Low Microwave Power Levels Research Articles

Design and performance of an open-ended converging microwave antenna in fracturing biotite diorite at low microwave power levels

Design and performance of an open-ended converging microwave antenna in fracturing biotite diorite at low microwave power levels

Prediction of the charge carriers stationary distribution in the active region of the p-i-n structures by the perturbation theory methods

The p-i-n diode is an electronic device that is widely used for switching a microwave signals. The theory of the p-i-n diode is based on linear mathematical models that satisfactorily explain the diodes switching properties at low microwave power levels. The developed methods for modeling the corresponding devices on p-i-n diodes turned out to be untenable when studying the properties of diodes and diode structures under the with high-power microwave signals (typical for high-power switches and protective devices). Here are faced with the need to take into account the mutual influence of diffusion-drift, wave, thermal processes, in which the nonlinear components of the mathematical models will dominate. The development of the computer technology and the corresponding mathematical methods (for example, the perturbation theory methods) determines the possibility of improving the existing p-i-n diodes mathematical models and the possibility of the new approaches developing to the analysis of the nonlinear processes in p-i-n diodes and similar electronic devices. The goal of this paper is to improve the mathematical model and methods for predicting the electron-hole plasma stationary distribution in the active region of surface-oriented p-i-n structures based on the use of the boundary functions method. The mathematical model of the electron-hole plasma stationary distribution in the integrated surface-oriented p-i-n structures active region is constructed in the form of the nonlinear singularly perturbed boundary value problem for the system of equations of the charge carriers current continuity and Poisson. An approximate solution of the corresponding boundary value problem is found in the form of the asymptotic series leading terms in powers of a small parameter. A scheme for finding the problem solution is proposed, which automatically includes the classical formulations of problems for modeling the p-i-n structures characteristics and allows you to make significant amendments to the solution. This ensures an increase in the level of adequacy of modeling and understanding of the features of a number of physical processes (diffusion-drift, recombinant, injection) in the p-i-n diodes active region. We consider the proposed approach a promising tool for studying nonlinear thermal, diffusion-drift, generation-recombination stationary and non-stationary processes occurring in the p-i-n structures elements under the action of the external microwave radiation, and predicting new physical effects in the studied systems, for example, due to the influence of local surface and bulk defects on the p-i-n structures characteristics.

Properties of concrete incorporating microwave treated coarse aggregate: An experimental study

Using microwave heating technology to assist concrete recycling have great potential for improve the quality of recycled concrete aggregate. Unfortunately, there are few studies explored on the properties of concrete incorporating microwave treated coarse aggregates. This study aims to investigate the performance of coarse aggregate after microwave treatment and to evaluate the properties of the resulting concrete incorporated microwave treatment aggregate. The water absorption and crushing value tests were carried out to study the effect of microwave heating treatment on the physical properties of coarse aggregates. Concrete incorporated aggregate with various microwave heating parameters was fabricated and tested. The results indicated that physical performance of microwave treated aggregate was within the range of specification requirement and water absorption and crushing value of coarse aggregate heated under low microwave power levels (2–3 kW) is very closed to the untreated samples, while coarse aggregates treated with high power levels showed a significant increase. It was also observed that concrete made out of low microwave power levels heated aggregates are able to achieve strength on par with natural coarse aggregate concrete. In addition, in case of similar strength, the elastic modulus of microwave treated aggregate concrete is similar to natural aggregate concrete. These results indicate that using low microwave power levels to assisted aggregates recycling can reduce the damage to coarse aggregates to some extent, which provide a further understanding for the application of microwave-assisted aggregates recycling.

Paramagnetic Centers of Mineral Component in Annealed Bone Tissue

The mineral components of powdered samples of cortical dense bone tissue of domestic animals (pig and cow) were studied by electron paramagnetic resonance (EPR). Bone tissue was heated in a temperature range 600-1000 ºС in an oven and then held for more than one year at room temperature. This was done to allow short-lived defects to decay and to let the mineral structures stabilize. In these heat-treated samples, within the limits of the sensitivity of the experiments, no EPR signals were detected, but after irradiation with X-rays, multicomponent EPR signals appeared. Spectra vary depending on the heating temperatures. Paramagnetic centers related to РО32–, NO42–, CO2–, CO33– and O– groups are observed. The g-factor of spectroscopic splitting and amplitude of ultrafine interaction of the indicated paramagnetic centers in the spectra were determined. Due to different relaxation behavior of the paramagnetic centers, and hence their saturation effects, the form of the total EPR signals significantly depends on the microwave power level at which the spectra are recorded. Therefore, EPR signals were recorded at high (5 mW) and low (0.13 mW) microwave power levels. The temperature dependency of EPR signal amplitude was determined for some paramagnetic centers in the range of 600-1000 ºС. The EPR signal properties of the heated bone samples and synthetic hydroxylapatites were compared. The spectra indicate that phosphates in bone tissue have a more complex structure than simple synthetic hydroxylapatite. The results of this work can be used to create synthetic analogs of bone tissue, to help in the manufacturing of implants that are used to treat bone tissue, and to study the processes related to the assimilation of mineralogic-based implants by living bone tissue.

Characterization of aroma and phenolic composition of carrot (Daucus carota ‘Nantes’) powders obtained from intermittent microwave drying using GC–MS and LC–MS/MS

Carrot is an important food in human nutrition. The effects of different microwave (MW) powers on the aroma, phenolic and antioxidant properties of MW-dried and powdered carrot samples were investigated. Fresh carrot samples (Daucus carota ‘Nantes’) were dried using seven different MW power levels of 150, 200, 250, 300, 350, 400 and 450W (0.50, 0.67, 0.83, 1.00, 1.17, 1.33 and 1.50W/g, respectively). The aroma compounds of the carrot samples were extracted by purge and trap method and determined by gas chromatography-mass spectrometry (GCMS). In fresh samples, terpenes were identified as the dominant aroma group and a significant reduction was found in the amount of these compounds depending on the applied MW power level. Terpinolene, (E)-γ-bisabolene, elemicin and myristicin were identified as dominant terpenes. In the dried samples, aldehydes, furans, alcohols, acids and pyrazines were determined as major components depending on the applied MW power levels. Hexanal, 3-methyl-butanal, acetic acid and hexanoic acid were dominant in the dried samples. For the determination of the phenolic compounds, the method of liquid chromatography coupled with tandem mass spectrometry (LCMS/MS) was employed. As a result of the analysis, a total of seven phenolic compounds, two of which were more dominant (3-caffeoylquinic acid and di-caffeic acid derivative) were identified. The total phenolic content of the carrot samples dried with lower MW power level of 150 and 200W (0.50 and 0.67W/g) was found to be relatively better preserved. When both aroma and phenolic compounds were evaluated, it was observed that they were better preserved in the carrot samples dried with low MW power levels (150 and 200W).

Model of microwave effects on bacterial spores

A recent application of microwaves in the healthcare area is for the rapid detection of bacterial spores, particularly of clinically significant spores such as Clostridium difficile. Here, we present a working model of C. difficile spore disruption by the action of a 2.45 GHz microwave electric field, independent of overall sample heating. The model shows how inner layers of the spore with the lower complex permittivity values will be subject to higher values of the electric field. The model also shows how the electric effects can be enhanced, focusing the electric field into “hotspots” using “angled” nanoparticles, yielding effective DNA release even at low microwave power levels. The model’s predictions have been validated through experimental results, which show an enhancement of DNA release from spores.

Effect of Temperature and Microwave Power Levels on Microwave Drying Kinetics of Zhaotong Lignite

Microwave drying is a promising and effective way to drying and upgrading lignite. The influence of temperature (100–140 °C) and microwave power levels (500–800 W) on thin-layer drying characteristics of Zhaotong lignite under microwave irradiation were investigated. Fourteen thin-layer drying models were used to analyze the microwave drying process while six thin-layer drying models were used to analyze the hot-air drying process. The microwave drying processes at all temperature (100–140 °C) or low microwave power levels (500–700 W) exhibited four periods: a warm-up period, a short constant period, the first and second falling rate period, while one falling rate period was found during hot-air drying. The effective diffusion coefficient of lignite were calculated and it increases with increasing temperature and microwave power levels. During microwave drying, the two-term exponential model is the most suitable model for all applied conditions, while the Modified Page model is the most suitable model to describe the hot-air drying experiments. The apparent activation energy were determined from Arrhenius equation and the values for the first and second falling rate period are 3.349 and 20.808 kJ·mol−1 at different temperatures, while they are 13.455 and 19.580 W·g−1 at different microwave power levels. This implies the apparent activation energy is higher during the second falling rate period, which suggest that the dewatering of absorbed water is more difficult than capillary water. The value of apparent activation energy in hot-air drying is between the first and second falling rate period of microwave drying. Results indicate that microwave drying is more suitable to dewatering free water and capillary water of lignite.

Overdense microwave plasma heating in the CNT stellarator

Overdense plasmas have been attained with 2.45 GHz microwave heating in the low-field, low-aspect-ratio CNT stellarator. Densities higher than four times the ordinary (O) mode cutoff density were measured with 8 kW of power injected in the O-mode and, alternatively, with 6.5 kW in the extraordinary (X) mode. The temperature profiles peak at the plasma edge. This was ascribed to collisional damping of the X-mode at the upper hybrid resonant layer. The X-mode reaches that location by tunneling, mode-conversions or after polarization-scrambling reflections off the wall and in-vessel coils, regardless of the initial launch being in O- or X-mode. This interpretation was confirmed by full-wave numerical simulations. Also, as the CNT plasma is not completely ionized at these low microwave power levels, electron density was shown to increase with power. A dependence on magnetic field strength was also observed, for O-mode launch.

Microwave torrefaction of sewage sludge and leucaena

Abstract This study attempted to investigate and compare sewage sludge and leucaena torrefaction using a single-mode microwave oven. To discuss reactivity, product characteristics, and energy recovery, microwave torrefaction of the two feedstocks was carried out at various microwave power levels. Besides, thermal properties of raw feedstocks and torrefied products were observed by the deconvolution of derivative thermogravimetric profiles. The volatile matter contents of the two biomass feedstocks were substantially reduced at relatively low microwave power levels. Because of high heating value and fuel ratio as well as low atomic H/C and O/C ratios, the biochar could replace coal. Compared with sewage sludge, microwave torrefaction of leucaena produced thermally stable biochar at lower microwave power levels, which means that the microwave heating performance of leucaena is better. Compared with conventional torrefaction, mass and energy yields of microwave torrefaction were lower, which could be attributable to the more severe reaction accomplished by microwave heating. The atomic H/C and O/C ratios of leucaena biochar were close to those of anthracite, so combustion or gasification of the biochar could form less smoke. After microwave torrefaction at relatively high microwave power levels, a high HHV (dry ash-free basis) of sewage sludge biochar was found.

Antiferromagnetic resonance in ferroborate NdFe 3(BO 3) 4

The AFMR spectra of the NdFe 3(BO 3) 4 crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields H a1 =1.14 kOe and H a2 =60 kOe and magnetic excitation gaps Δ ν 1 =101.9 GHz and Δ ν 2 =23.8 GHz. It is shown that commensurate–incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width. At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.

Microwave discharge electrodeless lamps (MDELs). VI. Performance evaluation of a novel microwave discharge granulated electrodeless lamp (MDGEL)—Photoassisted defluorination of perfluoroalkoxy acids in aqueous media

Novel microwave discharge granulated electrodeless lamps (MDGELs; 5 mm × 10 mm), fabricated using vacuum-UV (VUV) transparent quartz envelopes, have been examined for the treatment of model contaminated wastewaters. Evaluation of these MDGELs was performed through the self-ignition of the lamps at low microwave power levels and by the photoassisted decomposition and defluorination of trifluoroacetic acid (TFA), heptafluorobutyric acid (HFBA) and perfluorooctanoic acid (PFOA) in aqueous solutions in a flow-through microwave photoreactor (MW/Photo) containing 20 such MDGELs. A conventional single rod-shaped microwave discharge electrodeless lamp (MDEL) and a low-pressure electrode Hg lamp were also used to compare the performance of these novel MDGELs. The dominant performance of the MDGELs was established by the defluorination kinetics per surface area of the light source immersed in the solutions and by the applied electric power of the lamps. At pH 10, the kinetics correlated with the carbon chain length and with the number of C–F bonds in the three fluorinated acids. Microwave irradiation of the single rod-shaped MDEL device achieved a defluorination level of 51% (TFA), 67% (HFBA), and 37% (PFOA) for a 240-min VUV/UV irradiation of solutions at initial pHs = 4.7–5.2, whereas at pH = 10 defluorination of TFA and HFBA was 100% complete within 120–135 min of irradiation; defluorination of PFOA was ca. 80% complete after 200 min. With the 20 MDGEL system, 100% defluorination of TFA and HFBA was achieved after 105 min, whereas 100% defluorination of PFOA required ca. 400 min of irradiation. A decomposition/defluorination pathway is described for PFOA on the basis of intermediates identified by LC–MS techniques that involved initial VUV/UV irradiation (185 nm and 254 nm) of the PFOA contaminant followed by subsequent losses of CO 2 and –CF 2 units to complete defluorination and degradation.

Microwave discharge electrodeless lamps (MDEL) Part IV. Novel self-ignition system incorporating metallic microwave condensing cones to activate MDELs in photochemical reactions

A metallic condensing cone that concentrates microwave radiation (equivalent to an optical lens) has been developed and used as part of a system to activate a microwave discharge electrodeless lamp (MDEL) in the oxidative treatment of wastewaters by aiding the novel self-ignition of the lamp on irradiation at low microwave power levels. This approach to self-ignition can potentially lead to considerable energy savings in such treatments. System performance was examined for the ignition power of microwaves of such MDEL devices in water, whose usefulness was assessed by investigating the photolytic transformation of aqueous solutions of representatives of three classes of contaminants: chlorinated phenols, herbicides and endocrine disruptors, specifically 4-chlorophenol (4-CP), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4,4'-isopropylidenediphenol (bisphenol-A; BPA), respectively, taken as model wastewaters in air-equilibrated, in oxygen-saturated and in TiO2-containing aqueous media. The results are discussed in terms of the dynamics of the photo-induced degradation processes.

Microwave discharge electrodeless lamps (MDEL). III. A novel tungsten-triggered MDEL device emitting VUV and UVC radiation for use in wastewater treatment

Exposure to low doses of the xenoestrogen bisphenol A (BPA) and to the hormonal 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide, an environmental endocrine disruptor, can have serious health consequences such as the induction of mammary gland ductal hyperplasias and carcinoma (LaChapelle et al., Reprod. Toxicol., 2007, 23, 20; Murray et al., Reprod. Toxicol., 2007, 23, 383). To the extent that these toxins are present in wastewaters (Donald et al., Sci. Total Environ. 1999, 231, 173; Brotons et al., Environ. Health Perspect. 1994, 103, 608; Olea et al., Environ. Health Perspect. 1996, 104, 298; Biles et al., J. Agric. Food Chem. 1997, 45, 3541; Markey et al., J. Steroid Biochem. Mol. Biol., 2003, 83, 235), we examined their oxidative destruction in aqueous media by a novel light source. A tungsten-triggered microwave discharge electrodeless lamp (W-MDEL) was fabricated for possible use in wastewater treatment using vacuum UV-transparent quartz in which a tungsten trigger, also embedded in quartz, was attached to the MDEL to aid in the self-ignition of the lamp on irradiation at low microwave power levels. The quantity of mercury gas in the W-MDEL was optimized by monitoring the continuous radiation and peak intensities of the emitted light in the vacuum UV (VUV) and UVC regions. The usefulness of the W-MDEL device was assessed through the degradation of 2,4-D and BPA in air-equilibrated aqueous media and in oxygen-saturated aqueous media. Enhanced degradation of these two xenoestrogenic toxins was achieved by increasing the number of W-MDEL devices while keeping constant the microwave radiation feeding each W-MDEL lamp. This novel lamp provides an additional light source in the photooxidation of environmental contaminants without the need for a metal-oxide photocatalyst. Under our conditions, process dynamics using the W-MDEL light source are greater than with the more conventional photochemical methods that employ low-pressure Hg arc electrode lamps in synthetic quartz to degrade these two toxic contaminants.

Destruction of Toxic Volatile Organic Compounds (VOCs) in a Microwave-Assisted Catalyst Bed

Control of VOC emissions from various industrial emission sources in an environmentally acceptable and energy efficient manner has been a constant challenge to the chemical industry. A promising technology for the effective control of such emissions is to use microwave energy to destroy VOCs in a packed bed of microwave absorbing medium. This paper reports preliminary results from an experimental study of VOC destruction using microwave energy. In the study, trace amount of benzene vapor carried by air or nitrogen flowed through an 11 mm ID microwave-assisted packed-bed reactor and was destroyed in the reactor by the elevated temperatures generated by microwave irradiation. Granular activated carbon (GAC) and silicon carbide (SiC) foam were used as the packed bed materials as well as the microwave absorption mediums. The inlet and outlet streams were analyzed on-line by GC/MS (Gas Chromatography/Mass Spectrometry) and the DRE (Destruction and Removal Efficiency) was determined based on the observed inlet and outlet concentrations of benzene. The experimental parameters also included benzene concentration, packed bed height, carrier gas flow rate, and microwave power level. The current results have indicated that silicon carbide is a better microwave absorption medium than GAC. With silicon carbide being the packed bed medium and air being the carrier gas, the microwave VOC destruction system is capable of achieving a DRE of higher than 99% with low microwave power levels ranging from 200 to 400 watts. The reaction products have been found to be largely CO2, CO, and H2O with only trace amount of other byproducts.

Non-thermal microwave effects on protein dynamics? An X-ray diffraction study on tetragonal lysozyme crystals

X-ray diffraction (XRD) was used to investigate the structural and dynamical effects of microwave fields on tetragonal single crystals of hen egg-white lysozyme at a resolution of 2.0 A. Using a modified slab-line waveguide allows on-line XRD to be carried out while the protein crystal is exposed to well defined microwave fields. High microwave power levels mainly lead to increased, but largely recoverable, lattice defects owing to the evaporation of crystal water. At lower microwave power levels, the presence of the microwave field results in localized reproducible changes in the mean-square displacements (B factors). At particular sites, it is found that the B factors even decrease with increasing microwave power. Most of these effects can be explained by a comparison of the data obtained under microwave irradiation with data obtained at elevated temperature which simulate heating owing to microwave absorption by unbound crystal water. The data show no indication of large microwave-driven displacements of structural subunits in the protein that would be expected if microwaves were to be absorbed resonantly by protein vibrations. Rather, the observed changes in the atomic mean-square displacements suggest that if microwaves couple non-thermally to globular proteins at hydration levels at which they still function, their effect on protein dynamics and structure is very small.

10 V SINIS Josephson junction series arrays for programmable voltage standards

A programmable dc voltage standard for output voltages of up to 10 V has been realized. The series arrays, consisting of about 69120 overdamped superconductor/insulator/normal metal/insulator/superconductor (SINIS) Josephson junctions, have been fabricated using the reliable Nb-Al/Al/sub 2/O/sub 3/ technology. The arrays can be operated in conventional Josephson voltage standards at microwave frequencies from 70 GHz to 75 GHz. Steps of constant voltage are observed at very low microwave power levels, since the major part of the microwave power is generated by the junctions themselves. The operation of the arrays and the formation of Shapiro steps are discussed.

Large-diameter microwave plasma source excited by azimuthally symmetric surface waves

This article describes a large-diameter, surface-wave excited plasma (SWP) source designed for materials processing. The plasma reactor employs a launcher of 2.45 GHz azimuthally symmetric surface waves in the field-free region of 24-pole line-cusp magnetic fields, generated by a set of permanent magnets surrounding the reactor chamber walls; the magnets also provide an electron cyclotron resonance (ECR) magnetic field of 875 G near the chamber wall surfaces. Langmuir probe and optical emission measurements were made for characterizing the plasma produced in Ar. After the microwave power was turned on, the discharge was observed to start near the ECR region and then propagate toward the field-free region in the central area of the chamber. Moreover, the discharge was also observed to be excited by ECR at low microwave-power levels, and by surface waves in the field-free region at above a critical power strongly depending on the gas pressure. Such a transition of plasma excitation from ECR to SWP was found to occur under conditions where the plasma electron density exceeds a 2.45 GHz microwave cutoff value of ≈7.4×1010 cm−3. As a result, overdense plasmas with large diameters could be successfully produced with moderate microwave input powers ≲1 kW; the electron density was measured to be Ne∼1−3.5×1011 cm−3 in Ar plasmas, with its spatial variation being ≲5% over a 26-cm-diam area. Furthermore, preliminary experiments of SiO2 etching in pure CF4 plasmas showed that the SiO2 etch rate was typically ∼300 nm/min with a good uniformity ≲3% over an 8-in.-diam wafer.

The Bloch Equations in High-Gradient Magnetic Resonance Force Microscopy: Theory and Experiment

We report theory and observations of paramagnetic resonance in a measured field gradient of 44,000 T per meter by the technique of magnetic resonance force microscopy (MRFM). Resonance was induced in a dilute solid solution of diphenylpicrylhydrazyl in polystyrene at 77 and 10 K by an amplitude-modulated microwave field. This modulated the force between resonant sample spins and a micrometer-scale SmCo magnetic tip on a force microscope cantilever. The force signals were typically of order 10 fN, and were detected above a thermal noise floor of 80 aN per root hertz at 10 K, equivalent to a magnetic moment noise of 200 μB per root hertz of bandwidth. Resonance saturation was readily observed. Starting with the Bloch equations, we derived simple analytic expressions for the predicted cantilever signal amplitudes and T1-dependent phase lags, valid at low microwave power levels. For power levels below saturation, the data were in good agreement with the Bloch equation predictions, while above saturation the measured force increased more slowly with power than predicted. Several ESR mechanisms which might lead to non-Bloch dynamics in the MRFM environment are reviewed. Spin-relaxation mechanisms are also reviewed. A detailed description of the experimental apparatus is offered.

Nonlinear response of HTSC thin film microwave resonators in an applied DC magnetic field

The non-linear microwave surface impedance of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field of magnitude comparable to that of the microwave field. Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied DC field before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance.

Josephson voltage standard microwave circuits fabricated in a modified all-niobium technology for operation at low drive frequencies

A resonant type of cryoelectronic microwave circuit for the generation of frequency-scaled Josephson voltages is presented. The circuits consist of superconducting microstripline resonators with series arrays of Josephson tunnel junctions located on the microwave current antinodes. This microwave design was chosen to obtain highly accurate Josephson voltages at low microwave power levels and low drive frequencies. The resonator circuits have been fabricated in both niobium-lead and all-niobium technology. At present, we have observed long-term stable Josephson voltages up to 0.75 V for niobium-lead circuits with 1320 junctions and 0.66 V for all-niobium circuits with 1456 junctions, respectively.