Growth parameters for refractory carbide single crystal

Bartlett, R. W.

English

NASA Technical Reports Server

June 27, 1966 
Report N o .  9 
Quar t e r ly  S t a t u s  Report  No. IX/March 1, 1966 t o  June 1, 1966 
STUDY OF GROWTH PARAMETERS FOR 
REFRACTORY CARE3 IDE SINGLE CRYSTALS 
By: R .  W .  B a r t l e t t  
SRI P r o j e c t  N o .  FMU-4892 
Prepared for: 
Nat iona l  Aeronautics and Space Administration 
Washington, D . C .  
Contrac t  N o .  NASr-49 (19) 
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Approved : 
, 
F .  A.  Halden, D i r e c t o r  
Ceramics and Metal lurgy Div is ion  
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N66 30848 
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CONTENTS 
LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . .  iii 
I INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . .  1 
I1 SUMhLARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . .  2 
I11 CRYSTAL GROWTH STUDIES . . . . . . . . . . . . . . . . . . .  3 
A. Arc-Verneuil Growth . . . . . . . . . . . . . . . . . .  3 
1. Apparatus . . . . . . . . . . . . . . . . . . . .  3 
2. Experimental Results . . . . . . . . . . . . . . .  5 
B. Metal Solution Growth . . . . . . . . . . . . . . . . .  8 
IV FUTWUCWORK . . . . . . . . . . . . . . . . . . . . . . . .  10 
ii 
ILLUSTRATIONS 
F i g .  1 Tantalum Carbide Boule (TaC ) Grown i n  the 1-x 
Arc-Verneuil Furnace (5-27-66) . . . . . . . . . . . . . 4 
F i g .  2 Circuit  for  Superimposing a High Frequency 
S tab i l i z ing  Signal on the Arc-Electrode . . . . . . . . 7 
iii 
I . . .  
I INTRODUCTION 
I n t e r e s t  i n  t h e  r e f r a c t o r y  carb ides  has  increased  r e c e n t l y  i n  
a n t i c i p a t i o n  of many new app l i ca t ions  r e q u i r i n g  t h e  use  of super- 
r e f r a c t o r i e s .  However, dur ing  t h e  r e sea rch  and develoixient viork on 
t h e s e  m a t e r i a l s ,  d i f f i c u l t i e s  have been encountered i n  a t t a i n i n g  and 
reproducing des i r ed  phys ica l  p r o p e r t i e s .  L i t t l e  is  known about u l t i -  
mate i n t r i n s i c  phys ica l  p rope r t i e s  o r  about t he  inf luences  of stoi- 
chiometr ic  changes, impur i t i e s ,  and g ra in  boundaries on these  prop- 
e r t i e s .  In  obta in ing  t h i s  type  of information,  s i n g l e  c r y s t a l s  of 
va r ious  carb ide  compositions would be of g r e a t  va lue .  A t  p r e sen t ,  
t h e  only c r y s t a l s  r e a d i l y  ava i l ab le  a r e  of t i t an ium ca rb ide ,  grown 
by t h e  Verneui l  p rocess ,  and l i t t l e  is  known of t h e i r  s t r u c t u r e  and 
p e r f e c t i o n .  
Stanford Research I n s t i t u t e  has  been engaged by the  Nat ional  
Aeronautics and Space Administration t o  i n v e s t i g a t e  the  a p p l i c a t i o n  
of new techniques and procedures t o  t h e  growth of s i n g l e  c r y s t a l s  
of tantalum ca rb ide ,  hafnium carb ide ,  and s o l i d  s o l u t i o n s  of t hese  
ca rb ides .  The new techniques being inves t iga t ed  f a l l  i n t o  two 
c l a s s e s :  (1) u t i l i z a t i o n  of a-c a r c  mel t ing  f o r  Verneui l  c r y s t a l  
growth; and (2) a p p l i c a t i o n  of r ecen t ly  developed methods of l i q u i d  
metal  s o l u t i o n  growth of crystals.  
----..-- 
1 
, a . . .  
I1 SuNlMARY AND CONCLUSIONS 
During t h i s  r epor t ing  period t h e  new arc-Verneuil c r y s t a l  growing 
fu rnace  w a s  i n s t a l l e d  and tantalum ca rb ide  c r y s t a l  growth experiments 
w e r e  resmed. Mechanical opei-atiun of t h e  c r y s t a l  grower is satis- 
f a c t o r y ;  i t  produces uniform ope ra t ing  condi t ions and s y m m e t r i c a l l y  
shaped boules .  However, a major impediment t o  tantalum carbide (TaC) 
c r y s t a l  growth is  deca rbur i za t ion  of t h e  boule ,  which occurs i n  a l l  
atmospheres i n  which a low voltage a-c a r c  is s t a b l e .  Under ope ra t ing  
cond i t ions  which provide improved s toichiometry r a t i o s  (>l% hydrogen), 
a r c  s t a b i l i t y  i s  s t i l l  q u i t e  poor. Hafnium ca rb ide  and tantalum-hafnium 
carbide powders in  a s i z e  range s u i t a b l e  f o r  c r y s t a l  growth have been 
o rde red ;  c r y s t a l  growth of t h e s e  m a t e r i a l s  w i l l  be i n i t i a t e d  as  soon 
a s  t h e  powders are received.  
Metal s o l u t i o n  (menstruum) growth experiments have no t  yielded 
tantalum carbide c r y s t a l s  l a r g e r  than 0 .2  mm. Fu r the r  work i n  t h i s  
a r e a  has  t h e r e f o r e  been discontinued. 
2 
I11 CRYSTAL GROWTH STUDIES 
A .  Arc-Verneuil Growth 
1. Apparatus 
The new arc-Verneuil  furnace chamber was received and i n s t a l l e d  
dur ing  March. Axes of t h e  three  h o r i z o n t a l  e l e c t r o d e s  and the  seed 
ho lde r  of t he  c r y s t a l  grower form a common i n t e r s e c t i o n  which d e v i a t e s  
less than 0.010 inch during r o t a t i o n  of a l l  fou r  members. Some d i f f i -  
c u l t i e s  w e r e  encountered with the primary vacuum s e a l  f o r  t he  ho r i zon ta l  
e l e c t r o d e s .  Since de l ive ry  lead t i m e  f o r  t h e  Viton quad-rings used i n  
t h i s  s e a l  was 9 weeks, t h e  seal  was redesigned t o  employ O-rings,  
Operat ion with a Viton O-ring sea l  is s a t i s f a c t o r y  i f  p e r i o d i c  replace-  
ments of overheated seals are made. Larger motors f o r  r o t a t i n g  t h e  
e l e c t r o d e s  have been s u b s t i t u t e d  f o r  t h e  o r i g i n a l  motors, and some of 
t h e  o r i g i n a l  e l ec t rode  bushings have been replaced with bushings t h a t  
a r e  more concent r ic .  With these  minor changes, opera t ion  of the  furnace 
appara tus  is  good. The shape symmetry of boules  is  much b e t t e r  than 
was poss ib l e  with the  previous apparatus;  an example i s  shown i n  F ig .  1. 
The modified a-c a r c  power supply works s a t i s f a c t o r i l y ;  no d i f f i c u l t i e s  
have been encountered i n  providing s u f f i c i e n t  power. 
The i n i t i a l  tantalum carbide c r y s t a l  growth experiments w e r e  made 
wi th  the  audio-driven pulsed-bed p a r t i c l e  f eede r  descr ibed i n  the  
e igh th  Quar te r ly  S t a t u s  Report .  The p r i n c i p a l  disadvantage of t h i s  
powder feeder  is its small  powder s torage  capac i ty ,  which is  l imi t ed  
by t h e  tendency of excess  powder t o  overdamp t h e  v i b r a t i n g  s i e v e  and 
prevent  p a r t i c l e  flow. Since the s to rage  capacity of tantalum carb ide  
is  marginal f o r  growth of a complete boule ,  a METCO feeder  was even- 
t u a l l y  subs t i t u t ed  f o r  t h e  audio-driven f eede r .  
3 
. .  
FIG. 1 TANTALUM CARBIDE BOULE (ToC,-) GROWN IN THE ARC-VERNEUIL 
FURNACE (5-27-66) 
4 
2.  Experimental Resu l t s  
A series of runs was made t o  explore  ope ra t ing  parameters of t he  
new furnace and modified power supply, which are s u i t a b l e  f o r  t h e  growth 
of tantalum carbide (TaC) boules. I n i t i a l l y ,  i n  t hese  runs,  t he  hot- 
pressed tantalum carbide ”seeds” u s u a l l y  f r a c t u r e d  when the  arc w a s  
s t r u c k .  Th i s  apparent ly  w a s  caused by t h e  f a c t  t h a t  t h e  minimum power 
ou tpu t  of t h e  modified power supply is considerably h ighe r  t h a t  t h a t  
i n  the previous u n i t .  Thus, when t h e  a rc  is  i n i t i a t e d ,  t h e  seed is 
now severely shocked thermally.  On the  o t h e r  hand, growth of a boule 
d i r e c t l y  on a g raph i t e  f o o t  usual ly  r e s u l t s  i n  s p a l l i n g  of t h e  boule 
a t  t h e  i n t e r f a c e  because of poor adhesion. Th i s  problem h a s  been c i r -  
cumvented by using s h o r t  tantalum p lugs ,  1/8-inch i n  diameter , i n s e r t e d  
i n  the  top  of t h e  seed holder .  The tantalum m e l t s  t o  form a base upon 
which tantalum ca rb ide  is securely welded p r i o r  t o  growth of t he  boule.  
However, i f  it is  necessary to  r e s t r i k e  t h e  a r c  a f t e r  a temporary power 
f a i l u r e  of a l l  t h r e e  e l e c t r o d e s ,  t he  boule u s u a l l y  f r a c t u r e s .  
LOSS of carbon from t h e  carbide boule is  one of t h e  major o b s t a c l e s  
t o  be overcome i n  growing single-phase c rys t a l s .  Our previous work h a s  
ind ica t ed  t h a t  hydrogen add i t ions  t o  t h e  environment g r e a t l y  r e s t r i c t  
t h e  ex ten t  of carbon loss. However, hydrogen and ace ty lene  a d d i t i o n s  
t o  argon i n  excess of 5% a t  0.5 t o  1 a t m  t o t a l  p re s su re  cause consider- 
a b l e  a r c  i n s t a b i l i t y .  Although improved a r c  s t a b i l i t y  is a t t a i n e d  a t  
lower p re s su res ,  t h e  r a t e  of deca rbur i za t ion  of t h e  ca rb ide  simulta- 
neously inc reases .  In 5% t o  7% hydrogen a t  a t o t a l  p re s su re  of 0.5 atm 
a r c  performance i s  s t a b l e  and y i e l d s  a boule with very l i t t l e  subcarbide 
(Ta2C). 
boundaries l oca t ed  nea r  t h e  boule su r face .  Although t h e  e x t e r n a l  sur- 
f a c e s  of t h e s e  boules are y e l l o w ,  probably because of r e c a r b u r i z a t i o n  
during cool ing,  cross-sectioned su r faces  are m e t a l l i c  gray.  The carbon 
content  by X-ray l a t t i c e  parameter measurement is  i n  the lower region 
of t h e  tantalum ca rb ide  phase f i e l d .  Although boules  grown i n  ace ty l ene  
have no t  been sect ioned f o r  carbon a n a l y s i s ,  t h e r e  is  no apparent  
advantage t o  t h e  use of acetylene i n  s t a b l i z i n g  the  a r c .  The primary 
The subcarbide t h a t  is  present u s u a l l y  decora t e s  subgrain 
5 
j . t,' 
tantalum carbide g r a i n  size continues t o  be similar t o  t h e  g ra in  s i z e  
observed i n  our previous experiments. Usually t h r e e  t o  s i x  g r a i n s  
are i n t e r s e c t e d  by a long i tud ina l  c r o s s  sec t ion  of t h e  boule.  
Most of t he  p re sen t  e f f o r t  is d i r e c t e d  toward inc reas ing  the  
carbon content  of tantalum carbide boules .  For t h i s  purpose, s e v e r a l  
runs w e r e  made t o  determine t h e  f e a s i b i l i t y  of maintaining molten 
tantalum carbide w i t h i n  a small c r u c i b l e  cup d r i l l e d  i n  the  top of a 
g r a p h i t e  seedholder.  The graphi te  w a l l s  maintain a high carbon 
s toichiometry i n  tantalum carbide,  and conceivably one could u t i l i z e  
a h o t  g raph i t e  s h e l l  i n  c lose  proximity, surrounding t h e  molten cap 
of t h e  boule ,  t o  assist i n  control  of t h e  f i n a l  c r y s t a l  composition. 
Tantalum ca rb ide  bars 1/8-inch i n  diameter w e r e  melted in  a 1/4-inch 
OD g raph i t e  rod with t h e  p re sen t  saturable-core r e a c t o r  power supply.  
The c o l o r  of t he  tantalum carbide i n  c ros s  s e c t i o n  remained yellow, 
which i n d i c a t e s  a high carbon content .  
no t  p r e s e n t .  Unfortunately,  erosion of t he  g r a p h i t e  c r u c i b l e ,  even 
wi th  an argon b l anke t ,  i s  very rapid and t h e  g raph i t e  con ta ine r  
d i sappea r s  almost as f a s t  as  tantalum ca rb ide  melt ing occurs .  Con- 
sequent ly  i t  is very d i f f i c u l t  t o  s o l i d i f y  tantalum ca rb ide  a t  a slow 
r a t e  under con t ro l l ed  cond i t ions .  
The subcarbide (Ta2C) was 
In p r i n c i p l e ,  a high voltage a r c - s t a b i l i z i n g  c i r c u i t  can be super- 
imposed on t h e  low vol tage power supply c i r c u i t  t o  ensure t h a t  t he  a r c  
w i l l  not  ex t ingu i sh  under marginal ope ra t ing  cond i t ions .  This  may be 
done by us ing  a high frequency generator and a s u i t a b l e  inductance t o  
i s o l a t e  t h e  high frequency s igna l  from t h e  60-cycle a-c power supply,  
as shown i n  F i g .  2.  A 500-kc, 3000-volt, 10-watt o s c i l l a t o r  power 
Supply w a s  constructed t o  test i ts  capac i ty  t o  maintain an arc  on one 
of t h e  electrodes. The test r e s u l t s  i nd ica t ed  t h a t  t o o  much of t he  
high frequency power w a s  l o s t  i n  the inductance used t o  i s o l a t e  t h e  
low vo l t age  power supply.  A l a r g e r  generator  (300-watt r a d i o  t r ans -  
m i t t e r )  i s  being adapted f o r  a s i m i l a r  tes t  a t  a frequency of 2 mc. 
Vacuum-f i r e d ,  r e a c t o r  grade,  hafnium ca rb ide  and mixed s o l i d  
s o l u t i o n  carbide powders (-200 +325) have been ordered from Wah Chang 
Corporat ion,  Albany, Oregon, and a r e  expected t o  a r r i v e  by July 1, 1966. 
6 
v 
CPS 
POWER SUPPLY 
r------------ 
I 
I 
I 
I 
I 
I 
I 
I RF POWER 
i AMPLIFIER 
I (BC-610  E) 
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L- -------__ 
FIG. 2 CIRCUIT FOR SUPERIMPOSING A HIGH FREQUENCY STABILIZING SIGNAL 
ON THE ARC-ELECTRODE 
7 
€3. Metal Solu t ion  Growth 
Addit ional  experiments were made i n  an at tempt  t o  t r a n s f e r  
tantalum carb ide  from a n u t r i e n t  zone a c r o s s  a l i q u i d  metal  s o l u t i o n  
zone approximately 1/4-inch th ick  t o  a seed a rea  f o r  growth of tantalum 
carb ide .  These w e r e  ex t ens ions  of experiments descr ibed  i n  m o r e  d e t a i l  
i n  t he  eighth Quar te r ly  S t a t u s  Report. Liquid platinum d i s s o l v e s  
tantalum and s o m e  carbon, Platinum w a s  used a s  t h e  so lvent  i n  order  
t o  permit g r a v i t y  sepa ra t ion  of t he  seed,  which was a t tached  a t  t he  
bottom of t h e  con ta ine r ,  and t h e  tantalum carb ide  n u t r i e n t  p a r t i c l e s  
f l o a t i n g  on t h e  platinum so lven t .  Although the  experiment was ter- 
minated a f t e r  t h r e e  hours  because of a hea t ing  element f a i l u r e ,  t h e r e  
was no evidence of s i g n i f i c a n t  d i s so lu t ion  and t r a n s f e r  of tantalum 
carb ide  t o  the  seed.  I n s u f f i c i e n t  thermochemical d a t a  a r e  a v a i l a b l e  
t o  c a l c u l a t e  t he  platinum so lu t ion  concent ra t ion  of tantalum i n  
equi l ibr ium with tantalum carbide but  it is expected t o  be l e s s  than 
1 a t .  %. 
Temperature g rad ien t  so lu t ion  growth of tantalum carb ide  s i n g l e  
c r y s t a l s  l a r g e r  than 0 .2  mm and, i n  p a r t i c u l a r ,  growth on seed c r y s t a l s  
has  been prevented by homogeneous nuc lea t ion  wi th in  the  sa tu ra t ed  
s o l u t i o n  near  t he  seed. To  prevent nuc lea t ion ,  lower temperature 
g r a d i e n t s  a r e  ev iden t ly  requi red .  Because of t h e  l o w  tantalum carb ide  
equi l ibr ium s o l u b i l i t y  i n  l i q u i d  meta ls ,  long experiment t i m e s  a r e  
a l s o  requi red  t o  compensate for the slow growth r a t e .  A 66-hour 
experiment was conducted i n  an A 1  0 c r u c i b l e  us ing  an i ron  s o l u t i o n  2 3  
wi th  tantalum carb ide  powder as  a n u t r i e n t .  Seed c r y s t a l s  were placed 
on an A1203 d i s h  and submerged i n  t h e  s o l u t i o n .  A temperature d i f f e r -  
ence of about 100 C was maintained between t h e  coo le r  base of t h e  
c r u c i b l e  and t h e  s o l u t i o n  su r face .  This  was deemed t o  be t h e  minimum 
temperature d i f f e r e n c e  t h a t  would ensure a small  bu t  proper ly  d i r e c t e d  
temperature  g rad ien t  t o  the  seed c r y s t a l s .  N o  evidence f o r  growth 
of tantalum carb ide  on seeds  w a s  obtained.  
0 
8 
* -  
. 
The t r a n s f e r  experiment involving a boron n i t r i d e  c ruc ib l e  and a 
zone of l i q u i d  i ron  reported i n  Quar te r ly  S t a t u s  Report N o .  V I 1 1  was 
repea ted .  The tantalum carb ide  t r a n s f e r  from t h e  p o l y c r y s t a l l i n e  
n u t r i e n t  t o  t h e  seed occurs  because of i n t e r g r a n u l a r  so lu t ion  and 
sepa ra t ion  of n u t r i e n t  tantalum carbide g r a i n s  wi th in  the  hot-pressed 
tantalum carb ide  b i l l e t .  Thus, a major f r a c t i o n  of t h e  tantalum 
carb ide  is  t r a n s f e r r e d  a s  s o l i d  p a r t i c l e s  r a t h e r  than chemically by 
t h e  s o l u t i o n .  
Because of t h e  low equi l ibr ium s o l u b i l i t y  of tantalum carb ide  and 
hafnium carb ide  i n  l i q u i d  metals a t  temperatures  where then can be 
contained without  chemical s i d e  r eac t ions  i n  a v a i l a b l e  c r u c i b l e  
m a t e r i a l s ,  and because of t he  ease of homogeneous nuc lea t ion  of tantalum 
ca rb ide  wi th in  the  l i q u i d  metal  a l l o y s  inves t iga t ed ,  growth of tantalum 
ca rb ide  and hafnium carb ide  c r y s t a l s  a s  l a r g e  a s  one cm i n  l i q u i d  m e t a l  
s o l u t i o n s  does not  appear a s  promising as does growth by the  arc-Verneuil  
method. Work on t h e  so lu t ion  method is  t h e r e f o r e  being discont inued 
f o r  t h i s  i nves t  iga t ion. 
I 
9 
IV FUTURE WORK 
Future work w i l l  include growth of hafnium carbide and mixed 
s o l i d  s o l u t i o n  carbide boules ,  f o r  which t h e  r e l a t i v e  deca rbur i za t ion  
r a t e  is  not  expected t o  be a s  severe a s  t h a t  f o r  tantalum carb ide .  
The  work wi th  tantalum carb ide  w i l l  emphasize methods f o r  increas ing  
t h e  carbon content  of boules  and f o r  e l imina t ing  g ra in  boundaries  of 
t h e  primary tantalum carb ide  phase. These e f f o r t s  w i l l  include:  
(1) use  of t h e  h igh  frequency a r c  s t a b i l i z i n g  c i r c u i t ;  (3) use  of 
f l u i d i z e d  f i n e  carbon p a r t i c l e s  in  the  a r c  r eg ion ;  (3) mixing carbon 
p a r t i c l e s  with the  carb ide  feed powder; and (4 )  t he  u s e  of vapor 
sources  of carbon o t h e r  than hydrocarbons. O f  p a r t i c u l a r  i n t e r e s t  
a r e  f reon  (F C C 1  ) and carbon t e t r a c h l o r i d e  (CC1 1. 
2 2  4 
10 


Study of Growth Parameters for Refractory Carbide Single Crystals Quarterly Status Report No. 9, 1 Mar. - 1 Jun. 1966

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