Dielectric behaviour

Polymeric films have replaced paper as the dielectric medium in capacitors for high voltage or high frequency alternating current (AC) use. The relationship for the capacitance of a parallel plate capacitor 

When an AC voltage is applied across a polymer, its dielectric properties determine the current. The dielectric constant (or relative permittivity) is defined in terms of the electric field E and electric displacement D vectors by 

V can be represented on an Argand diagram (Fig. 12.13b) by a vector of length Vo rotating at an angular frequency co. For an ideal capacitor, of capacitance C, the current is given by 

In the Argand diagram, the current vector rotates 90° ahead of the voltage vector. For real dielectrics, the current leads the voltage by an angle 90° - 5, and is given by 

Dielectric data is usually presented as the real part of the dielectric constant s as a function of frequency, and the ratio 

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This is a test to monitor the quality and dielectric behaviour of the insulating system of high-voltage machines, 5 kV... 

SMYTH, c. R, Dielectric Behaviour and Structure, McGraw-Hill, New York (1955)... 

Dielectric behaviour and molecular structure of inorganic complexes. S. Sorriso, Chem. Rev., 1980, 80, 313-327 (240). 

Bohidar H. B., Maity S., Saxena A., Jena S. Dielectric behaviour of Gelatin solutions and Gels. Journal Colloid Polymer Science 276,1 (1998) 81-86. 

Grant, E. H., R. J. Shephard, and G. P. South. 1978. Dielectric Behaviour of Biological Macromolecules in Solution. Oxford, Clarendon Press. 

Cook, H. F., 1952. A comparison of the dielectric behaviour of pure water and human blood at microwave frequencies, Br. J. Appl. Phys., 3, 249-255. 

The most important application for bismaleimide resin is multilayer boards. The development in this area requires resins with low dielectric constants. It is well documented in the literature that fluorine containing linear polyimides show lower dielectric constants vis a vis their non-fluorinated counterparts. Recently, Hitachi Research Laboratory, Japan, reported the thermal and dielectric behaviour of fluorine-containing bismaleimides (29). The chemical structures of the fluorinated BMIs investigated are provided in Fig. 6. The non-fluorinated four aromatic rings containing BMI, 4,4 -bis(p-maleimidophenoxyphenyl) propane, was tested in comparison. 

We turn to the relaxation processes observed in smectic polymers with different attachment of mesogenic groups to the macromolecular backbone and compare dielectric behaviour of smectic and nematic polymers having identical mesogenic groups but different main chain structure. 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

As mentioned, the dielectric j3 peak is related to carbonate motions however, it is important to check whether the adjacent phenyl groups are involved or not in such motions. Concerning this feature, it is interesting to consider the dielectric behaviour of TMBPA-PC. The temperature dependence of the... 

However, it is interesting to perform a more direct comparison of the experimental results to check whether some differences between the mechanical and dielectric behaviours could exist as a function of temperature. The appropriate quantity is E" for the mechanics and, for the dielectric response, it is the dielectric loss modulus, m" (defined as e"/ sa + s"2)). Figure 112 shows the temperature dependence of E" and m" at 1 Hz, obtained by superposing the low-temperature part of the j3 transition. 

The dielectric relaxation of CMIMx copolymers has been studied and compared to the PMMA dielectric behaviour [75]. Figure 129 shows the temperature dependence of the dielectric loss, e > at 1 Hz. 

The dielectric behaviour of pure water has been the subject of study in numerous laboratories over the past fifty years. As a result there is a good understanding of how the complex permittivity t = E — varies with frequency from DC up to a few tens of GHz and it is generally agreed that the dielectric dispersion in this range can be represented either by the Debye equation or by some function involving a small distribution of relaxation times. 

The high frequency limit of for this second process is therefore n. The result of the fit is shown in Table III where the mean values of the various parameters and their associated 95% confidence intervals are given. Considering the small amplitude of the second dispersion both in absolute t rms and in relation to the main dispersion the parameters 6m, n and Y are quite well defined, and therefore it may be concluded that the double Debye representation is an acceptable description of the dielectric behaviour of water up to around 2THz. Other alternative interpretations are clearly possible but no attempt has been made here to follow these up at this stage. What is clear is that a small subsidiary dispersion region in the far infrared is necessary to account for all the presently available permittivity data, and that such a dispersion is centred around 650GHz and has an amplitude of about 2.4 in comparison with that of the principal dispersion which is approximately 75. 

Grant,E.H Sheppard,R.J South,G.P "Dielectric behaviour of biological molecules in solution" Oxford University Press,... 

Grant, E. H. Sheppard, R. J., South, G. P., "Dielectric Behaviour of Biological Molecules in Solution" Oxford University Press Oxford, 1978. 

TTie dielectric behaviour of the deuterium substituted crystal has also been studied. The Tc is drastically raised to about 500 K. This large isotope effect supports the interpretation that the dielectric response of squaric acid is derived from proton (deuterium) tautomerism. Similar isotope effects on... 

With this additional feature in mind, there are three aspects of polymeric molecules which must be considered in order to reach an understanding of their dielectric behaviour in the liquid state the type of dipole present on each repeat unit of the polymer, the equilibrium conformation of the individual molecules, and the flexibility of the molecular chains. 

As regards aqueous solutions of electrolytes, there has been some significant advance in the understanding of the dielectric behaviour of the hydration sheath around the ions. In the field of non-electrolyte solutions, no claim can yet be made that th e is good understanding. It is obvious that the data are still insufficient this is the fidd where measurements are most needed. 

Instead of reviewing the extensive experimental work on the dielectric behaviour of biomolecuiar systems, this diapter will be mainly concerned with a discussion of the basic aspects of various mechanisms which may yield relevant dielectric polarization. These result from the following effects of an applied external electric field ... 

Generally, all potential electric and dielectric behaviour of a mediiun is taken into account by writing... 

As we see, the parameter 1 results from Langevin reorientation of the polarizability ellipsoid and is always positive. The second of the above parameters, 2, corresponds to Bom s term in the Kerr effect and can be positive or negative, depending on the electric structure of the molecule. The third, the Debye parameter 3, has no counterpart in other phenomena of molecular orientation, and is specific to the non-linear dielectric behaviour of dipolar substances. 

Coming to the present volume, one aim has been to provide a basis on which the student and researcher in molecular science can build a sound appreciation of the present and future developments. Accordingly, the chapters do not presume too much previous knowledge of their subjects. Professor Scaife is concerned, inter alia, to make clear what is the character of those aspects of the macroscopic dielectric behaviour which can be precisely delineated in the theoretical representations which rest on Maxwell s analysis, and he relates these to some of the general microscopic features. The time-dependent aspects of these features are the particular concern of Chapter 2 in which Dr. Wyllie gives an exposition of the essentials of molecular correlation functions. As dielectric relaxation methods provided one of the clearest models of relaxation studies, there is reason to suggest that dipole reorientation provides one of the clearest examples of the correlational treatment. If only for this reason, Dr. Wyllie s chapter could well provide valuable insights for many whose primary interest is not in dielectrics. 

In principle complete information on the dielectric behaviour may be obtained by studying the noise output of a condenser. The advantage of testing with a substantial applied field is simply to produce signals wdl above the noise from other possible, uncontrolled, sources in the system. 

The dielectric behaviour of proteins in aqueous solutions was first extensively studied by Ondey and co-workers. They interpreted the data in terms of rotational polarization of permanent dipole moments. The latter were found to be in the range of 100—1000 D (1 D = 10- e.s.u.) while the relaxation times came out at ca. 10" s. Despite some ta-itidsm, the preferential-orientation effect must still be considered the prindpal dielectric-polarization mechanism of proteins. - This view is also supported by dieledric dispersion studies of various proteins in solvents of different viscosity. The measured relaxation times were indeed proportional to rj as predided by (29) and (30). Nevertheless, for very large molecules (M, > 10 ) indications of other mechanisms, whose relaxation does not depend on the bulk viscosity of the solvent, have been observed. ... 

It must be emphasized, however, that a AM which is directly proportional to E as specified by (67) rests on the implicit assumption that the rate of polarization is much faster than the rate of chemical relaxation. Such an assumption is very well justified for small molecular dipoles which can freely rotate in a liquid. Then we have a rotational relaxation time Tr 10 —10 s which is small compared with pertinent chemical relaxation times r, in almost all cases of practical interest. Under these drcumstances, chemically induced dielectric behaviour cannot be possible for small E. It can occur, on the other hand, in the reverse case of > tci,. This condition may actually be encountered in macromolecular reaction... 

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