Electrical breakdown in polar liquids

Polar liquids are being studied as switch media for the generation of sub-microsecond pulses with nanosecond or even shorter rise times. The inherent dielectric strength of polar liquids allows for small gap lengths, down to less than 1 mm even for voltages up to 100 kV, and consequently enables the reduction of the switch inductance. Most of the breakdown studies to date have been conducted for longer gap distances and pulse durations in the microsecond range. Empiric equations for the breakdown strength under these conditions, which do not include effects of electrode materials, have been developed by J. C. Martin. In recent studies, we found that for shorter, high voltage pulses of only 200 ns duration, the dielectric strength of sub-millimeter water gaps can be much higher than expected from these empirical formulas. Moreover, the dielectric strength strongly depends on the electrode condition. By polishing the stainless steel electrodes of a pin-to-plane geometry (diameter of the pin electrode: 1.7 mm) the breakdown strength in a 200 m gap was increased from 1 MV/cm up to 1.8 MV/cm. For propylene carbonate, a breakdown strength of 2.2 MV/cm was achieved instead of only 1 MV/cm for unpolished electrode surfaces. Experiments indicate that the initiation of breakdown is independent of the conductivity of the bulk, but depends on effects at the liquid-electrode interface. To explore this hypothesis further, experiments were conducted using electrode materials of different work functions (zirconium, silver, stainless steel). The conductivity of the liquid dielectric was varied by adding salt (NaCl) in different concentrations to distilled water. For comparison, similar experiments were carried out with propylene carbonate. In order to relate the breakdown strength in sub-millimeter water gaps to results for longer gap distances, increasingly longer pulses, of up to 5 s, were applied.