EFFECTS OF DETECTOR NONLINEARITY AND SPECIMEN SIZE ON THE APPARENT THERMAL-DIFFUSIVITY OF NIST-8425 GRAPHITE

With the use of a graphite thermal conductivity standard it is demonstrated that optical detector non-linearity, coupled with excessive laser pulse energies, is primarily responsible for the anomalous specimen size dependence of the thermal diffusivity measured by the laser-pulse technique. High laser pulse energies also result in an anomalous positive temperature dependence for thin specimens near room temperature, in contrast to the expected negative temperature dependence. Using moderately thick specimens and attenuated laser pulses yields excellent agreement with thermal diffusivity calculated from standard thermal conductivity data. © 1990 Plenum Publishing Corporation.