Microwave Heating Power Research Articles

Convenient, Rapid Synthesis of Ag Nanowires

The rapid, microwave-assisted aerobic synthesis of silver nanowires based on the polyol method is described. Benchtop dissolution of NaCl and AgNO3 (ratio 1:6 to 1:3) in ethylene glycol and subsequent heating using microwave irradiation (300 W) in the presence of polyvinylpyrrolidone generates Ag nanowires in ∼80% yield in 3.5 min. Upon purification, microscopy (TEM, SEM) and powder X-ray diffraction reveal a uniform set of crystalline Ag nanowires with dimensions of 45 nm × 4−12 μm. Wire formation is highly dependent upon the microwave heating power, time, and NaCl:AgNO3 ratio because of the rapid heating process and the presence of O2 as an etching coreagent. Extended microwave heating causes the wires to fuse if in proximity or degrade to shorter wires presumably via the etching reaction. In the absence of O2/Cl-, the wires melt upon extended microwave heating (>4 min), suggesting that nanowire melting may contribute to the observed morphology under etching conditions. Compared to existing wet-chemical methods using traditional heating techniques, microwave irradiation not only rapidly heats, but dielectric heating of the growing wires also occurs, resulting in accelerated deposition of Ag0 at the wire ends. Furthermore, this high yielding preparation does not require any external seed crystals, precursors, or mechanical stirring and is conducted under ambient O2 conditions, leading to significant potential for the large-scale fabrication of Ag nanowires using this simple approach.

Thin-layer drying of porous materials: Selection of the appropriate mathematical model and relationships between thin-layer models parameters

Experimental investigation of the drying kinetics of various types of materials was carried out in laboratory-scale dryers under different conditions of temperature, microwave heating power and pressure. Leather samples (mechanically and vacuum-dewatered bull napa and wet blue cutting), paperboards (grafopack, testliner), wood (alder, birch, willow) and two pharmaceutical powders (chlorpropamide and hydrochlorotiazide) were dried in a microwave dryer. Thin clay slabs, Al–Ni catalyst and chlorpropamide were dried in a convection dryer, while chlorpropamide and ketoprofen were dried in a vacuum dryer. In order to compare drying kinetics, experimentally obtained data, X = f( t), were correlated with the Lewis “thin-layer” equation, the modified Page equation and Fick's second law. The drying constant, effective diffusion coefficient, mass transfer coefficient and modified Page model parameters were estimated by fitting the selected mathematical models to experimental data. High levels of correlation between measured and calculated data were obtained for all materials and dryers using modified Page model. The application of the Lewis and Fick's equation is justified only for drying of clay, catalyst and leather. Mass transfer coefficient depends linearly on the drying constant. The relation between the modified Page model parameter and the drying constant can be represented by a unique power function.

Effects of Cooking Conditions on the Lycopene Content in Tomatoes

ABSTRACT: Raw tomato contains a high level of lycopene, which has been reported to have many important health benefits. However, information on the changes of the lycopene content in tomato during cooking is limited. In this study, the lycopene content in raw and thermally processed (baked, microwaved, and fried) tomato slurries was investigated and analyzed using a high‐performance liquid chromatography (HPLC) method. In the thermal stability study using a pure lycopene standard, 50% of lycopene was degraded at 100 °C after 60 min, 125 °C after 20 min, and 150 °C after less than 10 min. Only 64.1% and 51.5% lycopene was retained when the tomato slurry was baked at 177 °C and 218 °C for 15 min, respectively. At these temperatures, only 37.3% and 25.1% of lycopene was retained after baking for 45 min. In 1 min of the high power of microwave heating, 64.4% of lycopene still remained. However, more degradation of lycopene in the slurry was found in the frying study. Only 36.6% and 35.5% of lycopene was retained after frying at 145 and 165 °C for 1 min, respectively.

Emission spectroscopy diagnostics of rare gases in the PNX-U facility

Results are presented from measurements of the electron temperature and neutral atom density in a low-temperature microwave plasma by the method of emission spectroscopy. The measurements were conducted in the PNX-U facility—a magnetic confinement system with a “magnetic wall.” Multichord measurements of plasma radiation at a wavelength of 750.37 nm were performed with the help of an absolutely calibrated monochromator. The neutral atom density was calculated using the collisional-radiative model. The degree of plasma ionization near the axis of the facility was found to be close to unity. The electron temperature of the argon plasma was measured from the relative intensities of the spectral lines of neutral helium injected in small amounts into the plasma (the so-called helium thermometer method). At a low microwave heating power, the results of these measurements agree well with the results of probe measurements.

Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF) with silver colloids in 96-well plates: Application to ultra fast and sensitive immunoassays, High Throughput Screening and drug discovery

Fluorescence detection is the basis of most assays used in drug discovery and High Throughput Screening (HTS) today. In all of these assays, assay rapidity and sensitivity is a primary concern, the sensitivity determined by both the quantum yield of the fluorophores and efficiency of the detection system, while rapidity is determined by the physical and biophysical parameters of temperature, concentration, assay bioaffinity, etc. In this paper we describe a platform technology that promises to fundamentally address these two physical constraints of sensitivity and rapidity. By combining the use of Metal-Enhanced Fluorescence (MEF), a near-field effect that can significantly enhance fluorescence signatures, with low power microwave heating, we can significantly increase the sensitivity of surface assays as well as > 95% kinetically complete the assay within a few seconds. In addition, the metallic nanostructures used to facilitate MEF appear to be preferentially heated as compared to the surface assay fluid, advantageously localizing the MEF and heating around the nanostructures. To demonstrate proof of principle, a 96-well plate has been functionalized with silver nanostructures, and a model protein avidin–biotin assay studied. In our findings, a greater than 5-fold fluorescence enhancement coupled with a ≈ 90-fold increase in assay kinetics was observed, but with no assay washing steps needed due to the silver-enhanced evanescent field mode of excitation. These findings promise to strongly facilitate high throughput fluorescence-based processes, such as in biology, drug discovery and general compound screening.

Microwave-assisted hydrothermal synthesis of zeolite films on ceramic supports

Zeolite films were formed on ceramic supports by a novel microwave-assisted hydrothermal method and were characterized by XRD, SEM, FTIR and BET surface area. The two-stage synthesis technique consists of microwave heating of supports saturated with precursor solution to form a thin layer of zeolite nano-seeds, followed by hydrothermal treatment to crystallize MFI zeolite. The resulting zeolite formation time was drastically reduced and the BET surface area of the product was double that obtained using conventional hydrothermal method. The dipping time, microwave heating time, power and hydrothermal heating time influence the amount of zeolite formation, the BET surface area and preferred orientation of the zeolite film. This new method of seed-film growth using a simple domestic microwave oven has the advantages of a two-stage synthesis technique, allowing independent control of nucleation and crystallization to obtain an optimum zeolite film and reduce the synthesis time.

Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF): Application to Ultra Fast and Sensitive Clinical Assays

In this rapid communication we describe an exciting platform technology that promises to fundamentally address two underlying constraints of modern assays and immunoassays, namely sensitivity and rapidity. By combining the use of Metal-enhanced Fluorescence (MEF) with low power microwave heating (Mw), we can significantly increase the sensitivity of surface assays as well as >95% kinetically complete the assay within a few seconds. This technology is subsequently likely to find significant importance in certain clinical assays, such as in the clinical assessment of myoglobin, where both the assay rapidity and sensitivity are paramount for the assessment and treatment of acute myocardial infarction.

Microwave Irradiation: An Effective Method For The Modification Of The Surface Features Of Some Solids Currently Used In Heterogeneous Catalysis

This work investigates the effect of the variation in preparative procedure (sol-gel and precipitation methods using aluminium tri-sec-butoxide, tetraortosilane, zirconium nitrate, zirconium oxychloride salts and propionic, oxalic acids and ammonium hydroxide) and conventional (CH) or microwave heating (MW) on the structure and chemical-physical properties of solids. The BET surface area of the synthesized solids has been found to be dependent on the microwave heating power. Using microwave synthesis, X-ray diffraction shows clear evidence for tetragonal ZrO2 structure, good crystallinity degree and micropore materials together with small particle size while conventional heating mesoporous solids are observed.

Structural analysis of TiO 2 films grown using microwave-activated chemical bath deposition

TiO 2 layer films were grown using the microwave (MW)-activated chemical bath deposition technique on two different indium tin oxide substrates. The TiO 2 films are studied to determine their structural response when changing the MW heating power. Thickness (areal density), oxygen concentration profile, composition and surface homogeneity were determined using Rutherford backscattering spectrometry, nuclear reaction analysis and atomic force microscopy. The analysis showed that the composition, thickness and surface structure of the films are highly influenced by MW heating power. The substrate, acting as seed for nucleation, influences the layer thickness, indicating that a thinner layer of TiO 2 is obtained for the more conducting substrates. The oxygen concentration profile is constant in the TiO 2 layer at low MW heating, power (≈20%). The rugosity of the samples and the non-homogeneity increase with the MW heating power. If the MW heating power is high enough pinholes in the TiO 2 layer of the order of the sample thickness are produced.

Numerical study on three-region thawing problem during cryosurgical re-warming

Knowledge of the temperature transients of biological bodies during cryosurgical re-warming is critical for the survival of healthy tissues. To better understand the mechanisms thus involved, a one-dimensional numerical algorithm based on finite difference method was applied to simultaneously solve the thawing processes occurring in three regions with one thawing, another blood-perfused and the third frozen one sandwiched between them. Two typical surface heatings with heating plate or convective warm water and a spatial heating using microwave were particularly adopted to investigate the advancement of the two phase-change interfaces and the transient temperature field over the tissue. Differences among these results were compared and their implementation for the cryosurgical re-warming were discussed. Parametric studies were performed to explore influences of the blood perfusion, the microwave heating power, the surface heat convection coefficient, and the surface heating temperature to the thermal history of the biological bodies. Taking account of several typical blood re-flow patterns most probably occurred in the originally frozen and then thawed tissues after the two phase change interfaces meet together, four heat transfer equations were proposed to characterize the re-warming behavior of the biological body. Effect of the non-ideal solution property of the living tissues to the transient temperature field during cryo-surgical re-warming was also tested through introducing a simple however intuitive way.

Density control and plasma edge characterisation of ECRH heated plasmas in the TJ-II stellarator

In the 1999 experimental campaign, the Spanish stellarator, TJ-II ( R=1.5 m, a<0.22 m, B 0<1 T) has been operated under a broad range of parameters, including changes in the magnetic configuration, working gas (H 2 vs He), microwave heating power (100–600 kW, two independent lines at 53.2 GHz, second harmonic X-mode) and plasma–wall interaction conditions (wall conditioning, poloidal vs toroidal limiter). Although a close coupling between the plasmas and the TJ-II vacuum vessel is naturally present in most conditions, a good control of central plasma values has been achieved for both atomic species even under conditions close to the highest power density ( n e0 <1.7×10 13 cm −3 , T e0<1.3 keV). For this purpose, a careful control of wall conditions has been required. In addition, the low electron density and temperature of TJ-II edge plasmas have led to a significant reduction of the expected plasma–wall interaction. In this work, the issues of plasma density control and edge characteristics for the different plasma species and heating power are addressed. Results of new edge diagnostics, as a thermal lithium beam and a supersonic helium beam, among others are presented.

COMPARISON OF CONVECTIVE, VACUUM, AND MICROWAVE DRYING CHLORPROPAMIDE

Drying kinetics of convective, vacuum, and microwave drying of a pharmaceutical product, chlorpropamide, has been investigated on a laboratory scale, in the temperature interval from 40°C to 60°C, and the range of microwave heating power from 154 W/kg dm to 385 W/kg dm . The experimental data obtained were approximated with the “thin-layer” equation and a two parameter exponential model. In order to compare convective, vacuum, and microwave drying, effective diffusion coefficients and specific heat consumption were calculated for each drying method. Higher rates and shorter drying times were achieved at a higher temperature and microwave heating power. The highest drying rates and the lowest specific heat consumption were achieved with microwave drying. This leads to the conclusion that microwave heating is the most appropriate method for drying of chlorpropamide. The quality of product was not changed for all applied methods.

Study on dental diagnosis and treatment using millimeter waves

In order to diagnose dental caries noninvasively, the transmission coefficient of the dental caries is measured and compared to that of a sound tooth. It has been revealed that dental caries are significantly more lossy than a sound tooth in millimeter waves. This characteristic can be utilized as a new caries diagnosis method. This paper also presents microwave and millimeter-wave heating results for the lossy dental caries that can be used as a sterilization treatment. Temperature-distribution results from microwave power heating have revealed that dental caries are easily heated. Furthermore, the results of the calculated specific absorption rate distribution using the finite-difference time-domain method indicate the possibility of caries treatment by millimeter-wave heating. It is concluded that millimeter waves can be used for dental medical diagnosis and treatment.

Predicting temperature range in food slabs undergoing long term/low power microwave heating

The contribution of standing wave patterns to nonuniform heating in homogeneous slabs was examined through the development of equations for various heating situations relating temperature distribution to thermal, geometric and dielectric properties. The equations were solutions to the differential heat equation, which incorporated a volumetric heat generation term based on a full solution to Maxwell's equations. This analysis was based on steady-state microwave heating resulting in either steady temperatures or temperature profiles and was thus applicable only to very long term heating and, where food is concerned, low power heating as well. This latter stipulation allowed the additional assumption of temperature-independent thermal and dielectric properties. Results were reported in the form of the minimum-to-maximum temperature range, indicating the degree of nonuniformity. Parameter values for agar gel and cooked beef were used to illustrate the behavior of the equations. For both types of food it was found that the temperature range was a very sensitive sinusoidal function of slab width. Furthermore, for slab widths of 10 cm or less, it was found that insulation at the slab faces significantly reduced the temperature range. Future work will determine if this approach can be extended to short term/high power applications.

Microwave heating power supply operating in a high power-factor mode

The performance of a switched-mode magnetron power supply is considered in which a rectified, non-filtered mains voltage is applied to the converter input. The system features output voltage boosting at the input voltage zero crossings, and provides high power-factor operation. A closed-form expression for the system gain is obtained with and without active input current shaping. The analytical results show that the system tends to linearize the inherently nonlinear magnetron characteristic, and that a complete linearization could be achieved by frequency modulation within the mains period. The optimum converter component values are identified and the results are verified experimentally.

Improved power control during microwave heating in biological applications.

The heating performance of prototype variable power control units attached to two domestic microwave ovens, 500 W and 700 W, was investigated. The units employed the phase control method where rated voltage was obtained by switching AC supply at a particular angular position on the sinusoidal voltage. In order to create experimental conditions employed in a previous study, Sydney tap water (50 mL) was heated at a 10% power level in the 500 W oven. Water boiled at 10 min in both experiments. However, the precision and control of the heating was greatly improved with addition of the control unit. A disinfecting solution (50 mL) and a tissue fixative (10 mL) were included for experiments with the 700 W oven. A power level of only 1% was sufficient to maintain the temperature of the fixative at low temperatures (30-40 degrees C) where many biological reactions occur. The present results indicate that microwave heating power can be controlled by the variable power control method. This will make temperature control possible through the provision of an electronic feedback loop which links a thermocouple with the power control unit.

Basic Principles of Microwave Power Heating

This article outlines the various parameters that are employed in the design of a microwave power applicator as well as the physical principles of the microwave energy conversion process that underlie the optimization of energy consumption and product quality.

Dehydration and dielectric permittivity measurements of a porous, inorganic material (13X zeolite) heated with microwave power

The microwave drying of 13X zeolite is experimentally studied by continuously measuring the permittivity of a sample zeolite material during given microwave drying cycles. The results are interpreted in light of the dielectric theory of heterogeneous mixtures, making it possible to distinguish the water vapor in circulation within the zeolite structure from the water absorbed on the solid material. The results agree with previous data, showing that the adsorbed water molecules are much less mobile than the free water molecules. Time constants involved with the desorption and transport processes of water are obtained under microwave power heating conditions. >

Formation of relativistic electron plasmas in tokamaks using electron cyclotron heating

A small relative population of high energy electrons has been predicted to have important beneficial effects on tokamak plasmas. In particular, the efficiency of steady state RF current drive can be increased and the high beta second regime of stable operation can be achieved. Formation of suitable concentrations of relativistic electrons using electron cyclotron heating (ECH) can be adversely affected by the occurrence of relativistic heating gaps, i.e. electron energy intervals within which the relativistic reduction of the electron gyrofrequency prevents resonance with the applied microwave heating power. A fully relativistic one-dimensional Fokker-Planck model of ECH exhibits efficient generation of relativistic electrons in typical tokamak plasmas for suitably chosen spectra of propagation vector,,and frequency, ω, of the ECH waves. Narrow illumination patterns, symmetric about normal incidence, can readily yield 10% relative densities of relativistic electrons when three or more frequencies are used. On the other hand, oblique illumination or broad illumination patterns with angular half-widths of 45° can give similar results when a single frequency is used.

Measurement of the Complex Permittivity of Food Products during Microwave Power Heating Cycles

AbstractA single mode active cavity (SMAC) perturbation technique operating with high level microwave power has been developed to heat and measure, at the same time, the permittivity of sample materials at 2.2 GHz. The accuracy of the SMAC measurement technique is compared with a series of permittivity measurements made on a number of identical liquid dielectrics using time domain spectroscopy (TDS) techniques. It is found that SMAC microwave power heat cycles on food products produced temperature permittivity hysteresis effects. Such hysteresis effects were previously observed on materials heated inside microwave ovens. A more complete characterization of the microwave heating hysteresis effect for food products may be of some interest in microwave food processing.