Direct visualization of three dimensional transfer process of both heat and mass around a growing crystal and mono-molecular growth layers on the surface is possible in situ by means of high resolution Hoffman modulation contrast microscopy coupled with three wavelength two beam Mach-Zehnder interferometry. This in situ observation is very suitable for the verification of the growth mechanism of a crystal in a solution or a melt in microgravity

Tsukamoto, Katsuo

English

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-APPLICATION OF HOFFMAN MODULATION CONTRAST MICROSCOPY COUPLED WITH 
THREE-WAVELENGTH TWO-BEAM INTERFEROMETRY TO THE IN SITU DIRECT 
OBSERVATION OF THE GROWTH PROCESS O F  A CRYSTAL IN MICROGRAVITY- I 
Katsuo Tsukamoto 
Faculty of Science, Tohoku University 
Aoba, Sendai 980, JAPAN 
ABSTRACT 
Direct visualization of three-dimensional transfer processes of both heat and mass around a growing crys- 
tal and mono-molecular growth layers on the surface is possible in situ by means of high resolution Hoff- 
man modulation contrast microscopy (HMCM) coupled with three-wavelength two-beam Mach-Zehnder 
interferometry (TTMI). This in situ observation is very suitable for the verification of the growth mech- 
anism of a crystal in a solution or a melt in microgravity. 
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https://ntrs.nasa.gov/search.jsp?R=19880008320 2020-03-20T08:15:06+00:00Z
Crystal growth process. 
Crystal growth proceeds when molecules are integrated at the surface via the transport of molecules 
from the bulk solution t o  the crystal surface, at which latent heat would be released at the same time. 
Observation of the crystal surface has been one of the most powerful ways to  investigate the integra- 
tion mechanism of molecules at the surface. This has been achieved by means of special optical micros- 
copies like phase contrast microscopy or electron microscopy using decoration techniques. However, 
these observations were limited to  the crystal which has been taken out of the solution, followed by a 
special surface treatment. If one could directly observe the movement of mono-molecular growth steps 
in situ, it would contribute to  investigation of the fundamental growth mechanism of a crystal in more 
detail. Based upon this ideal, a high-resolution in situ observation method was developed by the author 
in 1983, although there had been several other in situ observations with much less resolution. This made 
it possible to  visualize the movement of mono-molecular growth steps with 1.4 nm height on a crystal 
growing in aqueous solutions, as shown in fig. 1. This method was recently extended to  the growth of 
a crystal in a solution at elevated temperatures as h g h  as 1800 OK. 
The transport of molecules is another important problem which has not fully been understood yet, 
since very complex convections easily appear near the crystal in gravity, which thus disturbs the trans- 
port of molecules. The same holds for the transport mechanism of the latent heat which is released dur- 
ing the growth of the crystal. There have been many attempts t o  measure the concentration gradient 
around a growing crystal by the Schlieren method or interferometries. However, the transport of heat 
has not been measured exactly except in a few experiments, in which they put the array of very thin 
thermocouples normal to  the crystal surface for the measurement. The visualization of the heat transfer 
around a growing crystal is more difficult and thus few experiments have been performed up to now. 
So far, experiments on the transfer problem were carried out in the system in which either heat transfer 
or mass transfer is negligible, although in actual crystal growth processes, both are important. More 
interesting is their mutual interactions due t o  the completely opposite direction of the transfer of the heat 
and the mass. This is believed, in gravity, t o  cause complex fluctuating flows near the crystal surface, 
leading to  periodic fluctuation of the growth rate, or periodic impurity incorporation into the crystal. 
This interaction has not yet been investigated, first because of its complex nature for theory, and second 
because of lack of a suitable experimental method. Since these phenomena (movement of growth layers 
and the transport of mass and heat) have the mutual interactions, it would be necessary t o  investigate all 
phenomena at the same time by an in situ method, because these phenomena are not time-stationary. 
Observation in gravity. 
Direct observation of both crystal surface and the mass transport phenomena in gravity for the case of 
aqueous solution growth have been extensively investigated in our group, although heat transfer has not 
been investigated. The result is summarized as follows. 
When the supersaturation is very low, < 0.5%, no convection appears and thus the crystal is surrounded 
by a diffusion boundary layer, fig. 2a. On increasing the supersaturation, a solutal convection plum starts 
to develop, fig. 2b, due to  the large concentration gradient in the diffusion layer, the critical supersatura- 
tion of which is typically 1 - 2%. Since the plum develops intermittently and is unstable, the dispersion of 
the growth rate of a crystal in this range is very large, -40%, which would give rise to  periodic growth 
striations even if the bulk supersaturation is accurately controlled, <O.Ol%. This unstable plum disappears 
when the supersaturation is increased further, which is followed by the development of a stable plum, 
fig. 2c. This again stabilizes the growth rate of a crystal. It may be important to  note that the supersatur- 
ation range, 1 - 2%, is often used to  grow large crystals in solutions without considering the hydrodynami- 
cal behavior of the convections. 
The development of this kind of solutal convection influences not only the growth rate of a crystal 
versus supersaturation, fig. 3, but also the surface state of a crystal, fig. 4. When the surface of a crystal 
90 
which was grown at, for instance 576, is observed in situ, many inclusions are observed to  be trapped on 
the surface. It is interesting t o  see that they are trapped along the periphery of the root of a convection 
plumb. As the plum position shifts, the array of inclusions also shifts. 
Although this is one of the examples t o  show that hydrodyynamical properties of the solution around 
a crystal have many influences on the growth kinetics and the perfection of a crystal, numbers of in situ 
observations have already been performed during the past several years and thus, enough data are avail- 
able for comparison with the experiments in mocrogravity. I t  is proposed here to  perform similar in situ 
direct observations of a crystal growth process in microgravity, so that one can find out the essential dif- 
ference between growth mechanisms in gravity and in microgravity in much more direct ways. Such 
high resolution in situ observations as proposed here were developed in Japan for use in gravity, but have 
not yet been proposed in any countries for use in microgravity. If this kind of in situ visualization of the 
phenomena were performed together with the coupled kinetical measurements, it would have wider 
applicability in other material sciences in microgravity. 
Proposed experiment. 
1. Crystal 
Several solution-grown or melt-grown crystals will be selected from inorganic and organic crystals; the 
saturation temperatures and the melting temperatures are 40 - 50 "C. By varying the chemical composi- 
tion, some crystals are grown as faceted, which are suitable for the direct observation of layer growth on 
the surface and the mass and heat transport phenomena. The other crystals are grown as dendrites, which 
are suitable for the visualization of the heat and mass transfer process and the mutual interaction. Need- 
less t o  say, experiments both in gravity and in microgravity are necessary for the analysis of the growth 
mechanism. 
2. Optical system 
Hoffman modulation contrast microscopy (HMCM) with the auto-focusing system is selected among 
varieties of microscopes because of its high contrast images for both isotropic and anisotropic crystals, 
by which growth layers as thin as 1 nm can be resolved, fig. 5 .  In order to  visualize the mass and heat 
transfer process around a growing crystal quantitatively, three-wavelength two-beam Mach-Zehnder inter- 
ferometry is used (fig. 5 ) .  The optical pass and the lenses are almost in common with HMCM. The reso- 
lution of the interferometry has much higher resolution than holographic interferometries if it is adjusted 
correctly. In order to avoid the problem of heat radiation and the electric energy consumption, LEDs 
with three different colors are used, which are switched on alternatively, in order t o  increase the S/N 
ratio of the optical images by reducing the background intensity of the scattering light in a crystal. These 
are synchronized with the CCD TV camera for recordings. The interference fringes from three different 
wavelengths and the HMCM images are recorded on video tape. 
So as to  obtain the three-dimensional information on the heat and mass transfer, two vertically-crossing 
optical axes will be used. 
3. Growth cell. 
A crystal is grown in a growth cell, the temperature of which is controlled by thermo-modules with the 
temperature stability of < 1.0 "C. The cell is made of an aluminum block, fig. 6, in which a degasser is 
used t o  remove possible air bubbles in the solution, a solution pump and thermo-modules are installed. 
Six growth cells are prepared to be exchanged for the observation of different crystals. 
Since a seed crystal is used, a remote monitor is used to see its shape through TV images. This is nec- 
essary so as not t o  dissolve the seed crystal completely just before the growth experiments. 
91 
Fig. 1 SPIRAL STEP WITH 1.4 nm HEIGHT OF C d I, GROWING IN AN AQUEOUS SOLUTION, 
BY IN SITU OBSERVATION, FROM TV 
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Fig. 3 (a) GROWTH RATE VS SUPERSATURATION, AND (b) ADVANCE RATE O F  GROWTH 
LAYERS VS SUPERSATURATION, Ba(N03)2. NOTE THE LARGER SLOPE WHEN A 
PLUM APPEARS. 
Fig. 4. TRAP O F  INCLUSIONS DUE TO THE DEVELOPMENT OF A CONVECTION PLUM. THE 
DISTRIBUTION O F  THE INCLUSIONS IS LARGELY INFLUENCED BY THE PATTERN OF 
A CONVECTION PLUM. THE DEVELOPMENT OF DISLOCATIONS IS ALSO INFLUENCED 
BY THE CONVECTION. 
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Application of Hoffman modulation contrast microscopy coupled with three-wavelength two-beam interferometry to the in situ direct observation of the growth process of a crystal in microgravity

Application of Hoffman modulation contrast microscopy coupled with three-wavelength two-beam interferometry to the in situ direct observation of the growth process of a crystal in microgravity

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