Key Engineering Materials Vols. 336-338

Abstract: A new environment friendly method to synthesize sialon composites using fly ash with high content of Al2O3 by carbon-thermal reduction-nitridation was presented. The results showed that sintering temperature and the pressure of N2 gas are in favor of the nitrification synthesis of Sialon composites. When Samples wear buried in silicon powder and sintered at 1400°C under N2 atmosphere, Sialon (Si4Al2O2N6) had been synthesized.

Abstract: A combustion synthesis process was developed for the synthesis of magnesium silicon nitride powders in the Mg-Si-N system. The effects of ammonium halide addition and the nitrogen presssure on the phase composition and microstructure of the final product were discussed in detail. It was found that ammonium halides were helpful to enhance the nitrogen infiltration, decrease the agglomeration degree and improve the nitridation reaction. Furthermore, the nitrogen presssure greatly influenced the reaction process and the properties of the final MgSiN2 products, especially the phase composition. MgSiN2 powders with low level of impurities (MgO and free Si) could be synthesized through combustion synthesis process in the Mg-Si-N system.

Abstract: A new method for producing silicon carbide platelets with low cost and high yield was introduced. The silicon carbide platelets were synthesized by powder-heating techniques with carbon black and SiO2 powder as raw materials and CoCl2 as catalyst. The starting mixtures were heated at a temperature in the range of 1800-2000°C for the duration of about 2-4h to produce substantially completely unagglomerated silicon carbide platelets with a thickness of 5-20μm and the average diameter of 50-200μm. Compared to the conventional heating, the powder-heating technique is advantageous of less investment on equipment, easy to manufacture and convenient to operate. Furthermore, it is very suitable for realizing the scaled production because of the lower synthesizing temperature, shorter reaction time and greater output.

Abstract: Ultra-fine titanium diboride (TiB2) powders have been prepared by carbothermal reduction reactions in TiO2-B2O3-C system using tetrabutyl titanate, boric acid and phenolic resin as the solution-derived precursors. The reactions were substantially completed at relatively lower temperature (<1400°C) and the resulting products had a smaller average crystallite size (< 200 nm). However, below 1100°C, titanium carbide was the predominant phase and the relative content decreased with the rise of temperature. The thermodynamic change in TiO2-B2O3-C system was mainly studied by TG-DTA and the mechanism of synthesis of TiB2 was discussed. The crystallite size and morphology of the synthesized powders were characterized by SEM and TEM.

Abstract: A synthesis method of high-pure Ti3AlC2 powder with a large scale was presented as pressureless synthesis using TiC, Ti and Al powders as starting materials at 1300~1450°C in flow Ar. The different raw material scales were tested from 2 g to 1.0 kg, and the results showed that the purities of Ti3AlC2 powders could be kept around 97 wt% at different scales of raw materials. SEM observation and particle size analysis revealed that the synthesized Ti3AlC2 powders were uniform and around 5 μm in particle size. This method can be used for mass production of Ti3AlC2 powders with high-purity.

Abstract: A search for Ti3Si1.2-xAlxC2 (x=0~1.2) solid solution was undertaken using precise X-ray diffraction measurements. The samples covering the whole concentration range were studied. Except very ends of the concentration range, the samples contained two phases, identified as Ti3Si1.2-xAlxC2 solid solution and TiC respectively. Lattice parameter, a increased, c increased, c/a increased, and cell volume increased with the increasing of Al concentration.

Abstract: Ti2SnC has been fabricated from Ti, Sn and graphite elemental powders by mechanically activated low-temperature synthesis (MALS) technique. Superfine powders were obtained after milling the elemental powders for only 1 h with a charge ratio of 20:1. The mechanically alloyed powders were then pressureless sintered at different temperatures at Ar atmosphere for 0.5 h. High content of Ti2SnC was obtained at 950 oC, which is lower than the previously reported temperatures of above 1200 oC by sintering the conventional mixture powders. The microstructure shows that Ti2SnC grains with plate-like shape and smooth surface are less than 5 μm in size. The result demonstrates that the MALS is a novel method for the synthesis of Ti2SnC or other ceramic powders.

Abstract: High-pure bulk Ti3SiC2 samples were fabricated by directly hot-pressing (HP) high-pure Ti3SiC2 powder without any additives at 1200°C to 1500°C for 0.5–2 hours in flow argon atmosphere. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase identification and microstructure evaluation. The fabricated Ti3SiC2 materials have relative high density with high purity, flexural strength of 500-700MPa and fracture toughness of 9-12MPa·m1/2. The influence of sintering temperature and soaking time on the mechanical properties of Ti3SiC2 materials was discussed. The sintering mechanism for Ti3SiC2 powder without any additives was considered to be related with the fragile-ductile transformation of Ti3SiC2 at 1100°C.

Abstract: Anhydrite powders with nanostructure surface were prepared in Ca(OH)2-H2O-CO2 system. The effects of preparation parameters such as Ca(OH)2 concentration, revolution speed of mixer, temperature on surface finish and reaction speed were investigated. SEM was used to observe the changes of the morphology of anhydrite powders before and after surface coating. The results show that the appropriate condition is: the concentration of Ca(OH)2 should be 2~3%, the temperature between 20~25°C is better, the revolution speed should be big enough and so on. In order to gain nanostructure surface, appropriate technological parameter should be chosen carefully. The SEM photographs show that Anhydrite powders had fine surface morphology after surface finish that the smooth cleavage planes were coated with nanometer particles and the sharp edge angles were blunt by the coating of CaCO3 nanometer particles.