Elastic modulus and internal friction of thermoelectric composites: Enthalpy–entropy compensation

Mechanical spectroscopy measurements are reported on rods of (Bi1−xSbx)2Te3 and (Bi1−xSbx)2(Te1−ySey)3 polycrystalline thermoelectric (TE) composite alloys produced by mechanical alloying and hot extrusion. Low internal friction (Q−1) and large stiffness coefficients obtained in the 293–573 K range corroborate the improved elastic properties of these hot extruded and highly textured polycrystalline materials that have enhanced TE properties. Their stiffness shows a quadratic (decreasing) temperature dependence, and the value for most composites is larger than that of conventional single-phase random alloys and hot extruded bismuth telluride. One particular damping mechanism in Q−1, believed to be due to grain boundary motion, is thermally activated at temperatures higher than 420 K. The prefactor of the Arrhenius law correlates with the activation enthalpy (Δh) for every composite. It varies exponentially with Δh for more than six orders of magnitude for activation enthalpies up to 0.9 eV. The observation of this enthalpy–entropy compensation effect results from the existence of a maximum in energy dissipation in a linear regime of weak perturbations. This simple explanation is proposed to apply to this ubiquitous effect which has been observed in many areas of physics, chemistry, and biology.