Microwave Transparent Research Articles

Investigating the Existence of Nonthermal/Specific Microwave Effects Using Silicon Carbide Heating Elements as Power Modulators

The use of passive heating elements made out of chemically inert sintered silicon carbide (SiC) allows microwave transparent or poorly absorbing reaction mixtures to be heated under microwave conditions. The cylindrical heating inserts efficiently absorb microwave energy and subsequently transfer the generated thermal energy via conduction phenomena to the reaction mixture. In the case of low to medium microwave absorbing reaction mixtures, the addition of SiC heating elements results in significant reductions (30-70%) in the required microwave power as compared to experiments performed without heating element at the same temperature. The method has been used to probe the influence of microwave power (electromagnetic field strength) on chemical reactions. Six diverse types of chemical transformations were performed in the presence or absence of a SiC heating element at the same reaction temperature but at different microwave power levels. In all six cases, the measured conversions/yields were similar regardless of whether a heating element was used or not. The applied microwave power had no influence on the reaction rate, and only the attained temperature governed the outcome of a specific chemical process under microwave conditions.

Novel synthetic approach in microwave-assisted solid-supported oxidations using ‘in situ’ generated molecular oxygen

Three different types of oxidation reactions were carried out under microwave (MW) conditions in dry media, with nearly quantitative yield, using ‘in situ’, yet separately generated molecular oxygen as the reactive gas. The latter is formed by a controlled decomposition of potassium chlorate (220–306 °C) adsorbed on zeolite support, and is used as a reactive oxidizing agent for the solid-supported oxidations. The MW-assisted oxidations include an oxidative decomplexation of (η 6-arene)Cr(CO) 3 complexes to the corresponding arenes using silica as solid support (100 °C), an oxidation of fluorene to fluorenone induced by KF-alumina support (150 °C), and oxidation of benzyl alcohol to benzaldehyde using a supported ruthenium catalyst (150 °C). This synthetic approach allows to carry out in synchronized manner two different solid-supported reactions (oxygen generation and oxidation) at different temperatures and on different solid supports together in the same sealed system. It was made possible by tuning the absorption efficiency of MWs through accurate selection of the solid supports employed in the reactions. The high feasibility of this novel synthetic approach resulted from a preliminary study of the interaction between MWs and mineral oxides such as alumina, silica, clay, and zeolite particularly when mixed with additives such as water, ionic liquids or graphite (5% w/w). The use of these MW absorber additives allows the MW transparent or poorly absorbing mineral oxides to be efficiently heated to very high temperatures in few minutes.

Silicon Carbide Passive Heating Elements in Microwave-Assisted Organic Synthesis

Microwave-assisted organic synthesis in nonpolar solvents is investigated utilizing cylinders of sintered silicon carbide (SiC)--a chemically inert and strongly microwave absorbing material--as passive heating elements (PHEs). These heating inserts absorb microwave energy and subsequently transfer the generated thermal energy via conduction phenomena to the reaction mixture. The use of passive heating elements allows otherwise microwave transparent or poorly absorbing solvents such as hexane, carbon tetrachloride, tetrahydrofuran, dioxane, or toluene to be effectively heated to temperatures far above their boiling points (200-250 degrees C) under sealed vessel microwave conditions. This opens up the possibility to perform microwave synthesis in unpolar solvent environments as demonstrated successfully for several organic transformations, such as Claisen rearrangements, Diels-Alder reactions, Michael additions, N-alkylations, and Dimroth rearrangements. This noninvasive technique is a particularly valuable tool in cases where other options to increase the microwave absorbance of the reaction medium, such as the addition of ionic liquids as heating aids, are not feasible due to an incompatibility of the ionic liquid with a particular substrate. The SiC heating elements are thermally and chemically resistant to 1500 degrees C and compatible with any solvent or reagent.

TLM simulation of microwave hybrid sintering of multiple samples in a multimode cavity

AbstractMicrowave heating technology is a powerful mean to ensure successful sintering of ceramic materials. In sintering experiments, low loss insulators, conductors and high‐loss ceramics are microwaved so as to get optimal mechanical and structural properties. It is known that low‐loss ceramic materials such alumina and zirconia exhibit long waiting time before reaching a critical coupling temperature at which microwaves can be readily absorbed. On the other hand, some ceramics such as silicon carbid have a high loss factor and therefore can be used as a process stimulus for microwave sintering of microwave transparent ceramics. Furthermore, successful sintering experiments often require the use of carefully designed insulating structure in order to minimize thermal gradients caused by heat loss from surfaces. All these problems have led to the introduction of microwave hybrid heating (MHH) schemes using higher dielectric loss susceptors, insulation or coating. Since MHH depend mainly on human expertise, the optimization of sintering experiments will certainly benefit from numerical simulations. The transmission line matrix (TLM) is used to study two MHH schemes where both a susceptor and an insulating matrix were, respectively, used as process stimulus for microwave heating of multiple alumina samples within a three‐dimensional multimode cavity. The effects of such MHH schemes and target settings on electric field distribution and power absorption rates are reported in this paper. Copyright © 2003 John Wiley & Sons, Ltd.

Microwave-accelerated ruthenium-catalyzed olefin metathesis.

[reaction: see text]. Microwave heating is an efficient method for the acceleration of ring-closing metathesis reactions using ruthenium-based catalysts. The reaction can be rapidly conducted in either ionic liquids, such as 1-butyl-3-methylimidazolium tetrafluoroborate (bmim), or in a microwave transparent solvent (MTS) such as dichloromethane.

Heat and Mass Transfer during Microwave Steam Treatment of Contaminated Soils

A time-dependent mechanistic model has been developed to describe the coupled heat and mass transfer during microwave steam treatment of contaminated soils. The model consists of multiphase mass and energy conservation equations, along with various temperature-dependent parameterizations of important physical state properties. The coupled nonlinear equations were solved numerically by applying a macroscopic control volume method with the appropriate boundary and initial conditions and using a finite-difference scheme. Microwave heating experiments of soil samples were performed. The effects of microwave power, soil sample depth, and soil type on water evaporation rate and temperature were measured with time. The modeled results are in good agreement with the experimental data. The results revealed that microwave power density and soil sample depth significantly affect the microwave evaporation process of water from soils. Water evaporation rate and temperature variation are complex and associated with the dielectric properties of liquid water distributed within the soil. The deeper the soil sample, the more nonuniform the evaporation process, although nonuniformities diminish as the evaporation process approaches completion. Finally it was verified that soil types and their thermal properties play a minor role, provided that the soil minerals are microwave transparent.

Microwave Power Effect on the Alkyl, Aromatic Hydrocarbons Oxidation with Molecular Oxygen

Liquid-phase oxidation of toluene, p-xylene and pt-butyl toluene in the presence of catalyst (Co and Mn acetates), initiators (KBr, methyl ethyl ketone) and solvent (acetic acid) was studied. An effort was made to obtain high oxidation rates at temperature between 90 and 100°C and at atmospheric pressure using microwave heating. A modified multimode cavity was used. The reaction vessel was made of Teflon (microwave transparent, chemically inert) and was equipped with a stirrer, temperature control, a reflwc condenser and a system for continuously bubbling of air.

MEMS orientational optomechanical media for microwave nonlinear applications

The fabrication and testing of orientational optomechanical media suitable for microwave phase conjugation is described. It consists of metal-coated dielectric elongated beams 1 mm×100 μm×10 μm suspended by nonconductive torsional springs attached to a microwave transparent frame. Rotation of single elements, in a polarized electromagnetic field at 15 GHz, was measured and found to be in a good agreement with theory. This first experimental implementation of using microelectromechanical structures (MEMS) for nonlinear microwave devices demonstrates the potential of an entirely new class of devices.

Sulfur at Mercury, Elemental at the Poles and Sulfides in the Regolith

We propose that the bright radar spots seen at Mercury's north and south poles are a result of volume scattering from elemental sulfur. Further we suggest that the high average index of refraction for surface materials and notable microwave transparency of the Mercurian regolith can be explained by the presence of iron sulfides (troilite, pyrrhotite) and low-iron sulfides (daubreelite and sphalerite). Sulfides deposited throughout the regolith are a result of fumarolic and magmatic events following initial accretion of meteoroids rich in troilite and pyrrhotite to the planet. Calculations are made illustrating how solar abundances of sulfur volatilized from such a surface can provide an adequate supply over the history of the planet to be cold-trapped at high latitudes, be covered partially with dust, and be seen as bright regions in radar images. To test our hypothesis, we suggest a series of observations in the ultraviolet to search for the SI multiplet centered at 1814 Å, in the midinfrared to search for emissivity features at 7.7 and 11.8 μm, and at millimeter wavelengths to search for sulfureous molecules like H2S and SO2.

Novel Routes to Microwave Processing of Ceramic Materials

ABSTRACTMaterial parameters important for microwave processing are identified in case of powder synthesis from precursor compounds and in case of sintering Al2O3- as well as SiC-matrix ceramics. By varying the spatial distribution of the precursor in microwave transparent materials, different pyrolysis temperatures are obtained, which can be attributed to different heating rates due to selective microwave heating. The microwave sintering behavior of oxide ceramics is strongly influenced by the specific surface area of the powder and by aliovalent dopants. In contrast, for covalent ceramics, like SiC+TiC, no experimental evidence for similar effects was obtained.

Synthesis, Structure, and Properties of Poly(4-Methyl-1 -Pentene)

Abstract Poly(4-methyl-l-pentene) (PMP) is a linear α-polyolefin with isobutylbranches and with very interesting properties. From a practical point of view. PMP presents high optical transparency. relatively high thermal stability and chemical resistance. in addition to excellent dielectric characteristics and microwave transparency. Consequently. a wide range of applications is possible for this polymer. The chemical resistance. temperature stability. and transparency provide uses in medical equipment such as hypodermic syringes. blood collection and transfusion equipment. pacemaker parts. and respiration equipment; in chemical laboratory equipment such as cells for spectroscopic and optical analysis; and in wire and cable coating. Its microwave transparency grants applications in microwave oven cookware, food packaging, and service trays.

Characterization of ZrO2-Al2O3 composites sintered in a 2.45 GHz electromagnetic field

The microwave heating process has shown potential for uniform material heating with rapid heating rates possible. The ceramic composite 80 wt % ZrO2 (+3 mol % Y2O3) 20 wt % Al2O3 was sintered in a Cober S6F microwave oven operating at 2.45 GHz even though it was considered to be microwave transparent at room temperature. The microwave-sintered sample was densified more rapidly and in a short time and lower temperature than the conventionally sintered sample. The fracture of the conventionally sintered sample was intergranular, but one of the microwave-sintered samples was transgranular. The fraction of the monoclinic-phase zirconia was different in the conventionally and microwave-sintered samples. The unusual microstructure of microwave-processed materials is explained by a microwave heating theory.

Microwave Transparency of Thin Metal Plates in Parallel Magnetic Field

It is a theoretical study of transparency oscillations in thin plates in magnetic field with the frequency widely ranging, including high frequencies where the “spikelike” mechanism changes to the “time-of-flight” mechanism of field penetration into the metal. The sensitivity of the oscillation amplitude to the surface state can be used to study the surface scattering of electrons.

Druckaufschluß im Mikrowellenofen

Decompositions of inorganic and organic materials can be carried out within a few minutes by means of the microwave technique. For dissolving substances with volatile components in the microwave oven, low pressure containers were developed from microwave transparent materials and an excess pressure security system. Results and limitations of the method are explained by examples.

Holographically steered millimeter wave antennas

A class of dynamic holographically addressed millimeter wave antennas becomes possible by combining the principle of holography with the notion of microwave transparency produced by excess carrier generation in semiconductor panels or preferably by reflectivity modulation of an interferometric structure consisting of a dielectric air gap metal structure. The theory of operation of the holographically addressed antenna is presented together with a discussion of possible methods for its implementation. The advantages of using a microwave Kinoform for this purpose are also pointed out.

A Multiple-Animal Array for Equal Power Density Microwave Irradiation (Short Papers)

The introduction of multiple subjects into a microwave field invariably results in perturbations and interference patterns which make it difficult to accurately determine power densities at any specified location. To overcome this problem, investigators have restricted the number of subjects, which is inefficient, or used techniques to illuminate large volumes, which still results in large variations in power density due to curvature of the microwave field. An exposure array has been devised that negates these disadvantages and enables simultaneous irradiations of multiple animals at uniform average power density (/spl plusmn/5 percent). The array consists of microwave transparent cages positioned in accordance with the natural characteristics of the microwave field and separated sufficiently to insure minimum interaction between animals due to microwave reflection. The results of testing the array in an anechoic chamber at a freqnency of 2450 MHz using an isotropic field probe are presented.

Submillimetre spectra of pure high and low density polyethylene

As part of a search for a dielectric of high microwave transparency for telecommunications applications, the spectra of pure high and low density polyethylene, prepared by a new process, have been recorded at submillimetre wavelengths. The background absorption is markedly lowered. A hitherto undiscussed feature has been observed at 190 cm −1 and assigned: spectra at liquid nitrogen temp. are also presented.