Mu Zhang

Synthesis and formation mechanisms of morphology-controllable indium-containing precursors and optical properties of the derived In2O3 particles

In2O3 particles with three distinctive morphologies of 1D rods, 2D disks and 3D cubes were converted from their respective precursors synthesized by a facile urea-based homogeneous precipitation method. Two kinds of precursor phases, including In(OH)3 and InOHSO4(H2O)2, were obtained. In the case of high urea concentration, mesocrystalline rod-like In(OH)3 particles were produced by oriented primary particle aggregation induced by the coordination of urea on {012} of the primary particles. In contrast, cube-like In(OH)3 was obtained by Ostwald ripening in low-concentration urea solution. The addition of K2SO4 facilitates the formation of an In–sulfate complex, and InOHSO4(H2O)2 precursor disks about 2 μm in diameter were formed. It is suggested that the adsorption of urea on the active growth sites of the precursor leads to the formation of round disks instead of hexagonal plates. Upon blue light excitation, the three types of In2O3 particles obtained from their respective precursors exhibited a morphology-dependent photoluminescence behavior (disks > cubes > rods), but did not differ greatly in the positions of the PLE and PL bands of the luminescence. The emission (in the range of 500–540 nm) from In2O3 is associated with oxygen vacancies and is highly dependent on the annealing atmosphere.

Fabrication and Luminescent Properties of YAG:Ce Transparent Microspheres by Laser Heating

In this paper, YAG:Ce microspheres were fabricated by a laser heating method. The microspheres are polycrystalline. The melting of the YAG:Ce powder due to the extremely high temperature provided by the laser heating may account for the fast synthesis of the transparent YAG:Ce microspheres. Due to the temperature gradient during cooling, equiaxed and columnar grains were found inside the microspheres. The equiaxed grains are always located at the outer part of the spheres where large under-cooling was created. However, columnar grains are always found at the inner part of powder where a steep temperature gradient presented during cooling. The average size of the equiaxied crystallites observed from the SEM images is about 5 μm. The columnar crystallites are about 5 μm wide and 20 μm long. In the luminescence investigation, the optimum concentration of Ce3+ was found to be 2 at%. Furthermore, transparent spheres of other chemical compositions are also possible to be fabricated by this method, indicating its wide range of potential applications.

Effect of SnO2 Particle Size on Properties of Ag-SnO2 Electrical Contact Materials Prepared by the Reductive Precipitation Method

Performances of Ag-SnO2 electrical contact materials can be strongly affected by the microstructure. In this work, Ag-SnO2 composite powders were synthesized by chemical reductive precipitation method. During the precipitation process, Ag particle was deposited onto the surface of SnO2 particle with the assistance of citric acid. The microstructure and properties were analyzed for the prepared Ag-SnO2 electrical contact materials. Our research reveals that the particle size of SnO2 has significant influence on the morphology of the Ag-SnO2 composite powders, and therefore on the microstructure and physical properties of the electrical contact materials. With the decrease of particle size of SnO2, hardness of the Ag-SnO2 electrical contact materials increases, while electrical conductivity decreases.

The Fabrication of Monoclinic Gd

Monoclinic Gd2O3 transparent ceramics with spherical shape were successfully fabricated by a laser heating method. A simple model was used to describe the heating depth and cooling rate during the laser scanning process. The XRD analysis shows that these microspheres exhibit monoclinic structure which is room temperature thermodynamically unstable but kinetically possible due to the fast cooling rate. The SEM images show that these micro-spheres are polycrystalline and composed of randomly oriented Gd2O3 grains about 10 μm in particle diameter. No cracks, pores or secondary phase were observed at the grain boundary or inside the grains. A close-packed Gd2O3 spheres single layer was designed and prepared, and the in-line transmittance of this layer is about 44% in the visible light range. Pore-free monoclinic Gd2O3 can be transparent due to the slight difference between its ordinary and extraordinary refractive index (n e and n o). Another reason for its transparency is the small amount of grain boundaries though which light pass. Medical radiograph needs scintillators with small volume size (to increase the image lateral resolution and minimize optical cross-talking) and good transparency (to avoid severe light scattering and increase signal intensity). Transparent ceramic spheres may provide the possibility to meet those criterions.

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Fine yttrium stearate powder was produced at a relatively low temperature using yttrium nitrate hexahydrate, ammonia and stearic acid as the raw materials. Dispersed Y2O3 nanopowder was synthesized by calcining the yttrium stearate. The formation mechanism of the precursor and the Y2O3 nanopowder was studied by means of XRD, TG-DTA, FT-IR, BET, FE-SEM and HR-TEM. Pure and dispersed Y2O3 nanopowder with an average particle size of 30 nm was produced by calcining the precursor at 600 °C. The particle size increases to about 60 nm with the increase of the calcination temperature to 1000 °C. In the preparation of Y2O3 from yttrium stearate, no water medium is involved, thus capillarity force and bridging of adjacent particles by hydrogen bonds can be avoided, resulting in good dispersion of the particles. The dispersed Y2O3 nanopowder prepared in this work has potential application in phosphors and transparent ceramic materials.

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Yb rare earth doped YAG ultrafine particles were synthesized by the stearate melting method using yttrium stearate, ytterbium stearate and aluminum tristearate as starting materials. The phase formation of Yb:YAG, the properties and the sintering activity of the powders were investigated by means of XRD, SEM, dilatometry and vacuum sintering. The results show that pure Yb:YAG nanopowders can be obtained by calcining the co-melted precursor at a relatively low temperature of 800 °C for 4 h. The powders calcined at 1000°C have better sintering activity than the powders calcined at other temperatures. For the Yb:YAG powders doping with 0.5% TEOS, the compact can be sintered to 99.2% of the theoretical density at 1600 °C and 99.7% at 1700 °C. The transparent Yb:YAG ceramics obtained by vacuum sintering at 1700 °C for 5 h exhibit a pore-free and uniform microstructure.

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Ca3Co4O9 powders were synthesized by a solid-state reaction method. Porous Ca3Co4O9 ceramics with parallel sheet shaped pores were prepared by a template sacrifice method using epispastic polystyrend (EPS) hollow spheres as the templates. During compaction of the green body, the EPS hollow spheres change into EPS discs due to the pressing force. After sintering, the pores in the Ca3Co4O9 ceramics are sheet shaped, well distributed and parallel to the pressing surface of compaction. The value of ZT merit of the porous Ca3Co4O9 sample obtained with 10 wt% EPS spheres is 0.0489. It was found that the ZT merit value can be improved by changing the density of sample to achieve a high ratio of electrical conductivity to thermal conductivity.