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.
Corrosion Behavior of Sintered Ti-Mo Alloy in NaOH Solution
SYNOPSIS The corrosion behavior of sintered Ti-(0, 10, 20, 30, 40)mass%Mo alloys in (10, 102, 103, 104)mol/m3 NaOH solution was studied by immersion test and electrochemical measurements. The corrosion rate of sintered Ti-Mo alloys in NaOH solution increased with increasing NaOH concentration and Mo content. The corrosion rate of Ti40mass%Mo alloy in 103mol/m3 NaOH solution, for example, exhibited 5•~10-3 mm/year, which was 10 times higher than that of pure Ti. But any corrosion rate of Ti-(10, 20, 30, 40)mass%Mo alloy in 103mol/m3 NaOH solution was less than 1•~10-2 mm/year. This fact showed that these alloys had a high corrosion resistance in 103 mol/m3 NaOH solution. Immersion test showed that dissolved Mo content was higher than Mo content in the alloy, in spite of Mo content or NaOH concentration. An intergranular corrosion was observed in SEM microstructure of Ti-(20, 30, 40)mass%Mo alloys after immersion test. Potentiodynamic polarization curves in 103mol/m3 NaOH solution showed that the open circuit potential (corrosion potential) shifted toward more noble potential and corrosion current density increased slightly as the alloy Mo content increased. This might be caused by the unstable spontaneously
Thermal Decomposition of Some Substituted Barium Titanyl Oxalates and Its Effect on the Semiconducting Properties of the Doped Materials
Thermal analysis has been performed on BaTiO(C2O4)2.4H2O, Ba0.6Sr0.4TiO(C2O4)2.4H2O, Sr(TiO(C2O4)2.4H2O, Ba0.9Pb0.1TiO(C2O4)2.4H2O, and BaTi0.9Zr0.1O(C2O4)2.4H2O. It was observed that the strontium compound decomposes differently than the others. Previous investigators have proposed conflicting mechanisms for the pyrolysis of the barium salt and these results are discussed in comparison with this work. The electrical resistivity and temperature coefficient of fired lanthanum-doped materials were found to vary with the calcination temperature. Maximum conductivity was observed in samples calcined at 900°C whereas maximum positive temperature coefficient was observed for materials calcined at 1050°C. Particle sizes of the calcined material were compared with grain sizes in the fired pieces and correlated with the electrical properties. A cursory examination was made on the effects of fabrication pressure, 1.25 to 15 tsi, on the electrical conductivity. Both the conductivity and positive temperature coefficient were found to increase with decreasing fabrication pressure.