Anne Aimable

Contribution of aggregation to the growth mechanism of seeded calcium carbonate precipitation in the presence of polyacrylic acid.

Our work investigates the precipitation mechanism of a seeded calcium carbonate reaction, by using cryogenic TEM to observe the early stages of the reaction. The early precipitation of a hydrated phase is proposed as an intermediate phase before transformation into calcite. Thermodynamic modeling in conjunction with pH, surface potential measurements, and colloidal stability modeling demonstrate that calcite growth is dominated by agglomeration. This is in agreement with the cryogenic TEM observations, which suggest oriented attachment dominates early aggregation. The final stage of the reaction is described by a ripening mechanism that is significantly inhibited when high concentrations of polyacrylic acid (PAA) are used. The different concentrations of PAA lead to significant differences in the final particle substructure observed using cross section TEM. At low PAA concentrations, single crystal particles result, coherent with the proposed early oriented attachment mechanism and interfacial energy calculations. A core shell model is proposed for high PAA concentrations, whereas internal ripening of nanosized pores has been observed for low PAA concentrations, suggesting trapped solvent during the rapid initial particle formation at the relatively high supersaturations (S = 30) investigated.

Comparison of two innovative precipitation systems for ZnO and Al-doped ZnO nanoparticle synthesis

This study presents a comparative approach to investigate the potentials of two innovative methods for the synthesis of ZnO and Al-doped ZnO. The first method is a precipitation system working in mild hydrothermal conditions (90°C) using a tubular reactor (Segmented Flow Tubular Reactor, SFTR). The second method is a microwave-assisted hydrothermal process working at 250°C - 38 atmospheres. Nanocrystalline ZnO with a high specific surface area (49–68 m 2 /g) was obtained with both systems. Smaller equiaxed particles (50–70 nm) were obtained with the SFTR, with an excellent homogeneity in size and morphology, which was attributed to an excellent control of the process parameters (mixing, temperature, volume of reaction). A higher luminescence signal was measured on these samples. The microwave method leads to particles with a higher crystallinity due to the temperature of the reaction. A significant effect of the aluminum was observed, which reduces the crystal growth to produce equiaxed morphologies. This effect was enhanced by adding poly(acrylic) acid (PAA).

Electron-microscopic observation of BaTiO3 prepared by additive assisted aqueous synthesis

Bulk barium titanate (BaTiO3) has found widespread applications especially in multi-layered ceramic capacitors (MLCCs) and embedded decoupling capacitors (EDC). In the last years, the interest in one-dimensional (1D) nanostructured ferroelectric systems (nanotubes, nanowires, nanorods, nanobelts, nanofibers) is increasing. Recently theoretical studies reported an enhancement of ferroelectricity in 1D systems [1]. Although the hydrothermal and aqueous synthesis of equiaxed barium titanate powders have been thoroughly investigated [2-4] the growth of barium titanate anisotropic nanoparticles still less known. Indeed it is particularly dificult to modify the crystal habit by hydrothermal and related methods. However it is expected that the presence of some additives during the synthesis will change the growth kinetics. In this work the electron microscopy study of the effect of poly(acrylic acid) (PAA) and hydroxypropylmethylcellulose (HPMC) additives on the anisotropic growth of BaTiO3 by aqueous synthesis is reported. Figure 1 shows the X-ray diffraction paterns of the samples prepared in the presence of a) PAA and b) HPMC. As can be noticed cubic barium titanate is formed independetly on the used additives or on their concentration. In addition with the increase of the concentration of HPMC more barium carbonate is formed (Fig. 1b). This seems to suggest that HPMC is interacting more with Ti source than PAA. When the synthesis is performed in the presence of high concentration PAA directed agregation of BaTiO3 particles is observed (Figure 2a). However when the synthesis of barium titanate occurs in the presence of high concentrations HPMC barium titanate particles with different morphologies are formed (Figure 2b). The SEM X-Ray mapping (Figure 3) confirms that Ti and Ba are homogeneously distributed in the obtained powders. Incipient dendritic growth was observed under the presence of low concentrations of PAA (Fig. 4a). Although the SAED indicates a crystal growth direction along <001> axis of the cubic structure it is difficult to say that this is the preferential growth direction of the dendritic particles. The directed aggregation observed by SEM and the incipient dendritic growth noticed by TEM analysis indicate that the PAA is probably inducing an anisotropic growth. By analyzing the SAED of the barium titanate particles obtained under low concentration of HPMC (Fig. 4b) the growth habit along <001> direction is also verified. As a conclusion, SEM and TEM studies proved that the used of such additives markedly affect the crystalization of barium titanate. Microsc Microanal 15 (supp 3), 2009 51 Copyright 2009, LASPM doi:10.1017/S1431927609990717