Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75-0.87.     

Effect of plasma surface treatment and heat treatment ambience on mechanical and corrosion protection properties of hybrid sol-gel coatings on aluminum

Hybrid sol-gel coatings derived from a base catalyzed hydrolysis of tetraethylorthosilicate and methyltriethoxysilane were deposited on aluminum substrates by a dip coating technique. Some of the coatings were deposited on substrates whose surfaces were pre-treated using atmospheric-air plasma prior to coating in order to study the effect of surface activation by plasma pre-treatment. The coated substrates were heat treated in different ambiences like air, flowing N₂ and vacuum to see the effect of heat treatment ambience on the properties of the coatings. Characterization of the coatings after heat treatment was carried out with respect to coating thickness, pencil scratch hardness, adhesion, water contact angle and their microstructure. Corrosion testing for all the coatings was carried out by electrochemical polarization measurements as well as electrochemical impedance spectroscopy in 3.5% NaCl solution for 1 h exposure time to investigate on their corrosion resistance. Coating thicknesses ranging from 1 μm-5 μm were obtained by varying the withdrawal speeds. Heat treatment in a controlled atmosphere with low oxygen content was seen to improve the hydrophobicity of coated surface, as measured by water contact angles (20^o — air; 71^o — N₂; 95^o — vacuum), thereby improving the corrosion resistance. Surface pre-treatment using open-air plasma was seen to improve the adhesion of the sol-gel coatings thus making it possible to obtain adherent and thick coatings in a single dip coating process. Both the methods of processing the coatings reduced the corrosion rate of aluminum from 1.95 mpy to 0.004 mpy in case of coatings densified in nitrogen and to 0.00068 mpy for coatings deposited on a plasma treated substrate and densified in air.     

Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium

We present results of an optical emission spectroscopic study of Plasma during Electrolytic Oxidation (PEO) of aluminium in citric acid and in sodium-tungstate water solutions. The line shape analysis of the first two hydrogen Balmer lines indicates presence of two PEO processes characterized by relatively low electron number densities N_e ≈ 0.8 × 10¹⁵ cm^− 3 and at 2.5 × 10¹⁶ cm^− 3. Apart from these two N_e values, N_e ≈ 6.0 × 10¹⁶ cm^− 3 is determined from the width of the Al II 704.2 nm line. Three considerably different N_e values imply presence of three types of discharge during PEO of aluminium. The electron temperature T_e is determined from relative line intensities of the O II (T_e ≈ 38,300 K) and W I (T_e ≈ 3300 K) lines. The use of two databases of transition probabilities for O II lines introduces systematic difference of reported T_e results.     

Near cathode optical emission spectroscopy in N2-H2 glow discharge plasma

Optical emission spectroscopy (OES) is a non-intrusive diagnostic technique, widely used to study different kinds of plasmas. In the present work, a locally resolved OES technique was used to obtain near cathode (substrate) emission spectra for N₂-H₂ glow discharges. It was observed that, along with N₂⁺ and N₂ lines, the characteristic atomic nitrogen lines at 742.3 nm (3p ⁴S⁰_3/2 → 3s ⁴P_1/2), 744.2 nm (3p ⁴S⁰_3/2 → 3s ⁴P_3/2), 746.8 nm (3p ⁴S⁰_3/2 → 3s ⁴P_5/2) and H_α (656.3 nm) were the main emissions coming from the sheath region that shrouded the cathode. A qualitative analysis of the spectral lines near the cathode has been done in order to understand the mechanism of plasma nitriding and the role played by the hydrogen in the nitriding process. The decrease in local intensity of these atomic lines with hydrogen composition suggests that the effect of hydrogen is to enhance the sticking/adsorption of N on the cathode surface.     

Cerium insertion in 316L passive film: Effect on conductivity and corrosion resistance performances of metallic bipolar plates for PEM fuel cell application

A surface treatment to modify passive film properties formed on 316L stainless steel has been designed for bipolar plate application in PEMFC systems. This one is based on cerium insertion from electrochemical technique. The effect of this element addition on the composition of the passive films was evaluated by X-ray photoelectron spectroscopy (XPS) and low energy ion spectroscopy (LEIS). Potentiodynamic and potentiostatic curves, and electrochemical impedance spectroscopy (EIS) were used for the characterisation of the corrosion protection in simulated PEMFC media. Interfacial contact resistance (ICR) was also evaluated. Results highlight an increase of the polarisation resistance of the cerium treated sample at ambient temperature, and a good corrosion resistance in anodic and cathodic simulated media. An important diminution of the IRC is obtained (80% with respect to the reference), which is attributed to the enhancement of the passive layer conductivity of the processed sample. Surface analyses show a great enrichment of cerium in first atomic layers of the films, which explains the better conductivity observed by an increase of charge carrier density in the passive film.     

Optical emission spectroscopy studies of discharge mechanism and plasma characteristics during plasma electrolytic oxidation of magnesium in different electrolytes

The discharge mechanism of the plasma electrolytic oxidation (PEO) process in different electrolytes was investigated by examining the variation of the optical emission spectra (OES). The spectrum of active species existed in the bubble layer. The bubble layer was initially broken down, followed by the breakdown of the dielectric barrier layer. Breakdown is the initial stage of discharge. A micro-discharge formation model, which assumes that the discharge ignition in the bubble layer developed at the oxide/electrolyte interface, was proposed. The active plasma species that appeared in different electrolytes during the PEO process were also studied. The appearance order of the excited active plasma species depended on the energy that the orbit transition of the species needed, but was not related to the anion concentration in the electrolyte. The anions in the electrolyte, except the OH⁻, also had little influence on the composition of the active plasma species during the PEO process. The active plasma species were mainly composed of metal atoms, metal cations, and gases produced by water decomposition. The electron temperature of the excited hydrogen was between 6 × 10³ and 3 × 10⁴ K. The high temperature provided the possibility of ceramic film melting and sintering. The source and transition of the active plasma species were also studied. They were found to undergo dissociation, ionization, and excitation processes.     

RF plasma-polymerization of acetylene: Correlation between plasma diagnostics and deposit characteristics

Clay-polymer nanocomposites are developed to obtain better mechanical, electrical or permeability properties controlled by the interaction energy between polymer chains and filler dispersed in polymeric blend. The dispersion of clay particles in polymers was achieved by chemical pre-treatment of the individual clay layers. The technique of acetylene plasma deposition is proposed as an alternative route to alter hydrophilic character of the clay. Since the clay is a complex material, plasma-polymer, coated silicon wafer was used as reference and characterized by different surface techniques. Different plasma parameters were chosen to lead to the most hydrophobic coating. Optical Emission Spectroscopy (OES) was also used to establish the relationship between the plasma chemistry and the properties of the deposit layer.     

Characterization of AC PEO coatings on magnesium alloys

Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys. The findings revealed initiation and gradual increase in the number of discharges after 2-4 ms of each anodic pulse once a critical voltage was reached. No discharges were observed during the cathodic half-cycles. The lifetimes of discharges were in the range of 0.05-4 ms. A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270-450 HV_0.05. The coatings grew at a rate in the range 4.0-7.5 µm min^− 1, depending on the substrate composition. Regardless of the substrate, the coatings consisted of MgO and Mg₂SiO₄, with incorporation of alloying element species. Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20-40% of the coating thickness. A thin barrier layer consisting of polycrystalline MgO was located next to the alloy. The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 3.5 wt.% NaCl was reduced by 2-4 orders of magnitude by the PEO treatment.     

Effect of absorbed moisture on the atmospheric plasma etching of polyamide fibers

A potential problem for the atmospheric pressure plasma treatment is that the moisture absorbed by the substrate may influence plasma surface modification processes. This study evaluated the effect of moisture regain on the surface morphology change of polyamide fibers by plasma etching. Polyamide 6, poly(p-phenylene terephthalamide) (PPTA, aromatic polyamide), wool (polyamide 2), and ultrahigh modulus polyethylene (UHMPE, polyamide infinity) fibers, were selected to represent various polyamide molecular structures. The fibers were plasma treated at three moisture regains corresponding to three different relative humidity levels (10, 65, and 100%). Scanning electron microscope (SEM) showed that no apparent morphology change was observed on the surface of UHMPE and PPTA fibers. Under the nano-scale surface analysis of atomic force microscopy (AFM), however, rougher surface of UHMPE and PPTA fibers appeared with elevated relative humidity or higher moisture regain. In terms of polyamide 6 and wool, SEM images revealed that compared to the slight plasma etching effect of fibers with the lowest moisture regain, a thin surface layer of the treated fibers with higher moisture regain was partially or completely peeled off. It may be concluded that fiber moisture regain plays an important role in atmospheric pressure plasma etching of polyamide fibers, which may be mainly due to the interaction between the absorbed water and the polymer molecules. It can be concluded that the etching rate of atmospheric pressure plasma for a polymer depends on its moisture regain, intermolecular forces, crystallinity, and molecular structure.     

Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Plasma spraying is a well developed and widely used technology, successfully applied for ceramic and metal coatings in many fields of applications such as aeronautics, gas turbine, automotive or medical. The coatings obtained are usually intentionally porous and thick (more than 100 μm). Presently, thin films (< µm) are deposited using various physical vapour deposition (PVD) or chemical vapour deposition (CVD) processes with low deposition rates. In this paper, we make use of the high enthalpy and high ionisation degree of the plasma jet of conventional plasma spraying guns operated at low pressure (mbar) to obtain dense coatings by CVD from gaseous and/or liquid precursors. The advantages of such thermal plasma CVD processes are the high deposition rates to obtain dense and thin layers, and the possibility of combining these thin films with thermally sprayed coatings using the same equipment.An efficient injection and mixing of the liquid and gaseous precursors in the plasma jet, which is especially challenging for liquids in low-pressure processes, has been obtained by extensive developments and proper equipment design. Results of several different coatings based on liquid and gaseous precursors are presented. In particular, SiO_x thin films from HMDSO (Hexamethyldisiloxane, C₆H₁₈OSi₂) precursor and oxygen can be deposited over large areas (50 cm diameter) at typical deposition rates of 35 nm/s, with a precursor-to-film conversion efficiency exceeding 50%. For the case of amorphous carbon deposited from CH₄ or C₂H₄, deposition rates exceeding 25 nm/s are obtained. Results from mass spectrometry of the gas sampled in the plasma jet by an enthalpy probe show that the depletion of hydrocarbon precursors can reach 95% and that higher hydrocarbon species are formed by secondary reactions. In the case of carbon-containing precursors, results from mapping of the optical emission intensity throughout the plasma jet volume are presented. The formation and transport of excited precursor-based species, such as CH, C, C₂, and H are addressed. These results show, in particular, that the very high dissociation efficiency of the precursors takes place through (dissociative) charge exchange from Ar⁺ ions and subsequent dissociative recombination with low energy electrons. The peculiarities of plasma chemistry taking place in the low-pressure plasma jet compared to conventional low-density non-equilibrium plasmas are outlined.     

Improvement of adhesion of conductive polypyrrole coating on wool and polyester fabrics using atmospheric plasma treatment

In this paper wool and polyester fabrics were pretreated with atmospheric plasma glow discharge (APGD) to improve the ability of the substrate to bond with anthraquinone-2-sulfonic acid doped conducting polypyrrole coating. A range of APGD gas mixtures and treatment times were investigated. APGD treated fabrics were tested for surface contact angle, wettability and surface energy change. Effect of the plasma treatment on the binding strength was analyzed by studying abrasion resistance, surface resistivity and reflectance. Investigations showed that treated fabrics exhibited better hydrophilicity and increased surface energy. Surface treatment by an APGD gas mixture of 95% helium/5% nitrogen yielded the best results with respect to coating uniformity, abrasion resistance and conductivity.     

Growth of uniform ZnO nanoparticles by a microwave plasma process

ZnO nanoparticle coatings with a controlled size distribution have been grown on quartz substrates by a novel microwave plasma assisted spray (MPAS) technique. This study presents the analysis of structure, photoluminescence, and magnetic properties of particle coatings with two distinctly different mean particle sizes (400 nm and 200 nm). X-ray diffraction patterns show a typical wurtzite structure without any impurity phases for the nanoparticle coatings. SEM and TEM investigations have shown the grown nanoparticles to be spherical and well separated with a narrow size distribution. Nanoparticles are polycrystalline with smaller grain sizes associated with the smaller particle sizes. Photoluminescence (PL) spectra reveal the presence of oxygen vacancy related defects in the 400 nm nanoparticles, which become less pronounced in the 200 nm nanoparticles. The 400 nm nanoparticles are found to exhibit room-temperature ferromagnetism with a clear hysteretic behavior, while the 200 nm nanoparticles are diamagnetic even down to 10 K. These results suggest the oxygen vacancies were the cause for defect-induced ferromagnetism in the 400 nm nanoparticles.     

Study by optical emission spectroscopy of a physical vapour deposition process for the synthesis of complex AlCuFe(B) coatings

We investigate the formation of complex Al-Cu metallic phases by magnetron dc sputtering of elemental targets followed by low temperature annealing. We show that all stable low temperature phases of the Al-Cu system can be grown this way and suggest that some of these phases could be used as an interfacial layer to improve the adhesion of quasicrystalline AlCuFe coatings on metal substrate. Then we introduce optical emission spectroscopy as a tool to monitor the composition of thin films obtained by sputtering Al₆₃Cu₂₅Fe₁₂ and Al_59.5Cu_25.3Fe_12.2B₃ targets. This method should improve the synthesis of complex metallic alloy phases as new coating materials of industrial interest.     

Role of side assisting gas on plasma and energy transmission during CO2 laser welding

In this paper, the effect of how the side assisting gas influences the laser-induced plasma and energy transmission in CO₂ laser welding is studied. Under different configurations of the side assisting gas, the optical emission and spectrum signals from the laser-induced plasma were obtained and compared. From signal analysis and evaluation of weld results, it shows that the application of side assisting gas greatly suppresses the laser-induced plasma and enhances the efficiency of laser energy transmission, which in turn results in a sound weld quality with full penetration. A physical model of plasma consisting of diffusive plasma and residual plasma was also proposed and validated to illustrate the role of side assisting gas. The results suggest that the diffusive plasma can be blown away by the side assisting gas. This observation provides a promising solution to increase the absorption rate and utilization efficiency of the laser energy during laser welding by diminishing the diffusive plasma through side assisting gas.     

Effect of carbon fiber surface treatment on Cu electrodeposition: The electrochemical behavior and the morphology of Cu deposits

The relationship between surface functional groups and electrochemical behaviors of polyacrylonitrile (PAN)-based carbon fibers (CFs) differentiated by oxidation treatment in air was studied. The chemical character of the CFs surface was estimated by X-ray photoelectron spectroscopy (XPS) and the electrochemical behavior of treated CFs in CuSO₄ plating solution was studied by electrochemical setup. The influence of functional groups on the morphology of copper deposits was characterized by field-emission scanning electron microscopy (FESEM). It was found that the O/C atomic ratio rose rapidly from 23.05% (as-received carbon fibers) to 42.83% as the oxidation temperature was increased to 400 °C and the content of -CO was the highest. Concentrations and types of the functional groups on CFs surface showed a close connection with the electrochemical response of CFs in CuSO₄ plating bath. It was showed that Cu electrodeposition was the interaction of applied voltage and the reduction of surface functional groups. With the functional groups increased, the quantities of the Cu nuclei increased, further the morphology of deposited Cu was affected.     

Characteristics of atmospheric pressure plasma jet generated by compact and inexpensive high voltage modulator

An atmospheric pressure plasma jet has been successfully generated using a compact high voltage modulator driven by 12 V alkaline batteries. A jet nozzle was composed of a quartz tube with two cylindrical electrodes. The grounded electrode was rolled on the tube and the powered one was inserted in the tube for discharging at lower voltage. V-Q Lissajous analysis of the plasma jet indicated that energy and power consumed for the plasma generation were linear along the distance between the electrodes. Length of the plasma plume from the tip of the tube was 11 mm for the gap length of 5 mm and the input voltage DC of 12 V. At the input voltage, the energy and power consumed for the plasma generation were 4.1 μJ and 0.12 W, respectively. Optical emission spectroscopy analysis of the plasma showed that the plasma contained hydroxyl radicals and exited nitrogen molecules which are chemically active species. The plasma jet can be applied to plasma cleaning for material surface though was generated with the alkaline batteries.     

Formation and structure of plasma sprayed manganese-cobalt spinel coatings on preheated metallic interconnector plates

Manganese-cobalt spinels are promising materials for protective coatings on metallic interconnector plates in solid oxide fuel cells. These protective coatings are used to prevent growth and evaporation of chromium oxide, chromium poisoning and ageing phenomena of the cathode side of the fuel cell. It is well known that chromium trioxide and chromium hydroxide on interconnector plate may easily evaporate at high temperatures and transform back to chromium oxide at the cathodes active area and cause degradation of the solid oxide fuel cell performance.In the present study, plasma spraying together with a substrate pre- and simultaneous heating was found to be an appropriate technique to control the formation and densification of the coating. When plasma spraying Mn-Co spinel powder for cold substrate, high cracking effect inside the splats and in the formed coating was noticed. When applying the molten drops to the substrate heated up to 450 °C amount of cracks in single splats and coatings could be mostly eliminated. The splats stayed in the liquid state longer, which effected for spreading properties and spinel structure could be better preserved by longer crystallize time.     

Effect of plasma surface tungstenising on the friction and wear of Ti2AlNb-based alloys

To obtain a strong bond between W coatings and the substrate, a novel graded tungstenised layer on Ti-Al-Nb alloys was produced using a double glow plasma surface alloying technology and a special graded tribological coating was designed. The microstructural results showed that the tungstenised layer was distributed in a graded manner and was mainly comprised of W- or Ti-rich Ti_xW_1?x phases. Varying the friction conditions indicated that an increase in the load and sliding speed led to an increase of the friction coefficient and wear rate of the tungstenised layer at room temperature. These changes were mainly caused by the graded distribution of the W composition and the change in surface contact status. The results indicated that the friction and wear properties of Ti-Al-Nb alloys were greatly improved by the surface tungstenising.     

Effect of Li2O on structure and optical properties of lithium bismosilicate glasses

Bismuth–silicate glasses containing lithium oxide having composition xLi₂O·(85 − x)Bi₂O₃·15SiO₂ (5 ≤ x ≤ 45 mol%) were prepared by melt quench technique. Density, molar volume and glass transition temperature for all the glass samples were measured. IR spectroscopy was used for structural studies of these glasses in the range from 400 to 1400 cm⁻¹. The increase of Li₂O content in glass matrix results in the decrease of the Si–O–Si bond angle and increase in the covalence nature of Bi–O bond. IR spectra suggest the presence of distorted [BiO₆] octahedral units and the degree of distortion increases with the addition of Li₂O in these glasses. The optical transmission spectra in the wavelength range from 200 to 3300 nm were recorded and optical band gap (E_g) was calculated. The values of E_g lie in between 2.81 and 2.98 eV. The values of average electronic oxide polarizability as well as optical basicity in these glasses were found to be dependent directly on Bi₂O₃/Li₂O ratio.     

等离子喷涂纳米氧化锆涂层的热震失效机理研究

采用合理的喷涂工艺参数制备了纳米氧化锆涂层并在1100度下测试了其热震性能,利用XRD,SEM和TEM对涂层的结构及表面形貌进行了分析。实验结果显示孔隙或早期存在的微裂纹附近的纳米颗粒在热震实验过程中会长大。通过对结构分析,我们提出了在循环应力作用下纳米涂层的失效机理。即随着纳米结构涂层中的大多数或者全部的纳米颗粒长大后,纳米结构随之变为准微米结构,其热震失效模式将类似于传统微米涂层的失效方式—裂纹形成,扩展直至最后涂层剥落。