John S. Beaty

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.

Thermoelectric properties of hot‐pressed fine particulate powder SiGe alloys

Several research groups have tried to reduce the thermal conductivity of thermoelectric materials in order to improve the thermoelectric’s figure‐of‐merit and conversion efficiency (Pisharody and Garvey 1978). Some of these efforts have successfully reduced thermal conductivity, but also have decreased the electrical properties of the thermoelectric. Hence there has been not net gain in figure‐of‐merit. During the past, year of novel material fabrication technique has been applied to the production of silicon germanium thermoelectric decreased the electrical properties of the thermoelectric. Hence there has been material. Ultra‐fine particulates (50 A to 100 A) have been hot pressed into boron doped, p‐type, 80/20 silicon germanium. The initial results have been promising. When compared to standard silicon germanium, a reduction in thermal conductivities of up to 40% and an increase in figure‐of‐merit of 10% to 15% has been achieved.

Reduced thermal conductivity due to scattering centers in p‐type SiGe alloys

Theory predicts that the addition of ultra‐fine, inert, phonon‐scattering centers to SiGe thermoelectric material will reduce its thermal conductivity (Klemens 1987). To investigate this prediction, ultra‐fine particulates (20A to 120A) of silicon nitride have been added to boron doped, p‐type, 80/20 SiGe. All previous SiGe samples produced from ultra‐fine SiGe powder without additions have lower thermal conductivities than standard SiGe, but high temperatures (1575°K) heat treatment increased their thermal conductivity back to the value for standard SiGe. However, the SiGe samples with inert silicon nitride, phonon‐scattering centers retained the lower thermal conductivity after multiple heat treatments at 1250°C. A reduction of approximately 25% in thermal conductivity has been achieved in these samples, which agrees well with theoretical predictions. Transmission Electron Microscopy has been used to confirm the presence of occluded particulates and X‐ray diffraction has been used to determine the compo...

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>>