Porous Alumina Ceramics Research Articles

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm 2. The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.

Fabrication of Porous Alumina Ceramics by New Eco-Friendly Process

We have developed a new eco-friendly 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 3-D network was formed by the hydration of HA in HA slurry. The HA slurry was hardened by the formation of this 3-D network. Even without the addition of an organic binder, green bodies containing the 3-D 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 fugitive materials. The results of evolved gas analysis-mass 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 eco-friendly.

Preparation and Properties of Porous Alumina Ceramics with Oriented Pores by Extrusion Method

Preparation and Properties of Porous Alumina Ceramics with Oriented Pores by Extrusion Method

ヒドラジン分解反応促進に対する放電プラズマの適用検討

Conventional monopropellant thrusters for spacecraft utilize the pelletized catalyst bed to decompose the propellant, typically Hydrazine. The catalyst bed consists of Iridium catalyst supported by porous Alumina ceramics pellets. During the long-term operation of the thruster, however, the catalyst is occasionally damaged, which causes several critical problems like a deterioration of the decomposition performance or choke of the capillary tube. In order to solve or mitigate these problems, a new decomposition device was designed and its preliminary model was fabricated and evaluated in our research group. Preliminary tests were conducted to examine the new reaction mechanism using pulsed or stationary AC discharge plasma instead of pelletized catalysts in a bucket type reaction chamber. In the reaction chamber filled with Hydrazine, sudden increase of the temperature and the pressure was observed immediately after the production of plasma discharge. These results show that the plasma is capable of the decomposition of Hydrazine, and acts as catalysts, and that it is worthwhile developing a new monopropellant thruster system using plasma assisted reaction in order to eliminate the disadvantages associated with the pelletized catalyst bed.

Indentation Damage of Porous Alumina Ceramice

The Hertzian indentation contact damage behavior of porous alumina with controlled pore shape was investigated by experiments. Porous alumina ceramics containing well-defined pore shape, size and distribution were prepared by incorporation of fugitive spherical starch. Porous alumina with isolated pore structure was prepared with porosity range up to 30%. The indentation stress-strain curves of porous alumina were constructed. Elastic modulus and yield stress can be obtained from the stress-strain relationship. Impulse excitation method for the measurement of elastic modulus was also conducted as well as Hertzian indentation and was confirmed as a useful tool to evaluate the elasticity of highly porous ceramics. Elastic modulus of the inter-connected pore structure is more sensitive to porosity than that of the isolated pore structure. When the specimen had isolated pore structure, higher yield point was obtained than it had inter-connected pore structure. This study proposed that the elastic modulus of porous ceramics is strongly related to not only porosity , but also the structure of pore.

Image-based Modeling and Stress Analysis by Homogenization Method for Porous Alumina Ceramics

Image-based Modeling and Stress Analysis by Homogenization Method for Porous Alumina Ceramics

Processing and Property Assessment of Porous Alumina and Mullite-Alumina Ceramics

Processing and Property Assessment of Porous Alumina and Mullite-Alumina Ceramics

OS08W0101 Image-based modeling with high resolution X-ray CT for porous alumina and elastic analysis with homogenization method

Three-dimensional elastic analysis for porous ceramic materials was studied by means of combining a image-based modeling and homogenization method. Image-based modeling for porous alumina ceramics was obtained with X-ray CT. The prepared alumina samples had small spherical pores of 20 and 60μm in diameters with the porosities of about 1.0 and 7.0%. After reconstruction of the three-dimensional images, representative volume elements (RVE) were obtained. Homogenization method was applied to the RVE images for calculation of homogenized elastic constants. Using the elastic constants, FEM macroscopic elastic calculation for a global 4-point bending test models was carried out, which gives global stress distributions for the specimens. Under the macroscopic stress conditions, the local stress distributions in the RVE were calculated. High local stresses were shown near the pores, which will be very useful for future fracture analyses of the porous ceramics.

Fabrication of bimodal porous alumina ceramics

An alumina foam with two kinds of pores was prepared by combining the sponge method and the pore-former method. The large pores were provided by the sponge method, and their sizes were in the range of 1–2 mm. The small pores were produced by the pore-former method with size in the micrometer range. The large pores offered a high porosity while the small pores offered a large surface area. The strength of samples sintered at different temperatures was measured, and the effect of sintering temperature on foam strength was analyzed by discussing porosity and grain bonding area. The sample sintered at 1550 °C has a compressive strength of 1.3 MPa and a porosity of 86%.

High‐Strength Porous Alumina Ceramics by the Pulse Electric Current Sintering Technique

High‐strength porous alumina has been fabricated with a microstructure control using the pulse electric current sintering (PECS) technique. During sintering the discharge, which is assumed to take place in the voids between the particles, is thought to promote the bridging of particles by neck growth in the initial stages of sintering, leaving high porosity. The effect of dopants (MgO, 200 ppm; TiO2, 1000 ppm) and of secondary inclusions (3 vol% 3Y‐TZP) on the constrained densification and the improvement in the mechanical behavior of porous alumina ceramics has been reported. The porosity of the fabricated porous alumina was controllable between 30% and 50% depending on the sintering temperature. The flexural strength of alumina having 30% and 42% porosity showed impressive values of 250 and 177 MPa, respectively. The dominance of the preferential neck growth of grains over densification significantly improved the mechanical properties of porous alumina, besides leaving high porosity.

Damping properties of porous alumina ceramics.

Damping properties of porous alumina ceramics.

Corrosion Resistance of Porous Alumina Ceramics in Acetic Acid Solution

The effect of binder composition on corrosion resistance of porous alumina was determined. Three materials with different binder compositions were compared. Aluminas were exposed to acetic acid solution (pH 4) at 110 C for three weeks. Corrosion solutions were analyzed by induction coupled plasma spectrometry (ICPS). Small amounts of alkalis and silica dissolved during the test. Exposed samples were characterized by scanning electron microscopy and X-ray diffractometry, and the results were compared with the results of unexposed samples. (orig.)

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al2O3‐based porous ceramics fabricated from pure Al2O3 powder and the mixtures with Al(OH)3 were investigated. The fracture strength of the porous Al2O3 specimens sintered from the mixture was substantially higher than that of the pure Al2O3 sintered specimens because of strong grain bonding that resulted from the fine Al2O3 grains produced by the decomposition of Al(OH)3. However, the elastic modulus of the porous Al2O3 specimens did not increase with the incorporation of Al(OH)3, so that the strain to failure of the porous Al2O3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)3 particles. Fracture toughness also increased with the addition of Al(OH)3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture‐mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.

Scratch damage in porous alumina ceramics

Scratch damage in porous alumina ceramics

Preparation and in vivo testing of porous alumina ceramics for cell carrier applications.

Microporous alumina was used to develop implantable cell carriers shaped as a hollow-sphere with a central opening to allow ingrowth of vascularised tissues. The carriers were produced by suspending the ceramic raw materials in water, homogenising and dropping the resulting slurry onto a heated plate (hot plate moulding, HPM). Morphological characteristics of the cell carriers were investigated by SEM and optical microscopy. Produced carriers had an average diameter of 4.9 mm. The material was highly porous (56 +/- 8%). For in vivo testing the cell carriers were implanted into abdominal wall of Zur: SIV rats for up to 50 weeks and investigated by light microscopy, SEM and TEM. The surface of the hollow carriers was in close contact with unirritated muscle tissue; no inflammation or capsule formation was observed. Loose connective tissue had grown into the hollow cell carrier, and after prolonged implantation >20 weeks adipocytes were observed. The absence of scar tissue formation around the implant and the vitality within the cavity of the hollow carriers indicate that porous alumina may be used for cell transplantation devices.

Fabrication of Porous Alumina Ceramics with Uni-Directionally-Arranged Continuous Pores Using a Magnetic Field.

In order to fabricate porous ceramics with uni-directionally-arranged continuous pores, alumina compacts containing nickel wires as a pore forming agent were prepared by casting the alumina slurry on the plaster mold, on which nickel wires were settled and uni-directionally arranged with applying a magnetic field. Acid-leaching the nickel wires and subsequent sintering the samples resulted in the formation of continuous and straight pores in dense alumina matrix. Size of continuous pores could be varied with using nickel wires with different size. Interval between pores could be controlled by coating the nickel wires with alumina prior to the arrangement in the magnetic field. Electrophoresis technique was useful to form a coated layer with controlled thickness.

Chemistry and kinetics of chemical vapor infiltration of pyrocarbon-IV. Investigation of methane/hydrogen mixtures

Chemical vapor infiltration of pyrolytic carbon was studied at a temperature of 1100°C and total pressures of 20, 30 and 50 kPa using methane/hydrogen mixtures with molar ratios of 7:1, 6:1, 5:1, and 4:1. Cylindrically shaped porous alumina ceramics, 20 mm in height and 16 mm in diameter, were used as the substrate. The pore diameters range from 1 to 36 μm; the total porosity amounts to 23%. The infiltration rates are lowered by hydrogen, but the degrees of pore filling obtained after quasi infinitely long infiltration times are higher than those obtainable without hydrogen. A detailed analysis of the results, based on the Weisz modulus, the pore effectiveness factor, in-pore and surface deposition rates, showed that the overwhelming effect of hydrogen has to be ascribed to strongly lowered surface deposition rates; thus an overgrowing of the pore entrances is suppressed and a complete filling of the pores is possible.

Evaluation of Fracture Strength of Porous Alumina Sintered by Hot Isostatic Pressing (HIPing)

Porous alumina ceramics were sintered by conventional sintering (CSing) and hot isostatic pressing (HIPing) without encapsulating at temperatures between 1200°C and 1400°C under pressures between 0.1MPa and 200MPa of Ar gas. Pending strength and Weibull modulus of sintered porous aluminas were evaluated by three-point bending tests. The HIPed samples have well-grown necks and roundish pore shape compared to the CSed ones due to enhanced surface diffusivity, which results in lower specific surface area of HIPed samples than that of CSed ones with the same porosity and pore size. As increasing gas pressure, the bending strength increased for porous materials with same porosity. The bending strength of samples HIPed under 200MPa was about 30% higher than that of CSed ones having the same porosity of about 40%. The increase in bending strength by HIPing was significant as decrease in sintering temperature. The enhancement of bending strength by HIPing is caused by well-grown necks that reduce stress concentration around necks. On Weibull plots, kinks were observed for HIPed samples. As increasing gas pressure, Weibull modulus for weak samples increased, while that of strong samples was constant. Enhanced surface diffusivity by high gas pressure causes deviation of defect distribution from single Weibull distribution. As a result, a kink appears on Weibull plots. The increase in Weibull modulus for weak samples is caused by removal of insufficiently grown necks by enhanced surface diffusivity.

Constrained densification in boehmite-alumina mixtures for the fabrication of porous alumina ceramics

It is demonstrated that inherent constrained densification characteristics of the bimodal size distributed powders can be utilized to fabricate porous alpha alumina. Seeded boehmite was mixed with coarse alpha alumina particles to produce various compositions of bimodal mixtures. The densification behaviour of the mixtures was correlated with shrinkage and microstructural evolution. The mixtures sintered at 1200 °C contained more than 30% porosity and the mechanical strength of the mixtures was increased by 2–4 fold relative to the coarse particles alone with similar porosity (<5 MPa). © 1998 Chapman & Hall