The ac-Heated Strip Technique for the Measurement of Thermal Properties of Thin, Solid Nonconducting Layers

Abstract

The differential equation describing the temperature oscillations generated by a metallic strip of thickness 2L1, coated with a thin nonconducting layer of thickness δ and heated by an ac current, was solved with and without reference to the heat transfer by radiation from the outer surface of the coating layer. The solution obtained shows that for L1/δ>5, the measurement of the amplitude (and phase) of the temperature oscillations of the strip allows the following thermal properties to be measured: thermal diffusivity (a), volumetric heat capacity (ρCp), and thermal conductivity (λ) coefficients of the thin nonconducting layers investigated when the layer thickness is less than the thickness (Λ) at which the temperature oscillations are practically damped in the material of the coating layer (δ<Λ). For thin films (δ≪Λ), a simple formula is obtained which allows the volumetric heat capacity of the film material to be determined alone. On the other hand, when the coating layer is very thick (δ≫Λ), only the thermal activity coefficient (λ/\(\sqrt a \)) can be determined. To test this technique, thin layers of polyethylene and polystyrene were investigated. Bulk specimens were investigated by the ac-heated wire technique. Both sets of results obtained are in good agreement with each other and with published data. The setup described was then used to measure the thermal properties of thin layers of Synthesite AC-43 (440 to 80°C) and Krylon (40 to 160°C). The results are reported. It is worth mentioning that this technique can be used for the estimation of the thickness of opaque thin layers of nonconducting solids, provided that their thermal properties are known. The thermal properties mentioned can be measured applying the ac-heated wire technique, but for bulk material.