Sublimation rate of rhenium measured for determining source of contamination - vaporization of thermionic refractory material

Plante, E. R.

English

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NATIONAL BUREAU OF STANDARDS REPORT 
8670 
VAPORIZATION OF THERMIONIC 
REFRACTORY MATERIALS 
February 28, 1965 
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Microfiche (M F) 
lli S. DEPARTMENT OF COMMERCE 
NATIONAL BUREAU OF STANDARDS 
https://ntrs.nasa.gov/search.jsp?R=19650015665 2020-03-17T01:45:58+00:00Z
‘P w E N A T I 0 N x1, 
L 
NATIONAL BUREAU OF STANDARDS REPORT 
NBS PROJECT 
3 13  043 1 28 February  1965 
VAPORIZATION OF THERMIONIC 
REFRACTORY MATERIALS 
E .  R. P l a n t e  
NBS REPORT 
8670 
F o u r t e e n t h  S t a t u s  Repor t  f o r  t h e  p e r i o d  December 1, 1964 - February  28, 1965 
t o  t h e  N a t i o n a l  Aeronau t i c s  and  Space A d m i n i s t r a t i o n  
NASA Order  No. R57 Amendment 2 
(Ex tens ion  of  Order  No. R6) 
IMPORTANT NOTICE 
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U. S. DEPARTMENT OF COMMERCE 
NATIONAL BUREAU OF STANDARDS 
The l as t  r e p o r t  (1)  l i s t e d  preli inii iary da ta  t ) b t a i n c d  j i l  i I J ( >  series  of measurements on 
r h d  r a t e  of subl imat ion  of rhenium. A t  t h e  t i m e  of w r i t i n g  that: r e p o r t ,  i t  was be l ieved  t h a t  
!>,e cause f o r  a d i f f e r e n c e  i n  t h e  average  rates of  sub1iinatio;i of rhenium of about  50% 
& m i n e d  from the  s e r i e s  I and series T I  da t a  was due l a r g e l y  t o  e r r o r  i n  ob ta in ing  the  wind:lw 
L~ r r cc t io i i  f a c t o r  f o r  t h e  series I d a t a .  I n  both o€ these aeries, d i f f e r e n c e s  i n  the  wlrtdow 
'ind pr ism c o r r e c t i o n  f a c t o r s  of about  5 , 7  x 10 corresponding t o  d i f  fereiices i n  tempera - -60K*-1 
i i i r e  of about  42'K a t  2700'K were rioted on tile h a s i s  L)f measurements of t h e  window f a c t o r s  
. I !L.~c~c hefore  and a f t e r  each series.  However, the p r e c i s i o n  o f  t h e  measurements i n  the  
i n d i v i d u a l  series was good and 110 s i g n i f i c a n t  d i f f e r e n c e  was observed between the  second law 
IleatR of Riiblimation. These two f a c t o r s  dippear t o  be a t  odds wi th  a change i n  t h e  window 
t i ic tor  as l a r g e  ao t h a t  obeerved. Also, 1t. wus noted  chat Latter agreement between occond niid 
t h i r d  law h e a t s  of sub l ima t ion  was obtained i f  the  witidow c o r r e c t i o n  obtained a t  t h e  end of tht-  
s e r i e 8  WBR used t o  c a l c u l a t e  t h o  t h i r d  law lieattc. It appeared t h a t  a l l  t h e  d i f f i c u l t i e s  could 
:le reso lved  i f  i t  was assumed t h a t  i n  both of these s e r i e e ,  a r ap id  change i n  t h e  window facto:: 
j ccur red .  It was suggested,  t h e r e f o r e ,  t h a t  contamination du r ing  e e r i e s  I had taken p l ace  
Ji irft ig the  f i r s t  experiment when rhenium moral was depos i t ed  on t h e  window because of t he  
Dr?l.r?c!tiori of t oo  h igh  on ope ra t ing  temperature  wh i l e  t h e  window had becane contaminated i n  
series I1 dur ing  t h e  pre l iminary  outgass ing  procedure by one of t h e  h ighly  v o l a t i l e  rhenium 
o x i d e s .  It was supposed t h a t  t h i s  oxide had go t t en  i n t o  t h e  vacuum system dur ing  glass blow in^; 
procedures  by o x i d a t i o n  of rhenium metal. 
On t h e  b a s i s  of t h e s e  assumptions,  t h e  microbalance appa ra tus  was c a r e f u l l y  c leaned  t o  
remove any remaining rhenium oxide  i n  the  h o t  zone of the  vacuum system and a t h i r d  s e r i e s  of 
mmsurenients was undertaken.  The r e s u l t s  of t h i s  t h i r d  series of measurements were p r a c t i c a l l y  
t h e  same as i n  t h e  f i r s t  two series i n  t h a t  the co r rzc t io t i  f a c t o r  f o r  t h e  window a g a i n  
inc reased  a l though by a l i t t l e  less than  i n  t h e  f i r s t  two seri.es. 
S i n c e  a change i n  t h e  window f a c t o r  of t h e  magnitude observed makes i t  imposs ib le  t o  
t r e a t  t h e  exper imenta l  d a t a  i n  a r e l i a b l e  fash ion  i t  was decided t o  a t t empt  t o  i s o l a t e  t h e  
cause  of t h e  change, The most l i k e l y  sources  of contaminant were considered t o  be; 
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1. Deposi t ion of  rhenium on che window dur ing  temperature  measurements; 
2 .  Evaporat ion of contaminant from the  s i i u t t e r ;  
3 .  Contamination of m i r r o r  o r  pr ism wirh  d u s t  du r ing  experiments  and d u r i n g  measuremencs 
. 
of t h e  window f a c t o r s ;  
4 .  Evaporat ion of contaminant from t h e  p la t inum-pla ted ,  vycor  i n s e r t  tube .  
The p o s s i b i l i t y  t h a t  o the r  sources  of a contaminat ing vapor  were important  were cons idered  
u n l i k e l y  because of unfavorable  geometry. There i s  a n  a d d i t i o n a l  p o s s i b i l i t y  t h a t  rhenium 
oxide could come from t h e  sample i t s e l f  b u t  t h i s  would r e q u i r e  a r e l a t i v e l y  temperature  
independent d i f f u s i o n  process  and would appear  t o  L e  r u l e d  ou t  by t h e  manufacturers  estimate 
of 12PpM oxygen impuri ty .  
An a t t e m p t  t o  r u l e  ou t  each of t h e  i t e m s  cons idered  w a s  made bu t  none y i e l d e d  r e s u l t s  
which i n d i c a t e d  tha t  i t  would be r e s p o n s i b l e  f o r  the change i n  t h e  window c o r r e c t i o n  f a c t o r  
observed du r ing  the  f i r s t  t h r e e  series of measurements. 
Examination of t h e  r e s i d u a l s ,  I logPobs[ - 1  l o g P c a l c [ ,  from t h e  least  squares  t r ea tmen t  
of t h e  series I1 and series 111 d a t a  shows a preponderance of p o s i t i v e  r e s i d u a l s  (p re s su res  
too l o w )  f o r  t h e  experiments a t  t h e  beginning  of a series and a preponderance of nega t ive  
r e s i d u a l s  (pressures  too h igh)  f o r  t h e  experiments a t  t h e  end of t h e  ser ies .  This shows t h a t  
t h e r e  was some change i n  the  window and pr ism c o r r e c t i o n  f a c t o r  du r ing  t h e  vapor p r e s s u r e  
measurements con t r a ry  t o  our prev ious  assumptions.  The r e s i d u a l s ,  expressed as percentages  
are,  +8, -I+, +5, -1, 0, -7, -3,  +2, -7 f o r  t h e  series I1 d a t a  and +19, 0, 0, +1, 0, +2, +1, -3, 
-11, -14, +9, -6 f o r  t h e  series 111 d a t a .  However, on t h e  b a s i s  of t h e  observed changes i n  
t h e  window and prism c o r r e c t i o n  f a c t o r  one would expec t  t o  see a range  of f34% i n  t h e  r e s i d u a l s  
f o r  series I1 and +28% f o r  t h e  series I11 d a t a .  Thus, w h i l e  some change i n  t h e  window 
c o r r e c t i o n  f a c t o r  may have occurred du r ing  t h e  series 11 and series 111 measurements, i t  i s  
ques t ionab le  t h a t  the t r end  i s  s u f f i c i e n t  t o  f o r c e  one t o  conclude t h a t  a l l  of  t h e  change took 
p l ace  du r ing  t h e  measurements and the  o r i g i n a l  assumptions made may have some v a l i d i t y .  
One f i n a l  series of measurements has  been undertaken.  I n  t h e s e  measurements i t  was 
planned t o  c a r r y  o u t  a l l  experiments a t  a tempera ture  of about  2650°K where 
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t h e  precisPoon of t h e  measurements would be s u f f i c i e n t l y  good t o  e a s i l y  d e t e c t  a change i n  t h e  
window c o r r e c t i o n  as l a r g e  as those  occurr ing  i n  the s e r i e s  I1 and s e r i e s  I11 measurements. 
A f t e r  seven experiments of 20 minutes du ra t ion  each w i t h  a t o t a l  weight l o s s  of 1900 micrograms 
t h e  appa ra tus  was opened and the  prism and window r e c a l i b r a t e d .  No change i n  the  window 
c a l i b r a t i o n  was observed and no change w a s  p red ic t ed  by t h e  d a t a  obtained.  I f  i t  t u r n s  out  
that  t n e  cause of the  change i n  t h e  window f a c t o r  has disappeared,  t h i s  s e r i e s  w i l l  L e  extended 
t o  h ighe r  and lower tempera t u r e s .  
h e  t e n t a t i v e  second l a w  h e a t s  from these  four  s e r i e s  are, 188.4, 187.5, 183.8, and 
137.4 kcal/mole and the  t e n t a t i v e  t h i r d  law neats c a l c u l a t e d  us ing  the  f r e e  energy func t ions  
g iven  by S t u l l  and Sinke (2 )  are  187.1, 185.1, 186.4, and 186.1 kcal/mole f o r  series I through 
I V ,  r e s p e c t i v e l y .  It should be noted t h a t  t h e  adopt ion of any cons tan t  window c o r r e c t i o n  
f a c t o r  w i l l  no t  e f f e c t  t h e  second l a w  h e a t  buc w i l l  have a n  e f f e c t  on the  v a l u e  of t h e  t h i r d  
l a w  h e a t .  S ince  t h e  window c o r r e c t i o n s  used t o  c a l c u l a t e  t h e  t h i r d  l a w  h e a t s  were those  
obta ined  a t  tne conclus ion  of t he  series, t h e  t h i r d  l a w  h e a t s  f o r  series I through I11 
represen t  upper l i m i t s  t o  t h e  Lhird l a w  h e a t .  
References 
(1 )  R. Szwarc and E .  R. P lan te ,  NBS Report 8573 
( 2 )  D .  R. S t u l l  and G: C.  Sinke, Thermodynamic P r o p e r t i e s  of t he  Elements, American Chemical 
Soc ie ty ,  Washington, D. C. (1956). 
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Vaporization of thermionic refractory materials fourteenth status report, 1 dec. 1964 - 28 feb. 1965

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