Search

Si2N2O matrix composites were fabricated by solid/gas reaction in air or N2 atmosphere. The effects of atmosphere on the phase and microstructure of the composites were investigated. The reaction mechanism of Si2N2O system was discussed by analysing the variation of the Gibbs free energy with temperature. The effect of N2 and air on sintering of Si2N2O matrix composites was discussed in relation to observed kinetics and thermodynamic calculations. The results showed that gradient structure of Si2N2O matrix composites were obtained in N2 atmosphere. While high N2 concentration was useful for the formation of the pure β-Si3N4 ceramics, low N2 concentration was proposed to form the pure Si2N2O ceramics. However, in the air atmosphere, structure of the Si3N4/SiO2 composites is homogeneous without the gradient structure appearing. Its composition is a little different as the O2 concentration changes.

In our previous study, high amounts of andalusite aggregates (about 19%) were used to improve the oxidation resistance of Al2O3-SiC-C castables, attributed to the transformation of andalusite. However, for andalusite aggregates with large particle size, only a small part of the silica-rich glass phase (about 20%) is extruded onto the surface of the particles to block the pores. Theoretically, the smaller particle size promotes the conversion of andalusite, and more silica-rich glass phase would be extruded to the particle surface to block the pores in the castables, which is speculated to favor the enhancement of the oxidation resistance of the castables. Therefore, in this work, andalusite fine powder (<0.055 mm) was employed to substitute the same-weight proportion of fused brown corundum fine powder in the castables. The results demonstrate that a small amount of andalusite fine powder can achieve the similar antioxidation enhancement compared with high addition content of andalusite aggregate. And the properties of castables fired at different temperatures proved that the thermal decomposition of andalusite fine powder into silica-rich glass and mullite is the key reason of elevated oxidation resistance as well as the improvement of strength and thermal shock stability. [ABSTRACT FROM AUTHOR]

Calcium aluminate cements (CAC) with 0.21 wt% B2O3 and without B2O3 were used as binders of corundum-based castables. The properties of the castables with and without B2O3 after heat treatment at 110 °C, 1,100 °C and 1,450 °C were investigated, with emphasis on studying the effect of B2O3 in CAC on the volume stability and high temperature strength of the castables. It is found that a very small amount (about 0.01 wt%) of B2O3 introduced by the cement alleviates the expansion of the castables after firing at 1,450 °C and decreases the hightemperature strength of castables as the presence of B2O3 should generate liquid phase. [ABSTRACT FROM AUTHOR]

The influences of an NH4F additive content and calcination time on the morphological evolution of α-Al2O3 were investigated by using commercial γ-Al2O3 precursor as raw material. The results showed that the morphological evolution of α-Al2O3 was highly related to the addition of NH4F, the additive content, and the calcination time. Square-like α-Al2O3 powders with a primary crystal size of ~200 nm were formed from the milled γ-Al2O3 precursor with 1 wt.% NH4F at 1300°C for 2 h, whereas round cake-like, hexagonal platelets, slenderlike, and other irregularly shaped α-Al2O3 pieces in the size range of 0.2-2 μm were synthesized from the milled γ-Al2O3 precursor with 20 wt.% NH4F addition under the same calcination. Both morphological regularity and particle size distribution of α-Al2O3 were greatly facilitated by increasing the calcination time from 2 to 5 h. The addition of NH4F may lead to an alteration of the α-Al2O3 growth process by a change in the scale of the developing microstructure. [ABSTRACT FROM AUTHOR]

The influence of impurities such as SiO2 and Fe2O3 on the hydration behavior of calcium aluminate cement (CAC) was studied in this work. The hydration of the cement was characterized by the exothermic profile of cement pastes, and the dissolution-precipitation rate of the cement aqueous suspension was evaluated by electrical conductivity measurement. In order to analyze the resulting hydration products, the cement hydration was terminated via the freeze-vacuum drying method at the designated times. Then the phase development and microstructure evolution during cement hydration were investigated by XRD and SEM, respectively. The results indicate that the hydration rate is faster when the cement contains a higher amount of impurities. It is proposed that the higher hydration rate of the cement is probably related to the lattice imperfection of the main calcium aluminates in CAC. [ABSTRACT FROM AUTHOR]

Al-O-Si bond formation in boehmite-fumed silica mixtures during mechanochemical activation was investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), magic angle spinning nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). The intrinsic structure deformation of the boehmite was examined with XRD. Formation of new Al-O-Si bonds was illustrated by the incorporation of Al3+ in Si-O-Si linkages, the change in the Al coordination number of boehmite, and the appearance of new resonance in the 29Si NMR spectrum of the ground mixture. The presence of Al-O units in silica frameworks was verified by the increase of Al 2p binding energy and the decrease of Si 2p binding energy. [ABSTRACT FROM AUTHOR]

The formation and oxidation of magnesium–aluminum oxynitride (MgAlON) were investigated with emphasis on the effect of impurities through using synthetic raw materials and natural raw materials. It was observed that the formation of MgAlON was not much influenced by the starting materials, but MgAlON prepared from the natural starting materials exhibited accelerated oxidation. The different oxidation behaviors of MgAlON prepared from the different raw materials were discussed from the viewpoint of impurities in the materials. The formation temperature and oxidation temperature of MgAlON produced in this work were compared with those of MgAlON prepared using other processes. [ABSTRACT FROM AUTHOR]

Abstract  Titanium carbide ultrafine powders were prepared from tetrabutyl titanate and sucrose by sol–gel and microwave carbothermal reduction. The influences of reaction temperature and molar ratio of Ti to C on the synthesis of titanium carbide were studied. The results show that excess amount of carbon plays a positive effect on the carbothermal reduction of TiO2 at low temperature. The inceptive carbothermal reduction temperature of TiO2 and formation of titanium oxycarbide was below 900 °C, and pure TiC can be prepared at 1,200 °C, which was considerably lower compared to that by conventional carbothermal reduction using a mixture of TiO2 and carbon powders as raw materials. The morphology and particle size of synthesized TiC powder were examined by field emission-scanning electron microscopy (FE-SEM) and the quantities of the phases of the powders were analyzed by Rietveld refinement method, the particle sizes of the TiC powders synthesized at 1,300 °C distribute over 0.1–0.5 μm. [ABSTRACT FROM AUTHOR]

MgAl2O4 spinel precursor was prepared by a novel method combining a sol–gel process with the “traditional” precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d50=600 nm, specific surface area=105 m2/g) was obtained after calcination at 900°C. [ABSTRACT FROM AUTHOR]

MgAl2O4 spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)3 dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl2O4 spinel formation from >1280° to ∼900°C. [ABSTRACT FROM AUTHOR]