Materials Dielectric

The absorption of electrical energy by a dielectric material that is subjected to an alternating electric field is termed dielectric loss. This loss may be important at electric field frequencies in the vicinity of the relaxation frequency for each of the operative dipole types for a specific material. A low dielectric loss is desired at the frequency of utiUzation. 

The magnitude of the dielectric constant for most polymers is less than for ceramics because the latter may exhibit greater dipole moments e, values for polymers generally lie between 2 and 5. These materials are commonly used for insulation of wires, cables, motors, generators, and so on and, in addition, for some capacitors. 

Two other relatively important and novel electrical characteristics that are found in some materials deserve brief mention—ferroelectricily and piezoelectricity. 

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Dielectric films Dielectric fluids Dielectric insulators Dielectric masking Dielectric materials Dielectric permittivity Dielectrics... 

Multilayer Capacitors. Multilayer capacitors (MLC), at greater than 30 biUion units per year, outnumber any other ferroelectric device in production. Multilayer capacitors consist of alternating layers of dielectric material and metal electrodes, as shown in Figure 7. The reason for this configuration is miniaturization of the capacitor. Capacitance is given by... 

In the electronics industry. Pis find wide appHcations as a dielectric material for semiconductors due to thermal stabiHty (up to 400°C) and low dielectric constant. Pis are being considered for use in bearings, gears, seals, and prosthetic human joints. The intended part can be machined or molded from the PI, or a film of PI can be appHed to a metallic part. Because of their superior adhesion, dielectric integrity, processing compatibUity, and lack of biological system impact. Pis have been used in many biological appHcations with particular success as body implants. 

Two parallel plates of conducting material separated by an insulation material, called the dielectric, constitutes an electrical condenser. The two plates may be electrically charged by connecting them to a source of direct current potential. The amount of electrical energy that can be stored in this manner is called the capacitance of the condenser, and is a function of the voltage, area of the plates, thickness of the dielectric, and the characteristic property of the dielectric material called dielectric constant. 

Another important use of dielectrics is as intermetal dielectrics (IMDs), where the dielectrics insulate metal lines from each other. The dielectric material must fill small gaps with high aspect ratios (depth to width) while maintaining all other dielectric properties. It is essential that the IMDs are void-free at submicrometer dimensions for both performance and rehabiUty. 

A. R. Von Hippel, ed.. Dielectric Materials and Applications, MIT Press, Cambridge, Mass., 1954. 

Electrical Properties. AH polyolefins have low dielectric constants and can be used as insulators in particular, PMP has the lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and alow dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon). These properties remain nearly constant over a wide temperature range. The dielectric characteristics of poly(vinylcyclohexane) are especially attractive its dielectric loss remains constant between —180 and 160°C, which makes it a prospective high frequency dielectric material of high thermal stabiUty. 

Antireflection coatings are used over the silicon surface which, without the coating, reflects ca 35% of incident sunlight. A typical coating consists of a single layer of a transparent dielectric material with a refractive index of ca 2, which is between the index of siUcon and ait or cover material. Materials such as titanium dioxide, tantalum pentoxide, Ta20, or siUcon nitride, Si N, ca 0.08-p.m thick are common. The coating and a physically textured... 

Strontium Titanate. Strontium titanate [12060-59-2], SrTiO, is a ceramic dielectric material that is insoluble in water and has a specific gravity of 4.81. It is made from strontium carbonate and is used in the form of 0.5-mm thick disks as electrical capacitors in television sets, radios, and computers. 

C = Q/V. In a vacuum, the charge density on the surfaces of the conductors is affected by the permittivity of free space, q. When a dielectric material is placed between the conductors, the capacitance increases because of the higher permittivity, e, of the material. The ratio of e and q gives the dielectric constant, K, of the material, k = e/eg The dielectric constant of siHca glass is 3.8. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Another method that has great potential for the preparation of advanced prepregs and has been explored extensively requites fine powders. The reinforcing fibers are coated with fine particles of the resin and, when heated, the resin flows over the fiber. This method requites finely divided particles either in aqueous solution, in an inert volatile solvent, or as high dielectric material that can be charged and coated by electrostatic attraction to the fiber. Synthetic methods that make fine particles, similar to that described for PEEK (23), are needed. 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

The insulating properties of polyethylene compare favourably with those of any other dielectric material. As it is a non-polar material, properties such as power factor and dielectric constant are almost independent of temperature and frequency. Dielectric constant is linearly dependent on density and a reduction of density on heating leads to a small reduction in dielectric constant. Some typical data are given in Table 10.6. 

Other fields for RBS analysis include optical and dielectric materials, hard and protective coatings, superconductors, and magnetic materials. 

The third group is the continuum, models, and these are based on simple concepts from classical electromagnetism. It is convenient to divide materials into two classes, electrical conductors and dielectrics. In a conductor such as metallic copper, the conduction electrons are free to move under the influence of an applied electric field. In a dielectric material such as glass, paraffin wax or paper, all the electrons are bound to the molecules as shown schematically in Figure 15.2. The black circles represent nuclei, and the electron clouds are represented as open circles. 

Capacitance is related to the area of the plates (yi), the distance between the plates (d), and the dielectric constant (e) of the material between the plates (Figure 2, equation I). The dielectric constant or permittivity of a material is the increased capacitance observed compared to the condition if a vacuum was present between the plates. Common dielectric materials are polystyrene (e = 2.5), mylar (e = 3), mica (e = 6), aluminum oxide (e = 7), tantalum oxide (e = 25), and titania (e = 100). In the Leyden jar the dielectric is silica. 

A capacitor, previously called a condenser, stores electrical energy based on the relationship between voltage (V) and stored charge (Q) in coulombs as shown in the equation C = QU. One farad of capacitance is a coulomb per volt of stored charge. The voltage limit of a capacitor is determined by the breakdown potential of the dielectric material. 

Capacitance (C) is the property that describes the quantity of electricity that can be stored when two conductors are separated by a dielectric material. The unit of capacitance is the farad. The capacitance of two equal-area, conducting parallel plates (see Figure 2-64) separated by a dielectric is given by... 

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