Frederick Dynys

Uncertainty Analysis for Common Seebeck and Electrical Resistivity Measurement Systems

This work establishes the level of uncertainty for electrical measurements commonly made on thermoelectric samples. The analysis targets measurement systems based on the four probe method. Sources of uncertainty for both electrical resistivity and Seebeck coefficient were identified and evaluated. Included are reasonable estimates on the magnitude of each source, and cumulative propagation of error. Uncertainty for the Seebeck coefficient includes the cold-finger effect which has been quantified with thermal finite element analysis. The cold-finger effect, which is a result of parasitic heat transfer down the thermocouple probes, leads to an asymmetric over-estimation of the Seebeck coefficient. A silicon germanium thermoelectric sample has been characterized to provide an understanding of the total measurement uncertainty. The electrical resistivity was determined to contain uncertainty of ±7.0% across any measurement temperature. The Seebeck coefficient of the system is +1.0%/-13.1% at high temperature and ±1.0% near room temperature. The power factor has a combined uncertainty of +7.3%/-27.0% at high temperature and ±7.5% near room temperature. These ranges are calculated to be typical values for a general four probe Seebeck and resistivity measurement configuration.

CoxNi4−xSb12−ySny skutterudites: processing and thermoelectric properties

N-type and p-type skutterudite samples with the composition CoxNi4−xSb12−ySny were synthesized with composition range 0xa0<xa0xxa0<xa02 and 3xa0<xa0yxa0<xa05. Samples were pre-processed by solidification into ingots. Skutterudite phase formation was achieved by mechanical alloying the crushed ingots. The milled powders were consolidated to dense pellets by hot pressing. Thermoelectric measurements showed limited high-temperature performance below 400xa0°C. Skutterudite decomposition above 250xa0°C was detrimental to Seebeck coefficient. The thermoelectric transport properties can be tuned by varying the Co and Sn level. The lowest lattice thermal conductivity measured was 1.0xa0Wxa0m−1xa0K−1 for the Co level of 1.5. The Seebeck coefficient was positive for Co levels >0.8 and negative otherwise. Seebeck coefficients were low, ranging from −40 to 58xa0µVxa0K−1. The combination of transmission electron microscopy with electron energy loss spectroscopy and powder X-ray diffraction established that Sn can substitute on 2a and 24g sites in the skutterudite structure. Due to the low Seebeck coefficients, the alloys exhibited low figure of merits (ZT) <0.05.

Composite ceramics for power beaming

In the search for candidate materials for a robust mm-wave heat-exchanger, composite materials composed of a thermally conductive “host” material and a small inclusion of a very lossy material are particularly promising. However, there is little data available on the properties of materials in the W-band, a frequency range which is particularly useful for long-range power transmission. In this work, we measure the dielectric properties of composite materials composed of aluminum oxide with a lossy material inclusion and subject these materials to heating by a 100 Watt W-band mm-wave source. The results of these measurements are presented and discussed.