Microwave Irradiation Power Research Articles

Spike leakage characteristic of limiter irradiated by high power microwave

In this work, the response characteristics of the microwave limiter under various high power microwave irradiation are investigated via both simulation and experiment, and the simulation results match those of the experiment well. When the input power exceeds 6 dBm, a spike leakage phenomenon is observed. As the input power increases, the rise time and pulse width of the leakage peak decrease, whereas the leakage power gradually increases. However, the leakage power of the plateau firstly linearly increases, then gradually decreases, and finally slightly increases. Moreover, the pulse width and repetition frequency have almost no influence on the characteristics of leakage pulse, and the leakage energy decreases as the injection power increases.

Microwave‐assisted rapid fabrication of robust polyetherimide bead foam parts

AbstractPolyetherimide (PEI) is an important candidate for fabrication of high‐performance polymer foams. Nevertheless, the manufacture of PEI foamed parts with specific three‐dimensional geometry shape and lightweight properties remains a big challenge worldwide. Herein, a microwave (MW)‐assisted foaming and selective sintering approach was proposed towards rapid fabrication of robust PEI bead foams. During the process, compressed carbon dioxide/tetrahydrofuran was used as physical co‐foaming agent, and the surface‐coated graphene nanoplatelets (GNPs) was used as MW absorbent. Upon MW irradiation, the GNPs selectively heated and served as solders that effectively facilitated the foaming of expandable PEI (EPEI) beads, and strongly promoted the local polymer melt and entanglement across the surface of EPEI beads. The MW irradiation power and time were considered as the important parameters to achieve fine inter‐bead bonding strength between foamed beads. As a model system, we successfully fabricated a 25‐mm‐thickness foamed part with an apparent density of 0.32 g/cm3and achieved excellent inter‐bead bonding performance. Specifically, we also utilized the COMSOL Multiphysics simulations to study the MW selective heating mechanism. This MW‐assisted fabrication strategy offers a new foaming approach toward high‐performance polymer foamed parts for many advanced applications.

Transformations in aluminum oxyhydroxide under powerful short-pulse microwave radiation

Modification of a material based on Al oxyhydroxide (boehmite/γ-Al2O3) with metal Al fragments (nanoparticles, clusters) has been achieved by the treatment of the material under powerful short-pulse microwave radiation (pulse duration 20 ns, pulse repetition 25 Hz, power density 8 kW/cm2, frequency 2.85 GHz, amplitude of the electric strength component ~2.5·106 V/m). The Al oxyhydroxide was precipitated from a mixture of AlCl3 with ammonia solution and heated at 250 °C for 4 hours. The prepared sample was of predominant amorphous structure, the overall water content (adsorbed, bound, structured) amounted to 15 wt.% in average. The microwave irradiation of the samples (1-3 min) resulted in formation of Al fragments smaller than 100 nm within the solid matrix. Formation of metal Al in the irradiated samples was detected by means of thermal analysis, X-ray diffraction, and transmission electron microscopy. The average content of Al in irradiated samples amounted to ~1.5 wt.%. A rational explanation of the observed metallization effect has been suggested.

Effect of microwave radiation on antioxidant capacities ofTartary buckwheat sprouts.

This study aimed to investigate the effects of different microwave radiation power and treatment time on the antioxidant enzyme activities and radical scavenging potency in Tartary buckwheat sprouts. The results indicated that the optimal microwave irradiation conditions for superoxide dismutase, catalase, peroxidise and ascorbate peroxidise antioxidant enzymes was the power 300W for 75s, and their activities were all higher than those of the control and the ungerminated seeds. In addition, under the above microwave conditions, the total reducing power and the ability to scavenge DPPH, ABTS, O2- and •OH were also optimal. These results indicated that suitable microwave treatment could effectively improve the antioxidant enzyme activity in Tartary buckwheat sprouts and enhance the antioxidant capacity of sprouts.

Rapid microwave-assisted bulk production of high-quality reduced graphene oxide for lithium ion batteries

Graphene-based advanced electrodes with improved electrochemical properties have received increasing attention for use in lithium ion batteries (LIBs). The conventional synthesis of graphene via liquid phase exfoliation or chemical reduction of graphene oxide (GO) approaches, however, either involves prolonged processing or leads to the retainment of high-concentration oxygen functional groups (OFGs). Herein, bulk synthesis of high-quality reduced graphene oxide using microwave irradiation (MWrGO) within few seconds is reported. The electromagnetic interaction of GO with microwaves is elucidated at molecular level using reactive molecular dynamic simulations. The simulation suggests that higher power microwave irradiation results in significantly less retainment of OFGs and the formation of structural voids. The synthesized MWrGO samples are thoroughly characterized in terms of structural evolution and physicochemical properties. Specifically, a modified ID/IG-in ratio metric for Raman spectrum, wherein the intensity contribution of D’ peak is deducted from the apparent G peak, is proposed to investigate the structural evolution of synthesized MWrGO, which yields a more reliable evaluation of structural disorder over traditional ID/IG ratio. Li-ion half-cell studies demonstrate that the MWrGO is an excellent candidate for usage as high capacity anode (750.0 mAh g−1 with near-zero capacity loss) and high-performance cathode (high capacity retention of ~70% for LiCoO2 at 10 C) for LIBs.

Alternate Catalyst Support from Microwave-Assisted Activation of Coconut Tree Fiber

Coconut tree fiber (CTF), representing a potentials feedstock from agricultural processing, was developed to porous carbon by microwave-assisted activation. CTF was transformed into CTF char before further activation. The CTF porous carbons (CTFCs) were obtained by the microwave-assisted activation using KOH as an activating agent at 700 W microwave radiation power. To investigate the effects of KOH on specific characteristics of CTFCs, the different ratios of KOH:CTFs char (i.e. 1:1, 1:2, 1:4, 1:8 w/w) were studied. CTFCs were extensively characterized on physicochemical properties including proximate & ultimate analysis, porous characteristics, and surface morphology. The porous carbon exhibits a combination of type I/type IV sorption isotherm representing the micro/mesopores in its pore structure. The optimal condition obtained the highest SBET of 913.56 m2/g with total pore volume of 0.42 cm3/g. CTFC were potentially applied for metal catalyst support in various chemical reaction due to its high surface area and porosity to enhance the catalyst dispersion.

PH/temperature‐responsive poly(dimethylaminoethyl methacrylate) grafted κ‐carrageenan copolymer: Synthesis and physicochemical properties

Abstractκ‐Carrageenan‐graft‐poly(dimethylaminoethyl methacrylate) (CRG‐g‐PDMA) was synthesized via free‐radical copolymerization under microwave irradiation. 13C‐Nuclear magnetic resonance spectroscopy (13C‐NMR), fourier‐transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X‐ray diffraction (XRD) methods were used for characterization of the CRG‐g‐PDMA structure. The effect of reaction variables including the concentrations of the initiator and dimethylaminoethyl methacrylate (DMA) as well as the microwave irradiation time and power on graft ratio and conversion was investigated, and their highest values were obtained to be 179 and 76%, respectively. The lower critical solution temperatures (LCSTs) of the CRG‐g‐PDMA copolymers were determined at various pH conditions, and their value in distilled water was found as 47°C. Besides, the swelling capacity of the copolymers under different pH levels was investigated and it was seen that the swelling capacity of the copolymer increased with the decrease in the pH value of the medium. The CRG‐g‐PDMA copolymer exhibited excellent swelling performance in acidic pH conditions and possessed responsiveness to pH and temperature stimuli.

CoMnMgAl mixed oxides prepared by a microwave assisted self-combustion synthesis for toluene total oxidation

Volatile organic compounds (VOCs) are one of the most relevant air pollutants as they act as precursor agents for photochemical smog, acid rain and other noxious compounds. Catalytic oxidation stands out as an abatement method of VOCs thanks to its high efficiency and low energy requirements. Within this approach, catalysts based on cobalt and manganese are remarkable systems due to their excellent redox properties and the oxygen mobility within the mixed oxides structures.In this work, CoMnMgAl-Ox type catalysts were prepared by a microwave (MW) assisted self-combustion synthesis method varying the Co/Mn molar ratio between 0 and 1, the MW irradiation power between 80 and 500 W and the impregnation or non-impregnation with platinum at 1.0 %. The catalysts were characterized by XRD, N2 adsorption-desorption isotherms, HR-TEM and XPS techniques and evaluated in the catalytic oxidation of toluene in diluted gas phase at atmospheric pressure. All solids prepared by MW showed higher catalytic activities than their equivalents prepared by traditional self-combustion, demonstrating that the use of microwaves in the synthesis enhances the catalytic activity of materials in the oxidation due to the prevalence of Co (III) species and a smaller particle size. Additionally, it was found that the Co/Mn = 0.6 M ratio in the oxide, gives rise to the most active catalyst, presenting conversions comparable to those of the catalysts impregnated with platinum.

A study of spin rectification effect enhancement of microwave field by a PIFA

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.

Parametric Study on Microwave-Assisted Pyrolysis Combined KOH Activation of Oil Palm Male Flowers Derived Nanoporous Carbons.

Oil palm male flowers (PMFs), an abundant agricultural waste from oil palm plantation in Thailand, have been utilized as an alternative precursor to develop nanoporous carbons (NPCs) via microwave-assisted pyrolysis combined potassium hydroxide (KOH) activation. The influences of relevant processing variables, such as activating agent ratio, microwave power, and activation time on the specific pore characteristics, surface morphology, and surface chemistry of PMFs derived nanoporous carbons (PMFCs) have been investigated to explore the optimum preparation condition. The optimum condition under a microwave radiation power of 700 W, activation holding time of 6 min, and activating agent ratio of 2:1 obtained the PMFC with the highest Brunauer–Emmett–Teller (BET) surface area and total pore volume approximately of 991 m2/g and 0.49 cm3/g, composed of a carbon content of 74.56%. Meanwhile, PMFCs have a highly microporous structure of about 71.12%. Moreover, activating agent ratio and microwave radiation power indicated a significant influence on the surface characteristics of PMFCs. This study revealed the potential of oil palm male flowers for the NPCs’ production via microwave-assisted KOH activation with a short operating-time condition.

A novel microwave-assisted steam distillation approach for separation of essential oil from tree peony (Paeonia suffruticosa Andrews) petals: Optimization, kinetic, chemical composition and antioxidant activity

Microwave-assisted steam distillation (MSD) was developed as an efficient technique for the separation of essential oil (EO) from tree peony (Paeonia suffruticosa Andrews) petals, on the basis of ensuring the extraction of flavonoids. During MSD process, the raw material was separated from water to keep it in a relatively dry state, thus contributing to the subsequent extraction of flavonoids. In this work, the optimization of MSD process was conducted to obtain the optimum conditions (1.7 mL/g water ratio for soaking, 1 h soaking time, 540 W microwave irradiation power and 30 min microwave irradiation time) and high yield of EO (0.66 ± 0.03 mg/g). The MSD technique was compared with other reference methods in terms of kinetic model fitting, economic value, environmental impact, EO composition and antioxidant activity. The result pointed out that the MSD method could be reasonably fitted by first-order kinetic model, was environmentally friendly and economically sustainable, provided higher separation efficiency, required lower energy consumption, and had certain advantages in the separation of EO with higher proportion of oxygenated components and better antioxidant activity. Furthermore, microwave-assisted extraction (MAE) of flavonoids from the petal residues obtained after MSD showed better result than that obtained after microwave hydrodistillation, and the result was comparable with MAE and heat reflux extraction. Overall, MSD is a potential and powerful alternative to traditional methods for the efficient distillation of EO, on the premise of reducing the loss of flavonoids etc. non-volatile components.

Cannabidiol-enriched hemp essential oil obtained by an optimized microwave-assisted extraction using a central composite design

The increase of cultivation of industrial hemp (Cannabis sativa L.) all over the world offers new opportunities for the industry to manufacture innovative products from this multipurpose crop. In this regard, the hemp essential oil represents a niche product with potential interest for the pharmaceutical, nutraceutical, cosmeceutical and agrochemical companies. On this basis, in the present work we used the microwave-assisted extraction (MAE) to get an essential oil enriched in bioactive compounds, especially cannabidiol (CBD), from the dry inflorescences of the Italian variety CS (Carmagnola Selezionata). For this purpose, the operative conditions to increase the essential oil yield and CBD concentration in terms of microwave irradiation power (W/g), extraction time (min) and water added to the plant matrix after moistening (%), were optimized using a central composite design (CCD) approach using a Milestone ETHOS X device. The conventional hydrodistillation (HD) performed for 240 min was used for comparative purposes. The qualitative compositions of essential oils obtained by MAE and HD were analysed by GC-MS, whereas the quantitative detection of CBD and main terpenoids (α-pinene, β-pinene, myrcene, limonene, terpinolene, (E)-caryophyllene, α-humulene and caryophyllene oxide) was achieved by GC-FID. Furthermore, the enantiomeric distribution of the chiral constituents (α-pinene, β-pinene, limonene, (E)-caryophyllene and caryophyllene oxide) was determined using chiral chromatography. Results showed that the MAE treatment, using high irradiation power and relatively long extraction times, increased significantly the content of CBD in the essential oil while maintaining high oil yield values when compared with conventional HD. The enantiomeric excess of three chiral monoterpenes (α-pinene, β-pinene and limonene) was determined, with the (+)-enantiomers being predominant, whereas (E)-caryophyllene and caryophyllene oxide were enantiomerically pure. In conclusion, the MAE was successfully applied to hemp dry inflorescences in order to obtain a CBD-rich essential oil which may be exploited in several industrial applications.

Effect of Microwave Pretreatment on Gaharu Essential Oil Using Hydrodistillation Method

Gaharu wood produces an expensive essential oil due to its fragrance and chemical compounds. Normally, the black color wood is used for the extraction of oil because it contains high resin. However, the cost of this resinous wood is very expensive. Therefore, in this study, the raw material with low resin content (identified by the white color of the wood) will be used for the extraction of oil. However, hydrodistillation alone will not produce satisfactory volume. Thus, pretreatment is needed to enhance the amount of essential oil. This paper aims to investigate the effect of microwave pretreatment on gaharu essential oil extraction using the hydrodistillation method for the lower grade wood. The gaharu wood was pre-treated with microwave irradiation power of 800 kW at three different durations which were 1, 2 and 3 min. Later, extraction was performed using the hydrodistillation method for 30 h. The chemical composition of the oil was analyzed using gas chromatography-mass spectrometry (GCMS). The results show that microwave pretreatment produced higher yield (0.0379 wt.%) of gaharu essential oil compared to the extraction of the non-pretreated wood (0.0286 wt.%) in which the increase was 24.55%. The three minutes pretreatment time produced the largest amount of oil (0.0877 wt.%). The outcomes of this study indicated the increase of chemical components of gaharu essential oils such as (+)-Epi-bicyclosesquiphellandrene, gurjunene, α-farnese, estragole, guaiene, valencene, spathulenol, α-cubebene and (-)-Spathulenol in the extraction of the pre-treated wood compared to the non-pretreated one.

Efficient biocatalytic strategy for one-pot Biginelli reaction via enhanced specific effects of microwave in a circulating reactor

A one-pot efficient biocatalytic strategy for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones was developed in a circulating microwave reactor selecting α-chymotrypsin as the promiscuous biocatalyst. In the circulating reaction system, the combination of microwave heating and external cooling could avoid the denaturation and inactivation of enzyme, and greatly improved the radiation power of microwave, thus improving the specific effects of microwave. During the reaction process, the microwave radiation power was automatically adjusted by adjusting the speed of the reaction mixture circulation. When the microwave power was maintained at 110W, the best results could be obtained with the highest yield of 96% at 55°C in 50min, and the reaction had a wide range of substrates. But no obvious product was detected in a tank microwave reactor at 55°C for 100min, under this condition, the microwave power was maintained at about 3W. As a contrast, the reaction only obtained 63% yield in 55°C oil bath for 96h.

Effect of Microwave on Changes of Gallic Acid and Resveratrol in a Model Extraction Solution

Microwave processing techniques have been extensively used in the food industry due to its significant reduction in time and energy consumption, but its effect on the potential degradation of the extracted target compounds cannot be ignored. In this paper, experiments were carried out to establish a model system of closed magnetic stirring and controlled temperature cooling microwave irradiation (CMCC-MI system); the model extraction solutions were constructed with the standards gallic acid and resveratrol as target compounds to be extracted; its type of free radicals were detected during microwave irradiation extraction, and its changes were monitored under different microwave extraction conditions, namely, solvent types, organic extractant concentrations, microwave irradiation power, extraction temperature, and extraction time, in order to understand the effect of microwave irradiation and provide an objective evaluation polyphenols under different microwave extraction conditions. The degradation and fragment ion peaks of gallic acid and resveratrol owing to hydroxyl free radicals imply that the extraction yield should not be over-focused in actual extraction applications of microwave irradiation; more attention should be paid to the potential degradation of the extracted target compounds induced by microwave irradiation.

Methods for Substantiating the Electrodynamic and Design Parameters of Absorbing Materials of Waveguide Type

The paper considers the issues related to the microwave diffraction on periodic structures of waveguide type. An approach used for assessing the reflective characteristics of absorbing material is proposed. This approach is based on the assumption that in accordance with the energy conservation law, the entire power of the incident microwave radiation on the absorbing material is distributed into two components. One of them characterizes electromagnetic radiation reflection from material surface and is determined through wave impedance at the interface “medium – absorbing material”, and the other, obeying the law of electromagnetic waves diffraction on nonperfectly conducting closed surfaces of waveguide type, propagates into the lattice, then it is partially absorbed in the bulk of the material and reflected from the object (metal substrate), after which it is summed with the first component at the output. A feature of the proposed technique is the design ratios, allowing to determine rational values of dielectric permeability of material, lattice elements conductivity as well as the period and thickness of the lattice which enable to achieve “a semi-infinite layer” at which the minimum value of material reflection coefficient is observed and its no longer depends on the presence or absence of an object (metal substrate)

Vacuum pyrolysis incorporating microwave heating and base mixture modification: An integrated approach to transform biowaste into eco-friendly bioenergy products

The escalating consumption of fossil fuels and dumping of palm kernel shells (PKS) drives biofuel production to improve supply and waste disposal. To convert PKS into modified biochar (MBC) value-added solid fuel, we use microwave vacuum pyrolysis accompanied by sodium-potassium hydroxide mixture modification. First, PKS underwent microwave vacuum pyrolysis to produce biochar, and then it was chemically activated using sodium-potassium hydroxide mixture. The MBC surface morphology, porous characteristics, proximate content, and energy properties depended on microwave irradiation period and power. High yields (79 ± 1.5 wt%) were recorded at microwave power 700 W and irradiation period of 10 min, giving a high BET surface area (1320 m2/g) and pore volume (0.70 cm3/g). The MBC had acceptable low content of ash, nitrogen, and no sulphur, demonstrating its potential as an environmental friendly fuel to replace conventional coal in combustion. The MBC shows high energy yield (≤90.5%), fuel ratio (≤26.47), and heating value (≤28.69 MJ/kg) comparable to conventional fuels, thus showing desirable solid fuel properties. Energy balance analysis shows positive energy ratio of up to 10 and net energy output of up to 24.47 MJ/kg, recovering a product with a higher energy content compared to electrical power input for the pyrolysis operation. These findings demonstrate the exceptional potential of the MBC produced by this innovative approach for bioenergy generation that per se will reduce the emissions of greenhouse gases and thereby reducing global warming and climate change.

The luminescence behavior of Zn1-xTaxO nanophotonic crystals prepared via Di-N-(2-hydroxyethyl)oleylamin assisted microwave approach

Semiconducting Zn1-xTaxO nanophotonic crystals were synthesized via Di-N-(2-hydroxyethyl)oleylamin assisted microwave approach using tantalum acetylacetonate, zinc acetylacetonate and 8-ethylquinoline N-oxide. The developed Zn1-xTaxO nanophotonic crystals exhibited a hexagonal crystalline structures and their lattice constant increased with increasing Ta atoms. The Zn1-xTaxO nanostructures demonstrated emission spectrum in the visible region from 354 to 430 nm. The inclusion of Ta atoms in the ZnO QDs enhanced the luminescent light and the quantum yields for ZnO nanocrystals by five orders at concentration of 5 wt%. The change of the power and time of microwave irradiation for the Zn0.95Ta0.05O nanocrystals instigated shift of the luminescence peak positions toward blue light and a modulation of the optical band gap. The increase of the microwave power to 850 W for 35 min for Ta-doped ZnO at concentration of 5 wt% comprised a remarkable increase of the luminescence intensity 7 times, decreased the emission band width by 5 times and enlarged the quantum yield to 92 %. These outstanding features may render the Zn0.95Ta0.05O nanocrystals to be used as laser material for the laser diode applications.

Preliminary investigation of a magnetically insulated relativistic backward wave oscillator operating in the C-band with low magnetic field

The design and preliminary results for a C-band relativistic backward wave oscillator (RBWO), which is magnetically well insulated, are presented. Under an external magnetic field of 0.36 T, the RBWO generated high power microwave radiation with a power of 3.3 GW and a frequency of 4.37 GHz for a diode voltage of 870 kV and a beam current of 13.5 kA. The electric field on the surface of the cathode holder was below the emission threshold, and an inlaid graphite cathode was designed to suppress the shunting current in the diode area. The device, operating with a low magnetic field and diode insulation, is a promising candidate for use as a permanent magnet package in high power microwave systems.

PENYERAPAN LOGAM BERAT Pb DAN Cu MENGGUNAKAN KARBON AKTIF BERBASIS MAHKOTA NANAS DENGAN VARIASI KONSENTRASI KALIUM HIDROKSIDA

This study aims to determine the influence of concentration of potassium hydroxide (KOH) on the adsorption of Pb2+ and Cu2+ metal ions in industrial used water. The activated carbon was prepared from pineapple crown waste. Pineapple crown was pre-carbonized at 180oC for 1 hours. Chemical activation was done using KOH with variations in concentrations of 2, 3, and 4 M. The samples were irradiated by using a microwave at 630 Watt of output power for 15 minutes. KOH activation and microwave irradiation power were influenced to the physical properties of the samples. Activated carbon obtained were characterized using SEM, EDX, FTIR and AAS. The results showed that highest adsorption were obtained from activated carbon that was activated by KOH 2 M. Adsorption efficiency for Pb2+ and Cu2+ metals were 81.15% and 49.71%, respectively. The carbon chain structure shows the functional groups C-H, C≡C, O-H, C=C at wavenumber 2889.49 cm-1, 2360.97 cm-1, 2339.97 cm-1.