Moderate Microwave Power Research Articles

Study on the underpinning mechanisms of microwave-induced synthesis of carbon-coated metal nanoparticles

Microwave-induced metal arc discharge provides an intriguing solution for the synthesis of carbon-coated metal nanoparticles (M@CNPs) due to its fast formation and improved quality of products, however, the underpinning reaction mechanism is not comprehensively revealed. In this work, the effect of arc discharge intensity on the product morphology is firstly investigated by adjusting microwave power. And then, the effects of the organic groups (i.e. cyclopentadienyl v. s Acetylacetonate groups) and metal catalysis on the product morphology are also investigated by selecting ferrocene, iron acetylacetonate, nickelocene, and nickel acetylacetonate as precursors. Specifically, moderate microwave power can not only destroy the precursor to form carbon nanosheets by inducing intense discharge heat release but also provide sufficient intermittency to allow the carbon nanosheets to deposit on the surface of the metal core, which is an important precondition in fabricating M@CNPs. The structure of organic groups in the precursor also plays a predominant role in adjusting product morphology. Cyclopentadienyl groups tend to encapsulate the metal core to form graphitized carbon shells as the coordination bond between cyclopentadienyl and metal is recognized as a very strong covalent bond that confines the cyclopentadienyl collapse to metal core, and the cyclic structure facilitates the formation of graphite. In contrast, the acetylacetonate groups intend to combine randomly due to their open-loop structure. Moreover, for open-loop structures such as acetylacetonate groups, metal catalysis also affects the growth trend, of which Ni is more likely to induce the formation of carbon nanotubes relative to Fe. This work can provide a good reference for the synthesis of M@CNPs with controllable morphology.

Microwave heating as a tool to enhance antioxidant activity and release soluble conjugates from Feutrell’s Early (citrus mandarin cultivar) peels

The phenolic constituents usually exist in free and covalently bound forms in the plant materials. This study reports the impact of microwave heating on certain phenolic compounds and antioxidant potential assessed by free radical-scavenging and reducing power of citrus peels. Different levels of microwave power (125, 250, 500 W) and heating times (5, 10, 15 min) were applied. The phenolic acids were fractionated into three forms namely; free, glycoside-bound, and ester-bound. Microwave power and heating time significantly increased the content of free phenolic fraction while the content of bound fractions decreased. Increasing microwave power significantly increased the antioxidant activity of heated samples. The microwave heating up to 10 min at 250 W increased the antioxidant activity of the samples but further heating to 15 min exerted an adverse effect. A moderate microwave power and heating time was helpful for improving the antioxidant potential of citrus peels. Practical applications A very simple method is used in this study to release the bound phenolics and to improve the antioxidant potential of citrus peels, thereby, maximizing the use of fruit byproducts in general and citrus peels in particular. By this way, it can be helpful to decrease the environmental pollution caused by citrus fruit waste products. The use of microwave heating might be helpful for the citrus processing industry to minimize the loss in terms of effluent management. It might be an economical processing technique for the pharmaceutical and functional food sector.

Double quantum magnetometry at large static magnetic fields

We present a protocol to achieve double quantum magnetometry at large static magnetic fields. This is a regime where sensitive sample parameters, such as the chemical shift, get enhanced facilitating their characterization. In particular, our method delivers two-tone stroboscopic radiation patterns with modulated Rabi frequencies to achieve larger spectral signals. Furthermore, it does not introduce inhomogeneous broadening in the sample spectrum preventing signal misinterpretation. Moreover, our protocol is designed to work under realistic conditions such as the presence of moderate microwave power and errors on the radiation fields. Albeit we particularise to nitrogen vacancy centers, our protocol is general, thus applicable to distinct quantum sensors.

Time-optimized pulsed dynamic nuclear polarization.

Pulsed dynamic nuclear polarization (DNP) techniques can accomplish electron-nuclear polarization transfer efficiently with an enhancement factor that is independent of the Zeeman field. However, they often require large Rabi frequencies and, therefore, high-power microwave irradiation. Here, we propose a new low-power DNP sequence for static samples that is composed of a train of microwave pulses of length τp spaced with delays d. A particularly robust DNP condition using a period τm = τp + d set to ~1.25 times the Larmor period τLarmor is investigated which is a time-optimized pulsed DNP sequence (TOP-DNP). At 0.35 T, we obtained an enhancement of ~200 using TOP-DNP compared to ~172 with nuclear spin orientation via electron spin locking (NOVEL), a commonly used pulsed DNP sequence, while using only ~7% microwave power required for NOVEL. Experimental data and simulations at higher fields suggest a field-independent enhancement factor, as predicted by the effective Hamiltonian.

Energy and exergy analysis, drying kinetics, modeling and quality parameters of microwave-dried turmeric slices

The aim of this study was to evaluate the effect of curing and microwave power levels on energy analysis, drying characteristics, modeling and quality parameters of turmeric slices in microwave dryer. Drying experiments were carried out for fresh and cured turmeric slices. Four microwave power levels of 30%, 50%, 80% and 100% with the total output power of 900 W were used for this study. From the drying rate curve, it was observed that the drying process mainly takes place in the warming up and falling rate periods. Among the eight models evaluated in the study, Midilli et al. and Page model had a good agreement with the experimental data. Moisture diffusivity values increased (1.83 × 10−08 to 1.59 × 10−07 m2/s) as the microwave power level increases. From the energy analysis, it was found that specific moisture extraction rate and specific energy consumption values varied in the range of 0.146–0.395 kg/kWh and 9.1093–24.6093 MJ/kg, respectively. Energy efficiency values (9.24–24.75%) were found to be higher than the exergy efficiency values (2.18–12.77%). Quality parameters such as color value and curcumin content of the fresh samples were found to be higher when compared to cured samples. SEM analysis revealed the porous internal structure of the dried samples. From this study, it is revealed that curing of turmeric slices has negligible effect on the parameters analyzed. A moderate microwave power level would be suitable for turmeric drying to produce high-quality product with lesser energy consumption.

Wheat Straw Bio-refining Part II: Optimization of Alkali Pre-treatment Method with Microwave Radiation for Fermentable Sugars Separation

Background: The potential of microwave-assisted heating for enhancing the alkaline pretreatment of lignocellulosic waste is highlighted in this work. Objective: To maximize separation of fermentable sugars from wheat straw. Methods: Wheat straw hydrolysis assisted by microwave was carried out by setting up a statistical experimental design method and further investigating the main process parameters, namely: temperature (°C), microwave power (W), NaOH concentration (M) and wheat straw pre-treatment time (min) towards maximization of fermentable sugars extraction from wheat straw. The reducing sugars yield (response) of the alkaline pre-treated and microwaved wheat straw in lab scale provided data for building a predictive model which reflected interactions, significance and impact of the process parameters (factors) on the wheat straw hydrolysis yield. Results: SEM and FTIR images of untreated and alkaline pre-treated wheat straw were studied for investigating the morphological changes of wheat straw surface quality and structure resulting from the microwave and/or alkaline pre-treatment. In addition reducing sugars yield of 87%wt. from wheat straw pre-treated at 180°C, 550W microwave power, 0.65M of NaOH for 25min was achieved. This result was significantly higher compared to the one from the straw which was only hydrolysed enzymatically (30 %wt). Conclusion: It was found that shorter pre-treatment times were obtained at higher temperatures, alkali concentration and moderate microwave power levels. The results obtained were further optimized and indicated that the microwave-assisted alkaline pre-treatment of wheat straw technique is an attractive pre-treatment method which reduces the wheat straw pre-treatment time and enhances the hydrolysis yield.

Torrefaction of biomass from municipal solid waste fractions II: Grindability characteristics, higher heating value, pelletability and moisture adsorption

The microwave-assisted torrefaction of woody construction demolition waste (CDW) and grass clippings (GC) from municipal solid waste (MSW) was investigated under nitrogen-activated inert condition with respect to microwave power (250, 500 and 750 W), residence time (15, 30 and 45 min), moisture content (12, 16 and 20% w.b.) for CDW and 64% w.b. for GC. The results show that grinding energy consumption decreases as the microwave power level increases, indicating a significant improvement in the grindability of the material after torrefaction. Depending on the treatment combination, grinding energy savings of about 29–70% and 30–68% resulted after microwave-assisted torrefaction of CDW and GC, respectively. The higher heating value (HHV) increases with increasing microwave power level and torrefaction residence time. For CDW, the HHV increased by about 3–39% having HHV of 25,691 MJ/t. While for GC, the HHV increased by about 8–24%, having HHV of 20,936 MJ/t. This investigation further demonstrates that torrefaction reduced moisture adsorption of CDW and GC, which significantly improves their storage capability in humid locations. The moisture uptake ratios for torrefied CDW were found to be 0.66–0.96 and about 0.92 for torrefied GC, against a moisture uptake ratio of 1.0 for raw CDW and GC. However, biomass subjected to severe torrefaction process conditions produced pellets having poor physical quality. Microwave irradiation can be used effectively for torrefaction of MSW investigated at moderate microwave power level and short torrefaction residence time.

Developmental Aspects of Microwave–Plasma Interaction Experiments: Phase-1

An experimental system, SYstem for Microwave PLasma Experiments (SYMPLE), is being developed in order to investigate the physics of linear and nonlinear interaction of a high-power microwave (HPM) with an overdense plasma (plasma frequency $f_{p} \,\, >$ microwave frequency $f_{\mu })$ . The physics of interaction has implications in inertial fusion. The developmental task is taken up in two phases: Phase-1 using moderate microwave power ( $\sim 3$ MW), satisfying the condition ${\scriptstyle{\raise2pt\hbox{$\scriptscriptstyle1$}\mkern-4mu/\mkern-2mu{\scriptscriptstyle 2}}}\varepsilon _{\textrm {o}}\textrm {E}_{\mu }^{2}/\textrm {n}_{\textrm {e}}\, \textrm {kT}_{\textrm {e}} \sim 1$ and Phase-2 involving HPM sources of much higher power. Here, $E_{\mu }$ is the microwave electric field, $n_{e}$ the plasma electron density, $T_{e}$ the electron temperature, $m$ the electron mass, $\varepsilon _{o}$ is the free space permittivity, and $k$ is the Boltzmann constant. The Phase-1 system consists of a pulsed ( $5~\mu \text{s}$ ), $S$ -band magnetron (3 MW, 3 GHz) coupled to a pulsed ( $100~\mu \text{s}$ ) washer-gun plasma ( $n_{e} \sim 1\times 10^{18}/\text{m}^{3})$ . The tasks associated with interfacing of the magnetron and plasma have involved the indigenous design and development of different subsystems, each satisfying the specific technical requisites of the Phase-1 system. These subsystems include a washer-gun plasma ( $n_{e} \sim 1\times 10^{18}/\text{m}^{3})$ driven by a pulse forming network, a line-type modulator (52-KV 120-A, 5- $\mu \text{s}$ pulsewidth, $\sim 450$ -ns rise time) that drives the $S$ -band magnetron, an HPM–plasma coupling system, and an HPM TE10 – TM01 mode converter. The developmental aspects of the subsystems form the subject of this paper.

Microwave Supported Treatment of Sewage Sludge

Abstract This work is focused on microwave treatment of sewage sludge. The aim of our experiments was to investigate the impact of microwave radiation upon different sewage sludge parameters such as concentration of nitrates and nitrites, phosphates, COD (Chemical Oxygen Demand), SVI (Sludge Volume Index) and the microscopic structure of sludge. The experiments with microwave irradiation of sewage sludge indicate that moderate microwave power causes visible effects on the chemical, physical and biological properties of the sludge. The calculation of profitability and energy efficiency is also presented.

Anti‐solvent effect of crystallization by feeding ethanol under microwave radiation

The nucleation and crystal growth of sodium chloride in aqueous solution during crystallization by ethanol droplet feeding were investigated by determining the suspension density and crystal size distribution. Crystallization was conducted in a microwave reactor, and the effect of the power, stirring speed, and ethanol concentration were investigated to clarify the anti‐solvent effect under microwave radiation. Molecular diffusion was accelerated by fine particle production due to microwave radiation when the stirring speed was low and ethanol concentration was high. However, moderate microwave power and stirring speed were required for efficient molecular diffusion.

Preparation of Calcium-Deficient Carbonated Hydroxyapatite Nanoparticles by Microwave Process

In this paper, carbonated calcium-deficient hydroxyapatite (d-HA) nanoparticles were synthesized by a chemical precipitation method assisted with microwave irradiation. The important factors which effected on the synthesis process, such as pH value, time and the power of microwave irradiation on the Ca/P molar ratios of d-HA were discussed. The experimental results showed that carbonated d-HA nanoparticles could be quickly synthesized by irradiating the reaction solution for about 1h under moderate microwave power and pH value ranging from 8 to 9.

Microwave photoconductivity of a two-dimensional electron gas: Mechanisms and their interplay at high radiation power

We develop a systematic theory of microwave-induced oscillations in the magnetoresistivity of a two-dimensional electron gas, focusing on the regime of strongly overlapping Landau levels. At linear order in microwave power, two mechanisms of the oscillations (``quadrupole'' and ``photovoltaic'') are identified, distinctly different from those studied before (``displacement'' and ``inelastic''). The quadrupole and photovoltaic mechanisms are shown to be the only ones that give rise to oscillations in the nondiagonal part of the photoconductivity tensor. In the diagonal part, the inelastic contribution dominates at moderate microwave power, while at elevated power the other mechanisms become relevant. We demonstrate the crucial role of feedback effects, which lead to a strong interplay of the four mechanisms in the nonlinear photoresponse and yield, in particular, a nonmonotonic power dependence of the photoconductivity, narrowing of the magnetoresonances, and a nontrivial structure of the Hall photoresponse. At ultrahigh power, all effects related to the Landau quantization decay due to a combination of the feedback and multiphoton effects, restoring the classical Drude conductivity.

Homoepitaxial CVD diamond: Raman and time-resolved PL characterization

In this work, we report on the structural characterization of homoepitaxial Microwave Plasma Enhanced CVD diamond grown onto Ib diamond substrates by varying systematically the methane to hydrogen ratio in the gas mixture (1–7% CH 4). X-ray diffraction, Raman spectroscopy and photoluminescence (PL) have been used to characterize the diamond samples. Raman measurements pointed out the excellent crystalline quality and phase purity of the specimens. PL measurements in the 1.7–2.7 eV energy range have shown completely flat spectra, excluding the presence of nitrogen-related optical centers. Such results show that the homoepitaxial CVD diamond can be grown, at moderate microwave power (720 W), and at growth rates not too low (∼ 1 μm/h) preserving a good quality. Moreover, the homoepitaxial crystals exhibited a strong free-exciton recombination radiation at room temperature even at the highest methane concentration used (7%). Preliminary measurements of the lifetime of the free exciton at room temperature have been also performed. The excitation was produced by a 5 ns pulsed laser irradiation at energies above the diamond band gap. The results have been compared with the structural properties of the samples and correlated with the growth conditions.

Efficient synthesis of diacetal of pentaerythritol under microwave irradiation using heteropoly acid H 3PW 12O 40

Tungstophosphoric acids (HPA) act as an efficient heterogeneous catalyst for the simple, and environmentally friendly synthesis of acetals in microwave at shorter reaction time with moderate microwave power superior to liquid phase involving HPA and various other heterogeneous catalysts, where the reactants were refluxed with benzene/toluene solvents for longer duration. While aryl aldehydes acetals are often easily prepared from their respective ketones, many acetals from aryl alkyl and diaryl ketones have been less accessible because of electronic and steric factor. This paper pays a greener alternative route for the direct transformation of such wide range of ketones and aldehydes into their respective acetals in short reaction time with high selectivity and yield, without the need for solvents in contrast to conventional method.

Characterization of homoepitaxial CVD diamond grown at moderate microwave power

In this paper, we report on the characterization of homoepitaxial CVD diamond grown onto HPHT Ib diamond substrates by varying systematically the methane to hydrogen ratio in the deposition gas mixture (1–7%) and the microwave power (520–720 W). Growth rates up to approximately 2.2 μm/h have been achieved. X-ray diffraction, Raman spectroscopy and photoluminescence (PL) have been used to characterize the diamond samples. Raman measurements point out an excellent crystalline quality and phase purity of the homoepitaxial specimens even at the highest CH 4 concentration used. Completely flat PL spectra registered in a wide energy range (1.7–2.7 eV) exclude impurity contamination of the diamond samples. Such results show that homoepitaxial CVD diamond can be grown, at moderate microwave power and with moderate growth rate, preserving a good crystalline quality.

Decomposition of phosgene by microwave plasma-torch generated at atmospheric pressure

The microwave plasma-torch and its application for decomposition of chemical warfare agent using a moderate microwave power are investigated. Decomposition of chemical warfare agents is demonstrated for phosgene (COCl/sub 2/) as a sample example. The main byproducts of phosgene decomposition are CO and Cl/sub 2/. The Cl/sub 2/ gas was completely converted to HCl by injecting NH/sub 3/ into the microwave plasma-torch as an additive gas. The HCl is, in turn, eliminated by passing the exhaust gas through a wet-scrubber. Detailed decomposition properties of phosgene by microwave plasma-torch at the atmospheric pressure are also investigated by making use of Fourier transform infrared, mass spectroscopy, and ion chromatography.

Characteristics of an Arc-Seeded Microwave Plasma Torch

The design and operation of a portable microwave plasma torch is presented. An arc plasma torch running at 60 Hz and generated by a torch module is installed on the bottom wall in the narrow section of a tapered S-band rectangular cavity, and is used to seed the microwave discharge at the location where the microwave electric field is at a maximum. This tapered cavity is designed to support the TE/sub 103/ mode. With seeding, only low Q cavity and moderate microwave power (time average power of 700 W) are needed. The microwave-enhanced discharge increases the size, cycle energy, and duty cycle of the torch plasma considerably. This torch can be run without introducing gas flow to stabilize the arc and microwave discharges. Adding gas flow can increase not only the size of the torch plasma, but also its cycle energy which reaches a plateau of about 12 J/per cycle for a gas flow rate exceeding 0.393 l/s. The electron density and excitation temperature, and the composition of torch species are determined by emission spectroscopy. It is shown that, at the bottom of the torch close to the cavity wall, electrons distribute quite uniformly across the core of the torch with density and excitation temperature determined to be about 7/spl times/10/sup 13/ cm/sup -3/ and 8000 K, respectively. It is also found that this torch produces an abundance of reactive atomic oxygen.

Probe diagnostics of high pressure microwave discharge in helium

A method for the determination of electron temperature and plasma density in high pressure helium plasmas is established using numerical results of the continuum probe model by Cohen [Phys. Fluids 6, 1492 (1963)]. Simple algebraic functions are derived to approximate the probe characteristics of high pressure plasmas calculated by Cohen and are applied to use iterative procedures for the determination of plasma parameters. The proposed fitting technique has allowed one to obtain reasonable plasma parameters even for the probe characteristics strongly affected by large secondary electron emission currents from the probe. Fitting of the ion saturation current may possibly be used to estimate the ion temperature, provided that the electron temperature and plasma density are known. Finally high pressure helium microwave discharges have been produced by moderate microwave power of 400 W and investigated by the present method.

Characterization of a new electron cyclotron resonance source working with permanent magnets

Most electron cyclotron resonance (ECR) sources available today are working with electromagnets. A new source, developed for etch applications, is supplied with permanent magnets. This leads to strong short-range magnetic fields and the ECR condition is fulfilled on a ring near the chamber wall. Having the maximum electric field strength of the 2.45 GHz along this ring in the microwave radiator leads to efficient plasma production. The principle source characteristics are demonstrated with oxygen plasmas. At low pressure (some 10−4—some 10−3 mbar) and moderate microwave power (100–500 W) the plasma burns as a ring plasma. Increasing pressure and/or microwave power over the above mentioned values leads to a ‘‘bulk’’-type plasma. The change from ‘‘ring’’-type to bulk-type plasma occurs discontinuously and is connected (a) with a discontinuity in the run of the reflected power and (b) with a sudden change in the relative ion flux densities of different ionic fragments measured by mass spectrometry. Absolute ion densities up to 5×1010 cm−3 have been found by Langmuir probe measurements in a bulk plasma at a pressure of 1×10−3 mbar. Anisotropic etching of polysilicon over SiO2 was performed in the ring mode with pure Cl2 as feed gas and applying a radio-frequency (rf) extraction to the ECR plasma. Low damage etching was achieved due to the low direct-current (dc)-bias voltage of 150–200 V with very good anisotropy. The poly-Si etchrate was 85 nm/min with a selectivity of 18:1 to SiO2 and a uniformity of ±7.5% over 200 mm wafers.

Sensitivity improvement of a Varian E-109 EPR spectrometer with a low noise microwave amplifier

The installation of a low noise GaAsFET microwave preamplifier into a Varian E-109 homodyne X-band EPR spectrometer is described, and measurements of the resulting improvement in sensitivity as a function of microwave power are presented. Particular attention is paid to modification of the automatic frequency control (AFC) system to compensate for the preamplifier gain, which would otherwise result in instability of the AFC servoloop and decreased spectrometer performance at moderate microwave power levels. Specific details are provided for modification of the Varian E-102 bridge, but the general approach is applicable to other spectrometers.