N. Parkansky

Development and application of pulsed-air-arc deposition

Abstract Pulsed-air-arc deposition (PAAD) is a process of depositing coatings using a chain of high current short-duration pulsed electrical arcs to melt and evaporate material from a source anode and to transport it to workpiece which is held in close proximity. The workpiece serves as the cathode, and the electrical discharge action at its surface removes surface contaminants so that an adhesive coating forms. The short distance between the source electrode and the workpiece, together with the high pressure of the plasma jet emitted from the source anode, excludes air from the vicinity of the deposition and minimize oxidation. The required equipment is very compact, having a volume similar to a lunch box, and can be readily transported to the work site for field service. The source electrode can be manually held and, since the process can work in atmospheric air, no process chamber or vacuum system is required. PAAD has been used to apply hard carbide coatings manually to metal and wood cutting tools, increasing their lifetime by factors of 2–6. The lifetime of punches has similarly been increased by factors of 2–7. Machinery parts have been reconditioned and hardened, and anticorrosion and heat-resistant coatings have been applied to mechanical parts. The maximum coating thickness which can be applied, about 100 μm, is limited by residual tensile stress (RTS), which ultimately causes surface damage and material loss. The RTS increases with increasing deposition time, until a maximum is reached, after which surface damage and mass loss occur. Ductile and brittle materials exhibit different surface damage patterns and have different post-RTS maximum behaviours. The maximum coating thickness can be increased by a factor of 2 by periodically interrupting the deposition process and annealing. A compressive stress externally applied to the source electrode decreases the erosion rate, and conversely tensile stresses increase the erosion rate. An external tensile stress applied to the workpiece decreases the maximum coating thickness and conversely an externally applied compressive stress can increase the maximum coating thickness by a factor of 2.

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.

Anode mass loss during pulsed air arc deposition

Anode erosion during Pulsed Air Arc Deposition was investigated. The depositions were conducted with a series of pulses with a current amplitude of 300 A, a pulse duration of 150 μs, and a pulse repetition rate of 100 Hz. Slab anodes of pure Al, Ti, Cu, Fe, W, and WC based hard alloys were used to coat low carbon steel and Ti cathode-substrates. The influence of the anode material and deposition time on the anode mass loss were studied. The anode mass loss increased in the following order: W, Cu, WC based hard alloys, Ti, Al. A criterion was proposed to explain the material influence on the anode mass loss, based on heat flux required for the anode to reach either the melting or boiling temperature. The anode material order obtained by this criteria agrees with the experimental data.

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.

Influence of transverse current during In−O vapor deposition

Abstract The effect of electrical current flow parallel to the surface of growing In−O thin films was investigated. The films were produced by thermal vapor deposition of In−O powder in a vacuum of 1.3 mPa onto glass substrates at room temperature. Seven 15 × 1.5 mm2 samples were deposited on each substrate through a mask, and silver paint electrodes were applied to the end of each sample. A potential difference of 0 to 110 V d.c. was applied to the central sample during deposition, while the remaining six films had no voltage applied. The current flowing during the deposition was monitored using a shunt. X-ray diffraction studies showed that all films were amorphous. It was observed that for the film grown with an applied voltage of 110 V, the radius of the first coordination sphere is 3% shorter than for the films grown without voltage. Optical microscopy and SEM showed an increased proclivity for a net-like microstructure to form with increasing applied voltage, with a typical cell dimension of 10 μm. The electrical conductivity of the In−O films grew rapidly with the applied voltage, reaching an improvement factor of 7 in comparison with films deposited without a transverse current.

Corrosion resistance of Zn coatings produced by air arc deposition

Abstract Pulsed air arc deposition (PAAD) is a process of depositing coatings using a chain of high current, short duration, pulsed electrical arcs to melt and evaporate material from a source anode and transport it to the workpiece-cathode which is held in close proximity. The depositions were conducted with a series of pulses with current magnitudes of 150 or 500 A, a pulse duration of 150 us, and a pulse repetition rate of 100 Hz. Zinc anodes of 2 and 5 mm diameters were employed. Coatings of Zn were applied to 18×18×5mm low carbon (0.2%) steel substrates. The influence of discharge energy and anode diameter on the coating process and the corrosion resistance of the Zn coating in a 3% aqueous NaCl solution were investigated. The coating thickness increased with increasing discharge energy and decreasing anode diameter. Coatings of 7 and 15 μm reduced the corrosion mass loss rate by factors of 4.2 and 8.5, respectively. First signs of corrosion were visible on uncoated samples after 1 d immersion in 3% aqueous NaCl solution. Application of a coat of Zn-containing spray paint extended the period to 4 d, while application of a 15 μm PAAD coating plus spray paint extended the period to 8 d. Salt fog testing showed that the PAAD coating had a higher corrosion resistance than a Zn spray paint coating. X-Ray diffraction studies showed the presence of metallic Zn, ZnO and the Fe3Zn10 intermetallic phase in the formed coatings.

Improvement of thin film semiconductor conductivities using a transverse current during deposition

Abstract The effect of electrical current flow parallel to the surface of a growing semiconductor thin film on the electrical conductivity and microstructure of the deposition was investigated. The depositions were produced by filtered vacuum arc deposition. Conducting Sn-O coatings were produced using a 160 A arc on the Sn cathode, with a background O2 pressure of 0.8 Pa, while B-doped amorphous Si depositions were produced with 20 A arcs on a B-doped Si cathode. The Sn or Si plasma was extracted through a 122 mm inner diameter annular anode and passed through a 160 mm minor diameter, 600 mm major diameter, quarter-torus magnetic filter with a 14 mT magnetic field in order to remove macroparticles. The thin films were deposited on glass microscope slide substrates held at room temperature. Prior to deposition, silver paint electrodes were applied to the substrate in the form of two parallel bands, separated by 20 mm. A potential difference of 0–36 V was applied between the electrodes during the deposition process, and the current flowing along the deposition could be monitored via a shunt resistor. Transparent Sn-O conductivity increased as a function of the applied voltage, reaching an improvement factor of 9 in comparison with films deposited without an imposed transverse current. Scanning electron microscopy examination of the coating surface revealed the presence of numerous round structures. When no current was rejected during film growth, these structures were arranged randomly on the surface, whereas the structures tended to be aligned in long strings parallel to the current field lines when a current was injected during deposition. X-ray diffraction patterns obtained with the sample aligned with the current injection direction parallel and perpedicular to the X-ray beam reveal distinct differences indicating an anisotropy in the microstructure. Preliminary experiments on B-doped Si films similarly showed an improvement by a factor of 2 in the conductivity with the imposition of a transverse potential difference of 36 V.

Anode erosion during pulsed arcing

An experimental study of the anode erosion rates of Cu, Zr, Ti, Mo, Ta, and W is presented under conditions similar to those used for electrodischarge coating. The arcs are conducted between a small anode and a larger cathode in air with pressures ranging from 10/sup -4/ to 10/sup 3/ torr. Unipolar arc pulses of 200-400-A peak current and 0.1-ms duration are produced at a 100-Hz pulse repetition rate by an RC circuit. For most materials, the electrode mass loss is primarily from the anode, and the mass loss is independent of pressure for pressures less than 0.1 torr, decreases steeply with increasing pressures in the range 0.1 to 10 torr, and decreases more gradually with increasing pressure above 10 torr. The experimental results are explained by using a limiting case of the integral conservation laws. In the low-pressure region the input energy is expended mainly in the acceleration of the metal vapor, and thus the erosion rate is independent of pressure. In the intermediate-pressure region the metal vapor jet is braked by its interaction with the surrounding gas. In the high-pressure region the vapor jet is completely halted, and vapor transport takes place only by diffusion through the surrounding gas. >

Pulsed Submerged Arc Plasma Disinfection of Water: Bacteriological Results and an Exploration of Possible Mechanisms

The pulsed submerged arc is a high-current electrical discharge between two electrodes in a liquid, in which the electrical current is conducted via a plasma bubble consisting of vaporized and partially ionized liquid and electrode material. The submerged arc discharge has the potential to kill harmful pathogens by a combination of several mechanisms. In this preliminary investigation, the ability of the pulsed arc to sterilize water was tested, hydroxyl radical (OH·) production and the radiation spectrum were measured, and shock wave production was estimated.