Determination of apparent thickness of graphite coating in flash method

When a sample for the measurement of thermal diffusivity is coated with graphite to enhance the absorbance of flash energy in the flash method, thermal resistance of the sample is increased due to the graphite layer itself and the contact resistance between the graphite layer and the sample surface. Such increased thermal resistance is considered as that of the graphite layer whose thickness is the apparent thickness ℓapgr of the graphite coating whose actual thickness is ℓgr. Using an equation for the thermal diffusivity proposed by Parker et al. [W.J. Parker, R.J. Jenkins, C.P. Butler, G.L. Abbott, J. Appl. Phys. 32 (1961) 1679], the present study found that the resistance factor defined as Greff = ℓapgr/ℓgr is a unique function of the half time, irrespective of the materials, which is given by a correlation equation, Greff=4.2454(ts+gr)1/2−0.465. Therefore, an accurate measurement of the half time enables us to find the thermal diffusivity of the graphite-coated sample material with an uncertainty level of about 0.6. It was found that the present method produces the thermal diffusivity of standard materials within 0.6% difference with respect to the standard data except very high thermal diffusivity materials such as copper and alumina. The advantage of this method lies in avoiding the lengthy mathematical equations (e.g. a three-layer analysis) needed to correct the errors caused by the graphite coating.