Low-temperature Radio-frequency Plasma Research Articles

Variations of plasma potential in RF discharges with DC-grounded electrode

The self-bias voltage Vbias and the plasma potential Vp are the key parameters to control plasma-wall interactions in radio-frequency (RF) asymmetric plasma discharges. Knowing these two parameters allows to monitor the ion energy distribution on the electrode surface. However, in a direct current (DC) coupled plasma, the plasma potential increased to hundreds of volts while no more Vbias developed on the driven electrode. In addition, the plasma potential is strongly impacted by the electrode-wall area ratio. Several analytical or semi-analytical models exist to explain this phenomenon and to approximate the plasma potential, knowing the electrode-wall area ratio considering capacitive sheaths or a combination of resistive and capacitive sheaths. The Vp and several other plasma parameters were investigated experimentally with a Langmuir probe and a Retarding field energy analyser for different electrode/wall area ratios in low-temperature RF plasma. The validity of the models was studied for an extensive range of area ratios with different pressures and RF driving amplitude. Moreover, the presence of strong stochastic heating of the plasma for area ratios below 6 was evidenced by an increased ion flux and an increased electron temperature.

Plasma-assisted catalytic formation of ammonia in N2-H2 plasma on a tungsten surface.

Plasma catalysis has drawn attention in the past few decades as a possible alternative to the Haber-Bosch process for ammonia production. In particular, radio frequency plasma assisted catalysis has the advantage of its adaptability to the industrial scale. However, in the past years, very few experimental studies have focused on the synthesis of ammonia from nitrogen/hydrogen radio frequency plasma. As a consequence, to date, there has been little agreement about the complex mechanisms underlying the radio frequency plasma-catalyst interactions. Gaining such an understanding is therefore essential for exploiting the potential of radio frequency plasma catalysis for ammonia production. In this study, we present results of ammonia formation from a nitrogen/hydrogen radio frequency plasma both without and with a tungsten catalyst for different initial nitrogen ratios. High yields of ammonia up to 32% at 25/75% of nitrogen/hydrogen were obtained using a combination of radio frequency low pressure plasma and a W surface as a catalyst. Furthermore, based on chemical analysis of the catalytic surface composition, a formation pathway of ammonia via the Eley-Rideal mechanism between adsorbed nitrogen and hydrogen from the gas phase is presented.

Enhanced recrystallization and dopant activation of P+ ion-implanted super-thin Ge layers by RF hydrogen plasma treatment

Radio-frequency (RF) hydrogen plasma treatment, thermal annealing in a furnace, and rapid thermal annealing of high-dose P+ ion implanted p-type Ge layers have been studied by Raman scattering spectroscopy, atomic force microscopy, secondary ion mass spectrometry, electrochemical capacitance–voltage profiling, four-point probes method, and x-ray reflectometry. It was shown that low-temperature RF plasma treatment at temperature about 200 °C resulted in full recrystallization of amorphous Ge layer implanted by P+ ions and activation of implanted impurity up to 6.5 × 1019 cm−3 with a maximum concentration at the depth of about 20 nm. Rapid thermal annealing (15 s) and thermal annealing (10 min) in nitrogen ambient required considerably higher temperatures for the recrystallization and activation processes that resulted in diffusion of implanted impurity inside the Ge bulk. It was demonstrated that RF plasma treatment from the samples with front (implanted) side resulted in considerable stronger effects of recrystallization and activation as compared with the same treatment from the back (unimplanted) side. The experiment shows that nonthermal processes play an important role in enhanced recrystallization and dopant activation during the RF plasma treatment. Mechanisms of enhanced modification of the subsurface implanted Ge layer under plasma treatment are analyzed.

Safety and Efficacy Evaluation of Degradation of Aflatoxin B1 by Low-temperature Radio Frequency Plasma

The purpose of this study was to explore the application of Low-Temperature Radio Frequency Plasma (LTRFP) in the degradation of Aflatoxin B1 (AFB1). After AFB1 was exposed to LTRFP, the degradation rate was analyzed by HPLC and the acute toxicity of the degradation products was evaluated in Wistar rats and Human Hepatocellular carcinoma cells (HepG2). The results showed that AFB1 could be effectively degraded by LTRFP and the degradation rate decreased with initial AFB1 concentration rise. In initial concentration 0.2 mg/L, the degradation rate of AFB1 reached up to 95.34%; nevertheless, as the AFB1 concentration increased to 100 mg/L, the degradation rate declined markedly to 29.32%. The degradation product with degradation rate 29.32% was subjected to toxicity evaluation in rats and a LD50 of 29.41 mg/kg•bw with 95% confidence limit 25.15-34.40 mg/kg•bw was recorded, which was much higher than that of untreated AFB1 (0.5 to 10 mg/kg•bw). Similar results were also found in the HepG2 cells and the acute toxicity of the degradation product was reversely proportional to the degradation rate. It could be concluded that LTRFP was a potential alternative for the detoxification of AFB1 due to the reduced toxicity of the degradation product.

Degradation of aflatoxin B1 by low-temperature radio frequency plasma and degradation product elucidation

Plasma is a partially ionized gas that contains abundant free radicals and UV photons. In this work, the application of low-temperature radio frequency plasma in aflatoxin B1 (AFB1) degradation was investigated and the possible structures of the degradation products were elucidated. It was revealed that AFB1 could be effectively degraded by plasma and exposure to 300 W plasma for 10 min resulted in a degradation rate of up to 88.3 %. Kinetic analysis showed that the degradation process followed a first-order reaction and the generator power was the dominative parameter for AFB1 decomposition. Five major compounds, including 3-(hydromethyl)-8, 9-dihydro-2H-furo[2, 3-h]chromen-8-ol, 1-hydroxy-2-oxo-5-(2, 4, 6-trihydroxy-2, 3, 3a, 8a-tetrahydrofuro[2, 3-b]benzofuran-5-yl)cyclopentanecarboxylic acid, 2-(2, 4-dihydroxy-2, 3, 3a, 8a-tetrahydrofuro[2, 3-b]benzofuran-5-yl)-5-hydroxycyclopent-1-enecarboxylic acid, 1-hydroxy-2-(4-hydroxy-6-methoxy-2-oxo-2, 3, 3a, 8a-tetrahydrofuro[2, 3-b]benzofuran-5-yl)-5-oxocyclopentanecarboxylic acid, and 1, 2-dihydroxy-5-(4-hydroxy-2-oxo-2, 3, 3a, 8a-tetrahydrofuro[2, 3-b]benzofuran-5-yl)cyclopentanecarboxylic acid, were revolved and identified from the degradation product by using UPLC–Q-TOP–MS, and the addition reaction was mainly responsible for AFB1 decomposition. All the five degradation products lost their double bonds in the terminal furan ring and hence theoretically had reduced toxicity compared with AFB1 according to the structure–activity relationship. For this reason, the low-temperature radio frequency plasma is proposed potential in the decontamination of AFB1.

Characterization of Capacitively Coupled Radio-Frequency Argon Plasma by Electrical Circuit Simulation

Low temperature radio frequency plasma is widely used in low temperature plasma processing medium for material processing in many fields including microelectronics, aerospace, and the biology. For proper utilization of the process, it is very much important to know the plasma parameters. In this paper a novel technique is used to determine the plasma parameters from the electrical discharge characteristic and the power balance method. The homogeneous discharge model is used to evaluate the relation between the plasma parameters with the discharge characteristics. The electron density and temperature is found to be well agree with the Langmuir probe data in the range of 0.5x1016 to 45x1016 cm-3 and 1.4 to 1.6 ev for wide range of rf power.

Methane conversion in the presence of oxygen under low-temperature radio frequency plasma

Methane conversion in the presence of oxygen under low-temperature radio frequency (RF) plasma was investigated. The experiment results indicated that the following four factors, i.e., discharge voltage, discharge area, O 2/CH 4 molar ratio and total gas flowrate, affected remarkably the reaction performance. The optimum reaction conditions of methane conversion in the presence of O 2 under RF plasma are as follows: discharge voltage 1050 V, discharge area 989.1mm 2, O 2/CH 4 molar ratio 1/10 and total gas flowrate 200 ml/min. A methane conversion of 91% could be reached under the optimum conditions. Oxygen is good for the breaking of C-H bonds and also acts as a sort of thinner. According to the low-temperature plasma characteristics, the macroscopic kinetics model of methane conversion in the presence of O 2 under radio frequency plasma was studied.

The Analysis of Morphology After the Treatment of Rabbit's Nasal Mucosa with High Intensity Focused Ultrasound and Low-temperature Radiofrequency Plasma Ablation

Objective To observe the influence degree of morphology after treating the rabbit's nasal muco-sa with focused ultrasound technology.Methods By using the High Intensity Focused Ultrasound,a straight line scan for a rabbit's turbinate under the certain dose parametem had been done.And then the rabbit's turbinate was treated with low temperature plasma radiofrequency ablation.A comparison test after one month and three months was carried out.A comparison between the changes of rabbit mucosa morphological of the two kinds of treatment methods through general morphology and light microscopy observation had been taken.Results The experimental results showed that there were no significant changes in appearance and color between the use of focused ultrasound treatment of rabbit's inferior turbinate and scan the rabbit's inferior turbinate under certain dose parametem after one month and three months later.Under the lisht microscope showed a complete rabbit nasal epithelium.And after the treatment by using low-temperature plasma mdiofrequeney ablation instrument,the rabbit's turbinate became narrow,gray in color and there were some secretions compared witk normal rabbit's turbinate.Under the light microscopy,the rabbit nasal epi-thelium showed broken and part of the conical slices completely disappeared.Conclusion Under certain parameters,the focused ultrasonic energy could not undermine the nasal epithelium;thereby the function of mucociliary system is fully retained Key words: Ultrasonic therapy; Isoclectric focusing; Nasal mucosa; Rabbits; Morphogenesis

Surface modification and characterization of cotton and polyamide fabrics by plasma polymerization of hexamethyldisilane and hexamethyldisiloxane

PurposePlasma polymerization is a very promising technique to produce functional textile materials for any textile end uses as well as for high performance clothing. It can be possible to obtain highly cross‐linked, pinhole free and very thin polymer films up to 1 μm thickness with unique physical and chemical properties. These films can be used as very effective barriers. The purpose of this paper is to investigate the influences of plasma polymerization of hexamethyldisilane (HMDS) and hexamethyldisiloxane (HMDSO) on the surface properties of cotton and polyamide fabrics.Design/methodology/approachThe methodology is based on the surface modification of the cotton and polyamide fabrics by plasma polymerization of HMDS and HMDSO. The fabrics are modified by low pressure low temperature RF (radio frequency −13.56 MHz) plasma polymerization system under different power and time conditions. The changes in surface structure and morphology of the fabrics are investigated by Fourier transform infrared spectroscopy‐attenuated total reflectance (FTIR‐ATR) analysis and atomic force microscopy (AFM).FindingsWater repellency of polyamide fabrics is strongly enhanced after plasma polymerization of both HMDS and HMDSO monomers. In addition to this, the treatments are found to slow down the vertical flame spread in cotton fabrics.Originality/valueIncreased water repellency and decreased vertical flame spread are achieved using plasma polymerization technique in a very short time with very little amount of chemical and without water and auxiliary agent.

Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term “polymorphous” is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.