Jan Vandersande

Thermal conductivity reduction in SiGe alloys by the addition of nanophase particles

Abstract Transport models have predicted that the thermal conductivity of SiGe alloys could be appreciably reduced by incorporating discrete 40 A particles within the SiGe grains. These particles would scatter the thermal phonons which transport most of the heat in these alloys. Such a thermal conductivity reduction would lead to substantial improvements in the figure-of-merit and efficiency of thermoelectric materials used in power generation applications. This paper reports on the results of adding 40 A particles to SiGe by using a spark erosion process. Thermal conductivity reductions consistent with the transport models have been achieved; however, the improvement in figure-of-merit has not been as large as predicted.

Thermoelectric properties of hot pressed p‐type SiGe alloys

This paper presents the results of meausrements of electrical resistivity (ρ), Seebeck coefficient (S), thermal conductivity (Λ); as well as Hall carrier concentration (n), and mobility (μ), for hot pressed SiGe 80 a/o Si‐20 a/o Ge (SiGe) thermoelectric materials containing 0.24–3.0 a/o boron.The carrier concentration was varied by annealing and quenching at different high temperatures. Figure‐of‐merit, Z, was found to be 0.60±0.03×10−3 K−1 over a carrier concentration range from 1.8–3.5×10−20 cm−3. This result is very encouraging from a production standpoint, since the dopant concentration is not critical.

Status of p‐type SiGe alloys with nanophase inclusions

Transport models have predicted that the thermal conductivity of SiGe alloys could be appreciably reduced by incorporating discrete 40 A particles within the SiGe grains. These particles would scatter the thermal phonons which transport most of the heat in these alloys. Such a thermal conductivity reduction would lead to substantial improvements in the figure‐of‐merit and efficiency of thermoelectric materials used in power generation applications. This paper reports on the results of adding 40 A particles to SiGe by using a spark erosion process. Thermal conductivity reductions consistent with the transport models have been achieved, however, the improvement in figure‐of‐merit has not been as large as predicted.

Progress in the optimization of n‐type and p‐type SiGe thermoelectric materials

A comprehensive experimental and theoretical work has been conducted in order to optimize the thermoelectric properties of Si80Ge20 materials and reach the goal of a combined figure of merit value of 0.85×10−3 K−1 averaged over 600–1000 C temperature range. Improvement for the n‐type material have been obtained by determining the optimum amounts of gallium and phosphorus dopants necessary to achieve optimum carrier mobility and concentration. The emphasis is now on the good reproducibility of these results through understanding and control of the processing parameters relating microstructure and composition to the transport properties. The optimum doping level has now been firmly established for p‐type materials, and work is concentrating on the reduction in thermal conductivity. BN ultra fine particles have been successfully incorporated into fully dense samples and have resulted in desired improvement of the figure of merit. Efforts are being made to reproduce these encouraging results.

Effect of Ga and P dopants on the thermoelectric properties of n-type SiGe

The purpose of this study was to hot-press improved n-type Si/sub 80/Ge/sub 20//GaP samples directly (without any heat treatment) and to confirm that a Ga/P ratio less than one increases the solubility of P and, hence, improves the power factor and Z. One of the three sample (Ga/P=0.43) had an improvement in Z of about 20% between 400 and 1000 degrees C over that for standard SiGe. This demonstrates that improved samples can be pressed directly and that a Ga/P ratio less than one is necessary. The other two samples (Ga/P=0.33 and 0.50) and Zs equal to or less than that of standard SiGe but had a lower hot-pressing temperature than the improved sample.<<ETX>>

Status of Thermal Conductivity Reduction in SiGe Using Ultrafine Particulates

A comprehensive experimental and theoretical work has been conducted in order to optimize the thermoelectric properties of Si80Ge20 materials and reach the goal of a combined figure‐of‐merit value of 0.85×10−3u2009K−1 averaged over a 600–1000 C temperature range. The optimum doping level has now been firmly established for p‐type materials, and work is concentrating on the reduction in thermal conductivity. BN nanophase particles have been successfully incorporated into fully dense samples and have resulted in desired improvement of the figure‐of‐merit. Recent work has focussed on improving process control via the use of blended electrodes in order to reproducibly achieve high figure‐of‐merit.

Status of the improved N‐type SiGe/GaP thermoelectric material

A comprehensive experimental and theoretical work has been conducted in order to optimize the thermoelectric properties of n‐type SiGe materials. A key factor to the improvement of the figure of merit Z of these materials has been the successful ability to substantially increase the electron concentrations by up to a factor of two at room temperature over current standard P‐doped n‐type SiGe materials using a Ga and P as dopants. Particular emphasis has been placed upon the understanding of the changes in P solid solubilities when multi‐doping with both Ga and P, and upon the relationships between electrical and thermal properties with microstructure and composition of heavily doped hot‐pressed SiGe materials. Application of the results obtained from a set of various experimental techniques coupled with thermodynamic theoretical considerations resulted in reproducible improvements for several SiGe/GaP samples with average Z values close to 1×10−3 K−1 over a 600–1000°C temperature range.

Novel fabrication technique for improving the figure-of-merit of thermoelectric materials

Reduction of the thermal conductivity of thermoelectric materials in order to improve the figure of merit and, hence, the conversion efficiency is discussed. A novel fabrication technique that reduces the thermal conductivity without too adverse an effect on the electrical properties is reported. This is achieved by producing an oxygen-free, very-fine-grain SiGe alloy with very small (on the order of 50 AA) precipitates.<<ETX>>

Thermal conductivity reductions in SiGe using nanophase particles

Transport models have predicted that the thermal conductivity of SiGe alloys could be appreciably reduced by incorporating discrete 40A particles within the SiGe grains. These particles would scatter the thermal phonons which transport most of the heat in these alloys. Such a thermal conductivity reduction would lead to substantial improvements in the figure‐of‐merit and efficiency of thermoelectric materials used in power generation applications. This paper reports on the results of adding 40A particles to SiGe by using a spark erosion process. Thermal conductivity reductions consistent with the transport models have been achieved, however, the improvement in figure‐of‐merit has not been as large as predicted.

Nanophase metals and semiconductors

A new procedure for fabricating nanophase powder was developed at ThermoTrex Corporation for application in thermoelectric materials. This process uses a high‐voltage discharge to produce powders with a particle size of approximately 4 nm. In thermoelectric materials these ultra‐fine powders are dispersed in a SiGe alloy matrix to effect a reduction in thermal conductivity. Since these particles are too fine to scatter electrons, the electrical properties are not affected and the figure‐of‐merit is improved. These powders can find application in a variety of other systems; some of these applications will be discussed below with an emphasis on metal and semiconductor powders.