Optical observations of unidirectional solidification in microgravity
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METALLURGICAL TRANSACTIONS A
due to variations in cooling rate. Thermocouple data o b tained from other test runs confirmed this. The interferograms in Figures 1 to 3 show temperature and concentration profiles. Figure 1 shows initial sample cooling and the onset of convection during ground based tests. Changes in the fluid sample temperature profile are clearly shown by the interferometric fringe build-up which occurs at the bottom of the cell as the sample is cooled. The final frame of Figure 1 shows the onset of convection caused by changes in fluid concentration ahead of the growth interface. While the resultant fringes then represent both temperature and concentration profiles, it becomes possible to separate the two by waiting until the growth plumes rise to a region with a uniform temperature, as can be seen in Figure 2. Figure 2 thus shows concentration profiles in growth plumes resulting from gravity-induced convection. Figure 3 shows interferograms taken on the KC-135 aircraft in a low-g environment. In the absence of gravity-induced convection, the concentration profiles are seen to be relatively stable. In order to analyze these interferograms, refractive index data were taken for NH4-C1-H20 using a Belligham and Stanley Model 60/ED refractometer. Test cell temperature was determined by a Neslab Model RTE-4 circulating bath controlled to _+0.02 °C. From these data, it was determined that in the range of interest, a 1 pct change in concentration ~ An = 0.00187, and a 1 °C change in temperature An = 0.00012, where An = change in sample refractive index. Assuming a two-dimensional test field, the refractive index at any point in the test cell is given by 3'0E n = -q- nref
L
where
3'0 = e = L = nref =
vacuum wavelength optical path length difference in terms of A length of test cell refractive index at reference point.
We may thus write n ~ m
L Using Y0 = 632.8 nm (He-Ne laser wavelength), the appropriate measured An, and L = 1 cm (test cell depth), we obtain a value of e = 1.95 corresponding to a 1 °C temperature change. Similarly, using L = 1 mm and An (1 pct concentration change) = 0.00187, we obtain e = 2.95 for a 1 pct change in concentration. Note that e = 1 corresponds to one fringe shift in our interferograms. These results were used to calculate the temperature and concentration profiles shown in Figures 4 and 5. The initial change in fluid temperature with time was determined from the thermal profiles and is shown in Figure 6. The low-g and 1-g cases parallel each other for the first fifteen seconds of cooling; then the 1-g fluid cooling rate begins to decrease while the low-g fluid continues to cool at the initial constant rate. Although nothing was visible in the interferometric photographs to explain this divergence, ground-based laser streak photographs using tracer particles showed convection cells at the base of the cuvette within twenty seconds after cooling began. Cool liquid was
U. S. GOVERNMENT WORK NOT PROTECTED BY U. S. COPYRIGHT
VOLUME 14A, OCTOBER 1983--2163
t-O
t = 30 seco
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