The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike P-Si,N, singlecrystal particles with a diameter of 1 pm and a length of 4 pm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of P-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation D" -Do" = kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4-8.7 MPa. m'", retaining high strength levels of about 1 GPa.

We have developed a new ecofriendly fabrication process for porous ceramics using hydraulic alumina (HA) and water. In the present study, we fabricated porous alumina ceramics using this new process. A boehmite gel 3D network was formed by the hydration of HA in HA slurry. The HA slurry was hardened by the formation of this 3D network. Even without the addition of an organic binder, green bodies containing the 3D network demonstrated high compressive strength. Furthermore, the water acted as a fugitive material in the green bodies.Consequently, the open porosity of sintered alumina ceramics could be controlled over a wide range of 56.6 69.1÷ by addition of water (ratio of water to HA powder: 1.0 to 2.0 by weight) without the use of organic fugi tive materials. The results of evolved gas analysismass spectrometry measurements showed that the emissions from the hardened green body mostly consisted of water. Consequently, the new fabrication process for porous alumina ceramics was confirmed to be ecofriendly.

A slurry used to produce dense green compacts by slip casting should exhibit low viscosity, high solids content, and good dispersion. Slurries with good characteristics were produced in the present study by adding oligosaccharide alcohol to an Al 2 O 3 slurry with an NH 4 ؉ salt of poly(methacrylic acid) (NH 4 ؉ -PMA). The role of NH 4 ؉ -PMA and oligosaccharide alcohol in the Al 2 O 3 slurry was examined by DTA, -potential measurement, high-pressure liquid chromatography, and viscometry. The viscosity of the slurry with NH 4 ؉ -PMA and oligosaccharide alcohol was lower than that of the slurry with NH 4 ؉ -PMA at a high solids content. Oligosaccharide alcohol did not interact with the Al 2 O 3 surface. However, the Al 2 O 3 slurry with NH 4 ؉ -PMA was influenced by the addition of oligosaccharide alcohol. We found that the dispersibility of the slurry was greatly improved by adding oligosaccharide alcohol. The transmittance of the Al 2 O 3 ceramics produced by slip casting using the slurry with both NH 4 ؉ -PMA and oligosaccharide alcohol was higher than that of ceramics produced by slip casting using the slurry with NH 4 ؉ -PMA alone. The increased optical property resulted from low viscosity, which was attributed to the addition of oligosaccharide alcohol, at a high solids content.

The extrusion behavior of hydraulic alumina HA pastes prepared by hydration of the HA was studied in order to fabricate the porous alumina ceramics by an extrusion technique. While all the HA paste samples cured for over 8 h were hardened by hydration, different extrusion behaviors were observed. In the early stage of hydration 24 h , water separation from the hardened HA paste was remarkable and the paste was not extruded clearly. On the other hand, even without the addition of an organic plasticizer, the hardened HA pastes cured for a long period 48 h showed good extrusion behavior. Furthermore, the water acted as a fugitive material in the extruded green bodies. Consequently, the open porosity of the sintered at 1400? C for 2 h alumina ceramics could be controlled over a range of 66.9-71.7! by the addition of water ratio of water to HA powder: 0.8 to 1.4 by weight without the use of organic fugitive materials.