Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.     

Effect of gas flow rates and nozzle throat width on deposition of α-alumina films of granule spray in vacuum

Deposition of α-alumina film by spraying alumina granules in a vacuum at room temperature was performed using a modified aerosol deposition system that had a supplemental gas flow line in addition to the carrier gas flow line. The pressure difference between the nozzle inlet and deposition chamber was increased by increasing the supplemental gas flow rate or by decreasing the nozzle throat width. For the same nozzle, the alumina film thickness per granule consumption increased as the pressure difference was increased. However, the film obtained by using the wide throat nozzle was thicker than that by using the narrow throat nozzle in spite of the smaller pressure difference except for the case without using the supplemental gas. The deposition behaviors according to the supplemental gas flow rate and the nozzle throat width were explained in part by the friction-affected layer near the nozzle throat wall and a critical granule velocity for film deposition. Alumina granules and films were characterized by scanning electron microscopy, X-ray diffractometry, surface profilometry, and transmission electron microscopy to assess and correlate the film quality to the deposition conditions.     

Comparison of freeze drying and spray drying to obtain porous nanostructured granules from nanosized suspensions

This work studies the spray drying and freeze drying of different nanosized ceramic materials and the physicochemical characteristics of the obtained granules. Colloidal suspensions of alumina, titania, and a 87/13 mixture were studied. The influence of temperature, pressure, nozzle diameter, and solids loading on the morphology and characteristics of dried granules were evaluated. It was demonstrated that these processing parameters have practically no influence, and the only parameter determining the granules characteristics is the solids content of the suspensions, in both processes. Spray drying leads to a monomodal distribution with higher granule size, while freeze drying produces more porous granules, with a bimodal intragranular distribution. The flowability of spray-dried powder is better than that of the freeze-dried powder and suit better the requirements of a feedstock targeted to obtain coatings by plasma thermal spraying whereas freeze drying can produce high porosity, softer granules.     

过滤器用氧化铝多孔陶瓷

以氧化铝为原料 ,用碳化硅和活化剂混合物作粘合剂 ,用碳酸氢铵作发泡剂制得了过滤器用氧化铝多孔陶瓷 ,并研究了添加剂量和烧成温度对陶瓷气孔率和强度的影响     

Growth of Alumina/Metal Composites into Porous Ceramics by the Oxidation of Aluminum

Alumina/metal composites were grown into the pores of porous alumina, porous aluminosilicate, and porous silicon carbide substrates through the oxidation of Al–Si (5 wt%) powder compacts coated with magnesia powder (11 mg/ cm²). The thickness of the resulting composite increased with oxidation time and temperature, and was proportional to (pore size)^0.5 on using porous alumina. The composite thickness was more than 2 times larger in the silicon carbide and about 4 times larger in the aluminosilicate than in the alumina at 1523 K for 1 h. The products using these three types of substrates consisted of alumina, aluminum, and silicon, except that a silicon carbide phase occurred when using the silicon carbide substrate. Silica and mullite in the aluminosilicate substrate changed to silicon and alumina, and silica in the silicon carbide substrate changed to silicon because of the reduction by aluminum.     

Cavitation wear of structural oxide ceramics and selected composite materials

The usage of ceramic materials in the applications endangered by intensive cavitation could limit erosion phenomena. In the presented work, cavitation erosion resistance of commonly used (in structural application), oxide phases (α-alumina, tetragonal zirconia) were investigated. Additionally, the behaviour under cavitation conditions of two composite materials, based on alumina and zirconia matrices, was tested.Significant difference in cavitation wear mechanisms for alumina and tetragonal zirconia materials was observed. Alumina was degraded by removing the whole grains from the large surface subjected to cavitation. Degradation of zirconia proceeded locally, along ribbon-like paths of removed grains. Cavitation wear of composites was strongly dependent on the residual stress state in the material. Alumina/zirconia composite with compressive stresses in the matrix showed a significant improvement of cavitation resistance. The zirconia/tungsten carbide composite with relatively high level of tensile stresses in the matrix was the worst of all investigated materials.     

PE-HD-g-MAH共混改性PA612性能研究

通过马来酸酐接枝高密度聚乙烯(PE-HD-g-MAH)熔融共混改性尼龙612(PA612),研究了玻纤增强改性后的PA612复合材料。结果发现,随着PE-HD-g-MAH含量增加,PA612的结晶温度逐渐下降。通过肉眼和光泽仪研究PE-HD-g-MAH对玻纤增强PA612表观的影响,结果发现随着接枝物的引入,相同条件下复合材料注塑后的样件浮纤更少,光泽度更高。尤其在低模温的条件下,这种差异更加明显。加入PE-HD-g-MAH共混改性后的玻纤增强PA612的复合材料对模温的要求更低,表观更好,改善了其成型性。同时,PE-HD-g-MAH的引入使拉伸强度有小幅度的下降(6%),同时可以提高其缺口冲击强度(18%),对PA612有一定的增韧效果。最后,研究了共混改性后对复合材料的吸水性及尺寸稳定性的影响。PE-HD-g-MAH可以有效降低PA612的吸水性,提高PA612复合材料制件的尺寸稳定性。     

Fabrication and characterization of alumina fiber by anodic oxidation and chemical dissolution processes

Alumina fiber was fabricated by an anodic oxidation process from pure aluminum or a chemical dissolution process from porous alumina. In the experiments, porous alumina layer was firstly formed on the surface of pure aluminum by anodic oxidation process in phosphoric acid electrolyte. The alumina fiber was obtained by either further anodic oxidation process or a chemical dissolution process from the porous alumina layer. The thickness of the porous alumina layer, the diameter and wall thickness of the pores in the porous alumina layer, and the length and diameter of the obtained alumina fiber were examined. The formation mechanism of the alumina fiber was discussed.     

Ceramic laminates with improved mechanical reliability by tailoring the porosity of the constituting layers

Two different ceramic laminates composed of porous alumina and alumina/zirconia layers were designed and produced in the present work. The two symmetrical architectures were selected whose fundamental difference is the presence on the surface of a porous layer in the first and a compact alumina/zirconia composite layer in the second. The residual stress profile and corresponding fracture toughness were tailored to promote the stable growth of surface defects prior to final failure to increase the mechanical reliability of the material. The laminates were realized by stacking together different green laminae (containing specific pore former content) in a specific order, thermo-compression and co-sintering. The results point out an important reduction of the strength scatter and a clear insensitivity to surface damage. It is also shown that the mechanical performances are strictly related to the specific architecture of the laminate, this allowing to tailor a priori the mechanical performances of the composite.     

Fabrication and characterization of composite sol-gel coatings on porous ceramic substrate

Highly porous ceramic materials were coated using a composite sol-gel method. Alumina powder is dispersed in a silica sol-gel solution and then dip-coated on the substrate. The resulting coatings present a composite microstructure in which crystalline alumina grains are linked to each other by an amorphous silica phase. In this work, we show that, by accurately controlling the sol-gel parameters (water, solvent and silica precursor (TEOS) ratio, pH and ageing time of the sol) and also the powder grain size distribution it is possible to obtain crack-free thick films (more than 20 μm in one step). These coatings present good adherence to the substrate, decrease the roughness and also close the surface porosity of the substrate. Coating mechanical properties have been evaluated thanks to micro-indentation measurements and linked to coating structural evolution with the thermal treatment temperature.     

氧化铝的表面改性及其对BR导热性能的影响

采用不同品种偶联剂对氧化铝进行表面湿法改性,制备改性氧化铝/BR复合材料,研究复合材料的物理性能和导热性能.结果表明:硅烷偶联剂KH-550湿法改性氧化铝/BR复合材料的物理性能最佳,硬脂酸湿法改性氧化铝/BR复合材料的导热性能最佳;随着硬脂酸于法改性氧化铝用量的增大,复合材料的硬度、拉断伸长率及导热系数明显增大;在硬脂酸干法改性氧化铝/BR复合材料中加入炭黑N330,复合材料的物理性能提高,导热性能基本不变.     

Waste recycling of coal fly ash: A novel approach to prepare hierarchically porous coal fly ash/Al2O3 ceramic composite with high porosity and high strength templated by emulsion-assisted self-assembly

The coal fly ash (CFA) produced from coal-fired power generation is classified as a common solid waste; thus, improving the recovery and utilization rate of CFA is highly desirable. In this study, a novel strategy using CFA and Al₂O₃ as raw materials, to prepare hierarchically porous ceramic composites that serve as potential candidates for future building materials is developed. In this process, the well-developed self-assembly method in which an anionic modifier is used to prepare hydrophobic powders that form an attractive oil/water network via electrostatic interactions, thereby yielding honeycomb-like structures. In order to explore the mechanism of preparation, five samples with different mixture ratios of alumina and CFA were prepared according to 1: 0, 2: 1, 1: 1, 1: 2, and 0: 1 (Alumina: CFA). Compared with the sample prepared with pure CFA, the as-prepared CFA/Al₂O₃ composite exhibited both superior porosity and high mechanical property. When the porosity is as high as 73 ± 0.17%, the compressive strength is as high as 80.9 ± 3.4mpa (alumina: CFA = 1:1). As the porosity decreases to 49.3 ± 0.7%, the compressive strength reaches 159.33 ± 36.89mpa (alumina: CFA = 1:2). Moreover, this work obtains the highest compressive strength-porosity related B-value in comparison to previously reported CFA-based composites and provides a new insight into the effective recycling of CFA and offers a novel approach to prepare CFA/Al₂O₃ composite with excellent overall mechanical properties.     

Phase transition and interface evolution of Al2O3/ZrO2 particles in plasma-sprayed coatings

Alumina and zirconia ceramic particles exhibit high hardness and excellent wear resistance at high temperature, and hence are used as ceramic reinforcement phases in some plasma sprayed coatings. In this study, the interface evolution of a zirconia/alumina eutectic ceramic and the phase transition of zirconia in a plasma-sprayed coating were investigated. Scanning electron microscopy and transmission electron microscopy combined with focused-ion beam and energy dispersive X-ray were used to analyze the microstructure and composition of the ceramic interface. The results showed that the eutectic ceramic particles consisted of alumina (outer) and columnar zirconia (inner) before and after the plasma spraying process. The inner zirconia part showed the martensitic transformation of t-type zirconia to stripe-like m-type zirconia. After the plasma spraying, the interface between alumina and zirconia changed significantly, which formed a new oxide layer. The phase transition mechanism in the ceramic particle and oxide layer formation mechanism at the alumina/zirconia interface were investigated.     

Influence of behaviour of alumina slurry on quality of alumina powder prepared by jet wheel impact atomization

Alumina slurries prepared in pilot plant scale have been characterized in terms of slurry rheology and stability and processed in a spray drier based on jet wheel impactor for atomization. Spheroidal alumina granules have been produced and their density, particle size and flowability behaviour are determined. Microstructure of the spheroidal granules has been observed under SEM and results are reported. The powders find application in plasma spray coatings, production of dense ceramic parts, components and tiles.     

模板法组装尼龙-66/铂同轴纳米电缆的研究

采用简单的溶液浸润多孔氧化铝模板的方法制备了PA66的纳米管,再以这种含有PA66纳米管的模板作为“二次模板”,通过电化学沉积技术,在聚合物纳米管内部沉积金属铂纳米线,制得了PA66/铂同轴纳米电缆。用SEM和TEM等测试手段对所制备的聚合物纳米管和同轴纳米电缆进行了表征。     

21世纪高性能纤维的发展应用前景及其挑战（Ⅱ）含铝氧化物陶瓷纤维

本文针对目前国内外高性能陶瓷纤维的发展现状,对含铝氧化物陶瓷纤维的发展概况及遇到的困难作了较为详细的回顾和概括。对含铝氧化物陶瓷纤维的发展前景和发展方向进行了较为深刻的探讨。     

Effect of coarse particles on the strength of alumina made by slip casting

The effect of coarse particles in alumina powder slurry on the microstructure and strength variation of the sintered alumina bodies made by slip casting was examined. A commercially available low-soda alumina powder was used as the raw material. Coarse agglomerated particles were added in the alumina powder slurry just before slip casting. Alumina ceramics were fabricated through slip casting process. The ceramic strength and fracture toughness were examined. Fracture origins and microstructure were observed with scanning electron microscopy (SEM). The internal structures of sintered bodies were examined by using mid-infrared microscope. As a result, direct observation technique using mid-infrared microscope enabled us to analyze a small amount of coarse agglomerated particles in sintered body. The strength variation of the sintered bodies was correlated with the size of coarse agglomerated particles detected by mid-infrared microscopy.     

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.     

相变增韧和层状复合协同强韧化Al2O3陶瓷

本文采用相变增韧和层状复合协同强韧化Ａｌ２Ｏ３复合陶瓷,相变增韧制备出ＺＴＡ陶瓷,在此基础上,采用层状复合工艺,制备出ＺＴＡ／ＢＮ陶瓷。研究结果表明：相变增韧和层状复合协同强韧化Ａｌ２Ｏ３能基本保持陶瓷抗弯强度,冲击韧性提高了３７倍。     

Fabrication of Ceramics with Complex Porous Structures by the Impregnate–Freeze-Casting Process

The ceramics with complex porous structures were fabricated by a combination of impregnating and freeze-casting process. The polyurethane sponge was impregnated in the mold with 20 vol% of aqueous alumina slurry, and then the bottom of the cast body was kept at a constant cooling rate of 6°C/min to induce unidirectional solidification. After drying and sintering of the green part, porous ceramic with obvious lamellar architectures was prepared. The lamellae thickness and interlayer distance were as large as ∼9 and ∼15 μm, respectively. The large pores, which resulted from the burn-up of sponge struts were homogeneously distributed in the sample. The use of the porous template introduced some local interruption of the lamellar structures. However, high compression strength for the porous ceramic can still be obtained.