Microwaves penetration depths

Table 2 Microwave penetration depth (245 GHz) in some common materials a...

Fig. 14 The effect of moisture on microwave penetration depth. (From Ref...

Z. Peng, J.-Y. Hwang, J. Mouris, R. Hutcheon, X. Huang, Microwave penetration depth of in materials with non-zero magnetic susceptibility, ISIJ Int. 2010, 50(1 I), 1590-1596. 

On the other hand, microwave penetration depth (Anlage et al. 1994) and Raman spectroscopy measurements (Stadloper et al. 1995) on the electron-doped superconductor Ndi 85Ceo.i5Cu04-, indicate that the superconducting order parameter of this material has s-wave symmetry. 

This shows that the penetration depth decreases dramatically with increasing conductivity of the medium to be penetrated. This has been plotted (Fig. 2) for different specific resistivities of the medium and the frequency of 10-40 Gc/s11 at which microwave conductivity measurements are typically performed. It can be seen that with a specific resistivity of 10 Q cm, a penetration depth of only 2 mm can be expected. Figure 2 furthermore shows the doping densities at which the respective penetration depths can be expected for silicon. Whereas the lower frequency X-band of microwaves (8-12.5 Gc/s) offers some advantages for materials with very low resistance, the high-frequency microwave Ka-band (26.5 10... 

Figure 2. Penetration depth of microwave energy as a function of specific resistivity or conductivity of the semiconductor material for microwave radiation of 10 and 40 Gc/s.11...

According to definition, the penetration depth is the point where 37% (1/e) of the initially irradiated microwave power is still present [6]. The penetration depth is in-... 

The penetration depth of waves is defined as the distance from the surface of the material at which the power drops to 1 /e from its value at the surface. The penetration depth of microwaves is equal to 15 mm for water at 20 °C. The electromagnetic energy transfer is ensured by matched alumina windows. The propagated mode within the reactor is theoretically the TEn mode. The interest of this system is to make very specific chemical reaction such as oxidation in aqueous medium under critical conditions. 

On the other hand, it must be kept in mind that a number of critical issues are linked with the application of microwave in chemical scale-up (Ondruschka et al. 2004). Firstly, penetration depth of microwaves in... 

Buschmuller et have demonstrated that microwave resonance can be used effectively as means to monitor the moisture levels in a fluidized-bed dryer during the granulation process. The penetration depth of microwave resonance may be limited to a few microns, and hence this technique may not have any real advantages over NIR which has also been used for monitoring moisture in dryers, and has the advantage of providing chemical information such as solvent levels in addition to water, and other important properties such as polymorphic form, and particle size. 

In a microwave cavity a standing wave pattern is generated, which depends of course on a multiple of the wavelength of the radiation (12.5 cm at 2.45 GHz), and therefore depending on the dielectric properties and size of the sample one can get considerable variations in temperature. The penetration depth, Dp, of the radiation is clearly an important consideration. 

The values of e and e" of a food material play a critical role in determining the interaction of the microwave electric field with the material. A discussion of these interactions follows. A "map" of foods plotted against their dielectric parameters was introduced by Bengtsson and Risman (1971). Table 1 gives values for the dielectric constant, loss factor and penetration depth, and Figure 1 shows a "map" of these values for common foods. 

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. 

The term, penetration depth, has three common, slightly different definitions. Their definitions and differences have been discussed (Buffler 1993). The internationally accepted definition is that distance in which the microwave power, once... 

The penetration depth (Dp) of the materials used to denote the depth at which the power density of microwave irradiation is reduced to 37% (i.e., 1/e) of its initial value at the surface of the material. It is proportional to the wavelength of the radiation and depends on the dielectric properties of the material. For lossy dielectrics (e"/e 1) the... 

Eventually, the sample size, penetration depth as well as heating rate are strongly coupled during microwave irradiation and may finally result in more homogeneous or heterogeneous heating of the material, which in turn can result in overheating of the material and creation of so called hot spots in the latter case. 

Microwave (2.45 GHz) penetration depth (Dp) in some common materials [11]. 

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