Key Engineering Materials Vols. 280-283

Abstract: Rheological behaviors of alumina aqueous suspension were investigated, and some methods to modify the rheological behaviors of the suspensions were studied. It was found that there is a critical solid volume fraction for alumina aqueous suspensions. When the volume fraction reaches or exceeds the critical value the suspensions show shear thinning behaviors all along, while above which the rheological behaviors of alumina suspensions change from shear thinning to shear thickening.

Abstract: The effects of solid volume fraction (SVF) on the gelation of alumina suspensions for gelcasting, debonding and sintering of the green body were studied. It was found that with SVF rising, the gelation of alumina suspension delayed; and the strength of green body decreased. On the other hand, high SVF resulted in that polymerized acrylamide split at a relative low temperature. These phenomena manifest that the fast polymerization of monomers in high SVF alumina suspension was inhibited, and the flexibility of the gelcasting was improved. However, Excessive solid volume fraction was prone to a bad rheological behavior of alumina suspension, and deteriorated the microstructure and properties of sintered body.

Abstract: In this paper, the α-Al2O3 microfiltration membrane modified by nano-ZnO coating was obtained using zinc nitrate and urea as raw materials via homogeneous precipitation method. The effects of reaction temperature and coating times on the modification of the membrane were emphatically studied. The test results show that the flux of modified membrane is raised by 43% when the nano-ZnO particles (5 ~ 10 nm) are smoothly and densely coated on the surface of the membrane. The mechanism of the modification was studied by means of TEM, FTIR, XPS, Zeta electric potential analyzer, etc.

Abstract: A commercial a-alumina powder was mixed with Zr(OH)4 gel synthesized by a precipitation method. The mixture was calcined at 600°C for 2h. The Al2O3−15 wt% ZrO2 composites were sintered for 2 h in the temperature range between 1400°C and 1650°C in air. At the sintering temperature of 1550°C the highest density, the best wear resistance, the maximum bending strength of 708 MPa and the highest fracture toughness of 5.8 MPa×m1/2 were achieved, respectively. Improvement of the mechanical and wear properties of the Al2O3 matrix should mainly be attributed to the addition of ZrO2.

Abstract: New nano-submicron textured Al2O3-ZrO2 ceramic eutectic composites were obtained from high-temperature melts produced by the SHS metallurgical process with intro-granular and 1-3 composite structures. Nano-micron textured ceramic composites were naturally self-assembled in one quick step by the SHS metallurgical process, in-situ synthesis and symbotic eutectic transformation under high degree of undercooling. The experimental results indicated that the Al2O3-ZrO2 ceramic eutectic composites with ZrO2 fibers at the nano-submicron scale are only suitable for the ceramic composite with hypoeutectic composition under the conditions of the experiment.

Abstract: Microstructure and mechanical properties of zirconia-toughened alumina (ZTA) composite ceramics prepared by gelcasting process were investigated. The results indicated that Al2O3-ZrO2 composites have high bending strength and fracture toughness for the solid loading of the suspension ³ 50vol.%. The bending strength and fracture toughness of sintered ZTA ceramics reach values as high as 631.5 MPa and 7.64 MPa·m1/2, respectively, with solid loading of suspension being 55vol.%. The t-ZrO2 content is increased, and the uniformity of ZrO2 particle distribution is improved in sintered samples prepared by gelcasting compared with those by dry pressing.

Abstract: A new kind of polymer/layered silicate (PLS) nanocomposites-containing slurry was used to join green bodies of ZrO2 (8YSZ) and 95Al2O3 ceramics, without applied pressure. After sintering, a fine joint could be obtained without obvious cracks, pores and other defects, and the average flexural strength of the joints was comparable to that of the 95Al2O3 matrix. Effects of PLS on joining process were analyzed and the results showed suitability of PLS-containing slurry for green state joining ZrO2 to Al2O3 ceramics.

Abstract: Freeze casting is a useful forming technique to produce complex-shaped ceramic parts. The colloidal processing stage is an essential step for preparation of freeze casting slurries. In order to optimize freeze casting slurries, several processing additives, such as dispersant, binder, surfactant, and cryo-protectant are usually employed. Dispersion and suspension rheology of aqueous alumina/ zirconia slurries with 55 and 60 vol.% solid loading have been characterized in terms of the effect of organic additives, and the resulting microstructures of the freeze cast green and sintered materials examined.

Abstract: The effect of microstructure on the wear resistance of two Al2O3 dispersed in ZrO2 matrix (ADZ) ceramics was investigated, Al2O3 dispersed in commercially co-precipitated 3mol% Y2O3-doped tetragonal zirconia polycrystal (CYADZ) and Al2O3 dispersed in mixed 3mol% Y2O3-doped tetragonal zirconia polycrystal (MYADZ). These materials were used as a pin sliding against a plate of polyurethane rubber (UR) in the presence of aqueous NaOH solution containing SiO2 as the abrasive medium at a sliding speed of 0.15m/s and a normal load of 100N. The results indicated that the CYADZ specimen had uniform microstructure and fine grain size, which accounted for its better abrasive wear-resistance than that of the MYADZ specimen with non-uniform microstructure and largely different grain sizes due to inhomogeneous distribution of Y2O3. The hardness and toughness of two ADZ specimens were close, but strength of the MYADZ specimen was lower than that of the CYADZ specimen, which indicated the defect was serious in the MYADZspecimen. The CYADZ specimen was characterized by plastic deformation and micro-ploughing, which the MYADZ specimen dominated by micro- cracking and brittle fracture.

Abstract: Stable oxygen concentration in exhausts is a decisive factor in catalytic purification of the vehicle exhausts. Nanostructured CeO2 is generally added into catalytic coating on a ceramic support to adjust the oxygen concentration because it possesses a unique oxygen storage capacity (OSC). However, a small amount of CeO2 gets insufficient when the oxygen concentration fluctuates in a large range. In the present work, the cordierite-CeO2 composite ceramic with oxygen storage capacity was prepared for a support material of catalytic converters. The oxygen storage-release performance of the ceramics, including temperature of releasing oxygen, adsorption process of oxygen and the oxygen storage, was examined by a temperature-programmed gas chromatography. The composition of the ceramics was analyzed by an X-ray powder diffraction (XRD) and a scanning electron microscope (SEM) to validate the reliability of the ceramic as the support material with OSC. Results show that the ceramics consist of α-cordierite and CeO2 phases, the latter of which is uniformly dispersed throughout the cordierite matrix. The ceramic powder with 10-20wt% of CeO2 possesses the expected oxygen storage capacity. It is suggested that this novel cordierite-CeO2 composite ceramic is helpful of improving the purification effect of vehicle exhausts.