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The molecular mechanism of the microwave nonthermal effect is still not clear. This work investigated the spatial orientation and kinetic energy of active site collision of carnosine, a natural bioactive dipeptide, under the weak microwave irradiation using the molecular dynamics simulation. Our results showed the influences of the temperature, microwave intensity, microwave frequency, and microwave polarization mode (linear polarization and circular polarization) on the spatial orientation and kinetic energy of active site collision of carnosine. First, under the constant intensity and frequency of linear polarization microwave irradiation, the increment of the collision probability between the 6N atom of carnosine and the 28H atom of the other carnosine at effective space angle decreases from 85.0% to 3.5% with increasing temperature. Second, with the increase of microwave intensity, the change of spatial orientation and kinetic energy becomes more and more significant. However, the change of circular polarization microwaves on the spatial orientation and kinetic energy of collision is weaker than that of linear polarization. Third, under the constant intensity of linear polarization microwave irradiation, the collision probability between the 6N atom and the 28H atom at effective space angle decreases from 70.2% to 14.7% with increasing frequency. Finally, under the microwave polarization, the spatial orientation and kinetic energy of molecular collision are changed, which is summarized as the microwave postpolarization effect (MWPPE). The dependence of MWPPE on temperature, microwave intensity, microwave frequency, and polarization mode is very complicated. In the end, this effect can provide a new insight into the molecular mechanism of the microwave nonthermal effect.

Different physicochemical properties between Cr(VI) and phenolic compounds pose serious challenges for the effective treatment of co-contamination. This study developed an electrodeless high-flow microwave atmospheric plasma jet for the single-step simultaneous degradation of p-nitrophenol (PNP) and reduction of Cr(VI). Following a 15 min treatment with microwave atmospheric pressure plasma, the removal efficiency of Cr(VI) and PNP reached 97.5% and 93.6%, respectively, whereas that of total organic carbon reached 30.2%. Adding PNP to the solution significantly improved Cr(VI) reduction, whereas PNP degradation increased slightly with Cr(VI). The results indicate that the PNP intermediates significantly affected Cr(VI) reduction. Additionally, long-lived H 2 O 2 and short-lived · H aided the reduction of Cr(VI) during plasma treatment. The addition of hydroxyl scavengers during treatment implied that · OH was largely responsible for PNP oxidation. High-performance liquid chromatography-mass spectroscopy (HPLC-MS) revealed that PNP intermediates, including p-nitrocatechol and 5-nitrobenzene-1,2,3-triol, function as Cr(VI) reductants. On the basis of the examined intermediate products, the potential PNP degradation pathway was investigated. The factors that could influence simultaneous dehgradation and reduction, including solution pH, gas velocity, and distance between the plasma outlet and the water surface were researched. Low pH supports Cr(VI) reduction, and the promotion of PNP for Cr(VI) reduction applies to all pH values. The degradation of PNP is insensitive to pH values with or without Cr(VI). The optimal gas velocity for PNP degradation and Cr(VI) reduction was revealed to be 6 L/min. The simultaneous removal of PNP and Cr(VI) benefits from a shorter distance between the plasma outlet and the water's surface., (Copyright © 2022. Published by Elsevier Ltd.)

The aim of this work was firstly to do a precise thermal analysis of microwave assisted production of biodiesel. In this paper, the effective permittivity of biodiesel synthesis was updated with two methods: a traditional method and a bivariate function of temperature and concentration of one component, then the thermal analysis of the reaction process were accomplished with multi-physics calculation. The results show that there exists large distinction in temperature between these two simulation results calculated by the two methods. The two hot spots locate in the opposite side and their temperature's difference is up to 9°C when the reaction is just carried out for 18s. But the temperature risings and distributions calculated by the new method are closer to the measured results. The thermal analysis based on the new method will be helpful for the industrial design of biodiesel production., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Microwave imaging for dielectric objects was considered in this paper. Applying Bayesian approach to represent prior information about permittivity distribution of observed object by prior probability density and combine measurements information of scattering field, we obtained posterior probability density that included synthetic information about the observed object. And then, Gibbs sampler, one of Markov Chain Monte Carlo method, was used to sample the posterior probability density. The sample mean was regarded as an evaluation of the permittivity distribution. The results of simulation imaging with "blocky" objects showed that this set of methods made good use of information and had the advantages of feasibility and very strong anti-noise ability. In addition,it is capable of describing (definite or indefinite) prior information in a convenient and controllable way, as well as capable of giving the "complete" solution, i.e., the occurrence probability of every permittivity distribution.

The microwave-to-plasma energy conversion efficiency and the ease of plasma self-ignition are critical factors affecting the applications for microwave plasma sources (MPSs). This study presents a novel MPS utilizing dielectric wedges for self-ignition and improved energy conversion. Firstly, we crafted a dielectric wedge with a gradient refractive index, guiding the electric field from air to dielectric materials and facilitating microwave propagation along the dielectric in a waveguide. Through electromagnetic simulation, we explored how the size and permittivity of the dielectric wedge affect the electric field distribution. Then, the MPS based on the dielectric wedge was designed. In this configuration, a dielectric tube encloses the discharge tube, connecting to dielectric wedges to guide electromagnetic waves to the plasma. We analyzed the MPS performance using the Drude model, evaluating microwave energy conversion efficiency across various electron densities and collision frequencies. The results were compared with a commonly used MPS based on a tapered waveguide, demonstrating the proposed MPS has wider applicability across different operation conditions. Finally, experiments under low pressures were conducted using various gases, showing an average energy conversion efficiency of approximately 40% higher than the tapered waveguide MPS. The experiments also indicate the proposed MPS has a greater capability of self-ignition at lower power levels. These findings highlight the efficacy of incorporating dielectric wedges to enhance MPS performance, making it conducive for broader industrial applications. [ABSTRACT FROM AUTHOR]

A compact high-power four-port circulator aiming to simplify the conventional, complex, and bulky injection-locking magnetron system is proposed. To reduce the performance deterioration and the risk of ferrite rupture under long-term high-microwave-power condition, the method of breaking a monolithic ferrite into three discrete ferrites in a conventional three-port circulator is proposed. To miniaturize the size and cost of the four-port circulator, a butterfly-shaped waveguide structure is proposed, with a stub inserted into the cavity at the central point and with no connecting waveguide. Multiphysics simulation results show that the temperature coefficient of variation (COV) at the surface of the discrete ferrites is 12.4% lower than that of a monolithic ferrite circulator, with input microwave power of 10 kW. The size of the proposed four-port waveguide circulator is 27% less than the assembly of two three-port circulators, and way smaller than a conventional differential phase shift circulator (DPSC). The simulated and measured S-parameters match well, and the measured power capacity of the fabricated circulator is higher than 3 kW (limited by the experimental condition). A magnetron is successfully locked using only one designed compact circulator. The research in this paper promotes the development of injection-locking magnetron and provides a design example for the compact, high-power circulator. [ABSTRACT FROM AUTHOR]

Cylindrical loads are commonly found in microwave heating. However, cylindrical loads tend to have a thermal focus, leading to uneven microwave heating. In this paper, a novel microwave heating method was proposed by employing resonant rings to mitigate the thermal focus of cylindrical loads. Firstly, a multiphysics simulation model for microwave heating of a cylindrical tube is established, and the influence of the resonant ring around the cylindrical tube on the temperature distribution of load was analyzed. Subsequently, microwave heating experiments of agar gels with and without resonant rings were carried out. In agreement with the simulation results, the use of resonant rings reduces the coefficient of temperature variation (COV) from 1.064 to 0.793. In addition, the parameters affecting the heating uniformity of the cylindrical tube were discussed through simulations. [ABSTRACT FROM AUTHOR]

A microwave atmospheric pressure plasma jet (MW‐APPJ) generator based on a substrate‐integrated waveguide (SIW) resonator is proposed. A thin, straight plasma needle with a length of 9 cm could be obtained. Higher than 90% of microwave feeding efficiency is obtained under a wide range of gas flow rates without a microwave matching network. As the gas flow increased from 1 to 14 L/min, a transition from laminar to turbulent mode is observed in the plasma jet's morphology. Furthermore, a preliminary surface treatment was tested for the device. The polyimide film became super hydrophilic after a 5‐min treatment with the developed SIW‐based plasma generator. Generally, the SIW‐based MW‐APPJ is low‐cost, electrodeless, and low‐profile along the plasma jet length direction. [ABSTRACT FROM AUTHOR]

Impedance matching plays a vital role in a microwave atmospheric pressure plasma jet (MW-APPJ). However, real-time tuning of MW-APPJ impedance matching has rarely been investigated. In this study, we have found that the dissipated microwave power and length of a MW-APPJ are effected by the application of a DC-bias magnetic field. By placing a MW-APPJ in a gap between two electromagnets, the consumed microwave power varied with the insertion position of the MW-APPJ. When the MW-APPJ was fixed at the center of the electromagnets, the consumed microwave power decreased as the DC magnetic field intensities increased, from 12 to 750 mT. The variation of consumed microwave power was much higher at lower gas velocities, which indicates that the DC magnetic field has a relatively strong effect on microwave plasma with lower gas velocity. The length of the plasma jet also decreased with increasing DC magnetic field, which was mainly due to the variation of the consumed microwave power. To explain these interesting phenomena, the effective permittivity of microwave plasma and the equivalent circuit model of a generator under DC-bias magnetic field have been analyzed. The effective permittivity of the plasma is related to its cyclotron frequency, which is a function of the DC-bias magnetic field. The variation of the effective permittivity changes the load impedance of the plasma generator. Therefore, the reflection coefficient of the plasma generator is adjusted and the length of the plasma jet varies with the change in consumed microwave power. [ABSTRACT FROM AUTHOR]

Microwave plasma chemical vapor deposition (MPCVD) has received tremendous research interest in fabrication of carbon nanotubes (CNTs) due to its unique advantages of high reactivity, rapid heating, no pollution, good controllability, etc. It would be meaningful to summarize the efforts that have been devoted in this area. Up to now, no such review has been seen in the literature. In this review, a summarization and discussion of the MPCVD devices applied in CNTs fabrication are firstly given, followed by the discussion on effect and affecting mechanisms of H2 plasma pretreatment, nitrogen atoms in the reacting gases and microwave power. Notably, the parameters of as-synthesized CNTs products and the corresponding synthesizing MPCVD conditions are listed out. Finally, the capabilities of MPCVD in facilitating atmospheric-pressure, low-temperature and in situ growth of CNTs are reviewed. This review can give a comprehensive grasp of current progress and understanding of MPCVD in preparation of CNTs and may provide a useful guidance for readers to design their fabricating systems to obtained required CNTs using MPCVD. [ABSTRACT FROM AUTHOR]

A dual-band sensor based on a planar rectangular cavity loaded with pairs of improved planar resonator is used to measure the permittivity difference of liquids with a small volume ($2.5~\mu \text{l}$). The planar cavity is connected by two microstrip transmission lines, constituting a two-port device with two branches. A pair of interdigital capacitor (IDC)-based split ring resonator (SRR) is coupled to the middle part of each branch. The distance between the IDC–SRRs is large, and their coupling can be neglected. Thus, two resonance frequencies are produced (4.15 and 9.18 GHz). One frequency is generated from the resonance of the two IDC-based SRRs (4.15 GHz), whereas the other results from the coupling of the rectangular cavity and the two IDC-based SRRs (9.18 GHz). Frequency splitting is used to detect changes in the dielectric characteristics of the liquids. If the two branches (IDC-based SRRs) of the proposed sensor are placed on the same liquid, only two resonance frequencies are obtained. When two different liquids are loaded by the sensor, the resonance frequency will be split into four. A planar rectangular cavity is used instead of a splitter configuration to motivate the two IDC–SRRs. This novel design affords a dual-band miniaturized sensor. [ABSTRACT FROM AUTHOR]

The development of microwave heating technology promotes the improvement of industry quality and efficiency, but the drawbacks of microwave heating, such as uneven heating, prevent its wide application in industry. In this paper, a double‐layer ring tube is proposed for improving the heating uniformity and achieving the target temperature. The gap between the inner and outer tubes is evenly divided into four regions, and the cooling liquid flows through the gap to control the temperature of the heating target placed in the inner tube. Maxwell's equation, heat conduction equation, Navier‐Stokes equation and the mass conservation equation are coupled to simulate the heating process, and the experimental data and simulation data are compared to verify the correctness of the model. Meanwhile, the heating uniformity of structure with or without heating exchanger is compared, and the effect of different fluid velocities, different sizes of the reactor and different dielectric constant of heating target on the heating performance is discussed. [ABSTRACT FROM AUTHOR]

This article investigated the discharge characteristics of Ar plasma sustained by a coaxial plasma jet under different microwave frequencies (433 MHz, 915 MHz, 2450 MHz, and 5800 MHz). Clear distinctions of the discharge characteristics such as excitation and plasma-sustaining power, shape, electron number density, electron temperature, and so on were experimentally obtained under these operating frequencies. The results demonstrated that microwave powers at different frequencies could excite distinct particles. The higher the microwave frequency, the lower the excitation and the plasma-sustaining power. However, these powers did not decline linearly to the increase in microwave frequency. The Ar plasma at a frequency of 5800 MHz exhibited an upside-down "bell" shape and a larger cross section, while those at other three frequencies seemed to have thin and long plumes. The investigation also revealed that the electron temperature increased and then decreased, while the electron number density and gas temperature elevated monotonously with increasing frequency. This study is intended to be helpful in determining the optimal frequency and design microwave plasma sources for certain applications. [ABSTRACT FROM AUTHOR]

Microwave selective/localized heating has been widely used in special areas, e.g., medical treatment and food processing, but the control of selective/localized heating in restricted space, e.g., microwave ovens, is difficult to accomplish. In this article, a Giuseppe Peano fractal phased-array antenna is designed, and the phase of each radiator is derived to achieve the arbitrary spatial division of the electromagnetic field in a cavity based on the superposition principle of waves. Then, a multiphysics model of microwave heating is established by coupling the electromagnetic field and heat transfer to simulate the directional control of microwave heating in a cavity. An experimental system is built, and the heating results are measured using an infrared thermal imager to verify the temperature distribution in different spatial regions. Finally, the feasibility of the model for different heating targets is discussed. [ABSTRACT FROM AUTHOR]

The progress of spin rectification effect, which has the potential in powering the nanoscopic devices wirelessly, has been impeded by its low conversion efficiency. To solve this problem, a 5.8 GHz planar inverted F‐shaped antenna, which is designed to harvest radiated microwave power as well as foster a field enhanced area surrounding a permalloy (Py, Ni80Fe20) monolayer, is presented. The electric and magnetic field are enhanced by ~17.5‐fold and ~ 45‐fold, respectively. With the proposed antenna, a photovoltage of 0.85 μV can be detected in the monolayer 1.5 m away from the transmitting antenna, with input power of 30 dBm. This work provides an effective method to enhance the spin rectification effect signal in the far field region and paves the way for spintronic devices to be potentially utilized in practical wireless applications, such as quantum information, photovoltaics, and nanogenerator. [ABSTRACT FROM AUTHOR]

A microwave power-combining technique based on magnetrons has been widely researched to solve the urgent demands for high-power, low-cost microwave sources in microwave industrial applications. To achieve high resultant efficiency, conventionally, injection locking with static frequency is utilized to drag the output frequency of each magnetron source to the exact same frequency, and a phase shifter is required to adjust the output phase of each combining source. In this article, we propose a novel phase shifterless microwave power system with two magnetrons utilizing a frequency-searching injection-locking technique. Coaxial cables with different physical lengths are used to realize phase adjustment under different injection frequencies. By sweeping the injection frequency in a small band, the phase difference between combining signals can be adjusted. Thus, high resultant efficiency can be obtained. When the injection frequency is swept, the phase difference between the output and injection signals has been analyzed theoretically, the phase difference between the different-length coaxial injection cables has also been analyzed. Furthermore, the relationship between the sweeping frequency bandwidth and requirements of the minimum length difference of the coaxial injection cables has been derived and analyzed. Experimental results show that a high resultant efficiency of 94.6% can be obtained with the proposed phase shifterless system. The system could maintain high resultant efficiency even when the free-running frequency and power of one magnetron are changed by adjusting its anode current. The experiments reveal that the proposed method works for different combining sources. [ABSTRACT FROM AUTHOR]

This work aims to improve the uniformity and efficiency of microwave heating. A dual-port simulation model based on the finite element method was built, and two pins were applied to the heating cavity. The impact of dual frequency and pins on the coefficient of variation (COV) and heating efficiency were investigated. The results demonstrated that the uniformity of dual-frequency heating can be improved by 15.4%. Moreover dual-frequency with double pins can maximally increase heating efficiency to 166.7% and the COV value can be reduced to 60.2%, compared with single-frequency heating. The heating effect of other materials was also analyzed. The uniform and efficient microwave heating were realized at dual frequency with pins. [ABSTRACT FROM AUTHOR]

The heating efficiency of sodium chloride solution would be increased with elliptically polarized microwave electric field for that a more frequent motion of water molecules and ions would be generated with multidirectional polarization electric field. [Display omitted] • The heating efficiency of sodium chloride solution with elliptically polarized and linearly polarized microwave electric field was compared. • Rotation autocorrelation function was used to study rotating motion of water molecules with different heating modes. • The hydrogen bonds between water molecules of sodium chloride solution were simulated. Our molecular dynamics simulation results show that the elliptically polarized microwave electric field would increase the heating efficiency of sodium chloride solution. The growth rate of the heating efficiency is positively correlated with the concentration of the sodium chloride solution and intensity of applied electric field. The rotation autocorrelation function results confirm that water molecules would do a more frequent rotational motion with the multidirectional polarization of elliptically polarized microwave electric field. The result of the more frequent rotational motion is consistent with that of the higher microwave energy absorption of energy analysis. The following analysis of hydrogen bond shows that the existence of ionic solutes would break the hydrogen bonds between water molecules and make it easier for water molecules to rotate with the applied electric field. These results indicate that elliptically polarized microwave electric field can indeed improve the heating efficiency of salt solution. [ABSTRACT FROM AUTHOR]

Microwaves have been widely used in organic chemical syntheses, but the mechanism is still unclear. The aim of this paper is to study the polarization of polar molecule reactions in the liquid phase. A novel model is proposed to describe the polarization of chemical reactions. In this model, the polarization is expressed by the summation of the orientational polarization of the liquid mixture and the polarization increment due to the chemical process. On the basis of the assumption of local equilibrium, the chemical process is divided into a series of equilibrium systems. We derive the general expression for the polarization increment according to the mass balance equations. By considering a simple chemical process, we give a specific expression for the polarization. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In some cases such as heating objects by microwave power, variation of the temperature in objects changes their dielectric permittivity and then leads to variation of the electromagnetic (EM) field. The commonly adopted method of simulation for the heating procedure is to repeatedly execute the full-wave simulation for each small time intervals, which results in a heavy computation burden. A fast algorithm for simulating 2-D sceneries is proposed for the first time. It starts with a full-wave simulation to get an initial EM field, and then corrects the EM field in an efficient way rather than repeating the full-wave simulation when the object's permittivity has slightly changed. A few sample numerical results are presented and analyzed, which verifies the accuracy, robustness, and effectiveness of the proposed fast algorithm. [ABSTRACT FROM PUBLISHER]

The microwave light emitting plasma (MLEP) is an important environmentally friendly UV light source. Finding the optimal condition (pressure, ratio and microwave power) of a MLEP spectral line which makes its intensity strongest is the key work to improve its efficiency in applications, but it has not been studied under the condition with multiple variables yet. In this paper, taking Ar+N 2 MLEP as an example, a novel method for optimizing the condition of MLEP by employing ANN to model the relationship between the condition and target spectral line intensity using experimental optical emission spectra data is proposed for the first time. The evaluation parameters showed that the ANN has good prediction accuracy. The optimal pressure, Ar concentration and microwave power were simultaneously predicted for UV spectral lines of 281.13 nm, 295.27 nm, 308.91 nm, 313.14 nm, 336.14 nm and 353.00 nm of Ar+N 2 MLEP. Their intensities were increased by 53%, 22%, 64%, 17%, 20% and 22% respectively under the predicted optimal condition compared with the maximum intensity in pure experimental method. Therefore, the proposed method is feasible for optimizing the condition of UV luminescence of MLEP, and has great significance in accurately improving the specific spectral line intensity of light sources. • A novel method for optimizing the condition of MLEP is presented for the first time. • The optimal condition of six UV spectral lines of Ar+N 2 MLEP were predicted. • The six UV spectral lines intensities were greatly improved after optimization. • This method is applicable to a variety of light sources without limitation. [ABSTRACT FROM AUTHOR]

A novel dual-frequency microwave rectifier that operates at 2.45 GHz alone, 5.8 GHz alone, or their combination has been developed in this paper. It is the first approach to microwave rectifying for a situation of the incoherent microwave power combination. We focused on the two primary components, i.e. the regulation of impedance matching and the recycling of the second harmonics. A quarter-wave short-ended microstrip line at 5.8 GHz is shunted at the input of the rectifier to enhance the impedance matching at 2.45 GHz. A microstrip line band-stop filter (BSF) and a quarter-wave short-ended microstrip line are introduced to improve the recycling of the second harmonics. The measured microwave to DC (MW-DC) conversion efficiencies are 57.5, 46.5, and 40.7% at 2.45, 5.8 GHz, and their combination, respectively, with an input microwave power at 17 dBm. It may be applied to a dual-frequency microwave power wireless transmission system. [ABSTRACT FROM AUTHOR]

In this article, the effects of microwave frequency (2450 MHz and 5800 MHz) and gas humidity (1%, 2%, 3%, 4%, 6% and 8%) on in vitro blood coagulation with cold atmospheric pressure plasma (CAPP) were investigated. The generation of reactive oxygen species (ROS, OH, O) was measured by optical emission spectra. The exposure temperature on blood droplets under treatment was below 55 °C in all cases, to avoid the thermal effect of plasma on the blood clotting. Investigations showed that, with the increase of frequency, the doses of ROS increased, the blood sample presented a more serious collapse and its surface became drier. The humidity of ionized gas can also accelerate the generation of ROS and the process of blood clotting. Our results propose a method to accelerate in vitro blood coagulation in CAPP by adjusting microwave frequency and gas humidity, and suggest a clinical benefit for plasma treatment as a coagulation device in surgery. [ABSTRACT FROM AUTHOR]

The temperature of water under the irradiation of two incoherent microwaves rise faster than that of one microwave. [Display omitted] • A microwave Separated Field Reconstructed (SFR) device is established. • Incoherent microwaves heat water at a higher heating rate than one microwave. • The higher heating rate is due to the more discrepant electric field directions. To improve microwave heating efficiency, a combined experimental and simulated investigation of incoherent microwaves heating of water was carried out. The experiment was conducted with Separated Field Reconstructed equipment, which can provide incoherent microwaves generated from two independent sources. Both the experimental and simulated results indicated that, with same total powers, the heating rate of water under the irradiation of two incoherent microwaves was increased than that of one microwave. The enhancement can be understood qualitatively that the more discrepant electric field directions of incoherent microwaves would stimulate more molecular collisions, which would result in a higher heating rate. [ABSTRACT FROM AUTHOR]

[Display omitted] • Evolution of hydration shells in NaCl solutions with microwave field was simulated. • The application of microwave field causes the EC variations in NaCl solutions. • The calculated results met with the measured results in the experiment. The slight electrical conductivity variations of sodium chloride aqueous solutions under the microwave field have been found in experiment, but the mechanism is still unclear. By applying molecular dynamics simulations, the number of water molecules in the hydration shells of sodium are examined quantitatively, we demonstrate that the outer water molecules of shell are stripped away due to the oscillations and collisions among water molecules under the microwave field, this is a microwave desolvation effect. By further study on calculating diffusion coefficient of ions, the electrical conductivity of solutions has been estimated. The result showed that as the diffusion coefficient increased by the desolvation effect, the electrical conductivity of solutions slightly enhanced. The electrical conductivity of solutions at different temperatures and concentrations agree with the measured results in the experiment. [ABSTRACT FROM AUTHOR]

The microwave non-thermal effect is still a controversial issue. Here, the influence of weak microwaves on the spatial collision and energy distribution of water molecules is studied by the molecular dynamics simulation. The results show that the polarization of microwaves makes the change of spatial collision and energy distribution. The spatial collision probability of water molecules increases obviously in the direction of electric field and the proportion of high energy molecules decreases while the proportion of intermediate energy molecules increases. This shows the constrains of microwave fields on the water molecules, and these consequences would be helpful to understanding the microwave non-thermal effect and the mechanism of microwaves accelerating chemical reactions. [ABSTRACT FROM AUTHOR]

Microwave heating has been widely used in the chemical industry because of its advantages, such as fast heating rate, selective and controllable heating, increasing reaction rate and reducing by-products in chemical reactions. The Special Issue contains research on microwave applications in chemical engineering. [ABSTRACT FROM AUTHOR]

Subcritical water has been widely used as the solvent in microwave‐assisted extractions, but the interaction between microwaves and subcritical water has been not yet fully understood. In this article, we find that there exists abnormal absorption of subcritical water posts in microwave heating at some temperatures with low loss factors. By calculating the absorbed power of a subcritical water post under exposure plane waves, the water post can still absorb microwave energies well at some temperatures even with low loss factors. It is different from the traditional view that materials can absorb little microwave power with low loss factors. By analyzing the characteristic equation, we show that the abnormal absorption is caused by dielectric resonance. This abnormal phenomenon can also be observed in applications with microwave cavities. It indicates that even with low loss factors, materials can be possibly heated by microwaves with good efficiency. [ABSTRACT FROM AUTHOR]

Multiphysics modeling is an essential tool to understand the heating characteristics of microwave processing food products. However, when the heated sample is moving and deforming during the heating process (e.g. the long-term microwave drying process), the huge mesh distortions caused by the sample's motion and deformation will make the simulation impossible. To solve this problem, a hybrid double-layer Arbitrary-Lagrange-Eulerian (ALE)/implicit algorithm is proposed in this paper, where the sample's motion and deformation is tracked by a double-layer ALE framework and convert to time-varying implicit variables. By doing so, the influence of the sample motion and deformation on the electromagnetic field distribution will be characterized by the evolution of time-varying implicit variables, rather than by explicit modeling, which reduces the mesh distortion and makes the calculation possible. A two-dimensional and a three-dimensional model of microwave cavities containing a rotating and deforming food are given to describe the algorithm in detail. Experiments for the three-dimensional model are also conducted to validate the proposed method. Results show that the shrinkage of the food may have a negative impact on the heating performance (especially energy efficiency) and corresponding remedies need to be performed for long-term microwave heating processes. • Double-layer ALE framework is used to track the deformation and movement of foods. • Mesh distortion caused by the food deformation and motion is weakened by the proposed hybrid algorithm. • This algorithm is applicable to the cases with the moving food expanding or shrinking. • Heating dynamics of microwave heating processes considering heat and mass transfer, deformation, and motion is analyzed. [ABSTRACT FROM AUTHOR]

• We propose an on-chip wideband microwave interferometry sensor for measuring minute variations in the dielectric properties of chemical liquids with a small volume (5 ul). • The relationship between the variations in transmission-zeros and the variations in dielectric properties of the sample are investigated. • The sensitivity of the wideband interferometry sensor is improved about 1.2 times by introducing the ESD at the sensing region. In this paper, we propose an on-chip wideband microwave interferometry sensor to measure minute variations in the dielectric properties of chemical liquids with a marginal volume (5 ul). The device is based on coplanar waveguide (CPW) and slot-line technology. It consists of two uniplanar broadband hybrid ring couplers and two branches. Each branch is loaded with an electrically small discontinuity (ESD), which is the sensing region. If the sensor is unloaded or loaded with the same material under test (MUT), we obtain a transmission-zero. When different MUTs are loaded into the two microfluidic channels, new interference frequencies occur. Variations in transmission-zeros are employed to express variations in the dielectric properties or concentrations of the MUT. Experiments show that the sensitivity of the ESD-inspired interference sensor is higher (average value from 13 GHz to 17 GHz) than that of the wideband interference sensor by 1.2 times. [ABSTRACT FROM AUTHOR]

• A multiphysics model for heating process with a slow- rotating heated sample and a fast-rotating mode stirrer is developed. • The calculation amount is reduced by transformation methods based on the symmetry of geometric structure. • The combined effect of mode stirrer and turntable on microwave heating is analyzed. • The effect of the distance between the mode stirrer and the heated material on the heating effect is analyzed. Moving elements have been widely used in microwave heating process, especially mode stirrers and turntables. However, the huge difference in speed of these two elements makes the simulation of this heating process difficult to achieve. In this work, the heating process is computed by a step-by-step algorithm based on implicit function and level set methods. In each time step, the slow- rotating heated sample is considered as static while the fast-rotating mode stirrer have rotated a certain angle. The heat source in this time step is obtained by averaging the dissipated powers with mode stirrer in different angles. Moreover, the calculation amount is reduced by transformation methods based on the symmetry of geometric structure. In addition, heating process with different time step lengths and numbers of mode stirrer angles have been computed to analyze the model sensitivity. Experiments are also carried out to validate the effectiveness of the proposed method. Simulated results are in accordance with experimental ones. The combined effect of mode stirrer and turntable on microwave heating is analyzed. This model can serve as an effective method to deal with the thermal analysis with unequal velocity moving elements during the microwave heating. [ABSTRACT FROM AUTHOR]

Abstract Two-dimensional (2D) nanostructures, though promising in energy storage, suffer from aggregation and subsequent deterioration of performance in practical applications. Hence, assembly of 2D nanostructures into three-dimensional (3D) architectures is highly desirable. Here, we report a microwave-assisted approach to the controllable synthesis of 2D materials with tunable 3D structures simply by adjusting the ratio of water/ethylene glycol (H 2 O/EG). Novel flower-on-sheet 3D hierarchical structures of nickel cobalt double hydroxide (NiCo LDHs) are obtained at EG content of 40%, while microspheres and 2D nanosheets are obtained when the EG content is 0% and 75%, respectively. We propose that under microwave irradiation, EG molecules disperse the nuclei and facilitate the initial formation of 2D sheets. Subsequently, the dominating hydrophobicity of the assembling results in the formation of nanoflowers on the sheets. When tested as electrode materials in supercapacitors, the flower-on-sheet NiCo LDH exhibits superior capacitance (1187.2 F g−1 at 1 A g−1), good rate capability (71% retention at 30 A g−1), and high stability (only 0.3% cyclic decay per cycle with respect to the first charge capacitance), which is ascribed to that 'sheet' could act as buffer substrate while 'flower' expose more active site. Our results demonstrate an energy-saving and one-pot approach for controllable construction of 2D derived 3D nanostructure that can be applied in next-generation energy storage materials. Highlights • Novel 3D flower-on-sheet structure of NiCo LDH was obtained via a microwave method. • The role of ethylene glycol in tuning the 3D morphology of NiCo LDHs was studied. • The flower-on-sheet structure enabled fast ion-diffusion and ion-available sites. • Highest capacitance and rate capability were gained for flower-on-sheet structure. [ABSTRACT FROM AUTHOR]

• Ozone is produced by self-designed microwave electrodeless UV lamp device. • O 3 +UV 254 /TiO 2 complex system is applied to degrade NOR. • Microwave UV lamp devices are expected to be applied to treat wastewater. • Four possible degradation pathways were proposed. This work designed a microwave electrodeless UV light source device integrating ozone generation and photocatalysis (O 3 + UV 254 /TiO 2), norfloxacin was selected to evaluate the degradation performance of the device. The degradation and mineralization efficiencies obtained for norfloxacin (10 mg/L, 5 L) treated by the O 3 + UV 254 /TiO 2 system for 30 min and 60 min were 82.12% ± 2.84%, 97.00% ± 2.55% and 67.93 ± 0.46%, 75.77 ± 0.26%, respectively, and the highest energy yield obtained at 5 min was 465.53 ± 67.99 mg/kWh. The experiments compared the degradation effects of O 3 , O 3 + UV 254 and O 3 + UV 254 /TiO 2 treatment of NOR solutions, and the degradation efficiencies obtained at 40 min were 74.51± 2.61%, 77.09 ± 2.12% and 91.63 ± 2.62%, respectively, which showed that O 3 + UV 254 /TiO 2 exhibited the best degradation effect. In addition, the effect of coexisting inorganic salts on the degradation efficiency of norfloxacin was investigated. The identification of intermediates by HPLC-Q-TOF-MS proposed four possible degradation pathways. The ring opening of the quinolone group represents a decrease in the antibacterial activity of the norfloxacin drug, indicating that the microwave electrodeless UV light device is a promising alternative strategy for treating effluent containing norfloxacin. [Display omitted] [ABSTRACT FROM AUTHOR]

Cylindrical containers are commonly used in microwave food processing applications. However, due to the shape effect, microwave energy mainly focuses in the center of the sample within the container, leading to a nonuniform temperature gradient between the inner and outer layers of the sample. Employing a lossy container may be an effective method to address this issue because the heat conduction from the container's wall or the microwave's interaction with the container edge can compensate for the lack of heating in the outer layer. Multiphysics modeling is a powerful tool for uncovering the effect of the lossy container on microwave heating performance. However, noncompliance between the fine and coarse mesh elements at the thin-walled boundary of the container will generate a huge number of irregular meshes, which will not only lead to excessive computational costs but also lead to the possibility of calculation error of the boundary flux. To overcome this difficulty, a fast simulation method based on transformation optics is proposed to analyze the effect of lossy containers on microwave food processing performance in this paper. By using the proposed method, the calculation efficiency can be increased by up to 800% with comparable or even better accuracy than the traditional method. Meanwhile, the results show that the lossy container can improve the temperature uniformity around microwaved food but it will slightly reduce the heating efficiency due to energy loss within the container. This paper offers an efficient and fast way to analyze microwave food heating processes with a thin-wall structure. Furthermore, the discussion on the feasibility of using lossy containers to improve the microwave heating uniformity will also contribute to the further application of microwave cylindrical food processing. • Microwave heating models with lossy thin-walled cylindrical containers were realized by transformation optics. • Compared with traditional method, this method can improve the computing efficiency by 8 times with a wall thickness of 1 mm. • The influence of container's material, size and wall thickness on microwave heating is analyzed. • Employing lossy container can weak the phenomenon of thermal focus during microwave heating cylinder food. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel microwave cavity was applied to degrade volatile organic compounds. • The relative permittivity of catalyst was measured by neural network. • Electric field and temperature distribution validated with experimental data. • High energy efficiency in degrading volatile organic compounds was achieved. Volatile organic compounds (VOCs) are common organic pollutants that are harmful to human and the environment. Degradation of VOCs has aroused widespread concern. Catalytic oxidation is considered as an effective method to degrade VOCs. Microwave heating is a promising technology for heterogeneous catalytic gas–solid reactions, which is considered as an alternative to conventional conductive heating. Here, microwave enhanced catalytic degradation of VOCs were carried out in a novel double ridge field compressed cavity, in which Cu-Mn/AC catalyst was applied. The relative permittivity of Cu-Mn/AC at different temperatures were obtained by neural network. Further, based on the relative permittivity obtained, numerical simulations for the double ridge field compressed cavity were conducted at 2.45 GHz by a coupled three-dimensional electromagnetic-thermal model using COMSOL Multiphysics software. The simulation results of electric field distribution and temperature distribution in cavity present similar trend to the experimental results. Experimentally, the degradation of ethanol, benzene and acetone were carried out in this reactor for 50 W and 70 W at 2.45 GHz with the concentration and gas flow rate of 1000 ppm and 18 L/min. At 70 W and 2.45 GHz, the degradation efficiency of ethanol, benzene and acetone reached 99%, 70% and 99%, leading higher energy efficiency than other VOCs degradation methods. [ABSTRACT FROM AUTHOR]

To understand the effects of shape, size and property changes in a spherical sample during microwave drying, a fundamentals-based coupled electromagnetics and multiphase porous media model is developed and associated experimental details are described. Microwave drying of different sized spheres is carried out in a domestic microwave oven operating at 10% power level. Maxwell's equations for electromagnetics are solved inside a three dimensional (3D) microwave oven to obtain the electric field distribution inside the oven cavity and the spheres. The drying samples are treated as a porous media consisting of three phases: solid (skeleton), liquid (water) and gas (water vapor and air). Modes of transport for the fluid phases include capillary flow, binary diffusion between vapor and air, gas pressure driven flow and phase change between liquid water and vapor which is spatially distributed. An elaborate experimental system comprising of infrared camera, optical fiber probe and digital balance is built to validate the model in terms of temperature distribution, point temperatures, gas pressure generation and moisture loss from the samples at different times during the drying process. Results, validation, sensitivity analysis and "what-if" scenarios are presented in the companion paper. The work together would provide tremendous benefits when designing and developing microwave drying processes and products through a novel synergy between physics-based modeling and detailed experimentation. [ABSTRACT FROM AUTHOR]

The coupled electromagnetics and multiphase porous media model, developed in the companion paper, has been applied to the microwave drying of potato spheres. Microwave energy absorption, temperature, pressure and moisture distribution were obtained in 3D samples to gain a comprehensive understanding of the process and address issues such as overheating resulting from microwave focusing. The model was validated against key process parameters and good agreement was found between experimental data and predicted values. The model and experiments demonstrated that the different sized spheres behaved quite differently under similar conditions of drying and general guidelines were obtained for drying sphere-shaped materials. Intermediate sized spheres were found to be more prone to excessive volumetric heating via focusing of microwave energy compared with larger sized spheres. This led to their explosion midway through the drying process; whereas, smaller sized spheres underwent uniform, low temperature drying without any quality loss. Sensitivity analysis showed that the model was highly sensitive to the mass transfer coefficient of the surrounding air inside the microwave oven while intrinsic permeability did not affect moisture loss from the material. This indicated that capillary diffusion is the dominant mode of transport in small sized spheres. Development of this physics-based model would go a long way in making and improving computer-aided design and optimization of microwave drying processes. [ABSTRACT FROM AUTHOR]

Making samples moving during microwave heating is popular way to improve the heating uniformity. However, the common methods, e.g. using turntable and conveyor belt, can only make samples move in one plane, resulting in limited effect on improving the heating uniformity. In order to improve the uniformity of microwave heating in both horizontal and vertical directions, a rotary lifting turntable is used to drive the sample to rotate and lift during heating in this work. To observe the improvement of heating uniformity caused by the rotary lifting motion, an algorithm based on level set methods and implicit function are used to model the process. The electromagnetic field distribution during the heating process obtained by the model is verified by the discrete locations. The transient-temperature of three points and the spatial temperature profile obtained by the model are verified by the experiments. With this model, the heating effect of samples in rotary lifting motion is compared to that of samples only in rotary or lifting motion. The results show that the heating effect of the samples in rotary lifting motion is better than that of most of the samples in static state and single motion mode. In addition, influences of rotating speed, lifting speed, sample properties, and diameter of spiral motion on the heating effect is also analyzed. Since the proposed motion mode is aimed to change the horizontal and vertical position of the sample in the electromagnetic field to make the sample absorb microwave uniformly in all directions, it can be applied to all kinds of microwave cavities regardless of geometric shapes. • A multiphysics model for heating processes with a solid sample in rotary lifting motion is developed. • Implicit function and level set methods are used to update the permittivity of the moving region. • Heating efficiency and uniformity are improved by the rotary lifting motion. • Influence of rotating and lifting speed, sample property, and diameter of spiral motion on the heating effect is analyzed. [ABSTRACT FROM AUTHOR]

Moving objects are widely applied in microwave heating process for improving heating uniformity. In order to improve the heating effect, various moving objects with complex shapes will be used in microwave heating. However, for objects with irregular shapes, it is difficult to model the object movement due to the difficulty of characterizing the object's boundaries. In this paper, by using Arbitrary Lagrange Eulerian (ALE) method to track the object movement, the position of the object is expressed by a time-varying implicit function. The impact of the moving object on the electromagnetic field distribution is therefore represented by the change of the implicit parameter. Meanwhile, after the heat source is transformed back to the initial position by a coordinate transformation method, the calculation of the heating process of the moving object can be realized in a fixed mesh structure, thus re-meshing is avoided. A 2-D model and a 3-D model with rotating objects in irregular shapes are used to illustrate the proposed method. Experiments are conducted to testify the validity of the proposed method. Good agreements have been obtained. Moreover, the results show that, for dealing with the cases with moving objects in complex shapes, this method is much more accurate and efficient than the ALE-remeshing method. • Instead of using mathematical definition, ALE method was used to update the implicit function of the moving object. • Boundaries of arbitrary shape moving objects during microwave heating can be simply traced by the proposed ALE scheme. • Remeshing was avoided due to the use of time-varying implicit function to express the motion of the object. • Compared with ALE-remeshing method, the proposed method was more efficient and accurate. [ABSTRACT FROM AUTHOR]

• A model of microwave heating process with translational motion and rotation was built. • Implicit function and level set method are used to express the elements' motion. • Effects of samples' moving speeds and shape on the heating uniformity are analyzed. • This method can analyze the industrial microwave heating process with complex motion. In practical chemical applications, in order to improve the microwave heating uniformity, moving elements, such as mode stirrers, turntables and conveyor belts, have been widely used as an easy way to implement. However, different moving elements may have different shapes, modes of motion and moving speed, which makes the simulation of the heating process very difficult. In this paper, a stepping algorithm based on implicit function and level set method is proposed to simulate the microwave heating process of multi-component with unequal moving speed. The accuracy of the algorithm is verified by discrete method and experiments. Moreover, the model was used to analyze the heating process with different components at different positions and speeds. The effect of different materials of mode stirrers and shapes of heated sample on heating process were also compared. The researches show that the proposed model can effectively improve the heating uniformity and heating efficiency. [ABSTRACT FROM AUTHOR]

Abstract: A facile and green method, in which rusty iron (Fe and Fe2O3) and deionized water were irradiated with a household microwave oven as the microwave source to prepare Fe3O4 nanoparticles, without alkali, acid or high-temperature, was proposed in this paper. The as-prepared Fe3O4 nanoparticles attached to the iron surface after the rusty iron was irradiated with microwaves in deionized water for 30min, which were separated from the surface by ultrasonic wave and magnet. The results show that this method is feasible, and has several advantages, such as convenience, harmless preparation, non-polluting raw materials, and the ability to reclaim and recycle scrap iron. [Copyright &y& Elsevier]