Materials Science Forum Vols. 561-565

Abstract: Machinable silicon nitride/ hexahedral boron nitride (Si3N4/h-BN) composites were in-situ synthesized in a nitrogen (N2) atmosphere by means of combustion synthesis gas-solid reaction with silicon (Si) powder and h-BN as raw materials. The effect of the volume fraction of h-BN on the machinable properties of Si3N4/BN composite was studied. The results show that Si powder was fully nitrified and no residual Si was found. Microstructures by a scanning electron microscopy (SEM) show Columnar crystals of β-Si3N4 are the main phase and acicular crystals of h-BN disperse β-Si3N4 intergranular. With the increasing of the volume content of h-BN, the machinability of the composite increases, but the bending strength of composite decreases firstly and then increases. The lowest bending strength is 84.96MPa at 25% volume fraction of h-BN.

Abstract: Grain growth behavior in Al2O3 with a small amount of ZrO2 (< 5 vol%) was examined. Grain growth of Al2O3 was retarded by zirconia particles, despite the small amount of added zirconia. The fraction of the zirconia particles embedded within alumina grains (intragranular zirconia particles) increased with decreasing zirconia content and increasing sintering temperature. Grain growth inhibition of alumina in the Al2O3-ZrO2 showed good agreement with the prediction of modified Zener’s pinning effect by the zirconia particles on grain boundaries.

Abstract: In this paper, the MgAlON ceramic was fabricated by Spark Plasma Sintering (SPS) and hot press sintering respectively. The results showed that highly pure and single-phase MgAlON could be fabricated at lower sintering temperature in a short period through SPS process, compared with the conventional Hot Press sintering (HP) process. The bending strength of MgAlON specimens prepared by SPS process was higher than 500MPa while bending strength of HP specimens was much lower. The open porosity was almost eliminated in SPS MgAlON specimens. Spark Plasma Sintered MgAlON had a single phase of MgAlON while Hot Press Sintered MgAlON had major MgAlON and minor AlN and Al2O3.

Abstract: A new wire mesh metallic catalyst support has been studied by using a stainless heat resistant steel of including aluminum. This catalyst support was improved for the metal honeycomb catalyst support that had been put to practical use. The wire mesh catalyst support was made in the following procedures. First, it was made from flat plate made by the stainless steel from the machining. Second, the low oxygen atmosphere in the heat treatment furnace did the aluminum extraction processing. Third, the aluminum oxide layer was made on the surface of catalyst support by furnace in air. Metal honeycomb catalyst has been made for several years by this method. The aim of this study was to evaluate the aluminum oxide layer on the surface of wire mesh catalyst support. The aluminum oxide surface was measured using scanning electron microscopy (SEM) and X-ray reflection diffraction (XRD). This catalyst support has the performance similar to the conventional metal honeycomb catalyst support.

Abstract: The effect of stoichiometry, i.e. Ca/Cu ratios (CaCu3xTi4O12, x = 0.8, 0.9, 1.0, 1.1 and 1.2) on the microstructure and electrical properties was investigated. The grain sizes of CaCu3xTi4O12 composition increased sharply with the increase of copper, from ~1 μm with x = 0.8 to ~50 μm with x = 1.2. The real part of dielectric permittivity changed dramatically, the pellet with x = 1.0 had the highest dielectric permittivity ~160, 000 at 1 kHz. Furthermore, the dielectric permittivity of all pellets was impressively large values (between 10, 000 to 1, 000,000 at 100 Hz) and was nearly constant over a wide frequency range between 100 Hz to ~100 MHz. However, the dielectric permittivity of CaCu3xTi4O12 composition is not consistent with the amount of copper and cell parameters and grain sizes. Impedance spectroscopy exhibited that the CaCu3xTi4O12 composition had two semicircle at least at high frequency (~ 107 Hz) and low frequency (<100 Hz), respectively. The grain and grain boundary of the compositions had different impedance and relaxation behavior.

Abstract: Mn3GaN has anti-perovskite structure and there exists an abnormal thermal expansion behavior in accompanying with a magnetic transition and variation of electronic transport properties. Substitution of Ga by Ge(Si) induces the change of the thermal expansion properties and the corresponding temperature range. The structure, heat capacity, magnetic and electronic transport properties of Mn3Ga(Ge,Si)N were investigated and discussed.

Abstract: The current-carrying wear characteristics of Ti3AlC2 sliding against low-carbon steel were investigated. Tests were carried out using a block-on-disk type friction tester, with sliding speeds of 20~60 m/s, normal pressures range in 0.4~ 0.8 MPa, and the current intensity of 0 A, 50 A and 100 A. The Ti3AlC2 showed good current-carrying wear properties. At the sliding speed of 20 m/s, the wear rate of the Ti3AlC2 (× 10-6 mm3/Nm) was varied in the range of (2.05 ~ 2.41), (2.64 ~ 2.39) and (6.26 ~ 3.62), under the current of 0 A, 50 A and 100 A, respectively. Both the surfaces of Ti3AlC2 and the steel were covered by a frictional film, which was consisted of iron titanate (Fe2.25Ti0.75O4) and aluminum iron oxide (AlFeO3). The wear rate of Ti3AlC2 with current was composed of two parts: the interaction of micro-arc ablation and mechanical friction, and the coupled action of thermal and mechanical effect. Which one will be the main mechanism depends on the material parameters of Ti3AlC2 and the mechanical parameters such as the normal pressure or the sliding speed.