Mixed dielectric capacitors

Until the 1970s the chemical used as the impregnating and dielectric medium for capacitor units was PCB (polychlorinated biphenyl) liquid. It was found to be toxic and unsafe for humans as well as contamination of the environment. For this reason, it is no longer used. The latest trend is to use a non-PCB, non-toxic, phenyl xylyl ethane (PXE-oil), which is a synthetic dielectric liquid of extremely low loss for insulation and impregnation of the capacitor elements or to use mixed polypropylene or allpolypropylene (PP) liquids as the dielectric. A non-oil dielectric, such as epoxy resin, is also used. 

The dielectric constant and breakdown field of a material are not the only considerations. Because a capacitor is used for charge storage, the leakage properties of the film are also critical. These are largely controlled by the defects in the films which can include oxygen (anion) vacancies or mixed valence effects in the cations. It is important that films that are as fully oxidized as possible be studied. 

Most steatite ceramics are either dry mixed and pressed, or wet-mixed, spray-dried and pressed, but they can also be wet-mixed and extruded. Typical firing temperatures lie close to 1300 °C. Many small parts are made for the electronics components industry where low dielectric losses are required, for example for tie-bars and other parts for ganged capacitors, small trimmer capacitors, high-power capacitors (see Section 5.6.3), coil formers, lead-throughs and substrates for... 

The vast growth in electronic equipment owes to capacitors which are essential in almost all the devices. Barium titanate forms the heart of the capacitors. The perovskite structure contains a small ion of high charge at the centre of an MC>6 octahedron. The high polarizability is the basis of a high dielectric constant in the capacitor. The addition of Nd to the mixed titanate gives a stable capacitance over a wide temperature range. 

The dielectric materials used in power capacitors include paper, polypropylene, and mixed vacuums impregnated with mineral or synthetic oil. The most reliable dielectric material is polypropylene. The metal electrodes can be aluminum foils or thin metal films deposited onto the surfaces of the dielectrics. The reliabilities and lifetimes of power capacitors are always concerns because of the possibility of overheating. 

Two processes for forming multilayer capacitors (MFCs) are shown in Fig. 3.24. In both processes, the dielectric powders are mixed in a solvent solution with dispersant, organic binders, and plasticizers. In the tape process, the slurry is de-aired and cast into thin sheets using a doctor blade. The tape-casting process is shown schematically in Fig. 3.25. The slurry is typically pumped into a reservoir. The leading edge of the reservoir has a small. 

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