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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  i n the Department of MINING AND METALLURGY  We accept t h i s thesis as conforming to the standard required from candidates f o r the degree of MASTER OF APPLIED SCIENCE.  Members of the Department of Mining and Metallurgy  THE UNIVERSITY OF BRITISH COLUMBIA December, I960  In p r e s e n t i n g  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 r e q u i r e m e n t s f o r an advanced degree a t the  University  o f B r i t i s h C o l u m b i a , I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e f o r r e f e r e n c e and  agree t h a t p e r m i s s i o n f o r e x t e n s i v e f o r s c h o l a r l y purposes may  study.  I further  c o p y i n g of t h i s  be g r a n t e d by the Head of  Department o r by h i s r e p r e s e n t a t i v e s .  g a i n s h a l l not  be a l l o w e d w i t h o u t my w r i t t e n  Department o f  M i n i n g and  The U n i v e r s i t y o f B r i t i s h Vancouver 3 , Canada. Date  January  6th, 1961.  Columbia,  my  I t i s understood  t h a t c o p y i n g o r 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  Metallurgy  thesis  financial  permission.  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"" num. of 1+  i n contact with the s i l i c a . graphically.  Hg.)  The interface was examined metallo-  The formation of compounds was examined by X-ray  d i f f r a c t i o n technique. A l i q u i d layer was formed by reaction of the metals with s i l i c a .  This reaction was responsible f o r 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 t h e o r e t i c a l consideratations indicated that i n most cases the l i q u i d was a low f u s i b l e silicate.  The nature of the reaction was found to be e s s e n t i a l l y  oxidation of the metal followed by solution of the oxides i n the glass. Vanadium oxides are r a p i d l y dissolved i n s i l i c a causing extensive corrosion of the metal.  The oxides of niobium and tan-  talum do not dissolve so r a p i d l y .  Only molybdenum showed good  corrosion resistance under experimental conditions. The oxides found at room temperature were respectively v o  0 . 9 » N 2°5' b  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 s i l i c o n , s i l i c a , sub-oxides, s i l i c i d e s and c r y s t a l l i s e d  silicates.  ACKNOWLEDGEMENT  The author wishes to g r a t e f u l l y acknowledge the assistance given by members of the Department of Mining and Metallurgy.  He i s especially grateful to Prof. W  0  M  0  Armstrong  and Dr. A„ C. D„ Chaklader f o r their supervision and encouragement, also to Mr Ko G o Davis f o r h i s c r i t i c a l discussions and 0  untiring help i n the preparation of the t h e s i s . 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  0  . » . . oo .  Ac  General purpose and scope .  Bo  Factors a f f e c t i n g bonding . „  e  «  o  1. Wetting properties . . o • . . » 0  3.  II.  1  o  1  0  2  . o  0  2  Chemical reactions „ . . • © . o o o . ©  4  Adhesion of oxides to metal. . . . . . .  6  Review of similar systems  D.  Purpose of the present investigation. „ . ©  EXPERIMENTAL  .  6  C.  0  0  . © . < >o  0  11  o . o . o . o o o o . o e o o o o © .  Materials . © • « . .  8  . . * 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  .  C.  Preparation of Materials. . . .  D.  Experimental procedure. .  Ao  III.  .  © © . . « .  0  2  o  .  0  .  0  0  0  0  0  0  .  9  .  0  . . . . . . . .  15  .  0  0  0  A.  Observation on  Bo  Observations on the Interface . . »  C.  Microscopic observations  0  o o  0  .  Molybdenum - s i l i c a .  .  .  0  0  lo  0  .  4  0  5»  silica©  Niobium s i l i c a  .  0  18  0  .  22  0  . . 0 . . • . • .  24  .  .  24  .  o  .  .  .  .  .  0  0  0  0  0  0  0  0  .  o  0  .  0  o . o . o o . o . o o o o  Tantalum s i l i c a ,  18  0  o 0  0  2. Titanium and zirconium silica„ . . . 3o Vanadium  11  0  14  EXPERIMENTAL RESULTS AND DISCUSSION. heatingo  0  . . . . .  0  0  0  o . o . o o o o o o . o  26 27 29 30  TABLE OF CONTENTS (cont'd) Page  Do IV.  X-Ray investigation on the tantalum  VI.  0  39  . o  THERMODYNAMIC CALCULATIONS. . . . . . . . . . . . A.  Pressure i n the system . . . . . . . . . .  Bo  S t a b i l i t y of the oxides. . . . . „ . .  Oo  SxlicicL©So  Do  Sllil.C3"t@So  0  hi  0  .  . .  *+l *+3  o o o o o o o o o o o o o o o o o o o o o o o o » e o o  ^4" 9  S i l i c a t e s with tantalum . . . . . . . . .  *+9  2 . S i l i c a t e s with other metals . . „ . . . .  50  1  V.  0  0  CONCLUSIONS  o o © o o e  RECOMMENDATION FOR FUTURE WORK. . . . . . . . . .  VII.  APPENDICES.  VIII  BIBLIOGRAPHY. .  53  « • • • • • • . . . • • • • . o .  . . . . . . . . . .  55 56 77  LIST OF FIGURES No.  Page  1.  Surface tension forces acting on a s e s s i l e drop . . .  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  «  3  20  o  (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  20  o  (c) - Same s i l i c a drop on vanadium a f t e r 15 minutes fit X6^O^C (XOx)  e  *  0  »  «  e  a  o  o  o  o  9  e  o  o  3.  Cross-section through s i l i c a drop on niobium (15x)  k.  Cross-section through s i l i c a drop on tnoXylDcl ©ntim (X^x)  •  «  *  *  o  «  o  o  »  o  *  *  o  o  9  «  «  20  o  .  o  21  2X  ©  5o  Molybdenum - s i l i c a i n t e r f a c e .  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) .  (b) - Same as Figure 7 (a). Dark f i e l d . 8.  9. 10.  (300x) . . .  (a) - Niobium - s i l i c a interface.  Etched (300x) . .  (b) - Same as Figure 8 (a). Dark  field.  Etchedo  (300x)  •  o  o  «  Oxygen p r e c i p i t a t i o n i n Ta.  «  «  o  o  o  o  o  o  o  «  o  28 30  3X  o  Etched (250x) . . . . . . 32  (a) - S i l i c a t e penetration i n tantalum oxide scale.  Unetched (250x)  0  . . . . .  0  (b) - Same as Figure 10 (a). Dark f i e l d . 11.  28  (a) - S i l i c a t e - oxide interface. ...(b)  0  . . . .  (250x) . .  Unetched (450x)  - Same as Figure 11 (a). Dark f i e l d .  3*+  (45x) „  3^  0  35  0  35  LIST OF FIGURES (cont'd)  No.  Page  12.  Outside layers on Ta.  Unetched  13.  Outside layers on Ta.  Polarized l i g h t .  TTn©tci*i@cl 1H-.  (X200x)  o  «  o  »  «  o  «  e  M e t a l l i c p r e c i p i t a t i o n i n Ta. Ull©t CtlS(3.  ( X200x)  o  o  e  o  o  (550x) . . . . .  »  o  «  «  >  o  o  o  o  36  0  37  o  Polarized l i g h t .  e  o  e  e  t  t  e  o  o  e  15.  Surface layers on Ta.  16.  Eutectic temperatures i n s i l i c a - metal oxide  o  o  3^  o  (^5x). . . . . .  Unpolished  38  .  52  17.  Surface tension forces acting on a s e s s i l e drop. . .  52  l8o  Relation between cos ©, y  systems vs cation f i e l d strength  . . . . . . . a^  S  and  L  for a solid  with surface energy 1 , 0 0 0 ergs/cm .  52  . . . . . . . .  19.  Complete surface equilibrium f o r imperfect wetting .  20.  Section through a s o l i d p a r t i c l e resting on a  58  plate showing a lens of l i q u i d at the point of contact 5 9 21.  Section through the middle of the drop. . . . . . . .  22.  S i l i c a tantalum interface t i l t e d after 2 tnitlU*fc©S  3 t X6  C  e  *  e  *  o  o  «  e  o  o  o  o  o  o  o  o  o  60  6X  o  23.  Keying on of a l i q u i d between two solids . . . . . . .  2h  Determination of the l i m i t of s o l u b i l i t y  a  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  62  76  LIST OF TABLES  Table Io  Page Position of the refractory metals i n the O O O O O O O O O> O O O O © o o o  XO  of refractory metals„ . „ . . o . . » .  10  P@r!!LoCllCl3clTDX©o  II o III. IV. V.  Properties  Typical analysis of metals . . . . . . . . . . . .  12  Experimental variables. . . . . . . . . . . . . .  18  S t a b i l i t y of oxides.  kk  VI.  a  1650°C f o r xMe + S i 0 ~ * Me 0 at 1650°C f o r yMe + S i — M e ^ i  ( g )  IX. X.  o  o  o  o  e  e  o  o  «  . . . . .  e  *  *  «  o  o  o  o  «  o  h6  ^0  o  Comparison of A.S.T.M. Standard Mo and MoOo with sample layer X-R f i l m 163k . . . . . . . . . Comparison of A.S.T.M. Standard V and V 0 Q  6k 65  Q  163^+ . . . . . . . . .  Comparison of AoS.T.M. Standard Nb, C I * i S "t 0 " b 3 XX  XII.  2  Surface energies and interface energies . . . . .  with sample layer X-R f i l m XI.  x  Maximum s o l u b i l i t y of oxygen i n niobium and 13ri"fccl XUltl ©  VIII.  - Si. . .  t  2  AF° VII.  Vapour pressures. . . . . .  "t © o  o  o  o  o  o  o  o  o  c  o  66  Nb 0^, 2  o  o  o  o  o  o  o  Typical patterns of the tantalum system. . . . . .  ^7  69  REACTIONS BETWEEN REFRACTORY METALS AND SILICA AT ELEVATED TEMPERATURE  I. A.  INTRODUCTION  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 f o r use as structu r a l parts at elevated temperatures.  I t 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 i n developing these new materials depends l a r g e l y on our knowledge of the bond between the two d i s s i m i l a r phases. The f a c t o r s ^ ^ a f f e c t i n g ceramics to metal bonding are: a)  Chemical factors including wetting phenomena. (This includes selective d i f f u s i o n of one component into others).  b)  Mechanical factors related to the thermal expansion of the i n d i v i d u a l phases and t h e i r bulk strength.  c)  Design factors involving s i z e , shape and uses.  The f i r s t of these factors w i l l be considered.  - 2 -  B.  Factors a f f e c t i n g bonding. 1.  Wetting properties. The degree of wetting of a s o l i d phase by a l i q u i d i s  governed primarily by the surface i n t e r a c t i o n , where surface energies are playing a part, and the formation of s o l i d solution or chemical compounds by i n t e r f a c i a l reactions. I n t e r f a c i a l energies at elevated temperature and under low pressure can be most conveniently studied by a s e s s i l e drop technique as used extensively by Kingery, Huraenick and Pask > 3, 4, 5, 6, 7, 8 ) . (2  When a l i q u i d i s brought into contact with a s o l i d surface the spreading of the l i q u i d i s determined  by the condi-  t i o n that the free energy of the system as a whole should decrease.  At constant temperature and volume there are two  factors c o n t r o l l i n g the free energy, namely the change i n i n t e r n a l energy AH and the change i n entropy &S.  The l a s t 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 i n t e r n a l energy term.  I f the atoms of the  s o l i d a t t r a c t those of the l i q u i d drop, heat w i l l be liberated and, AH being negative, wetting w i l l be c e r t a i n . hand,  On the other  when the atoms of the s o l i d f a i l to a t t r a c t those of the  l i q u i d as much as the l i q u i d atoms attract each other, wetting w i l l depend on the magnitude of AH -TAS.  - 3 -  Atoms i n the-'-discontinuous regions or at the i n t e r faces do not have t h e i r normal number of neighbouring  atoms at  normal distances; they are In higher energy states compared to atoms i n homogeneous  phases  ,  S p e c i f i c surface energy i s by d e f i -  n i t i o n the Increase of free energy of a system per unit increase of area under the condition that the new energy configuration.  surface has i t s minimum  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 i n the c a l c u l a t i o n s  0  The tensions around the s e s s i l e drop resting on a s o l i d r e s i s t i n g penetration are related by Young's equation (Figure 1): V  where  S L = sv  Y  ~ LV °os  y  L v  y  e*  = surface tension of the l i q u i d ,  y gy=  surface tension of the s o l i d ,  Yg^ = i n t e r f a c i a l tension between l i q u i d and  solid,  6 = the contact angle measured between vectors y y L  and VAPOUR  -SOLID Figure 1. Surface tension forces acting on a s e s s i l e drop. * The v e r t i c a l component of the liquid-gas and interface vector i s balanced by e l a s t i c forces In the s o l i d phase.  -  The i n t e r r e l a t i o n  If  -  of parameters i n Young's equation  and i t s dependence on the nature of the reaction has been discussed graphically and thoroughly i n Appendix I.  The  equil-  ibrium of forces i n a system where low melting point compounds are formed has also been treated i n Appendix I I . Contact angle measurements i n a system with considerable reaction w i l l lead to an erroneous r e s u l t unless they are done through cross-sectionsof the specimens, and the value of y  'is modified to take care of changes i n the chemical compo-  L  s i t i o n of the l i q u i d . In his review on surface energies based on the data of the other workers, K i n g e r y ^ ) l i s t e d values f o r i n t e r f a c i a l energies i n the systems A^O-^  - l i q u i d metal,fused quartz -  l i q u i d metal and Ag or Cu - l i q u i d sodium s i l i c a t e .  He also  included values for surface energies of E^O-^, FeO,  PbO,  A1 0^ 2  and S i 0 . 2  Many of these are relevant to the present  investi-  gation and are shown i n Appendix I I I . 2.  Chemical reactions. The temperature  at which a ceramic w i l l react with  a metal depends on the co-ordination values f o r the solute and  - 5-  the solvent, thermodynamic properties, d i f f u s i b i l i t i e s of components into each other e t c . The factors involved i n such reactions are summarised as follows by Kingery^ ^:  (a) The p o s s i b i l i t y of forming a  1  s o l i d solution of Cr^O-^ - AlgO^ i n the case of Cr - AI2O3 system i s quite l i k e l y and has been found.  Similarly a solid  solution  of FeO - MgO i n the system Fe - MgO has also been reported. The formation of fofsterite (2MgO - SiO^) can also be expected i n the system S i - MgO^ ^. 1  (b) Changes i n the composition of  both the ceramic and the metal phase have been reported(e.g. NbC and Zr i n the system ,Nb - ZrC.) (c) One of the most important factors a f f e c t i n g such chemical reactions i s the s o l u b i l i t y of oxygen or other gases i n the metal and ceramic phase. Coenen^-9) considers that the r e l a t i o n s h i p between the w e t t a b i l i t y of Rd-Pt, Au-Pt, Be-Pt a l l o y s by soda-lime glass i s related to the r e v e r s i b i l i t y of the electrochemical properties of these a l l o y s , caused by d i f f u s i o n of oxygen atoms i n the a l l o y s ,  (d)  The s o l u b i l i t y of the ceramic phase or one component of the phase i n the metal i s also very important f o r any chemical ' reaction,  (e.g. WC i n a binder)  (e) The formation of an  oxide f i l m , p a r t i c u l a r l y on the molten metal phase helps the reaction to take place. of the atmosphere.  This i s primarily due to the influence  3  Adhesion of oxide to metal.  0  (A short note on the mechanism of enameling) Adherence of porcelain enamels i s generally attributed to chemical bonds or to mechanical "keying" of the enamel•  If  the surface roughness i s a factor a f f e c t i n g adherence i t i s due to the oxide phase f i l l i n g cracks i n the surfaces  The oxide  produces l o c a l i s e d stresses f o r i t s large volume and exerts a keying effect on the i n t e r f a c e  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)„  a c t u a l l y a measure of the quantity of oxide  The roughness i s  produced.  Most versions of the chemical theory r e l y on the presence of an oxide f i l m as a t r a n s i t i o n zone between the steel and the glass, with the oxide f i l m attached to each C.  a  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 m i s c l b i l i t y gap, but no systematic investigation has been reported on the behaviour of the refractory metals with silica  e  Chemical compatibility between possible crucible Mlf)  materials and s i l i c a has been investigated by F. Bacon et a l at 2500°Co  v  The s i l i c a ' was found to react with ir^idium, tanta-  lum and molybdenum but not with tungsten.  No further d e t a i l s  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 " using internal oxidation of copper v  s i l i c o n powder compacts i n a study of dispersion hardening. Economos and K i n g e r y ^ s t u d i e d the behaviour of s i l i c o n i n 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 s i l i c a t e glasses i n vacuo and under d i f f e r e n t atmospheres at 900°C (7) studied by Pask et a l  0  9  has been  No apparent c o r r e l a t i o n between the  metal - sodium s i l i c a t e contact angles was observed,.  The v a r i a -  tions i n the contact angle from metal to metal could be related to the p o l a r i s i n g power of the metal, because of the lowering of the  i n t e r f a c i a l tension, which i s i n agreement with Weyl's  theory  (17  »  l 8 )  „  I t 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 i n 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©m  Hg  0  D  on Mo and W  .  In the case of these  two metals surface absorbed gases played a s i g n i f i c a n t role In the wetting c h a r a c t e r i s t i c 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 m.m.of Hg and a considerable reaction was y  D  observed i n t h i s system when heated to 1000°C for 12 hours, with the formation of Na Ta 0^ (sodium metatantalate)in the Interface,, 2  2  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 i n He  0  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 i n a number of f i e l d s . I t i s true that considerable work was done on compound  - 9 -  (20  glasses  21)  »  v  ' for use i n electronic or mechanical applications  but the s e l e c t i v e reaction of the components of the glass makes i t very d i f f i c u l t action^ c  to analyse the fundamental mechanism of re-  No attempt has been made to evaluate the part played  by s i l i c a i n 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 r e s i s t a n t coatings on refractory metals and s i l i c i d e s have been suggested^^) <j an  (2^ a coating has been patented ' v  a  process involving such  2 *) 1  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 the same objectives can be achieved  whether  by spraying s i l i c a d i r e c t l y  i s not yet known• L a s t l y , cermet as a b r i t t l e  systems based on carbides or oxides  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 i n v e s t i g a t i o n using  refractory metals and s i l i c a glass to find out the nature of reactions between them.  -10-  In t h i s study, the refractory metals u t i l i s e d were limited to the hard b.c.c. metals of the t r a n s i t i o n groups and VI^ with more emphasis on the former'group,, Their positions i n the periodic table are shown i n Table I and selected physical properties i n Table I I . Table I . -  P o s i t i o n of the refractory metals i n the periodic table. Group  Series I V  A  VA  VI  A  VII  A  vin  A  vin  1st long  Ti  V  Cr  2nd long  Zr  Nb  Mo  Tc  Ru  Rh  3rd longi  Hf  Ta  w•  Re  Os  Ir  Table I I .  -  B  Properties of Refractory Metals.  MP. °C  BP. °C  Cryst. Structure  Density gms/cm^  Surface tensions at melting point dynes/cm  V  1900  3350  B 0C •C 0  7.2p  1697  Nb.  2hl5  3300  B «C •G a  8.56  2030  Ta  2996  6100  B 0C 0C •  16.60  2860  Mo  2610  *f800  B 00 00 0  10.20  2500  Ti  I667  3252  H •C «P e  h.50  1^60  Zr  1850  ^27  H©C P  6A9  2080  Hiiemenu  * **  B.C.C. above 882°C B.C.Co above 870°C  0  0  - 11 -  II.  EXPERIMENTAL  A. Materials. 1.  Silicao Two d i f f e r e n t kinds of s i l i c o n dioxide were used i n  t h i s investigation. gel  Preliminary work was carried out with s i l i c a  (> 99.98$ of S I 0 ) 2  supplied by Murex L t d .  In the major part of this work high purity Vycor glass c a p i l l a r i e s 3 m.m.  diameter were used.  This material, supplied  by Corning Glass Co., has a nominal composition of 99.99$ 2.  silica.  Metals. The metals used i n t h i s work were obtained i n sheet or  c r y s t a l 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. f o i l s 0 . 0 0 5 " thick.  Molybdenum and zirconium were i n  A. D. McKay Company supplied the titanium  (iodide c r y s t a l bar)as well as the zirconium. A t y p i c a l analysis of these materials i s given i n Table I I I . B.  Apparatus. The experimental set-up was the same as that used  by previous workers and i s already described i n d e t a i l ^ ^ ' 2  In Figurel.Aa drawing of the apparatus i s shown.  2  ^.  - 12 -  Table I I I .  -  Typical analysis of Metals.  Analysis (max. percent)  M.ets. C  °2  H  V  2  0.09 0.01 0.05 Bal.  V  o.o5  Nb  0.05 0.07  Ta  0 . 0 5 0.02 0.02 0 . 0 1  Mo  0.03  Zr  0.1  Ti  0.03 0.01  0.01  -  -  0.03  -  -  -  Nb  Ta  Mo  Zr  Ti  Fe  -  -  -  -  -  -  0.01 0.01  0 . 0 5 B a l . 0.18  -  0 . 0 1 0 0i+ 0 . 0 1  Bal.. 0 . 1 5  0.05  -  -  0.05  -  -  -  -  Bal.  -  Si  - 0.01 -  o  -  -  - -  0.05  B a l . 0.06 0.12 0 . 0 5 2  -  -  B a l . 0.0*+ 0 . 0 3  A furnace-tube assembly, consisting of a fused s i l i c a tube 2-1/2 inches i n diameter and 18 inches i n length, formed the vacuum t i g h t envelope i n which a 10~%i.m,of Hg. vacuum could be obtained by a two stage o i l d i f f u s i o n pump backed by a mecha n i c a l fore pump.  Hf  A molybdenum susceptor 0 . 7 5 inches i n diameter  and 3-1/2 inches long, shielded by an open loop of molybdenum could bring the specimen up to 1900°C i n vacuo.  A light  source  and an o p t i c a l pyrometer were placed outside a window i n such a way that either of them could be aligned with the furnace.  An  o p t i c a l 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  -  FURNACE ASSEMBLY  OPTICAL SYSTEM (1)  Ground g l a a s o r photographic p l a t e .  (12)  (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  (1M  Heating element, r a d i a t i o n s h i e l d and  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 .  track.  Vycor tube.  speclme  (7)  O b j e c t i v e l e n s , a h u t t e r and I r i s  (18)  Viewing window.  (8)  V e r t i c a l adjustment screw.  (19)  Brass  (9)  Water-cooled o p t i c a l f l a t .  (SO)  Optical  (10)  Water-cooled brass f i t t i n g .  (21)  L i g h t source i n t e r c h a n g e a b l e w i t h pyrometer.  (11)  Magnetic a h u t t e r .  diaphram.  F i g u r e 1,A.  fittings. pyrometer.  - 14 -  a water cooled copper Induction c o i l outside the s i l i c a tube© The temperature i n the furnace was measured by a Hartmann and Braun o p t i c a l pyrometer, Model TO-lOe. was made.  No emissivity correction  However, a v e r i f i c a t i o n of the melting point of Ni  made i n t h i s apparatus gave a value close to 1455°C and a comparison with Pt/Pt-Rd thermocouple  i n a r e s i s t o r furnace at 1600°C  gave a difference i n reading of less than Co  10°C  o  Preparation of Materials. The metal sheets and c r y s t a l rods were cut to a  diameter of approximately 1 cm.square and mounted i n bakelite. Standard metallographic techniques were applied to p o l i s h the f l a t surface.  After dismounting the specimens, they were care-  f u l l y washed i n d i l u t e HCl, d i s t i l l e d water and alcohol.  Care  was taken i n 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 c y l i n d r i c a l die of 1/4  inch  diameter, under a pressure of f o r t y tons per square inch.  After  compacting, the edges-on one end of the p e l l e t s were rounded to ensure an advancing contact angle. Where Vycor glass was used, the glass c a p i l l a r y was melted i n 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 i n contact with the metal.  The drop was  - 15  -  washed i n 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 p e l l e t s would cause considerable bulging and bloating under vacuum above 1500°C. The s e s s i l e drop picture could not be obtained i n t h i s condition.  To overcome t h i s d i f f i c u l t y , the s i l i c a gel was pre-  heated to 250°C i n dry a i r to remove hydration water.  This  prevented the drop from bulging, but evaporation rate remained high.  The condensation on the window made the experiment  difficult  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 l e v e l l e d on the metal p l a t e l e t with the aid of the l i g h t source.  The furnace was  and pumped to a pressure of 10~3 m.m.  of Hg.  The  assembled temperature  was brought to 750°C.  P u r i f i e d 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 i n 5 minutes u n t i l the temperature of the specimen reached 1500°C© At t h i s temperature neither melting nor reaction appeared to take p l a c e  The specimen was kept at t h i s temper-  D  ature at least 5 minutes for homogen^ization and to obtain the dynamic pressure equilibrium  The temperature was then slowly  0  raised to the required temperature around 1650°C, maintaining the vacuum at the lowest possible value, generally i n the range of 1 0  - 1 +  to lO'^m.m.of Hg  0  The specimen was cooled quite r a p i d l y  to 600°C In about 3 minutes  To eliminate further error due to  0  vapours and condensation on the window when reading the temperature, a l l runs were carried out at the same power input f o r a p a r t i c u l a r temperature©  Checks at the beginning of each run  showed fluctuations within 10 C© o  The drop was observed by  c a r e f u l l y focusing the image on the ground glass of the o p t i c a l system©  Several pictures were taken of the successful runs© For the X-ray d i f f r a c t i o n investigation to determine  the formation of any compounds or s o l i d solution at the i n t e r face, the drop was sheared from the metal.  Material from the  drop was removed by chipping o f f small coloured parts on' the bottom of the glass©  On the metal side, successive layers of  reaction product were s e l e c t i v e l y removed with a sharp diamond point.  The layer under investigation was crushed and prepared  - 17 -  for i d e n t i f i c a t i o n by the X-ray powder method.  The powder  patterns were obtained by exposure to copper  radiation  (A = l <?+0k X) at 25°C with Ni f i l t e r (power input: o  15 m„Ao) f o r 2 to 3 hours.  1+0 KV -  - 18 -  III. A.  EXPERIMENTAL RESULTS AND DISCUSSION.  O b s e r v a t i o n s on h e a t i n g . V a l u e s o f major v a r i a b l e s a f f e c t i n g t h e b e h a v i o u r o f  the specimen a r e shown f o r a few t y p i c a l runs i n T a b l e I V .  Table IV.  -  Experimental  Variables.  Run  Metal  Silica  T.emperature  2  Mo  Silica gel  1600  5  Mo  Silica gel  1680  26  Mo  Glass  16^0  28  Mo  Glass  1650  2. 5xl0~  lf  13  V  Glass  1650  1x10  *  20  V  Glass  1650  17  Nb  Glass  lk50  -k 2x10 ^ 5xio-5  18  Nb  Glass  1650  5x10"^  6  Ta  Silica gel  1750  7  Ta  Silica gel  1650  5x10"^  22  Ta  Glass  1650  1x10^  kk  Ti  Glass  1650  2.5x10"^  k9  Zr  Glass  1650  5x10"^  Vacuum m.m. Hg. 5xio~k  Remarks  Condensation Ibid.  1x10-^  3.5x10"  3  Condensation Ibid.  Condensation  Condensation  3  - 19 -  In a l l cases In presence of s i l i c a , a vacuum of 10"  3  0  f Hg,  mm. 0  could not be obtained, whereas with metals alone, the  affl&e order of pressure was reached quite e a s i l y .  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 s p e c i f i c surface. Reaction between s i l i c a and metal occurred i n a l l the systems to some extent.  A sequence of three stages i n the  reaction process of the same drop as observed on the ground glass of the o p t i c a l system i s shown Figure 2 - a the  0  b» C*  In  case of vanadium i n contact with s i l i c a glass, no sensible  change i n the drop shape took place up to l 5 0 0 ° C  o  Between 1550°C  and 165*0°C softening of the glass at the point of contact with the  metal i s s u f f i c i e n t to f i l l the neck around the drop with  l i q u i d materials.  This l i q u i d phase climbed along the glass  surface and eventually etched a groove i n the metal p l a t e l e t . With the formation of a considerable amount of l i q u i d 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  f l o a t i n g on the l i q u i d layer. The o r i g i n a l round shape at the lower part of the drop remained long after the l i q u i d phase grooved into the metal.  This can be seen from Figures 3 and 4**where cross-  sections of t h i s specimen showed two extreme cases after 5 minutes heating at 1650°C. the  Niobium reacted extensively with  glass (Figure 3 ) , whereas molybdenum•developed only a few  * - see Page 20 ** - see Page 21  - 20 -  F i g u r e 2 ( a ) . S i l i c a drop on vanadium ' 1 minute a t 1650°C.  after (lOx)  F i g u r e 2 ( b ) . Same s i l i c a drop on vanadium a f t e r 5 minutes a t 1650°C„ ( l O x )  F i g u r e 2 ( c ) . Same s i l i c a drop on vanadium a f t e r 15 minutes a t 1650°C. ( l O x )  - 21 -  anchorage p o i n t s around t h e neck ( F i g u r e  h),  F i g u r e 3.  C r o s s - s e c t i o n t h r o u g h s i l i c a drop on n i o b i u m . (I5x)  Figure  Cross-section through s i l i c a drop on molybdenum. (15x)  The observations reported above ruled out the use of s e s s i l e drop techniques as a convenient way of measuring contact angles i n these systems. A new mathematical model has been proposed (and given i n Appendix I I ) .  The calculations are carried out with  - 22 ~  the assumption that i n the l a s t stage of reaction, the l i q u i d surface tension i s s u f f i c i e n t 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 v a r i a t i o n s , from deep penetration into the metal (Nb, V) to a very limited corrosion  (Mo)» Most of the reactions may  be considered  place between the l i q u i d and s o l i d phases.  to have taken  No traces of gaseous  reactions with the metals wa-s revealed by metallographic  or  X-ray examination outside the contact of the l i q u i d l a y e r  0  In a l l the investigated systems, an adherent layer of glass was  observed  This t h i n layer of glass had d i f f e r e n t  0  o p t i c a l properties under the microscope i n comparison with the s i l i c a glass©  This new  glassy phase was  darker than Vycor  glass under normal r e f l e c t e d l i g h t and showed numerous bright spots and cracks i n a dark f i e l d the polarised l i g h t .  0  Both glasses were dark i n  This o p t i c a l l y i s o t r o p i c 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 -  E v i d e n c e o f l o c a l i s e d s t r e s s e s a r e 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 I n e x p a n s i o n c o e f f i c i e n t s o f t h e two phases i s most p r o b a b l y r e s p o n s i b l e f o r t h e c r a c k s observed i n t h e g l a s s . S t r e s s e s a r e a l s o r e v e a l e d by p o l a r i z e d l i g h t as can be seen i n F i g u r e 13 w i t h 1200x m a g n i f i c a t i o n o f a s o l i d i f i e d l i q u i d  phase.  A s l o w e r r a t e o f c o o l i n g was t r i e d i n a few r u n s t o t e s t t h e I n f l u e n c e o f t h e r m a l shock on b o n d i n g .  The temperature  was reduced t o 1300°C i n 15 m i n u t e s , and t h e n t o 700°C i n 10 more m i n u t e s . The p r o p o r t i o n o f f a i l u r e s remained about t h e same w i t h no s i g n i f i c a n t improvement o f t h e ceramic - m e t a l bond. When f a i l u r e o c c u r r e d on c o o l i n g , t h e drop was found to be sheared from t h e m e t a l a t t h e 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  glass.  Where t h e g l a s s drop was s e p a r a t e d 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 glass away from t h e boundary between t h e two g l a s s e s and a t about lO/i  from t h e m e t a l o r i t s compounds.  Where s i l i c a  g e l was  used I n s t e a d o f V y c o r g l a s s , b e t t e r bonding o c c u r r e d .  The  inhomogeneity o f t h e g l a s s p r o b a b l y reduced t h e 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  strain.  C r i t i c a l o b s e r v a t i o n o f s e v e r a l samples showed t h a t extensive p e n e t r a t i o n o f the g l a s s i n t o the metal  Improved  the apparent bond s t r e n g t h o n l y by s e a l i n g t h e drop w i t h m e t a l o x i d e as i n F i g u r e 3.  On t h e o t h e r hand i t has been observed  2 h  t h a t an e x c e s s i v e amount o f s i l i c a t e g l a s s a t t h e i n t e r f a c e I s d e t r i m e n t a l t o t h e bond s i n c e t h e f r a c t u r e always s t a r t s and d e v e l o p s i n t h a t phase. The systems i n v o l v i n g vanadium and t a n t a l u m 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 t h e case o f vanadium  good bonding was caused by t h e f o r m a t i o n o f a amount o f l i q u i d l a y e r t h a t n e a r l y c o m p l e t e l y I n t h e case o f t a n t a l u m helped  t o achieve  showed  considerable embedded t h e d r o p .  t h e e x t e n t o f o x i d a t i o n and d i f f u s i o n  t h i s bonding.  S i l i c a g l a s s adhered t o n i o b i u m 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 t o produce m e c h a n i c a l l o c k i n g as i n F i g u r e 3.  Molybdenum showed v e r y poor adherent p r o p e r t i e s w i t h  V y c o r g l a s s under t h e e x p e r i m e n t a l better properties with s i l i c a  gel.  c o n d i t i o n s and somewhat Zirconium  and t i t a n i u m  showed poor adherence i n a l l c a s e s , m o s t l y because o f t h e b r i t t l e n e s s of the i n t e r f a c i a l C.  Microscopic 1.  oxides.  Observations.•  Molybdenum - s i l i c a . The r e a c t i o n w i t h molybdenum was n o t e x t e n s i v e ; most  o f t h e i n t e r f a c e was c l e a n and f r e e from c o r r o s i o n ( F i g u r e 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. (300x).  Only a t a m a g n i f i c a t i o n g r e a t e r t h a n 900x c o u l d a roughened i n t e r f a c e be observed ( F i g u r e 6 ) .  Figure 6.  Molybdenum - s i l i c a i n t e r f a c e . Unetched. (900x).  When t h i s o c c u r r e d , a v e r y t h i n (ICyu) porous l a y e r suggests t h e presence o f d i s s o l v e d oxygen i n t h e m e t a l .  26  The formation of cracks i n the S i 0  2  glass i s generally-  found to be associated with regions of deeper corrosion,,  Care-  f u l etching with Fe C I 3 (10 gm of crystals f o r 100 c „ c of water) f a i l e d to give any evidence of oxide  formation.  The presence of s i l i c a t e glass i n t h i s system was d i f f i c u l t to ascertain because of the small amount of reaction. No l i q u i d 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 i n the bottom of Figure §  a  The major part of  the dark area v i s i b l e at the interface was most probably voids introduced during the preparation of the sample for metallographic examination, as they do not show extinction i n the polarized  light. 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 i n 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. the T i 0 - S i 0 2  2  This i s attributed to the eutectic i n  system at  l5h0 C. 0  Considerable  opacification  of the drop took place though no bond was observed.  - 27  Zirconium  -  oxidised r e a d i l y at the point of contact  between the metal and the s i l i c a . a whole range of coloration was  The metal became b r i t t l e  and  observed i n the reaction zone  0  No further i n v e s t i g a t i o n has been carried out with these two metals« 3.  Vanadium - s i l i c a . Reaction i n t h i s system was  the drop was  extensive.  In most cases,  half embedded i n the l i q u i d formed by i n t e r f a c i a l  reactions.  The layer of l i q u i d materials as seen on the ground  glass at elevated temperature was  about two millimeters thick.  This layer s o l i d i f i e d on cooling and held the s i l i c a glass drop f i r m l y  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 the metal.  deeply i n  From l e f t to r i g h t the s i l i c a glass, the s i l i c a t e  and the vanadium metal are c l e a r l y v i s i b l e .  Around the  sili-  cate a sponge-like texture i n vanadium suggests extensive oxidation which was  l a t e r ascertained by etching.  (Note the  d i f f e r e n t nature of the two glasses and the shining traces of metallic materials embedded i n the s i l i c a t e ) . These metallic p a r t i c l e s i n 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 Unetched.  interface. (300x).  Figure 7 (b). Vanadium - s i l i c a interface. Same as Figure 7 (a). Dark f i e l d . (300x). the  melt on cooling.  I t seems reasonable to assume that t h i s  second glass contains oxides of vanadium and i s probably some kind of vanadium  silicate.  The only phases detectable by X-ray powder methods at room temperature at the metal interface were vanadium and  - 29 -  VO  n  o (Appendix IV-B).  This compound has a range of oxygen to  metal r a t i o from 0 , 5 to 0 , 9 ,  Above this value, a defect (35) l a t t i c e with vacancies i n vanadium atom sites f o r m s o w  Formation of such low melting compound i n the system of V - S i 0  2  could not be properly accounted f o r , but there are  very low melting e^tectics i n both the V 0 ^ - S i 0 2  and the V 0 ~ S i 0 2  2  2  s  v  s  t  e  m  (650°C)  system (around 1350°C),  A study of the extent of reaction showed that the vanadium oxide dissolved i n 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 t h i s conclusion, as no evidence of large amounts of vanadium oxide was ever encountered i n the quenched specimens, k.  Niobium - silica„ The nature of the reaction with niobium was d i f f e r e n t  from that with vanadium.  The s i l i c a t e glass was present i n  considerable amounts as can be seen i n Figure 3  ?  but because  of the protective action of the oxide, the penetration i n the metal was much more l i m i t e d . Even f o r a specimen heated for h a l f an hour at 1650°C where the groove was deep, an oxide layer of 50 to 2 50/awas always found i n the reaction zone.  - 30 -  Figure 8 (a) shows the oxide darkened by etching with HpSO^ (95%) - HNO3 (70%) and HF (h-8%) i n the r a t i o ( 5 : 2 : 2 ) ,  Figure 8 (a). Oxide layer on niobium. Etched. (300x).  Figure 8 (b). Oxide layer on niobium. Same as Figure 8 (a). Dark f i e l d . (300s). The oxide i s Nb 0^, as found by X-ray at room 2  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 i n the reaction zone are possibly saturated with oxygen.  (Note also i n the dark f i e l d  (Figure 8 (b)) the highly  fissured s i l i c a t e glass i n contact with the oxide. glass appears immediately behind t h i s phase ) 0  The s i l i c a  The only other  compound traced by X-ray powder technique i s /3 c r i s t o b a l i t e which i s the high temperature stable form of s i l i c a but is unstable at room temperature.  This shows that p a r t i a l c r y s t a l l i s a t i o n  of the glass took place i n the region of s t a 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  workers^^\  The general pattern of the penetration of the oxide into the metal and the s i l i c a t e into the oxide formed suggests that the t o t a l reaction was a two step process: the  metal with oxygen, then d i s s o l u t i o n 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 . the  saturation of  As revealed i n Figure i i ,  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 Nb C>5 was reported to be as low as 1 5 1 2 ° C 2  5.  ( 3 0 )  (or 1 ^ 6 0 ° C  ( 3 1  ^)  Tantalum - s i l i c a . The tantalum system showed some features i n common  with niobium as may be expected from the nearly i d e n t i c a l properties of those two metals.  The oxide, found by X-ray to  0  - 32 -  "be Ta 0^, r e a c t s w i t h t h e m e t a l p r e f e r e n t i a l l y along g r a i n 2  boundaries,  l e a v i n g oxygen s a t u r a t e d m e t a l i n t h e o x i d e s c a l e  0  I n t h e v i c i n i t y o f t h e o x i d e l a y e r , but n o t always i n c o n t a c t w i t h i t , p l a t e l e t - l i k e sheets a r e v i s i b l e ( F i g u r e 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 o f R^SO^ (96<£) HNO^  (70£) and HF (k$$) i n r a t i o  F i g u r e 9°  (2:1:1).  Oxide p r e c i p i t a t i o n i n Ta. F.tched, (250x).  L e n t i c u l a r p l a t e l e t s were observed a l o n g t h e c u b i c planes o f tantalum Cathart  ( 3 3 )  ,  a t lower temperature by G e b h a r d t ^ ^ , 3  and B a k i s h  ( 3 l + )  and a t 2500°C by B a c o n  ( l l + )  .  I t seems t h a t more t h a n one cause i s r e s p o n s i b l e f o r t h e i r appearance.  A t 500°C, B a k i s b / ^ 3  concluded  that  t h i s o x i d e was T a 0 ^ , but G e b h a r d t ^ ^ a t 900°C c o u l d n o t 32  2  i d e n t i f y I t as any known t a n t a l u m  oxide.  - 33 -  Gebhardt  proposed three explanations f o r the  formation of p l a t e l e t - l i k e oxides.  F i r s t , the formation by  engassing when the formation of p l a t e l e t s occurs simultaneously with the solution of oxygen; second, formation by dissolving T a 0 ^ from the outer skin into the metal and, t h i r d , p r e c i p i t a 2  t i o n from supersaturated solution. Microhardness  (50 gm load) has been measured on  specimens annealed at 1650°C and on the same specimens i n p  contact with s i l i c a . lum and 450 kg/mm throughout the  The value was 130 kg/mm  for pure tanta-  for tantalum with s i l i c a nearly uniformly  cross-section.  This increase i n hardness, i f attributed  solely to  oxygen, corresponds to 2 . 5 atom % of 0 (22) metal according to available data the dissolved  2  i n the  v  The s o l u b i l i t y of oxygen at 1000°C i s approximately 2 . 5 atom % and decreases with lower  temperatures.  Out of the three suggestions made by Gebhardt, p r e c i p i t a t i o n from supersaturated solution seems to explain best the presence of p l a t e l e t s i n t h i s  system.  The main evidence i n favour of p r e c i p i t a t i o n i s the random d i s t r i b u t i o n of the p l a t e l e t s below the oxide layer, and not necessarily  i n 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 i n the oxide scale between the metal and the drop,  Figure 10 (b). Same as 10 ( a ) Dark f i e l d .  0  At the l e f t i s a portion of s i l i c a t e  (250x). penetrating  the oxide revealed by i t s clear appearance under dark f i e l d and also a m e t a l l i c p r e c i p i t a t i o n covering the oxide interface.  silicate  At the r i g h t , grain boundary d i f f u s i o n can be  seen as well as the dark n u c l e i for oxidation inside the  - 35 g r a i n s ( s e e a l s o F i g u r e 13),  The l e n t i c u l a r appearance o f t h e  s i l i c a t e may be a t t r i b u t e d t o c r y s t a l l i s a t i o n o f t h e g l a s s . F i g u r e 11 (a) and 11 (b) show c l e a r l y t h e d i f f e r e n c e between t h e o x i d e s and t h e 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 F i g u r e 11 ( b ) , A l s o v i s i b l e a r e porous spaces I n t h e s c a l e t h a t makes p o s s i b l e t h e oxygen d i f f u s i o n towards t h e 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 - o x i d e i n t e r f a c e . ' Unetched. (450x).  F i g u r e 11 ( b ) . Same as F i g u r e 11 ( a ) . Dark f i e l d .  0*56x).  -  -  36  Observations on the l i q u i d formed around the neck between drop and the metal r e v e a l i n t e r e s t i n g Figure  12  features  0  shows such a l a y e r , a t some d i s t a n c e  from  the neck.  Figure  12.  Outside l a y e r s on Ta„ Unet ched, (550x).  From top t o bottom, the f o l l o w i n g recognisable,,  Tantalum d i s p e r s e d  features are  i n a grey T a 0 ^ matrix, 2  a r i b b o n - l i k e zone almost f r e e from oxides and g r a i n boundary oxidation, transported  and above the r i b b o n - l i k e l a y e r , m a t e r i a l s by the l i q u i d d r o p  samples r e v e a l e d metal s u r f a c e  D  n  Close observation  being  of various  that the r i b b o n - l i k e l a y e r was the o r i g i n a l O x i d a t i o n progressed below t h i s l a y e r without  much r e a c t i o n i n t h i s p a r t i c u l a r phase.  Stable  a l l o y forma-  t i o n s by Ta with S i might have acted as a p r o t e c t i v e b a r r i e r to f u r t h e r o x i d a t i o n  o f the phase.  A l l metallic particles  i n the matrix became dark on e t c h i n g ,  i n d i c a t i n g the presence  - 37 of oxygen i n s o l u t i o n . F i g u r e 13 under p o l a r i z e d 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 l a y e r s .  This  f i g u r e also reveals that the tantalum i n the matrix, as w e l l as i n the r i b b o n - l i k e layer i s free from p i t s .  Figure 13,  Outside l a y e r s on Ta, Polarized l i g h t . Unetched. (1200x).  Figure l^f shows m e t a l l i c p r e c i p i t a t i o n both i n s i d e the grains and i n the g r a i n 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. P o l a r i z e d l i g h t . Unetched. (1200x)  - 38 -  An attempt has been, made t o see t h e n a t u r e o f t h e r e a c t i o n by o b s e r v a t i o n o f t h e u n p o l i s h e d s u r f a c e o f t h e r e a c t i o n zone as i n F i g u r e  F i g u r e 15.  15.  S u r f a c e l a y e r s on T a . (*+5x). Unpolished.  D i s t i n c t d i f f e r e n t zones o f r e a c t i o n p r o d u c t s a r e visible. place.  C o n s i d e r a b l e 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  No compound c o u l d 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  investigation.  I t can be seen t h a t t h e c r y s t a l l i s a t i o n i s  much more e x t e n s i v e near t h e s i l i c a g l a s s drop than a t a c e r t a i n d i s t a n c e from i t . B a c o n ^ ^ p u b l i s h e d w i t h o u t comment a microphotograph o f a 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 prolonged h e a t i n g at  2500°C.  H i s observations confirm ours.  Clearly visible  a r e t h e grey m a t r i x 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 , t h e r i b b o n l i k e phase, and, i m m e d i a t e l y b e h i n d , t h e l e n t i c u l a r platelets.  oxide  - 39 -  Do  X-Ray Investigation on the Tantalum Interface„ X-Ray d i f f r a c t i o n powder patterns from various  layers at the interface indicate that the oxide present at room temperature i s T a 0 ^ . 2  The remaining l i n e s of the sample  layer patterns could be interpreted as tantalum, s i l i c a , and s i l i c o n but because of overlapping of many l i n e s their presence could not be ascertained. Besides these l i n e s are also others common to nearly a l l the samples, (see pattern 1 ^ 7 7 Appendix I V - D )  These l i n e s  0  could be attributed to one or more of the nine A o S T M 0  Index patterns f o r the f i v e s i l i c i d e s l i s t e d Ta S i  Q o 5  , Ta S i  0 > 1 +  ,  0  0  Card  (Ta S i , Ta S i g 2  Q  Ta S i ^ ) .  Tantalum oxides corresponding to the formulae Ta^O, Ta 0 , Ta 0 , Ta 0 > 2  22  hatfebeen found by X-ray techniques  by Shonberg^35) on heating hydrides or carbides i n 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 t h e i r formation. I t i s f e l t that at the present time not enough observations have been gathered on tantalum oxides and  - ho  silicides.  The investigations i n t h i s f i e l d are not yet  completed and the opinions of the workers are often controversial© Various attempts have been made to f i n d better evidence for s i l i c i d e s or sub-oxides,  (a)  Mixtures of Ta  and S i ( < 3 0 0 mesh) have been sintered i n vacuo f o r two at 1 6 5 0 ° C and analysed by X-ray method.  hours  There i s a certain  analogy between the pattern of these mixtures with that obtained i n the sample layer, s p e c i a l l y i n the low d spacings, as shown i n Appendix IV-C.  (b)  S i - T a 0 ^ ( a n a l y t i c a l purity) and S i 0 2  2  Ta mixtures have been sintered at 1 6 5 0 ° C  i n vacuo.  Their  pattern reveals also some of the extra l i n e s of the sample layer pattern,  (c)  F i n a l l y , mixtures of T a 0 ^ and S i 0 2  2  have  been f i r e d at 1 6 0 0 ° C under oxidising atmosphere i n a graphite r e s i s t o r furnace; the r e s u l t i n g X-ray patterns did not produce further information on any p a r t i c u l a r compound. In the course of t h i s study, cones were prepared with mixtures of T a 0 t j and S i 0 2  of various proportions.  2  melting point of these cones was noted.  The  Mixtures corresponding  to nominal(weight percent) 8% T a 0 ^ and 20% T a 0 ^ melted at 2  about 1 5 8 0  2  ± 10°C. In conclusion, i t may be said that there i s strong  evidence f o r the presence of T a 0 ^ as the major constituent, 2  and some evidence f o r the possible presence of Ta and S i 0 . 2  The remaining l i n e s could probably be attributed to s i l i c o n , s i l i c i d e s , s i l i c a t e s or sub-oxides.  -1+1  IV.  -  THERMODYNAMIC CALCULATIONS.  Observations during t h i s investigation showed a sens i b l e rate of vaporisation and the presence of oxides,  Silicides  might be also present i n the case of tantalum. Each of these observations w i l l be discussed with the aid of available thermodynamic data, A,  Pressure i n 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 r e s u l t of the reducing action of the metal whereby SiO i s formed. In the f i r s t case, the following reactions must be considered.  (The references and calculations are i n  Appendix V-A).  (  1  )  ( 2 )  (3)  S  l  S i  2  (gl)~*  S 1  °2  (gl)-"  S i  0  SI0  2  (  g  l  )  CD * ° 2  °(g)  S i 0  2  (  +  g  1  )  /  2  (g)  AF  °1650°  °2(g)^ °l650°C F  p  s i 0 2  = + C  =  =  +  1 6 3  °  7 6 0  °  6 8 o 3 6  K c a l  K  c  a  °  1  '  7 x l 0 - atm. 7  The t o t a l pressure above a system i n equilibrium where only the reaction given by equation (1) takes place is 7 . 5 x l O ~  7  m.m. of Hg.; f o r equation ( 2 ) the t o t a l pressure  - 42  i s 1.05x10 num.  of Hg. and f o r equation (3)  num.  i t i s 7.6x10""  of Hg. The t o t a l vapour pressure i s obtained by the addi-  t i o n of the p a r t i a l pressures of a l l the gaseous molecules produced.  In t h i s case the t o t a l pressure w i l l be controlled  by equation (2). The t o t a l pressure of the system as indicated by the i o n i z a t i o n gauge i s 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.  I f the pumping capacity i s  s u f f i c i e n t to balance the gas evolution, low readings w i l l occur.  (b)  The SiO(g) i s unstable and w i l l condense on the  walls forming S i 0 , thereby removing oxygen from the atmosphere, 2  (c)  The vaporisation process i s not e n t i r e l y i n 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 v o l a t i l e metallic oxide forms at the i n t e r face, the t o t a l equilibrium pressure could be much higher. However the small area of reaction probably does not produce enough gaseous products to affect the furnace.  sensibly the pressure i n  -  -  43  I t seems that a slow d i s s o c i a t i o n of s i l i c a at into SiCLand 0  i s responsible for the major part of the raise  o  S  S  1650°C  cr  i n pressure above the nominal working pressure of 10" mm. 7  of Hg,  0  Bo  S t a b i l i t y of the oxides„ . The presence of oxides i n the specimens cooled to room  temperature  i s an Indication that reactions took place at higher  temperatures. The r e l a t i v e s t a b i l i t y of various oxides has been evaluated by calculating the pressure of the gaseous species i n equilibrium with the oxide at a given temperature. are shown i n Table V (see Page 44)„  The results  The references f o r the  data and the calculations are i n Appendix V-B. The conclusions of these calculations are? Mo0 and MoO^ 2  conditions  (1650°C  and  are v o l a t i l e under experimental 1.3x10"^  atm,),  Any oxide i n 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.  'was found at room  Only MoO^  although i n theory both MoO^ Several factors may  temperature,  and Mo0 could be present© 2  account f o r a greater s t 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:  The layer between the drop and the metal Is subjected to  (a)  Table V,  S t a b i l i t y of oxides.  1650°G  Reaction M o 0  3(s,l,g) —  3 M o 0  M o 0  M  o  0  2(s,l)  3 ( s , l ) -^-(Mo0 ) 3  2(s,l)  ^-Mo  +  1  M o  V  2  2 ° 5 ( s , l ) -- ^  2 V 0  M  o  g  V( )  V 0  (g)  V 0  A  T  5»6xl0~ atm.  °  M  o  6 8xl0~  atm.  b  %00  s  +  1  /  2  +  1  /  2  P0  °2 °2  P n  3  =  "  1  0  h  x  l  2  "  •*»•.  6  2  =  0 ?  0  '  h  a  t  -  1  1  90 2 x 1 0 - 1 + 3 t m  2,lxl0"  ll+  PQ = 2,1x10 2  2  m  So^xlO"- - atm.  = 2  p =  f 1/2 Oo  °  l O " ^ atm. 5 x l O ~ atm.  3.1xl0  atm.  Wb0  — *> 2 N b 0 ( ) + 1/2 0 NbO( ) + 1/2 0  2 ( s )  NbO( ) s  s  atra.  10~35  s  -Hj-  N b ( ) + 1/2 0 s  2  2  o 2  PQ =  1.65xl0~°atm.  p -  2 9xlO  PQ =  5,75xl0  2  0 2  2  - 1 1  0  atm,  = 1 2  atm  8xl0" atm. 8  1  lO  0  -lf  3.8xlO" ^atm.  atm,  Py = 8x10-7 atm,  (g)  2  atm.  atm.  2 2  o  - 3 2 a  a  tm t  0  f f l o  1.6xl0" 3atm, 1  No q u a n t i t a t i v e data a r e a v a i l a b l e f o r the v a p o r i s a t i o n N 205(s)  10  3  ( )  + 1/2 0  s  S  5  2  2 ( s , l ) -- ^ 2 ° 3 ( s , l )  V0( )  =  PQ^-U-XIO"  g)  PMc0  2(s,l)  s)  atm.  10  a  lf  g  2 ° 3 C s , l ) "—e>2V0^  h 5x10" PM0O3  2  V  27°C  727°C  °2(g)  ° 2 ( s , l ) -— M o 0 ( )  2 V 0  V  2  3 ( g )  3 °°2(s,l) '—*- 2 M o 0 ( ) + 3  /  + 0 (  ( S j l )  M  Vapour p r e s s u r e s  o f V 0^, 2  h5  surface forces.  The r e s u l t i n g p r e s s u r e c o u l d be o f t h e o r d e r  o f h a l f an atmosphere as c a l c u l a t e d i n Appendix I I „  The  o x i d e s c o u l d be s o l u b l e i n t h e m e t a l o r i n t h e g l a s s , t h e r e b y increasing their  stability.  The c a l c u l a t e d e q u i l i b r i u m oxygen p r e s s u r e s f o r d e c o m p o s i t i o n o f vanadium o x i d e s show t h a t VgO^ and V 0 a r e 2  u n s t a b l e a t l 6 5 0 ° C under t h e e x p e r i m e n t a l 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 a t t h a t temperature and may c o e x i s t w i t h As observed m e t a l l o g r a p h i c a l l y t h e o x i d e i s n e a r l y  V 0^° 2  always i n c o n t a c t w i t h t h e g l a s s .  S o l u t i o n o f t h e o x i d e may  account f o r a g r e a t e r s t a b i l i t y than c a l c u l a t e d .  The o x i d e  found a t room temperature does n o t c o r r e s p o n d t o t h e f o r m u l a VO b u t t o a s o l i d s o l u t i o n r a n g i n g form 0 . 6 t o 0 . 9 oxygen, thus i n c l u d i n g t h e two more t h e r m o d y n a m i c a l l y s t a b l e o x i d e s . A l l t h e o x i d e forms o f n i o b i u m a r e s t a b l e a t 1 6 5 0 ° C and s h o u l d i n t h e o r y be p r e s e n t , but o n l y N b 0 r ; was found i n 2  d e t e c t a b l e amounts a t room t e m p e r a t u r e . on Nb a t 1 0 0 0 ° C by o t h e r w o r k e r s ^ ^  l e d t o t h e same c o n c l u s i o n ,  2  Nb0  2  Oxidation studies  was observed o n l y i n t r a c e amount and t h e major component  o f t h e adherent p o r t i o n o f t h e s c a l e was N b 0 ^ . 2  Two t a n t a l u m  o x i d e s a r e known t o e x i s t a t e l e v a t e d t e m p e r a t u r e : TaO  ( 3 7 )  2  . Ta 0r; 2  and a t  T a 0 ^ and  1  and TaO a r e i n e q u i l i b r i u m a t 1 8 0 0 ° C and 2 0 0 ° C  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 , face hut might  also  be  glass with silicao  TaO  should escape from the  stabilised  On c o o l i n g ,  by f o r m a t i o n o f  the monoxide  inter-  a compound  transforms  to  pent-  oxide: 5 TaO  T a 0 ^ •+•  This behaviour mental observation of  the  the  3 Teu,  2  is  i n good a g r e e m e n t w i t h t h e  of m e t a l l i c  + y  The t o t a l at  1650°C, a s s u m i n g  atm  0  Samples of  This is  of  the the  Co  Silicideso  (  6  )  2  (SiOp)  of  pressure  for  TaO = 2 / 5 A F ° T a 0 t j ,  0  2  condensation showed  (glass)  products  the  collected  presence of S i 0  the presence of  2  ( ? )  equation is  on  2X10-  (U-)  1  cooler  and  Ta 0^o 2  g a s e o u s TaO and S i O  temperature,  Many o f  the  c o u l d be i d e n t i f i e d  lines  i n the patterns  as p o s s i b l e  following discussion w i l l i s not  (TaO)  (Si0 )  thatAF  evidence  elevated  reaction  kind:  * + SIO, » (g)  e q u i l i b r i u m vapour  furnace  at  this  of  (g)  d  (TaO)  The  2  of  part  constituent  t o be T a 0 ^ »  c a n b e assumed  Ta + S i O p - ^ T a O , x  The m a j o r  found by X - r a y t e c h n i q u e  b e t w e e n t a n t a l u m and s i l i c a  experi-  p r e c i p i t a t i o n i n the outer  s c a l e r u n n i n g from the d r o p .  s c a l e was  parts  ^  thermodynamically  lines  show t h a t impossible  for  the  of  tantalum  suicides.  The  formation of  i n the  case of  scale  silicides  tantalum.  _ i+  -  7  The process could not Involve gaseous species since the suspected s i l i c i d e s are found d i s t r i b u t e d i n the oxide scale©  The t o t a l reaction may be written, with Me for-any  p a r t i c u l a r metal© (x + y) Me  + S I 0 — • Me 2  0  x  2  + Me  y  Si  ( 6 )  i e© the sum of two reactions: 0  (1)  xMe + S i 0 - * M e  (2)  yMe + S i —*• Me  2  x  y  0  2  + Si  ^  Si  ( 8 )  Calculations have been carried out for Mo, V, and Ta,  The source of data i s given i n Appendix V-C.  Nb, Oxides  s t a b l e / at 1650°C are indicated i n the column "oxide" and s i l i c i d e s with highest and lowest free energy of formation i n column " s i l i c i d e s " .  (See Table VI - Page 48)©  I f the sum of A F ( y ) and A F 0  s i l i c i d e s are expected©  0  ^  i s p o s i t i v e , no  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 s i l i c i d e s i s impossible according to equation ( 6 ) ,  The large errors i n free energies i n 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 ° (  and Element  Mo  V  Nb  Ta  A F ° / Q \  Oxide  7  )  at 1 6 5 0 ° C f o r xMe + S I 0  Me 0  2  x  2  +Si  at 1 6 5 0 ° C f o r yMe +> Si--* Me Si v  AF°( ) Kcal/mole of 0  Silicides  7  2  Z^F°(8) Kcal/atom of SI  + 61  Mo Si  - 17 - 4  M0O3  ¥ 67  Mo #'3 4  - 26 t 3  VO  + 2  V Si  - 41 i 5  Mo0  2  v o 2  3  Nb0  2  2  r * 22  Nb20c;  t 38  Ta 2 0c;  t 10  Ta 0  3  10  TMb  lo82  Si  -  25  - 10  - 19 i 7 Ta Sip  - 17 - 3 - 36 ± 4  been observed to dissolve e a s i l y i n the glass, i t i s not impossible that the a c t i v i t y of oxides has been lowered by solution. This c a l c u l a t i o n gives some reasons to expect  sili-  cides i n tantalum rather than i n 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 a l t e r the r e l a t i v e 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 f a c t o r ;  they w i l l not form any extensive s o l i d solution,, The interatomic distance i n Nb and Ta ( 2,86 S) i s large enough to allow some i n t e r s t i t i a l solution of small atoms l i k e C, N, Oo Do  Silicates, 1,  S i l i c a t e s with tantalum. The major components at the tantalum-silica interface are  oxides, at elevated temperature, and TaO  vaporises.  at that temperature.  T a 0 ^ melts at 1877°C - 100°C ?  Thus one should not expect a l i q u i d layer Among the other compounds l i q u i d at that  temperature are S i (melting point lU-20°C) and Ta Si2 (melting point l^+OO^),  X-Ray i n v e s t i g a t i o n , however, showed that i f  present they are only i n small amounts. To account f o r the presence of a l i q u i d layer at 1650°C two p o s s i b i l i t i e s have been considered:  oxygen s o l u b i l i t y  i n tantalum and a possible eutectic i n the Ta 05-Si02 system. 2  The lowering of the melting point of tantalum with increasing amount of oxygen i n solution has been calculated using the Clausius-Clapeyron T -T Q  = -  equation:  (T x T ) AH  R  n  x  2  -  N  g  T p  where N  s  i s the mole f r a c t i o n of any substance i n solution and  A HTP  t  n  checked  e  heat of f u s i o n o f t h e m e t a l .  The procedure has been  on t h e 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  e x p e r i m e n t a l v a l u e s a r e g i v e n i n T a b l e V I I and t h e c a l c u l a t i o n s a r e i n Appendix V-D.  T a b l e VII„  Maximum s o l u b i l i t y o f oxygen i n n i o b i u m and t a n t a l u m .  Calculated Nb  Maximum  solubility  Maximum  solubility  0,72  weight %  1915°C  11 atom % 1920°C  Temperature  The  0 . 7 3 5 weight % 1920°C  Temperature Ta  Observed  e x t r a p o l a t e d maximum s o l u b i l i t y o f oxygen i n  t a n t a l u m i s 9 atom % and t h e c o r r e s p o n d i n g m e l t i n g 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 o f oxygen cannot  f o r t h e l i q u i d phase 2.  Silicates Attempts  account  observed.  with other metals. have been made t o f i n d a r e l a t i o n between  g l a s s f o r m a t i o n w i t h SiOp and c a t i o n f i e l d approach v e r y s i m i l a r t o D i e t z e l ' s ^ 3 9 ) h W  0  s t r e n g t h , an f  o u n (  j  a  correlation  between t h e m e l t i n g p o i n t o f o x i d e s 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 Z" i s t h e v a l e n c e o f t h e c a t i o n and r,  "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 c o r r e l a t i o n was found between the temperature of the lowest eutectic i n various s i l i c a - oxide binaries and the cation f i e l d  strength as shown i n Figure 16.  (See Page 52)  Systems with more than one oxide show various cation ,  field Fe  + + +  strength c o e f f i c i e n t and melting temperature  , V^ ,  V >  +  ++  + +•  (Fe  ,  e t c ) . More interesting are the liquidus temTa 0^  peratures f o r unknown or incompletely studied systems. and Nb 0^ show a melting point around 1550°C, 2  600°C,  around  2  Mo 6+around  650°C.  It i s also quite clear that the metal reacting extensively with s i l i c a i n t h i s 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 D ^ and MoO^ 2  are too v o l a t i l e at 1650°C to play a  s i g n i f i c a n t r o l e i n forming eutectics under the experimental conditions of t h i s 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  -  V.  Bond formation involves the formation  53  -  CONCLUSIONS.  between s i l i c a and r e f r a c t o r y metals  of oxides and of s i l i c a t e glass at the  interface. These two mechanisms have been studied with, d i f f e r e n t approaches:  v i s u a l observations  temperature, metallographic  of the interface at elevated  techniques on tapered  sections  and  analysis of the bonded interface by X-ray technique. The reaction s t a r t s above 1500°C with the  formation  of a l i q u i d layer at the s i l i c a - metal contact points.  Geo-  metrical factors and r e l a t i v e high temperature properties of the various S i 0 2 ~ r e f r a c t o r y metal compounds influence the bonding of the two phases. Thermal stresses resulted i n f a i l u r e of the bond at the glass - s i l i c a t e boundary i n most cases.  F a i l u r e occurred  generally inside the s i l i c a t e glass on the a p p l i c a t i o n of external forces. The major I n t e r f a c i a l reaction i s oxygen corrosion by the oxygen r i c h  silicate.  X-Ray data show the oxides are the p r i n c i p a l components at the i n t e r f a c e .  Although X-ray evidence se&s; inconclu-  sive about the presence of c r y s t a l l i s e d s i l i c a t e s or s i l i c i d e s , there are indications that they might be present,  particularly  - 9+ i n the case of Ta-S10  2  system.  The lack of data on s i l i c a t e s of refractory metals i n general, and especially tantalum s i l i c a t e s and  sub-oxides,  put serious l i m i t a t i o n s to the interpretation of the microscopic observations and the interpretation of X-ray  investigations,  A limited study of the liquidus i n the Ta C>5-Si0 2  2  binary showed that mixtures with a nominal composition around Si and 20% by weight T a 0 c ; melt at about 1 5 8 0 ° C - 1 0 ° C 2  0  - 55 -  VI.  RECOMMENDATIONS FOR FUTURE WORK.  This investigation has opened up new avenues of research p a r t i c u l a r l y concerning the formation of s i l i c a t e s from refractory metal oxides with s i l i c a glass.  Crystalline  c h a r a c t e r i s t i c s and properties of these s i l i c a t e s should be studied i n more d e t a i l . Titanium and zirconium deserve, further attention. They could give a broader base to interpret the behaviour of the refractory metals i n contact with s i l i c a . The oxides and s i l i c a t e layers i n the tantalum system should be suitable f o r further study with an electron probe micro-analyser. Further studies i n the Ta-0 and Ta 0^-Si02 binary 2  systems, by X-ray and metallographic techniques are also necessary to interpret the tantalum- - s i l i c a  system.  -  56 -  APPENDIX I Discussion of Young' equation. Young's equation gives a r e l a t i o n between 7 ^ , the surface tension of the l i q u i d , X g , the surface tension of the s o l i d , 6 is  and /g^, the i n t e r f a c i a l tension between l i q u i d and s o l i d . the contact angle measured between vectors y^y and )^ as i n 1  Figure 1 7 .  SOLID Figure 1 7 . Surface tension forces acting on a s e s s i l e drop. This equation i s shown g r a p h i c a l l y at coni^ei /  g  in  Figure 1 8 .  F i g u r e 1 8 . R e l a t i o n between cos 8 , a n d for a s o l i d w i t h surface energy 1 0 0 0 ergs/cm  :  _ 57 -  Murray^* " ) pointed out the following f a c t s : 4  2  (1)  When 0 = 9 0 ° , y  (2)  At constant r  S  L  i n 9 i f 0 < 90°,  s  =^  S  L  whatever the value of  andXg, a decrease o f c a u s e s and an increase i n 6 i f 6 > 90°.  practice changes InYg^  a decrease In  o f f s e t t h i s 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  (h)  For low energy surfaces, 0 can be zero only with a l i q u i d  erg/cm . 2  of low surface tension, provided y g ^ <Y$° (5)  I f y-^ i s low (100 erg/cm ) then y 2  values, approaching (6)  that o f X g ,  S  L  can have quite high  even when 0 i s zero.  The lower the values of y-^j the greater the s e n s i t i v i t y of 0 for a given change i n y g ^ .  S e s s i l e drop measurements and surface tension calcul a t i o n s loose much of t h e i r significance as soon as a considerable amount of chemical reaction has taken place between the l i q u i d and. the s o l i d phase.  The complicated  geometry of the drop  brought about by such a reaction and change i n the value o f Y ^ increase the chances of error i n the measurement of 0 .  58  Equation  (9) becomes i n that case (Figure 19)  ~ >SL X  -  L  C 0 S  0  s i n 9 - 7-1$  y  - >L  cos 6 =  sin 0 =  0  0  VAPOUR  SOLID  Figure 19»  Complete surface equilibrium for imperfect wetting.  (10)  - 59 APPENDIX II  Static  equilibrium of a p a r t i a l l y molten drop on a s o l i d . The mechanism by which a l i q u i d phase promotes cohesion (If •>)  between solids has been discussed by Ford and White"; l i q u i d wets the two solids, they w i l l be attracted by  .  I f the  capillary  forces at the point of contact as i l l u s t r a t e d i n Figure 2 0 .  Figure 2 0 .  Section through a s o l i d p a r t i c l e resting on a plate showing lens of l i q u i d at point of contact.  Figure\ 20 i s a representation of a drop at the beginning of the- reaction at the neck.  Growth of the neck  at the point of contact i s generally attributed to p l a s t i c flow, viscous flow or d i f f u s i o n .  I f the bottom of the drop  i s a portion of a spherical sector, the c a p i l l a r y i n the lens w i l l be given by S =X  L  (_!  r where  3  1_) r  2  i s the surface tension of the l i q u i d .  suction  - 60 -  The curvature r ^ l i e s i n the horizontal plane and r  2  i s at r i g h t angles to t h i s curvature i n 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 t o t a l cohesive force through the whole area 27vr of A r |  3  exerted by the lens i s the sum  S owing to suction and 27\r^y^  owing to the tension  on the surface.  Let t h i s formula be applied to a t y p i c a l drop soon after reaction has started: Data:  r  2  T3  =  0 . 0 1 cm.  *  0.125  (assumed):  cm. 5 0 0 dynes/cm  2  Density of the drop at room temperature: 2.2.  - 61 -  Area o f t h e neck: VT r ^ =  5.10"^ cm  Approximate volume o f t h e drop: The downwards p u l l  f =  10  - 3  cm . 3  (2)  5.10 x500 ( T - ^ T + T T T ^ p ) u.Ol 0.12? _2  2,700 dynes 2 . 7 5 gm Downwards  - -75 5x10-2 2  pressure  This indicates a rather the molten l a y e r i n c r e a s e s  = 5^0 gm/cm . (±1/2 atm.) 2  s t r o n g bonding f o r c e .  i n thickness,  When  the influence of r  2  d e c r e a s e s , b u t w e l l b e f o r e r ^ reaches a v a l u e comparable t o r the s u r f a c e o f t h e l i q u i d i s d i s t o r t e d by a k e y i n g e f f e c t o f the  edges. Figure  22 shows a drop t h a t 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 .  Figure  22.  Silica-tantalum interface a f t e r 2 minutes a t l650°C.  (10 X )  2  -  A sharp edge i s c l e a r l y visible„  62  -  When t h e t h i n  interfacial  l a y e r wants t o expand beyond t h e edges, i t s s u r f a c e 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 2 3 , wha-fc i s i m p o s s i b l e .  SOLID  F i g u r e 23» K e y i n g on o f a l i q u i d two s o l i d s o  between  P o s i t i o n 3 w i l l be t h e p o s i t i o n i n t o which t h e l a y e r w i l l be lockedo  A t t h e p o i n t A, r  2  = oo „  The c u r v a t u r e I s  p o s i t i v e above t h a t p o i n t , and n e g a t i v e below. I t may be assumed t h a t t h e t o t a l c o n t r i b u t i o n o f r  2  ing  w i l l be n e g l i g i b l e i n t h a t c a s e . cohesive  The v a l u e o f t h e r e m a i n -  f o r c e w i l l be: f  =  5xl0~  =  200 dynes  =  0 , 2 2 gm  ?  x 500 x (  By t h a t time t h e m e t a l w i l l be corroded  0 o  \ ^ 2  by t h e l i q u i d as shown  i n F i g u r e 3 , and t h e c u r v a t u r e a t t h e bottom w i l l g i v e an upwards p u l l o f magnitude f'„  63  Data: - h = R =  1*3 «  0 . 0 3 cm. 0,5  cnio  0 , 1 2 5 cm,  A = area of the spherical zone = 27^R h=2x3,l +x0 5x3xl0" 5  o  = 9,4  f' =  =  2 x 9  oT°  = ^ =  10"  2  cm , 2  -  &do  dynes  1,9 gm.  This value Is well i n excess o.v4-r the weight of the drop: V  3,14 x „ 3  =^Ar =  Weight  3  =  2 , 2 x ,116 =  3  =  ,116 cm . 3  . 2 5 5 gm.  I t may be expected that the drop w i l l have a tendency to f l o a t on a viscous layer after a certain time during the experiment.  Slow movements of the drop from one side to the  other hatyebeen observed i n some cases.  2  _ 64 -  APPENDIX I I I  Table VIII.  Surface energies and interface energies i n (9)  vacuo and inert atmosphere.  Oxides  2° (s)  "  A 1  2°3(s)  "  A 1  2° (s)  "  A 1  3  3  P  F  Temperature °C b  e  (D  ( D  Fused q u a r t z ( ) - CU(i) s  Surface energy erg/cm * 2  400  1450  1000  1770  1570  2300  1120  1370  Ag( ) - sodium s i l i c a t e ^ )  900  1040  CU( )  900  1500  s  - sodium s i l i c a t e ^ )  S  Be 0 2  PeO  ( 1 )  PbO  (1)  A1 0 2  A 1  900  3 ( 1 )  3 ( 1 )  2°3(s)  Fused q u a r t z ( ) s  79.5  1420  585  900  132  2080  700  1850  905  1100  250  - 65 -  APPENDIX IV A.  Comparison of A.S.T.M. standard Mo and  M0O3  w  *th  sample  layer X R-film 16 #3. Table No.  Sample layer  IX.  Mo St 'd.  M0O3  5 - 0508  h - 0809  d g  I/l!  1 2 3  7.0100 3.8100 3.^600 3.2600  Halo 10  5  2.65VO  10  6  2.3100  10  7  2.2200  100  8  1.9800  *+0  9  1.7^00  5  10  1.5700  11 12  h  13  1^  d  2  I/l!  5  10  2.2250  100  80  1.57^0  21  1.281+0  80  1.0000 .9080  20 t+OB 60B  1.2850 1.1120 0.9950 0.9085 0.84-05  39 11 17 7 26  .8^+05  St'd.  d  8 •  i/ix  6.930 3>+ 3.810 82 61 3.>+63 3.260 100 3.006 13 2.702 19 35 2.655 3 lines < 1 2 2.309 31 18 2.7H 2.190 3 V 1.996 k lines < 21 1.733 17 < 15 5 lines 6 1.587 8 lines < 1 5  - 66 -  B.  Comparison of A.S.T.M. Standard V - and - VOQ  with  sample layer X-R f i l m 1634,  Table " X  V St «d„  Sample layer No.  d  v  1 - 1224  2  I / I ,  1  2.377  20B  2  2.194  60  3  2.144  100  4  2.085  60  5  1.502  30  6  1.438  60  7  1.247  20  8  1.231  40  9  1.196  10  1.032  d  8  0  I/l!  o  St'do 10 - 313 0 o 9  dX  2.38  2.14  20  100 2.06  80  1.45  100  1.24  60  40  1.19  60  20B  1.03  50  0.94  50  0.92  70  1.51  1.24  0.96  7  20  3  >  0.88  1  0.81  3  0.76  1  0.71  1  - 67 -  C.  Comparison o f A.S.T.M. Standard Nb, N b 0 ^ , c r i s t o b a l l i t e 2  with sample l a y e r X-R f i l m 1595. Table  Sample l a y e r  Nb S t ' d .  (1-1185) No.  1 2  d  8  i / ld % 1  I/I  50 VO 20 30B  I  V.lVO 3.928 3.V86 3.1V5  5 6 7 8 9 10  2.852 2.731 2.586 2.53V 2 . WO 2.329  20 20 20 20 20 100  11 12 13  1.972 1.900 1.828  20 5 10  IV  1.655  30  1.65  20  15 16  1.537 1.VV2 1.3^8  10 10 80  1.3>+  32  17 18  19 20  1.270  20  1.170  20B  1.060,  10B  21  0.992  10  22  0.888 0.835  VOB 20B.  23  2.33  1.16 1.0V  .88  100  6  10B  6  XI;.  Nb 0^ S t ' d . (5-0352)  . /3Cri. ( 2-588)  2  d I  I/I  5.2VO V.320  Halo 50  3.931 3.V8V 3.1V0  100 50 100  2.855 2.728 2.590  50 50 50  2.VV7  70  2.120 1.962 1.908 1.820 1.790  50 50 70 70  1.662 1.572 1.5V3 1.V57 1.336 1.322  70 70 70 70 70 70  1.226 1.209 1.197 1.1VV  70 70  1.022 0.999  50 50  1  d 1  I/Ix  V.lVO  100  2.920  5  2.530  90  2.170  10  1.691 1.639  5 70  1.V57  60  1.376  20  1.263  30  1.206  50  1.127 1.087 1.030  20 10 10  0.927  10  0.838  5  50  50  70  - 68  Do  Comparison  o f Ta sample l a y e r p a t t e r n s .  Four t y p i c a l p a t t e r n s are reproduced i n T a b l e X I . F i l m No. 1577 i s a p i c t u r e o f the tantalum i n t e r f a c e . F i l m No. 1632 i s a p i c t u r e o f mixture o f 50% by weight Ta and S i heated f o r two hours at 1650°C i n vacuo. F i l m No. 1610 i s a p i c t u r e o f a mixture o f 50% by weight T a 0 ^ and S i s i n t e r e d a t 1650°C i n vacuo. 2  F i l m Wo. I6h2 i s a mixture o f 8% S i 0  2  and 72$ T a 0 ^ by weight 2  heated f o r 1 hour a t l650°C i n a i r . A.S.T.M. Card 8-255 p a t t e r n has been added f o r comparison. Dotted l i n e s i n d i c a t e t h a t a few l i n e s w i t h low i n t e n s i t i e s have been omitted i n t h i s p a t t e r n f o r convenience.  - 69 T a b l e XIL A .S.T.M. 8-225 Ta 0^ 2  d2  I/h  d  3.15 3.09  2.55 2.46 2.43 2.36  90  100 50  20 100 60 20  2.11 2.02 1.95 1.83 1.80 1.76  10 30 50 40 40 20  1.66  100  1.33  70  1610  1577  1632  Interface  Ta & S i  8  5.220  3-8?  T y p i c a l patterns of the t a n t a l u m system.  d  8  I/I,  TapO^&Si d A  I/l!  10  3.850 3.680 3.390  90 5 5  3.140 3.090  95 40  2.830 2.720 2.540 2.450 2.420  5 5 10 70 100  2.240 2.180 2.110 2.020 1.930  5 10 45 5 40  1.800  3.400 3.280  1642  4.27  5  3.84  5  30 30  Ta 0 5&Si0 2  d  8  6.120  10  4.500 4.240 4.040 3.840  5 5 100 90  3.390  20  3.120  5  3.00  5  2.83  5  2.830 2.730  -5 5  2.43 2.40  50 100  2.440  10  2.250  10  2.120 2.040 1.930  10 5 40  1.800 1.760  20 20  1.660 1.530 1.470 1.340  90 33 30 40  1.230 1.210  40 40  1.150  20  2.970  100  2.550  5  2.420  10  2.190 2.110  40 30  20  1.930 1.860 1.820 1.760  100 30 10 40  1.640 •  70  1.650  100  1.350 1.310 1.290 1.220 1.210 1.190 1.170 1.150 1.140 1.020 1.000 .977  "5 30 20 30 5 40 10 10 10 5 20 5  1.3^0 1.300 1.280  5 60 20  1.36  30  1.23  20  1.170  20  1.17  10  1.130 1.020 1.000 .976  70 30 10 80  .99  5  2.11  70  1.94  20  2  70 (cont'd)  Table x i i  T y p i c a l p a t t e r n s of t h e t a n t a l u m system,,  AoS ,T.Mo  1577  1632  Ta 0c;  Interface  Ta & SI  8-225 2  d A  1/^  d 8  d 8  o930 o925  .933  20  5  .903  .891  60  ,858  5 5  10  .852  20 10  .834 .816  .805  .799 .796 .79^  5  10  5  40  30  4 lines  I/I  5 5 5  a 916 .908 .800  .851 .833 .828 .819  .817 .805  ,800  x  40 20 40  10 50  10  etc. 6 lines  1642  1610 TagOc^&Si d 8 I/I  Ta 0 5&S10 2  1  2  d X  .933  10  .892  10  ,853  10  .826  10  ,810  60 10 10  ,806 .798  APPENDIX V Thermodynamic Calculations A.  Calculations on silica© The data f o r £ F ° are taken from E l l i o t  , except  for the vapour pressure of SiO taken from Tombs and Welch 6  F  °Si0 (gi) 2  = - 36.018 Kcal/mole  °Si0( )  " 58.316 Kcal/mole  =  g  1.  .  " " 126,685 Kcal/mole  ^F°si(g) A F  v  Calculations of the t o t a l vapour pressure at equilibrium  for equation: S i 0  2(gl) ~* (g) * S i  log 1  1 0 0  p °2 0 o  From stoAchiometry:  =-  p  Q  °2(g>  163-760 . if. 575x1.923 = p  s l  - ——  2 log p  S i  = -18.615  log p  S i  = - 9.3075  log p p  t o t e  tot.  A F  ?650O  =  +l6  C  3 . 7 6 0 Kcal  - 18.615  s  P  t o t i  = - 9.3075 + 0.301 = - 9.006, =  1 0  ~  9  a t r a  «  72  2.  C a l c u l a t i o n of the t o t a l vapour pressure a t equilibrium  for equation: S  1  0  2 ( l ) "  log  1  >  S  i  p p  l o g  (  0  tot  F  x p *) - -  2 p ^ Q  S  3  p  p  6 8  ° 3 5 9 Kcal s  -  7-77  0  0 * 2  3 / 2  0 2  10  s  0  0  2 x (p )  1 Q  02(g) ^ ° 1 6 5 0 ° C  /2  P S 1 0  -  S i Q  r - 7.770  t o t . = " § " 770=0.301) - 0.V78 = - V.856 (  p. ,  B.  1  + 1-  )  g  K r iog  0  From sto&chiometrys  log  (  0  -  l.VxlO"^ atm.  C a l c u l a t i o n of s t a b i l i t y of oxides. AF°  for V 0 ^ , V 0 2  2 ?  V^,  VO, N b 0 , Nb 0^ are taken 2  2  from G l a s s n e r ^ ^ ) . the pressure of sublimation of VO and M0O3 and £ F° f o r Mo0 , M0O3, from E l l i o t t P \ 8  2  Extrapolations, when  necessary were carried out using Figure h - 8 .  (p.290).  As an example, calculations of the s t 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° V 0  =  - 132 Kcal/mole.  "  - 231 Kcal/mole.  2  a F° V 0 2  r\ F° VO  3  =  -  79 Kcal/mole.  73  1.  V 0 2  2 V  5 ( g )  V2  log  1 0  P  0  log  1 0  p  0 2  2V0  2  (  S  )  1/2 l o g  2  ^ V 0 2  P  1 0  0  2  s  )  SlO Po  V 0 2  V2  3 ( s )  iog l o  V.  V0  (  s  )  P  -  V  1/2 log p  2  2  PQ  2  (  s  )  1/2 0  +  "  -  1 8  (  s  h  1/2 0  ) +  = -  g  )  *° „ V.575x1.923  ™  =  (  1 3  °  6 1  =  +  l  8  K c a l  l 6 5 o  oc  r + 6  °  K G a l  °  = -  114  1/2 0 <  ^ °1650°G F  g)  -§2 _ V 575x1.923  = = 6.82  13.6V  2  = - 2.1xl0~  l l f  atm.  )  =  °  2.1xlO" ' atm,  PQ  ho  50 C o  l 6  '"  = -  p  F O  - 2.0V8  A F 2  2  g  = - 2 Kcal,  c  14  =  (  o  1.25X10" " atm.  PQ  v c y - v o  0  6  0  5.  5  '°3  "  2  log  6  2 2 ?  ^  2 ( g )  -  =  +  n  = °°  r  l  1  =  1 0  Po  ^°  )  3 . 5 X 1 0 " atm.  2  2 V0  Sio  g  V.575x1.923  -  0  (  -  S  PQ^ 3.  2  0.V5V  2  l o  1/2 0  +  =  3 ( s )  O  (  u ^ x T ^  =  2  PQ 2.  0  2  8.05x10"  /  atm.  = 4  6  Q  K c a X  '  °  7h  C.  C a l c u l a t i o n on  sHicides.  Values of A F ° 0nlyAH° ^g 2  f o r oxides are taken from  - or the s i l i c i d e s o f Mo,V, f  O  K  published„  Wb,  Elliot{?K Q  Ta have been  The v a l u e s d i f f e r w i d e l y between workers.  The  fH-6) v a l u e s p u b l i s h e d by Searcy\_  have been chosen because they  are the r e s u l t o f p e r s o n a l d a t a and c r i t i c a l e v a l u a t i o n o f the l i t e r a t u r e . supposing A H  Values f o r A F  have been evaluated by  0  constant and an entropy o f f o r m a t i o n of  0  2 cal/mole x  °Ko  D„  o f the m e l t i n g p o i n t o f Ta by s o l u t i o n o f  Lowering  The  lowering i n m e l t i n g temperature  T -T^ Q  0 ° 2  o f any  substance r e s u l t i n g from the presence o f s o l u t e i s T O  where H  s  - T, 1  =  R AA H tl  (  T  l  x  T  Q>  <  N . ^  T p  i s the mole f r a c t i o n o f any  s  s o l u t e and AH^p  the heat  of m e l t i n g o f the s o l v e n t . The  l i m i t of s o l u b i l i t y i s determined  by the  inter-  s e c t i o n o f the l i n e showing the lowering o f m e l t i n g p o i n t w i t h amount o f s o l u t e , and the experimental v a l u e s o f s o l u b i lities  o f Q>2 * a  lower temperature.  the e x i s t i n g Nb-0  system  T h i s method was  and the r e s u l t was  agreement w i t h p u b l i s h e d d a t a .  tried  on  found i n good  S i n c e tantalum and  have c l o s e chemical p r o p e r t i e s , i t i s expected  that  niobium this  method w i l l be a p p l i c a b l e t o tantalum a l s o w i t h good accuracy,  - 75 a. - Nb-0 s o l u b i l i t y of 0  2  a t 500°C  :  0 2 5 weight %  1650°C  t  O 68 weight #  o  0  A H p = 6 ,,430 cal/mole T  T  = 2690°K  Q  If N  1)  = 1CT  o  2  1 98xl0"  (2 690xTi)  2  0  0  2)  I f N * = 4xl0~ T  -T °"  b  0  2  0  - Ta-O  1600°C :  6  atora%  1800°C i  7o5  atom%  = 10~  T -T 1  2  a 1.98xlO~ "  If N  -  T -T  =  °  3»75 atom#  s 32 50°K  Q  If N 0  a t 1200°C :  2  = 7.500 cal/mole  T p  T  3)  1  6.430  1  A H  2)  2  - 1O98XHX10^ (2 690XT )  s o l u b i l i t y of 0  1)  p  6,430  1  (3250xTi)  2  7,500 3xlO  - 2  1.98X3X10"  2  (325OXT1)  7 500  1  P  If N  - 6xl0~  mm  „ 1.98x6xlO~ (3250xTi)  2  2  - 76 -  Figure 2k.  Determination of l i m i t of s o l u b i l i t y of 0 i n Ta and Nb. 2  1 2 3 h  -  Liquidus l i n e Liquidus l i n e S o l u b i l i t y of S o l u b i l i t y of  for Ta. for Nb. O2 i n * T a . O2 i n ^ N b .  -  77  BIBLIOGRAPHY  lo  Kingery, W D ; J . Am Ceram. S o c ,  2,  Kingery, W.D. and Humenik, M ; Jo Phys  3.  A l l e n , BoGo, and Kingery, W D ; A I o M E  ho  Kurkjian, C R . and Kingery, W.D.; JoPhySoChem.  5o  Halden,  6  Humenik,  0  7o  8.  0  0  c  (1953)  36  0  0  F.Ao  Mo  o  0  0  10.  Chem., £Z  D  0  Trans , 0  0  0  0  Zackay, V.,Pask A , et a l , J . Am. Ceram. Soc. 2§. e  557  18  (195V) 8V  (1953)  0  0  Soc.  VO (1957)  269  Kingery, W D ; Property measurements at high temperature. J . Wiley & Sons ( 1 9 5 9 ) 0  0  Coenen, M ; Paper given to the meeting of the German Soc. of Glass Technology.  Freudenstadt  (1958)  llo  Rodin, T.N.; J . Am. Chem S o c ,  12  Moore, D.G. and Embawks, A.G.; JoAmoCeramoSoc. 21  (1956)  13.  King, B.W., Tripp, H.P., et a l JoAmoCeramoSoc. h2  (1959) 50V  lh  Bacon, J.F., Hasapsis, A A  0  0  31  961  (1955)  0  Fulrath, R.M., M i t o f f , P., and Pask, A J . Am Ceram  359  6 0 (1956)  and Kingery, W D ; JoAm GeramoSoCo 21 0  (1953)  2 1 f (1959)  £2  and Kingery, W D ; J.phySoChem.  0  9o  362  0  0  0  £ 2  (1950)  V3V3 357  and Wholley, J.W.;  Physics and Chemistry of Glass  1 (I960)  90  59-777  15.  Adashi, M. and Grant, N.J.; W.AoD.C. Technical Report  160  Economos, G. and Kingery, W„D ; J.AmoCeram.Soc. 2§. ( 1 9 5 3 )  17.  Weyl, W.A.;  180  Weyl, W.A.; Coloured Glasses,p.331-Soc.of Glass Techn-Sheffield  19.  Yaeger, L W ;  20.  Pincus, AoG.;  J . Am. Ceram. Soc.  21.  Navias, L.;  J Am. Ceram. Soc. 21  0  0  0  A.S.ToM. Stand.  V6 (19V6)  Adhesion and Adhesive,  0  ^6  p.8l  *+03  1506  Wiley  (1953)  152  (195V-)  329  (L95V)  - 78 BIBLIOGRAPHY (cont'd)  Proc. Plasee 2  F i t z e r , E.;  23o  Campbell, E . et a l ; U.S. Pat. 2,771,666  24.  N a c h t i g a l l , E. and K i e f f e r ; German Pat. 895,848(Nov.5, 1958)  25.  K i e f e r , R., Benesovsky, F. and Konopicky, F ;  d  Seminar  Reutte (1956)  22.  (1956)  0  Ber. Deutc keram. Ges. ^1 (1954) 223 26.  Clarke, J.F.;  Thesis, Univ. of B r i t i s h Columbia (1959)  27.  Hasselman, P.H.; Thesis, Univ. of B r i t i s h Columbia (1959)  28o  Shoenberg, N.;  29.  Klopp, W.D., Sims , C.T. and Jaffee, R.I.;  Acta Chem. Scand.  8/1 (1954) 221  Trans. A.S.M. 30.  Orr, R.L.;  31.  Kubaschewski, 0. and Hopkins, B;  J . Am. Ceram. Soc.  %L (1959)  282.  (1953) 2808  21  Oxidation of (1953) p . 9 Butterworth  Metals and A l l o y s . 32.  Gebhardt, E.;  3 r d Plansee Seminar  33. 34. 36.  Cathcart, J . , Bakish, R. et al;J.Electrochem.Soc. 107 (1950) 668 Bakish, R.; J . Electrochem. Soc. 10J? (1958) 71 Shafer, H., Brewer, L.; Private communications reported by M i l l e r : Tantalum and Niobium p.602 Butterworths (1959)  35.  Shonberg, N.;  37.  Peters, E.;  38.  E l l i o t t , J . F . and Gleiser, M.; Thermochemistry f o r  Acta. Chem. S cand 8/2 Private communication  Reutte -Austria  (1954)  0  Steelmaking V o l . 1. Reading MA 39.  D i e t z e l , A.;  240  Zeit.Fur Electrochemie  (I960)  }+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)  (I960)  Powder Metallurgy  64  42„  Murray, P.;  43o  Ford, W.P. and White, J„; J.Trans Brit Ceram SoCoj>6 (1957) 309 0  44 < > Tombs, N o C and Welch, A J ; J 0  0  0  0  0  Iron and S t e e l Insto  172 (1952) 45«  Glassner, A„;  46o  Searcy, A W„; 0  69 - 78  A„NoL. - 5750 J  a  Am  0  Cerara S o c 0  _)+0  (1957)  431  

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