Nucleation and growth processes in interstellar grain formatio

Hudson, J. B.

English

NASA Technical Reports Server

F i n a l  Report 
to t h e  
Of f i ce  of Grants  and Contracts 
Nat ional  Aeronautics and Space Agency 
on 
Grant No. NsG 663 
I 
i Spat ia l  Nucleation and Crys t a l  Growth 
I 
0 
John B. Hudson 
Materials Divis ion 
Rensselaer Polytechnic  I n s t i t u t e  
Troy, New York 12151 
/ 
(CODE) 
a 
P 
(CATEGORY) 
https://ntrs.nasa.gov/search.jsp?R=19680010170 2020-03-12T08:47:25+00:00Z
INTRODUCTION 
For the pe r iod  from 1 June 1964, t o  30 September 1967, 
NASA has  supported a program i n  the  Materials Engineering De-  
partment a t  Rensselaer aimed a t  ob ta in ing  t h e o r e t i c a l  and ex- 
per imental  understanding of t h e  processes  involved i n  t h e  
formation and growth of s o l i d  p a r t i c l e s  i n  t h e  s p a t i a l  en- 
vironment. 
measurements of energy t ransfer ,  adsorpt ion,  and nuc lea t ion  
phenomena i n  a number of vapor-solid systems and t h e o r e t i c a l  
s t u d i e s  of t h e  relevance of classical  t h e o r i e s  of nuc lea t ion  
and growth t o  t h e  process  of i n t e r s t e l l a r  g r a i n  formation. 
This  program has involved both experimental  
This  r e p o r t  summarizes t h e  accomplishments of t h a t  per iod  
and o u t l i n e s  planned continued e f f o r t s  along t h e  same l i n e s .  
THE SCIENTIFIC  PROBLEM 
I t  is  w e l l  known from astronomical observa t ions  t h a t  l a r g e  
numbers of p a r t i c l e s ,  covering a f a i r l y  wide range of par t ic le  
s i z e  and presumably composition and morphology, e x i s t  i n  var ious 
reg ions  of  i n t e r p l a n e t a r y  and i n t e r s t e l l a r  space. 
of t h e s e  p a r t i c l e s  and t h e i r  subsequent l i f e  h i s t o r y  are, 
however, no t  a t  a l l  w e l l  understood. Consequently, it is  
The o r i g i n  
a ques t ion  of considerable  importance, t o  a s t r o p h y s i c i s t s  i n  
g e n e r a l  and cosmologists i n  par t icular ,  t o  develop an under- 
. .  
, 
-2- 
s tanding  of t h e  processes involved i n  t h e  b i r t h ,  l i f e ,  and 
eventua l  death of t h e s e  p a r t i c l e s .  
When the p r e s e n t  study w a s  begun, it w a s  i m p l i c i t l y  
assumed t h a t  t hese  p a r t i c l e s ,  a t  least  i n  most cases, were 
formed by a homogeneous nucleat ion mechanism i n  i n t e r s t e l l a r  
space. Calcu la t ions  based on the c l a s s i c a l  theory  of nuclea- 
t i o n ,  however, i n d i c a t e  that it is un l ike ly  t h a t  t h e  requi red  
nuc lea t ion  rate would be achieved under the condi t ions  e x t a n t  
i n  t h i s  environment. More r e c e n t l y  it has been proposed 
t h a t  t h e  p a r t i c l e  nuc lea t ion  process t a k e s  p l ace  i n  t h e  
1 
v i c i n i t y  of cool ,  carbon-rich stars. Calcu la t ions  by Donn 
-- e t  al. 2 i n d i c a t e  t h i s  t o  be a much more l i k e l y  mechanism. 
Consequently, it is of i n t e re s t  t o  make measurements of 
homogeneous nuc lea t ion  k i n e t i c s  under condi t ions  appropr i a t e  
t o  t h e s e  s te l lar  atmospheres, i n  o r d e r  t o  t e s t  t h e  v a l i d i t y  
o f  t h e s e  ca l cu la t ions .  
Assuming f o r  t h e  moment t h a t  the nuc lea t ion  of t h e s e  
particles can be descr ibed by the  above explanat ion,  ques t ions  
s t i l l  remain concerning t h e  subsequent growth processes  as a 
p a r t i c l e  moves about i n  an atmosphere of condensible atomic 
or molecular species, and about t h e  var ious  processes  of 
a t t r i t i o n ,  such as ion,  e l e c t r o n  or  photon bombardment and 
s u r f a c e  chemical r eac t ion  which may eventua l ly  reduce t h e  
- 3 -  
. p a r t i c l e  s ize  o r  even result i n  complete d e s t r u c t i o n  of t h e  
p a r t i c l e .  Very l i t t l e  is  known about t h e  growth of s o l i d  
p a r t i c l e s  i n  a d i l u t e ,  multicomponent vapor such as i s  in-  
volved here .  Even less i s  known about t h e  l o s s e s  i n  mass t o  
be expected under t h e  inf luence of t h e  bombardment process  
descr ibed above, espec ia l ly  when these  processes  t a k e  p lace  
i n  t h e  extreme vacuum environment involved here .  
Thus, it i s  seen t h a t  many problems of fundamental 
importance t o  an understanding of t he  production and pro- 
p e r t i e s  of s o l i d  p a r t i c l e s  i n  space either cannot be char-  
a c t e r i z e d ,  O r  a t  
in t h e  l i g h t  of present  knowledge. The sec t ions  t h a t  follow 
con ta in  a summary of work performed under t h e  c u r r e n t  g ran t ,  
which is  aimed a t  increasing our knowledge of t h e  above- 
mentioned processes ,  and ind ica t e  t h e  type of a d d i t i o n a l  
s t u d i e s  along these  same l i n e s  which m u s t  be performed before  
anything approaching complete understanding w i l l  be possible .  
best can be charac te r ized  only incompletely 
PROGRESS TO DATE-THEORY 
I n  t h e  course of the  g r a n t  per iod t h e  p r i n c i p l e  inves- 
t i g a t o r  has col labora ted  with HASA personnel1 i n  developing 
a theory of t h e  o r i g i n  of i n t e r s t e l l a r  g r a i n s  by nuc lea t ion  
of g r a p h i t e  p a r t i c l e s  i n  the  atmospheres of carbon-rich s t a r s .  
. 
i 
The r e s u l t i n g  theory is emboc?ied i n  a paper which h a s  been 
s u b m i t t e d  f o r  publ ica t ion  i n  t he  Astrophysical  Journal .  111) 
PROGRESS TO DATE-EXPERIMENTAL 
Five experimental  programs w e r e  pursued i n  t h e  course  
of  t h e  g r a n t  period. Of these one h a s  been followed t o  t h e  
l i m i t  of  i t s  c a p a b i l i t i e s  and i s  no longer  c u r r e n t l y  ac t ive .  
Three o t h e r s  have been developed t o  t h e  po in t  where va luable  
r e s u l t s  a r e  be ing  obtained a t  present .  The f i f t h  is  s t i l l  
i n  i t s  formative s tages .  ITork on t h e s e  l a s t  four  i s  con- 
t inu  ing. 
A The Study of Heteroqeneous Nucleation b y  V i s u a l  Obser- 
va t ion  
The program which has been completed involved the  
measurement of t h e  c r i t i c a l  supe r sa tu ra t ion  for nuc lea t ion  
of selenium on pyrex g l a s s  under var ious  experimental  con- 
d i t i o n s .  The experiments were performed i n  two small. g l a s s  
u l t r a h i g h  vacuum systems in vhich measurements of t h e  
c r i t i c a l  supersa tura t ion  for  nuc lea t ion  of  vapor on a s o l i d  
s u b s t r a t e  could be measured by e i t h e r  s t a t i c  vapor ormolecu la r  
beam techniques.  I n  both c a s e s  t h e  onse t  of nuc lea t ion  
was cha rac t e r i zed  by t h e  v i s u a l  observat ion of t h e  condensed 
phase on t h e  su r face  of i n t e r e s t .  This  permitted measurements 
- 5 -  
. which could d e l i n e a t e  e f f e c t s  of  vapor temperature and sub-  
s t r a t e  c l e a n l i n e s s  on t h e  nucleat ion process.  
w e r e  used for  an extensive s tudy  of t h e  nuc lea t ion  behavior  
of selenium on pyrex glass .  
ve rba l ly  on two occasions (see s e c t i o n  on Verbal Presenta t ions)  
and have been published i n  t h e  Jou rna l  of Chemical Physics. 
These systems 
R e s u l t s  have been reported 
B Mass Spectrometr ic  Molecular Beam Studies  
I n  t h e  e a r l y  s tages  of t h e  g r a n t  per iod,  development 
was begun on a system for t h e  measurement of gas-surface 
i n t e r a c t i o n s  by molecular Beam-mass spectrometr ic  techniques.  
This  system used a s m a l l m s s  spectrometer  t o  monitor t h e  
f l u x  of ma te r i a l  evaporated o r  desorbed from a metal  sur face  
in u l t r ah igh  vacuum. 
could be made of t h e  c o e f f i c i e n t  of thermal accommodation of 
vapor beam molecules a t  t h e  surface,  t he  mean stay-time f o r  
adsorpt ion,  and t h e  k i n e t i c s  of t h e  nuc lea t ion  of t h e  bulk 
phase of t h e  vapor species  on t h e  subs t r a t e .  R e s u l t s  ob- 
t a i n e d  f o r  cadmium vapor impinging on a tungsten su r face  have 
been published i n  t h e  Journal  of Vacuum Science and Technology 
By s u i t a b l e  manipulation, measurements 
and t h e  book 
- 6 -  
T h i s  work i s  c u r r e n t l y  cont inuing  under sponsorship o f  
another  agency. 
C Electron-Bombardment-Induced Desorption S t u d i e s  
Another p r o j e c t  sponsored i n  p a r t  by t h e  p re sen t  g r a n t  
involved the development of a system f o r  the study of 
electron-bombardment-induced desorp t ion  of gases from so l id  
surfaces .  
used t o  monitor t h e  ion  c u r r e n t  r e s u l t i n g  from t h e  desorp t ion  
of gas  molecules (as ions)  C r o m  a s u r f a c e  i n  u l t r a h i g h  vacuum 
I n  this system a small  mass spectrometer i s  
under the in f luence  of low energy e l e c t r o n  bombardment. 
These measurements provide d a t a  on t h e  ad layer  coverage as 
a func t ion  of s u b s t r a t e  temperature  and exposure t o  va r ious  
atmospheres, a s  w e l l  as information on t h e  binding energy 
i n  t h e  adsorbed s t a t e  and t h e  c ross -sec t ion  f o r  t h e  de- 
so rp t ion  reac t ion .  This system i s  j u s t  now beginning t o  
y i e l d  p o s i t i v e  r e s u l t s  for t h e  desorp t ion  of var ious  r e s i d u a l  
gases  from a tungs ten  surface.  This  work i s  c u r r e n t l y  con- 
t i n u i n g  under sponsorship 2rom another  group. 
D Gas-Surface I n t e r a c t i o n  S t u d i e s  b y  S t a t i c  Vapor-Mass 
Spectrometr ic  Techniques 
One of the conclusions f r o m  the r e s u l t s  obtained from 
t h e  measurements described i n  p a r t  A of t h i s  s e c t i o n  was t h a t  
- 7 -  
I . 
more c a r e f u l  c o n t r o l  and rneasurement of the environment was 
going t o  be requi red  i n  order t o  o b t a i n  completely unambiguous 
results on t h e  nuc lea t ion  02 vapors on substrates from a 
s t a t i c  vapor source. The system discussed h e r e  was b u i l t  i n  
response t o  t h a t  need. It c o n s i s t s  of a g l a s s  u l t r ah igh  
vacuum chamber, conta in ing  t h e  su r face  t o  be s tud ied  (up t o  
t h i s  t i m e  a blown Pyrex glass  t i p ) ,  and connected t o  a 
second u l t r a h i g h  vacuum chamber through a small  o r i f i c e ,  The 
second chamber con ta ins  a mass spectrometer,  mounted so t h a t  
t h e  only m a t e r i a l  from t h e  C i r s t  chamber t h a t  can reach 
the spectrometer i o n  source i s  m a t e r i a l  which made i t s  
l a s t  c o l l i s i o n  before passing t h e  o r i f i c e  on t h e  surzace under  
study. That is, t h e  mass spectrometer ' 'sees" only m a t e r i a l  
desorbing o r  evaporating from t h e  s u b s t r a t e ,  Y5th this 
system one can do experiments s i m i l a r  t o  those described 
under B above, b u t  without t h e  complication of angle-of-in- 
c idence  e f f e c t s  t h a t  a r e  present  when a molecular beam tech- 
n ique  is used. 
measure t h e  thermal accommodation and nuc lea t ion  behavior 
of mercury on g l a s s ,  a s  a func t ion  of t h e  c l e a n l i n e s s  of the  
g l a s s  surface.  
E Homoqeneous Nucleation in a Supersonic Molecular Beam 
e 
This  system i s  p resen t ly  be ing  used t o  
It was recognized a t  t h e  o u t s e t  of t h e  present g r a n t  
- 0 -  
per iod  t h a t  t h e  nuc lea t ion  process  involved i n  t h e  formation of 
i n t e r s t e l l a r  g r a i n s  must  be a homogeneous nuc lea t ion  process  
of some s o r t .  However, a t  t h a t  t i m e ,  no experimental  methods 
for measuring homogeneous nucleat ion i n  systems of i n t e r e s t  
w e r e  ava i lab le .  Consequently t h e  i n i t i a l  s t u d i e s  repor ted  
above involved heterogeneous nuc lea t ion  processes .  Recently, 
the developement of  systems i n  which one could form, con- 
t inuous ly ,  molecular beams of high d e n s i t y  and h igh  super- 
s a t u r a t i o n  have o f fe red  a technique whereby t h e  desired 
homogeneous nuc lea t ion  measurenents could be made. 
system is  now under cons t ruc t ion ,  A t  t h i s  po in t ,  cons t ruc t ion  
h a s  proceeded t o  t h e  poin t  where t h e  systemhaxdw-e is nea r ly  
complete and i n i t i a l  t e s t s  of the beam forming process  vi11 
be made soon. When t h i s  system i s  completed, it can be used 
t o  s tudy p a r t i c l e  formation i n  gas  mixtures  s i m i l a r  t o  those  
be l i eved  t o  ex is t  i n  regions where the formation of i n t e r s t e l l a r  
g r a i n s  may be t ak ing  place, i n  t h e  hope of providing d i rec t  
evidence i n  support  of t h e  t h e o r e t i c a l  conclusions discussed 
earlier i n  t h i s  r e p o r t  and erribodied i n  t h e  paper presented 
i n  Appendix A. Work on t h i s  system i s  continuing. 
Such a 
VERBAL PRESENTATIONS 
The following verba l  t e c h n i c a l  p re sen ta t ions  of work 
0 
supported under t h e  cu r ren t  g r a n t  have been made: 
"The Nucleation of Non-Emilibrium Phases From t h e  Vapor", 
Nat ional  Vacuum Symposium of t h e  AVS, New York, New York, 
October 1965 
"The E f f e c t  of Surfaces i n  Heterogeneous Nucleation of 
Condensed Phases From t h e  Vapor", Frankl in  I n s t i t u t e  
Research Laboratories.  
May 1966 ( i n v i t e d )  . Seminar Series - Phi lade lphia  - 
"The Observation of Adsorption and C r y s t a l  Nucleation 
by Mass Spectrometric Techniques", Th i r t een th  Nat ional  
Vacuum Symposium of  t h e  AVS - San Francisco - October 
1966 ( i n v i t e d )  . 
"The Adsorption of Cadmium on Tungsten", Symposium on 
Gas-Surface In t e rac t ions ,  San Diego-December 1966. 
"Nucleation and Growth by Mass Spectrometric Techniques" 
Gordon Conference on Thin Films-Tilton, N.H. - August 
1967 . 
PLANNED FUTURE EFFORT 
The experimental  and t h e o r e t i c a l  s t u d i e s  of t h e  p a s t  
pe r iod  have served t o  pu t  t he  k i n e t i c  and thermodynamic pro- 
cesses of i n t e r e s t  i n  t h e  formation of p a r t i c l e s  i n  space 
i n t o  much sharper  focus then was poss ib l e  a t  t h e  beginning of 
the period. I t  i s  thus  poss ib l e  t o  r ede f ine  t h e  o b j e c t i v e s  
of t h e  program, and t o  suggest f u r t h e r  experiments w h i c h d l  lead 
to r e a l i z a t i o n  of t h e s e  objec t ives .  
The most s i g n i f i c a n t  r e o r i e n t a t i o n  of  e f f o r t  w i l l  be 
away from s t u d i e s  of heterogeneous nuc lea t ion  and toward 
- 10 - 
E 
s t u d i e s  of homogeneous nucleat ion.  I t  was r e a l i z e d  a t  the out- 
set of t h i s  work t h a t  homogeneous nuc lea t ion  were of g r e a t e s t  
re levance  t o  t h e  s p a t i a l  nuc lea t ion  process. However, at 
t h a t  t i m e  no experimental  method was a v a i l a b l e  by which 
homogeneous nuc lea t ion  could be s tud ied  i n  s y s t e m o f  i n t e r e s t .  
Recently, t h e  technique of sampling t h e  a d i a b a t i c a l l y  expand- 
ing  flow from a supersonic  nozzle discussed above has  been 
developed and shows g r e a t  promise a s  a t o o l  f o r  s tudying 
homogeneous nucleat ion.  
f u r t h e r .  
This technique w i l l  be pursued 
While nuc lea t ion  measurements such as t h e s e  can provide 
i n s i g h t  i n t o  t h e  b i r t h  mechanism of i n t e r s t e l l a r  p a r t i c l e s ,  
o t h e r  techniques a r e  required t o  i n v e s t i g a t e  t h e  processes  
mentioned e a r l i e r  which c o n t r i b u t e  t o  t h e i r  subsequent l i f e  
h i s t o r y .  
t h e  p a r t i c l e s  e i t h e r  a s  r e l a t i v e l y  pure p a r t i c l e s  or  a s  
"mantled" p a r t i c l e s ,  w i t h  an overgrowth of some o t h e r  chemical 
s p e c i e s  occurr ing on t h e  o r i g i n a l l y  nucleated p a r t i c l e ,  and 
t h e  e ros ive  processes  assoc ia ted  with exposure t o  f l u x e s  of 
i ons ,  ene rge t i c  n e u t r a l  atoms, e l ec t rons ,  cosmic ray  pr imaries ,  
and photons. Thus, i n  order  t o  understand completely t h e  l i f e  
h i s t o r y  of t h e  p a r t i c l e s  one m u s t  make measurements of both 
These processes  i n c l u d e  t h e  continued growth of 
- 11 - 
t h e  growth and comminution processes  under appropr i a t e  
condi t ions  of exposure, 
Two of t h e  experimental systems developed t o  date, with 
some modif icat ions,  w i l l  be used t o  study those  processes ,  
The system developed t o  study thermal accommodation and 
nuc lea t ion  from a s t a t i c  vapor can be modified t o  s tudy both 
c r y s t a l  growth processes  and sur face  chemical r e a c t i o n s  
wi th  n e u t r a l  spec ies ,  poss ib ly  even with a c t i v a t e d  n e u t r a l s  
such a s  atomic oxygen. 
deso rp t ion  t o  s tudy adsorbed l a y e r s  can be used a s  i s  f o r  
s tudying t h e  i n t e r a c t i o n  of e l e c t r o n s  with p a r t i c l e  sur faces ,  
and with some modif icat ion t o  s tudy t h e  e f f e c t s  of ion  and photon 
bodardment .  
The system developed t o  u s e  e l e c t r o n  
Thus,  a t  t h i s  po in t ,  techniques a r e  e i t h e r  a v a i l a b l e  
o r  i n  a developmental s t age  for studying v i r t u a l l y  a l l  of t h e  
processes  involved i n  t h e  b i r t h ,  l i f e t i m e  and dea th  of cosmic 
p a r t i c l e s ,  
t h e  c u r r e n t  g r a n t  period, we a r e  i n  a pos i t i on ,  i n  t h e  f u t u r e ,  
t o  ob ta in  d e f i n i t i v e  d a t a  d i r e c t l y  app l i cab le  t o  s i t u a t i o n s  
Of a s t rophys ica l  s i g n i g i c  ance. 
Through t h e  information and i n s i g h t s  gathered i n  
- 12 - 
RE F'E RENCE S 
1. Hoyle, F. and Wickramasinghe, LJ. C., M.N., 124 417 (1962) 
2. Donn, B. Wickrarnasinghe, N. C. Hudson, J. B., and 
Stecher ,  T. P., "On t h e  Formation of Graphi te  Grains 
i n  Cool Stars", submitted t o  Astrophy. J. 


Spatial nucleation and crystal growth Final report, 1 Jun. 1964 - 30 Sep. 1967

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