Low-pressure Microwave Plasma Research Articles

Selection of reference genes for expression profiling in biostimulation research of soybean

BackgroundPlant biostimulants constitute a promising environmentally friendly alternative for increasing crop yield and tolerance to unfavorable conditions. Among various types of such formulations, botanical extracts are gaining more recognition as products supporting plant performance. Moreover, novel tools such as cold-plasma or low-pressure microwave plasma discharge are being proposed as techniques that might improve their efficacy. Elucidation of the biostimulant’s mode of action requires complex research at a molecular level. Transcriptional changes occurring after biostimulant spraying might be investigated using RT-qPCR. However, this technique requires data normalization against stable endogenous controls.ResultsHere, we tested the expression stability of ten candidate genes in soybean plants exposed to various biostimulants treatment. Selection of the best-performing reference genes was conducted using four algorithms (geNorm, NormFinder, BestKeeper, and ΔCt method). According to the obtained results, Bic-C2 (RNA-binding protein Bicaudal-C) and CYP (cyclophilin type peptidyl-prolyl cis–trans isomerase) showed highest expression stability, while expression of EF1B (elongation factor 1-beta) fluctuated the most among a tested set of candidate genes.ConclusionsOverall, we recommend using Bic-C2 together with CYP for the RT-qPCR data normalization in soybean biostimulation experiments. To our best knowledge, this is the first comprehensive study of reference genes stability in plants subjected to biostimulant treatment. The results of this study will aid in further biostimulant research in crop plants, facilitating analyses performed on the transcriptional level.Graphical

The Impact of Plasma Surface Treatments on the Mechanical Properties and Magnetic Performance of FDM-Printed NdFeB/PA12 Magnets.

This study presents a novel approach for improving the interfacial adhesion between Nd-Fe-B spherical magnetic powders and polyamide 12 (PA12) in polymer-bonded magnets using plasma treatments. By applying radio frequency plasma to the magnetic powder and low-pressure microwave plasma to PA12, we achieved a notable enhancement in the mechanical and environmental stability of fused deposition modeling (FDM)-printed Nd-Fe-B/PA12 magnets. The densities of the FDM-printed materials ranged from 92% to 94% of their theoretical values, with magnetic remanence (Br) ranging from 85% to 89% of the theoretical values across all batches. The dual plasma-treated batch demonstrated an optimal mechanical profile with an elastic modulus of 578 MPa and the highest ductility at 21%, along with a tensile strength range of 6 to 7 MPa across all batches. Flexural testing indicated that this batch also achieved the highest flexural strength of 15 MPa with a strain of 5%. Environmental stability assessments confirmed that applied plasma treatments did not compromise resistance to corrosion, evidenced by negligible flux loss in both hygrothermal and bulk corrosion tests. These results highlight plasma treatment's potential to enhance mechanical strength, magnetic performance, and environmental stability.

Nanoparticles synthesis in microwave plasmas: peculiarities and comprehensive insight

Low-pressure plasma processes are routinely used to grow, functionalize or etch materials, and thanks to some of its unique attributes, plasma has become a major player for some applications such as microelectronics. Plasma processes are however still at a research level when it comes to the synthesis and functionalization of nanoparticles. Yet plasma processes can offer a particularly suitable solution to produce nanoparticles having very peculiar features since they enable to: (i) reach particle with a variety of chemical compositions, (ii) tune the size and density of the particle cloud by acting on the transport dynamics of neutral or charged particles through a convenient setting of the thermal gradients or the electric field topology in the reactor chamber and (iii) manipulate nanoparticles and deposit them directly onto a substrate, or codeposit them along with a continuous film to produce nanocomposites or (iv) use them as a template to produce 1D materials. In this article, we present an experimental investigation of nanoparticles synthesis and dynamics in low-pressure microwave plasmas by combining time-resolved and in-situ laser extinction and scattering diagnostics, QCL absorption spectroscopy, mass spectrometry, optical emission spectroscopy and SEM along with a particle transport model. We showed for the first time the thermophoresis-driven dynamic of particle cloud in electrodless microwave plasmas. We showed that this effect is linked to particular fluctuations in the plasma composition and results in the formation of a void region in the bulk of the plasma surrounded by a particle cloud in the peripherical post-discharge. We also reveals and analyze the kinetics of precursor dissociation and molecular growth that result in the observed nanoparticle nucleation.

Low-Pressure Microwave Plasma Reduction of Iron and Copper Salt Compounds at Low Temperatures for Oxidation State Alteration and Functional Applications.

The influence of plasma-reduction treatment on iron and copper compounds at different oxidation states was investigated in this study. For this purpose, reduction experiments were carried out with artificially generated patina on metal sheets and with metal salt crystals of iron(II) sulfate (FeSO4), iron(III) chloride (FeCl3), and copper(II) chloride (CuCl2), as well as with the metal salt thin films of these compounds. All the experiments were carried out under cold low-pressure microwave plasma conditions; the main focus was on plasma reduction at a low pressure in order to evaluate an implementable process in a parylene-coating device. Usually, plasma is used within the parylene-coating process as a supporting tool for adhesion improvement and micro-cleaning efforts. This article offers another useful application for implementing plasma treatment as a reactive medium in order to apply different functionalities by an alteration in the oxidation state. The effect of microwave plasmas on metal surfaces and metal composite materials has been widely studied. In contrast, this work deals with metal salt surfaces generated from a solution and the influence of microwave plasma on metal chlorides and sulfates. While the plasma reduction of metal compounds commonly succeeds with hydrogen-containing plasmas at high temperatures, this study shows a new reduction process that reduces iron salts at temperatures between 30 and 50 °C. A novelty of this study is the alteration in the redox state of the base and noble metal materials within a parylene-coating device with the help of an implemented microwave generator. Another novelty of this study is treating metal salt thin layers for reduction purposes in order to provide the opportunity to include subsequent coating experiments to create parylene metal multilayers. Another new aspect of this study is the adapted reduction process of thin metal salt layers consisting of either noble or base metals, with an air plasma pre-treatment prior to the hydrogen-containing plasma-reduction procedure.

Investigation of nitrogen fixation in low-pressure microwave plasma via rotational–vibrational NO and N2 kinetics

A pulsed microwave surfaguide-type discharge used for nitrogen fixation in N2–O2 gas mixtures is characterized by optical emission spectroscopy. Results show that both rotational and vibrational temperatures are elevated in the active zone near the waveguide, decaying along the discharge tube in both upstream and downstream. The characteristic length of optical emission from NO(A-X) transition gets contracted when pressure increases, specifically at P≥2 Torr. The degree of vibrational non-equilibrium (defined as the ratio between vibrational and rotational temperatures) is decreased by a factor of two when pressure changes from 0.6 to 10 Torr. Non-equilibrium likely disappears as the discharge pressure rises, resulting in a gas temperature elevation. A correlation between gas residence time, pulse duration, and characteristic times for different energy transfer channels is discussed. The rotational–vibrational dynamics differs for NO and N2 during the pulse. Both species lose vibrational excitation at the beginning of the pulse, whereas N2 gets re-excited again during the second half of the pulse, which may occur as a result of an efficient pumping-up effect through the vibrational–vibrational energy transfer. At the same time, vibrational relaxation of NO takes place primarily due to a strong vibrational–translational exchange via NO–O2 and NO–O collisions.

Experimental Investigation of Low-Pressure Microwave Plasma with Oxygen and Argon for Internal Sterilization of Medical Devices with Slender Catheters

This study explores the application of microwave plasma sterilization methods to prevent surgical site infections, specifically targeting the challenge of sterilizing slender catheters used in medical devices. An experimental setup was designed to verify the sterilization effectiveness by varying input power levels, treatment times, and the ratio of injected gases. The temperature inside the sterilization setup was closely monitored to ensure that the death of bacterial spores was not caused by thermal effects. The results demonstrate that microwave plasma has a robust sterilization effect when the microwave power is 200 W, the treatment time is 10 minutes, and the ratio of oxygen to argon is higher than 14.45%. The study highlights that the sterilization effect of plasma significantly depends on the composition of the discharge gas. The experimental setup is capable of simulating sterilization inside slender catheters, indicating the potential of microwave plasma sterilization for such medical devices.

Synthesis of Plasma-Polymerized Toluene Coatings by Microwave Discharge

Plasma- polymerized coatings were successfully applied on aluminum alloy, AA2024, surface for corrosion protection. The plasma polymerization process was carried out by low pressure microwave plasma at room temperature. The effect of microwave plasma power on the corrosion resistance of polymer coatings was investigated using the potentiodynamic polarization technique. As the microwave plasma power increased, the relative protective efficiency increased. Polymer coatings on alloy surfaces suppressed both anodic and cathodic reactions. The increment in protective efficiency was due to a higher degree of cross-linking in the coating. These findings suggest that the toluene polymer coatings provide a considerable protection barrier for aluminum alloys.

New chemical pathway for large-area deposition of doped diamond films by linear antenna microwave plasma chemical vapor deposition

Today, boron-doped diamond (BDD) is among the key materials for electrochemical sensing and advanced oxidation of micropollutants. Therefore, a cost-effective fabrication of large-area BDD electrodes is of high interest. We present an implementation of a liquid-phase boron precursor trimethyl borate for large-area deposition of boron-doped diamond films by linear antenna microwave plasma CVD. Trimethyl borate vapors were used not only as a source of boron for doping but also as the only source of carbon and oxygen, while completely saturating the requirements for the growth of high-quality boron-doped diamond films. However, to allow for control over the doping level through maintaining the B/C and B/O ratios, carbon dioxide was employed as an additional source of carbon and oxygen. The film morphology was controllable from microcrystalline to ultra-nanocrystalline by changing the concentrations of trimethyl borate. Using this unique precursor system, we were able to grow diamond films with a doping level in range from 8 × 1017 cm−3 to 2 × 1022 cm−3 and resistivity as low as 1.16 × 10−2 Ω·cm. Low activation energies were calculated from the Arrhenius plot and growth rates as high as up to 170 nm/h for the low pressure microwave plasma CVD were reached. The investigation of the plasma emission spectrum revealed a chemical composition similar to that of hydrogen-rich plasmas with methane, and the proposed chemical reactions indicate that the diamond growth takes place via the hydroxyl radical. The results demonstrated that trimethyl borate is a suitable source of carbon and boron for the large-area growth of highly boron-doped diamond via low pressure microwave plasma CVD methods.

Experimental Investigation into the Influence of Plasma Technology on Seed Surface Wettability

The influence of atmospheric plasma discharge (APD) of the Gliding Arc type and low-pressure microwave plasma discharge (LPMD) on the class of various seeds was investigated. Pea, wheat, and sunflower seeds, representing legumes, cereals, and oilseeds, respectively, were selected for the treatment. Our study aimed to verify the effect of plasma treatment on the water contact angle and the wettability of the seeds. Treatment time, working gas flow and microwave power were varied to determine their impact. All treated and untreated variants were used to conduct the water contact angle measurements to compare their wettability. APD treatment seemed to be utterly ineffective for improving the wettability for most process parameters. On the contrary, LPMD manifested a much more efficient impact. The maximum effect was found for the 800 W microwave power applied to the pea seed for 30 s. The contact angle achieved decreased by approximately 50% compared to the untreated sample. These results indicate that LPMD may be an effective alternative to traditional pre-sowing seed treatments used in agriculture for water intake enhancement. Still, it is strongly dependent on the seed’s type and the used process parameters.

Effect of microwave plasma treatment on magnetic and photocatalytic response of manganese ferrite nanoparticles for wastewater treatment

The conventionally synthesized nano-ferrite materials do not possess bulk properties, generally required for their use in mainstream industry. To make ferrite nanoparticles clinically applicable materials, it is important to have good control over morphology and optical properties of these materials. In this study, low-pressure microwave plasma was used to tailor the structural properties and surface chemistry of manganese ferrite nanoparticles. A facile sol-gel method was used to prepare cubic spinal structures of manganese ferrite nanoparticles. These nanoparticles were exposed to oxygen plasma sustained with a microwave source for improving their magnetic and photocatalytic activities. The techniques like XRD, SEM, PL, UV-Vis DRS, transient photocurrent response and EIS were used to characterize the samples. The plasma treated nanoparticles were used to degraded methyl blue (MB) dye in the solution. The photocatalytic activity showed 85% degradation of MB after 100 min of exposure of visible light. The second part of the paper studied the magnetic properties of the nanoparticles. The saturation magnetization decreased from 0.78 emu/g to 0.68 emu/g after plasma treatment of nanoparticles.

Highly Hydrophobic Organosilane-Functionalized Cellulose: A Promising Filler for Thermoplastic Composites.

The aim of this work is to design and optimize the process of functionalization of cellulose fibers by organosilane functional groups using low-pressure microwave plasma discharge with hexamethyldisiloxane (HMDSO) precursor in order to prepare a compatible hydrophobic filler for composites with nonpolar thermoplastic matrices. Particular attention was paid to the study of agglomeration of cellulose fibers in the mixture with polypropylene. In our contribution, the dependence of the surface wettability on used process gas and treatment time was investigated. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses were applied to characterize the surface morphology and chemical composition of the cellulose fibers. It was observed that the plasma treatment in oxygen process gas led to the functionalization of cellulose fibers by organosilane functional groups without degradation. In addition, the treated cellulose was highly hydrophobic with water contact angle up to 143°. The use of treated cellulose allowed to obtain a homogeneous mixture with polypropylene powder due to the significantly lower tendency of the functionalized cellulose fibers to agglomerate.

Localized breaking of parameter uniformity by macroscopically-negative-permeability metamaterial in low-pressure microwave plasma

Measurements of plasma parameters and transmitted microwaves are performed in a composite of double-split-ring resonators (DSRRs) making magnetic permeability negative and plasmas excited by a 2.45 GHz microwave. We launch the microwave at 100–200 W, modulated by a pulse wave at a low frequency and a low duty ratio to detect time evolutions of plasma parameters. We perform the measurements at different positions with varying distance from the supporting plate of the DSRRs, and find enhanced wave transmission in the composite with non-uniform profiles of electron temperature. To reinforce the measurements of electron temperature, we evaluate the electron energy distribution function (EEDF) by deriving the second deviation of electron current, upon which data smoothing methods are applied, and the estimated EEDF becomes spatially non-uniform with a similar profile to electron temperature. In terms of the validity of results on the enhanced wave transmission, one-dimensional electromagnetic particle simulations reveal sneaking microwaves inside the composite. In comparison with cases of the negative-permeability and the passive (positive-permeability) DSRRs, we confirm that the non-uniformity is attributed to the magnetic resonance of the DSRRs, which is a microscopic effect of the DSRRs to plasma generation, whereas their macroscopic effect makes permeability negative and enhances wave energy transfer inside the composite.

Correlation between the surface chemistry, the surface free energy and the adhesion of metallic coatings onto plasma-treated Poly(ether ether ketone)

The effects of the surface modification of poly(ether ether ketone) (PEEK) by plasma treatment have been characterized by X-ray Photoelectron Spectroscopy and wettability measurement, while the adhesion of metallic thin films grown by PVD has been assessed by the pull-off test. To investigate the role of the modified surface chemistry on the practical adhesion stress of aluminium thin films, two different plasma technologies were considered: (i) atmospheric pressure plasma and (ii) low pressure microwave plasma. Though on different scales, both of these plasmas under oxidative conditions induced a significant increase of the surface concentration of CO (ketones) and COO (esters and acidic) functional groups, while COC (ether) groups remained constant. It is shown that after plasma functionalization, when the surface concentration of CO and COO are above a critical value, a strong correlation appears between the concentration of these polar groups, the wettability and the adhesion potential.

Role of Electron–Ion Dissociative Recombination in $$\hbox {CH}_{4}$$ Microwave Plasma on Basis of Simulations and Measurements of Electron Energy

C–H bond activation was studied in low pressure microwave plasma discharges in methane. Electron energy loss channels were analyzed in view of promoting vibrational excitation. The molecular dissociative recombination (DR) channel is concluded to play multiple roles in the hydrocarbon plasma chemistry. DR increases the electron temperature with input power density and simultaneously breaks the hydrocarbon chains. Depending on the ionic species considered, plasma density $$\hbox {n}_e$$ in the range of $$10^{17}-10^{19}\,\hbox {m}^{-3}$$ ($$10^{-6}-10^{-4}$$ ionization degree) and the electron mean energy $$ $$ in the range of $$2-4\hbox { eV}$$ were estimated on basis of a Boltzman solver. $$ $$ from $$2-3\hbox { eV}$$ measured with Thomson scattering anchored the microwave discharges in a preferential regime for vibrational excitation. The best agreement with experiments was obtained when $$\hbox {C}^{+}$$ is the dominant ion in the $$\hbox {CH}_{4}$$ microwave plasma, formed through successive DR and charge exchange reactions from molecular ions.

Controlling surface properties of electrospun polyphenylsulfone using plasma treatment and X-ray photoelectron spectroscopy

Plasma treatment is an efficient method to modify organic surfaces. In this work electrospun polyphenylsulfone was systematically subjected to low-pressure microwave plasma and atmospheric-pressure coplanar barrier discharge in order to control the surface chemistry, which is important for controlling surface properties. Polar anchor groups such as keto/aldehyde groups and especially carboxylic acid groups affect hydrophilicity. The composition of plasma-induced chemical anchor groups can be monitored (and thus controlled) by X-ray photoelectron spectroscopy. The atmospheric-pressure plasma provided subtle oxidation, and the low-pressure plasma provided significant oxidation that resulted in polyphenylsulfone surfaces with a very high hydrophilicity. The low-pressure plasma treated polyphenylsulfone exhibited a significant age effect over 212 days, which was attributed to a diffusion phenomenon where the polyphenylsulfone surface becomes enriched in non-oxidated polyphenylsulfone. It was shown that the improved hydrophilicity will diminish but not vanish in time.

Characterisation of titanium oxide layers using Raman spectroscopy and optical profilometry: Influence of oxide properties

Abstract This study evaluates the use of a combination of Raman spectroscopy and optical profilometry as a surface characterisation technique for the examination of oxide layers grown on titanium metal substrates. The titanium oxide layers with thickness of up to 8 µm, were obtained using a low-pressure oxygen microwave plasma treatment of the titanium metal substrate. The effect of the microwave plasma processing conditions (input power, pressure and treatment time) on the Raman bandwidth, intensity and peak position was evaluated. Also, the effect of these processing conditions on the surface roughness parameters (Sa, Sdq, Ssk and Sku) of the oxide layers was investigated. Analysis of the peak positions of Eg and A1g modes indicated that the effects of input power and chamber pressure was to induce a shift towards the lower frequency with increasing input power and pressure (1–2 kPa). The intensity of the Raman bands was found to be dependent on the morphology and surface chemistry of the oxide layer. The intensity of Raman band (A1g), was found to be particularly influenced by the average surface roughness (Sa) and the crystallite size. Exponential and polynomial relations were found to correlate with these properties. A two-latent variable Partial Least Squares Regression model developed on Raman spectral data could predict surface roughness with a coefficient of determination (R2) of approx. 0.87 when applied to the testing of an independent set of titanium oxide test coatings.

Research progress on atmospheric pressure lower-power microwave plasma sources and their application prospects

In comparison with low pressure microwave plasma, atmospheric pressure microwave plasma has some excellent properties such as high density, high activity, simple production equipment, and so on. However, the microwave plasma excited at high gas pressure is often appeared as the form of streamer discharge and the very higher incident power is needed for atmospheric pressure microwave discharges, thence it is difficult in realizing some special applications because of the expensive cost for microwave discharges at atmospheric gas pressure. Aiming at the difficult problem of low power microwave discharge in atmospheric pressure, the research status and application prospect of atmospheric pressure low power microwave plasma source are presented in this paper. Firstly, the research and development status of four kinds of international typical atmospheric pressure low power microwave plasma sources are summarized. Followed the research progress of numerical calculation is introduced. And then the physical mechanism of atmospheric pressure low power microwave discharge is discussed. Finally, several engineering application examples are recommended and their potential applications are prospected.

Study on electronic properties of diamond/SiNx-coated AlGaN/GaN high electron mobility transistors operating up to 500 °C

We report on the research related to integration and characterization of AlGaN/GaN heterostructure-based circular high electron mobility transistors (c-HEMTs) with diamond thin films. Prior to diamond deposition the transistors were coated by thin SiNx layer in order to protect AlGaN/GaN heterostructure from deposition process impact. The diamond thin film was selectively grown on the top of SiNx-coated c-HEMTs using low-pressure low-temperature microwave plasma deposition equipment with substrate table 20 × 30 cm2 from C/O/H gas mixture. We demonstrate the functionality of the diamond/SiNx-coated and the SiNx-coated AlGaN/GaN-based c-HEMTs in the temperature range from room temperature to 500 °C. The electrical measurements revealed increase of c-HEMTs' leakage currents at reversed gate voltages after diamond deposition, although the transistors remained operable in the tested temperature range. The increase of gate leakage currents and thus more negative pinch-off voltages were attributed to hydrogen penetrated towards AlGaN/GaN interface during the 60 h deposition process despite the SiNx protection. The influence of temperature rise on electronic properties of the diamond/SiNx-coated and the SiNx-coated AlGaN/GaN-based c-HEMTs is discussed.

Ejector-based sampling from low-pressure aerosol reactors

Online measurements of nanoparticles are necessary when rapid information about the particle size and mass distribution is needed. Currently, the application of online measurement techniques with commonly used instruments such as SMPS, CPMA and ELPI+ is not possible at low-pressure conditions. In this work, a commercial vacuum ejector is used as a simple tool to transfer nanoparticles from a low-pressure region to atmospheric pressure. The vacuum ejector is investigated for different process pressures between 120 and 170 mbar to measure size-selected aerosols in the range from 10 to 100 nm. It was found that the sampling with the vacuum ejector does not change the particle size. The gas and particle dilution factors as well as the particle losses are determined, so that quantitative measurements of the aerosol size distribution can be obtained. Additionally, the applicability of the vacuum ejector is tested during particle synthesis in a low-pressure microwave plasma reactor with a combination of online instrumentation. The direct transfer of the aerosol to atmospheric pressure allows real-time measurements. The primary particle size, mass mobility exponent and effective density are calculated exemplary based on parallel online ELPI+, SMPS and CPMA measurements and are compared to offline TEM analysis.

Composition, Mixing State and Water Affinity of Meteoric Smoke Analogue Nanoparticles Produced in a Non-Thermal Microwave Plasma Source

Abstract The article reports on the composition, mixing state and water affinity of iron silicate particles which were produced in a non-thermal low-pressure microwave plasma reactor. The particles are intended to be used as meteoric smoke particle analogues. We used the organometallic precursors ferrocene (Fe(C5H5)2) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) in various mixing ratios to produce nanoparticles with radii between 1 nm and 4 nm. The nanoparticles were deposited on sample grids and their stoichiometric composition was analyzed in an electron microscope using energy dispersive X-ray spectroscopy (EDS). We show that the pure silicon oxide and iron oxide particles consist of SiO2 and Fe2O3, respectively. For Fe:(Fe+Si) ratios between 0.2 and 0.8 our reactor produces (in contrast to other particle sources) mixed iron silicates with a stoichiometric composition according to FexSi(1−x)O3 (0≤x≤1). This indicates that the particles are formed by polymerization of FeO3 and SiO3 and that rearrangement to the more stable silicates ferrosilite (FeSiO3) and fayalite (Fe2SiO4) does not occur at these conditions. To investigate the internal mixing state of the particles, the H2O surface desorption energy of the particles was measured. We found that the nanoparticles are internally mixed and that differential coating resulting in a core-shell structure does not occur.