, the term dielectric strength has the following meanings:
*for a pure electrically insulating
material, the maximum electric field
that the material can withstand under ideal conditions without undergoing electrical breakdown
and becoming electrically conductive (i.e. without failure of its insulating properties).
*For a specific piece of dielectric material and location of electrode
s, the minimum applied electric field (i.e. the applied voltage divided by electrode separation distance) that results in breakdown. This is the concept of breakdown voltage
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 be accelerated to velocities that can liberate additional electrons by collisions with neutral atoms or molecules, in a process known as 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. In a solid material, a breakdown event severely degrades, or even destroys, its insulating capability.
is a flow of electrically charged particle
s in a material caused by an electric field
. The mobile charged particles responsible for electric current are called charge carrier
s. In different substances different particles serve as charge carriers: in metals and other solids some of the outer electron
s of each atom (conduction electron
s) are able to move about the material; in electrolyte
s and plasma
it is ion
s, electrically charged atom
s or molecule
s, and electrons. A substance that has a high concentration of charge carriers available for conduction will conduct a large current with the given electric field created by a given voltage
applied across it, and thus has a low electrical resistivity
; this is called an electrical conductor
. A material that has few charge carriers will conduct very little current with a given electric field and has a high resistivity; this is called an electrical insulator
However when a large enough electric field is applied to any insulating substance, at a certain field strength the concentration of charge carriers in the material suddenly increases by many orders of magnitude, so its resistance drops and it becomes a conductor. This is called ''electrical breakdown''. The physical mechanism causing breakdown differs in different substances. In a solid, it usually occurs when the electric field becomes strong enough to pull outer valence electron
s away from their atoms, so they become mobile. The field strength at which breakdown occurs is an intrinsic property of the material called its ''dielectric strength''.
In practical electric circuit
s electrical breakdown is often an unwanted occurrence, a failure of insulating material causing a short circuit
, resulting in a catastrophic failure of the equipment. The sudden drop in resistance causes a high current to flow through the material, and the sudden extreme Joule heating
may cause the material or other parts of the circuit to melt or vaporize explosively. However, breakdown itself is reversible. If the current supplied by the external circuit is sufficiently limited, no damage is done to the material, and reducing the applied voltage causes a transition back to the material's insulating state.
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 as ions in water can provide conductive channels.
*For gases it increases with pressure according to Paschen's law
*For air, dielectric strength increases slightly as the absolute humidity increases but decreases with an increase in relative humidity
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 of the applied electric field. Because dielectric materials usually contain minute defects, the practical dielectric strength will be a significantly less than 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 thickness around 1 μm
is about 0.5GV/m. However very thin layers (below, say, ) 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 10⋅volt/meter) of various common materials:
, the unit of dielectric strength is volt
s 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
[For one of many examples, see ''Polyimides: materials, processing and applications'', by A.J. Kirby]
google books link
/ref> The conversion is:
* Breakdown voltage
* Relative permittivity
* Rotational Brownian motion
* Paschen's law - variation of dielectric strength of gas related to pressure
* Electrical treeing
* Lichtenberg figure
Article "The maximum dielectric strength of thin silicon oxide films" from ''IEEE Transactions on Electron Devices''