Guy Blaise

Breakdown phenomena related to trapping/detrapping processes in wide band gap insulators

Mechanisms of breakdown phenomena related to charge carrier trapping and detrapping are investigated in wide band gap insulators. Trapping occurs in potential wells due to the presence of polarization charge at the border of domains having different permittivities and conductivities. Findings in this work are related to (1) the calculation of the polarization energy of the medium surrounding a trapped charge, (2) the investigation of the mechanism of space charge destabilization, and (3) the explanation of the breakdown mechanism from the polarization energy dissipation after detrapping. An experimental method to produce breakdowns is recalled. In this method electron trapping is produced under keV energy electron irradiation and the space charge destabilization is obtained under low energy electron irradiation. Depending on the experimental arrangement, the method makes it possible to produce flashover or bulk breakdown. The formation of a plasma, the melting of the sample, and the formation of treeing or thermal etched pits are observed. >

Charging of MgO

MgO has been well known as a material with high secondary electron yield. The escape depth of electrons for MgO with the ordered structure is relatively large, therefore the electrons are usually re-emitted from bulk MgO. In addition, when the electrons are injected deep into a perfect MgO crystal, polarons are formed, because the electrons have polarized its surroundings. When the density of the polaron sites is high enough, a narrow energy band is established. In the space charge physics, such a band in perfect crystal MgO has been predicted to have a very low barrier between adjacent sites, thus the polarons in the crystal are usually free to move or hop. Any excess electrons will eventually escape to the surface and later they will be neutralized by positive ions or removed by air molecules. Therefore, the material is normally positively charged under electron beam bombardment. In this report, we will verify this proposal in the space charge physics by the well-known mirror method experiment.

Insulator damages related to charge detrapping

It is shown that a critical space charge variation is the cause of insulator sample damage ranging from atom size to macroscopic size. This finding has implications for space charge characterization. Correlation of the space charge characteristics to breakdown strength requires the critical conditions for initiating breakdown. These conditions are related to such material intrinsic parameters as the trap energy distribution, the trap spatial distribution, the schubweg, and the modification of the free energy of the charged sample. The crucial point is therefore not how to measure the charge distribution but how to characterize trapping/detrapping critical parameters.<<ETX>>

Charging and discharging phenomena in oxides, electrical and mechanical applications

Research was conducted to investigate (1) the origin of the correlation which exists between the charging of insulators and their properties (breakdown, fracture toughness, wear, etc.); (2) the origin of the analogy which exists between electric properties and mechanical properties of insulators; and (3) the origin of electron and photoemissions which occur prior to or at the instant of the catastrophe (breakdown, fracture). A theoretical approach to charging and discharging phenomena has made it possible to develop an experimental method giving characteristic dielectric parameters: permittivity, distribution of defects, and energy which can be dissipated inside the insulator without initiating a catastrophe. This method was used to correlate these parameters with the breakdown voltage of oxides and of polymers, and with mechanical properties of ceramics. New high performance materials were prepared.<<ETX>>

An oscillation phenomenon in charging/discharging of small solid insulators

An oscillation phenomenon of charging/discharging in insulators is observed using the scanning electron microscope (SEM) mirror-image technique. After implanting electrons in an insulating material, bright rings are observed on the insulator surface in SEM images. Traces of flashover from the rings are also found. These observations suggest that charge is distributed or oscillated in rings. The mechanism of this oscillation behavior is discussed.