Researchers have developed a new low-cost electrical contact material for thermoelectric devices that is stable at high temperatures. The thermoelectric material can generate electricity by using a temperature difference between its two faces. The thermoelectric device can also function as a small heat pump, moving heat from one side of the device to the other.
Thermoelectric materials convert thermal energy directly into electricity through a process that involves a process of diffusion of electrons and phonons in the solid state. Although the principle has been known for two centuries, it had limited utility because the energy conversion efficiency of most known thermoelectric materials is very low. Nanotechnology has brought innovations to improve the efficiency of materials, but the mass market application of these innovations has remained limited due to the low conversion efficiency of devices of 6-10%. This makes the electricity produced more expensive than other technologies.
Researchers at the International Advanced Research Center for Powder Metallurgy & New Materials (ARCI), an independent institute of the Department of Science and Technology, designed and developed thermoelectric modules using lead telluride (PbTe) and stannite silicide from magnesium (Mg2Si1 timeSnX), which gives a conversion efficiency greater than 10%. This work was published in the journal‘Materials Research Bulletin‘ recently.
The thermoelectric device, which requires a metal electrode electrically connected to semiconductor thermoelectric materials, requires two essential functional requirements. The seal must be stable at a high operating temperature (300-600 ° C) and must not undergo any thermochemical degradation. It must have the least contact resistance so that the current flow and the delivered power reach a maximum.
The ARCI team explored various metallization methods and diffusion barrier materials with a systems approach to make stable bismuth doped Mg2(Yes1 timeSnX) -Cu joint to make a thermoelectric generator. A single step direct vacuum hot pressing of bismuth doped magnesium silicide stannide (Mg2(Yes0.38Sn0.6)Bi0.02) the powder produced a dense pellet with a mechanically stable metallized layer suitable for use in thermoelectric modules up to 400 ° C.
The bismuth doped magnesium silicide stannide powder synthesized by induction melting of the building blocks and ball milling of the solidified ingots was hot pressed with SS304 and copper (Cu) powders. The specific contact resistance of the seal is approximately 4.4 µΩ.cm2, which is the lowest value reported so far in this document.
The seal showed excellent thermal stability up to a temperature of 450 ° C when tested for 15 days, with insignificant change in the thickness of the reaction layers at the interface. Using these gaskets, a thermoelectric device has been successfully developed and tested for its performance.
Figure 1 shows TE legs, SEM micrograph and the change in contact resistance with Mg annealing2(Yes0.38Sn0.6) Bi0.02-Cu seal with SS 304 barrier layer.
Figure 1 SEM micrograph of Mg2(Yes0.38Sn0.6) Bi0.02-Cu seal with SS 304 diffusion barrier layer (left) and the variation of the specific contact resistance with annealing time at 450 ° C. The interfaces are free from mechanical defects showing a perfect diffusion bond of Cu with SS304. At SS304 / Mg2Yes0.38Sn0.6Bi0.02 interface, a continuous reaction layer with two sublayers was developed. The combined thickness of the reaction layers varied from 12 to 30 µm. The good bond between Mg2(Yes0.38Sn0.6) Bi0.02 and SS 304 due to these reaction layers makes the hot pressed pellets suitable for mechanical dicing up to 3mm x 3mm of cross sections usually used in TE modules. Thus, research has demonstrated a low-cost one-step process to fabricate a reliable metal-thermoelectric seal with less than 5 µΩ.cm2 contact resistance to make the TEG.
Post link: https://doi.org/10.1016/j.materresbull.2020.111147