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.
Solution-based synthesis of BaZrO3 nanoparticles: conventional versus microwave synthesis
In this paper, we report on the synthesis of BaZrO3 nanostructures by novel bottom-up synthesis methods. Nanocrystals with diameters ranging from 5 to 10 nm are prepared from aqueous or multiple phase precursor solutions. In order to transform the precursor solutions into nanocrystal containing suspensions, both conventional and microwave-assisted solvothermal treatments are used. An additional heat treatment was necessary to obtain crystalline particles starting from the aqueous precursor, while crystalline particles are directly obtained after solvothermal treatment of the multiple phase precursor. The crystallinity and size of the obtained nanoparticles are investigated by means of dynamic light scattering, X-ray diffraction and transmission electron microscopy. We found that the nature of the bases used in the multiple phase precursor have an effect on the particle morphology. In general, the microwave-assisted solvothermal synthesis renders the best prospects towards small particle sizes between 3 and 5 nm in diameter with a narrow size distribution. In addition, the process exhibits higher energy efficiency, resulting in lower reaction times (5 min–2 h) in comparison with the conventional solvothermal treatment (4–24 h).
Solidified structure and leaching properties of metallurgical wastewater treatment sludge after solidification/stabilization process
ABSTRACT The presented study investigates solidification/stabilization process of hazardous heavy metals/arsenic sludge, generated after the treatment of the wastewater from a primary copper smelter. Fly ash and fly ash with addition of hydrated lime and Portland composite cement were studied as potential binders. The effectiveness of the process was evaluated by unconfined compressive strength (UCS) testing, leaching tests (EN 12457-4 and TCLP) and acid neutralization capacity (ANC) test. It was found that introduction of cement into the systems increased the UCS, led to reduced leaching of Cu, Ni and Zn, but had a negative effect on the ANC. Gradual addition of lime resulted in decreased UCS, significant reduction of metals leaching and high ANC, due to the excess of lime that remained unreacted in pozzolanic reaction. Stabilization of more than 99% of heavy metals and 90% of arsenic has been achieved. All the samples had UCS above required value for safe disposal. In addition to standard leaching tests, solidificates were exposed to atmospheric conditions during one year in order to determine the actual leaching level of metals in real environment. It can be concluded that the EN 12457-4 test is more similar to the real environmental conditions, while the TCLP test highly exaggerates the leaching of metals. The paper also presents results of differential acid neutralization (d-AN) analysis compared with mineralogical study done by scanning electron microscopy and X-ray diffraction analysis. The d-AN coupled with Eh-pH (Pourbaix) diagrams were proven to be a new effective method for analysis of amorphous solidified structure.