Dielectric strength

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 conducting 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 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 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 decreases 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. 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)	0.15
Air	3.0
Alumina	13.4
Window glass	9.8 - 13.8
Borosilicate glass	20 - 40
Silicone oil, mineral oil	10 - 15
Benzene	163
Polystyrene	19.7
Polyethylene	19 - 160
Neoprene rubber	15.7 - 26.7
Distilled water	65 - 70
High vacuum (field emission limited)	20 - 40 (depends on electrode shape)
Fused silica	25–40 at 20 °C
Waxed paper	40 - 60
PTFE (Teflon, extruded )	19.7
PTFE (Teflon, insulating film)	60 - 173
Mica	118
Diamond	2000
PZT	10–25
Vacuum	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). The conversion is:

$1{\text{ V/m}}=2.54\times 10^{-5}{\text{ V/mil}}$

$1{\text{ V/mil}}=3.94\times 10^{4}{\text{ V/m}}$

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

•  This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

External links

• Dielectric Strength of Air (with multiple references)
• 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