The purpose is to examine solidification in the LiCl-KCl system to determine if phenomena such as solute rejection can be obseved by laser schlieren imaging. Molten salts have attributes that make them attractive as physical models in solidification studies. With optical techniques of investigation such as schlieren imaging, it is possible to study fluid flow phenomena in molten salts and to watch the trajectory of the solid-liquid interface

Sadoway, D. R.

English

NASA Technical Reports Server

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Probes and Monitors for the Study of Solidification 
of Molten Semiconductors 
Principal Investigator: Professor D.R. Sadoway -4 
. ' Abstract 
Product morphology in metal casting and semiconductor crystal growth is strongly influenced by 
the nature of the solid-liquid interface during solidification. Since in situ observation of solidification 
in these technologically important systems is not possible, investigators have resorted to the use of 
physical models, such as water and organic liquids, which while they are transparent to visible light, 
suffer from the fact that their Prandtl numbers are too high. Molten alkali halides are better physical 
models in this respect, and are transparent to visible light. The purpose of the present study is to 
examine solidification in the LiCl-KCI system to determine if phenomena such as  solute rejection can 
be observed by laser schlieren imaging. 
Introduction 
The properties and performance of a material are determined by its structure and composition, 
both of which are to a large extent controlled through processing. In semiconductor crystal growth, 
for example, product morphology and dopant concentration are strongly influenced by the behavior of 
the solid-liquid interface during solidification, yet it is difficult to observe directly. Accordingly, i t  is 
necessary to resort to mathematical models and physical models must be chosen with a high degree of 
similarity to the system of interest, Le., Prandtl number, Jackson a factor, etc. Significant deviations 
in such properties from those of the system of interest can weaken the physical model to the extent 
that it cannot be assumed that certain phenomena observed in the model occur a t  all in the real 
system. This is particularly true for nontrivial solidification processes, such as solute redistribution 
in a multicomponent system. As another example, the problem of dendrite growth in metals has long 
been the subject of study. Many theories have been enunciated [ 1,2,3,1; however, it is extremely 
difficult to test them. Since in situ observation of solidification in molten metals is not possible, 
investigators have used physical models such as water and organic liquids, which are transparent to 
visible light. Table I compares the physical properties of several melts of technological significance to 
those of various analogs. Metals and elemental semiconductors have low Prandtl numbers (Pr C 10- 
2), while water and organic liquids have high Prandtl numbers (Pr > 10). In the solidification of 
elemental semiconductors, the Jackson a factor (approximated here as AH fusion /RT melting and 
designated p) is greater than 2, while in the solidification of many organic liquids p C 2. Such 
disparities in the Prandtl number and 1.1 are unacceptable for the reasons given above. 
With these considerations in mind, the present study has turned to the use of molten alkali 
chlorides as physical models representative of systems of commercial interest. 
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https://ntrs.nasa.gov/search.jsp?R=19890004304 2020-03-20T04:20:59+00:00Z
* 
Table 1. Physical Properties 
Property cu Si 
b 
molecular weight 63.55 28.09 
melting temp, T mP 1083 141 2 
( g  mol-’) 
(“C) ,. 
heat of fusion, 13,050 50,550 
AHf ( 3  mol-’) 
1.18 3.58 AHf -RT,p . 
viscosity, n 4.0 1 
(Paas) x 10’ 
thermal conductivity, 165.6 
K~,,, (Wm-IK-’) 
( k g  m- 
heat capacity, cp 31.46 26.12 
( 3  rnol-’K-’) 
Prandtl Number * 1.2 x 10-2 3 x 10-2 
31 
density, 0 8000 2520 
b 
r 
crystal srructure f cc diamond 
of Selected Liquids 
v 
S CN KC1 -Li C1 “ZO 
18 80.09 55.47 
0 58.08 3 55 
6008 3700 13,083 
2.65 1.41 2.71-3.06 
1.787 4.6 1.46 
0.561 0.223 0.6903 
1000 970 1577 
75.91 160 72.31 
> 13 41 < 6  
hexagonal bcc sc 
The Present Study 
As part of an  investigation of the use of molten salts as physical models in solidification processes, 
fluid flow patterns in the melt are being studied by laser schlieren imaging. 
Figure 1 is a schematic of the experimental apparatus. The salt melt is contained in a fused 
quartz cell which has a square cross-section, 2” on edge. The cell is set inside an electrical resistance 
furnace with two viewing ports. A water cooled copper chill makes contact with the cell wall along 
one side. Schlieren imaging is accomplished through the use of a 5 mW helium-neon laser. The 
initial 0.8 mm diameter beam is expanded to 22 mm diameter before entering the cell. A lens focuses 
the light leaving the cell. At the focal point there is a removable knife edge mounted on an  x-y-z 
translator. A 35 mm camera body is positioned so that its film plane coincides with the image plane of 
the cell. Finally, a thin, straight wire is interposed between the laser source and the cell to act as a 
reference against which dserences in index of refraction can be viewed. 
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EDGE 
Figure 1 .  Schematic drawing of the experimental set-up. 
In a typical experiment the purified salts are charged and melted in the cell. Then cooling water 
is fed to the chill and the resulting convection patterns are observed. 
Two melt compositions have been chosen both in the LiCl-KCl system: the eutectic (41.5 mole% 
KCl) and an off-eutectic solution (46.2 mole8 KCI). 
Figure 2 shows a laser shadowgraph image of solidification of the eutectic melt. In a 
shadowgraph the schlieren knife edge has been removed from the light path. The chill plate is to the 
right of the field of view. Growth of solid phase is occurring in two directions: topdown and bottom 
up. Dendrites near the cell wall are faceted; interior dendrites are not. Instead, they are thinner with 
multiple branches which sometimes broke off and fell (see Fig. 2(a). Weak convective motion could be 
detected by schlieren imaging as seen in Figure 3. 
Figures 4 and 5 show images selected during the solidification of the off-eutectic melt. The 
process was different in this example. First, there was only one growth direction, horizontally 
outward from the chill. Secondly, turbulent convection was obstzrved. The general behavior of the 
schlieren images can be characterized as horizontal, wavelike motion at a frequency of - 0.5 Hz. The 
source of these oscillations is presumed to be double diffusive instability. Since denser KCl is 
, crystallizing the colder liquid close to the solid-liquid interface is richer in the less dense LiCl. 
Initially, the temperature effect on density dominates and the melt sinks. However, it appears that  
heat transfer is more rapid than mass transfer: the melt temperature rises and the compositional 
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ORIGINAL PAGE IS 
OF POOR QBATm. 
93A 1 e m  & 2 6  8 
Figure 2. Shadowgraphs of solidifying KCI-LiCI eutectic melt. Chill placed beyond field of view to the right. Time of 
photograph reported in minutes from beginning ofexperiment (a) 23, (b) 24.17, (c) 25.75, and td) 28.67. 
effect begins to dominate. This is evident in the "plumes" of the schlieren and shadowgraph images in 
Figure 5. 
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Conclusions 
Molten salts have attributes that make them attractive as physical models in solidification 
studies. With optical techniques of investigation such as schlieren imaging it is possible to study 
fluid flow phenomena in molten salts and to watch the trajectory of the solid-liquid interface. 
Experiments are continuing. 
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ORIGINAL PAGE IS 
OE POOR QUALITY 
Figure 3. Sehlieren images of didifying eutectic melt. Chill placed beyond field ofview to the right. Time of photograph 
reported in minutes from beginning of experiment: (a)  6. (b) 17.5. (c) 20.33. and (d) 24.5. 
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-24- ORIGINAL PAGE IS 
OE POOR QUALITY 
Figure 4. Schlieren images of solidifying off-eutectic (LiCI - 46.2mo18 KCI) melt. Chilled from the righthand side. Visible 
dilrtorted image of reference wire. Time of photograph reported in minutes from beginning of experiment: (a)  10.08, 
(b) 11.17.and (c) 15.42. 
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.ORIGINAL PAGE IS 
DIE: POOR QUALITY 
Figure 5. Shadowgraph of solidifying off-eutectic melt. Visible rising plumes caused by double diffusive phenomena. Chill 
placed beyond field ofview to the rightJ Time of photograph reported in minutes from beginning of experiment: ( a )  
20 and (b120.17. 
Reference List 
1. J.S. Langer, Metall. Trans. 15A (1984): 961. 
2. M.E. Clicksman, R.J. Schaefer, and J.D. Ayers, Metall. Trans. 7AJ1976): 1747. 
3.  S.R. Coriell, M.R. Coreles, W.J. Boettinger, and R.F. Sekerka, J .  CrystuZGrowth 49 (1980): 13-28. 
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Probes and monitors for the study of solidification of molten semiconductors

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