Some common dielectric materials include mica, plastics, glass, porcelain and various metal oxides. Dry air also serves as a superior dielectric and performs that role in some transmission line types and variable capacitors. Distilled water serves as a fair dielectric, while a vacuum has considerable efficiency in this role.
Dielectric materials conduct electricity poorly but provide support for electrostatic fields. Minimizing the current flow between opposing electric poles while allowing electrostatic flux lines to run freely permits an electrostatic field to store energy. This has applications with capacitors at radio frequencies and in constructing radio-frequency transmission lines.
The reason why substances with high dielectric constants (such as aluminum oxide) are so valuable is that they allow the construction of high-value capacitors with minimal volume. However, these materials usually lack the ability to survive intense electrostatic fields with the same strength as air, which has a low dielectric constant. If voltage running across a dielectric material spikes too high, making the electrostatic field too intense, the material begins to conduct current. Known as dielectric breakdown, this process reverses itself if the dielectric is a gas or liquid, bringing voltage back below the critical level. If the dielectric is solid, breakdown often leads to lasting damage.