Insulators and Dielectrics

To structure a discussion around all these developments is not an easy task and we have attempted to assist in intelligibility by presenting it in two parts. 

Electroceramics Materials, Properties, Applications. 2nd Edition. Edited by A. J. Moulson and J. M. Herbert. 2003 John Wiley Sons, Ltd ISBN 0 471 49747 9 (hardback) 0 471 49748 7 (paperback) 

Part I is focused on capacitors, providing the oportunity to introduce the many important ideas relating to their performance and, indeed, to the wider application of dielectrics and insulators. Part II is concerned with the important ceramic types and their applications. 

Dielectrics and insulators can be defined as materials with high electrical resistivities. Dielectrics fulfil circuit functions for which their permittivities e and dissipation factors tan S are also of primary importance. Insulators are used principally to hold conductive elements in position and to prevent them from coming in contact with one another. A good dielectric is, of course, necessarily a good insulator, but the converse is by no means true. 

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As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

UHMWPEs exhibit excellent dielectric and insulating properties. The base polymer is an effective electrical insulator with a dielectric constant of 2.3 at 2 MHz. The high surface resistivity may cause electrostatic discharges. It can be reduced by the addition of carbon black. 

Before embarking on a detailed consideration of the application of dielectrics and insulators, it is opportune to focus attention briefly on dielectric strength and thermal shock resistance . Both properties demand careful consideration in certain applications of dielectrics and insulators. They are by no means simple to define and, generally speaking, it is necessary only to develop some appreciation of how component and operational parameters determine them. 

Ceramic dielectrics and insulators cover a wide range of properties, from steatite with a relative permittivity of 6 to complex ferroelectric compositions with relative permittivities exceeding 20000. For the purposes of this discussion insulators will be classed with low permittivity dielectrics, although their dielectric loss may be too high for use in capacitors. Reference should be made to Table 5.10 and Fig. 5.40. 

Thermoplastic polyolefin blends can be produced as quite soft materials (as low as 55 Shore A) or as harder (above 95 Shore A), flexible products. They are particularly useful when a combination of good weatherabi1ity and rubbery behavior is required. Applications include automotive parts, weather stripping, hose, and sporting goods. Because of their good dielectric and insulating properties, polyolefin blends may be particularly useful in wire and cable insulation. 

The measurement of bulk resistivity of a powder includes volume and surface conduction mechanisms. It is generally not possible to separate out the two effects so that the effective powder resistivity, either the volume or surface resistivity, for dielectric and insulating particles such as glass depends on such factors as the presence of surface impurities and the relative humidity. For clean metal powders, the volume resistivity will dominate conduction in a bed of particles whereas, the presence of a surface oxide film can dominate conduction via the contact resistance for only lightly compacted powders. 

C TOOLS More about dielectrics and insulators An insulator is a material through which no steady conduction current can flow when it is submitted to an electric field. Consequently, an insulator can accumulate electric charge, hence electrostatic energy. The word dielectric, especially if it is used as adjective, covers a wide range of materials including electrolytes and even metals. 

Among these electrical elements, and represent the properties of the bulk solution and the diffusion of the redox probe, respectively, whereas Cji and / et depend on the dielectric and insulating features at the electrode/electrolyte interface, respectively. The electrochemical impedance spectrum in Fig. 4d shows a typical Bode plot for the IME system with mixed kinetic and charge transfer control. The agreement between the measured data (solid line) and the fitting spectrum (broken Une) indicates that this... 

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