Pressure Microwave Research Articles

AN EFFICIENT HIGH POWER WATER DUMMY LOAD AT 2.45 GHz

A water dummy load developed as an element of equipment in the composition of a microwave atmospheric pressure plasmatron operating at a frequency of (2450±50) MHz is described. The load is created on the base of a rectangular waveguide with a cross-section of 90x45 mm. The simulation results carried out by CST Microwave Studio shows that the return losses reach -42 dB at 2.45 GHz. In the frequency band from 2 to 2.9 GHz, the load provides a voltage standing wave ratio (VSWR) of at least 1.2. The effect of changes in water temperature and salinity on the device characteristics is studied. The load is manufactured and successfully used as part of experimental waveguide paths.

The Effects of Different Cooking Systems on Changes in the Bioactive Compounds, Polyphenol Profiles, Biogenic Elements, and Protein Contents of Cauliflower Florets

In the current study, we examined the effects of boiling cauliflower in a pressure cooker, conventional boiling, conventional heating, and microwave heating on the chemical components, total phenol, flavonoids, antioxidant capacity (DPPH test), phenolic compounds, and mineral contents of cauliflower florets to reveal the differences between these cooking methods. Cauliflower is generally consumed either boiled or cooked in dry heat. In this study, different boiling and dry heat cooking methods were tried to reveal the changes in phytochemical composition and protein and mineral contents of cauliflower florets. Depending on the cooking methods of the cauliflower florets, the total phenolic and flavonoid contents of the cauliflower florets were determined to be between 273.72 (conventional heating) and 731.01 mg GAE/100 g (microwave heating) and 142.02 (conventional heating) and 797.10 mg/100 g (conventional boiling), respectively. The antioxidant capacity results of cauliflowers were found to be between 8.30 (conventional heating) and 33.69 mmol/kg (fresh). Statistically significant differences were detected in the moisture, total phenol, total flavonoid, and antioxidant activity values of cauliflower depending on the cooking techniques applied (p < 0.05). The gallic acid and 3,4-dihydroxybenzoic acid values of fresh and cooked cauliflowers were identified to be between 10.93 (microwave heating) and 194.79 mg/100 g (boiling in pressure cooker) and 17.58 (conventional heating) and 145.80 mg/100 g (boiling in pressure cooker), respectively. In general, the lowest amounts of phenolic compounds were defined in cauliflower samples boiled with a conventional heating system, followed by cauliflower samples cooked with the microwave heating method. Considering the component amounts as a result of cooking, the highest phenolic component amounts were specified in the cauliflower sample cooked by boiling in a pressure cooker. The protein quantities of fresh and cooked cauliflowers were determined to be between 16.11 (fresh) and 19.79% (microwave heating). The K and S contents of fresh cauliflowers and cauliflowers cooked with different blanching methods were specified to be between 19,647.62 (conventional boiling) and 35,130.01 mg/kg (conventional heating) and 3196.54 (boiling in pressure cooker) and 5105.65 mg/kg (microwave heating), respectively. The K, Mg, S, Fe, Cu, Mn, and Zn results of cauliflowers cooked in an oven and microwave were higher than those cooked using the control and boiling methods.

Nitrogen Fixation by Atmospheric Microwave Plasma with Local Electric Field Enhancement

Abstract Nitrogen fixation by atmospheric microwave plasma is a promising green technology. However, the energy consumption of nitrogen fixation driven by microwave plasma is still too high in existing reports, and there is a lack of effective methods to reduce it. In this paper, a method of local electric field enhancement in atmospheric microwave plasma is reported, which is achieved in an optimized metal array structure that can increase the electric field intensity by 385 times. The experimental results show that the reduction of energy consumption reaches up to 20% and the nitrogen oxide production increases by up to 26% with the local electric field enhancement. The achieved energy consumption is as low as 1.78 MJ/mol with a corresponding nitrogen oxide production of 3.53%. To the best of our knowledge, this is the first study to explore the role of local electric field enhancement on nitrogen fixation, and the energy consumption in this study is the lowest among the existing studies of nitrogen fixation by atmospheric pressure microwave air plasma.

Synthesis and structural analysis of graphene nanosheets via microwave atmospheric pressure plasma

This study reports the effective synthesis of multilayered graphene sheets via microwave atmospheric pressure plasma. This innovative approach streamlines and expedites graphene production and other carbon nanostructures, eliminating the need for catalysts, solvents, or complex processing conditions. Ethanol is directly injected into a microwave-generated argon plasma plume, leading to the formation of graphene. Raman spectroscopy revealed characteristic peaks (2D, G, and D bands) confirming graphene composition, with defects indicated by the D band. X-ray diffraction analysis supported these findings, indicating a broad peak at 25∘ corresponding to the (002) plane, affirming a multi-layered graphene structure. Scanning electron microscopy exhibited crumpled, randomly oriented graphene sheets, albeit with uneven structures suggesting impurity incorporation. The presence of defects was quantified through the intensity ratio of the D to G band (ID/IG) in Raman spectroscopy, revealing a value of 0.80, signifying the presence of defects in the synthesized graphene. The 2D to G band intensity ratio (I2D/IG) suggested the existence of 7–10 graphene layers, highlighting the need for further optimization for enhanced graphene quality and purity.

Continuous batch synthesis with atmospheric-pressure microwave plasmas

Plasmas under atmospheric pressure offer a high-temperature environment for material synthesis, but electrode ablation compromises purity. Here, we introduce an atmospheric-pressure microwave plasma (AMP) operated without electrodes to overcome the existing limitations in pure material synthesis. The distribution of the electrostatic field intensity inside a waveguide during AMP excitation was examined via electrostatic field simulations. The lateral and radial gas temperature distributions were also studied using optical emission spectroscopy. The AMP exhibited a uniform ultrahigh temperature (9,000 K), a large volume (102-104 cm3), and a response time on the millisecond level. AMP efficiently synthesized silicon nanoparticles, graphene, and graphene@Si-Fe core-shell nanoparticles within tens of milliseconds, ensuring purity and size control. We propose the "heat impulse" metric for evaluating the plasma characteristics (n a, T g, and t) in material synthesis, extended to other high-temperature plasmas. AMP is compact, cost-effective, and easy to assemble, promising for eco-friendly mass production of pure materials.

Atmospheric pressure microwave (915 MHz) plasma for hydrogen production from steam reforming of ethanol

This work presents experimental results on the energy efficiency in hydrogen production using atmospheric microwave plasma (915 MHz) through steam reforming of ethanol. Ethanol was chosen as a liquid hydrogen carrier due to its high hydrogen atom content, low cost, and wide availability. The experimental work began with the maximization of an energy efficiency of the used microwave plasma source. The process of maximization involved determining a position of a movable plunger that ensures the most efficient transfer of microwave energy from a microwave source to the generated plasma in the microwave plasma source. The aim of the investigations was to test the following working conditions of the microwave plasma source: absorbed microwave power PA by the generated plasma (up to 5.4 kW), the carrier gas volumetric flow rate (up to 3900 Nl/h), and the amount of the introduced ethanol vapours on the efficiency of hydrogen production (up to 2.4 kg/h). In the range of tested working conditions, the highest energy yield for hydrogen production achieved a rate of 26.9 g(H2)/kWh, while the highest hydrogen production was 99.3 g(H2)/h.

Investigation of a Novel Atmospheric Pressure Microwave Cold Plasma Torch and Its Characteristics

Investigation of a Novel Atmospheric Pressure Microwave Cold Plasma Torch and Its Characteristics

Pressurized microwave-assisted hydrothermal treatment with various salts for efficient production of monosaccharides from rice straw

This study proposed a two-stage pressurized microwave hydrothermal treatment with a catalyst, followed by enzymatic saccharification, as a pretreatment method for efficiently converting cellulose and hemicellulose from rice straw into glucose and xylose. The use of various inorganic salts and dilute sulfuric acid as catalysts enhances sugar production. Using 1 wt% sulfuric acid as a catalyst at 150 °C for 5 min for the first-stage and then 180 °C for 5 min for the second-stage yielded the highest sugar production from rice straw compared with other inorganic salts tested. The filtrate and enzymatic saccharification solution contained a total sugar of 0.434 g/g-untreated rice straw (i.e. 0.302 g-glucose/g-untreated rice straw and 0.132 g-xylose/g-untreated rice straw). When inorganic salts such as NaCl, MgCl2, CaCl2, and FeCl3 were used as catalysts, the highest sugar yield of 0.414 g/g-untreated rice straw (i.e. 0.310 g-glucose/g-untreated rice straw and 0.104 g-xylose/g-untreated rice straw) was obtained when using 1 wt% FeCl3 at 170 °C for 5 min in the first-stage and 190 °C for 5 min in the second-stage, with a value close to that of 1 wt% sulfuric acid. These findings suggest that two-stage treatment with a catalyst is a suitable pretreatment method for the production of glucose and xylose from rice straw owing to the different hydrolysis temperatures of cellulose and hemicellulose.

Green synthesis of graphene for targeted recovery of silver from photovoltaic waste

Atmospheric pressure microwave plasma can synthesize freestanding graphene in a few seconds at ambient conditions. Recent research has explored this method for the synthesis of graphene yet constrained by the utilization of toxic or non-renewable resources. This study aimed to substitute environmentally benign and sustainable precursors, synthesizing graphene from expired tangerine peel oil, an abundant natural source globally. The Raman spectrum of synthesized material showed a characteristic graphene-related 2D peak at microwave powers varied between 200 and 1000 W. The images of transmission electron microscopy revealed interstitial spacing of 0.34, which matched the value of X-ray diffraction calculated through Bragg's law. However, marginal variations in lattice spacing owing to the presence of oxygen functional groups were also observed. Additionally, the as-synthesized graphene deposited on a screen-printed electrode was used to selectively recover silver from spent photovoltaics. Our approach of creating a graphene-silver composite directly from waste material offers environmental benefits, resource utilization, waste reduction, and versatile applications in electrochemistry.

In-flight iron ore reduction and nanoparticle formation in an atmospheric pressure hydrogen microwave plasma

The in-flight reduction of iron ore particles using an atmospheric pressure hydrogen plasma is investigated. Iron ore particles with a size less than 75 µm are aerosolized and carried with an argon-hydrogen (90%–10%) gas mixture through an atmospheric pressure microwave plasma. After the treatment, the collected particles are observed to follow three distinct populations: (i) fully reduced nanoparticles, (ii) partially reduced spheres, larger than the feedstock, and (iii) partially melted, partly reduced agglomerates. A model is developed to explain the possible mechanism for the origin of the three populations. The nanoparticles (i) are found to be likely formed from the previously evaporated material whereas the particles (ii) and (iii) result from the partial/complete melting of the particles and agglomerates flowing through the reactor. The gas temperature is estimated to be more than 2000 K, which enables the rapid melting, evaporation, and reduction of these particles within residence times of only a few 10 ms.

Validation of the thermo-chemical approach to modelling of CO2 conversion in sub-atmospheric pressure microwave gas discharges

The CO 2→ CO + 12 O2 conversion experiment [D’Isa et al 2020 Plasma Sources Sci. Technol. 29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with a straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as a prescribed heat source. The parameter range covered is the specific energy input (SEI) 0.3–5 eV/molecule at pressure p = 0.9 bar, and SEI = 0.6–2 eV/molecule at p = 0.5, 0.2 bar. The calculated conversion χ is always close to the experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p = 0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. The net conversion in the model is driven to a large extent by the radial diffusion of CO and O from the hot core towards the wall and steep radial temperature gradients. The main factor that reduces the energy efficiency is the re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of a realistic bulk flow field on the chemical process cannot be studied. Hence, the results must be considered preliminary and have to be confirmed with a more elaborate and accurate model of the vortex-stabilized flow inside the reactor.

Analysis of Gas Composition of a Cold Plasma Jet Generated on the Basis of Atmospheric Pressure Microwave Discharge

Analysis of Gas Composition of a Cold Plasma Jet Generated on the Basis of Atmospheric Pressure Microwave Discharge

Precise surface machining of fused silica using high stability atmospheric pressure microwave plasma jet with a new internal electrode

Plasma is a promising approach for machining of fused glass substrates. Developing high-performance microwave torch is the core element to ensure the processing accuracy of this technology. In this study, a spline curve-based microwave torch internal electrode structure was proposed, which increased the internal electric field strength by 54 %. The discharge dynamics simulation based on this electrode shows that the plasma is excited at the inner electrode tip and the bottom of the resonant cavity, then it expands to form a linear discharge region and eventually fill the discharge tube. It was revealed by CCD images that with the increase of carrier gas flow rate, turbulent flow state appears in the tail of the jet. OES data shows that microwave Ar plasma can effectively dissociate CF4, and due to the oxidation effect of oxygen, CFx and C2 content decrease significantly with the addition of oxygen. The machining experiment on fused silica shows that the material removal stability of this torch reaches 3.69 %, and the removal rate increases nYarly with dwell time due to thermal effect. Finally, the machining performance of the atmospheric pressure microwave plasma jet was verified by figuring of a planar fused silica surface reducing the form error from 108.1 nm RMS to 16.5 nm RMS.

Research on ionization characteristics of atmospheric pressure pulse-modulated microwave He/air plasma jet

The atmospheric pressure pulsed microwave He plasma jet has the advantages of high electron density and abundant active particles, but its shrinking on the discharge electrode morphology limits its application range. In order to modulate a He plasma jet with a longer plume and study its ionization development characteristics, we constructed a dual-channel pulsed microwave coaxial discharge device. He and air were, respectively, injected into the inner and outer gas channels of the resonator to generate a double-layer atmospheric pressure microwave plasma jet with a longer plume. It is observed that the bifurcation of the stratified plasma jet will occur by changing the gas flow. The ionization development of plasma jet was observed by using enhanced charge-coupled device and microwave Rayleigh scattering apparatus measured the space-time evolution of plasma and observed the three times ionization enhancement process of plasma jet development. The spectral lines of the active products associated with Penning ionization were observed by using a fiber optic spectrometer. A fluid model was constructed to simulate and analyze that under the condition of sufficient He flow rate (He flow rate is above 0.6 slm), there will be sufficient and stable He mole fraction (64%) at the stratification of the plasma jet. The experimental and simulation results show that the jet profile of the microwave He plasma is related to the inlet structure of the discharger and He flow rate. Stratified intake structure can produce stratified He plasma jet, and the unique appearance of bifurcation of jet can be produced by changing the flow rate of He. In the bifurcation process of the plasma jet, the product of Penning ionization inhibits the development of the main branch of the plasma jet, and the secondary electron avalanche of the local electric field promotes the formation of the branch of the plasma jet and is accompanied by the enhancement of the second ionization. The ionization mechanism of microwave He plasma is the resonance excitation of local enhanced electric field, the advance of ionizing waves, and the interaction between the spatially distributed active particles.

A Novel Three-Layer Coaxial Atmospheric Pressure Microwave Plasma Jet

A Novel Three-Layer Coaxial Atmospheric Pressure Microwave Plasma Jet

Improvement of gelation properties of indica rice flour with stabilized rice bran from different thermal treatment

AbstractBackground and ObjectivesRice bran is enriched with bioactive compounds, which makes it more competitive as a functional food raw material. The stabilization of rice bran can inhibit the activity of endogenous enzymes, reduce its oxidation sensitivity, and change its physical and chemical properties, providing more possibilities for its application in the food industry. In this study, rice bran from five stabilization treatments (atmospheric pressure cooking, high‐pressure cooking, atmospheric steam, microwave heating, and extrusion cooking) were investigated for their effect on the gel properties of indica rice flour.FindingsStabilized rice bran could enhance the hydration properties and increase the enthalpy value of rice flour. The gelatinization properties indicated that the viscosity and fluidity of the rice paste were increased, and the aggregation and rearrangement of rice starch were promoted. From the rheological properties, stabilized rice bran enhanced the viscoelasticity of the rice flour gel. In addition, the stabilized rice bran could also reduce the yield of freeze‐thawing water of the rice flour gel. Rice bran with thermal treatments can significantly improve the gel properties of indica rice flour.ConclusionsIndica rice flour mixed with stabilized rice bran showed significant improvements in hydration, gelatinization, rheological properties, and freeze‐thaw stability.Significance and NoveltyIt is potential for the application of the gel from rice flour with stabilized rice bran in the starch‐based food industry.

Oxidation of Zinc Microparticles by Microwave Plasma to Form Effective Solar-Sensitive Photocatalysts

The presented work studies the processes of synthesis of ZnO microstructures using atmospheric-pressure microwave nitrogen plasma and investigates their photocatalytic activity in the processes of degradation of 2,4-dinitrophenol and the antibiotic ciprofloxacin when irradiated with sunlight. The work proposes an effective method for formation of photosensitive ZnO powders. Due to the features of plasma treatment in the open atmosphere of zinc metal microparticles, ZnO structures are formed with sizes from hundreds of nanometers to several micrometers with various micromorphologies. The lattice parameters of ZnO structures are characteristic of a hexagonal unit with a = 3.258 Å and c = 5.21 Å, volume 47.95 Å3. The size of the crystallites is 48 nm. The plasma treatment was performed by means of a 2.45-GHz plasmatron at a power input of 1 kW in nitrogen flow at a rate of 1–10 L/min. Zn microparticles were injected into the microwave plasma at a mass rate of 20 g/min. High photoactivity was demonstrated (rate constants 0.036 min−1 and 0.051 min−1) of synthesized ZnO structures during photo-degradation of 2,4-dinitrophenol and ciprofloxacin, respectively, when exposed to solar radiation. Photo-active structures of ZnO synthesized using microwave plasma can find application in processes of mineralization of toxic organic compounds. Structures of ZnO synthesized using microwave plasma can find application in processes of mineralization of toxic organic compounds, and also in scintillation detectors, phosphors.

The changed multiscale structures of tight nut (Cyperus esculentus) starch decide its modified physicochemical properties: The effects of non-thermal and thermal treatments

Non-thermal dielectric barrier discharge plasma (DBDP) and four thermal treatments, including baking (BT), high pressure cooking (HPC), radio frequency (RF) and microwave (MW) were applied to modify the structural and physicochemical properties of Cyperus esculentus starch (CES). The results showed that the thermal treatments remarkably disordered the crystalline structures of CES through weakening the double-helix conformation of amylopectin, while DBDP caused much more gentle influence on the starch structures than them. Specifically, MW induced the high-frequency displacement of polar molecules and intensive collisions between starch and water molecules, causing the largest stretching and swelling extents of amylopectin, resulting in the highest pasting and rheological viscosity of CES in four thermal treatments. As DBDP did not favor the aggregation of amylopectin chains, the deaggregated starch chains promoted the hydration effects with water molecules, boosting the final pasting viscosity, apparent rheological viscosity, freeze-thaw stability and digestion velocity of CES. Besides, the gelatinization-retrogradation process in the thermal treatments regulated starch digestion velocity and produced type III resistant starch in CES. Conclusively, the modified physicochemical properties of CES resulted from the altered molecular structures of starch by the applied treatments.

Microwave plasma-based dry reforming of methane: Reaction performance and carbon formation

We investigate atmospheric pressure microwave (MW) plasma (2.45 GHz) conversion in CO2 and CH4 mixtures (i.e., dry reforming of methane, DRM) focusing on reaction performance and carbon formation. Promising energy costs of ∼2.8–3.0 eV/molecule or ∼11.1–11.9 kJ/L are amongst the best performance to date considering the current state-of-the-art for plasma-based DRM for all types of plasma. The conversion is in the range of ∼46–49% and ∼55–67% for CO2 and CH4, respectively, producing primarily syngas (i.e., H2 and CO) with H2/CO ratios of ∼0.6–1 at CH4 fractions ranging from 30% to 45%. Water is the largest byproduct with levels ranging ∼7–14% in the exhaust. Carbon particles visibly impact the plasma at higher CH4 fractions (> 30%), where they become heated and incandescent. Particle luminosity increases with increasing CH4 fractions, with the plasma becoming unstable near a 1:1 mixture (i.e., > 45% CH4). Electron microscopy of the carbon material reveals an agglomerated morphology of pure carbon nanoparticles. The mean particle size is determined as ∼20 nm, free of any metal contamination, consistent with the electrode-less MW design.

Plasma decomposition of methanol to produce hydrogen with an atmospheric-pressure nitrogen microwave plasma torch

An atmospheric-pressure microwave plasma torch is employed to generate hydrogen by injecting methanol aerosols into the near afterglow region of a nitrogen microwave plasma. Two types of reaction chambers are proposed to study the influence of different boundary conditions of reaction chambers on hydrogen production by comparison. Fourier transformation infrared spectrometer and gas chromatography are used to measure and determine the hydrogen production rate, energy efficiency, and hydrogen yield. Also, the effects of conditions of methanol injection and discharge parameters on methanol decomposition are investigated, respectively. It is found that the active species originated from collision with the excited and ionized N2 in the high gas temperature in the plasma discharge plays an important role in the conversion of methanol to hydrogen. The gas flow pattern in the reaction chamber is closely related to boundary conditions and significantly affects the reaction time of methanol in it, which is analyzed with the software of computational fluid dynamics. The achievements of hydrogen production in our work are as follows: production rate up to 921 l/h, energy yield up to 371 l/kWh, and hydrogen yield up to 70%, respectively. Furthermore, the reaction mechanism is discussed in detail with respect to the formation of outlet products.