THERMOELECTRIC PROPERTIES AND MICROSTRUCTURE EVOLUTION OF Cu-Bi-Te and Ag-Bi-Te TERNARY EUTECTIC ALLOYS

Abstract

Waste heat is the regular problem in today's industrialized and urbanized society. The ability of thermoelectric (TE) materials to effectively transform waste heat into useful electrical energy and vice versa, offering a solid-state energy solution, has attracted a lot of attention. For waste heat conversion, set-ups in the automotive, military, aerospace, and industrial sectors have taken notice of their environmental friendliness and noiselessness. The efficiency of thermoelectric materials is still restricted; however, developments in thermoelectric materials and increased figure merit performance are the concepts that provide opportunities to improve power efficiency. This dissertation was focused on designing, synthesizing, and examining the microstructure and thermoelectric properties of Cu-Bi-Te and Ag-Bi-Te alloys by using flame melting processing. The Ag-Bi-Te and Cu-Bi-Te systems are made by flame melting, which is a simple and affordable way to create extremely homogeneous materials. The synthesized samples were analyzed, including crystal structure analysis, elemental status, chemical composition, surface morphology, Seebeck coefficient, electrical conductivity, thermal diffusion, and thermodynamic calculations. These analyses were conducted using X-ray diffraction (Rigaku Smart Lab), electron probe micro-analysis with electron dispersion spectroscopy (EPMA-EDS), scanning electron microscopy (SEM), ULVAC-Rico-ZEM-3, and Netzsch LFA 447. NanoFlash systems, Scheil cooling, and CALPHAD techniques. A microstructure of the solidified alloys revealed phase fluctuation with variations copper fraction in the Bi-Cu-Te alloys. EPMA-EDS analysis suggested the formation of Bi2Te3 and gammaCu3Te2 phases in all the alloys (A1 to A5). However, alloy A1 showed the highest ZT value of 0.33 at 530K due to its higher Seebeck coefficient. The hardness increased from ~1.0GPa to 1.21GPa until alloy A4, i.e., a fully eutectic alloy, and alloy A5, which consisted of primary ℽCu3Te2 dendrites and eutectics, showed decreased hardness from ~1.21GPa to 1.13GPa. Backscattered secondary electron (BSE) images of the microstructures of the Ag-Bi-Te alloys showed express lamellar and dendritic eutectic morphology in the alloys under investigation. The thermal conductivity was observed in alloy B2 at 0.9W/mK, with a corresponding ZT of 0.42. The mechanical properties of optimized composition alloys (B1 and B3) showed hardness values of

1.1GPa and 1.2GPa, respectively. In this work, self-consistent thermodynamic databases of Cu Bi-Te and Ag-Bi-Te alloys were built, and the effects of adding Ag and Cu to Bi2Te3 on the microstructure and thermoelectric properties of ternary eutectic Cu-Bi-Te and Ag-Bi-Te alloys were obtained.