The effect of substrate holder size on the electric field and discharge plasma on diamond-film formation at high deposition rates during MPCVD

The effect of the substrate holder feature dimensions on plasma density(ne), power density(Qmw) and gas temperature(T) of a discharge marginal plasma(a plasma caused by marginal discharge) and homogeneous plasma were investigated for the microwave plasma chemical vapor deposition process. Our simulations show that decreasing the dimensions of the substrate holder in a radical direction and increasing its dimension in the direction of the axis helps to produce marginally inhomogeneous plasma. When the marginal discharge appears, the maximum plasma density and power density appear at the edge of the substrate. The gas temperature increases until a marginally inhomogeneous plasma develops. The marginally inhomogeneous plasma can be avoided using a movable substrate holder that can tune the plasma density, power density and gas temperature. It can also ensure that the power density and electron density are as high as possible with uniform distribution of plasma. Moreover, both inhomogeneous and homogeneous diamond films were prepared using a new substrate holder with a diameter of 30 mm. The observation of inhomogeneous diamond films indicates that the marginal discharge can limit the deposition rate in the central part of the diamond film. The successfully produced homogeneous diamond films show that by using a substrate holder it is possible to deposit diamond film at 7.2 μm h~(–1)at 2.5 kW microwave power.     

Electron Heating of LHCD Plasma in HT-7 Tokamak

Electron heating via lower hybrid current drive （LHCD） has been investigated in HT-7 superconducting tokamak. Experiments show that the central electron temperature Te0, the volume averaged electron temperature 〈 Te 〉 and the peaking factor of the electron temperature QTe = Teo/〈 Te 〉 increase with the lower hybrid wave （LHW） power. Simultaneously the electron heating efficiency and the electron temperature as the function of the central line-averaged electron density （ne） and the plasma current （Ip） have also been investigated. The experimental results are in a good agreement with those of the classical collision theory and the LHW power deposition theory.     

Study on Modification of Nano-Sized Anatase Titanium Dioxide by Nitrogen-Plasma

The nano-sized particles of anatase titanium oxide （TiO2） were obtained by hydrolysis of titanium ester （TNB） in basic media and dehydrated in acid media. And then the anatase titanium oxide was treated with nitrogen plasma. The effect of nitrogen plasma treating time on the activity of photo-catalytic reduction of the Cr2O7^2- for sample obtained was investigated. The samples were characterized by X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS）, Transmission Electron Microscope （TEM） and Ultraviolet （UV）. A peak of 396 eV in the N 1 s XPS spectra of sample obtained with nitrogen plasma treated TiO2 showed that nitrogen-doped titanium oxide （TiO2-zNx） has been obtained. The spectra of UV showed that the light absorption of TiO2-xNz obtained by nitrogen plasma treated TiO2 for 10min. had moved to the visible region. The picture of TEM and spectra of XRD indicated that the crystallographic forms and particle dimension had no apparent change for both the modified and the unmodified TiO2. When the TiO2 sample was treated for 7 min with nitrogen plasma, it exhibited best photo-catalytic activity.     

Monte Carlo Simulation of Electron Velocity Distribution and Gas Phase Process in Electron-Assisted Chemical Vapor Deposition

The gas phase process of diamond film deposition from CH4/H2 gas mixture by electron-assisted chemical vapor deposition is simulated by the Monte-Carlo method. The electron velocity distribution under different E/P (the ratio of the electric field to gas pressure) is obtained, and the velocity profile is asymmetric. The variation of the number density of CH3 and H with different CH4 concentrations and gas pressure is investigated, and the optimal experimental parameters are obtained: the gas pressure is in the range of 2.5 kPa ~ 15 kPa and the CH4 concentration is in the range of 0.5% ~ 1%. The energy carried by the fragment CH3 as the function of the experiment parameters is investigated to explain the diamond growth at low temperature. These results will be helpful to the selection of optimum experimental conditions for high quality diamond films deposition in EACVD and the modeling of plasma chemical vapor deposition.     

The Gridless Plasma Ion Source （GIS） for Plasma Ion Assisted Optical Coating

High-quality optical coating is a key technology for modern optics. Ion-assisted deposition technology was used to improve the vaporized coating in 1980's. The GIS (gridless ion source), which is an advanced plasma source for producing a high-quality optical coating in large area, can produce a large area uniformity＞1000 mm(diameter), a high ion current density ～ 0.5mA/cm2, 20 eV ～ 200 eV energetic plasma ions and can activate reactive gas and film atoms. Now we have developed a GIS system. The GIS and the plasma ion-assisted deposition technology are investigated to achieve a high-quality optical coating. The GIS is a high power and high current source with a power of I kW ～ 7.5 kW, a current of 10 A ～ 70 A and an ion density of 200μA/cm2 ～ 500μA/cm2. Because of the special magnetic structure, the plasma-ion extraction efficiency has been improved to obtain a maximum ion density of 500μA/cm2 in the medium power (～ 4 kW) level. The GIS applied is of a special cathode structure, so that the GIS operation can be maintained under a rather low power and the lifetime of cathode will be extended. The GIS has been installed in the LPSX-1200 type box coating system. The coated TiO2, SiO2 films such as antireflective films with the system have the same performance reported by Leybold Co, 1992, along with a controllable refractive index and film structure.     

Microwave Plasma Chemical Vapor Deposition of Diamond Films on Silicon From Ethanol and Hydrogen

Diamond films with very smooth surface and good optical quality have been deposited onto silicon substrate using microwave plasma chemical vapor deposition(MPCVD)from a gas mixture of ethanol and hydrogen at a low substrate temperature of 450℃.The effects of the substrate temperature on the diamond nucleation and the morphology of the diamond film have been investigated and observed with scanning electron microscopy(SEM).The microstructure and the phase of the film have been characterized using Raman spectroscopy and X-ray diffraction(XRD).The diamond nucleation density significantly decreases with the increasing of the substrate temperature.There are only sparse nuclei when the substrate temperature is higher than 800℃ although the ethanol concentration in hydrogen is very high.That the characteristic diamond peak in the Raman spectrum of a diamond film prepared at a low substrate temperature of 450℃ extends into broadban indicates that the film is of nanophase.No graphite peak appeared in the XRD pattern confirms that the film is mainly composed of SP^3 carbon.The diamond peak in the XRD pattern also broadens due to the nanocrystalline of the film.     

Microstructure and Mechanical Properties of CrN Films Deposited by Inductively Coupled Plasma Enhanced Radio Frequency Magnetron Sputtering

CrN films have been synthesized on Si（100） wafer by inductively coupled plasma （ICP）-enhanced radio frequency （RF） magnetron sputtering. The effects of ICP power on microstructure, crystal orientation, nanohardness and stress of the CrN films have been investigated. With the increase of ICP power, the current density at substrate increases and the films exhibit denser structure, while the DC self-bias of target and the deposition rate of films decrease. The films change from crystal structure to amorphous structure with the increase of ICP power. The measured nanohardness and the compressive stress of films reach the topmost at ICP power of 150 W and 200 W, respectively. The mechanical properties of films show strong dependence on the crystalline structure and the density influenced by the ICP power.     

The Instability of Terahertz Plasma Waves in Two Dimensional Gated and Ungated Quantum Electron Gas

The instability of terahertz(THz)plasma waves in two-dimensional(2D)quantum electron gas in a nanometer field effect transistor(FET)with asymmetrical boundary conditions has been investigated.We analyze THz plasma waves of two parts of the 2D quantum electron gas:gated and ungated regions.The results show that the radiation frequency and the increment(radiation power)in 2D ungated quantum electron gas are much higher than that in 2D gated quantum electron gas.The quantum effects always enhance the radiation power and enlarge the region of instability in both cases.This allows us to conclude that 2D quantum electron gas in the transistor channel is important for the emission and detection process and both gated and ungated parts take part in that process.     

Numerical and Experimental Investigation of Electron Beam Air Plasma Properties at Moderate Pressure简

Large size of air plasma at near atmospheric pressure has specific effects in aerospace applications. In this paper, a two dimensional multi-fluid model coupled with Monte Carlo （MC） model is established, and some experiments were carried out to investigate the characteristics of electron beam air plasma at pressure of 100-170 Torr. Based on the model, the properties of electron beam air plasma are acquired. The electron density is of the order of 1016 m-3 and the longitudinal size can exceed 1.2 m. The profiles of charged particles demonstrate that the oxygen molecule is very important for air plasma and its elementary processes play a key role in plasma equilibrium processes. The potential is almost negative and a very low potential belt is observed at the edge of plasma acting as a protection shell. A series of experiments were carried out in a low pressure vacuum facility and the beam plasma densities were diagnosed. The experimental results demonstrate that electron density increased with the electron beam energy, and the relatively low pressure was favorable for gaining high density plasma. Hence in order to achieve high density and large size plasma, it requires the researchers to choose proper discharge parameters.     

Characterization of Crystalline Nanoparticles/Nanorods Synthesized by Atmospheric Plasma Enhanced Chemical Vapor Deposition of Perfluorohexane

A mass of nanoparticles/nanorods were formed on a simultaneously deposited gran- ular film by plasma enhanced chemical vapor deposition （PECVD） of perfluorohexane at atmo- spheric pressure without any catalysts or templates. Scanning electron microscopy （SEM） and X-ray photoelectron spectroscopy （XPS） were used to characterize the morphology and the chem- ical compositions of nanoparticles. The average size of particles is about 100 nm and the length of synthesized nanorods is between 1 μm and 2.5/tm. The analyses of transmission electron microscopy （TEM）, high-resolution transmission electron microscopy （HRTEM）, selected area electron diffraction（SAED） and X-ray diffraction （XRD） reveals that the nanoparticles and nanorods are crystalline.     

Study of Multi-Function Micro-Plasma Spraying Technology

A multi-functional micro-arc plasma spraying system was developed according to aerodynamics and plasma spray theory. The soft switch IGBT （Insulated Gate Bipolar Transistor） invert technique, micro-computer control technique, convergent-divergent nozzle structure and axial powder feeding techniques have been adopted in the design of the micro-arc plasma spraying system. It is not only characterized by a small volume, a light weight, highly accurate control, high deposition efficiency and high reliability, but also has multi-functions in plasma spraying, welding and quenching. The experimental results showed that the system can produce a supersonic flame at a low power, spray Al2O3 particles at an average speed up to 430 m/s, and make nanostructured AT13 coatings with an average bonding strength of 42.7 MPa. Compared to conventional 9M plasma spraying with a higher power, the coatings with almost the same properties as those by conventional plasma spray can be deposited by multi-functional micro-arc plasma spraying with a lower power plasma arc due to an improved power supply design, spray gun structure and powder feeding method. Moreover, this system is suitable for working with thin parts and undertaking on site repairs, and as a result, the application of plasma spraying will be greatly extended.     

Experimental Study on Indoor Air Cleaning Technique of Nano-Titania Catalysis Under Plasma Discharge

In this study, a new technique of air cleaning by plasma combined with catalyst was proposed, which consisted of electrostatic precipitation, volatile organic compounds （VOCs） decomposition and sterilization. A novel indoor air purifier based on this technique was adopted. The experimental results showed that formaldehyde decomposition by the plasma-catalyst hybrid system was more efficient than that by plasma only, Positive discharge was better than negative discharge in formaldehyde removal. Meanwhile, the outlet concentration of ozone byproduct was effectively reduced by the nano-titania catalyst.     

Deposition of SiOx on Metal Surface with a DBD Plasma Gun at Atmospheric Pressure for Corrosion Prevention

In this study, SiOx films were deposited by a dielectric barrier discharge （DBD） plasma gun at an atmospheric pressure. The relationship of the film structures with plasma powers was investigated by Fourier transform infrared spectroscopy （FTIR）, and scanning electron microscope （SEM）. It was shown that an uniform and cross-linking structure film was formed by the DBD gun. As an application, the SiOx films were deposited on a carbon steel surface for the anti-corrosion purpose. The experiment was carried out in a 0.1 M NaCl solution. It was found that a very good anti-corrosive property was obtained, i.e., the corrosion rate was decreased c.a. 15 times in 5% NaCl solution compared to the non-SiOx coated steel, as detected by the potentiodynamic polarization measurement.     

The Oblique Incident Effects of Electromagnetic Wave in Atmospheric Pressure Plasma Layers

The propagating behaviours, i.e. phase shift, transmissivity, reflectivity and absorp- tivity, of an electromagnetic （EM） wave in a two-dimensional atmospheric pressure plasma layer are described by the numerical solutions of integral-differential Maxwell＇s equations through a generalized finite-difference-time-domain （FDTD） algorithm. These propagating behaviours are found to be strongly affected by five factors： two EM wave characteristics relevant to the oblique incident and three dimensionless factors. The two EM wave factors are the polarization mode （TM mode or TE mode） and its incident angle. The three dimensionless factors are： the ratio of the maximum electron density to the critical density no/nor, the ratio of the plasma layer width to the wave length d/λ, and the ratio of the collision frequency between electrons and neutrals to the incident wave frequency veo/f.     

Comparison of Sterilizing Effect of Nonequilibrium Atmospheric-Pressure He/O2 and Ar/O2 Plasma Jets

The sterilizing effect of the non-equilibrium atmospheric pressure plasma jet by applying it to the Bacillus subtilis spores is invesigated. A stable glow discharge in argon or helium gas fed with active gas （oxygen）, was generated in the coaxial cylindrical reactor powered by the radio-frequency power supply at atmospheric pressure. The experimental results indicated that the efficiency of killing spores by making use of an Ar/O2 plasma jet was much better than with a He/O2 plasma jet. The decimal reduction value of Ar/O2 and He/O2 plasma jets under the same experimental conditions was 4.5 seconds and 125 seconds, respectively. It was found that there exists an optimum oxygen concentration for a certain input power, at which the sterilization efficiency reaches a maximum value. It is believed that the oxygen radicals are generated most efficiently under this optimum condition.     

D.C.Plasma—Sprayed Coatings of Nanostructured Alumina—Titania—Silica

Nanocrystalline powders of w(Al2O3)=95%,w(TiO2)=3%,and w(SiO2)=2%,were reprocessed into agglomerated particles for plasma spraying,by using consecutive steps of ball milling,slurry forming,spray drying,and heat treatent,D.C.Plasma was used to spray the agglomerated nanocrystalline powders,and resultant coatings were depostied on the substrate of stainless steel,Scanning electron microscopy(SEM) was used to examine the morphology of the agglomerated powders and the cross section of the alumina-titania-silica coatings.Experimental results show that the agglomerated nanocrystalline particles are spherical,with a size from(10-90)um,The flow ability of the nanocrystalline powders is greatly improved after the reprocessing.The coatings deposited by the plasma spraying are mainly of nanostructure.Unlike conventional plasma-sprayed coatings,no laminar layer could be found in the nanostructured coatings.Although the nanostructured coatings have a lower microhardness than conventional microstructured coatings,the toughness of the nanostructured ceramic coatings is significantly improved.     

A New Atmospheric Pressure Microwave Plasma Source （APMPS）

An atmospheric pressure microwave plasma source （APMPS） that can generate a large volume of plasma at an atmospheric pressure has been developed at Tsinghua University. This paper presents the design of this APMPS, the theoretical consideration of microwave plasma ignition and the simulation results, including the distributions of the electric field and power density inside the cavity as well as the accuracy of the simulation results. In addition, a method of producing an atmospheric pressure microwave plasma and some relevant observations of the plasma are also provided. It is expected that this research would be useful for further developing atmospheric pressure microwave plasma sources and expanding the scope of their applications.     

Atmospheric Pressure Plasma Jet in Organic Solution: Spectra,Degradation Effects of Solution Flow Rate and Initial pH Value

The organic compounds of p-nitrophenol （PNP） solution was treated by the active species generated in a stirred reactor by an atmospheric pressure plasma jet （APPJ）. The emission intensities of hydroxyl （OH）, oxygen （O）, nitric oxide （NO）, hydrogen （H） and molecular （N2） were measured by optical emission spectroscopy （OES）. The relations between the flow rates of the PNP solution and degradation, the degradation effects and initial pH value of the solution were also investigated. Experimental results show that there exist intense emissions of O （777.1 nm）, N（337.1 nm）, OH （306-310 nm） and NO band （200-290 nm） in the region of plasma. Given the treatment time and gas flow rate, the degradation increased as a function of discharge energy and solution flow rate, respectively. The solution flow rate for the most efficient degradation ranged from 1.414 m/s to 1.702 m/s, and contributed very little when it exceeded 2.199 m/s. This indicates the existence of diffusion-controlled reactions at a low solution flow rate and activation- controlled reactions at a high solution flow rate. Moreover, increasing or decreasing the initial pH value of neutral PNP solution （pH=5.95） could improve the degradation efficiency. Treated by APPJ, the PNP solutions with different initial pH values of 5.95, 7.47 and 2.78 turned more acidic in the end, while the neutral solution had the lowest degradation efficiency. This work clearly demonstrates the close coupling of active species, photolysis of ultraviolet, the organic solution flow rate and the initial pH value, and thus is helpful in the study of the mechanism and application of plasma in wastewater treatment.