Key Engineering Materials Vols. 317-318

Abstract: The sintering stress is related to the thermal stability of porous structure. The sintering stress for a given porous structure in equilibrium can be calculated by three methods theoretically; the energy difference method, the curvature method, and the force balance method. The sintering stresses by three different methods were exactly the same for the idealized porous materials in equilibrium, in which the pore surface had a constant curvature at any point. The porous material does not spontaneously shrink when the sintering stress becomes zero or negative. The sintering stress will be used to design optimal porous structures with improved thermal stability.

Abstract: Porous alumina ceramics with unidirectionally oriented pores were prepared by an extrusion method. Pastes for extrusion were prepared by mixing alumina powder with nylon 66 fibers (av. 19 μm in diameter), together with dispersants and binders. The resulting pastes were extruded, dried at room temperature, freed from the binder at 600 °C and fired at 1500 °C for 2 h. The pore size distribution of the samples showed pores of 16 μm diameter, in good agreement with the original nylon 66 fibers. The pore size observed from SEM photographs of the sintered body was 16 μm corresponding to the burnt-out nylon 66 fibers. A very high degree of cylindrical pore orientation was seen in the SEM micrographs of the samples. The orientation increases with decreasing fiber loading.

Abstract: Porous alumina green bodies were fabricated by a gel-casting method, for which the slurries of alumina (AKP-30) and PMMA (polymethylmetacrylate) bead were mixed and dispersed at 1:1, 1:2, 1:3, and 1:4 volume ratios. PMMA bead as precursor of carbon source was used for growing SiC nano-fiber or nano-whisker during a VLS (vapor-liquid-solid) reaction at 1450°C for 9hrs. PMMA beads are converted to carbon particles after calcination at 1300°C for 9hrs in static argon (Ar) atmosphere. Finally, carbon particles remain in pores of alumina bodies connected by continuous pore channels. Fe solution was infiltrated into the porous alumina bodies, which are absorbed on the carbon particles in pores. Fe precursor functions as a seed to develop the SiC nano-fiber or nano-whisker in the porous alumina bodies. The liquid droplets formed at the end of the SiC fiber or whisker are evident for the typical VLS mechanism. The microstructure of the SiC fiber or whisker grown by the VLS reaction was observed by SEM. The porosity was measured by mercury porosimeter. The formation behavior of SiC fiber or whisker is dependent on volume ratio of carbon converted from PMMA bead.

Abstract: Ceramic honeycomb filter supported zeolite membrane modules were prepared by a novel microwave-assisted in-situ crystallization method. The synthesis was done in two stages; microwave heating of substrates saturated with precursor solution to form a thin layer of zeolite nuclei (seeds) on the porous substrates followed by hydrothermal treatment to crystallize the ZSM-5 zeolite on the substrates. Zeolite formation was significantly enhanced by comparison with the conventional hydrothermal method, as evident from XRD and BET surface area results. The degree of film formation was varied with varying the dipping time of the substrates in the precursor solution prior to microwave heating, thereby controlling film formation inside the pores of the substrates. This new method of seeding using a simple domestic microwave oven was employed to zeolite membrane modules by forming a zeolite film on the thin walls of a honeycomb filter. The permeability of single gases and the separation of mixed gases were evaluated, showing a low pressure-drop and high permeating flux of light molecule gases.

Abstract: In this work, cordierite whiskers were successfully coated on cordierite honeycomb support by in-situ method. Whisker coating was performed on walls of the pore channels of cordierite support from aqueous slurry. Sintering was carried out at 1300 °C for 4 hours. XRD measurements revealed a complete formation of cordierite phase, no traces of other phases were identified. A well adherence of coating on the support was identified from the microstructure. SEM analysis also revealed the formation of three-dimensional network of cordierite whiskers with open pores between them on the cordierite support.

Abstract: Porous Ni
ZrO2 has been widely used as anode in SOFC. However, it has been reported that there was reduction in cell performance at high temperature since Ni grains in the porous Ni
ZrO2 sintered during cell operation. In this study, NiO
ZrO2 composite powders in which NiO powder were covered with ZrO2 particles were prepared with controlled microstructure of porous Ni
ZrO2 which can prevent sintering of Ni grains during cell operation. NiO
ZrO2 composite powders (HNZ) were prepared using NiO powder of high specific surface area and ZrO2 sol of average particle size of 50 nm by ball milling (sol-coating method). SEM and EPMA analysis of prepared NiO
ZrO2 composite powders showed that NiO particles were covered with ZrO2 particles. The crystallite phases, crystallite sizes, BET specific surface area of NiO
ZrO2 composite powders and porosities of fabricated porous Ni
ZrO2 were characterized. SEM and EPMA analysis showed that ZrO2 was distributed around Ni of a few μm range in the porous Ni
ZrO2 (HNZ) fabricated by the sol-coating method. Mechanical properties of porous Ni
ZrO2 (HNZ) are required to be improved.

Abstract: As the oxygen permeation flux of La0.7Sr0.3Ga0.6Fe0.4O3-δ (LSGF) membranes is lower than commercial membranes, we coated the nano porous La0.6Sr0.4CoO3-δ (LSC) particles to enhance the oxygen permeation flux. The nano porous LSC particles were synthesized in an advanced process to increase the volume fraction of nano pores by ultrasonic spray pyrolysis. The synthesized LSC particles consisted of nano sized primary particles and pores. They also had remarkably high surface area (22 m2/g). It was found that the LSGF membrane coated by the nano porous LSC resulted in significant improvement in the oxygen permeability.

Abstract: A porous filter was manufactured using cordierite powder by an extrusion method. The filter had an apparent porosity of 58% and a compressive strength of 10 MPa. Pulse cleaning process affected the durability of the filter. We investigated the effect of operating conditions of a hot gas cleaning system using ceramic filter, thermal shock and pressure shock on durability. When pulse cleaning was repeated periodically, the baseline differential pressure was relatively higher. On the other hand, when we repeated pulse cleaning per specific maximum differential pressure we could improve the durability of the filter. In our test conditions, thermal shock is the most important factor decreasing the durability of the filter. Pressure shock didn’t deteriorate the durability of the filter. However thermal shock caused the formation of cracks at the filter surface. This will lead to an increasing of the baseline differential pressure by depth filtration of dust and decrease the durability of the filter.

Abstract: Mesoporous silica with 5~10 nm pore size was coated over recycled ceramic, prepared from lake sludge, with pore size 100-200μm. The mesoporous silica coated ceramic was characterized by XRD, N2 sorption, SEM and TEM. The results clearly showed the formation of mesoporous coating on the macroporous ceramics. These ceramics with the mesoporous coating are specially suitable for stabilization of enzymes. The mesopore stabilized enzymes are recyclable catalysts and showed comparable performance with the free enzyme.

Abstract: A bimodal porous hydroxyapatite (HAp) body with high flexural strength was prepared through slip casting. HAp fine powder used in this study was synthesized by wet milling, drying and heating of a mixture of calcium hydrogen phosphate di-hydrate and calcium carbonate. The synthesized HAp powder was 0.32
0.05 μm in size and 38.1
0.8m2/g in specific surface area. The slip was prepared by adding deflocculant and foaming reagent. The optimum value for the minimum viscosity in the present HAp slip with respect to its solid loading and the optimum amount of the deflocculant were studied. The total porosity of the specimens obtained from a slip of 48 wt% HAp solid loading is in the range of 49 – 61vol %, and the resultant porous HAp sintered body had large spherical pores of 300 -m with interconnecting rectangular voids. Many small pores in the size range of 2-3 -m or below were observed in the specimen obtained by heating at 1100, and 1200 . The flexural strength of the bimodal porous HAp ceramics sintered at 1200 C showed a large value of 17.6 MPa, with a porosity of 60.5vol.