Key Engineering Materials Vols. 512-515

Abstract: The preparation of interconnected mesoporous carbon monoliths (MCMs) derived from phenolic resin/ethylene glycol mixtures based on polymerization-induced phase separation have been investigated for fabrication of complex-shape SiC ceramics. The effect of the ethylene glycol content, curing catalyst and the curing temperature on the pore structure and pore distribution of carbon monoliths has also been studied, with emphasis on controlling the apparent porosity and pore size distribution. Fractal dimensions (DF) was proposed to evaluate the morphologies of carbon monoliths by using the box counting method. The results show that interconnected mesoporous carbon monoliths with narrow pore size distribution were obtained by changing the curing temperature and the content of ethylene glycol, curing catalyst in the resin mixtures and its mechanism was discussed in this paper. In this paper, interconnected mesoporous structure was attributed to the mechanism of spinodal decomposition (SD), which was discussed in detail. Carbon monoliths inherit their porosity from cured resins where it was formed as a result of phase separation of resin-rich and glycol- rich phases.

Abstract: The paper mainly focused on the two issues that restricted the practical application of superplastic ceramics, which were the low strain rate in superplastic forming as well as resulted severely cavitation in deformed materials. The alumina-based composites Al₂O₃-ZrO₂ (3Y) and Al₂O₃-30mol%ZrO₂(3Y)-30mol%MgAl₂O₄ (AZ30S30) were selected as research materials. The nano-sized composite powders were synthesized by heating of ethanol-aqueous salt solutions method. The superplastic forming tests under the compressive stress state were carried out to evaluate the superplastic formability of the as-sintered materials. The results demonstrate that the following conditions are the essentials for attaining high-strain-rate superplastic forming in alumina based ceramic composites: reduction in the initial grain size by second phase dispersion and insurance of a homogeneous microstructure, enhanced diffusivity by co-doped certain elements, suppressed dynamic grain growth in deformation, as well as provide new rate-controlling accommodation process in superplastic forming. The results also indicate during the superplastic forming the cavitation damage was eliminated because of compression stress state, which ensured the mechanical properties after deformation. Therefore, the postdeformation mechanical properties after superplastic forming were enhanced in some extent.

Abstract: Experiments were carried out to join reaction-bonded silicon carbide (RBSC) ceramic in green body. SiC green bodies were fabricated by gel-casting method. The slurries for joint were prepared by different grain size of SiC. The results indicate the solid loading and the grain size of SiC influence the strength of welding. There is no clear welding interface between the two substrates in an appropriate solid volume loading and grain size. The average bending strength of welding after sintering can reach 300MPa. SEM was used to observe the cross-section morphology and microstructure of binding sites. The test result shows that its microstructure is similar to the RB-SiC ceramic. The mechanical properties of binding sites were tested. The result shows a desirable mechanical property.

Abstract: C/C composite and TC4 alloy were successfully brazed using 70Ag28Cu2Ti (wt. %) as filler metal at brazing temperature 820 º°C~920 °C for soaking time 5 min ~30 min. The effects of brazing parameters on the microstructures and phase composition and the fracture modes of the brazed joints were investigated by SEM and XRD. The mechanical performances of the brazed joints were measured by a universal mechanical testing machine. The results show that the maximum shear strength of the brazed joint is 28 MPa at brazing temperature 860°C and soaking time 10min. Fracture surface analysis of the brazed joints indicates that the position of the fracture surface is related to the orientation of carbon fiber. The brazed joints are fractured in the C/C composite when the carbon fiber is parallel to the joined surface, and the brazed joints are fractured at the C/C composite / 70Ag28Cu2Ti interface when the carbon fiber is vertical to the joined surface.

Abstract: The cold ramming paste for aluminum electrolysis has been studied using electrically calcined anthracite as aggregate, medium temperature pitch and anthracene oil as binder, metallic silicon powder as additive in the paper. The effects of addition of silicon on volume density, compressive strength, electrical resistivity, shrinkage value have been studied. The results show that: volume density, compressive strength of cold ramming paste are increased firstly and then decreased with the addition of silicon increasing, and electrical resistivity, shrinkage value are decreased firstly and then increased with the addition of silicon increasing. So when the addition of silicon is 3%, the physical properties are preferable.

Abstract: The SiC_3D/Al composite with high SiC content were prepared by vacuum– pressure infiltration process, and the prediction model of infiltration pressure threshold involving capillary pressure and viscous resistance was established. The effects of model parameters on the critical infiltration pressure were analyzed. The results show that the main factors are the permeability coefficient of SiC performs and the temperature of molten aluminum alloy. Temperature affects the melt viscosity and the wetting angle between SiC and molten alloy. The ingiltration pressure threshold of the specific infiltration system had been calculated by this model, which is in agreement with experimental results.

Abstract: In this study, low-cost kerosene with low toxicity was used as organic solvent to research the solvent debinding behavior of ceramic compact produced by injection molding. The Influences of time, temperature and solid loading on the debinding rate were investigated, the mechanism of which was also discussed. The results show that: The debinding rate increased with increasing temperature and decreased with extended time; Diffusion is the controlling factor at the initial stage and temperature was the main factor influencing the reaction rate; Dissolution is the controlling factor and concentration difference was the main factor at the later stage; The higher the solid loading of green body, the slower the debinding rate and the lower the final proportion of binder removal.

Abstract: This paper compares the low-temperature aging performance of 5Y-TZP and 8Y-TZP by coprecipitation and solid-phase synthesis methods in the steam. Analysis the weight changes of samples using analytical balance, and the sample surface microstructure and phase changes with SEM, XRD. The results show that the content of Y2O3 and synthesis method all affect the aging properties of zirconia ceramics; solid-phase synthesis of 8Y-TZP has the best anti-aging performance.

Abstract: Semiconductor gas sensors utilize porous polycrystalline resistors made of semiconducting oxides.The working principle involves the receptor function played by the surface of each oxide grain and the transducer function played by each grain boundary. In addition, the utility factor of the sensing body also takes part in determining the gas response. Therefore, the concepts of sensor design are determined by considering each of these three key factors.ZrO2 is one of the most commonly used sensitive material, The performances of the different components Y2O3-ZrO2 are studied in this paper.The performances of YSZ Prepared with different Y2O3 powder(untreated and 1300°C preburned) and the Mechanism are explored in this paper.

Abstract: Electrospinning is an effective appoach to get one-dimensional nano structured materials.Thin PVA/zirconium oxychloride and yttrium nitrate composite fibers were prepared by using precurssor solution and electrospinning technique. Influence factors in PVA content and concentration of inorganic salts on nanofibers were discussed in this paper.After calcinations of the above precursor fibres, ZrO₂ nanofibres could be successfully obtained.The fibres were characterized by SEM, XRD, respectively.