View source for Dielectric strength

{{Short description|Degree of insulation}}
In [[physics]], the term '''dielectric strength''' has the following meanings:

*for a pure [[electrical insulator|electrically insulating]] material, the maximum [[electric field]] that the material can withstand under ideal conditions without undergoing [[electrical breakdown]] and becoming electrically conductive (i.e. without failure of its insulating properties).
*For a specific piece of dielectric material and location of [[electrode]]s, 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.

==Electrical breakdown==
[[Electric current]] is a flow of electrically [[charged particle]]s in a material caused by an [[electric field]].  The mobile charged particles responsible for electric current are called [[charge carrier]]s.  In different substances different particles serve as charge carriers: in metals and other solids some of the outer [[electron]]s of each atom ([[conduction electron]]s) are able to move about the material; in [[electrolyte]]s and [[plasma (physics)|plasma]] it is [[ion]]s, electrically charged [[atom]]s or [[molecule]]s, 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 electron]]s 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 circuit]]s 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.<ref>{{cite web |author =DuPont Teijin Films
|title =Mylar polyester film |year = 2003 |url =http://usa.dupontteijinfilms.com/informationcenter/downloads/Electrical_Properties.pdf}}</ref> (see "defects" below)
*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<ref>{{cite journal|title=Durchschlagfeldstärke des homogenen Feldes in Luft |date=1932 |doi=10.1007/BF01657189 |volume=26 |issue=4 |journal=Archiv für Elektrotechnik |pages=219–232|last1=Ritz |first1=Hans |s2cid=108697400 }}</ref>

== 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 [[micron|μm]] is about 0.5{{nbsp}}GV/m.<ref>
{{cite journal
 | title=Electrical insulation properties of sputter-deposited SiO<sub>2</sub>, Si<sub>3</sub>N<sub>4</sub> and Al<sub>2</sub>O<sub>3</sub> films at room temperature and 400 °C
 | date=2009-01-21
 | doi=10.1002/pssa.200880481
 | volume=206
 | issue=3
 | journal=Physica Status Solidi A
 | pages=514–519
 | bibcode=2009PSSAR.206..514B
|last1 = Bartzsch|first1 = Hagen| last2=Glöß
 | first2=Daniel
 | last3=Frach
 | first3=Peter
 | last4=Gittner
 | first4=Matthias
 | last5=Schultheiß
 | first5=Eberhard
 | last6=Brode
 | first6=Wolfgang
 | last7=Hartung
 | first7=Johannes
 | s2cid=93228294
 }}</ref> However very thin layers (below, say, {{nowrap|100 nm}}) become partially conductive because of [[electron tunneling]].{{clarify|reason=does this not seem to contradict the above 'films tend to exhibit greater dielectric strength than thicker samples of the same material.' since samples 'a few hundred nm to a few μm thick is approximately 0.5 GV/m' and samples less than '100nm are partially conductive...'|date=November 2018}} Multiple layers of thin dielectric films are used where maximum practical dielectric strength is required, such as high voltage [[capacitor]]s and pulse [[transformer]]s. Since the dielectric strength of gases varies depending on the shape and configuration of the electrodes,<ref>
{{cite journal
 | last1=Lyon | first1=David
 |display-authors=et al.
 | title=Gap size dependence of the dielectric strength in nano vacuum gaps
 | journal=IEEE
 | volume=20
 | issue=4
 | pages=1467–1471
 | date=2013
 | doi=10.1109/TDEI.2013.6571470
| s2cid=709782
 }}</ref> it is usually measured as a fraction of the dielectric strength of [[nitrogen gas]].

Dielectric strength (in MV/m, or 10{{sup|6}}&sdot;volt/meter) of various common materials:

{| class="wikitable sortable"
|-
! Substance
! data-sort-type=number | Dielectric strength<br>(MV/m) or (Volts/micron)
|-
| [[Helium]] (relative to nitrogen)<ref name="CRC">''[[CRC Handbook of Chemistry and Physics]]''</ref><br>{{clarify|date=January 2019}}
| {{nts|0.15}}
|-
| [[Air]]<ref>{{cite web
| url=https://hypertextbook.com/facts/2000/AliceHong.shtml
| title=Dielectric Strength of Air
| first=Alice
| last=Hong
| year=2000
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| access-date=2020-06-18
}}</ref>
| {{nts|3}}
|-
| [[Sulfur hexafluoride]]<ref name="CRC"/>
| {{ntsh|9.15}}8.5–9.8
|-
| [[Alumina]]<ref name="CRC"/>
| {{nts|13.4}}
|-
| Window [[glass]]<ref name="CRC"/>
| {{ntsh|11.8}} 9.8–13.8
|-
| [[Borosilicate glass]]<ref name="CRC"/>
| {{ntsh|30}} 20–40
|-
| [[Silicone oil]], [[mineral oil]]<ref name="CRC"/><ref>{{cite web| last =Föll |first =H. |url=http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_5_1.html |title=3.5.1 Electrical Breakdown and Failure |publisher=Tf.uni-kiel.de |access-date=2020-06-18}}</ref>
| {{ntsh|12.5}} 10–15
|-
| [[Benzene]]<ref name="CRC"/>
| {{nts|163}}
|-
| [[Polystyrene]]<ref name="CRC"/>
| {{nts|19.7}}
|-
| [[Polyethylene]]<ref>{{cite web
| url=https://hypertextbook.com/facts/2009/CherryXu.shtml
| title=Dielectric strength of polyethylene
| first=Cherry
| last=Xu
| year=2009
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| access-date=2020-06-18
}}</ref>
| {{ntsh|20.3}} 19–160
|-
| [[Neoprene]] rubber<ref name="CRC"/>
| {{ntsh|21.65}} 15.7–26.7
|-
| Distilled [[water]]<ref name="CRC"/>
| {{ntsh|67.5}} 65–70
|-
|-
| [[Beryllium oxide]]<ref>"[https://www.azom.com/properties.aspx?ArticleID=263 Azom Materials - Beryllium Oxide Properties]". azom.com. Retrieved 2023-12-05.</ref>
| {{ntsh|29}} 27-31
|-
| High [[vacuum]] (200 [[Pascal (unit)|μPa]])<br>(field emission limited)<ref>{{Cite conference |conference =20th International Symposium on Discharges and Electrical Insulation in Vacuum |url=http://www.htee.tu-bs.de/forschung/veroeffentlichungen/giere2002.pdf |title=HV dielectric strength of shielding electrodes in vacuum circuit-breakers |last1=Giere |first1=Stefan |last2=Kurrat |first2=Michael  |last3= Schümann |first3=Ulf  | access-date=2020-06-18 |archive-url=https://web.archive.org/web/20120301112907/http://www.htee.tu-bs.de/forschung/veroeffentlichungen/giere2002.pdf |archive-date=2012-03-01 |url-status=dead }}</ref>
| {{ntsh|30}} 20–40<br>(depends on electrode shape)
|-
| [[Fused silica]]<ref name="CRC"/>
| {{ntsh|570}} 470–670
|-
| Waxed paper<ref>{{cite web
| url=https://hypertextbook.com/facts/2007/DashaMulyukova.shtml
| title=Dielectric strength of waxed paper
| first=Dasha
| last=Mulyakhova
| year=2007
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| access-date=2020-06-18
}}</ref>
| {{ntsh|50}} 40–60
|-
| [[PTFE]] (Teflon, [[Extrusion|extruded]] )<ref name="CRC"/>
| {{nts|19.7}}
|-
| [[PTFE]] (Teflon, insulating film)<ref name="CRC"/><ref>{{cite web|author=Glenn Elert |url=https://physics.info/dielectrics/ |title=Dielectrics - The Physics Hypertextbook |publisher=Physics.info |access-date=2020-06-18}}</ref>
| {{ntsh|116.5}} 60–173
|-
| [[PEEK]] (Polyether ether ketone)
| {{nts|23}}
|-
| [[Mica]]<ref name="CRC"/>
| {{nts|118}}
|-
| [[Diamond]]<ref>{{cite web|url=https://www.researchgate.net/publication/267937297|title=Electronic properties of diamond|publisher=el.angstrom.uu.se|access-date=2013-08-10}}</ref>
| {{nts|2000}}
|-
| [[PZT]]
| {{ntsh|17}} 10–25<ref>
{{cite journal
 | title = Electrical Characteristics of Ferroelectric PZT Thin Films for DRAM Applications
 | last = Moazzami | first = Reza |author2=Chenming Hu |author3=William H. Shepherd
 | journal = IEEE Transactions on Electron Devices
 | date=September 1992 
 | volume = 39 | issue = 9 | page = 2044
 | url = http://www.eecs.berkeley.edu/~hu/PUBLICATIONS/Hu_papers/Hu_JNL/HuC_JNL_114.pdf
 | bibcode = 1992ITED...39.2044M | doi = 10.1109/16.155876
}}</ref><ref>
{{cite journal 
 | url = https://www.researchgate.net/publication/237514139
 | title = Performance of Piezoelectric Ceramic Multilayer Components Based on Hard and Soft PZT
 | author = B. Andersen
 | author2 = E. Ringgaard
 | author3 = T. Bove
 | author4 = A. Albareda
 | author5 = R. Pérez
 | name-list-style = amp
 | journal = Proceedings of Actuator 2000
 | year = 2000
 | pages = 419–422
}}</ref>
|-
| [[Perfect vacuum]]
| [[Schwinger limit|10<sup>12</sup>]]
|}

==Units==
In [[SI]], the unit of dielectric strength is [[volt]]s 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 [[Thou (length)|mil]] (a mil is 1/1000 [[inch]]).<ref>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]</ref> The conversion is:
:<math>\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}</math>

== See also ==
* [[Breakdown voltage]]
* [[Relative permittivity]]
* [[Rotational Brownian motion]]
* [[Paschen's law]] - variation of dielectric strength of gas related to pressure
* [[Electrical treeing]]
* [[Lichtenberg figure]]

==References==
{{Reflist|30em}}
*{{FS1037C MS188}}

==External links==
* [http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1474271 Article "The maximum dielectric strength of thin silicon oxide films" from ''IEEE Transactions on Electron Devices'']

[[Category:Electricity]]