Jin Zengsun

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.

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.

Field emission from amorphous carbon nitride films deposited on silicon tip arrays

Amorphous carbon nitride films (a-CNx) were deposited on silicon tip arrays by rf magnetron sputtering in pure nitrogen atmosphere. The field emission property of carbon nitride films on Si tips was compared with that of carbon nitride on silicon wafer. The results show that field emission property of carbon nitride films deposited on silicon tips can be improved significantly in contrast with that on wafer. It can be explained that field emission is sensitive to the local curvature and geometry, thus silicon tips can effectively promote field emission property of a-CNx films. In addition, the films deposited on silicon tips have a smaller effective work function (F = 0.024 eV) of electron field emission than that on silicon wafer (F = 0.060 eV), which indicates a significant enhancement of the ability of electron field emission from a-CNx films.

Thermal Conductivity of Diamond-Based Silicon-on-Insulator Structures

Diamond films of various thickness (1-300 μm) were deposited on single-crystal Si active (300 μm) by a microwave plasma chemical vapor deposition method using gaseous mixtures of methane and hydrogen. After thinning of the Si layer by machine and ion-beam polishing a diamond-based silicon-on-insulator structure with final Si layer thickness of about 1 μm is formed. Thermal conductivity of this structure material with various thicknesses of diamond and Si layer was measured. Compared with bulk silicon, the thermal conductivity of the silicon-on-diamond structure with 300 μm diamond and 1 μm silicon increases by 850%.

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.