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

摘要

Layered double hydroxides (LDHs) with the hydrotalcite-type structure containing Co and Al, or Zn,Coand 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 variedfrom 2/1 to 1/2. The samples have been characterised by element chemical analysis, powder X-raydiffraction, differential thermal and thermogravimetric analysis, temperature-programmed reductionand 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 anddawsonite and probably an additional amorphous phase. Calcination at 1200°C in air led to formation ofspinels; a small amount of NaAIO_2was observed in the Al-rich samples, which was removed bywashing. The nature of the spinels formed (containing Co~(II), Co~(III), Al~(III) and Zn~(III)) strongly depends on thecations molar ratio in the starting materials and the calcination treatment, leading to a partial oxidationof Co~(II) species to Co~(III) ones. Colour properties(L~*a~*b~*)of the original and calcined solids have beenmeasured. While the original samples show a pink colour (lighter for the series containing Zn), thecalcined Co,Al samples show a dark blue colour and the Zn,Co,AI ones a green colour. Changes due tothe different molar ratios within a given calcined series are less evident than between samples with thesame composition in different series. These calcined materials could be usable as ceramic pigments.