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Improvement of the Cowan method for thermal diffusivity determination
Dominique Gosset, Michel Colin

A two-step procedure, equivalent to the Cowan method, has been developed for determining the thermal diffusivity of homogeneous materials by the flash method. First, a mathematical description of the system (radial symmetry) is presented which allows the theoretical calculation of thermograms, from which a general relationship can be deduced. The formal description of the system includes partial or heterogeneous illumination of the front face of the sample, recording of the thermal history (thermogram) at the rear face with a thermocouple or an infrared sensor, and assuming different heat loss coefficients for the front, rear, and peripheral surfaces of the samples. Second, the relationship is used to deduce from the analysis of experimental thermograms the g coefficient in the classical expression a = ge2/t1/2, where a is thermal diffusivity, e is sample thickness, and t1/2 is the time at which the temperature increase reaches half of its maximum. We have then used the thermogram equation to compare some classical methods. The calculation has been validated by analysis of an iron standard (SRM 8421). The measurements were performed in a diffusimeter constructed by us which allowed measurements in the temperature range 20 to 1000°C.

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