Functionally graded materials for sensor and energy applications

Principles, preparation, characterisation, and application of functional materials containing a gradient of their functional properties are surveyed, with main emphasis on thermoelectric (TE) materials for application in sensors and thermogenerators. Further examples of the implementation of functionally graded materials (FGM) presented are dielectric thin-film stacks for capacitors with low temperature coefficient, microwave-processed structural gradients in fuel cell electrodes, and zone-melted graded (Bi1 − xSbx)2Te3 materials for Peltier coolers. Preparation and properties of compositional gradients in TE solid solutions (FeSi2 doped by alloying, (Bi1 − xSbx)2Te3, Mg2(Si,Ge,Sn), PbTe) are analysed, as well as composites joining thermoelectrics of dissimilar chemistry and joints to metallic contacts and interlayers. Thermal spraying of doping-graded FeSi2 was developed as a preparation technique of TE silicide-based FGM. Design, preparation and test of a layered heat-flux sensor based on FeSi2 are described. A calibration test evidenced the feasibility of linearisation of thermal sensor characteristics. A theoretical design tool for functionally graded and segmented thermoelectric structures was based on a local selection criterion to identify the optimal spatial compositional distribution.