Creep Deformation Of Thermoelectric Materials

Abstract

As more thermoelectric materials/devices make it into the market for various applications, several aspects need to be explored and optimized, beyond simply targeting high conversion efficiency at the material levels. One critical aspect is the guarantee of mechanical stability at both the material and the device level, which demands deeper understanding of the stresses affecting a thermoelectric (TE) material/module and the concomitant mechanical deformation modes. In this thesis, we elaborated on characterizing the stresses impacting a TE module, both the manufacturing residual and the operational categories. Such stresses collectively are capable of inducing time-dependent high-temperature deformation (i.e. creep), which is shown to negatively impact the thermoelectric performance, seen as a drop in the power factor and the figure of merit zT. Among the mid-high temperature TE materials tested for creep, the half-Heusler alloy Hf0.3Zr0.7NiSn0.98Sb0.02 depicted the highest creep resistance reported so far, sustaining stresses up to ∼ 360 MPa at 600 ℃ without notable failure. The creep resistance of the Skutterudite alloy Yb0.3Co4Sb12 is midway between that of low-mid temperature TE materials, such as Bi2Te3, PbTe and TAGS-85, and that of midhigh temperatures thermoelectrics, including silicides and half-Heusler alloys. The in situ electrical resistivity-creep experiments on doped PbTe confirmed that dislocations multiplication continuously increases the electrical resistivity in a semi-linear trend with the engineering strain, despite the high temperature recovery processes. The resistivity increase was seen to be controlled by the rate of change of the immobile dislocations density with the engineering strain, such dislocations were seen to form complex networks and subgrain structures, as revealed by TEM analysis. Lastly, in an effort to explore 3D printability of ternary and multinary TE alloys, the n-type half-Heusler alloy Nb1-xCoSb was successfully shown to be printable utilizing fully prealloyed powder and saturation annealing under Sb atmosphere to mitigate issues such as volatility of Sb and porosity. A zTmax ∼ 0.1 was achieved at 600 ℃.