菱镁矿粒度对堇青石陶瓷烧结性能的影响

以菱镁矿、高岭土和石英为主要原料,采用固相反应法制备了堇青石陶瓷。采用TG-DSC、XRD、SEM能谱分析等方法,研究了菱镁矿粒度对堇青石陶瓷烧结性能的影响以及堇青石合成过程的反应机理。结果表明,降低菱镁矿的粒度可以有效降低堇青石陶瓷的烧成温度;在该体系中,堇青石陶瓷的烧成受固相扩散传质控制,降低原料粒度可以促进堇青石陶瓷的烧成。     

原料对堇青石陶瓷性能的影响

指出了用国内原料可以制得符合世界领先公司堇青石组成所要求的各种堇青石配料复合物。当主要氧化物和杂质停含量相同时,观察至堇青石性能与配料组成的关系。研究了滑石预烧温度对一步烧结工艺制得堇青石试样性能的影响。     

增硬加强堇青石陶瓷材料的研究

采用粘土、滑石、氧化铝为主要原料合成的堇青石作为基相,加入适量复合添加剂,使堇青石陶瓷材料达到致密烧结,并使其抗折强度和硬度大幅提高.     

利用晋城煤矸石合成堇青石的实验研究

以煤矸石、滑石粉和氧化铝为原料,按堇青石理论比配料,在1 150℃~1 220℃下烧结2h~4h,合成了堇青石;正交实验分析表明,对于样品体积收缩率,影响最大的因素是烧结温度;SEM表征显示,产品颗粒形貌不规则;XRD分析发现,杂质存在能显著降低烧结温度,但同时会导致出现较多杂质晶相,因此要特别注意控制原料中杂质的含量.合成堇青石的最佳条件为烧结温度1 220℃,保温时间4h,磨料粒度45μm.     

MgO对多孔堇青石材料组成结构的影响

以菱镁矿轻烧粉、废弃水口和硅灰为原料制备多孔堇青石材料,研究分析菱镁矿轻烧粉中氧化镁对制备多孔堇青石材料致密度、结晶相组成及显微结构的影响。用XRD法和SEM法表征多孔堇青石材料中的结晶相和显微结构。结果表明:利用高温固相反应烧结,可以制备出以堇青石为主晶相的多孔堇青石材料。但菱镁矿轻烧粉的过量引入会降低多孔堇青石材料的孔隙率,导致合成材料中镁橄榄石相增多。     

堇青石原料粒径对堇青石陶瓷烧结性能的影响

为了研究堇青石粉体原料粒径对制备堇青石陶瓷性能的影响,以平均粒径分别为10、23、37、58和99μm的5种堇青石粉料为原料,平均粒径为15μm的土豆淀粉为造孔剂,分别制备了堇青石致密陶瓷和多孔陶瓷,并对比研究了粉体粒径对两种陶瓷烧结性能的影响。结果表明:在相同的烧成制度下,堇青石粉体的平均粒径越大,纯堇青石陶瓷的显气孔率和线收缩率越大,体积密度越小;添加淀粉后,陶瓷材料的显气孔率明显提高,体积密度降低,且因原料粒径不同,增加和降低的幅度不同;但淀粉加入后,多孔陶瓷材料的性能随原料粒径的增大无明显变化。     

堇青石粉末合成及其烧结特性的研究

以热喷雾法制备的MgAl2.6(OH)x粉末、水玻璃和MgCb为原料,用沉淀包裹法制备堇青石前躯体粉末。通过控制沉淀过程中体系的pH值,在MgAl2.6(OH),粉末粒子上形成无定形连续包裹层。烧结过程中,在700℃之前,包裹层与被包裹粒子反应形成无定形相;在850℃,从无定形体系中析出的纳米尖晶石对致密化烧结过程没有影响,相反,方石英的析出将阻碍烧结。堇青石的烧结属液相烧结,其致密化过程发生在堇青石合成之前。700℃煅烧的堇青石前躯体在1250℃烧结2h,其相对密度可达96．4％。     

堇青石的红外辐射特性

堇青石是一种硅酸盐矿物，因其具有优良的红外辐射特性、较高的热稳定性和化学稳定性等优点而成为一种重要的红外辐射材料。分析了改善堇青石材料红外性能的几个主要途径：适当减小原料的粒度、合理选择堇青石合成方法、在堇青石基质中添加适量过渡金属氧化物、选取适宜的堇青石烧结温度等。介绍了堇青石在红外辐射导电陶瓷、红外泡沫陶瓷、高温红外辐射涂料和常温红外辐射涂料中的应用，展示了堇青石作为红外材料的发展前景。     

碳铬渣合成堇青石的反应机理及结构表征

以高碳铬铁合金渣、工业氧化铝粉和硅微粉为主要原料,通过高温固相法合成制备堇青石。利用X射线衍射和扫描电子显微镜等研究了烧成温度、保温时间对堇青石材料结晶度、晶粒尺寸、晶相组成和显微结构的影响规律。结果表明:样品中堇青石主要由镁橄榄石进一步反应生成,其最佳烧结制度为1 350℃,保温3 h,堇青石含量在87%左右;随着烧结温度和保温时间升高,样品相对结晶度降低,堇青石晶粒尺寸则逐渐增加,晶胞参数沿a轴晶向轴方向增大,c轴方向减小,晶胞体积变大。铬元素固溶进堇青石晶格是使其晶胞参数变化的主要原因。     

Bi_2O_3对堇青石陶瓷的烧结行为、相变和热膨胀性能的影响

采用X射线衍射仪、差热分析仪和热膨胀仪等手段研究了由氧化物粉末 (MgO、Al2 O3 和SiO2 )制备堇青石陶瓷时 ,添加Bi2 O3 对堇青石陶瓷相变和性能的影响 ,分析了Bi2 O3 在烧结过程中的作用机理是低温产生液相促进烧结。试验表明 ,Bi2 O3 能明显降低烧结温度 ,在 12 5 0℃烧成 3h后的陶瓷是由α堇青石和少量的 μ堇青石组成。随着Bi2 O3 含量增加 ,陶瓷的致密度和热膨胀系数逐渐升高。Bi2 O3 的添加量 (质量分数 )为 0 .0 4 ,原料相石英消失。Bi-O膨胀系数较Si-O的大和Bi3 + 离子进入堇青石晶格中是引起堇青石陶瓷热膨胀系数升高的主要原因。     

Thermal conductivity and thermal expansion behavior of glass fiber-reinforced rigid polyurethane foam

The thermal conductivities and the thermal expansion curves of glass fiber-reinforced rigid polyurethane foams with various fiber lengths, various fiber volume fractions and various matrix densities were determined experimentally. Additionally the thermal expansion coefficients of these materials at room temperature were examined in terms of the interaction between fiber and matrix. The thermal expansion properties were analyzed successfully with the analogous treatment which is applied to the mechanical tensile behavior.     

Germanium-Modified Cordierite Ceramics with Low Thermal Expansion

Magnesium cordierite ceramics modified by ionic substitution of Ge for Si were synthesized, fabricated, and characterized. The effect of the amount of ionic substitution on thermal expansion was investigated. Ionic substitution of ∼20% germanium for silicon reduces thermal expansion to zero at room temperature. A processing schedule to achieve nearly theoretical densities in germanium cordierite specimens was developed, and the microstructures of sintered samples were examined by scanning electron microscopy. Other thermal properties such as specific heat and thermal conductivity were also measured for the germanium-modified cordierites.     

Thermal Expansion of High Filler Content Cellulose-Plastic Composites

Abstract

The effects of resin content (10–30%), cellulose fiber length (120 and 300 micrometer), and molding conditions (press, injection, and extrusion molding) on the thermal expansion of high filler content cellulose/polypropylene composites were evaluated. Other physical properties such as densities, bending strengths, and water absorption of composites were also determined. The results indicated that thermal expansion of composites is dependent on the above conditions. Small thermal expansion was observed for composites with low resin (high cellulose) content. Composites with long fibers (300 micrometer) showed smaller thermal expansion than those of 120 micrometer, except for injection-molded composites with 20% resin content. Composites prepared by injection and extrusion molding showed anisotropy of thermal expansion depending on the parallel and perpendicular directions of molding. The results of thermal expansion are discussed in the light of other physical properties and the interaction of fibers in the composites.     

改善氧化锆陶瓷材料抗热震性的探讨

通过分析氧化锆的热膨胀行为和相变特征 ,对改善氧化锆陶瓷材料抗热震性的途径进行了探讨。重点讨论了利用相变提高氧化锆材料抗热震性能的方法 ,并给出了实验结果。在所提到的改善氧化锆材料抗热震性的三种方法中 ,都不是以牺牲材料的致密度为代价的。     

Molecular modeling of crosslinked epoxy polymers: The effect of crosslink density on thermomechanical properties

Molecular dynamics and molecular mechanics simulations are used to establish well-equilibrated, validated molecular models of the EPON 862-DETDA epoxy system with a range of crosslink densities using a united atom force field. Molecular dynamics simulations are subsequently used to predict the glass transition temperature, thermal expansion coefficients, and elastic properties of each of the crosslinked systems. The results indicate that glass transition temperature and elastic properties increase with increasing levels of crosslink density and the thermal expansion coefficient decreases with crosslink density, both above and below the glass transition temperature. The results demonstrate reasonable agreement with thermomechanical properties in the literature. The results also indicate that there may be a range of crosslink densities in epoxy systems beyond which there are limited changes in thermomechanical properties.     

Network mechanical properties of amine-cured epoxies

The technique of Impulse Viscoelasticity was used to characterize the network mechanical properties of amine-cured epoxies during cure. The effects of amine molecular weight, functionality and stoichiometry were investigated. Among the properties which were obtained were the equilibrium tensile modulus, gelation time, cure and thermal stresses, volumetric changes during cure, glass transition temperature, thermal expansion coefficient, and molecular weight between cress-links. It was found that these networks cured elastically and agreed closely with the predictions of rubber elasticity theory over a wide range of crosslink densities.     

Properties of Glasses in the System AgI-Ag2O-B2O3

Glasses with AgI contents between 75 and 50 mol% were prepared in the pseudo-binary system AgI-Ag₂O-B₂O₃ by different methods and at varied cooling rates. Glass transition temperatures, densities, and thermal expansion coefficients were measured and compared with literature data. Thermal history markedly injuences the properties .     

Thermal expansion of silicate glass-forming systems at high temperatures from topological pruning of ring structures

The thermal expansion of a silicate glass system containing four types of modifiers at temperatures above the glass transition can be correlated with changes in medium range order (viz., the ring size distribution) without significant modifications in short range order such as Si coordination and Q-speciation. An empirical linear relation between the network densities of the two groups of silicate glass-forming systems and their characteristic ring size has been found using the topological pruning picture proposed by Stixrude and Bukowinski (Am Mineral, 1990, 75:1159-69). Although variation in thermal expansion among different samples exists, for individual composition of the glass-forming liquids, a strong linear correlation exists between the ring size distribution and the thermal expansion contributed by both configurational change and thermal vibration. The ring evolution of the glass-forming liquid driven by temperature has been modeled by assuming an Arrhenius-like activation picture for small member rings and treated as constant numbers for large rings. The compositional dependence of the ring structure can be reflected in the mixed-alkali or alkaline-earth effect on the small ring activation energy.     

Residual Stress and Grain Deformation in Extruded Polyerystalline BeO Ceramics

Residual strain measurements on a series of sintered BeO tubes indicate that significant stresses developed during processing from the thermal expansion anisotropy. X-ray topographic studies show that the resulting strains are preferentially located near grain and subgrain boundaries. Fine-grained samples, 20μ and less, do not show pronounced subgrain structure and exhibit lower dislocation densities in the grain interiors than do coarse-grained samples.     

Thermal Expansion of BaO · 2B<Subscript>2</Subscript>O<Subscript>3</Subscript> in the Vitreous and Crystalline States: I. Thermal Expansion and Density of Barium Diborate Prepared from Monolithic Glass of the Same Composition

The thermal expansion and density of vitreous and polycrystalline barium diborates prepared by crystallization of monolithic glasses of stoichiometric composition are investigated in the temperature range 20–700°C. The glass crystallizes in the form of the α and β modifications depending on the heat treatment conditions. The α phase nucleates on the surface of the monolithic sample and grows inward the sample in the form of textured layers. The β phase nucleates in the melt bulk and grows in the form of spherulites. Textured polycrystalline barium diborate in the form of the α phase possesses a pronounced anisotropy of thermal expansion with respect to the direction of crystalline-layer growth. This is associated with the negative expansion along one of the crystallographic axes. The effective temperature coefficients of thermal expansion in the temperature range 20–600°C increase in the order: α phase-glass-β phase. Upon heating above 690°C, the β phase transforms partially into the α phase. This is accompanied by a considerable increase in the volume. The densities of the glass and polycrystalline samples in the temperature range 20–700°C increase in the order: glass-α phase–β phase.