Space charge injection and extraction in high divergent fields

Applied Physics Letters

Okamoto

2004

It is suggested that an electric field above a given value eliminates the barriers to the transport of trapped charge carriers so as to produce an extended state in the form of a percolation cluster, and that the consequent current multiplication results in electrical breakdown. This model provides an estimated value of intrinsic breakdown strength close to the actual value. By considering the interactions between trap barrier potentials, the effect of electrical aging can be explained in terms of an increase in trap density. Many phenomena, such as the effect of weak points and the change of breakdown strength with the content of co-monomers or additives, can also be explained using The mechanism of intrinsic breakdown associated with the transition of electron behavior at high electrical field in insulating polymers is still not clearly understood. Their conduction band is near or above the vacuum level, a large energy gap ͑ϳ8 eV͒ exists between the valence and the conduction bands, 1 and the mean free path is less than 3 nm. 2,3These factors cause the breakdown strength calculated from the classical models for electron driven breakdown to be above 10 9 V/m, 2 much larger than experimental values of the order of 10 8 V / m. It is usually argued that this reduction is related to weak points, such as free volume, submicrovoids, and low-density regions. In the model of free-volume breakdown 4,5 it is noted that the largest empty spaces in polymers may be as large as several decades of nanometers, 5 the longest free path of electron can therefore be large enough for electrons to overcome the trap barrier or to induce impact ionization. However, this model seems only able to describe the development of degradation around the large free volume regions, it is unsatisfactory to explain the overall current multiplication in an intrinsic breakdown process. A contrary example is partial discharge in which breakdown may not occur at all even though the internal gas cavity has been discharged.In respect of electrical aging it has been suggested that the creation of low-density regions near the electrodes at high fields is a necessary step leading to breakdown. 6,7 The formation of such regions is associated with molecular dissociation due to the energy released in the trapping process of hot electrons, 2 in kinetic energy transfer during spacecharge injection and extraction processes, 8 or lattice deformation on charge trapping.9 However, these aging models do not seem suitable for instantaneous impulse breakdown when low-density regions have no time to form. It is also still not very clear how the molecular dissociation affects the electron behavior and the breakdown strength. Other models, such as the filamentary theories 10,11 for the creation of a breakdown path and the simulation models 12,13 for the shape of breakdown paths, use the breakdown strength as a parameter. They are not concerned with the physics behind the transition of the electron state in the breakdown process. This letter proposes a percolation model f...

mentioning

confidence: 99%

Percolation model for electrical breakdown in insulating polymers

Applied Physics Letters

Okamoto

2004

“…In this case the decay follows equation (2) N is the density of trap states per unit energy interval, and ∆ min and ∆ max the minimum and maximum trap depths, which can be obtained from the times at which the plot deviates from a straight line, as long as the detrapping attempt frequency can be estimated. Here h kT = ν is used for the estimation as this frequency is related to incoherent thermal vibrations of the trap state [10]. (3), the minimum depth of filled traps can be estimated for both samples, and are 0.89eV and 0.95eV for the control and irradiated sample respectively.…”

Section: Charge De-trapping Analysis Based On Space Charge Decaymentioning

confidence: 99%

“…The space charge decay yields a way to investigate the change introduced by irradiation by supplying a means to evaluate trap depths and trap densities [9,10]. The total charge in the bulk of the sample, during the decay period, has been calculated from equation (1) and its value plotted against the logarithm of the decay time, as shown in figure 5.…”

Section: Charge De-trapping Analysis Based On Space Charge Decaymentioning

confidence: 99%

See 1 more Smart Citation

The Effect of Gamma Irradiation on Space Charge Behaviour and Dielectric Spectroscopy of Low-density Polyethylene

Chen

2007 IEEE International Conference on Solid Dielectrics

et al. 2007

Abstract:The influence of gamma irradiation on low-density polyethylene (LDPE) has been studied through space charge measurement and dielectric spectroscopy. Commercial LDPE film with a thickness of 100µm was irradiated with γ-rays at a dose rate of approximately 10kGy/h to different doses up to 100kGy in an atmosphere of air, vacuum and nitrogen respectively. Space charge profiles vary depending on irradiation environments and radiation dose. The space charge profile of a sample after irradiation at 100kGy in air is more complicated than those irradiated in vacuum and nitrogen at same dose. More importantly, the space charge decay rate for the irradiated samples is slower than that of the virgin sample, indicating that the irradiation had changed the material by the generation of deep traps for space charge. Irradiation in air also produced changes in the dielectric spectrum in the frequency range between 10 -3 Hz and 10 6 Hz where the imaginary part of the permittivity was found to be about one order of magnitude higher than that of the virgin sample (~10 -4 ). This was attributed to oxidation as samples with the same (or lower) radiation dose in vacuum or nitrogen showed no pronounced difference to that of virgin samples.

“…The experiments reported in [7] could not distinguish between a transport mechanism controlled by field-assisted thermally activated hopping between neutral sites or a Poole-Frenkel process [6]. It was shown in [9] however, that the decay of space charge in an epoxy resin was controlled by the time dependent de-trapping of the injected charge from trap sites whose trap depths uniformly covered a range of energies. In this case a geometrically divergent field was investigated and therefore it could be expected that on de-trapping the main part of the injected charge would move rapidly to the neighbouring injecting electrode for extraction, rather than travel to the planar counter-electrode.…”

Section: Introductionmentioning

confidence: 99%

Decay of space charge in a glassy epoxy resin following voltage removal

IEEE Trans. Dielect. Electr. Insul.

Griseri

Peasgood

et al. 2006

Self Cite

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTThe PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 hours after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 10 2 s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature.