Microwave energy absorption

Table 6. Microwave Energy Absorption and the Factor of Merit for Various Types of Glasses and Glass-Ceramics...

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

The intensity of the absorption of microwave energy is a measure the abundance of that isotope. The potency of the NMR spectroscopy is not only its ability to quantify the concentration of an isotope, but to check the enviromnent into which an isotope is embedded. This is possible because the magnetic resonance and thus the absorption frequency prove to be sensitive to the spins of neighboring atoms and to structural features of the probe. Therefore, NMR spectroscopy is more a tool for scientific structural analyses than for daily food (colorant) inspection. For a detailed study of the NMR techniques used in food science we recommend books by Macomber and Pochapsky. - ... 

In order to perform extraction of additives or dissolution of polymers, solvents that absorb microwave energy are necessary. This is more important than direct absorption of microwave energy by the polymer or additives. When microwave extraction of additives... 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

On the other hand, solvents usually show a decrease in dielectric constant with temperature. Efficiency of microwave absorption diminishes with temperature rise and can lead to poor matching of the microwave load, particularly as fluids approach the supercritical state. Solvents and reaction temperatures should be selected with these considerations in mind, as excess input microwave energy can lead to arcing. If allowed to continue unchecked, arcing could result in vessel rupture or perhaps an explosion, if flammable compounds are involved. Therefore it is important in microwave-assisted organic reactions, that the forward and reverse power can be monitored and the energy input be reduced (or the load matching device adjusted) if the reflected power becomes appreciable. 

A completely different tactic is the use of microwave energy absorption of this energy by a peptide results in decreased aggregation. Lithium salts facilitate the solution of peptides in these polar solvents, increase the rate of swelling of polar resins, and improve coupling yields. Sometimes the order of addition of two solvents to a peptide makes a difference. The inclusion of pyridine can also have a beneficial... 

Cembrola56 has compared microscope, stylus and resistivity methods and concludes that no one method is universally the best. Guerbe and Freakley57 compared four methods including a microwave energy absorption method. 

Electrons can be made to resonate between these two states by the application of microwave energy. In EPR spectroscopy samples are subjected to microwave radiation of constant frequency and the magnetic field strength is increased until energy absorption is detected - this occurs when the energy difference between the two spin states matches the energy of the microwave radiation. 

The more absorptive a material, i.e., the higher the loss factor e", the less deep microwave energy will penetrate into that material. A parameter, penetration depth, dp, has been defined which measurers this penetration, dp is a function of both e and e" and serves as a guideline to the heating effectivity of a material. 

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