Capacitor materials typical capacitor construction

CAPACITOR MATERIALS TYPICAL CAPACITOR CONSTRUCTION

Dielectric materials are used to manufacture capacitors of different ranges. The selection of right dielectric materials for capacitor applications is based on various parameters like value of capacitance, frequency, tolerance size and operating voltage.

Dielectric materials such as paper and plastic film, mica film, single layer ceramic, multilayer ceramic, solid electrolytic (Al, Ta) and electrolytic (Al, Ta) are used in capacitors to obtain a range of capacitance 1 pf to 10⁴ mf.

1. Single and multilayer dielectric capacitors

The diagramatic representation of a single layer capacitor is shown in fig. 1.22. It consists of a thin ceramic disk or plate placed in-between the metal electrodes. The leads for the electrical connections are taken from the metal electrode.

In order to prevent the degradation of dielectric ceramic plate it is encapsulated in an epoxy coating.

The capacitance of single layer capacitor for an area of 1 cm² is 885 pf. The capacitance of the capacitor by connecting N number of ceramic plate in parallel by providing a sufficient space as shown in fig. 1.22(b).

The capacitor with stacking of ceramic plates in different layers is known as multilayer capacitor. The capacitance of multilayer capacitor is in the range of a few hundred microfarads.

2. Polymeric film capacitors

Polymer thin film capacitors are as mid-frequency capacitors. The two metal coated polymer films are placed in parallel. In order to get the capacitor, the two metal sheets are rolled together like Swiss roll.

The electrical connections are taken from the opposite sides of the metals by proper soldering. A multilayer polymeric thin film capacitors like multilayer ceramic capacitors can also be produced.

3. Electrolytic capacitors

A large values capacitors are achieved using electrolytic capacitors. These capacitors are of different types namely, aluminium electrolytic and solid electrolyte tantalum capacitors.

A cross-sectional view of aluminium electrolytic capacitors is shown in Fig. 1.23. It consists of two aluminium (Al) foils.

A thin layer of Al₂O₃ as a dielectric medium is grown on the rough surface of one of the foils. The two foils are wound together in the form of cylinder. They kept inside the cylindrical case as shown in fig. 1.23.

The thickness of Al₂O3, grown on the foil is 0.1 μm and hence, it gives large capacitance value (i.e., small thickness and large area.

 The Al₂O₃ is grown on the Al foil electrolytically and hence, the capacitor is known as electrolytic capacitor.

A thin film of Al₂O₃ dielectric coated on the surface of Al foil act as anode. Al foil acts as a cathode.

The structure of a solid electrolyte tantalum capacitor is shown in fig. 1.24(a). The anode is made up of tantalum (Ta) pellet. The surface of Ta pellet is anodized to get the tantalum pentoxide (Ta₂O₃).

A thick solid electrolyte like MnO₂ is coated above Ta₂O₂. The process is completed by coating graphite and silver paste as shown in fig. 1.25. The above system is then coated by an epoxy. Tantalum capacitors are widely used in electronic applications.