Directionally Solidified Cobalt-Doped MgO-MgAl₂O₄ Eutectic Composites for Selective Emitters

Cobalt-doped MgO-MgAl2O4 eutectic composites were explored for their use as selective emitters for thermophotovoltaic devices. Eutectic ceramic rods with different cobalt content were directionally solidified by using the laser floating zone technique at two processing rates to obtain microstructures with different domain sizes. Thermal emission between 1000 °C and 1500 °C and optical properties (reflectance and transmittance) at room temperature were measured in the Co-doped composites and the effect on microstructure and cobalt content was investigated. Thermal emission consisted of an intense broad band at about 1.67 µm matching with the bandgap of the InGaAs cell. The emission was ascribed to the de-excitation from the 4T1(F) multiplet to the 4A2(F) ground state of the thermally excited Co ions located in the tetrahedral sites of the MgAl2O4 phase. The selectivity of the thermal emission showed a decrease with the cobalt content due to the enhancement of other electronic transitions, which leads to keeping the cobalt content in these composites at low levels (<0.15% at Co) for their use as selective emitters.