Flashover in wide-band-gap high-purity insulators: Methodology and mechanisms

Abstract

The electron beam of a scanning electron microscope (SEM) is used to charge an unmetallized insulator (Al2O3, Y2O3, SiO2) in vacuum. The charging is found to be stable in time after the e-beam is switched off. The SEM is also used to measure the implanted charge by measuring the resulting electrostatic potential. The distribution of potential around the trapped charges is determined by the classical laws of electrostatics. The electrostatic energy stored in the polarized dielectric can thus be determined. The implanted charge can be removed by the introduction of carriers into the polarized sample by flooding the surface with electron beams of varying energies. Slow or rapid relaxation occurs depending on the operating conditions of the flood gun (energy, intensity, etc.) which introduces the electrons. When the relaxation kinetics are slow, the electrostatic charge decreases slowly as a function of time. On the other hand, a rapid relaxation of the dielectric leads to the appearance of a high-density plasma which spreads over the insulator surface, resulting in treeing on the insulator surface. Along the path of the arc, the transfer of energy to the lattice triggers the sublimation of the ceramic and its mechanical fracture through thermal shock. At particular flood gun setup conditions, we have observed the formation of parallel fracture lines along the ceramic surface, away from the treeing region. These results constitute the basis for a new approach to understanding flashover along ceramics-vacuum interfaces. The important step is the plasma initiation, which we interpret on the basis of dielectric relaxation mechanisms. The parameter of the insulator that determines its breakdown initiation is its complex dielectric constant. It is concluded that the insulator’s band gap, the nature and density of defects localized in the band gap, and the dipolar relaxation induced by a variation of the electric field and connected with the presence of defects determine the holdoff level of insulators.

Effects of the Microstructure Induced by Sintering on the Dielectric Properties of Alumina

“…Charge trapping can occur around intrinsic point defects, defects induced by the dissolution of impurities, defects associated with grain boundaries interfaces and dislocations (Kolk & Heasell, 1980). We must also keep in mind that trapping in insulators gives rise to polarization and lattice deformation allowing energy accumulation within the material Le Gressus et al, 1991;Stoneham, 1997). As a result, if some critical density of trapped charges (or some critical electric field) is reached, external stresses (thermal, electrical or mechanical) can trigger a collective relaxation process corresponding to a release of stored energy.…”

“…This parameter is intimately linked to the charging properties as breakdown originates from the enhancement of the density of trapped charges, which stems from the competition between charge trapping and conduction Le Gressus et al, 1991;Liebault et al, 2001;Haddour et al, 2009). The charges can be either generated by irradiation or injected through interfaces via an applied voltage.…”

Charging compensation of alumina samples by using an oxygen microinjector in the environmental scanning electron microscope

“…The charging effects cause beam deflection and increase imaging aberration and error of the x-ray analysis (Goldstein et al 1992, Le Gressus et al 1991.…”

A novel technique for visualizing electron beam induced charging

“…For example, the distribution of the fields, around the trapped charges and the corresponding surface potential, can be defined by determining the incident beam energy at which the surface acts as an electron mirror (Le Gressus et al 1991). The surface potential V s can also be measured using Auger electron spectroscopy (AES) by determining the energy shift ∆E (= eV s ) between the measured and tabulated energy of a given Auger peak (Hoffmann 1992, McDonald et al 1976.…”