# Dielectric strength

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

## Contents

• 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 called 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 strength

• It decreases with increased sample thickness. [1] (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. [2] [ citation needed ]
• For air, dielectric strength increases slightly as the absolute humidity increases but decreases with an increase in relative humidity [3]

## 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 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. [4] 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, [5] it is usually measured as a fraction of the dielectric strength of nitrogen gas.

Dielectric strength (in MV/m, or 106volt/meter) of various common materials:

SubstanceDielectric strength
(MV/m)
Helium (relative to nitrogen) [6]
[ clarification needed ]
0.15
Air [7] 3.0
Alumina [6] 13.4
Window glass [6]
Borosilicate glass [6]
Silicone oil, mineral oil [6] [8]
Benzene [6] 163
Polystyrene [6] 19.7
Polyethylene [9]
Neoprene rubber [6]
Distilled water [6]
High vacuum
(field emission limited) [10]
Fused silica [6]
Waxed paper [11]
PTFE (Teflon, extruded ) [6] 19.7
PTFE (Teflon, insulating film) [6] [12]
Mica [6] 118
Diamond [13] 2,000
PZT

## Units

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

In United States customary units, dielectric strength is often specified in volts per mil (a mil is 1/1000 inch). [16] The conversion is:

{\displaystyle {\begin{aligned}1{\text{ V/m}}&=2.54\times 10^{-5}{\text{ V/mil}}\\1{\text{ V/mil}}&=3.94\times 10^{4}{\text{ V/m}}\end{aligned}}}

## References

1. "Vacuum Circuit Breaker - PowerPoint Slides". www.learnpick.in. Retrieved 2018-01-22.
2. Ritz, Hans (1932). "Durchschlagfeldstärke des homogenen Feldes in Luft" (PDF). Archiv für Elektrotechnik. 26 (4): 219–232. doi:10.1007/BF01657189 . Retrieved 2017-06-29.
3. Bartzsch, Hagen; Glöß, Daniel; Frach, Peter; Gittner, Matthias; Schultheiß, Eberhard; Brode, Wolfgang; Hartung, Johannes (2009-01-21). "Electrical insulation properties of sputter-deposited SiO2, Si3N4 and Al2O3 films at room temperature and 400 °C". Physica Status Solidi A. 206 (3): 514–519. Bibcode:2009PSSAR.206..514B. doi:10.1002/pssa.200880481.
4. Lyon, David; et al. (2013). "Gap size dependence of the dielectric strength in nano vacuum gaps". IEEE. 20 (4): 1467–1471. doi:10.1109/TDEI.2013.6571470.
5. CRC Handbook of Chemistry and Physics
6. "3.5.1 Electrical Breakdown and Failure". Tf.uni-kiel.de. Retrieved 2011-11-08.
7. "Dielectric Strength of Polyethylene". Hypertextbook.com. Retrieved 2011-11-08.
8. "Dielectric Strength of Waxed Paper". Hypertextbook.com. Retrieved 2011-11-08.
9. Glenn Elert. "Dielectrics - The Physics Hypertextbook". Physics.info. Retrieved 2011-11-08.
10. "Electronic properties of diamond". el.angstrom.uu.se. Retrieved 2013-08-10.
11. Moazzami, Reza; Chenming Hu; William H. Shepherd (September 1992). "Electrical Characteristics of Ferroelectric PZT Thin Films for DRAM Applications" (PDF). IEEE Transactions on Electron Devices. 39 (9): 2044. Bibcode:1992ITED...39.2044M. doi:10.1109/16.155876.
12. B. Andersen; E. Ringgaard; T. Bove; A. Albareda & R. Pérez (2000). "Performance of Piezoelectric Ceramic Multilayer Components Based on Hard and Soft PZT". Proceedings of Actuator 2000: 419–422.
13. For one of many examples, see Polyimides: materials, processing and applications, by A.J. Kirby, google books link