Ultrahigh-strength alumina ceramic foams via gelation of foamed boehmite sol

Ceramic foams with significant porosity and robust mechanical properties have received extensive attentions. However, it is still difficult to achieve excellent compressive strength at high porosity levels. In this work, a gelation of foamed boehmite sol method is proposed to settle this issue. The technological parameters during preparation process are systematically investigated. As-prepared alumina ceramic foams possess unprecedentedly high compressive strength of 34.1-89.1 MPa at high porosity levels of 66.0%-87.2%, which is attributed to the present of hierarchical pore structure, small grain size and pore size. This work demonstrates a facile and novel method for the fabrication of high-performance alumina ceramic foams toward practical applications.     

Near-zero-shrinkage Al2O3 ceramic foams with coral-like and hollow-sphere structures via selective laser sintering and reaction bonding

The large shrinkage that ceramics undergo during sintering is a severe challenge for high-performance porous ceramics. In this study, we report a powder-based selective laser sintering (SLS) approach to prepare Al₂O₃ ceramic foams with near-zero shrinkage, high porosity, and outstanding strength. The ceramic foams consist of specific coral-like and hollow-sphere structures derived from the raw Al₂O₃/Al composite powders via reaction bonding (RB). A near-zero shrinkage of 0.91 ± 0.15 % and a high porosity of 73.7 ± 0.2 % can be achieved based on the Kirkendall effect during the oxidation of Al particles. Meanwhile, the reinforced sintering necks and robust bond-bridge connections between hollow-sphere and coral-like structures result in a remarkable bending strength of 7.37 ± 0.37 MPa. This measured strength is more than six times higher than other fabricated samples from spherical Al₂O₃ powders, and the comprehensive performance of ceramic foams prepared by this novel SLS/RB strategy is exceptionally remarkable versus that via conventional forming methods.     

Mechanically robust ZrO2 foams with 3D reticular architecture prepared from chemical-modified ZrO2 powder

Engineering mechanically robust ZrO₂ foams with three-dimensional (3D) reticular architecture and high porosity is extremely challenging. In current work, we presented an approach for constructing such cellular ceramics via a combined polyurethane foaming and chemical grafting method. Ester functional groups were chemically grafted on the ZrO₂ powder surface to enhance its dispersibility. This approach caused the micron-sized windows to assemble onto the cell wall of porous framework, contributing to 3D interconnected reticular architecture. The resulting products with such cellular architecture possessed a high porosity level of 89.2% and maintained an ultrahigh compressive strength of 8.5 MPa. Our results open up new opportunities for fabricating high-performance ZrO₂ foams toward practical applications.     

A novel fabrication method for glass foams with small pore size and controllable pore structure

In this paper, a novel fabrication method for glass foams has been introduced based on colloidal suspension foaming method using waste glass as starting materials. It is demonstrated for the first time that foam stabilization via in‐situ hydrophobization of waste glass particles is possible. The fabrication conditions for stable glass particle‐stabilized foams are optimized by investigating the influences of pH value, concentration of propyl gallate and ball milling time, respectively. It is demonstrated that this is a versatile method for fabrication of stable foamed glass suspension and final glass foams with small pore size of dozens of microns, which is much smaller than that of most glass foams in previous literatures. This novel method enables both closed pore structure and open pore structure simply by tailoring solid loading of glass suspension. A “sieve‐like” hierarchical pore structure can be achieved by adjusting sintering temperature. The glass foams with controllable structure could be applied in thermal insulation fields for closed pores, and in catalyst loading, filtration, and separation fields for open pores and hierarchical pore structure.     

Al2O3 hollow nanospheres prepared by fluidized bed chemical vapor deposition and further heat treatment

Hollow-structured Al₂O₃ nanospheres were prepared by chemical vapor deposition (CVD) and further heat-treatment process. For the CVD process, a unique cooling gas loop was designed in the precursor delivery system and amorphous Al₂O₃ nanospheres with incomplete pyrolysis intermediated products were obtained in a vertical fluidized bed at 500℃. The microstructure of the nanospheres tended to hollow after subsequent heat treatment at 1300℃ in air. It was found that the obtained Al₂O₃ hollow nanospheres showed a size of 100-450 nm in diameter and 20-80 nm in shell thickness. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis confirmed that the Al₂O₃ hollow nanospheres were α-Al₂O₃. The possible formation mechanism was proposed based on the results of the thermal gravimetric (TG) measurement connected with a Fourier transform infrared spectrometry (FTIR). The novel method to prepare Al₂O₃ hollow nanospheres can inspire ideas for the design and synthesis of other hollow-structured nanospheres.     

利用核桃壳的形态结构优化Al2O3多孔材料的孔结构和性能

首先将质量分数为5%的ZrO₂溶胶、7%的Al₂O₃溶胶、3%的SiO₂溶胶作为浸渍试剂对核桃壳粉（WSP）浸渍处理。然后以α-Al₂O₃微粉为主原料,以处理后的WSP为造孔剂,制备了Al₂O₃多孔材料。研究了溶胶浸渍处理后WSP对多孔材料孔结构、热导率和力学性能的影响。结果表明,在Al₂O₃多孔材料的孔中可以清楚地观察到WSP的形变,这是优化陶瓷孔结构的重要因素。通过使用质量分数为3%的SiO₂溶胶浸渍处理的WSP,可以获得低热导率(200℃,0.297 W·m^-1·K^-1)和高耐压强度（43.5 MPa）的Al₂O₃多孔材料,并在孔中发现了莫来石的交叉网络结构。     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Environment-oriented low-cost Al2O3 ceramics with hierarchical pore structure fabricated from SiC solid waste

Environment-oriented low-cost Al₂O₃ reticulated porous ceramics with hierarchical pore structure were fabricated by the polymer sponge replica method combined with vacuum infiltration methods, using Al₂O₃ powders and SiC solid waste (SCSW) as raw material and a pore-forming agent. The effects of SCSW addition amount on mechanical properties, microstructure and pore size of Ceramics were investigated. The results showed that the thermal shock resistance of specimens increased gradually with addition of SCSW, however, the median pore diameter increased firstly and then decreased, due to the generation of mullite and liquid phase. After calcination, the residual stress was generated within the coating layer because of the difference in the thermal expansion coefficients of ceramic matrix and coating layer, which could improve the properties of Ceramics by deflecting and bifurcating crack growth path. The results showed that the best dosage of SCSW was 30 wt%.     

A novel design of Al2O3-ZrO2 reticulated porous ceramics with hierarchical pore structures and excellent properties

The use of reticulated porous ceramics(RPCs) was of great interest in high-temperature catalytic application owing to their high surface area. In order to further optimize the pore structure and mechanical properties of RPCs, vacuum infiltration with CaCO₃-Al₂O₃ slurries process was applied to fabricate Al₂O₃-ZrO₂ RPCs with hierarchical pore structures. The pores within ceramic struts were prepared by processes of CaCO₃ decomposition and calcium hexaluminate grains growth. And the compressive residual stress was formed within multi-layered struts owing to the difference in the thermal expansion of coating layer and ceramic struts, which was established as a key factor in improving the mechanical properties and thermal shock resistance of RPCs. Furthermore, the size of pores within struts ranging from 5 μm to 14 μm affected the thermal shock resistance of RPCs significantly based on grey incidence analysis. And the potential of this materials as high-temperature catalyst supports was demonstrated.     

Preparation and characterization of polyimide/Al2O3 hybrid films by sol–gel process

A sol–gel process has been developed to prepare polyimide (PI)/Al₂O₃ hybrid films with different contents of Al₂O₃ based on pyromellitic dianhydride (PMDA) and 4,4′‐oxydianiline (ODA) as monomers. FESEM and TEM images indicated that Al₂O₃ particles are relatively well dispersed in the polyimide matrix after ultrasonic treatment of the sol from aluminum isopropoxide and thermal imidization of the gel film. The dimensional stability, thermal stability, mechanical properties of hybrid PI films were improved obviously by an addition of adequate Al₂O₃ content, whereas, dielectric property and the elongation at break decreased with the increase of Al₂O₃ content. Surprisingly, the corona‐resistance property of hybrid film was improved greatly with increasing Al₂O₃ content within certain range as compared with pure PI film. Especially, the hybrid film with 15 wt % of Al₂O₃ content exhibited obviously enhanced corona‐resistance property, which was explained by the formation of compact Al₂O₃ network in hybrid film. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

鱼雷罐用微气孔高强度Al2O3-SiC砖的研制与应用

以特级矾土(粒度分别为5~3mm、3~1mm、≤1mm、≤0.088mm)、工业SiC(粒度≤1mm)、电熔刚玉(粒度分别为3~1mm和≤0.088mm)和软质黏土粉(粒度≤0.088mm)为原料,按常规生产工艺制备鱼雷罐用烧成Al2O3-SiC砖,研究了粒度组成以及硅微粉、活性氧化铝微粉、蓝晶石微粉和红柱石微粉4种添加剂及其加入量(分别为3%、5%和8%)对Al2O3-SiC砖显气孔率、孔径分布和强度的影响,并在钢厂鱼雷罐上对研制的Al2O3-SiC砖进行了实际使用试验。结果表明:以颗粒紧密堆积理论为基础设计原料的粒度组成,加入合适种类和数量的添加剂,能够生产出强度大,显气孔率低,气孔孔径小且分布窄的Al2O3-SiC砖;研制的Al2O3-SiC砖在钢厂鱼雷罐上使用时具有良好的抗渣侵蚀性和抗铁水冲刷性,粘渣少且易清除,使用寿命比普通Al2O3-SiC-C砖延长34.5%。     

Mechanical properties of Al2O3 and Al2O3/Al with Gyroid structure obtained by stereolithographic additive manufacturing and melt infiltration

To obtain both plasticity and toughness of the material at the same time, various manufacturing techniques of ceramic-metal composites and structures have been studied. In this work, a bio-inspired Al₂O₃ ceramic scaffold with Gyroid structure was designed and prepared by stereolithographic (SL) additive manufacturing, then the Al₂O₃/Al ceramic-metal hybrid structure was prepared by infiltrating molten Al into the Al₂O₃ ceramic structure. The performances of the Al₂O₃ ceramic scaffold and the Al₂O₃/Al ceramic-metal hybrid structure were compared and analyzed by a quasi-static compression experiment. The quasi-static compressive strength of the pristine Al₂O₃ scaffold was 14.36 MPa, while that of the Al₂O₃/Al ceramic-metal hybrid structure was up to 89.06 MPa. Moreover, the plasticity of the Al₂O₃/Al ceramic-metal hybrid structure was much higher than that of the Al₂O₃ scaffold. During compression, the Al₂O₃/Al ceramic-metal hybrid structure had excellent energy absorption, reaching up to 2569.16 KJ/m³, 15 times that of the Al₂O₃ scaffold. Therefore, this method can obtain materials with excellent ductility and toughness.     

A facile approach for preparing Al2O3 reticulated porous ceramics with optimized closed-cell struts and excellent mechanical properties

Reticulated porous ceramics (RPCs) are of great value in applications such as non-ferrous alloys and flue gas filtration. For the purpose of further enhancing the mechanical properties of RPCs without drastically reducing their apparent porosity, a new route to improve the mechanical properties of the strut by constructing a closed-cell structure was proposed to fabricate Al₂O₃ reticulated porous ceramics (ARPCs). The effects of cryolite on the generation of closed pores, the microstructure, and the mechanical performance of struts were investigated. It was found that the addition of cryolite promoted the densification of the pore walls within the struts during sintering. Furthermore, ARPCs added with 10 wt% cryolite achieved an apparent porosity of nearly 80%, while exhibiting excellent mechanical strength and thermal-shock resistance, with compressive strength and residual strength ratio significantly increased by 56% and 11%, respectively, compared to the samples without cryolite addition.     

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.     

A novel approach to improve flexural strength of Al2O3‐20 wt% ZrO2 composites by oscillatory pressure sintering

In this communication, Al₂O₃‐20 wt% ZrO₂ composites (ZTA) were prepared by a novel oscillatory pressure sintering (OPS) process. An almost perfectly full density of 99.94% and a relatively high flexural strength of 1145 MPa can be achieved for the OPS‐fabricated specimen, which were higher than those prepared by pressureless sintering (98.24% and 682 MPa) and hot pressing (99.53% and 990 MPa). The enhancement of flexural strength of the OPS‐fabricated specimen was correlated with its smaller flaw size and higher fracture surface energy. It was suggested that OPS was an effective approach to fabricate ZTA composites with ultrahigh density and high strength.     

豆荚结构聚乙烯醇缩丁醛/氧化铝复合相变纤维的制备及性能

利用微流控技术成功制备了聚乙烯醇缩丁醛（PVB）基材中共混氧化铝（Al₂O₃）纳米颗粒，其内部包封正十五烷的豆荚结构复合相变纤维。系统考察了Al₂O₃的含量对复合相变纤维的微观结构、相变性能和导热性能的影响规律。研究结果表明，复合相变纤维的致密表面可将正十五烷良好地包封于其内部的独立腔室内，包封率约为30%。在PVB基材中添加Al₂O₃纳米颗粒对复合相变纤维的豆荚结构和相变焓均无明显影响，但可显著提高相变纤维的导热性能。在模拟太阳光照射下，添加10% Al₂O₃纳米颗粒的复合相变纤维的表面温度比不含Al₂O₃纳米颗粒的相变纤维的表面温度升高更快，前者的熔融时间比后者缩短了25%。研究结果将为制备结构可控、具有良好、稳定导热性能和高效、快速调温性能的相变纤维提供重要的实验参考和指导。     

Synthesis and characterization of Al2O3-TiC-ZrO2 ceramics with intragranular nanostructure by spark plasma sintering

In this paper, Al₂O₃-TiC ceramic composites with intragranular nano-ZrO₂ were prepared in vacuum by spark plasma sintering (SPS). The effect of ZrO₂ particles with different nano-sizes on the microstructure and mechanical properties of ceramics was studied. The results show that SPS can achieve relative densification of materials without generating new impurity phases. At the same time, the sintering densification temperature of ceramic materials can be reduced by adding ZrO₂ (20 nm) particles. Under the action of SPS strong electric field, the nano-ZrO₂ adsorbed on the surface of the matrix particles can enter the interior of matrix grains, and form intragranular nanostructures when the grain boundaries move and the particles merge. The microstructure and mechanical properties of ceramic materials can be improved through the intragranular structure formed by nanoparticles. The main reasons for the increased strength and toughness of ceramic materials are crack deflection, crack bridging and transgranular fracture.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.     

Directly fabricated Al2O3/GdAlO3 eutectic ceramic with large smooth surface by selective laser melting: Rapid solidification behavior and thermal field simulation

Selective laser melting (SLM), a novel approach for one-step melting and solidifying ceramic powder beds layer by layer without post-process of degreasing and sintering, has been developed to directly prepare highly dense (>95 %) Al₂O₃/GdAlO₃(GAP) eutectic composite ceramics with large smooth surfaces. Compact net-shaped plates with the maximum size of 73 × 24 × 5 mm³ are obtained by different strategies of laser pre-heating and multi-tracks’ deposition without any binders. Combined with the finite element thermodynamic coupling simulation results, it is proved that the stress between the substrate and depositions during SLM can be greatly reduced by the step-up preheating, and thus effectively improving the ceramic forming quality. The macro-morphology, microstructure evolution, rapid solidification behavior and mechanical properties of the SLM-ed eutectic ceramics are systematically investigated at different laser processing parameters. The microstructure transforms from ultra-fine irregular eutectic to complex regular eutectic with the increase of the scanning rate. The average eutectic spacing, and solidification rate has an approximately linear relationship consistent with the Jackson-Hunt (JH) model. The microhardness and fracture toughness can reach 17.1 ± 0.2 GPa and 4.5 ± 0.1 MPa·m^1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.