Simultaneous measurement of in-plane and through-plane thermal conductivity using beam-offset frequency domain thermoreflectance

Transient thermoreflectance (TTR) techniques are ubiquitous methods for measuring thermal conductivity of bulk materials and thin-films. Both through-plane thermal conductivity k(perpendicular to) and in-plane thermal conductivity k(parallel to) should be independently measured in transversely anisotropic materials. When these properties are measured using conventional TTR techniques, the accuracy of the k(parallel to) measurement is dependent on the accuracy of measuring k(perpendicular to) and vice versa. This is especially problematic for thin-films measurements as uncertainty in k(perpendicular to) (similar to 5%) can propagate and grow for uncertainty in k(parallel to). In this paper, we present a method for the simultaneous measurement of k(perpendicular to) and k(parallel to) using beam-offset frequency domain thermoreflectance (FDTR) with robust uncertainty estimation. The conventional diffusive heat transfer solution is analyzed to show that offset and heating frequency can independently control the sensitivity to directional thermal conductivity and extract values for k(parallel to) and k(perpendicular to). Numerical uncertainty analyses demonstrate that sweeping both heating frequency and beam offset results in a reduction of measurement uncertainty. This modified measurement technique is demonstrated on crystalline alumina (c-Al2O3), amorphous alumina (a-Al2O3), quartz, fused silica, and highly oriented pyrolytic graphite. Published by AIP Publishing.