inductor L, stored energy is 112/2. The SMES is inherently low-loss,with —95% efficiency expected. Small SMES systems could be used by industries, hospitals,office complexes, and shopping centers to reduce electricity costs during peak demandperiods. It might also be used as a power backup to protect sensitive equipment, such aslarge computer or telecommunication systems, against sudden power failures.Other superconducting magnet applications include magnets used to reduce the sizeor to increase the energy of accelerators for high energy physics, high power motors andgenerators, magnetic levitation for fast ground transportation systems and airplane runways,etc.111.2.4 Electric Power ApplicationsElectric power applications include power transmission and distribution. The trans-mission of electrical energy over long distances with very small losses is an importantproblem for power industry. Superconducting AC and DC transmission lines are certainlyideal for this purpose. The major impact of the new high temperature superconductors wouldbe in terms of low refrigeration energy cost due to the use of liquid nitrogen instead of liquidhelium.1.3 Difficulties of Practical Applications of Bulk YBa2Cu30. CompoundsDespite the impressive progress in high temperature superconductors, researchers havesoon realized that there are many tough problems which must be solved before the commer-cialization of ceramic superconductors is possible. The complexities of the problems dampenedalmost all enthusiasms of study on the bulk materials, at least by the physicists, who have nowturned their research activities to thin films. These problems are left to the ceramic communityto handle1.3.1 Low Critical Current DensityThere had been much hype in the media about these materials which was initiallydirected towards increasing the critical temperature (Tc) at which these ceramics becomesuperconducting. However, it has become clear that the critical current density (J o) is thelimiting factor for their applications and not the T c. The relatively low .Tc in the polycrys-talline, bulk superconductors and its significant deterioration in weak magnetic fields181 havebeen the major roadblocks to the rapid technical advancement towards commercialization.For large scale applications such as high-field magnetic coils, the materials must be fabricated12in the form of wires or tapes with critical current densities (J o) of the order of 105 A/cm2 inmagnetic fields higher than 10T at 77K.'9' Measurements on the thin films indicates that at77K, critical current densities (Jr) as large as 106 A/cm2 in zero field are possible."' However,the best ceramic polycrystals have Jc values at least two orders of magnitude lower than thatof thin films or single crystals. Usually, it is only about 100 A/cm 2 for ceramic sampleswithout any specific treatment during processing. The poor J c of bulk YBa2Cu30„ is theresult from a combination of several factors associated with this material.The YBa2Cu30„ compound is very anisotropic. It is virtually a two dimensionalconductor. Measurements on single crystals of YBa 2Cu30„ showed that the Jc in the crystal'sa-b plane was at least an order of magnitude higher than that along the c axis. 111] Becauseof this anisotropy, it is expected that the Jc will be poor in three dimensions for bulk ceramicYBa2Cu30„ materials in which the crystals are randomly oriented.One of the important parameters of superconductivity is the coherence length Thecoherence length may be thought of as a measure of the maximum distance between membersof a Cooper pair which carries supercurrent. Within the coherence length, the electrons orholes are said to be "correlated", and one may think of as being the "size" of the Cooperpair. The value of of YBa2Cu30„ was measured to be approximately 13A and 2A in thea-b plane and in the c direction, respectively. [12-13] These are extremely short compared tothe values of the order of 1,000.E for conventional metallic superconductors. Since E is thedistance over which coherence can be maintained across a normal state region, it is a measureof the scale at which lattice interruptions can disrupt the supercurrent. The inherent char-acteristic is that even grain boundaries or twinning planes (twins are abundant in thesuperconducting YBa2Cu30„ phase) may have sufficiently large dimensions, i.e. widths, toact as weak links in the superconducting state and, therefore, hinder the movement of Cooper13pairs resulting in a poor transport J, in bulk materials. Direct measurements of critical currentdensities of individual grain boundaries in thin films as a function of their relative mis-orientation were reported." 41 The result (Figure 5) showed that the critical density across agrain boundary fell rapidly with misorientation angle, until it had fallen by about a factorof 50 for 0>15° relative to the intragranular J.1.000.500.300.20..---.LC)0■., 0.102.LC)-a^0-3•• •••• •• •i^I^ I0.00^10.00^20.00 30.00^40.00Misorientation angle (degree)Figure 5 Ratio of the grain boundary critical current density to the average value of thecritical current density in the two adjacent grains as a function of the mis-orientation angle in the basal plane. Measurements were made at 5K.mYBa2Cu3Ox is virtually a point compound at any particular oxygen potential (Figure6). Any deviation from its stoichiometrically cationic composition will lead to non-14superconducting phases. These phases usually segregate in grain boundaries forming weaklinks and limiting the Jc values. The worst case is that these insulating phases form acontinuous film coating each individual superconducting grain and severely deterioratingthe Jc.CuOBa A4Ba,CuO,Ba2CuO -,thati41udO,Cu0 2BaO^Ba4Y20 7^BaY20 4^YO IsFigure 6 Y1.50-BaO-CuO phase diagram at 950°C.E15]1.3.2 Brittleness and WeaknessThe applications of superconductors often require the materials to have a sufficientflexibility to be fabricated into complex shapes, e.g. wound into a solenoid. Unfortunately,brittleness is an inherent characteristic of ceramics. It has been found to be exceptionallydifficult to fabricate YBa2Cu30„ into structurally sound bulk forms such as tapes, wires, ormore intricate configurations while maintaining a high Jc. Compared to other ceramicmaterials, YBa2Cu3Ox is very weak in terms of fracture toughness (-1 MPa•m 12). The15material is usually porous with only about 85% of theoretical density. Lower sinteringtemperature is responsible for such a high porosity. As shown in Figure 7, YBa2Cu30„ meltsperitectically at a temperature of 1,015°C to form a liquid (of approximate compositionYomBa0.25Cuo.740) and the crystalline phase Y2BaCuO5. Both of them are non-superconducting although the presence of the liquid phase can enhance the sintering. Toavoid the formation of insulating phases, the sintering temperatures typically employed arelow and lie in a range between 925 and 975°C, yielding poor density in the final compact.So far, no sintering aid which does not affect the superconducting properties of the materialhas been found. High porosity not only gives low strength but also generates poor electricalconnections between individual grains. There is a contradictory requirement about the grainsize: good mechanical properties require fine grains but finer grains are accompanied by alarger amount of non-superconducting grain boundaries (even without a liquid phase), i.e.poorer Jc values.I.1015 t•,..,7. 123+L123+011+001200+1211+L123+211211+200+2105Y0..+Ba0YBa.Cus0. Y,BaCuO .Figure 7^Section through the ternary phase diagram along the join YBa 2Cu30,-Y2BaCu05. Positions of all lines are approximate, except for the incongruentmelting point of YBa2Cu30x.f16I16Highly dense YBa2Cu30„ samples can be produced by applying external forces on thesamples during sintering such as hot-pressingn, sintering-forging t181 and hot isostaticpressing (HIP) [l9] Densities over 90%, even 99.3% of theoretical, were reportedly achieved.But as will be discussed in the next section, another serious problem will arise for sampleswith low porosity.1.3.3 Oxygenation BehaviorAccording to Gallagher and othersim-231 , YBa2Cu3Ox will go from non-superconductingto superconducting and to non-superconducting again as oxygen is removed or added to thestructure. This phenomenon is associated with the orthorhombic+4tetragonal phase trans-formation. Those materials with x=6.38 are non-superconducting and tetragonal, and when)07.1 they also are non-superconducting. The best superconductors, which must beorthorhombic, were found with x=6.98. X is a function of both temperature and oxygenpartial pressure.The control of oxygen content in a YBa2Cu3Ox sample is very important. As theoxygen content, x, decreases, critical temperature, T„ decreases as well. This is shown inFigure 8. Both orthorhombic and tetragonal phases show a monotonic decrease in oxygencontent with temperature above 400°C. This has allowed the use of thermogravimetricanalysis (TGA) to determine the x values of YBa 2Cu3Ox samples. It has been demonstrated mthat the weight loss experienced by YBa2Cu30x upon heating is due solely to changes inoxygen stoichiometry. A TGA trace of orthorhombic YBa 2Cu3Ox heated and cooled under1 atmosphere of oxygen is shown in Figure 9. It can be seen that at high temperature thevalue of x is low (about 6.2 at 950°C) and the YBa 2Cu3Ox phase is tetragonal. This indicatesthat the sintering of the YBa2Cu3Ox compound is always performed in the tetragonal region.17Since the samples are totally tetragonal after sintering these samples must be converted tothe orthorhombic phase, which is superconducting at lower temperatures, by oxygenation.From the same figure (TGA trace) it can also be seen that oxygen stoichiometry is completelyreversible. This provides a processing window to regain high oxygen content, which isrequired for good superconductivity, by annealing the samples in an oxygen atmosphere.10080toQ.E 06a)040207 0^6.8^6.6^6.4^62Oxygen content(x)Figure 8 Transition temperature versus oxygen content of YBa 2Cu30„. (211To ensure an oxygen content around 6.98 for the YBa 2Cu30x compound, the materialmust be annealed in an oxygen atmosphere. Annealing temperature is usually below about500°C since the orthorhombic phase is stable until —600°0 22-231 . The diffusion of oxygeninto the structure is limited by the low temperature and a very low diffusion coefficient of10-14 cm2/sec at 500°C. (241 It was found experimentally that a 1 mm thick specimen could1. ^0-^Heating and cooling in oxygen(1 atm)200 400 600 800 100018be completely oxygenated in several days at 600°0'53 otherwise the interior of each grainwas tetragonal, i.e. non-superconducting. For samples of larger dimensions and high density,the time requirement for complete oxygenation would be impractical. An additional com-plication encountered is the finding that the activation energy for oxygen diffusion intoYBa2Cu3Ox is dependent on the oxygen stoichiometry of the material such that the greaterthe oxygen content the lower the oxygen diffusion. Specifically, Tu, et ai.E251 found that theactivation energy was only —0.5eV for a composition YBa 2Cu306.62 and rose to —1.3eV forthe fully oxygenated composition YBa 2Cu3O7. Reoxidation of bulk material is, therefore,believed[24• 263 to require open porosity so that the necessary oxygen diffusion path is shorter.But this is contrary to the higher density requirement to promote the critical current densityJ, and to obtain good mechanical properties.Temperature(C)Figure 9 TGA trace for YBa 2Cu30„ heated and cooled under 1 atmosphere oxygen,showing orthorhombic-tetragonal phase transformation and completelyreversible stoichiometry.191.3.4 Weak Flux PinningOne of the most fundamental properties of superconductors which was discovered byMeissner and Ochsenfeld in 1933 is that magnetic fields do not penetrate into a supercon-ducting sample, i.e. a perfect diamagnetism, if the external magnetic field is lower than thecritical magnetic field (HO of the sample. This is known as the Meissner effect. Since zeroresistivity is, of course, impossible to be measured in an absolute sense, reports of newsuperconductors must meet Meissner effect criterion prior to acceptance by the scientificcommunity Superconductors can be divided into two broad categories: Type I and TypeH. Type I superconductors are generally pure metals which exhibit perfect diamagnetismbelow Most compound or alloy superconductor, including the YBa 2Cu3Ox compound,are Type H superconductors. As illustrated in Figure 10, Type II superconductors allowadditional magnetic flux penetration without losing the ability of carrying supercurrentbetween their lower critical field H (  and upper critical field Ha. When an electrical currentis passed through a Type II superconductor which contains magnetic flux, the current createsa Lorentz force (JxB) which acts on the flux lines. The movement of the flux lines underthe Lorentz force causes dissipation of the energy of the current, resulting in a lower criticalcurrent density The movement of flux lines may be pinned by the presence of impuritiesand defects.TinkhamEni theoretically analyzed the critical currents in high Tc superconductors andargued that high T, materials may have an intrinsic weakness of flux pinning due to a varietyof unfortunate parametric combinations. To further improve the critical current density ofthe YBa2Cu3Ox compound, intragranular pinning is necessary. Oxygen point defects arebelieved to be the most important pinning sites in the YBa 2Cu3Ox compound, and the critical20density is higher in more oxygen-disordered crystals. 12830.1^1^10^100APPLIED FIELD H (T)Figure 10 Plot of magnetization M versus magnetic field H. The values of Ha and Haare typical for high-Tc superconductors.1.4 Approaches to Overcome the ProblemsTo overcome these difficult and contradictory problems, two approaches have beenexplored. One is to develop microstructural textures so that the grains are connected by pre-dominantly low-angle grain boundaries. The methods reported are mechanical alignment underuniaxial pressure such as hot forging, (9291 melt texturing'301 and alignment of YBa2Cu30„ powdersin the presence of a magnetic field. 1311 Another method is to add a certain amount of silver intoYBa2Cu30x. The latter method will be reviewed in detail in the next section.10p201N0.100.010.01Type IH^Type II211.5 Previous Work on Silver Addition to the YBa 2Cu3Ox CompoundBecause silver addition may provide a large number of beneficial effects to YBa 2Cu3Oxit has been extensively studied, especially to improve the mechanical and superconductingproperties of the YBa2Cu3Ox compound. Only noble elements can be used, as other elementsreduce or remove oxygen from the system.1.5.1 Improved Mechanical Properties by Silver AdditionFor most practical applications, both good superconducting (high critical temperature,current density and magnetic field) and mechanical properties (strength and fracturetoughness) are desirable. Previous authors have observed that YBa 2Cu3Ox specimens arevery brittle, with unacceptably low strength. 132-331 This owes not only to the intrinsic natureof ceramics but also to the unavoidable presence of porosity. To overcome this, silver wasemployed as a reinforcement by a number of authors.Mechanical and electrical properties of rectangular bars and thin tapes ofYBa2Cu30x-Ag composites with 10-30 vol% of Ag or Ag 2O were reported by Singh, et al.. [341The addition of 20 vol% Ag resulted in an increase in strength from 40 to 75 MPa and atwofold increase in critical current density from 150 to 300 A/cm2 at 77K in the absence ofan applied magnetic field was observed. They believed that these results were primarilydue to the increased density (p th up to 98%). Silver phase was also believed to relax residualstresses resulting from the expansion anisotropy of grains and to provide increased resistanceto crack propagation by pinning the propagating cracks. The strength was increased furtherto 87 MPa by an addition of 30 vol% Ag, while the elastic modulus was improved from 75to 117 GPa and hardness from 137 to 243 kg/mm2 But the critical current density dropped22to 50 A/cm2 for samples containing 30 vol% Ag whereas the critical temperature wasunaffected. In samples with high silver content, a continuous silver phase was formed. Thiscontinuous, non-superconducting phase may be responsible for the low critical currentdensity in these samples. No difference between Ag and Ag2O additions was found by theseauthors.Nishio, et al. (35) prepared composite superconductors by sintering a mixture ofYBa2Cu3Ox and Ag powders at various proportions (0-95.1 wt%) and were able to enhancethe mechanical strength up to 226 MPa from 42 MPa for sintered pure YBa 2Cu3Ox sampleswithout seriously lowering the critical temperature. It was surprising that the porosity ofthese samples was high (25% for samples without silver and 16% for samples containing25.9 wt% Ag) while still attaining high strengths. They claimed that silver did not reactwith YBa2Cu3Ox at all and reported that for samples containing even 50 vol% silver, fineYBa2Cu3Ox grains formed continuous networks around homogeneously distributed silverparticles, which was contrary to the observation of Singh, et al.A YBa2Cu30„-Ag composite tape with 13 vol% silver was reportedly fabricated bytape casting technique followed by appropriate sintering.i 361 The deformation strain couldbe up to 2% without breaking, as compared with a typical pure YBa 2Cu3Ox material, whichfractured at a strain of —0.1-0.2%. The silver addition did not adversely affect thesuperconducting properties of the composite tapes. Small changes of critical temperaturewere demonstrated and actual change on critical current density was not reported.A most recent report from the University of Houston1381 shows that by the refinementsof grain size and porosity, in addition to the control of silver morphology and distribution,a fracture toughness of 3.6 MPa-m 1/2 could be obtained from YBa2Cu30,/Ag composites for23silver composition near 20%. This is the highest bulk fracture toughness value publishedto date. This value is comparable with many structural ceramic materials. The supercon-ducting properties of these composites are not mentioned in this paper.Although the data about the mechanical properties of silver reinforced YBa 2Cu3Oxcomposites are scattered, the reinforcing effect is not questionable. The introduction of aductile second phase can certainly enhance the material as a result of crack blunting andcrack arrest mechanisms. f371 The problem is how to improve the mechanical properties whilemaintaining good superconducting properties.1.5.2 Enhanced Critical Current Density by Silver AdditionAs mentioned in previous sections, the transport supercurrent density of bulkYBa2Cu30„ is limited by non-superconducting grain boundaries. These grain boundariesare also called weak links. A weak link in a superconductor generally means a junctionbetween two bulk superconductors. The supercurrent carrying capacity across such ajunction is generally lower than that in bulk materials and depends on the material at theinterface.E391 The interfacial material can be an insulator (I), a normal conductor (N) or evena second superconductor (S') which has a lower critical temperature. (391 Thus S/I/S, S/N/Sand S/S' weak links may be formed. In a polycrystalline material, in order to transport acurrent through the bulk of the material, supercurrent must pass through these weak links.Therefore the properties of grain boundaries are of vital importance to the critical currentdensity. Since the maximum critical current is inversely proportional to the normal stateresistance of the S/I/S junction,m and silver can scavenge the impurities at the grainboundaries and thereby reduce the thickness of non-superconducting grains," silver was24introduced into the YBa 2Cu3Ox compound by a number of researchers under the hypothesisthat silver can reduce grain boundary resistance and improve the coupling condition of theweak links at the grain boundaries.It is necessary to clarify that there are two different methods of J c measurements. Oneis the DC four-lead method which will be described in detail in Section 2.5.1. This methodis a direct method and the critical current density measured by this method is called transportJ, Another method is an indirect one, which uses a magnetometer to measure the magne-tization (M) versus Magnetic field (H) hysteresis loop. The critical current density is cal-culated through the Bean model using the following equation: 1421Jc=30AMIdwhere AM is magnetization hysteresis loop width in A/cm and d is the particle dimensionin cm perpendicular to the applied H. Average grain size is usually used as d. The J c valuesfrom this method are called magnetization currents. Usually magnetization currents aremuch higher than transport currents because magnetization currents are not limited by thegrain boundaries. But the magnetization currents may be affected by microcracks and twinwalls within the grains.Attempts of adding silver to enhance the critical current density of the YBa 2Cu30„compound can be divided into two groups according to the starting composition formulasused: YBa2Cu30„Agx and YBa2(Cu3_yAgy)0x. The first one is a YBa2Cu30x+Ag compositeand the second one is a substitutional solid solution.Most researchers prepared samples in the form of composites. The optimum contentof silver for enhancing the critical current density given in published papers are very con-troversial, ranging from 0.8 to 20 wt%. The best transport critical current densities with25silver vary from 215 to 700 A/cm2 at 77K without any applied magnetic field. The bestmagnetization currents reported are from 1.24x104 to 105 A/cm2. These published data arelisted in Table 3, and also data measured by the same authors from the samples withoutsilver (if provided) are also included for comparison.Although almost all authors claimed that critical current densities of YBa 2Cu30x withsilver were enhanced, the data were in apparent contradiction. Overall, most authors agreedthat a small portion of silver would be beneficial to the critical current density. The onlypaper that claimed that silver had no substantial enhancement in critical current density waspublished by Jin, et a/.. E523 They mixed 25 wt% Ag20 powders with YBa2Cu30„ powdersfollowed by normal sintering and annealing processes. They found the magnetization currentwas 4,800 A/cm2 for the composite whereas it was 5,200 A/cm 2 for the sintered samplewithout silver.Some authors [53-56] substituted copper by various amount of silver in the form ofYBa2(Cu3_,,Agy)Ox and got similarly contradictory results. For example, Motsumoto, et al.t551believed that the best critical current density was obtained at y3.3 although they noticedthat silver could not completely substitute copper when y>0.1. Gangopadhyay and Masont 561argued that critical current densities were nearly unchanged for y up to 0.12.Most of the authors stated that silver did not have any reactivity with YBa2Cu30x andno silver was detectable within YBa 2Cu30, grains although they did not provide sufficientevidence about that. In the cases of substitution, the content of silver which truly substitutedat the copper sites was not checked by most of the authors. Only a few authors [56-571 inferredthat there was a certain amount of silver really existing within YBa 2Cu3Ox grains.Weinberger, et a/. (571 examined the microscopic composition inside the grains of26Table 3 Summary of the Data on Critical Current Densities of YBa 2Cu30x-EAg Com-posites and YBa2Cu30. at 77K and Zero Magnetic FieldOptimumAg (wt%)J, (A/cm2) Referencetransport magnet. pure 1230.8 300 150 [43]5 215 105 [44]5 12,400 [37]5 105 [45]7 450 50 [46]10 700 450 [47]-[48]10 52,480 20,000 [49]15 22,000 6,000 [50]20 300 150 [34]10-50 250-350 2 orderslower[51]YBa2Cu307./Ag samples by WDX and found the compositions for YBa2Cu30x/Agol andYBa2Cu30,1Agi .0 samples to be YBa2,02Cu286Ago.0370x and YBamiCu3.09Ago.o500., respec-tively. This result indicates that the silver content within the grains is lower than the nominalcontent. These authors also claimed that the Cu (1), i.e. Cu, was substituted by silver basedon the information that the length of Ag-O bond was found to be 2.0±0.1À from extended27X-ray absorption fine structure (EXAFS) measurements. However, there are severalCu(1)-O and Cu(2)-O bonds ranging from 1.931 to 1.955A in the YBa 2Cu3Ox lattice. Hence,this experimental result is insufficient to support their claim. Recently, Gangopadhyay andMason [561 have made an attempt to determine the silver solubility in YBa 2Cu30„ by usingelectron probe microanalysis (EPMA). The nominal composition of the samples they usedwas YBa2Cu3,Ag„064.3, (x up to 0.75). They also found that the silver content within thegrains is lower than the nominal content. Their EPMA results are shown in Figure 11.However, silver can be either in the form of precipitates or within the YBa2Cu30„ lattice.They just assumed that the silver substituted copper in YBa 2Cu306,3, and did not provideany evidence for it.0.06R.'dc 0.040a)<0a)N>, 0.02criC<wt% ----..-3.2^6.30.00 ^0.00, 1^,0.20 0.40Nominal Ag conc. (X)Figure 11 Electron probe microanalysis (EPMA) Ag contents in 123 vs. doping inYBa2Cu3_„Agx06,y (re-plotted from Gangopadhyay and Masont 56').281.53 Improved Flux Melting Temperatures by Silver AdditionCritical temperature of a superconductor is also limited by a magnetic field (H). Asthe applied magnetic field increases, temperatures lower than TT are required to maintain alower thermodynamic state, i.e., the superconducting state. The variation of the criticalmagnetic field, 1-1, with temperature for T<rc is empirically shown to approximate a parabola.If He(0) is the critical field at absolute zero temperature, a useful approximation of H e attemperature T is:11 2= Hc(0){1 —( )}T,If the applied field is greater than H e(0), it is impossible to produce a superconducting stateeven at absolute zero temperature. As the applied magnetic field increases, temperatureslower than Te are required to maintain a lower thermodynamic state, i.e., the superconductingstate. A critical temperature under a specific magnetic field lower than He(0) is called theflux melting temperature (or flux depinning temperature), T e(H), at this magnetic field.The influence of silver addition to the YBa2Cu3Ox compound on fluxline dynamicswas studied by Sayer, et al.. 158  They reported that the flux melting temperatures ofYBa2Cu30„•Agy samples were substantially improved for y=0.38. Their results are sum-marized in Table 4.29Table 4 Flux Melting Temperatures of YBa 2Cu30x-Agy Samples 1581Magneticfield(Tesla)Drivecurrent(mA)Flux melting temp. (K)Y=0^y=0.38^y=0.771 0.2 74 83 83.51.0 74 80 802 0.2 71 811.0 71 783 0.2 70 801.0 70 781.5.4 Other Studies on Silver Addition to YBa2Cu3OxThe effect of silver addition on flux pinning was observed by some researchers [45-51 '591 from the same group headed by M.K. Wu, who discovered the first 123 sample, in theirmagnetic studies of YBa2Cu30x-Ag composites. Large magnetization and hysteresis werefound to be present in samples containing 2.2, 3.1 and 5.1 wt% silver.E 451 This was believedto be the main evidence of the existence of strong pinning forces. But such a strong pinningforce was not observed in the sample containing 14% silver by the same authors because itcontained too much silver. Similar results were also reported by Huang, et al.. 1591Another observation found in silver-containing YBa2Cu3Ox samples is the significantgrain growth promoted by the presence of silver. Although the microstructure of a ceramicmaterial depends on the processing conditions, the silver-promoted grain growth wascommonly reported by a number of authors who used various processing routes and differentamounts of silver additions. 159-663 The promotion of the grain growth results from the30increased amount of liquid phase present at high temperature. It has been reported that silveraddition causes partial melting of the YBa2Cu3Ox phase above 931°C where the silver leachescopper from YBa2Cu3Ox resulting in its decomposition. f671It has also been reported that silver addition improves normal state resistance (49. 51. 62'64.681 and resistance to water144.361 , and silver acts as an internal oxygen donorf 691 and a fasterdiffusion path for oxygen. 168,70jIn summary, the role of silver addition to YBa2Cu3Ox compound is multifold.1) Silver can improve the mechanical properties of YBa 2Cu30x by filling the poresand voids among YBa2Cu30 7, grains and forming a dense composite .2) Silver may reduce the resistance of the junctions, i.e. grain boundaries, betweensuperconducting grains to enhance J, since the maximum in a weak link isinversely proportional to the junction resistance. Silver can also scavenge theimpurities at the grain boundaries and therefore reduce the thickness of non-superconducting regions.3) The raw material of Ag2O for silver addition can act as an internal oxidant tosupply the necessary oxygen to form the orthorhombic phase. Silver can alsoprovide a faster diffusion route of oxygen as well as act as a catalyst to promotethe low temperature decomposition of the barium carbonate species added as rawmaterial for synthesis of YBa2Cu3Ox .4) Dissolved and precipitated silver within YBa2Cu3Ox grains can pin magnetic fluxlines and therefore increase J c. It may also increase flux melting temperature.311.6 Objective of Present StudySilver additions to the YBa 2Cu3Ox compound have already been extensively studied. Thedata published in the improvement of the mechanical properties of YBa 2Cu3Ox compounds areconvincing because the reinforcing effects of silver in YBa2Cu3Ox compounds are primarilyintergranular. However, published data on silver enhanced electrical and magnetic propertiesof YBa2Cu3Ox compounds are in an apparent contradiction. Since the electrical and magneticproperties can be affected by the presence of both intergranularly and intragranularly silver, thefact that the contradiction is existing may suggest that not only the quantities, but also thedistribution of silver may have a significant effect on the properties of bulk YBa 2Cu3Ox com-pounds. Published papers predominantly assert that silver does not have any reaction withYBa2Cu3Ox compounds and all silver added is intergranular. Little attention has been paid tothe interaction between the YBa2Cu3Ox compound and silver at the molecular level, and itseffect on the intragranular and intergranular properties. The questions of whether silver maygo into the YBa2Cu3Ox lattice, to what extent silver may enter and which site silver may occupyare still unclear. Moreover, because silver has been widely employed as the cladding materialin the fabrication of YBa2Cu3Ox wire, tape, etc, it is of extreme importance to know the reactivitybetween the YBa2Cu30x compound and silver, and the solid solubility of silver in the YBa 2Cu3O xcompound.To shed some light on these issues, attempts were made to study the solid solubility ofsilver in the YBa2Cu3Ox compound. This is carried out by studying a number of silver dopedYBa2Cu3Ox samples using X-ray diffraction (XRD), optical microscopy, scanning electronmicroscopy (SEM), energy dispersive X-ray spectroscopy (EDX), wavelength dispersive X-rayspectroscopy (WDX) and secondary ion mass spectrometry (SIMS). In addition, precise lattice32parameter determination of YBa2Cu3Ox samples with and without silver may allow the identi-fication of sites within the unit cell, which may be occupied by silver ions. And also, the criticalcurrent density (Jo), normal state resistance and the critical temperature (T) of thesilver-containing YBa2Cu30„ samples are measured.332 EXPERIMENTAL PROCEDURE2.1 Sample PreparationIn this study a number of samples were prepared with nominal silver content from 0 to10 % which was added using two separate routes-wet and dry mixing. All silver concentrationsreferred to in this thesis, unless otherwise noted, are in weight percent. Two batches of thesamples were made for the purpose of testing the reproducibility of experimental data. Acommercial powder of YBa 2Cu307 (particle sizes: 211m - provided by SSC, Inc., Seattle,U.S.A.) was used as the raw material. A silver nitrate (AgNO 3) solution and a silver oxide(Ag20) powder were used as the starting materials of silver addition for wet and dry mixing,respectively.The wet mixing method was employed to produce samples with nominal silver concen-trations of 0.5, 1.0, 1.5 and 2.5%. An appropriate quantity of analytical grade AgNO 3 crystalwas dried in air at 120°C for two hours, weighed and dissolved completely in a small amountof distilled water for each nominal silver concentration. The aqueous solutions were then dilutedby an appropriate amount of denatured alcohol. These solutions were mixed thoroughly withthe YBa2Cu30„ commercial powder by manual stirring to form homogeneous slurries. Theslurries obtained were put into a dryer at 120°C. The dried powder was pulverized and regroundby pestle and mortar.In the sample preparation route of dry mixing, a silver oxide powder was added to thesamples in the nominal concentration of 5 and 10%. Raw powders of Ag 20 and YBa2Cu3O xwere dry mixed in a vibratory mixer for 20 minutes. The mixed powders were ground manuallyand repeatedly. The final powders from both wet and dry routes were subsequently pelletized34into discs under an uniaxial force about 300 MPa. The pellets were about 9.5 mm in diameterand 1g in weight. These pellets were sintered in air at 950°C for 24 hours followed by anannealing process at 500°C in pure 0 2 for the same amount of time for the purpose of reoxy-genation. After the annealing process, samples were furnace cooled to room temperature.In order to study the silver solubility at higher temperature, one sample of each of the Agnominal concentrations was sintered in a suspended crucible in a vertical tube furnace andquenched by dropping it into liquid nitrogen immediately from 950°C after sintering. The flowchart of the sample processing procedure is illustrated in Figure 12.The commercial powder used in this study was produced by decomposition of organo-metallics and contains a significant amount of residual carbon ini . This carbon contaminationcould deteriorate the transport properties of YBa2Cu30„ by reducing the oxygen content in areasof high carbon to the point where the material may be locally non-superconducting. To overcomethis problem, raw YBa 2Cu30x powders were synthesized using Y203, BaCO3 and CuO powdersas precursors and calcining. The samples processed through this route were used for electricalproperty measurements. 35silver nitrate solution commercial 123 powder^ silver oxide/ \silver nitrate/123 slurry1drying at 120Cpowder mixingpulverizing/grindingdry pressing 30GPasintering in air 950C/24hzannealing in oxygen 500C/24hifurnace cooling to RT^ quenching to liquid nitrogenFigure 12 Flow chart of the processing steps for sample preparation.362.2 Scanning Electron MicroscopyA scanning electron microscope (SEM) with both energy dispersive X-ray spectroscopic(EDX) and wavelength dispersive X-ray spectroscopic (WDX) attachment was used to examinesilver doped YBa2Cu3Ox samples. The SEM scans the surface of a sample with a beam ofelectrons which produces secondary electrons, backscattered electrons, X-ray photons, etc. fromthe surface layer of the sample which is detected. From the detected secondary electron densityan image of the surface can be generated. In this investigation, the backscattered electronimaging technique was also used to enhance the contrast between YBa 2Cu30„ and silver phases.X-ray signals generated by the bombardment of the sample with incident electrons were usedto identify the silver phase and determine the quantity of silver dissolved and/or precipitatedwithin YBa2Cu3Ox grains. EDX is suitable for a quick qualitative examination of the sampleswhereas WDX is more sensitive and can do the quantitative analysis more precisely. Thesensitivity of WDX varies element by element, and depends on the operating conditions andthe standard chosen. X-ray emission obeys the statistical rules of random processes. In thisstudy the detection sensitivity for silver was estimated at about 0.05% by using the followingequation r751 :34--B-ci .c ^st P—Bwhere^CI :^detection sensitivity (in mass%);Cxt:^concentration of analyzed element in a standard (in mass%);P:^total peak counts;B:^total background counts.37SEM samples were polished by diamond powders down to 1p.m and carbon-coated. PureY, Cu, and Ag metals, and a BaSO4 crystal were employed as the standards for quantitativeanalysis by wavelength dispersive X-ray spectroscopy. The SEM equipment with the WDXdetector was operated at 20kV. The counting time was 10 seconds. In order for the X-rayemitting efficiency of silver to be high enough and detectable, a constant beam current of 40nA, which was the standard beam current for analyzing silver, was used. The diameter of theincident beam was about 0.3 gm. But the area where the X-ray signal was generated was larger.Castaine suggested that the spot size can be estimated by following equation:S(.1111) = 0.033(V"7 — pZwhere V: accelerating voltage (kV);Vc: critical exciting voltage (kV);A: atomic weight (g/mol);Z: atomic number;p: density (g/cm3).Since A/Z is approximately constant, from the Monte Carlo calculation of Bishop, [731 ReedL74lgave the following general expression for deducing the lateral extent of the X-ray distribution:0.077 S(pn) —^(17L5 — .5)pIn this experiment, V was 20 kV and Vc was 3.8 kV for silver La,. Assuming x=6.9 for theYBa2Cu30„ compound and taking the lattice parameters obtained from X-ray diffraction in thisstudy, the theoretical density (p) of YBa 2Cu3Ox was estimated to be 6.34 g/cm3 . By substituting38these numbers into Reed's equation, the resulting S was about 1 gm. This was small enoughcompared to the width of elongated YBa2Cu3Ox grains, which was about 5 to 10 gm, so that thegrain boundaries where the silver segregated would not be stricken by the bombardment ofincident electrons during the quantitative analysis. For the data of quantitative analysis by WDXto be statistically reliable, at least 25 grains of various morphologies were analyzed from eachsample. Computerized ZAF corrections were applied to the concentrations of Y, Ba, Cu andAg in the area analyzed.2.3 Secondary Ion Mass SpectroscopySecondary ion mass spectroscopy (SIMS) is mainly a surface analytical technique whichwas used in this study to detect the existence of silver within the YBa 2Cu30, grains. Sputteringis the physical process on which SIMS analysis is based. A beam from an ion gun is directedto a point on the sample surface. The high energy ions strike the surface causing atoms andions of the surface material to be ejected from the surface. These secondary ions are collectedand passed through a mass spectrometer which separates the ions of different mass numbers.The number of ions at each mass number are measured over a given time period. The massnumber identifies the ion, and the measured number of ions is related to the concentration ofthat element in the sample.In this investigation, the SIMS data was collected by using a V.G. SCIENTIFIC LTD.QUADRUPOLE MASS SPECTROMETER mounted on a Micro Lab MK II chamber. Thesystem was fitted with a gallium liquid metal ion gun (LMIG), with an angle of incidence tothe sample of 50° off normal and an MM12-12 quadrupole mass spectrometer with one atomicmass unit (a.m.u.) resolution. The primary ion used was Ge and 89y  used as the internalstandard. The instrument was operated with an accelerating voltage of 10 kV and a beam current39of 2 nA. The mass span was set from 106 to 112 and the scan time was 40 minutes. For thesame reason given in the WDX analysis, the magnification was set at 20,000 to avoid the grainboundaries being hit by primary ions. Because SIMS is mainly a surface analytical techniqueand the current interest in this research was in the bulk solubility of silver within YBa 2Cu30„grains, the surface of each sample was sputtered off before the sample was used for detection.2.4 Precise Determination of Lattice ParametersThe lattice parameters of the samples were determined by X-ray powder diffraction atroom temperature on a Philips diffractometer (type 1011/80). Copper radiation filtered withnickel was used at the setting of 36 kV and 30 mA. The divergent, scattering and receiving slitsused were 1, 0.1 and 1 degree, respectively. The wavelength used for calculation was X=1.5405A.Diffraction spectra were scanned between 20 of 14° and 70° at the speed of 0.25 degree perminute using the maximum time constant. The diffraction spectra were recorded on a chartrecorder and the chart speed was set at 10 mm per minute. The combination of slow scanningand fast chart speeds set here was to improve the resolution of the 20 values measured from thediffraction peaks.By differentiating the Bragg Law, 2dsin0 =X., with respect to 0, the following equation isobtained.Ad/d=-A0.cot0As shown in Figure 13, since cot0 approaches zero as 0 approaches 90°, the calculatedlattice parameters from the various lines on the diffraction pattern approach the true value moreclosely as 20 increases. Unfortunately, for the YBa 2Cu30„ compound the line with the highest20 and known index is (026) for which 20 is —68.8°. This was the reason why the scanning wasstopped at 20=70°.A SINGA SIN 015^30^45^60^90AO^e (degree) AO40Figure 13 O-sing relationshipThe main systematic errors in the measurement of interplanar spacing, d, by a diffrac-tometer are the following:1) Displacement of the sample from the diffractometer axis;2) Misalignment of the instrument;3) Use of a flat sample instead of a sample curved to conform to the focusingcircle.These systematic errors can be minimized by using an internal standard material forcalibration. The selection of an internal standard is normally based on the following consid-erations:1) Its d values are well known and reliable;412) Its peaks do not overlap or interfere with the peaks of the sample to bemeasured;3) It does not react significantly with the sample at the experimental temperature.The standards typically employed in X-ray diffraction are silicon, rocksalt, diamond,quartz, fluorophlogopite, etc. cm] In this study, an analytical grade and crystallized NaCI (rocksalt)powder was chosen as the internal standard because it was of the least overlapping with thesamples under analysis. NaC1 powder was mixed with YBa 2Cu3Ox powder by the weight ratioof 1:3 in the experiments.In addition to the systematic errors mentioned above, extra caution was needed for theslow scanning and fast chart speeds which were used in the experiments. It was found duringthe experiments that the scale on the chart paper sometimes deviated away from its nominalscale. The deviation was very small, for example, 0.5 mm short for a length of 300 mm (<0.2%),which might be due to the uneven shrinkage of the paper during manufacturing. But in thisstudy, over 2 meters of the chart paper was used to record the diffraction spectrum in eachexperiment. Hence the cumulative error from it could not be ignored in order to get highlyprecise lattice parameters. In each X-ray experiment a calibration curve was constructed byplotting 0(20), the peak shift from its theoretical value, against 20 of NaCl lines measured fromthe experimental spectrum. The curves were used to correct the peak positions, i.e. 20 values,of the YBa2Cu30„ phase. It was found that these curves were not always horizontal. Two ofthem are shown in Figure 14 on the following page.-0.06ca) -0.09cv-0.12-0.15-0.18■ 1 23+0.5%Ag run #2• 123+1.0%Ag run #1420.00-0.2110^20^30^40^50^60^7020Figure 14 Calibration curves for 20 measurements of X-ray diffraction-0.030.00 cv-‹)-0.03-0.06-0.09-0.1280Because of the use of slow scanning and fast chart speeds diffraction peaks were broad-ened. For the diffractometer method, there are several factors (e.g., absorption of X-ray by thesample and vertical divergence of the X-ray beam) which affect the peak profile asymmetrically.Due to this inherent nature, broadened peaks are usually obviously asymmetrical. The methodsusually used to determine the positions of these broadened and/or asymmetrical peaks are f76-793 :1) Use of dl/d0=0 (0 1 );2) Extrapolating the linear portions of a peak and taking the intersection pointas the peak position (02);Dotted line: centroid lineDashed line: parabola0,\0 5 0, 0 30 243Figure 15 Methods to determine X-ray diffraction peak positions3) Extrapolating the center at 1/2, 3/4, 7/8,^ peak heights and taking theintersection point of the extrapolation line and the peak profile as the peakposition (03);4) Use of centroids (04);5) Constructing and refining a parabolic equation with a vertical axis by usingseveral points on the profile of a peak, and taking the axis of the parabola asthe peak position (0 5);6) Use of peak-hunting software.44The methods 1-5 are illustrated in Figure 15. Mathematically, method 4 is the most precise.Since most of the JCPDS powder diffraction cards were established by using method 3n,method 3 was employed in this study.By taking a combination of the above experimental routines, an accuracy of ±0.01° formeasuring 20 was reached. A USGS computer program (courtesy of the Department ofGeological Science, UBC) using an iterative least square solution was then employed to calculatethe lattice parameters and their standard deviations from the measured 20 values. After 9 cyclesof iteration, the final values were normally accurate to within 4 decimal places for latticeparameters a and b, and 3 decimal places for lattice parameter c.2.5 Electrical Property Measurements2.5.1 T, MeasurementThe critical temperature (Tc) of the samples were determined by a standard DCfour-lead technique measuring the resistance as a function of temperature. The apparatusused is shown in Figure 16. Two leads touching a cylindrical sample (-49.5x3 mm) carrya known constant DC current, 10 mA here, into and out of the samples, and the other twoleads measure the potential drop between two equipotential surfaces resulting from thecurrent flow. Electrical contacts were prepared with a silver paint. A diode temperaturesensor was used to measure the temperature of the sample. The sample was spring-loadconnected to a superconductor resistance bridge (Department of Physics, UBC).The measurements were done in the temperature range between room temperatureand liquid nitrogen temperature. During measurements, a quartz tube containing the samplewas gradually lowered into a liquid nitrogen bath. The quartz tube was also filled withARTTop viewThermalsensorNSuperconductorresistancebridgeSample111111110*A3110Silver electrodesChart recorder00 o 0 Di45nitrogen in order to obtain good heat transfer and keep the sample away from water to ensuregood electrical contact between leads and electrical contacts. Both temperature andresistance were recorded by a chart recorder (Goerz Metrawatt SE780). The chart recorderwas zeroed with respect to the temperature and resistance before each experiment. Theaccuracy of temperature (AT) was ±0.5 K.Figure 16 Schematic diagram of the Sample and apparatus for Tc measurements2.5.2 Normal State ResistanceQuality of superconductors can also be determined by their normal state electricalproperties f463 . To describe electrical properties independently of the sample shape, resistivity(p) is normally used. In this study, because the samples were not perfectly cylindrical, a46normalized resistance, which is the ratio of the resistances at 295K and 100K (R 295K/RlooK),was used instead of the resistivity ratio. Data were taken from the extrapolation of the linearparts of the curves obtained in 'r e measurements, and the resistance data obtained directlyfrom the plots were also used.2.5.3 J, MeasurementFigure 17 The geometry of the sample used for Jc measurementCritical current density (Jo) measurements were performed at a temperature of 4.2Kusing liquid helium as the cryogen by the Department of Physics, UBC. The four-leadtechnique was also employed here. Rectangular samples (-10x5x1 mm) were used so thatthe dimensions of the samples could be measured more precisely. The measurements wereperformed without a magnetic field. The geometry of the sample used for J c measurementsis shown in Figure 17. A voltage versus current was obtained in each experiment. Thecurrent value at 111V/cm was used to compute the critical current density, J„ of the samplemeasured.473 RESULTS3.1 X-ray Diffraction Phase IdentificationSintered pellets with various compositions and post-sintering heat treatment conditions(annealing or quenching) were reground into powders separately. These powders were char-acterized qualitatively with respect to the phases present by a Phillips X-ray powder diffrac-tometer operated at 36 kV/20 mA. A normal scanning speed of 20=1° per minute was used.Room temperature X-ray diffraction spectra between 20 values of 14 and 70° for annealedsamples, and 14 and 60° for quenched samples were obtained. All samples were tested underidentical experimental conditions. The major phase was identified as orthorhombic YBa 2Cu3Oxin annealed samples whereas that of quenched samples was tetragonal YBa 2Cu3Ox which has alower x value. Some typical X-ray diffraction spectra which were reproduced are shown inFigures. 18-25. A trace amount of Y 2BaCuO5 phase, which was conventionally called 211 orgreen phase (due to its color), was detected in all samples containing silver. But this greenphase was not found in pure YBa2Cu3Ox samples. Metallic silver phase was not found in samplescontaining 1% or less silver in annealed samples. Silver phase was difficult to detect in quenchedsamples because the strongest peak of silver, (111), is at 20=38.12° which overlaps the (005)peak of tetragonal YBa2Cu3Ox at 38.20°. It seemed that the apparent solid solubility of silverin YBa2Cu30x was between 1 and 1.5%. But this may be an overestimation since it is close tothe detection limit, which is 1 to 2% for most materials, [781 by the X-ray diffraction method.Comparing the X-ray diffraction spectra, one feature can be noted. The diffractionintensities of the orthorhombic YBa2Cu3Ox phase of silver doped samples are always apparentlylower than that of pure YBa2Cu3Ox sample even if the amount of silver dopant is as small asPure orthorhombic 123Slowly cooled014 16 20 24 28 32 36 40 44 48 52 56 60 64 68 7020Figure 18 Diffraction pattern of pure orthorhombic YBa 2Cu301 0.5% Ag^ 211Slowly cooled000^1.^I^. 14 16 20 24 28 32 36 40 44 48 52 56 60 64 68 7020Figure 19 Diffraction pattern of orthorhombic YBa 2Cu30„+0.5%Ag481008060402010080604020• 2111.5% AgSlowly cooled^ • Ag00I , I^.^I10080 491.0% Ag^ • 211Slowly cooled60= 4020O ^14 16 20 24 28 32 36 40 44 48 52 56 60 64 68 7010080604002eFigure 20 Diffraction pattern of orthorhombic YBa2Cu30x+1.0%Ag14 16 20 24 28 32 36 40 44 48 52 56 60 64 68 7020Figure 21 Diffraction pattern of orthorhombic YBa 2Cu30x+1.5%AgPure tetragonal 123Quenched16 20 24 28 32 36 40 44 48 52 56 60 64 68 7020Figure 22 Diffraction pattern of orthorhombic YBa2Cu300-2.5%Ag50100806024020O^I . 14^16 20 24 28 32 36 40 44 48 52 56 602 0Figure 23 Diffraction pattern of pure tetragonal YBa2Cu3Ox10% AgQuenched• 2111 0080608• 40 511.5% AgQuenched• 211200 ^.^I^I^ I^.^I^.^I^I^.^. 14^16 20 24 28 32 36 40 44 48 52 56 6020Figure 24 Diffraction pattern of tetragonal YBa2Cu30x+1.5%Ag14^16 20 24 28 32 36 40 44 48 52 56 6020Figure 25 Diffraction pattern of tetragonal YBa 2Cu30„+10%Ag100806040200520.5%. By assuming that a linear relationship between the amount of phase present and thediffraction intensity of the phase exists and ignoring the presence of Y2BaCu0s, the normalizedheights of the strongest peak of orthorhombic YBa2Cu3Ox were obtained after deducting thecorresponding background counts. The peak is a combination of (103) and (110) lines. Theresults are listed in Table 5. Although it is difficult to make a quantitative judgement here, thedifference is obvious. There are several factors, e.g. absorption and scattering of X-rays, whichaffect the diffraction intensity for the diffractometer method. The most sensitive one is thestructural factor. Detailed analysis of an absolute diffraction intensity is very laborious withoutthe help of a computer software due to the complex crystal structure of YBa 2Cu3Ox. No suchan attempt has been made in this study. But the change of the diffraction intensity of YBa 2Cu3Ox ,at least, provided the information that the crystal structure of YBa 2Cu3Ox had been changed,even if minutely, after the introduction of silver. It was possible that a YBa 2(Cu, Ag)30x solidsolution had been formed.Table 5 Intensities of the Strongest Peak of Orthorhombic YBa 2Cu3Ox Phase in Ag+Y-Ba2Cu3Ox SamplesNominal Ag content(%) 0 0.5 1.0 1.5 2.5 5.0 10Normalized heights of thestrongest peak of orthorhom-bic YBa2Cu30x(%)  100 76 73 82 82 78 79533.2 Microstructure Studies3.2.1 Optical Metallographic ExaminationBoth annealed and quenched samples were polished and viewed in a reflected polarizedoptical microscope. Optical micrographs of samples with various nominal silver concen-trations and heat treatment conditions are shown in Figure 26a to 26h.In these pictures YBa 2Cu30„ grains exhibit needle-like shapes. Following ASTMStandard E112-80, Estimating the Average Grain Size of Metal, no attempt was made todetermine the average grain size of this type of elongated grain. For most YBa2Cu30„ grains,the thickness was in a range between 5 and 10 gm and the aspect ratio was about 4 to 6.Liquid phase, which appears in bright contrast, can be easily seen at grain boundaries. Inslowly cooled and annealed samples, fine twin structures can be clearly seen withinYBa2Cu3Ox grains because these grains underwent the tetragonal-orthorhombic phasetransformation around 650°C during cooling. Twin structure is the distinct feature of theorthorhombic phase. YBa2Cu30„ material is relatively weak compared to other ceramicmaterials. The samples tend to crumble on polishing, leading to a poor finish. Thereforesome irregular voids can been seen in these micrographs because of pull-out of grains whilepores left behind during sintering are a more spherical shape. Similar to the X-ray diffractionexamination, no silver phase can be readily seen for samples containing 1.5% silver or less.Figure 26a Pure YBa2Cu30x, annealed (bar=2011m).Figure 26b YBa2Cu30x+0.5%Ag, annealed (bar, 26c YBa2Cu30x+1.0%Ag, annealed (bar=20pm).Figure 26d YBa2Cu30„+1.5%Ag, annealed (bar=201.1m).Figure 26e YBa2Cu30x+1.5%Ag, quenched (bar=20gin).Figure 26f YBa2Cu30x+2.5%Ag, annealed ( 26g YBa2Cu30„+5%Ag, annealed (bar=20pm).Figure 26h YBa2Cu30x+10%Ag, quenched (bar=2Own).57583.2.2 Electron Metallographic ExaminationSince 1.5% would be below the detectability of the optical metallographic method,these samples were coated with a thin layer of carbon and observed under a scanning electronmicroscope. The accelerating voltage used was 20 kV. To produce a more striking contrastbetween silver and other phases, the backscattered electron imaging (BEI) method wasemployed. Selected pictures obtained from the electron metallographic observation areshown in Figure 27a to 27e.Figure 27a YBa2Cu30„+0.5%Ag, annealed (bar=20iim).Figure 27b YBa2Cu3014-1.0%Ag, quenched (bar=20pm).59Figure 27c YBa2Cu30,(+2.5%Ag, annealed (bar=20pm).-410,0""•1 /r, t\nk ,■4‘•.^ •^- -^"P;Figure 27d YBa2Cu3Q+5.0%Ag, annealed ( 27e YBa2Cu30x+10%Ag, quenched (bar=201.un).61It can be evidently seen from these BEI pictures that bright particles are present atgrain boundaries and triple grain junctions in all samples containing silver. Energy dispersiveX-ray spectroscopic (EDX) analyses revealed that these particles were pure silver particles.An EDX spectrum from one of the particles is given in Figure 28. For samples with highernominal silver concentrations, silver grains are of comparable grain size to that of YBa 2Cu30 ),grains. The fact that a significant number of silver particles was detected in samples con-taining even only 0.5% nominal silver suggests that the solid solubility of silver inYBa2Cu30„, if any, is very low.5000400030004E'=0(..)2000100000.000^2.000^4.000^6.000^8.000^10.000keVFigure 28 Energy dispersive X-ray spectrum taken from a bright particle at the grainboundary of a quenched YBa2Cu30„ sample containing 1% Ag.62A notable discovery is that a few tiny silver particles were spotted within YBa 2Cu3Oxgrains in annealed samples at a higher magnification. One of them is shown in Figure 29.These particles may be the silver which precipitated within the grains during cooling. Thisindicated that the solubility would be higher at higher temperatures.Figure 29 Scanning electron micrograph of an annealed YBa 2Cu30x+5%Ag sample. Thewhite spot marked AG is a silver precipitate detected by EDX (bar=11.1m).633.3 Secondary Ion Mass Spectroscopic StudyTo confirm the existence of silver within YBa2Cu30„ grains, a secondary ion massspectrometer was employed using 89Y+ as the internal standard. Since SIMS is not a quantitativetechnique as the ion yield is strongly affected by the matrix,t 809 the SIMS examination was apreliminary study. Selected samples were examined and the spectra were digitized andreproduced. Every precaution was taken to avoid the grain boundary. Only large grains wereanalyzed for detecting the silver within the grain. Some reproduced positive SIMS spectra areshown in Figure 30. As seen from the spectra, both 167Agl. and "AS' isotopic peaks are presentin the samples under examination. These results confirmed that silver was definitely presentwithin YBa2Cu30. grains.3.4 Quantitative WDX AnalysisQuantitative wavelength dispersive X-ray spectroscopic (WDX) analyses were carriedout on both annealed and quenched samples to probe the extent that silver goes into YBa 2Cu3Oxgrains. A large number of YBa2Cu30„ grains with different morphologies were analyzed. Theselection of grains was random. The results are listed in Table 6. In addition, Y, Ba and Cu inall YBa2Cu3Ox grains were analyzed and found to be in the atomic ratio of 1:2:3. Deviationsfrom the ratio were never exceeded by more than 10%.The intragranular silver concentrations detected by WDX against the nominal silverconcentrations are plotted in Figure 31. By assuming Cu is substitued by Ag and x=7, weightpercent of the intragranular concentration is converted to they of YBa2Cu3.1Agy0x. The y-valuescale is also included in the WDX plot. An unexpected feature of the WDX results is that nomatter how low the original silver concentration is, only a small amount of silver can go into10% Ag(quenched)108^109^110^111106Atomic mass unit64Figure 30 Positive SIMS spectra of YBa2Cu 3Ox samples containing silver.65YBa2Cu30„ grains. The intragranular concentration of silver increases linearly with the increaseof the nominal concentration but reaches a saturation level after the nominal concentration isover 2.5%. As predicted by electron microscopic observations, at the same nominal concen-tration level, quenched samples always have higher intragranular concentration of silver thanthe annealed samples. The saturation suggests that the amount of silver that can be dissolvedin YBa2Cu3Ox has a maximum value. Neglecting the precipitation of silver, this value wouldbe the silver solubility in YBa2Cu30x. The room temperature and 950°C solubilities are estimatedto be 0.37% and 0.42%, respectively (as can be seen from the figure).Table 6 Intragranular Silver Concentrations Obtained from WDX AnalysesNominal Intragranular Ag(%)Ag (%) Annealed samples  QuenchedsamplesBatch #1 Batch #2 In-house made powders0.5 0.12(4) 0.14(3) 0.15(5) 0.15(2)1.0 0.15(3) 0.16(2) 0.17(4) 0.20(3)1.5 0.26(3) 0.32(4) 0.28(3) 0.28(4)2.5 0.32(5) 0.30(4) 0.35(6) 0.42(5)5.0 0.37(4) 0.41(5) 0.39(4) 0.41(4)10 0.37(6) 0.35(4) 0.40(10) 0.42(5)Note: The bracketed numbers indicate the standard deviations of the last decimal.0.0300.0250.0200.0150.0100.005660.6zg 0.5"E'a)c.)c8 0.3CDas= 0.2cEcmasc 0.100.0000 2.0 4.0 6.0^8.0 10.0 12.0Annealed SamplesBatch #1Batch #2In-house synthesizedpowder0.500.400.300.200.100^1^2^3^4^5^6^7^8^9^10^11Nominal Ag concentration(wt.%)Figure 31 Intragranular vs nominal silver concentrationsNominal silver concentration(wt%)Figure 32 Reproducibility of WDX data for determining the intragranular silver concentra-tions67To check the reproducibility of WDX data, another batch of annealed samples was pre-pared and analyzed. Nearly the same results were yielded as shown in Figure 32. Next, aseparate set of samples containing 0.5, 1.5 and 5% nominal silver was heat-treated two dayseach at 950, 800, 600, 400 and 200°C. The WDX results from these samples did not producea statistical difference from those processed with normal annealing procedures. Uncertaintiesresulting from kinetic problems could, therefore, be eliminated.3.5 Precise Determination of Lattice ParametersAs stated earlier, silver in a YBa2Cu3Ox grain could be in solid solution as well as in theform of precipitates. The size of these possible precipitates could be very tiny and thereforeinvisible in a scanning electron microscope. The formation of a solid solution between silverand YBa2Cu3Ox, possibly in a substitutional type of YBa 2(Cu, Ag)30„, will change the dimen-sions of the unit cell because size of silver ion is much larger than that of copper ion. To ascertainif the silver within the YBa2Cu3Ox phase is really incorporated within the lattice, experimentswere performed to determine lattice parameters of the orthorhombic YBa 2Cu3Ox phase by themethod detailed in section 2.4. As mentioned in section 2.1, two batches of the samples weremade for the purpose of testing the reproducibility of experimental data. Powder samples fromboth batches were tested and the data were averaged. Some selected 29 values measured fromthe experiments and used for the calculations of the lattice parameters are listed in the Appendix.The resulting lattice parameters are listed in Table 7. The bracketed numbers indicate thestandard deviations of the last digit.For the orthorhombic YBa2Cu3Ox phase in annealed samples, plots of lattice parametersversus nominal silver concentrations are shown in Figures 33, 34 and 35. It can be seen thatlattice parameters a and c increased with the nominal silver concentration and appeared to reach68constant values at high silver concentrations. Also, as shown in Figure 36, the unit cell volumecalculated from the lattice parameters of the orthorhombic YBa2Cu30„ increased with the silverconcentration. These results are in agreement with the WDX results indicating that copper ionswere partially substituted by silver ions and the substitution has a upper limit level. The latticeparameter b remained relatively unchanged within the statistical deviations of the measurements.The information provided by the lattice parameter measurements implied that the dissolvedsilver ions occupied some specific crystallographic positions in the lattice and modified slightlythe crystallographic parameters of the lattice in such a way that only the a and c parameters aresignificantly altered but not the b parameter. A possible explanation is offered in the discussionsection.Table 7 Lattice Parameters of Orthorhombic YBa 2Cu30 Phase from X-ray DiffractionNominal Ag(%) a(A) b(A) c(A) V(A3)0 3.8195(5) 3.8894(5) 11.670(1) 173.36(6)0.5 3.8225(4) 3.8892(4) 11.676(1) 173.58(5)1.0 3.8229(8) 3.8894(8) 11.676(2) 173.61(9)1.5 3.8231(5) 3.8901(5) 11.681(1) 173.72(6)2.5 3.8227(7) 3.8904(7) 11.679(1) 173.69(6)5.0 3.8240(10) 3.8900(10) 11.681(3) 173.76(13)10 3.8247(8) 3.8888(8) 11.681(2) 173.74(10)3.8273.826 -3.825-iris  3.824Ea) 3.823 -a 3.822 -a)3.8213.8203.819 am-3.818 ^0 0 2.0^4.0^6.0^8.0^10.0Nominal silver concentration(wt%)12.0Figure 33 Lattice constant a of orthorhombic YBa 2Cu301 vs nominal silver concentration11.69011.685 -.."6- 11.680 -'a5a)11.675caa.11.670ca—J11.665 -11.660^I^I^I^.^I^I0 0 2.0 4.0 6.0 8.0 10.0^12.0Nominal silver concentration(wt%)69Figure 34 Lattice constant c of orthorhombic YBa2Cu30„ vs nominal silver concentration3.889510.0 12.000^2.0^4.0^6.0^8.0Nominal silver concentration(wt%)Figure 35 Lattice constant b of orthorhombic YBa 2Cu30„ vs nominal silver concentration0 0 2.0^4.0^6.0^8.0Nominal silver concentration(wt%)Figure 36 Unit cell volume of orthorhombic YBa2Cu3Ox vs nominal silver concentration10.0 12.0•173.80173.70735 173.600a>c.) 173.50173.40173.303.9003.895■••cts 3.890cts3.8853.88070713.6 Electrical Property MeasurementsAs mentioned in section 2.1, in-house synthesized YBa2Cu3Ox powders were used toprepare samples for electrical property measurements, instead of the commercial YBa 2Cu3Oxpowders. These samples were also examined by WDX and the data are included in Figure 32.No statistical discrepancy of intragranular silver contents between these samples and that of thesamples made by the commercial powders was found.Figure 37 shows typical resistance versus temperature curves. From these curves mid-point superconducting transition temperatures (T cm) were determined and are included in Table8. Tan values are similar for all YBa2Cu30x samples containing silver. It should be noted thatin all cases the transition from normal to superconducting state remained sharp. Overall, silverdoes not have much influence on the transition temperature.It is thought that the introduction of silver within the lattice (in the place of Cu) may alterthe inherent resistance of the system. As the substitution of silver into the YBa 2Cu3Ox latticereached a constant value, the change with resistivity may also reflect this effect of the solubilitylimit.The normal state resistance of silver doped YBa 2Cu3Ox compound was investigated byusing the resistance as obtained directly from the plots and by extrapolating the linear portionof the resistance versus temperature curve to determine the resistances at 295K and 100K. Thenormalized resistance ratio, R 295/Rioo , versus nominal silver concentration was plotted in Figure38. It was found that the behavior of normalized resistance had a similar behavior to those ofthe lattice parameters and intragranular silver concentration. Though there is a significant scatterin the data, R295/Rwo increased linearly with increasing silver concentration and then reached a2.502.000.5072constant value at high silver concentration. The results also strongly suggest that it was theintragranular, not the intergranular, silver that might be playing a more important role in changingthe electrical properties of YBa 2Cu30. compounds.0.00 ^01^t. 150^100^150^200^250^300T(K)350Figure 37 Resistance of YBa2Cu3O,, as a function of temperatureA A■ A••i Data were taken directly from R vs T curves■ Data were taken by extrapolating linear partson R vs T curvesi^i^1^1■73Typical plots from the Jc measurements are shown in Figure 39 and 40. J c data at 11.1,V/cmare also contained in Table 8. Silver reduced J c values in all cases except the nominal silverconcentration of 0.5%. The J c values were determined by the Physics Department, UBC.3.202.805Caa 2.40CClecc:n2.001.601.2000^2.0 4.0 6.0 8.0^10.0^12.0Nominal silver concentration(wt%)Figure 38 Normalized resistence of YBa 2Cu30,c+Ag as a function of nominal silver concen-trationTable 8 Electrical Properties of Pure and Silver Doped YBa 2Cu30„ CompoundsNominal CAg(%) Intragranular CAg(%) Teni(K) Jc(A/cm2) R295K/R1OOK0 0 90 88 1.740.5 0.12 91 92.3 1.701.0 0.15 92 42 1.891.5 0.26 88 - 1.992.5 0.32 89 28 2.155.0 0.37 91 72.3 2.0310 0.37 92 86.3 2.18741.6 18 201 402 04 06 08 10 1210.■ an•al •Temperature=4.2K 123+1% Ag75Current(A)Figure 39 Voltage as a function of electric current of YBa2Cu30x+1.0%AgTemperature=4.2K 123+10% AgII• _IN^Sample AA^Sample 13AIl•A_ MIA-A_- MIE=IIIIAIII A00^05^10^15^20^25^30^35^40^45Current(A)Figure 40 Voltage as a function of electric current of YBa2Cu30 7,+10%Ag10. DISCUSSION4.1 The Preferential Site of Silver SubstitutionThere is no doubt that silver ions are present within the YBa 2Cu3Ox lattice, as has beenconfirmed by the change of the lattice parameters, although the levels of the concentration arevery low. The maximum concentration of silver in the YBa2Cu3Ox compound is found to beabout 0.37 wt% at room temperature. Figure 31 is re-plotted as Figure 41 using the atomicfraction (y) of silver (of YBa2Cu3-3,,Agy0x). The maximum y value is about 0.023 at roomtemperature. There are two possible ways that the solid solution may be formed: interstitial orsubstitutional solid solutions.0.030co 0.025itsEmoc 0.020ooE,Toi6 0.015=c2ca2 0.010''''0.0050 0^2.0^4.0^6.0^8.0^10.0^12.0Nominal silver concentration(wt%)Figure 41 Analyzed silver content as a function of the nominal silver content410111 ^a=3.8195ASpace group: PmmmYCut0 040 0Cu 101^0^b=3.8894A 0577Figure 42 Crystal structure of orthorhombic YBa2Cu307 .It has been confirmed that the superconductivity of YBa2Cu3Ox compound is directlyrelated to its specific crystal structure. Since all doped samples demonstrate clear supercon-ducting properties in the T c and Jc measurements, the dissolved foreign atoms or ions shouldnot significantly modify the crystal structure of YBa 2Cu3Ox lattice. The YBa2Cu30, is derivedfrom the perovskite structure, i.e. 02  along with Y 3+ and Ba2+ are close-packed. All equivalentoxygen octahedral interstitial sites are already filled by copper ions and tetrahedral intersititial78sites are empty. The ionic radii (tetrahedral coordination) of silver are 0.89 to 1.14A, dependingon valence and that of oxygen is 1.24A. By comparing the radius ratio of silver and oxygenions, it is obvious that if interstitial solid solution were formed, the crystal structure would betotally changed because the oxygen tetrahedra can not accommodate such large silver ions or,at least, the increase of the lattice parameters would be much larger than that has been observedin this study. The radius ratio of the tetrahedral and octahedral sites within oxygen ion packingare 0.225 and 0.414. Therefore the likelihood of silver occupying the interstitial sites is negli-gible.For a substitutional solid solution, several possibilities exist. As can be seen in Figure42, there are four crystallographically distinct cationic sites in a YBa2Cu3Ox lattice: Y, Ba, Cu(1)and Cu(2). At the Cu(1) site the valence of the copper ion is +3 with a square coordination(C.N.=4), whereas it is +2 with a pyramidal (C.N.=5) coordination at the Cu(2) site.Silver has a similar outer shell electron configuration to copper (Ag: 44105s 1 and Cu:3d1°4s1) and both of them tend to lose the single electron in the outermost shell to form an ionicbond and may use the s, p and d orbitals of that shell to form co-linear sp, tetrahedral sp a orsquare dsp2 hybrid bonds. But in spite of the general similarity in the electron structures of theiratoms, silver and copper differ very considerably in their chemical behavior.E8n First, thevalences exhibited in their common compounds are: Cu, 1 and 2, and Ag, 1. Second, the moststable ion of Cu is the Cu2+ ion, whereas that of silver is Agt Both Cu and Ag may be oxidizedto the higher valences Cu, Ag2+ and Aglt Unlike Cu, the Ag2+ state can only exist in a verystrong electrophilic environment, such as in fluorides, e.g. AgF2 in which the Ag2+ has anoctahedral coordination (C.N.=6). Oxides with Ag 2+ are rarely found in naturally occurringminerals. This is because Ag2+ has a stronger tendency to disproportionate to Ag+ and Ag3+ than79CU2+. The oxide AgO is actually a Ag iAgm02. In AgO, Ag' has two co-linear 0 neighbors whileAg" has four co-planar 0 neighbors. This is in contrast with CuO in which all Cu atoms have4 co-planar 0 neighbors, i.e. square coordination.As silver is rarely found in the +2 formal valence state in oxides, the probability that silversubstitutes Ba2+ and/or Cu2+ in YBa2Cu30x lattice is very low. This is coincident with the factthat the only coordinations of Ag 2+ which have been found are co-planar square (C.N.=4) andoctahedral (C.N.=6) whereas the coordination numbers of Ba 2+ and Cu2+ in YBa2Cu3Ox are 10and 5, respectively. Hence, it is reasonable to conclude that the silver ions in the YBa 2Cu30„lattice are trivalent. The preference for square coordination of trivalent Ag makes it very unlikelythat Ag/Y substitution will occur. From the above analyses, based on structural chemistry, itcan be speculated that if silver substitution occurs anywhere in the YBa 2Cu30„ lattice it choosesthe Cu(1) site, because of not only the chemical similarity of silver and copper but also, andmore importantly, the same coordination environment exists for both silver and copper ions atthe Cu(1) site.This speculation is consistent with the results of the precise determination of latticeparameters in this study. First, the occupancy of oxygen in the orthorhombic phase at 0(5) siteson the a axis is very low, only about 0.06 [82] and this site is usually thought to be empty for theorthorhombic phase. Therefore, in YBa2Cu30, the equilibrium distance between Cu(1) ionsalong a axis, i.e. the length of lattice parameter a, is mainly balanced by the coulombic forcebetween these two Cu(1) ions. As Ag3+ occupy the Cu(1) sites instead of Cu, the increasedrepulsion force due to larger ion size of Ag3+ (0.811 for Ag3+ and 0.68A for Cu3+ with squarecoordinations (831) pushes the Ag3+ ions apart until a new balance is reached, resulting in anenlargement of the lattice parameter a. Second, it can be found from Table 9 that the Cu(1)-O(4)bond along the c axis is the shortest Cu-0 bond in the lattice. This may imply that Cu(l) and800(4) are strongly bonded and there is little room in this direction to allow any larger substitutionalion. The introduction of larger Ag3+ ions might not be completely accommodated and thereforecauses a slight increase in the c direction of the lattice whereas these larger ions could be readilyaccommodated by the 0(1) ions on the b axis without significantly changing the bond length.It has been observed' 821 that the OW is most easily lost upon heating which indicates Cu(1)-0(1)bonds are the weakest. The maximum increase of the lattice parameter c observed in this studyis only 0.011A compared to the bond length difference between Cu(1)-0(1) and Cu(1)- 0(4)of 0.11A. This may suggest that the space along the b direction of the unit cell is big enoughto accommodate the Ag-0(1) bonds. Moreover, the facts that at high temperature 0(1) ions arecompletely removed resulting in the tetragonal phase of YBa2Cu3O6 and Cu+ at the Cu(1) sitewith two-fold coordination, which was found in the isostructure of Ag2O and Cu2O, also supportthe agreement that Cu(1) ions are substituted by silver.Table 9 Bond Length in YBa2Cu3071841Bond Length(A)Cu(1)-0(1) 1.941-0(4) 1.831Cu(2)-0(4) 2.285-0(2) 1.931-0(3) 1.955Ba-0(1) 2.891-0(2) 2.976-0(3) 2.963-0(4) 2.747Y-0(2) 2.404-0(3) 2.383814.2 Impurity Phase and Its Effect on the Electrical PropertiesAs mentioned earlier, it has been reported r671 that an Ag-O eutectic liquid forms above931°C, which is lower than the sintering temperature of 950°C used in this study, and dissolvescopper from YBa2Cu3Ox phase leading to the decomposition of YBa2Cu3Ox into the Y2BaCuO5phase. In this study, the Y2BaCuO5 phase was detected in all samples containing silver and theamount of the Y2BaCuO5 phase increased with increasing nominal silver content. This is alsoevident from the microstructural observation of this study. Significant amounts of liquid phasecan be seen in the quenched samples under the optical microscope(e.g. Figure 26e). Thenon-superconducting liquid and Y2BaCuO5 phases accumulated at the grain boundaries andtherefore considerably degraded the transport supercurrent at high silver levels resulting in lowcritical current densities which are observed in this study. Unlike which is mainly the functionof microstructure of a bulk material, critical temperature, T c, is primarily an intrinsic propertyand sensitive only to the crystal structure of the superconducting phase. These results suggestthat silver can be used beneficially only for low levels of addition.4.3 Previously Reported Results of Silver Addition to YBa 2Cu30„CompoundsAs referred in the introduction, Gangopadhyay and Masonm have determined the silversolubility in YBa2Cu306.. The trend of their EPMA results are similar to the WDX results ofthis study except that all the analyzed silver contents in their study are higher than those of thisstudy at any given nominal silver content. However, it is worth noting that in both studies thesaturation of silver substitution occurs at the nominal concentration of about 3 wt%. The smallgrain size in their samples may be responsible for the discrepancy. The "average grain size"82reported in their study was in the range between 2 and 6 p.m. The authors did not specify themethod to determine the "average grain size". It has been well documented that YBa 2Cu3Oxgrains have rectangular cross sections with the (001) plane parallel to the long edge. If the linearintercept method was used as is the usual practice, the average width of these grains would beeven smaller depending on the aspect ratio of the grains. Since the effective probe samplingarea of electron microanalysis is approximately 1p in diameter, it is most likely that the incidentelectron beam hit the grain boundaries surrounding and/or underneath the grain under analysisand generated signals from these silver-rich boundaries leading to exaggerated results.Both Weinberger, et al.157' and Cahen, et a t" have measured the lattice parameters ofYBa2Cu3Ox samples with and without silver doping by X-ray diffraction. The result from theformer is that all three crystalline axes of the orthorhombic unit cell expand by about 0.08% forthe sample YBa2Cu2.95Ago.050x, whereas the latter claimed that a and b remained constant whilethe lattice parameter c increases by 0.24% for the sample YBa2Cu2.977Ag0.0230x. Both of themused only the (200), (020) and (006) lines to calculate the lattice parameters. Lattice parametersmay be precisely measured by minimum lines (1 or 2 or 3 lines, depending on the symmetry ofthe sample to be measured) provided the lines are at a very high angle (0>80°) and the lines canbe clearly resolved. P81 However, (200), (020) and (006) lines of the orthorhombic YBa2Cu30xphase lie in the range of 0 between 22 and 24°. Also, the (006) and (020) peaks are seriouslyoverlapped in the orthorhombic YBa2Cu3Ox phase. For example, the 20 values are 46.633 and46.725° for (006) and (020) planes in YBa2Cu3O7 at room temperature. 185' It is very difficult toseparate them by an ordinary X-ray scanning method. Therefore the method employed by theseauthors to estimate the lattice parameters may not be very accurate.83The similar behaviors of intragranular silver concentration, lattice parameters a and c,volume of the unit cell, and the normalized normal state resistance with the increasing nominalsilver concentration observed in this study strongly support that silver is dissolved intoYBa2Cu3Ox lattice. The different changes in the lattice parameters a, b and c favor that thedissolved silver substitutes copper at the Cu(1) site. The substitution is limited by the large sizedifference between silver and copper (-19%). The maximum solubility is about 0.37 wt% or2.3 atomic percent, i.e. at that level, the chemical formula of 123 grains is YBa2Cu2.977Ago.0230x.845 SUMMARY AND CONCLUSIONS1. At a lower level of nominal silver addition, the concentration of silver in the YBa 2Cu3Qcompound increases with increasing the nominal concentration.2. Quenched samples always have higher silver contents than annealed samples for allnominal silver concentrations.3. The solid solubility of silver in YBa 2Cu30„ has a saturation level, —0.37 wt% at roomtemperature and —0.41 wt% at 950°C, corresponding to y values 0.023 and 0.025 ofYBa2Cu3_yAgy0„, respectively.4. No matter how low the nominal silver addition is, silver could not be completelydissolved into the YBa2Cu30„ lattice.5. Lattice constants a and c, as well as the unit cell volume, increase as intragranularsilver content increases and have reached constant values at high silver concentrationlevels. Lattice constant b remains relatively unchanged for all silver concentrations.6. It is confirmed for the first time by experiments that the dissolved silver ions favorablyoccupy specific crystallographic sites in the lattice i.e. Cu(1) sites and only Cu ionsare substituted by dissolved silver ions.7. Normalized normal state resistance is improved as nominal silver content increases.It also has reached a constant value at high nominal silver concentration levels.8. Silver addition has little effect on the critical temperature, T c. The critical currentdensity, Jc is slightly affected. Only small amounts of silver addition can be usedbeneficially for the improvement of the bulk electrical properties.856 REFERENCES1. J.G. Bendnorz and K.A. Muller, 'Possible High Tc Superconductivity in the Ba-La-Cu-O System', Z. Phys., B64, 189(1986).2. M.K. Wu, et al., 'Superconductivity at 93K in a New Mixed-Phase Y-Ba-Cu-O Com-pound System at Ambient Pressure', Phys. Rev. Lett., 58, 908(1987).3. J. Narayan, et al., 'Microstructure and Properties of YBa 2Cu309_,5 Superconductors withTransition at 90 and near 290K', Appl. Phys. Lett., 51, 940(1987).4. C.W. Chu, 'Superconductivity above 90K', Proc. Nall. Acad. Sci. USA, 84,4681(1987).5. W.H. Poisl, 'Sintering Kinetics of the Superconducting YBa2Cu3Ox Compound',M.A.Sc. Thesis, Department of Metals and Materials Engineering, The University ofBritish Columbia, August, 1989.6. A.C.D. Chaklader, private communication.7. L. Giaever, 'Energy Gap in Superconductors Measured by Electron Tunneling', Phys.Rev. Lett., 5, 147(1960).8. S. Jin, et al., 'Critical Current Density of the YBa 2Cu307_8 Superconductor as Affectedby Microstructural Control', Mater. Res. Soc. Symp. Proc., 99, 773(1988).9. L.M. Sheppard, 'Superconductors: Slowly Moving to Commercialization', CeramicBulletin, 70, 1479(1991).10. P. Chaudhari, et al., 'Critical Current Measurements in Epitaxial Films of YBa 2Cu307.„Compound' , Phys. Rev. Lett., 58, 2684(1987).11. T.K. Wothington, et al., `Anisotropic Nature of Hihg-Temperature Superconductivityin Single-Crystal Y 1Ba2Cu307_„', Phys Rev. Lett., 59, 1160(1987).12. B. 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Ambegaokar & A. Baratoff, 'Tunneling between Supeconductors', Phys. Rev. Lett.,10, 468(1963).41. M. Sayer, et al., Quarterly Report No. 5, CUICAC Superconductor Consortium,Queen's University, 1990.42. C.P. Bean, 'Magnetization of Hard Superconductors', Phys. Rev. Lett., 8, 250(1962).43. M. Sayer, et al., Quarterly Report No. 2, CUICAC Superconductor Consortium,Queen's University, 1989.44. Y. Hikichi, et al., 'Property and Structure of YBa 2Cu30x-Ag Composites Prepared fromNitrate Solution', Jpn. J. Appl. Phys., 29, L1615(1988).45. Y. Shapira, et al., 'Magnetization and Magnetic Suspension of YBa 2Cu30x-Ag0Ceramic Superconductors', J. Mag. Mag. Mater., 78, 19(1989).46. M. Itoh and H. Ishigaki, 'Influence of Silver on Critical Current of the Y-Ba-Cu-OSuperconductor', J. Mater. Res., 6, 2272(1991).47. B. Dwir, et al., 'Critical Current and Electronic Properties of YBCO-Ag Compound',Physica C., 162-164, 351(1989).8848. B. Dwir, et al., 'Evidence for Enhancement of Critical Current by Intergrain Ag inYBaCuO-Ag Ceramics', Appl. Phys. Lett., 55, 399(1989).49. D. Pavuna, et al., 'Electronic Properties and Critical Current Densities of Supercon-ducting (YBa2Cu306.9)1-.Agx Compounds', Solid State Commun., 68, 535(1988).50. K. Salama, Processing of High Current YBa2Cu3Ox Superconductors, 3rd CnadianConference on High Temperature Superconductivity, Montreal, May 4-5, 1989.51. P.N. Peters, et al., 'Observation of Enhanced Properties in Samples of Silver OxideDoped YBa2Cu30„', Appl. Phys. Lett., 52, 2066(1988).52. S. Jin, et al., 'Large Magnetic Hysteresis in a Melt-textured Y-Ba-Cu-O Superconduc-tor', Appl. Phys. Lett., 54, 584(1989).53. Y. Saito, et al., 'Composition Dependence of Superconductivity on Y-Ba-(Ag, Cu)-0System', Jpn. J. Appl. Phys., 26, L832(1987).54. P. Strobel, et al., 'Superconducting Properties of Substituted YBa2Cu30.-"x 07-8',Solid State Commun., 65, 585(1988).55. Y. Motsumoto, et al., `Ag Doping Effect on the Superconduction of YBa 2Cu30yCeramics', Mater. Res. Bull., 24, 1231(1989).56. A.K. Gangopadhyay and T.O. Mason, 'Solubility of Ag in YBa 2Cu306,y and Its Effecton the Superconducting Properties', Physica C, 178, 64(1991).57. B.R. Weinberger, et al., `Y-Ba-Cu-O/Silver Composites: An Experimental Study ofMicrostructure and Superconductivity', Physica C, 161, 91(1989).58. M. Sayer, et al., Quarterly Report No. 7, CUICAC Superconductor Consortium,Queen's University, 1990.59. C.Y. Huang, et al., 'High Field and Microstructures Studies of SuperconductingnYBa2Cu30y :Ag0 Composites', Modern Phys. Lett. B, 3, 525(1989).60. C.Y. Huang and M.K. Wu, 'Resistive Transitions of Some Superconducting 123-AgOComposites in High Magnetic Fields', Modern Phys. Lett. B, 3, 805(1989).61. 0. Ishii, et al., 'Reduction of the Surface Resistance of YBa 2Cu307_x Pellets and ThickFilms by Adding Ag', Jpn. J. Appl. Phys., 29, L1075(1990).62. J.J. Lin, et al., 'Superconducting Property and Structure Studies of YBa 2Cu307-Ag20Composites', Jpn. J. Appl. Phys., 29, 497(1987).63. 0. Laborde, et al., 'Magnetic and Transport Properties of Superconducting(YBa2Cu307 _8) 1 _xAgx Ceramics', Physica C, 162-164, 827(1989).8964. H.K. Verma, et al., `Silver-YBCO Composites derived from Citrate Gel', Supercon.Sci. Technol., 3, 73(1990).65. M. Miller, 'Improvement of YBa 2Cu307.x Thick Film by Doping with Silver', et al.,Appl. Phys. Lett., 54, 2256(1989).66. G.G. Peterson, et al., 'Improvement of Polycrystalline Y-Ba-Cu-O by the Addition ofSilver', J. Mater. Res., 3, 605(1988).67. Loehman, et al., 'Wetting and Reactions between Silver and Bulk YBa2Cu307-:,Physica C, 170, 1(1990).68. S. Zannella, et al., 'Comparative Study between the Electric and Magnetic Propertiesand the Grain Boundary Structure of YBa 2Cu307_„-Ag2O Superconductors', Physica C,162-164, 1179(1989).69. S. 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Li, X-ray Diffraction and Electron Microanalysis of Metals, Metallurgical Press,Beijing, 1980, p. 132.78. H.P. Mug and L.E. Alexander, X-ray Diffraction Procedures, John Wiley and Sons,New York, 1967.79. B.D. Cullity, Elements of X-ray Diffraction, Addison-Wiley, New York, 1969, p. 447.9080. Z.X. Shen, L.A. Frederick and A.C.D. Chaklader, Superconductivity and CeramicSuperconductors, Ceramic Transaction, 13, eds. K.M. Nair and E.A. Giess, TheAmerican Ceramic Society, Inc., Westerville, Ohio, 1990, p. 467.81. A.F. Wells, Structural Inorganic Chemistry, 4th ed., Oxford, London, 1975, p. 875.82. J.D. Jorgensen, et al., 'Oxygen Ordering and the Orthorhombic-to-Tetragonal PhaseTransition in YBa2Cu307.x ', Phys. Rev., B36, 3608(1987).83. R.D. Shannon, 'Revised Effective Ionic Radii in Halides and Chalcogenides', ActaCryst., A32, 751(1976).84. D. Cahen, et al., 'Effects of Ag/Cu Substitution in YBa2Cu3O7 Superconductors',Mater. Res. Bull., 22, 1581(1987).85. A.M.T. Bell, 'Calculated X-ray Powder Diffraction Patterns and Theoretical Densitiesfor Phases Encountered in Investigations of Y-Ba-Cu-O Superconductors', Supercond.Sci. Technol., 3, 55(1990).APPENDIXSelected X-ray Diffraction Data of the Orthorhombic YBa 2Cu30„ Phase and the Calculated Lat-tice Parameters from USGS Computer ProgramTable 10 Pure YBa2Cu30„a=3.8196±0.0005A^b=3.8883±0.0005A^c=11.671±0.001Ahid 28 (degree)003 22.84100 23.25102 27.88013 32.53103 32.85110 32.85112 36.36005 38.50113 40.38020 46.68006 46.68200 47.58210 53.40121 53.40123 58.23116 58.23213 58.80026 68.10108 68.84206 68.8491Table 11 YBa2Cu300-0.5%Aga=3.8225±0.0004A^b=3.8892±0.0004A^c=11.6763±0.0009Ahkl 20 (degree)002 15.17003 22.84100 23.25102 27.88013 32.52103 32.82110 32.82112 36.34005 38.51113 40.36020 46.67006 46.67200 47.54115 51.48123 58.21116 58.21213 58.77026 68.11108 68.78206 68.7892Table 12 YBa2Cu300-1.5%Aga=3.8231±0.0004A^b=3.8901±0.0005A c=11.681±0.001Ahkl 20 (degree)002 15.15003 22.82100 23.24102 27.86013 32.52103 32.80110 32.80112 36.34005 38.50113 40.34020 46.66006 46.66200 47.53115 51.46203 53.35210 53.35123 58.19116 58.19213 58.77026 68.08108 68.75206 68.7593Table 13 YBa2Cu30x+10%Aga=3.8247±0.0008A^b=3.8888±0.0008A^c=11.681±0.002Ahid 20 (degree)003 22.81100 23.21102 27.81013 32.53103 32.81110 32.81005 38.50113 40.33020 46.69006 46.69200 47.51115 51.41123 58.20116 58.20213 58.75026 68.08108 68.75206 68.7594


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