Low Pressure Radio Frequency Plasma Research Articles

Plasma Bromination of Graphene for Covalent Bonding of Organic Molecules

Plasma-chemical bromination applied to graphitic materials, in particular to highly ordered pyrolytic graphite is reviewed. The resulting bromination efficiency of the plasma-chemical treatment was subject of systematic process optimization. The plasma of elemental bromine vapour produced bromine concentrations on graphene surfaces of more than 30 % Br/C using either inductively or capacitively coupled low-pressure radio-frequency plasmas. Plasma brominated graphite surfaces have been studied by Near Edge X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The introduction of bromine into graphene-like graphite layers and its binding situation were investigated. To study the physical effect of the plasma bromination process, Kr plasma was also used because of its chemical inertness but similar atomic mass. Covering the samples with a Faraday cage or with a LiF window should help to differentiate between physical, chemical and radiation effects of the plasma. Bromination was assigned to radical or electrophilic addition of bromine onto fully substituted aromatic double bonds (sp2 C) with exothermal reaction enthalpy. Low bromination shows a strong decay of aromatic double bonds, higher bromination percentage let disappear all aromatic rings. The formed C–Br bonds were well suited for efficient grafting of organic molecules by post-plasma wet-chemical nucleophilic substitution. This grafting onto the graphene surface was demonstrated using aminosilane and different diamines. The bromination of double bonds changes the hybridization of carbon atoms from plane sp2 to tetrahedral sp3 hybridization. Thus, the plane topography of graphene is destroyed and the conductivity is lost.

Optimization of Plasma Treatment for Enhanced Filler Matrix Adhesion in Manufacturing Green Composites with Rice Husk

The objective of the current work is to study the effect of plasma treatment of rice husk (RH) to reduce the water absorption of natural filler (RH) reinforced unsaturated polyester matrix composite. RH was treated by low-pressure radio frequency plasma in a capacitively coupled plasma reactor at room temperature using nitrogen gas. It was observed that the composites manufactured from plasma-treated RH had lower water absorption as compared with non-treated ones. An experimental matrix for nitrogen plasma treatment of RH with the key process variables such as RF power, treatment time and reactor pressure was determined and experiments were performed according to the experimental matrix to optimize the process parameters by response surface methodology of a central composite design. A quadratic model was developed to predict the percentage reduction of water absorption owing to change in plasma process variables. The process variables were optimized using the predicted model and the optimum conditions were verified by experimental data.

On the reactivity of plasma-treated photo-catalytic TiO2 surfaces for oxidation of C2H2 and CO

The objective of this study is to understand fundamental aspects of interactions of plasmas with catalytic surfaces. Based on this approach the reactivity of plasma treated and stimulated catalytic surfaces of TiO2 is studied by analysing the oxidation (i) of C2H2 to CO and CO2 and (ii) of CO to CO2. The inner surface of a Pyrex discharge tube is coated with TiO2 films impregnated with TiO2 nanoparticles, which provides a surface area of about 4 m2. In addition to the exposure of the TiO2 surface by low-pressure radio-frequency plasmas using O2, Ar or N2 (f = 13.56 MHz, p = 0.53 mbar, P = 17 W) the surfaces are stimulated by heating and UV radiation treatment. The temporal development of the concentrations of the precursor gases C2H2 or CO and of the reaction products is monitored using quantum cascade laser absorption spectroscopy, which provides multi-component detection in the mid-infrared spectral range. The C2H2 concentration was found to be nearly constant over time after a pre-treatment with Ar or N2 discharges using an initial gas mixture of 1% C2H2 in Ar. However, a strong decay of the concentration of C2H2 is observed for pure O2 plasma pre-treatment. In general, the decay is found to be nearly exponential with time constant in the order of about 10 min. The reactive adsorption of C2H2 molecules on the inner surface of the tube reactor showed a density of about 7.5 × 1012 C2H2 molecules cm−2. This behaviour demonstrates that the reaction produces some adsorbed intermediates, which can be thermally or photo-catalytically oxidized to CO2. In contrast, when 1% CO in Ar is used as an initial gas mixture no adsorption processes on the TiO2 surface could be detected. An effective destruction of CO took part via photo-catalytic oxidation.

Determination of the elemental composition of micrometric and submicrometric particles levitating in a low pressure Radio-Frequency plasma discharge using Laser-Induced Breakdown Spectroscopy

The LIBS (Laser-Induced Breakdown Spectroscopy) technique has shown its potential in many fields of applications including that of aerosol analysis. The latter is usually carried out on the particle flow, thereby allowing quantitative detection in various experimental conditions such as ambient air analysis or exhaust stack monitoring, to name but a few. A possible alternative method for particle analysis has been experimented combining a low pressure RF (Radio-Frequency) plasma discharge with the LIBS technique. Such approach has two peculiar features in comparison to the usual LIBS analysis. First, the particles injected in the RF plasma discharge are trapped in levitation. Second, the analysis is performed at a reduced pressure of around 1mbar. LIBS detection at such low pressure has this peculiarity that particle vaporization is assumed to be achieved through direct laser particle interaction whereas it is caused by laser-induced plasma ignited in the gas at atmospheric pressure. The use of such particle trap could allow improving particle sampling, making organic particle analysis possible (by using an inert gas for RF plasma ignition) and even (depending on the pressure) obtaining a better signal to noise ratio. Detection of the elements of nanoparticle agglomerates made following their injection in the RF discharge has demonstrated the feasibility of such approach. Future experiments are intended to explore its potentialities when tackling issues such as process control or ambient air monitoring.

Surface functionalization of graphenelike materials by carbon monoxide atmospheric plasma treatment for improved wetting without structural degradation

Oxygen plasma treatment has been extensively used to functionalize the surface of graphenelike materials. However, functionalization is usually accompanied by degradation of the structure, which may affect mechanical and electrical performance. Atmospheric plasma treatment (APT) of HOPG was performed to compare the effect of surface modification using carbon monoxide (CO) as the active gas, in comparison to O2. Both Raman and STM demonstrated nanoscale degradation of the structure when using the O2 treatment. CO treated specimens exhibited no observable damage to the material with high levels of oxygen incorporation. Instead, a well ordered monolayer of oxygen-containing film was observed on the surface of the specimens which could accommodate high levels of surface oxygen. Changes in surface characteristics were analyzed using x-ray photoelectron spectroscopy (XPS) as a function of duration. The results indicated that the use of O2 plasma resulted in only a limited oxygen uptake (O/C = 0.11), while CO APT resulted in tailorable surface O/C ratios as high as 0.65, a result not observed even when using low-pressure radio frequency plasmas. XPS analysis and Auger spectroscopy confirmed that a tailorable level of carbonyl functional groups could be evenly distributed throughout the surface. Contact angle measurements verified the formation of a highly stable hydrophilic surface. The CO treatment was also successfully applied to other nanocarbon materials such as graphene nanoplatelets with similar results.

Study on mechanism of etching in low pressure radio-frequency plasmas

In order to investigate the mechanism of etching in low pressure radio-frequency (rf) plasmas, we adopt a two-dimensional Monte-Carlo (MC) cellular method to simulate the profile evolution, in which the ion reflection, the ion angular distribution (IAD) etc. are considered. It is found that the phenomenon of microtrenching results from the reflection of particles on the sidewall and the IAD is responsible for round corners at the bottom of the trench. The deeper the trench is, the more easily microtrenching or notching appear. In addition, the neutral particles can destroy the anisotropic etching of the trench and lead to bowling. On the other hand, by means of solving the Laplace equation near and in the trench with an iterative procedure, we obtain a stable or periodic stable electric field biased by a dc or rf source, in which the charging effect on the photoresist sidewall of the trench is considered. It is observed that both the dc and rf biases applied to the substrate can influence the profiles of etching, and the better etching profiles on the sidewall of the trench can be obtained with the rf bias than those with the dc bias.

Magnetic layer formation on plasma nitrided CoCrMo alloy

In this study structural and magnetic character of the expanded austenite phase (γN) layer formed on a medical grade CoCrMo alloy by a low-pressure Radio-Frequency plasma nitriding process was investigated. The formation of the expanded austenite phase is facilitated at a substrate temperature near 400°C for 1, 2, 4, 6 and 20h under a gas mixture of 60% N2–40% H2. The magnetic state of the γN layers was determined by a surface sensitive technique, magneto-optic Kerr effect (MOKE), and with a scanning probe microscope in magnetic force mode (MFM). Strong evidence for the ferromagnetic nature of the γN-(Co,Cr,Mo) phase is provided by the observation of stripe domain structures and the hysteresis loops. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions (~10%) due to high N contents (~30at.%). As an interstitial impurity, nitrogen dilates the host lattice i.e. the Co–Co (or Fe–Fe) distance is increased, which strongly influences the magnetic interactions. An analogy between the magnetic properties of the expanded phases, γN-(Fe,Cr,Ni) and γN-(Co,Cr,Mo), formed in austenitic stainless steel alloys and the CoCrMo alloy of this study is made, and it is suggested that the ferromagnetic states for the γN-(Co,Cr,Mo) and γN-(Fe,Cr,Ni) phases may be correlated with the volume dependence of the magnetic properties of fcc-Co/Co4N and fcc-Fe/Fe4N, respectively.

Diagnostic of Capacitively Coupled Low Pressure Radio Frequency Plasma: An Approach through Electrical Discharge Characteristic

Diagnostic of Capacitively Coupled Low Pressure Radio Frequency Plasma: An Approach through Electrical Discharge Characteristic

Coagulation of nanoparticles in a plasma

Coagulation of nanoparticles in a low-pressure radio frequency plasma was studied by means of a detailed numerical model for the spatiotemporal evolution of the nanoparticle-plasma system. Simulation results indicate that the occurrence of coagulation to any significant degree in such systems requires the existence of two effects: first, gas-phase nucleation is not limited to a brief burst, but rather continues in regions that are sufficiently free of nanoparticles; and second, coagulation coefficients for collisions between neutral and negatively charged nanoparticles are enhanced by the image potential induced in the neutral particle. Accounting for these effects, coagulation is predicted to be dominated by coagulation between very small (approximately 1 or 2 nm in diameter) neutral particles and larger negatively charged particles that are trapped in the plasma. Coagulation ceases when the spreading of the nanoparticle cloud across the plasma quenches gas-phase nucleation.

Numerical Investigation for $\hbox{CF}_{4}$ Decomposition Using RF Low-Pressure Plasma

Among perfluorocompounds, is considered to be one of the most stable and difficult-to-decompose gases. We investigated the decomposition of CF4 using the inductively coupled plasma (ICP) reactor with radio frequency (RF) power supply, which was used for semiconductor cleaning process as well as decomposition. This technology was confirmed to achieve an extremely high efficiency and more economical system in comparison with the conventional system. The purpose of this paper is to perform numerical simulation of CF4 decomposition using RF low-pressure plasma. The experimental investigation of CF4 the decomposition was performed to validate the computed results, and the reaction products such as CO, CO2, and COF2 upon the CF4 decomposition were measured CO2 using a analyzer and a Fourier transform infrared spectrophotometer. Then, the numerical simulations of the CF4 decomposition using commercially available code were performed to obtain the gas temperature, electron temperature, electron number density, gas velocity, and chemical species number density distribution to cope with CF4 the decomposition in the ICP reactor.

06/01132 Coupling and reforming of methane by means of low pressure radio-frequency plasmas: Patino, P. et al. Fuel, 2005, 84, (16), 2008–2014

06/01132 Coupling and reforming of methane by means of low pressure radio-frequency plasmas: Patino, P. et al. Fuel, 2005, 84, (16), 2008–2014

Self-excited instability occurring during the nanoparticle formation in an Ar–SiH4 low pressure radio frequency plasma

An experimental investigation of an instability occurring during dust nanoparticle formation is presented in this paper. The present study has been performed in radio frequency low pressure plasma in an argon-silane mixture. The formation and growth of nanoparticles is followed, thanks to the analysis of the amplitude of the third harmonics (40.68MHz) of the discharge current and the self-bias voltage (Vdc). In some cases, at the end of the accumulation phase of the nanocrystallites an instability occurs. It seems to be an attachment induced ionization instability as observed in electronegative plasmas. A detailed study of the influence of different operating conditions (injected power, gas temperature, and silane flow rate) on this instability behavior and frequencies is presented. The paper concludes by examining a very particular case of the instability.

Influence of plasma pulsing on the deposition kinetics and film structure in low pressure oxygen/hexamethyldisiloxane radiofrequency plasmas

The deposition rate and structure of films deposited in an oxygen/hexamethyldisiloxane 85 : 15 low pressure radiofrequency plasma are investigated as a function of the pulse parameters. The variations of the deposition rate with the plasma-off and plasma-on times are compared to a model involving characteristic rise and decay times for the deposition, which are found to be equal to 2 and 80 ms, respectively. Such a long decay time suggests the presence of long lifetime species, which act as precursors to the film growth. These latter allow to reduce the average applied power without significantly reducing the deposition rate. On the other hand, the structure and the properties of the deposited film can be controlled and varied from SiO 2-like to SiO x C y H z organic film by tuning the pulse parameters. Pulsed plasmas are shown to be very attractive in order to deposit graded composition layers without changing the oxygen to organosilicon flow rate ratio.

Towards a methodology for the effective surface modification of porous polymer scaffolds

A novel low-pressure radio-frequency plasma treatment protocol was developed to achieve the effective through-thickness surface modification of large porous poly ( d,l-lactide) (PDLLA) polymer scaffolds using air or water: ammonia plasma treatments. Polymer films were modified as controls. Scanning electron micrographs and maximum bubble point measurements demonstrated that the PDLLA foams have the high porosity, void fraction and interconnected pores required for use as tissue engineering scaffolds. The polymer surface of the virgin polymer does contain acidic functional groups but is hydrophobic. Following exposure to air or water: ammonia plasma, an increased number of polar functional groups and improved wetting behaviour, i.e. hydrophilicity, of wet surfaces was detected. The number of polar surface functional groups increased (hence the decrease in water contact angles) with increasing exposure time to plasma. The change in surface composition and wettablility of wet polymer constructs was characterised by zeta potential and contact angle measurements. The hydrophobic recovery of the treated PDLLA polymer surfaces was also studied. Storage of the treated polymer constructs in ambient air caused an appreciable hydrophobic recovery, whereas in water only partial hydrophobic recovery occurred. However, in both cases the initial surface characteristics decay as function of time.

Coupling and reforming of methane by means of low pressure radio-frequency plasmas

Non-oxidative coupling of CH 4/H 2 mixtures was carried out by means of radio frequency (rf) glow discharges for the first time. A central composite design was employed to determine the best experimental conditions for methane transformation into higher hydrocarbons and to fit the experimental data. rf power was the factor showing the highest effect on the results while CH 4/H 2 mole ratio showed the lowest. Conversion was 46.4% at 100 W, 0.07 mbar and CH 4/H 2 mole ratio of 1/2. Selectivity was 56.9% for C 2, 6.9% for C 3, and 36.2% for C 4 hydrocarbons. Least squares fits of quadratic equations yielded approximating functions permitting to predict results of random experiments with errors of about 5%. The same rf system was used for the reforming of methane with CO 2, O 2, and steam plasmas, respectively. The highest oxidation was observed with oxygen whilst steam plasma produced the best results. H 2/CO mole ratio was adjusted by setting specific experimental parameters of the latter. CO 2 free synthesis gas was produced at higher H 2O and CH 4 flow rates, i.e. 0.8 mmol/h and higher power, i.e. 100 W. CO 2 and CO free H 2 was produced at 0.3 and 0.6 mmol/h flow rates of H 2O and CH 4, respectively, and 50 W.

Particle coating in seeded dusty plasma reactor: Distribution of deposition rates

We seed a low-pressure radio-frequency plasma with micron and submicron silica particles and induce surface deposition of plasma-generated solid films. This process produces surface coatings ranging from few nm to several hundred nm. The size distribution of the coated particles (core plus shell) is monitored as a function of time and is found to increase both in mean size as well as in variance. To explain the broadening of the distribution we propose a deposition model which assumes that the reaction rate is nonuniform within the plasma. This model predicts that the average particle radius increases linearly with deposition time with a slope equal to the average deposition rate while the variance increases as a quadratic function of time. These predictions compare favorably with experiments conducted with seeds ranging 0.1–2.3 μm in radius. Based on this model we are able to obtain the distribution of film deposition rates in the plasma. We find that this distribution is wide with a substantial fraction of particles experiencing very low deposition rates.

Low-Pressure Radio-Frequency Plasma for Surface Decontamination of Artillery Shell Casings. 1. Dinitrotoluene

A novel low-pressure radio-frequency nonthermal plasma system has been successfully designed, installed, and tested for the decontamination of full-size shell surfaces. The destruction removal efficiency (DRE) of the new plasma system for dinitrotoluene (DNT), a simulant of high explosives, destruction depends on various plasma parameters including gases, power outputs, on/off cycles, and treatment times. Under the same decontamination conditions, H2 plasma is more uniform and efficient for all surface decontamination of DNT than O2 plasma. The best three decontamination conditions investigated so far are 100 W with a 6 ms/3 ms cycle, 75 W with continuous wave, and 200 W with a 3 ms/6 ms cycle of H2 plasma. The best DRE achieved is 99.86%. No unreacted DNT was detected bypassing the plasma treatment in the gas stream. N2, CO2, and H2O are the major products detected by the online mass spectrometer, from the process of plasmas reacting with DNT. There is some NOx, mainly NO, detected in the product stream, but it is a minor product of the process. It appears that the current nondestructive decontamination process is economical and environmentally friendly and has a great potential to replace the open burning/open detonation process currently practiced by the military for obsolete artillery shell casings.

Observation of submicron dust particles trapped in a diffused region of a low pressure radio frequency plasma

Submicron dust particles (Cu, φ 50 nm) were observed to be trapped in an rf plasma at relatively low pressure. The laser light scattering by particles shows the time evolution of the dust particles for several hours after pulsed injection of the particles. The dust particles were localized in the diffused region as a large volume cloud. The suspension mechanism of the dust particles in the diffused plasma was explained by an electrostatic force due to the potential structure. The spatial distribution of the charge of the dust particles was determined by this potential structure. The dispersion relations of the self-excited dust acoustic waves were adopted to confirm the spatial distribution and the time evolution of the charge of the dust particles.

Nanoparticles and Nanocomposites in RF Plasma

AbstractThe use of low pressure radio-frequency (rf) plasma for nanoparticle formation and the deposition of thin film on particulate substrates are reported. Plasma polymer particles are synthesized in a capacitively-coupled Ar/monomer discharge at rf power of 15-30 W. A variety of particle structures are observed, including monodispersed nanospheres and liquid-like viscous nano-droplets. Styrene in particular is observed to produce hollow nanospheres. By manipulating the process parameters, films of plasma polymers can be deposited onto suspended submicron particles. We take advantage of the electrostatic trapping of “dusty plasma” to suspend small particles in plasma for extended periods of time until the desired coating thickness is achieved. Sub-micron silica particles introduced into a low pressure rf Ar/monomer plasma are coated with film of thickness ranging from 2 to 70 nm.

The chemical structure and stability of plasma-deposited thin hydrocarbon layers on polyethylene

AbThe surfaces of polyethylene (PE) films were modified by deposition of layers from acetylene/ethylene monomer gases in a low-pressure radio-frequency plasma. The chemical structure of the plasma-deposited layers and their long-term stability were studied by X-ray photoelectron spectroscopy (XPS), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and contact angle measurements. These studies have shown that the plasma-deposited layers consist mainly of amorphous, short-chain, functional CxHy structures with aryl units. As the reactive radical centers formed during the plasma process are susceptible to further reaction with atmospheric oxygen and water, the chemical stability of these layers was investigated. This stability is ensured over the long-term, although an increase in the number of functional groups is obtained over time. It was demonstrated that the post-reactions reach a state of equilibrium after a few weeks.