Materials Science Forum Vols. 475-479

Abstract: In this paper, we have employed a kinetic Potts model to simulate sintering of ceramics in which grain growth and densification of microstructure are usually concerned about. Grain growth and pore evolution in three-dimension are simulated using the kinetic Potts model. Quantitative information about grain size distribution, density and surface area are also evaluated in evolution of the model. Morphologic changes and densification kinetics are used to verify the model.

Abstract: AlN-SiC Composites were obtained by hot-pressing at 1900ı for 1h in a nitrogen atmosphere. The influences of the amount of SiC on densification behavior and microwave attenuation characteristics were studied. The results showed that although the densification of composites decreased along with the increasing SiC content, it had no distinct influence on microwave attenuation characteristics. When the content of SiC increased, the attenuation of AlN-SiC composites increased within the range of 2 to 8 GHz. SiC is a kind of wide frequency attenuation agent.

Abstract: A2O3/YAG/ZrO2 eutectic Melt-Growth-Composites (MGCs) were unidirectionally solidified by the modified-pulling-down method (MPD) and the Bridgman type method, in which a crucible was brought down at different speeds. The microstructures and crystallographic textures were studied by field emission scanning electron microscopy (FE-SEM) and electron backscattered pattern (EBSP) method. The high-temperature strength was investigated by compression tests. All MGC rods show strong preferred growing orientation, although the structural size of eutectic microstructure among MGC rods was different. The high-temperature strength of MGC rods is dependent on orientation, compression temperature and strain rate. The high-temperature strength of MGC rods is controlled by the anisotropic strength of constituent Al2O3, as well as the structural size of eutectic microstructure.

Abstract: An all-oxide composite consisting of alumina fiber, alumina matrix, and barium zirconate interphase has been investigated. The barium zirconate interphase was applied on alumina fibers through coating via a sol-gel route. The incorporation of the coatings did not significantly influence the densification behavior of the composite under hot-pressing conditions. During the processing of the composite, the barium zirconate reacted in situ with alumina fiber and alumina matrix to form Ba-β-alumina platelets with an elongated morphology, which is propitious for crack deflection and thus toughness enhancement. The results reveal that it is possible to reduce fiber strength degradation by controlling the coating and densification processes.

Abstract: A process of fast uniform mixing & controllable colloidal forming of ceramics was invented to thoroughly avoid the crucial effect of temperature on colloidal forming of ceramics. The ceramic suspension was divided into two components A and B. The monomer was added into A, and the initiator was added into B. Under this condition, there were no reactions in separate A or B because of segregation of the monomer and the initiator. Both of the suspensions could keep good fluidity until they were mixed quickly and uniformly. The mixture would fast solidify under a determined pressure. The highly automatic equipment also had been developed for this process.

Abstract: A new ceramic gelcasting system was developed in this paper. The rheological behavior of alumina suspension with an addition of polymer was examined. Particular attention is given to the flexural strength of green body prepared from gelcasting in the presence of polymer.

Abstract: The origin of the inner stress in the ceramics green bodies prepared by colloidal forming was studied. The results show that the inner stress originates from the nonuniform shrinkage during the solidification of the precursor suspension and the drying of the green bodies, and the gradients of temperature, initiator concentration and moisture are the important original factors causing the inner stress. The inner stress could result in cracks in the green bodies during debinding.

Abstract: Ceramic gelcasting has to be performed in nitrogen to avoid surface-exfoliation and crack of the green body. The rapid drying of gelled bodies can cause nonuniform shrinkage. Non-uniform drying in various regions due to the solvent gradient, induces structural and residual stresses which cause defects, such as cracking, warpage and the other malformations. These malformations can be minimized or eliminated via adding a proper amount of water-soluble polymer polyethylene glycol(PEG).This study concentrates attention on dispersion, rheological property and gelation behavior in the new system, The flexural strength and microstructure of ZrO2 green bodies were measured and observed.

Abstract: Gelcasting is a novel ceramic forming technology that could be used to manufacture high-performance ceramics with complex shape. However, there are some problems in debinding of large-size green body made by gelcasting. In this paper, a new pretreatment way before binder burn-out was introduced. By means of this pretreatment processing, large-size green bodies with no defects were successfully obtained.

Abstract: Controllable forming technology is one of the key problems for the industrial application of gelcasting. In order to solve this problem, various kinds of influence factors in gelcasting were investigated. Trace ions, ionic strength, pH, dispersant agent, premix, amount of initiator and catalyst, temperature, pressure, materials of container and inhibitor all have influence on solidification process of ceramic slurry. The forming process of gelcasting can be controlled effectively by adjusting these factors.