Microstructure And Ther Moelecttric Properties Of The Bi₂₃In₁₇Te₆₀Alloy Via Zone Melting Directional Solidification

Thermoelectric materials, which can directly generate electricity from waste heat such as automotive exhaust gas and industrial waste heat or provide energy benefits in refrigeration, could play an important role in energy and environment solutions. The use of thermoelectric devices is seriously limited by their low efficiencies because of the lower ZT of thermoelectric materials. Nanostructure introduced into bulk thermoelectric materials induces the quantum-confinement effects, which shows high potential in enhancing thermoelectric figure of merit. In this paper, eutectic Bi40-xInxTe60 alloys with typical anisotropy in property were prepared by zone melting directional solidification experiments. Preferential growth of crystals along optimal performance direction was investigated. Further nanostructure was introduced into the orientation-growth crystals through solid-state phase transformation to lower the lattice thermal conductivity.Bi23In17Te60 alloy was directionally solidified at growth rates of 3, 5 and 10 μm/s respectively. Uniform lamellar eutectic microstructure was derived at different growth rates. A relatively high growth rate induces a strong thermal-solute convention which results in the appearance of impure crystals. At 3 μm/s, a fully lamellar structure is derived which is well-aligned to the growth direction. The average lamellar space and the average thickness of In2Te3 phase at 10μm/s are 1.41 μm and 2.42μm, respectively. This reduces to 0.68 μm to and 1.40μm at 3μm/s, respectively.Annealing was carried out on Bi23In17Te60 alloy prepared by directional solidification at 673 K for 72, 168 and 240 hours, respectively. The precipitates of In2Te3 phase always showed lamellar morphology, and the orientation relationship between the precipitates and matrix is the same as that in eutectic microstructure. Large amounts of nanoscale In2Te3, precipitated from the Bi2Te3 matrix, were found after anneling of 168 hours. The average phase space and thickness were 52 nm and 31 nm, respectively. With increasing of the annealing time to 240 hours, coarsening of nanoscale In2Te3 was found. The average phase space and thickness slightly increased to 77 nm and 43 nm, respectively. The size of matrix of Bi2Te3 phases had little influence on the precitates of In2Te3 phase.The Bi23In17Te60 alloy shows n-type conduction because high doping concentration of In change the type and concentration of point defects. The electric properties of Bi23In17Te60 alloy were improved after zone melting directional solidification, which reach the highest power factor 1.1×10-4 wm-1k-2 at 3μm/s. This is 2.3 times higher than the as-cast Bi23In17Te60 alloy with equaxied crystals. However, the conduction type changes from p-type to n-type after the precipitation of nanoscale In2Te3 from the matrix of Bi2Te3 by annealing. This results in a substantial decrease in Seebeck coefficient and thus the thermoelectric properties. Therefore, a further appropriate doping is needed in the future to enhance carrier concentration and thus the electric properties. It is expected that a low thermal conductivity by the present directional solidification and nano-precipatation strategy and further considering the appropriate doping, a high thermoelectric property of Bi-In-Te can be reached.