Conductive-dielectric efficiency

In general, the power at the terminals of the antenna, Pt, and the radiated power Prad are different, with Prad < due to losses in the dielectric and conductive parts of the antenna. Such a relationship is expressed by the conductive-dielectric efficiency ecd, being Prad = Scd Pt, with 0 < ecd < 1- Hence, the relation between gain and directivity is ... 

Efficiency to maximize the radiated power, the antenna designer wiU aim at a large radiation efficiency. As previously discussed, the total efficiency is the product of two terms. The first one, the impedance mismatch factor M, can he maximized by minimizing the reflection coefficient. The second term, the conductive-dielectric efficiency Ced can be maximized by using textile materials with low ohmic and dielectric losses, ie, electrotextiles with high conductivity and textile dielectric substrates with low tan 6. 

Microwave energy is not transferred primarily by conduction or convection as with conventional heating, but by dielectric loss [28]. The dielectric loss factor (loss factor, e") and the dielectric constant (e ) of a material are two determinants of the efficiency of heat transfer to the sample. Their quotient is the dissipation factor (tan 8),... 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Several workers have employed monomodal cavities for microwave chemistry on the sub-gram scale. In some cases in which monomodal cavities have been used7, special benefits of so-called focussed microwaves have been claimed. As mentioned earlier, the dielectric properties of a sample can alter substantially with temperature and/or with changing chemical composition. Hence, regardless of whether multi-modal or unimodal cavities are employed, frequent tuning may be necessary if heating efficiency is to be retained. This aspect has often been overlooked by proponents of focussed microwaves. The nett result is that transfer of microwave conditions between monomodal to multi-modal cavities is usually facile. With the MBR (which had a tunable multimodal cavity), Cablewski et al. performed five reactions that had been conducted earlier on the gram scale or below with focussed microwaves (T. Cablewski, B. Heilman, P. Pilotti, J. Thorn, and C.R. Strauss, personal communication see also Ref. 117 for conference poster). These were scaled-up between 40- and 60-fold and reaction conditions... 

Differences in sample size and composition can also affect heating rates. In the latter case, this particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low-molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. When working under pressure, it is essential to measure pressure. This can be used for reaction control. If pressures fall beyond acceptable upper and lower limits or the rate of pressure rise exceeds a tolerable value, operating software should automatically shut down the machine. In combination with efficient cooling this approach can avoid thermal runaways near their onset. 

The dielectric constant e can be estimated to be of the order of 100, and the donor concentration N can be estimated from the measured conductivity, activation energy.and Hall mobility to be of the order of 10 cm. Then W = 10 cm and L is even smaller, so that expansion of the exponential yielSs a quantum efficiency porportional to the optical absorption coefficient even for large values of a. 

As was mentioned above, every efficient application of microwave energy to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in the most microwave reactors enable a synthesis to be carried out without knowing the dielectric properties or/and conductive properties of all the components of the reaction mixture in detail. On the other hand, temperature control during microwave irradiation is a major problem that one faces during microwave-assisted chemical reactions. In general, temperature in microwave field can be measured by means of ... 

MAOS is mainly based on the efficient heating of materials by the microwave dielectric heating effect [15] mediated by dipolar polarization and ionic conduction. When irradiated at microwave frequencies, the dipoles (e.g., the polar solvent... 

It is known that in some cases the modulus representation M (oo) of dielectric data is more efficient for dc-conductivity analysis, since it changes the power law behavior of the dc-conductivity into a clearly defined peak [134]. However, there is no significant advantage of the modulus representation when the relaxation process peak overlaps the conductivity peak. Moreover, the shape and position of the relaxation peak will then depend on the conductivity. In such a situation, the real component of the modulus, containing the dc-conductivity as an integral part, does not help to distinguish between different relaxation processes. 

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