Dielectric strength

In physics, 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 electrodes, 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.

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In most capacitors, a non-conducting material is placed between the two metal pieces that make up that capacitor. There's two reasons for this. For one, the non-conducting material prevents the pieces of metal from touching each other, which is important because if the pieces of metal were touching, no charge would ever get stored since you've completed the circuit. But there's another bonus to inserting a non-conducting material between the plates of a capacitor. It will always increase the capacitance of that capacitor. As long as the material is non-conducting, it doesn't even matter what it is. As long as you don't change the area or separation between the plates, inserting a non-conducting material will always increase the capacitance. The name we give to non-conducting materials place between capacitor plates is a dielectric. But why does a dielectric increase the capacitance? To find out, let's look at this example. When you hook up a battery of voltage V to a capacitor, charge will get separated. Now let's say you remove the battery. The charge is stuck on the plate since the negatives don't have a path in which to get back to the positives. So even after removing the battery, the charge on the plates is going to remain the same. And the voltage will also remain the same as the voltage of the battery that charged it up. Now imagine placing a dielectric in between the plates of the capacitor. The dielectric material is made out of atoms and molecules, and when placed in between the plates of this charged up capacitor, the negative charges in the dielectric are going to get attracted to the positive plate of the capacitor. But those negatives can't travel to the positive plate since this dielectric is a non-conducting material. However, the negatives can shift or lean towards the positive plate. This causes the charge in the atoms and molecules within the dielectric to become polarized. To put it another way, the atom kind of stretches and one end becomes overall negative and the other end becomes overall positive. It's also possible that the dielectric material started off polarized because some molecules are just naturally polarized like water. In this case, when the dielectric is placed between the charged up capacitor plates, the attraction between the negative side of the polarized molecule and the positive plate of the capacitor would cause the polarized molecules to rotate, allowing the negatives to be a little bit closer to the positively charged capacitor plate. Either way, the end result is that the negatives in the atoms and molecules are going to face the positive capacitor plate and the positives in the atoms and molecules are going to face the negative capacitor plate. So how does this increase the capacitance? The reason this increases the capacitance is because it reduces the voltage between the capacitor plates. It reduces the voltage because even though there's still just as many charges on the capacitor plates, their contribution to the voltage across the plates is being partially cancelled. In other words, some of the positive charges on the capacitor plate are having their contribution to the voltage negated by the fact that there's a negative charge right next to them now. Similarly, on the negative side there's just as much negative charge as there ever was, but some of the negative charges are having their contribution to the voltage canceled by the fact that there's a positive charge right next to them. So the total charge on this capacitor has remained the same, but the voltage across the plates has been decreased because of the polarization of the dielectric. If we look at the definition of capacitance, we see that if the charge stays the same and the voltage decreases, the capacitance is going to increase, because dividing by a smaller number for the voltage is going to result in a larger value for the capacitance. So inserting a dielectric in this case, increase the capacitance by lowering the voltage. Let's look at another case of inserting a dielectric. Imagine we, again, let a battery of voltage V fully charge this capacitor. And let's insert a dielectric between the plates. But this time, let's leave the battery connected. Now what's going to happen? Well, just like before, the atoms and molecules in the dielectric are going to stretch and orient themselves so that the negatives are facing the positive plate and the positives are facing the negative plate, which again reduces the voltage between the two capacitor plates. But remember, we left the battery connected and this battery is going to try to do whatever it has to do in order to make sure the voltage across the capacitor is the same as the voltage of the battery V. Because that's just what batteries do. They try to maintain a constant voltage. So since the dielectric reduced the voltage by canceling the contributions from some of the charges, the battery's just going to cause even more charges to get separated until the voltage across the capacitor is again the same as the voltage of the battery. So the charge stored on the capacitor is going to increase, but the voltage is going to stay the same. Looking at the definition of capacitance, the charge on the capacitor increased after we inserted the dielectric. But the voltage across the capacitor plates stayed the same, since it's still hooked up to the same battery. So the effect of inserting a dielectric again is to increase the capacitance, this time by storing more charge for the same amount of voltage. To figure out how much you've increased the capacitance, you just need to know what's called the dielectric constant of the material that you've inserted between the capacitor plates. The dielectric constant is often represented with a Greek letter kappa or simply a K. The formula for finding out how the dielectric will change the capacitance is simple. If the capacitance of a capacitor before inserting a dielectric was C, then the capacitance after inserting a dielectric is just going to be k times C. We should note that since a dielectric always increases the capacitance, the dielectric constant k for a non-conducting material is always greater than 1. So for example, if a capacitor as a capacitance of 4 farads, when you insert a dialect with dielectric constant 3, the capacitance will become 12 farads.

Electrical breakdown

Electric current is a flow of electrically charged particles in a material caused by an electric field. The mobile charged particles responsible for electric current are called charge carriers. In different substances different particles serve as charge carriers: in metals and other solids some of the outer electrons of each atom (conduction electrons) are able to move about the material; in electrolytes and plasma it is ions, electrically charged atoms or molecules, 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 electrons away from their atoms, so they become mobile. The field strength at which break down occurs is an intrinsic property of the material called its dielectric strength.

In practical electric circuits 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 may vary with sample thickness.^[1] (see "defects" below)
It may vary with operating temperature.
It may vary with 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^[2]

Break down field strength

The field strength at which break down 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 0.5 GV/m.^[3] However very thin layers (below, say, 100 nm) become partially conductive because of electron tunneling.^{[clarification needed]} 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,^[4] 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) or (Volts/micron)
Helium (relative to nitrogen)^[5] ^{[clarification needed]}	0.15
Air^[6]	3
Sulfur hexafluoride^[5]	8.5–9.8
Alumina^[5]	13.4
Window glass^[5]	9.8–13.8
Borosilicate glass^[5]	20–40
Silicone oil, mineral oil^[5]^[7]	10–15
Benzene^[5]	163
Polystyrene^[5]	19.7
Polyethylene^[8]	19–160
Neoprene rubber^[5]	15.7–26.7
Distilled water^[5]	65–70
Beryllium oxide^[9]	27-31
High vacuum (200 μPa) (field emission limited)^[10]	20–40 (depends on electrode shape)
Fused silica^[5]	470–670
Waxed paper^[11]	40–60
PTFE (Teflon, extruded )^[5]	19.7
PTFE (Teflon, insulating film)^[5]^[12]	60–173
PEEK (Polyether ether ketone)	23
Mica^[5]	118
Diamond^[13]	2,000
PZT	10–25^[14]^[15]
Perfect 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).^[16] The conversion is:

{\begin{aligned}1{\text{ V/mil}}&=3.94\times 10^{4}{\text{ V/m}}\\1{\text{ V/m}}&=2.54\times 10^{-5}{\text{ V/mil}}\end{aligned}}

References

^ DuPont Teijin Films (2003). "Mylar polyester film" (PDF).
^ Ritz, Hans (1932). "Durchschlagfeldstärke des homogenen Feldes in Luft". Archiv für Elektrotechnik. 26 (4): 219–232. doi:10.1007/BF01657189. S2CID 108697400.
^ Bartzsch, Hagen; Glöß, Daniel; Frach, Peter; Gittner, Matthias; Schultheiß, Eberhard; Brode, Wolfgang; Hartung, Johannes (2009-01-21). "Electrical insulation properties of sputter-deposited SiO₂, Si₃N₄ and Al₂O₃ films at room temperature and 400 °C". Physica Status Solidi A. 206 (3): 514–519. Bibcode:2009PSSAR.206..514B. doi:10.1002/pssa.200880481. S2CID 93228294.
^ Lyon, David; et al. (2013). "Gap size dependence of the dielectric strength in nano vacuum gaps". IEEE. 20 (4): 1467–1471. doi:10.1109/TDEI.2013.6571470. S2CID 709782.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ CRC Handbook of Chemistry and Physics
^ Hong, Alice (2000). Elert, Glenn (ed.). "Dielectric Strength of Air". The Physics Factbook. Retrieved 2020-06-18.
^ Föll, H. "3.5.1 Electrical Breakdown and Failure". Tf.uni-kiel.de. Retrieved 2020-06-18.
^ Xu, Cherry (2009). Elert, Glenn (ed.). "Dielectric strength of polyethylene". The Physics Factbook. Retrieved 2020-06-18.
^ "Azom Materials - Beryllium Oxide Properties". azom.com. Retrieved 2023-12-05.
^ Giere, Stefan; Kurrat, Michael; Schümann, Ulf. HV dielectric strength of shielding electrodes in vacuum circuit-breakers (PDF). 20th International Symposium on Discharges and Electrical Insulation in Vacuum. Archived from the original (PDF) on 2012-03-01. Retrieved 2020-06-18.
^ Mulyakhova, Dasha (2007). Elert, Glenn (ed.). "Dielectric strength of waxed paper". The Physics Factbook. Retrieved 2020-06-18.
^ Glenn Elert. "Dielectrics - The Physics Hypertextbook". Physics.info. Retrieved 2020-06-18.
^ "Electronic properties of diamond". el.angstrom.uu.se. Retrieved 2013-08-10.
^ Moazzami, Reza; Chenming Hu; William H. Shepherd (September 1992). "Electrical Characteristics of Ferroelectric PZT Thin Films for DRAM Applications" (PDF). IEEE Transactions on Electron Devices. 39 (9): 2044. Bibcode:1992ITED...39.2044M. doi:10.1109/16.155876.
^ B. Andersen; E. Ringgaard; T. Bove; A. Albareda & R. Pérez (2000). "Performance of Piezoelectric Ceramic Multilayer Components Based on Hard and Soft PZT". Proceedings of Actuator 2000: 419–422.
^ For one of many examples, see Polyimides: materials, processing and applications, by A.J. Kirby, google books link