Determination of the Local Thermal Diffusivity of Inhomogeneous Samples by a Modified Laser-Flash Method

The local thermal diffusivity is of special interest for quality control of materials grown by physical vapor transport. A typical specimen of these materials consists of single crystals with sizes up to 1 mm. The conventional laser-flash method delivers only an average value of the thermal diffusivity of these polycrystalline materials. A local sensitive measurement system is desirable to determine the thermal diffusivity of single grains with diameters of 100 μm and above. In this work a modification of a standard laser-flash apparatus is presented. The key feature is the position control of the specimen in the plane perpendicular to the laser beam and the IR-detection unit. The mechanical precision of the position control is better than 100 μm. The IR-detection unit consists of a MCT-detector, a polycrystalline IR-fiber, and a system to focus on the sample surface. To study the experimental potential of the modified laser-flash method, measurements of the local thermal diffusivity of a multiphase specimen with known microscopic thermal properties are presented. The obtained results are discussed with respect to the energy profile of the laser beam and the alignment of the IR-detection unit. It is shown that the thermal diffusivity of a small specimen area with a diameter of 2 mm can be determined with an uncertainty of ±5 %. For a polycrystalline aluminum nitride (AlN) specimen with grain sizes of the order of 1 mm, a mean value for the thermal diffusivity of (72.1 ± 3.6) m² · s⁻¹ at room temperature is determined. A possible local variation of the thermal diffusivity cannot yet be observed. An improvement of the resolution is in progress. Paper presented at the Seventeenth European Conference on Thermopysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.