Donald T. Morelli

Low temperature properties of the filled skutterudite CeFe4Sb12

CeFe4Sb12 is a member of a class of advanced thermoelectric materials. In order to evaluate this material’s potential for such applications, we have measured a variety of its properties at low temperature, including thermal conductivity, thermoelectric power, electrical resistivity, Hall coefficient, and magnetic susceptibility.

The effect of rare‐earth filling on the lattice thermal conductivity of skutterudites

Polycrystalline samples of Ir4LaGe3Sb9, Ir4NdGe3Sb9, and Ir4SmGe3Sb9 have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb3; thus, void filling shows promise as a method for improving the thermoelectric properties of these materials. We present the lattice thermal conductivity of these filled skutterudites in an effort to quantify the impact of void filling in this structure. It is believed that the atoms ‘‘rattle’’ in the voids of the structure and therefore interact with a broad spectrum of lattice phonons, reducing their mean free paths substantially below that in the ‘‘unfilled’’ skutterudites. An additional phonon scattering mechanism is caused by phonon‐stimulated transitions between the low‐lying energy levels of the 4f electron configurations in the case of Nd3+ and Sm3+. Magnetic susceptibility and Hall‐effect measurements are also presented.

When thermoelectrics reached the nanoscale

The theoretical work done by Lyndon Hicks and Mildred Dresselhaus 20 years ago on the effect of reduced dimensionality on thermoelectric efficiency has had deep implications beyond the initial expectations.

Magnetocaloric properties of doped lanthanum manganite films

LaMnO3 films doped with Ca, Ba, or Sr have been fabricated using the metalorganic decomposition technique. These films exhibit paramagnetic‐to‐ferromagnetic phase transitions at 250, 300, and 350 K, respectively. By measuring the film magnetization as a function of field and temperature we have determined the entropy change associated with these transitions. The large magnetization of these materials results in a total entropy change a factor of five less than that of gadolinium, the prototypical high‐temperature magnetocaloric material. Improvements in film morphology and composition may provide a further increase in the magnetization and total entropy change in these materials.

Thermal conductivity of synthetic diamond films

We have measured the thermal conductivity of two diamond films grown by a chemical vapor deposition process. At room temperature the thermal conductivity is of the order of 10 Wu2009cm−1u2009K−1, i.e., as high as naturally occurring (type Ia) single‐crystal diamonds. This value exceeds the thermal conductivity of copper at room temperature by a factor of 2. Since these films consist of microcrystallites of diamonds of dimensions on the order of 2 μm, boundary scattering of phonons is expected to be large. The expected effect of boundary scattering on the lattice thermal conductivity is calculated, and is in qualitative agreement with the observed results above about 30 K. However, important differences between the measured conductivity and that expected for boundary scattering are observed below this temperature. It is proposed that a small amount of disorder present in the lattice (identified through Raman studies) can account for this unusual behavior.

High thermoelectric figure of merit in the Cu3SbSe4-Cu3SbS4 solid solution

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

High thermoelectric figure of merit in the Cu 3 SbSe 4 -Cu 3 SbS 4 solid solution

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

Vacancy phonon scattering in thermoelectric In2Te3–InSb solid solutions

Solid solution formation is a common and effective approach to reduce the lattice thermal conductivity for thermoelectric materials because of additional phonon scattering by point defects due to mass and strain fluctuations. This scattering is maximal for vacancies. In this paper we present an example of the strong effect of phonon-vacancy scattering for the InSb–In2Te3 system. The measured transport properties show a reduction by more than an order of magnitude of the lattice thermal conductivity. Further efforts are also made on optimization for thermoelectric applications through doping. Our results indicate that these solid solutions are a prospective new-type thermoelectric material.

Phonon‐defect scattering in high thermal conductivity diamond films

We have investigated the thermal conductivity of large diamond samples grown by both hot filament and microwave plasma assisted chemical vapor deposition in order to study in detail the processes limiting heat conduction in this system. For samples containing nearly 100% diamond material and no apparent defects, the thermal conductivity is consistent with that expected for polycrystalline diamond with a given crystallite size. In films prepared by the hot filament technique, we observe an additional scattering of phonons near 60 K, which we attribute to either a resonant phonon‐defect interaction, or a crossover from geometrical to Rayleigh phonon‐defect scattering.

Lattice thermal conductivity of the Cu3SbSe4-Cu3SbS4 solid solution

The compositional dependence of the crystal structure and lattice thermal conductivity in the Cu3SbSe4-Cu3SbS4 system has been studied. The lattice parameters of the Cu3SbSe4-xSx compounds decrease linearly with x, and the tetragonal structure (space group 14−2m no. 121) of the end compounds is maintained at all compositions. The lattice thermal conductivity is much lower than that predicted by a simple rule of mixtures, which is typical for a solid solution. The Debye model produces a very reasonable fit to the experimental lattice thermal conductivity data when phonon scattering due to atomic mass and size differences between Se and S is taken into account. Compounds in this series are likely to improve upon the thermoelectric performance of Cu3SbSe4, which has shown ZTu2009=u20090.72 when optimized.