Laminar Plasma Research Articles

Structured La0.6Sr0.4Co0.2Fe0.8O3-δ cathode with large-scale vertical cracks by atmospheric laminar plasma spraying for IT-SOFCs

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is a promising cathode material for solid oxide fuel cells (SOFCs). Hence, structured LSCF coatings were deposited in this study by atmospheric laminar plasma spraying (ALPS) at the spraying distances from 150 mm to 250 mm. Particles with high temperature were obtained at low plasma arc power of only 15 kW. The microstructural characterization of the coatings showed cluster-like morphologies on the top surfaces and columnar features with large-scale vertical cracks at the cross section. An evolution mechanism of the coatings was proposed based on particle size. The movement of particles during laminar plasma jet depended on particle size, which was studied experimentally and by simulation to explain the selective deposition. The coatings showed lower polarization resistance when compared to LSCF coatings deposited by atmospheric plasma spraying (APS) due to large vertical crack densities. The high output performance of the SOFCs with the LSCF cathode also proved this phenomenon. The results suggest that ALPS provides extensive options for SOFCs manufacturing.

A Highly Cost-Efficient Large-Scale Uniform Laminar Plasma Jet Array Enhanced by V-I Characteristic Modulation in a Non-Self-Sustained Atmospheric Discharge.

Developing cost‐efficient large‐scale uniform plasma jets represents a significant challenge for high performance in material processing and plasma medicine. Here, a V–I characteristic modulation approach is proposed to reduce the discharge power and increase the plasma scale and chemical activity in non‐self‐sustained atmospheric direct‐current discharges. The electric field in discharge space is optimized to fundamentally empower simultaneously initiating all discharge cells far below Townsend breakdown potential and stably sustaining each plasma jet at low voltage. These strategies create a crucial step to fabricating a flexible and compact low‐power large‐scale uniform laminar plasma jet array (LPJA) with high activity in cheap argon. The mechanisms behind the discharge enhancement are revealed by combining V–I characteristic examination and a modulation model. Compared with conventional arrays, this LPJA possesses the widest size (90 mm) and raises its uniformity from 30% to 97%. Comparing different discharge modes shows that the LPJA scale is surprisingly increased nearly by 4 times with the discharge power reduced from 7.4 to 4.8 W. The methodology provides a highly cost‐efficient roadmap to break through the bottleneck of restricting low‐power discharge, large‐gap discharge, large‐scale discharge, parallel‐multi‐electrode discharge, and uniform discharge together. This advance will meet the urgent need for various plasma applications.

Тепловые характеристики столба дуги в зоне стабилизации ламинарного плазмотрона, работающего на воздухе

The method of universal characteristics of an electric arc is described as a mathematical basis for the heat exchange model of an electric arc in the processes of electric arc heating in electrical engineering. Application of this method for calculation of the temperature characteristics of an arc column compressed by the walls of the discharge channel in the stabilization zone of a laminar plasma torch operating in air is considered. An example of the temperature profile calculation of a wall-stabilized arc for a plasma generator with a discharge channel diameter of 5 mm operating in air at an arc current of 60 A is presented. Calculated results are compared with the results of experimental studies of this plasma generator.

Design and characteristics of a new type laminar plasma torch for materials processing

Laminar plasma jet (LPJ) generated by laminar plasma torch (LPT) has a favorable temperature and velocity distribution. Thus, it is superior to the turbulent plasma jet in material processing. However, most of the reported LPTs usually operate at a relatively low output power with a relatively low arc voltage and thermal efficiency, which limits its capabilities. In this context, this paper attempts to design a new type of high-power LPT with a relatively low arc current and a high thermal efficiency. In the first section, the design principle of the main components is studied and discussed in detail, and a new high-power LPT is proposed. Then, the experimental characteristics of the proposed high-power LPT are examined. Experimental results reveal the following characteristics of the proposed LPT. (1) The max jet length of the proposed LPT reaches at 540 mm. (2) Its mean arc voltage is higher than 290 V when the LPT works with arc currents lower than 200 A, leading to an output power greater than 50 kW. (3) The mean thermal efficiency is higher than 50%. Lastly, the proposed LPT has been applied to spheroidize the aluminum oxide powers. The experiment results for the production of spherical powders show that the proposed LPT has a good characteristic for material processing.

Microstructure and mechanical properties of 308L stainless steel fabricated by laminar plasma additive manufacturing

In the present work, a novel direct energy deposition method for metal additive manufacturing is developed employing laminar plasma as the heat source. With a combination of modified process parameters, a high-performance 308L stainless steel component with four hollow straight walls is prepared. The behavior of phase formation, microstructure, density and mechanical properties of the samples with different heights were investigated. Transformation from columnar to equiaxed dendrites can be observed as the height of wall increases from the substrate to about 30 mm. In the top region of the deposited wall, the equiaxed dendrites are dominant. The average density of the sample reaches 98.3%. Anisotropic property is observed in the bottom and middle regions, while the top region is isotropic.

1D time evolving electric field profile measurements with sub-ns resolution using the E-FISH method.

We present an approach for the measurement of time evolving electric field profiles in atmospheric pressure plasma discharges using electric field induced second harmonic generation (E-FISH). While the E-FISH effect has been known of for some time, recent advances in laser and detection technology have allowed the method to be utilized for spatial measurements of an arbitrarily applied electric field. A cylindrical lens is used to focus the femtosecond laser light to a line and an intensified charge coupled device is used for detection, allowing for one-dimensional (1D) spatial resolution on the order of ∼50 μm. Measurements have been carried out verifying the spatial resolution using a spatially periodic, localized electric field. Calibrated 1D electric field measurements have been completed with a time resolution of 500ps in a laminar cold atmospheric pressure plasma jet with argon core flow and N2 co-flow powered by a nanosecond (ns) pulse dielectric barrier discharge. The field was shown to propagate as an ionization wave, with a velocity of ∼0.3 mm/ns.

Conditions for Plasma Jet Formation in a Laminar Plasmatron

The laminar plasma jet is an effective tool of plasma technologies for material processing thanks to great values of bulk temperature and high penetration of a plasma stream. However, laminar stream formation in an axial jet plasmatron is connected with a number of difficulties caused by physical restrictions. Conditions of formation of a laminar stream at the exit from a plasmatron nozzle with interelectrode inserts are found when using the most widespread plasma-forming gases. With use of a method of universal characteristics of arc and heat-transfer model of arc of jet plasmatrons, calculation of characteristics of the arc compressed by walls of the discharge channel is carried out; the necessary arc compression ratio in the stable laminar stream mode and its extreme values taking into account the permissible thermal load on walls of the discharge channel in the arc stabilization zone are defined. It is shown that reduction of the diameter of the plasmatron discharge channel reducing diameter of the laminar plasma stream exhausting from a plasmatron nozzle reduces the permissible values of the arc compression ratio. Influence of the cooling mode of walls of the plasmatron arc channel formed by interelectrode inserts on the arc compression ratio at separate water cooling of each interelectrode insert and at their joint cooling is shown. All calculations were carried out for the values of gas consumption and discharge channel length meeting a basic condition of laminar stream formation of Re < 150. The arc current values necessary for a laminar plasma stream formation, depending on the discharge channel section and a plasma-forming gas consumption taking into account thermal properties of the gas environment, are determined. The method proposed in this article makes it possible to calculate an arc column temperature profile in the stabilization zone of a laminar plasmatron and can be used when developing plasmatrons with a stable laminar stream of set plasma-forming gas.

Development and diagnosis of an atmospheric pressure plasma torch for investigating magnetohydrodynamic instabilities

The development and diagnosis of an atmospheric pressure, non-transferred, DC arc plasma torch for investigating magnetohydrodynamic experiments is presented. Utilizing a custom AC starter circuit coupled with a high power DC power supply a long, laminar plasma jet was successfully generated by adjusting voltage and current characteristics, gas flow rates, and torch geometry (nozzle diameter and length). A complete and detailed description of the design and operation conditions of our torch is presented such that our results may be reproduced. Utilizing copper, aluminum, and tungsten alloy electrode components in conjunction with an electrically isolated water cooling system allows for continuous torch operation. The 50 kV AC starter circuit is coupled with a 15 kW DC power supply via a series injection transformer as a part of the custom arc starter circuit components. Axial and tangential gas flow, with nitrogen as the working gas, is regulated to generate a stable, laminar, atmospheric plasma torch with a jet length of over 80 cm under ideal operating conditions. To visualize the plasma interface in magnetohydrodynamics experiments, a technique was developed to image the cylindrical plasma jet in both the R-Z plane (axial) and R-θ plane (cross sectional) using planar-laser Mie scattering from particles added to the surrounding gas. Plasma emission wavelengths are filtered out allowing scattered light from the particle laden-flow field illuminated at 532 nm by an Nd:YAG laser, to be isolated. Plasma electron temperature and degree of ionization were estimated using a technique based on the well-known Fowler–Milne spectroscopic method. Electron temperatures of up to 14 000 K and degree of ionization up to 27% at the torch exit were obtained.

Microstructural evolution of alumina coatings by a novel long laminar plasma spraying method

A super long and stable laminar plasma jet was used in the atmospheric plasma spray process in this work. Microstructures evolution and properties of alumina coatings that were obtained using a long spraying distance ranging from 200 mm to 350 mm in an atmospheric environment were studied in this work. These processes were carried out by using a total gas flow rate of 14 slpm with 70% nitrogen and 30% argon in volume and output power of 25.4 kW (current of 160 A). A long particle residence time was obtained in this study, which compared with other current atmospheric plasma spray methods. The microstructures of coatings showed the multi-island protrusions on the top surfaces and overlapped lamellar splats at the fracture surfaces. The microstructural evolution was significantly affected by the heating and motion behaviors of alumina particles when these were flowing in the long laminar plasma jet at different spraying distances.

Generation of Long Laminar Plasma Jets: Experimental and Numerical Analyses

A novel direct current non-transferred arc plasma torch that can generate silent, stable and super-long laminar plasma jets in atmospheric air is investigated. The results showed that laminar plasma jets of length ranging from 100 to 720 mm in length can be generated by controlling the gas input rate ranging from 8.5 to 15 L min−1 and the output power from 8.5 to 28 kW. The length of the plasma jets generally increased with the output power and gas flow rate. Observations of temporal evolution of the plasma jet appearance and the voltage demonstrated that the jet is highly stable in the atmospheric environment. The fluid dynamic properties of the laminar plasma jet were studied using a numerical simulation incorporating a laminar flow model and an RNG turbulent flow model. Simulation results show the expansion of a high temperature region close to the torch nozzle exit, corresponding to a bright region observed in experiments.

Effect of Surface Hardening on Dynamic Frictional Rolling Contact Behavior and Degradation of Corrugated Rail

The present study was undertaken to evaluate the effect of surface hardening technology on dynamic frictional rolling contact behavior and degradation of corrugated rail in Shenzhen. Characteristic parameters such as length and depth of corrugation were analyzed by means of a continuous measurement method based on the corrugation analysis trolley. The explicit finite element method for material hardening characteristics and real contact geometry was adopted to set up the 3D transient FE model of wheel and rail, after which the value and distribution of stress/strain as well as contact solutions could be obtained during frictional contact, and then the Archard wear model and simplified wear superposition method are integrated as a numerical simulation tool for rail wear after hardening. The simulation results show that laminar plasma surface hardening technology can increase residual stress and shear stress in quenched zones, leading to local stress concentration at their boundaries; the plastic strain in the matrix material is higher than that in the quenched zones, while the strain concentration is mainly focused on the matrix material. The hardening can remarkably reduce the rail wear along the corrugation wave, and the wear depth of material with hardening technology is about 36% of that of nonhardening material. Laminar plasma surface hardening technology can therefore restrain the development of rail corrugation.

Study of the Synchronous Injection in a Controlled Pulsed Arc Plasma

In the field of plasma spray technologies, new processes are developing to obtain coatings with nanostructured architectures. Difficulties of understanding and controlling the process originate from the continuous injection of a liquid material and the power instabilities of the current torches which strongly affect the heat and momentum transfers to the nanometric particles. This paper reports an original method to make TiO2 coatings by suspension plasma spraying. A direct current (DC) power supply applying time-modulated current amplitude to a custom DC torch is used to generate at low power (1.5 kW) a pulsed laminar plasma jet with periodic oscillations of its properties. To make best use of this pulsed mode, a synchronization device was developed. It allows triggering from the arc voltage an inkjet nozzle to deliver at a precise moment a single droplet to improve the control of plasma/material interaction. An ink of TiO2 anatase solid particles is formulated to be compatible with a drop-on-demand printhead dispenser. In-flight diagnostic is made by optical emission spectroscopy and a fast shutter camera. TiO2 coatings are characterized by scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Results show that homogeneous TiO2 coatings of nanostructured cauliflowers shapes are obtained thanks to the controlled injection system. A competition between nucleation mechanism and liquid particles deposition are also observed. These deposits correspond to a mixture of anatase and rutile phases.

Influences of the Arc Chamber Length on the Jet Characteristics of Laminar Plasma Torch

Laminar plasma jet (LPJ) which is generated by laminar plasma torch (LPT) has more advantages than the turbulent plasma jet on applications, such as materials processing and plasma spraying. The jet characteristics of LPT are influenced by many factors, e.g., the working parameters, plasma torch construction, gas injection angle, and arc chamber length. A lot of research has been done in recent years. However, a better understanding of how the arc chamber length influences LPT’s jet characteristics is still required. Thus, this paper aims to study the influences of the arc chamber length on LPT’s jet characteristics. The first part mainly focuses on the introduction of the LPT setup we used in this paper, and this setup can switch the chamber length from 67 to 84 mm or 101 mm by adjusting interelectrodes’ number from 3 to 5. The second part introduces the methods and equipment used in this paper. The last part is results and discussions. The results show that when the arc chamber length increases from 67 to 84 mm and from 84 to 101 mm: 1) the corresponding arc voltage and working power of the experimental LPT increase 20% and 12%, respectively; 2) the corresponding thermal efficiency increases 30% and 8%, respectively; 3) the corresponding mean specific enthalpy of the LPJ increases 50% and 20%, respectively; and 4) on the one hand, the LPT’s jet length increases slightly with increasing the arc chamber length, and on the other hand, it increases first and then decreases with increasing the working gas; additionally, the plasma jet changes from laminar flow state to turbulent flow state.

Laminar and turbulent flow modes of cold atmospheric pressure argon plasma jet

Laminar and turbulent flow modes of a cold atmospheric pressure argon plasma jet are investigated in this work. The effects of the gas flow rate, applied voltage, and frequency on each plasma mode and on intermodal transitions are characterized using photographic, electrical, and spectroscopic techniques. Increasing the gas flow rate increases the plasma jet length in the laminar mode. Upon transition to the turbulent mode, increasing the gas flow rate leads to a decrease in the plasma jet length. The flow rate at which the jet transitions from laminar to turbulent increases with the applied voltage. The presence of nitric oxide (NO) radicals is indicated by the emission spectra of the turbulent plasmas only, while excited Ar, N2, OH, and O excited species are produced in both laminar and turbulent modes. With no distinctive behavior observed upon transition between the two operating modes, the power consumption was found to be insensitive to gas flow rate variation, while the energy density was found to decrease exponentially with the gas flow rate. Rotational and vibrational temperature measurements of the two plasma modes indicated that they are of the non-thermal equilibrium plasma type. Since they offer NO radicals while maintaining the benefits of the laminar plasma jet, the turbulent plasma jet is more useful than its laminar counterpart in biomedical applications.

A novel structure of YSZ coatings by atmospheric laminar plasma spraying technology

A novel structure of yttria stabilized zirconia coatings was developed in this study by a newly laminar plasma spraying technology in atmospheric environment. The unique microstructures of coatings showed the multi-island protrusions feature at the top surface, quasi-columnar structures along the cross-section distributed as a certain interval and hybrid droplet/vapor deposited structures at the fracture surface. The distributions of particle velocity and surface temperature along the axial direction of long laminar plasma jet were investigated and compared with other plasma spraying methods. The effects of different microstructures to thermal conductivities comparing with other current plasma spray methods were also demonstrated.

Characterization of a Laminar Plasma Plume Based on Dielectric-Barrier Discharge at Atmospheric Pressure

With argon used as working gas, a laminar plasma plume with a fairly large volume is flushed out of a dielectric-barrier discharge device into the ambient air at atmospheric pressure. Discharge characteristics are investigated through electrical and optical methods. Results indicate that discharge power, based on Lissajous plot, increases with increasing the peak voltage. High-speed imaging indicates that the formation mechanism of the plasma plume corresponds to a series of microdischarge filaments leaving outlet of the gas duct, whose probable lifetime is about several milliseconds.

Development of long laminar plasma jet on thermal spraying process: Microstructures of zirconia coatings

A novel direct current non-transferred arc plasma torch that can generate long, silent and stable laminar plasma jet in air is applied to thermal spraying process in this article. Such long laminar plasma jet, making a low level of noise (<80dB) in air, features a jet length over 350mm, and presents regular variation in length with the increase of output power and gas flow rate. Microstructures and properties of YSZ coatings obtained under spraying distances ranging from 130mm to 300mm are discussed in the following. The results show different intervals of vertical cracks across the transverse sections of the coatings and orderly distributed island-protrusions on the top surface of the coatings. A mass of vapor deposited structures can be found in the samples, that means the refractory 8YSZ powders can be melted sufficiently when flowing in long laminar plasma jet, although the stable output power of the laminar plasma torch is only 25–26kW during the spraying process. This may provide extensive options for different plasma spraying applications and improve the controllability of plasma spraying technology.

Determining the heat flux distribution of laminar plasma jet impinging upon a flat surface: An indirect method using surface transformation hardening

Laminar plasma jet (LPJ) is a promising high energy beam for various engineering applications. However, there lacks of a method for measuring the heat flux distribution of the LPJ impinging upon a flat surface (qH(r)), with high accuracy, little limitations on the axial distance from the torch exit and the power of the LPJ. Therefore, this paper proposes an indirect method for determining qH(r) using surface transformation hardening. It makes use of the fact that when LPJ is used in the surface transformation hardening of a ferrous material, the size of the heat affected zone is directly correlated with the heat flux distribution. It combines the experiment and numerical simulation on the heat transfer process of the surface transformation hardening to indirectly obtain qH(r) at a certain axial distance from the torch exit. The proposed method was used to determine qH(r) of the nitrogen LPJ generated by a customized laminar plasma torch at different axial distance from the torch exit. It was found that the proposed method exhibits high accuracy through benchmarking with the results obtained by numerical simulation and by direct measurement using the double calorimetric technique reported in literature. Furthermore, the proposed indirect method has little limitations on the axial distance from the torch exit and the power level of the LPJ. In addition, it was found that that axial gradient of the LPJ’s heat flux is low, with less than 50% drop after an axial distance of 100 mm from the torch exit. Finally, the determined qH(r) was directly verified by the numerical simulations and experimental observation on plasma surface transformation hardening of a different ferrous material.

Synthesis of thick photocatalytic titania surface layers by solution plasma spraying and subsequent treatment by pulsed laminar plasma jet

The two-stage method of synthesis of TiO2 photocatalytic surface layers with improved adhesion and cohesion, and photocatalytic properties based on vortex atmosphere solution precursor plasma spraying (SPPS) of thick anatase-rutile mixed coatings and their subsequent melting by pulsed laminar argon-helium plasma jet followed by auto-quenching and solidification is offered and approved for the first time. The titania coatings with increased rutile/anatase ratio characterized by high degree of crystallinity, increased specific surface area and permeability, and also advanced cohesion and bond strength with metal substrate, are sufficiently obtained for practice. The properties of the coatings were studied using X-ray diffraction (XRD) analysis, scanning electron and optical microscopy, BET- assisted measuring specific surface area and methylene-blue (MB) decoloration testing in an ultra-violet (UV) light room. Since the experimental determination of conditions of the post-treatment of the as-sprayed coatings by pulse plasma jet is rather labor- and power-consuming procedure, preliminary computer-aided evaluation has been carried out.

A Versatile Laminar Flow Atmospheric Pressure Plasma Jet Using a Double Coaxial Glass Tube

We report the characteristics of laminar flow atmospheric pressure plasma jets (APPJs) using a double coaxial glass tube. Two gases can be introduced independently into the tube, and the gas and plasma flows form a laminar flow and then interact outside of the glass tube. In the case of helium (He) as an inner gas flow with an outer nitrogen (N2) gas flow, emission lines of nitric oxide, the first negative and second positive system bands of N2 were observed, while only the second positive system band was observed in the case of argon (Ar) as the inner one. Considering the Penning effects of each excited state, the metastable state of He has a higher energy than that of Ar, resulting in energetic nitrogenous productions in the former case. The excited state in the core plasma is one of the important conditions to determine the characteristics of the APPJ produced with a double coaxial glass tube.