High Microwave Field Research Articles

The synergy of transformation of isomorphous phyllosilicate structures

The article presents an analysis of the synergy of structural transformations of phyllosilicates subjected to high temperatures and microwave radiation in terms of the destruction and formation of crystal structures. Studying the synergy of transformations of crystal structures is essential for the development of new materials and technologies, because it helps to create materials with unique properties, which cannot be obtained when using the same factors separately. The material used in the study was a polymineral complex, which contained quartz, montmorillonite, kaolinite, chlorite, paragonite, and iron oxides (listed from the largest to the smallest mass fraction). The methods used in the study allowed us to assess the structural transformations. Averaged structural formulas of the studied phyllosilicates were calculated using the oxygen method combined with a recalculation method based on the results of a microprobe analysis. Differential thermal analysis demonstrated a synergistic effect of high temperature and microwave fields registered as a decrease inthe temperature gradient of the ceramics and initiation of the sintering process at lower temperatures and with greater intensity. All the treated samples contained the amorphous phase in significant concentrations: from 15 to 25 vol. %. Half of the three-layer phyllosilicates (chlorite and montmorillonite) were destroyed by the microwave field. Kaolinite and paragonite practically did not react to external factors. The synergistic effect was the most obvious in the structural transformations of silicon and iron oxides. Our experiments demonstrated for the first time the mechanism of formation of magnetite and hematite crystals from X-ray amorphous iron-containing films covering the particles of clay minerals. The use of the MW field and the resulting dehydration led to the formation of crystals of nuclei of iron oxides. The following high-temperature processes activated an increase in aggregated iron (magnetite and hematite). The X-ray diffraction analysis determined the presence of a synergistic effect in the evolution of structures, which could not be identified by means of IR-spectroscopy. The EPR spectroscopy allowed us to register the states of rare irregular cells with foreign paramagnetic atoms. The shift of the foreign Fe3+ from the geometric centers of octahedral crystal cells towards the minima of potential energy caused by the Jahn-Teller effect decreased the potential energy of the crystal lattice. At the same time, some chemical bonds forming the crystal cellbecame stronger, while others weakened

High‐Speed, microwave ignition: Antenna igniters with frequency and bandwidth selectivity

AbstractThis effort demonstrates the development of (1) dielectric breakdown igniters and (2) bridgewire igniters powered by half‐wavelength dipole microwave microstrip antennas with bandwidth selectivity for use in standoff ignition applications. Dielectric igniters utilized a dielectric epoxy filled with up to 30 wt. % nanothermite cast between the gap of two quarter wavelength microstrips. Bridgewire igniters employed a wire soldered across the gap of two quarter wavelength microstrips. A variety of different bridgewire materials were tested in diameters ranging from 76 μm to 203 μm. The ignition delay of resonant microwave igniters is measured from high‐speed video observation of the microwave illumination process within a free‐space anechoic microwave cavity driven by a 2 kW continuous magnetron source. Dielectric breakdown igniters produced ignition delays ranging from 400 ms to 850 ms. Ignition delay decreased with higher loading of thermite. Bridgewires were found to be highly efficient and measured ignition delays ranged from 17 ms to 271 ms (24.4 J to 457.1 J ignition energy). Smaller diameter bridgewires and high resistivity metals were found to produce the shortest ignition delays. We demonstrate through S‐parameter measurements the bandwidth selectivity of bridgewire igniters and show that control of dipole geometry enables tuning of igniter center frequency and bandwidth and demonstrate experimentally the ability of bridgewire igniters to reject accidental ignition from an off frequency high power microwave field. The tunability, bandwidth selectivity, and low energy requirements, in particular of bridgewires, make them attractive for a number of new and challenging ignition applications.

Microwave enhanced precision in 2D and 3D atom localization at nonzero temperatures

Two- and three-dimensional (2D and 3D) atom localization is analyzed by monitoring the probe absorption spectrum in a microwave driven X-type scheme. It is found that for both stationary and moving atom cases, the precision and certainty in atomic position can be significantly improved by proper adjustment of the system parameters. Our results also reveal that the high microwave field strength curbs the Doppler broadening effect to a large extent and enhances detection probability to 100% in 2D and 3D subspace at nonzero temperatures. Our proposed scheme may be helpful for experimental realization of high precision position measurement and atom nanolithography at room temperature.

Application of the PLP-01M microwave laboratory system using control samples to assess the accuracy of the results of studies of cadmium content

This paper considers and analyzes the results of examining samples for cadmium content by determining them on a Kvant-2AT atomic absorption spectrometer, taking into account the use of the PLP-01M microwave laboratory system and during sample preparation in accordance with GOST 26929 “Raw materials and food products. Sample preparation. Mineralization to determine the content of toxic elements”. The PLP-01M microwave laboratory system uses a fundamentally new method of sample preparation. The decomposition was carried out in a closed system - sealed fluoroplastic vessels under the influence of high temperature, pressure and microwave field. The microwave field in the working chamber of the furnace was created by a special generator-magnetron. Based on the results of the assessment of the operational control of the measurement procedure using samples for cadmium control during the implementation of the PLP-01M microwave laboratory system in the laboratory, the analysis procedure was recognized as satisfactory.

Control by the accuracy of the results of studies for the lead content in samples applying the microwave laboratory system PLP-01M

The article presents the analysis of the study results of samples for lead content by their determination on the atomic absorption spectrometer “Kvant-2AT”, taking into account the use of the microwave laboratory system PLP-01M and while the sample preparation in accordance with GOST 26929 “Raw materials and food product. Sample preparation. Mineralization to determine the content of toxic elements”. A fundamentally new method of the sample preparation is used in the microwave laboratory system PLP-01M. The decomposition was carried out in a closed system, i.e. sealed fluoroplastic vessels under the influence of high temperature, pressure and microwave field. The microwave field was created by a special generator-magnetron in the operating chamber of the furnace. According to the assessment results of the operational control of the measurement procedure applying samples for the lead control when the microwave laboratory system PLP-01M was introduced in the laboratory; the analysis procedure was found satisfactory.

Broadband multi-magnon relaxometry using a quantum spin sensor for high frequency ferromagnetic dynamics sensing

Development of sensitive local probes of magnon dynamics is essential to further understand the physical processes that govern magnon generation, propagation, scattering, and relaxation. Quantum spin sensors like the NV center in diamond have long spin lifetimes and their relaxation can be used to sense magnetic field noise at gigahertz frequencies. Thus far, NV sensing of ferromagnetic dynamics has been constrained to the case where the NV spin is resonant with a magnon mode in the sample meaning that the NV frequency provides an upper bound to detection. In this work we demonstrate ensemble NV detection of spinwaves generated via a nonlinear instability process where spinwaves of nonzero wavevector are parametrically driven by a high amplitude microwave field. NV relaxation caused by these driven spinwaves can be divided into two regimes; one- and multi-magnon NV relaxometry. In the one-magnon NV relaxometry regime the driven spinwave frequency is below the NV frequencies. The driven spinwave undergoes four-magnon scattering resulting in an increase in the population of magnons which are frequency matched to the NVs. The dipole magnetic fields of the NV-resonant magnons couple to and relax nearby NV spins. The amplitude of the NV relaxation increases with the wavevector of the driven spinwave mode which we are able to vary up to 3 × 106 m−1, well into the part of the spinwave spectrum dominated by the exchange interaction. Increasing the strength of the applied magnetic field brings all spinwave modes to higher frequencies than the NV frequencies. We find that the NVs are relaxed by the driven spinwave instability despite the absence of any individual NV-resonant magnons, suggesting that multiple magnons participate in creating magnetic field noise below the ferromagnetic gap frequency which causes NV spin relaxation.

Operation of bolometer system using Pt foil on SiN substrate detector for EAST tokamak.

The foil resistive bolometer diagnostic on experimental advanced superconducting tokamak has been upgraded partly with a new generation of detectors. The new detectors have faster response time. However, the microwave interference is still a serious issue for the bolometer system. The system response to microwave is tested, and the test results show that the closed Wheatstone bridge circuit in the detector is the most sensitive component to high power microwave field. Simulation results of microwave transmission by the high frequency structure simulator software and shielding design are also presented.

Direct production of levulinic acid in high yield from cellulose: joint effect of high ion strength and microwave field

Cellulose without any pretreatment was directly converted into levulinic acid (LA) in a microwave-assisted acidic catalytic system with a high ionic strength. The highest LA yield could reach 67.3 mol% within 60 min even when the cellulose concentration was as high as 10 wt%. It is concluded that high ion strength and microwave irradiation were jointly responsible for the fast cellulose conversion and high LA yield, and a cooperative acceleration mechanism is finally proposed. The high ion concentration provided by alkali metal halides not only accelerated the cellulose hydrolysis but also facilitated glucose conversion into LA by shifting the weak acid ionization equilibria, and microwave irradiation further promoted this salt effect by its characteristic heating way of ion conduction. Such a one-pot catalytic system provides a possibility of practical application for direct highly efficient conversion of cellulose due to its green properties, low cost and efficient characteristics.

Photon-assisted tunneling through a topological superconductor with Majorana bound states

Employing the Keldysh Nonequilibrium Green’s function method, we investigate time-dependent transport through a topological superconductor with Majorana bound states in the presence of a high frequency microwave field. It is found that Majorana bound states driven by photon-assisted tunneling can absorb(emit) photons and the resulting photon-assisted tunneling side band peaks can split the Majorana bound state that then appears at non-zero bias. This splitting breaks from the current opinion that Majorana bound states appear only at zero bias and thus provides a new experimental method for detecting Majorana bound states in the Non-zero-energy mode. We not only demonstrate that the photon-assisted tunneling side band peaks are due to Non-zero-energy Majorana bound states, but also that the height of the photon-assisted tunneling side band peaks is related to the intensity of the microwave field. It is further shown that the time-varying conductance induced by the Majorana bound states shows negative values for a certain period of time, which corresponds to a manifestation of the phase coherent time-varying behavior in mesoscopic systems.

Experimental investigation of a compact relativistic magnetron with axial TE11 mode radiation

As one of the relativistic electron tubes having compact configuration and high efficient output, the relativistic magnetron with direct axial radiation is very attractive in pulsed power and high power microwave fields for industrial and military applications. In this paper, the experimental investigation of a relativistic magnetron with axial TE11 mode radiation is reported. Under a total length of ∼ 0.3 m, volume of ∼ 0.014 m3, working at an applied voltage of 508 kV and a magnetic field of ∼ 0.31 T, the relativistic magnetron radiates a microwave of 540 MW with the TE11 mode at 2.35 GHz in the axial direction. The power conversion efficiency is 15.0%. After a lot of shots, the detected amplitudes of microwaves are nearly the same. The fluctuations of wave amplitudes are less than 0.3 dB.

Eddy currents appearing in a p-n junction in a high microwave field

The influence of a high-frequency quasi-potential on the electromotive force generated by a microwave field and on eddy currents in a p-n junction has been studied. It is shown that the microwave quasi-potential reduces the eddy currents appearing in the p-n junction.

The influence of microwave pre-treatment of the leach behaviour of disseminated sulphide ore

Heap leaching has become a widely used method of extracting metals from low grade ores. The simplicity of heap leach process design and low capital and operating costs are the main drivers. However, heap leaching in general has the drawbacks of low recoveries and long extraction times. In this regard, rock permeability is a key requirement for this technology to be viable. The aim of this study is to investigate whether heap leaching efficiency can be improved by introducing micro fractures and cracks into the ore by the application of high power microwave energy, which has already been shown to enhance comminution. This paper presents the effect of microwave pre-treatment on copper extraction during leaching. The study shows that for a low grade ore containing finely disseminated sulphide mineral phases, the pre-treatment of ore particles in a high intensity microwave field increases the copper extraction over that for un-treated ore particles. It is suggested that microwave pre-treatment increases the internal surface area of the ore particles by creating fractures along grain boundaries, so exposing locked Cu-bearing mineral phases to the leach solution. It is proposed that leaching of exposed mineral phases will enhance the extraction of copper as compared to leaching of untreated ore particles. Further, the addition of wetting agents increases the copper extraction, as compared to samples of microwave pre-treated ore particles leached without wetting agents. The influence of microwave pre-treatment on the recovery vs. residence time of heap leaching of a low grade ore, with finely disseminated sulphide mineral phases, is discussed.

Bipolaron Formation in Organic Solar Cells Observed by Pulsed Electrically Detected Magnetic Resonance

We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a π-conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g = 2.0028(3) is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons.

Nonlinear spin-wave phenomena in a [Mn{(R/S)-pn}2]2[Mn{(R/S)-pn}2H2O][Cr(CN)6] chiral molecular ferrimagnet

The instability of ferromagnetic resonance (FMR) spectra initiated by microwave power was observed in [Mn{(R/S)-pn}2]2[Mn{(R/S)-pn}2H2O][Cr(CN)6] single crystals. It was established that the value of threshold microwave power Pth = 0.2−0.9 mW depends on the orientation of the easy magnetization axis relative to the sweeping magnetic field of the spectrometer. P > Pth spin-wave bistability occurs in the region of high microwave fields.

Detection of concealed and buried chemicals by using multifrequency excitations

In this paper, we present a new type of concealed and buried chemical detection system by stimulating and enhancing spectroscopic signatures with multifrequency excitations, which includes a low frequency gradient dc electric field, a high frequency microwave field, and higher frequency infrared (IR) radiations. Each excitation frequency plays a unique role. The microwave, which can penetrate into the underground and/or pass through the dielectric covers with low attenuation, could effectively transform its energy into the concealed and buried chemicals and increases its evaporation rate from the sample source. Subsequently, a gradient dc electric field, generated by a Van De Graaff generator, not only serves as a vapor accelerator for efficiently expediting the transportation process of the vapor release from the concealed and buried chemicals but also acts as a vapor concentrator for increasing the chemical concentrations in the detection area, which enables the trace level chemical detection. Finally, the stimulated and enhanced vapors on the surface are detected by the IR spectroscopic fingerprints. Our theoretical and experimental results demonstrate that more than sixfold increase in detection signal can be achieved by using this proposed technology. The proposed technology can also be used for standoff detection of concealed and buried chemicals by adding the remote IR and/or thermal spectroscopic and imaging detection systems.

The nonideality coefficient of current-voltage characteristics for p-n junctions in a high ultrahigh-frequency (microwave) field

The effect of heating of electrons and holes on the nonideality coefficient of the current-voltage characteristic for a p-n junction in a high microwave field is studied. It is established that the nonideality coefficient for a diode depends on the type of charge carriers that make the major contribution to the current in the p-n junction. It is shown that, in some cases in silicon samples, the nonideality coefficient for the diode is governed by the temperature for holes in spite of the fact that the temperature for electrons is higher than the temperature for holes.

Microwave assisted sintering of green metal parts

Microwave processing has gained worldwide acceptance as a novel method for heating and sintering a variety of materials, as it offers specific advantages in terms of speed, energy efficiency, process simplicity, finer microstructures, and lower environmental hazards. Microwave assisted post-treatment of green metal parts (AISI 420 metal powder with 2–3 wt% polymeric binder) proved to be a new method, suitable for near-net-shape and net-shape manufacturing. Green parts exposed to high intensity microwave fields, using either single mode or multimode microwave applicators operating at 2.45 GHz ISM frequency, were investigated in terms of densification behaviour, microstructure and joining micromechanism. The results, in terms of microstructure and shape retention are comparable to the conventional heating ones, reported in literature, but they have been obtained in much shorter time.

Kerr microscopy studies of microwave assisted switching

We study microwave assisted magnetization reversal of elliptical Ni81Fe19 thin film elements employing Kerr microscopy. A microwave field is applied along the short axis of the element and orthogonally to a quasistatic magnetic field, which is parallel to the long axis of the element. The reversal process is characterized by the formation of a complex multidomain structure. We demonstrate that the reversal process in total is compressed by applying a sufficiently high microwave field, which assists to overcome the effective energy barrier of the domain nucleation, and thus reduces the coercive field.

Investigation of energetic electrons in a 915 MHz microwave discharge produced in Ar

The electrical field distribution and plasma parameters in the vicinity of the quartz plate of a 915 MHz microwave discharge are measured using Langmuir probe, thermal probe and microwave field antenna. Energetic electrons with energies exceeding 15 eV are observed at locations of high microwave field that are correlated with high temperatures by thermal probe and high effective electron temperature. Plasma parameters show a spatial dependence in the skin depth while are almost uniform in plasma volume. Present results confirm that the kinetic of the energetic electrons is strongly related to the complex distribution of the microwave electric filed near the dielectric. Measurements are performed in Ar for different pressures and discharge powers.

Microwave Protocols for Plant and Animal Paraffin Microtechnique

Abstract The microwave oven is a valuable tool for light and electron microscopy microtechnique labs. Tissue processing times, traditionally taking up to two weeks, have been reduced to a few hours as a result of the implementation of microwave technology (Kok et al., 1988, Gibberson and Demaree, 2001). In addition, the quality of the tissue preparations has improved dramatically. Microwave ovens have also evolved since their first use in the laboratory. Early experiments were conducted using relatively crude commercial microwave ovens. Now, labs use microwave ovens with temperature probes, strict control over the magnetron (which generates the microwaves), variable power supplies, chamber cooling, and high microwave field uniformity.