Low-pressure Gas Discharge Plasma Research Articles

Structure and properties of high-chromium steel irradiated with a pulsed electron beam and nitrided in a low-pressure gas discharge plasma

Ion-plasma saturation of the surface of machine parts and mechanisms with gas elements (nitrogen, oxygen, carbon) is currently one of the most effective and widely used methods of surface hardening of metal products for various purposes in the industry of developed countries. The aim of this research is to develop a complex method for modifying the surface layer of AISI 310 steel, combining irradiation with an intense pulsed electron beam and subsequent nitriding in the plasma of a low-pressure gas discharge. As a result of the studies performed, the optimal parameters of modification were revealed, which make it possible to increase the hardness of the surface layer of steel by more than 11 times, relative to the hardness of the initial material, and 8 times, relative to the hardness of steel irradiated with a pulsed electron beam. In this case, the wear resistance of the steel exceeds the wear resistance of the original and irradiated material by more than 100 times. It has been established that the high strength and tribological properties of the modified steel are due to the formation of a two-phase (iron nitride and chromium nitride) layered nanoscale structure in the surface layer.

Radar cross-section reduction by tunable low-pressure gas discharge plasma

In this experimental research, the effect of tunable low-pressure gas discharge plasma was examined on radar cross-section (RCS) reduction. A quintet array of plasma tubes (PAr = 10 torr, PHg ≈ 2–6 × 10−3 torr, Rin = 18 mm) next to each other was used to reduce the RCS of a planar metallic target. Plasma columns were optimized to have minimum axial and temporal inhomogeneity. The plasma electron density, plasma frequency, and collision frequency of the plasma tubes were measured by fast time-resolved microwave interferometry with simultaneous measurement of current and voltage. The influence of the discharge current (Irms = 50–1500 mA) as a regulator of the plasma parameters was investigated. RCS reduction changes were observed by tuning plasma parameters at a frequency range of 2–18 GHz for the normal and oblique incidence of TE and TM polarizations. The assembled plasma tubes depicted broadband RCS reduction (<−10 dB) in TM polarization for a 300 mA discharge current (42.5% total bandwidth) and in TE polarization for a 1400 mA discharge current (34% total bandwidth) for normal incidence. It was shown that the measurement is in good agreement with the simulation. It was concluded that by performing bistatic measurements, the plasma cylinders behave as a wave absorber and scattering object in the TE and TM polarizations, respectively.

The influence of electrical waveform on the electron density and collision frequency of low-pressure gas discharge plasma

In this experimental research, the effect of electrical waveforms including direct current (DC), sinusoidal, sawtooth, triangular, modified sinusoidal and square wave on the plasma electron density and collision frequency of the positive column of low-pressure gas discharge (PAr = 10 Torr, PHg = 6 × 10−3 Torr, R = 18 mm, Irms = 400 mA) was examined. Time-resolved plasma electron density and collision frequency were measured by fast time-resolved microwave interferometry with simultaneous current and voltage measurements. In the case of direct current excitation, the striation with 800 Hz fluctuation was observed. In a sine wave, the striation was observed at 50 Hz while it was disappeared at 5 kHz. In the square waveform, the collision frequency that keeps the plasma density constant could be controlled by tuning the excitation frequency. It was also found that a rapid change in the electrical current signal, like the sawtooth waveform, causes a total reduction in the electron density and collision frequency. Studying the rectangular pulse periods, showed that the maximum density and collision frequency, as well as their evolutions, could be controlled. Regarding the obtained results, it was concluded that one could effectively control (increase or reduce) the electron density and collision frequency with respect to the critical time intervals for practical applications such as plasma antennas and plasma RCS reduction.

Strengthening of cutting tools using beams of fast neutral atoms in a low-pressure gas discharge plasma

The surface of a 26-mm-diameter reamer made of AISI M2 high-speed steel has been nitrided in a low-pressure glow discharge plasma. Then a 3-μm-thick wear-resistant TiN coating has been magnetron sputtered on the hardened surface. After the combined processing the reamer’s cutting edges radii grew from the initial value of ∼8 μm up to ∼37 μm. The reason for the strengthened tool blunting is selective sputtering of the edges by heating ions accelerated from the plasma. Another reamer under nitriding was heated by a beam of fast atoms arriving at the tool surface from an immersed in the plasma concave grid negatively biased to 6 kV. Due to the uniform sputtering of the tool surface by the beam, the edges radii after nitriding and 3-μm-thick TiN coating deposition did not exceed the initial value.

Laser-Induced Phase Transition in a Monolayer of Polymer Particles Levitating in a Low-Pressure Gas-Discharge Plasma

We report on the results of analysis of the mean kinetic energy and the pair correlation function of polymer particles in a plasma–dust structure under the action of laser radiation. We have observed experimentally the crystal–liquid phase transition in the monolayer of particles levitating in the near-electrode layer of a capacitive high-frequency discharge. The coupling parameter of the dust system has been estimated. The results of analysis of the modification of the polymer dust particle surface after holding in the plasma are considered. We propose an explanation of the phase transition taking into account the role of the photophoretic force in the motion of macroparticles. The effect of the photophoretic force is associated with the modification of the dust particle surface in the plasma, as a result of which the particles can effectively absorb laser radiation.

Structure, phase composition, and properties of the titanium surface modified by electron-ion-plasma methods

The results of studying the phase and elemental compositions, imperfect substructure, and mechanical and tribological properties of commercially pure VT1-0 titanium subjected to combined treatment, which involves nitriding in low-pressure gas-discharge plasma and nitride coating deposition, are presented. The regularities are analyzed, the physical mechanisms of structural modification are discussed, and the optimal impact modes allowing a multiple increase in the microhardness and wear resistance of the material are revealed.

Plasma-deposited fluorocarbon polymer films on titanium for preventing cell adhesion: a surface finishing for temporarily used orthopaedic implants

The design of a titanium implant surface should ideally support its later application in clinical use. Temporarily used implants have to fulfil requirements different from permanent implants: they should ensure the mechanical stabilization of the bone stock but in trauma surgery they should not be integrated into the bone because they will be removed after fracture healing. Finishing of the implant surface by a plasma-fluorocarbon-polymer (PFP) coating is a possible approach for preventing cell adhesion of osteoblasts. Two different low pressure gas-discharge plasma processes, microwave (MW 2.45 GHz) and capacitively coupled radio frequency (RF 13.56 MHz) plasma, were applied for the deposition of the PFP film using a mixture of the precursor octafluoropropane (C3F8) and hydrogen (H2). The thin films were characterized by x-ray photoelectron spectroscopy, Fourier transform infrared reflection absorption spectroscopy, and water contact angle measurements. Cell culture experiments show that cell adhesion and spreading of MG-63 osteoblasts were clearly reduced or nonexistent on these surfaces, also after 24 h of storage in the cell culture medium. In vivo data demonstrated that the local inflammatory tissue response for the PFP films deposited in MW and RF plasma were comparable to uncoated controls.

Mechanisms of low-temperature diffusion saturation of structural steel with nitrogen in the plasma of a low-pressure arc discharge

The structure and the phase composition of the diffusion saturation zone of 40Cr- and 12Cr2Ni4N-type steels subjected to nitriding a low-pressure gas discharge plasma have been studied by the methods of optical, scanning, and diffraction electron microscopy. An essential possibility of nitriding steel at low (~200°C) temperatures has been revealed. The structural-phase state of the nitrided layer has been shown to depend strongly on the morphology and phase composition of the initial material.

On the effect of ion-neutral collisions on dust grain screening in a low-pressure gas discharge plasma

The effect of ion-neutral collisions on charging of micrometer-sized dust grains immersed in a low-pressure argon discharge plasma is studied on the basis of the Vlasov-Bhatnagar-Gross-Krook kinetic equations. The equations are solved numerically using the method described in our previous work [I. L. Semenov et al., Phys. Plasmas 18, 103707 (2011)]. A modified version of the numerical method is proposed to reduce the required computational time. Numerical calculations are carried out for typical plasma parameters used in laboratory investigations of dusty plasma. On the basis of the obtained results, the influence of collisions on the ion flux and grain charge is analyzed. A comparison of our results with those obtained using different analytical models proposed earlier is presented. In addition, applicability of simple kinetic models describing the influence of collisions on the electric potential around a dust grain [S. A. Khrapak et al., Phys. Rev. Lett. 100, 225003 (2008); A. G. Zagorodny et al. Ukr. J. Phys. 54, 1089 (2009)] is examined. The influence of ion-neutral collisions on the distribution of plasma macroparameters near the grain surface is also demonstrated.

Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + Uc) (where U is voltage between the grid and the vacuum chamber and Uc is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.

10.1007/s11455-008-1022-x

Conditions for the synthesis of nanodimensional silicon islands on noncrystalline substrates in microwave low-pressure gas discharge plasma have been studied in the case of weak interactions at the deposit-substrate interface. It is established that the formation of silicon nanoislands proceeds via the overgrowth (healing) of depressions on the initial substrate surface. The effect of temperature on the kinetics of nanoisland growth and the possibilities of controlling the parameters of nanoisland morphology are determined.

Formation of a Boundary-Free Dust Cluster in a Low-Pressure Gas-Discharge Plasma

An attraction between negatively charged micron-sized plastic particles was observed in the bulk of a low-pressure gas-discharge plasma under microgravity conditions. This attraction had led to the formation of a boundary-free dust cluster, containing one big central particle with a radius of about 6 microm and about 30 1 microm-sized particles situated on a sphere with a radius of 190 microm and with the big particle in the center. The stability of this boundary-free dust cluster was possible due to its confinement by the plasma flux on the central dust particle.

Effect of temperature on the formation of silicon nanoislands on noncrystalline substrates in microwave low-pressure gas discharge plasma

Conditions for the synthesis of nanodimensional silicon islands on noncrystalline substrates in microwave low-pressure gas discharge plasma have been studied in the case of weak interactions at the deposit-substrate interface. It is established that the formation of silicon nanoislands proceeds via the overgrowth (healing) of depressions on the initial substrate surface. The effect of temperature on the kinetics of nanoisland growth and the possibilities of controlling the parameters of nanoisland morphology are determined.

Migration healing of surface relief during the formation of nanocrystals in a microwave low-pressure gas discharge

We have studied conditions for the synthesis of nanodimensional silicon islands (quantum dots) in a microwave low-pressure gas discharge plasma on noncrystalline substrates with a weak interaction at the deposit-substrate interface. It is established that the formation of nanoislands proceeds via the leveling (healing) of depressions on the substrate surface. A mechanism of the influence of the parameters of deposition on the kinetics of formation of nanoislands is proposed.

Effect of elastic interactions on the formation of silicon nanocrystals on noncrystalline substrates in microwave low-pressure gas discharge plasma

Conditions for the synthesis of nanodimensional silicon islands on noncrystalline substrates in a microwave low-pressure gas discharge plasma have been studied for various energies of elastic interactions at the deposit-substrate interface. It is established that the formation of nanoislands proceeds via the leveling (healing) of depressions on the substrate surface. The size of nanoislands grown under optimum conditions is determined by the composition and temperature of a substrate on which silicon is deposited.

Detailed comparison between probe density measurements of glow discharge in argon and in helium

In this article we shall look a bit more closely at some of the fundamental plasma parameters obtained by a cylindrical Langmuir probe within low-pressure electrical gas discharge plasma. The presented measurements were made in argon and in helium glow discharge plasmas. We are mainly concerned with the densities of the charged particles (electrons and ions) within the plasma and the effect of the discharge conditions upon them. The electron density is calculated from the electron current at the space potential and from the integration over the EEDF. The ion density is calculated by using the OML collisionless theory. The parameterization of Laframboise's numerical results is also used for the ion density calculation. In the range of our experimental conditions the results of plasma density, for both gases, tend to show that the ion densities measured with the OML and Laframboise theories exceeds the measured electron densities. The results also show that the plasma electron and ion densities increased with both discharge power and gas pressure.

Dust-acoustic wave instability at the diffuse edge of radio frequency inductive low-pressure gas discharge plasma

A spontaneous excitation of a grain density wave in a dusty cloud of monodisperse particles suspended at the diffuse edge of an rf inductive gas discharge has been discovered. The main physical parameters of this wave (phase velocity, wavelength, and growth rate) and of the background plasma (distributions of the electron density, electron temperature, and space potential) were measured. A theoretical model of the observable phenomenon based on the theory of dust acoustic waves in a collisional dusty plasma correlates well with the experimental data in a broad range of experimental conditions. The influence of a varying dust grain charge on the development of the observed dusty plasma instability has been analyzed. It is shown that the necessary condition for the instability excitation is the availability of a permanent electrical field (E0 ⩾3 V/cm) in the dusty cloud region.

Dust-acoustic instability in an inductive gas-discharge plasma

Spontaneous excitation of a dust-particle density wave is observed in a dust cloud levitating in the region of the diffused edge of an rf inductive low-pressure gas-discharge plasma. The main physical parameters of this wave and of the background plasma are measured. The analytic model proposed for the observed phenomenon is based on the theory of dust sound and successfully correlates with experimental data in a wide range of experimental conditions. The effect of variable charge of dust particles on the evolution of the observed dust-plasma instability is studied analytically. It is shown that the necessary condition for the development of the dust-acoustic instability is the presence of a dc electric field in the dust cloud region.

On the charge of dust particles in a low-pressure gas discharge plasma

Self-consistent molecular-dynamics calculations of the charge of micron-size particles in a low-pressure gas-discharge plasma are performed. It is shown that charge exchange of ions on neutrals starts to affect the charge of dust particles at pressures corresponding to ion mean free paths much greater than the Debye radius. The computational results show that the potential of a particle depends nonmonotonically on the pressure and on the particle size.

The fundamentals of plasma.

Various kinds of plasma are used for industrial applications. Among those, low pressure gas discharge plasma has been recognized to be a most elegant and efficient means for dry processing, such as treatment and deposition of material, etching, oxidation and polymerization.For this reason, special attention is directed toward low pressure gas discharge, which is driven by direct current (d.c.) or high frequency (h.f.) electric field. The plasma thus produced is a partially ionized one with low-temperature and low-density. Therefore, the properties of the discharge as well as the plasma are governed by various collision processes.In this note, the initation and sustainment of the d.c. and h.f. discharges are considered. Also, the positive ion sheath formed around the electrode and the positive column with charge neutrality are separately discussed. Further, detailed descriptions are given to elucidate peculiar phenomena appearing in the h.f. discharge. These include the trapping of the charged particles, the sheath potential as affected by the asymmetrical electrode area and the self-bias voltage.