Reliability Of Thermoelectric Device Research Articles

Effect of Temperature and Environmental Aging on the Mechanical Reliability of n- and p-Type Thermoelectric Couples

Bismuth telluride-based alloys are the most widely used thermoelectric materials, which can obtain the conversion between heat energy and electricity. As a kind of lead-free solder alloy, Sn96.5Ag3.0Cu0.5 (SAC305) shows great potential in soldering between the thermoelectric element and electrode in thermoelectric couples. Cu/ SAC305 /Ni-plated Bi0.5Sb1.5Te3 (n-type) and Cu/ SAC305/Ni-plated Bi1.8Sb0.2Se0.15Te2.85 (p-type) couples were connected at 250°C by reflow soldering to investigate effects of temperature and thermall aging on the efficiency and reliability of thermoelectric device. Effects of temperature and environmental aging on shear force of thermoelectric couples were studied respectively in the temperature range of 20°C to 180°C. At room temperature of 20°C, n-type thermoelectric couples showed higher shear force than p-type thermoelectric couples. N-type thermoelectric couple showed higher shear reliability than p-type thermoelectric couples after synchronized environmental aging. The shear force of both types of thermoelectric couples decreased with the growth of thermal aging cycles. Finally, the fracture surfaces of two types of thermoelectric couples were investigated respectively using SEM to further analyse the failure mechanism.

Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hot-press. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb_3Te_4 mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb_3Te_4 grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb_3Te_4 layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb_3Te_4 and within the PbTe matrix, indicating that the bonding between Nb and Nb_3Te_4 is weak.

Interfacial reaction between n- and p-type thermoelectric materials and SAC305 solders

Abstract Sn96.5Ag3.0Cu0.5 (SAC305) is one of the most promising lead-free solder alloys and bismuth telluride-based compounds are the best known thermoelectric materials. Interfacial reactions of the SAC305/Bi0.5Sb1.5Te3 and SAC305/Bi1.8Sb0.2Se0.15Te2.85 couples at 260 °C are studied to investigate the effect of reaction on the efficiency and reliability of thermoelectric device. Initially, the major products in SAC305/Bi0.5Sb1.5Te3 couple reaction for a short time are a strip-type Ag-rich IMC layer and a dense SnTe–SbSn mixture layer. With the reaction time increasing, a two-phase layer, which is SnTe–SbSn mixture with pores with 200–500 nm size which are occupied by solder originally, appears next to the dense layer. However, the reaction products do not change significantly. It is observed that the growth rate of reaction phase layer is about 4.05 μm/min. As for the SAC305/Bi1.8Sb0.2Se0.15Te2.85 couple reaction, besides SnTe phase, Bi–Te phase is also formed at the solder/Bi1.8Sb0.2Se0.15Te2.85 interface. As the reaction time increases, the morphology of IMC layer becomes rough, and then the stable scallop-type IMC appears; the growth rate of reaction phase layer is about 0.39 μm/min.

High temperature reliability evaluation of CoSb 3/electrode thermoelectric joints

The CoSb 3/electrode thermoelectric (TE) joints were fabricated with the insertion of Ti foil by spark plasma sintering (SPS). The interfacial microstructure evolution and reliability of joints were investigated during accelerated thermal aging. After thermal aging, a multi-layer structure, which was composed of intermetallic compounds (IMCs), was observed at the CoSb 3/Ti interface. The growth kinetics of IMC layers was studied. The theoretic life of CoSb 3/electrode joints was predicted using the growth rate of IMC layers. The shear strength of aged CoSb 3/electrode joints was also tested and the results showed the joints had sufficient strength for keeping the reliability of TE device. Scanning electron microscopy (SEM) showed that, in the aged sample, all fracture occurred inside of the IMC layers, while the fracture occurred in the Ti layer or Ti/TiSb interface in the un-aged sample. The results of four-probe tests showed that the electrical contact of CoSb 3/electrode joint was good.