A transient heating method for measuring thermal diffusivity of a solid disk with thermal radiation losses from all faces, (1). Step-wise heating

A theoretical study was undertaken for developing a procedure for measuring by transient methods the thermal diffusivity of a small solid disk whose diameter is not very large compared with its thickness. The disk is placed in a vacuum furnace and heated on one face by a radiant heat source whose intensity is varied step-wise. Thermal radiation losses not only from the front and rear faces but also from the lateral face of the disk are considered. The results established that, if the ratio 2γ₀/l (2γ₀: diameter, l: thickness) of the disk is larger than 6, the thermal diffusivity can be determined within 1.0% error by reckoning with the disk regarded as a infinite slab. If 2γ₀/l≲6, the thermal diffusivity should be determined by the present theory which takes into account thermal radiation losses from all faces of the disk. We further find from the present theory that if 2γ₀/l≲2 and that at the same time the thermal radiation losses are large, it should be difficult to measure thermal diffusivity with high accuracy. (author)