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 (relative to nitrogen) | 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 18.9 - 21.7 |
| Neoprene rubber | 7001216500000000000 15.7 - 26.7 |
| Distilled Water | 7001300000000000000 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 ) | 7001600000000000000 19.7 |
| PTFE (Teflon, Insulating Film) | 7001600000000000000 60 - 173 |
| Mica | 7001450000000000000 118 |
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