Preparation and characterisation of closed-pore Al2O3-MgAl2O4 refractory aggregate utilising superplasticity

ABSTRACT

A novel high closed porosity Al₂O₃-MgAl₂O₄ refractory aggregate has been successfully fabricated by utilising superplasticity with Al₂O₃ and MgO as raw materials, SiC as high temperature pore-forming agent. The effects of the addition amounts of MgO and SiC on porosity, sintering behaviours, phase composition, pore size distribution and microstructure of the refractory aggregate have been investigated. The formation mechanism of the closed pore in the refractory aggregate has been discussed. The results showed that the MgO can improve the superplastic deformation ability of Al₂O₃-based ceramic at high temperature. With the content of MgO and SiC increased, the closed porosity and the pore size increased. The oxidation of SiC improved the sinterability of materials at the initial stage of sintering, and then the released gases due to the further oxidation of SiC promoted the formation of closed pores by motivating the superplastic deformation ability of Al₂O₃-based materials.     

Al2O3基陶瓷摩擦材料的研制及性能研究

以Al2O3为主要原料,用粘土作烧结助剂,再加入其他辅助原料制备了Al2O3基陶瓷摩擦材料。研究了不同配比的原料对Al2O3陶瓷摩擦材料的体积密度、气孔率、吸水率、抗压强度、摩擦性能以及显微结构的影响。对氧化铝基陶瓷摩擦材料的实际应用有重要的指导意义。     

α-Al_2O_3微粉对低碳MgO-C材料性能的影响

以电熔镁砂及鳞片石墨为主要原料,热塑性酚醛树脂为结合剂,研究了α-Al_2O_3微粉对低碳MgO-C材料性能的影响.结果表明:(1)Al_2O_3微粉的加入提高了低碳MgO-C材料的体积密度,降低了显气孔率,提高了强度,显著改善了MgO-C砖的抗氧化性;(2)高温下Al_2O_3微粉与MgO原位反应生成连续的尖晶石相,改善了材料基质的显微结构,增强了陶瓷结合,将MgO颗粒与鳞片石墨紧密地结合起来,提高了组织结构的整体性,从而提高了材料的致密度和强度;(3)高温下原位反应生成连续的尖晶石相堵塞脱碳层中的部分气孔,降低了氧的扩散速度,同时提高了MgO的沉积速度,使加入Al_2O_3微粉的低碳MgO-C材料抗氧化性能得以提高.     

Fabrication of porous magnesium spinel with directional pores by unidirectional solidification

A porous magnesium spinel (MgAl₂O₄) with directional pores was fabricated by unidirectional solidification in pressurized argon and hydrogen mixed gases. Two different kinds of pores with large directional and small facet shape were formed in the solidified samples. The former pores were dominant in the porous structure. A small amount of free corundum phase was formed in the solidified porous spinel as a secondary phase by vaporization of MgO component during the solidification process. With increasing total gas pressure, the pore size of the solidified samples decreased while no change in the porosity. The porosity and pore size of the samples increased with increasing hydrogen partial pressure. The porosities of the samples fabricated under 10%H₂–90%Ar and 1%H₂–99%Ar mixed gases were 30 and 10%, respectively, and that under Ar atmosphere was very low, non-porous.     

氧化铝多孔支撑体的制备工艺

叙述了用挤出成型法制备无机膜支撑体的工艺,研究了原料粒径、成孔剂用量和烧结温度对所制得多孔氧化铝支撑体性能的影响。研究结果表明,用粒径小于10μm的-αA l2O3粉体,以7%碳粉为成孔剂,烧结温度为1300℃,可以成功制得孔径分布较窄、平均孔径为2.1μm、孔隙率为48.9%的多通道无机膜支撑体。     

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%.     

矿化剂对Al2O3-SiO2质陶瓷辊棒性能的影响

研究了含MgO的单一型矿化剂及含MgO和TiO2 的复合型矿化剂对Al2 O3-SiO2 质陶瓷辊棒性能的影响。结果表明 :基质中Al2 O3与SiO2 的质量比 (Al2 O3/SiO2 )为 2 .33时 ,引入 0 .6 %的复合型矿化剂可促进莫来石的发育和基质的烧结 ,明显改善了陶瓷辊棒的抗热震性和机械强度。     

二氧化硅和氧化铝微粉加入量对MgO基浇注料流变性的影响

采用电熔镁砂颗粒作骨料,电熔镁砂粉、电熔白刚玉粉、Al2O3微粉和SiO2微粉作基质料,固定骨料与基质料质量比为65:35,且固定基质料中Al2O3微粉与SiO2微粉的总加入量为8%,改变两种微粉的加入比例(Al2O3微粉的加入量分别为8%、6%、4%、2%、0,SiO2微粉的相应为0、2%、4%、6%、8%),分别加6.4%的水制成MgO基浇注料,并采用浇注料流变仪研究了Al2O3微粉与SiO2微粉的加入量对浇注料流变性(扭矩、流动阻力和粘度)的影响。研究结果表明:随着SiO2微粉加入量的增加,浇注料的扭矩、流动阻力和粘度呈降低趋势,但SiO2微粉加入量超过4%后变化很小。     

Hierarchically porous ceria with tunable pore structure from particle-stabilized foams

We firstly fabricated CeO₂ ceramic foams with tunable structure by using particle-stabilized bubbles as template, and designed their interconnected porous structure and even hierarchically porous structure, which endows them the penetration ability for gases or liquids. Hollow spheres with single-layer shell were innovatively selected as the pore-former, which allows for the formation of open pores on the cell wall. Moreover, 3D printing CeO₂ particle-stabilized foams are realized with the aid of direct ink writing, which enables the production of CeO₂ ceramic foams with complex shape. Highly porous CeO₂ with relatively high compressive strength have been fabricated, the porosity of which varies from 81.0% to 92.0% while their excellent compressive strength ranges from 5.0 MPa to 20.0 MPa. Attributed to the hierarchical porous structure, uniform pore size distribution as well as densely assembled cell wall, 3D printing CeO₂ ceramic foams possess superior mechanical performance at high porosity level.     

SiC晶须增强Al2O3—TiC复相陶瓷的研究

运用品须补强和粒子弥散双重手段,对氧化铝材料的性能进行改进。比较了Al_2O_3-TiC(AT),Al_2O_3/SiC_w(As),Al_2O_3-TiC/SiC_w(ATS)材料的力学性能,证实ATS材料有明显的迭加增强增韧效果,并具有良好的高温力学性能。抗氧化实验证实,将SiC晶须加入AT中以后,材料的抗氧化性能有明显改进。从热膨胀失配角度分析和SEM显微结构观察证实,ATS材料的主要增韧机理是界面解离和裂纹偏转。     

Fabrication and characterization of cordierite-bonded porous SiC ceramics

A reaction bonding technique was used for the preparation of cordierite-bonded porous SiC ceramics in air from α-SiC, α-Al₂O₃ and MgO, using graphite as the pore-forming agent. Graphite was burned out to produce pores and the surface of SiC was oxidized to SiO₂ at high temperature. With further increasing the temperature, SiO₂ reacted with α-Al₂O₃ and MgO to form cordierite. SiC particles were bonded by the cordierite and oxidation-derived SiO₂. The reaction bonding characteristics, phase composition, open porosity, pore size distribution and mechanical strength as well as microstructure of porous SiC ceramics were investigated. The pore size and porosity were strongly dependent, respectively, on graphite particle size and volume fraction. The porous SiC ceramics sintered at 1350 °C for 2 h exhibited excellent combination properties, the flexural strength of 26.0 MPa was achieved at an open porosity of 44.51%.     

Porous Mullite Preforms via Fused Deposition

Nonrandom porous mullite ceramic preforms were fabricated by the indirect fused deposition process. MgO was added to commercial mullite powders as a sintering aid. The influence of porosity and the amount of MgO on the densification and mechanical properties was studied. The shrinkage and compressive strength of the porous samples were found to increase with increasing MgO content. At constant MgO concentration, shrinkage was found to decrease linearly as the volume fraction of porosity increased. The compressive strength of porous samples decreased with an increase in the volume fraction of porosity at constant MgO content.     

Processing of Porous SiC by Aluminum‐Derived Binders and Sacrificial Porogen Leaching Method

Porous SiC was successfully fabricated by a facile and energy efficient sacrificial porogen leaching method using in situ synthesized aluminum‐based binders by reaction bonding at low sintering temperatures of 600–1000°C. Porous SiC ceramics with porosity in the range of 30–58% and compressive strength of 1–33 MPa were obtained. Interconnected bimodal pores were produced by both stacking of SiC particles and leach out of salt. During sintering, the aluminum binder experienced metal to ceramic transformations forming various alumina polymorphs (γ, δ, θ and α‐Al₂O₃). The porogen content and sintering temperatures significantly influence the properties of porous SiC.     

Effect of technological parameters on densification of reaction bonded Si/SiC ceramics

Si/SiC composite ceramics was produced by reaction sintering method in process of molten silicon infiltration into porous C/SiC preform fabricated by powder injection molding followed by impregnation with phenolic resin and carbonization. To optimize the ceramics densification process, effect of slurry composition, debinding conditions and the key parameters of all technological stages on the Si/SiC composite characteristics was studied. At the stage of molding the value of solid loading 87.5% was achieved using bimodal SiC powder and paraffin-based binder. It was found that the optimal conditions of fast thermal debinding correspond to the heating rate of 10?°C/min in air. The porous C/SiC ceramic preform carbonized at 1200?°C contained 4% of pyrolytic carbon and ～25% of open pores. The bulk density of Si/SiC ceramics reached 3.04?g/cm³, silicon carbide content was 83–85?wt.% and residual porosity did not exceed 2%.     

Preparation of closed-pore MgO-MgFe2O4 aggregates with low thermal conductivity and high mechanical properties

MgO-MgFe₂O₄ refractory aggregates with high closed porosity were fabricated using MgO agglomerates and Mg(OH)₂ with introducing Fe₂O₃ additive. The evolutions of pores and microstructure and their relationship with the properties of the specimens were studied. The addition of Fe₂O₃ obviously promoted the MgO grain growth and conversion of large open pores into small closed pores, attributing to the formation of cationic vacancies and intergranular MgFe₂O₄ bonding phase. Owing to the presence of closed pores and networks of intergranular MgFe₂O₄, both thermal insulation and strength were enhanced significantly. Besides, the formed closed pores and MgFe₂O₄ phase could accommodate thermal stress and induce transgranular fracture and crack deflection, therefore effectively improving the thermal shock resistance. The specimen with 15 wt% Fe₂O₃ showed a apparent/closed porosity of 0.7%/10.1%, median pore diameter of 4.37 µm, thermal conductivity of 9.3 W/(m·K) (500 °C), flexural strength of 143.5 MPa, and residual flexural strength of 24.1 MPa after thermal shock.     

Prediction of crack deflection in porous/dense ceramic laminates

The arrangement of ceramic layers in laminated structures is an interesting way to enhance the flaw tolerance of brittle ceramic materials. The interfaces are expected to deflect cracks, increasing the fracture energy of the laminate compared to a monolithic material and thus raising the toughness.Laminates have been fabricated with alternating dense and porous layers of the same material, i.e. SiC or B₄C, in order to obtain a good chemical compatibility between the laminas and almost no thermal residual stresses. Porosity, in the porous layers, is achieved by incorporating organic particles which are removed during the debinding step. In this context, the target of this study is to predict the volume fraction of pores, in the porous layer, required to cause crack deflection.The proposed criterion derives from an energy balance. It relies on a two-scale analysis taking into account the laminated structure of the material. It can be written in terms of two relevant material parameters: the ratio of Young's moduli of the dense and porous materials and toughness ratios. A unique function depending on the volume fraction of pores can be used to express the two above-mentioned ratios. Assuming a cubic lattice of spherical voids, the parameters of the porous ceramic depend linearly on the porosity and vanish at the point of percolation of pores. As a consequence, the criterion can be rewritten in term of a single parameter: the porosity.Crack deflection is permitted only for very high values of the porosity. Predicted values agree satisfactorily with experiments on SiC and B₄C. The comparison with the He and Hutchinson criterion shows that this latter underestimates the correct value.     

Al2O3/BN复相陶瓷的R曲线特性

采用压痕-强度法测定了Al2O3／BN复相陶瓷的阻力曲线。结果表明;该复相陶瓷具有良好的耐接触损伤性能,呈现出上升的阻力曲线效应,显示出增强的抗裂纹扩展能力。而对应的Al2O3单相陶瓷的阻力曲线为一水平直线。分析认为：超细弥散的h—BN与基体之间模量及热膨胀失配会在h—BN的弱结合层间产生微开裂,有助于抑制主裂纹的失稳扩展,这是Al2O3／BN复相陶瓷具有优良耐损伤性能和上升型R曲线效应的主要原因。     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Fabricating porous ceramic materials via phase separations in blends of cellulose acetate and ceramic nanoparticles

As applications of porous ceramic materials have gradually expanded, the novel technologies for the fabrication of porous ceramic materials with a delicate and controllable structure are still attractive. In this work, three types of porous monolithic ceramic materials, including Al₂O₃–SiO₂, TiO₂, and SiC, have been fabricated by thermally impacted and non-solvent-induced phase separations in blends of cellulose acetate and ceramic nanoparticles. These materials possessed three-dimensional interconnected porous structures with low densities, high porosities, and hierarchical pores ranging from 5 nm to 6 μm. The relationships between microstructures and phase separations were systematically investigated. Furthermore, electromagnetic shielding effectiveness of 20 dB from 5 to 18 GHz in porous SiC materials has been achieved, revealing that those materials have potential applications in the electromagnetic shielding. This work provides a powerful and general approach to fabricate porous monolithic ceramic materials with a wide range of various ceramic nanoparticles.     

Al_2O_3-Si材料加热过程中物相组成和显微结构研究

以电熔刚玉骨料和细粉为主要原料,引入5%～8%的Si粉,以纸浆废液为结合剂制备了Al_2O_3-Si材料.研究了Al_2O_3-Si材料埋碳加热(1000～1600 ℃)过程中物相组成和显微结构的变化.结果表明: 1200 ℃烧后,Si开始与CO反应生成晶须状SiC,且晶须相互交叉连锁.1300～1400 ℃烧后,SiC晶须生成量明显增加,且发育长大,形成交叉连锁的网络填充在刚玉骨架结构中.1400 ℃烧后试样中还出现少量O'-SiAlON晶体.1500～1600 ℃烧后,SiC晶体发育长大,呈枝杈状和弯曲状.1400～1500 ℃烧后残留Si已熔化,发生塑性变形,起助烧结作用.