Jia-Min Wu

Thermal stability and microstructure characterization of MgAl2O4 nanoparticles synthesized by reverse microemulsion method

Magnesium aluminate (MgAl2O4) spinel nanoparticles were synthesized by reverse microemulsion process in cyclohexane by using two kinds of surfactants, n-amyl alcohol as cosurfactant and mixture of aluminic/magnesic salt aqueous solution as basic reagents. The effects of surfactant types and titration methods on the morphologies and sizes of the MgAl2O4 nanoparticles were characterized by TEM, TGA-DTA, XRD, HR-TEM and FT-IR. TEM images show that the particles prepared by forward-titration method with SPAN-80/Triton X-100 compound emulsifier have uniform spherical shape and good monodispersity with an average size of 9.5 nm. However, the average size of the particles prepared by reverse-titration method was about 10 nm and some particles have irregular platelike appearance. The products prepared with NP-40 surfactant and forward-titration method were agglomerated with an average size of 13 nm. TGA and XRD results show that the reverse microemulsion method has dramatically lowered the calcination temperature of MgAl2O4 with a degree of 700xa0°C, and the precursor can transform to single spinel phase at 900xa0°C.

Microstructures and dielectric properties of Ba[(Co0.7Zn0.3)1/3Nb2/3]O3-based ceramics prepared by aqueous gelcasting-assisted solid-state method

The aqueous gelcasting-assisted solid-state method (AGAS) and traditional solid-state reaction method (TSSR) were used to prepare Ba[(Co0.7Zn0.3)1/3Nb2/3]O3 (BCZN)-based ceramics. The effects of different powder-preparation methods on the microstructures and dielectric properties of the BCZN-based ceramics were investigated. X-ray diffraction analysis showed completely the same phase compositions regardless of the preparation method adopted. The relative permittivity (εr) did not significantly vary between the two methods. However, BCZN with CeO2-added ceramics prepared by AGAS had higher and more uniform density (6.374xa0g/cm3) and high quality factor of resonant frequency (Qxa0×xa0f) value (75,843xa0GHz) than those prepared by TSSR because of the more uniform microstructures, as shown by scanning electron microscopy images. The temperature coefficients of resonant frequency (τfxa0=xa07.4xa0ppm/°C) of the ceramics prepared by AGAS were also closer to zero than those prepared by TSSR.

High-performance ceramic parts with complex shape prepared by selective laser sintering: a review

ABSTRACT Selective laser sintering (SLS) is an additive manufacturing technology which has shown great advantages in direct formation of the polymer, metal and their composites. However, ceramic parts prepared by the SLS still exhibit some fatal defects, including low density and poor mechanical properties. In this respect, recent advances for preparing ceramics have improved the final density and performance by adopting post-processing methods. In this review, three commonly used powder preparation approaches (i.e. mechanical mixing, solvent evaporation and dissolution-precipitation process) and two powder sintering mechanisms for the SLS are introduced. Porous ceramic parts are prepared directly through the SLS by virtue of their high porosity. And dense, high-performance Al2O3, ZrO2, kaolin and SiC ceramic parts with complex shape are prepared by introducing CIP technology into the SLS, indicating that the hybrid technology could be the promising route for preparing high-performance ceramic parts used in various fields.

Study of 0.9Al2O3–0.1TiO2 ceramics prepared by a novel DCC-HVCI method

In this paper, in-situ coagulation of 0.9Al2O3–0.1TiO2 suspension and microwave dielectric properties of 0.9Al2O3–0.1TiO2 ceramics prepared by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method were proposed. The 0.9Al2O3–0.1TiO2 suspension could be coagulated via controlled release of calcium ions from calcium iodate at an elevated temperature. The influence of tri-ammonium citrate (TAC) content, solid loading, and calcium iodate content on the rheological properties of the suspension was investigated. In addition, the influence of coagulation temperature on coagulation time and properties of green bodies was also studied. It was found that the stable 0.9Al2O3–0.1TiO2 suspension could be successfully prepared by adding 0.3 wt% TAC and adjusting pH value to 10–12 at room temperature. 0.9Al2O3–0.1TiO2 green bodies with uniform microstructures were coagulated by adding 8.0 g/L calcium iodate after treating at 70 °C for 1 h. 0.9Al2O3–0.1TiO2 ceramics, sintered at 1500 for 4 h and annealed at 1100°C for 5 h, showed °Cuniform microstructures with density of 3.62±0.02 g/cm3. The microwave dielectric properties of 0.9Al2O3–0.1TiO2 ceramics prepared by DCC-HVCI method were: εr = 11.26±0.06, Q×f = 11569±629 GHz, τf= 0.93±0.60 ppm/°C. The DCC-HVCI method is a novel and promising route without binder removal process to prepare complex-shaped microwave dielectric ceramics with uniform microstructures and good microwave dielectric properties.

Dynamic image of vacuum arcs

This paper describes the principle and structure of a high-speed image sampling computer system to observe vacuum arcs (VAIS). In addition, the initial testing results are presented. It is found that the first concentrating of cathode spots in an axial magnetic field in a short gap can also lead to the appearance of anode spots, and the reignition point arises at the former cathode surface.

Electron density of vacuum arcs

The radial electron density distribution in a vacuum arc was deduced from the basic equations of plasma. The decay of the electron density is proportional to the square of the distance from the arc center inside the arc, and to the exponential function of that outside. In addition, a definition of arc radius was presented and determined. The theoretical calculation is in good agreement with experimental data.