Effect of synthesis methods and aging on synthesis of uvarovite garnet by ceramic and sol-gel processes

The uvarovite garnet (CaO)3 (Cr2O3 )(SiO2)3 has been synthesised (mineralised with borax to facilitate diffusion of precursors) by several sol-gel methods. Two routes for uvarovite formation have been observed: where CaCrO4 forms as an intermediate phase; and where metastable pseudowollastonite (alfa-CaSiO3) forms as an intermediate phase. Synthesis via CaCrO4 appears to be more reactive in unaged samples. The reactivity of samples can be directly related to the chemical homogeneity ofraw powders, and two methods of synthesis, Pechini and alkoxide, were found to stand out for their reactivity. Aging of raw powders for 6 months inhibits nucleation. Consequently, samples without nucleating agents (i.e. chlorides), such as gels from alkoxides, lead to the stabilisation of amorphous material or metastable phases (alfa-CaSiO3) and, as a result, hinder uvarovite formation. In contrast, aged samples involving heterogeneous nucleation agents, as in the Pechini method with CaCl2 as precursor, enhance reactivity. BCT/319