The emission of electrons from glass induced by a strong electric field and the mechanism of the silent electric discharge

Abstract

Ordinary borosilicate (Pyrex) glass was found to emit electrons in vacuum when a sufficiently strong electric field was applied. Two distinct phenomena were observed. First, emission from the surface of a sharp glass tip occurred with a macroscopic field of about 5 × 10 9 V m -1 applied, but the electric stress was then great enough to shatter the glass in a short time. Second, emission from active sites on glass tubes occurred at macroscopic fields ( F ) of about 5 × 10 6 V m -1 . The number of emitting sites increased with applied field and was about 0.5 mm -2 with a field of 10 7 V m -1 . Emission from these sites was essentially the same under ordinary vacuum conditions and under ultra-high vacuum conditions. The current from active sites under vacuum conditions increased nearly exponentially with F ½ at constant temperature, and nearly exponentially with temperature at constant field. Active emitting sites on glass were very stable to chemical cleaning processes and to ion bombardment and electric discharges, in sharp contrast to emitting sites on thin metal wires. The sites on glass also emitted electrons in the presence of gases, but no emission of negative ions was noted in magnetic deflexion experiments. Current-voltage relations in the presence of gases were measured. With oxygen present, ozone was formed using glass tubes with either a. c. or d. c. applied. However, with a silica tubes ozonizer, ozone was formed with a. c. applied but not with d. c. applied. Ozone was also formed in an electric discharge between a thin metal wire (thin enough to act as a field emitter) and an outer metal cylinder. This ozonizer was compared with the glass ozonizer and energy efficiencies were measured. It is proposed that the external electric field causes emission of single electrons from isolated sites on glass and that the emitted electron interacts with the electron hole left behind at the surface. Equations are deduced for the case when the interaction is mainly between the electron and the electron hole rather than between the electron and its image. A model of field enhanced thermionic emission from sites with low work function (Schottky emission) is found to explain the results for emission from active centres on glass, and the field intensification factor β appears to be about unity. Although Schottky emission is considered more important than field emission for the fields used with glass tubes, calculations show that field emission becomes more important at higher electric fields. The emission of electrons is considered to be important in initiating the silent electric discharge.