Lü Xian-Yi

The effects of current density on the phase composition and microstructure properties of micro-arc oxidation coating

Ceramic coatings have been synthesized on the Al alloy substrate by micro-arc oxidation (MAO) technique. The effects of current density on the mechanical and chemical properties of MAO coatings have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), and micro-hardness test. The samples fabricated at high current density consist mainly of α-Al2O3, while the samples fabricated at low current density are almost composed of γ-Al2O3. XRD analysis at different depths from coating surface towards substrate indicates that there is a complicated variation of α-Al2O3 content as a function of depth, which is different from the results of others. Micro-hardness test shows that these kinds of samples have high hardness. However, in the plane parallel to the substrate, there is a non-uniform distribution of hardness for the samples deposited at high current density, which can be attributed to the non-uniform distribution of pores in the same plane parallel to the substrate.

Characterization of microarc oxidation process on aluminium alloy

The specific features of electric current variation against time were studied by keeping the anode and cathode pulse voltage to be constant. The maximum thickness of the films fabricated by microarc oxidation was found to be closely related to the voltage. The surface morphology of the coatings can be divided into different stages according to the variation of electric current. A method for obtaining high micro hardness coating has been discussed.

Synthesizing of ZnO Micro/Nanostructures at Low Temperature with New Reducing Agents

Instead of the conventional graphite, new additional reducing agents (diamond, silicon, metal elements, etc.) have been mixed with zinc oxide (ZnO) powder to fabricate ZnO micro/nanostructures by a thermal vapour transport method. Due to the strong reducibility for those additions, the corresponding heating temperature is decreased by 100-500° C compared to the case of graphite, which subsequently decreases the corresponding growth temperature for the products. Being placed separately for the powder sources of ZnO and addition, a vapour-vapour reduction-oxidation reaction mechanism between the sources is proposed as an important channel to fabricate ZnO. Photoluminescence and magnetic examinations indicate that the ZnO products synthesized have strong ultraviolet (visible) emissions and are room-temperature ferromagnetic, meaning that the products are available for applications.

High-Rate Growth and Nitrogen Distribution in Homoepitaxial Chemical Vapour Deposited Single-crystal Diamond

High rate (> 50 μm/h) growth of homoepitaxial single-crystal diamond (SCD) is carried out by microwave plasma chemical vapour deposition (MPCVD) with added nitrogen in the reactant gases of methane and hydrogen, using a polycrystalline-CVD-diamond-film-made seed holder. Photoluminescence results indicate that the nitrogen concentration is spatially inhomogeneous in a large scale, either on the top surface or in the bulk of those as-grown SCDs. The presence of N-distribution is attributed to the facts: (i) a difference in N-incorporation efficiency and (ii) N-diffusion, resulting from the local growth temperatures changed during the high-rate deposition process. In addition, the formed nitrogen-vacancy centres play a crucial role in N-diffusion through the growing crystal. Based on the N-distribution observed in the as-grown crystals, we propose a simple method to distinguish natural diamonds and man-made CVD SCDs. Finally, the disappearance of void defect on the top surface of SCDs is discussed to be related to a filling-in mechanism.

Synthesis of Thick Diamond Films by Direct Current Hot-Cathode Plasma Chemical Vapour Deposition

The method of direct current hot-cathode plasma chemical vapour deposition has been established. A long-time stable glow discharge at large discharge current and high gas pressure has been achieved by using a hot cathode in the temperature range from 1100°C to 1500°C and nonsymmetrical configuration of the poles, in which the diameter of the cathode is larger than that of anode. High-quality thick diamond films, with a diameter of 40-50 mm and thickness of 0.5-4.2 mm, have been synthesized by this method. Transparent thick diamond films were grown over a range of growth rates between 5-10 µm/h. Most of the thick diamond films have thermal conductivities of 10-12 W/Kcm. The thick diamond films with high thermal conductivity can be used as a heat sink of semiconducting laser diode array and as a heat spreading and isolation substrate of multichip modules. The performance can be obviously improved.

Characterization of Microarc Oxidation Discharge Process for Depositing Ceramic Coating

The specific features of current variation during different stages of the microarc oxidation process were studied on the timescale. The cathodic current was found to be closely related to the microarc discharge. The discharge was responsible for the formation of a dense coating with unique physical characteristics. A method for obtaining high-quality coating has been discussed.

Superconductivity in Heavily Boron-Doped Diamond Films Prepared by Electron Assisted Chemical Vapour Deposition Method

Heavily boron-doped thick diamond films with higher superconducting transition temperatures have been prepared by election assisted chemical vapour deposition method. The results of scanning electron microscopy, Raman spectroscopy, x-ray diffraction, and Hall effect indicate that the Elms have nice crystalline facets, a notable decrease in the growth rate, and an increase in the tensile stress. Meanwhile, the film resistivity decreases with the increase of the carrier concentration. Our measurements show that the Elms with 4.88 x 10(20) cm(-3) and 1.61 x 10(21) cm(-3) carrier concentration have superconductivity, with onset temperatures of 9.7K (8.9K for zero resistance) and 7.8K (6.1K for zero resistance), respectively.

Study of Diamond Nucleation on Porous Silicon

Diamond films have been deposited on porous silicon with microwave plasma chemical vapour deposition method. Nucleation density and site have been observed and analyzed by using scanning electron micrography. Experimental results showed that the nucleation density changes with the porosity and the nucleation occurs mostly on the edge of the pores. Reasons of these phenomena are discussed.

Selective deposition of diamond films on silicon wafer with SiO2 masks by tungsten filament chemical vapor deposition

Abstract Polycrystalline diamond films were selectively deposited on silicon wafers by use of a SiO2 mask and tungsten filament chemical vapor deposition from a gas mixture of methane and hydrogen. The evaporation of the SiO2 mask during diamond synthesis and deposition was successfully controlled by optimizing the synthesis parameters.

Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium (TA2) substrates in a Na2SiO3–NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope (SEM) and x-ray diffraction (XRD) are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile (TiO2). With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.