Pulsed Hollow Cathode Research Articles

Numerical Analysis of Thermal Flow Dynamics of Arc Plasma and Molten Pool in Hollow Cathode Arc Welding with Oxygen Content

The mechanism of the pulsed hollow cathode arc welding (HCAW) process was revealed using a fully coupled model with a hollow cathode. We solved the governing equations with the Marangoni effect to study the dynamic behaviors of a molten pool with a square pulsing current (200~400 A, 900 Hz) and varying O2 content; the dynamics of the arc plasma and the weld pool in the HCAW process were investigated quantitatively. The results show that the intensity of the arc plasma was more significantly weakened by the design of the hollow cathode in HCAW than that in GTAW with a solid hollow cathode. We could obtain a stable molten pool even with a large pulsing current section (200 A–400 A) at higher frequencies. The flow dynamics of the molten pool were mainly dominated by the Marangoni effect with varying oxygen content, and we could promote penetration by increasing O2 content in HCAW.

Characteristics of a pulsed hollow cathode discharge operated in an Ar+O[formula omitted] gas mixture and deposition of copper nickel oxide thin films

A hollow cathode discharge with a CuNi (Cu50Ni50) cathode is operated inside a vacuum chamber with Ar gas flowing through its nozzle. O2 gas is admitted to the vaccum chamber. Typical Ar+O2 gas pressures are in the range of 2–50 Pa. The energy distribution of plasma ions is investigated with the help of energy-resolved mass spectrometry. Singly charged Ar+ and molecular O2+ ions are the most abundant ionic species. Deposition rate and heat flux to a substrate increase as function of discharge current. At high pressures, the deposition rate is further increased by the directional gas flow, which becomes more focused onto the substrate. Deposited and annealed thin films are analysed by X-ray diffraction and Raman spectroscopy. As-deposited films are composed of a mixed CuxNi1−xO cubic phase with a preferred (111) orientation. Upon annealing at 600 oC, the mixed CuxNi1−xO phase separates into two sub-phases composed of NiO and CuO. Annealed films display a photoelectrochemical (PEC) activity as a photocathode. The PEC activity deteriorates with time, however. This behaviour is related to the reduction of CuO to Cu2O.

Effect of the energy of hydrocarbon ions on diamond-like carbon films deposited on alumina microparticles through repeated pulsed discharge in hollow cathode with methane gas

Diamond-like carbon (DLC)-modified alumina microparticles are expected to be an ideal filler that can greatly improve the thermal conductivity of epoxy-alumina composites with a high fill factor, while the composites can still maintain high dielectric properties. Plasma-enhanced chemical vapor deposition (PECVD) has been mostly used to prepare DLC film on bulk material, and the ion energy in the plasma shows a crucial influence on the properties of the DLC. However, the properties of the DLC on solid microparticles prepared through the PECVD method as well as the effect of the ion energy are still unclear. In this paper, DLC on alumina microparticles has been prepared through a highly efficient method of repeated pulsed hollow cathode discharge in methane gas, and the characteristics of the DLC on the alumina particles have been studied and analyzed. The morphology and bond composition of DLC on particles have been studied through electron microscopy, Raman spectrometer and x-ray photoelectron spectroscopy. The ion energy distribution in the discharge plasma was diagnosed and used to analyze its effect on the hybrid bond content of the DLC through correlation analysis. The correlation of the ion energies and the hybrid bond content of the DLC shows that hydrocarbon ions in the discharge plasma with an energy of 100–200 eV present the most benefit to the formation of the sp3 hybrid bond content of DLC film on alumina microparticles. In addition, hydrogen ions with an energy of 700–1000 eV are also beneficial to improve the sp3 hybrid bond content of the DLC film.

Simulation of the spatio-temporal evolution of the electron energy distribution function in a pulsed hollow-cathode discharge

This article presents the 2D simulation results of a nanosecond pulsed hollow cathode discharge obtained through a combination of fluid and kinetic models. The spatio-temporal evolution of the electron energy distribution function (EEDF) of the plasma column and electrical characteristics of the nanosecond pulsed hollow cathode discharge at a gas pressure of 5 Torr are studied. The results show that the discharge development starts with the formation of an ionization front at the anode surface. The ionization front splits into two parts in the cathode cavity while propagating along its lateral surfaces. The ionization front formation leads to an increase in the fast isotropic EEDF component at its front, as well as in the anisotropic EEDF component. The accelerated electrons enter the cathode cavity, which significantly contributes to the formation of the high-energy EEDF component and EEDF anisotropy.

Generation of Plasma with Increased Ionization Degree in a Pulsed High-Current Low-Pressure Hollow Cathode Discharge

The paper presents the results of research of the processes of generation of pulsed beam-plasma formations in a high-current non-self-sustained glow discharge with a hollow cathode at low (0.025–0.25 Pa) pressures. In the hollow cathode volume of 0.34 m3, the glow discharge pulsed currents up to 800 A were obtained at injected electron currents up to 150 A, discharge burning voltages up to 400 V, and a pulse duration of 1 ms. The plasma with a density of about 1012 cm-3 was formed with ionization degree up to 16%. It has been shown that the electron free path length λC between the Coulomb interactions in the pressure range 0.025–0.05 Pa is by a factor of about 3.5 less than the electron free path length λe when interacting with neutrals at high (10–15%) degree of plasma ionization. The measured saturation electron current density on the single electric probe was 3.7 (0.025 Pa), 5 (0.05 Pa), and 6.7 A/cm2 (0.25 Pa). Relatively homogeneous beam-plasma formations generated in the glow discharge are promising for the formation of intensive pulsed electron beams of large cross section.

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

Glass-like carbon films grown by pulsed hollow cathode via plasma immersion ion implantation and deposition

Plasma Immersion Ion Implantation and Deposition (PIII&D) systems based on pulsed metal tubes are a very promising technique to deposit diamond-like carbon and correlated thin films. A hollow cathode plasma can be appropriately used to locally deposit glass-like carbon films on small substrates by means of PIII&D. The morphology, microstructure, roughness as well as the investigation of the chemical structure of the as-deposited graphitic films outside the pulsed tube are reported. By the described PIII&D setup, the power of the discharge plasma can be properly adjusted to produce a range of disordered carbon films. But their structures are not so extensively changed since high temperatures (around 900 °C) are inherent in these conditions. Furthermore, there is a minimum power required to sustain the hollow cathode discharge when small tubes are used. Moreover, an additional effect caused by the intense bombardment of secondary electrons outside the tube can account for the growing of more ordered and rougher graphitic carbon coatings. We have then found that those nanostructured glass-like carbon films are strongly dependent on the power of the produced plasma that impinges on the substrate surface increasing its temperature.

Growth of semi-coherent Ni and NiO dual-phase nanoparticles using hollow cathode sputtering

Anisotropic heterogenous Ni/NiO nanoparticles with controlled compositions are grown using a high-power pulsed hollow cathode process. These novel particles can be tuned to consist of single-phase Ni via two-phase Ni/NiO to fully oxidized NiO, with a size range of 5–25 nm for individual crystals. A novelty of this approach is the ability to assemble multiple particles of Ni and NiO into a single complex structure, increasing the Ni-NiO interface density. This type of particle growth is not seen before and is explained to be due to the fact that the process operates in a single-step approach, where both Ni and O can arrive at the formed nanoparticle nuclei and aid in the continuous particle growth. The finished particle will then be a consequence of the initially formed crystal, as well as the arrival rate ratio of the two species. These particles hold great potential for applications in fields, such as electro- and photocatalysis, where the ability to control the level of oxidation and/or interface density is of great importance.Graphical abstract

Distinctive features of the high-current glow discharge with the hollow cathode at low gas pressures

A pulsed hollow cathode glow discharge with current up to 200 A, with the pulse width amounting to a few milliseconds, is studied in a wide range of the gas pressure p from 0.01 to 10 Pa. The high current discharge is established due to the application of a high-voltage pulse with amplitude Uo up to 5 kV between the anode and the hollow cathode of a continuous discharge with a current of Io = 0.5 A. The establishing time τ ranges from 10 to 300 μs, diminishes with the pulse amplitude growth, increases in the gas pressure, and decreases in the output orifice area of the hollow cathode. At Uo > 2.5 kV, an abnormal mode of the discharge establishing process is observed with intensive oscillations of the discharge current and voltage in the frequency range from 1 to 10 MHz. In the abnormal mode, the high current establishing time τ can be reduced by hundred times down to τ ∼ 1 μs. At a low gas pressure p < 0.1 Pa, the high current can be reached only in the abnormal mode. Without oscillations at the pulse front, only low-current high-voltage discharge is established.

Experimental investigations of enhanced glow based on a pulsed hollow-cathode discharge

In this work, the pulsed hollow cathode discharges at low pressure argon with an axial magnetic field were studied. The results indicate that the pulsed discharge is operated in an enhanced glow (EG) mode. Under the same conditions, the discharge current of the pulsed discharge is two or three orders higher than that of the direct current discharge. The spatial and temporal evolution of the light emission shows that, the current fluctuation at the rising edge of the pulse plays an important role for the EG discharge of pulsed hollow cathode, which forms a high-density, high-current and long-distance plasma column outside the cavity.

Study of plasma dynamics in a pulsed hollow cathode magnetron

Study of the hollow cathode magnetron (HCM) discharge, which is supplied by a source of powerful current pulses, was carried out. By means of probe and spectral measurements, the dynamics of a pulsed discharge was studied. The results show that a high density plasma (ne > 1012 cm-3) is generated at a distance of 20 cm from the HCM. Based on the data obtained, a mechanism for the formation and development of a discharge was proposed.

Structure and properties of titanium after nitriding in a plasma of pulsed hollow cathode glow discharge

The paper reports on a study of the structure, phase composition and mechanical characteristics of a modified surface layer of VT1-0 titanium after nitriding in the plasma of pulsed and stationary non-self-sustained glow discharge. It was revealed that after nitriding in a pulsed glow discharge, the volume fraction of nitride phase (Ti2N, TiN) in the surface layer of VT1-0 titanium is higher than that in the nitrited layer obtained by exposure to a plasma of steady glow discharge at the same nitriding temperatures, average ion current density at the surface and ion energy. As a result, the hardness of the surface layer of samples obtained by nitriding in pulsed discharge mode exceeds that of samples treated in steady glow discharge mode. Wear resistance of titanium after nitriding in the plasma of a pulsed glow discharge is approximately four times higher than that of titanium without treatment and 1.5 times higher than that achieved by nitriding in a steady glow discharge plasma.

Nucleation of titanium nanoparticles in an oxygen-starved environment. I: experiments

A constant supply of oxygen has been assumed to be necessary for the growth of titanium nanoparticles by sputtering. This oxygen supply can arise from a high background pressure in the vacuum system or from a purposely supplied gas. The supply of oxygen makes it difficult to grow metallic nanoparticles of titanium and can cause process problems by reacting with the target. We here report that growth of titanium nanoparticles in the metallic hexagonal titanium (αTi) phase is possible using a pulsed hollow cathode sputter plasma and adding a high partial pressure of helium to the process instead of trace amounts of oxygen. The helium cools the process gas in which the nanoparticles nucleate. This is important both for the first dimer formation and the continued growth to a thermodynamically stable size. The parameter region, inside which the synthesis of nanoparticles is possible, is mapped out experimentally and the theory of the physical processes behind this process window is outlined. A pressure limit below which no nanoparticles were produced was found at 200 Pa, and could be attributed to a low dimer formation rate, mainly caused by a more rapid dilution of the growth material. Nanoparticle production also disappeared at argon gas flows above 25 sccm. In this case, the main reason was identified as a gas temperature increase within the nucleation zone, giving a too high evaporation rate from nanoparticles (clusters) in the stage of growth from dimers to stable nuclei. These two mechanisms are in depth explored in a companion paper. A process stability limit was also found at low argon gas partial pressures, and could be attributed to a transition from a hollow cathode discharge to a glow discharge.

Overcoming sheaths overlapping in a small diameter metallic tube with one end closed and using a high density plasma from a high power pulsed hollow cathode discharge

High voltage sheaths are formed when plasmas are produced by application of high negative voltage pulses to conductive supports or components, as in Plasma Immersion Ion Implantation (PIII) treatments of materials surfaces. For parts with concave shape, as inside metal tubes, these sheaths behave quite differently according to the tube configuration and size, as well as, PIII treatment pressure of operation and pulsing parameters. In this work, an SS304 tube of 1.1 cm internal diameter and 20 cm length was pulsed typically at -0.5 to -2.6 kV, 20 μs pulse length, 500 Hz repetition rate, nitrogen pressure of 5x10-2 mbar and with one side closed configuration. Different currents (between 10 and 30 A) were used to produce plasmas with sheaths that overlapped or not, depending on the currents used. To study these sheath behaviors, a simple plasma diagnostic technique based on a bi-dimensional mapping of the deposition of sputtered materials and by etching via the plasma on a Si wafer target surface, both coming out from the tube, was used. This mapping showed clearly the border line situation between overlapping and non-overlapping sheaths in that small tube which allowed to estimate the plasma density to be around 1011 cm-3 at such a sheath condition, as previously anticipated by Sheridan. Above that border condition, nitrogen PIII was successfully obtained in such a small tube of SS304, producing TiN and Ti2N in samples of Ti6Al4V placed inside the tube, when temperatures higher than 800°C were reached there. Below the border, no significant uptake of nitrogen was possible. Using this type of experimental set-up, it is now possible to explore different hollow cathode behaviors, efficient or high temperature (above 800°C) PIII conditions and also new utilizations of the plasma ejected from the tube.

Influence of pressure on the morphology and structure of surfaces sintered in pulsed DC annular hollow cathode discharge

Modification of the morphology and structure of surfaces sintered in pulsed DC annular hollow cathode discharge (AHCD) at high temperature, subjected to continuous sputtering, was investigated for different gas pressures. The studied AHCD cylindrical surfaces concentrically placed each other comprised the surface of a pressed (green) iron powder sample (bcc, α-Fe structure), at the central cathode, and the internal surface of an austenitic stainless steel tubular part (fcc, γ-Fe structure), at the external cathode. In this case, metal atoms sputtered from the central cathode are condensed, clustered and sintered (in solid state) at the external cathode internal surface, and atoms sputtered from the external cathode tend to undergo the same events in the opposite direction. Sintering was performed at 1150 °C, for 60 min, at 1, 3, 6, and 9 Torr (133, 400, 800 and 1200 Pa, respectively). Results show that the pressure is the key parameter to control the modification of the AHCD processed surface characteristics subjected to the sintering treatment. Sputtering is strongly decreased at high pressures (6–9 Torr) keeping the original-surface structural and morphological characteristics of both the cathodes practically unaltered. On the other hand, significant changes on the surface characteristics of both the cathodes were observed for smaller pressures (1–3 Torr). The most important is that the austenite (γ-Fe) phase initially present in the AISI 310 austenitic stainless steel gives place to a deposited growing layer, mainly constituted by Fe atoms, and thus by the ferrite (α-Fe) phase, in the steel cathode internal surface, associated with a morphological modification from an apparently plasma etched (austenitic) to a rough (ferritic) surface, as the pressure is changed from 9 to 1 Torr.

Investigation of the antimicrobial properties of modified multilayer diamond-like carbon coatings on 316 stainless steel

Modified diamond-like carbon (DLC) coatings were deposited onto 25mm diameter 316 stainless steel discs by pulsed (direct current) hollow cathode plasma enhanced chemical vapour deposition (HC-PECVD). Multilayer films of total thickness 1–2μm were deposited, both with and without germanium dopant. Characterisation of the coatings was performed by SEM/EDX, surface energy/contact angle analysis, and assessment of possible biofilm-inhibiting properties. Both modified DLC and germanium-doped DLC (Ge-DLC) coatings showed a significant anti-biofouling effect on P. aeruginosa, a Gram-negative bacterium. A 90% reduction in P. aeruginosa biomass was observed compared to control for both DLC and Ge-DLC, however this effect could not be attributed to germanium incorporation alone. Neither modified DLC nor Ge-DLC showed a significant inhibitory effect on S. aureus, a Gram-positive bacterium. Scanning electron microscopy of P. aeruginosa biofilms on Ge-DLC coated 316 stainless steel clearly displayed disruption of the cellular wall, as well as leakage of cellular components; this effect was not observed with modified DLC coating. This suggests that germanium-doped DLC coatings may potentially exhibit a cidal mode of action versus P. aeruginosa biofilms.

Properties of Copper nitride thin films deposited using a pulsed hollow cathode discharge

Copper nitride (Cu3N) thin films were deposited on a glass substrates using pulsed hollow cathode discharge (PHCD). The deposition was performed at 4kV charging voltage, and nitrogen gas pressure of 10-2 torr. The structure of the thin films was identified by X-Ray diffraction (XRD) technique. The XRD measurements indicated that the thin films have a nanocrystalline nature and exhibit orientation at (111) phase of Cu3N. The grain size of the nanocrystalline films ranged from 41 nm to 80 nm. The optical band gaps were measured using UV–Vis–NIR spectrophotometer and Touc's equation. The optical band gaps of the films decreased from 2.55 eV to 2.25 eV by increasing the number of deposition shots from 20 to 80 shots. The surface morphology was studied by scanning electron microscopy (SEM). The SEM image indicates that the film have facetted surface morphology packed particles.Â

Nanoparticle growth by collection of ions: orbital motion limited theory and collision-enhanced collection

The growth of nanoparticles in plasma is modeled for situations where the growth is mainly due to the collection of ions of the growth material. The model is based on the classical orbit motion limited (OML) theory with the addition of a collision-enhanced collection (CEC) of ions. The limits for this type of model are assessed with respect to three processes that are not included: evaporation of the growth material, electron field emission, and thermionic emission of electrons. It is found that both evaporation and thermionic emission can be disregarded below a temperature that depends on the nanoparticle material and on the plasma parameters; for copper in our high-density plasma this limit is about K. Electron field emission can be disregarded above a critical nanoparticle radius, in our case around nm. The model is benchmarked, with good agreement, to the growth of copper nanoparticles from a radius of nm– nm in a pulsed power hollow cathode discharge. Ion collection by collisions contributes with approximately 10% of the total current to particle growth, in spite of the fact that the collision mean free path is four orders of magnitude longer than the nanoparticle radius.

Ionized sputtering with a pulsed hollow cathode magnetron

A hollow cathode magnetron has been operated with a HiPIMS power supply at duty cycles around 1%, and the non-reactive sputtering of several target materials such as copper, aluminum and carbon has been investigated. Systematically varying pulse length, frequency, discharge voltage, pressure and geometry of the magnetic field, waveforms of discharge voltage and current have been recorded. Beyond a material-specific voltage level, the pulse current always shows a runaway behavior stopped only at current densities above 10A/cm2 due to the limits of the power supply. Furthermore, the plasma has been characterized by time-averaged and time-resolved optical emission spectroscopy as well as by energy-resolved ion mass spectroscopy. The optical spectra showed the contribution of self-sputtering and the portion of metal ions rising with elapsing time during the pulse. Measurement of ion energy distribution functions of different species mostly revealed a low (below 30eV) and a high (around 100eV) energy part. Moreover, the portion of singly and doubly charged target ions is increased with the pulsed discharge current. Deposited layers show a dense, fine-grained structure, indicating a high degree of ionization of the layer-forming particles.

Deposition of hematite Fe2O3 thin film by DC pulsed magnetron and DC pulsed hollow cathode sputtering system

Semiconducting hematite Fe2O3 thin films were fabricated by means of reactive sputtering in a high power DC pulsed magnetron (HIPIMS) and in a DC pulsed hollow cathode plasma jet sputtering system. Fused silica slides (quartz) were used as substrates. Both plasma processes were monitored with the help of a quartz crystal monitor (QCM) that was also fitted with magnetic field electron suppression filter and biased collecting electrode. This set up measured the deposition rate and the ratio of ionized to neutral fluxes of depositing particles on the substrate during the coating process. The deposition methods were compared in terms of the properties of produced films such as crystalline structure, optical absorption and surface topography. The as-deposited hematite Fe2O3 thin films (without annealing) and after their thermal treatment (with annealing) were examined.