Liquid Dielectrics

liquids whose specific electric resistivity is more than 10¹⁰ ohms-cm. In an electrical field, both liquid and solid dielectrics are characterized by their polarization and dielectric losses; in strong fields, breakdown occurs. The electrical conductivity of liquid dielectrics is brought about by the ions formed as a result of the dissociation of the intrinsic and extrinsic molecules of the liquid. The breakdown of a liquid dielectric in a strong electric field is associated primarily with impurities contained in the liquid.

Liquid dielectrics are of great importance in electrical engineering and laboratory practice. They have a higher electric strength, dielectric constant ε, and thermal conductivity than air or other gases at pressures close to atmospheric pressure. Therefore, the permissible operational voltage of electrical devices is increased if the air is removed from fibrous or other porous insulation and the resulting voids are then filled with a liquid dielectric. An analogous effect may be achieved by potting with a liquid dielectric such parts as transformer casings, capacitors, or units of radio components or by impregnating with a liquid dielectric the paper insulation of capacitors and high-voltage power cables. Impregnation of the paper insulation of a capacitor with liquid dielectric results in a significant increase in capacitance.

Electrically insulating mineral (petroleum) oils are the most widely used type of liquid dielectrics. Their chemical composition is a mixture of various hydrocarbons, with ε ≈ 2.2–2.4 and a low dielectric loss angle δ (after a thorough cleaning and at normal temperature, tan δ < 0.001).

Chlorinated hydrocarbons with an asymmetric molecular structure (in the USSR, sovol and sovtol} are polar dielectrics with high values of ε (3–6), which exhibit a characteristic dependence of ε and tan δ on temperature and frequency. Synthetic liquid dielectrics (organosilicon and organic fluoride liquids) are also widely used.