A study of the thermal diffusivities of carbons using optical beam deflection

The thermal diffusivity is a property which completely characterizes the thermal wave in a material. The thermal diffusivity determines the speed, the wavelength, and the rate of extinction of the thermal wave. Optical beam deflection (OBD) is an experimental method of directly measuring the thermal diffusivity of a material parallel to the sample surface.; Chopped light impingent upon the surface of a sample generates thermal waves in the material. The thermal waves propagate outward in all directions from the illuminated region. The temperature of the layer of gas immediately above the sample is driven by the temperature of the surface of the material. The time-dependent gradient of the index of refraction of the gas corresponds to the gradient of the time-dependent temperature field. Therefore, a probe laser beam propagating in this layer of gas is deflected by each passing heat pulse. The time required for the heat pulse to reach the probe laser beam causes a phase shift in the deflection of the probe beam measured relative to the phase of the illumination. This phase shift increases with probe-beam distance from the region of illumination. The thermal diffusivity is determined from the change of the phase with respect to the displacement of the probe beam. At sufficient probe-beam distances from the illuminated region, OBD theory predicts a linear dependence of the phase on the probe-beam displacement.; The thermal diffusivities of five carbon materials were determined by OBD at room temperature after each was heat treated at six temperatures ranging from 1000{dollar}spcirc{dollar}C to 2600{dollar}spcirc{dollar}C. Two materials, an isotropic carbon and an anisotropic carbon, were made by chemical vapor deposition (CVD). After the anisotropic carbon was heat-treated beyond 2400{dollar}spcirc{dollar}C, an increase in the carbon network was evidenced by a significant rise in the thermal diffusivity. The thermal diffusivity of the isotropic CVD carbon remained unchanged with heat treatment. Like the anisotropic CVD carbon, the thermal diffusivities of polyamide carbon films significantly increased after heat treatment beyond 2400{dollar}spcirc{dollar}C. The thermal diffusivity of carbon films previously heat-treated beyond 2400{dollar}spcirc{dollar}C decreased by a factor of five after a few minutes of surface oxidation.