Nanosecond Breakdown of Porous Alumina Ceramics Saturated with Perfluorinated Liquids

Abstract

Porous ceramics can be used to supply liquid dielectric to a discharge area in liquid propellant pulsed plasma thrusters [1]. It is interesting to use perfluorinated liquids as propellants, because they have relatively low specific plasma formation energy. Also, it is an attractive idea to use the liquid both as an insulator and propellant. Under nanosecond pulsed conditions, the combination of porous insulating media and liquid dielectric may demonstrate unexpected behavior [2]. In this work, we present the results of measurements of the anode-initiated electrical breakdown front velocity in porous alumina ceramics saturated with perfluorinated esters, alkanes and other liquids. The experimental setup used comprises of a nanosecond pulse generator, breakdown cell and digital oscilloscope. The generator is a Tesla transformer, built into a coaxial forming line with a stored energy of 0.8 J, controlled by a gas-filled spark gap. The generator impedance is 50 $\Omega$ , and voltage under no-load is 140 kV. The pulse duration is 8 ns under load-matched conditions, and the rise time is less than 0.5 ns. The configuration of electrodes is point-to-plane with positive point. Samples were made by uniaxial quasistatic compression of alumina nanopowder with specific surface area of 30 m2/g and annealed in air at a temperature of 1900 K [3]. The density of the samples was 66% of that of monolithic alumina. We used liquids that have relatively a high breakdown channel velocity in the single-pulse mode of voltage application $(\text{up} \text{to} 1.5\cdot 10^{5}\mathrm{m}/\mathrm{s})$ . Ceramics saturated with propyl perfluoropentanoate demonstrates a significant decrease in breakdown channel speed (3–4 times) when the samples are thicker than those broken within a single pulse. This behavior may be attributed to the multi-pulse voltage application, and to the properties of the heterogeneous insulating media. Discharge cannot be initiated in a liquid within pores as easily as it can be in a free liquid, nor does it go as quickly as in monolithic alumina or sapphire. Pores with liquid between the grains of powder serve as barriers, which significantly reduce the mobility of charge carriers. Therefore, a combination of porous ceramics and liquid dielectric may be used as an effective insulator in the multipulse mode even in the case of easy-to-breakdown liquids.