THIN FILM THERMOELECTRIC CHARACTERIZATION TECHNIQUES
The key thin film thermoelectric characterization techniques are described. Because of the small dimensions, a careful examination of electrical and thermal paths in the device is necessary. We describe both in-plane and cross-plane measurement methodologies. Sample requirements for four-probe and van der Pauw methods for in-plane electrical conductivity measurement are discussed. The in-plane Seebeck coefficient is characterized under a temperature gradient that generates a voltage. Precise measurements of temperature and voltage at the same location in the sample are very important. For the cross-plane electrical conductivity, the modified transmission line method is evaluated. To eliminate parasitic contact and substrate resistances, several samples with varying thicknesses are required. Two approaches, a DC method and the 3ω method, are described in detail for the cross-plane Seebeck coefficient characterization. Next, we focus on the transient Harman method to directly measure the cross-plane thermoelectric figure of merit of a thin film. The device requirements for reducing parasitic heat losses and current nonuniformity are presented. Thermoreflectance imaging can be used together with transient Harman in order to extract electrical and thermal conductivities and the Seebeck coefficient simultaneously. Finally, Z-meters are described for directly determining the figure of merit and efficiency of a thermoelectric element or module under a large temperature gradient. Recent developments have significantly reduced thermal and electrical parasitics, as well as radiation heat loss in the system, enabling ZT measurement of legs as thin as one hundred microns.
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Illustration of composite TIMs with a percolation of spherical nanoparticles, and high aspect ratio nanowires. NANOSTRUCTURED THERMAL INTERFACES
Photograph of copper/diamond sintered wick structure. RECENT ADVANCES IN TWO-PHASE THERMAL GROUND PLANES
The microchannel with a single pillar used by Jung et al., and an SEM image of the pillar with a flow control slit at 180 deg (facing downstream). ADVANCED CHIP-LEVEL LIQUID HEAT EXCHANGERS
Schematics of thermal boundary conductance calculations. NONEQUILIRIUM MOLECULAR DYNAMICS METHODS FOR LATTICE HEAT CONDUCTION CALCULATIONS
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