Dielectric strength

Electrical breakdown

Electric current is a flow of electrically charged particles in a material caused by an electric field. The mobile charged particles responsible for electric current are called charge carriers. In different substances different particles serve as charge carriers: in metals and other solids some of the outer electrons of each atom (conduction electrons) are able to move about the material; in electrolytes and plasma it is ions, electrically charged atoms or molecules, and electrons. A substance that has a high concentration of charge carriers available for conduction will conduct a large current with the given electric field created by a given voltage applied across it, and thus has a low electrical resistivity; this is called an electrical conductor. A material that has few charge carriers will conduct very little current with a given electric field and has a high resistivity; this is called an electrical insulator.

However, when a large enough electric field is applied to any insulating substance, at a certain field strength the concentration of charge carriers in the material suddenly increases by many orders of magnitude, so its resistance drops and it becomes a conductor. This is called electrical breakdown. The physical mechanism causing breakdown differs in different substances. In a solid, it usually occurs when the electric field becomes strong enough to pull outer valence electrons away from their atoms, so they become mobile. The field strength at which break down occurs is an intrinsic property of the material called its dielectric strength.

In practical electric circuits electrical breakdown is often an unwanted occurrence, a failure of insulating material causing a short circuit, resulting in a catastrophic failure of the equipment. The sudden drop in resistance causes a high current to flow through the material, and the sudden extreme Joule heating may cause the material or other parts of the circuit to melt or vaporize explosively. However, breakdown itself is reversible. If the current supplied by the external circuit is sufficiently limited, no damage is done to the material, and reducing the applied voltage causes a transition back to the material's insulating state.

Factors affecting apparent dielectric strength

• It may vary with sample thickness.{{cite web |author =DuPont Teijin Films
• It may vary with operating temperature.
• It may vary with frequency.
• For gases (e.g. nitrogen, sulfur hexafluoride) it normally decreases with increased humidity as ions in water can provide conductive channels.
• For gases it increases with pressure according to Paschen's law
• For air, dielectric strength increases slightly as the absolute humidity increases but decreases with an increase in relative humidity

Break down field strength

The field strength at which break down 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 | display-authors=et al.

Dielectric strength (in MV/m, or 10 V/m) of various common materials:

Units

In SI, the unit of dielectric strength is volts per meter (V/m). It is also common to see related units such as volt per centimeter (V/cm), megavolts per meter (MV/m), and so on.

In United States customary units, dielectric strength is often specified in volt per mil (a mil is 1/1000 inch). The conversion is: :\begin{align} 1 \text{ V/mil} &= 3.94\times 10^{4} \text{ V/m} \ 1 \text{ V/m} &= 2.54\times 10^{-5} \text{ V/mil} \end{align}

References

References

1. (1932). "Durchschlagfeldstärke des homogenen Feldes in Luft". Archiv für Elektrotechnik.
2. ''[[CRC Handbook of Chemistry and Physics]]''
3. Föll, H.. "3.5.1 Electrical Breakdown and Failure". Tf.uni-kiel.de.
4. "[https://www.azom.com/properties.aspx?ArticleID=263 Azom Materials - Beryllium Oxide Properties]". azom.com. Retrieved 2023-12-05.
5. "HV dielectric strength of shielding electrodes in vacuum circuit-breakers".
6. Glenn Elert. "Dielectrics - The Physics Hypertextbook". Physics.info.
7. "Electronic properties of diamond". el.angstrom.uu.se.
8. (November 2006). "Past the Schwinger limit". Nature Physics.
9. (November 2010). "On the Schwinger limit attainability with extreme power lasers". Phys. Rev. Lett..
10. For one of many examples, see ''Polyimides: materials, processing and applications'', by A.J. Kirby, [https://books.google.com/books?id=N7EigauKuTIC&pg=PA19 google books link]