Controlling plasma composition during carbon film deposition in microwave discharges by means of optical-emission spectroscopy

Optical emission spectra from the microwave discharge plasma that is used to activate gas-phase deposition of carbon films are systematically investigated under various deposition conditions. The line emission intensities from CH and C₂ radicals, which are responsible for the growth of the diamond and graphite phases, respectively, are studied as functions of the main macroparameters of the process. To find the relation between the features of the emission spectra and the composition of the films obtained, the films were examined using Raman spectroscopy and electron microscopy. It is shown that monitoring the relative intensities of the spectral lines can be used to obtain the desired type of film, in which case the state of the substrate surface and the presence of a catalyst on it also play an important role. Experiments on the deposition of carbon films in the pulsed regime of plasma excitation show the possibility of changing the phase composition of the film by varying both the pulse repetition rate and the off-duty factor. At the same average microwave power, the rate of film deposition in the pulsed regime of plasma excitation is lower than that in a continuous discharge; however, the growth rate of the graphite phase decreases insignificantly.     

Hydrocarbon plasma chemistry in a continuous microwave discharge

A study is made of the relation between the kinetic processes involving carbon-containing species and the intensity ratios of different emission lines in synthesizing diamond films in a microwave discharge plasma. The intensity ratios of the emission lines are measured as functions of the pressure, composition, and flow rate of the gas mixture. The kinetic processes involving carbon-containing components are simulated under conditions close to the experimental ones. It is shown that the intensity ratios of different pairs of lines can be used to control diamond film deposition.     

Accelerated esterification of free fatty acid using pulsed microwaves

It was demonstrated that pulsed microwave irradiation is a more effective method to accelerate the esterification of free fatty acid with a heterogeneous catalyst than continuous microwave irradiation. A square-pulsed microwave with a 400 Hz repetition rate and a 10-20% duty cycle with the same energy as the continuous microwave were used in this study. The pulsed microwaves improved the esterification conversion from 39.9% to 66.1% after 15 min in comparison with the continuous microwave under the same reaction conditions. These results indicated that pulsed microwaves with repetitive strong power could enhance the efficiency of biodiesel production relative to the use of continuous microwave with mild power.     

Polycrystal diamond growth in a microwave plasma torch

Diamond films of different structures were deposited on quartz, WC-Co, and molybdenum substrates in a microwave plasma torch discharge in an argon-hydrogen-methane gas mixture in a sealed chamber at pressures close to atmospheric by using the chemical vapor deposition technique. Images of diamond polycrystal films and separate crystals, as well as results of Raman spectroscopy, are presented. The spectra of optical plasma radiation recorded during film deposition demonstrate the presence of intense H_α hydrogen and C₂ radical bands known as Swan bands.     

Self-consistent simulation of pulsed and continuous microwave discharges in hydrogen

Results are presented from numerical simulations of pulse-periodic and continuous microwave discharges in hydrogen that are used in CVD reactors for chemical vapor deposition of diamond films. Attention is focused on the processes that should be taken into account in order to construct the simplest possible adequate numerical model. It is shown that the processes of vibrational excitation of hydrogen molecules, as well as chemical reactions, play an important role in the establishment of energy balance within the discharges. The results of numerical simulations are compared to the experimental data.     

Experimental study of a multipactor discharge on a dielectrics surface in a high-Q microwave cavity

Results from experimental studies of multipactor discharges on the surfaces of various dielectrics placed in a high-Q cylindrical microwave cavity excited at the TE₀₁₃ mode in the X-band are presented. The thresholds for the onset and maintenance of a multipactor discharge on quartz, polycrystalline diamond, lithium fluoride, and Teflon surfaces possessing different roughness are determined. It is shown that, in such a resonance system, a steady multipactor discharge can operate without transition into the stage of microwave breakdown of the desorbed gas. It is found that, due to long-term action of the discharge, a thin carbon-containing film is deposited on the dielectric surface, which leads to an increase in the breakdown threshold.     

Beam-plasma interaction in a hybrid plasma waveguide in the pulsed mode of microwave generation

Results are presented from experimental studies of the energy spectra of an electron beam in a model beam-plasma oscillator based on a hybrid plasma waveguide in the pulsed mode of microwave generation with a pulse duration of 1 µs or shorter. The beam energy spent on sustaining the beam-plasma discharge in a slow-wave structure is measured. A correlation between the type of excited waves and the generation of a group of accelerated beam electrons with energies exceeding the injection energy is revealed. It is shown that the pulsed mode of microwave generation is related to the time variations in the plasma density profile in the waveguide and the trapping of beam electrons by the excited microwave field.     

Epitaxial synthesis of diamond layers on a monocrystalline diamond substrate in a torch microwave plasmatron

The epitaxial growth of a diamond single-crystal film in a torch microwave discharge excited by a magnetron of a domestic microwave oven with the power of ≤1 kW in an argon-hydrogen-methane mixture with a high concentration of methane (up to 25% with respect to hydrogen) at atmospheric pressure on a sub-strate of a synthetic diamond single crystal (HPHP) with the orientation (100) and 4 × 4 mm in size is obtained. A discharge with the torch diameter of ∼2 mm and the concentration of the microwave power absorbed in the torch volume of >10³ W/cm³ is shown to be effective for epitaxial enlargement of a single crystal of synthetic diamond. The structure of the deposited film with the thickness up to 10 μm with high-quality morphology is investigated with an optical microscope as well as using the methods of the Raman scattering and scanning electron microscopy.     

Pulsed microwave discharge in a capillary filled with atmospheric-pressure gas

A pulsed microwave coaxial capillary plasma source generating a thin plasma filament along the capillary axis in an atmospheric-pressure argon flow is described. The dynamics of filament formation is studied, and the parameters of the gas and plasma in the contraction region are determined. A physical model of discharge formation and propagation is proposed. The model is based on the assumption that, under the conditions in which the electric fields is substantially below the threshold value, the discharge operates in a specific form known as a self-sustained-non-self-sustained (SNS) microwave discharge.     

Atmospheric-Pressure Microwave Plasma Torch for CVD Technology of Diamond Synthesis

Plasma Physics Reports - An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension...     

Pulsed microwave induced light, sound, and electrical discharge enhanced by a biopolymer

Intense flashes of light were observed in sodium bicarbonate and hydrogen peroxide solutions when they were exposed to pulsed microwave radiation, and the response was greatly enhanced by a microwave-absorbing, biosynthesized polymer, diazoluminomelanin. A FPS-7B radar transmitter, operating at 1.25 GHz provided pulses of 5.73 +/- 0.09 micros in duration at 10.00 +/- 0.03 pulses/s with 2.07 +/- 0.08 MW forward power (mean +/- standard deviation), induced the effect but only when the appropriate chemical interaction was present. This phenomenon involves acoustic wave generation, bubble formation, pulsed luminescence, ionized gas ejection, and electrical discharge. The use of pulsed microwave radiation to generate highly focused energy deposition opens up the possibility of a variety of biomedical applications, including targeting killing of microbes or eukaryotic cells. The full range of microwave intensities and frequencies that induce these effects has yet to be explored and, therefore, the health and safety implications of generating the phenomena in living tissues remain an open question.     

Microwave and Optical Gas Discharges in Superstrong Pulsed Fields

A review is given of theoretical papers on gas breakdown in high-power pulsed microwave and optical fields under conditions such that the electron oscillatory energy in the wave field is much higher than the ionization energy of gas atoms. In microwave fields, which are much weaker than the atomic field, the ionization mechanism for gas atoms is governed by electron-impact avalanche ionization. In high-power optical fields that are comparable in strength to the atomic field, the gas atoms are ionized via the tunneling of the bound electrons. It is shown that, in both cases, the electrons obey similar, highly anisotropic distributions, thereby strongly affecting the stability of the discharge plasma.     

Absence of Non-Thermal Microwave Effects on the Function of Giant Nerve Fibers

Functioning of the giant axon of the isolated earthworm ventral nerve cord was examined during exposure to 6.45 GHz microwaves. We used continuous wave and pulsed irradiation, either synchronized with stimuli or asynchronous, lasting for 10–50 min at specific absorption rate from 30 to 230 W/kg. Action potential (AP) conduction velocity and the capability of nerve fiber to answer long-lasting high-frequency stimulation served as indices of microwave effect. Under some experimental conditions the nerve appeared to have extreme sensitivity to subtle temperature changes, induced by irradiation, but no non-thermal microwave effects were detected.     

Initiation of solid-phase chemical reactions in powder mixtures by microwave discharges

The initiation of exothermic chemical reactions in powder (metal-dielectric) mixtures by irradiating them with a high-power microwave beam is investigated. The initial stage of microwave breakdown is accompanied by the emission in the atomic lines of the metal component of the mixture (Ti, Mo, Sn, Al, etc.). The subsequent microwave discharge generates a continuous optical spectrum, the temperature of the effective Planckian radiator being 2000–3000 K. A prolonged radiation of the mixture after the end of the microwave pulse is caused by the energy release in chemical reactions.     

Comparative estimation of the effects of continuous and intermittent cyclical microwave radiation on the behavior of rats in the extraordinary situation

Research has been carried out to investigate the effects of pulsed microwave exposure without pause (7 GHz, 400 pps, 100 microseconds, 70-150 mW/cm2, exposure 10 min) and pulsed interrupted cyclical microwave exposure (5 min exposure--4 min pause--5 min exposure) on learned behaviors of rats in the paradigm of extraordinary situation (the rescue of the life). It was shown that reductions in conditioned behavior after acute pulsed microwave exposure occurred at SAR of 21 W/kg (100 mW/cm2) and after cyclical pulsed microwave exposure at SAR of 28.4 W/kg (135 mW/cm2).     

Biological effects of high-frequency electromagnetic fields on Salmonella typhimurium and Drosophila melanogaster

Salmonella typhimurium and Drosophila melanogaster were exposed to continuous wave (CW) 2.45-GHz electromagnetic radiation, pulsed 3.10-GHz electromagnetic radiation, CW 27.12-MHz magnetic fields, or CW 27.12-MHz electric fields (only Drosophila). The temperatures of the treated sample and the nonexposed control sample were kept constant. The temperature difference between exposed and control samples was less than +/- 0.3 degrees C. Ames' assays were made on bacteria that had been exposed to microwaves (SAR 60-130 W/kg) or RF fields (SAR up to 20 W/kg) when growing exponentially in nutrient broth. Survival and number of induced revertants to histidine prototrophy were determined by common plating techniques on rich and minimal agar plates. The Drosophila test consisted of a sensitive somatic system where the mutagenicity was measured by means of mutations in a gene-controlling eye pigmentation. In none of these test systems did microwave or radiofrequency fields induce an elevated mutation frequency. However, a significantly higher concentration of cells was found in the bacterial cultures exposed to the 27-MHz magnetic field or 2.45-GHz CW and 3.10-GHz pulsed microwave radiation.     

Controlling the characteristics of a repetitive volume discharge in CFC-12 and its mixtures with argon

The parameters of a repetitive volume discharge in CF₂Cl₂ (CFC-12) and its mixtures with argon at pressures of P(CF₂Cl₂)≤0.4 kPa and P(Ar)≤1.2 kPa are studied. The discharge was ignited in an electrode system consisting of a spherical anode and a plane cathode by applying a dc voltage U_ch≤1 kV to the anode. The electrical and optical characteristics of a volume discharge (such as the current-voltage characteristics; the plasma emission spectra; and the waveforms of the discharge voltage, the discharge current, and the total intensity of plasma emission) are investigated. It is found that, by shunting the discharge gap with a pulsed capacitor with a capacitance of C₀≤3.5 nF, it is possible to control the amplitude and duration of the discharge current pulses, as well as the characteristics of the pulsed plasma emission. The increase in the capacitance C₀ from 20 to 3500 pF leads to a significant increase in the amplitude and duration of the discharge current pulses, whereas the pulse repetition rate decreases from 70 to 3 kHz. The glow discharge exists in the form of a domain with a height of up to 3 cm and diameter of 0.5–3.0 cm. The results obtained can be used to design an untriggered repetitive germicidal lamp emitting in the Cl₂(257/200 nm) and ArCl (175 nm) molecular bands and to develop plasmachemical methods for depositing amorphous fluorocarbon and chlorocarbon films.     

B16 melanoma development in black mice exposed to low-level microwave radiation

The effect of low-level microwave exposure, 2,450 MHz, at a power density of 1 mW/cm2 and specific absorption rate of 1.2 mW/g, continuous waves (CW) or pulsed waves (PW), 2.5 h/day, 6 sessions/week until death (up to 690 h of irradiation), has been studied in black C57/6J mice with B16 melanoma. The results show that no significant effects are observed on tumor development or on survival times compared to controls, or between CW- and PW-treated animals.     

Spectroscopic and probe measurements of the electron temperature in the plasma of a pulse-periodic microwave discharge in argon

A pulse-periodic 2.45-GHz electron-cyclotron resonance plasma source on the basis of a permanent- magnet mirror trap has been constructed and tested. Variations in the discharge parameters and the electron temperature of argon plasma have been investigated in the argon pressure range of 1 × 10^–4 to 4 × 10^–3 Torr at a net pulsed input microwave power of up to 600 W. The plasma electron temperature in the above ranges of gas pressures and input powers has been measured by a Langmuir probe and determined using optical emission spectroscopy (OES) from the intensity ratios of spectral lines. The OES results agree qualitatively and quantitatively with the data obtained using the double probe.     

Propagation of a surface microwave along the afterglow plasma column of a high-current pulsed discharge

It is demonstrated experimentally that the lifetime of the afterglow plasma of a high-current pulsed discharge in a dielectric tube filled with a mixture of argon with saturated mercury vapor is longer than 1 ms. Such a long lifetime, during which the electron density decreases from 10¹⁴ to 10¹² cm⁻³, is explained by the chemi-ionization of mercury vapor by long-lived metastable argon atoms. During this time, the afterglow plasma can serve as a microwave waveguide for a weakly damped low-noise E ₀-type axisymmetric surface mode, which allows one to use it for transmission of signals in the centimeter wavelength range.