Preparation of nanosized barium zirconate powder by precipitation in aqueous solution

Several ways were explored to synthesize barium zirconate by soft chemistry methods in aqueous solution. In the first method the synthesis of barium zirconate was initiated by urea decomposition, through an homogeneous precipitation of barium and zirconium salts followed by a “low temperature” thermal treatment. The kinetic of the reaction and the optimum urea/cation ratio have been determined by means of X-ray diffraction and Inductive Coupled Plasma analyses. It has been demonstrated that an amorphous zirconium hydrated oxide starts to precipitate followed by the precipitation of barium carbonate[1]. A calcination at 1200°C during 2 hours gives rise to the formation of a pure barium zirconate phase. In the other methods, barium zirconate was synthesized, in one step without any thermal treatments, by precipitation in highly basic aqueous solutions containing barium and zirconium salts. The effect of the hydroxide concentration was discussed in relation to the barium zirconate phase formation, the particles size and the particles size distribution. For each powder, microstructural characterisations have been performed on sintered bodies in order to evaluate the influence of the thermal treatment on the final density. Dilatometric measurements have been also performed in order to quantify the densification process. Important informations were obtained by these techniques, as for example the existence of an internal porosity which severely limits the final density of the material, even if sintering was performed at high temperature. Thus a careful control of the heating profile seems to be necessary in order to produce dense materials. 1.Preparation of nanosized barium zirconate powder by thermal decomposition of urea in aqueous solution containing barium and zirconium, and calcination of the precipitate. It is well know that the pH of a salt solution can be increased homogeneously by using the thermal decomposition of urea at about 90°C. The decomposition of urea gives rise to a controlled release of ammonia and carbon dioxide into the solution. OH and CO3 ions induce the precipitation of metal hydroxides and/or hydroxycarbonates [2] which are the precursors for the perovskite compounds. In order to produce a precursor with a barium/zirconium ratio equal to 1 from solutions containing barium and zirconium chloride, ICP measures were performed on the solid phase for different molar ratios urea/cations and different decomposition time. Quantitative precipitation is observed for a urea/cations molar ratio equals to 30 and a decomposition time of 24 hours. The precursor is a mixture of crystalline barium carbonate and an amorphous hydrated zirconia phase. This reactive mixture was calcined to produce barium zirconate and the phase evolution was followed as a function of the temperature by XRD measurements as shown on figure 1. Pure barium zirconate phase is formed at about 1100°C . The particles are spherical and their size lies in the range of 50 to 120 nm with some bigger aggregates characterized by a diameter close to 300 nm. A lot of neck-formation between particles can be seen on figure 2. Sintering of the powder. As shown by the dilatometric studies, the onset of shrinkage starts between 1250 and 1300°C. SEM photomicrographs of the pellets surface sintered at 1200°C (A), 1300°C (B), 1400°C (C) and 1500°C (D), for 2 hours with a heating rate of 3°C/min (figure 3) were taken to study the temperature effect on the densification of the material. Shrinkage at 1300 °C are confirmed by the photomicrographs. No open porosity can be observed after a sintering temperature at 1500°C for 2 hours. Although, Archimede's measurements on the pellets indicated an important open porosity equal to 26 % at 1200°C which decreases to 3.6 % by rising the sintering temperature at 1500°C (Figure 4). Moreover, BaZrO3 sintered at 1400 and 1500°C shows a strong enlargement in grain size (figures 3). Figure 3: SEM photomicrographs of the pellets surface sintered at 1200°C (A),1300°C (B), 1400°C (C) and 1500°C (D), for 2 hours. Figure 4: SEM photomicrographs of polished cross section of BaZrO3 pellets sintered at 1200°C (A), 1300°C (B), 1400°C (C), and 1500°C(D) for 2 hours. Magnification rate 2500X. Figure 1: Evolution of the reacting mixture calcined between 600°C and 1100°C Figure 2: photomicrographs of calcined BaZrO3 powder at 1200°C for 2 hours. Magnification rate 50000X.