Modeling of Inner Surface Modification of a Cylindrical Tube by Plasma-Based Low-Energy Ion Implantation

The inner surface modification process by plasma-based low-energy ion implantation（PBLEII）with an electron cyclotron resonance（ECR）microwave plasma source located at the central axis of a cylindrical tube is modeled to optimize the low-energy ion implantation parameters for industrial applications.In this paper,a magnetized plasma diffusion fluid model has been established to describe the plasma nonuniformity caused by plasma diffusion under an axial magnetic field during the pulse-off time of low pulsed negative bias.Using this plasma density distribution as the initial condition,a sheath collisional fluid model is built up to describe the sheath evolution and ion implantation during the pulse-on time.The plasma nonuniformity at the end of the pulse-off time is more apparent along the radial direction compared with that in the axial direction due to the geometry of the linear plasma source in the center and the difference between perpendicular and parallel plasma diffusion coefficients with respect to the magnetic field.The normalized nitrogen plasma densities on the inner and outer surfaces of the tube are observed to be about 0.39 and 0.24,respectively,of which the value is 1 at the central plasma source.After a 5μs pulse-on time,in the area less than 2 cm from the end of the tube,the nitrogen ion implantation energy decreases from 1.5 keV to 1.3 keV and the ion implantation angle increases from several degrees to more than 40°;both variations reduce the nitrogen ion implantation depth.However,the nitrogen ion implantation dose peaks of about 2×10^（10）-7×10^（10）ions/cm^2 in this area are 2-4 times higher than that of 1.18×10^（10）ions/cm^2 and 1.63×10^（10）ions/cm^2 on the inner and outer surfaces of the tube.The sufficient ion implantation dose ensures an acceptable modification effect near the end of the tube under the low energy and large angle conditions for nitrogen ion implantation,because the modification effect is mainly determined by the ion implantation dose,just as the mass transfer process in PBLEII is dominated by low-energy ion implantation and thermal diffusion.Therefore,a comparatively uniform surface modification by the low-energy nitrogen ion implantation is achieved along the cylindrical tube on both the inner and outer surfaces.

涂层和未涂层铝基体上接触应力的有限元模拟

基于对涂层/基体系统上产生的接触应力的分析，研究了软基体上薄膜的磨损行为。应用有限元法对由压头向涂层或未涂层铝基体施加正压力的情况进行模拟。先在没有薄膜的系统上对模型的精确性进行检验。然后通过改变薄膜的弹性模量比，开发出适用于铝基体上有两层硬薄膜试样的模型。同时还研究了薄膜厚度和载荷的影响。较厚的薄膜降低了模型上某些点的应力，但是增加了另一些点的应力。从弹性模量的角度来说，薄膜内的计算应力主要与薄膜的弹性模量有关，而通过改变薄膜的弹性模量比几乎不会使应力产生差异。

Ultrasonic characterization of modified Cr_2O_3 coatings by reflection coefficient spectroscopy

Pores,microcracks and density of plasma sprayed Cr2O3 coatings before and after high-intensity pulsed ion beam(HIPIB) irradiation were investigated using the ultrasonic reflection coefficient spectroscopy(URCS).The URCS was analyzed based on an acoustic transmission model for the multi-layered structure.The longitudinal velocity in the coatings was calculated from the experimental URCS,and the attenuation coefficient expression was deduced by comparing the experimental and numerical fitting amplitude spectral lines.The longitudinal velocity of as-sprayed Cr2O3 coating is 2 002 m/s,and increases to 2 099 and 2 148 m/s after being irradiated by HIPIB with 1 and 5 shots.Correspondingly,the factor A changes from 0.046 to 0.026 and 0.020 and n from 1.702 to 1.658 and 1.649 in the attenuation coefficient expression of α=Af n.It is observed that the surface morphology of Cr2O3 coatings changes from rough and porous to smooth and uniform with the increase of shot number,which accords with the ultrasonic analyses reasonably.The URCS seems to provide a convenient and nondestructive method to characterize surface modification of the plasma sprayed coatings.

Thickness measurement approach for plasma sprayed coatings using ultrasonic testing technique

The special ultrasonic testing system has been developed for thickness measurement of plasma sprayed coatings. The ultrasonic immersion method was used to obtain stable coupling condition and avoid other disadvantages of contact method. Spherical acoustic lens were designed to focus ultrasonic beam so as to improve beam directivity and concentrate ultrasonic energy. To increase testing precision and avoid mussy wave signals, moderate pulse width and frequency of the transducer has been selected. The displacement of transducer in X-Y-Z directions was precisely manipulated by step-controlled system to insure the accuracy of focus length and repetition of measurement. Optimized testing conditions (with the transducer of center frequency of 10 MHz and crystal diameter of 8 mm, focus length of 9.5 mm, diameter of focal column of 0. 1 mm and length of focal column of 0.27 mm) were selected to determine the thickness between 285 -414 μm of ZrO2 coatings plasma sprayed on the nickel based superalloy. The frequency interval of the periodic extremums in ultrasonic power spectra decreases with increasing coating thickness. The ultrasonic results accord with those of metallographical method.

Characteristics and properties of metal aluminum thin films prepared by electron cyclotron resonance plasma-assisted atomic layer deposition technology

Metal aluminum （A1） thin films are prepared by 2450 MHz electron cyclotron resonance plasma-assisted atomic layer deposition on glass and p-Si substrates using trimethylaluminum as the precursor and hydrogen as the reductive gas. We focus our attention on the plasma source for the thin-film preparation and annealing of the as-deposited films relative to the surface square resistivity. The square resistivity of as-deposited A1 films is greatly reduced after annealing and almost reaches the value of bulk metal. Through chemical and structural analysis, we conclude that the square resistivity is determined by neither the contaminant concentration nor the surface morphology, but by both the crystallinity and crystal size in this process.

Comprehensive analysis of pulsed plasma nitriding preconditions on the fatigue behavior of AISI 304 austenitic stainless steel

This study aims to draw an exact boundary for microstructural and mechanical behaviors in terms of pulsed plasma nitriding conditions.The pulsed plasma nitriding treatment was applied to AISI 304 austenitic stainless steel at different temperatures and durations.Results reveal that nitriding depth increased as process temperature and duration increase.The nitriding depth remarkably increased at 475℃for 8 h and at 550℃for 4 h.An austenite structure was transformed into a metastable nitrogen-oversaturated body-centered tetragonal expanded austenite(S-phase)during low-temperature plasma nitriding.The S-phase was converted to CrN precipitation at 475℃for 8 h and at 550℃for 4 h.Surface hardness and fatigue limit increased through plasma nitriding regardless of process conditions.The best surface hardness and fatigue limit were obtained at 550℃for 4 h because of the occurrence of CrN precipitation.

Lattice structures and electronic properties of CIGS/CdS interface:First-principles calculations

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective （2VCu＋ Incu） CulnGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of （2VCu＋ InCu） interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the （2VCu＋InCu） interface the interface states near the Fermi level in CulnGaSe2 and CdS band gap regions are mainly composed of interracial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CulnGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.

First-principles calculations of 5d atoms doped hexagonal-AlN sheets: Geometry, magnetic property and the influence of symmetry and symmetry-breaking on the electronic structure

The geometry, electronic structure and magnetic property of the hexagonal AlN(h-AlN) sheet doped by 5d atoms(Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au and Hg) are investigated by first-principles calculations based on the density functional theory. The influence of symmetry and symmetry-breaking is also studied. There are two types of local symmetries of the doped systems: C3v and D3h. The symmetry will deviate from exact C3v and D3h for some particular dopants after optimization. The total magnetic moments of the doped systems are 0μBfor Lu, Ta and Ir; 1μB for Hf, W, Pt and Hg; 2μB for Re and Au; and 3μB for Os and Al-vacancy. The total densities of state are presented, where impurity energy levels exist. The impurity energy levels and total magnetic moments can be explained by the splitting of 5d orbitals or molecular orbitals under different symmetries.

Numerical study on the thermo-stress of ZrO_2 thermal barrier coatings by high-intensity pulsed ion beam irradiation

This paper studies numerically the thermo-mechanical effects of ZrO2 thermal barrier coatings （TBCs） irradiated by a high-intensity pulsed ion beam in consideration of the surface structure. Taking the deposited energy of ion beams in TBCs as the source term in the thermal conduction equation, the distribution of temperature in TBCs was simulated. Then, based on the distribution, the evolution of thermal stress was calculated by the finite element method. The results show that tensile radial stress formed at the valley of TBC surfaces after irradiation by HIPIB. Therefore, if cracks happen, they must be at valleys instead of peaks. As for the stress waves, no matter whether through peak or valley position, tensile and compressive stresses are present alternately inside TBCs along the depth direction, and the strength of stress decreases with time.

Research on ZrO2 Thermal Barrier Coatings Modified by High-Intensity Pulsed Ion Beam

We report a modification method for ZrO2 thermal barrier coatings （TBCs） by high-intensity pulsed ion beam （HIPIB） irradiation. Based on the temporal and spatial distribution models of the ion beam density detected by Faraday cup in the chamber and the ions accelerating voltage, the energy deposition of the beam ions in ZrO2 is calculated by Monte Carlo method. Taking this time-dependent nonlinear deposited energy as the source term of two-dimensional thermal conduction equation, we obtain the temporal and spatial ablation process of ZrO2 thermal barrier coatings during a pulse time. The top-layer TBC material in thickness of about 0.2 μm is ablated by vaporization and the coating in thickness of 1 μm is melted after one shot at the ion current density of 200 A/cm^2. This calculation is in reasonable agreement with those measured by HIPIB irradiation experiments. The melted top coat becoming a dense modification layer due to HIPIB irradiation seals the gaps among ZrO2 crystal dusters, and hence barrels the direct tunnel of oxygen.

Electron Cyclotron Resonance Plasma-Assisted Atomic Layer Deposition of Amorphous Al_2O_3 Thin Films

Without extra heating, Al2O3 thin films were deposited on a hydrogen-terminated Si substrate etched in hydrofluoric acid by using a self-built electron cyclotron resonance （ECR） plasma-assisted atomic layer deposition （ALD） device with Al（CH3）3 （trimethylaluminum; TMA） and O2 used as precursor and oxidant, respectively. During the deposition process, Ar was in- troduced as a carrier and purging gas. The chemical composition and microstructure of the as-deposited Al2O3 films were characterized by using X-ray diffraction （XRD）, an X-ray photo- electric spectroscope （XPS）, a scanning electron microscope （SEM）, an atomic force microscope （AFM） and a high-resolution transmission electron microscope （HRTEM）. It achieved a growth rate of 0.24 nm/cycle, which is much higher than that deposited by thermal ALD. It was found that the smooth surface thin film was amorphous alumina, and an interfacial layer formed with a thickness of ca. 2 nm was observed between theAl2O3 film and substrate Si by HRTEM. We conclude that ECR plasma-assisted ALD can growAl2O3 films with an excellent quality at a high growth rate at ambient temperature.

Lattice structures and electronic properties of WZ-CuInS2/WZ-CdS interface from first-principles calculations

Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS （100）/WZ-CdS （100） interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-CdS interfaces can exist stably, when the interface bonding energies are -0.481 J/m2 （bi-layer terminated interface） and -0.677 J/m2 （monolayer terminated interface）. Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level. The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface.

Optimum Duty Cycle of Unsteady Plasma Aerodynamic Actuation for NACA0015 Airfoil Stall Separation Control

Unsteady dielectric barrier discharge（DBD） plasma aerodynamic actuation technology is employed to suppress airfoil stall separation and the technical parameters are explored with wind tunnel experiments on an NACA0015 airfoil by measuring the surface pressure distribution of the airfoil.The performance of the DBD aerodynamic actuation for airfoil stall separation suppression is evaluated under DBD voltages from 2000 V to 4000 V and the duty cycles varied in the range of 0.1 to 1.0.It is found that higher lift coefficients and lower threshold voltages are achieved under the unsteady DBD aerodynamic actuation with the duty cycles less than 0.5as compared to that of the steady plasma actuation at the same free-stream speeds and attack angles,indicating a better flow control performance.By comparing the lift coefficients and the threshold voltages,an optimum duty cycle is determined as 0.25 by which the maximum lift coefficient and the minimum threshold voltage are obtained at the same free-stream speed and attack angle.The non-uniform DBD discharge with stronger discharge in the positive half cycle due to electrons deposition on the dielectric slabs and the suppression of opposite momentum transfer due to the intermittent discharge with cutoff of the negative half cycle are responsible for the observed optimum duty cycle.

Numerical Study on the Two-Dimensional Temperature Fields of Titanium/Aluminum Double-Layer Target Irradiated by High-Intensity Pulsed Ion Beam

Interaction between high-intensity pulsed ion beam （HIPIB） and a double-layer target with titanium film on top of aluminum substrate was simulated. The two-dimensional nonlinear thermal conduction equations, with the deposited energy in the target taken as source term, were derived and solved by finite differential method. As a result, the two-dimensional spatial and temporal evolution profiles of temperature were obtained for a titanium/aluminum double-layer target irradiated by a pulse of HIPIB. The effects of ion beam current density on the phase state of the target materials near the film and substrate interface were analyzed. Both titanium and aluminum were melted near the interface after a shot when the ion beam current density fell in the range of 100 A/cm2 to 200 A/cm2.

Phase equilibrium of Cd_(1-x)Zn_xS alloys studied by first-principles calculations and Monte Carlo simulations

The first-principles calculations based on density functional theory combined with cluster expansion techniques and Monte Carlo （MC） simulations were used to study the phase diagrams of both wurtzite （WZ） and zinc-blende （ZB） Cdl_xZnxS alloys. All formation energies are positive for WZ and ZB Cdl-xZnxS alloys, which means that the Cdl-xZnxS alloys are unstable and have a tendency to phase separation. For WZ and ZB Cdl_xZnxS alloys, the consolute temperatures are 655 K and 604 K, respectively, and they both have an asymmetric miscibility gap. We obtained the spatial distributions of Cd and Zn atoms in WZ and ZB Cd0.sZn0.sS alloys at different temperatures by MC simulations. We found that both WZ and ZB phases of Cdo.sZn0.sS alloy exhibit phase segregation of Cd and Zn atoms at low temperature, which is consistent with the phase diagrams.

A GaN p-i-p-i-n Ultraviolet Avalanche Photodiode

A separated absorption and multiplication GaN p–i–p–i–n avalanche photo-diode(APD)with a 25μm diameter mesa is proposed and demonstrated.Compared to the conventional p–i–n APD,the p–i–p–i–n structure reduces the probability of premature micro-plasma breakdown,raises the gain from 30 to 400 and reduces the work voltage from 93 to 48 V.The temperature test is set on p–i–p–i–n APDs,and the positive coefficient of 30 mV/K shows that avalanche breakdown happens in the devices.The peak responsivity of p–i–p–i–n APDs is 0.11 A/W under a wavelength of 358 nm.

Deposition of Titanium Layer on Steel Substrate Using PECVD Method: A Parametric Study

Metallic titanium film was deposited on H-13 steel substrate at 470°C - 530°C using plasma enhanced chemical vapor deposition (PECVD) method. In this paper, the effects of manufacturing parameters were investigated on deposited titanium coating characteristics. XRD, FESEM, XPS and AFM were used in order to study coating characteristics. Increasing hydrogen flow rate from 200 to 360 sccm, resulted in a 72% decrease in oxygen content and 38% decrease in chlorine content of the film. Applied plasma voltage has a severe effect on nanohardness of coating. Pressure of the deposition chamber has a negative effect on titanium characteristics.

Preface for the Special Column of Methane Transformation

Methane is the main constituent of natural gas, coal-bed gas, landfill gas and methane hydrate resources. These resources may be used more efficiently as clean fuels or as chemical feedstocks if methane can be effectively transformed into liquid fuels or chemicals. However, methane only possesses C-H bonds and is a very stable organic molecule hard to functionalize. The C-H activation, particularly the selective functionalization of C-H bonds in saturated hydrocarbons, remains a difficult challenge in chemistry. The present technology for chemical utilization of methane involves the steam reforming of methane to synthesis gas and the subsequent transformation of synthesis gas to methanol or hydrocarbon fuels via methanol synthesis or Fischer-Tropsch synthesis. However, the steam reforming of methane is a high-cost process. The development of more efficient and economical processes for methane transformation is a dream of all chemists and chemical engineers. I think that this is also one of the most important themes of the Journal of Natural Gas Chemistry.

Nanotexturing and Wettability Ageing of Polypropylene Surfaces Modified by Oxygen Capacitively Coupled Radio Frequency Plasma

Polypropylene （PP） was treated by an oxygen capacitively coupled radio frequency plasma （CCP） under a radio frequency （RF） power of 200 W for exposure time of 1, 5, and 10 rain. The ageing process of the plasma- treated PP was studied at an ageing temperature of 90 ~C during an ageing time up to 25 h. The formation of the nanotextures with different geometry and aspect ratio and the grafting of large number of oxygen- containing groups were achieved on as-treated PP surfaces under the oxygen CCP treatment for the increased exposure time. The hydrophilicity on the as-treated PP surfaces with the stable nanotextures was rapidly depressed during the ageing process at 90 ℃ due to the restructuring of chemical composition. The surface restructuring rate was dependent on the aspect ratio and the oxygen-containing groups on the nanotextured PP with increasing exposure time. The hydrophobic over-recovery to high hydrophobicity and superhydrophobicity were observed on the post-aged surfaces with the stable nanofibrils from as-treated hydrophilic surfaces. The superhydrophobicity with the low water adhesion was achieved on the post-aged surfaces preserving the nanofibrils with high aspect ratio and large distance due to the decrease of the oxygen-containing groups after the surface restructuring.

Adsorption and Diffusion Behavior of Cl^- on Sputtering Fe–20Cr Nanocrystalline Thin Film in Acid Solution(pH = 2)

The adsorption and diffusion behavior of Cl^- on sputtering Fe-20 Cr nanocrystalline（NC） thin film compared with corresponding coarse crystalline（CC） alloy has been studied in HCl ＋ NaCl solution（pH = 2,[Cl^-]=0.1 mol/L） by electrochemical techniques,X-ray photoelectron spectroscopy（XPS） and the firstprinciples calculations.The XPS results show that adsorption and diffusion of Cl^- in the passive film has been inhibited on NC thin film.Ultra-violet photoelectron spectroscopy（UPS） results show that the work function Φ s of NC thin film（4.7 eV） is higher than that of CC alloy（4.5 eV）.The theoretical calculations and valence electron structure analysis were used to understand the effect of nanocrystallization in this work.