Microwave Thermal Effect Research Articles

Effect of Microwave Power on Performance of Microwave-Assisted Spark Ignition under CO2-Diluted Condition

To explore the application potential of microwave assisted ignition under EGR conditions, the effect of microwave peak power on MAI performance under 8% CO2 and non-diluted condition is investigated in a constant volume combustion chamber with peak power varying in 0∼1000W under equivalence 1.0 and ambient pressure 0.2 MPa. Results show that increasing the microwave peak power can strengthen the MAI enhancement, however, increasing peak power presents a linear relation in non-diluted cases indicating the thermal effect of microwave. In 8% CO2 case, it tends to be a threshold process, indicating that microwave exhibits more in chemical kinetics effect.

Separation and extraction of non-thermal effects of strong microwave electric field on dielectric properties of materials based on time modulation and cavity perturbation method

The influence of strong microwave electric field (SMEF) on the dielectric properties of materials is the result of the joint action of microwave thermal effect and microwave non-thermal effect. Generally, the thermal effect of SMEF is stronger than the non-thermal effect, which makes the non-thermal effect of SMEF difficult to detect. Moreover, it is difficult to distinguish the influence of these two factors from each other. Therefore, the formation mechanism and characteristics of the non-thermal effect of SMEF have not been elucidated so far. In this paper, a separation and extraction model of the non-thermal effect of SMEF on the dielectric property of material is proposed based on the time modulation method and cavity perturbation method. By adjusting the interaction time between SMEF and materials, reducing the influence of microwave thermal effect, and strengthening the proportion of microwave non-thermal effect, the separation and extraction of the non-thermal effect of SMEF is realized. Through the designed re-entrant coaxial cavity, the corresponding test system is constructed and the typical materials are tested. Experimental results show that the proposed research method is feasible. The research method proposed in this paper provides an effective way for the follow-up study on the formation mechanism and characteristics of the non-thermal effect of SMEF on the dielectric properties of materials.

Na+ inserted metal-organic framework for rapid therapy of bacteria-infected osteomyelitis through microwave strengthened Fenton reaction and thermal effects

The poor penetration ability of light in tissues makes phototherapy be difficult to treat bacteria-induced osteomyelitis effectively. Herein, we developed a rapid and noninvasive therapeutic strategy by employing Prussian blue (PB) as a microwave-responsive material. The PB was excited by microwave through dielectric loss of PB and the changed spin state of iron ions, which led to the heat generation and then weakened the bond energy of Fe II‒(CN) and Fe III‒(NC) in physiological saline. At the same time, the insertion of Na + in PB made the bond energy become irreversible, which accelerated the release of Fe 2+ and Fe 3+ from PB. The released Fe 2+ and Fe 3+ could be easy to penetrate bacterial membrane with reduced permeability by microwave to react with H 2 O 2 and GSH in the inside of bacteria, leading to the final death of the bacteria due to the synergistic action of microwave, microwave thermal effect and Fe 2+ /Fe 3+ induced Fenton reaction/GSH consumption. This study will provide novel insight for designing remote and noninvasive antibacterial systems for treating deep bacteria-infected diseases. • This is the first report of therapy of bacteria-induced osteomyelitis by Na + inserted PB under microwave (MW) radiation. • PB can response MW via dielectric loss and spin crossover structure, accelerating Fe ions release in NaCl soloution. • Na + insertion makes the spin state of the Fe ions irreversible and further accelerates the release of Fe ions under MW. • The increased membrane permeability under MW favors the entrance of Fe 2+ , accelerating Fenton reaction in bacteria.

Thermal and Nonthermal Microwave Effects of Ethanol and Hexane-Mixed Solution as Revealed by In Situ Microwave Irradiation Nuclear Magnetic Resonance Spectroscopy and Molecular Dynamics Simulation.

Microwave heating is widely used to accelerate the organic synthesis reaction. However, the role of the nonthermal microwave effect in the chemical reaction has not yet been well characterized. The microwave heating processes of an ethanol-hexane mixed solution were investigated using in situ microwave irradiation nuclear magnetic resonance spectroscopy and molecular dynamics (MD) simulation. The temperature of the solution under microwave irradiation was estimated from the temperature dependence of the 1H chemical shifts (chemical shift calibrated (CSC)-temperature). The CSC-temperature increased to 58 °C for CH2 and CH3 protons, while it increased to 42 °C for OH protons during microwave irradiation. The CSC-temperature of CH2 and CH3 protons reflects the bulk temperature of solution by the thermal microwave effect. The lower CSC-temperature of the OH proton can be attributed to a nonthermal microwave effect. MD simulation revealed that electron dipole moments of OH groups ordered along the oscillated electric field decreased the entropy by absorbing microwave energy and simultaneously increased the entropy by dissipating energy to the solution as the thermal and nonthermal microwave effect. Ordered polar molecules interact to increase hydrogen bonds between OH groups as the nonthermal microwave effect, which explains the lower CSC-temperature of the OH protons. The nonthermal microwave effects contribute to the intrinsic acceleration of the organic reaction.

Solid-Phase Microwave Reduction of WO3 by GO for Enhanced Synergistic Photo-Fenton Catalytic Degradation of Bisphenol A.

The synergistic photocatalytic Fenton reaction is a powerful advanced oxidation technique for the degradation of persistent organic pollutants. However, microwave-induced thermal effects on the formation of novel structures facilitating the photocatalytic degradation have been rarely reported. Herein, a two-step microwave thermal strategy was developed to synthesize a new hybrid catalyst comprising defective WO3-x nanowires coupled with reduced graphene oxides (rGOs). Conventionally, microwave methods could induce superhot spots on the GO surface, resulting in the site-specific crystallization and oriented growth of WO3. However, in the solid phase, localized microwave thermal effects could reduce the interfacial area between WO3 and rGO and enhance the bonding between them. As for the unique structure and surface properties, the synthesized catalyst enhanced the light absorption, promoted the interfacial charge separation, and increased the carrier density in the photocatalytic processes. In addition, surface formation of W4+ provided a new pathway for Fe3+/Fe2+ cycling which linked the photocatalytic reaction and the Fenton process. The optimized catalyst exhibited a remarkable performance in the degradation of bisphenol A with a ∼83% removal yield via a photo-Fenton route. These microwave-induced functionalities of materials for synergistic reactions could also give a new avenue to other photoelectrocatalytic fields and solar cells.

Microwave Thermal Effect on Diels-Alder Reaction of Furan and Maleimide

Background: Higher temperature regions (hot spots) have been observed in organic reactions and are attributed to microwave selective heating. The accumulated heat in reaction systems accelerates certain reactions. Methods: The theoretical calculation was applied to select a suitable Diels-Alder reaction as a molecular probe to determine the microwave thermal effect on Diels-Alder reaction, one class of bimolecular reactions. The kinetic investigations were utilized to determine the reaction activation energies and further to calculate the actual reaction temperatures under different microwave conditions from the Arrhenius equation. Results: On the basis of the theoretical calculational results, Diels-Alder reaction of furan and maleimide was selected as a molecular probe to determine the microwave thermal effect in Diels- Alder reaction. Their activation energies under thermal conditions were determined from kinetic data by using the Arrhenius equation. The actual reaction temperatures under different microwave conditions were further deduced from their activation energies and the Arrhenius equation. Conclusion: Higher temperature regions (hot spots) were observed in Diels-Alder reaction, and they are more obvious in less polar solvents than those in more polar solvents in the microwave irradiated reactions.

Decoupling thermal and non-thermal effects of the microwaves for lignocellulosic biomass pretreatment

In order to develop an efficient biorefinery concept based on the sustainable use of biomass, pretreatments are a major issue as they consume a considerable amount of energy, among others. Competitive and sustainable bioconversion technologies are required and microwave (MW) treatment has emerged as an interesting option during the recent years. Nevertheless, MW action mode on lignocellulosic biomass is not yet fully understood. With some exceptions, experimental investigations dealing with MW pretreatment do not address this important aspect nor energy and power level effects on biomass conversion.To study the action mode of MW on lignocellulosic biomass and its energy absorption behavior, MW assisted pretreatments of three types of grass biomass of industrial interest were performed at several solid to liquid (S/L) ratios, i.e. 1/21 or 1/9, with different solvents (water, ethanol, acid water or alkaline water) and different MW power densities (2.38 or 4.76 W/g). Incident, reflected and absorbed MW power levels and the reaction temperatures were monitored on-line. Thermal and non-thermal MW effects were studied by comparing experiments performed with a glass reactor and an in-house-made jacketed glass reactor designed to avoid the heating of sample. Under the tested conditions, only thermal MW effects have been detected. In an attempt to optimize the bio-solubilization of biomass components, microwave pretreatment with increased pressure and incident power density (7.5 W/g) was also tested. The cell wall was significantly impacted with a solubilization of 33% of hemicelluloses, which opens the way to industrial applications.To our knowledge, this is the first reported experimental study that addresses the non-thermal MW effects on biomass by decoupling heating effect from irradiation.

Experimental and theoretical study of microwave enhanced catalytic hydrodesulfurization of thiophene in a continuous-flow reactor

Hydrodesulfurization (HDS) of thiophene, as a gasoline model oil, over an industrial Ni-Mo/Al2O3 catalyst was investigated in a continuous system under microwave irradiation. The HDS efficiency was much higher (5%–14%) under microwave irradiation than conventional heating. It was proved that the reaction was enhanced by both microwave thermal and non-thermal effects. Microwave selective heating caused hot spots inside the catalyst, thus improved the reaction rate. From the analysis of the non-thermal effect, the molecular collisions were significantly increased under microwave irradiation. However, instead of being reduced, the apparent activation energy increased. This may be due to the microwave treatment hindering the adsorption though upright S-bind (η1) and enhancing the parallel adsorption (η5), both adsorptions were considered to favor to the direct desulfurization route and the hydrogenation route respectively. Therefore, the HDS process was considered to proceed along the hydrogenation route under microwave irradiation.

Bismuth oxide self-standing anodes with concomitant carbon dots welded graphene layer for enhanced performance supercapacitor-battery hybrid devices

To meet the energy storage demand of developing electronic information technology, the supercapacitor-battery hybrid devices based on Bi2O3 have attracted much attention due to their superior energy storage characteristics. However, the poor cycling stability limits its application. Herein, through the nano-welding technique induced by the microwave thermal effect of graphene, the carbon nanotube film-supported Bi2O3 electrode with a graphene/carbon dots encapsulation structure is fabricated. The as-prepared composite electrode delivers a remarkable capacity of 1.90 mAh cm−2 at 1 mA cm−2 as well as high rate performance (1.57 mAh cm−2 at 100 mA cm−2). Assembled device with above composite electrode as anode and the carbon nanotube loaded nickel cobalt as cathode exhibits a maximum energy density of 589.3 μWh cm−2 (98.2 Wh kg−1) at power density of 0.8 mW cm−2 and maintain 288.3 μWh cm−2 (48.1 Wh kg−1) at 57.1 mW cm−2. Most notably, by virtue of the flexible confinement of graphene, the device exhibits impressive cycle stability (80.1% capacity retention after 8000 cycles, ∼4.5 times than that of a device assembled with a raw anode). This work opens a new pathway to the design and fabrication for high energy density and durable cyclic stability energy storage devices.

Thermal and non-thermal effects of microwaves in synthesis

Thermal and non-thermal effects of microwaves in synthesis

Investigation on preparation of Na0.5K0.5NbO3 nanoparticles by microwave heating method

Lead-free piezoelectric (K0.5Na0.5)NbO3 nanoparticles were successfully synthesized via microwave heating. The phase structure, morphology and particle size of the samples synthesized by microwave and conventional method were characterized by TG-DSC, XRD and FTIR. Microwave thermal effect and the reaction mechanism were analyzed. The results revealed that single-phase (K0.5Na0.5)NbO3 nanoparticles could be obtained at relatively low temperature and short time of 650 °C for 10 min by microwave method. The particles possess an orthorhombic crystal structure and the composition is consistent with stoichiometric ratio. The obtained particles are fine, uniform and with little agglomeration. With the microwave synthesis temperature reducing from 800 °C to 650 °C, the average diameter of particles decreases from 600 nm to 100 nm and weakens the agglomeration. The possible formation mechanism of (K0.5Na0.5)NbO3 nanoparticles affected by microwave thermal effect was proposed. Due to the selective microwave heating, low temperature eutectic reaction between Na+, K+ solid solution and Nb2O5 occurs and the intermediate phase (K, Na)2Nb4O11 is formed. Subsequently, as the reaction temperature increased, high temperature eutectic reaction arises and the Na2Nb4O11 and K4Nb6O17 intermediate phases are formed. Ultimately, all the intermediate phases disappear and (K0.5Na0.5)NbO3 nanoparticles are synthesized.

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO2). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30 × 3 mm2. The pre-forming pressure was selected at 0 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa and 5 MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4 MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.

Microwave irradiation’s effect on promoting coalbed methane desorption and analysis of desorption kinetics

To enhance desorption of coalbed methane and increase extraction efficiency, a technique using microwave irradiation is proposed. Methane desorption experiments without and with microwave irradiation were carried out in the laboratory using an experimental system developed to study methane desorption. The experimental results show that microwave irradiation causes the total quantity of methane desorbed to increase from 1.91 to 3.92 times larger than the quantity desorbed without MI. Two hour trials were performed, applying microwaves for 4, 8 or 16 min, equivalent to output energies of 192, 384 and 768 kJ. These results show that 1 kJ of microwave energy can cause methane desorption to increase by 0.0088 ml per gram of coal. Under successive microwave irradiations of 40 s, the desorption rate increases by a factor of 10.2. Kinetic analysis also found that microwave irradiation enhances the methane diffusion coefficient and decreases the attenuation coefficient. Regardless of which diffusion model is used to determine the diffusion coefficient, the quantity of gas desorbed increases as the microwave irradiation time increases. The pore and surface features of coal samples exposed to microwaves were determined by using mercury intrusion porosimetry and nitrogen adsorption and specimens were examined by scanning electron microscopy. Methane desorption evidently results from both the microwave thermal effect and structural damage to the coal. The results reported herein reveal the mechanisms that promote methane desorption by microwave irradiation and suggest a new field technique for extracting coalbed methane.

Research progress on the effect of microwave sterilization on agricultural products quality

Different sterilization methods have different effects on the quality of agricultural products, microwave sterilization inhibited or eliminated microorganism by the use of microwave thermal effects and non-thermal. In this paper, the effects of microwave sterilization on the quality of fruits and vegetables, dairy, meat, grain, aquatic products and other agricultural products were introduced, and the possible development trends of microwave sterilization in agricultural products processing application were put forward.

Experimental Investigation on the Interaction Mechanism Between Microwave Field and Semiconductor Material

An experimental method and apparatus for investigating the interaction mechanism between microwave field and semiconductor material are presented in this paper. The investigation is based on a dual-mode compressed rectangular cavity, which includes the TE102 and TE103 modes. The TE102 mode is used for providing the stimulus signal, which supplies the microwave field during the experiment, and the TE103 mode is for testing. The two modes are individually coupled to the cavity, and two band-stop filters are introduced to isolate the signals coming from both the modes. A directional coupler is utilized to monitor the power level of the stimulus signal. Experimental results demonstrate that microwave field may affect the inherent characteristics of indium phosphate, a typical kind of semiconductor materials, which eventually leads to the nonlinear behavior of the dielectric property. After the microwave field strength exceeds a critical value, the nonlinear behavior of the microwave materials becomes more and more drastic along with further increasing field strength, and according to the particular experiment setup, we conclude that the nonlinear behavior is not caused by microwave thermal effect, but some inherent characteristics of the material itself. In addition, the experimental method and apparatus can also be employed to predict the consequences of the non-thermal action of microwave effect of other high-power microwave material under microwave field.

Microwave-assisted synthesis of zirconium-based metal organic frameworks (MOFs): Optimization and gas adsorption

Microwave-assisted synthesis of zirconium (Zr) based metal organic frameworks (MOFs) were performed and the yield and porous property of UiO-67 was optimized by varying the quantity of the modulator (benzoic acid, BenAc and hydrochloric acid, HCl), reaction time and temperature. It was found that (i) an increase in the amount of modulator enhanced the specific surface area and pore volume of UiO-67 due to the promotion of linker deficiency; and (ii) the presence of modulators influenced the number of nuclei (and hence the crystal size) and nucleation time (and hence the yield). Optimum amounts of BenAc and HCl for synthesizing UiO-67 under microwave irradiation were determined as 40 mol equivalent and 185 mol equivalent (to Zr salt), respectively. In comparison to conventional solvothermal synthesis, which normally takes 24 h, microwave methods promoted faster syntheses with a reaction time of 2–2.5 h (at similar temperatures of 120 °C and 80 °C for BenAc and HCl, respectively). The thermal effect of microwave is believed to contribute to the fast synthesis of UiO-67 in the microwave-assisted synthesis. The reaction mass efficiency and space-time yield show that microwave heating promoted the simple yet highly efficient preparation of Zr-based MOFs. In addition, UiO-67 MOFs from different synthesis methods (i.e. the microwave-assisted and solvothermal method) were evaluated using single-component (CO2 and CH4) adsorption, showing comparable gas uptakes.

Microwave fast-tracking biodiesel production

Three kinds of biodiesel production processes are compared and it is shown that the microwave heated acid process has higher productivity than the conventional heating alkaline or acid processes. The reaction time to reach the maximum yield in microwave-irradiated process is reduced to 50% of the time required by conventional heating processes. An optimization of process variables was done to identify the optimum microwave process irradiation. Searching an explanation for this non thermal microwave effect, a statistical design to stablish the complete chemical kinetic equation for soya oil transesterification with ethanol catalyzed by methane sulfonic acid was realized. The kinetical results show that this process has a complex mechanism that can be seen as a pseudo -1.6 global order reaction with low activation energy. Precisely, the order is 0.004 with respect to ethanol and -1.6 to soya oil. An analysis of this negative order was done and it brings a little more information to understand how to get best returns from biodiesel operations.

Investigation of Selective Microwave Heating Phenomena in the Reactions of 2-Substituted Pyridines

Debated selective microwave heating effects were investigated in a rearrangement and a benzylation reaction involving 2-substituted pyridines. An accurate, reproducible comparison technique and simultaneous temperature measurement using both external infrared and internal fibre optic sensors were utilized. The experimental details of the benzylation reaction were thoroughly addressed to resolve the inconsistencies that have been discussed previously in the literature. Hidden inhomogeneities in temperature and concentration were revealed within the reaction mixtures during microwave heating, which could be prevented by the modification of the vessel wall using an inert fluoropolymer liner. Instead of the previously proposed microscopic thermal microwave effect, the enhanced reaction rate could be explained by macroscopic-scale localized heating in the boundary layers close to the vessel surface in the microwave-heated experiment.

Multisynergistic Platform for Tumor Therapy by Mild Microwave Irradiation-Activated Chemotherapy and Enhanced Ablation.

Microwave (MW) therapy, as a promising type of thermal therapy, has been attracting more and more attention from scientists. The combination of thermal and chemotherapy is of great significance in the latest studies of synergistic tumor therapy. However, the research on the MW therapy mechanism, especially the nonthermal effect applied in the combined cancer therapy, is not thorough enough. Pleasantly, we have discovered that nonthermal MW irradiation can promote the cellular uptake of nanoparticles and anticancer drugs via experiments in vitro and in vivo. Therefore, multifunctional nanoplatforms have been designed for enhanced tumor inhibition by loading ionic liquids (ILs), doxorubicin hydrochloride (DOX), and phase change materials (PCMs) into ZrO2 hollow nanoparticles. PCMs act as MW switches. The as-made IL-DOX-PCM@ZrO2 nanoplatforms were injected into H22-tumor-bearing mice via the tail vein. Mild microwave irradiation (0.9 W, 450 MHz) was then applied. The thermal effect of MW could cause the temperature of the tumor site to rise (58 °C). On the other hand, it will trigger the MW switch to open and release DOX when the temperature is high enough. At the same time as drug release, a MW nonthermal effect could improve the cellular uptake of nanomaterials and anticancer drugs. The multisynergistic effect can promote the survival rate of the IL-DOX-PCM@ZrO2+MW group to 100%. The results of the tumor therapy experiment in vivo demonstrated that as-made multifunctional IL-DOX-PCM@ZrO2 nanoplatforms could enhance the therapeutic outcome of combined thermal and chemotherapy under mild MW irradiation.

Microwave Effect versus Thermal Effect on the Synthesis of 4-[(substituted benzylidene)amino]-5-(pyridin-4-yl)-4H-1,2,4-Triazole-3-Thiol Candidates

A series of Schiff base derivatives 4a-k were synthesized from 4-amino-3-(4-pyridyl)-5-mercapto-4H-1,2,4-triazole 3 as starting materials by using microwave irradiation technique. The synthesized compounds were fully characterized by NMR, FT-IR, elemental analysis, and HR-mass spectra. The results indicated that the microwave irradiation technique gave higher yields of pure products with short time of the reaction in compare to the thermal method.