Synthesis of hexacelsian barium aluminosilicate by film boiling chemical vapour process

An original oxide/oxide ceramic-matrix composite containing mullite-based fibers and a barium aluminosilicate matrix has been synthesized by the film boiling chemical vapour infiltration process. Alkoxides were used as liquid precursors for aluminum, silicon and barium oxides. The structure and microstructure of the oxide matrix were characterized by Scanning Electron Microscopy, Energy Dispersive Spectroscopy and X-ray diffraction. Apart from small residual mullite and amorphous phase amounts, the oxide matrix is composed of the hexacelsian phase, conferring to the material interesting perspectives for high-temperature electromagnetic and structural applications.     

Mössbauer studies of iron in Lurgi gasification ashes and power plant fly and bottom ash

Iron Mössbauer spectroscopy and X-ray diffraction methods were applied to the study of a selection of ashes produced in a Lurgi gasification plant and fly ash from a pulverized coal combustion. The ashes contained hematite, magnetite, and goethite. Sixty percent or more of the iron in these ashes was in the oxide form, with the remainder present in mullite and other silicate phases. Iron was divalent in the latter, and present as both Fe⁺² and Fe⁺³ in mullite. Ratios of Fe⁺² and Fe⁺³ varied from 0.3 to 0.7. By comparison, a water-quenched molten bottom ash was free of iron oxides and contained only amorphous silicate phases with virtually all iron in the divalent state.     

负载型NiO和CoO催化剂上N2O分解研究

本研究在对多种金属氧化物催化剂进行初步筛选的基础上，研制出以莫来石为载体的负载型NiO、CoO催化剂，考察了分解温度、催化剂组成和负载量对N2O分解率的影响，并对其分解反应动力学进行了研究。结果表明莫来石负载NiO、CoO催化剂对N2O分解有良好的催化性能；其反应速度对N2O均为一级反应；同样负载量下NiO有更好的催化分解活性；这一研究为开发阻力低、催化性能好的工业用蜂窝型规整填料奠定了基础。     

Reaction between zinc oxide and chamotte refractories

Conclusions As a result of chemical reaction between chamotte and zinc oxide, zinc aluminate ZnAl₂O₄ and zinc silicate Zn₂SiO₄ are formed.The mullite is not chemically resistant under the action of zinc oxide. Even at 900°C mullite reacts with zinc oxide, forming ZnAl₂O₄ and SiO₂.In accordance with the quantity of melt formed in the refractory under the action of zinc oxide, high-alumina products should be more resistant to the action of vapors and oxides of zinc at elevated temperatures than chamotte and semiacid refractories.Translated from Ogneupory, No. 7, pp. 36–39, July, 1971.     

Evaluation of novel interphases for oxide/oxide ceramic matrix composites

Abstract

Stoichiometric MXO₄ type compounds, where M represents a rare earth or yttrium ion and X a pentavalent cation, have been prepared using mixed oxide and liquid precursor methods. Their stability in relation to Al₂O₃, mullite, and yttrium–aluminium garnet (YAG) has been determined by examining interfaces exposed in reaction couples after heating to 1400°C. Complex oxides of the phosphate and vanadate type are shown to possess the desired chemical stability with some of the candidate oxides and can be considered as suitable interphase materials. Close control over composition and homogeneity is shown to be important in determining their performance as potential interphases due to the possible formation of a liquid phase which can react readily with the oxide matrix or fibre. Selected MXO₄ compounds have also been successfully deposited on to oxide substrates and woven oxide fibres using liquid precursors and RF magnetron sputtering techniques, yielding controlled and uniform fibre coatings.     

Reactions between Synthetic Mica and Simple Oxide Compounds with Application to Oxidation-Resistant Ceramic Composites

Reaction-couple experiments have been pursued in order to evaluate the potential of a phyllosiiicate to act as a chemically protective, fracture-deflecting, oxidation-resistant interphase for oxide fiber–oxide matrix composites. The synthetic mica fluorophiogopite (KMg₃[AlSi₃]O₁₀F₂) was reacted with single-phase substrates of alumina (Al₂O₃), mullite (3Al₂O₃·2SiO₂), forsterite (Mg₂SiO₄), or enstatite (MgSiO₃). X-ray spectroscopy, X-ray diffraction, and scanning electron and transmitted polarized light microscopy were applied to the analysis of the reaction couples. Fluorophlogopite reacts strongly with alumina, mullite, and enstatite, resulting in substantial damage to the substrate as well as the breakdown of the mica. The chemical reactions between mica–alumina and mica–mullite are examined critically. In the case of alumina, the reaction results in the formation of a planar spinel (MgAl₂O₄) layer separating the substrate from the breakdown products of the mica. This unvarying result suggests, therefore, that a spinel diffusion barrier would prove effective in protecting alumina from fluorophlogopite. Experiments revealed such effectiveness: local equilibrium is established in the layer sequence alumina–spinel–fluorophlogopite; i.e., planar interfaces are established amongst these phases that are stable under conditions of high temperature and high oxygen fugacity. A similar chemical approach for protection of mullite is not obvious. Based on an understanding of its intrinsic fracture energy, the fluoromica interphase is expected to be effective in mechanically protecting adjacent oxides from propagating cracks, a behavior qualitatively demonstrated by indentation experiments on the kinetically persistent alumina–spinel–fluorophlogopite–spinel–alumina laminates.     

SOME PYROCHEMICAL PROPERTIES OF PYROPHYLLITE *

Two samples of pyrophyllite from North Carolina were heated at temperatures from 400° to 1400°C. Studies of weight loss, volume change, and other heat effects were made. X-ray examinations were made of the unheated and heated samples to determine the minerals present. Pyrophyllite lost its water of constitution between 400° and 700°C without any heat effect or breakdown of the crystal lattice. There was, however, an increased separation of the alumino-silicate layers reflected in a permanent linear expansion. Between 1000° and 1100°C, the dehydrated pyrophyllite decomposed, one of the products being mullite. In the presence of small quantities of vanadic and tungstic oxides, mullite and cristo-balite were noticed at a lower temperature; boric oxide had no effect.     

Effects of inorganic oxides on polymer binder burnout. I. Poly(vinyl butyral)

The effects of silica, mullite, α-alumina and γ on the nonoxidative thermal degradation of poly(vinyl butyral) are described. To varying degrees, all of the inorganic oxides catalyzed reactions that produced butanal. Other significant products included: water, butenal, acetic acid, and alkyl aromatics. Two distinct evolution steps were detected for samples containing mullite and α-alumina, suggesting that multiple interactions existed between these oxides and the polymer. The relative amounts of volatile aromatic products evolved by heating polymer/oxide samples were greater than the amounts generated from the neat polymer. For the polymer/γ-alumina sample, carboxylate species were detected on oxide surfaces above 250°C, indicating that a reaction between the polymer and γ-alumina occurred. © 1996 John Wiley & Sons, Inc.     

Physicochemical factors that affect the pseudocapacitance and cyclic stability of Mn oxide electrodes

Manganese (Mn) oxide was prepared by anodizing metallic Mn film that was electrodeposited in n-butylmethylpyrrolidinium bis(trifluoromethylsulfony)imide (BMP-NTf₂) ionic liquid. Different anodization courses, namely potentiostatic and cyclic voltammetric methods, led to variations in physical and chemical characteristics of the Mn oxides, and therefore in their pseudocapacitive performance. Evolution of the microstructure, residual weight, and chemical state of the Mn oxides with the charge-discharge cycling number was studied using a scanning electron microscope (SEM), an atomic absorption spectroscope, and an X-ray photoelectron spectroscope, respectively. The analytical results indicate that the electrochemical stability of Mn oxide is mainly determined by its microstructure; the more fibrous (or porous) oxide had a greater durability against cyclic charge-discharge. Moreover, the chemically hydrous state was found to be the most crucial factor that governed the mass specific capacitance of Mn oxide.     

Solid Solutions of Silicon Oxide in Mullite

An explanation is proposed to account for the possibility of formation of solid solutions of silicon oxide in mullite. The difference in the diffusion coefficients of cations in complex oxides leads to the formation of a solid solution of the oxide containing the cation with the lower diffusion coefficient in the complex oxide and a phase containing the cation with the higher diffusion coefficient or rich in this oxide. Using yttrium or scandium additives, which are chemisorbed on the surface of aerosil (SiO₂ source), it is possible to make the rate of diffusion mass transfer of aluminum cations higher than that of silicon cations and thus obtain a solid solution of silicon oxide in mullite.     

卫鲁蓝晶石的特性研究

实验所用原料来自于卫鲁蓝晶石精矿。将卫鲁蓝晶石原料湿磨后成型并进行烧结。利用湿化学方法、X射线衍射分析仪(XRD)、粒度分析仪和扫描电子显微镜(SEM)对卫鲁蓝晶石的化学成分,矿物组成、湿磨后粒径和微观形貌进行了表征,并考察了其烧结性能。研究表明:在1 100℃左右就开始了蓝晶石向莫来石的转化过程,在1 250℃时,可以在SEM照片中明显的看到一次莫来石化形成的鳞片状莫来石。     

Hydration of magnesium oxides in the presence of water vapour

The uptake of water vapour on to magnesium oxide has been studied using a silica spring technique. In the experiment reported here, porous magnesium oxides were investigated, and the amount adsorbed at equilibrium at various vapour pressures of water was determined. The magnesium oxides used were characterised using low temperature nitrogen adsorption to give a pore size distribution and a specific surface area, and chemical analysis from X-ray fluorescence data. The nitrogen adsorption isotherms obtained were a mixture of type II and type IV using the BET classification. The oxides sorbed water vapour to give an equilibrium isotherm which was sigmoid in character. Rehydration of the samples after desorption and degassing at 298 K produced samples with an enhanced capacity for water vapour sorption. This is associated with a change in the surface of the oxide from a hydrophobic to a hydrophilic character upon the second adsorption cycle. This is of importance in assessing the water sorption capacity of industrial produced porous magnesium oxides and their subsequent use as desiccants.     

10-mol%-Gd2O3-Doped CeO2 Solid Solutions via Carbonate Coprecipitation: A Comparative Study

10mol% Gd₂O₃-doped CeO₂ solid solutions (20GDC) have been synthesized via carbonate coprecipitation using ammonium bicarbonate (AHC) and urea as the precipitants. The precursors and the resultant oxide powders were characterized via chemical analysis, X-ray diffractometry (XRD), Brunauer–Emmett–Teller (BET) analysis, and high-resolution scanning electron microscopy (HRSEM). Sinterabilities of the 20GDC oxides in air were studied by constant-rate-of-heating (CRH) sintering and the conventional ramp-and-holding sintering methods. The precursor processed by both methods is hydroxyl carbonate but shows quite different particle morphologies in the two cases. Highly sinterable 20GDC oxides that can be densified to >99% of the theoretical at 1050°C within 4 h have been obtained via the AHC method.     

Role of Structural Fe(III) and Iron Oxide Nanophases in Mullite Coloration

Six mullite samples, derived from heat-treated natural kaolinites with various iron content, were investigated and compared to synthetic monophasic mullite. They were analyzed by X-ray diffraction, diffuse reflectance spectroscopy, and electron paramagnetic resonance. To quantify mullite coloration, the CIE colorimetric system was used. In contrast to synthetic mullite, samples showed charge transfer bands involving O²⁻ and Fe³⁺ ions as well as ferric crystal-field transitions due to Fe³⁺ ions in iron oxide nanoparticles. Absorption edges showed red shifts. The resulting yellowness, saturation of which increased with the content of iron oxide nanoparticles, is direct evidence for the coloring effect of Fe³⁺ ions.     

Oxide CMCs: interphase synthesis and novel fibre development

Strategies which have been used in the synthesis of high temperature interphases with debond capability in oxide/oxide systems have been: (i) the formation of layered oxides (β-aluminas or magnetoplumbites) with preferred crystal orientation due to in-situ interface reactions between phases deposited from vapour or liquid precursors and (ii) the deposition on fibre surfaces of complex oxides (vanadates and phosphates, principally of the rare earths) from colloidal precursors or by magnetron sputtering. This paper is primarily concerned with the latter. The constitution and thermal stability of these interphases has been studied with reference to potential fibres and matrices (alumina, YAG and mullite). Included within the program are newly developed single phase mullite fibres and examples of these are also presented.     

Constitution of Fireclays at High Temperatures: II, Mineralogical Composition

Quantitative X-ray diffraction analysis of quenched specimens, supplemented by HF solution and X-ray fluorescence, was used to determine the mineralogical composition of three refractory clays after heating at 1150° to 1500°C. The three clays represent a range of purity found in high and superduty fireclay refractories: an impure plastic clay, a flint fireclay, and a kaolin. The results show the percentages of mullite, cristobalite, quartz, and liquid. The quantity and composition of the liquid phase were estimated by difference. Iron and titanium oxide impurities in the mullite decreased with increased firing temperatures.     

Sm2O3掺杂对纳米莫来石前驱粉体微观结构和烧结性能的影响

为了改善纳米莫来石粉体烧结性能,以硫酸铝和硅酸钠为主要合成原料,添加不同含量Sm2O3,采用共沉淀工艺制备莫来石前驱粉体,经过煅烧得到莫来石纳米粉体,研究了Sm2O3掺杂量对莫来石粉体微观结构和烧结性能的影响.研究表明:当Sm2O3的加入量为4wt%时,合成温度可由传统的1300 ℃左右降低至1000 ℃,晶粒尺寸约为39 nm,比表面积达到95.265 m2/g.说明适当掺杂Sm3+对于合成纳米莫来石具有改善微观结构,促进烧结,促进莫来石晶相形成的作用.     

Porous mullite blocks with compositions containing kaolin and alumina waste

In the production of alumina by the Bayer process, the calcination step generates a waste containing ~90% aluminum oxide (Al₂O₃). Due to the high content of this oxide, this waste can be used as a source of alumina in porcelain formulations, especially those used in the synthesis of mullite. The purpose of this study was to produce porous mullite blocks using compositions containing kaolin and alumina waste. The compositions were formulated based on a mullite stoichiometry of 3:2. Heat treatments were carried out in a conventional furnace at temperatures of 1450 to 1500 °C, applying a heating rate of 5 °C/min and a 1-h hold time at the firing temperature threshold. The powders were characterized by means of X-ray fluorescence (XRF); X-ray diffraction (XRD); thermal analysis (TGA-DTA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physic mechanical properties of the test specimens: water absorption, apparent porosity, linear shrinkage and flexural strength were also evaluated. The XRD results revealed the formation of mullite as the major phase. The morphological analysis by SEM revealed typical mullite needles originating from clay minerals. The size of the mullite needles was calculated based on the TEM analysis, which indicated diameters smaller than 400 nm, confirming the nanometric dimensions of the needles. The flexural strength test of the specimens indicated that this parameter ​​tends to increase as the temperature is raised.     

Preparation of Mullite Ceramics from Coprecipitated Aluminum Hydroxide and Kaolinite Using Hexamethylenediamine

Mullite ceramics have been prepared from an aqueous suspension of kaolinite (raw or ground) and aluminum hydroxide. The precursor was coprecipitated in the mixture using hexamethylenediamine (HMDA) or ammonium hydroxide, and a solution of aluminum chloride from acid dissolution of wastes of aluminum metal. The precursor and the resulting materials were characterized and studied by X-ray diffraction, thermal methods, and mechanical strength and porosity measurements. The feasibility of the proposed chemical processing route for mullite preparation was demonstrated, in particular using HMDA as a precipitating agent for aluminum hydroxide instead of ammonium hydroxide, which adversely affects the system reactivity. The use of HMDA, as compared with ammonium hydroxide, and ground kaolinite produces single-phase mullite and enhances the flexural strength (maximum of 49 MPa) of the resultant ceramic porous bodies (porosity ca. 52–45 vol%) fired at 1550–1600°C for 30 min.