UCLA

There are several possible approaches to enhancing the thermoelectric properties of skutterudite materials, most of which strive to reduce the total thermal conductivity of the structure in some way. Formulations of CoSb₃ skutterudite that incorporate alkali metals, alkaline-earth elements, or rare-earth elements as void fillers have achieved the highest reported thermoelectric figure of merit (ZT) values for n-type CoSb₃. For thermoelectric applications, devices that use IrSb₃-based materials as full legs or segments of legs could enable the utilization of a greater maximum temperature, as compared to CoSb₃-based devices. A series of n-type Ba_yIr₄Sb₁₂ compositions have been synthesized. Phase purity and elemental compositions are analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Room temperature and high temperature thermoelectric properties are reported with a maximum ZT of about 0.8 at 750 ^oC (1023 K) and EPMA results suggest the maximum filling fraction for barium in Ba_yIr₄Sb₁₂ is around y = 0.3.

In light of the success of single-element filling of iridium-based skutterudite, double-element filling of the structure was investigated. The high temperature thermoelectric properties for the double-filled Ba_xYb_yIr₄Sb₁₂ and Ba_xEu_zIr₄Sb₁₂ skutterudites were compared to Ba_xIr₄Sb₁₂ single-filled skutterudites. Maximum ZT values for double-filled skutterudites increased as much as 25% over that of single-filled compositions. Phase purity and elemental compositions were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Room temperature and high temperature thermoelectric properties are reported with a maximum ZT of 1.0 at 750 ^oC (1023 K) for the Ba_0.15Eu_0.15Ir₄Sb₁₂ nominal composition, but EPMA results show that these skutterudites exhibit preferential filling for certain elements in the case of double-filling, such that Ba > Eu > Yb.

Skutterudite materials were processed by high energy ball milling to produce nanostructured powders. CoSb_{3 was used as a proof-of-principle material along with Co0.955Pd0.045Sb2.955Te0.045, Ba0.05Yb0.15Co4Sb12, and Ce0.9Fe3.5Co0.5Sb12. Each skutterudite was milled to a fine powder containing crystallites as small as 1 nm, with sample averages as low as ~20 nm. The powders were characterized by XRD, SEM, and TEM, then hot-pressed and measured for their high temperature thermoelectric properties. Hot-pressing conditions necessary for densification led to significant grain growth. Improvements in reduced thermal conductivity in each of the skutterudites examined were off-set by changes in the electrical properties of the samples. All observed increases in ZT over bulk materials were within the error of the measurement. Nanostructured skutterudite materials often behaved inconsistently and a majority of ball milled samples undergo reactions or phase transformations during high temperature measurements. Improving the ZT of skutterudite materials by ball milling to create nanostructures remains a challenge.}

In addition to further studies of new filler atoms in IrSb₃ skutterudites, composites and alloys offer promise of reduced thermal conductivities for enhancement of ZT. Preliminary results of alloying small amounts of cobalt in iridium-based skutterudite indicate that with optimization, these alloys have the potential to reduce thermal conductivity by the introduction of point defects in the structure. With more accurate band modeling, it could be possible to incorporate multiple approaches to reducing thermal conductivity into one sample, taking advantages of several different modes of phonon scattering.