dielectric breakdown[‚dī·ə′lek·trik ′brāk‚dau̇n]
a sharp reduction in the electric resistance (an increase in the current density j) of a dielectric that occurs when the intensity of the applied electric field Eap reaches a certain value, usually approximately 105–106volts/cm. It is associated with the formation in a dielectric crystal of a conducting path in which the current density is substantially higher than the average for the specimen. The Joule heat generated because of the high-density current in the path leads to the destruction of the material, including melting; the appearance of an air channel as a result of volatilization; and the extensive formation of crystal defects, or cracking. Thus, dielectric breakdown is an irreversible phenomenon.
In an ideal, homogeneous, crystalline dielectric, a conducting path is formed as a result of a current pinch, which unavoidably arises when the differential electric resistance ρ = ∂E/∂j of the dielectric becomes negative. In actual solid dielectrics, the breakdown occurs at values of E lower than those in the ideal case because various inhomogeneities facilitate its onset.
Breakdown in fluid dielectrics is also due to the formation of a conducting path; Eap depends on the purity of the fluid.
REFERENCESSee references under DIELECTRICS.
A. P. LEVANIUK