Abstract
Cryogenic specific heats are measured for the evaluation of the surface state of alumina powders. Specific heats of commercial powders are measured in a temperature range between 15K and 50K. For specific heat measurements, powder particles are covered by Apiezon N grease to increase the thermal conduction of powder compacted bodies. Excess specific heats are observed below 30K for each sample. For the commercial powders, the total entropy becomes higher as the specific surface area increases. The specific surface entropies of commercial powders are higher than those of porous samples. A possible reason of the decrease of entropy upon sintering process is the phase transformation of hydroxide alumina on the surface of commercial powders to other phases like the α-phase by sintering process. The weight change by the heat treatment at 1200°C is in agreement with the calculated weight change for the phase transformation, assuming that the surface thickness is about 1nm. The high entropy of commercial powders may be due to the presence of hydroxide alumina which exists on the surface of the alumina crystals and is not detectable by X-ray diffraction.
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