Guotian Ye

Effect of B2O3 on Volume Stability and Strength of Corundum-based Castables

Abstract 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 high-temperature strength of castables as the presence of B2O3 should generate liquid phase.

Al-O-Si Bond Formation in Boehmite-Fumed Silica Mixtures during Mechanochemical Activation

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.

Influences of NH4F additive and calcination time on the morphological evolution of α-Al2O3 from a milled γ-Al2O3 precursor

Abstract 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, slender-like, 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.

Hydration Behavior of Calcium Alumina Cements with Different Amounts of Impurities

The influence of impurities such as SiO2 and F2O3 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.

Preparation of Sphere-like α-Al2O3 from Commercial Al(OH)3 with a Mechanical Ball Milling Process

In comparison to the typically worm-like α-Al2O3 powders of 20–30 µm size from unmilled Al(OH)3 precursor fired at 1450°C, almost-spherical α-Al2O3 powders of ∼0.5 µm size were synthesized from 9 h-milled Al(OH)3 precursor with the same heat treatment. The effect of the commercial Al(OH)3 precursor milling treatment on phase transformation and morphology of α-Al2O3 was studied by XRD and SEM. It was found that the milling treatment of the commercial Al(OH)3 precursor not only led ho an accleration of α-transformation during calcination, but also changed the morphology and particle size distribution of the α-Al2O3 powders. As the Al(OH)3 precursors were broken into irregularly shaped pieces with a ≤1 µm size range by the milling treatment, the resulting sharp corners with high vapor pressure would alter the evaporation-condensation mechanism into one that is mainly based on diffusion in the α-Al2O3 calcination process and promotes the tendency to form spherical α-Al2O3 powders.

Mg-O-Si chemical bond formation in light burned magnesia and fumed silica mixture during mechanical activation

Mg-O-Si chemical bond formation in a light burned magnesia (MgO) and fumed silica (SiO2) mixture during mechanical activation was investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), magic angle spinning nuclear magnetic resonance (MAS-NMR), and X-ray photoelectron spectroscopy (XPS). Crystallinity and intrinsic structure changes of the starting mixture during high-energy milling were examined by XRD. The formation of new Mg-O-Si chemical bonds of the ground mixture was illustrated by the incorporation of Mg2+ in Si-O-Si linkages, the appearance of new resonance in the 29Si NMR spectrum and the decrease of the Si 2p binding energy. The formation of Mg-O-Si chemical bonds created during grinding partly contributed to the lowered temperature of complete forsterite formation from 1400 to 1100°C.