In physics, the term dielectric strength has the following meanings:
- Of a pure insulating material, the maximum electric field that the material can withstand under ideal conditions without breaking down (i.e. without failure of its insulating properties).
- For a specific configuration of dielectric material and electrodes, the minimum applied electric field (i.e. the applied voltage divided by electrode separation distance) that results in breakdown. This is the concept of breakdown voltage.
The theoretical dielectric strength of a material is an intrinsic property of the bulk material, and is independent of the configuration of the material or the electrodes with which the field is applied. This "intrinsic dielectric strength" corresponds to what would be measured using pure materials under ideal laboratory conditions. At breakdown, the electric field frees bound electrons. If the applied electric field is sufficiently high, free electrons from background radiation may be accelerated to velocities that can liberate additional electrons by collisions with neutral atoms or molecules, in a process known as 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. In a solid material, a breakdown event severely degrades, or even destroys, its insulating capability.
Factors affecting apparent dielectric strengthEdit
- It decreases with increased sample thickness. (see "defects" below)
- It decreases with increased operating temperature.
- It decreases with increased frequency.
- For gases (e.g. nitrogen, sulfur hexafluoride) it normally decreases with increased humidity.
- For air, dielectric strength increases slightly as the absolute humidity increases but decreases with an increase in relative humidity
Breakdown field strengthEdit
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 of the applied electric field. Because dielectric materials usually contain minute defects, the practical dielectric strength will be a significantly less than 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 thickness around 1 μm is about 0.5 GV/m. However very thin layers (below, say, 100 nm) become partially conductive because of electron tunneling.[clarification needed] Multiple layers of thin dielectric films are used where maximum practical dielectric strength is required, such as high voltage capacitors and pulse transformers. Since the dielectric strength of gases varies depending on the shape and configuration of the electrodes, it is usually measured as a fraction of the dielectric strength of nitrogen gas.
Dielectric strength (in MV/m, or 106⋅volt/meter) of various common materials:
|Helium (relative to nitrogen)
|Sulfur hexafluoride (uniform electric field)||7|
|Sulfur hexafluoride (non-uniform electric field)||9|
|Silicone oil, mineral oil||10–15|
(field emission limited)
(depends on electrode shape)
|PTFE (Teflon, extruded )||19.7|
|PTFE (Teflon, insulating film)||60–173|
|PEEK (Polyether ether ketone)||23|
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- CRC Handbook of Chemistry and Physics
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- For one of many examples, see Polyimides: materials, processing and applications, by A.J. Kirby, google books link
- Dielectric Strength of Air (with multiple references)
- Dielectric Strength and Insulation Materials of Mineral Insulated Cable
- Article "The maximum dielectric strength of thin silicon oxide films" from IEEE Transactions on Electron Devices
- Properties of silicon dioxide and silicon nitride, from semiconductorglossary.com