Search

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]

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]

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]

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]