Preparation by hydrolysis of aerosols and colour properties of Cr-doped and Co-doped zircon powders

Abstract

Cr-doped and Co-doped zircon powders consisting of spherical particles with broad size distribution (0.1–5 μm) and different M Zr atomic ratios (M = Cr or Co) were prepared by hydrolysis of aerosols generated from mixtures of silicon and zirconium alkoxides and ethanolic solutions of the corresponding metal salt. The thermal evolution of these powders was studied up to zircon crystallisation and the colour properties of the resulting pigments were evaluated and related to the nature and localisation of the chromophore in the zircon matrix. The effects of the addition of a flux agent (NaF) on the colour properties of the pigments was also investigated. It was found that green Cr-doped pigments can only be obtained by heating the starting powders at 1300 °C in the presence of NaF, since in the absence of the flux, the whole amount of the Cr ions was volatilised during zircon formation (1500 °C). This colour was mainly due to Cr 3+ ions located out of the zircon lattice, probably forming a sodium chromium (III) silicate. In Co doped zircon, the addition of NaF had no influence on the temperature of zircon formation (1100 °C), although it affected the colour properties of the resulting pigments. Thus, the colour observed in the absence and the presence of the flux was green and blue, respectively. Although the only Co phase detected in both cases was Co 2SiO 4, the green colour was tentatively attributed to the presence of a small amount of unreacted CoO, whereas the blue one, to the formation of a certain amount of a sodium cobalt (II) silicate.