Densification of nanocrystalline MgO ceramics by hot-pressing

Densification of pure nanocrystalline MgO powder with 10 nm particle size by hot-pressing was investigated in the temperature range 700–800 °C, applied pressure range 100–200 MPa, and for durations of up to 240 min. It was shown that significant densification under the pressure begins above 440 °C. Densities higher than 99.5% with grain size of 73 nm were achieved at 790 °C and 150 MPa for a 30 min duration. Remarkable densification from 90 to 99.5% was observed by temperature change from 700 to 790 °C, for which the grain size was doubled only. The final grain size decreased with increasing the applied pressure. Higher shrinkage rates and cumulative shrinkages were recorded by the application of pressure at 550 °C rather than from room temperature. The temperature at which the pressure was applied is crucial in determining the maximum shrinkage rate in the nanocrystalline compacts. This effect was related to the morphological changes of the particles caused by plastic deformation at lower temperatures. Analysis of the densification rate and its comparison to the literature data was in agreement with Coble creep, where self-diffusion of Mg²⁺ cations along the grain boundaries acts as a main densification mechanism.     

聚氯乙烯多乙烯胺树脂催化合1-碘戊烷及其催化原理

本文报道了聚氯乙烯多乙烯胺(PVC—PP)树脂催化合成l—碘戊烷并简述了PVC—PP树脂的三相催化机理。     

Thin water films and particle morphology evolution in nanocrystalline MgO

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X‐ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing‐induced morphology changes on powders of MgO nanocubes in different gaseous H₂O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H₂O) = 10^?5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two‐dimensional solvent for Mg²⁺ ion hydration. In addition, we obtained direct evidence for hydroxylation‐induced stabilization of (110) faces and step edges of the grain surfaces.     

固相法合成纳米氧化镁

以草酸和醋酸镁为原料 ,用室温固相化学反应合成出前驱配合物 ,前驱配合物在 60 0℃热分解 3h ,得到产物纳米MgO。用X射线粉末衍射、透射电镜对得到的产物进行表征。结果表明 ,固相法得到的纳米氧化镁为立方晶系结构 ,产物平均粒径为 15nm。作者以硝酸镁、碳酸钠为原料 ,用直接沉淀法合成得到纳米MgO ,平均粒径为 3 0nm。比较两种方法所得纳米氧化镁 ,固相法得到产物平均粒径比液相法小     

Reaction thermodynamics for ZrO2, MgO,and CaO with refractory metals

Conclusions We made a thermodynamic analysis of the reactions between ZrO₂, MgO, CaO with Mo, W, Nb,Ta, and Zr at 2000–3000°K on the basis of which the basic reactions of this interaction were determined. We determined the relative intensity of the reactions of ZrO₂, MgO and CaO with refractory metals Mo, W, Nb, Ta, Zr and also the temperatures of initial reaction in regard to the pressure of the surroundings.Translated from Ogneupory, No.7, pp.46–52, July, 1967.     

Synthesis of high surface area nanocrystalline MgO by pluronic P123 triblock copolymer surfactant

Nanocrystalline magnesium oxide with high surface area was prepared by a simple precipitation method using pluronic P123 triblock copolymer (Poly(ethylene glycol)-block, Poly(propylene glycol)-block, Poly(ethylene glycol)) as surfactant. The prepared samples were characterized by X-ray Diffraction (XRD), N₂ adsorption (BET), Fourier transform infrared spectroscopy (FTIR), Thermal, differential thermal gravimetric and differential thermal analyses (TG/DTG/DTA) and Scanning electron microscopy (SEM). The obtained results revealed that the addition of surfactant is effective to prepare magnesium oxide with high surface area and affects the morphology of the prepared samples. The results showed that the magnesium oxide calcined at different temperatures ranging from 600 to 800 °C possessed a high surface area in the range of 133.9–78.1 m² g^?1. In addition, the magnesium oxide prepared with the addition of surfactant showed a narrower pore size distribution compared to the sample prepared without the addition of a surfactant.     

金属／MgO上的乙醇水蒸气重整制氢

研究了氧化镁负载镍、铁、钴、锰、钼、铜和锡等金属催化剂在乙醇水蒸气重整反应的性能,结果表明在650℃,101．3kpa条件下,所有催化剂的活性都较高,乙醇接近完全转化,而对氢的选择性顺序为：Ni〉Co〉Sn〉Cu〉Fe〉Mo〉Mn。除镍的选择性是随温度上升之外,其他催化剂的选择性都随温度变化有个最佳值。镍催化剂的TPR和XRD表征表明,催化剂中存在3种形态的镍。     

Hot-Pressing of MgO with NaF

The results of using NaF additives in hot-pressing MgO are described and compared with the results when LiF is used. To obtain nearly theoretical density, the temperatures required for hot-pressing with NaF are ∼100°C higher than those required with LiF. Specimens made with NaF were generally translucent after hot-pressing or subsequent annealing. Sodium fluoride inhibits grain growth and can produce a duplex grain structure, whereas LiF enhances grain growth during annealing. Both NaF and LiF wet MgO and lubricate it well. The present study and other investigations of fluoride densification aids indicate that the formation of a liquid phase, with resultant wetting and lubrication, is important, especially in the early stages of densification. Possible solution of anions and cations is discussed.     

Reaction mechanism between MgO crucible and AerMet100 steel during vacuum induction melting

AerMet100 steel has strict composition and inclusion requirements. Therefore, its reaction with MgO refractory during vacuum induction melting cannot be ignored. In this study, the reaction mechanism between the MgO refractory and AerMet100 steel during the refining stage was investigated using a MgO crucible. The influence of the MgO crucible on AerMet100 steel composition and inclusions under refining vacuum pressures of 50–100 and 5–10 Pa was compared. The results indicate that SiO₂, Al₂O₃, and MgO in the crucible decompose and are reduced by C in the liquid steel, which results in the increase of Si, dissolved Al (Als), and dissolved Mg (Mgs) content in the liquid steel. The increase of Ca content is due to the reduction of CaO in the crucible by C in the liquid steel. The reaction of Al₂O₃ inclusions and Mgs in the liquid steel is the primary generation method of MgO·Al₂O₃ spinel inclusions. As the Mgs content in the liquid steel increases, Al₂O₃ inclusions transform into MgO·Al₂O₃ spinel inclusions along the path Al₂O₃ + Mgs → Al₂O₃ with a small amount of MgO + Mgs → MgO·Al₂O₃ spinel. In contrast, the vacuum pressure of 50–100 Pa is more effective at controlling the composition and inclusions of AerMet100 steel and is a more appropriate choice for the refining vacuum pressure.     

电沉积纳米晶镍-铁-铬合金

采用直流和脉冲电沉积方式从三价铬的氯化物镀液中沉积出镍-铁-铬纳米合金镀层，利用扫描电镜分析镀层形貌及晶粒尺寸，研究了沉积速率，电流效率，三价铬浓度及pH值随沉积时间的变化关系，结果表明，脉冲电沉积所得镀层的结构和性能均优于直流电沉积，这是由于脉冲电沉积存在断电时间，使得电极表面扩散层中金属离子的浓度得到及时恢复。     

Gas transport properties of MgO filled poly(1-trimethylsilyl-1-propyne) nanocomposites

Magnesium oxide (MgO) nanoparticles were dispersed via solution processing in poly(1-trimethylsilyl-1-propyne) (PTMSP) to form polymer nanocomposites. Transmission electron microscopy was used to determine the extent of particle aggregation in the composites. Both nanocomposite density and CO₂, CH₄, N₂, and H₂ permeability were influenced by nanoparticle loading. Nanocomposite densities were markedly lower than predicted by a two phase additive model. For example, in films containing 75 nominal volume percent MgO, the polymer-particle composite density was 68 percent lower than expected based on an additive model. At this loading, gas permeability coefficients were, depending on the gas, 17-50 times higher than in unfilled PTMSP at similar conditions. The changes in permeability with particle content were interpreted in terms of measured changes in gas solubility with particle content and diffusion coefficients calculated from the permeability and solubility data.     

高氧化镁石灰石配料方案的调整

平凉祁连山水泥有限公司2 500t／d新型干法熟料水泥生产线于2005年3月点火投产,投产之初设定熟料的配料方案是KH:0．900;SM:2．70;SM:1．50,但从半年多的运行状况来看,由于石灰石中MgO含量高,存在生料、熟料中MgO含量高,水泥窑烧成带煅烧温度低,物料煅烧范围窄,熟料产量低,熟料强度低的状况。     

Fast coating of ultramicroelectrodes with boron-doped nanocrystalline diamond

The present work is focused on the deposition of thin boron-doped nanocrystalline diamond (B-NCD) films on electrochemically etched tungsten wires by the hot filament assisted CVD method. The goal is the manufacturing of robust inert ultramicroelectrodes (UMEs) with a superior performance to be used for localised electrochemical analysis. The conductive diamond films can confer high stability of chemical and physical properties as well as low background current.Filament and substrate temperatures were kept constant at 2350 °C and 670 °C, respectively. The total system pressure was equal to 50 mbar and the CH₄/H₂ gas flow ratio was 0.07. Boron was used as the doping agent by solving B₂O₃ in ethanol, with a B/C content of 15000 ppm, and the solution was then dragged with argon gas flowing through a bubbler. The (Ar+B)/H₂ ratio values varied within the range of 0.06–0.21. The film growth rate decreases with the boron content increasing, but larger (Ar+B)/H₂ ratios result in smoother surfaces. UMEs insulation was carried out with epoxy resin in a home built device.The production of very sharp tungsten tips fully coated with B-NCD after just 30 min, for a (Ar+B)/H₂ ratio of 0.21, is one of the main outcomes of this work. The cyclic voltammetry showed a stable behavior with a wide electrochemical window of ~ 2.25 V in a 0.05 M NaCl solution proving the applicability of the developed UME for localized electroanalytical studies in biomedical and corrosion applications.     

Preparation of gelatin-based films modified with nanocrystalline cellulose

Gelatin is a natural biological macromolecule derived from the collagen in the connective tissue of the skin, bone and other tissues. It has been widely used in medicine, food and industrial production and other fields for easy molding, excellent compatibility and biodegradability. However, physical and chemical disadvantages impede its further application, seriously. Therefore, modification of the gelatin films becomes more and more important. In this study, the gelatin/nanocrystalline cellulose (NCC) composite films were prepared by casting method with 4% glycerol as plasticizer. The effect of NCC on the properties of the composite films was investigated by the characterization of its morphology and mechanical, thermal, and optical properties and water adsorption. The results showed that mechanical, thermal stability and water absorption properties of the gelatin/NCC composite film were obviously improved. The composite films showed the highest tensile strength (13.56?±?0.25 MPa) when the mass concentration of NCC was 0.6%. Adding NCC to gelatin benefited the thermal stability of composite films. The gelatin/NCC composite film of 0.4% NCC had the highest melting transition temperature (138.9 °C). The composite films exhibited the lower water absorption (271.1%) when mass concentration of NCC was 1.0%. Thus, these results indicated that NCC could affect the properties of gelatin-based composite films, and showed it has potential for application in food packing.     

Reaction of the benzoyloxy radical with α-methylstyrene

Benzoyl peroxide labelled with carbon-14 and tritium has been used for study of the end-groups in copolymers of α-methylstyrene (MST) with methyl methacrylate, in particular determination of the relative numbers of benzoate and phenyl end-groups derived from the initiator. MST is found to be quite reactive towards the benzoyloxy radical. It is deduced that combination is the dominant mode of termination in the copolymerization, that transfer reactions are not important and that there is little abstraction of hydrogen from MST by the benzoyloxy radical.