Dielectric strength

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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.[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 106 Volt/meter) of various common materials:

Substance Dielectric Strength (MV/m)
Helium (relative to nitrogen)[2] Template:Ntsh 0.15
Air [3] Template:Ntsh 3.0
Alumina[2] Template:Ntsh 13.4
Window glass[2] Template:Ntsh 9.8 - 13.8
Silicone oil, mineral oil[2][4] Template:Ntsh 10 - 15
Benzene[2] Template:Ntsh 163
Polystyrene[2] Template:Ntsh 19.7
Polyethylene[5] Template:Ntsh 19 - 160
Neoprene rubber[2] Template:Ntsh 15.7 - 26.7
Distilled water[2] Template:Ntsh 65 - 70
High vacuum (field emission limited)[6] Template:Ntsh 20 - 40 (depends on electrode shape)
Fused silica[7] Template:Ntsh 25–40 at 20 °C
Waxed paper[8] Template:Ntsh 40 - 60
PTFE (Teflon, extruded )[2] Template:Ntsh 19.7
PTFE (Teflon, insulating film)[2][9] Template:Ntsh 60 - 173
Mica[2] Template:Ntsh 118
Diamond[10] Template:Ntsh 2000
PZT 10–25[11] [12]
Vacuum Template:Ntsh 1012

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).[13] The conversion is:

See also

References

  1. Template:Cite web
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 CRC Handbook of Chemistry and Physics
  3. http://hypertextbook.com/facts/2000/AliceHong.shtml
  4. Template:Cite web
  5. Template:Cite web
  6. http://www.htee.tu-bs.de/forschung/veroeffentlichungen/giere2002.pdf
  7. Fused silica datapage
  8. Template:Cite web
  9. Template:Cite web
  10. Template:Cite web
  11. {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  12. {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  13. For one of many examples, see Polyimides: materials, processing and applications, by A.J. Kirby, google books link

External links