Study of discharge after electron irradiation in sapphires and polycrystalline alumina

Abstract

Transmission electron microscopy study of platinum clusters on Al 2 O 3 /NiAl(110) under the influence of electron irradiationThe fraction R of charges undergoing discharge during the time separating two electron pulses is derived from the induced current method developed in a scanning electron microscope. Irradiation is performed via a 10 keV defocused electron beam and low current density. The evolution of R with temperature ͑in the range 300-663 K͒ obeys to an Arrhenius type relation. Activation energies connected with the processes involved are deduced. In sapphire, no discernible discharge is observed due to the dominance of deep traps. In silver doped sapphire, R increases sharply from 10% to 70% as the temperature rises from 360 to 420 K, with a corresponding activation energy of 0.51 eV. In contrast, in polycrystalline alumina processed by solid state sintering ͑grain diameters of 1.7, 2.7, and 4.5 m͒ the degree of discharge increases continuously with temperature and grain size. The enhancement with grain size indicates that the sintering conditions influence strongly the efficiency of a gettering effect. The activation energy below 573 K is about 0.12 eV independently of grain size. Above 573 K, a second activation energy of 0.26 eV appears for the smallest grain size sample. The results suggest that discharge may stem from a density of trapping states, associated to grain boundaries in sintered samples, rather than from a single trapping level linked to the doping element as Ag in sapphire.

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

“…The activation energy that arises from the semi-logarithmic plot of the recovery parameter versus reciprocal temperature in the range 600-663 K is 0.53 eV (Moya et al, 2007). These results are an indication that detrapping occurs, at about 600 K, from a dominant efficient trap having a well-defined energy level in the gap as in the case of silver doped single crystal (Zarbout et al 2010). …”

“…The experimental set up, described below, provides means for measuring the evolution of the net amount of trapped charges Q t , which characterize the charging state of the insulator. For this purpose, we use the Induced Current Measurements "ICM" method (called also the Displacement Current Measurements), which we have recently improved (Zarbout et al, 2008(Zarbout et al, , 2010.…”

“…The dielectric properties of undoped corundum, α-alumina, of different kinds (single crystal or polycrystalline obtained by solid state sintering) have been the subject of numerous studies concerning, in particular, the breakdown strength and the charging behavior (Haddour et al, 2009;Liebault et al, 2001;Si Ahmed et al, 2005;Suharyanto et al, 2006;Thome et al, 2004;Touzin et al, 2010;Zarbout et al, 2008Zarbout et al, , 2010. The common feature pointed out by most of these investigations is the conspicuous role played by the microstructure and the impurities.…”

An excitonic model for the electron–hole plasma relaxation in proton-irradiated insulators

“…However, the effect of detrapping is minor in a-SiO 2 because of the large activation energy of about 1.5 eV [52]. This behaviour must be contrasted against the case of sapphire (Al 2 O 3 ) for which a much longer (about 100 ps) free electron-hole pair relaxation time has been reported [53], along with a much smaller activation energy for detrapping, between 0.04 an 0.51 eV [54]. The case of borosilicate crown glass BK7 is also relevant because long (exceeding 10 ps) free electron-hole pair relaxation times-longer than for a-SiO 2 -have been reported both using optical [15] and proton [24,[26][27][28] probes.…”

Dielectric breakdown of alumina single crystals

“…Defects of the microstructure such as pores, grain boundaries, impurities, crystallographic defects and secondary phases are considered as trapping sites. Therefore, it is generally believed that the resistance against dielectric breakdown is directly connected to microstructure properties 7,8,9,10 . In the present study, the focus is on the dielectric breakdown behaviour of alumina single crystals.…”