High-intensity Microwave Radiation Research Articles

Microstructural characteristics of larch wood treated by high-intensity microwave

High-intensity microwave radiation was applied to treat and modify larch wood. Using scanning electron microscopy, changes in the microstructure of larch wood due to microwave irradiation were analyzed. After the microwave treatment, the microstructure of the larch wood was varied in its weakest section. The changes of the microstructure, in the pits of the tracheid wall, the plane of the tracheid wall, and the wood ray tissue of the radial section, were notable. The cracks were located among the latewood tracheid, and some of them were throughout the wood ray section, so wood rays were broken along the vertical cell arrangement. The results showed that high-intensity microwave treatment effectively penetrates the narrow channels of larch wood.

Flash Sintering of Oxide Ceramics under Microwave Heating

We report on the results of the analysis of the effect of flash sintering, which is observed upon heating compacted powder materials by high-intensity microwave radiation. Ceramic samples of Y2O3, MgAl2O4, and Yb: (LaO)2O3 were sintered to a density exceeding 98–99% of the theoretical value during 0.5–5 min without isothermal hold. The specific microwave power absorbed volumetrically in the samples was 20–400 W/cm3. Based on the analysis of the experimental data (microwave radiation power and heating and cooling rates) and of the microstructure of the obtained materials, we propose a mechanism of flash sintering based on the evolution of the thermal instability and softening (melting) of the grain boundaries. The proposed mechanism also explains the flash sintering effect observed when a dc or a low-frequency ac voltage is applied to the samples. The microwave heating makes it possible to implement flash sintering without using electrodes for supplying energy to the articles being sintered.

Modulation of high-intensity microwave radiation during its resonant interaction with counterflow of nonexcited cyclotron oscillators

It is shown that high-intensity continuous microwave radiation can be transformed into a periodic sequence of short pulses during resonant interaction with a counterflow of nonexcited cyclotron oscillators. This phenomenon can be created in microwave-radiation modulators.

Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN) 6 3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.

Microwave Induced Jet Boiling Investigated via Voltammetry at Ring−Disk Microelectrodes

High intensity microwave radiation is (self-)focused at metal electrodes immersed in aqueous electrolyte solutions to generate highly localized superheating and convection effects. It is shown that, for an electrode pointing downward, low intensity microwave radiation causes density driven convective flow (upward), which at the onset of boiling abruptly switches to a fast jet of liquid moving away from the electrode surface (downward). This "jet-boiling" phenomenon allows extremely high rates of mass transport and mixing to be realized at the electrode surface. Cyclic voltammograms obtained at electrodes placed into a microwave field show very strong mass transport enhancement effects. Cyclic voltammograms recorded at a Pt/Pt ring-disk electrode system (r(1) = 25 microm, r(2) = 32 microm, r(3) = 32.4 microm) in the presence of microwave radiation are employed to further explore mass transport effects under microwave conditions. Mass transport coefficients, collection efficiencies, and temperatures are determined as a function of microwave intensity.

Focused microwaves in electrochemical processes

The effects of high intensity microwave radiation in electrochemistry are summarized and discussed. In situ microwave activation of electrochemical processes has been introduced recently and is possible by placing a carefully designed electrochemical cell directly into a microwave cavity. Self-focusing of intense microwave radiation occurs into a region close to the electrode | solution (electrolyte) interface of a microelectrode placed into the electrochemical cell. The electrode diameter and the electrode material strongly affect the observed mass transport enhancement and temperature effects. Experiments have been conducted to determine the temperature at the electrode surface electrochemically and to quantify the rate of chemical processes which occur in the vicinity of the electrode under high intensity microwave conditions. The effects of microwaves in a wide range of solvent systems from aqueous solutions to organic solvents (DMSO, acetonitrile, DMF, formamide) and in an ionic liquid (BMIM +PF 6 −) have been investigated. Considerable current and temperature enhancements are observed in all solvents and are explained based on the interaction of microwaves with the liquid (electrolyte) and the physical properties of the liquids or solutions.

Microwave-enhanced electrochemical processes in micellar surfactant media

High intensity microwave radiation effects are demonstrated for electron transfer processes at 25 or 50-μm diameter platinum electrodes immersed in micellar sodium dodecylsulfate (SDS) solutions. First, a solution containing 2 mM Fe(CN)63− and 2 mM Fe(CN)64− in aqueous 0.1 M NaCl with and without SDS is employed to calibrate the electrode temperature and mass transport conditions. Addition of 0.1 M SDS has only a small effect on the microwave enhanced voltammetry for the Fe(CN)63-/4− system. Next, two highly water-insoluble redox systems are studied. A solution of 1 mM tert-butylferrocene in aqueous 0.1 M NaCl containing 0.1 M SDS is shown to give no current response in the absence of microwaves. In the presence of focused microwaves at a platinum disc electrode, a strong current for the one electron oxidation of tert-butylferrocene is detected presumably due to localized disruption of the micellar solution. Concentrations of tert-butylferrocene down to the micromolar level are detected. α-Tocopherol, a lipophilic vitamin and antioxidant, is soluble in aqueous 0.1 M SDS/0.1 M NaCl. In the presence of microwave radiation, a strong and concentration dependent anodic current response consistent with the two-electron oxidation of α-tocopherol is observed. A heptode array of seven individual 50 μm diameter platinum microelectrodes placed in ca. 720 μm distance of each other is shown to allow microwave enhanced currents to be increased sevenfold with each electrode exhibiting the same microwave effect.

Microwave activation of the electro-oxidation of glucose in alkaline media

The oxidation of glucose is a complex process usually requiring catalytically active electrode surfaces or enzyme-modified electrodes. In this study the effect of high intensity microwave radiation on the oxidation of glucose in alkaline solution at Au, Cu, and Ni electrodes is reported. Calibration experiments with the Fe(CN)(6)(3-/4-) redox system in aqueous 0.1 M NaOH indicate that strong thermal effects occur at both 50 and 500 microm diameter electrodes with temperatures reaching 380 K. Extreme mass transport effects with mass transport coefficients of k(mt) > 0.01 m s(-1)(or k(mt) > 1.0 cm s(-1)) are observed at 50 microm diameter electrodes in the presence of microwaves. The electrocatalytic oxidation of glucose at 500 microm diameter Au, Cu, or Ni electrodes immersed in 0.1 M NaOH and in the presence of microwave radiation is shown to be dominated by kinetic control. The magnitude of glucose oxidation currents at Cu electrodes is shown to depend on the thickness of a pre-formed oxide layer. At 50 microm diameter Au, Cu, or Ni electrodes microwave enhanced current densities are generally higher, but only at Au electrodes is a significantly increased rate for the electrocatalytic oxidation of glucose to gluconolactone observed. This rate enhancement appears to be independent of temperature but microwave intensity dependent, and therefore non-thermal in nature. Voltammetric currents observed at Ni electrodes in the presence of microwaves show the best correlation with glucose concentration and are therefore analytically most useful.

Microwave activation of electrochemical processes: enhanced electrodehalogenation in organic solvent media.

The effect of high-intensity microwave radiation focused into a "hot spot" region in the vicinity of an electrode on electrochemical processes with and without coupled chemical reaction steps has been investigated in organic solvent media. First, the electrochemically reversible oxidation of ferrocene in acetonitrile and DMF is shown to be affected by microwave-induced thermal activation, resulting in increased currents and voltammetric wave shape effects. A FIDAP simulation investigation allows quantitative insight into the temperature distribution and concentration gradients at the electrode / solution interface. Next, the effect of intense microwave radiation on electroorganic reactions is considered for the case of ECE processes. Experimental data for the reduction of p-bromonitrobenzene, o-bromonitrobenzene, and m-iodonitrobenzene in DMF and acetonitrile are analyzed in terms of an electron transfer (E), followed by a chemical dehalogenation step (C), and finally followed by another electron-transfer step (E). The presence of the "hot spot" in the solution phase favors processes with high activation barriers.

Collapse of electron plasma wave in microwave plasma interaction

During the interaction of obliquely incident microwaves with an unmagnetized inhomogeneous laboratory plasma, the Langmuir wave energy is observed to narrow spatially and to rapidly grow to its peak value followed by a sudden collapse. This phenomenon has been referred to as the 'Soliton Flash' and occurs at rather high intensity microwave radiation (P0>or=600 W, eta 0=E02/8 pi n kappa T>or=4*10-3). The observed flash can be interpreted using the wavebreaking model, and the electric field strength at the wavebreaking time has been estimated by several methods, these methods showing reasonable agreement.

Nonlinear absorption and generation of microwave radiation due to interaction of ammonia molecules with high-power infrared radiation

An investigation was made of the excitation of 14NH3 molecules at pressures of 10−2–1 Torr by CO2 laser pulses in a wide range of laser radiation energy densities (Φ=10−3–10 J/cm2) and of offsets of the laser frequency from the absorption lines of the vibrational–rotational transition in the ν2 band of ammonia. Effective excitation of ammonia as a result of nonresonant interaction with infrared radiation was due to field broadening. Calculations dispensing with fitting parameters accounted for the experimental results (within the limits of the measurement error). The anomalously strong absorption of ammonia due to resonant pumping was attributed to generation of high-intensity microwave radiation which suppressed the rotational bottleneck in the excitation of molecules.

Microwave-Oven Design for Radiation Safety

A brief review of present knowledge on the biological hazards associated with extremely high-intensity microwave radiation is presented. Existing and proposed safety standards are discussed, including the proposed standard of the U. S. Department of Health, Education, and Welfare and the Bureau of Radiological Health. Instrumentation and measurement techniques are presented. Various door closure systems are discussed including 1) door-seal designs tested under conditions of wear and abuse; 2) door viewing screens; and 3) interlock, or safety switch, systems.

Effect of High-Intensity Microwave Radiation on Optical Transitions of Atomic Hydrogen

Interaction of atomic systems with high intensity monochromatic microwave radiation leads to characteristic splittings of optical spectral lines