Porous Garnet Coatings Tailoring the Emissivity of Thermostructural Materials

The power radiated by a surface influences the energy transfer processes following a T⁴ slope. It is therefore clear that, at high temperature, the study and control of spectral emissivity of materials play a key role in many important technologies: re-entry-vehicle thermal protection shields, high temperature radiators, selective emitters in thermophotovoltaic generators, etc. We have developed a class of thick porous garnet coatings that may raise or lower the spectral emissivity of thermostructural materials in the VIS, NIR, and IR regions. The porosity of the coatings nearly cancels any emission contribution from the underlying materials due to the scattering in the inhomogeneous system: pore/high refraction garnet. The yttrium aluminum garnet polycrystals vary their emissivity as a function of the doping rare earth elements they contain. We achieved an emission control capability in the range 700–3000 nm. Porous coatings have been prepared from ceramic slips containing a reactive colloidal phase and rare earth garnet powders prepared by drying and calcining mixed oxide aqueous gels. Garnet coatings containing Er, Yb, and Tm were prepared with thickness ranging between 50 and 400 microns. The coatings have been investigated by morphological and microstructural studies. A dedicated experimental set up has been developed to measure the spectral emissivity up to 1700 K under different heating conditions.