Phase diagrams of thermoelectric Bi-In-Se-Te quaternary system

• Main Authors: Lin, Yi-Cheng, 林奕丞
• Other Authors: Chen, Sinn-Wen
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/8mwq86

碩士 === 國立清華大學 === 化學工程學系所 === 106 === Bi2(Se,Te)3-based alloys are the most common commercial thermoelectric materials. Literatures results have indicated that their thermoelectric properties could be enhanced with indium doping. Phase diagrams provide phase equilibria information and are fundamentally important for the understanding of phase transformation and microstructural evolution. This study investigates the phase diagrams of Bi-In-Se-Te quaternary system to provide necessary data for further development of Bi2(Se,Te)3-based thermoelectric materials. There are four constituent ternary systems, Bi-In-Se, Bi-In-Te, Bi-Se-Te and In-Se-Te in the Bi-In-Se-Te quaternary system. The phase diagrams of Bi-In-Se and Bi-In-Te systems are available in the literatures. This study experimentally determines the phase diagrams of In-Se-Te and Bi-Se-Te systems which are lacking in the literatures. The Bi-In-Se-Te quaternary phase diagram is then constructed based on the phase diagrams of their four constituent ternary systems. The temperature examined in this study is 250oC which is in the application temperature range of Bi2(Se,Te)3-based thermoelectrics. Ternary In-Se-Te and Bi-Se-Te alloys were prepared with pure bismuth, indium, selenium and tellurium elements. The phases of the equilibrated samples were analyzed by SEM (Scanning Electron Microscopy), EPMA (Electron Probe Microanalysis) and XRD (X-ray Diffraction). The experimental results were used together with the phase diagrams of the constituent binary systems to construct the phase diagrams of ternary systems. In the ternary In-Se-Te ternary system at 250oC, there are terminal phases, liquid (In) phase, liquid (Se) phase, and (Se,Te) solid phase, binary phases, In4Se3, In6Se7, In2Se3, InSe, In4Te3, InTe, In3Te4, In2Te3 and In2Te5 phases, and two ternary phases, In4Se7Te9 and In4SeTe2 phases. The phase boundaries and compositions of these two ternary compounds need further examinations. There are 15 three-phase regions in the In-Se-Te 250oC isothermal section, and they are In2Se3+(Se,Te)+Liquid(Se), Te+In2Se3+In6Se7, Te+InSe+In6Se7, Te+InSe+ In7Se4Te9, Te+In2Te5+In7Se4Te9, In2Te3+In2Te5+In7Se4Te9, In2Te3+In2Te5+ In7Se4Te9, In3Te4+In2Te3+ In7Se4Te9, InTe+ n3Te4+ In7Se4Te9, InTe+InSe+ In7Se4Te9, InTe+InSe+In4Te3, In4SeTe2+InSe+In4Te3, In4SeTe2+In+In4Te3, In4SeTe2+In+In4Se3 and In4SeTe2+ InSe+ In4Se3. There is no ternary compound in the Bi-Se-Te ternary system at 250oC. The complicated binary phases in the Bi-Se and Bi-Te binary systems are grouped as Bi2Se3 and (Bi2)n(Bi2Se3)m and Bi2Te3 and (Bi2)n(Bi2Te3)m phases. The (Bi2)n(Bi2Se3)m and (Bi2)n(Bi2Te3)m form a continuous solid solution phase, (Bi2)n(Bi2(Se,Te)3)m. Both Bi2Te3 and Bi2Te3 phases have significant ternary solubilities, but they do not form a continuous solid solution. In the 250oC Bi-Se-Te isothermal section, there are 3 ternary-phase region, and are (Bi2)n(Bi2(Se,Te)3)m+ Bi2Te3+ Bi2Se3, Bi2Te3+ Bi2Se3+ (Se,Te) and Bi2Se3+ (Se,Te)+ Liquid.