UC Davis

Radioisotope thermoelectric generators (RTG) have been providing reliable, long-term power to space missions for over 50 years. These devices consist of a radioisotope heat source at the core which is surrounded by many p- and n-type thermoelectric legs. Radiative fins on the outside of the enclosure provide the cold side of thermal gradient across the thermoelectric legs. The efficiency at which these materials then covert that thermal gradient into usable electricity is related to the unitless thermoelectric figure of merit, zT = σS2T/κ, where sigma (σ) is the electrical conductivity, S is the Seebeck coefficient, or voltage per degree of thermal gradient, T is the absolute temperature, and kappa (κ) is the thermal conductivity.Yb14MnSb11 and its analog, Yb14MgSb11, are two p-type thermoelectric materials which have zT’s greater than 1.0 at high temperatures. Because of this, these materials are of interest for use in the next generation of RTG However, polycrystalline samples of these materials can be difficult to make with high purity and those impurities tend to negatively impact the zT. In addition to impacts on thermoelectric properties, impurities can also have an impact on physical and chemical properties such as thermal stability and oxidation. In the first work presented, the metal hydrides, YbH2 and MgH2, were investigated for use as reactive precursors to make Yb14MgSb11. While YbH2 proved effective in reducing the cold-welding of Yb during ball milling, a large excess of Mg (50%) was required to make high purity product due to Mg being 1/26 atoms within the structure. MgH2 provided a higher dispersity Mg source allowing for a reduction in Mg excess to 20%, ultimately leading to improved thermoelectric figure of merit, reaching a peak zT of 1.26 at 1200 K. To better understand the reaction pathway taken by polycrystalline reactions to form Yb14MSb11 (M = Al, Mn, Mg) the next work utilizes a series of quenching studies to investigate the phases formed after ball milling and during heating before the final product forms. These studies revealed that utilization of the binary intermediate Yb4Sb3 as a precursor with YbH2 and either MnSb or Mg3Sb2 allowed for direct formation of high purity Yb14MnSb11 and Yb14MgSb11 through balanced, stoichiometric reactions. The improved synthetic route yielded Yb14MgSb11 and Yb14MnSb11 with further improved thermoelectric properties, reaching peak zT’s of 1.28 at 1175 K and 1.24 at 1275 K respectively. While Yb14MgSb11 and Yb14MnSb11 have been proven to be high performing thermoelectric materials, another analog, Yb14ZnSb11, was initially reported as having very poor thermoelectric performance. As only 1/26 atoms are being exchanged in Yb14ZnSb11, the overall bonding and resultant density of states (DOS) within the structure should change very little. In this work, computed DOS and band structures for Yb14ZnSb11 and other analogs are presented. As expected, the band structure of Yb14ZnSb11 was shown to indeed have the light band at gamma and the highly degenerate (Nv = 8) pocket of bands between N and P which are responsible for the high thermoelectric performance of the Mn and Mg containing analogs. To investigate whether an improved synthetic route could bring the predictions of high zT into a reality, the route previously developed for Yb14MgSb11 and Yb14MnSb11 was adapted to make Yb14ZnSb11. Pure phase Yb14ZnSb11 was obtained through reactions with Yb4Sb3, YbH2, and ZnSb in a matter of hours via spark plasma sintering (SPS). This high purity Yb14ZnSb11 confirmed the predicted high thermoelectric performance (zTmax = 1.2 at 1175 K), placing Yb14ZnSb11 amongst the other high performing analogs of this structure type and suggests that other less explored analogs may have similar performance when made in high purity. The final three studies within this dissertation focus on the high temperature oxidation of Yb14MnSb11, Yb14MgSb11, and Yb14ZnSb11 along with chemical substitution of Lu for Yb within Yb14-xLuxMnSb11 and Yb14-xLuxZnSb11 (x = 0 – 0.9) to investigate the possibility of improving oxidation properties along with its effect on thermoelectric properties.