tai apparatus is shown in Fig. 5. The electric heater was made of carbon cloth since it has good X-ray transmissivity. Temperature was measured with a platinum-rhodium thermocouple located just under the containment canister. The maximum operating temperature was approximately 1500 K. The outer cylindrical vessel (made of aluminum) was 150 mm in diameter and about 200 mm in height. An X-ray scan of the general view is shown in Fig. 6. An example of the continuous visualization results of the solidification process is shown in Fig. 7. A lithium fluoride and calcium fluoride eutectic mixture (80.5-19.5 mol%) was placed within the open carbon canister. The melting temperature was first stabilized at 1070 K. At this temperature, the salt is a liquid (see Fig. 7, first picture). From this setting, the input power of the heater was gradually decreased. Snapshots of the changing meniscus (intervals 90 s) as the salt underwent solidification are shown in Fig. 7. The last picture shows the frozen PCM at 1010 K. By examining the results (though accurate estimation is difficult), the volumetric change of this eutectic salt can be calculated to be 24%. This measured value almost agrees with the data presented in Ref. [1]. A cross section of the cylindrical canister plus carbon fin is shown in Fig. 8. The white semicircular area indicates the solid lithium fluoride while the interior black area indicates the void. The outer circular dashed line delineates the electric heater. This
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