Synthesis and Characterization of Paintable Bi2Te3-based Thermoelectric Materials

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Synthesis and Characterization of Paintable Bi2Te3-based Thermoelectric Materials
Park, Sung Hoon
Son, Jae Sung
Issue Date
Graduate School of UNIST
The thermoelectric (TE) effect has attracted considerable attention from a number of different research areas, as its ability to directly convert between thermal and electrical energy offers a unique solution for sustainable power generation from waste heat sources. The power generation performance of solid-state TE devices largely depends on the characteristics of the TE materials from which they are made, such as the energy conversion efficiency. This efficiency is estimated from a dimensionless figure-of-merit: ZT = (S2σT/κ), where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. The shape and dimensions of TE materials are also crucial to efficient energy conversion in system-level TE modules with minimum heat loss. Typically, TE legs chipped into planar-structured TE devices are fabricated by means of a top-down dicing process to produce cube or cuboid-shaped TE blocks, in which TE ingots are synthesized through energy intensive processes such as zone-melting or hot-pressing. Although these conventional technologies can produce bulk-scale TE legs with well-established TE properties and moderately high ZT values, a key constraint lies in the difficulty in engineering the shapes and dimensions of the TE legs. This restricts the flexibility in designing TE devices for efficient thermal energy transfer from heat sources with various shapes. A few attempts have been made to design and fabricate ring-shaped TE legs chipped into tubular TE devices for energy harvesting from exhaust pipes, but achieving suitable performance and process simplicity remains a challenge. This dissertation describes the synthesis and characterization of Bi2Te3-based TE paints. In particular, the bulk-level TE properties of the painted materials are reported. In addition, the power generating performance of TE devices fabricated on curved heat sources via the painting process is discussed. First, the background of TE research area is briefly described. The basic principles of TE phenomenon such as the Seebeck effect, the Peltier effect, and the Thompson effect are described. Furthermore, the structural and TE characteristics of Bi2Te3-based TE materials, arguably the best TE materials at near room temperature are discussed. Finally, the measurement methods of TE devices and their types are described. Second, it has been described that the Bi2Te3-based TE paints aided by Sb2Te3-based molecular chalcogenidometalate (ChaM) are synthesized and their TE properties are characterized. ChaM ions are known for soluble precursor and widely utilized these molecules as inorganic ligands and solders for nano- and meso-scale semiconductor particles, and so I simply expand this concept to TE paints. Molecular Sb2Te3 based ChaM is used as a solder or a sintering aid for n-type BiTeSe and p-type BiSbTe TE particles. The Sb2Te3-ChaM easily fills the voids and interfaces between these TE particles, forming interconnecting crystalline phases without the need for external pressure. The soldering effect substantially influence TE properties of the painted materials, of which ZT values increase up to 1.21 and 0.69 for p-and n-type materials. Furthermore, the fabricated in-plane TE power generators via the painting process exhibits remarkably high output power density of 4.0 mW/cm2 under the temperature difference of 50 oC. In particular, the thourgh-plane TE power generator chipped with the molded TE blocks shows ~30 mW/cm2 under the temperature difference of 50 oC, competing the commercial planar-structured TE module. This painting approach therefore provides a simple and cost-effective way to design and fabricate TE devices directly onto any shaped heat source using a brush, thereby eliminating the need for additional equipment. What makes this painting process suitable for preparing TE devices is the fact that they are less sensitive to the resolution of the mm-scale TE legs than other electronic devices.
Department of Materials Science Engineering
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