Study and optimization of ceramic coatings for thermophotovoltaic applications

The use of ceramic porous coatings as selective emitters for thermophotovoltaic applications is proposed. The coatings have been produced from ceramic slips containing a reactive colloidal phase and rare earth garnet powders. Garnet coatings containing Er, Yb, TM, have been prepared with thickness ranging between 50 and 400 microns. Good adhesion has been evidenced on silicon carbide and alumina/alumina ceramic composites that are both good candidate for combustor cylinder of thermophotovoltaic generators. Microstructural and functional properties of the coatings have been investigated including Hg porosimetry and electron microscopy. An investigation on the scattering and the absorption coefficient has been performed with the aim of studying and tailoring their high temperature emissivity. As expected from Kirchoff's law, emission and absorption appear to be strictly linked but also hardly influenced by the material porosity. In fact, the coating porosity is useful both to compensate the difference of thermal expansion with the substrate and to shield the radiation emerging from it with an enhancement of the emitter efficiency. By considering the coating-substrate system, the emission spectrum results from the sum of the emission spectrum of the coating and the radiation emitted by the substrate and transmitted by the coating