Key Engineering Materials Vols. 512-515

Abstract: SiC powder can be produced generally through the Acheson process and it is required long carbothermic reaction time of SiO2 with carbon powder around 2200 °C ~ 2400 °C. Due to the high reaction temperature and long reaction time of the process, the powders produced have a large particle size and consist of mostly alpha phase SiC. Synthetic temperature of beta phase SiC powder is known to produce at 1700 °C ~ 1900 °C which is lower temperature than that of alpha phase SiC powder. We prepared β-SiC powder by heating precursor derived from the mixture of phenolic resin and tetraethyl orthosilicate. The precursor was heated at 1800 °C for 4 h in an Ar atmosphere. In order to examine the pyrolysis residue after the heat treatment, the SiC powder was analyzed with XRD and SEM. The X-ray diffraction result of the SiC powder shows the diffraction peaks around 35°, 60°, and 73° corresponded to the beta SiC phase. β-SiC powder prepared in this study contains lower metallic impurities compare than that of α-SiC powder prepared from Acheson method and is able to use as a good starting material for SiC single crystal growing.

Abstract: Fiber-like SiC had been synthesized using used silica bricks and carbon powder as raw materials by carbothermal reduction. A new kind of beaded SiC also had been found in the specimen. The result showed that well-crystallized β-SiC fibers began to appear at 1550 °C with excessive of 100 wt% content of carbon. While at 1600 °C for 6 h with excessive of 100 wt% content of carbon, the as-synthesized SiC fibers were grown with some beads in the fiber strings. The beaded SiC consisted of strings with diameters of less than 0.5 μm (or even 0.1μm) and periodic beads with diameters of 0.5-1.0 μm. Vapor-solid (VS) mechanism and vapor-liquid-solid (VLS) mechanism were also discussed in the synthesizing process of fiber-like SiC. This kind of fiber-like SiC may used as reinforce materials in ceramic composites, the fracture toughness of brittle ceramics can be effectively improved via the incorporation of strong SiC fibers into the matrix due to crack deflection, bridging and fiber pull-out. Moreover, these toughening mechanisms do not diminish as temperature increases.

Abstract: Superconductor samples Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) are synthesized from nano SiC, Mg and amorphous boron powders by microwave direct synthesis in a short time. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO and Mg₂Si. The main peaks of MgB₂, (100), (101), (002) and (110) are shift to the higher diffraction angle position and the width of half height of the diffraction plane is broaden for the SiC doping Mg(B_1-2x(SiC)_x)₂, which show that the B positions of MgB₂ are partly substituted and the grains of MgB₂ are fine. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is very small and the sample is tightness (compact). The onset superconducting transition temperature of the Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples measured by magnetization measurement are about 37.6 K, 37.0 K, 36.8 K respectively. The critical current density J_c are calculated according to the Bean model from the magnetization hysteresis loop of the slab Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples. The critical current density Jc of nano SiC doping Mg(B_1-2x(SiC)_x)₂ samples are greatly enhanced. In higher external magnetic field, the J_c of 10% SiC doped sample is the highest; in lower external magnetic field, the Jc of 5% SiC doped sample is the highest; while in the whole external magnetic field, the J_c of undoped sample is the lowest.

Abstract: α-Si₃N₄ possesses excellent sintering activity, which is used to prepare high performance Si₃N₄-based ceramics and composite refractory. Si₃N₄ powder is always synthesized by nitriding silicon in controlled-atmosphere furnace whose furnace volume is very small(effective volume: 1840×1420×1660mm), the extreme reaction heat is difficult to diffuse, which leads to high reaction temperature and conversion of α-Si₃N₄ to β-Si₃N₄, thus α-Si₃N₄ is difficult to be obtained in controlled-atmosphere furnace. While flame-isolation nitridation shuttle kiln has much larger furnace volume to conduct reaction heat (effective volume: 11500×4190×1684mm), so it owns homogeneous temperature field and stable low-temperature environment which benefits the preparation of α-Si₃N₄. Thermodynamic analysis of Si-N system is shown that Si₃N₄ can be formed by two formats: direct nitridation of Si(s) and indirect nitridation of SiO(g); to ensure completely nitridation, the particle size of silicon powder should be less than 88μm. With reclaimed powder from polysilicon cutting slurry as starting materials, both reactive α-Si₃N₄ and SiC mixed powder were successfully prepared in flame-isolation nitridation shuttle kiln. Because of the gas-gas reaction between SiO(g) and N₂(g), α-Si₃N₄ is fiber-like and in favor of processing high quality Si₃N₄-based materials.

Abstract: Abstract. Aluminum nitride(AlN) powders were synthesized by carbonthermal reduction of Ammonium aluminum carbonate hydroxide(AACH). The AACH were prepared from ammonium alum and ammonium hydrogen carbonate by precipitation method and AACH adhere to carbon black during precipitation. The precursor has a high reaction activity (near 100% of nitridation ratio after heated in flow nitrogen at 1400°C, 2h). After carbon remove in muffle furnace at 700 °C, white color Aluminum nitride (AlN) powder attained and mean size is 100 nm. The specific surface area of the powders decreased with increasing of concentration of ammonium alum and ammonium hydrogen carbonate, which range from 22 m2/g to 7 m2/g, particle size vary from 58 to 120 nm. Phase presents in the products during heating in flowed nitrogen were observed by X-ray diffraction. The -Al2O3 formed when the precursor heated to 1200°C, AlN was founded at 1300°C and the reaction ended at 1400°C, the reaction temperature and annealing time were much lower than nitridate the mixture of Al2O3 and carbon black. Particle size was increased when reaction temperature increased from 1400°C to 1550°C.

Abstract: A high purity of Ti₂AlC powder has been synthesized by pressureless sintering a mixture of Ti-Al-TiC-Sn (Sn as an additive) powders. Four recipes with different mole ratios of Ti-Al-TiC-Sn were examined at sintering temperature from 1400°C to 1480°C. A high purity of Ti₂AlC powder can be obtained by sintering all these four recipes at temperature 1450°C for 10 min in an Ar atmosphere. The synthesis of Ti₂AlC on this large mole ratio scale of starting materials is associated with the evaporation of Al at high temperature and the structure stability of Ti₂AlC. From the X-ray diffraction analysis, a reaction path for the Ti₂AlC formation is proposed. Scanning electron microscopy was also used to characterize the samples.

Abstract: Ti(C,N) powder was prepared via carbothermal reduction nitridation (CRN) using rutile and carbon black as the raw material. The phase evolution and the reaction mechanism during the CRN synthesis of Ti(C,N) were investigated, and the effect of reaction temperature and C/TiO₂ molar ratio on the phase composition and x value in TiC_1–xN_x was analyzed. The XRD and SEM results show that: Ti(C,N) powder was synthesized at 1500°C for 4h with the C/TiO₂ molar ratio of 2.2, under the nitrogen pressure of 0.2MPa. Irregular granular structure and the growth stripes were observed in the final products. The growth of Ti(C,N) grains in CRN process was followed by the gas-solid mechanism.The phase compositions of the products were quite dependent on the reaction temperature and the C/TiO₂ molar ratio. The TiN content in Ti(C,N) decreased with the increase of reaction temperature. TiC_1–xN_x powder with different x values can be synthesized by optimizing the experiment conditions including the synthesis temperature and the C/TiO₂ molar ratio.

Abstract: The high-pure alpha-alumina powders with some agglomerate were treated by bead milling and their sintering behaviors were studied in this paper. During the treatment, the effect of different types and different amounts of dispersants was also investigated. It was shown that optimum dispersant content was 0.5% citric acid and optimum milling time is 6 hours. The sintering property of alumina powder after bead milling was significantly improved. Their relative density after sintering at 1500 degrees can reach 99.3% while the raw powders’ can only reach 90% at same condition.

Abstract: This paper describes the synthesis and surface characterization of a new silica white prepared from a high Si:Al fine powder by using the sulfuric acid precipitation method. In the beginning, water and alkali are added to the sieved high Si:Al fine powder. The mixed liquid is then heated until it is boiling and left alone at the same temperature for 2.5 hours before it is filtrated. Applying liquid NaCl (20%) to the filtrated solution before it is heated to 80°C. Applying certain amount of surfactant and H₂SO₄ (20%) while maintaining the temperature at 80°C for 3 hours before it is washed, precipitated, filtrated, and dried to produce the final product of silica white. Examinations with XRD and IR show that the silica white is amorphous silicon. It has a PH value of 7-7.5 determined by aqueous suspension method, a white index of 96% and above, a volume average particle diameter of 4.6 - 7.1 µm, a specific surface area of 95.5-103.1 m2/g, an oil absorption rate of 4.34-4.87 ml/g, and a LOI (loss on ignition) of 5.12% at 1000°C. The synthesized silica white has a satisfying uniformity of particle sizes, meeting standard of HG/T 3061-1999.