Evaluation of critical thermal stresses at the interface between silicide thermoelectric elements and ceramic substrates

Silicide-based thermoelectric modules are thought to operate with mid-high temperature gradients. In the operating conditions, thermal stresses in materials with different coefficient of thermal expansion (thermoelectric legs, connectors, ceramic substrates) may reduce the mechanical strength of the modules. In particular, a rigid connection of thermoelectric elements with the ceramic substrates leads to critical tensile/shear stress and to the potential failure of thermoelectric materials. The comprehension of potential failure mechanisms and the evaluation of maximum admissible stresses are important to identify suitable strategies for the realization of the modules, in order to avoid local damage and reduction of thermoelectric materials strength. In this work, available failure criteria for brittle materials are compared to identify properly the ultimate strength of silicide legs. Some results of finite element simulation of mechanical behavior of a single Mg2Si and of a 16 Mg2Si - HMS legs module under different thermal gradients are presented. The numerically evaluated distribution of critical stresses is finally compared with the damage pattern observed during the test of a module prototype.