The Design, Fabrication And Characterization Of Electrode For Mg-Si-Sn Unicouple

Mg-Si-Sn is one of the important thermoelectric materials used in medium temperature, and it shows a borad application prospect in the the field of industrial and automobile waste heat recovery. As little research has been doing on Mg-Si-Sn thermoelectric generators, this paper focuses on Mg2.16(Si0.3Sn0.7)0.98Sb0.02 TE leg for power generation in medium temperature. We have designed a multi-layer electrode, which matches well with the Mg-Si-Sn thermoelectric material. The process of connection between the multi-layer electrode and the Mg-Si-Sn material has been optimized. We have also characterized the joint between the multi-layer electrode and Mg-Si-Sn thermoelectric material, including the interface microstructure, chemical composition, contact resistance, shearing strength and high temperature stability. Further more, we have assembled a n-type Mg-Si-Sn/p-type Cu2 Se unicouple. The unicouple has been evaluated in terms of TE power convension efficiency and output power. Some research have been carried out on the high temperature stability of the solder layer. The following conclusions have been obtained in this research:The multi-layer electrode, formed by the first Ni-Al alloy layer, the second Ni-Al alloy layer and the Ag layer, meets the Mg-Si-Sn unicouple design requirements. The thermal expansion coefficient of the first Ni-Al alloy layer and the Mg2.16(Si0.3Sn0.7)0.98Sb0.02 thermoelectric material match each other well in most temperature range of interest. The Ag layer shows good solderability with Pb-Sn alloy. The joint between the multi-layer electrode and the thermoelectric materials shows no obvious cracks or mutual element diffusions. The contact resistance of the joint is between 0.07 m? and 0.13 m?, taking accout 3.5 ~ 8.3% of the thermoelectric material internal resistance, The shearing strength between the multi-layer electrode and the thermoelectric material is higher than the thermoelectric material itself, as the failure always occurs in the thermoelectric materials part during the shearing strength test.The thermal stability of the joint between the multi-layer electrode and the thermoelectric materials was invesitigated with different annealing temperatures for a week. Results show that when the annealing temperature is above 673 K, the first Ni-Al alloy layer will evolve from a multiphase structure of Al, NiAl3, Ni2Al3 and Ni into a stable two-phase structure of Al and NiAl3. With the annealing temperature increased, much more micro-holes were formed in the surface of the joint, and the diffusion of Mg into the multi-layer electrode side would be more obvious. The contact resistance increased from 0.10 m? for sample annealed at 573 K to 0.21 m? while annealed at 773 K.The thermal stability of the joint between the multi-layer electrode and the thermoelectric materials was invesitigated with different annealing time at 773 K. Results show that when the annealing time increased to 27 h, the phase composition of the first Ni-Al alloy layer become stable. With the annealing time increased, a large number of holes were observed within the first Ni-Al alloy layer and near the joint, and some holes distribute on the interface. On the other hand, the content of Mg in the first Ni-Al alloy layer increased gradually with increased annealing temperature. Contact resistance increases from 0.12 to 0.16 m?, with its growth rate up to 33%.An n-type Mg-Si-Sn/p-type Cu2 Se unicouple has been successfully assembled by using the Pb-Sn soldering at low temperature. The good electrical connection of n-type Mg-Si-Sn/p-type Cu2 Se unicouple can be verified by a four point probe method, which is an efficient way to screen the failure samples. The thermoelectric conversion performance of the n-type Mg-Si-Sn/p-type Cu2 Se unicouple shows a peak efficiency of 6.33% with ?T at 410 K, and the maximum output power reach 264.39 mW with ?T at 477 K.The thermal stability of the solder layer between Mg-Si-Sn leg and Cu stripes was invesitigated with different treatment time at 773 K. Resluts show that, the Pb-Sn alloy in solder layer would evaporate after annealing for 1 day. The Ag layer and the Cu layer would be closely connected, and Sn will diffuse into both sides. The thermal stability of the solder layer between p-type Cu2 Se leg and Cu stripes was also invesitigated. Resluts show that, as annealing time increase, Ni and Cu would diffuse into the soldering layer, forming Ni-Pb-Sn-Cu alloy, which is stable at 773 K. A large number of holes in the Ni-Pb-Sn-Cu alloy soliding layer can be seen, leading to the departion of p-type Cu2 Se leg from the Cu stripes.