Dielectric strength

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

• Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties.
• For a given configuration of dielectric material and electrodes, the minimum electric field that produces breakdown.
• the maximum electric stress the dielectric material can withstand without 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. 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 dielectric strength

• it increases with the increase in thickness of the specimen. (Directly proportional)
• it decreases with the increase in operating temperature. (Inversely proportional)
• it decreases with the increase in frequency. (Inversely proportional)
• it decreases with the increase in humidity. (Inversely proportional)

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.^[1] 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 10^6 Volts/meter) of various common materials:

Substance	Dielectric Strength (MV/m)
Helium (relative to nitrogen)[2]	&10000000000000000150000 0.15
Air (relative to nitrogen)[3]	&10000000000000003000000 0.97[4]
Alumina[2]	&10000000000000013400000 13.4
Window glass[2]	&10000000000000011800000 9.8 - 13.8
Silicone oil, Mineral oil[2][5]	&10000000000000012500000 10 - 15
Benzene[2]	&10000000000000016000000 163
Polystyrene[2]	&10000000000000019699999 19.7
Polyethylene[6]	&10000000000000020300000 18.9 - 21.7
Neoprene rubber[2]	&10000000000000021649999 15.7 - 26.7
Distilled Water[2]	&10000000000000030000000 65 - 70
High Vacuum (field emission limited)[7]	&10000000000000030000000 20 - 40 (depends on electrode shape)
Fused silica[8]	&10000000000000032500000 25–40 at 20 °C
Waxed paper[9]	&10000000000000050000000 40 - 60
PTFE (Teflon, Extruded )[2]	&10000000000000060000000 19.7
PTFE (Teflon, Insulating Film)[2][10]	&10000000000000060000000 60 - 173
Mica[2]	&10000000000000045000000 118

See also

• Relative permittivity
• Rotational Brownian motion

References

1. ^ "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. http://onlinelibrary.wiley.com/doi/10.1002/pssa.200880481/pdf. Retrieved 2011-11-08. 
2. ^ ^{a b c d e f g h i j k} CRC Handbook of Chemistry and Physics
3. ^ [1]^{[dead link]}
4. ^ http://blazelabs.com/l-vacuum.asp
5. ^ "3.5.1 Electrical Breakdown and Failure". Tf.uni-kiel.de. http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_5_1.html. Retrieved 2011-11-08. 
6. ^ "Dielectric Strength of Polyethylene". Hypertextbook.com. http://hypertextbook.com/facts/2009/CherryXu.shtml. Retrieved 2011-11-08. 
7. ^ http://www.htee.tu-bs.de/forschung/veroeffentlichungen/giere2002.pdf
8. ^ Fused silica datapage
9. ^ "Dielectric Strength of Waxed Paper". Hypertextbook.com. http://hypertextbook.com/facts/2007/DashaMulyukova.shtml. Retrieved 2011-11-08. 
10. ^ Glenn Elert. "Dielectrics - The Physics Hypertextbook". Physics.info. http://physics.info/dielectrics/. Retrieved 2011-11-08. 

•  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

External links

• 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