Micro- and nanomachined tools for measuring in-plane thermal conductivity of thermoelectric thin films

Many of the potential next-generation thermoelectric materials being studied are either thin films or nanostructures that are expected to have anisotropic properties. For example, the thermal conductivity of a layered thin film in the plane of the film, $k_{\parallel}$, is likely to be different from that perpendicular to the layers, $k_{\perp}$. Techniques such as the $3\omega$ method and picosecond thermoreflectance allow accurate measurements of $k_{\perp}$ at temperatures relevant to thermoelectrics, but measuring $k_{\parallel}$ is often difficult. In this talk we discuss our efforts to design and demonstrate accurate measurements of $k_{\parallel}$ of thin films from $77-475$ K using micro- and nanomachined thermal isolation platforms. Using thin-film structures to support the thin-film sample reduces background contributions, and careful control of the geometry keeps radiation errors small. We will also discuss plans for a next-generation device that will simultaneously measure thermal conductivity, thermopower, and electrical conductivity of a thin-film or nanostructure, allowing determination of the thermoelectric figure-of-merit, $ZT$.