Demand factor

2013-11-28T12:59:19Z

180.234.78.55: /* Electrical engineering */

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.<ref>{{cite web|url=http://onlinelibrary.wiley.com/doi/10.1002/pssa.200880481/pdf |title=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 |publisher=Onlinelibrary.wiley.com |date=2009-01-21 |accessdate=2011-11-08}}</ref> However very thin layers (below, say, {{nowrap|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 [[capacitor]]s and pulse [[transformer]]s. 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:

{| class="wikitable sortable"
|-
! Substance
! Dielectric Strength (MV/m)
|-
| [[Helium]] (relative to nitrogen)<ref name="CRC">''[[CRC Handbook of Chemistry and Physics]]''</ref>
| {{ntsh|0.15}} 0.15
|-
| [[Air]] <ref>http://hypertextbook.com/facts/2000/AliceHong.shtml</ref>
| {{ntsh|3.0}} 3.0
|-
| [[Alumina]]<ref name="CRC"/>
| {{ntsh|13.4}} 13.4
|-
| Window [[glass]]<ref name="CRC"/>
| {{ntsh|11.8}} 9.8 - 13.8
|-
| [[Silicone oil]], [[Mineral oil]]<ref name="CRC"/><ref>{{cite web|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 |date= |accessdate=2011-11-08}}</ref>
| {{ntsh|12.5}} 10 - 15
|-
| [[Benzene]]<ref name="CRC"/>
| {{ntsh|16}} 163
|-
| [[Polystyrene]]<ref name="CRC"/>
| {{ntsh|19.7}} 19.7
|-
| [[Polyethylene]]<ref>{{cite web|url=http://hypertextbook.com/facts/2009/CherryXu.shtml |title=Dielectric Strength of Polyethylene |publisher=Hypertextbook.com |date= |accessdate=2011-11-08}}</ref>
| {{ntsh|20.3}} 18.9 - 21.7
|-
| [[Neoprene]] rubber<ref name="CRC"/>
| {{ntsh|21.65}} 15.7 - 26.7
|-
| Distilled [[Water]]<ref name="CRC"/>
| {{ntsh|67.5}} 65 - 70
|-
| High [[Vacuum]] (field emission limited)<ref>http://www.htee.tu-bs.de/forschung/veroeffentlichungen/giere2002.pdf</ref>
| {{ntsh|30}} 20 - 40 (depends on electrode shape)
|-
| [[Fused silica]]<ref>[http://www.sciner.com/Opticsland/FS.htm Fused silica datapage]</ref>
| {{ntsh|32.5}} 25–40 at 20 °C
|-
| Waxed paper<ref>{{cite web|url=http://hypertextbook.com/facts/2007/DashaMulyukova.shtml |title=Dielectric Strength of Waxed Paper |publisher=Hypertextbook.com |date= |accessdate=2011-11-08}}</ref>
| {{ntsh|50}} 40 - 60
|-
| [[PTFE]] (Teflon, [[Extrusion|Extruded]] )<ref name="CRC"/>
| {{ntsh|19.7}} 19.7
|-
| [[PTFE]] (Teflon, Insulating Film)<ref name="CRC"/><ref>{{cite web|author=Glenn Elert |url=http://physics.info/dielectrics/ |title=Dielectrics - The Physics Hypertextbook |publisher=Physics.info |date= |accessdate=2011-11-08}}</ref>
| {{ntsh|116.5}} 60 - 173
|-
| [[Mica]]<ref name="CRC"/>
| {{ntsh|118}} 118
|-
| [[Diamond]]<ref>{{cite web|url=http://www.el.angstrom.uu.se/Seminar/MGabrysch_Lic.pdf |title=Electronic properties of diamond |publisher=el.angstrom.uu.se |date= |accessdate=2013-08-10}}</ref>
| {{ntsh|2000}} 2000
|-
| [[Vacuum]]
| {{ntsh|2000}} [[Schwinger limit|1012]]
|}

==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 [[volt]]s per [[centimeter]] (V/cm), [[volt|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, [http://books.google.com/books?id=N7EigauKuTIC&pg=PA19 google books link]</ref> The conversion is:
:<math>1 \text{ V/m} = 2.54\times 10^{-5} \text{ V/mil}</math>
:<math>1 \text{ V/mil} = 3.94\times 10^{4} \text{ V/m}</math>

== See also ==
* [[Breakdown voltage]]
* [[Relative permittivity]]
* [[Rotational Brownian motion]]

==References==
<references/>
*{{FS1037C MS188}}

==External links==
* [http://hypertextbook.com/facts/2000/AliceHong.shtml Dielectric Strength of Air (with multiple references) ]
* [http://www.isomil.de/en/thermocouple-dielectric-strength.htm Dielectric Strength and Insulation Materials of Mineral Insulated Cable]
* [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'']
* [http://semiconductorglossary.com/default.asp?searchterm=silicon+dioxide%2C+SiO2 Properties of silicon dioxide and silicon nitride, from semiconductorglossary.com]

[[Category:Electricity]]

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Demand factor