Papers by Author: Jian Feng Tong

Abstract: A new technology, gel solid-state reaction, for synthesis of SrTiO₃ ceramic composite powders is described in the present paper. This process, valuable for a great number of composite powders, is of great simplicity and has a relative low synthetic temperature of powders. The calcined lumped powder is porous and easy to grind. XRD and d-spacing-component figures of the solid solution powder demonstrated that the compounds were mutually miscible in the solid solutions, and SEM showed that they were uniform and substantially spherical with an average size of 1.0 μm in diameter. The remarkable advantages of the method are high yields, solvent-free, and environmental benign.

Abstract: Platelet alumina powders have been synthesized by introducing AlF₃ as crystalline transformation additive. The influence of the additive contents on the phase transformation of the start material Al (OH)₃ was studied by SEM, the effect of calcination temperatures on the phase transformation and microstructure of alumina powder were investigated by TG-DSC, XRD and SEM. The results indicate that uniform platelet powders could be abtained while more than 15% AlF₃ was introduced. When the calcination temperature higher than 1100°C pure α-Al₂O₃ could be synthesized, comparing to the tranditional preparation of platelet alumina powders, the remarkable advantages of the method are low calsination temperature, small particle size and simple process.

Abstract: BaTiO₃ ceramic composite powders were prepared by a gel-solid reaction method using the raw materials of two particle sizes, DSC was used to study the thermal decomposition of the green body. The effect of the calcination temperatures on the phase structure of the powders was investigated. The influence of the particle sizes of the raw materials on the reaction temperature and the microstructure of the powders was explored. The results showed that reaction temperature of BaTiO₃ ceramic powders reacted from BaCO₃ of the large size was about 933.0°C, and the one from BaCO₃ of the smaller size was about 903.5°C, respectively. XRD results indicated that BaTiO₃ powders could be prepared at a temperature range of 900°C ~ 1000°C using the raw materials of either size. The particle size of the synthesized powders was determined by the particle size of the raw material. Therefore, the raw materials of small size should be chosen to prepare the synthesized powders of small size, and at a lower reaction temperature.

Abstract: Barium strontium titanate/magnesium oxide composite ceramic plates were prepared by non-aqueous gel-casting using barium strontium titanate and MgO as the raw materials. The rheological properties of the non-aqueous slurries, the dosages of the catalyst and initiator, micro structures and phase structures of the sintering ceramic plates were investigated and analyzed. The results showed that the slurry with solid content of 50wt% in non-aqueous gel-casting had excellent rheological properties and operating characteristics. Ba0.6Sr0.4TiO3/MgO plates with a smooth surface and dense structure were fabricated through desiccation and sintering at 1300°C, and the relative density is up to 4.14g/cm3. The polished surface of the ceramic substrate was composed of 35% MgO and 65% Ba0.6Sr0.4TiO3, and the content of Ba0.6Sr0.4TiO3 was obviously higher compared to the content in the raw materials which was explained by the phenomena of surface phase enrichment.

Abstract: Arc-shaped integrated alumina ceramic armor for body bulletproof with the size of 300 × 250 × 6 mm, R400mm were prepared by semi-aqueous gel-casting. The excellent characters and advantages of the semi-aqueous gel-casting were analyzed; ballistic experiments were carried out to test the protection coefficient of 92%, 96%and 99 alumina ceramics. The experimental results showed that the 92% alumina ceramic and the 99% alumina ceramic had favorable performance while the 96% alumina ceramic had the worst one. But the 92% alumina ceramic had the highest protection coefficient which is 9.96.

Abstract: A technique for the preparation of porous ceramic materials by gelcasting mothod is studied using a water-soluble epoxy resin combined with a amine hardener. The increase in the volume and rheological behavior of slurry , TGA and DSC of the green bodies are investigated. The influence of solid content on the relative density, the cell size distribution and the compressive strenghth is studied. The experimental results show that in the measurable shear rate range the rheology of slurry exhibt a shear thinning behavior, the relative density increase with the solid content, with the decrease of the relative densities the cell size increase. The compressive strength increased exponentially with the increase relative density, and the exponent, m, is 1.094.

Abstract: A process is adopted to grow whiskers on carbon fibers. The whisker growth is carried out in a two-step process. Plasma spraying, electroplating and magnetron sputtering are used to pretreat the surface of carbon fibers and then ZnO whiskers are grown by hydrothermal method. By comparing three different methods of surface pretreatment, it is concluded that magnetron sputtering is most available and has no damage to the carbon fibers. Combining preparation method of the ZnO whiskers, a new process is developed to surface modified of carbon fibers which is expected apply to carbon fiber composites.

Abstract: The influence of Lu2O3 on phase transformation and seeds morphology was investigated. The result showed that the β-Si3N4 seeds with up to 95% β phase content could be obtained with 2wt% Lu2O3 as the additive content under 1750°C for two hours. The microstructure and mechanical properties of hot-pressed Si3N4 ceramics, using 9wt.% of Lu2O3• additives were investigated by the means of MTS measurements and Vickers indentation crack size measurements, as well as XRD and SEM. It was known that the high fracture toughness of Si3N4 ceramics was attributed to the rodlike morphology of β-Si3N4 grains. And the reinforcement effect and mechanism of β-Si3N4 seed were studied. It was found that the grain size and its distribution influence the property and microstructure of Si3N4 ceramics, namely, the relative narrow distribution of grain diameter in some extent and relative wide range of bimodal distribution of grain aspect ratio could improve the property of Si3N4 ceramics. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.The high temperature properties of self-reinforced Si3N4 with different additives were studied. By this method, self-reinforced Si3N4 ceramics with an increment of 10~20 percent of fracture toughness was successfully fabricated.

Abstract: The dynamic process of crack initiation and propagation in a SiC/BN-Al2O3 laminated composite was observed in situ by scanning electron microscopy. During a bending test with a single-edge notched-beam specimen, an interfacial crack first initiated in the interlayer near the notch tip, after which a through-thickness crack formed in the matrix layer at the notch tip. After the through-thickness crack had grown across the first matrix layer, it was deflected by the next interlayer and again became an interfacial crack. Interfacial cracks and through-thickness cracks were generated alternately until the composite failed. The load-displacement plot of the laminated composite exhibited several peaks, each caused by one propagation of a through-thickness crack. The toughening mechanisms of the laminated composite included crack deflection, interfacial cracking, and through-thickness branch cracking.

Abstract: A new technique for the fabrication of yttria stabilized zirconia (YSZ) substrates using aqueous gel-casting has been developed. This technique has been used to fabricate planar thin-substrate YSZ fuel cells. A thin-substrate YSZ electrolyte with high density and low porosity was prepared by this method with 57vol% solid content slurry. The character of the technique was discussed based on the influence of dispersant and pH value on slurry. After sintering, the YSZ electrolyte thickness is between 100 and 200µm, and the electrolyte area is 100 × 100 mm2. The research shows that aqueous gel-casting allows fabricate thin YSZ substrate with high density and homogenous structure. The method is suitable for preparing thin-substrate electrolyte of yttria stabilized zirconia.