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