Phase diagram determinations of the N-type Bi-Se-Te-Ga alloys

• Main Authors: Lin, Po Han, 林伯翰
• Other Authors: Chen, Sinn Wen
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/89733933061116520064

碩士 === 國立清華大學 === 化學工程學系 === 103 === Energy-related issues are the world-wide focus nowadays. Thermoelectric materials have attracted tremendous research interests because of their abilities to convert waste heat directly into electricity and thus increase the energy usage efficiencies. N-type-(Bi2Se3)x(Bi2Te3)1-x alloys have good thermoelectric properties, and recent studies indicate their properties can be further enhanced with Ga doping. Phase diagrams are basic materials information, and are fundamentally important for the processing routes selection and understanding of related phenomena. This study determines the phase diagrams of the Bi-Se- Te-Ga system since there are only limited information available. The research efforts of this study include: (1) determination the isothermal sections at 200oC, based on the available related literatures, of the four constituent ternary systems of the Bi-Se-Te-Ga system, Bi-Se-Ga、Bi-Te-Ga、Ga-Se-Te and Bi-Se-Te, (2) experimental determination the 200oC isothermal sections of Bi-Se-Te-Ga at composition at 40 at.% Bi, (3) experimental determination the liquidus projections of Bi-Se-Te and Bi-Se-Ga systems. The item (1) has been completed, and the Bi-Se-Ga、Bi-Te-Ga、Ga-Se-Te and Bi-Se-Te at 200oC isothermal sections have been determined. Two four-phase regions, Bi+Liquid+GaSe+GaTe and Bi+Bi2Te+GaSe+GaTe, are determined in the 200oC isothermal section of Bi-Se-Te-Ga quaternary system at the Ga-rich corner. Since most of the binary compounds at the Bi-rich side are not clearly determined, only the phase relationships at the Se-Te side are determined. Two primary solidification phase regions, Bi2(Se,Te)3 and (Se,Te), were confirmed. No ternary compound has been found. There is a saddle point at the boundary of Bi2(Se,Te)3/(Se,Te). In the part of Bi-Se-Ga liquidus projection, there are eight primary solidification phase regions: Bi, Ga, GaSe, α-Ga2Se3, β-Ga2Se3, Se, Bi2Se3 and unknown Bi-Se binary compounds. There are one class III reaction: Liquid+Bi+GaSe=Ga at 222oC and one class II reaction: Liquid+β-Ga2Se3=Bi+GaSe at 260oC.