In physics, the term dielectric strength has the following meanings:
- Of an insulating material, the maximum electric field that a pure material can withstand under ideal conditions without breaking down (i.e., without experiencing 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.
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. The "intrinsic dielectric strength" is 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 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 apparent dielectric strength
- it increases slightly 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 humidity increases
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 0.5GV/m. However very thin layers (below, say, 100 nm) 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. 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:
|Substance||Dielectric Strength (MV/m)|
|Helium (relative to nitrogen)||0.15|
|Window glass||9.8 - 13.8|
|Silicone oil, Mineral oil||10 - 15|
|Polyethylene||18.9 - 21.7|
|Neoprene rubber||15.7 - 26.7|
|Distilled Water||65 - 70|
|High Vacuum (field emission limited)||20 - 40 (depends on electrode shape)|
|Fused silica||25–40 at 20 °C|
|Waxed paper||40 - 60|
|PTFE (Teflon, Extruded )||19.7|
|PTFE (Teflon, Insulating Film)||60 - 173|
- "Electrical insulation properties of sputter-deposited SiO2, Si3N4 and Al2O3 films at room temperature and 400 °C - Bartzsch - 2009 - physica status solidi (a) - Wiley Online Library". Onlinelibrary.wiley.com. 2009-01-21. Retrieved 2011-11-08.
- CRC Handbook of Chemistry and Physics
- "3.5.1 Electrical Breakdown and Failure". Tf.uni-kiel.de. Retrieved 2011-11-08.
- "Dielectric Strength of Polyethylene". Hypertextbook.com. Retrieved 2011-11-08.
- Fused silica datapage
- "Dielectric Strength of Waxed Paper". Hypertextbook.com. Retrieved 2011-11-08.
- Glenn Elert. "Dielectrics - The Physics Hypertextbook". Physics.info. Retrieved 2011-11-08.
- "Electronic properties of diamond". el.angstrom.uu.se. Retrieved 2013-08-10.
- For one of many examples, see Polyimides: materials, processing and applications, by A.J. Kirby, google books link
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
- 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