Novel freeze-casting fabrication of aligned lamellar porous alumina with a centrosymmetric structure

A novel freeze-casting method is used to fabricate aligned lamellar porous alumina with a centrosymmetric structure from aqueous alumina slurries. Two cold fronts oriented perpendicularly to each other, originating from the bottom and side of the cylindrical copper mold, induce the growth of ice crystals in specific directions along the radius of the cylindrical mold. Lamellar channels of porous alumina are arranged centrosymmetrically along the radial axis. The pore distribution of the currently prepared porous ceramics is more regular when compared with that of porous ceramics prepared by conventional freeze casting. This affords porous ceramics with improved mechanical properties and stability. The current method addresses the issue of partial failure as induced by the randomly distributed channels in lamellar porous ceramics.     

High porous ceramics with isometric pores by a novel saponification/gelation/freeze-casting combined route

A new route based on the combination of saponification/gelation/freeze-casting processes, in which the water was used as the solvent, was proposed to produce highly porous ceramics with isometric pores. The saponification reaction using gelatin promoted an inter-connected and equiaxial pore structure with the absence of lamellar pores, even using water as a solvent. The final porosity was up to ∼95 %, and the average pore size ranged from ≈200 up to 500 μm. The amount of saponifying agent had a significant influence on the pore size, while the contents of solids had a more substantial effect on the open porosity. The results indicate that the in situ production of surfactant combined with freeze casting technique allows obtaining ceramics with high porosity and isometric pores even using water as a solvent.     

A novel approach to fabricate porous alumina ceramics with excellent properties via pore-forming agent combined with sol impregnation technique

An innovative approach for fabricating porous alumina ceramics (PACs) with improved mechanical and thermal properties using walnut shell powders as pore-forming agent combined with alumina sol impregnation is reported in the present work. It is demonstrated that uniform distribution of spherical pores can be observed in as-prepared PACs by using above technical route. The decrease of walnut shell powder sizes significantly promotes the enhancement of crushing strength and reduction of thermal conductivity of the PACs. Meanwhile, the impregnated alumina sol is favoring for the formation of spherical micro-pores, then further improves their mechanical and thermal insulation performances. The lowest thermal conductivity and highest crushing strength of resulting sample reach 0.16?W/m?K and 29.2?MPa, respectively. This novel method offers new possibilities to fabricate high-quality PACs.     

Production of highly aligned porous alumina ceramics by extruding frozen alumina/camphene body

We herein propose a new technique for producing highly aligned porous ceramics by extruding a frozen ceramic/camphene body. To accomplish this, an alumina/camphene slurry with an initial alumina content of 10 vol% was first frozen unidirectionally in a 20 mm × 20 mm mold and extruded through a reduction die with a cross-section of 5 mm × 5 mm at room-temperature. This simple process enabled the formation of porous alumina ceramics with highly aligned pores as a replica of the camphene dendrites with a preferential orientation parallel to the extrusion direction. The sample showed much higher compressive strength of 280 ± 80 kPa with a porosity of 83 vol% when tested parallel to the direction of pore alignment. In addition, these materials could be used as a valuable framework for the production of ceramic/epoxy composites, particularly with a lamellar structure, which would result in a remarkable increase in mechanical properties.     

Compound surfactant-based emulsion method for the preparation of high-porosity alumina microspheres

Porous alumina microspheres have attracted significant attention owing to their high mechanical strength and excellent chemical and thermal stability. The emulsion method is considered as a simple and controllable method for the preparation of inorganic microspheres. However, preparing alumina microspheres with the emulsion method is challenging because the emulsification of the precursor is inhibited by the rapid hydrolysis of aluminum alkoxide. Herein, we report a new emulsion method for the preparation of high-porosity alumina microspheres using a combination of ionic and non-ionic surfactants; in this method, the compound surfactants act as a template agent to guide aluminum alkoxide to form a lamellar structure through self-assembly. The decomposition of the templating agent and transformation of the alumina crystal at a high temperature result in structural collapse and formation of lamellar pores. Compound surfactants increased the spheroidization rate of the emulsion from 47% to 63% after hydrolysis, whereas the particle size was decreased by almost half. Additionally, the morphology and porosity of the alumina microspheres were changed. With increasing anionic surfactant content, the porosity increased initially and then decreased. The porosity of the alumina microspheres reached a maximum value of 76% at the 1:1 mass ratio of the non-ionic to anionic surfactants. Heat treatment was found to change the size of lamellar pores, with the pore diameter reaching maximum value at 1300 °C. The compound surfactants also increased the compressive stress and specific surface area of the porous alumina microspheres.     

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.     

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.     

Fabrication of wood-like porous silicon carbide ceramics without templates

The porous silicon carbide ceramics with wood-like structure have been fabricated via high temperature recrystallization process by mimicking the formation mechanism of the cellular structure of woods. Silicon carbide decomposes to produce the gas mixture of Si, Si₂C and SiC₂ at high temperature, and silicon gas plays a role of a transport medium for carbon and silicon carbide. The directional flow of gas mixture in the porous green body induces the surface ablation, rearrangement and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar fibrous silicon carbide crystals and tubular pores in the axial direction. The orientation degree of silicon carbide crystals and pores in the axial direction strongly depends on the temperature and furnace pressure such as it increases with increasing temperature while it decreases with increasing furnace pressure.     

Effects of pores on dielectric breakdown of alumina ceramics under AC electric field

The relationship between pores and dielectric breakdown strength of alumina ceramics has been investigated. Alumina specimens with different internal pores in size and quantity were obtained with polyvinyl alcohol (PVA) granules additives being removed after sintering. Pores inside the ceramic samples were characterized by density measurement and micro computed tomography (Micro CT) analysis. We tested the breakdown strength of different alumina sheets under AC electric field, with the same electrodes and voltage loading methods being adopted. The experimental results indicated that the dielectric breakdown strength of alumina specimens decreased with the increment of their thickness. Interestingly, a great deal of large pores influences the breakdown strength significantly. And the breakdown channel passes through the large hole when there is a large hole in the sample. Small pores have little effects on the insulating properties. Further discussion and analysis reveal that the measured results of breakdown strength conform to the Weibull distribution. In addition, the experimental results can be well explained by space charge injection models and electric field simulation.     

Effects of B4C addition on the microstructure and properties of porous alumina ceramics fabricated by direct selective laser sintering

Direct selective laser sintering (dSLS) is a promising method for the fabrication of complex-shaped ceramic parts. In this paper, boron carbide (B₄C) was used as an inorganic additive to improve the laser sintering behavior of alumina. The effects of B₄C addition on the microstructure and mechanical properties of porous alumina ceramics were investigated. Mixture of alumina powders and different amount of B₄C were directly sintered using different SLS parameters. Results indicated that the process window of alumina could be expanded by the addition of B₄C. Furthermore, the amount of B₄C played an important role in surface morphologies of alumina ceramics. It could be explained by the increase of mass transfer due to the addition of B₄C, which enhanced the densification process. The compressive strength of sintered samples increased with the increase of B₄C, which reached its maximum value when the content of B₄C was 7?wt% and the density of the samples after post treatment could reach 1.4?g/cm³. In addition, a size expansion phenomenon was observed. The size expansion could reach 5% after SLS, which could be attributed to the pin effects and oxidation behavior of B₄C particles.     

Porous alumina ceramics with highly aligned pores by heat-treating extruded alumina/camphene body at temperature near its solidification point

This study reports a novel way of increasing the pore size of highly aligned porous alumina ceramics by heat-treating an extruded alumina/camphene body at a temperature near its solidification point. The pore size obtained increased remarkably from 51 ± 8 to 125 ± 27 μm with increasing heat-treatment time from 1 to 24 h, due to the continuative overgrowth of the camphene dendrites during heat-treatment, while a highly aligned porous structure was preserved. In addition, interestingly, this heat-treatment enabled alumina walls to be densified quite well, whereas porous walls were observed in the sample produced without heat-treatment, which led to a considerable increase in compressive strength. The sample produced with a heat-treatment time of 12 h showed a high compressive strength of 11.6 ± 1.2 MPa at a porosity of approximately 84 vol%, which was much higher than that (0.28 ± 0.1 MPa) of the sample produced without heat-treatment.     

Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics

Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.     

Formation,microstructure and properties of aluminum borate ceramics obtained from alumina and boric acid

The formation of aluminum borates (Al₁₈B₄O₃₃ and Al₄B₂O₉) from alumina and boron oxide occurs between 600 and 800 °C. These materials have refractory properties and corrosion resistance. The objective of this work is to develop materials from the Al₂O₃-B₂O₃ system, employing alumina and boric acid as starting powders, to study the critical processing variables and describe the developed microstructure and properties.Three formulations (13, 19.5 and 26 wt% B₂O₃) were studied. In order to confirm the formation of borates, the differential thermal analysis and thermogravimetric analysis were carried out. Afterwards, uniaxially pressed disc-shaped specimens were fired at four temperatures above the formation temperature. The textural properties of the ceramics were evaluated by the immersion method, this permit to evaluate the sintering processes. Then the degree of borate formation was confirmed by X-ray diffraction.Finally, the developed microstructures were characterized by scanning electron microscopy, and the diametral compression behavior was evaluated.A series of porous (≈50%) refractory materials from the Al₂O₃-B₂O₃ system were developed. The processing strategy resulted in materials with Al₁₈B₄O₃₃ as the main crystalline phase. Needle grains with diameters between 0.2 and 1 µm and an aspect ratio over 20:1 were obtained. Thus, based on the information gathered from our research, aluminum borate ceramic materials can be designed for structural, insulating or filtering applications employing only alumina and boric acid as boron oxide source.     

泡沫陶瓷的研究进展

介绍了泡沫陶瓷的特性、制造工艺、用途及在冶金工业中的应用。指出除现有的堇青石质、氧化铝质、堇青石－氧化铝质过滤器外，还开发了氮化硅质、碳化硅质、氧化铝质产品。     

Foam gel-casting preparation of SiC bonded ZrB2 porous ceramics for high-performance thermal insulation

Lightweight SiC-ZrB₂ porous ceramics is of great potential as thermal insulation material used in aerospace, chemical and energy industries. In this work, a series of SiC bonded ZrB₂ (SiC_b-ZrB₂) porous ceramics with porosity high up to 86.9% were prepared by a simple foam gel-casting method. The SiC_b-ZrB₂ porous ceramic prepared at 1573 K exhibited a low thermal conductivity of 0.280 W/(m?K) and a reasonable compressive strength of 0.52 MPa. It could maintain the original geometric shape and microstructure after a secondary heat treatment at 1473 K in inert atmosphere. When heating the samples with thickness of 30 mm for 12 min with an alcohol spray lamp (～1273 K), the temperatures of the cold sides of SiC_b-ZrB₂ ceramics were all lower than 432 K, demonstrating their exceptional insulation capabilities. The present work provides a simple route to produce robust and thermally-insulating non-oxide porous ceramics for use under high temperature.     

Phase-inversion tape casting and synchrotron-radiation computed tomography analysis of porous alumina

A variant of tape casting based on the phase inversion phenomenon was adopted for fabrication of porous ceramic wafer. A slurry was prepared by dispersing alumina powder in an N-methyl-2-pyrrolidone (NMP) solution of the polymers polyethersulfone (PES) and polyvinylpyrrolidone (PVP). The slurry was cast using a doctor blade, and immersed in water to solidify the polymer solution via phase inversion. The green tape was dried and sintered at 1500 °C. The as-prepared ceramic wafer was characterized using synchrotron-radiation computed tomography (SR-CT). It was revealed that the ceramic wafer contained typical finger-like macrovoids, and the porosity resulting from these macrovoids was ~30%. The overall porosity of the wafer was 59%, as derived from the density data measured by Archimedes method in mercury. It is concluded that the phase inversion tape casting is a simple and effective method for preparation of porous ceramics.     

A novel and green preparation of porous forsterite ceramics with excellent thermal isolation properties

This work provides a novel and green approach to preparing porous forsterite ceramics by a transient liquid phase diffusion process based on fused magnesia and quartz powders without detrimental additives. The size of quartz particles markedly affected the sintering behaviors, phase composition, microstructure and properties of the porous forsterite ceramics. Fine quartz particles (D₅₀, 3.87?µm) accelerated the rate of the forsterite formation at elevated temperatures and promoted solid-state sintering behavior of the porous ceramics. Conversely, coarse quartz particles (D₅₀, 25.38?µm) reduced the rate of the solid state reaction and a large amount of unreacted SiO₂ and enstatite (MgSiO₃) phases transformed into a transient liquid-phase during the firing process. This effect resulted in a high porosity (approximately 58.89%) and formation of many large pores (mean pore size of 42.36?µm). These features contributed to the excellent thermal isolation properties of the prepared porous forsterite ceramics. The strength of the obtained porous ceramics (about 23.6?MPa) is relatively high compared with those of conventional ceramics.     

A novel method for the preparation of porous zirconia ceramics with multimodal pore size distribution

This work presents a new route of processing porous ceramic materials by vacuum induction melting of metallic ternary alloys from the system Ni–Zr–Y. Following internal oxidation results in an interpenetrating network of nickel and oxide phase consisting of yttria and zirconia. After dissolution of the metallic nickel matrix the oxide phase remains as a stable porous ceramic material with a multimodal pore size distribution. The porosity, pore size distribution and specific surface area can be controlled by alloy composition, solidification conditions and oxidation parameters.     

叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷

以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.