Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.