Advanced Materials Research Vols. 399-401

Abstract: MgO-SiC refractory powders were prepared by using high-silicon magnesite and carbon black powders as the starting materials calcined at 1550 to 1650 °C, respectively, for 3 hours under argon atmosphere. The phase compositions and microstructure of samples are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The effects of the calcining temperature and the proportion of carbon black on the synthesis of MgO-SiC refractory powders were investigated. The results show that high-silicon magnesite reacts with carbon black to form MgO and SiC as the main phases, and the minor phases include FeSi, Ca₃Al₂Si₃O₁₂ and Mg₂SiO₄. Carbon black content is the key factor to affect the synthesis of MgO-SiC composite powders.

Abstract: Tabular structure of Al₄SiC₄-Al₈SiC₇ composites was successfully synthesized using a mixture of calcined bauxite, SiC and carbon black by a carbothermal reduction process. The effects of the amount of SiC addition and the heating temperature on synthesis of Al₄SiC₄-Al₈SiC₇ composites by carbothermic reduction were investigated. The results show that SiC amount played an important role in the content of the final products. With the increasing of heating temperature, the Al₄SiC₄ content increased and Al₈SiC₇ content decreased in the products, which indicated the formation and growth of Al₄SiC₄ were promoted.

Abstract: In this paper, A1₄SiC₄ was synthesized by using flint, aluminum and carbon black as raw materials，and they mass ratio was 28:50:22. The samples were fired at 1500°C、1600°C and 1700°C in an argon atmosphere. The phase compositions were determined by X-ray diffraction (XRD), the microstructures were examined by scanning electron microscope (SEM) and the elemental and quantitative compositions were determined by the energy dispersive X-ray spectroscopy (EDX). The results showed that the flint reacted with the carbon black, and produced Al₂O₃ and SiC firstly; meanwhile Al₄C₃ as intermediate product formed by the reaction between Al and C. Then a certain Al4C3 reacted with SiC, and produced A1₄SiC₄; most Al₄C₃ reacted with the Al₂O₃, and produced Al₄O₄C at higher temperature. Subsequently, the Al₄O₄C and the SiC reacted with carbon black, and produced A1₄SiC₄. The formed quantity of A1₄SiC₄ increased as the heating temperature was raised, and crystal structure was the flake structure of 1-2μm thickness and about 10μm length. The formation mechanism of A1₄SiC₄ was also discussed.

Abstract: Polycrystalline alumina fibers were prepared by sol-gel method. The influence of heating rate on average tensile strength of fibers as well as the influence of calcining temperature on average tensile strength of fibers, crystal phases form, specific surface area of fibers were studied. The results show that when the heating rate (below 600°C) is 4°C/min,the average tensile strength of fibers over 1000Mpa The fibers pulverize obviously when the heating rate＞7°C/min. The average tensile strength and the specific surface area of fibers decreased with calcining temperature rising. When the calcining temperature reaches over 1300°C, the average tensile strength of fibers decreased quickly and the specific surface area of fibers decreases slowly for the crystalline growth.

Abstract: The centrifugal spray drying process of bauxite slurry was studied to investigate the influence of slurry solid content on the particle size distribution and the sphericity of the microspheres, and so was operation technology on average diameter and the sphericity of the microspheres. The optimal operation parameters are obtained to prepare high spherical microspheres bauxite with the diameter distribution of 100 μm and whose particle size distribution is narrow: the slurry solid content is 70 wt%, the dry air temperature is 250 °C, the rotation rate of spray head is 6000 rpm and the feed flow rate is 1 L/h in the experiments.

Abstract: MgAl₂O₄-SiC composites were prepared by using MgAl₂O₄ and SiC powder as main starting materials, being pressed to cylindrical specimens of Φ30mm×30mm at 200 MPa and heated respectively at 1400°C and 1500°C for 3 hours in air atmosphere. The room temperature physical properties of sintered samples were tested according to related national standards. The phase compositions were analyzed by XRD. The microstructure of the fracture of the samples was observed by SEM. The results showed that MgAl₂O₄-SiC materials could sintered well in air atmosphere. The mechanism is that SiC was oxidized to form active SiO₂, which reacted with MgAl₂O₄ to produce liquid cordierite promoting the sintering of the materials. The sinterability of MgAl₂O₄-SiC materials was overall improved with increasing the sintering temperature. When the content of SiC of MgAl₂O₄-SiC materials was 10%, the cold crushing strength and bulk density of the samples reached maximums and apparent porosity reached a minimum; the content of SiC was more than 10%, the sinterability of samples deteriorated.

Abstract: In this paper, the synthesis of MgAl₂O₄-SiC-C refractory composite powders by forsterite, alumina and carbon black was studied. The samples were calcined under reducing atmosphere and argon atmosphere at 1650 °C for 4 hrs respectively. The phase compositions and microstructure of the synthesized powders were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The result shows that adding excess carbon can enhance the formation of SiC. The argon atmosphere can promote the carbothermal reaction of forsterite and the growth of spinel crystal.

Abstract: Boron-doped BaTi_1-xB_2xO_3+X materials were prepared via organic chelating chemistry using alcohols as solvents/ligands. When sintered under air at 900 °C, those materials were light-yellow colored, and yielded the dielectric properties comparable to those of pristine BaTiO₃ sintered at 1300 °C. However, they displayed blue/gray color after sinterd under Ar or a reductive atmosphere like 5%H2/Ar at T > 850 °C, and the dielectric properties dramatically changed compared with the samples sintered under air, suggesting the transformation of insulating BaTiO3 into an semiconductor. By contrast, pristine BaTiO₃ samples differed slightly in the dielectric properties when the sintering atmosphere was shifted from air to Ar or 5%H2/Ar. The possible mechanism of the “reduction” behavior in BaTi_1-xB_2xO_3+X materials (the role of boron in reduction of Ti⁴⁺ to Ti³⁺ in BaTiO₃ lattice) is under investigation.

Abstract: Praseodymium salt was used as additives for the first time in preparing anodic aluminum oxide (AAO) films to improve its performance. AAO films were prepared by anodization method from a 15 vol.% sulphuric acid solution containing praseodymium salt. The effects of Pr concentration on microhardness and thickness of AAO film were investigated, respectively. The effect of heat treatment temperature on structure of AAO film was investigated. AAO films were characterized by XRD, EDAX and SEM techniques. AAO films showed higher microhardness and thickness, the surface of AAO film was smoother and the aperture of AAO film was more uniform than those of films prepared in 15 vol. % sulphuric acid anodization solution. The microhardness and thickness of AAO film were 355.7HV and 84μm respectively. The apertures of AAO film were in 25～30nm. There was not praseodymium in AAO film. AAO films were amorphous when heat treatment temperatures of AAO film were below 800°C. When heat treatment temperature of AAO film were 850°C and 1000°C respectively, AAO films were γ-Al2O3 and α-Al2O3 film respectively.

Abstract: Magnesia-magnesium aluminate (MA) spinel composite refractory was prepared by using fused magnesia, industrial alumina and AlCl₃•6H₂O as the raw materials. The effects of calcining temperature and holding time on phase compositions of MgO-MgAl₂O₄ system samples were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). With the increase of calcining temperature, the hydration mass increasing rate of the sample decreases significantly. The results show that MgO-MgAl₂O₄ composite refractory has quite excellent hydration resistance because of periclase grains with exsolved spinel phase.