Experimental Analysis of Sintering of MgO Compacts

Experimental sintering studies On undoped and cao-doped Mgo powder compacts in Static air and flowing Water Vapor atmospheres were conducted at 1230° to 1600°C. Corresponding microstructural changes of specimens during sintering were examined by scanning electron microscopy. Kinetic and microstructural data were analyzed to determine sintering mechanisms during the initial and intermediate stages of sintering.     

A Perspective on Counting Catalytic Active Sites and Rates of Reaction Using X-Ray Spectroscopy

Identification of active sites and phases in heterogeneous catalysts and the understanding of the reaction mechanism remain highly challenging. In most catalysts, the existence of a multitude of surface species, which are dynamic in relation to reaction conditions, presents a challenge of distinguishing those that are involved in the catalytic cycle from those which are spectators. The emergence of the field of single-site catalysts potentially eliminates these issues, although it can be argued that these systems remain dynamic and that multiple speciation, each a candidate for the active site, often remains a consideration. A perspective on how X-ray spectroscopy and characterization tools in general, can be used to correlate the number of active sites and the rate of their formation, in single-site and redox catalyst systems, is presented. The importance of observing proportionality between spectra features and the reaction rate, to differentiate between active sites and spectator species is discussed. Performing characterisation under catalyticly relevant conditions on structures that are demonstrably representative of actual catalysts is essential.

     

X-Ray Absorption and Photoemission Studies of the Active Oxygen for Ethylene Epoxidation over Silver

Activation of clean polycrystalline silver by a C₂H₄ + O₂ reaction mixture has been studied by XANES, XPS, and UPS. In situ monitoring of the O K-edge XAS spectrum of the pre-treated silver surface revealed a broad signal at 10–20 eV above the threshold. The comparison of the X-ray absorption spectra with O 1s and valence band photoemission data allowed us to attribute this XAS signal to electrophilic oxygen (E _b(O 1s) = 530.4 eV) which is known to be active in ethylene epoxidation. The complete absence of XAS features in the photon energy range typical for ^* and ^* transitions of molecular oxygen (530–535 eV) indicates both the atomic origin of the electrophilic oxygen and the absence of molecular species on the catalyst surface under the present reaction conditions.     

Active centres of magnesium oxide surface and calculations of dissociative chemisorption of methane on modified MgO

In the framework of supermolecular approach using MINDO/3 method the various channels of dissociative chemisorption of methane molecule on pure and lithium doped magnesium oxide surfaces are considered. On the basis of the obtained calculation results the possible ways of methane activation on MgO and Li/MgO are discussed.     

Structural Analysis of Amorphous Alloys by X-Ray Diffraction

The structure of amorphous alloys has been studied by using various techniques, such as x-ray, neutron, and electron diffraction experiments. X-ray diffraction is the most conventional of all the techniques and is based on EXAFS (extended x-ray absorption fine structure) [1-3] and AXS (anomalous x-ray scattering) [4-5] experiments, which are used for the investigation of the local environment around a species of atom. Recently, x-ray diffraction has been applied to the structural analysis of some quasicrystals [5,7]. It is, of course, hard to determine the local structure of a noncrystalline material only from ordinary x-ray diffraction experiments. However, the role of x-ray diffraction in structural analysis is still important from the viewpoint that this supplies a relatively convenient method, and the appropriate selection of a Sample makes it possible to extract the dominant atomic correlation in the material. In the present study, a conventional θ-2θ diffractometer was used with a monochrometor in the diffracted beam. First, the theoretical background is discussed, the data analysis procedure is given in detail, and error estimation in the data analysis is presented last.     

Comparison of Sizes of Magnesia Crystals Calculated from X-ray Diffraction Line-Broadening and Transmission Electron Microscopy Observations

When MgO is formed by decomposition of MgCO₃ under vacuum, crystal sizes calculated from XRD line breadths are in reasonable agreement with direct TEM observations, 4.4 ± 0.6 nm compared with 3.2 ± 0.8 nm. For aligned MgO crystallites from Mg(OH)₂, the sizes calculated are discordant, 14.0 ± 3.0 nm and 2.6 ± 0.7 nm. The sizes calculated from XRD data are too large because the MgO crystallites form from Mg(OH)₂ in near-perfect alignment.     

铝锌系列变形铝合金的X射线荧光光谱分析

介绍了采用射线荧光法测定铝锌系列铝合金中各元素含量的方法及相关条件,以俄罗斯BилC公司研制的Al-Zn-Mg-Cu系列铝合金标准物质作为光谱标准,基体效应校正采用基本参数法,建立了X射线光谱测定铝锌系列变形铝合 金中Cu、Mn、Mg、Fe、Si、Zn、Ni、Cr、Ti和Zr1O种元素的方法.该方法分析实际样品,结果...     

X-Ray Spectroscopic Method for Chemical Analysis of Periclase Powders

An x-ray fluorescence spectroscopic analysis in the incoming inspection of raw materials for the production of periclase powders is developed. Optimum conditions for sample preparation are described and a metrological characterization of certified mixtures is given. The advantages of the newly-developed method, based on the use of a Spectroscan-V x-ray spectrometer, are emphasized.     

水泥熟料的X射线定量物相分析研究

对比Bogue法和Rietveld全谱拟合法定量水泥熟料矿相的优缺点,比较两种方法定量来自不同水泥企业的工业水泥熟料结果.结果表明,Rietveld法定量熟料中C3S含量高于Bogue法计算的含量,而C2S和C4AF矿相的含量则偏低.     

Surface Diffusion in MgO and Cr-Doped MgO

The surface diffusion of MgO and Cr-doped MgO was examined using grain-boundary grooving. Measurements were taken at T=1100° to 1400°C for a concentration range of 0 to 0.442 at.% Cr. The activation energy for surface diffusion of MgO was found to be 239± kj/mol. The surface diffusivity did not change markedly with chromium concentration. How-ever, an abrupt change in activation energy from ∼240 to ∼180 kj/mol occurred between 0.038 and 0.254 at % Cr, which may be associated with a change in surface structure.     

MgO低碳水泥的碳化性能分析和数学模型建立

针对目前水泥行业的高碳排放,国外提出MgO低碳水泥,它是由硅酸盐水泥、粉煤灰和活性MgO组成。本文经碳化实验表明,该水泥能够有效吸收CO2,对于低碳减排具有很好的实用意义。且可利用粉煤灰,进一步提升了能源的有效利用。同时基于Fick第一扩散定律对该水泥的碳化能力进行数学建模,具有很强的理论指导意义。     

X-Ray Stress Analysis of WC-Co Cermets: I,Procedures

The factors that influence the measurement of stress by the “two-exposure” X-ray diffractometer technique in the WC phase of WC-Co cermets were studied. Chromium Karadiation was used on the (1012) line of WC (2θ= 135.8°). The X-ray values of Young's modulus and Poisson's ratio are 105×10⁶ psi and 0.19, respectively, and the X-ray stress factor is 1.51 × 10⁸ psi/Å. The measured stresses are biaxial. Because penetration is limited to less than 5 μm for CrKa radiation, extreme care in surface preparation and heat treatment is required. A suitable preparation is metallographic polishing with 6 μm diamond followed by annealing in hydrogen at 750°C. This treatment allows measurement of about 40, 000 psi compressive temperature stress inherent in the WC particles resulting from differences in coefficients of thermal expansion between WC and Co. Mechanical stresses can also be measured; they arise from external forces such as grinding, polishing, and sand blasting. Mechanical and temperature stresses cannot always be clearly distinguished, however.     

X-Ray Diffraction Analysis of Phase Formation in Synthesis of Actinide Matrices

The products of synthesis of crystalline matrices promising for immobilization of actinides were studied by the method of x-ray phase analysis. The experiments were performed on compositions corresponding to the phase stoichiometry of structural types of zirconolite (CaZrTi₂O₇) and pyrochlore (CaCeTi₂O₇, Gd₂Ti₂O₇, Gd₂Zr₂O₇). The experiments were carried out within a temperature range of 800 – 1600°C for a sintering time of 5 – 50 h, in air and in an oxygen temperature. The phase formation conditions in matrices of different compositions are identified. Practical recommendations are issued.