B. Alterkop

Recent progress in filtered vacuum arc deposition

Abstract During this decade significant advances have been made both in the understanding and implementation of filtered vacuum are deposition. Rigid rotor models have been analyzed statistically, and new models which treat the mutual influence of the electrons and ions on each other self-consistently, take into account the centrifugal force on the ions, and take into consideration collisions, have been formulated. It was shown that the plasma transport efficiency is limited by drifts caused by the centrifugal force and by the electric field generated by charge separation in the plasma. For a range of magnetic fields strengths for which the ions are not magnetized, i.e., confined to a Larmor radius less than the duct radius, the transport efficiency for Cu plasma is about 10%, and depends only weakly on the magnetic field strength. Increased transmission is found when the ions are magnetized, reaching about 50% for a 36–60 mT field in typical configurations. The plasma transport efficiency and spatial distribution has been measured over a large parameter range, and correlated with the various theories. The plasma beam may be approximated as a Gaussian distribution which is displaced in the B × G direction, where G is in the direction of the centrifugal force, while a displacement in the plane of symmetry is surprisingly found in the − G direction. The total convected ion current decreases exponentially with distance from the toroidal filter entrance. Macroparticle transport within the magnetic filter has been analyzed, and it has been shown that electrostatic reflection from the walls can occur if the magnetic field is weak. Filtered arc sources with improved throughput performance and novel geometries have been built, and are now available commercially. The range of coatings deposited with FVAD has been expanded to include metals, oxides, and nitrides, as well as diamond-like carbon. In several cases, coatings having the highest quality reported in the literature have been fabricated with the FVAD technique, and one commercial application has been reported.

Vacuum arc plasma jet propagation in a toroidal duct

A two fluid magneto‐hydrodynamic theory of vacuum arc plasma jet propagation in a magnetized toroidal duct is developed. The physical mechanisms of jet transverse displacement and plasma losses are analyzed and the centrifugal force on the ions is shown to play the principle role in these processes. Optimal conditions for jet propagation occur when the centrifugal force is balanced by the electrical force on the ions. An analytical solution of the nonlinear problem of plasma beam transport through a toroidal duct is obtained for the two cases where ions are magnetized or not magnetized. The ion mass current decreases with the azimuthal distance along the torus as (1+φ/φ0)−1 where φ0 is a characteristic angular distance, for the case when ions are magnetized, and exponentially when the ions are not magnetized. Numerical calculations show that the decrease of plasma density leads to a longitudinal electric field and current. This current, together with the current due to the centrifugal drift, form a curren...

On a sheath between a plasma and a conducting surface

The influence of plasma inertia and temperature and wall polarity on the formation of a stationary charged sheath between the plasma and a conducting wall was considered. A criterion for stationary sheath existence for both negative and positive wall polarities was formulated and applied to several special cases. It was shown, in particular, that a sheath is not formed in a two-beam plasma with cold electrons and cold or warm supersonic ions. The formation of a discontinuity-like a shock wave at a thermal singularity was analyzed. Consideration of special cases showed that conditions for the steady sheath are compatible with the condition for shock wave formation (supersonic ions) only in the case of positive wall potential, hot electrons and warm ions, moving to the wall with velocity greater then the ion-sound velocity. The determined maximum wall positive potential at which no discontinuity forms is φc=[ZTe−Ti ln(1+(ZTe/Ti))]/2Ze, where Te is the electron temperature, and Ti and Ze are the ion temperat...

Influence of gas pressure on the ion current and its distribution in a filtered vacuum arc deposition system

Abstract A filtered vacuum arc deposition system consisted of a cathode, an annular anode, a quarter torus duct macroparticle filter, and a deposition chamber. Arcs were sustained on a Ti cathode in the presence of noble background gases helium and argon, and reactive gases nitrogen and oxygen. The gas pressure was continuously varied from 3 × 10 −5 Torr (4 mPa) to 0.1 Torr (13.3 Pa). A toroidal magnetic field of up to 20 mT, and a straight field in the deposition chamber of up to 10 mT were imposed for plasma guiding. The total saturation ion current was measured with a 130 mm diameter probe. The ion current density distribution was measured with a nine-segment multi-probe, and the individual probe element currents were fitted to a two-dimensional Gaussian distribution. It was shown that in the presence of a noble gas the total saturation ion current at first increases with increasing the background gas pressure, then achieves a maximum at a pressure of 10 mTorr (1.3 Pa) for He, and at 2 mTorr (0.26 Pa) for Ar, where its value is 1.4–2 times greater than in vacuum. With further increase in the pressure the ion current strongly decreases. In the presence of reactive gases this maximum is not observed, and the total ion current strongly decreases at pressures greater than 2–3 mTorr (0.26–0.4 Pa). In contrast to this, a maximum is observed in the ion current collected on small diameter individual probes of multi-probe, positioned towards the direction of the plasma beam displacement. With increasing gas pressure, the distribution width decreases, and a displacement of the beam center is observed both in the − g direction and in the ( B × g ) direction, where B and g are vectors of the toroidal field and centrifugal acceleration, respectively. The present results show that proper substrate positioning in the deposition chamber must take into account the beam displacement due to the background gas.

The numerical calculation of plasma beam propagation in a toroidal duct with magnetized electrons and unmagnetized ions

Electron-magnetized vacuum arc plasma transport in a magnetic toroidal duct is calculated numerically taking in account electron - ion collisions, electron and ion temperatures, and the high conductivity of the duct wall. The longitudinal magnetic field in the duct, the fully ionized plasma density and the electric potential distribution at the torus entrance are given, while the plasma density, electrical field and current, and macroscopic plasma velocity across the magnetic field inside the duct are calculated. Toroidal coordinates are used to describe plasma beam propagation. A Runge - Kutta routine is used for the calculations along the torus while a finite difference method is used across the torus cross section. It is found that plasma loss due to particle flux to the duct wall depends on the electron and ion temperatures and the plasma density distribution at the torus entrance cross section. With an electron temperature of , 30 000 K and 50 000 K, an ion temperature and a Gaussian distribution of plasma density at the torus entrance with a maximum value , we found that the duct efficiency was less than 10% for longitudinal magnetic field strengths of 10 mT and 20 mT. In the case where only the electrons are magnetized, filter efficiency depends only weakly on the magnetic field strength, on , and on .

Single-pulse arc production of carbon nanotubes in ambient air

Multi-wall nanotubes (MWNTs) of carbon were produced by pulsed arc discharges between a room temperature sample and a counter-electrode, with peak currents of 7–100 A, and pulse lengths of 0.2–26 µs, in open air at selected locations on the sample. The samples were 10 × 10 mm2 graphite plates, carbon-coated 200 mesh copper grids, and Ni-coated glass slides. The counter-electrodes were graphite in the form of 1 × 4 mm2 bars or 4 mm diameter rods with a cone tip of 28°, or 0.1 mm diameter steel rods. Randomly oriented MWNTs (typically 5–15 walls) with a diameter of ~ 10 nm and lengths of up to 3 µm were produced on the samples with a single 0.2 µs pulse, implying linear growth rates of up to 15 m s−1. MWNTs were produced with both polarities and with all types of counter-electrodes used when the substrate contained carbon. Near vertically oriented MWNTs were deposited on the Ni/glass samples using a graphite counter-electrode. The simplicity, rapidity and selectivity of the process may facilitate wider study and practical application.

Effect of air annealing on opto-electrical properties of amorphous tin oxide films

Amorphous tin oxide films, 100?800?nm thick and of resistivity ~6?8?m??cm, were deposited on glass substrates using a filtered vacuum arc with an oxygen background gas pressure of 4.0?mTorr. The films were annealed in air at a temperature of 300?C for 1, 3, 5, 7, and 10?min. Film morphology, structure, composition, roughness, and light transmission were determined before and after the annealing, on cold samples, with atomic force microscopy, x-ray diffraction diagnostics, x-ray photoelectron spectroscopy, and light transmission meter. The roughness depended weakly on the annealing time, and decreased with the thickness of the film. The film transmission in the visible region was practically independent of the annealing time. Film conductivity increased with the annealing time, reaching a maximum value after 3?7?min, larger by a factor of 2.0?2.9 than that measured before annealing. The oxygen to tin density ratio on the film surface decreased relative to its value before annealing and reached a minimum after annealing for 7?min. After annealing for 10?min, the O/Sn ratio increased relative to the minimum value but was lower than the ratio before annealing. The O/Sn ratio in the bulk decreased monotonically for annealing times longer than 1?min. The film conductivity before and after annealing depended linearly on the film thickness. A model is proposed to elucidate the dependence of the conductivity on the annealing time and on the film thickness.

Propagation of a magnetized plasma beam in a toroidal filter

A two-fluid magneto-hydrodynamic model for the motion of a vacuum-arc-produced magnetized plasma beam in a toroidal magnetic filter is presented. The model takes into account in a self-consistent way electron-ion collisions and electrical, magnetic, centrifugal and pressure forces. An analytical solution is obtained that describes the distribution of the plasma density, the electron and ion velocities, the electric field and the current in the plasma. Analytical expressions for the filter efficiency as a function of the toroidal magnetic field are also derived. The effect of the centrifugal and diamagnetic ion drifts on the polarization electric field is studied. It is shown that the efficiency increases exponentially when the polarization field decreases. A decrease of the polarization field can take place when a current path outside the plasma short-circuits the electrical currents generated by the magnetic field in the plasma. When there is no polarization electric field, the filter efficiency increases with the magnetic field as where , is the ion mass, is the total input ion flux, is the transverse plasma conductivity and R and are the major and minor radii of the torus, respectively.

Influence of a parallel electric field on the conductivity of a growing indium oxide film

Abstract Thin films of amorphous InO were produced by thermal vapor deposition of InO powder in vacuum (P = 10−5 Torr) at room temperature onto glass substrates. A potential difference in the range of 0–110 V DC was applied to the sample during deposition. The effect of the applied voltage on the film conductivity was monitored by the current through the film. The film mass increase was linearly proportional to the deposition time (5–6 min). A non-linear increase of the current of several orders of magnitude was observed. The film resistance was measured in the range 100−108 Ω. To explain the experimental data it was hypothesized that when the voltage across the sample is applied tunneling and percolation are responsible for the growing film conductivity. The variation of the characteristic parameters of tunneling and percolative conductance with the applied voltage were derived by fitting the dependence of the current on the voltage with analytical expressions corresponding to tunneling conductance at the initial phase of the coating process, followed by percolative conductance. It was found that the transition from tunneling conductance to percolative conductance takes place at earlier times as a function of the applied voltage, and that the critical percolation exponent increases with the applied voltage.

Propagation of vacuum arc plasma beam in a toroidal filter

An analytical solution to the problem of plasma beam transport in a toroidal magnetic filter for unmagnetized ions is derived. A two-fluid model taking into account electromagnetic and pressure forces, electron-ion collisions, magnetic force line curvature, and radial dependence of centrifugal force is used. From comparison with experimental data it is shown that the obtained solution describes well the main properties of plasma beam behavior in the filter, e.g. (1) the relative value of the ion current along the torus decreases exponentially, (2) the deflection of the plasma beam from the center of the torus correlates with the centrifugal drift of the plasma beam across a magnetic field, and (3) experiment and theory agree well on the weak correlation between magnetic field strength and filter efficiency.