Journal of University of Science and Technology Beijing, Mineral, Metallurgy, Material最新文献

Metal and nano-ceramic nanocomposite coatings were prepared on the gray cast iron surface by the electrodeposition method. The Ni-Co was used as the metal matrix, and nano-Al₂O₃ was chosen as the second-phase particulates. To avoid poor interface bonding and stress distribution, the gradient structure of biology materials was found as the model and therefore the gradient composite coating was prepared. The morphology of the composite coatings was flatter and the microstructure was denser than that of pure Ni-Co coatings. The composite coatings were prepared by different current densities, and the 2-D and 3-D morphologies of the surface coatings were observed. The result indicated that the 2-D structure became rougher and the 3-D surface density of apices became less when the current density was increased. The content of nanoparticulates reached a maximum value at the current density of 40 mA·cm⁻², at the same time the properties including microhardness and wear-resistance were analyzed. The microhardness reached a maximum value and the wear volume was also less at the current density of 40 mA·cm⁻². The reason was that nano-Al₂O₃ particles caused dispersive strengthening and grain refining.

Submicron copper microcrystal was synthesized by reducing Cu₂O with hydrazine hydrate as reducer in aqueous solution, and was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The shapes of copper microcrystal depend on additives. Cubic copper particles were observed when the inorganic salt containing Cl⁻, such as NH₄Cl, NaCl, or KCl, was added into the reaction system. By combined use of NH₄Cl and polyvinylpyrrolidone (PVP), the proportion of cubic copper particle number exceeded 90%, and the particle size is 0.1∼0.5 μm. While other inorganic salt without Cl⁻, such as Na₂SO₄ or (NH₄)₂SO₄, had little effect on the shapes of the copper particles. The growth mechanism of metallic copper crystal in aqueous solution was analyzed. It is suggested that the formation of cubic copper crystals is ascribed to the selective adsorption of Cl⁻ on copper crystal (100) faces.

To investigate the influence of stress alone on the stray field signals of ferromagnetic materials, the static tensile tests of 0.45%C steel and 45CrNiMoVA steel flat-shaped specimens were performed on an MTS810 hydraulic testing machine. H_p(y) signals, the normal component of spontaneous stray field, were measured during the testing process by an EMS-2003 metal magnetic memory diagnostic apparatus cooperated with a non-magnetic electric control displacement instrument. Fracture and microstructure were observed by a scanning electronic microscope and a transmission electron microscope, respectively. The relationships between axial applied stress or residual stress measured by X-ray diffraction method and H_p(y) were analyzed. The results indicate that some regular pattern of the magnetic curve is displayed only between applied stress and stray field signals. The magnetic ordering process stops due to dislocation pinning magnetic domain structure, and H_p(y) value unvaried nearly with applied load increasing in the plastic deformation stage.

The fatigue crack propagation behavior of the LY12CZ aluminum alloy fastener involving a central hole in air or in 3.5wt% NaCl solution was investigated. The experimental results indicated that the corrosion fatigue crack growth rate decreased with the increasing loading frequency, and in a corrosive environment, the crack growth rate was slightly larger than that in air. Based on the experimental results, the virtual corrosion fatigue crack propagation tests were investigated and the stochastic process method and the AFGROW simulation method were presented. The normal process and lognormal process were considered for the stochastic process method based on the numerically fitted Paris equation. The distribution of crack size and the corresponding probabilistic model of crack length distribution for a given number of cycles can be found by integrating the stochastic process over time. Using the AFGROW software, the virtual simulation was carried out to analyze the corrosion fatigue crack growth behavior and the predicted crack growth curve was in good agreement with the experimental results.

A new coating on lining in industrial furnace for energy saving has been developed. Properties and microstructure of the coatings were revealed by emissivity instrument, X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. The result indicates that the emissivity of coatings is higher than 0.90 and the thickness of coatings is about 200 μm. ZrO₂, Cr₂O₃ and SiC in the coating benefit practical applications of coatings at high temperature with durable high emissivity and the continuous structure between the coatings and the substrate makes the coatings high cohesion and excellent adhesion for both specimens with and without sintering at high temperature. Result from laboratory experiment shows that the heating speed of specimen with coating is higher than that of controlled specimen and the temperature increases 30°C during the heating. The average temperature drop of specimen with coatings has a 13.5% improvement in the cooling speed. The application of coatings on the checker brick in a blast furnace of 1750 m³ indicates that the coating causes the blast temperature to an average increase of 28°C, reduces the fluctuation of blast temperature before the blowing-in and leads to a fuel saving of 10% approximately.

Micropipe and step structures on 6H-SiC (0001) surface were investigated by an atomic force microscopy (AFM). On the facet, all micropipes examined are the origins of spiral steps, indicating that dislocations intersect the surface at these points. Micro-pipes are empty-core super-dislocations as originally described by Frank. The micropipe radius increases with the square of the dis-location Burgers vector. From the center to the periphery, step structures change with different surface inclinations. Regular step is observed within the central faceted area. Step bunching and atomically rough surfaces are observed within the peripheral convex area. If the inclination with respect to the (0001) plane is large enough, step bunching of 15R-SiC can be observed.

In an effort to incorporate the low thermal conductivity of the silica aerogel and the superior structure strength of the xonotlite, a composite material of these two was produced. It was synthesized under vacuum condition and dried by supercritical drying technique. The thermal conductivity of the new material, which is at 298 K with the gas pressure ranging from 1.01×10⁵ to 1×10⁻² Pa, was measured using the transient hot-strip method. The mechanism of the low thermal conductivity was studied. The results indicate that the low thermal conductivity mainly results from the significant decrease of gaseous thermal conductivity of the new material due to the restriction of the motion of gas molecules in its fine structures. The formation of the fine structures is because the new material takes the pore structure of the silica aerogel which consists of mainly nanometer-sized pores. © 2008 University of Science and Technology Beijing. All rights reserved.

A novel approach to modelling phase-transition processes in phase change materials used for optical and electrical data storage applications is presented. The model is based on a cellular automaton (CA) approach to predict crystallization behaviour that is linked to thermal and electrical simulations to enable the study of the data writing and erasing processes. The CA approach is shown to be able to predict the evolution of the microstructure during the rapid heating and cooling cycles pertinent to data storage technology, and maps crystallization behaviour on the nanoscale. A simple example based on possible future nonvolatile phase-change random access solid-state memory is presented.