Wenchao Li

Effect of Ti content on the martensitic transformation in zirconia for Ti-ZrO2 composites

Abstract Ti–ZrO 2 metal-ceramic composites with a series of graded compositions have been prepared by powder metallurgical process. The effect of Ti on the martensitic transformation in ZrO 2 from tetragonal to monoclinic structure has been studied by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results show that the phases of the sintered Ti–ZrO 2 system composites are α-Ti, tetragonal zirconia (t-ZrO 2 ) and monoclinic zirconia (m-ZrO 2 ). The volume fraction of m-ZrO 2 in ZrO 2 increases with the increase of Ti content when the volume fraction of Ti is higher than 12.5vol.% in the Ti–ZrO 2 composites. The nucleation of m-ZrO 2 occurs preferentially at the Ti–ZrO 2 interface. The interfacial stress arising from the thermal expansion mismatch and the plastic deformation of Ti enhance the driving force for the t-ZrO 2 →m-ZrO 2 transformation. It is noted that the content of Y 2 O 3 stabilizing agent in ZrO 2 should be increased as the Ti content increases in the preparation of Ti–ZrO 2 functionally graded materials (FGMs) to appropriately control the volume fraction of m-ZrO 2 .

Influence of different seeds on transformation of aluminum hydroxides and morphology of alumina grains by hot-pressing

The influence of different seeds on transformation of aluminum hydroxides and morphology of alumina grains was investigated. There are two kinds of alumina seeds used separately, alumina milling ball abrasive seeds and ultra-fine alumina powder seeds. Abrasive seeds were introduced to the starting materials by wet-grinding of high-purity alumina milling balls. The alumina powder seeds were directly added into raw materials. First, the introduction of different seeds can lower the transformation temperature from κ phase to α phase. The phase transformation from aluminum hydroxide to α-Al2O3 was completed at 1100 °C in 2 h when a certain amount of seeds were employed. Second, seeds also changed the sintering behaviour and microstructure of alumina ceramics. For the hot-pressed sample with alumina powder seeds added directly, the microstructure with platelets was observed. However, the anisotropic growth of alumina grains is favored in the presence of alumina abrasives as seeds. The elongated grains were developed in the sample with abrasive seeds introduced.

Synthesis and characterisation of MgAlON

This paper deals with the synthesis of MgAlON ceramics. The thermodynamic properties of this system were evaluated and the phase stability diagram of MgAlON was established. The synthesis of this material was successfully carried out by a careful choice of the experimental parameters in accordance with the phase stability diagram. The procedure adopted was to subject the starting materials to sinter under 25 MPa. A microstructure evaluation of the synthesised material was carried out. The mechanical properties of MgAlON were determined. The synthesised material was also subjected to X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy analyses. The crystallographic data of MgAlON were generated as a result of the present studies. A JCPSD file of MgAlON has been suggested.

Synthesis and characterization of MgAlON-BN composites

Abstract In the present paper the Gibbs energy of formation of MgAlON was evaluated, and on this basis the phase stability diagram of Mg – Al – O – N – B was established. Dense magnesium aluminium oxynitride – boron nitride (MgAlON – BN) composites with 0 – 30 vol.% BN were synthesized by hot pressing in the temperature range 1750 – 1850 °C in the phase stability region. The phase compositions of the composites analyzed by X-ray diffraction indicated that the main phases were MgAlON and h-BN and no impurities were found. The microstructures of the composites analyzed by transmission electron microscopy, scanning electron microscopy, and high-resolution electron microscopy showed that the MgAlON did not react with BN, and the latter was distributed on the grain boundary of MgAlON homogeneously. Excessive BN (> 20 vol.%) resulted in a discontinuous microstructure for MgAlON. A matrix-flushing method was employed in the quantitative X-ray diffraction analysis for the multi-component MgAlON – BN composites. The results showed that the relative MgAlON content decreased in the process of sintering MgAlON – BN composites. Thermodynamic analysis showed that some Al2O3, AlN, and MgO escaped under the reaction conditions. The lattice parameters of MgAlON in the composites have also been evaluated.

Silica photonic crystals with quasi-full band gap in the visible region prepared in ethanol

Monodisperse silica spheres of 252 not with a standard deviation of 5.7% are prepared by Stober method. By comparison of both of media, ethanol instead of water is used to assemble opal, and the artificial opal has been prepared by the sedimentation in ethanol of silica spheres. The structure of the opal prepared has been examined and discussed. The results show that the artificial opal has a structure similar to the face-centered cubic (fcc) type packed system with silica spheres. Transmission measurements of the artificial opal have been conducted, which shows that the artificial opal is quasi-full band gap silica photonic crystals in the visible region.