Effect of Sn co-doping on new yellowish ceramic pigments based on Ni-MgTi2O5 pseudobrookites and Ni-MgTiO3 ilmenites

Abstract

This investigation deals with the optimization of new yellowish-orange ceramic pigments of Ni-doped Mg titanates with pseudobrookite (MgTi2O5) and ilmenite (MgTiO3) structures. The main goal of the study has been to analyze the effect of tin co-doping on these ceramic pigments, observing its influence on their stability against decomposition, the possible modification of their optical (colour) properties and also their pigmenting performance (stability) within ceramic glazes. For this purpose several solid solutions of Mg1-xNixTi2-ySnyO5 pseudobrookite (karrooite) and Mg1-xNixTi1-ySnyO3 ilmenite (geikelite) doped with Sn and Ni (x = 0.2, 0.4 and y= 0, 0.05, 0.1, 0.2) were prepared by traditional ceramic route using rapid firing conditions (1200-1400ºC/3h). In order to assess this Sn effect, the following characterization techniques have been performed: XRD, SEM/EDX, UV-vis-NIR and colour measurement (CIE-L*a*b*) techniques. XDR results showed that the Sn solid solubility was quite limited and temperature dependent for both phases and Ni-doping had a destabilizing effect on these phases. The yellow colorations were more intense or saturated (lower L*) and also more red and yellow (higher a* and b* hues) with the increase of firing temperature and Ni-doping, whereas Sn-doping had and opposite effect on saturation (L*) and red (a*) values. In the last part of this investigation selected compositions were also prepared through the use of a metalorganic decomposition route (MOD-citrate) as non-conventional synthesis method, using rapid firing conditions (1000-1200ºC) too. The main goal of this part was analyse the advantages in reactivity and different properties like thermal and chemical stability, microstructure and optical or colorimetric properties. Unfortunately, although the presence of secondary phases (such as ilmenite and/or rutile) was considerably reduced with the use of this alternative route, the Sn solid solubility was considerably lower, observing a much higher amount of free SnO2 in fired pigments.