Application of thermomechanical characterization techniques to bismuth telluride-based thermoelectric materials

The thermoelectric properties of bismuth telluride based thermoelectric (TE) materials are well-characterized, but comparatively little has been published on the mechanical and thermomechanical properties of these materials. Material structures range from mica-like to sandstone-like depending on the processing conditions and TE materials exhibit varying degrees of anisotropy thermoelectric and mechanical behavior. In this paper, the initial dynamic mechanical analysis (DMA) data for n-type and p-type bismuth telluride based TE materials is presented. The TE materials' tan delta values, indicative of viscoelastic energy dissipation modes, approach that of glassy or crystalline polymers and are greater than ten times the tan delta of structural metals. TE samples measured perpendicular to the van der Waals planes have higher tan delta values. Thermal scans in the DMA compressive mode showed changes in mechanical properties versus temperature with clear hysteresis effects. These changes were correlated to differential scanning calorimetry (DSC) thermal transitions. The expected anisotropy was shown in flexural 3-point bending results for one n-type material that showed a storage modulus of 0.10 to 0.45 GPa in the direction parallel to the van der Waals planes and 0.07 to 0.2 GPa in the perpendicular direction. The DMA results showed that the application of the DMA techniques are useful in the evaluation of thermophysical and thermomechanical properties of these TE materials.