Thermal Diffusivity Investigations on Alumina Substrates using Laser Flash Analysis

This paper describes a procedure to measure the thermal diffusivity and thus the thermal conductivity of alumina substrates for power electronics. While the laser flash analysis usually obtains precise results regarding the thermal diffusivity, the measurements could be inaccurate for thinner thicknesses of the specimens. This is due to the necessity of applying a coating before running the measurements of thermal diffusivity which subsequently has a significant impact on the results at smaller thicknesses. Therefore, the manufacturer of the used LFA 447 measurement device by Netzsch GmbH recommends using platinum or gold sputtering at thinner thicknesses instead of graphite spraying. The latter is the standard coating material for almost every application because of its excellent absorption and emission properties. The following describes a process to obtain the thermal diffusivity of alumina substrates still using a graphite coating for the measurement applied by graphite spray cans and thus saving costs compared to sputter systems. The measurement procedure utilizes a linear correlation between the thermal diffusivity of the alumina specimens and the graphite coating masses at room temperature that was discovered in the following investigations.