Mechanical and microstructural properties of cordierite-bonded porous SiC ceramics processed by infiltration technique using various pore formers

Cordierite-bonded porous SiC ceramics were prepared by air sintering of cordierite sol infiltrated porous powder compacts of SiC with graphite and polymer microbeads as pore-forming agents. The effect of sintering temperature, type of pore former and its morphology on microstructure, mechanical strength, phase composition, porosity and pore size distribution pattern of porous SiC ceramics were investigated. Depending on type and size of pore former, the average pore diameter, porosities and flexural strength of the final ceramics sintered at 1400 °C varied in the range of ~ 7.6 to 10.1 µm, 34–49 vol% and 34–15 MPa, respectively. The strength–porosity relationship was explained by the minimum solid area (MSA) model. After mechanical stress was applied to the porous SiC ceramics, microstructures of fracture surface appeared without affecting dense struts of thickness ~ 2 to 10 µm showing restriction in crack propagation through interfacial zone of SiC particles. The effect of corrosion on oxide bond phases was investigated in strong acid and basic salt medium at 90 °C. The residual mechanical strength, SEM micrographs and EDX analyses were conducted on the corroded samples and explained the corrosion mechanisms.     

Preparation of mullite bonded porous SiC ceramics by an infiltration method

A powder compact of α-SiC and α-Al₂O₃ was infiltrated with a liquid precursor of SiO₂, which on subsequent heat treatment at 1500 °C produced a mullite bonded porous SiC ceramics. Results showed that infiltration rate could be estimated by using weight gain measurements and theoretical analysis. The bond phase was composed of needle-shaped mullite which was observed to be grown from a siliceous melt formed during the process of oxide bonding. The porous SiC ceramics exhibited a density and porosity of 2 g cm⁻³ and 30 vol%, respectively, and also a pore size distribution in a range of 2–15 μm with an average pore size of 5 μm. No appreciable degradation of room temperature flexural strength (51 MPa) was observed at high temperatures (1100 °C).     

新型塑性加工制备球形孔多孔金属及其性能

将铝板塑性加工成半球孔层金属结构薄层,以此为基本单元按一定方式连接形成球形孔多孔金属,研究了孔结构对球形孔多孔金属性能的影响.结果表明,平板层相连的球形孔结构,使其强度高于同样条件下堆积的空心金属球结构.这种新型球形孔多孔金属的强度较高,能够有效地吸收能量,铝板的厚度和孔隙率对其压缩和能量吸能性能有明显的影响.     

Fabrication of porous ceramics with controlled pore size by colloidal processing

Well-defined macroporous ceramics consisting of TiO₂ and ZrO₂ have been fabricated by two methods. One is via a template-assisted colloidal processing technique and the other is via a hetero-coagulation of template/ceramic particle colloidal processing. The former technique is as follows. Close-packed polymer spheres were first prepared as a template using centrifugation or gravitational sedimentation, followed by infiltration with alkoxide precursors. Then the removal of the template beads was achieved by calcination of the organic–inorganic hybrids at appropriate temperatures, yielding well-ordered macroporous ceramics. The latter technique is as follows. Core–shell composites of polymer/ceramic were obtained by mixing the oppositely charged two suspensions via electrostatic attraction following by filtration and calcination to produce macroporous ceramic materials. SEM images revealed that macroporous TiO₂ and ZrO₂ with ordered and uniform macropores have been obtained by both procedures.     

Some properties of mullite powders prepared by chemical vapour deposition

The sinterability of mullite (3Al₂O₃·2SiO₂) powder prepared by chemical vapour deposition was examined to improve the conditions for fabricating dense mullite ceramics. The starting powder contained not only mullite, but also a small amount of -Al₂O₃ (Al-Si spinel) and amorphous material. Although the compressed powder was fired at a temperature between 1550 and 1700 °C for 1, 3 and 5 h, the relative densities of the sintered compacts were limited to 90%: (i) due to the creation of pores/microcracks during the solid state reaction (1100–1350 °C), and (ii) due to restriction on the rearrangement of grains because the amount of liquid phase (1550–1700 °C) was insufficient. Calcination of the starting powder was effective for preparation of easily sinterable powder with homogeneous composition. When the compact formed by compressing the calcined powder at 1400 °C for 1 h was fired at 1650 °C for 3 h, the relative density was raised up to 97.2%; moreover, mullite was the only phase detected from the sintered compact. The sintered compact was composed of polyhedral grains with sizes of 1–2 m and elongated grains with long axes of 6 m.     

Microstructure and properties of porous silicon nitride ceramics prepared by gel-casting and gas pressure sintering

Wave-transparent porous Si₃N₄ ceramics were prepared by gel-casting and gas pressure sintering, and the effects of solid loading on microstructure, mechanical and dielectric properties were investigated. Microstructures with interlocked elongated β-Si₃N₄ grains and uniformly distributed pores were observed, while both the β-Si₃N₄ phase content and grain aspect ratio reduced as the solid loading increased due to the restrained anisotropic growth of β-Si₃N₄ grains. As the solid loading increased from 30 to 45 vol.%, the porosity of ceramics declined from 57.6% to 36.4%. The flexural strength increased linearly from 108.3 to 235.1 MPa, and the dielectric constant and loss tangent of ceramics increased from 2.63 and 2.85 × 10⁻³ to 3.68 and 3.56 × 10⁻³ (10 GHz), respectively.     

Synthesis and properties of spodumene-modified mullite ceramics formed by sol-gel processing

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Electro-conductive porous ceramics prepared by chemical vapor infiltration of TiN

Using pressure-pulsed chemical vapor infiltration (PCVI) method, TiN was partially infiltrated at 850 °C from gas system of TiCl₄ (1%)–N₂ (10%)–H₂ into the highly porous carbon substrates prepared by the carbonization of cotton-wool, filter paper, and wood at 1,000 °C in Ar for 4 h. After 10,000 pulses of PCVI, electro-conductive porous ceramics having the three-dimensionally continuous current paths were obtained, which had the porosity of 80% and more, the resistivity of 10⁻⁵–10⁻⁶ Ω m, and the average pore sizes of 10–40 μm. The geometric surface area per unit volume of the sample was higher than that of the conventional foil-type current collector for lithium-ion battery. The surface area showed the highest value for the sample obtained from carbonized wood substrate.     

孔径可控的多孔羟基磷灰石的制备工艺研究

采用添加造孔剂法,选择合适的造孔剂聚甲基丙烯酸甲酯（PMMA）,通过严格筛分,可烧结制得孔径可控的多孔基磷灰石陶瓷,气孔率可从20％到50％变化。并对烧结多孔体中孔的结构、孔径分布与特征,影响气孔率和力学性能的因素进行了研究与讨论。     

Strength and pore morphology of porous aluminum and porous copper with directional pores deformed by equal channel angular extrusion

Porous aluminum with a porosity of 17.6% and porous copper with a porosity of 39.7% (the pores of both aluminum and copper were cylindrical and oriented in one direction) were deformed by equal channel angular extrusion using a 150° die with sequential 180° rotations (route C), and the mechanical strength and pore morphology after the extrusions were investigated. In the case of porous aluminum with low porosity, the pores were collapsed by the extrusions that were both parallel and perpendicular to the orientation direction of the pores. In contrast, the porosity of porous copper decreased slightly after extrusions that were parallel to the orientation direction of the pores, and the pores thus remained even after four extrusions. The yield strength after the second extrusion was 7.3 times greater than it was before the extrusion, even though there was a decrease in porosity of only 8%. On the other hand, almost all the pores of the porous copper collapsed after the fourth extrusion, when the extrusion direction was perpendicular to the orientation direction of the pores. Thus, the yield stress cannot be enhanced without being accompanied by progressive densification.     

Preparation of ZnO bulk porous nanosolids of different pore diameters by a novel solvothermal hot press (STHP) method

ZnO bulk porous nanosolids were prepared by a novel solvothermal hot press (STHP) method with different pore-forming agents using ZnO nanoparticles. The samples were characterized with SEM, XRD, FTIR, a mercury intrusion porosimeter and N₂ adsorption–desorption method. Experimental results showed that pore size of the nanosolids was between 20–32 and 20–100 nm when de-ionized water and CTAB solution were used as pore-forming agents, respectively. There was no obvious morphological change in the samples when all pore-forming agents were removed from the autoclave.     

真空烧结SiC多孔陶瓷的组织与性能

以SiC为主要原料,以羧甲基纤维素(CMC)为造孔剂,分别以Al2O3-Y2O3和Al2O3-高岭土为烧结助剂,在1550℃真空烧结制备出孔洞均匀的SiC多孔陶瓷,研究了造孔剂和烧结助剂对其抗折强度和抗弯强度的影响.结果表明:随着造孔剂用量的增加,气孔率和线收缩率提高,抗折强度下降.对于经预氧化处理以Al2O3-Y2O3为助烧剂的样品,因发生低温液相反应烧结,其结构致密,气孔率较小,线收缩率和抗折强度较高;而在以Al2O3-高岭土为助烧剂的样品中发生固-固反应,其结构疏松,因而线收缩率和抗折强度较低.但是这种疏松结构有利于热应力的释放,使样品具有较高的抗热震性能.     

莰烯基冷冻浇注成型法制备定向通孔氧化铝陶瓷

由莰烯、氧化铝、Texaphor 963组成的陶瓷料浆,在温度梯度的诱导下,用冷冻浇注成型法制备了氧化铝陶瓷坯体,经干燥、烧结后,制备出了高孔隙率、高强度的定向通孔氧化铝陶瓷.通过扫描电镜观察其孔结构,并对其孔隙率、抗压强度和收缩率进行分析测试.结果表明:料浆的固体含量和烧结温度显著影响试样的气孔率、抗压强度和收缩率;...     

Effect of YSZ fiber addition on microstructure and properties of porous YSZ ceramics

Yttria-stabilized zirconia (YSZ) fiber was introduced as the reinforcement for porous YSZ ceramics fabricated by tert-butyl alcohol-based gel-casting process and pressureless sintering. Effect of YSZ fiber addition on the microstructure and properties of porous YSZ ceramics was studied systematically. Results showed that YSZ fiber obviously obstructed densification during the sintering process and therefore higher porosity could be achieved with the same solid loading of the initial slurry. Mean pore size regularly increased with increasing fiber addition. The reinforcing effect reached its optimum with 10 wt% YSZ fiber addition, yet decreased with increasing porosity. Fiber addition significantly changed the fracture mode of the porous ceramics from brittle to quasi-ductile with increasing fiber additions; fiber pull-out and crack deflection play major roles in the process. Compared with the porous ceramics without fibers, the thermal conductivity decreased a little. With improved mechanical and thermal properties, YSZ fiber-reinforced porous YSZ ceramics are more applicable in thermal insulation materials.     

Pechini法制备铌酸锂陶瓷的结晶性能研究

以Li2CO3、Nb2O5、K2CO3为初始原料,采用Pechini法制备了铌酸锂(LiNbO3)陶瓷。用XRD、SEM等分析了LiNbO3陶瓷的结晶特性。分析结果表明Li2O对于铌酸锂陶瓷的烧结及致密度有着重要的影响,经980℃烧结得到的LiNbO3陶瓷晶粒发育良好,当烧结温度>1100℃时,Li2O有较大的损失,所得到的LiNbO3陶瓷组分与化学计量比有一定偏差。因此应采用多种技术方法来减少Li2O的损失以得到致密、结晶性良好的铌酸锂陶瓷。     

多孔硅的电化学制备与研究

以电化学方法制备了多孔硅材料并通过表面轮廓测试仪、原子力显微镜、显微拉曼光谱仪等设备对制备多孔硅的孔隙率、厚度、表面形貌、以及热导率进行了表征.结果发现,本实验制备的多孔硅属于介孔硅(15～20nm),其孔隙率随腐蚀时间和腐蚀电流的变化有先增大后减小的趋势.增加多孔硅的厚度和孔隙率,可以使得多孔硅的热导率显著降低(最低可低至0.62W/m·K).     

Mechanical and tribological properties of ceramic-matrix friction materials with steel fiber and mullite fiber

The purpose of the present work was to investigate and compare the mechanical and tribological behaviors of ceramic-matrix friction material (CMFM) with steel fiber (SF), mullite fiber (MF), and mixing SF and MF. The CMFM was prepared by hot-pressing sintering, and the tribological behaviors were determined using a constant speed friction tester. The worn surfaces and wear debris were observed by a scanning electron microscopy (SEM). Experiment results show that the combination of SF and MF can improve the mechanical properties that each single fiber does not have. The sever fade for the specimen reinforced by single MF during the whole friction testing can be attributed to the poor interface cohesive strength between MF and matrix. Mixing the SF and MF can improve the friction stability, and the friction coefficients for friction material with a mixture of the SF and MF increases with increasing MF content. For all specimens, increasing in the friction temperatures result in the increase of wear rates.     

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si₃N₄ ceramics were prepared by freeze casting using liquid N₂ as refrigerant. The pore structure, porosity, α → β-Si₃N₄ transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α → β-Si₃N₄ phase transformation, but was beneficial to the mechanical properties of the obtained porous Si₃N₄ due to its unique pore structure.