This paper describes a method for the determination of thermal diffusivities which has been developed specifically for substances which are poor conductors and which have high melting points. Materials which are useful for thermal energy storage fall into this category. The method has several unique features. The basic principle involved consists of raising the surface temperature of a solid specimen at a uniform rate. After the initial transients have died out, the diffusivity can be determined from temperature measurements alone. The advantages of the method are: (a) Heat flux measurements are not needed; (b) materials can be tested right up to the melting point, since the specimens can be encapsulated and softening can be tolerated; (c) large temperature ranges can be tested quickly; (d) precision and accuracy are good. The method has been extended to the liquid range, and results will be published as Part II. Results of measurements are reported for alumina and lithium fluoride. The results for alumina (Lucalox) check results reported previously. The results on LiF differ from published results. Data on other substances are still being produced and results will be published at a later date.

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