Polar Polymer Dielectric Capacitors

Polar polymer dielectric capacitors are miniaturized devices that are widely used in miniaturized circuits in modern electronics. In fabricating polar polymer dielectric capacitors, a physical vapor deposition process is used to coat polycarbonate foil with an A1 layer ( 0.5 pm) to act as the electrode plates. Melted metal is then deposited by an airbrush method on either a cylindrical or flattened roll to provide contacts for the terminal connections. By changing the dielectric thickness, the capacitor s voltage can be changed from 60 to 250 V. 

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Depending on the dielectric materials and applications, various types of capacitors have been designed, fabricated, and commercialized. These capacitors are categorized as fixed, variable, power, high voltage, interference suppression, ferrodielectric, polar polymer dielectric, linear, and nonlinear capacitors [7]. The following sections will briefly discuss each type. 

The capacitance is expressed in nanofarads (10 ) or picofarads (10 ). Often the dielectric constant is measured in an alternating field. The reason more electrons can be stored on one plate of the capacitor at a given voltage with a solid dielectric is that electron fields in the material are distorted and counteract the impressed stress. Even a nonpolar polymer such as polyethylene can have valence electrons displaced to give an electronic displacement with a corresponding component of dielectric constant of about 2.0-2.5. A polar polymer such as poly(vinyl chloride) also can respond to an imposed field by actual orientation of molecular segments, provided it is above 7. ... 

Capacitors can be polarized or non-polarized, depending on the - dielectric. Non-polarized devices have dielectrics consisting of ceramics or polymers (such as polystyrene, polyester, or polypropylene). They are normally box-shaped and their capacity is usually in the range from pF to pF, the maximum voltage up to 1000 V. Polarized capacitors are electrochemical devices the dielectric is an anodic oxide of A1 (pF to 100 mF, potentials up to 1000 V), Ta (capacities pF to 100 pF, potentials up to 20 V), or Nb (- electrolytic capacitor) or a double layer (- supercapacitor, capacities up to some 10 F and potentials up to 2.5 V or 5 V). Aluminum electrolytic capacitors are normally of cylindrical shape with radial or axial leads. Tantalum capacitors are of spherical shape and super capacitors form flat cylinders. 

The uses of dielectrics range from capacitors for storing charge to ultrasound imaging for medical applications. We separate dielectrics from insulators, which we described in Chapter 30, because dielectrics have permanent electric dipoles. If the resultant polarization is spontaneous we have ferroelectrics. This topic is essentially exclusive ceramic materials. Although some polymers are ferroelectric they do not find as wide use as ceramics. And metals cannot be ferroelectric because the charge is not localized. 

The dielectric loss constant, e", or its associated tan 5 can be measured by placing the sample between parallel plate capacitors and alternating the electric field. Polar groups on the polymer chain respond to the alternating field. When the average frequency of molecular motion equals the electric field frequency, absorption maxima will occur. 

Inorganic-polymer nanocomposites characterized by exceptional dielectric constant are often called artificial dielectrics . Artificial dielectrics are created when isolated particles become polarized due to the presence of an applied electric field. These novel nanocomposite artificial dielectrics have the potential to posses high dielectric constants (>100) at high frequencies and the low processing temperature associated with polymers. Such a combination of properties is not found in other capacitor materials [180]. Polymer matrices like PMMA, poly(vinylidene fluoride) (PVDF), PS, and polyurethane (PU) have been used. Owing to their physicochemical properties, they represent suitable polymer components for embedding nanoscopic functional inorganic fillers (Table 2). 

If a polymer mass containing only truly covalent bonds free from dipoles is placed in an electric field there is an instantaneous electron shift (electron polarization) but no actual movement of the molecules themselves. If this field is between two charged plates the polymer acts as a dielectric in a capacitor and the capacity of the charged plates is increased. The factor by which the capacity is increased is known as the dielectric constant or permittivity. Because it depends on virtually instantaneous electron movement the value of the dielectric constant is not dependent on temperature nor on the frequency (if the field is subject to alternation). The dielectric constant of such materials is frequently equal to the square of the refractive index and both properties may be calculated from a knowledge of the chemical bonds present. The method of making such a computation is given in most standard texts on physical chemistry. 

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