Growth parameters for refractory carbide single crystal

Bartlett, R. W.

English

NASA Technical Reports Server

A p r i l  5 ,  1966 
R e p o r t  No. 8 
Q u a r t e r l y  Status R e p o r t  No. V I I I / D e c e m b e r  1 ,  1965 to March 1, 1966 
STUDY OF GROWTH PARAMETERS FOR 
REFRACTORY CARBIDE SINGLE CRYSTALS 
B y :  R .  W .  B a r t l e t t  
S R I  Project No. FMU-4892 
Prepared for : 
N a t i o n a l  A e r o n a u t i c s  and Space A d m i n i s t r a t i o n  
W a s h i n g t o n ,  D . C .  
C o n t r a c t  No. N A S r - 4 9  (19) 
A p p r o v e d  : 
s-2 234&?L/ 
F .  A .  H a l d e n ,  D i r e c t o r  
C e r a m i c s  and Metallurgy D i v i s i o n  
C o p y  No. 4 , 
V 
1 
https://ntrs.nasa.gov/search.jsp?R=19660014829 2020-03-16T19:39:21+00:00Z
CONTENTS 
I I N T R O D U C T I O N . .  . . . . . . . . . . . . . . . . . . . . . .  1 
I1 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . .  2 
111 CRYSTAL GROWTH STUDIES . . . . . . . . . . . . . . . . . . .  3 
A .  A r c - V e r n e u i l  G r o w t h  . . . . . . . . . . . . . . . . . .  3 
1. A p p a r a t u s  Modifications . . . . . . . . . . . . . .  3 
2 .  Future Work . . . . . . . . . . . . . . . . . . . .  4 
B .  Metal Solution G r o w t h  . . . . . . . . . . . . . . . . .  5 
1 .  E x p e r i m e n t a l  R e s u l t s  . . . . . . . . . . . . . . . .  5 
2 .  Future Work . . . . . . . . . . . . . . . . . . . .  9 
ii 
I INTRODUCTION 
I n t e r e s t  i n  the  r e f r ac to ry  carbides  has increased recent ly  i n  
a n t i c i p a t i o n  of many new appl ica t ions  r equ i r ing  the  use of super- 
r e f r a c t o r i e s .  However, during the  research and development work on 
these  ma te 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 red  physical  p roper t ies .  L i t t l e  is  known about u l t i -  
mate i n t r i n s i c  physical  p roper t ies  o r  about t he  inf luences  of stoi-  
chiometr ic  changes, impur i t ies ,  and gra in  boundaries on these  prop- 
er t ies .  I n  obta in ing  t h i s  t y p e  of information, s ing le  c r y s t a l s  of 
var ious carbide compositions would be of g rea t  value.  A t  p resent ,  
the  only c r y s t a l s  r ead i ly  ava i lab le  are of t i t an ium carb ide ,  grown 
by the  Verneuil  process,  and l i t t l e  i s  known of their s t r u c t u r e  and 
pe r fec t  ion.  
Stanford Research I n s t i t u t e  has  been engaged by the Nat ional  
Aeronautics and Space Administration t o  inves t iga t e  appl ica t ion  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 carb ide ,  hafnium carbide,  and so l id  so lu t ions  of these 
carb ides .  The new techniques being inves t iga ted  f a l l  i n t o  two 
classes: (1) app l i ca t ion  of recent ly  developed methods of l i q u i d  
metal so lu t ion  growth of crystals,  and (2) u t i l i z a t i o n  of a-c a r c  
mel t ing f o r  Verneuil  c r y s t a l  growth. 
1 
I1 SUMMARY AND CONCLUSIONS 
Arc-Verneuil c r y s t a l  growth experiments w e r e  suspended during 
the  qua r t e r  while the  new crystal-growing furnace chamber was being 
constructed.  Machining of the  furnace p a r t s  w a s  completed during 
the  qua r t e r ,  and de l ivery  of t he  assembled furnace is expected e a r l y  
i n  the  following qua r t e r .  Crys ta l  growth experiments w i l l  be resumed 
a f t e r  t h e  furnace is  i n s t a l l e d .  
Solut ion growth experiments were c u r t a i l e d  during p a r t  of the 
qua r t e r  i n  order  t o  complete designs f o r  the modified arc-Verneuil 
furnace.  Some experiments were conducted using the  thin-f i lm,  solu- 
t i o n - c r y s t a l l i z a t i o n  method, which involves a narrow l i q u i d  zone of 
solvent  metal and a s t e e p  temperature gradient  between the  n u t r i e n t  
and seed materials. 
2 
I11 CRYSTAL GROWTH STUDIES 
A. Arc-Verneuil G r o w t h  
1. Apparatus Modif icat ions 
One of t he  major problems encountered i n  the growth of carbide crys- 
t a l s  using the  Verneuil  technique i s  the  feeding of powder t o  the  boule 
a t  the required slow, well-control led r a t e s .  The ind iv idua l  carbide 
p a r t i c l e s  a r e  near ly  equidimensional and have a high s p e c i f i c  g rav i ty .  
As a r e s u l t ,  contrary t o  condi t ions encountered handling oxide ma te r i a l s ,  
the  carbide powders a r e  too  free-f lowing. When conventional,  mechani- 
c a l l y  ac t iva t ed  screen feeders  are  used, i t  is  necessary t o  restrict the  
screen  a rea  t o  such an exten t  t ha t  s e n s i t i v i t y  of the powder flaw r a t e  
t o  the  tapping r a t e  i s  near ly  lost .  The powder flow may s t o p  completely 
or may g r e a t l y  acce le ra t e ,  w i t h  no apparent change i n  the f eede r  con t ro l  
sys t em.  
I n  order  t o  gain improved cont ro l  of p a r t i c l e  feeding f o r  the  arc- 
Verneuil  furnace,  an audio-driven p a r t i c l e  feeder  w a s  designed and con- 
s t r u c t e d .  T h i s  feeder  employs t h e  d r i v e r  element of an audio speaker t o  
pressure  pulse  the column of gas between the  speaker and a bed of carbide 
p a r t i c l e s  r e s t i n g  on a metal s ieve .  A s  the  p a r t i c l e  bed expands and 
c o n t r a c t s  during the  audio pulsing, p a r t i c l e s  t r ickle  through the s i eve .  
The r a t e  of flow i s  regulated by c o n t r o l l i n g  the amplitude and frequency 
of the audio s i g n a l .  S imi la r  devices a r e  being used f o r  growing oxide 
c r y s t a l s  a t  the  Insu la t ion  Laboratory of MIT. 
The new feeder  w a s  t e s t e d  i n  a i r ,  us ing  tantalum carbide powder, 
-200+270 mesh. This  powder cons i s t s  of approximately sphe r i ca l  p a r t i c l e s  
2 and i s  extremely free-flowing. Using a 200 mesh s ieve  w i t h  a l l  but  1 cm 
of the  s i eve  a rea  bl inded,  w e  found it  poss ib le  t o  maintain the des i red  
s m a l l  p a r t i c l e  feed rates. The feed r a t e  i s  highly dependent on the 
he igh t  of the  p a r t i c l e  bed. A s  the he ight  decreases ,  t h e  p a r t i c l e  feed 
3 
rate inc reases ,  un less  compensating changes i n  the frequency and ampli- 
tude of the audio s igna l  a r e  made. The  increas ing  r a t e  i s  caused by a 
decrease i n  damping of the audio s igna l  by the  p a r t i c l e  bed a s  the  t o t a l  
mass of p a r t i c l e s  i s  reduced. T h i s  e f f e c t  is  p a r t i c u l a r l y  prominent w i t h  
3 tantalum carbide because of i ts  high s p e c i f i c  g rav i ty ,  14.5 g/cm . The 
audio feeder  appears t o  work i n  a i r  as w e l l  as, o r  b e t t e r  than, o the r  
feeders ,  and i t  w i l l  be tested w i t h  t he  new arc-Verneuil furnace as soon 
as assembly is  complete. 
The new arc-Verneuil furnace chamber is being f ab r i ca t ed  by an out- 
s i d e  machine shop. A l l  p a r t s  t o  the furnace w e r e  completed and de l ivered  
t o  S R I  i n  mid-February. However, t h e  furnace w a s  not  i n  a s u f f i c i e n t l y  
assembled condi t ion t o  v e r i f y  the  c r i t i c a l  to le rances  f o r  i n t e r s e c t i o n  
of the  three hor i zon ta l  e lec t rodes  and the  v e r t i c a l  seed holder .  Conse- 
quent ly ,  t he  furnace p a r t s  were returned t o  the  f a b r i c a t o r  f o r  assembly 
and spec i f i ed  to le rance  v e r i f i c a t i o n  p r i o r  t o  acceptance. Some minor 
co r rec t ions  were a l s o  made, and the furnace was returned t o  SRI on 
March 7 ,  1966. The furnace w a s  constructed i n  conformance w i t h  d e t a i l e d  
drawings submitted by S R I ,  and it fol lows t h e  desc r ip t ion  and sketches 
submitted i n  the seventh quar te r ly  s t a t u s  r epor t .  
2.  Future Work 
The  new furnace chamber w i l l  be i n s t a l l e d  i n  place of t he  previous 
arc-Verneuil furnace and w i l l  be tested. Connections must be made f o r  
water, e l e c t r i c  power, vacuum and gas i n l e t s ,  the  p a r t i c l e  feed tube,  
and both ro t a ry  and t ransverse  motion f o r  the th ree  ho r i zon ta l  e l ec t rodes  
and the  v e r t i c a l  seed holder .  Crys t a l  growth experiments w i l l  resume i n  
conjunct ion w i t h  furnace t e s t i n g ,  using tantalum carbide f o r  t he  i n i t i a l  
experiments. Bids on supplying mixed s o l i d  so lu t ion  tantalum carbide- 
hafnium carbide powders have been received,  and add i t iona l  hafnium car- 
b ide  powders and mixed s o l i d  so lu t ion  powders w i l l  be secured i n  the  
nea r  f u t u r e .  
4 
B. Metal Solu t ion  Growth 
1. Experimental Resul t s  
Some i n i t i a l  experiments were conducted w i t h  the thin-f i lm,  solut ion-  
c r y s t a l l i z a t i o n  method. These experiments involved an attempt t o  t r a n s f e r  
tantalum carbide from a n u t r i e n t  zone across  a t h i n  l i q u i d  metal  zone, ap- 
proximately 4 inch th i ck ,  t o  a seed a rea  f o r  growth of tantalum carbide.  
A l l  of the  experiments involved t r a n s f e r  caused by a s t e e p  temperature 
grad ien t  along the v e r t i c a l  ax i s .  A small g raphi te  hea t ing  element, 0,850 
inch bore,  was constructed t h a t  was capable of a t t a i n i n g  temperatures t o  
230OOC. 
used i n  the  furnace f o r  which  t h i s  power supply was o r i g i n a l l y  se lec ted .  
Hence, some d i f f i c u l t i e s  were encountered i n  designing an optimum hea t ing  
element t h a t  w a s  compatible w i t h  the low voltage-high cur ren t  character-  
i s t i c s  of the  ava i l ab le  furnace power supply. Y e t  i t  was necessary t o  
design a small  hea t ing  element t h a t  would be c lose  t o  the  c ruc ib le  i n  
order  t o  provide the des i red  s t eep  temperature g rad ien t .  Five test  runs,  
each leading t o  modif icat ions i n  the  furnace design,  w e r e  required before  
a s u i t a b l e  hea t ing  element was produced. 
This  p a r t i c u l a r  hea t ing  element was much smaller  than i s  normally 
I 
The f i r s t  tantalum carbide t r a n s f e r  experiment used a graphi te  cru- 
c i b l e  with a tapered bottom. The c ruc ib l e  was p a r t i a l l y  f i l l e d  w i t h  i r on .  
A hot-pressed tantalum carbide b i l l e t  immersed i n  the i ron  was used a s  the  
n u t r i e n t  f o r  t r a n s f e r r i n g  tantalum carbide t o  the cooler  bottom of the  
c ruc ib l e .  T h i s  approach is s imi l a r  t o  Stockbarger 's  method of m e l t  growth. 
The experiment was unsuccessful because the  tantalum carbide b i l l e t  sepa- 
r a t e d  from the  i ron  m e l t .  
Another experiment conducted i n  a graphi te  c ruc ib l e  involved a tan- 
talum carbide boule as t h e  seed, separated from the  tantalum carbide powder 
source by a l i q u i d  platinum zone. The  tantalum carbide p a r t i c l e s  f l o a t e d  
on the  more dense platinum m e l t .  This  experiment w a s  aborted because the 
g raph i t e  h e a t e r  element f a i l e d .  The experiment w i l l  be repeated.  
A t h i r d  t r a n s f e r  experiment employed a boron n i t r i d e  c ruc ib l e  with 
t w o  hot-pressed tantalum carbide b i l l e t s  separa ted  b y  a boron n i t r i d e  
c o l l a r .  Liquid i ron  was used t o  f i l l  the  region separa t ing  the tantalum 
5 
. 
~ 
i . 
carbide b i l le t s ,  and t o  f i l l  the  surrounding region.  The b i l l e t s  w e r e  
i n i t i a l l y  of equal  s i z e ,  and t h e  temperature gradient  was designed t o  
t r a n s f e r  the tantalum carbide from the  upper t o  the lower b i l l e t .  A 
c ross  sec t ion  of the  tantalum carbide b i l l e t s  i n  the  cruc b l e ,  following 
the  run, i s  shown i n  F ig .  1. The  volume of tantalum carb de i n  t h e  upper 
b i l l e t  has  been reduced, and tantalum carbide has been deposited i n  the  
intermediate  zone between the t w o  b i l l e t s .  T h i s  experiment w i l l  be re- 
peated, us ing  tantalum carbide boules i n  place of the hot-pressed poly- 
c r y s t a l l i n e  b i l l e t s ,  t o  determine whether tantalum carbide w a s  t rans-  
f e r r e d  by a so lu t ion-prec ip i ta t ion  mechanism a s  des i red ,  or by l i q u i d  
i n f i l t r a t i o n  and separa t ion  of ind iv idua l  tantalum carbide g ra ins  from 
the  hot-pressed b i l l e t s .  
Most of the  recent  c r y s t a l  growing experiments i n  metal  so lu t ions  
have employed po lyc rys t a l l i ne  tantalum carbide as a n u t r i e n t  f o r  growth 
of s i n g l e  c r y s t a l s  of t h i s  mater ia l  ( r e v e r s i b l e  growth). On the  o the r  
hand, tantalum and carbon dissolved i n  the menstruum i n  excess  of the 
tantalum carbide s o l u b i l i t y  product w i l l  r eac t  i r r e v e r s i b l y .  I f  the re- 
a c t a n t  concentrat ion i n  excess  of t he  s o l u b i l i t y  product is  l a rge ,  nu- 
c l e a t i o n  and growth of many small c r y s t a l s  a r e  expected t o  occur.  How- 
e v e r ,  if a menstruum is used i n  which the  s o l u b i l i t y  of both tantalum 
and carbon i s  very low, growth of l a r g e r  crystals  may be poss ib le .  
Tantalum carbide was grown by two i r r e v e r s i b l e  experiments i n  which 
tantalum metal and carbon were included with the  aluminum m e l t  i n  an 
aluminum oxide c ruc ib l e .  A l l  of the components of the m e l t  w e r e  premixed 
i n  powder form. These experiments w e r e  conducted a t  1550 C and 1100 C. 
The tantalum carbide c r y s t a l  y i e l d  a t  the lower temperature was much 
h ighe r  than the y i e l d  from the high temperature experiment. Average 
c rys ta l  s i z e  was 0.2 mm. 
0 0 
A mixed so l id-so lu t ion  crystal-growth experiment was performed using 
an aluminum menstruum with the primary goal of providing mixed c r y s t a l s  
f o r  the  arc-Verneuil experiments. The crystals  were grown a t  1550 C. I n  
add i t ion  t o  aluminum, the  menstruum contained a 50-50 s to ich iometr ic  r a t i o  
of hafnium and tantalum,and carbon i n  excess of the amount required t o  
0 
6 
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0 
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0 
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W 
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7 
form the  monocarbides. The hafnium w a s  introduced t o  the  c ruc ib l e  as 
sponge, and tantalum was introduced a s  -100 mesh powder. The experiment 
w a s  conducted i n  a helium atmosphere. 
A f t e r  cooling the  m e l t  and d isso lv ing  the  aluminum i n  hydrochloric 
ac id ,  w e  obtained a purple res idue of f i n e  c r y s t a l s .  This  res idue w a s  
separated i n t o  the  following s ieve f r a c t i o n s :  +48, -49+150, -150+325, 
and -325 mesh. Approximately equal amounts of ma te r i a l  w e r e  re ta ined  
i n  each f r a c t i o n .  The -325 mesh mater ia l  w a s  examined by X-ray f luores-  
cence, and equal  amounts of hafnium and tantalum were present .  X-ray 
d i f f r a c t i o n  ana lys i s  of t h i s  mater ia l  showed t h a t  an i n s i g n i f i c a n t  amount 
of f r e e  hafnium and l a rge  amounts of = s e p a r a t e  carbide phases were 
present .  These carbide phases had the  f ace  centered cubic s t r u c t u r e  of 
0' hafnium carb ide  and tantalum carbide,  but  t h e i r  l a t t i c e  parameters, a 
w e r e  4.519 d? and 4.586 g .  These values  were determined i n  the  back re- 
f l e c t i o n  region (2 8 > 120°) by d i f f r a c t i o n  from (333) l a t t i c e  planes, 
and they a re  intermediate  values  between the  l a t t i c e  parameters of tan- 
talum carbide (ao = 4.456 g) and hafnium carbide (ao = 4.640 g) . Thus, 
two s o l i d  so lu t ions  with d i s t i n c t  compositions w e r e  formed, suggest ing 
p a r t i a l  s o l i d  immisc ib i l i ty  i n  these monocarbides. I f  w e  assume a l i n e a r  
r e l a t i o n  between l a t t i c e  parameter and mole f r a c t i o n  (Vegard's Law), the  
compositions of the two mixed so lu t ion  compounds by atomic weight a re :  
(1) 34% HfC - 66% TaC 
(2) 7% HfC 30% TaC 
If Rudy's data* f o r  the  r e l a t i o n  between. l a t t i c e  parameter and mole 
f r a c t i o n  f o r  t hese  mixed carbides  a r e  used, the  ca l cu la t ed  compositions 
are : 
(1) 50% HfC * 50% TaC 
(2) 82% HfC 18% TaC 
Th i s  apparent immisc ib i l i ty  has not  been previously reported.  
* 
Rudy, E . ,  "Ternary Phase Equ i l ib r i a  i n  Trans i t i on  Metal-Boron-Carbon- 
S i l i c o n  Systems, P a r t  I ,  Ternary Systems, V o l .  1, Ta-Hf-C System, Tech. 
Rept. N o .  AFML-TR-65-2, June 1965, Aerojet  General Corporation, p. 36. 
11 
8 
-. . .  
2. Future Work 
Work on so lu t ion  growth w i l l  be continued a t  a reduced l e v e l  of 
e f f o r t ,  and w i l l  be r e s t r i c t e d  t o  (a) f u r t h e r  explora t ion  of the  thin-  
f i lm ,  so lu t ion -c rys t a l l i za t ion  method, and (b) a l i m i t e d  number of ir- 
r eve r s ib l e  long-time experiments conducted i n  aluminum oxide c ruc ib l e s  
us ing  aluminum-tin menstruums of various compositions t o  r egu la t e  tan- 
talum and carbon s o l u b i l i t y .  Depending on the  r e s u l t s  of these  experi-  
ments, work on so lu t ion  growth may be terminated. Although c r y s t a l l i n e  
per fec t ion  and growth of s to ich iometr ic  compositions have been q u i t e  
encouraging using t h i s  technique, it appears doubtful  t h a t  c r y s t a l s  of 
the des i red  s i z e  w i l l  be obtained wi th in  the remaining t i m e  on t h i s  
p r o j e c t  . 
9 


Study of growth parameters for refractory carbide single crystals Quarterly status report no. VIII, 1 Dec. 1965 - 1 Mar. 1966

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