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.     

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.     

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.     

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.     

Effect of dispersant on paste rheology in preparation of porous alumina with oriented pores by extrusion method

The effect of additives on paste rheology was investigated for preparation of porous ceramics with unidirectionally aligned cylindrical pores. Ammonium poly-carboxylic acid (APA) used as a dispersant and it was adsorbed on alumina powder surface. The adsorption isotherm of APA was fitted by Langmuir equation. The saturated monolayer adsorption was 5.9 mg/g. The apparent viscosity became a minimum at 0.8 mass % of APA corresponding to 71.2 mPa⋅s. This APA amount of 5.6 mg/g, is in good agreement with the observed APA amount. Since the nylon 66 fibers (0–35 vol. %) mixed with the alumina powder have a strong interaction with each other, they became twisted and agglomerated. This agglomeration increased with increasing fiber content but decreased by adding oleic acid. The pastes with added oleic acid were capable of being extruded at higher pressure. The obtained porous alumina ceramics showed highly oriented cylindrical pores parallel to the extrusion direction. The pore orientation was higher in the oleic acid added pastes than those without oleic acid. The added nylon 66 fibers are mostly converted to pores while maintaining the original shape after sintering. The pore size distribution of the obtained porous ceramics measured by mercury porosimetry method showed a peak at about 4 μm which is apparently smaller than that observed in the SEM photographs and the obtained result is considered to be corresponding to the necks formed by fiber contacts.     

Effect of dispersant on paste rheology in preparation of porous alumina with oriented pores by extrusion method

The effect of additives on paste rheology was investigated for preparation of porous ceramics with unidirectionally aligned cylindrical pores. Ammonium poly-carboxylic acid (APA) used as a dispersant and it was adsorbed on alumina powder surface. The adsorption isotherm of APA was fitted by Langmuir equation. The saturated monolayer adsorption was 5.9 mg/g. The apparent viscosity became a minimum at 0.8 mass % of APA corresponding to 71.2 mPa?s. This APA amount of 5.6 mg/g, is in good agreement with the observed APA amount. Since the nylon 66 fibers (0–35 vol. %) mixed with the alumina powder have a strong interaction with each other, they became twisted and agglomerated. This agglomeration increased with increasing fiber content but decreased by adding oleic acid. The pastes with added oleic acid were capable of being extruded at higher pressure. The obtained porous alumina ceramics showed highly oriented cylindrical pores parallel to the extrusion direction. The pore orientation was higher in the oleic acid added pastes than those without oleic acid. The added nylon 66 fibers are mostly converted to pores while maintaining the original shape after sintering. The pore size distribution of the obtained porous ceramics measured by mercury porosimetry method showed a peak at about 4 μm which is apparently smaller than that observed in the SEM photographs and the obtained result is considered to be corresponding to the necks formed by fiber contacts.     

Formation of porous alumina with oriented pores

Adopting the theories describing the bubble forming in the metal–hydrogen solidification process, porous alumina with oriented pores was prepared by combining a foaming method with sol-gel technology. The bubble forming process in the sol-gel is different from that in the metal–hydrogen system. Samples were calcined at temperature from 800 to 1200C. The volume-shrinkage and compressive-strength increased with increasing calcination temperature. The porous alumina exhibited a bimodal pore structure when prepared below 1200°C.     

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.     

多孔氧化铝陶瓷的制备、表征及性能研究

以蔗糖溶液为低温介质，采用冷冻干燥法和退火工艺制备多孔Al2O3陶瓷。研究了烧结温度和退火时间对多孔陶瓷孔隙结构、开孔率和力学性能的影响。结果表明，随着烧结温度的升高，试样的线性烧成收缩率有明显的升高，开孔率和维氏硬度先缓慢降低，当烧结温度为1600℃时迅速下降。平均晶粒尺寸是影响甚至决定多孔氧化铝陶瓷维氏硬度的主要原因。随着退火时间的延长，多孔陶瓷的孔径显著增大，多孔陶瓷开孔率范围为40.35%～64.58%，退火处理后的孔隙率比未退火处理提高了60.05%。多孔陶瓷的抗压强度随退火时间的延长而降低，而在最长的24h退火时间后，多孔陶瓷的抗压强度仍能达到25.9MPa，可以满足许多应用领域的强度要求。可以通过调节退火时间来控制多孔陶瓷的孔隙结构、开孔率和抗压强度。     

助烧剂和造孔剂对真空烧结SiC多孔陶瓷性能的影响

以粒度≤0.063mm的SiC为主要原料,分别加入30%(质量分数)的Al2O3-Y2O3与10%的Al2O3-高岭土复合助烧剂,并外加不同量(分别为12.8%、26.3%、30.0%和36.4%)的造孔剂羧甲基纤维素钠(CMC),制样后首先在空气炉中经过300℃2h或1100℃4h的预烧,然后在真空炉中于1550℃4h真空烧结而制备成SiC多孔陶瓷,并研究了助烧剂种类以及造孔剂CMC外加量对SiC多孔陶瓷显微组织、显气孔率及抗折强度的影响。结果显示:采用Al2O3-Y2O3作为助烧剂的SiC多孔陶瓷比Al2O3-高岭土作助烧剂的具有较高的抗折强度,显气孔率稍有减小;随着羧甲基纤维素钠量的增加,加入两种助烧剂的SiC多孔陶瓷均表现为显气孔率增加,抗折强度降低。     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Microstructure and properties of pore-created SiC ceramics with calcium chloride as pore former

Pore-created silicon carbide (SiC) ceramics were liquid phase sintered (LPS) by using Al₂O₃–Y₂O₃ as sintering additive and calcium chloride (CaCl₂) as pore former. The CaCl₂ did not react with other compositions, and accumulatively formed CaCl₂ crystals on the grain boundary of SiC ceramics. The addition of CaCl₂ decreased the sintering and mechanical properties of SiC ceramics, but obviously reduced the dry friction coefficient. The pores on the surface and inside of SiC ceramics could be continuously created by the solubility and non-volatility of calcium chloride.     

A urea crystal templating method for the preparation of porous alumina ceramics with the aligned pores

A novel process for the preparation of porous alumina ceramics with an aligned pore structure has been reported. The urea dissolved in the aqueous alumina slurry at higher temperature formed rod shaped crystals aligned in the direction of gravity when cooled to room temperature. The gelatin used to set the slurry, controls the crystallization of urea such that the urea crystals produced in the suspension containing the gelatin had much lower width and thickness compared to that produced in the suspension without the gelatin. The alumina powder catalyzed the thermal decomposition of urea that enabled the removal of the majority of the urea crystals from the alumina green body by isothermal heating at temperature much lower than the melting point of urea. The void space created by the removal of the urea crystals remained as pores in the sintered ceramics.     

Tailored pore structures and mechanical properties of porous alumina ceramics prepared with corn cob pore-forming agent

In this work, the effects of porosity and different particle sizes of pore-forming agent on the mechanical properties of porous alumina ceramics have been reported. Different grades of porous alumina ceramics were developed using corn cob (CC) of different weight contents (5, 10, 15, and 20 wt%) and particle sizes (<63 µm, 63-125 µm and 125-250 µm) as the pore-forming agent. Experimental results showed that total porosity and pore cavity size of the porous alumina ceramics increased with rising addition of CC pore former. Total porosity increased with increasing particle size of CC with the Al₂O₃-^<63CC₅ sample exhibiting the lowest total porosity of 41.3 vol% while the highest total porosity of 68.1 vol% was exhibited by the Al₂O₃-^125-250CC₂₀. The particle size effect of CC on the mechanical properties revealed that diametral tensile strength and hardness of the porous alumina ceramics deteriorated with increasing particle size of CC pore former. The Al₂O₃-^<63CC₅ sample exhibited the highest diametral tensile strength and hardness of 25.1 MPa and 768.2 HV, respectively, while Al₂O₃-^125-250CC₂₀ exhibited the lowest values of 1.1 MPa and 35.9 HV. Overall, porous alumina ceramics with the smallest pore sizes under each particle size category exhibited superior mechanical properties in their respective categories.     

Investigating the effect of porosity level and pore former type on the mechanical and corrosion resistance properties of agro-waste shaped porous alumina ceramics

The strength integrity and chemical stability of porous alumina ceramics operating under extreme service conditions are of major importance in understanding their service behavior if they are to stand the test of time. In the present study, the effect of porosity and different pore former type on the mechanical strength and corrosion resistance properties of porous alumina ceramics have been studied. Given the potential of agricultural wastes as pore-forming agents (PFAs), a series of porous alumina ceramics (Al₂O₃-xPFA; x=5, 10, 15 and 20 wt%) were successfully prepared from rice husk (RH) and sugarcane bagasse (SCB) through the powder metallurgy technique. Experimental results showed that the porosity (44–67%) and the pore size (70–178 µm) of porous alumina samples maintained a linear relationship with the PFA loading. Comprehensive mechanical strength characterization of the porous alumina samples was conducted not just as a function of porosity but also as a function of the different PFA type used. Overall, the mechanical properties showed an inverse relationship with the porosity as the developed porous alumina samples exhibited tensile and compressive strengths of 20.4–1.5 MPa and 179.5–10.9 MPa respectively. Moreover, higher strengths were observed in the SCB shaped samples up to the 15 wt% PFA mark, while beyond this point, the silica peak observed in the XRD pattern of the RH shaped samples favored their relatively high strength. The corrosion resistance characterization of the porous alumina samples in hot 10 wt% NaOH and 20 wt% H₂SO₄ solutions was also investigated by considering sample formulations with 5–15 wt% PFA addition. With increasing porosity, the mass loss range in RH and SCB shaped samples after corrosion in NaOH solution for 8 h were 1.25–3.6% and 0.44–2.9% respectively; on the other hand, after corrosion in H₂SO₄ solution for 8 h, the mass loss range in RH and SCB shaped samples were 0.62–1.5% and 0.68–3.3% respectively.     

Permeability and corrosion resistance of the porous alumina doped by Y2O3 with monodispersed PMMA as pore former

Porous alumina, with monodispersed PMMA as pore former and Y₂O₃ as sintering additive, was prepared via a gel casting route with Isobam as a gelling agent. The effects of PMMA addition on its properties, including apparent porosity, bulk density, strength, permeability, and corrosion resistance to acid/alkali, were investigated. With PMMA addition increased, the apparent porosity and permeability were increased obviously, while strength and corrosion resistance to acid/alkali were deteriorated due to increased porosity. Higher firing temperature resulted in lower porosity, higher strength, lower permeability, and better corrosion resistance to acid/alkali. Coarser raw powders resulted in lower strength and higher permeability due to the coarser structure and larger pores of the fabricated samples. Because Y₂O₃ was used as a sintering additive, and no silica was introduced, the resulting samples possess better corrosion resistance to acid and alkali, especially much better corrosion resistance to alkali, than those reported with silica introduced.     

Multi-functional mullite fiber-based porous ceramics with a multilevel pore structure assembled by alumina platelets and mullite whiskers

Mullite fiber-based porous ceramics have been widely used in the field of heat insulation. To further broaden their applications in other fields, such as filtration and sound absorption, mullite whiskers and alumina platelets were introduced as the secondary structural materials in mullite fiber-based porous ceramics by a sol-gel combining heat-treating method, and new fiber-based porous ceramics with a unique multilevel pore structure were developed. By adjusting the molar ratios of aluminium tri-sec-butoxide to aluminium fluoride and calcination temperature, these fiber-based porous ceramics not only presented the characteristics of lightweight (maximum density of 0.38 g/cm³) and good heat insulation (minimum thermal conductivity of 0.11 W/mK) comparable to traditional fiber-based porous ceramics, but also showed a superior specific surface area (up to 11.5 g/m²) and excellent sound absorption performance (average sound absorption coefficient as high as 0.728). Owing to these outstanding characteristics, the corresponding porous ceramics are expected to be promising multifunctional materials in diverse fields, especially thermal insulation and sound absorption.     

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.     

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.     

Preparation of porous alumina ceramic with ultra-high porosity and long straight pores by freeze casting

Porous alumina ceramic was prepared by freeze casting method using tert-butyl alcohol as the solvent. The as?Cprepared porous alumina ceramic possessed long straight porous structure. The non-dendrite pore feature was quite distinguished from that prepared based on common solvents such as water and camphene. The porosity of the ceramic could be regulated through the solid loading. When the solid loading in the slurry was 20?vol%, the porosity of the alumina ceramic was 65%. With decreasing the solid loading, the porosity of the alumina ceramic increased linearly. The relationship between the total porosity (P) and initial solid loading (X) can be expressed as P?=?98.8?1.7X. The ultra-high porosity of 82% could be achieved when the solid loading was 10?vol%. Moreover, the density of the porous alumina ceramic with the porosity of 82% was even lower than water??s. The compressive strength of the porous alumina ceramic with the porosity of 63 and 82% was determined to be 37.0 and 2.6?MPa, respectively.