Section: 15 | Dielectric Strength of Insulating Materials |

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DIELECTRIC STRENGTH OF INSULATING MATERIALS

L. I. Berger

The loss of the dielectric properties by a sample of a gaseous, liquid, or solid insulator as a result of application to the sample of an electric field^* greater than a certain critical magnitude is called dielectric breakdown. The critical magnitude of electric field at which the breakdown of a material takes place is called the dielectric strength of the material (or breakdown voltage). The dielectric strength of a material depends on the specimen thickness (as a rule, thin films have greater dielectric strength than that of thicker samples of a material), the electrode shape,** the rate of the applied voltage increase, the shape of the voltage vs. time curve, and the medium surrounding the sample, e.g., air or other gas (or a liquid — for solid materials only).

Below are given the dielectric strength of gases (Table 1), liquids (Table 2), and solids (Table 3) commonly used in insulating situations.

Breakdown in Gases

The current carriers in gases are free electrons and ions generated by external radiation. The equilibrium concentration of these particles at normal pressure is about 10³ cm^-3, and hence the electrical conductivity is very small, of the order of 10^-16 – 10^-15 S cm^-1. But in a strong electric field, these particles acquire kinetic energy along their free path, large enough to ionize the gas molecules. The new charged particles ionize more molecules; this avalanche-like process leads to formation between the electrodes of channels of conducting plasma (streamers), and the electrical resistance of the space between the electrodes decreases virtually to zero.

Because the dielectric strength (breakdown voltage) of gases strongly depends on the electrode geometry and surface condition and the gas pressure, it is generally accepted to present the data for a particular gas as a fraction of the dielectric strength of either nitrogen or sulfur hexafluoride measured at the same conditions. In Table 1, the data are presented in comparison with the dielectric strength of nitrogen, which is considered equal to 1.00. For convenience to the reader, a few average magnitudes of the dielectric strength of some gases are expressed in kilovolts per millimeter. The data in the table relate to the standard conditions, unless indicated otherwise.

Breakdown in Liquids

If a liquid is pure, the breakdown mechanism in it is similar to that in gases. If a liquid contains liquid impurities in the form of small drops with greater dielectric constant than that of the main liquid, the breakdown is the result of formation of ellipsoids from these drops by the electric field. In a strong enough electric field, these ellipsoids merge and form a high-conductivity channel between the electrodes. The current increases the temperature in the channel, liquid boils, and the current along the steam canal leads to breakdown. Formation of a conductive channel (bridge) between the electrodes is observed also in liquids with solid impurities. If a liquid contains gas impurities in the form of small bubbles, breakdown is the result of heating of the liquid in strong electric fields. In the locations with the highest current density, the liquid boils, the size of the gas bubbles increases, they merge and form gaseous channels between the electrodes, and the breakdown medium is again the gas plasma.

Breakdown in Solids

It is known that the current in solid insulators does not obey Ohm’s law in strong electric fields. The current density increases almost exponentially with the electric field, and at a certain field magnitude it jumps to very high magnitudes at which a specimen of a material is destroyed. The two known kinds of electric breakdown are thermal and electrical breakdowns. The former is the result of material heating by the electric current. Destruction of a sample of a material happens when, at a certain voltage, the amount of heat produced by the current exceeds the heat release through the sample surface; the breakdown voltage in this case is proportional to the square root of the ratio of the thermal conductivity and electrical conductivity of the material. A semi-empirical expression for dependence of the breakdown voltage, V_B, on the physical properties and geometry of a sample of a solid material for the one-dimensional case is

V_B = [Aρκ/aφ(d)]^½

where A is a numerical constant related to the system of units used, ρ and κ are the volume resistivity and thermal conductivity of the sample material, a is a constant related to the chemical bond nature and crystal structure of the sample material, and φ(d) is a function of the sample geometry, first of all, thickness, d (see, e.g., Ref. R6). In the majority of materials, φ(d) increases with d, hence, the magnitude of V_B is greater in the thinner samples of a particular material.

The electrical breakdown results from the tunneling of the charge carriers from electrodes or from the valence band or from the impurity levels into the conduction band, or by the impact ionization. The tunnel effect breakdown happens mainly in thin layers, e.g., in thin p-n junctions. Otherwise, the impact ionization mechanism dominates. For this mechanism, the dielectric strength of an insulator can be estimated using Boltzmann’s kinetic equation for electrons in a crystal.

In the following tables, the dielectric strength values are for room temperature and normal atmospheric pressure, unless indicated otherwise.

^*The unit of electric field in the SI system is newton per coulomb or volt per meter.
^**For example, the U.S. standard ASTM D149 is based on use of symmetrical electrodes, while per U.K. standard BS2918 one electrode is a plane and the other is a rod with the axis normal to the plane.

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TABLE 1. Dielectric Strength of Gases

Material	Formula	Dielectric strength*	Ref.
Continued on next page...
Nitrogen	N₂	1.00
Hydrogen	H₂	0.50	1,2
Helium	He	0.15	1
Oxygen	O₂	0.92	2
Air		0.97	6
Air^b		3.0^a	3
Air		0.4-0.7^a	4
Air		1.40^a	5
Neon	Ne	0.25	1
Neon	Ne	0.16	2
Argon	Ar	0.18	2
Chlorine	Cl₂	1.55	1
Carbon monoxide	CO	1.02	1
Carbon monoxide	CO	1.05	2
Carbon dioxide	CO₂	0.88	1
Carbon dioxide	CO₂	0.82	2
Carbon dioxide	CO₂	0.84	6
Nitrous oxide	N₂O	1.24	2
Sulfur dioxide	SO₂	2.63	2

^*Relative to nitrogen, unless units of kV/mm are indicated.
^aIn kV/mm.
^bFlat electrodes.
^cAt 12.5 torr.
^dAt 370 torr.

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