There are several problems with TE modules used in waste heat recovery applications. First, the TE modules themselves are not very efficient; although some heat energy gets converted into electricity, still much of it is wasted.
The next problems have to do with construction of the module itself and durability in its desired environment. Inside a TE module, semiconductors are soldered in series to copper pads to conduct the electricity. They are limited by temperature; in many situations where waste heat recovery would be the most desirable, higher temperatures exist than the melting point of the solder used to hold the modules together. The modules would literally melt and fall apart.
Another problem has to do with the coefficient of thermal expansion (CTE). When the module is heated, brittle intermetallics form due to the diffusion of Ni (the coating) into the substrate (Bi2TE3). The CTE of the newly formed Ni2Te3 is drastically different from the CTE of the surrounding materials. It is easy to see how the shear stresses at the interface generated by high or cyclic temperatures would have a tendency to crack the brittle intermetallic that forms in the modules.
Currently, most of the processes commonly used in bridge fabrication are fusion welding processes. The problem with fusion welding is that molten and resolidified metal in the weld deposit remains as-cast microstructure in the structure and has mechanical properties greatly dependent on chemistry/dilution and cooling rates.
These processes inherently have a potential for creating weld discontinuities such as solidification cracking in the fusion zone [figure 1] or gas and slag entrapment. Generic concerns remain in HAZ grain coarsening and loss of toughness in fusion welds.