Transformation of tridymite to cristobalite below 1470° C in silica refractories

Because silica refractory has good volume stability and creep properties at high temperature, it has been used in several furnaces. However, silica has three polymorphs (quartz, tridymite and cristobalite) and each polymorph has an- type transformation. It is known that cristobalite is the stable phase of silica between 1470° C and the melting point of silica refractories. However, sometimes cristobalite was found in silica refractories used in the stable temperature region of tridymite. Therefore, the cause and mechanism of this tridymite-to-cristobalite transformation below 1470° C was studied. Although the transformation temperature of tridymite to cristobalite was also 1470° C in the sample used in this research, it decreased on addition of Al₂O₃. The apparent activation energy of the tridymite-to-cristobalite transformation was found to be 787 kJ mol^–1 above 1470° C and 176 kJ mol^–1 below 1470° C with Al₂O₃ by measuring the transformation rate. It was also observed using EPIVIA that the tridymite included CaO; however, CaO and Al₂O₃ were located on the outside of the cristobalite which was produced below 1470°C. Therefore, it is supposed that the liquid phase was produced by the penetration of Al₂O₃, and impurities in the tridymite crystal diffused outside and then silica was precipitated as cristobalite.     

Treatment of electrostatic interactions at the Si(1 0 0)-SiO2 interface

Silica in equilibrium with silicon is known to be partly crystalline. Because of crystallographic constraints, models proposed so far (cristobalite as well as tridymite) deal with polar surfaces of silica, which are expected to be highly unstable. In this paper, using both exact treatments of simplified models and Monte-Carlo simulations of the real system with effective potentials, we show how the system compromises to decrease the electrostatic energy. We also investigate the role of electrostatic interactions on the formation of oxygen vacancies at the Si-SiO₂ interface. Using a tight binding approach, implications of these interactions on mid-gap surface states in silica are also discussed.     

Physical,chemical and electric characterization of thermally treated rice husk ash and its potential application as ceramic raw material

Rice husk ash (RHA) is composed essentially by carbon and high levels of silica that justify the economically viable use of RHA in production of ceramic materials. Therefore, this study presents the physical, chemical, and electric characterization of two RHAs submitted to thermal treatment at different temperatures. Grain size, specific weight, specific surface area, total carbon analysis as well as X-ray fluorescence spectrometry, scanning electron microscopy, X-ray diffraction, and complex impedance spectroscopy were carried out. The results indicate that the thermal treatments afforded to produce high-silica RHA with cristobalite and tridymite crystalline phases that indicate the potential of RHA to be used as raw material to obtain advanced silica-based ceramic products. RHA particle size increased with treatment temperature, followed by a slight increase in specific weight and a drastic drop in surface area. Electrical characterization showed that silica particles are closely packed to a carbon film.     

碱硅酸反应与碱碳酸盐反应

碱集料反应（AAR）可分为两类,即碱硅酸反应（ASR）与碱碳酸盐反应（ACR）。二者的共同点是与碱发生的化学反应可导致混凝土中集料的体积增大,从而可能使混凝土甚至整个建筑物或构筑物发生膨胀开裂。文章着重从膨胀过程和机理以及岩石的结构特征探讨二者的特性与差异。ASR类型岩石具有碱活性的前提条件是较低的二氧化硅结晶完整度。只有隐晶质、微晶质、玻璃质或发生过应变的二氧化硅才会具有较高的化学活性,导致混凝土破坏。通过系统研究证实,对碱碳酸盐反应,虽然结晶的完整程度以及白云石（CaCO3·MgCO3）分子式中Ca/Mg比也将影响其与碱反应的速率,但起决定作用的是白云石晶体的尺寸及其在岩石中的分布状态和被基质包围的紧密程度。从微观结构得出的这些特征将有助于加深对碱集料反应膨胀机理的认识。文中还介绍了形成活性白云石的地质环境和碱硅酸反应与碱碳酸盐反应的区分方法。     

碳纤维/石英复合材料中石英玻璃的析晶

碳纤维补强石英复合材料具有很高的强度和断裂韧性,是纤维补强陶瓷(玻璃)基复合材料中比较成功的一个系统。在碳纤维/石英复合材料中,石英玻璃的析晶将明显地影响复合材料的烧结及其性能。本文通过热分析、X 射线衍射和分析电镜等手段,对不向温度下热压烧结的碳纤维补强石英复合材料的析晶性状进行研究。对所析出的方石英、鳞石英和硫方石英的显微结构进行了讨论,同时亦讨论了石英玻璃的析晶对复合材料烧结的影响。这一研究与讨论将为复合材料制备工艺提供依据。     

Exposure profiles and source identifications for workers exposed to crystalline silica during a municipal waste incinerator relining period

In this study, respirable crystalline silica exposures to furnace relining workers of 7 exposure groups were assessed by conducting personal respirable dust samplings. All possible pollutant sources were identified for each exposure group through field observations, and bulk samples were randomly collected from each identified pollutant source. All collected samples were analyzed for their tridymite, cristobalite, and quartz contents by using the X-ray diffraction method. Results show that quartz was the only detectable crystalline silica content. We found that the resultant respirable quartz exposure levels presented in sequence for the 7 exposure groups (sand blasting>bottom ash cleaning>wall demolishing>relining>others>grid repairing>scaffold establishing) were different from that of the corresponding respirable dust exposure levels (bottom ash cleaning>wall demolishing>sand blasting>relining>grid repairing>scaffold establishing>others). 87.3-100% of workers' respirable quartz exposures of the 7 exposure groups exceeded the TLV-TWA (0.025 mg m(-3)) indicating appropriate control strategies should be taken immediately. By comparing the fractions of quartz contained in personal respirable dust samples with that contained in all possible pollutant sources for each exposure group, this study identified main pollutant sources for each exposure group as: bottom ash cleaning and scaffold establishing: bottom ash; sand blasting: blasting sand; wall demolishing: refractory cement+wall ash; wall relining: refractory brick; grid repairing: wall ash+refractory cement; grid repairing: wall ash+refractory cement; others: blasting sand+bottom ash. Finally, effective control strategies were proposed for exposure reduction by using above information together with our field observations.     

Characterization of Rice Hull Ash and Its Performance in Turbidity Removal From Water

This study characterizes the locally obtained samples of rice hull ash and investigates its performance on turbidity removal from water. Four samples of this material were studied, namely, unwashed parboiled rice hull ash (UPRHA), washed parboiled rice hull ash (WPRHA), unwashed unparboiled rice hull ash (UUPRHA), and washed unparboiled rice hull ash (WUPRHA). Scanning electron microscopy (SEM), x-ray diffractometer (XRD), and Fourier infrared spectroscopy (FTIR) were carried out to characterize these samples. A filtration process was carried out to investigate the effectiveness of the rice hull ash medium in removing water turbidity. The XRD results showed the silica, which is present in the ashes, to be cristobalite, quartz, and tridymite. The silica contents of the UUPRHA and WUPRHA were observed to be 77.10% and 98.24%, respectively, while those of UPRHA and WPRHA were 79.07% and 94.97%, respectively. The SEM images showed agglomeration of ash particles after the ashes were washed. The washed RHA samples showed improved pH, a good percentage of turbidity removal (<5 NTU) from water sample. Washing RHA with distilled water increased the efficiency of RHA in turbidity removal from water and regulated water pH to an acceptable range.     

硅基陶瓷型芯方石英含量与叶片浇注不露芯率的关系EI

本文从硅基型芯的强度与方石英含量、方石英含量与叶片浇注不露芯率之间的关系进行了分析，重点讨论了对方石英含量的控制，为提高叶片不露芯率提出可信依据。     

The formation of zircon from amorphous ZrO2 · SiO2 powders

The different factors affecting the mechanism of zircon formation from amorphous ZrO₂ · SiO₂ powders have been studied. It was shown that zircon was formed by solid state reaction between tetragonal zirconia and silica (amorphous and cristobalite). The previously suggested Hedvall effect associated with the crystallization of amorphous silica into cristobalite did not play any role in this reaction. A high degree of Si-Zr mixing in the starting amorphous powders did not affect the mechanism of zircon formation, but speeded up the reaction rate due to the small particle size of the zirconia and silica particles segregated previously to zircon formation. It was also found that the formation reaction was retarded by the presence of carbonaceous species coming from the alkoxide precursors, which were probably retained at grain boundaries after calcination, acting as a diffusion barrier. These factors can explain the observed differences in the temperatures of zircon formation previously reported.     

方石英的析晶与无定形化

石英是一种重要的结构和功能材料, 在航天飞行器、半导体、电子通讯和光学器件等领域得到重要应用, 但是高温下方石英的析晶限制了它的进一步应用. 本文综述了不同研究人员对不同形态石英与石英基复合材料析晶行为及其差异的研究成果和所持的不同观点, 再从“表面析晶”的角度分别对不同形态石英析晶行为的差异性与不同工艺因素对石英基复合材料的影响机制进行了系统阐述. 最后, 结合作者的工作, 进一步介绍了方石英无定形化方面的最新研究成果.     

Synthesis of zeolite omega. Influence of the temperature and the reagents on the crystallization kinetics

The kinetics of the crystallization of zeolite omega in the system Na₂O, TMA₂O, Al₂O₃, SiO₂, H₂O have been determined as a function of the temperature and the nature of the sources of silicon and aluminium. At 100°C, faujasite appears first and is then dissolved while crystals of zeolite omega develop. Sodalite is also formed but pure zeolite omega can be isolated after 15 days. At 135°C the transient formation of faujasite is not observed. The crystallization kinetics of zeolite omega are not sensitive to the nature of the reagents but those of the secondary products (sodalite, analcite, mordenite, cristobalite) depend on their reactivity. Porous silica gel favours the nucleation of sodalite. Reaction mixtures prepared from kaolinite lead to die-shaped monocrystals which are stable in the medium whereas sodium aluminate produces polycrystalline aggregates which are progressively dissolved. Under these conditions pure zeolite omega was obtained after only one day of reaction using sodium aluminate and a non-porous silica gel as starting materials.     

Infra-red studies of vitreous silica at elevated temperatures

The purpose of this paper is to present a new approach to the problem of structure of the ordered regions in X-ray amorphous silica glass. The problem is open because of the differences in scientific opinion as to which polymorph of silica is the true crystalline model of vitreous silica. Infra-red absorption spectra of cristobalite and silica glass taken at temperatures ranging from room temperature to 500° C were used to identify a characteristic low-high temperature polymorphic transition relaxed in X-ray amorphous silica after heating at 210° C for at least 10 h. The infra-red spectrum of silica glass from various sources was identified as the spectrum of high-cristobalite, the spectrum after the polymorphic transition in silica glass is identical with that of low-cristobalite. The results give a strong support to the high-cristobalite/cubic/model of normal silica glass, with ordered regions in metastable form at room temperature.     

The luminescent properties, thermal stability of phthalic acid and energy transfer from phthalic acid to Tb in sol—gel derived silica xerogels

In recent years, the energy transfer between different rare earth ions and the luminescent properties of rare earth ions have been studied extensively. However, the research of luminescent properties, thermal stability and structural variations of organic compounds, as well as their energy transfer to rare earth ions in silica xerogel, have not been reported. Organic—inorganic composites have even more efficient luminescence than pure rare earth ions. We describe the fabrication of silica xerogels doped with phthalic acid and Tb³⁺, their luminescent properties and thermal stability. The results show that, in silica xerogels, phthalic acid can exist steadily at 300 °C, and it can transfer the energy that it has absorbed to Tb³⁺.     

Hydrothermal stability of pure and modified microporous silica membranes

The hydrothermal stability of microporous (0.6 nm) silica membranes prepared by the sol-gel process was studied at 600 and 800 °C in a 50 mol% steam atmosphere. The membranes remained microporous after calcination and hydrothermal treatment at 600 °C for 30 h but a substantial reduction in the specific surface area (48%) accompanied by a 77% decline in the micropore volume was observed. Hydrothermal treatment at 800 °C for 30 h resulted in complete densification of the membranes. The effect of alumina and magnesia on the hydrothermal stability of the membranes was investigated. Both Al₂O₃ and MgO were introduced into the membranes by doping the starting silica sol with controlled amounts of the corresponding nitrate salts. Alumina did not change the pore structure of the silica membranes which retained a large part of their microporosity after hydrothermal treatment at 600 °C compared to pure silica membranes. Doping with magnesia, however, resulted in lower specific surface areas relative to those of pure and alumina-doped silica membranes after drying and calcination. These effects on the stability of the membranes are explained by assuming structural changes in the membranes catalysed by magnesia.     

Solid-state NMR elucidation of the role of mineralizers in the thermal stability and phase transformations of kaolinite

The effect of lithium nitrate mineralizer on the thermal stability and phase transformations of kaolinite has been studied using⁷Li,²⁷Al and²⁹Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) in tandem with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and TEM. As the temperature is raised, lithium nitrate melts, wets the surface of the kaolinite particles, and Li⁺ diffuses into the bulk of the crystals. The mineralizer retards the dehydroxylation of kaolinite by 15–20 °C.²⁷Al MAS NMR indicates that metakaolinite originating from mineralized kaolinite contains less five-coordinated and slightly more four-coordinated Al. This promotes the transformation of metakaolinite into hightemperature phases which contain only four- and six-coordinated Al. Thus at 920 °C, mineralized samples (MS) contain-alumina type spinel but non-mineralized samples (NMS) do not. At higher temperatures, MS and NMS contain different amounts of spinel phase, mullite and cristobalite.²⁹Si MAS NMR shows that cristobalite crystallizes from segregated amorphous silica. The process is reversed between 1300 and 1400 °C in MS (but not in NMS) and cristobalite vitrifies. This is probably caused by the lowering of the liquid temperature and subsequent rapid quenching of the melt.     

Transformation pathways of silica under high pressure

Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.     

Fabrication of transparent silica glass by powder sintering

Transparent silica glasses were obtained by sintering a green compact (fabricated by slip-casting methods for high-purity silica glass powder) in diverse atmospheres. The relationships between sintering atmosphere and sintering temperature that result in transparent, sintered silica glass were shown. The results indicate that there are four forming phases for each sintering atmosphere and temperature: (1) nontransparent glass resulting from an overabundance of pores (2) crystal, such as cristobalite or β-quartz, (3) moganite, and (4) transparent glass. Optimum sintering temperature for fabricating transparent silica glass was above 1673 K in a high-vacuum (10^–4 Pa: p(O₂) = 10^–14) atmosphere. We investigate the fabrication of transparent and hydroxyl-free silica glass by a powder-sintered method. After studying the effect of sintering schedule on residual [OH^–] concentration for transparent, sintered silica glasses, we sintered a green compact prepared by silica powders with a mean particle size of 1.6 µm, first heating it to 1523 K for dehydration and then to 1873 K for densification. This typical fabricated condition resulted in a transparent, sintered silica glass with <1 ppm [OH^–] concentration.     

改性二氧化硅对SiO2/PLA复合膜性能的影响

目的研究改性二氧化硅对聚乳酸力学性能、氧气透过性能和水蒸气透过性能的影响。方法选择粒径为50nm的工业级二氧化硅为添加剂,使用KH570硅烷偶联剂对其进行改性,然后通过溶液浇铸法将改性后的二氧化硅与聚乳酸共混制备成膜。测试分析拉伸性能、透氧性能和透水蒸气性能,表征复合膜的力学性和阻隔性能。结果与纯PLA膜相比,改性复合膜的拉伸强度和弹性模量分别提高了18.65%和19.91%;玻璃化转变温度比纯PLA膜高11℃左右,热稳定性得到增强。与纯PLA膜相比,改性复合膜的氧气透过系数和水蒸气透过系数分别降低了29.89%和43.76%,阻隔性明显提高。结论经KH570硅烷偶联剂改性的二氧化硅对聚乳酸材料性能的增强效果更佳,为聚乳酸材料在包装领域的应用提供了依据。     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

Sodium-assisted oxidation of reaction-bonded silicon nitride

The oxidation of reaction-bonded silicon nitride in air, and with small amounts of sodium carbonate applied to the sample surface, has been studied. The action of the alkali is to cause short-term enhanced oxidation,which is terminated when specific compositions of the product sodium silicate glass are attained. These correspond closely to liquidus compositions in the Na₂O-SiO₂ system, and it is postulated that the retardation in the oxidation rate at this stage is due to the formation of a stable tridymite film at the silicon nitride-glass interface. The implications for the high temperature stability of reactionbonded silicon nitride components in alkali contaminated atmospheres are discussed.