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Reactions between refractory metals and silica at elevated temperature De Cleene, Michel Louis André 1960

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REACTIONS BETWEEN REFRACTORY METALS AND SILICA AT ELEVATED TEMPERATURE by MICHEL LOUIS ANDRE DE CL5ENE A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of MINING AND METALLURGY We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE. Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA December, I960 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree th a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n permission. Department of Mining and Metallurgy The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 3 , Canada. Date January 6th, 1961. ABSTRACT An investigation was conducted on the reaction between s i l i c a glass and the refractory metals, Mo, V, Nb, Ta, T i and Zr. The metals were heated to 1650°C under vacuum (5xlO""1+ num. of Hg.) in contact with the s i l i c a . The interface was examined metallo-graphically. The formation of compounds was examined by X-ray diffraction technique. A liquid layer was formed by reaction of the metals with s i l i c a . This reaction was responsible for bonding between the two phases. Vanadium and tantalum showed the best bonding characteristics. Niobium formed only a f a i r bond. In the case of molybdenum and titanium l i t t l e adhesion occurred. Experimental observations and theoretical considerata-tions indicated that in most cases the liquid was a low fusible s i l i c a t e . The nature of the reaction was found to be essentially oxidation of the metal followed by solution of the oxides in the glass. Vanadium oxides are rapidly dissolved in s i l i c a causing extensive corrosion of the metal. The oxides of niobium and tan-talum do not dissolve so rapidly. Only molybdenum showed good corrosion resistance under experimental conditions. The oxides found at room temperature were respectively v o 0 . 9 » N b 2 ° 5 ' T a 2 ° 5 a n d M o 0 3 * X-Ray data showed that tantalum - s i l i c a interface contained other species that could be si l i c o n , s i l i c a , sub-oxides, s i l i c i d e s and crystallised s i l i c a t e s . ACKNOWLEDGEMENT The author wishes to gratefully acknowledge the assistance given by members of the Department of Mining and Metallurgy. He is especially grateful to Prof. W0 M0 Armstrong and Dr. A„ C. D„ Chaklader for their supervision and encourage-ment, also to Mr0 Ko G o Davis for his c r i t i c a l discussions and untiring help in the preparation of the thesis. The work was financed by Research Grant 7510-32 provided by the Defence Research Board of Canada. TABLE OF CONTENTS Page I a INTRODUCTION . 9 o o « » . » o o « o o e « o o . o 1 Ac General purpose and scope . 0 . » . . o o . 1 Bo Factors affecting bonding . „ 0 © © . . « . 2 1. Wetting properties . . o • . . » 0 0 . o 2 2 0 Chemical reactions „ . . • © . o o o . © 4 3. Adhesion of oxides to metal. . . . . . . 6 C. Review of similar systems 0 . 0 0 . © . <> o 6 D. Purpose of the present investigation. „ . © 8 II. EXPERIMENTAL o . o . o . o o o o . o e o o o o © . 11 Ao Materials . © • « . . . . * o » « . . o e o 11 1. S i l i C a e o . . e o . o . o o o o o o e c 11 2© Metals © © o © « © . . o © o . © © . e o 11 B o Apparatus . . 0 . 0 0 0 0 0 0 . 9 . 0 0 0 0 11 C. Preparation of Materials. . . . 0 . . . . . 14 D. Experimental procedure. . 0 . . . . . . . . 15 III. EXPERIMENTAL RESULTS AND DISCUSSION. . 0 0 0 . 0 0 1 8 A. Observation on h e a t i n g o 0 o o 0 . 0 . . . 0 18 Bo Observations on the Interface . . » 0 0 o 0 22 C. Microscopic observations 0 . . 0 . . • . • . 24 l o Molybdenum - s i l i c a . . . . o . . . . . . 24 2. Titanium and zirconium silica„ . . . 0 o 26 3o Vanadium silica© . 0 0 0 0 0 0 0 0 0 . 0 27 4 0 Niobium s i l i c a o . o . o o . o . o o o o 29 5» Tantalum s i l i c a , o . o . o o o o o o . o 30 TABLE OF CONTENTS (cont'd) Page Do X-Ray investigation on the tantalum 0 0 . o 3 9 IV. THERMODYNAMIC CALCULATIONS. . . . . . . . . . . . 0 hi A. Pressure in the system . . . . . . . . . . . *+l Bo Sta b i l i t y of the oxides. . . . . „ . . 0 . . *+3 Oo S x l i c i c L© S o o o o o o o o o o o o o o o o o o D o S l l i l . C 3 " t@ S o o o © o o e o o o o o o o » e o o 4^" 9 1 0 Silicates with tantalum . . . . . . . . . *+9 2 . Silicates with other metals . . „ . . . . 5 0 V. CONCLUSIONS « • • • • • • . . . • • • • . o 5 3 VI. RECOMMENDATION FOR FUTURE WORK. . . . . . . . . . . 5 5 VII. APPENDICES. . . . . . . . . . . 5 6 VIII BIBLIOGRAPHY. . 77 LIST OF FIGURES N o . Page 1. Surface tension forces acting on a sessile drop . . . 3 2. (a) - S i l i c a drop on vanadium after 1 minute fitt X6 0 ( XOx) « o o « o « d o o e o o « o « o 20 (b) - Same s i l i c a drop on vanadium after 5 minutes 3*fc X6 j?0^ C (XOx) o * * o e « e o e o o o o o e o 20 (c) - Same s i l i c a drop on vanadium after 15 minutes fit X6^ O^ C (XOx) e 0 * » « e a o o o o 9 e o o o 20 3. Cross-section through s i l i c a drop on niobium (15x) . 21 k. Cross-section through s i l i c a drop on tnoXylDcl ©ntim (X^x) • « * * o « o o » o * * o o 9 « « o © 2X 5o Molybdenum - s i l i c a interface. Etched (300x) . . . . 25 6. Molybdenum- s i l i c a Interface. Unetched (900x) . . . 25 7. (a) - Vanadium - s i l i c a interface. Unetched (300x) . 28 (b) - Same as Figure 7 (a). Dark f i e l d . (300x) . . . 28 8. (a) - Niobium - s i l i c a interface. Etched (300x) . . 30 (b) - Same as Figure 8 (a). Dark f i e l d . Etchedo (300x) • o o « « « o o o o o o o « o o 3X 9. Oxygen precipitation in Ta. Etched (250x) . . . . . . 32 10. (a) - Sil i c a t e penetration in tantalum oxide scale. Unetched (250x) 0 . . . . . 0 0 . . . . 3*+ (b) - Same as Figure 10 (a). Dark f i e l d . (250x) . . 3^ 11. (a) - Si l i c a t e - oxide interface. Unetched (450x) 0 35 . . . ( b ) - Same as Figure 11 (a). Dark f i e l d . (45x) „ 0 35 LIST OF FIGURES (cont'd) No. Page 1 2 . Outside layers on Ta. Unetched (550x) . . . . . 0 3 6 1 3 . Outside layers on Ta. Polarized light. TTn©tci*i@cl ( X 2 0 0 x ) o « o » « o e « » o « « > o o o o o 3 7 1H- . Metallic precipitation in Ta. Polarized light. Ull©t CtlS(3. ( X200x) o o e o o e o e e t t e o o e o o o 3 ^ 1 5 . Surface layers on Ta. Unpolished (^5x). . . . . . 3 8 1 6 . Eutectic temperatures in s i l i c a - metal oxide systems vs cation f i e l d strength . . . . . . . . 5 2 a^ 1 7 . Surface tension forces acting on a sessile drop. . . 5 2 l 8 o Relation between cos ©, y S L and for a solid with surface energy 1 , 0 0 0 ergs/cm . . . . . . . . . 5 2 1 9 . Complete surface equilibrium for imperfect wetting . 5 8 2 0 . Section through a solid particle resting on a plate showing a lens of liquid at the point of contact 5 9 2 1 . Section through the middle of the drop. . . . . . . . 6 0 2 2 . S i l i c a tantalum interface t i l t e d after 2 tnitlU*fc©S 3t X6 C e * e * o o « e o o o o o o o o o o 6X 2 3 . Keying on of a liquid between two solids . . . . . . . 6 2 2ha Determination of the limit of solubility Of 0^ ) i-H. TS 311(3. NID 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 6 LIST OF TABLES Table Page Io Position of the refractory metals in the P@r!!LoCllCl3clTDX©o O O O O O O O O O> O O O O © o o o XO II o Properties of refractory metals„ . „ . . o . . » . 10 III. Typical analysis of metals . . . . . . . . . . . . 12 IV. Experimental variables. . . . . . . . . . . . . . 18 V. Stab i l i t y of oxides. Vapour pressures. . . . . . kk VI. a t 1650°C for xMe + S i 0 2 ~ * Mex02 - S i . . . A F ° ( g ) at 1650°C for yMe + S i — M e ^ i . . . . . h6 VII. Maximum solubility of oxygen in niobium and 13ri"fccl XUltl © o o o o e e o o « e * * « o o o o « o o ^0 VIII. Surface energies and interface energies . . . . . 6k IX. Comparison of A.S.T.M. Standard Mo and MoOo with sample layer X-R film 163k . . . . . . . . . 65 X. Comparison of A.S.T.M. Standard V and V0 Q Q with sample layer X-R film 163^+ ......... 66 XI. Comparison of AoS.T.M. Standard Nb, Nb20^, C I * i S "t 0 " b 3 XX "t © o o o o o o o o o c o o o o o o o o ^7 XII. Typical patterns of the tantalum system. . . . . . 69 REACTIONS BETWEEN REFRACTORY METALS AND SILICA AT ELEVATED TEMPERATURE I. INTRODUCTION A. General Purpose and Scope Lately cermets - metal bonded carbides, borides, oxides, s i l l c i d e s etc., fabricated by powder metallurgy methods - have drawn considerable attention for use as struct-ural parts at elevated temperatures. It i s hoped ultimately to combine the strength and s t a b i l i t y of ceramics at high temperatures with the d u c t i l i t y and shock resistance properties of metals. Success in developing these new materials depends largely on our knowledge of the bond between the two dissimilar phases. The f a c t o r s ^ ^ affecting ceramics to metal bonding are: a) Chemical factors including wetting phenomena. (This includes selective diffusion of one component into others). b) Mechanical factors related to the thermal expansion of the individual phases and their bulk strength. c) Design factors involving size, shape and uses. The f i r s t of these factors w i l l be considered. - 2 -B. Factors affecting bonding. 1. Wetting properties. The degree of wetting of a solid phase by a liquid i s governed primarily by the surface interaction, where surface energies are playing a part, and the formation of solid solution or chemical compounds by interfacial reactions. Interfacial energies at elevated temperature and under low pressure can be most conveniently studied by a sessile drop technique as used extensively by Kingery, Huraenick and Pask ( 2> 3, 4, 5, 6, 7, 8). When a liquid i s brought into contact with a solid surface the spreading of the liquid i s determined by the condi-tion that the free energy of the system as a whole should decrease. At constant temperature and volume there are two factors controlling the free energy, namely the change in internal energy AH and the change in entropy &S. The last term always tends to reach a maximum value and thereby promotes the wetting process. Whether wetting occurs or not depends on the magnitude of the internal energy term. If the atoms of the solid attract those of the liquid drop, heat w i l l be liberated and, AH being negative, wetting w i l l be certain. On the other hand, when the atoms of the solid f a i l to attract those of the liquid as much as the liquid atoms attract each other, wetting w i l l depend on the magnitude of AH -TAS. - 3 -Atoms in the-'-discontinuous regions or at the inter-faces do not have their normal number of neighbouring atoms at normal distances; they are In higher energy states compared to atoms in homogeneous phases , Specific surface energy i s by defi-nition the Increase of free energy of a system per unit increase of area under the condition that the new surface has i t s minimum energy configuration. Surface tensions are f i c t i t i o u s forces per unit length which are assumed to replace the free energies per unit area of interface in the calculations 0 The tensions around the sessile drop resting on a solid resisting penetration are related by Young's equation (Figure 1): VSL= ysv ~ yLV °os e* where Y L v = surface tension of the liquid, y gy= surface tension of the solid, Yg^ = interfacial tension between liquid and solid, 6 = the contact angle measured between vectors y L y and VAPOUR -SOLID Figure 1. Surface tension forces acting on a sessile drop. * The vertical component of the liquid-gas and interface vector i s balanced by elastic forces In the solid phase. - If -The interrelation of parameters in Young's equation and i t s dependence on the nature of the reaction has been discussed graphically and thoroughly in Appendix I. The equil-ibrium of forces in a system where low melting point compounds are formed has also been treated in Appendix II. Contact angle measurements in a system with consider-able reaction w i l l lead to an erroneous result unless they are done through cross-sectionsof the specimens, and the value of y L 'is modified to take care of changes in the chemical compo-sition of the li q u i d . In his review on surface energies based on the data of the other workers, Kingery^) listed values for interfacial energies in the systems A^O-^ - liquid metal,fused quartz -liquid metal and Ag or Cu - liquid sodium s i l i c a t e . He also included values for surface energies of E^O-^, FeO, PbO, A120^ and S i 0 2 . Many of these are relevant to the present investi-gation and are shown in Appendix III. 2. Chemical reactions. The temperature at which a ceramic w i l l react with a metal depends on the co-ordination values for the solute and - 5 -the solvent, thermodynamic properties, d i f f u s i b i l i t i e s of compo-nents into each other etc. The factors involved in such reactions are summarised as follows by Kingery^ 1^: (a) The possibility of forming a solid solution of Cr^ O-^  - AlgO^ in the case of Cr - AI2O3 system is quite l i k e l y and has been found. Similarly a solid solution of FeO - MgO in the system Fe - MgO has also been reported. The formation of fofsterite (2MgO - SiO^) can also be expected in the system S i - MgO^1^. (b) Changes in the composition of both the ceramic and the metal phase have been reported(e.g. NbC and Zr in the system ,Nb - ZrC.) (c) One of the most impor-tant factors affecting such chemical reactions i s the solubility of oxygen or other gases in the metal and ceramic phase. Coenen^-9) considers that the relationship between the wettability of Rd-Pt, Au-Pt, Be-Pt alloys by soda-lime glass i s related to the re v e r s i b i l i t y of the electrochemical properties of these alloys, caused by diffusion of oxygen atoms in the alloys, (d) The solubility of the ceramic phase or one component of the phase in the metal is also very important for any chemical ' reaction, (e.g. WC in a binder) (e) The formation of an oxide film, particularly on the molten metal phase helps the reaction to take place. This i s primarily due to the influence of the atmosphere. 3 0 Adhesion of oxide to metal. (A short note on the mechanism of enameling) Adherence of porcelain enamels is generally attributed to chemical bonds or to mechanical "keying" of the enamel• If the surface roughness is a factor affecting adherence i t is due to the oxide phase f i l l i n g cracks in the surfaces The oxide produces localised stresses for i t s large volume and exerts a keying effect on the interface e Adherence of vitreous coatings to stainless steel was long believed to be due to roughness of surface, but was later found to be dependent on the amount of oxide formed by a plating out action of the iron by the enamel(12» 13)„ The roughness i s actually a measure of the quantity of oxide produced. Most versions of the chemical theory rely on the presence of an oxide film as a transition zone between the steel and the glass, with the oxide film attached to eacha C. Review of similar systems. Extensive work has been carried out on the systems Involving s i l i c a and oxides of Cr, Zr, T i ; a l l of these systems show a misclbility gap, but no systematic investigation has been reported on the behaviour of the refractory metals with s i l i c a e Chemical compatibility between possible crucible M l f ) materials and s i l i c a has been investigated by F. Bacon et a l v at 2500°Co The silica' was found to react with ir^idium, tanta-lum and molybdenum but not with tungsten. No further details have been published. The copper s i l i c a system was investigated at 1000°C by Adashi and G r a n t v " using internal oxidation of copper -sil i c o n powder compacts in a study of dispersion hardening. Economos and K i n g e r y ^ s t u d i e d the behaviour of sil i c o n in contact with various oxides at 1600°C and found there were considerable reactions with oxides such as BeO, TI1O2, MgO and kl^Oy The degree of wetting of group I (Cu, Ag, and Au) and group VIII (Nl, Pd and pt) metals by sodium sil i c a t e glasses in vacuo and under different atmospheres at 900°C 9 has been (7) studied by Pask et a l 0 No apparent correlation between the metal - sodium s i l i c a t e contact angles was observed,. The varia-tions in the contact angle from metal to metal could be related to the polarising power of the metal, because of the lowering of the interfacial tension, which i s in agreement with Weyl's theory ( 1 7» l 8 ) „ It can be concluded from their work that the pure metals studied did not react with sodium s i l i c a t e glasses in the inert atmospheres. On the other hand, i t has been found that binary sodium s i l i c a t e glasses spread considerably at 8 1000°C under 10~5 m©m0 HgD on Mo and W . In the case of these two metals surface absorbed gases played a significant role In the wetting characteristic of these glasses v . At the commencement of the reaction between tantalum and sodium - s i l i c a t e glass, the contact angle was rather high (around 80°) but decreased with time v . A stable angle was obtained under 10 ym.m.of HgD and a considerable reaction was observed in this system when heated to 1000°C for 12 hours, with the formation of Na 2Ta 2 0^ (sodium metatantalate)in the Interface,, Reaction between Zr and T i with sodium s i l i c a glass was observed, with a contact angle of about 135°© Nl i s reported to form TTiO when sintered with sodium s i l i c a glass powder at 900°C in He0 Considerable attention has been paid to the production of,good seals between metals and ceramic systems© Pincus^ 2"^ gave a detailed account of a process where slag was fused at lM+9°C to the interface between the base metal and the ceramic„ Do Purpose of the present investigation. The mechanism of reactions between molten s i l i c a and refractory metals at elevated temperature has never been thoroughly studied, although these systems are of fundamental importance in a number of f i e l d s . It i s true that considerable work was done on compound - 9 -(20 21) glasses v » ' for use in electronic or mechanical applications but the selective reaction of the components of the glass makes i t very d i f f i c u l t to analyse the fundamental mechanism of re-a c t i o n ^ c No attempt has been made to evaluate the part played by s i l i c a in the metal - compound glass system, because of the complex nature of the reaction, and also the temperatures of most investigations were low enough so that the action of the s i l i c a could be neglected. Oxidation resistant coatings on refractory metals and s i l i c i d e s have been suggested^^) an<j a process involving such (2^ 21*) a coating has been patented v J ' S i l i c i d e s of metals have been oxidised to a small extent to promote a protective oxide c o a t i n g b u t whether the same objectives can be achieved by spraying s i l i c a directly is not yet known• Lastly, cermet systems based on carbides or oxides as a b r i t t l e phase have been considerably studied and extensively developed, but the reverse approach - using oxides with lower fusing temperatures and refractory metals - has never been attempted. It was decided to start an investigation using refractory metals and s i l i c a glass to find out the nature of reactions between them. - 1 0 -In this study, the refractory metals utilised were limited to the hard b.c.c. metals of the transition groups and VI^ with more emphasis on the former'group,, Their positions in the periodic table are shown in Table I and selected physical properties in Table II. Table I. - Position of the refractory metals in the periodic table. Series Group I VA VA VIA V I IA vin A v i n B 1st long T i V Cr 2nd long Zr Nb Mo Tc Ru Rh 3rd longi Hf Ta w • Re Os Ir Table II. - Properties of Refractory Metals. MP. BP. Cryst. Density Surface tensions Hiiemenu °C °C Structure gms/cm^ at melting point dynes/cm V 1900 3350 B 0 C • C 0 7.2p 1697 Nb. 2hl5 3300 B « C • G a 8.56 2030 Ta 2996 6100 B 0 C 0 C • 16.60 2860 Mo 2610 *f800 B 0 0 0 0 0 10.20 2500 T i I667 3252 H • C « P e h.50 1^60 Zr 1850 ^ 2 7 H © C 0 P 0 6 A 9 2080 * B.C.C. above 882°C ** B.C.Co above 870°C - 11 -II. EXPERIMENTAL A. Materials. 1. Silicao Two different kinds of silicon dioxide were used in this investigation. Preliminary work was carried out with s i l i c a gel (> 99.98$ of S I 0 2 ) supplied by Murex Ltd. In the major part of this work high purity Vycor glass capillaries 3 m.m. diameter were used. This material, supplied by Corning Glass Co., has a nominal composition of 99.99$ s i l i c a . 2. Metals. The metals used in this work were obtained in sheet or crystal rod form. Vanadium and niobium supplied by the Union Carbide Co. (Metals Company) and tantalum, supplied by Murex Ltd., were In sheets 0 . 0 5 " thick. Molybdenum and zirconium were in f o i l s 0 . 0 0 5 " thick. A. D. McKay Company supplied the titanium (iodide crystal bar)as well as the zirconium. A typical analysis of these materials i s given in Table III. B. Apparatus. The experimental set-up was the same as that used by previous workers and i s already described in d e t a i l ^ 2 ^ ' 2 ^ . In Figurel.Aa drawing of the apparatus i s shown. - 12 -Table III. - Typical analysis of Metals. M.ets. Analysis (max. percent) C °2 H 2 V Nb Ta Mo Zr T i Fe Si Hf V o.o5 0.09 0.01 0 . 0 5 Bal. - - - - - - - -Nb 0 . 0 5 0.07 - 0 . 0 3 - Bal.. 0.15 - - 0.01 0 .01 0.01 -Ta 0 . 0 5 0.02 0.02 0.01 - 0 . 0 5 Bal. 0.18 - 0.01 0o0i+ 0.01 -Mo 0 . 0 3 - - - - - - Bal. - - 0 . 0 5 - -Zr 0.1 0 .01 - 0 . 0 5 - - - - Bal. 0.06 0.12 0 . 0 5 2 T i 0 . 0 3 0 .01 - 0 . 0 5 - - - - - Bal. 0.0*+ 0.03 -A furnace-tube assembly, consisting of a fused s i l i c a tube 2-1/2 inches in diameter and 18 inches in length, formed the vacuum tight envelope in which a 10~%i.m,of Hg. vacuum could be obtained by a two stage o i l diffusion pump backed by a mech-anical fore pump. A molybdenum susceptor 0 .75 inches in diameter and 3-1/2 inches long, shielded by an open loop of molybdenum could bring the specimen up to 1900°C in vacuo. A light source and an optical pyrometer were placed outside a window in such a way that either of them could be aligned with the furnace. An optical system, giving 10 times magnification was positioned at one.end of the furnace tube. Power was supplied by the Lepel Model T-10-3 ( 2 3 . 5 KWA-i+O Kilocycle) high frequency induction generator with OPTICAL SYSTEM FURNACE ASSEMBLY (1) Ground glaas or photographic plate. (12) Vycor tube. (2) V e r t i c a l adjustment screw. (13) Induction c o i l . (3! H o r i z o n t a l adjustment track. (1M Heating element, r a d i a t i o n s h i e l d and speclme CO Focussing screw. (15) Thermocouple gauge. (5) Adjustable bellows. (16) I o n i z a t i o n gauge. <« Ocular l e n s . (17) Oos I n l e t c o n t r o l . (7) Objective l e n s , ahutter and I r i s diaphram. (18) Viewing window. (8) V e r t i c a l adjustment screw. (19) Brass f i t t i n g s . (9) Water-cooled o p t i c a l f l a t . (SO) O p t i c a l pyrometer. (10) Water-cooled brass f i t t i n g . (21) Light source interchangeable with pyrometer. (11) Magnetic ahutter. Figure 1,A. - 14 -a water cooled copper Induction c o i l outside the s i l i c a tube© The temperature in the furnace was measured by a Hartmann and Braun optical pyrometer, Model TO-lOe. No emissivity correction was made. However, a verification of the melting point of Ni made in this apparatus gave a value close to 1455°C and a compar-ison with Pt/Pt-Rd thermocouple in a resistor furnace at 1600°C gave a difference in reading of less than 10°C o Co Preparation of Materials. The metal sheets and crystal rods were cut to a diameter of approximately 1 cm.square and mounted in bakelite. Standard metallographic techniques were applied to polish the f l a t surface. After dismounting the specimens, they were care-f u l l y washed in dilute HCl, d i s t i l l e d water and alcohol. Care was taken in handling the metals to avoid contamination of the surfaces. Tweezers were used whenever possible. When s i l i c a powder was used, the powder was crushed to < 300 mesh and compacted i n a cylindrical die of 1/4 inch diameter, under a pressure of forty tons per square inch. After compacting, the edges-on one end of the pellets were rounded to ensure an advancing contact angle. Where Vycor glass was used, the glass capillary was melted in a'rvoxy-gas flame, shaped into a bead, cooled and cut from the rod. This ensured a f a i r l y spherical surface where the drop would be in contact with the metal. The drop was - 15 -washed in concentrated HC1, water and alcohol, always being handled with tweezers. Do Experimental Procedure. Preliminary experiments showed that without precautions, s i l i c a gel pellets would cause considerable bulging and bloating under vacuum above 1500°C. The sessile drop picture could not be obtained in this condition. To overcome this d i f f i c u l t y , the s i l i c a gel was pre-heated to 250°C in dry air to remove hydration water. This prevented the drop from bulging, but evaporation rate remained high. The condensation on the window made the experiment d i f f i c u l t to follow but this was overcome to some extent by the use of s i l i c a (Vycor) glass. In most of the experiments, the following procedure vvy he re. was used except/;otherwise stated: The s i l i c a drop was levelled on the metal platelet with the aid of the light source. The furnace was assembled and pumped to a pressure of 10~3 m.m. of Hg. The temperature was brought to 750°C. Purified hydrogen was then flushed through the furnace. Reduction of the thin oxide layer on the surface of the metal produced by mechanical treatment was expected to take place and should approach a reproducible state of surface condition of the metal. - 16 -After a cleansing period of 10 minutes, the system was pumped down to a vacuum of 10~-'m.m©of Hg© The temperature was then slowly Increased in 5 minutes until the temperature of the specimen reached 1500°C© At this temperature neither melting nor reaction appeared to take place D The specimen was kept at this temper-ature at least 5 minutes for homogen^ization and to obtain the dynamic pressure equilibrium 0 The temperature was then slowly raised to the required temperature around 1650°C, maintaining the vacuum at the lowest possible value, generally in the range of 1 0 - 1 + to lO'^m.m.of Hg0 The specimen was cooled quite rapidly to 600°C In about 3 minutes0 To eliminate further error due to vapours and condensation on the window when reading the temper-ature, a l l runs were carried out at the same power input for a particular temperature© Checks at the beginning of each run showed fluctuations within 10oC© The drop was observed by carefully focusing the image on the ground glass of the optical system© Several pictures were taken of the successful runs© For the X-ray diffraction investigation to determine the formation of any compounds or solid solution at the inter-face, the drop was sheared from the metal. Material from the drop was removed by chipping off small coloured parts on' the bottom of the glass© On the metal side, successive layers of reaction product were selectively removed with a sharp diamond point. The layer under investigation was crushed and prepared - 17 -for identification by the X-ray powder method. The powder patterns were obtained by exposure to copper radiation (A = lo<?+0k X) at 25°C with Ni f i l t e r (power input: 1+0 KV -15 m„Ao) for 2 to 3 hours. - 18 -III. EXPERIMENTAL RESULTS AND DISCUSSION. A. Observations on he a t i n g . Values of major v a r i a b l e s a f f e c t i n g the behaviour of the specimen are shown f o r a few t y p i c a l runs i n Table IV. Table IV. - Experimental V a r i a b l e s . Run Metal S i l i c a T.emperature Vacuum m.m. Hg. Remarks 2 Mo S i l i c a g e l 1600 5xio~k Condensation 3 5 Mo S i l i c a g e l 1680 I b i d . 26 Mo Glass 16^0 1x10-^ 28 Mo Glass 1650 2 . 5 x l 0 ~ l f 13 V Glass 1650 1x10 * Condensation 20 V Glass 1650 -k 2x10 ^ I b i d . 17 Nb Glass lk50 5xio-5 18 Nb Glass 1650 5x10"^ 6 Ta S i l i c a g e l 1750 3 . 5 x 1 0 " 3 Condensation 7 Ta S i l i c a g e l 1650 5x10"^ 22 Ta Glass 1650 1x10^ kk T i Glass 1650 2.5x10"^ Condensation k9 Zr Glass 1650 5x10"^ - 19 -In a l l cases In presence of s i l i c a , a vacuum of 10" 3 m0m. 0 f Hg, could not be obtained, whereas with metals alone, the affl&e order of pressure was reached quite easily. Condensation was more pronounced when using s i l i c a gel than s i l i c a glass, probably because of i t s greater specific surface. Reaction between s i l i c a and metal occurred in a l l the systems to some extent. A sequence of three stages in the reaction process of the same drop as observed on the ground glass of the optical system i s shown Figure 2 - a 0 b» C* In the case of vanadium in contact with s i l i c a glass, no sensible change in the drop shape took place up to l500°C o Between 1550°C and 165*0°C softening of the glass at the point of contact with the metal i s sufficient to f i l l the neck around the drop with liquid materials. This liquid phase climbed along the glass surface and eventually etched a groove in the metal platelet. With the formation of a considerable amount of liquid at the interface, the upper part of the s i l i c a drop was separated from the metal. In some cases the s i l i c a cap was observed floating on the liquid layer. The original round shape at the lower part of the drop remained long after the liquid phase grooved into the metal. This can be seen from Figures 3 and 4**where cross-sections of this specimen showed two extreme cases after 5 minutes heating at 1650°C. Niobium reacted extensively with the glass (Figure 3 ) , whereas molybdenum•developed only a few * - see Page 20 ** - see Page 21 - 20 -Figur e 2 ( a ) . S i l i c a drop on vanadium a f t e r ' 1 minute at 1650°C. (lOx) Figur e 2 ( b ) . Same s i l i c a drop on vanadium a f t e r 5 minutes at 1650°C„ (lOx) Fig u r e 2 ( c ) . Same s i l i c a drop on vanadium a f t e r 15 minutes at 1650°C. (lOx) - 21 -anchorage p o i n t s around the neck (Figure h), F i g u r e 3. C r o s s - s e c t i o n through s i l i c a drop on niobium. (I5x) Figure Cross-section through s i l i c a drop on molybdenum. (15x) The observations reported above ruled out the use of sessile drop techniques as a convenient way of measuring contact angles in these systems. A new mathematical model has been proposed (and given in Appendix II). The calculations are carried out with - 22 ~ the assumption that in the last stage of reaction, the liquid surface tension i s sufficient to l i f t the drop from the metalo Bo Observations on the Interface. Investigations on normal and tapered sections at the interface revealed reactions with considerable variations, from deep penetration into the metal (Nb, V) to a very limited corrosion (Mo)» Most of the reactions may be considered to have taken place between the liquid and solid phases. No traces of gaseous reactions with the metals wa-s revealed by metallographic or X-ray examination outside the contact of the liquid layer 0 In a l l the investigated systems, an adherent layer of glass was observed0 This thin layer of glass had different optical properties under the microscope in comparison with the s i l i c a glass© This new glassy phase was darker than Vycor glass under normal reflected light and showed numerous bright spots and cracks in a dark f i e l d 0 Both glasses were dark in the polarised light. This optically isotropic phase may be a s i l i c a t e glass formed by the reaction of s i l i c a glass with the metalo For convenience i t w i l l be referred to as " s i l i c a t e " glass In the rest of the text. Several drops were found to have sheared from the metallic surface while removing the specimen from the furnace,, - 23 -Evidence of l o c a l i s e d s t r e s s e s are found i n c r o s s - s e c t i o n s . The d i f f e r e n c e In expansion c o e f f i c i e n t s of the two phases i s most probably r e s p o n s i b l e f o r the cracks observed i n the g l a s s . S t r e s s e s are a l s o revealed by p o l a r i z e d l i g h t as can be seen i n Fi g u r e 13 w i t h 1200x m a g n i f i c a t i o n of a s o l i d i f i e d l i q u i d phase. A slower r a t e of c o o l i n g was t r i e d i n a few runs to t e s t the Influence of thermal shock on bonding. The temperature was reduced to 1300°C i n 15 minutes, and then to 700°C i n 10 more minutes. The p r o p o r t i o n of f a i l u r e s remained about the same w i t h no s i g n i f i c a n t improvement of the ceramic - metal bond. When f a i l u r e occurred on c o o l i n g , the drop was found to be sheared from the metal at the I n t e r f a c e between s i l i c a t e and s i l i c a g l a s s . Where the gl a s s drop was separated by e x t e r n a l f o r c e s , f a i l u r e occurred w i t h i n the s i l i c a t e g l a s s away from the boundary between the two glasses and at about lO/i from the metal or i t s compounds. Where s i l i c a g e l was used Instead of Vycor g l a s s , b e t t e r bonding occurred. The inhomogeneity of the gl a s s probably reduced the l o c a l i s e d s t r e s s e s and o v e r a l l s t r a i n . C r i t i c a l observation of s e v e r a l samples showed that extensive p e n e t r a t i o n of the gl a s s i n t o the metal Improved the apparent bond streng t h o n l y by s e a l i n g the drop w i t h metal oxide as i n F i g u r e 3. On the other hand i t has been observed 2 h that an excessive amount of s i l i c a t e g l a s s at the i n t e r f a c e I s d e t r i m e n t a l to the bond since the f r a c t u r e always s t a r t s and develops i n that phase. The systems i n v o l v i n g vanadium and tantalum showed the b e t t e r bonding c h a r a c t e r i s t i c s . I n the case of vanadium good bonding was caused by the formation of a considerable amount of l i q u i d l a y e r that n e a r l y completely embedded the drop. I n the case of tantalum the extent of o x i d a t i o n and d i f f u s i o n helped to achieve t h i s bonding. S i l i c a g l a s s adhered to niobium i n a few cases where c o r r o s i o n was s u f f i c i e n t to produce mechanical l o c k i n g as i n F i g u r e 3. Molybdenum showed very poor adherent p r o p e r t i e s with Vycor g l a s s under the experimental c o n d i t i o n s and somewhat b e t t e r p r o p e r t i e s with s i l i c a g e l . Zirconium and t i t a n i u m showed poor adherence i n a l l cases, mostly because of the b r i t t l e n e s s of the i n t e r f a c i a l o xides. C. Mic r o s c o p i c Observations.• 1. Molybdenum - s i l i c a . The r e a c t i o n w i t h molybdenum was not extensive; most of the i n t e r f a c e was clean and f r e e from c o r r o s i o n (Figure 5). F i g u r e 5- Molybdenum - s i l i c a i n t e r f a c e . Etched. ( 3 0 0 x ) . Only at a m a g n i f i c a t i o n greater than 900x could a roughened i n t e r f a c e be observed (Figure 6 ) . F i g u r e 6 . Molybdenum - s i l i c a i n t e r f a c e . Unetched. ( 9 0 0 x ) . When t h i s occurred, a very t h i n (ICyu) porous l a y e r suggests the presence of d i s s o l v e d oxygen i n the metal. 26 The formation of cracks in the Si0 2 glass is generally-found to be associated with regions of deeper corrosion,, Care-f u l etching with Fe C I 3 (10 gm of crystals for 100 c„c of water) failed to give any evidence of oxide formation. The presence of s i l i c a t e glass in this system was d i f f i c u l t to ascertain because of the small amount of reaction. No liquid phase could be observed during f i r i n g but on some specimens wetting of the metal surface by s i l i c a glass could be seen, as in the bottom of Figure § a The major part of the dark area visi b l e at the interface was most probably voids introduced during the preparation of the sample for metallo-graphic examination, as they do not show extinction in the polarized l i g h t . An X-ray investigation on the powder of the interface indicated the presence of traces of M0O3 (Appendix IV-A) at room temperature. The oxygen present in the metal at high temperature could have been rejected from solution on cooling with the ultimate formation of M 0 O 3 , 2, Titanium and zirconium - s i l i c a , Softening of the titanium appears without much apparent reaction. This is attributed to the eutectic in the T i 0 2 - S i 0 2 system at l5h00C. Considerable opacification of the drop took place though no bond was observed. - 27 -Zirconium oxidised readily at the point of contact between the metal and the s i l i c a . The metal became b r i t t l e and a whole range of coloration was observed in the reaction zone0 No further investigation has been carried out with these two metals« 3. Vanadium - s i l i c a . Reaction in this system was extensive. In most cases, the drop was half embedded in the liquid formed by interfacial reactions. The layer of liquid materials as seen on the ground glass at elevated temperature was about two millimeters thick. This layer so l i d i f i e d on cooling and held the s i l i c a glass drop firmly 0 Figure 7 (a) and 7 (b) show the nature and the extent of reaction. (See Page 2 8 ) . A s i l i c a t e glass (dark areas) penetrated deeply in the metal. From l e f t to right the s i l i c a glass, the s i l i c a t e and the vanadium metal are clearly v i s i b l e . Around the s i l i -cate a sponge-like texture in vanadium suggests extensive oxidation which was later ascertained by etching. (Note the different nature of the two glasses and the shining traces of metallic materials embedded in the sil i c a t e ) . These metallic particles in the glass could have been l e f t over due to the incomplete reaction and/or could have been precipitated from - 28 Figure 7 ( a ) . Vanadium - s i l i c a interface. Unetched. ( 3 0 0 x ) . Figure 7 (b). Vanadium - s i l i c a interface. Same as Figure 7 (a). Dark f i e l d . ( 3 0 0 x ) . the melt on cooling. It seems reasonable to assume that this second glass contains oxides of vanadium and is probably some kind of vanadium s i l i c a t e . The only phases detectable by X-ray powder methods at room temperature at the metal interface were vanadium and - 29 -VOn o (Appendix IV-B). This compound has a range of oxygen to metal ratio from 0 , 5 to 0 , 9 , Above this value, a defect (35) lattice with vacancies in vanadium atom sites f o r m s w o Formation of such low melting compound in the system of V-Si0 2 could not be properly accounted for, but there are very low melting e^tectics in both the V 2 0 ^ - S i 0 2 s v s t e m (650°C) and the V0 2~Si0 2 system (around 1350°C), A study of the extent of reaction showed that the vanadium oxide dissolved in the s i l i c a glass to form s i l i c a t e and never acted as a protection against further reaction, X-Ray study on the interface also supports this conclusion, as no evidence of large amounts of vanadium oxide was ever encountered in the quenched specimens, k. Niobium - silica„ The nature of the reaction with niobium was different from that with vanadium. The si l i c a t e glass was present in considerable amounts as can be seen in Figure 3 ? but because of the protective action of the oxide, the penetration in the metal was much more limited. Even for a specimen heated for half an hour at 1650°C where the groove was deep, an oxide layer of 50 to 2 50/awas always found in the reaction zone. - 30 -Figure 8 (a) shows the oxide darkened by etching with HpSO^ (95%) - HNO3 (70%) and HF (h-8%) in the ratio ( 5 : 2 : 2 ) , Figure 8 (a). Oxide layer on niobium. Etched. ( 3 0 0 x ) . Figure 8 (b). Oxide layer on niobium. Same as Figure 8 (a). Dark f i e l d . (300s). The oxide is Nb20^, as found by X-ray at room temperature (Appendix IV-C). The reaction does not seem to follow the grain boundaries; instead the reaction front - 31 -advances more or less evenly. Most of the shiny metallic inclusions in the reaction zone are possibly saturated with oxygen. (Note also in the dark f i e l d (Figure 8 (b)) the highly fissured s i l i c a t e glass in contact with the oxide. The s i l i c a glass appears immediately behind this phase0) The only other compound traced by X-ray powder technique is /3 cristobalite which is the high temperature stable form of s i l i c a but is unstable at room temperature. This shows that partial crystallisation of the glass took place in the region of sta b i l i t y of /3 c r i s t -obalite. These findings are similar to those made on gaseous oxidation of niobium at 1000°C by other worker s ^ ^ \ The general pattern of the penetration of the oxide into the metal and the si l i c a t e into the oxide formed suggests that the total reaction was a two step process: saturation of the metal with oxygen, then dissolution of the s i l i c a into the a oxide to form a layer of s i l i c a t e . As revealed in Figure i i , the reaction between s i l i c a and the oxide seems slower than the.oxidation of the metal phase though the melting point of Nb2C>5 was reported to be as low as 1 5 1 2°C ( 3 0 ) (or 1^60°C ( 3 1^) 0 5. Tantalum - s i l i c a . The tantalum system showed some features in common with niobium as may be expected from the nearly identical properties of those two metals. The oxide, found by X-ray to - 32 -"be Ta 20^, r e a c t s with the metal p r e f e r e n t i a l l y along g r a i n boundaries, l e a v i n g oxygen saturated metal i n the oxide s c a l e 0 I n the v i c i n i t y of the oxide l a y e r , but not always i n contact w i t h i t , p l a t e l e t - l i k e sheets are v i s i b l e (Figure 9) on specimens etched w i t h a s o l u t i o n c o n s i s t i n g of R^SO^ (96<£) -HNO^ (70£) and HF (k$$) i n r a t i o (2:1:1). F i g u r e 9° Oxide p r e c i p i t a t i o n i n Ta. F.tched, ( 2 5 0 x ) . L e n t i c u l a r p l a t e l e t s were observed along the cubic planes of tantalum at lower temperature by G e b h a r d t ^ 3 ^ , C a t h a r t ( 3 3 ) , and B a k i s h ( 3 l + ) and at 2500°C by B a c o n ( l l + ) . I t seems that more than one cause i s r e s p o n s i b l e f o r t h e i r appearance. At 500°C, B a k i s b / 3 ^ concluded that t h i s oxide was Ta 2 0 ^ , but Gebhardt^ 3 2^ at 900°C could not i d e n t i f y I t as any known tantalum oxide. - 33 -Gebhardt proposed three explanations for the formation of platelet-like oxides. F i r s t , the formation by engassing when the formation of platelets occurs simultaneously with the solution of oxygen; second, formation by dissolving Ta 2 0 ^ from the outer skin into the metal and, third, precipita-tion from supersaturated solution. Microhardness (50 gm load) has been measured on specimens annealed at 1650°C and on the same specimens in p contact with s i l i c a . The value was 130 kg/mm for pure tanta-lum and 450 kg/mm for tantalum with s i l i c a nearly uniformly throughout the cross-section. This increase in hardness, i f attributed solely to the dissolved oxygen, corresponds to 2 . 5 atom % of 0 2 in the (22) metal according to available data v The solubility of oxygen at 1000°C i s approximately 2 . 5 atom % and decreases with lower temperatures. Out of the three suggestions made by Gebhardt, precipitation from supersaturated solution seems to explain best the presence of platelets in this system. The main evidence in favour of precipitation i s the random distribution of the platelets below the oxide layer, and not necessarily in contact with i t , and oxygen solution of tantalum as shown by microhardness measurements. - 3k -Figure 10 (a) and 10 (b) show the main features in the oxide scale between the metal and the drop, Figure 10 (b). Same as 10 (a) 0 Dark f i e l d . ( 2 5 0 x ) . At the l e f t is a portion of s i l i c a t e penetrating the oxide revealed by i t s clear appearance under dark f i e l d and also a metallic precipitation covering the oxide - s i l i c a t e interface. At the right, grain boundary diffusion can be seen as well as the dark nuclei for oxidation inside the - 35 -grains (see a l s o F i g u r e 13), The l e n t i c u l a r appearance of the s i l i c a t e may be a t t r i b u t e d to c r y s t a l l i s a t i o n of the g l a s s . F i g u r e 11 (a) and 11 (b) show c l e a r l y the d i f f e r e n c e between the oxides and the s i l i c a t e . This i s p a r t i c u l a r l y t r u e i n Figure 11 ( b ) , Also v i s i b l e are porous spaces In the sc a l e that makes p o s s i b l e the oxygen d i f f u s i o n towards the metal oxide I n t e r f a c e . F i g u r e 11 ( a ) . S i l i c a t e - oxide i n t e r f a c e . ' Unetched. (450x). F i g u r e 11 ( b ) . Same as Figu r e 11 ( a ) . Dark f i e l d . 0*56x). - 3 6 -Observations on the l i q u i d formed around the neck between drop and the metal reveal i n t e r e s t i n g features 0 Figure 12 shows such a layer, at some distance from the neck. Figure 12. Outside layers on Ta„ Unet ched, (550x). From top to bottom, the following features are recognisable,, Tantalum dispersed i n a grey Ta 20^ matrix, a ribbon-like zone almost free from oxides and grain boundary oxidation, and above the ribbon-like layer, materials being transported by the l i q u i d drop n Close observation of various samples revealed that the ribbon-like layer was the o r i g i n a l metal surface D Oxidation progressed below t h i s layer without much reaction i n th i s p a r t i c u l a r phase. Stable a l l o y forma-tions by Ta with S i might have acted as a protective b a r r i e r to further oxidation of the phase. A l l metallic p a r t i c l e s i n the matrix became dark on etching, i n d i c a t i n g the presence - 37 -of oxygen i n solution. Figure 13 under polarized l i g h t shows traces of cooling stresses around metal p a r t i c l e s i n the outer layers. This figure also reveals that the tantalum i n the matrix, as w e l l as i n the ribbon-like layer i s free from p i t s . Figure 13 , Outside layers on Ta, Polarized l i g h t . Unetched. ( 1200x) . Figure l^f shows metallic p r e c i p i t a t i o n both inside the grains and i n the grain boundaries. Figure lW. M e t a l l i c p r e c i p i t a t i o n i n Ta. Polarized l i g h t . Unetched. (1200x) - 38 -An attempt has been, made to see the nature of the r e a c t i o n by observation of the unpolished surface of the r e a c t i o n zone as i n Figu r e 15. F i g u r e 15. Surface l a y e r s on Ta. (*+5x). Unpolished. D i s t i n c t d i f f e r e n t zones of r e a c t i o n products are v i s i b l e . Considerable p r e c i p i t a t i o n and c r y s t a l l i s a t i o n took p l a c e . No compound could be c o n c l u s i v e l y i d e n t i f i e d i n t h i s i n v e s t i g a t i o n . I t can be seen that the c r y s t a l l i s a t i o n i s much more extensive near the s i l i c a g l a s s drop than at a c e r t a i n d i s t a n c e from i t . B a c o n ^ ^ published without comment a microphoto-graph of a s i l i c a - tantalum i n t e r f a c e a f t e r prolonged heating at 2500°C. His observations confirm ours. C l e a r l y v i s i b l e are the grey matrix c o n t a i n i n g m e t a l l i c i n c l u s i o n s , the r i b b o n -l i k e phase, and, immediately behind, the l e n t i c u l a r oxide p l a t e l e t s . - 39 -Do X-Ray Investigation on the Tantalum Interface„ X-Ray diffraction powder patterns from various layers at the interface indicate that the oxide present at room temperature is Ta 2 0 ^ . The remaining lines of the sample layer patterns could be interpreted as tantalum, s i l i c a , and silic o n but because of overlapping of many lines their presence could not be ascertained. Besides these lines are also others common to nearly a l l the samples, (see pattern 1^77 Appendix IV-D) 0 These lines could be attributed to one or more of the nine A o S 0 T 0 M 0 Card Index patterns for the five s i l i c i d e s listed (Ta S i 2 , Ta S i Q g Ta S i Q o 5 , Ta S i 0 > 1 + , Ta S i ^ ) . Tantalum oxides corresponding to the formulae Ta^O, Ta 0 , Ta 0 2 , Ta 0 2 2> hatfebeen found by X-ray techniques by Shonberg^35) on heating hydrides or carbides in oxidising atmospheres. No pattern of these sub-oxides has been published. Shafer and B r e w e r c o n s i d e r that these sub-oxides of tantalum are indubitably ternary or quaternary phases which require the presence of carbon or nitrogen for their formation. It is f e l t that at the present time not enough observations have been gathered on tantalum oxides and - ho s i l i c i d e s . The investigations in this f i e l d are not yet completed and the opinions of the workers are often contro-versial© Various attempts have been made to find better evidence for s i l i c i d e s or sub-oxides, (a) Mixtures of Ta and Si ( < 3 0 0 mesh) have been sintered in vacuo for two hours at 1 6 5 0 ° C and analysed by X-ray method. There is a certain analogy between the pattern of these mixtures with that obtained in the sample layer, specially in the low d spacings, as shown in Appendix IV-C. (b) S i - T a 2 0 ^ (analytical purity) and Si 0 2 -Ta mixtures have been sintered at 1 6 5 0 ° C in vacuo. Their pattern reveals also some of the extra lines of the sample layer pattern, (c) Finally, mixtures of T a 2 0 ^ and S i 0 2 have been fired at 1 6 0 0 ° C under oxidising atmosphere in a graphite resistor furnace; the resulting X-ray patterns did not produce further information on any particular compound. In the course of this study, cones were prepared with mixtures of T a 2 0 t j and S i 0 2 of various proportions. The melting point of these cones was noted. Mixtures corresponding to nominal(weight percent) 8% T a 2 0 ^ and 20% T a 2 0 ^ melted at about 1 5 8 0 ± 10°C. In conclusion, i t may be said that there i s strong evidence for the presence of T a 2 0 ^ as the major constituent, and some evidence for the possible presence of Ta and S i 0 2 . The remaining lines could probably be attributed to silicon, s i l i c i d e s , silicates or sub-oxides. -1+1 -IV. THERMODYNAMIC CALCULATIONS. Observations during this investigation showed a sen-sible rate of vaporisation and the presence of oxides, Sili c i d e s might be also present in the case of tantalum. Each of these observations w i l l be discussed with the aid of available thermodynamic data, A, Pressure in the system. Two alternatives are offered as possible explanation of the vaporisation of s i l i c a In contact with the metal: i t can be due either to the inherent high vapour pressure of the s i l i c a , or i t can be the result of the reducing action of the metal whereby SiO is formed. In the f i r s t case, the following reactions must be considered. (The references and calculations are in Appendix V-A). ( 1 ) S l 0 2 (gl)~* S 1 CD * ° 2 (g) AF°1650°C = + 1 6 3 ° 7 6 0 K c a l° ( 2 ) S i ° 2 ( g l ) - " S i°(g) + 1 / 2 ° 2 ( g ) ^ F ° l 6 5 0 ° C = + 6 8 o 3 6 ° K c a 1 ' ( 3 ) S I 0 2 ( g l ) S i 0 2 ( g ) p s i 0 2 = 7 x l 0 - 7 atm. The total pressure above a system in equilibrium where only the reaction given by equation (1) takes place is 7 . 5 x l O ~ 7 m.m. of Hg.; for equation (2) the total pressure - 42 is 1.05x10 num. of Hg. and for equation (3) i t is 7 .6x10"" num. of Hg. The total vapour pressure i s obtained by the addi-tion of the partial pressures of a l l the gaseous molecules produced. In this case the total pressure w i l l be controlled by equation (2). The total pressure of the system as indicated by the ionization gauge is several orders of magnitude lower, 4 approximately 10 m.m. of Hg. There are several reasons why the pressure can be lower than the calculated equilibrium pressure. (a) The readings correspond to a dynamic equilibrium between the rate of gas evolution and pumping. If the pumping capacity is sufficient to balance the gas evolution, low readings w i l l occur. (b) The SiO(g) is unstable and w i l l condense on the walls forming S i 0 2 , thereby removing oxygen from the atmosphere, (c) The vaporisation process i s not entirely in equilibrium. The rate of vaporisation depends on the surface area and on the polymeric nature of s i l i c a , and was observed to be higher with s i l i c a gel than with s i l i c a glass. When a volatile metallic oxide forms at the inter-face, the total equilibrium pressure could be much higher. However the small area of reaction probably does not produce enough gaseous products to affect sensibly the pressure in the furnace. - 4 3 -It seems that a slow dissociation of s i l i c a at 1650°C into SiCLand 0 o is responsible for the major part of the raise S S cr in pressure above the nominal working pressure of 10" 7 m0m. of Hg, B o Stability of the oxides„ . The presence of oxides in the specimens cooled to room temperature is an Indication that reactions took place at higher temperatures. The relative s t a b i l i t y of various oxides has been evaluated by calculating the pressure of the gaseous species in equilibrium with the oxide at a given temperature. The results are shown in Table V (see Page 44)„ The references for the data and the calculations are in Appendix V-B. The conclusions of these calculations are? Mo02 and MoO^  are volatile under experimental conditions ( 1650°C and 1 . 3 x 1 0 " ^ atm,), Any oxide in contact with the atmosphere around the specimen would escape at the Interface. The amount of oxide found at room temperature was small, but enough to be detected by X-ray analysis. Only MoO^  'was found at room temperature, although in theory both MoO^  and Mo02 could be present© Several factors may account for a greater st a b i l i t y of the oxides under the experimental conditions than predicted on the basis of equilibrium data. The main factors are: (a) The layer between the drop and the metal Is subjected to Table V, S t a b i l i t y of oxides. Vapour pressures Reaction 1650°G 727°C 27°C M o 0 3 ( s , l , g ) — M o 0 2 ( s , l ) + 1 / 2 °2(g) ha5x10"10 atm. 3 M o 0 3 ( s , l ) - ^ - ( M o 0 3 ) 3 ( g ) PM0O3 = 5 A T M ° 5»6xl0~lf atm. 10 atm. M o 0 2 ( s , l ) ^ - M o ( S j l ) + 0 2( g ) o PQ^-U-XIO" atm. 6 o 8 x l 0 ~ 2 2 atm. 3 M°°2(s,l) ' —*- 2Mo0 3( g) + M o ( s ) % 0 0 3 " 1 0 " 6 •*»•. l O "3 ^ atm. M o°2(s,l) -— M o 0 2 ( g ) PMc02 = h x l 0 ' h a t m - 5xlO~2 atm. V 2 ° 5 ( s , l ) -- ^ 2 V 0 2 ( s , l ) + 1 / 2 °2 P 0 2 = So^xlO"-1- atm. 3.1xl0 - l f atm. 8xl0"8atm. 2 V 0 2 ( s , l ) -- ^ V 2 ° 3 ( s , l ) + 1 / 2 °2 P n 2 = 1 9 0 2 x 1 0 - 1 + 3 t m ° 3.8xlO" 1^atm. V 2 ° 3 C s,l) " —e>2V0^ s ) f 1/2 Oo p 0 ?= 2,lxl0"l l +atm, l O - 3 2 atm 0 V0( S) V( s) + 1/2 0 2 PQ 2= 2,1x10 atra. 10~35 a t f f l o V 0 ( g ) V 0 ( g ) Py0= 8x10-7 atm, 1.6xl0 " 1 3 a t m , No quantitative data are available for the vaporisation of V20^, N b 205(s) — *> 2Nb0 2( s) + 1/2 0 2 o PQ 2= 1 .65xl0~°atm. Wb0 2 ( s ) NbO(s) + 1/2 02 p 0 2 - 2 0 9 x l O - 1 1atm, NbO( s) -Hj- Nb( s) + 1/2 0 2 PQ 2= 5 , 7 5 x l 0 = 1 2atm h5 surface f o r c e s . The r e s u l t i n g pressure could be of the order of h a l f an atmosphere as c a l c u l a t e d i n Appendix II „ The oxides could be s o l u b l e i n the metal or i n the g l a s s , thereby i n c r e a s i n g t h e i r s t a b i l i t y . The c a l c u l a t e d e q u i l i b r i u m oxygen pressures f o r decomposition of vanadium oxides show that VgO^ and V 0 2 are unstable at l 6 5 0 ° C under the experimental c o n d i t i o n s . VO i s s l i g h t l y v o l a t i l e at that temperature and may c o e x i s t w i t h V 2 0 ^ ° As observed m e t a l l o g r a p h i c a l l y the oxide i s n e a r l y always i n contact w i t h the g l a s s . S o l u t i o n of the oxide may account f o r a greater s t a b i l i t y than c a l c u l a t e d . The oxide found at room temperature does not correspond to the formula VO but to a s o l i d s o l u t i o n ranging form 0 . 6 to 0 . 9 oxygen, thus i n c l u d i n g the two more thermodynamically s t a b l e oxides. A l l the oxide forms of niobium are s t a b l e at 1 6 5 0°C and should i n theory be present, but only Nb 2 0 r ; was found i n det e c t a b l e amounts at room temperature. O x i d a t i o n studies on Nb at 1 0 0 0°C by other w o r k e r s ^ 2 ^ l e d to the same conclusion, Nb0 2 was observed only i n t r a c e amount and the major component of the adherent p o r t i o n of the s c a l e was Nb 2 0 ^ . Two tantalum oxides are known to e x i s t at elevated temperature: T a 2 0 ^ and T a O ( 3 7 ) . T a 2 0 r ; and TaO are i n e q u i l i b r i u m at 1 8 0 0°C and 2 0 0°C and at 1 atm: T a 2 ° 5 ( s ) + 3 T a ( s ) " ?TaO( g) - he A t 1650°C and l o w p r e s s u r e , TaO s h o u l d e s c a p e f r o m t h e i n t e r -f a c e h u t m i g h t a l s o be s t a b i l i s e d by f o r m a t i o n o f a compound g l a s s w i t h s i l i c a o On c o o l i n g , t h e monoxide t r a n s f o r m s t o p e n t -o x i d e : 5 TaO T a 2 0 ^ •+• 3 Teu, ^ T h i s b e h a v i o u r i s i n good agreement w i t h t h e e x p e r i -m e n t a l o b s e r v a t i o n o f m e t a l l i c p r e c i p i t a t i o n i n t h e o u t e r p a r t o f t h e s c a l e r u n n i n g f r o m t h e d r o p . The m a j o r c o n s t i t u e n t o f t h e s c a l e was f o u n d b y X - r a y t e c h n i q u e t o be Ta 2 0^» The r e a c t i o n b e t w e e n t a n t a l u m and s i l i c a c a n be assumed o f t h i s k i n d : Ta + S i O p - ^ T a O , * + S I O , » ( 6 )  d (g) (g) x (TaO) + y (Si0 2) (TaO) ( S i O p ) ( g l a s s ) ( ? ) The t o t a l e q u i l i b r i u m v a p o u r p r e s s u r e f o r e q u a t i o n (U-) a t 1650°C, a s s u m i n g t h a t A F 0 TaO = 2 / 5 A F ° T a 2 0 t j , i s 2X10-1 a t m 0 S a m p l e s o f c o n d e n s a t i o n p r o d u c t s c o l l e c t e d on c o o l e r p a r t s o f t h e f u r n a c e showed t h e p r e s e n c e o f S i 0 2 and T a 2 0 ^ o T h i s i s t h e e v i d e n c e o f t h e p r e s e n c e o f gaseous TaO and S i O a t e l e v a t e d t e m p e r a t u r e , C o S i l i c i d e s o Many o f t h e l i n e s i n t h e p a t t e r n s o f t a n t a l u m s c a l e c o u l d be i d e n t i f i e d as p o s s i b l e l i n e s f o r s u i c i d e s . The f o l l o w i n g d i s c u s s i o n w i l l show t h a t t h e f o r m a t i o n o f s i l i c i d e s i s n o t t h e r m o d y n a m i c a l l y i m p o s s i b l e i n t h e c a s e o f t a n t a l u m . _ i+ 7 -The process could not Involve gaseous species since the suspected s i l i c i d e s are found distributed in the oxide scale© The total reaction may be written, with Me for-any particular metal© (x + y) Me + S I 0 2 — • Mex 0 2 + Mey Si ( 6 ) i0e© the sum of two reactions: ( 1 ) xMe + Si0 2-*Me x 0 2 + Si ^ (2) yMe + Si —*• Mey Si ( 8 ) Calculations have been carried out for Mo, V, Nb, and Ta, The source of data i s given in Appendix V-C. Oxides stable/ at 1650°C are indicated in the column "oxide" and si l i c i d e s with highest and lowest free energy of formation in column " s i l i c i d e s " . (See Table VI - Page 48)© If the sum of A F 0 ( y ) and A F 0 ^ i s positive, no si l i c i d e s are expected© Although these calculations s t r i c t l y apply for pure elements and compounds only, i t may be seen that formation of molybdenum si l i c i d e s is impossible according to equation ( 6 ) , The large errors in free energies in niobium does not permit clear conclusions, but the formation of s i l i -cides with niobium i s much less probable than with tantalum - only tantalum and vanadium s i l i c i d e s w i l l be favoured assuming the proposed equations to be valid© In the case of vanadium, where the oxides have - 48 -Table VI. A F ° ( 7 ) at 1650°C for xMe + S I 0 2 Me x 0 2 + Si and A F ° / Q \ at 1650°C for yMe +> Si--* Me vSi Element Oxide AF°( 7) Kcal/mole of 0 2 S i l i c i d e s Z^F°(8) Kcal/atom of SI Mo Mo0 2 + 61 Mo 3Si - 17 - 4 M0O3 ¥ 67 Mo #'3 4 - 26 t 3 V VO v 2o 3 + 2 r V 2Si - 41 i 5 Nb N b 0 2 * 22 TMb l o 8 2Si - 25 - 10 Nb20c; t 38 - 19 i 7 Ta Ta20c; t 10 Ta Sip - 17 - 3 Ta 0 10 - 36 ± 4 been observed to dissolve easily in the glass, i t i s not impossible that the activity of oxides has been lowered by solution. This calculation gives some reasons to expect s i l i -cides in tantalum rather than in any of the other studied systems. The work of B r e w e r e t a l show that carbon, nitrogen and oxygen alter the relative s t a b i l i t y of the different s i l i c i d e s . This i s due probably to the fact that - 1+9 -carbon, nitrogen and oxygen have an unfavourable size factor; they w i l l not form any extensive solid solution,, The interatomic distance in Nb and Ta ( 2,86 S) is large enough to allow some i n t e r s t i t i a l solution of small atoms like C, N, Oo Do Silicates, 1, Silicates with tantalum. The major components at the tantalum-silica interface are oxides, at elevated temperature, Ta ? 0^ melts at 1877°C - 100°C and TaO vaporises. Thus one should not expect a liquid layer at that temperature. Among the other compounds liquid at that temperature are Si (melting point lU-20°C) and Ta Si2 (melting point l^+OO^), X-Ray investigation, however, showed that i f present they are only in small amounts. To account for the presence of a liquid layer at 1650°C two possibilities have been considered: oxygen solubility in tantalum and a possible eutectic in the Ta205-Si02 system. The lowering of the melting point of tantalum with increasing amount of oxygen in solution has been calculated using the Clausius-Clapeyron equation: TQ-T = - R (T x T n) x 2 - N g A H T p where N s i s the mole fraction of any substance in solution and A H T P t n e heat of f u s i o n of the metal. The procedure has been checked on the Nb-0 e q u i l i b r i u m diagram. The c a l c u l a t e d and experimental values are given i n Table V I I and the c a l c u l a t i o n s are i n Appendix V-D. Table VII„ Maximum s o l u b i l i t y of oxygen i n niobium and tantalum. C a l c u l a t e d Observed Nb Maximum s o l u b i l i t y Temperature 0 . 7 3 5 weight % 1920°C 0 , 7 2 weight % 1915°C Ta Maximum s o l u b i l i t y Temperature 11 atom % 1920°C The extrapolated maximum s o l u b i l i t y of oxygen i n tantalum i s 9 atom % and the corresponding melting p o i n t i s 1920°C. Thus i n c r e a s i n g s o l u b i l i t y of oxygen cannot account f o r the l i q u i d phase observed. 2. S i l i c a t e s w i t h other metals. Attempts have been made to f i n d a r e l a t i o n between gl a s s formation w i t h SiOp and c a t i o n f i e l d s t r e n g t h , an approach very s i m i l a r to D i e t z e l ' s ^ 3 9 ) W h 0 f o u n ( j a c o r r e l a t i o n between the melting p o i n t of oxides and a c a t i o n f i e l d strength p c o e f f i c i e n t Z/a , where r ,Z" i s the valence of the c a t i o n and "a" the a c t u a l cation-oxygen d i s t a n c e i n the oxide. - 51 -Using the data of & l b r i g h t ( I f 0 ) , Levin and McMurdie ( l + 1 ) a good correlation was found between the temperature of the lowest eutectic in various s i l i c a - oxide binaries and the cation f i e l d strength as shown in Figure 16. (See Page 52) Systems with more than one oxide show various cation , + +• f i e l d strength coefficient and melting temperature (Fe , F e + + + , V^+, V + +> etc). More interesting are the liquidus tem-peratures for unknown or incompletely studied systems. Ta 2 0 ^ and Nb20^ show a melting point around 1550°C, Mo 6+around 600°C, around 650°C. It i s also quite clear that the metal reacting extensively with s i l i c a in this study are those whose oxides form low melting eutectics with s i l i c a : e.g. vanadium (1350°C), titanium ( 1 ^ 0°C), and tantalum (1550°C), but not zirconium (1675°C). V 2D^ and MoO^  are too volatile at 1650°C to play a significant role in forming eutectics under the experimental conditions of this investigation. - 52 -Figure 1 6 . Eutectic temperature In s i l i c a -metal oxide systems vs cation f i e l d strength Z/a 2. - 53 -V. CONCLUSIONS. Bond formation between s i l i c a and refractory metals involves the formation of oxides and of s i l i c a t e glass at the interface. These two mechanisms have been studied with, different approaches: visual observations of the interface at elevated temperature, metallographic techniques on tapered sections and analysis of the bonded interface by X-ray technique. The reaction starts above 1500°C with the formation of a liquid layer at the s i l i c a - metal contact points. Geo-metrical factors and relative high temperature properties of the various Si02~refractory metal compounds influence the bonding of the two phases. Thermal stresses resulted in failure of the bond at the glass - s i l i c a t e boundary in most cases. Failure occurred generally inside the s i l i c a t e glass on the application of external forces. The major Interfacial reaction is oxygen corrosion by the oxygen rich s i l i c a t e . X-Ray data show the oxides are the principal compo-nents at the interface. Although X-ray evidence se&s; inconclu-sive about the presence of crystallised silicates or s i l i c i d e s , there are indications that they might be present, particularly - 9+ -in the case of Ta-S102 system. The lack of data on silicates of refractory metals in general, and especially tantalum silicates and sub-oxides, put serious limitations to the interpretation of the microscopic observations and the interpretation of X-ray investigations, A limited study of the liquidus in the Ta 2C>5-Si0 2 binary showed that mixtures with a nominal composition around Si and 20% by weight Ta 2 0 c ; melt at about 1 5 8 0°C - 1 0°C 0 - 55 -VI. RECOMMENDATIONS FOR FUTURE WORK. This investigation has opened up new avenues of research particularly concerning the formation of silicates from refractory metal oxides with s i l i c a glass. Crystalline characteristics and properties of these silicates should be studied in more detail. Titanium and zirconium deserve, further attention. They could give a broader base to interpret the behaviour of the refractory metals in contact with s i l i c a . The oxides and s i l i c a t e layers in the tantalum system should be suitable for further study with an electron probe micro-analyser. Further studies in the Ta-0 and Ta20^-Si02 binary systems, by X-ray and metallographic techniques are also necessary to interpret the tantalum- - s i l i c a system. - 5 6 -APPENDIX I Discussion of Young' equation. Young's equation gives a relation between 7 ^ , the sur-face tension of the liq u i d , X g , the surface tension of the solid, and /g^, the interfacial tension between liquid and solid. 6 i s the contact angle measured between vectors y^y and 1)^ as in Figure 1 7 . SOLID Figure 1 7 . Surface tension forces acting on a sessile drop. This equation i s shown graphically at coni^ei / g i n Figure 1 8 . Figure 1 8 . Relation between cos 8 , a n d for a s o l i d with surface energy 1 0 0 0 ergs/cm: _ 57 -Murray^*4"2) pointed out the following facts: (1) When 0 = 90° , y s = ^ S L whatever the value of (2) At constant r S L andXg, a decrease o f c a u s e s a decrease in 9 i f 0 < 90°, and an increase in 6 i f 6 > 90°. In practice changes InYg^ offset this increase and i t can be generally assumed that a lower promotes wetting. (3) 9 can never be zero i f 1 0 0 0 erg/cm2. (h) For low energy surfaces, 0 can be zero only with a liquid of low surface tension, provided yg^ <Y$° (5) If y-^ is low (100 erg/cm2) then y S L can have quite high values, approaching that ofXg, even when 0 is zero. (6) The lower the values of y-^j the greater the sensitivity of 0 for a given change i n y g ^ . Sessile drop measurements and surface tension calcu-lations loose much of their significance as soon as a considerable amount of chemical reaction has taken place between the liquid and. the solid phase. The complicated geometry of the drop brought about by such a reaction and change in the value ofY ^ increase the chances of error in the measurement of 0 . 58 -Equation (9) becomes in that case (Figure 19) ~ >SL C 0 S 0 - >L cos 6 = 0 X L sin 9 - 7-1$ sin 0 = 0 (10) y VAPOUR SOLID Figure 19» Complete surface equilibrium for imperfect wetting. - 59 -APPENDIX II Static equilibrium of a partially molten drop on a solid. The mechanism by which a liquid phase promotes cohesion (If •>) between solids has been discussed by Ford and White"; . If the liquid wets the two solids, they w i l l be attracted by capillary forces at the point of contact as illustrated in Figure 2 0 . Figure 2 0 . Section through a solid particle resting on a plate showing lens of liquid at point of contact. Figure\ 20 is a representation of a drop at the beginning of the- reaction at the neck. Growth of the neck at the point of contact is generally attributed to plastic flow, viscous flow or diffusion. If the bottom of the drop is a portion of a spherical sector, the capillary suction in the lens w i l l be given by S = X L (_! 1_) r 3 r 2 where i s the surface tension of the li q u i d . - 60 -The curvature r^ li e s in the horizontal plane and r 2 i s at right angles to this curvature in a plane perpendicular passing through the center of the spherical section, Figure 21. Figure 21. Section through the middle of the drop. Supposing complete wetting, the total cohesive force through the whole area 27vr 3 exerted by the lens i s the sum of A r | S owing to suction and 27\r^y^ owing to the tension on the surface. Let this formula be applied to a typical drop soon after reaction has started: Data: r 2 = 0 . 0 1 cm. T3 * 0 . 1 2 5 cm. (assumed): 5 0 0 dynes/cm2 Density of the drop at room temperature: 2.2. - 61 -Area of the neck: VT r ^ = 5.10"^ cm Approximate volume of the drop: 10 - 3 cm 3. The downwards p u l l f = 5.10_2x500 ( T - ^ T + TTT^p) u.Ol 0.12? (2) Downwards pressure 2,700 dynes 2.75 gm - 2-75 = 5^0 gm/cm2. (±1/2 atm.) 5x10-2 This i n d i c a t e s a r a t h e r strong bonding f o r c e . When the molten l a y e r increases i n t h i c k n e s s , the i n f l u e n c e of r 2 decreases,but w e l l before r ^ reaches a value comparable to r 2 the surface of the l i q u i d i s d i s t o r t e d by a keying e f f e c t of the edges. F i g u r e 22 shows a drop that was t i l t e d immediately a f t e r the beginning of the r e a c t i o n . F i g u r e 2 2 . S i l i c a - t a n t a l u m i n t e r f a c e a f t e r 2 minutes at l650°C. (10 X ) - 6 2 -A sharp edge i s c l e a r l y visible„ When the t h i n i n t e r f a c i a l l a y e r wants to expand beyond the edges, i t s surface must d i s t o r t which as shown s c h e m a t i c a l l y i n F i g u r e 23 , wha-fc i s impossible. SOLID F i g u r e 23» Keying on of a l i q u i d between two solidso P o s i t i o n 3 w i l l be the p o s i t i o n i n t o which the l a y e r w i l l be lockedo At the po i n t A, r 2 = oo „ The curvature I s p o s i t i v e above that p o i n t , and negative below. I t may be assumed that the t o t a l c o n t r i b u t i o n of r 2 w i l l be n e g l i g i b l e i n that case. The value of the remain-ing cohesive f o r c e w i l l be: f = 5 x l 0 ~ ? x 500 x ( 0 o \ 2 ^ = 200 dynes = 0,22 gm By that time the metal w i l l be corroded by the l i q u i d as shown i n F i g u r e 3 , and the curvature at the bottom w i l l give an upwards p u l l of magnitude f'„ 63 Data: - h = 0 .03 cm. R = 0 ,5 cnio 1*3 « 0,125 cm, A = area of the spherical zone = 27^R h=2x3,l 5 +x0 o 5x3xl0" 2 = 9,4 1 0 " 2 cm2, f' = = 2 x 9 oT° = ^&do dynes-= 1,9 gm. This value Is well in excess o.v4-r the weight of the drop: V = ^ A r 3 = 3,14 x „ 3 3 = ,116 cm3. Weight = 2,2 x ,116 = .255 gm. It may be expected that the drop w i l l have a tendency to float on a viscous layer after a certain time during the experiment. Slow movements of the drop from one side to the other hatyebeen observed in some cases. _ 64 -APPENDIX III Table VIII. Surface energies and interface energies in (9) vacuo and inert atmosphere. Oxides Temperature °C Surface energy erg/cm2* A 1 2° 3(s) " P b ( D 400 1450 A 1 2 ° 3(s) " 1000 1770 A 1 2° 3(s) " F e ( D 1570 2300 Fused quartz( s) - CU(i) 1120 1370 Ag( s) - sodium s i l i c a t e ^ ) 900 1040 CU( S) - sodium s i l i c a t e ^ ) 900 1500 B e 2 0 3 ( 1 ) 900 7 9 . 5 PeO ( 1 ) 1420 585 PbO ( 1 ) 900 132 A 1 2 0 3 ( 1 ) 2080 700 A 1 2 ° 3 ( s ) 1850 905 Fused quartz( s) 1100 250 - 65 -APPENDIX IV A. Comparison of A.S.T.M. standard Mo and M 0 O 3 w * t h sample layer X R-film 16 #3. Table IX. No. Sample layer Mo St 'd. M0O3 St'd. h - 0809 5 - 0508 d g I / l ! d 2 I / l ! d 8 • i / i x 1 7.0100 Halo 6.930 3>+ 2 3.8100 10 3.810 82 3 3.^600 5 3.>+63 61 h 3.2600 10 3.260 100 3.006 13 2.65VO 2.702 19 5 10 2.655 35 3 lines < 1 2 6 2.3100 10 2.309 31 2 . 7 H 18 7 2.2200 100 2.2250 100 *+0 2.190 3 8 1.9800 1.996 V 1.7^00 k lines < 21 9 5 1.733 17 5 lines < 15 10 1.5700 80 1.57^0 21 1.587 6 1.281+0 8 lines < 1 5 11 80 1.2850 39 1.1120 11 12 1.0000 20 0.9950 17 13 .9080 t+OB 0.9085 7 1^ .8^ +05 60B 0.84-05 26 - 66 -B. Comparison of A.S.T.M. Standard V - and - VOQ with sample layer X-R film 1634, Table " X 0 No. Sample d 2 layer I / I , V St 1 -d 8 «d„ 1224 I / l ! v o 0 o 9 10 -d X St'do 313 1 2.377 20B 2.38 20 2 2.194 60 3 2.144 100 2.14 100 4 2 .085 60 2.06 80 5 1.502 30 1.51 7 6 1.438 60 1.45 100 7 1.247 20 1.24 20 1.24 60 8 1.231 40 9 1.196 40 1.19 60 10 1.032 20B 1 .03 50 0.96 3 0.94 50 > 0 .92 70 0.88 1 0.81 3 0.76 1 0.71 1 - 67 -C. Comparison of A.S.T.M. Standard Nb, N b 2 0 ^ , c r i s t o b a l l i t e with sample layer X-R f i l m 1595. Table X I ; . No. Sample layer d 8 i / l 1 Nb St'd. (1-1185) d % I / I Nb 2 0^ St'd. (5-0352) d I I / I 1 . /3Cri. ( 2-588) d 1 I / I x 1 2 I 5 6 7 8 9 10 11 12 13 IV 15 16 17 18 19 20 21 22 2 3 V.lVO 50 3.928 VO 3.V86 20 3.1V5 30B 2.852 20 2.731 20 2.586 20 2.53V 20 2. WO 20 2.329 100 1.972 20 1.900 5 1.828 10 1.655 30 1.537 10 1.VV2 10 1.3^8 80 1.270 20 1.170 20B 1.060, 10B 0.992 10 0.888 VOB 0.835 20B. 2.33 100 1.65 20 1.3>+ 32 1.16 6 1.0V 10B .88 6 5.2VO Halo V .320 50 3.931 100 3.V8V 50 3.1V0 100 2.855 50 2.728 50 2.590 50 2.VV7 70 2.120 50 1.962 50 1.908 5 0 1.820 70 1.790 70 1.662 70 1.572 70 1.5V3 70 1.V57 70 1.336 70 1.322 70 1.226 70 1.209 70 1.197 5 0 1.1VV 70 1.022 50 0.999 50 V.lVO 100 2.920 5 2.530 90 2.170 10 1.691 5 1.639 70 1.V57 60 1.376 20 1.263 30 1.206 50 1.127 20 1.087 10 1.030 10 0.927 10 0.838 5 - 68 Do Comparison of Ta sample layer patterns. Four t y p i c a l patterns are reproduced i n Table XI. Film No. 1577 i s a picture of the tantalum in t e r f a c e . Film No. 1632 i s a picture of mixture of 50% by weight Ta and S i heated for two hours at 1650°C i n vacuo. Film No. 1610 i s a picture of a mixture of 50% by weight Ta 20^ and S i sintered at 1650°C i n vacuo. Film Wo. I6h2 i s a mixture of 8% S i 0 2 and 72$ Ta 20^ by weight heated for 1 hour at l650°C i n a i r . A.S.T.M. Card 8-255 pattern has been added for comparison. Dotted l i n e s indicate that a few l i n e s with low i n t e n s i t i e s have been omitted i n t h i s pattern for convenience. - 69 -Table XIL T y p i c a l patterns of the tantalum system. A .S.T.M. 1577 1632 1610 1642 8-225 Ta 20^ I n t e r f a c e Ta & S i TapO^&Si Ta 20 5&Si0 2 d 2 I/h d 8 d 8 I / I , d A I / l ! d 8 6.120 10 5.220 10 4 .500 5 4 .27 5 4.240 5 4.040 100 3-8? 90 3.850 90 3.84 5 3.840 90 3.680 5 3.390 5 3.400 30 3.390 20 3.280 30 3.15 100 3.140 95 3.120 5 3.09 50 3.090 40 3.00 5 2 .970 100 2.830 5 2.83 5 2.830 -5 2.720 5 2.730 5 2 . 5 5 20 2.540 10 2 .550 5 2.46 100 2.450 70 2.43 50 2.440 10 2.43 60 2.420 100 2.420 10 2.40 100 2.36 20 2.240 5 2 .250 10 2.180 10 2.190 40 2.11 10 2.110 45 2.110 30 2.11 70 2.120 10 2.02 30 2.020 5 2.040 5 1 . 9 5 50 1 .930 40 1 .930 100 1.94 20 1 .930 40 1.83 40 1.860 30 1.80 40 1.800 20 1.820 10 1.800 20 1.76 20 1.760 40 1.760 20 1.66 100 1.640 • 70 1.650 100 1 .660 90 1 .530 33 1.470 30 1 .33 70 1 .350 "5 1.3^0 5 1.36 30 1.340 40 1.310 30 1 .300 60 1.290 20 1 .280 20 1.220 30 1 .23 20 1 .230 40 1.210 5 1.210 40 1.190 40 1.170 10 1.170 20 1.17 10 1 .150 10 1 .150 20 1.140 10 1.130 70 1.020 5 1.020 30 .99 5 1.000 20 1.000 10 .977 5 .976 80 70 (cont'd) Table x i i T y p i c a l p a t t e r n s of the tantalum system,, AoS ,T.Mo 1577 1632 1610 1642 8-225 Ta20c; Interface Ta & SI TagOc^&Si Ta 20 5&S102 d A 1/^ d 8 d 8 I / I x d 8 I / I 1 d X o930 5 .933 20 .933 10 o925 5 a 916 5 .908 10 .903 60 .800 5 .891 .892 10 ,858 5 .852 20 .851 5 ,853 10 .834 10 .833 40 .828 20 .826 10 .819 40 .816 5 .817 10 ,810 60 .805 10 .805 50 ,806 10 .799 5 ,800 10 .798 10 .796 40 .79^ 30 etc. 4 lines 6 lines APPENDIX V Thermodynamic Calculations A. Calculations on silica© The data for£F° are taken from E l l i o t , except for the vapour pressure of SiO taken from Tombs and Welchv . 6 F°Si0 2 ( g i ) " " 126,685 Kcal/mole ^ F ° s i ( g ) = - 36.018 Kcal/mole A F ° S i 0 ( g ) = " 58.316 Kcal/mole 1. Calculations of the total vapour pressure at equilibrium for equation: S i 0 2 ( g l ) ~* S i(g) * °2 ( g > A F ? 6 5 0 O C = + l 63.760 Kcal l o g 1 0 p 0 o =- 163-760 . s - 18.615 1 0 °2 if. 575x1.923 From stoAchiometry: p Q = p s l - — — P t o t i 2 log p S i = -18.615 log p S i = - 9 .3075 log p t o t e = - 9 .3075 + 0 .301 = - 9.006, p t o t . = 1 0 ~ 9 a t r a« 72 2. Calculation of the total vapour pressure a t equilibrium for equation: S 1 0 2 ( l ) " > S i 0 ( g ) + 1-/ 2 02(g) ^ F ° 1 6 5 0 ° C s 6 8 ° 3 5 9 Kcal l o g 1 0 K r i o g 1 0 ( P S 1 0 x p 0 * ) - - s - 7 - 7 7 From sto&chiometrys p S i Q - 2 p Q^ 0 p t o t 0 S 3 p 0 2 * l o g 1 Q 2 x (p 0 2) 3 / 2 r - 7.770 l o g 1 0 p t o t . = " § ( " 770=0.301) - 0.V78 = - V.856 p. , - l.VxlO"^ atm. B. Calculation of s t a b i l i t y of oxides. AF° for V 2 0 ^ , V 0 2 ? V ^ , VO, Nb 2 0, Nb 20^ are taken from Glassner^^) . the pressure of sublimation of VO and M0O3 and £ F° for Mo02, M0O3, from E l l i o t t P 8 \ Extrapolations, when necessary were carried out using Figure h - 8 . (p.290). As an example, calculations of the st a b i l i t y of vanadium oxides at 1650°C are shown here. Data at 1650°C £ F° V2O5 = - 200 Kcal/mole. L\ F° V0 2 = - 132 Kcal/mole. a F° V 2 0 3 " - 231 Kcal/mole. r\ F° VO = - 79 Kcal/mole. 73 1. V 2 0 5 ( g ) 2 V 2 0 2 ( s ) + 1/2 0 2 ( g ) ^ ° l 6 5 0 o c = - 2 Kcal, V 2 l o g 1 0 P 0 2 = u ^ x T ^ r = ° ° 2 2 ? l o g 1 0 p 0 2 = 0.V5V P Q 2 - 3 .5X10" 1 atm. 2. 2 V 0 2 ( S ) ^ V 2 0 3 ( s ) + 1/2 0 2 ( g ) ^ F O l 6 5 0 o C = + l 8 K c a l ° 1/2 l o g 1 0 P 0 O S - 1 8 - - 2.0V8 2 V.575x1.923 l oSlO P o 2 " " h'°36 PQ^ = 1.25X10"14" atm. 3 . V 2 0 3 ( s ) - 2 V 0 ( s ) + 1/2 0 2 ( g ) A F ° l 6 5 o o c r + 6 ° K G a l ° V 2 i o g 1 0 P 0 2 = - * ° „ = -V.575x1.923 l o S i o P o 2 = ™ 1 3 ° 6 1 ' " P Q 2 = 2.1xlO"114' atm, V. V 0 ( s ) - V ( s ) + 1/2 0< g ) ^ F ° 1 6 5 0°G = 4 6 Q K c a X ' 1/2 log p n = -§2 _ = = 6.82 2 V 0575x1.923 log P Q 2 = - 13.6V P Q 2 = - 2 . 1 x l 0 ~ l l f atm. 5. v c y - v o ( g ) ho p = 8 . 0 5 x 1 0 " / atm. 7h C. Calcu l a t i o n on s H i c i d e s . Values of A F ° for oxides are taken from Elliot{?QK 0 n l y A H ° 2 ^ g O K -for the s i l i c i d e s of Mo,V, Wb, Ta have been published„ The values d i f f e r widely between workers. The fH-6) values published by Searcy\_ have been chosen because they are the r e s u l t of personal data and c r i t i c a l evaluation of the l i t e r a t u r e . Values f o r A F 0 have been evaluated by supposing A H 0 constant and an entropy of formation of 2 cal/mole x °Ko D„ Lowering of the melting point of Ta by solution of 0 2 ° The lowering i n melting temperature T Q-T^ of any substance r e s u l t i n g from the presence of solute i s T - T, = R ( T l x TQ> < N . O 1 A t l ^ s H T p where H s i s the mole f r a c t i o n of any solute and AH^p the heat of melting of the solvent. The l i m i t of s o l u b i l i t y i s determined by the i n t e r -section of the l i n e showing the lowering of melting point with amount of solute, and the experimental values of so l u b i -l i t i e s of Q>2 a * lower temperature. This method was t r i e d on the existing Nb-0 system and the r e s u l t was found i n good agreement with published data. Since tantalum and niobium have close chemical properties, i t i s expected that t h i s method w i l l be applicable to tantalum also with good accuracy, - 75 a. - Nb-0 s o l u b i l i t y of 0 2 at 500°C : 0 o 2 5 weight % 1650°C t O068 weight # AH Tp = 6 ,,430 cal/mole T Q = 2690°K o 1) I f N = 1CT 2 1 098xl0" 2 (2 p690xTi) 0 1 6,430 2) I f N * = 4 x l 0 ~ 2 T -T - 1O98XHX10^ 2(2 0690XT 1) °" 1 6.430 b 0 - Ta-O s o l u b i l i t y of 0 2 at 1200°C : 3»75 atom# 1600°C : 6 atora% 1800°C i 7o5 atom% A H T p = 7.500 cal/mole T Q s 32 50°K 1) If N = 10~ 2 T -T a 1.98xlO~2 (3250xTi) 0 1 " 7,500 2) I f N - 3 x l O - 2 T -T = 1.98X3X10"2 (325OXT1) ° 1 7 P 500 3) I f N - 6 x l 0 ~ 2 mm „ 1.98x6xlO~ 2 (3250xTi) - 76 -Figure 2k. Determination of limit of solubility of 0 2 in Ta and Nb. 1 - Liquidus line for Ta. 2 - Liquidus line for Nb. 3 - Solubility of O2 i n * T a . h - Solubility of O2 in^Nb. - 7 7 BIBLIOGRAPHY lo Kingery, W0D0; J. Amc Ceram. Soc, 3 6 ( 1 9 5 3 ) 3 6 2 2 , Kingery, W.D. and Humenik, M0; Jo PhysD Chem., £Z ( 1 9 5 3 ) 3 5 9 3 . Allen, BoGo, and Kingery, W0Do; A 0IoM 0E 0 Trans 0, 2 1 f ( 1 9 5 9 ) 3 1 ho Kurkjian, CR. and Kingery, W.D.; JoPhySoChem. 6 0 ( 1 9 5 6 ) 9 6 1 5o Halden, F . A o and Kingery, W0D0; J.phySoChem. £2 ( 1 9 5 5 ) 5 5 7 6 0 Humenik, Mo and Kingery, W0D0; JoAm0GeramoSoCo 21 ( 1 9 5 V ) 1 8 7o Zackay, V.,Pask A e, et a l , J. Am. Ceram. Soc. 2§. ( 1 9 5 3 ) 8 V 8 . Fulrath, R.M., Mitoff, P., and Pask, A 0 J. Am0 Ceram0 Soc. V O ( 1 9 5 7 ) 2 6 9 9o Kingery, W0D0; Property measurements at high temperature. J. Wiley & Sons ( 1 9 5 9 ) 1 0 . Coenen, M ; Paper given to the meeting of the German Soc. of Glass Technology. Freudenstadt ( 1 9 5 8 ) l l o Rodin, T.N.; J. Am. Chem0 Soc, £ 2 ( 1 9 5 0 ) V 3 V 3 1 2 0 Moore, D.G. and Embawks, A.G.; JoAmoCeramoSoc. 21 ( 1 9 5 6 ) 3 5 7 1 3 . King, B.W., Tripp, H.P., et al JoAmoCeramoSoc. h2 ( 1 9 5 9 ) 5 0 V lh0 Bacon, J.F., Hasapsis, A 0A 0 and Wholley, J.W.; Physics and Chemistry of Glass 1 ( I 9 6 0 ) 9 0 1 5 . Adashi, M. and Grant, N.J.; W.AoD.C. Technical Report 59-777 160 Economos, G. and Kingery, W„D 0; J.AmoCeram.Soc. 2§. ( 1 9 5 3 ) *+03 1 7 . Weyl, W.A.; A.S.ToM. Stand. V 6 ( 1 9 V 6 ) 1 5 0 6 180 Weyl, W.A.; Coloured Glasses,p.331-Soc.of Glass Techn-Sheffield 1 9 . Yaeger, L0W0; Adhesion and Adhesive, p . 8 l Wiley (L95V) 2 0 . Pincus, AoG.; J. Am. Ceram. Soc. ^ 6 ( 1 9 5 3 ) 1 5 2 2 1 . Navias, L.; J 0 Am. Ceram. Soc. 21 ( 1 9 5 V - ) 3 2 9 - 78 -BIBLIOGRAPHY (cont'd) 22. Fitzer, E.; Proc. Plasee 2 d Seminar Reutte (1956) 23o Campbell, E . et a l ; U.S. Pat. 2,771,666 (1956) 24. Nachtigall, E. and Kieffer; German Pat. 895,848(Nov.5, 1958) 2 5 . Kiefer, R., Benesovsky, F. and Konopicky, F 0; Ber. Deutc keram. Ges. ^1 (1954) 223 2 6 . Clarke, J.F.; Thesis, Univ. of British Columbia (1959) 27. Hasselman, P.H.; Thesis, Univ. of Brit i s h Columbia (1959) 2 8 o Shoenberg, N.; Acta Chem. Scand. 8/1 (1954) 221 2 9 . Klopp, W.D., Sims , C.T. and Jaffee, R.I.; Trans. A.S.M. %L (1959) 282. 3 0 . Orr, R.L.; J. Am. Ceram. Soc. 21 (1953) 2808 31 . Kubaschewski, 0. and Hopkins, B; Oxidation of Metals and Alloys. (1953) p .9 Butterworth 3 2 . Gebhardt, E.; 3rd Plansee Seminar Reutte -Austria 3 3 . Cathcart, J., Bakish, R. et al;J.Electrochem.Soc. 107 (1950) 668 34. Bakish, R.; J. Electrochem. Soc. 10J? (1958) 71 36. Shafer, H., Brewer, L.; Private communications reported by Miller: Tantalum and Niobium p.602 Butterworths (1959) 35. Shonberg, N.; Acta. Chem. S cand 8/2 (1954) 240 37. Peters, E.; Private communication0 38. E l l i o t t , J . F . and Gleiser, M.; Thermochemistry for Steelmaking Vol. 1. Reading MA (I960) 39. Dietzel, A.; Zeit.Fur Electrochemie }+8 (1942) 9 40.. Albright, J.G.; Handbook of Chemistry and Physics 41st Ed. p.2678 41. Levin, E. and McMurdie, H.F.; Phase Diagrams For Ceramists. (Am. Ceram. S o c 1959) - 79 -BIBLIOGRAPHY (cont'd) 42„ Murray, P.; Powder Metallurgy (I960) 64 43o Ford, W.P. and White, J„; J.Trans0Brit0Ceram0SoCoj>6 (1957) 309 44 <> Tombs, NoC and Welch, A 0 J 0 ; J 0 Iron and Steel Insto 172 (1952) 69 - 78 45« Glassner, A„; A„NoL. - 5750 46o Searcy, A 0W„; J a Am0 Cerara0 Soc _)+0 (1957) 431 

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