De-Zheng Yang

Preparation, structure and properties of TaN and TaC films obtained by ion beam assisted deposition

Abstract Tantalum nitride and tantalum carbide films were prepared by the method of ion beam assisted deposition (IBAD). The results of tranmission electron microscopy (TEM) and X-ray diffraction showed that the films are all fcc structures and the grain sizes in the films are very small (about 10 to 20 nm). The composition depth profile and chemical binding character of films were determined by Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS). Measurements of film properties, such as microhardness, adhesion of film to substrate, and surface resistance, were made on a microhardness tester, a scratch tester and a bridge of resistance measurement with four-pole probe, respectively. The residual stress, dislocation density and dislocation distribution in the films were calculated by a method called X-ray diffraction line profile analysis developed by Wilkens and Wang et al. According to the elastic theory of dislocations, the microhardness of films was also calculated by using the parameters of dislocations obtained.

The enhancement of solid solubility limits of AlCo intermetallic compound by high-energy ball milling

In this work the AlCo intermetallic compound has been prepared by high‐energy ball milling of Co and Al powder mixtures with the composition of Co‐50 at.% Al, Co‐71.4 at.% Al, Co‐76.5 at.% Al, and Co‐81.8 at.% Al, respectively. X‐ray diffraction and transmission electron microscopy analysis of powders milled for different times proved that the AlCo phase formed directly during milling of the powder mixture of the former three compositions. In the Co‐81.8 at.% Al powder mixture the amorphous phase was first formed and then crystallized to the AlCo phase. The result of energy dispersion x ray showed that the composition of AlCo phase formed during milling of the above four compositions is in agreement with that of the powder mixture, although their crystal structure is that of the AlCo intermetallic compound. The results indicate that the solubility range of intermetallic compound during mechanical alloying processes can be greatly enhanced by high‐energy ball milling. The enhancement of solubility is discu...

Strain effect on lattice vibration, heat capacity, and thermal conductivity of graphene

First-principle calculation based on density functional theory is performed to study the lattice vibration, heat capacity, and thermal conductivity of graphene under strain. Two degenerate optical branches in the phonon dispersion curves split near the G points due to the reduced crystal symmetry, and the frequencies of the optical phonon modes shift down thus inducing more phonon modes at a given temperature. The heat capacity is increased, but the thermal conductivity is reduced because of enhanced Umklapp scattering among more phonons. This phenomenon should be considered when determining the heat management of graphene-based devices.

A homogeneous dielectric barrier discharge plasma excited by a bipolar nanosecond pulse in nitrogen and air

In this paper, a nanosecond bipolar pulse voltage with 20?ns rise time is employed to generate a low gas temperature homogeneous dielectric barrier discharge plasma both in nitrogen and air at atmospheric pressure. Images of the discharge, waveforms of pulse voltage and discharge current, and the optical emission spectra emitted from the discharge are recorded successfully under severe electromagnetic interference. The effects of pulse peak voltage, pulse repetition rate and the gap distance between electrodes on gas temperature and the emission intensities of NO (A?2????X?2?), OH (A?2????X?2?) and N2 are discussed. It is found that the emission intensities of NO (A?2????X?2?), OH (A?2????X?2?) and N2 rise with increasing both pulse peak voltage and pulse repetition rate but decrease with gap distance between the electrodes when it is above 2.5?mm. The effect of concentrations of O2 on the emission intensities of NO (A?2????X?2?), OH (A?2????X?2?) and N2 is also investigated, and it is found that the emission intensities of both NO (A?2????X?2?) and OH (A?2????X?2?) reach maximum values when the O2 concentration is 0.3%.

A diffusive air plasma in bi-directional nanosecond pulsed dielectric barrier discharge

In this study, a diffuse dielectric barrier discharge in air is generated by a bi-directional nanosecond pulsed power supply using a needle–plate electrode configuration at atmospheric pressure. Time-resolved spectra of N2(C 3Πu → B 3Πg, 0–0, 337.1 nm) for both positive and negative pulse discharges are recorded under severe electromagnetic interference. It is found that the lagged time of the photocurrent pulse compared with the voltage pulse in the negative pulse discharge is about 50 times longer than that in the positive pulse discharge at 16 kV pulse peak voltage and becomes shorter with an increase in pulse peak voltage. In addition, the gas temperature of the air plasma is determined to be approximately 395 K by measuring the optical emission spectra of the first negative band of , 0–0, 391.4 nm).

Production of atmospheric pressure diffuse nanosecond pulsed dielectric barrier discharge using the array needles-plate electrode in air

In this paper, a bidirectional high pulse voltage with 20 ns rising time is employed to generate an atmospheric pressure diffuse dielectric barrier discharge using the array needles-plate electrode configuration. Both double needle and multiple needle electrode configurations nanosecond pulsed dielectric barrier discharges are investigated. It is found that a diffuse discharge plasma with low gas temperature can be obtained, and the plasma volume increases with the increase of the pulse peak voltage, but remains almost constant with the increase of the pulse repetition rate. In addition to showing the potential application on a topographically nonuniform surface treatment of the discharge, the multiple needle-plate electrode configuration with different needle-plate electrode gaps are also employed to generate diffuse discharge plasma.

Multiple current peaks in room-temperature atmospheric pressure homogenous dielectric barrier discharge plasma excited by high-voltage tunable nanosecond pulse in air

Room temperature homogenous dielectric barrier discharge plasma with high instantaneous energy efficiency is acquired by using nanosecond pulse voltage with 20-200 ns tunable pulse width. Increasing the voltage pulse width can lead to the generation of regular and stable multiple current peaks in each discharge sequence. When the voltage pulse width is 200 ns, more than 5 organized current peaks can be observed under 26 kV peak voltage. Investigation also shows that the organized multiple current peaks only appear in homogenous discharge mode. When the discharge is filament mode, organized multiple current peaks are replaced by chaotic filament current peaks.

Comparison of atmospheric air plasmas excited by high-voltage nanosecond pulsed discharge and sinusoidal alternating current discharge

In this paper, atmospheric pressure air discharge plasma in quartz tube is excited by 15 ns high-voltage nanosecond pulsed discharge (HVNPD) and sinusoidal alternating current discharge (SACD), respectively, and a comparison study of these two kinds of discharges is made through visual imaging, electrical characterization, optical detection of active species, and plasma gas temperature. The peak voltage of the power supplies is kept at 16 kV while the pulse repetition rate of nanosecond pulse power supply is 100 Hz, and the frequency of sinusoidal power supply is 10 kHz. Results show that the HVNPD is uniform while the SACD presents filamentary mode. For exciting the same cycles of discharge, the average energy consumption in HVNPD is about 1/13 of the SACD. However, the chemical active species generated by the HVNPD is about 2–9 times than that excited by the SACD. Meanwhile, the rotational and vibrational temperatures have been obtained via fitting the simulated spectrum of N2 (C3Πu → B3Πg, 0-2) with the measured one, and the results show that the plasma gas temperature in the HVNPD remains close to room temperature whereas the plasma gas temperature in the SACD is about 200 K higher than that in HVNPD in the initial phase and continually increases as discharge exposure time goes on.In this paper, atmospheric pressure air discharge plasma in quartz tube is excited by 15 ns high-voltage nanosecond pulsed discharge (HVNPD) and sinusoidal alternating current discharge (SACD), respectively, and a comparison study of these two kinds of discharges is made through visual imaging, electrical characterization, optical detection of active species, and plasma gas temperature. The peak voltage of the power supplies is kept at 16 kV while the pulse repetition rate of nanosecond pulse power supply is 100 Hz, and the frequency of sinusoidal power supply is 10 kHz. Results show that the HVNPD is uniform while the SACD presents filamentary mode. For exciting the same cycles of discharge, the average energy consumption in HVNPD is about 1/13 of the SACD. However, the chemical active species generated by the HVNPD is about 2–9 times than that excited by the SACD. Meanwhile, the rotational and vibrational temperatures have been obtained via fitting the simulated spectrum of N2 (C3Πu → B3Πg, 0-2) with th...

Optical study of diffuse bi-directional nanosecond pulsed dielectric barrier discharge in nitrogen

In this study, a bi-directional high voltage pulse with 20 ns rising time is employed to generate diffuse glow-like dielectric barrier discharge plasma with very low gas temperature in N2 using needle-plate electrode configuration at atmospheric pressure. Both the diffuse nanosecond pulsed dielectric barrier discharge images and the optical emission spectra of the discharge are recorded successfully under severe electromagnetic interference. The effects of pulse peak voltage, pulse repetition rate, and the concentrations of Ar and O2 on the emission intensities of NO (A2Σ→X2Π), OH (A2Σ→X2Π, 0-0), N2 (C3Πu→B3Πg, 0-0, 337.1 nm), and N2+ (B2Σu+→X2Σg+, 0-0, 391.4 nm) are investigated. The effects of the concentrations of Ar and O2 on the discharge diffuse performance are also studied. It is found that the emission intensities of NO (A2Σ→X2Π), OH (A2Σ→X2Π, 0-0), N2 (C3Πu→B3Πg, 0-0, 337.1 nm), and N2+ (B2Σu+→X2Σg+, 0-0, 391.4 nm) rise with increasing pulse peak voltage, pulse repetition rate, and the concentration of Ar, but decrease with increasing the concentration of O2. The main physicochemical processes involved are also discussed.

Investigation on properties of ceramic coatings of ZrN

Abstract The ceramic coatings of zirconium nitride were deposited on M2 high speed steel by reactive magnetron sputtering ion plating. The microhardness and the scratch critical load (Lc) of the coatings with different nitrogen partial pressures have been discussed. The adhesion and toughness of the coatings as well as the scratch failure have also been studied by means of the scratch test.