CuGaTe(2) with a chalcopyrite structure demonstrates promising thermoelectric properties. The maximum figure of merit ZT is 1.4 at 950 K. CuGaTe(2) and related chalcopyrites are a new class of high-efficiency bulk thermoelectric material for high-temperature applications.

Radioisotope thermoelectric generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a thermoelectric converter. RTGs have been successfully used to power a number of space missions and have demonstrated their reliability over an extended period of time (tens of years) and are compact, rugged, radiation resistant, scalable, and produce no noise, vibration or torque during operation. System conversion efficiency for state-of-practice RTGs is about 6% and specific power £5.1 W/kg. A higher specific power would result in more onboard power for the same RTG mass, or less RTG mass for the same onboard power. The Jet Propulsion Laboratory has been leading, under the advanced thermoelectric converter (ATEC) project, the development of new high-temperature thermoelectric materials and components for integration into advanced, more efficient RTGs. Thermoelectric materials investigated to date include skutterudites, the Yb 14 MnSb 11 compound, and SiGe alloys. The development of long-lived thermoelectric couples based on some of these materials has been initiated and is assisted by a thermomechanical stress analysis to ensure that all stresses under both fabrication and operation conditions will be within yield limits for those materials. Several physical parameters are needed as input to this analysis. Among those parameters, the coefficient of thermal expansion (CTE) is critically important. Thermal expansion coefficient measurements of several thermoelectric materials under consideration for ATEC are described in this paper. The stress response at the interfaces in material stacks subjected to changes in temperature is discussed, drawing on work from the literature and project-specific tools developed here. The degree of CTE mismatch and the associated effect on the formation of stress is highlighted.

The phonon density of states ͑DOS͒ of La 3−x Te 4 compounds ͑x = 0.0, 0.18, 0.32͒ was measured at 300, 520, and 780 K, using inelastic neutron scattering. A significant stiffening of the phonon DOS and a large broadening of features were observed upon introduction of vacancies on La sites ͑increasing x͒. Heat-capacity measurements were performed at temperatures 1.85Յ T Յ 1200 K and were analyzed to quantify the contributions of phonons and electrons. The Debye temperature and the electronic coefficient of heat capacity determined from these measurements are consistent with the neutron-scattering results, and with previously reported first-principles calculations. Our results indicate that La vacancies in La 3−x Te 4 strongly scatter phonons and this source of scattering appears to be independent of temperature. The stiffening of the phonon DOS induced by the introduction of vacancies is explained in terms of the electronic structure and the change in bonding character. The temperature dependence of the phonon DOS is captured satisfactorily by the quasiharmonic approximation.

Thin films of oxides, phosphates, fluorides and other analogous materials on lithium-ion cathode particles are well known to improve cathode performance in terms of cycle life and rate performance. Explanations for this phenomenon abound, but the underlying mechanisms that dictate the nature of these effects are still in question, which motivates the work herein. We have carried out systematic PITT, EIS, Tafel, and cycling experiments as a function of temperature for Al 2 O 3 -coated and uncoated layered solid solution Li 2 MnO 3 ÀLiMO 2 (M ¼ Mn, Co, Ni) cathode materials and shown that we can reproduce the well-documented improvement in performance with surface coatings. In particular the effects are most pronounced at reduced temperatures and after temperature cycling (23 to 0 C to 30 to 0 C). Interestingly, we find the activation energies for the diffusion coefficients estimated from PITT data are nearly identical to the activation energy for exchange current measured from Tafel polarization data. This finding may provide some insight into the relative control of the mass transfer and the charge transfer processes on the overall cathode reaction. Alternately, it may be the due to inadequate correction for the mass transfer effects in the Tafel and PITT analyses.

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