Effect of fuels in combustion synthesis of CoCr2O4 pigments

Abstract

The CoCr2O4 is a bluish-green pigment characterized by a high thermal stability, good resistance to atmospheric effects and chemical corrosion [1]. This work is aimed to present the process of production of CoCr2O4 oxide by means of the original one-step stoichiometric combustion method starting from metallic nitrates and four fuels: aspartic acid (Asp) or lysine (Lys) or ethylenediaminetetraacetic acid (Edta) or tris(hydroxymethyl)aminomethane (Tris). In the production of CoCr2O4, once obtained the ashes by combustion processes, they are calcined for 2 hours at 800°C and 1000°C in air. The pigments are characterized by X-ray diffraction (XRD) to determine their crystalline structure, average crystallite size from peak (2 = 36°) using Scherrer equation. The diffraction data were analysed by the Rietveld method, using the FULLPROF refinement program. Additionally, L*a*b* colour parameters of samples were measured, following the CIEL*a*b* method [1]. In addition, the specific object of this paper is to analyse the influence of the crystalline structure on the pigment colour, then, these pigments will be applied to insensitive spectrally selective paints for coloured solar absorbers [2]. The diffractograms of all the pigments calcined at 1000°C (1000°Cpigment) correspond to CoCr2O4 which crystallizes in a cubic spinel structure with space group (No. 227) and a face-centred lattice defined in the cubic Fd- 3തm. In the case of Tris-1000°C pigment, the unit-cell parameter is: a= 8.224 Å. For this model, it was obtained a very good fit between observed graphics and the calculated XRD profiles producing good agreement factors as shown in Fig.1. The average crystallite size ranged between 30 and 50 nm for 1000°C-pigments. In the case of 800°C calcined pigments (800°C-pigments), the stabilized CoCr2O4 structure was also observed but with a lower crystallite size than 1000°C-pigments. Coordinate L* for 1000°C-pigments was lower than the one corresponding to 800°C-pigments. Additionally, coordinate b* is negative for 1000 °C- pigments, indicating a slight blue contribution, while for 800°C-pigments is positive indicating a slight yellow contribution. Probably, the increase of the calcination temperature would cause a higher crystallite size and more solar absorption. In conclusion 1000 °C- pigments are darker than 800°C-pigments and have a major aptitude to be used in spectrally selective paints. It would be necessary to carry out further specific technics to confirm this relation.