Experimental and Modeling Study of Thermoelectric Materials for High Figure of Merit

Abstract

Half-Heusler alloys RNiSn (R=Hf, Zr, Ti) were made by arc melting under argon atmosphere followed by the consolidation using Spark Plasma Sintering (SPS) technique. The thermoelectric properties were measured from room temperature up to 1170 K. The temperature dependences of thermoelectric properties and structures were investigated. A frequency-dependent differential effective medium (DEM) model was developed to evaluate the lattice thermal conductivity. This model was also applied to calculate the thermal conductivity of nanostructured Si and Si 80 Ge 20 systems. The calculation results were compared with the experimental data measured with timedomain thermoreflectance to investigate length scale effects on phonon wavelength, mean free path, as well as the resulting impact on the thermal conductivity.In the case of n-type Hf 0.6 Zr 0.4 NiSb 0.995 Sb 0.005 , thermoelectric properties of this composition were conducted by annealing this material near the melting point in order to improve the structural order. The reduction in the lattice strain was observed. It also led to a significant enhancement of the power factor, and thus, the figure of merit (ZT) was raised to 1.2 around 850 K without the need of nanostructures. This large increase in power factor was related to the decrease of carrier concentration and the increase of mobility. This approach can be applied to the broader class of Heusler alloys, and may lead to the improvement of their thermoelectric properties. The elemental substitution and embedment of nanoparticles in the half-Heusler matrix techniques were also investigated to achieve higher ZT. Thermoelectric properties of the substitution of Ti for (Hf, Zr) sites and the addition of nano-ZrO 2 in n-type (Hf, Zr)NiSn were measured. The thermal conductivity of the composition was significantly reduced due to the enhanced phonon At the end, to evaluate the lattice thermal conductivity, a non-gray differential effective medium (DEM) model was developed and applied to half-Heusler systems. This model not only expanded the applicability of effective medium approach from small volume fractions to the whole range of volume fraction from 0 to 1, but also accounted for the multiple scattering. Then this frequency-dependent effective medium approach was utilized to calculate the lattice thermal conductivity of nanostructured Si and Si 80 Ge 20 systems. The model accurately predicted a large reduction in the thermal conductivity of these systems measured with time-domain 4 thermoreflectance (TDTR). Then the phonon spectral analysis was conducted to investigate length scale effects, and the insight into the role of long wavelength phonons on the thermal conductivity of the nanograined Si and Si-Ge alloys was gained. It was shown that phonons with wavelength much greater than the averaged grain size would not be impacted by the grain boundary scattering. 5