Microwave-induced Plasma Research Articles

Shock Exfoliation of Graphene Fluoride in Microwave.

An unprecedented microwave-based strategy is developed to facilitate solid-phase, instantaneous delamination and decomposition of graphite fluoride (GF) into few-layer, partially fluorinated graphene. The shock reaction occurs (and completes in few seconds) under microwave irradiation upon exposing GF to either "microwave-induced plasma" generated in vacuum or "catalyst effect" caused by intense sparking of graphite at ambient conditions. A detailed analysis of the structural and compositional transformations in these processes indicates that the GF experiences considerable exfoliation and defluorination, during which sp2 -bonded carbon is partially recovered despite significant structural defects being introduced. The exfoliated fluorinated graphene shows excellent electrochemical performance as anode materials in potassium ion batteries and as catalysts for the conversion of O2 to H2 O2 . This simple and scalable method requires minimal energy input and does not involve the use of other chemicals, which is attractive for extensive research in fluorine-containing graphene and its derivatives in laboratories and industrial applications.

Effects of a microwave-induced corona discharge plasma on premixed methane-air flames

A microwave resonator integrated with a floated electrode was used to initiate an additional plasma source to examine the effect on a flame plasma at atmospheric pressure. In the current system, the corona discharge occurred at 200 W of microwave power. The effect of a microwave-induced corona discharge plasma on a flame plasma was investigated using a double Langmuir probe. Through this probe, the electron temperature around the electrode tip was found to be significantly increased by over 350% relative to its original value, and the plasma concentration increased by over 400%. In addition to the effects of the microwave field, a DC field was observed after the plasma discharge, which resulted in a slight bending of the original profile of the ion concentration. Langmuir probe measurements along the flame axis revealed the boundary of effective region of electron acceleration by the microwave field. From the optical emission spectroscopy results measured from the gas where the discharge occurred, the mechanism underlying the flame speed enhancement as well as its application for flame stabilization could be rationalized.

Determination of Trace Elements in Meat and Fish Samples by MIP OES Using Solid-Phase Extraction

A new procedure for the determination of As, Cd, Cr, Cu, Hg, Pb, and Zn in fish and meat (bovine and ovine) samples by microwave-induced plasma optical emission spectrometry (MIP OES) was developed. The procedure is based on separation and preconcentration of analytes by solid-phase extraction (SPE), composed from the functionalization of silica with p-aminobenzoic acid (PABA-MCM-41). Factors that directly affect the SPE procedure as pH, buffer concentration, sample flow, and elution flow were evaluated through a factorial design (24). The optimized conditions employed at the proposed procedure were as follows: pH 9.5, 0.01 mol L−1 borate buffer concentration, sampling, and 6.0 mL min−1 of elution flow. Linear models were treated regarding the weighted regression, an optional strategy of the equipment, the weighted fit. The proposed procedure provides limits of quantification (LOQ) of 9.6, 2.8, 0.1, 0.2, 1.5, 13.3, and 3.9 μg g−1 for As, Cd, Cr, Cu, Hg, Pb, and Zn, respectively. Accuracy was assessed from the analysis of the certified reference materials (CRM) of dogfish liver (DOLT-5, NRC) and bovine liver (1577c, NIST), and a reference material of bovine liver (RM-Agro E3001a, EMBRAPA). The procedure was applied in local market meats (bovine and ovine) and fish (tilapia) samples, and it showed adequate for the determination of As, Cd, Cr, Cu, Hg, Pb, and Zn by MIP OES.

Application of microwave-induced plasma for extremely fast synthesis of large surface area activated carbon

This research applied microwave-induced plasma with KOH activation to obtain activated carbon. It was discovered that activated carbon with a large surface area can be synthesized by this treatment in an extremely short time, where activated carbon with a Brunauer–Emmett–Teller surface area of 1007, 1888, 2084 m2 g−1 was obtained within 80, 270, 330 s, respectively. It is remarkable that the investigated method can reduce the activation time by one to two order of magnitude in comparison to conventional thermal activation. Emission spectra from the plasma and pore structures of the products depending on KOH concentration are discussed here.

Antimicrobial effects of microwave-induced plasma torch (MiniMIP) treatment on Candida albicans biofilms.

SummaryThe susceptibility of Candida albicans biofilms to a non‐thermal plasma treatment has been investigated in terms of growth, survival and cell viability by a series of in vitro experiments. For different time periods, the C. albicans strain SC5314 was treated with a microwave‐induced plasma torch (MiniMIP). The MiniMIP treatment had a strong effect (reduction factor (RF) = 2.97 after 50 s treatment) at a distance of 3 cm between the nozzle and the superior regions of the biofilms. In addition, a viability reduction of 77% after a 20 s plasma treatment and a metabolism reduction of 90% after a 40 s plasma treatment time were observed for C. albicans. After such a treatment, the biofilms revealed an altered morphology of their cells by atomic force microscopy (AFM). Additionally, fluorescence microscopy and confocal laser scanning microscopy (CLSM) analyses of plasma‐treated biofilms showed that an inactivation of cells mainly appeared on the bottom side of the biofilms. Thus, the plasma inactivation of the overgrown surface reveals a new possibility to combat biofilms.

A Microwave-Induced Room-Temperature Atmospheric-Pressure Plasma Jet

A 2450-MHz microwave-induced room-temperature atmospheric-pressure plasma jet (RTAPPJ) based on a coaxial structure is proposed for applications of plasma biomedicine. This device is a simple structure with small size, low-power requirement and high controllability. It can induce an argon discharge at atmospheric pressure with as low as 15-W microwave power. The optical emission spectrum of the RTAPPJ shows various highly reactive radicals such as OH and O. The appropriate operation condition can reduce the temperature at the downstream of the flame to be around 30 °C. This portable device is designed to be suitable to manipulate different medical treatments.

Measurement of C2 and CH temperatures in argon‐acetylene low‐pressure microwave‐induced plasma

The emission spectra of C2(d3Πg–a3Πu), CH(A2Δ–X2Π), and CH(B2Σ–X2Π) bands are analysed to measure rotational Trot, vibrational Tvib, and gas temperature Tg from Ar/C2H2 (5–20% C2H2) microwave‐induced plasma (MIP). In case when helium and hydrogen are used in the gas mixture instead of argon, no significant change in Trot is noticed. Both studied temperatures are insensitive in terms of the C2H2 percentage. From CH(0–0, A2Δ–X2Π) band R2 branch lines, two Trot (Trot ∼ 520–580 K for J′ = 3–9 and Trot ∼ 1,700–1,800 K for J′ = 10–17) are determined. The lower Trot equals the Tg (500–700 K) measured from C2 bands in this study. The H2 Fulcher‐α diagonal bands are recorded as well in the H2/C2H2 mixtures and Trot∼750–900 K of the H2 ground state measured. Tvib ∼ 6,000 K in Ar/C2H2 MIP is calculated from the integral intensity ratio of C2(2,1) and C2(3,2) bands.

Plasma-Treated Air and Water-Assessment of Synergistic Antimicrobial Effects for Sanitation of Food Processing Surfaces and Environment.

The synergistic antimicrobial effects of plasma-processed air (PPA) and plasma-treated water (PTW), which are indirectly generated by a microwave-induced non-atmospheric pressure plasma, were investigated with the aid of proliferation assays. For this purpose, microorganisms (Listeria monocytogenes, Escherichia coli, Pectobacterium carotovorum, sporulated Bacillus atrophaeus) were cultivated as monocultures on specimens with polymeric surface structures. Both the distinct and synergistic antimicrobial potential of PPA and PTW were governed by the plasma-on time (5–50 s) and the treatment time of the specimens with PPA/PTW (1–5 min). In single PTW treatment of the bacteria, an elevation of the reduction factor with increasing treatment time could be observed (e.g., reduction factor of 2.4 to 3.0 for P. carotovorum). In comparison, the combination of PTW and subsequent PPA treatment leads to synergistic effects that are clearly not induced by longer treatment times. These findings have been valid for all bacteria (L. monocytogenes > P. carotovorum = E. coli). Controversially, the effect is reversed for endospores of B. atrophaeus. With pure PPA treatment, a strong inactivation at 50 s plasma-on time is detectable, whereas single PTW treatment shows no effect even with increasing treatment parameters. The use of synergistic effects of PTW for cleaning and PPA for drying shows a clear alternative for currently used sanitation methods in production plants. Highlights: Non-thermal atmospheric pressure microwave plasma source used indirect in two different modes—gaseous and liquid; Measurement of short and long-living nitrite and nitrate in corrosive gas PPA (plasma-processed air) and complex liquid PTW (plasma-treated water); Application of PTW and PPA in single and combined use for biological decontamination of different microorganisms.

Non-Isothermal Crystallisation Kinetics of Carbon Black- Graphene-Based Multimodal-Polyethylene Nanocomposites.

The effect of carbon black (CB) and microwave-induced plasma graphene (g) on the crystallisation kinetics of the multimodal high-density polyethylene was studied under non-isothermal conditions. The non-isothermal crystallisation behaviour of the multimodal-high-density polyethylene (HDPE), containing up to 5 wt.% graphene, was compared with that of neat multimodal-HDPE and its carbon black based nanocomposites. The results suggested that the non-isothermal crystallisation behaviour of polyethylene (PE)-g nanocomposites relied significantly on both the graphene content and the cooling rate. The addition of graphene caused a change in the mechanism of the nucleation and the crystal growth of the multimodal-HDPE, while carbon black was shown to have little effect. Combined Avrami and Ozawa equations were shown to be effective in describing the non-isothermal crystallisation behaviour of the neat multimodal-HDPE and its nanocomposites. The mean activation energy barrier (ΔE), required for the transportation of the molecular chains from the melt state to the growing crystal surface, gradually diminished as the graphene content increased, which is attributable to the nucleating agent effect of graphene platelets. On the contrary, the synergistic effect resulting from the PE-CB nanocomposite decreased the ΔE of the neat multimodal-HDPE significantly at the lowest carbon black content.

Optical analysis during reduction of nitric oxide in microwave-induced plasma promoted by activated cokes at atmospheric pressure

Emission from microwave (MW)-induced argon plasma promoted by a bed of activated coke granules was analyzed spectroscopically during non-catalytic reduction of nitric oxide (NO) at atmospheric pressure. The spectral analysis with a high resolution revealed that strong 5 peaks were recognized in a similar wavelength region to N2+ first negative band when NO molecules were included in argon plasma. The strongest peak is caused by the Ar(II) ions excited with energy corresponding to 388.033 nm in the progress of atomic transition 3s23p4(3P)4d 4D1/2 → 3s23p4(3P)4p 4D°1/2. This transition must be enhanced by an interaction with any intermediates or radical species yielded during NO reduction reactions. The plasma temperatures were determined by Boltzmann plots from intensities of any two peaks in the spectral band. The temperature was significantly different dependently on choice of two peaks, whose fact is implying non-equilibrium plasma. The maximum temperature was recognized at a high level of 40,000 K due to the activation during the reactions. The images of the plasma observed by high-speed camera showed dynamic features so that MW discharge like streamer was generated frequently in the reactor. The reaction rate could be correlated with the ratio of the area dominated by dynamic plasma to the cross section in the reactor.

Rapidly monitoring the quality of flavoring essence based on microwave-induced plasma ionization mass spectrometry and multivariate statistical analysis

Microwave-induced plasma ionization mass spectrometry (MIPI-MS) combined with multivariate statistical analysis was first applied to rapidly monitor the quality of tobacco flavoring essence. A small isolation and reaction chamber was set up between the ion source and the injection port of mass spectrometer to effectively eliminate the interference of external environment in the process of analyzing samples. The improved experimental apparatus (MIPI-MS) could achieve online and high-throughput analysis, with minimal sample preparation steps. Further, two types of tobacco flavoring essences with the similar appearance and physicochemical parameters were employed to verify the usability of the promising method in the field of quality monitoring. Firstly, the mass spectral fingerprint of each essence was established by the improved MIPI-MS method within 2 min. Then, two multivariate statistical processes were carried out to analyze mass spectral data. The similarity results indicated that the thresholds of tobacco flavoring essences from different batches were 1.512 and 2.638, respectively. The first three principal components of the established PLS-DA described 93.6% of the total variability, and provided a visualized comparison for the two types of flavoring materials. Finally, the adulterated samples were successfully distinguished by employing the two multivariate statistical processes.

Multi-flow calibration applied to microwave-induced plasma optical emission spectrometry

A novel calibration method based on multiple nebulization gas flow rates is used to improve accuracy and sample throughput in microwave-induced plasma optical emission spectrometry.

Microwave-Assisted Digestion Procedures Using Diluted Oxidant Mixture for Elemental Analysis of Crustaceans by MIP OES.

The aim of this study was to evaluate three digestion procedures using a diluted oxidant mixture of nitric acid, hydrogen peroxide, and water for determination of Cu, Fe, K, Mg, Mn, and Zn in shrimp and crab samples from Northern Brazil by microwave-induced plasma optical emission spectrometry. The efficiency of the digestion procedure was evaluated by comparing the residual carbon content, residual acidity, and elemental concentrations obtained in the samples. Digestion procedure 2 (4mL HNO3 + 4mL H2O2 + 4mL H2O) was chosen due the adequate residual carbon content (5.85%) and low residual acidity (2.94molL-1). The recoveries obtained using the certified reference material (DORM-4) varied from 90 to 105%. High contents of K and Mg were found in the studied crustaceans. Also, it can give an excellent contribution to the recommended daily intake of Cu and Zn, indicating that these foods can be considered a good source of these minerals for the human diet.

Advancing the Use of High-Performance Graphene-Based Multimodal Polymer Nanocomposite at Scale.

The production of an innovative, high-performance graphene-based polymer nanocomposite using cost-effective techniques was pursued in this study. Well-dispersed and uniformly distributed graphene platelets within a polymer matrix, with strong interfacial bonding between the platelets and the matrix, provided an optimal nanocomposite system for industrial interest. This study reports on the reinforcement of high molecular weight multimodal-high-density polyethylene reinforced by a microwave-induced plasma graphene, using melt intercalation. The tailored process included designing a suitable screw configuration, paired with coordinating extruder conditions and blending techniques. This enabled the polymer to sufficiently degrade, predominantly through thermomechanical-degradation, as well as thermo-oxidative degradation, which subsequently created a suitable medium for the graphene sheets to disperse readily and distribute evenly within the polymer matrix. Different microscopy techniques were employed to prove the effectiveness. This was then qualitatively assessed by Raman spectroscopy, X-ray diffraction, rheology, mechanical testing, density measurements, thermal expansion, and thermogravimetric analysis, confirming both the originality as well as the effectiveness of the process.

Evaluating root cause: The distinct roles of hydrogen and firing in activating light- and elevated temperature-induced degradation

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Determination of Fluorine Chemical Structure in Slag by Thermal Hydrolysis He-MIP-OES.

A novel microwave-induced helium plasma optical emission spectrometry (He-MIP-OES) combined with a thermal hydrolysis sample introduction system was newly established for the determination of fluorine (F) compounds in slags. The MIP maintained through an Okamoto cavity was adopted, which was quite stable and provided a higher detection capability of F by helium plasma. The basic analytical performance of He-MIP-OES combined with an ultrasonic nebulizer for F analysis was also examined, and expected results could be obtained. The thermal hydrolysis behavior of CaF2, Ca4(Si2O7)(F,OH)2 and Ca5(PO4)3F, as known F chemical structures in slag, was also examined by thermal hydrolysis He-MIP-OES; F compounds in actual slag could be successfully identified and determined by this developed system.

Excitation of O(1S) auroral green line from ambient air in a microwave-induced plasma jet at atmospheric pressure

Strong emission from the auroral green line O(1S) → O(1D) at 557.7 nm was observed in the afterglow of an atmospheric-pressure 2.45 GHz microwave-induced plasma jet. The device used Ar as the working gas for the plasma, and a surrounding N2 gas flow to stabilize the discharge. The characteristic line-shape of the peak at 557.7 nm indicated that the emission was from the O(1S)N2 excimer. Vibrational temperatures of NO and N2, estimated based on the emission spectra of the afterglow, were compared against the intensity of the green line and the concentration of N2 gas flowing in the system. The data points yielding the most intense green lines fall at vibrational temperatures of around 2000 K for both NO and N2. It was observed that the amount of N2 gas flowing in the system positively contributed to the intensity of the green line. Using fluid simulation, the O2 concentration in the afterglow was estimated to be in the range of 1–5.5%.

Multielement Determination of Trace Heavy Metals in Water by Microwave-Induced Plasma Atomic Emission Spectrometry after Extraction in Unconventional Single-Salt Aqueous Biphasic System.

For the first time liquid-liquid extraction was used for the preconcentration of heavy metals prior to their determination in water by microwave-induced plasma atomic emission spectrometry (MP-AES). Extraction of Pb(II), Cd(II), Co(II), Ni(II), Zn(II), and Cu(II) was performed in unconventional aqueous biphasic system (ABS) formed by addition of hydrophobic solid salt, namely, tetrahexylammonium bromide, to aqueous sample, with neither organic solvents nor salting-out agents being used. The metal ions were quantitatively recovered with 4-(2-pyridylazo)-resorcinol (PAR). The extract was diluted with ethanol/HCl and introduced directly into an MP-AES instrument. The factors influencing extraction (pH, reagent concentration, phase contact time, etc.) and MP-AES detection parameters were studied and optimized. For the developed method, limits of detection of 1.3, 4.9, 0.06, 1.2, 4.2, and 3.2 μg L-1 were obtained for cadmium, cobalt, copper, nickel, lead, and zinc, respectively, providing from 60- to 500-fold improvement as compared with the analysis without preconcentration. The method was applied for the analysis of two certified reference materials (CRM) of wastewater and surface water as well as the samples of well and seawater. Coupling MP-AES with ABS extraction significantly extends the capabilities of the method, especially for the analysis of high salinity waters.

Direct Measurement of Elemental Mercury Using Multidimensional Gas Chromatography with Microwave-Induced Helium Plasma Atomic Emission Spectroscopy

Microwave-induced helium plasma atomic emission spectroscopy permits direct measurement of picogram levels of elemental mercury in various matrices when combined with multidimensional gas chromatography. Two columns with different stationary phases provide excellent separation for elemental mercury, and multidimensional analysis improves the reliability, performance, and system cleanliness of atomic emission detection. The possibility of false positive identification is substantially eliminated, and excellent sensitivity for the target compound was attained with the use of two selective columns and atomic emission detection at 254 nm. A flame ionization detector was incorporated as part of the system configuration to increase analytical platform capability and flexibility. Elemental mercury was measured in gas matrices over a range of 0.1–170 μg/m3 having a correlation coefficient of R2 = 0.9995, a precision of less than 5% relative standard deviation (n = 10), and a measured recovery exceeding 99% in natural gas as a model matrix. The total analysis time is less than 10 min. Only a small 1 mL sample volume is needed, and the described approach does not rely on any form of sample enrichment. The utility of multidimensional gas chromatography with microwave-induced helium plasma atomic emission spectroscopy is demonstrated with challenging industrial applications, such as the measurement of elemental mercury in natural gas, industrial solvents, and vapor generated from ruptured compact fluorescent light bulbs.

Ligand free green plasma-in-liquid synthesis of Au/Ag alloy nanoparticles

Au/Ag alloy nanoparticles were successfully prepared by a microwave-induced plasma in liquid process without any organic protecting or reducing agents.