Effect of Oxide Additions on Sintering of Magnesia

The effect of transition metal, alkali, alkaline-earth, and heavy metal oxide additions on the sintering of magnesia was determined by measuring bulk density and shrinkage. Polished specimens were made on selected compositions and the grain structure was photographed. The unusual effect of titanium, zirconium, silicon, lithium, iron, and zinc oxides in promoting high sintered densities is explained on the basis of the defective crystal structure produced by these additions.     

Effect of Minor Additions on Sintering of MgO

Fourteen selected metal ions were added to magnesium basic carbonate, and the MgO obtained after calcining was fired at various temperatures to determine the effect of the additives on the sintering behavior. The magnesia without additives showed rapid increase in density with firing temperature near 1200° C. The majority of the additives caused increased density, at a given firing treatment, but some were without effect and one (Cr) dramatically inhibited sintering in certain percentages. It is believed that most of the additives which aided sintering did so by entering into the magnesia lattice and creating defects, although at least one (V) clearly promoted sintering by liquid formation.     

Effect of Temperature and Oxygen Partial Pressure on the Oxidation of Titanium Carbide

The oxidation of powdered titanium carbide was followed by weight gain measurement, using a thermogravimetric apparatus, between 600° and 850°C. The oxygen partial pressure effect was investigated between 6.5 and 645 mm Hg. The oxidation was parabolic with an activation energy of 46 ± 2 kcal/mole which was essentially independent of the overpressure. The rate of oxidation was dependent on p o₂¼ below approximately 800°C. Above this temperature the rate was proportional to p o₂¼. The oxidation of single-crystal TiC above 850° C produced porous rutile films. The rates of oxidation were between linear and parabolic time dependence.     

烧结压力对生物医用Ti-Nb-Zr-Mn合金显微组织与力学性能的影响

以单质金属粉末为原料,采用机械合金化(MA)和放电等离子烧结(SPS)法制备了力学性能优异的生物医用Ti24Nb4Zr3Mn合金,研究了烧结压力对Ti24Nb4Zr3Mn合金显微组织和力学性能的影响.结果表明,不同烧结压力下获得的Ti24Nb4Zr3Mn合金均为体心立方β相和斜方马氏体α"相双相合金.随着烧结压力的增加...     

Effect of Oxygen Pressure on Microstructure and Coercive Force of Magnesium Ferrite

The precipitate formed during partial reduction of magnesium ferrite raises the coercive force by setting up high stresses in the ferrite matrix as a result of the difference in thermal expansion of the two phases. The slower the cooling rate, the smaller is the final stress and the smaller the influence on the coercive force. Only the magnitude and not the sign of the difference in thermal expansion is of importance, as is shown by making additions of inert phases with thermal expansions both larger and smaller than that of the ferrite. The maximum radial tensile stress set up in the ferrite by the reduced phase is approximately 40,000 1b. per sq. in. in a specimen air-quenched from 1400° C. The formation of a reduced phase in magnesium ferrite is accompanied by a fourfold reduction of the residual porosity. It is not known whether this is the result of a change in stoichiometry of the ferrite with the development of an oxygen-deficient structure and hence enhanced oxygen diffusion rate, or whether the reduced phase prevents grain growth until most of the vacancies have collapsed. The reoxidation of such a two-phase material gives a polycrystalline material with greatly improved magnetic properties.     

Effect of Metal Additions on the Microstructure of Zirconia

The diffusion of titanium into zirconia was studied to determine the mechanism by which increased sintering occurred in the metal-modified zirconias. Results revealed that the increased sintering occurred only when compositions above the solubility limit were fired considerably above the melting point of the titanium. Examination of the system Ti-Zr-0 revealed that considerable liquid was present at these firing temperatures. This liquid phase promoted the sintering of the zirconia. The increased sintering did not occur for compositions above the solubility limit fired below the temperature of liquid phase formation, or for compositions below the solubility limit fired considerably above or below the melting point of titanium.     

Effect of Surface Impurities on the Microstructure Development during Sintering of Alumina

Microstructural evolution during sintering of alumina powder compacts prepared by cold isostatic pressing (CIP) was monitored. For CIP, rubber molds lubricated with silicone oil were used so that a very small amount of impurity was introduced to the surface of the powder compacts. During sintering at 1600°C, grain growth in the surface region was inhibited up to sintering for 1 h, but subsequently abnormal grain growth occurred. In the inner region, however, the grains grew uniformly without abnormal grain growth. Impurities that initially drag the boundary migration but form liquid at the end are suggested to cause abnormal grain growth.     

多种外加剂对红柱石制品烧结性能影响的研究

研究了多种外加剂对红柱石原料烧结性能的影响，分析了其作用机理。结果表明：加入氧化镁可明显促进红柱石的烧结，在高温阶段其作用更加明显，但其质量分数必须严格控制，不宜超过2％。添加氧化钛也可促进红柱石的烧结，其质量分数应该控制在4％以内。而添加氧化铬和氧化锆则不利于红柱石的烧结。     

尿素低压吸收塔高氧分压操作论述

华鹤尿素装置自开车以来,低压吸收塔尾气排放中氧含量值波动比较大,甚至有时超过了斯塔米卡帮公司设计的安全限值4%。采用CO_2汽提法工艺的尿素装置,该指标的波动极易进入爆炸区间,影响生产稳定运行和人员安全,为此结合本装置生产实际情况对氧含量超出设计值是否会发生爆炸进行针对性的论述。     

Effects of Oxygen Partial Pressure on the Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of oxygen. The diffusion rate constant was found to vary with the logarithm of the partial pressure of oxygen according to the theory of oxidation of thin films as proposed by Engell and Hauffe. An alternative explanation based on the change of free energy with surface coverage was also found to fit the data.     

Effect of Oxygen Partial Pressure on the Dielectric Properties and Microstructures of Cofired Base-Metal-Electrode Multilayer Ceramic Capacitors

The dielectric properties, dopant distributions, and microstructures of BaTiO₃-based multilayer ceramic capacitors (MLCCs) sintered in H₂–N₂–H₂O atmospheres with     =10^−7.5 Pa (BMX-7.5) and     =10^−9.5 Pa (BMX-9.5) were studied, and the effects of oxygen partial pressures were analyzed. Dielectric measurements showed that BMX-7.5 had a lower dielectric constant at temperatures above 20°C, but a higher dielectric constant at temperatures below 10°C when compared with BMX-9.5. The coexistence of core–shell and core grains was observed in bright field (BF) transmission electron microscopy images in both types of capacitors. Triple-point and grain boundary phases were observed more frequently in BMX-9.5 than in BMX-7.5, and energy-dispersive X-ray spectrometer point-by-point analysis revealed that these second phases contained high concentrations of dopants such as Si, Y, and Ca. The dopant concentration in the shell regions in BMX-7.5 was higher than that in similar regions in BMX-9.5. Smeared and twisted grain boundaries with fringes observed in both types of MLCCs indicated that the shell regions in both samples were formed either by diffusion of foreign ions into BaTiO₃ or by crystallization of grain boundary and triple-point liquid phases. It was deduced that the partial pressure of oxygen in the sintering atmosphere influenced the microstructures, dopant distributions, and core–shell ratios of the grains in these materials.     

Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite

The substitution of between 0 and 1.6 wt% silicon (Si-HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si-HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si-HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.     

The Effect of Sintering Temperature on Porous Silica Composite Strength

The effect of sintering temperature on porous silica composite strength was studied by discussing three factors, namely crystal phase, glassy phase and porosity. The fired products of clay and silica are composed of crystalline phase and glassy phase. The crystalline phases consist of alpha-quartz and mullite and the glassy phase contains a disordered silica network. With the increase of sintering temperature up to 1360°C, the crystalline silica decreased gradually. The disappearing silica dissolved into the glass and became a part of glass network and resulted in the enhancement of glass strength. This change in glass played an important role in the improvement of sample strength. At the same time, the increase of sintering temperature promoted the densification of samples and reduced the porosity of products, which also contribute to the increase of sample strength. When the sintering temperature is up to 1390°C, the silica in glass tended to convert to cristobalite with the expansion of glassy phase. This expansion weakened the connection of atoms in glass network and brought some closed pores into products, which led to the decrease of sample strength.     

Effect of SrO Additions on the Grain-Boundary Microstructure and Mechanical Properties of Magnesia-Partially-Stabilized Zirconia

Improvements in the mechanical properties of magnesia-partially-stabilized zirconia are obtained by the addition of SrO. Evidence is presented which indicates that the added SrO effectively neutralizes the detrimental effects of SiO₂ contaminant by forming a glass phase which is ejected from the bulk of the ceramic during sintering. This combined effect results in the retardation of the subeutectoid decomposition reaction while minimizing retention of glass phases at the grain boundaries.     

烧成制度及杂质对Mg—PSZ材料极出体形态和力学性能的影响

以阳泉产ＺｒＯ２原料为研究对象,通过控制烧成制度,即在烧结温度前保温（１６００℃）,再在Ｃ相区烧结固溶,经慢速至１１００℃后再自然冷却到室温,可使基体中长出大小合适的析出体,获得较佳的力学性能,含１０ｍｏｌ％ＭｇＯ的Ｍｇ－ＰＳＺ材料强度 可ａ韧 可达１２．３ＭＰａ·ｍ＾１／２,与宜兴料相比较,可知步量杂质（Ｆｅ２Ｏ３）可略降低材料的烧结温度,但对材料的力学性能无影响,通过ＳＥＭ对选用不同烧成制度材     

氧气分压对甲苯液相氧化的影响

在直径为48 mm的鼓泡塔反应器中,模拟工业条件研究了氧气分压对甲苯液相催化氧化生成苯甲酸反应的影响.结果表明,甲苯液相催化氧化反应在低氧气分压下对氧气是1级反应,随氧气分压增加,氧气的影响逐渐减弱,在高氧气分压区过渡为对氧气是0级反应.根据甲苯氧化的自由基链式反应机理,推导出了覆盖整个氧气浓度区的动力学方程,不仅可以很好地解释在不同氧气分压区氧气级数变化的现象,而且对反应过程的描述具有很高的计算精度,在145℃和155℃反应温度下的计算误差小于4%.     

Kinetics of Self-Crack-Healing of Alumina/Silicon Carbide Composite Including Oxygen Partial Pressure Effect

Self-crack-healing behavior under a combustion gas atmosphere with a low oxygen partial pressure ,     , is important for actualizing ceramic gas turbines, but to date only self-crack-healing behavior in air has been investigated. In this study, we investigated crack-healing behaviors at 1273–1773 K under several levels of     . Crack-healing in atmospheres with     gave rise to the complete strength recovery of cracked specimens, resulting from passive oxidation. Based on the obtained results, the kinetics for strength recovery by self-crack-healing was expressed as a function of healing temperature, T _H (K), and     (Pa). The strength recovery rate for complete crack-healing, v _H (s⁻¹), could be expressed as Using this rate equation, one can evaluate the healing time for complete strength recovery under combustion gas in a gas turbine.