Pulsed Microplasma Research Articles

Ultrasound harmonic generation and atomic layer deposition of multilayer, deep-UV mirrors and filters with microcavity plasma arrays

In honor of Professor Kurt Becker’s pioneering contributions to microplasma physics and applications, we report the capabilities of arrays of microcavity plasmas in two emerging and disparate applications. The first of these is the generation of ultrasound radiation in the 20–240 kHz spectral range with microplasmas in either a static or jet configuration. When a 10times 10 array of microplasma jets is driven by a 20-kHz sinusoidal voltage, for example, harmonics as high as m = 12 are detected and fractional harmonics are produced by controlling the spatial symmetry of the emitter array. The preferential emission of ultrasound in an inverted cone having an angle of pm ,45^circ with respect to the surface normal of the jet array’s exit face is attributed to interference between spatially periodic, outward-propagating waves generated by the arrays. The spatial distribution of ultrasound generated by the arrays is analogous to the radiation patterns produced by Yagi-Uda phased array antennas at RF frequencies for which radiation is emitted broadside to arrays of parallel electric dipoles. Also, the nonperturbative envelope of the ultrasound harmonic spectrum resembles that for high-order harmonic generation at optical frequencies in rare gas plasmas and attests to the strong nonlinearity provided by the pulsed microplasmas in the sub-250-kHz region. Specifically, the relative intensities of the second and third harmonics exceed that for the fundamental, and a “plateau” region is observed extending from the 5th through the 8th harmonics. A strong plasma nonlinearity appears to be responsible for both the appearance of fractional harmonics and the nonperturbative nature of the acoustic harmonic spectrum. Multilayer metal-oxide optical filters designed to have peak transmission near 222 nm in the deep-UV region of the spectrum have been fabricated by microplasma-assisted atomic layer deposition. Alternating layers of ZrO_2 and Al_2O_3, each having a thickness in the 20–50 nm range, were grown on quartz and silicon substrates by successively exposing the substrate to the Zr or Al precursor (tetrakis(dimethylamino) zirconium or trimethylaluminum, respectively) and the products of an oxygen microplasma while maintaining the substrate temperature at 300 K. Bandpass filters comprising 9 cycles of 30-nm-thick ZrO_2/50-nm-thick Al_2O_3 film pairs transmit 80% at 235 nm but < 35% in the 250–280 nm interval. Such multilayer reflectors appear to be of significant value in several applications, including bandpass filters suppressing long wavelength (240–270 nm) radiation emitted by KrCl (222) lamps.Graphical abstract

Metastable argon dynamics in a pulsed microplasma at 43 GHz

Short pulses of millimeter wave (MMW) radiation at 43 GHz create microplasma within a photonic crystal for pressures from 40 to 600 Torr (1.3 × 103–8.0 × 104 Pa). Gas breakdown occurs within a photonic crystal, which acts as an electromagnetic resonator to create a strong initial electric field. The time response of the argon metastable density is experimentally determined during the pulse and in the afterglow using laser absorption. The metastable density overshoots the steady-state condition at the beginning of the pulse and during the afterglow. Modeling is presented to understand these observations. The overproduction of argon 1s5 at the beginning of each pulse is due to a concurrent overshoot in the MMW electric field within the photonic crystal. This field overheats the plasma electrons and enhances the production of excited states. The burst of argon metastables observed in the afterglow is due to the pooled energy of the plasma stored in electrons, ions, and excited states of argon. Understanding metastable production is an important intermediate step to ionization and is also critical in the study of diode-pumped rare gas lasers.

Study on arc characteristics of different defects in pulsed micro-plasma arc welding

The shape of welding arc is closely related to the quality of welding. For micro plasma arc welding (MPAW), however, the change of arc shape is not obvious enough to evaluate the quality of welding, due to the small size of welding arc. Thus, it needs to extract the specific features of arc shape through image processing algorithm to judge its change quantitatively. Furthermore, for workpiece with bright surface, it is difficult to extract the arc shape directly from the arc images because of the strong arc reflection on the workpiece surface. In this paper, a new method based on vertical Sobel operator was proposed to overcome the above-mentioned challenges, and to extract the clear arc shapes from the arc images of pulse micro plasma arc welding (P-MPAW). The arc length, arc width and the height difference between two sides of arc were extracted as characteristic parameters to reveal the influence of different welding defects on the arc shape. The changes of arc characteristic parameters have been studied in depth, when there are different defects such as burn through, misalignment, undercut, concave deformation, excessive gap and overlap. The results show that, excessive welding gap and misalignment, caused by the assembly and machining accuracy, have obvious characteristics in the instantaneous arc shape. The defects caused by welding thermal process like burn through, concave deformation, overlap and undercut have their own distinctive characteristics in the changing process of the arc shape.

Microstructural evolution and mechanical properties of Inconel 625 superalloy fabricated by pulsed microplasma rapid additive manufacturing

An innovative pulsed microplasma additive manufacturing (MPAM) system was proposed for fabricating thin-walled Inconel 625 superalloy parts to improve the deposition and heat efficiency of additive manufacturing (AM). During deposition, the heat accumulation from the microplasma was effectively controlled by adjusting the AM current, pulse length of the plasma arc, and scanning speed. Subsequently, the microstructural evolution and mechanical properties were characterized. The Inconel 625 thin-walled parts manufactured using pulsed microplasma rapid AM consisted of γ, γ″, δ, MC, and Laves phases; the latter exhibited a rod-like aggregation distribution. The microhardness ranged from 210.4 HV0.5 to 279 HV0.5, and the tensile strength was 834.77–855.61 MPa. Furthermore, the yield strength was 533.84–543.79 MPa, and the elongation was 31.34%. An Nb-rich phase fraction (31.4%) was observed in the middle region of the as-fabricated sample. Furthermore, the mechanical properties of Inconel 625 superalloy parts produced using pulsed MPAM satisfied the requirements for standard forged Inconel 625 components.

Numerical and experimental study on oscillation mechanism of molten pool during pulsed microplasma arc welding ultrathin sheets

Molten pool oscillation behaviors during pulsed microplasma arc welding ultrathin stainless steel sheets were studied based on the numerical simulation, experimentally visual detection, and theoretical calculation. Without altering the average heat input, the effects of different pulse parameters of welding current on weld pool oscillation frequency and oscillation amplitude were discussed. It showed that the changes either in the weld pool temperature and liquid metal flow rate or in weld pool size cannot keep up with the rapid variations of welding current from base current to peak current or from peak current to base current. The mainly periodic changes of molten pool size and liquid metal flow rate are caused by pulsed current. However, on these main periodic changes, small amplitude fluctuations are superimposed due to weld pool oscillations. The oscillation frequency decreases and the oscillation amplitude increases slightly with the growth of molten pool. Oscillation frequency increased with an elevating in pulsed current frequency, and it rose firstly and then fell when duty cycle of pulsed current increased. Moreover, concerning the fluctuations in weld pool size, the oscillation amplitude reduced with the enhancement in pulsed current frequency though there were no obvious variations in it with the variations in duty cycle of welding current.

Methods for the study and control of microplasma processes in the synthesis of coatings

The physical bases for determining the ignition and quenching potentials of microplasma discharges in solutions have been developed. Chronopotentiometric, chronoamperometric dependences on the conditions for obtaining coatings and the dependence of the time constant of the transient process on the voltage and duration of the process, as well as current-voltage dependences at the rates of rise and fall of the potential voltage from 106 to 108 V/s, are obtained. The developed method is applicable to control the process of deposition of microplasma non-metallic inorganic coatings and is the basis of the technique for in situ control of coating deposition. Key words: pulsed microplasma oxidation, current-voltage dependences, controlled synthesis of oxide coatings. Keywords: pulsed microplasma oxidation, current-voltage dependences, controlled synthesis of oxide coatings.

Методы исследования и управления микроплазменными процессами при синтезе покрытий

The physical bases for determining the ignition and quenching potentials of microplasma discharges in solutions have been developed. Chronopotentiometric, chronoamperometric dependences on the conditions for obtaining coatings and the dependence of the time constant of the transient process on the voltage and duration of the process, as well as current-voltage dependences at the rates of rise and fall of the potential voltage from 106 to 108 V/s, are obtained. The developed method is applicable to control the process of deposition of microplasma non-metallic inorganic coatings and is the basis of the technique for in situ control of coating deposition. Key words: pulsed microplasma oxidation, current–voltage dependences, controlled synthesis of oxide coatings.

Innovative non-thermal plasma coating for “core–shell” CaCu3Ti4O12 material

Colossal permittivity of CaCu3Ti4O12 (CCTO) makes it a very interesting candidate for capacitor applications. To improve its properties, an innovative physical method based on a pulsed non-thermal micro-plasma treatment was set up to realize the coating of CCTO's grains (core) with silicon oxide (shell). This method is adapted to control the thickness and homogeneity of the shell, which will allow a better control of grain–grain boundary interfaces and improve the properties of this material. Best result is obtained for the set of gas mixture: Ar/O2/hexamethyldisiloxane (HMDSO) = 2028 N cm3 min−1/7.84 N cm3 min−1/523 mg h−1, respectively, in plasma with a shell thickness of 50 nm. This study offers a new opportunity to quickly synthetize core–shell materials with a dry technique and without almost no secondary product resulting from the chemical reaction because it is in the gaseous state. A complete analysis of the plasma by optical emission spectroscopy in the UV-visible range shows that HMDSO molecules are totally dissociated in atomic (Si, C, and O) or simple radical species (C2 and CH) in the plasma phase. In addition, several thermometer species (OH°, CH, CN, N2, and N2+) are used to estimate excitation temperatures of the plasma (Trot, Tvib, and Te = 300 K, 2400–3700 K, and 5.3 eV, respectively) that clearly shows the non-equilibrium character and the efficiency of this plasma.

Merging the Sol–Gel Technique with the Pulsed Microplasma Cluster Source Deposition to Improve Control over the Memristive Response of TiO2 Thin Films

Metal oxide thin films show promising resistive switching properties, making them materials of reference for the development of memristive devices. TiO2 is probably one of the most studied materials and is being synthesized using various techniques, each of them having specific optimizable characteristics. In this paper, we report on an innovative approach by combining the sol–gel and the pulsed microplasma cluster source (PMCS) methods, exploiting the low temperature and low cost of the former process and precise control over nanocristallinity of the latter. We show that this approach overcomes the reported limitations that each technique shows in fabricating memristive devices when independently used. A side-by-side comparison of the TiO2 thin films produced by the PMCS, sol–gel, and PMCS/sol–gel hybrid methods (HM) demonstrates an improvement of the memristive properties and a reduction of the electrical shorts in the TiO2 based devices.

Heterogeneous Metal Oxide Coatings with Magnetoactive Nickel, Cobalt, and Iron Phases Formed by the Method of Pulsed Microplasma Oxidation for Radiation Absorption in the Middle and Near-IR Regions

Heterogeneous coatings with high ability to absorb electromagnetic radiation in the middle (4000–400 cm–1) and near-IR regions (11000–2000 cm–1) are formed by the method of pulsed microplasma oxidation on aluminum and titanium alloys in inorganic electrolytes containing nickel, cobalt, and iron compounds. The electromagnetic radiation absorption is provided not only by the oxide coating itself, but also mainly by the heterogeneous metal particles of nickel, cobalt, and iron forming a dispersed metal phase and having magnetic properties that are incorporated into the coating pores by the method of microplasma oxidation under the action of a constant magnetic field. The influence of the electrolyte compositions and of the parameters of pulsed microplasma oxidation on the structure, composition, and radiation absorption properties of coatings is studied. It has been revealed that coatings formed in a phosphate-borate electrolyte with addition of nickel at a voltage of 300 V, a pulse duration of 100 μs, and a process duration of 5 min have the best absorption ability.

Application of cold filamentary microplasma pretreatment assisted by thermionic emission for potato drying

Cold filamentary microplasma (CFM) pretreatment assisted by thermionic emission (TE) is a promising, cold drying technology for agricultural products. Such novel pretreatment method based on local electroporation process in the air gap. Brief theory and treatment mechanism are explained. A fractional factorial design 33 was used to determine the effect of CFM treatment assisted by TE parameters such as pulse frequency (F = 40, 80 and 120 Hz), pretreatment duration (D = 60, 120 and 180 s) and temperature of thermionic source (T = 700, 850 and 1000 °C) on drying time and drying rate of potato samples. Maximal CFM pretreatment effect was observed at frequency of 80 Hz, treatment duration of 120 s and temperature of thermionic source of 1000 °C, and the corresponding drying rate and drying time were 35·10−5 (kg/kg s−1) and 130 min. The use of CFM treatment had minor effects on the main quality characteristics (total sugar and starch content) of potato samples. Fourier-transformed infrared spectroscopy confirmed the absence of any modifications of functional group composition of potato samples after CFM pretreatment. Results indicate that cold pulsed filamentary microplasma can enhance the drying rate and decrease drying time without changes in quality characteristics. Practical applicationsCold filamentary microplasma pretreatment assisted by thermionic emission can improve the efficiency of potato drying. Furthermore, it has potential application for reducing energy consumption in drying. CFM pretreatment could be potentially applied for agricultural products drying.

Structure of microplasma discharge on titanium surface covered with thin dielectric film

The spatial structure of the pulsed microplasma discharge (discharge duration is 100 μs, the amplitude of the discharge current is 200 A) was experimentally studied. The discharge is initiated on the titanium sample surface covered with the thin dielectric (titanium oxide) film with a thickness of ~10 nm. The sample was exposed to the pulsed plasma flow (the plasma density is 2 × 1013 cm–3, and the pulse duration is 25 μs). It was found that the microplasma discharge glow has the branched dendrite structure, which consists of a large number of cathode spots (brightly glowing localized formations) randomly distributed over the metal surface. As a result of interaction between the microplasma discharge and the titanium sample, erosion occurs on its surface. The erosion structure is visually “identical” to the discharge glow structure. It was ascertained that the erosion pattern on the titanium surface consists of a large number of individual microcraters with characteristic sizes from 0.3 to 10 μm randomly distributed over an area of approximately 1 cm2. All together, these microcraters form the branched dendrite structure. It was also ascertained that the microplasma discharge propagates along the titanium surface covered with a thin dielectric (titanium oxide) film at an average velocity of approximately 70 m/s, and this propagation has a “jumping” character: the plasma of burning cathode spots initiates the excitation of new cathode spots at distances of 3–30 microns from them.

Drying Technology Assisted by Nonthermal Pulsed Filamentary Microplasma Treatment: Theory and Practice

Nonthermal pulsed filamentary microplasma treatment for drying is a nonthermal technology with promising perspectives to dehydrate plant agricultural materials. The modified set of Luikov’s equations for heat, mass and pressure transfer, has been used to analyze nonthermal pulsed filamentary microplasma treatment effects. The finite element method in combination with the step-by-step finite-difference method for a coupled system of differential equations in partial derivatives was used for numerical simulation of heat, humidity and pressure potentials transfer. The drying time of samples treated by nonthermal pulsed filamentary microplasma treatment assisted by thermionic emission was reduced up to 20.6% (5 kV/cm; 1200 discharges) in comparison to intact tissue. The effect of the obtained approach is very useful for studying process mechanisms and for explaining nonthermal pulsed filamentary microplasma treatment effects. Refined transfer kinetic coefficients from a set of equations based on experimental drying curve can be used for the quantitative determination of thermodynamic coefficients. The agreement of the simulation data with the analytical equation and experimental results is satisfactory (discrepancy less than 3%). Obtained results showed that the proposed model with the refined transfer kinetic coefficients adequately describe the experimental data.

Electric wind generation by nanosecond repetitively pulsed microplasmas

A nanosecond repetitively pulsed microplasma generated between two tungsten electrodes in a pin-to-pin configuration is used as a supply of charged species to produce an electrohydrodynamic phenomenon when placed near a DC-biased electrode. We employed high-frequency particle image velocimetry in order to study the produced flow. The generated flow velocities are around 2 m · s−1 with a deposited average power on the order of 1 W. The velocity and the geometry of this flow are discussed as well as the impact of the repetition frequency and the DC field. We also explain the impact of each microplasma event on flow modification.

Spatio-temporal distribution of absolute densities of argon metastable 1s5 state in the diffuse area of an atmospheric pressure nanosecond pulsed argon microplasma jet propagating into ambient air

Atmospheric microplasma jets (MPJs) sustained in rare gases have gained increased attention due to their potential to generate highly reactive species. In this paper, we present space- and time-resolved argon metastable densities, Ar(1s5), measured in an atmospheric pressure MPJ operated in Ar and propagating into ambient air using tunable diode laser absorption spectroscopy (1s5 → 2p9 optical transition). The MPJ was produced using a dielectric barrier discharge energized by short duration (230 ns) high-voltage positive pulses (4.2–6.2 kV) at a repetition frequency of 20 kHz. The spectral absorption line profile was recorded and allowed measurements of the absolute metastable Ar(1s5) density integrated in the line-of-sight of the laser beam under various operating conditions of the MPJ. The results reveal a sensitive dependence of the Ar(1s5) density on spatial coordinates, i.e., distance from the exit of the capillary tube of the discharge and from the axis of the argon jet. The highest Ar(1s5) densities of about 3 × 1013 cm−3 were measured at the axis of the argon jet at longitudinal distances between 4 and 5.5 mm downstream from the nozzle of the tube. The temporal distribution of the Ar(1s5) density, which presents three maxima, is thoroughly discussed in this paper. The spatial distribution of the effective Ar(1s5) lifetime (&amp;lt;250 ns) is also reported, giving some insight into the surrounding environment of the argon metastable atoms. The determined spatiotemporal distributions of the Ar(1s5) density can be useful for the optimization of argon MPJs for different applications like surface or biomedical processes.

Investigations on thin SS sheets joining by pulsed micro-plasma transferred arc process

Joining of thin sheets is challenging due to difficulty in maintaining localized melting and solidification. This becomes more difficult while thin sheet joining of stainless steel due to its poor weld/joining properties. This article reports on autogenous joining of 0.4 mm thickness sheets of 316L austenitic stainless steel (SS) using pulsed micro-plasma transferred arc (P-µPTA) process. Decision of using autogenous joining was taken on the basis of observations of experimentation on homogenous joining of SS thin sheets using 0.274 mm diameter SS filler wire. Effects of pulse-on time, pulse-off time, peak current and torch travel speed on the joint strength of weld were investigated with an objective to identify the optimum values for the maximum tensile strength of the joint. It was found that 50% duty cycle, 10% pulse frequency, 3.5 A peak current and torch travel speed 250 mm/min yielded maximum joint strength 553 MPa. The fusion zone was found to have finer grain size. Non-existence of heat-affected zone absence was assured by very minor dissimilarity in the values of microhardness over the fusion zone. This study establishes the capability of pulse micro-plasma transferred arc process for joining thin sheets of stainless steel without any filler material and producing very good quality joint with very insignificant heat-affected zone (HAZ), thermal distortion and porosity using much less power as compared to conventional fusion joining processes.

A Comparative Study on Direct and Pulsed Current Micro-plasma Arc Welding of Alloy Ti–6Al–4V

In this project, the sheet of Ti–6Al–4V alloy (annealed) with a thickness of 1 mm was welded with butt joints by means of pulsed current micro-plasma arc welding without using a filler metal. The purpose of this study is to optimize parameters in the event of increasing tensile strength by using pulse current and its comparison with the pulse-free system. Optical microscope and scanning electron microscope were used to investigate the type of phases, microstructure of weld metal, base metal and heat-affected zone. Results illustrated that due to the low thickness of the sheet, the non-use of filler metal in the welding process and the lower heat input of this process, all contributed to a higher cooling rate of the molten pool.

Ionization and recombination in nanosecond repetitively pulsed microplasmas in air at atmospheric pressure

We confine the nanosecond repetitively pulsed discharge to the micrometer scale, in a 200 µm discharge gap in air at atmospheric pressure and room temperature, focusing on measurements of the electron number density and electron temperature. The Stark broadening of H, O and N atomic lines and electrical conductivity both show that the electron number density reaches a maximum value of 1 × 1019 cm−3. Boltzmann plots show the electron temperature to be 72 kK several nanoseconds after the end of the pulse of applied electric field. We will use these results to determine the mechanism responsible for electron loss during the early recombination phase (t < 500 ns) and comment on the degree of ionization and dissociation.

Advances in the research of techniques for traumatic scars intervention with photoelectric acoustic therapy

Traumatic scar is a common complication of skin injury, such as burn, trauma, and surgery. The mechanism for scar formation still remains unclear. Traumatic scar has a negative impact on the quality of patients' life, due to the appearance of scar always causes physical or/and psychological problems to patients. The treatments for scar include surgery, chemotherapy, radiotherapy, and pressure, depending on the kind of scar present. Recently, a great progress in treating scar has been achieved by novel techniques with laser, intense pulsed light, micro-plasma radiofrequency, and ultrasound. The aim of this review is introducing the advances of these techniques for traumatic scars intervention.

Cluster Beam Deposition of Ultrafine Cobalt and Ruthenium Clusters for Efficient and Stable Oxygen Evolution Reaction

Ultrafine cobalt and ruthenium clusters are deposited on carbon paper substrates by cluster beam deposition using a matrix assembly cluster source and a pulsed microplasma cluster source, respectively. When used to catalyze the oxygen evolution reaction (OER), the cobalt and ruthenium clusters show electrocatalytic performance superior to the state-of-the-art Ru/C and RuO2 nanoparticle catalysts on both a mass and a specific-surface-area basis. Typically, the cobalt clusters can deliver 10 mA cm–2 at a low overpotential of 320 mV and show a small Tafel slope of 50 mV dec–1 and a mass-based turnover frequency of 0.01 s–1 at an overpotential of 300 mV, outperforming many cobalt-based OER catalysts.