Inhibiting intermetallic compound growth by minor Ga addition in the Bi2Te3-based thermoelectric system

• Main Authors: Li, Mei-Hua, 李美樺
• Other Authors: Wang, Chao-Hong
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/f86c46

碩士 === 國立中正大學 === 化學工程研究所 === 105 === Thermoelectric materials(TE) play an important role in energy issue because they can generate electricity from waste heat. Among various kinds of thermoelectric materials, Bi2Te3-based materials are the most popular and have been widely used for commercial thermoelectric applications. Sb and Se are doped in Bi2Te3-based alloys for p- and n-type, respectively, i.e., Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3. Thermoelectric devices consist of many pairs of p- and n-type TE semiconductor elements, which were connected electrically with by soldering technology. It was known that the intermetallic compound (IMC) formation, i.e., SnTe phase, between Sn solder and Bi2Te3 material is very fast in packaging process, resulting in solder joint reliability is reduced. To avoid fast reactions during soldering, Ni is usually deposited on the surface of thermoelectric elements as a diffusion barrier layer. In this study, we investigated the solid/solid and liquid/solid reactions between lead-free solder and Bi2Te3-based thermoelectric materials. The SnTe growth rate of p-type TE couples were faster than the n-type TE.The growth kinetics were also investigated. For the solid-state reactions and liquid-state reactions, the IMC layer growth is linear for p-type TE reaction and the n-type TE case followed a parabolic law. Finally, the activation energy of SnTe was calculated to be 155.53 kJ / mol.Sn-3wt%Ag-0.5wt%Cu(SAC305) is one of the most promising lead-free solder alloys, investigated the solid/solid and liquid/solid reactions between SAC305 solder and p-type TE. the IMC layer growth is linear for p-type TE reaction and Ag element has the effect of suppressing SnTe growth, obviously. In addition, n-type TE growth is a parabolic law for the solid-state reactions and liquid-state reactions . In this study, we aimed to develop a new approach of adding minor element in solder to directly inhibit the IMC growth and investigated the relevant interface reactions. Liquid-state soldering reactions with Bi2Te3-type materials at 250oC were examined with addition of Ga ranging from 0.1wt.% to 1wt.%. For the p-type TE substrate, the growth of the SnTe layer with addition of 0.45wt%Ga was significantly suppressed, which was one-ninth thinner than the case with pure Sn without Ga addition. When the Ga content increased to 0.5wt%~1wt%, the IMC growth was greatly inhibited. The SnTe phase had no significant growth, ~1 μm, even after aging for a long time, but some thermoelectric materials dissolved into the solder. solid-state reactions with p-type materials at 180oC were examined with addition of Ga 0.5wt.% for 50 days. The main phase was transformed from SnTe phase to GaTe phase, ~4 μm, Thermoelectric materials were some dissolved into the solder. This study simulated the solder joints of the thermoelectric device, using the electroless deposition of Ni or Co as a diffusion barrier. In the solid / solid reaction, the reaction was inhibited significantly between Sn and p-type TE, but Ni and solder reaction to formed Ni3Sn4, itself also react with the thermoelectric material to formed NiTe.