Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from
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SiGe alloys belong to the class of classic high temperature thermoelectric materials. By the means of nanostructuring, the performance of this well-known material can be further enhanced. Additional grain boundaries and point defects added to the alloy structure result in a strong decrease in thermal conductivity because of reduced lattice contribution to the overall thermal conductivity. Hence, the figure of merit can be increased. To obtain a nanostructured bulk material, a nanosized raw material is essential. In this work, a new approach toward nanostructured SiGe alloys is presented where alloyed nanoparticles are synthesized from a homogeneous mixture of the respective precursors in a microwave plasma reactor. As-prepared nanoparticles are compacted to a dense bulk material by a field assisted sintering technique. A figure of merit of zT 5 0.5 6 0.09 at 450 °C and a peak zT of 0.8 6 0.15 at 1000 °C could be achieved for a nanostructured, 0.8% phosphorus-doped Si80Ge20 alloy without any further optimization.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.117
compactifying the material under preservation of the nanofeatures, recently reviewed in Ref. 3. Successful optimization of the figure of merit into a technologically relevant range was demonstrated for ball milled and sintered nanocomposites of classical thermoelectric compositions3 and has recently been realized for a half-Heusler compound, Zr0.5Hf0.5CoSb0.8Sn0.2.4 Thus, the approach of employing nanostructuring to increase the thermoelectric efficiency allows for an engineering of materials, which combines a high figure of merit and sustainability. This work will focus on the synthesis of nanoparticles of the well-known alloy Si1-xGex in an up-scalable gas phase synthesis and the application of these engineered nanostructures for thermoelectricity. SiGe has a suitable power factor and also a moderate thermal conductivity. SiGe represents a classic high temperature thermoelectric material, which is found in applications such as space missions, where sunlight is not sufficient as an energy source. Since germanium is an expensive element, a broad mass application on earth is still lacking. However, we will approve our new technology based on this well-known material to gain suitable results and a reasonable database to judge whether our approach vindicates further investigation in gas phase based bottom-up technologies. Besides the disadvantage of high germanium costs, a silicon-based material system for thermoelectric mass application would have the big advantage of a clear margin in processing issues, as industry has already accumulated a broad knowledge on processing silicon-based devices. Furthermore, silicon is a cheap and almost unlimitedly available material. Additionally, in combination with germanium and also in form of silicides it is mostly nontoxic, which is
1872
Ó Materials Research Society 2011
Thermoelectrics is not a new topic in the field of materials research, but the increasing
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