Unipolar Pulsed Dc Research Articles

Contrasting the characteristics of atmospheric pressure plasma jets operated with single and double dielectric material: physicochemical characteristics and application to bacterial killing

This study reports an experimental comparison of two types of atmospheric pressure plasma jets in terms of their fundamental plasma characteristics and efficacy in bacterial sterilization. The plasma jets are fabricated by inserting a high voltage electrode inside a one-end closed (double DBD plasma jet) or both ends open (single DBD plasma jet) quartz tubes which are further enclosed inside a second quartz tube containing a ground electrode. Both plasma jets are operated in contact with water surface by using a unipolar pulsed DC power supply with helium as the working gas. Results from electrical and time-resolved imaging show that the single DBD configuration induces 3–4 times higher accumulation of charges onto the water surface with significantly faster propagation of plasma bullets. These results are accompanied by the higher discharge intensity as well as stronger emissions from short-lived reactive species which were analyzed through optical emission spectroscopy at the plasma-water interface. The rotational temperature for the single DBD configuration was observed to be higher making it unsafe for direct treatments of sensitive biological targets. These characteristics of the single DBD configuration result in the production of more than two times higher concentration of H2O2 in plasma activated water. Shielding of the HV electrode reduces the plasma potential which in turn reduces the electric field & electron energy at the plasma-water interface. The reduced electric field for the double DBD configuration was lower by ≈463 Td than the single DBD configuration. The bactericidal efficacy of the two configurations of the plasma jets were tested against Escherichia coli, a well studied Gram-negative bacterium that can be commensal and pathogenic in human body. Our results demonstrate that although single DBD plasma jet result in stronger antibacterial effects, the double DBD configuration could be safer.

Resilient moth-eye nanoimprinted antireflective and self-cleaning TiO2 sputter-coated PMMA films

Moth-eye nanostructures are amongst the most remarkable surfaces in nature because of their multi-functionality including antireflection, self-cleaning and bactericidal ability. Moth-eye surfaces consist of subwavelength arrays of tapered nanostructures, which are challenging to reproduce artificially. Nanoimprint lithography is probably one of the most suited technologies for this purpose. However, the poor mechanical resilience and durability of the polymeric nanocones when exposed to the environment, hinders their use in actual applications. To overcome these limitations, this work demonstrates the use of a thin oxide coating over the polymer moth-eye features imprinted on poly methyl methacrylate (PMMA) films. Particularly TiO2 conformal thin film coatings are deposited by unipolar pulsed dc magnetron sputtering over the antireflective nanopatterns acting as encapsulant. The coating, while preserving the antireflective properties, protects the nanostructures against mechanical scratching and improves substantially their thermal stability to over 250 °C. Furthermore, the TiO2 layer provides additional photoinduced self-cleaning functionality and at the same time it protects the matrix from UV photodegradation. The robust and durable antireflective surfaces developed here may find application on solar cells covers, flat panel displays or on optical components.

Characterization of Ti–Zr–V thin films deposited by DC and unipolar pulsed DC magnetron sputtering

Ternary Ti–Zr–V getter films with approximately equal atomic concentrations were deposited on oxygen free copper substrates by magnetron sputtering using DC and unipolar pulsed DC current modes. The thin film morphologies were studied using scanning electron microscopy, atomic force microscopy, while their chemical and phase compositions were investigated by energy dispersive spectroscopy and X-ray diffraction, respectively. The chemical states within the superficial regions of the films were characterized by X-ray photoelectron spectroscopy in ultra-high vacuum. The results indicate that both current modes promote formation of porous thin films, while the microstructure of the DC film is much finer than that of the unipolar pulsed DC film. Initially, the thin films were covered by oxides of Ti, Zr and V that are reduced upon annealing treatment; those within the DC film present faster reduction kinetics as compared with that of the unipolar pulsed DC film.

Plasma Electrolytic Oxidation (PEO) Layers from Silicate/Phosphate Baths on Ti-6Al-4V for Biomedical Components: Influence of Deposition Conditions and Surface Finishing on Dry Sliding Behaviour

Plasma Electrolytic Oxidation (PEO) layers were produced on Ti-6Al-4V in different conditions, so as to assess the influence of layer structure, current mode, duty cycle and surface finishing on microstructural features and tribological behaviour. In DC regime, the double-layer structure (silicate bath followed by phosphate bath) beneficially affected wear resistance. In unipolar pulsed DC (phosphate bath), the wear resistance of single layers improved with increasing duty cycle, due to improved microstructure and adhesion: high duty cycle single layers can be considered an alternative to double-layer deposition. Surface finishing by abrasive blasting with spheroidal glass beads leads to surface roughness decrease and hence to decreased friction and improved wear resistance. The best-performing PEO layers showed promising results in the comparison with reference materials such as CoCrMo (both uncoated and (Ti,Nb)N PVD-coated) and PVD-coated Ti-6Al-4V up to 30 N normal load.

Influence of process parameters on growth behaviour and properties of coatings obtained by plasma electrolytic oxidation (PEO) on AA 6061

Plasma electrolytic oxidation coating is obtained on AA6061 alloy using positive uni-polar pulsed DC in a sodium silicate-based electrolyte. The effect of process parameters such as solution concentration, process time, average current density, pulse frequency and positive on-time is investigated systematically and the corresponding voltage–time response is correlated with the coating growth rate. Surface morphology of the coatings is studied using scanning electron microscopy and the elemental distribution on the coating is investigated using energy dispersive X-ray spectroscopy. The concentration of sodium silicate in the solution is found to play a key role in determining the morphology and composition of the coating. X-ray diffraction studies indicate a transition from crystalline to amorphous nature of the coating with increase in silicate content of the electrolyte. Effect of pulse frequency on the voltage–time response and the corresponding coating growth rate is highly dependent on the solution concentration.

Surface modification of Ti 13Nb 13Zr by plasma electrolytic oxidation

Titania coatings were produced on Ti 13Zr 13Nb by a unipolar pulsed DC plasma electrolytic oxidation (PEO) process in an electrolyte containing 1M H2SO4+0.1M H3PO4. The samples were galvanostatically anodized under a constant charge-carrier density of 63Ccm−2 and current-densities reaching from 14mAcm−2 up to 700mAcm−2. Both a constant current mode and a unipolar pulsed mode with a duty-cycle of 50% and frequencies of 1Hz to 50Hz were performed. The surface layers were characterized by scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and nanoindentation. Titanium oxide coatings with a thickness of several micrometers and a porous structure could be generated. The coatings morphology was evaluated. It can be shown that the porosity can be reduced with increasing frequency. The coatings consist of anatase and rutile, the phase composition of the coatings can be adjusted by the PEO current density. The corrosion resistance of the layers was tested in a simulated body fluid (SBF) with the addition of 0.1M H2O2 by open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). EIS data fitting indicates a composed layer structure of an outer porous layer and an inner barrier layer. Higher current densities during the PEO process formed thicker coatings, which consequently resulted in lower corrosion currents. The addition of zirconia nanoparticles into the electrolyte increased the surface hardness to an average value of 12.8GPa, compared to that of an untreated sample (4.1GPa) and PEO samples without particle addition (8.5GPa).

Correlation between plasma electrolytic oxidation treatment stages and coating microstructure on aluminum under unipolar pulsed DC mode

Alumina coatings were deposited on 6061 aluminum alloy substrates using plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Different electrical parameters, including frequency, duty cycle, and current density were applied to obtain PEO coatings. Scanning electron microscopy (SEM) and microhardness tests were used to investigate the coating microstructure and properties. For each sample, the voltage–time response of the PEO process was obtained and compared with surface morphology, and coating cross sections and thicknesses. Special consideration was given to the connection between the electrical parameters, different stages of the PEO process, and the morphology and microstructure of the coatings. Applying different electrical parameters changed the duration and ratio of the PEO stages in the voltage–time response curve and affected the growth rate, surface morphology, microstructure and microhardness of the coatings. We report the new finding of a correlation between the stage of the PEO process and the microstructure and morphology of the coating.

Erratum to: Growth Behavior of Cubic Boron Nitride (cBN) Phase in B-C-N Film Deposited on Si Substrate with Non-Uniform Ion Flux

B-C-N film with a ∼450 nm thickness was prepared on a (100) oriented silicon wafer (4 cm × 4 cm) by unbalanced magnetron sputtering of a B4C target with a unipolar pulsed DC substrate bias. At a substrate bias of −250 V, a cubic boron nitride (cBN) phase appeared in the B-C-N film when the nitrogen content was over 6.7 vol% in Ar-N2 reactive gases. For the B-C-N film deposited on unipolar pulsed DC biased Si substrate with Ar-16.7%N2 reactive gas, the content and formation behavior of the cBN phase along with the film thickness were critically dependent on the substrate position due to the difference in ion flux. In contrast to the circular central region of a substrate 2.5 cm in diameter, where cBN was nucleated and grown on hexagonal boron nitride (hBN) in a layered manner, the cBN phase was co-grown with the hBN phase and was surrounded by hBN at the circumference region of the circle. Only hBN was observed at the outer region of the circle. The microstructure of B-C-N film in which cBN is surrounded by hBN is believed to be effective in reducing residual stress developed by the nucleation of a cBN phase.

Synthesis and characteristics of Ag/Pt bimetallic nanocomposites by arc-discharge solution plasma processing

Arc discharge in solution, generated by applying a high voltage of unipolar pulsed dc to electrodes of Ag and Pt, was used as a method to form Ag/Pt bimetallic nanocomposites via electrode erosion by the effects of the electric arc at the cathode (Ag rod) and the sputtering at the anode (Pt rod). Ag/Pt bimetallic nanocomposites were formed as colloidal particles dispersed in solution via the reduction of hydrogen radicals generated during discharge without the addition of chemical precursor or reducing agent. At a discharge time of 30 s, the fine bimetallic nanoparticles with a mean particle size of approximately 5 nm were observed by transmission electron microscopy (TEM). With increasing discharge time, the bimetallic nanoparticle size tended to increase by forming an agglomeration. The presence of the relatively small amount of Pt dispersed in the Ag matrix could be observed by the analytical mapping mode of energy-dispersive x-ray spectroscopy and high-resolution TEM. This demonstrated that the synthesized particle was in the form of a nanocomposite. No contamination of other chemical substances was detected by x-ray photoelectron spectroscopy. Hence, solution plasma could be a clean and simple process to effectively synthesize Ag/Pt bimetallic nanocomposites and it is expected to be widely applicable in the preparation of several types of nanoparticle.

Low Frictional TiO<sub>2</sub> and MoS<sub>2</sub>/TiO<sub>2</sub> Coatings on Ti-6Al-4V Alloy

TiO2 and MoS2/TiO2 coatings were prepared using a Plasma Electrolytic Oxidation (PEO) process at a unipolar pulsed-DC mode. Both coatings, mainly comprised of anatase TiO2, exhibited a low coefficient of friction, C.O.F < 0.21 for the TiO2 coating and C.O.F < 0.16 for the MoS2/TiO2 coating, while the MoS2/TiO2 coating had a longer wear life than TiO2-coated and uncoated Ti alloy (Ti-6Al-4V). This result showed that the solid lubricant MoS2 may play a significant role in the tribological behavior and it can be applied to many high load-bearing systems in aerospace and automotive industries.

Coating growth behavior during the plasma electrolytic oxidation process

In this study, aluminum oxide was deposited on an Al alloy substrate to produce hard ceramic coatings using a plasma electrolytic oxidation (PEO) process working at atmospheric pressure. The process utilizes dc and unipolar pulsed dc in the frequency range 0.2–20 kHz. Optical emission spectroscopy was employed to study the species and electron temperature of the plasma. The morphology and microstructure of the coatings were investigated using scanning electron microscopy. It was found that in the first 12 min of the PEO process, the plasma electron temperature increased with the applied voltage during the experiments, the plasma electron temperature was found to be in the range 4000–9000 K, and the applied voltage to the electrodes ranged up to 550–600 V for the different current modes. The plasma temperature profile exhibits a wider peak temperature spike for the dc power mode than for the pulsed dc mode, indicating that the dc plasma discharges would provide longer sintering time. The pulsed dc mode increases the spike temperature up to 8700 K but does not necessarily enhance the coating growth. The high spike temperature generated by strong discharges likely melts the oxide and then traps gas into the melt pool, resulting in some porosity at the interface. By eliminating the high temperature spike, a denser interface layer and homogenous coating morphology are produced.

Effect of duty cycle on plasma parameters in the pulsed dc magnetron argon discharge

The time-resolved probe measurements of the plasma parameters and the electron energy distribution function are carried out in a unipolar pulsed dc magnetron argon discharge. The cathode target is driven by the 20kHz midfrequency unipolar dc pulses at three operating modes, such as constant voltage, constant power, and constant current with the duty cycles ranging from 10% to 90%. It is observed that as the duty cycle is reduced, the electron temperature averaged during the pulse-on period rapidly increases irrespective of the operating mode although the average electron density strongly depends on the operating mode. The comparison of the measured electron energy distribution functions shows that the electron heating during the pulse-on period becomes efficient in the pulse operation with short duty cycle, which is closely related to the deep penetration of the high-voltage sheath into the bulk during the pulse-on period.

Characterization of pulsed plasma in unbalanced magnetron argon discharge

Time-resolved probe measurements have been conducted in a unipolar pulsed dc magnetron system to investigate the temporal evolution of plasma parameters, such as the electron density and the electron temperature, and to determine how the driving frequency and the duty cycle of the dc pulse affect these parameters. A new probe measurement system was employed to perform the time-resolved measurements of I-V characteristic curves and their second derivatives with a maximum time resolution of 100 ns. The measurements were performed in a constant voltage mode, constant current mode, and constant power mode, with various pulse frequencies ranging from 5 kHz to 50 kHz and duty cycles ranging from 10% to 90%, to investigate the detailed temporal evolutions of the electron energy distribution function and the plasma parameters. The results show that as the pulse frequency increases, the electron density and the electron temperature exhibit insignificant changes. However, the reduction of the duty cycles results in a significant increase of the electron temperature, irrespective of the operating mode. A comparison of the measured electron energy distribution functions shows that the increase in the electron temperature is caused by a decrease in the population of trapped low-energy electrons and∕or by an increase in the population of drifting high-energy electrons. This result can be explained by considering the electron heating due to the deep-penetrating cathode sheath.

Pulsed dc Magnetron Discharges and their Utilization in Plasma Surface Engineering

AbstractFundamental aspects of magnetron operation in two types of practically interesting discharges, i.e. in highpower unipolar pulsed dc magnetron discharges for ionized high‐rate sputtering and in asymmetric bipolar pulsed dc magnetron discharges for reactive sputtering, were investigated on the basis of time‐resolved plasma diagnostics. We present the results obtained using time‐resolved optical emission spectroscopy and time‐ and energy‐resolved mass spectroscopy in high‐power unipolar pulsed dc magnetron discharges at a repetition frequency of 1 kHz, an argon pressure of 1 Pa and an average target power loading in a pulse up to 500 W/cm2. The main aim was to study complex ionization mechanisms of sputtered Cu atoms and working gas (Ar) atoms, and to characterize energy distributions of Cu+ and Ar+ ions and their fractions in ion fluxes to a substrate during pulses. Some results achieved with a grooved target at the frequencies up to 50 kHz are discussed. We report also on the results obtained using energy‐resolved mass spectroscopy and time‐resolved Langmuir probe measurements in asymmetric bipolar pulsed dc magnetron discharges at repetition frequencies of 100 kHz and 350 kHz, an argon pressure of 0.53 Pa and a mean discharge power of 300W. They explain a complex correlation between the pulsed plasma dynamics and the enhanced ion bombardment of growing films in these discharges. Some new development trends in the field of the dual pulsed dc magnetron sputtering are outlined. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)