Combustion Syntheses of CoAl2O4 Powders Using Different Fuels

This research intends to analyse the potential use of CoAl₂O₄ as an opaque pigment for solar collector selective paints. The opacity of the pigment occurs when the average crystallite size is micrometric. The CoAl₂O₄ samples were synthesized through combustion methods using aspartic acid (Asp) or lysine (Lys) as fuels. The powders obtained were calcined at temperatures between 600 °C and 1100 °C. Afterwards, the product was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). It was observed a CoAl₂O₄ phase in all the samples. The lowest average crystallite size was ≈24 nm corresponding to the sample obtained with Asp and calcined at 600 °C, while both powders calcined at 1100 °C showed sizes higher than 200 nm. In the light of these results it is suggested the use of even higher calcination temperatures so as to obtain opaque pigments for solar collectors selective paints.