Advancing Thermoelectric Materials with a New Fabrication Method

March 17th, 2014

Classic Materials, New Techniques

Researchers are constantly searching for new thermoelectric materials, driven by the promise of economically harnessing wasted heat energy. One team at UC Santa Barbara, led by Professors Galen Stucky and Martin Moskovits, has found success with a unique approach. Instead of searching for new compounds, they have developed a new method to produce silicon germanium alloys, a well-understood class of thermoelectric materials. Silicon germanium (SiGe) thermoelectrics are currently used for NASA space missions and are the best candidates for high temperature applications.

Thermoelectrics: Converting Waste Heat into Electrical Energy

As deep space probes drift towards the edge of the solar system, the fading glow of the sun becomes too weak for the spacecraft to generate electrical energy with solar panels. To solve this problem, NASA engineers generate heat with a decaying radioactive material and convert the heat into electrical energy. This is achieved by placing a thermoelectric between the heat source and the cold emptiness of space. With their unique ability to convert heat into electricity, thermoelectric materials are increasingly sought after for terrestrial uses. The falling cost and rising performance of these materials are already beginning to enable the conversion of waste heat from automobile exhausts and industrial processes into electricity.

Lowering Synthesis Time and Temperature with Magnesiothermic Reduction

Hot pressing
Figure 1: 
The SiGe-based thermoelectric materials in this work were produced
by the magnesiothermic reduction of a silica-germania nanocomposite.
Hot pressing was used to fuse the resulting powders into a single pellet.

Producing silicon germanium (SiGe) alloys is difficult today because it requires pure germanium and silicon. Purifying these materials is energy intensive, requiring expensive equipment and temperatures of 2000ºC. Utilizing a process called magnesiothermic reduction, the researchers synthesized the same material from more abundant germanium dioxide and silicon dioxide, also known as quartz. The technique works at dramatically lower temperatures, closer to 650ºC, and takes significantly less time than traditional preparation. Even more importantly, a new doping strategy was developed to introduce small amounts of boron and increase the electrical conductivity of the materials. This approach offers excellent control and may have applications beyond thermoelectrics.  

zT: The Thermoelectric Figure of Merit

Most thermoelectric materials are semiconductors with high electrical conductivity and low thermal conductivity, two qualities that are difficult to achieve simultaneously. As the ratio of these properties increases, a material converts heat to electrical energy more efficiently. This efficiency is known as the thermoelectric figure of merit, or zT. By optimizing the magnesiothermic reduction process and the concentration of the dopant Boron, the UC Santa Barbara team produced SiGe thermoelectrics that have a zT of 0.5, comparable to materials used by NASA. The next steps, according to research team member Matt Snedaker, are to lower the thermal conductivity of the alloy, improve the efficiency of the magnesiothermic reduction, and apply the doping strategy to new dopants and materials.

with graph
Figure 2: 
The performance of thermoelectric materials is quantified by the thermoelectric figure of merit (zT).
Increasing the concentration of boron in the thermoelectric materials led to an optimal dopant concentration with a high zT.


Author: Brian Evanko, February 2014
Materials Department, UC Santa Barbara

To read the full article, click here.

Authors and publication details:
M. L. Snedaker, Y. Zhang, C. S. Birkel, H. Wang, T. Day, Y. Shi, X. Ji, S. Kraemer, C. E. Mills, A. Moosazadeh, M. Moskovits, G. Je, and G. D. Stucky, “Silicon-Based Thermoelectrics Made from a Boron-Doped Silicon Dioxide Nanocomposite.” Chemistry of Materials. 2013, 25, 4867−4873.

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