Mechanical properties of cement foams in shear

Existing models indicate that the mechanical properties of brittle foams in shear depend on relative density, cell size, cell geometry and the modulus of rupture of solid cell walls. A series of mechanical testing on alumina cement foams with various relative densities were conducted to measure their shear modulus, shear strength and fracture toughness. Experimental data are compared to the existing theoretical models. Results suggest that the existing models for closed-cell foams are applicable to describe the mechanical properties of alumina cement foams in shear. Also, the microstructure coefficients included in the theoretical expressions for mechanical properties of alumina cement foams have been determined.     

氧化铝性能对环氧浇注的影响

氧化铝性能不仅影响环氧树脂酸酐体系的粘度、填料添加量,而且杂质还影响浇注件的表观及电气绝缘性能,以及对氧化铝环氧树脂中的分散性也有很大的影响。为了改善氧化铝的物化性能以对改善环氧浇注工艺和提高浇注制件质量,本文中从粒度及粒度分布、晶体结构、微观形貌、杂质含量等方面,对比分析了电工用氧化铝填料的各项物化指标对环氧树脂浇注性能及浇注件机电性能的影响,得出合适的粒度和粒度分布、提高α相转化率、改善微观形貌、提高纯度等是提高环氧浇注用填料氧化铝性能的途径。     

Effects of industrial scale production on the chemical composition of novel coupling agents and its relationship to the mechanical properties of chopped glass fibre mat reinforced thermoset composites

Previously successfully applied polyalkenyl-poly(maleic-anhydride)-ester/amide type additives synthesized in laboratory scale have gone through selection steps then the selected coupling additives were produced in industrial scale process. Additives have been applied for treating the surfaces of glass fibres with and then mechanical properties of the laminated polyester and vinyl–ester based composites have been determined. Results have been focussed on the effect of the size increasing of additive production on the final properties of the laminates. The structures of the additives have been compared based on their FT-IR spectra. Improvement of mechanical properties of composites treated by coupling additives has been found manufactured in industrial scale either. Tensile properties could be improved by 3.2–51.3% with additives from industrial scale related to the same properties of untreated laminates. The Charpy impact strength of laminates treated with coupling agents from industrial scale was higher than that of from lab scale. Fibre–matrix interaction has been studied on SEM micrographs of the fractured faces of the composites. Similarly, the unfavourable results had been caused by the fibres slipping out of the ester matrix.     

A colloidal method for manganese oxide addition to alumina powder and investigation of properties

Various methods can be applied to introduce additives to ceramic materials. Of these methods, mechanical mixing may not always be suitable to obtain a uniform distribution of the small quantities of additive within the structure, requiring colloidal methods to be applied for the purpose. The addition of manganese oxide to alumina powder has been studied using a colloidal method. The effect of the manganese addition on alumina microstructure and the later stages of the densification behaviour was investigated, together with the hardness and mechanical strength. No evidence of secondary phase formation was detected between the manganese cation and alumina powder for up to 0.5wt% manganese addition, suggesting that manganese is in solid solution with alumina. The microstructural evidence presented suggests that small quantities of a manganese addition to alumina enhance the densification process through the formation of fast diffusion paths within the crystalline structure, similar to the effect of TiO₂ addition.     

Processing high-purity and liquid-phase-sintered alumina ceramics using locally synthesized alumina powders

The feasibility of using locally synthesized powders in the development of high-purity and liquid-phase-sintered (LPS) aluminas, for potential use as ceramics in erosive wear environments, was explored. The principal purpose of the project was the development of processing philosophies for alumina-based ceramics that meet the dual, and usually conflicting needs, for improved properties at lower cost, with special attention to the Western Australian mining industry. The incorporation of glassy grain-boundary phases via sintering additives makes the achievement of low-cost LPS aluminas possible, but also threatens to compromise properties. Sintering behaviour, phase assemblage, microstructural evolution and mechanical properties were explored as key variables in this goal. The results indicate that the locally sourced alumina and mineral additives can be combined to produce LPS aluminas which are comparable in mechanical performance to currently-available commercial LPS alumina ceramics. © 1998 Chapman & Hall     

Hardness and flexural strength of single-walled carbon nanotube/alumina composites

This work adds new experimental facts on room temperature hardness and flexural strength of alumina and composites with 1, 2, 5 and 10 vol% single-walled carbon nanotubes (SWNT) with similar grain size. Monolithic Al₂O₃ and composites were spark plasma sintered (SPS) in identical conditions at 1300 °C, achieving high density, submicrometric grain size and a reasonably homogeneous distribution of SWNT along grain boundaries for all compositions with residual agglomerates. Vickers hardness values comparable to monolithic alumina were obtained for composites with low (1 vol%) SWNT content, though they decreased for higher concentrations, attributed to the fact that SWNT constitute a softer phase. Three-point bending flexural strength also decreased with increasing SWNT content. Correlation between experimental results and microstructural analysis by electron microscopy indicates that although SWNT agglomerates have often been blamed for detrimental effects on the mechanical properties of these composites, they are not the main cause for the reported decay in flexural strength.     

3D打印氧化铝陶瓷的气氛脱脂热处理工艺研究

脱脂热处理工艺对于3D打印陶瓷的成形质量具有重要的影响。目前光固化3D打印制备得到的氧化铝生坯经过在空气中的脱脂热处理工艺后烧结最终得到的氧化铝陶瓷存在的微观裂纹等缺陷, 将导致其力学性能较差。本工作研究了基于数字光处理(Digital light processing, DLP)技术的氧化铝陶瓷打印热处理工艺, 将3D打印制备得到的氧化铝陶瓷生坯分别在空气与氩气中脱脂后比较其宏观形貌, 发现在空气下脱脂的氧化铝生坯存在微观裂纹。再将脱脂后的生坯在空气下烧结得到氧化铝陶瓷, 并对其微观形貌和宏观性能进行表征, 发现在氩气下脱脂的氧化铝陶瓷平均晶粒尺寸要比直接在空气中脱脂得到的氧化铝陶瓷平均晶粒尺寸大, 而且晶粒结构致密, 无明显气孔和杂相, 而且具有更高的抗压强度。这说明在氩气中脱脂后烧结得到的氧化铝陶瓷性能更好。在氩气中脱脂的氧化铝致密度最高可达到96.72%, 抗压强度可达到761.7 MPa, 相比于只在空气中脱脂的氧化铝陶瓷性能得到显著提升。     

Effects of porosity and pore distribution on static strength properties of porous zirconia

Bulk porosity, along with size and spatial distribution of pores, play key roles in strength of porous ceramics, as reported in a study on porous alumina. Hence, a fracture mechanics procedure was proposed to evaluate their strength by presuming that behavior of pore distribution is equivalent to that of crack distribution, and each pore is surrounded by virtual crack. In contrast to alumina, zirconia has distinct spherical‐shaped pores. Moreover, its strength properties vary with stabilizing additives. In this research, strength properties of yttria‐stabilized zirconia ceramics were studied to verify applicability of the procedure proposed for simulating strength of porous ceramics. The effect of pore characteristics on static strength properties was determined experimentally and confirmed by Monte‐Carlo simulations. It was revealed that simulated strength coincided with experimental results within a narrow scatter band, ie, factor of 2^1/2. Therefore, the proposed procedure was found to be appropriate for estimating strength of porous zirconia.     

Transparent Polycrystalline Alumina Ceramic with Sub‐Micrometre Microstructure by Means of Electrophoretic Deposition

The optical quality attainable in coarse‐grained polycrystalline alumina is severely limited by grain‐boundary scattering, which is inherent to non‐cubic materials. The optical properties of sub‐micrometre polycrystalline alumina are of growing interest triggered by the fact that a decrease in the grain sizes of the final sintered material yields an improvement in the optical quality while the scattering mechanism changes as the grain size becomes comparable with the wavelength of light. To achieve transparent alumina ceramics with a fine‐grained microstructure, however, porosity and other defects must be avoided. This necessitates the optimization of processing and sintering procedures. Electrophoretic deposition (EPD) is a colloidal process in which ceramic bodies are directly shaped from a stable suspension by application of an electric field. Electrophoretic deposition enables the formation of homogeneous, uniform green microstructures with high density, which can be sintered to transparency. It is a simple and precise technique to synthesize not only monoliths, but also composites with complex geometries [1]. Alumina green bodies were deposited from stabilized aqueous suspensions with and without doping. Green alumina compacts were evaluated based on their pore size distribution and density. Densification behaviour was characterized by dilatometric studies conducted at constant heating rate. Samples were sintered at different temperatures with subsequent post‐densification by hot isostatic pressing. Transparency was evaluated by means of spectroscopic measurements. The measured in‐line transmission of the samples at 645 nm was more than 50 % and that is 58 % of the value of sapphire. The influence of dopings on transparency was investigated. The mechanical properties of the samples were tested.     

Liquid nucleating additives for improving thermal insulating properties and mechanical strength of polyisocyanurate foams

The effects of liquid-type nucleating silane additives on the cell structure, mechanical strength, and thermal insulating properties of the polyisocyanurate (PIR) foams have been studied. The PIR foams synthesized with hexamethyldisilazane (HMDS) as a silane additive showed the smaller average cell size and lower thermal conductivity than those of the PIR foams prepared with the hexamethyldisiloxane, dimethoxydimethylsilane, and hexadecyltrimethoxysilane. When HMDS was added, average cell size of the PIR foam was becoming smaller due to lower surface tension of the polyol solution, thereby the nucleation rate and number of bubbles produced were increased and then the cell size becomes smaller. The additives likely act as nucleating agents during the formation of PIR foams. The smaller cell size appears to be one of the major reasons for the improvement of thermal insulation properties and mechanical properties of the PIR foams. From the results of cell size, thermal conductivity, and mechanical strength of the PIR foams, it is suggested that the HMDS may be the efficient liquid-type additive for the reduction of cell size and improvement of the thermal insulation property of the PIR foams.     

低温烧结细晶Y-TZP

通过在Ｙ－ＴＺＰ中加入适量的硅酸盐玻璃添加剂,使其烧结温度明显降低,并且制备出具有细晶粒、高强度的四方相氧化锆增韧陶瓷材料．分析了添加剂含量及烧结温度与材料致密度、显微结构及力学性能的关系,发现在Ｙ－ＴＺＰ材料中加入１ｗｔ％的添加剂,可以使材料在１４００℃下烧结,氧化锆晶粒尺寸约为１００～２００ｎｍ;其抗折强度可达９５０ＭＰａ．     

氧化铝粉料的颗粒级配对成型行为和烧结行为的影响

研究了颗粒级配对超细氧化铝粉体成型行为和烧结行为的影响．发现两种颗粒直径之比约为2的氧化铝粉体,一定比例混合后可获得比同样条件下单独的粉体高得多的成型密度.在细颗粒体积百分数约为33％时,应用压滤成型工艺（45MPa）获得的素坯相对密度高达72％．研究了压滤和干压成型方法对成型素坯密度及其烧结的影响．压滤成型的素坯,由于成型密度高、气孔分布窄、孔径小而有利于烧结,在较低的温度下可以达到理论密度,烧结体晶粒细小均匀,无明显缺陷;这一条件下于压成型（300MPa）得到的素坯由于有较宽的气孔尺寸分布,影响烧结,并且烧结体中有较大的气孔,不能完全致密．应用新的烧结理论对此进行了解释．     

Improving the microstructural and physical properties of alumina electrical porcelain with Cr2O3, MnO2 and ZnO additives

The optimum concentrations of Cr₂O₃, MnO₂ and ZnO were added separately and combined to the basic composition of the alumina electrical porcelain body. The samples were prepared from wet and plastic porcelain bodies according to IEC standard methods. After drying and firing under appropriate conditions, the effects of added oxides on the microstructural and physical properties of sintered porcelain were studied. It was found that the microstructure of this material was remarkably improved compared to one without additives. The sintering temperature was decreased by 60 °C, and at the same time closed porosity decreased considerably resulting in a density increase. The mullite content increased slightly, but both the size and shape of its acicular crystals were limited to values providing formation of a very strong network connecting other crystalline phases in the glassy matrix without any significant internal microcracks. It was concluded that these oxides acted as densification catalysts, partly as mineralizers, and as crystal growth inhibitors causing significant improvement of mechanical, electrical and thermal properties of the alumina electrical porcelain investigated.     

Zirconia toughened alumina ceramic foams for potential bone graft applications: fabrication,bioactivation, and cellular responses

Zirconia toughened alumina (ZTA) has been regarded as the next generation orthopedic graft material due to its excellent mechanical properties and biocompatibility. Porous ZTA ceramics with good interconnectivity can potentially be used as bone grafts for load-bearing applications. In this work, three-dimensional (3D) interconnected porous ZTA ceramics were fabricated using a direct foaming method with egg white protein as binder and foaming agent. The results showed that the porous ZTA ceramics possessed a bimodal pore size distribution. Their mechanical properties were comparable to those of cancellous bone. Due to the bio-inertness of alumina and zirconia ceramics, surface bioactivation of the ZTA foams was carried out in order to improve their bioactivity. A simple NaOH soaking method was employed to change the surface chemistry of ZTA through hydroxylation. Treated samples were tested by conducting osteoblast-like cell culture in vitro. Improvement on cells response was observed and the strength of porous ZTA has not been deteriorated after the NaOH treatment. The porous 'bioactivated' ZTA ceramics produced here could be potentially used as non-degradable bone grafts for load-bearing applications.     

Processing of alumina-based composites via conventional sintering and their characterization

The present work relates to the processing of dense alumina-based composites, their microstructural characterization and study of mechanical properties. Alumina ceramic material and alumina-based composites with m-Zirconia and Ceria addition are sintered at 1600°C, 1650°C and 1700°C temperatures via conventional sintering. Solid-state diffusion during sintering led to volume diffusion in alumina, and volume and grain boundary diffusion in alumina composite. In the present sintering conditions alumina is found to be the least dense as improper solid-state diffusion resulted in porosity, whereas alumina–zirconia composite achieved the highest density of 97%. Scanning electron microscope (SEM) micrograph shows homogeneous distribution of fine zirconia particles inside the alumina matrix, filling the voids of the alumina skeletal structure. Zirconia connects to alumina particles, restricting its abnormal grain growth. It results in strong bonding and grain refinement. Alumina–zirconia composite exhibits the highest hardness and fracture toughness of 14.37?GPa and 4.6?MPa?·?m^1/2 at 1700°C. Alumina suppresses the transformation of m-t zirconia, resulting in high toughness of alumina composites. Alumina–zirconia–ceria composite revealed the presence of porosity, which led to less densification and low mechanical properties.     

两种通电快速烧结方法制备超细晶粒纯钨

采用在超高压力下通电快速烧结新方法在不添加任何烧结助剂的条件下制备出相对密度为97.9%、晶粒尺寸小于1 μm的超细晶粒纯钨块体材料,研究了细钨粉块体的致密化行为.在超高压力下通电烧结过程中,超高压力使烧结样品具有高密度,而样品的力学性能则主要得益于通电烧结.与"放电等离子体烧结"方法相比,超高压力下通电烧结不但能更好的保持材料的原始晶粒尺寸,还能进一步细化晶粒.     

Preparation of alumina–silica–nickel nanocomposite by in situ reduction through sol–gel route

Ceramic based composites with dispersion of nano sized metal/metal carbide particles have generated wide technological interest for their improved mechanical properties — hardness, fracture strength as well as fracture toughness, superior electrical properties and magnetic properties. In the present investigation alumina–silica gels have been prepared along with nickel chloride and dextrose distributed in the nanometric pores of the gel. The gels are prepared with different molar proportions of alumina and silica containing 5 wt% of nickel chloride and 50 wt% excess dextrose. During heat treatment at a temperature of 9008C for half an hour in nitrogen atmosphere, nickel chloride is reduced to metallic nickel by in situ generated hydrogen in the silica–alumina matrix. X-ray analyses indicate that no nickel chloride reduction is possible upto 50 mol% silica in alumina–silica matrix. Beyond this range, higher the silica content, higher is the reduction of nickel chloride. The presence of metallic nickel has been substantiated further by SAD analysis. Particle size analysis based on X-ray diffraction as well as transmission electron micrograph shows the presence of nickel particles of size ,20 nm distributed in the alumina–silica nanocomposite.     

Sintering and oxidation in gel-coated SiC w /Al2O3 composites

Boehmite gel-coated SiC whiskers have been studied as discs and as dispersed in gel and commercial alumina. The coated whiskers oxidize and react to some extent in alumina-rich composite compacts when heated in vacuum. In the absence of alumina as matrix, discs of coated whiskers turned brittle on heating above 1600°C. A density of 3.7g/cc was obtained for compacts of SiC whiskers in submicron alumina when sintered pressureless for 2 h at 1600°C. A smooth interface, compatible with the matrix, has resulted in good mechanical properties.     

The dependence of mechanical properties on structure in low alloy steel forgings

The mechanical properties of two heat-treated steel forgings, manufactured from medium carbon Ni-Cr-Mo-V steels have been measured and compared. Microstructural characteristics such as martensite packet and lath size, dislocation density and precipitate size and distribution were measured and used in a detailed quantitative analysis of the relation between microstructural characteristics and proof stress. It was found that the microstructural differences identified could well account for the differences in tensile strength between the materials. Electron microcopy indicated that temper embrittlement leading to intergranular fracture may be responsible for the differences in transition temperature.