Dielectric strength

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In physics, the term dielectric strength has the following meanings:

  • Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties.
  • For a given configuration of dielectric material and electrodes, the minimum electric field that produces breakdown.
  • the maximum electric stress the dielectric material can withstand without breakdown

The theoretical dielectric strength of a material is an intrinsic property of the bulk material and is dependent on the configuration of the material or the electrodes with which the field is applied. At breakdown, the electric field frees bound electrons. If the applied electric field is sufficiently high, free electrons may become accelerated to velocities that can liberate additional electrons during collisions with neutral atoms or molecules in a process called avalanche breakdown. Breakdown occurs quite abruptly (typically in nanoseconds)., resulting in the formation of an electrically conductive path and a disruptive discharge through the material. For solid materials, a breakdown event severely degrades, or even destroys, its insulating capability.

Factors affecting dielectric strength

  • it increases with the increase in thickness of the specimen. (Directly proportional)
  • it decreases with the increase in operating temperature. (Inversely proportional)
  • it decreases with the increase in frequency. (Inversely proportional)
  • it decreases with the increase in humidity. (Inversely proportional)

Contents

Breakdown field strength

The field strength at which breakdown occurs depends on the respective geometries of the dielectric (insulator) and the electrodes with which the electric field is applied, as well as the rate of increase at which the electric field is applied. Because dielectric materials usually contain minute defects, the practical dielectric strength will be a fraction of the intrinsic dielectric strength of an ideal, defect-free, material. Dielectric films tend to exhibit greater dielectric strength than thicker samples of the same material. For instance, the dielectric strength of silicon dioxide films of a few hundred nm to a few μm thick is approximately 0.1 MV/m[citation needed]. However very thin layers ( below, say, 100nm ) become partially conductive because of electron tunneling. Multiple layers of thin dielectric films are used where maximum practical dielectric strength is required, such as high voltage capacitors and pulse transformers.

Dielectric strength (in MV/m) of various common materials:

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See also

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