화학적 접근을 통한 저온용 열전소재 Bi0.5Sb1.5Te3의 열전특성 향상

Abstract

Fossil fuels have been providing substantial benefits for humankind, and supporting the development of economy. However, they also lead to serious problems such as an air pollution, and even worse, their usage is not circulated but it could be exhausted to dead end. In order to overcome issues, renewable energies such as photovoltaic, geothermal, wind, and tidal power are being intensively researched and developed. Nevertheless, the pattern of our energy consumption is not expected to be rapidly changed according to the projections of primary energy use reported by U.S. Department of Energy. Therefore, it would be very crucial to enhance the energy efficiency of fossil fuels for managing the energy related problems. A large fraction of the energy losses from the industry are released as a form of waste heat. For example, approximately 40% energy of gasoline in a car is emitted as exhaust gas in high temperature. If the waste heat can be harvested into useful energy, the energy efficiency of fossil fuels will be significantly increased. This energy harvesting from waste heat is able to be facilitated by thermoelectric technology. Because of this potentials, since the year 2013, U.S. Department of Energy was funded from various countries to develop high performance automotive thermoelectric generators (TEGs) in association with other governmental institutes, universities, and major automakers. Thermoelectric phenomena could be explained with two effects, which are Seebeck effect for the energy conversion from heat to electricity, and Peltier effect vice versa. The conversion efficiency can be evaluated by dimensionless figure of merit ZT, which can be expressed as ZT=(σS2/κ)T, where σ is the electrical conductivity; S is the Seebeck coefficient; κ is the thermal conductivity; and T is the average temperature. Because the behaviors of parameters are inversely interrelated, it is not easy to develop ZT. Thermoelectric materials have their own temperature range to reveal the highest ZT value such as Bi-Te system in low temperature (~100°C), Pb-Te system in medium temperature (~500°C), and Si-Ge system in high temperature (~900°C). Thermoelectric devices comprise array of n-type and p-type materials. In this study, efforts have been made to improve thermoelectric properties of Bi0.5Sb1.5Te3 alloys, which are well-known for the best p-type materials in Bi-Te systems for low temperature operations, via chemical approaches. Although the research direction was modified, because some problems, which were arisen during realizing the initial ideas, they provided another chance to find new phenomena. Using such findings, the research achievements were obtained as following below. First, Bi0.5Sb1.5Te3 powders in nanoscale particle size were synthesized by the solid state reaction from Te nanowires. The size of Bi0.5Sb1.5Te3 particle was determined by the morphology of Te. ZT of synthesized materials was improved by reducing lattice thermal conductivity arisen from small grain size after sintering process. Second, Ag, which was coated on Bi0.5Sb1.5Te3 powders by chemical displacement reaction, was doped into Bi0.5Sb1.5Te3 during sintering process. The efficiency of doping was 4–8 times higher than those in the previous studies. Although the ZT improvement was not observed, it was found that the temperature of maximum ZT was varied from 50 to 250°C maintaining the peak ZT value. Such a finding was discussed with the position change of Fermi level. Third, in order to remove the surface oxidation on Bi0.5Sb1.5Te3 powders, hydrogen annealing was performed. Although the thermal conductivity was increased with increasing the annealing temperature, ZT was improved as 18.2% by the enhanced power factor as 27.0%. Such an enhancement was described with the effect of minority carriers decreased by oxygen reduction.