Kai-Kun Yu

Effect of surface shallow traps on flashover characteristics across machinable ceramic in vacuum

The electrical strength of the surface of low-temperature melting machinable glass ceramic, prepared by using several different methods, is investigated before and after treatment with diluted hydrofluoric acid (HF). Using the surface potential decay method, the surface trap density and the energy of each sample are measured. The influence of acid treatment on the trap distribution is analyzed. Concurrent optical and electrical measurements allow surface flashover phenomena in vacuum under impulse voltage to be investigated, along with the influence of surface trapping parameters on flashover performance. It is shown that the glass phase on the surface of machinable ceramic greatly influences the shallow traps, which in turn degrade the flashover characteristics. Removing the glass phase with HF treatment reduces the concentration of shallow traps and stabilizes the flashover voltage.

Monte Carlo Simulation of Surface-Charging Phenomena on Insulators Prior to Flashover in Vacuum

Before flashover across an insulator under high electric field in vacuum, there are charging phenomena occurring on the insulator surface, which significantly affect the developing process of flashover. Based on the secondary-electron-emission-avalanche model and by using the Monte Carlo method, a 2-D analysis of surface charge density on cylindrical and conical insulators prior to flashover in vacuum has been performed under unipolar voltage. Different materials are employed, i.e., alumina ceramic, PTFE, PMMA, and PI. The influences of materials, voltage amplitudes, and coning angles on charge distribution are investigated. The results reveal that negative charges exist in a small surface region near the cathode, while the surface charges positive in a larger region away from the cathode. With increasing applied voltage, both the negative charge density and region decrease, and even vanish, whereas both the positive charge density and region increase, and the peaks of both regions move toward the cathode. For the conical insulator with a negative angle, the positive charge density is greater than that with a positive angle, and the simulation describes well experimental data relating the coning angle, the surface charge, and the flashover voltage.

Pulsed flashover developing across alumina ceramic in vacuum

The flashover phenomena in vacuum have been an interesting issue among many researchers in the fields of both electrical and electronic systems for more than half a century. However, the phenomena are so complicated and up to now the related mechanisms are still not understood clearly. In this paper the ICCD camera was used to grasp the dynamic flashover images on the surface of alumina ceramic in vacuum under pulsed voltage excitation. These images show the developing process of the flashover phenomena. The results indicated that the flashover develops rapidly in less than 50 ns and the electrode edge shows the highest light intensity. The sample with sputtered metal film electrode has also been employed to obtain the flashover photos with ICCD camera, and these photos were compared with those samples without sputtered electrode. And these two groups of photos show a different optical distribution, possibly due to their different local electric field distribution near the electrode edge.

Influences of preparation technology on the flashover characteristics of machinable ceramics in vacuum

This paper probes into the influences of preparation technology, mainly crystallization time, on the flashover characteristics of machinable ceramics under 0.4/2.5 mus impulse voltage in vacuum. Several machinable ceramic samples are prepared with the same procedure under different crystallization time. To study the association between crystallization time and flashover characteristics, the surface microstructures of samples are observed by SEM, while the surface electric strength of each sample is tested by flashover experiments in vacuum. It is revealed that the flashover characteristics vary with their crystallization time, and the sample with less crystallization time has a better flashover performance. From the SEM analysis of the samples, the crystallization time of machinable ceramic can affect its microstructure. The longer crystallization time is, the bigger grains will grow. And this will contribute to an influence on the flashover performance of machinable ceramics.

Influence of electrode contacts on flashover characteristics of alumina and PTFE under pulsed voltage in vacuum

The surface flashover phenomena across insulating materials always lead to the failure of system insulation and may cause serious accidents. Based on the synchronized measurement of optical and electrical signals, the flashover phenomena along alumina ceramics and polytetrafluoroethylene (PTFE) are investigated under impulse voltage in vacuum. The planar electrode system is used with the advantage of optical observation. Different electrode contact manners are employed, i.e., with the traditional electrode butt contact and with the sputtered metal films for adjusting the contact behavior. The fundamental characteristics of flashover phenomena are discussed, and the difference of the flashover characteristics between alumina and PTFE is also compared. It is also revealed that sputtering metal film electrode has obvious influence on the flashover characteristics of insulator. Based on the band theory of solid, these phenomena can be reasonably explained by the charge carrier injection and transport behaviors.

Investigation on novel parameter characterizing surface flashover phenomena in vacuum

Surface flashover in vacuum is a kind of complicated surface and interface phenomena, which seriously restricts the development of high voltage electro-vacuum devices. On the basis of a novel material as machinable ceramic (MC) for vacuum insulation system, which has excellent machinable performance and good surface electric strength, the surface electric property of samples with different preparation technologies is investigated under pulsed voltage excitation before and after erosion with dilute hydrofluoric (HF) acid. Using the surface potential decay method, the surface trap density and energy of each sample are measured, and the influence of acid erosion on trap distribution is analyzed. Based on the concurrent optical and electrical measurements, the surface electric strength of samples with/without erosion is tested in vacuum, and the influence of surface trap density and energy level on flashover performance is analyzed. It is revealed that, the glass phase on MC surface is an important factor for the largely existed traps, and the shallow trap parameters can be employed as a novel parameter to characterize the flashover phenomena. By eroding off the glass phase on the sample with hydrofluoric acid erosion, the concentration of shallow traps can be reduced, thus its flashover stability is greatly improved.

Measurement of surface charge distribution on insulating material under pulsed voltage

Since the beginning of the 20th century, surface flashover phenomena have been investigated from a viewpoint of scientific interest. The surface flashover can cause serious accident and limit the development of the electric power and high voltage equipment. The measurement of surface charge is important to understand the discharge of insulating materials and may help to find new ways to improve the flashover voltage. Based on the principle of the electrostatic probe, a system for the measurement of charge distribution on insulating material in vacuum is invented; the surface charge distribution on insulating material before and after the surface flashover is compared. The relationship between the flashover characteristics and the surface charge is discussed.

Effect of surface ion exchange of machinable ceramic on its flashover characteristics in vacuum

As well known, the surface conditions of solid materials strongly affect the flashover phenomena under high electric field in vacuum. On the basis of the novel low melting temperature machinable glass ceramics for vacuum insulation system, which has excellent machinable performance and good electrical properties, the ion-exchange method was used to change the samples surface condition, surface electric resistivity and surface trap density. Different alkali metal ions were used in this paper, and this may change the samples surface microscopic structure. We have measured the different samples surface flashover in vacuum, and investigated the relationship between the ion-exchange method and samples surface electricity characteristic. The results indicated that while using suitable ion-exchange method, the samples surface flashover performance can be improved.

Monte Carlo based simulation on surface charging phenomena of insulators prior to flashover in vacuum

As we know, before flashover across an insulator surface under high electric field in vacuum, there are charging phenomena occurring on the insulator surface, which significantly affect the developing process of flashover. Based on the secondary electron emission avalanche (SEEA) model, and by using the Monte Carlo method, a two-dimensional analysis of surface charge density on cylindrical and conical insulators prior to flashover in vacuum has been performed under unipolar voltage. Typical insulating materials are employed, i.e., machinable ceramic, alumina ceramic, PTFE and PMMA. The influences of secondary electron yield, voltage amplitudes and coning angles on charge distribution are investigated. The results reveal that negative charges exist in a small surface region near the cathode, while the surface charges positive in a larger region away from the cathode. With increasing applied voltage, both the negative charge density and region decrease, and even vanish, whereas both the positive charge density and region increase, and the peaks of both regions move towards the cathode.

Surface and interface physical phenomena induced by flashover across solid dielectrics in vacuum

Essentially surface flashover across a solid in vacuum is considered as a kind of complicated surface and interface physical phenomena. Based on the concurrent electrical and optical measurements and microscopic observations, the research works of our group concentrate on the relationship between flashover and surface/interface condition of insulating materials. There are some different optical phenomena detected from the surface of alumina and polymers, and we attribute the preflashover phenomena to the differences between the surfaces of different insulating materials and different electrode contacts. The experimental results reveal that, under low electric field, prior to field electron emission from the cathode triple junction, electroluminescence phenomena occur due to the radiative recombination of electrons and holes injected into the surface states from the electrodes. According to the results, the phenomena mentioned above are related to the surface states in the surface layer of a material. This work is a contribution to the traditional secondary electron emission avalanche (SEEA) model.