Microstructure evolution and thermoelectric behaviour of directionally solidified Bi₂Te₃–Ga₂Te₃ eutectic alloy

Abstract Thermoelectric (TE) materials are known for efficiently converting thermal to electrical energy, and vice versa. Multiphase TEs offer better freedom to tune the TE properties of such materials by varying individual phase fractions, microstructural morphology, interface density controls, etc. These controlled changes in microstructural features can be achieved by directional solidification. Bi 2 Te 3 -Ga 2 Te 3 based TEs have been reported with enhanced TE performance. In the present study, alloys with a combination of these phases were developed using a eutectic composition from the pseudo-binary phase diagram of Bi 2 Te 3 –Ga 2 Te 3 . The alloys were fabricated using the vertical Bridgman method with three different solidification velocities ( V ), namely 1000 (E1), 200 (E2) and 10 (E3) μ m s −1 , at a constant temperature gradient of 35 K cm −1 at the solid/liquid interface. Microstructural features show a significant change in the morphology and the interlamellar spacing between two adjacent Ga 2 Te 3 lamellae in alloys E1, E2 and E3. In addition, x-ray diffraction patterns suggest changes in the lattice parameters of Bi 2 Te 3 and Ga 2 Te 3 for all alloys. Significant change in the macrotexture of the different alloys was observed along the growth direction. The highest power factor of 1.6 mW mK −2 was recorded for alloy E2 at room temperature. Alloy E2 shows the highest figure of merit among all the alloys investigated (0.38 at room temperature), attributed to the highest S value of 201.7 μ V K −1 at room temperature. In addition, the total thermal conductivity of all alloys was around 1.33 W mK −1 at 307 K, while alloy E1 has the smallest lattice and bipolar thermal conductivity.