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 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 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)	6999150000000000000♠ 0.15
Air	7000300000000000000♠ 3.0
Alumina	7001134000000000000♠ 13.4
Window glass	7001118000000000000♠ 9.8 - 13.8
Silicone oil, mineral oil	7001125000000000000♠ 10 - 15
Benzene	7001160000000000000♠ 163
Polystyrene	7001197000000000000♠ 19.7
Polyethylene	7001203000000000000♠ 19 - 160
Neoprene rubber	7001216500000000000♠ 15.7 - 26.7
Distilled water	7001675000000000000♠ 65 - 70
High vacuum (field emission limited)	7001300000000000000♠ 20 - 40 (depends on electrode shape)
Fused silica	7001325000000000000♠ 25–40 at 20 °C
Waxed paper	7001500000000000000♠ 40 - 60
PTFE (Teflon, extruded )	7001197000000000000♠ 19.7
PTFE (Teflon, insulating film)	7002116500000000000♠ 60 - 173
Mica	7002118000000000000♠ 118
Diamond	7003200000000000000♠ 2000
PZT	10–25
Vacuum	7003200000000000000♠ 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

References

•  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 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