Characteristics of microbubbles generated by porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method using rayon fibers as the pore formers and the characteristics of microbubbles generated by these porous ceramics were investigated. The 1200 mm long ceramics were tubular and of thick or thin types of 20–30 mm inner diameter and 30–50 mm outer diameter, respectively. The thin and thick samples had porosities of 47 and 49% and average pore radii of 7.8 μm. The gas permeabilities of the thick and thin samples were 4.1 × 10^?14 and 5.4 × 10^?14 m², respectively. Microbubbles were generated by introducing N₂ gas through the ceramic tube by immersing it into water. The minimum pressure (bubble point pressure) for generation of microbubbles was 20 kPa, much lower than for other bubble-forming methods. The average microbubble radii ranged from about 70 to 105 μm at flow rates of 0.15–0.25 L/min in the thin sample and 0.3–0.7 L/min in the thick sample. These bubble sizes are much smaller than calculated for a Fritz-type bubble such as generally formed by bubbling from pores and/or orifices. However, the present bubble sizes agree well with the calculated value based on nanobubbles, indicating that bubble formation occurs by mixing the gas with water in small pores. Since microbubbles enhance the dissolution rate of a gas phase in water, they are potentially useful for improving water environments, especially oxygen-deficient water. The effectiveness of gas dissolution in water was confirmed by determining the dissolution behavior of CO₂ gas using these porous ceramics.     

Capillary rise properties of porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method to investigate their capillary rise properties. Rayon fibers 16.5 μm in diameter and 800 μm long were used as the pore formers by kneading with alumina powder, kaolin clay, China earthen clay and binder with varying Fe₂O₃ contents of 0, 5 and 7 mass%. The resulting pastes were extruded into cylindrical tubes (outer diameter (OD) 30–50 mm and inner diameter (ID) 20–30 mm), dried at room temperature and fired at 1500 °C for 4 h. The bulk densities of the resulting porous ceramics ranged from 1.31 to 1.67 g/cm³, with apparent porosities of 43.2–59.3%. The pore size distributions measured by Hg porosimetry showed a sharp peak at 10.0 μm in the sample without Fe₂O₃ and at 15.6 μm in the samples containing Fe₂O₃; these pores, which arose from the burnt-out rayon fibers, corresponded to total pore volumes ranging from 0.24 to 0.34 ml/g. SEM showed a microstructure consisting of unidirectionally oriented pores in a porous mullite matrix. Prismatic mullite crystals were well developed on the surfaces of the pore walls owing to the liquid phase formed by the Fe₂O₃ component added to color the samples. The bending strengths of the tubular samples ranged from 15.6 to 26.3 MPa. The height of capillary rise, measured under controlled relative humidities (RH) of 50, 65 and 85%, was greater in the ceramics containing Fe₂O₃ than in those without Fe₂O₃, especially in the thinner samples. The maximum capillary rise reached about 1300 mm, much higher than previously reported. This excellent capillary rise ability is thought to be due to the controlled pore size, pore distribution and pore orientation in these porous mullite ceramics.     

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/mm². 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 silicon carbide ceramics with high porosity and high strength

Unique porous SiC ceramics with a honeycomb structure were fabricated by a sintering-decarburization process. In this new process, first a SiC ceramic bonded carbon (SiC/CBC) is sintered in vacuum by spark plasma sintering, and then carbon particles in SiC/CBC are volatized by heating in air at 1000 °C without shrinkage. The honeycomb structure has at least two different sizes of pores; ∼20 μm in size resulting from carbon removal; and smaller open pores of 2.1 μm remaining in the sintered SiC shell. The total porosity is around 70% and the bulk density is 0.93 mg/m³. The bending and compressive strengths are 26 MPa, and 105 MPa, respectively.     

High optical quality Y2O3 transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment

High optical quality Y₂O₃ transparent ceramics with fine grain size were successfully fabricated by air pre-sintering at various temperature ranging from 1500 to 1600 °C combined with a post-hot-isostatic pressing (HIP) treatment using co-precipitated powders as the starting material. The fully dense Y₂O₃ transparent ceramic with highest transparency was obtained by pre-sintered at 1550 °C for 4 h in air and post-HIPed at 1600 °C for 3 h (the pressure of HIP 200 MPa), and it had fine microstructure and the average grain size was 0.96 μm. In addition, the in-line transmittance of the ceramic reached 81.7% at 1064 nm (1 mm thickness). By this approach, the transparent Y₂O₃ ceramics with fine grain size (<1.6 μm) were elaborated without any sintering aid.     

CO2 laser peeling of Al2O3 ceramic and an application for the polishing of laser cut surfaces

A laser controlled fracture peeling technique is demonstrated to smooth the Al₂O₃ ceramic surface without thermal damages. It was found that a chip can be separated and curled from the ceramic surface during a focused CO₂ continuous wave (CW) laser dual-scanning. The thickness of the curled chip is ～50 μm and the formed subsurface roughness (R_a ≈ 2 μm) is close to the surface machined by mechanical breaking (R_a = 1.84 μm). The chip formation is attributed to the controlled fracture by the residual tensile stress in the recast layer, whereas the chip curling only occurs when the melting depth is shallower than the position of lateral cracks. The peeling technique can be applied to polish the cut surface of laser fusion cutting in ceramics. The polished cut surface (R_a = 2.18 μm) is free from recast, crack and heat effects. The microstructure is similar to the base material. The material removal rate during polishing is up to 0.125 mm³/s.     

Preparation and properties of porous alumina ceramics with uni-directionally oriented pores by extrusion method using a plastic substance as a pore former

Porous alumina ceramics with uni-directionally aligned pores were prepared by an extrusion method using 0–40 vol.% poly (vinyl acetate) (PVAC) as the pore former. A paste was prepared by mixing 25 mass% distilled water, 4 mass% methylcellulose, 8 mass% oleic acid and 0.8 mass% ammonium poly (carboxylic acid). This paste was molded into a 10 mm Ø body using a ram-type extruder, dried at room temperature for 24 h, calcined at 600 °C for 1 h and sintered at 1500 °C for 2 h in air. The PVAC added to the paste was homogeneously dispersed and formed particles 0.1–150 μm in size which extended in the extrusion direction and were converted to through-hole pores after sintering. The resulting pore size distribution in the samples was bimodal, centered at about 0.4 μm with a broad peak at about 70 μm dia. The resulting porous alumina ceramics showed high gas permeability because of their uni-directionally oriented through-hole pore structure.     

Extrusion method using nylon 66 fibers for the preparation of porous alumina ceramics with oriented pores

Porous alumina ceramics with unidirectionally oriented pores were prepared using an extrusion method. The paste for extrusion was prepared by mixing alumina and nylon 66 fibers with binder and dispersant. The resulting paste was extruded, dried at room temperature, and after removal of the binder at 600 °C, fired at 1500 °C for 2 h. The pore size in the sintered body, determined from SEM micrographs, was 16 μm, corresponding to the size of the burnt-out nylon 66 fibers. The degree of orientation of the cylindrical pores was evaluated from SEM micrographs to be highly aligned to the extrusion direction. The orientation of the pores decreased with increasing fiber loading because of strong interaction between the fibers. The pore size distribution of the extruded samples showed a peak at 16 μm corresponding to the cylindrical pore diameter and also at 4 and 6 μm corresponding to the pores formed by connection of the fibers.     

Preparation and characterization of porous MgAl2O4 spinel ceramic supports from bauxite and magnesite

Porous magnesium aluminate spinel (MgAl₂O₄) ceramic supports were fabricated by reactive sintering from low-cost bauxite and magnesite at different temperatures ranging from 1100 to 1400 °C and their sintering behavior and phase evolution were evaluated. The effects of sintering temperature on the pore structure, size and distribution as well as on the main properties of spinel ceramic supports such as flexural strength, nitrogen permeation flux and chemical resistance were investigated. The supports prepared at 1300 °C showed a homogeneous pore structure with the average pore size of 4.42 μm, and exhibited high flexural strength (35.6 MPa), high gas permeability (with nitrogen gas flux of 3057 m³ m⁻² h⁻¹ under a trans-membrane pressure of 0.1 MPa) and excellent chemical resistance.     

Synthesis of hierarchical porous silicon oxycarbide ceramics from preceramic polymer and wood biomass composites

Hierarchical porous SiOC ceramics were successfully prepared using a polysiloxane preceramic polymer mixed with wood biomass by annealing at different temperatures under Ar atmosphere. These SiOC ceramics display a trimodal pore size distribution in the micro-, meso- (micropores + mesopores, 1.7–14 nm) and macro-size scale (1–15 μm). The mesopores and micropores mainly originate from the formation of large amounts of SiC crystals and nanowires, graphite-like microcrystallites, and nm-scale pores of ray parenchyma cells and pits of the wood biomass. The SiOC sample prepared at a higher temperature processes the specific surface area up to 180.5 m²/g. The specific surface area, pore volume and average pore width of the samples can be controlled by adjusting the pyrolysis temperature.     

A highly efficient and versatile carbon nanotube/ceramic composite filter

Carbon nanotubes were grown in the open pores of a commercial porous ceramic matrix consisting of mainly Al₂O₃ and SiO₂ with an average pore size of 300 and 500 μm, using a pre-formed nickel catalyst inside the pores and a camphor solution precursor at 780 °C. The resulting composites, ∅27 mm × 10 mm discs containing about 3 wt.% of carbon nanotubes, were then assessed as a filter for the removal of yeast cells and different heavy metal ions from water, and for the removal of particulates from air. The results showed that the carbon nanotube containing composite filter demonstrated a high efficiency of yeast filtration (98%), ca. 100% heavy metal ion removal from water and excellent particulate filtration from air. The composite filter also exhibited good reusability for these applications, owing to the excellent thermal and chemical stability of the carbon nanotubes.     

Effects of inert filler addition on the structure and properties of porous SiOC ceramics derived from silicone resin

Porous SiOC ceramics were obtained from a new self-blowing precursor silicone resin DC217, by pyrolysis at 1200 °C in argon. Silicon carbide powders were incorporated into the silicone resin as inert fillers. The effects of the mean particle size of SiC fillers on the porosity, compressive strength and microstructure of the porous ceramics were investigated. With the mean particle size of SiC powders increasing from 5 μm to 10 μm, the porosity (total and open) of the porous ceramic increased and the compressive strength decreased. However, the porosity, compressive strength and cell morphology of the porous ceramics showed no evident changes when the mean particle size of fillers increased from 10 μm to 15 μm. Micrographs indicated that, when the mean particle size of fillers exceeded 5 μm, the porous ceramics could have a well-defined and regular pore structure. Furthermore, comparing with the porous ceramics which fabricated under the same condition with the SiOC powders as fillers, the cell morphology was similar. But the compressive strength and the oxidation resistance of the porous ceramics with SiC powders as fillers were much better.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Effects of pore structure on thermal conductivity and strength of alumina porous ceramics using carbon black as pore-forming agent

In the present work, carbon black (CB) works as a pore-forming agent in the preparation of alumina porous ceramics. The pore structures (i.e. mean pore size, pore size distribution and various pores size proportions) were characterized by means of Micro-image Analysis and Process System (MIAPS) software and mercury intrusion porosimetry. Then their correlation and thermal conductivity as well as strength were determined using grey relation theory. The results showed that the porosity and mean pore size increased against the amount of CB, whereas the thermal conductivity, cold crushing strength and cold modulus of rupture reduced. The <2 μm pores were helpful for enhancing the strength and decreasing the thermal conductivity whereas the >14 μm pores had the opposite effects.     

Processing and properties of low cost macroporous alumina ceramics with tailored porosity and pore size fabricated using rice husk and sucrose

The present study demonstrates a cost effective way to fabricate porous ceramics with tailored microstructures using rice husk (RH) of various range of particle sizes as a pore former and sucrose as a binder as well as a pore former. Sample microstructures reveal randomly oriented elongated coarse pores and fine pores (avg. size 4 μm) created due to burnout of RH and sucrose, respectively. Porous alumina ceramics with 20–66 vol% porosity and 50–516 μm avg. pore size (length), having isolated and/or interconnected pores, were fabricated using this process. Mechanical properties of the porous samples were determined as a function of porosity and pore microstructure. The obtained porous ceramics exhibited flexural strength of 207.6–22.3 MPa, compressive strength of 180–9.18 MPa, elastic modulus of 250–18 GPa and hardness of 149–18 HRD. Suggested application area includes thermal, filtration, gas purging etc.     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

Camphene-based freeze casting technique was adopted to fabricate ZrB₂–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB₂–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB₂–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.     

Preparation and characterization of porous,biomorphic SiC ceramic with hybrid pore structure

Bio-carbon template (charcoal) was prepared by carbonizing pine wood at 1200 °C under vacuum, and was impregnated with phenolic resin/SiO₂ sol mixture by vacuum/pressure processing. Porous SiC ceramics with hybrid pore structure, a combination of tubular pores and network SiC struts in the tubular pores, were fabricated via sol–gel conversion, carbonization and carbothermal reduction reaction at elevated temperatures in Ar atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were employed to characterize the phase identification and microstructural changes during the C/SiO₂ composites-to-porous SiC ceramic conversion. Experimental results show that the density of C/SiO₂ composite increases with the number of impregnation procedure, and increases from 0.32 g cm⁻³ of pine-derived charcoal to 1.5 g cm⁻³ of C/SiO₂ composite after the sixth impregnation. The conversion degree of charcoal to porous SiC ceramic increases as reaction time is lengthened. The resulting SiC ceramic consists of β-SiC with a small amount of α-SiC. The conversion from pine charcoal to porous SiC ceramic with hybrid pore structure improves bending strength from 16.4 to 42.2 MPa, and decreases porosity from 76.1% to 48.3%.     

Preparation and characterization of mesoporous silicon oxycarbide ceramics without free carbon from polysiloxane

Mesoporous silicon oxycarbide ceramics without free carbon were prepared by pyrolysis of cross-linked polysiloxane at different temperatures (1300–1450 °C) followed by post treatments. The post treatments comprised two steps (HF etching and oxidation at 650 °C in air). The sample pyrolyzed at 1300 °C after post treatments exhibits the largest specific surface area (SSA) reaching up to 204 m²/g and the biggest total pore volume (0.58 cm³/g) with an average pore size of 11.4 nm. Increasing pyrolysis temperature will lead a quick decline of SSA and total pore volume. The thermal stability of pore structure of the sample pyrolyzed at 1300 °C with post treatments was investigated in air. The SSA and total pore volume almost keeps the same up to 750 °C, and subsequently decreases with a high speed. The most possible reason is the pores are severely closed by viscous flow of SiO₂ produced from SiC nanocrystallites.     

Porous alumina ceramics prepared with wheat flour

It is shown that wheat flour can be used as a pore-forming and body-forming agent in ceramic technology. In contrast to pure native starch, however, the pores do not result from the swelling starch granules alone but are mainly due to protein-assisted foaming. Therefore the porosity is significantly higher and the pore size larger than that resulting from the starch granules alone, and the wet milling time applied for homogenizing the ceramic suspensions becomes the most critical process parameter. Alumina suspensions with 70 wt.% alumina and 20–30 vol.% wheat flour with different initial particle size (fine grade and semolina, respectively) have been prepared using milling times of up to 8 h. Porosities of up to approx. 60% can be achieved with only 20 vol.% of flour or semolina after 8 h of milling time, with the cell sizes (diameters of pore cavities resulting from foam bubbles) being essentially independent of the milling time (median diameters of 120–240 μm). Effective pore throat sizes (i.e. diameters of cell windows or channels between cells), measured via mercury porosimetry, are 1–2 μm for short milling times (2–3 h), but for long milling times (8 h) they change by more than one order of magnitude to median sizes of 20–30 μm, closely corresponding to the median size of wheat starch granules (approx. 20 μm).     

A novel approach for the fabrication of carbon nanofibre/ceramic porous structures

This paper describes the fabrication of hybrid ceramic/carbon scaffolds in which carbon nanofibres and multi-walled carbon nanotubes fully cover the internal walls of a microporous ceramic structure that provides mechanical stability. Freeze casting is used to fabricate a porous, lamellar ceramic (Al₂O₃) structure with aligned pores whose width can be controlled between 10 and 90 μm. Subsequently, a two step chemical vapour deposition process that uses iron as a catalyst is used to grow the carbon nanostructures inside the scaffold. This catalyst remains in the scaffold after the growth process. The formation of the alumina scaffold and the influence of its structure on the growth of nanofibres and tubes are investigated. A set of growth conditions is determined to produce a dense covering of the internal walls of the porous ceramic with the carbon nanostructures. The limiting pore size for this process is located around 25 μm.