Directionally solidified Al2O3-ME3Al5O12 (ME: Y, Er and Yb) eutectic coatings for thermophotovoltaic systems

Selective emitters for thermophotovoltaic systems consisting of directionally solidified Al2O3-ME3Al5O12 (ME: Y, Er and Yb) eutectic coatings on Al2O3 substrates were produced and characterizated. Coatings were deposited by dip-coating on cylindrical substrates. After sintering, a continuous-wave CO2 laser was used to produce the surface resolidification. The optimization of the processing parameters yielded dense eutectic coatings with good adhesion to the substrate and with 90-200 µm in thickness. All coatings were free of voids and showed a eutectic microstructure consisting of a three dimensional interpenetrated network of Al2O3 and ME3Al5O12. The mechanical properties of the coatings (hardness and fracture toughness) were evaluated by indentation techniques. Thermal emission was studied by heating the rods with a CO2 laser at temperatures between 1000 and 1400 °C. Selective emission was observed in Er3+ and Yb3+ based coatings and attributed to the electronic transitions of the rare earth ions. Er3+-coatings showed the best emission properties as selective emitters for thermophotovoltaic converters.