Treball de Final de Grau en Química. Codi: QU0943. Curs acadèmic 2015-2016This investigation is focused on the analysis of the effect of Al co-doping on the reactivity, formation, color properties and performance as ceramic pigments of cobalt-doped karrooite (MgTi2O5), geikielite (MgTiO3) and qandilite (MgTi2O4) solid solutions.
With this aim, different solid solutions (Mg0,8-xCox)Al0,4Ti1,8O5, (Mg0,8-xCox)Al0,4Ti0,8O3 and (Mg1,8-xCox)Al0,4Ti0,8O4 were prepared with x=0,1 and x=0,2 through the use of citrates-gel MOD method. The homogenous raw xerogels were first charred at 500 ºC/1h and then fired directly at 1000 and 1200 ºC. The fired pigments were characterized by means of X-ray diffraction (XRD), optical spectroscopy (UV-vis-NIR) and CIE-L*a*b* colorimetry (also with samples enameled in different glazes).
XRD confirmed the total formation of karroite solid solutions of (Mg0,8-xCox)Al0,4Ti1,8O5 at 1200 ºC and the presence of secondary phase rutile at 1000 ºC. On the other hand, regarding to geikielite, the solid solutions (Mg0,8-xCox)Al0,4Ti0,8O3 did not form completely on account of the formation of MgAl2O4 spinel. The large amount of Al3+ favors the formation of Mg spinel, geikielite is form accompanied by two secondary phases karroite and MgAl2O4 spinel.
Finally, qandilite solid solutions (Mg1,8-xCox)Al0,4Ti0,8O4) also formed successfully, although accompanied by minor amounts of secondary phases, geikielite in greater proportion and MgAl2O4 spinel. In this case, the samples were subsequently fired at 1400 ºC, leading to a higher amount of qandilite phase, with only small amounts of residual geikielite.
The optical properties and performance as ceramic pigments were evaluated by UV-vis-NIR spectroscopy and CIE-L*a*b* color measurements applying them in different industrial double-firing transparent glazes (glaze A and B). The pigments exhibit slight greater stability using the glaze with high temperature (1050 ºC, high Ca and Zn content and free of Pb), these present a greenish-blue color. The blue color is associated with Co2+ in tetrahedral environments
