Enhancement in Figure of Merit (ZT) by Annealing of BiTe Nanostructures Synthesized by Microwave-Assisted Flash Combustion HARJEET KAUR, 1,2 LALIT SHARMA, 1 SIMRJIT SINGH, 2 BATHULA SIVAIAH, 1 G.B. REDDY, 2 and T.D. SENGUTTUVAN 1,3,4 1.—CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India. 2.—Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India. 3.—e-mail: tdsen@mail.nplindia.ernet.in. 4.—e-mail: tdsen@nplindia.org Uniform polycrystalline bismuth telluride (BiTe) nanowires of diameter 100 nm to 150 nm and hexagonal nanoplates with thickness of 50 nm to 100 nm have been successfully synthesized by the microwave-assisted flash combustion technique. The formation of BiTe nanostructures depends on the type of fuel and the oxidant-to-fuel ratio, which in turn affect the reaction time and reaction temperature. Spark plasma sintering has been employed for compaction and sintering of both as-synthesized as well as annealed BiTe powders. Increasing the sintering temperature while using faster sintering cycles reduced the porosity, resulting in high densification while preserving the nanostructures. The dimensionless figure of merit (ZT) was evaluated from the Seebeck coefficient, electrical resistivity, and thermal conductivity values over the range from 300 K to 600 K. The effect of annealing on the enhancement of ZT is discussed. These evaluations suggest that the rarely studied BiTe is a potential candidate for thermoelectric applications at low temperatures. Key words: Spark plasma sintering, BiTe, thermoelectric INTRODUCTION Waste-heat recovery by power generation and refrigeration by solid-state cooling are based on the thermoelectric effects, having the potential to reduce dependence on fossil fuels and thereby suppress greenhouse gas emissions. Thermoelectric device efficiency depends on the dimensionless figure of merit (ZT) of the material, given by ZT ¼ a 2 T qj ; where a, q, j, and T are the Seebeck coefficient, elec- trical resistivity, thermal conductivity, and absolute temperature, respectively. 1–3 Nanostructuring of thermoelectric materials has pro- ven to be effective for improving ZT, owing to both a high density of states giving an enhanced power factor and increased phonon scattering causing a reduced thermal conductivity. 4–7 Among the high-ZT materials, PbTe-based mate- rials are the most efficient for power generation applications at high temperature. 8 However, for relatively lower-temperature applications, Bi 2 Te 3 - based materials still dominate in terms of effi- ciency. 5 Other phases of the Bi-Te system such as BiTe and Bi 4 Te 3 , although less investigated, could be treated as candidate thermoelectric materi- als. 9–12 As proposed by Stasova, 10 BiTe comprises a Bi-Bi sequence sandwiched between two Te-Bi-Te- Bi-Te sequences. It has a layered structure having hexagonal symmetry with lattice constants a = 4.426 A ˚ and c = 24.069 A ˚ . It is composed of 12 atomic layers in the sequence (Received July 1, 2013; accepted October 11, 2013) Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-013-2864-9 Ó 2013 TMS