The phase equilibrium diagram of the system CaO-Al2O3-CaF2

The phase equilibria of the system CaO-Al₂O₃-CaF₂ have been studied by quenching in sealed platinum capsules followed by microscopic and X-ray examination of quenched products. The established phase diagram contains five ternary eutectics, two ternary peritectics and two invariant points of four-phase monotectic transformations. The system Al₂O₃-CaF₂ has been established to be the stable diagonal of the reversible reciprocal system Al₂O₃ + 3CaF₂ = 3CaO + 2AlF₃. In the high-fluoride region a wide zone of liquid immiscibility has been found. The system also shows such rarely noticed but theoretically possible phenomena as the transformation of peritecticals into eutecticals and occurrence of three different primary fields under single continuous zone of liquid immiscibility.     

SiO2-Al2O3干凝胶的制备与性能研究

采用二步酸碱催化溶胶-凝胶和常压干燥法,制备了低密度多孔xSiO2-(1-x)Al2O3干凝胶,其中摩尔比x分别取0.9、0.8、0.7和0.6.SiO2凝胶按照n(正硅酸乙酯(TEOS)):n(H2O):n(无水乙醇):n(HCl):n(氨水)=1: 4:7:7.5×10-4:0.0375制备,主要作为二元复合氧化物的网络骨架.Al2O3由廉价的Al(NO3)3·9H2O与NH4OH反应制得.采用BET、XRD、IR、SEM和TEM等实验手段,对xSiO2-(1-x)Al2O3干凝胶在300、600、900、1200℃4个热处理温度下的组织结构进行了对比和分析.结果表明,最终的二元干凝胶是由纳米颗粒组成的非晶网络与Al2O3晶体共同构成的纳米多孔结构,具有高的比表面积和窄的孔径分布.该复合干凝胶中,Al2O3进入SiO2骨架中,形成一定数量的Si-O-Al键,并随着Al含量的增加而增多.其中的Al2O3随着温度的升高,经历γ-AlOOH、γ-Al2O3,最终完全转化成α-Al2O3,SiO2则始终以非晶态形式存在.由于干凝胶中Al2O3的存在,大大提高了材料的热稳定性.     

Fracture mechanisms in a stoichiometric 3Al2O32SiO2 mullite

The mechanical behaviour from room temperature up to 1400°C (strength, toughness, Young’s modulus) of a 3Al₂O₃·2SiO₂ dense mullite material containing 0.2 wt% alkali has been studied. Microstructure has been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Toughness, bend strength and static Young’s modulus have been determined from room temperature up to 1400°C. The influence of strain rate on fracture behaviour has been investigated and a correlation of the mechanical parameters to fractographic observations by SEM has been stated. A strong influence of loading rate on microstructural modifications during fracture at 1300°C has been found. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characteristics of mullite prepared from co-precipitated 3Al2O3·2SiO2 powders

Diphasic gels were prepared by a precipitated process from the mixed solutions of colloidal silica and aluminium nitrate. High-purity stoichiometric mullite powders were produced by calcination of the products of these gels, with an exothermic reaction occurring at 1300°C. The chemical and structural evolutions, as a function of thermal treatment, have been characterized by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, infrared spectroscopy, and electron microscopy. The components of the co-precipitated gels react independently with increasing temperature up to the formation of mullite. The composition of mullite also varies with the temperature of thermal treatment. The calcined powders treated by ball-milling could be sintered into a high-density body, in spite of the occurrence of a small amount of glassy phase during sintering.     

Laser processing of ceramics of the SiO2-Al2O3 system

An investigation has been made of the constitution of laser-processed ceramics from the SiO₂-Al₂O₃ system. Samples were produced in the form of pellets a few millimetres in diameter by pulsed laser melting of mixtures of silica and alumina powders containing 40, 60 and 70 mol % Al₂O₃. X-ray diffraction identified the main crystalline phases as Al₂O₃ in the pellets produced from the 70 mol % Al₂O₃ mixture and mullite from the 60 and 40 mol % Al₂O₃ mixtures. The proportion of glassy phase present increased with increasing SiO₂ content. Microstructural observations on the 60 mol % Al₂O₃ pellet showed primary mullite crystals and a lamellar structure interpreted as a eutectic of Al₂O₃ and mullite. Pellets prepared by melting kaolin powder consisted essentially of a glassy phase and much porosity. Cladding of an alumina substrate, carried out using a continuous powder feed into a laser-generated melt pool, was carried out using the same silica-alumina mixtures as those employed for pellet production. A clad layer was also produced by preplacing a kaolin coat on the alumina substrate prior to laser processing. The effects of traverse speed over the range 3.7 to 7.4 mm s^?1 inclusive, power density (44.4 and 111 W mm^?2) and powder flow rate (0.13 to 0.47 g s^?1 inclusive) were investigated. It was found that the phases present in the clad layer depended on the composition of the precursor powder and the processing conditions. Microstructural examination of the clad layers produced from SiO₂-60 mol % Al₂O₃ and kaolin that had completely melted during processing exhibited various growth morphologies.     

SiO2-Al2O3-MgO-F系微晶玻璃的形变析晶

采用高温熔融法制备了SiO2 -Al2O3 -MgO-F系玻璃样品,应用X射线衍射(XRD)、扫描电子显微镜(SEM)研究了该微晶玻璃在自由形变时的析晶特征,得到了微晶玻璃自由形变析晶的规律.结果表明:在热处理过程中随着温度的升高,基础玻璃的黏度逐渐降低,可以在析晶前施加压力使之发生变形.在自由形变的前提下,微晶玻璃析出的晶体在垂直压力和平行压力方向上没有出现定向析晶.显微硬度测试结果也表明在微晶玻璃中没有明显的各向异性特征,不会带来材料性能在不同方向上的差异,为形状复杂、均质同性微晶陶瓷的制备提供了一个有效途径.     

Crystallization of gels in the SiO2-Al2O3-ZrO2 system

The preparation of glasses and ceramics from gels in the ternary system of SiO₂-Al₂O₃-ZrO₂ has been investigated. The phase relations in this system suggest the existence of a stable joint between mullite and ZrO₂ in addition to the joint between mullite and zircon. The gel technique allows metastable ZrO₂ particles to be readily dispersed in alumino-silicate matrices.     

LiCl/SiO2-Al2O3薄膜的纳米结构和湿敏特性

通过溶胶－凝胶工艺制得具有良好湿敏特性的LiCl／SiO₂－Al₂O₃薄膜材料,利用XRD和AFM对这类薄膜进行了结构表征．结果表明,具有纳米分相结构的薄膜在全湿范围内阻抗值的变化＞3个数量级,阻抗的对数值与相对湿度的关系具有较好的线性度,吸湿响应＜30s,脱湿响应＜60s．     

An investigation of phase stability in the Y2O3-Al2O3 system

The stability of the three known phases in the Y₂O₃-Al₂O₃ pseudo-binary system has been investigated. YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) decompose at elevated temperatures, the products of the reaction being the third compound Y₃Al₅O₁₂ (YAG) and an unknown phase (designated X). The decomposition is most evident in powders but can also be initiated on the surface of bulk single crystals. X-ray diffraction studies have been performed in an attempt to identify the structure and composition of the unknown phase. The thermal decomposition has been found to be surface controlled and an optical and scanning electron microscope study of the associated morphological changes in YAlO₃ indicates that the reaction involves localized surface melting, probably with the loss of oxygen which effectively moves the composition off the binary join.     

The phase equilibrium diagram of the ternary subsystem CaO-CaO·Al2O3-11 CaO·7 Al2O3·CaF2

On the basis of the experimental results obtained from the study of high-temperature equilibrium relations of seven pertinent joins the phase diagram of the subsystem CaO-CaO·Al₂O₃-11 CaO·7 Al₂O₃·CaF₂ has been constructed. In this diagram the delineation of the boundary curves of the primary fields of CaO, 3 CaO·Al₂O₃, CaO·Al₂O₃ and the 11 CaO·7 Al₂O₃·CaF₂ solid solution has been improved over our previously published phase diagram of the system CaO-Al₂O₃-CaF₂ [1]. The isotherms have also been drawn more precisely to give a better idea about the topography of the portion investigated.     

Glass transition temperature of glasses in the SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 system

A total of 24 glasses in or near the bioactive region in the system SiO₂-Na₂O-CaO-P₂O₅-Al₂O₃-B₂O₃ were studied. By differential thermal analysis their glass transition temperatures,T _g, were determined. On basis of an experimental plan for 16 glasses, two phenomenological equations describing the relationship betweenT _g and glass composition were developed. The equations describeT _g within the compositional ranges: SiO₂, 38.0–65.5 Na₂O, 15.0–30.0; CaO, 10.0–25.0; P₂O₅, 0–8.0; B₂O₃, 0–3.0; Al₂O₃, 0–3.0 wt%. The glass transition temperature shows a linear dependence of the Na₂O content. The higher the Na₂O content, the lower theT _g. Compositional alterations not including Na₂O influencesT _g little in comparison with changes in the Na₂O content.     

In vivo behaviour of glasses in the SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 system

Sixteen glasses in the SiO₂-Na₂O-CaO-P₂O₅-Al₂O₃-B₂O₃ system were studied. The glasses were implanted in rabbit tibia. According to theirin vivo behaviour, they were divided into five groups. A phenomenological equation for thein vivo behaviour was developed. The solubility of the glasses was determinedin vitro as weight loss in Tris buffer solution. The tissue response is discussed in relation to the glass composition and the solubility. For bone-bonding glasses calcium phosphate formation takes place within a silica-gel at the glass surface. The gel must be sufficiently hydrated and flexible to allow calcium phosphate to build up. The results suggest that alumina can inhibit bone bonding by retarding the formation rate of a silica-rich layer, by stabilizing the silica structure enough to prevent calcium phosphate build-up within the layer, or by either disturbance of the bone mineralization or bone incompatibility of an alumina-containing calcium- and phosphorus-rich surface layer. The mechanism responsible for the lack of bone adherence is determined by the glass composition. Up to about 1.5 wt % Al₂O₃ can be included in the glass without destroying the bioactivity.     

Assessment of in vitro bioactivity of SiO2-BaO-ZnO-B2O3-Al2O3 glasses: An optico-analytical approach

Bioactive glasses are an important subclass of biomaterials. The bioactivity of a glass depends on its initial constituents and their respective amounts. In the present investigation, five barium-zinc-borosilicate glass samples have been studied by varying Al₂O₃ mol% to check their bioactivity. The optical and bioactive properties of pristine glasses are compared with glasses soaked in Simulated Body Fluid (SBF) for 10 and 30 days using pH measurement, Ultraviolet–visible-Near Infrared-red (UV–vis–NIR), Fourier Transform Infra-Red (FTIR) spectroscopy, X-ray diffraction and Scanning Electron Microscopy (SEM) techniques. Although calcium is not present as an initial constituent in glass composition, yet bioactivity is observed in some glass samples after dipping them in SBF.     

SiO2-CaO-B2O3-Al2O3微晶玻璃在平板式ITSOFC中密封性能的研究

平板式中温固体氧化物燃料电池(ITSOFC)的密封材料在工作温度下,与其接触的电池材料应具备以下特性:(1)气密性;(2)尺寸稳定性;(3)热匹配性;(4)化学稳定性;(5)绝缘性.采用SiO₂-CaO-B₂O₃-Al₂O₃系统微晶玻璃制备出一种适用于850℃的密封材料.该材料在850℃保证一定尺寸的前提下,能够与8YSZ电解质和Ni-Cr双极板紧密黏附,热膨胀系数8.9×10^-6)/℃和8YSZ接近,电导率约为10^-8S/cm有良好的电绝缘性能,在O₂和H₂气氛下保温100h没有气体泄漏,且密封后的黏附界面边界分明,元素扩散层厚度<10μm.实验证明该材料适用于ITSOFC 850℃密封.     

Preparation of pressureless-sintered SiC-Y2O3-Al2O3

Sintering additives were prepared from aluminium hydroxide and yttrium hydroxide. These additives were soluble in water and resulted in a binder. A -SiC powder was mixed with the additive solution and sintered at 2150° C without pressure. The oxides formed from the additive promoted sintering. The sintered body contained no pores. Aluminium, silicon, and yttrium oxide were precipitated in the sintered body.