Polystyrene capacitor dielectric

End uses for all types of polystyrene are packaging, toys, and recreational products, housewares, bottles, lenses, novelties, electronic appliances, capacitor dielectrics, low-cost insulators, musical instrument reeds, light-duty industrial components, furniture, refrigeration, and building and construction uses (insulation). 

The variations of this basic process depend on the dielectric film used. For example, in some capacitors two foil layers can be dividedbyaplainfilm (polystyrene capacitors) (RIFA, 1990/91) or paper (impregnated with oil) two metalKzed offseted films can be used as well (polypropylene capacitors). There are many other sealing procedures as well. 

Many polymer films have been examined as capacitor dielectrics but those principally used are polyethylene terephthalate, polystyrene, polycarbonate, polytetrafluoroethylene, polyimides and polypara-xylylene. A number of these materials are more familiar by their trade names ... 

Polystyrene (PS) is another popular capacitor dielectric and this material has a lower dielectric constant than PET because it does not contain any polar groups in its molecular structure which is based upon the repeat unit... 

Polyester capacitors are an example of the plastic film capacitor. Polypropylene, polycarbonate and polystyrene capacitors are other types of plastic film capacitor. The capacitor value may be marked on the plastic film, or the capacitor colour code given in Table 3.5 may be used. This dielectric material gives a compact capacitor with good electrical and temperature characteristics. They are used in many electronic circuits, but are not suitable for high-frequency use. 

Essentially polymer-film capacitors comprise dielectric films (polymer or paper or both together) interleaved with aluminium electrodes, either as aluminium foil or, more commonly, in the form of a layer evaporated directly on the dielectric, and rolled together. They are sealed in an aluminium can or in epoxy resin. Because the dielectric films and evaporated electrodes have thicknesses of only a few microns and about 0.025 /mi respectively, volumetric efficiencies can be high. The dielectric films are polystyrene, polypropylene, polyester, polycarbonate or paper paper dielectrics are always impregnated with an insulating liquid. 

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. 

Polyethylene and polystyrene with their exceptionally low dissipation factors (<0.0005) and low dielectric constant (2.3-2.5) are the most suitable for high-frequency apphcations, as in television and radar. For dielectrics used in capacitors, however, a high dielectric constant is desirable. 

Figure 12.12 Percentage change in capacitance with temperature for capacitors with polystyrene and polyethylene terephthalate dielectric (from Bruins, R, Ed., Plastics for Electrical Insulation, Wiley Interscience, 1968).

The loss is converted to heat (dissipation). For low values of 6, the losses are low so that in the limit 6 = 0, there is no loss (this is the ideal capacitor). Low values of tan 6 may be approximated as tan 6 sin 6 cos 6, where cos 6 is defined as the power factor, 6 represents the angle between the direction of the voltage and current in an alternating current. Then Power = Voltage X Current x Power Factor. The loss factor approximately equals the product of the power factor and the dielectric constant, all values taken at a frequency of 60 Hertz. In nonpolar polymers (Teflon, polyolefins, polystyrene)... 

Polyvinylidene fluoride, is a fluorinated semi-crystalline thermoplastic which has a continuous use service temperature of up to 150 °C and a very low dielectric constant. Current manufacturers include Arkema which has become legally separated from its former owner. Total. PVDF applications include its use as fuel cell membrane material, film material in capacitors and as electrolyte material in sodium sulfur batteries. In the US, NASA has employed a crosslinked polystyrene sulfonic acid (PSSA)/PVDF composite as a PEM. 

Dielectric materials vary by cost and the capacitance needed for a specific application. Glass, ceramic, and mica papers are high quality, low capacitance dielectrics with extremely high breakdown resistance. Conversely, metal-ized polymer foils such as polystyrene, polyethylene terephthalate (PET), and Teflon (PTFE) are single-piece dielectric films that offer better capacitor performance. In recent years, polymer foils have come to dominate the static capacitor market because they have better stability at high temperatures, can be manufactured at lower cost, and age better than dielectric papers. 

Polystyrene also has a low dissipation factor that, along with its dielectric constant, is virtually independent of frequency making it suitable for high-frequency applications. Some of the properties important in capacitor design are compared for polystyrene and PET in Table 10.7. 

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