Di Huo

Preparation of transparent Y2O3 ceramic by slip casting and vacuum sintering

Abstract In the present work transparent Y 2 O 3 ceramics were made by slip casting and vacuum sintering of nanopowders with sodium polyacrylic acid (PAA-Na) as dispersant. The rheological properties of Y 2 O 3 nanopowder slurry were investigated using different amounts of dispersant and solid contents. The microstructures and transmittance of the sintered ceramics were also studied by means of scanning electron microscopy (SEM) and ultra-violet visible spectrometry. The results showed that rheological behaviors of the Y 2 O 3 nanopowder slurry were effectively promoted by sodium polyacrylic acid. Highly dispersive and stable slurries were obtained as the dispersant was added over 1.0 dwb% under the fixed conditions of pH 11 and 45 wt.% solid content. All the slip cast green bodies were sintered into highly dense ceramics after sintering at 1700 °C for 5 h in vacuum, wherein the sample added with 1.1% sodium polyacrylic acid exhibited the highest relative density of 99.36% and transmission of 30% at 800 nm wavelength.

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.

Homogeneous precipitation synthesis and magnetic properties of cobalt ferrite nanoparticles

Magnetic nanoparticles (NPs) of cobalt ferrite have been synthesized via a homogeneous precipitation route using hexamethylenetetramine (HMT) as the precipitant. The particle size, crystal structure, and magnetic properties of the synthesized particles were investigated by X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. The NPs are of cubic inverse spinel structure and nearly spherical shape. With the increase of oxidation time from 30 to 180 minutes in the reaction solution at 90°C, the average particle size increases from ∼30nm to ∼45 nm. The as-synthesized NPs ∼30nm in size show higher Ms (61.5 emu/g) and moderate Hc (945 Oe) and Mr/Ms (0.45) value compared with the materials synthesized by coprecipitation method using NaOH as precipitate at high pH value.

Fabrication and Properties of Hydroxyapatite-Ti Network Composites

Hydroxyapatite (HA) powder was synthesized by a sol-gel method with Ca(OH)2 and H3PO4 as reactants. The HA granules were then coated with TiH2 powder using a mechanical mixing method. The HA-TiH2 material system produced HA-Ti composites after hot-pressing at 1050°C. The HA-Ti composites are mainly composed of HA and Ti, with small amounts of Ca2P2O7 and Ca3(PO4)2 phases. Fracture toughness and bending strength are 2.4 MPa·m1/2 and 54.3 MPa, respectively for the HA-20vol%Ti composite, higher than those of the pure HA ceramic. The improvement in properties is because of the unique 3D network structure of Ti, which is an ideal reinforcement structure for the weak and brittle HA. According to ISO/TR 7405-1984, hemolysis test was performed to evaluate the blood compatibility of the material. The results show that the hemolysis rate of the HA-20vol%Ti composite is 0.56%. Relative growth rates (RGR) of L-929 cells soaked after 6 days in the HA-20vol%Ti group, pure Ti group, black group and pure Pb group were 132%, 100%, 90% and 6% respectively, while the level of cytotoxicity was grade 0 in HA-Ti composite group. These results imply that the HA-20vol%Ti composite has good biocompatibility and bioactivity.

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.

Fabrication of Transparent YAG Ceramics from Co-Precipitation Synthesized Nanopowders

YAG nanopowders were synthesized by a co-precipitation method using ammonium hydrocarbonate and ammonia water as the precipitants respectively. The influences of precipitants on chemical compositions, phase transformation and sinterability of the prepared powders, and transmittance of the vacuum-sintered YAG ceramics were studied. The sinterability of powders synthesized using ammonium hydrocarbonate as precipitant is better than that with ammonia water. Pure YAG phase can be obtained by calcining the hydrate precursor at 1200°C, while some impurity phases exist when calcining the carbonate precursor at the same temperature. Transparent YAG ceramics were fabricated by vacuum sintering at 1700°C for 5 h using the YAG nanopowders, and their in-line transmittance is about 60% in the visible light range.

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.