Oxidation Behavior of a Si-C-O-Fiber-Reinforced Magnesium Aluminosilicate

The oxidation behavior of a Si-C-O-fiber-reinforced magnesium aluminosilicate has been investigated in the temperature range 1000°-1200°C. Parabolic weight gain, together with other experimental observations, has suggested that the oxidation process was controlled by the outward diffusion of carbon monoxide gas. The matrix and fibers reacted to form enstatite and forsterite. During the initial stages of oxidation, an interlayer containing a mixture of silica and turbostratic carbon was formed. This interlayer was modified progressively over time to create interface morphologies near the surface and near the center of the oxidized samples that were different from one another. A mechanism explaining the morphological evolution of the fiber-matrix interfaces has been proposed.     

Effect of Processing on Microstructure and Mechanical Behavior of Magnesium Oxide

Polycrystalline MgO specimens were fabricated from two powders by hot-pressing in graphite or alumina dies and annealing in air, in vacuum, or within the graphite die in vacuum. Parameters affected were density, grain size, and visual appearance, which was considered to be dependent on grain boundary structure. The formation of a boundary liquid phase with one type of powder tended to eliminate the effect of the processing variables. The character (microstructure) of the specimens was correlated with their mechanical behavior in compression at a constant strain rate at 1200°C.     

Effect of Crystal Orientation on the Mechanical Behavior of Magnesium Oxide at High Temperatures

The high-temperature deformation of magnesia single crystals with a [110] tensile axis is described and related to previous observations on [001] crystals. The [110] tensile axis favors slip on systems with oblique vectors (i.e. at 120°). Two main modes of deformation are identified and distinguished by the interpenetrability of oblique slip: (1) Between 1400° and 1700°C interpenetration does not occur. The crystal becomes subdivided into distinct blocks slipping on different systems. These blocks are separated by sharp kink boundaries which act as barriers to further slip. Voids form in the kink interfaces and lead to brittle fracture. (2) Above 1700°C slip on all systems interpenetrates and a stable substructure develops throughout the gage section. At high strains the single crystal recrystallizes. The specimen work hardens and elongates 100% before necking down to completely ductile fracture. This behavior is discussed in terms of dislocation interactions.     

Crystallization in a Barium-Containing Magnesium Aluminosilicate Glass-Ceramic

Crystallization in a Ba-containing magnesium aluminosilicate (Ba-MAS) glass-ceramic was studied using transmission electron microscopy. Ba-MAS slabs were hot-pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ-cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba-rich silicate glass. The high-quartz solid solution generally acted as a precursor for the nucleation of the stable β -phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygonization of β -cordierite.     

Effect of Composition on the Mechanical Properties of Aluminosilicate and Borosilicate Glasses

Elastic moduli and fracture toughness were determined for several glasses in the systems soda-alumina-silica, calcia-alumina-silica, and soda-boric oxide-silica. Results for the aluminosilicates are analyzed in terms of Al³⁺:Na⁺ ratios. The mechanical properties do not show maxima or minima at the Al³⁺:Na⁺ ratio of 1, in contrast to conductivity, helium permeability, and refractive index. The moduli and toughness increase with Al³⁺:Na⁺ ratio, which is consistent with increased coherency of the glass network. Glasses which contain B₂O₃ instead of Al₂O₃ have slightly higher moduli but are considerably tougher. The moduli of calcium aluminosilicate glasses are ∼25% greater than sodium aluminosilicates, whereas the fracture toughnesses are similar.     

High-Temperature Mechanical Behavior of Stoichiometric Magnesium Spinel

The elastic and mechanical behavior, from room temperature up to 1300^deg;C, of Stoichiometric polycrystalline magnesium aluminum spinel is studied. Elastic modulus, fracture toughness, and modulus of rupture measurements and observations of polished and fracture surfaces have been performed. Two well-differentiated regions of fracture behavior as a function of temperature have been found. In the low-temperature region, this material behaves elastically, whereas in the high-temperature (>800^deg;C) region, plastic phenomena take place.     

Microstructural Study of the Hot Corrosion of a Calcium Aluminosilicate Glass-Ceramic and a Si-C-O-Fiber-Reinforced Calcium Aluminosilicate Matrix Composite via Sodium Sulfate in Air and Argon at 900°C

The hot corrosion of a calcium aluminosilicate (CAS) glass-ceramic and a composite of CAS matrix that has been reinforced with Si-C-O (Nicalon) fiber has been investigated by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Samples of the monolithic CAS and the Si-C-O-CAS composite were subjected to corrosion using liquid sodium sulfate at 900°C for 50 h in air and argon environments. The monolithic and composite samples both were corroded by sodium sulfate, and corrosion damage in the composites was more severe than in the monolithic CAS, irrespective of the gaseous environment. The increased corrosion damage in the composites was due to the presence of Si-C-O fibers, which changes the mechanism of corrosion. The corrosion products in monolithic CAS were different from those in the composites; this disparity was also due to the presence of the fibers in the composite. The corrosion zones in all the samples were severely cracked, and the cracks extended into the unaffected regions of the samples. Mechanisms of hot corrosion have been proposed and discussed for both the monolithic and composite samples.     

水化热调控剂与氧化镁复掺对混凝土抗裂行为的影响

为了研究氧化镁膨胀剂与水化热调控剂复掺对混凝土抗裂性能的影响,探讨水化热调控剂的作用机理,开展了单掺氧化镁及水化热调控剂与氧化镁复掺对混凝土水化热、绝热温升、限制膨胀率、体积变形、自收缩、弹性模量及微观结构的影响研究。结果表明:水化热调控剂可显著改变水泥水化放热历程,净浆抑温率可达31.2%,随入模温度升高,抑温效果增强;水化热调控剂可激发氧化镁的水化,促进膨胀能的释放,随养护温度提升,作用效果更显著;水化热调控剂可改变混凝土温升历程,特别是降低混凝土早期水化热释放速率;水化热调控剂与氧化镁复掺可降低混凝土自收缩,当水化热调控剂掺量为0.2%和0.4%(质量分数)时,混凝土收缩值分别减少了84 με和54 με;水化热调控剂会导致混凝土早龄期弹性模量降低,但对长龄期弹性模量影响不大;水化热调控剂对水泥水化历程、水化产物生成及微观结构密实程度的改变是提高混凝土抗裂性能的根本原因。     

Mechanical Behavior of Magnesium Oxide at High Temperatures

Magnesium oxide crystals show a wide variety of deformation and fracture modes under tension. These modes are determined by the number of slip systems operating concurrently in a given volume. (1) At low temperatures, slip is confined to a single (110) (110) system and plasticity is limited by stress concentrations which develop where slip switches from one plane to another. (2) At intermediate temperatures, (110) (110) slip systems at 90° can interpenetrate but those at 60° cannot. Mechanical properties then depend on the initial slip distribution. When slip is confined to 90° systems there is little work hardening and crystals neck down to a knife-edge ductile fracture. When slip is confined to 60° systems, crystals work harden and fracture by cleavage. (3) At high temperatures, dislocations can interpenetrate on all systems and polygonization can occur. After easy glide the crystals work harden and elongate over 100% before fracturing in a completely ductile manner. The transition temperature from one mode to another depends on strain rate.     

温度对镁合金微弧氧化膜在乙二醇水溶液中腐蚀行为的影响

在硅酸盐溶液中,利用微弧氧化(MAO)技术在镁合金AZ91D表面制备了氧化陶瓷膜。采用动电位极化和电化学阻抗谱的方法,研究AZ91D微弧氧化镁合金微弧氧化膜在含有NaCl的乙二醇水溶液中在不同温度的腐蚀行为,并用扫描电子显微镜观察了试样腐蚀前后的表面形貌。结果表明：随温度的升高,微弧氧化膜上的乙二醇分子逐步解析,氯离子代替乙二醇分子吸附至MAO膜层表面而导致腐蚀产生。     

氧化锆对锂铝硅酸盐玻璃化学强化性能的影响

采用熔融淬冷法制备了含不同摩尔分数Zr O₂的锂铝硅酸盐玻璃,通过两步化学强化法对玻璃样片进行了化学强化,研究了Zr O₂对玻璃的稳定性、硬度和化学强化效果的影响。结果表明:随着Zr O₂的摩尔分数从0增加至5%,玻璃化转变温度随之升高,玻璃稳定无析晶现象。引入适量的Zr O₂会促进Li⁺-Na⁺离子交换,提高应力层深度,表面压应力随着Zr O₂浓度的增加而增加,并在摩尔分数为4%时达到最大值,为1 055.6 MPa。中心张应力随着Zr O₂浓度的增加先增加后缓慢减小,表明该系列样品具有较好的抗冲击能力。Zr O₂的浓度变化对强化后玻璃的硬度影响较小,在引入Zr O₂后其抗裂性有所降低,但仍具有较好的抗裂能力。     

硅铝原料预处理对地聚合物早期水化反应行为的影响

采用低活性高硅尾矿预处理后作为硅铝原料(基质)与铝校正料偏高岭土复合,在碱硅酸盐溶液激发作用下制备地聚合物.通过ESEM和ATR-FTIR表征地聚合物净浆的早期水化反应机理,并对地聚合物砂浆试样后期的抗压强度及微观形貌进一步分析.结果表明:通过碱熔融与机械研磨复合的方式对尾矿进行预处理,可以有效促进地聚合物体系的早期水化反应进程,有利于铝硅酸盐凝胶相的形成,进而改善地聚合物后期的微结构及强度性能.本研究说明有可能通过改变硅铝原料性质的方式对地聚合物早期水化反应行为进行控制,从而调整地聚合物的后期性能(如强度性能、施工性能等),使其满足特定的工程应用需求.     

Mechanical Behavior of Polycrystalline Magnesium Oxide at High Temperatures

High-temperature tensile deformation of paolycrystalline magnesia prepared by ( a ) single-crystal recrystallization and ( b ) hot-pressing, is described. Recrystallized polycrystalline magnesia goes through a brittle-ductile transition at 1700°C (strain rate 10⁻⁴ sec⁻¹). The brittleness below 1700°C is due to a lack of slip systems and grain boundary sliding. At 1700°C grain boundary migration produces corrugations in the interface which interfere with sliding. Above 1700°C the matrix becomes sufficiently plastic through multiple slip and polygonization to accommodate any distortion. Polycrystalline specimens then neck down for completely ductile fracture. Hot-pressed magnesia starts through a transition at 2200°C, i.e. 500°C higher. The increase is attributed to pores and impurity. Porosity is considered to promote grain boundary sliding by ( a ) providing the source for intergranular sliding, ( b ) decreasing the interfacial contact area, and ( c ) preventing grain boundary migration and corrugation. These observations confirm that high-temperature deformation occurs by dislocation glide and climb and by grain boundary sliding and migration.     

Effect of Heat Treatments on the Wetting Behavior of Bismuth-Rich Intergranular Phases in ZnO:Bi:Co Varistors

The effect of annealing on the wetting behavior of Bi-rich intergranular phases in ZnO:Bi:Co varistors was studied. The intergranular phase exhibits temperature-dependent grain-boundary wetting, with an average equilibrium dihedral angle of 0° at 1140°C and over 55° at 610°C. The temperature-dependent wetting may be related to the temperature dependence of the ZnO concentration in the Bi₂O₃ liquid phase. The effect of the intergranular phase distribation on the electrical properties of ZnO varistors is discussed.     

Internal Reduction of an Iron-Doped Magnesium Aluminosilicate Melt

Optical and electron microscopies are used to analyze the mechanism and kinetics of internal reduction of an Fe²⁺-doped magnesium aluminosilicate melt. Melt samples are heated to temperatures in the range of 1300°–1400°C under a flowing gas mixture of CO/CO₂, which corresponds to a p _O2 range of 1 × 10⁻¹³–4 × 10⁻¹³ atm. The melt experiences an internal reaction in which a dispersion of nanometer-scale iron-metal precipitates forms at an internal interface. The metal precipitates show no signs of coarsening within the samples; however, the crystals at the surface (which formed in the initial part of the reaction) do grow via vapor phase transport. The overall reaction is characterized by parabolic kinetics, which is indicative of chemical diffusion being the rate-limiting step. The diffusion of network-modifier divalent cations—particularly Mg²⁺ cations—is demonstrated to be the rate-limiting factor, and its diffusion coefficient is calculated to be ∼1 × 10⁻⁶ cm²/s within the temperature range of the experiments.     

Improvement of Mechanical Properties of Machinable Glass-Ceramics through Postmachining Heat Treatments

A mica-based machinable glass-ceramic has been converted to a glass-ceramic containing cordiertie and body-centered cubic chondrodite through heat treatment at temperatures higher than that used for the crystallization of mica. The machinability is lost, microhardness increases, and the thermal expansion coefficient decreases with the change of the crystalline phase from mica to cordierite and chondrodite.     

超高摩尔质量聚乙烯氧化行为对生物摩擦学性能的影响

采用热压成型工艺制备了超高摩尔质量聚乙烯(UHMWPE),测试了加速老化前后辐照交联UHMWPE的力学和生物摩擦学性能的变化.DSC测试结果显示,加速老化后辐照交联UHMWPE的结晶度有所提高;红外光谱分析表明,加速老化后辐照交联UHMWPE的氧化指数大幅度提高;冲压剪切强度测试结果显示,加速老化导致辐照交联UHMWPE的弹性和塑性性能显著降低;摩擦磨损测试结果表明,加速老化后辐照交联UHMWPE的耐磨性能显著降低,并出现大量的压片状破裂.