Effect of added zinc on the properties of cobalt-containing ceramic pigments prepared from layered double hydroxides

Layered double hydroxides( LDHs) with the hydrotalcite-type structure containing Co and Al, or Zn, Co and Al in the brucite-like layers and carbonate in the interlayer have been prepared by coprecipitation. The Zn/Co molar ratio was kept to 1 in all samples, while the divalent/trivalent molar ratio was varied from 2/1 to 1/2. The samples have been characterised by element chemical analysis, powder X-ray diffraction, differential thermal and thermogravimetric analysis, temperature-programmed reduction and FT-IR spectroscopy. A single hydrotalcite-like phase is formed for samples with molar ratio 2/1, which crystallinity decreases as the Al content is increased, developing small amounts of diaspore and dawsonite and probably an additional amorphous phase. Calcination at 1200 degrees C in air led to formation of spinels; a small amount of NaAlO2 was observed in the Al-rich samples, which was removed by washing. The nature of the spinels formed(containing CoII, CoIII, AlIII and ZnII) strongly depends on the cations molar ratio in the starting materials and the calcination treatment, leading to a partial oxidation of Co-II species to Co-III ones. Colour properties(L*a*b*) of the original and calcined solids have been measured. While the original samples show a pink colour( lighter for the series containing Zn), the calcined Co, Al samples show a dark blue colour and the Zn, Co, Al ones a green colour. Changes due to the different molar ratios with in a given calcined series are less evident than between samples with the same composition in different series. These calcined materials could be usable as ceramic pigments. (C) 2009 Elsevier Inc. All rights reserved.