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Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α- Si3 N4 powder with a various content ratios. The effects of the contents of the fibrous α- Si3 N4 powder (0%-100%) on the microstructure and mechanical properties of porous Si3 N4 ceramics were studied. As the increase of the fibrous α- Si3 N4 powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si3 N4 particle. XRD analysis proved the complete formation of single β- Si3 N4 phase. SEM analysis revealed that the microstructure of the low content of fibrous α- Si3 N4 porous ceramics was almost composed of fine elongated β- Si3 N4 grains with high aspect ratio while numerous coarse elongated β- Si3 N4 grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α- Si3 N4 powder increased. With the increase in content of the fibrous α- Si3 N4 powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties. [ABSTRACT FROM AUTHOR]

Porous β- Si3 N4 ceramics are sintered at 1600°C in N2 and postheat treated at 1500°C under vacuum using Li2 O and Y2 O3 as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li2 O. The addition of Y2 O3 is advantageous for the formation of high aspect ratio β- Si3 N4 grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β- Si3 N4 porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li2 O and 0.33 wt% Y2 O3, and it is suitable for high-temperature applications. [ABSTRACT FROM AUTHOR]

We show that water-based porosimetry ( WBP), a facile, simple, and nondestructive porosimetry technique, accurately evaluates both the pore size distribution and throat size distribution of sacrificially templated macroporous alumina. The pore size distribution and throat size distribution derived from the WBP evaluation in uptake (imbibition) and release (drainage) mode, respectively, were corroborated by mercury porosimetry and X-ray micro-computed tomography (μ-CT). In contrast with mercury porosimetry, the WBP also provided information on the presence of 'dead-end pores' in the macroporous alumina. [ABSTRACT FROM AUTHOR]

This study describes the results of an investigation into the effects of the addition of rod-like silica nanoparticles on the properties of freeze-cast and sintered bodies formed from silica nanospheres. Rod-like silica particles with ∼220 nm diameter and tunable aspect ratio from ∼1 to ∼12 (length/diameter) were prepared and added to aqueous suspensions containing 22 nm spherical silica particles. After freeze casting, porous composites were created with all suspensions, which is shown to be consistent with the results of a simple analysis in which the experimental freezing rate is compared with the critical rate at which the dispersed particles can no longer be expelled from the growing ice front. The composites have elongated spherical pores, and the pore size changes slightly with increasing aspect ratio of the nanorods. Finally, it was found that the rod-like particles improve the flexural strength of the composites at both green and sintered states and this strengthening effect intensifies with increasing aspect ratio. This study provides a strategy for fabricating porous materials of improved properties and performance without compromising the porosity or changing the material composition. [ABSTRACT FROM AUTHOR]

This investigation focused on the synthesis and characterization of a modified polyvinylsilazane ( PVSZ) polymer. Previously, our group synthesized the PVSZ polymer through the ammonolysis of vinyltrichlorosilane ( VTS) in tetrahydrofuran ( THF). This material showed promise as a precursor for silicon nitride-silicon carbide-based ceramic materials. Due to the structure of the PVSZ polymer, the ability to stabilize (shelf-life) and dope with metals and metalloids has proven to be difficult. To overcome these problems, a second chlorosilane precursor, dichloromethylsilane, was introduced into the ammonolysis reaction. This polymer proved to be more stable (>3 months before cross-linking) and added a potential doping site on the backbone of the polymer while maintaining ceramic yields greater than 80%. The polymethylvinylsilazane ( PMVSZ) polymer was characterized using attenuated total reflectance ( ATR) spectroscopy, Fourier transform infrared ( FT- IR) spectroscopy, gel permeation chromatography ( GPC), thermogravimetric analysis ( TGA), X-ray diffraction ( XRD), and elemental analysis. The results indicated that the ratio of chlorosilane precursors used plays an important role in the composition of crystallized ceramic material. [ABSTRACT FROM AUTHOR]

Nonoxide Si CN hollow nanospheres were prepared from a block-copolymer comprising of a precursor block and a sacrificial block. The copolymer was first synthesized and then self-assembled into core-shell structured micelles with the sacrificial block forming the core and the precursor block forming the shell. The micelles were subsequently pyrolyzed at high temperatures to transform the shell into ceramics and to completely decompose the core to form a hole. The technique is probably applicable to other material systems. [ABSTRACT FROM AUTHOR]

We report the utility of three-dimensional ceramic/camphene-based coextrusion, newly developed in this study, for the production of unidirectionally macrochanneled alumina ceramics with three-dimensionally interconnected porous alumina walls. In this technique, a continuous ceramic/camphene filament with a diameter of 1 mm, comprised of a pure camphene core and a frozen alumina/camphene shell, was produced by the coextrusion process and then deposited in a layer-by-layer sequence using a computer-controlled 3-axis moving table. Unidirectionally aligned macrochannels (~400 μm in diameter) and three-dimensionally interconnected pores (several tens of micrometers in size) in the alumina walls were created by removing the camphene core and the camphene dendrites formed in the alumina/camphene region, respectively. The sample showed much higher compressive strength in the macrochannel direction than in the perpendicular direction. In addition, the compressive strength of the sample could increase with an increase in initial alumina content owing to a decrease in the total porosity. [ABSTRACT FROM AUTHOR]