Guodong Meng

Electrical characteristics of nanometer gaps in vacuum under direct voltage

The electrical characteristics of nanometer gaps in vacuum were studied with the tungsten electrodes under dc and pulsed voltage. A novel experimental technique to study the electrical characteristics of nanometer gaps was presented in the paper. In the experimental setup, the tungsten was fabricated and shaped to be a perfect sphere through the electrochemical etch and Joule melting method, and the nanogap was controlled precisely by the scanning electron microscope (SEM) and nanometer manipulator. The effects of electrode geometry, gap separation and injected voltage waveform were investigated. The current-voltage curves and Fowler-Nordheim plots showed the difference of the field emission process before breakdown between the sphere-sphere electrodes and needle-sphere electrodes. The gap separation dependence of dielectric strength demonstrated the similar trend to the previous work but better performance. The breakdown voltage for pulsed voltage was 4-5 times higher than that for the dc voltage. The analysis of the physical damage indicated that the current, duration time and electrode geometry played important roles in electrode modification. In addition, a possible mechanism of nanoscale vacuum breakdown was also proposed in the paper.

Variation of surface charge distribution in PD sequences

Partial discharge (PD) occurring in gas void introduced during the manufacturing process of insulating constructions forms a serious threat to the life of insulation, hence its necessary to clarify its mechanism. In our paper, the surface charge measurement system based on Pockels effect was constructed to detect the residual surface charge distribution after PD occurred in a cylindrical void enveloped by PE. The real discharge current (ns level) of the PD sequences was obtained simultaneously. Depending on the combined measurement system, the current wave of a PD and corresponding discharge area were clearly observed. During continuous discharge sequences, PDs tended to take place at the sites having heterocharges at dielectric surface after voltage polarity reversed. Sometimes, the pulse current waveforms of more than two discharges showed overlapped. This phenomenon could be easily observed at initial discharges after voltage was applied. This may be due to the discharge time lag and photoionization effect. Besides, there were no discharges observed at the decreasing part of applied voltage though some area without surface charges existing.

Simulation of space charge distribution in polyethylene under a temperature gradient

It has been reported that a temperature gradient can have a major influence upon the space charge distribution in polyethylene (PE) and oil-impregnated paper however details of the mechanism by which these modifications are produced have not yet been clarified. The experimental results for PE under a temperature gradient show clear evidence of heterocharge next to the electrodes. This feature has been simulated previously using a bipolar injection model that includes blocked extraction. A direct comparison with PEA experiments however is difficult because the finite spatial resolution of the measurement causes the bulk space charge to overlap with that of the electrodes leaving the experimental values as a mixture of the two quantities that are generated separately in the simulation. Here we allow for the finite spatial resolution of the experimental technique by broadening the simulated charge density values using a Gaussian function. This allows us to compare the simulation and the measurements more effectively. The space charge distribution in PE is simulated by including a temperature dependent charge injection and charge transportation. Under these conditions it is found that the simulation results give a detailed match to the experimental results. They also show that both the temperature dependent charge injection and temperature dependent charge transportation are of importance in determining the space charge distribution in PE under a temperature gradient.

Discharge behaviors of electrical breakdown across nanometer vacuum gaps

Micro-electro-mechanical systems (MEMS) and Nano-electro-mechanical systems (NEMS) are emerging technologies that uses tools and techniques in the microelectronics industry to build microscopic machines. Electrostatic force is often employed to drive the motion components in MEMS and NEMS devices, which could cause extremely high electric field (more than 108 V/m) between two metal conductors. However, the high field intensity may result in electrical breakdown across the conductors in case of improper operations or overvoltage. Therefore, this paper presented a novel experimental technique to study the discharge behaviors across nanometer gaps between 20 nm and 300 nm. The influence of gap separations on breakdown characteristics and the voltage contrast effect in the gap spacing were both investigated. Results showed that the field electron emission did not play a dominate role in the electrical breakdown process across nanometer gaps, which was different from the classical theory of vacuum breakdown, and the breakdown voltage increased as the increase of gap separations. Besides, the voltage contrast effect in the gap spacing was also observed through the scanning electron microscope, which was related to the electric field intensity.

Study on the vacuum surface flashover characteristics of epoxy composites with microTiO 2 fillers under steep high-voltage impulse

With the tendency towards higher power, higher capacity and less scale of the pulsed-power devices, the vacuum surface flashover strength turns to be a primary limitation of the development of the pulsed-power equipments, as it is much lower than the vacuum breakdown strength. To improve the different capabilities of the epoxy composite material, different kinds of micron inorganics are filled to the epoxy resin. TiO2 (TO) is a typical kind of oxide ceramic, as well as an important sort of broad band-gap oxide semiconductor material. With its unique optical and electrical properties, excellent chemical stability, high dielectric constant and semi-conductive properties, it is widely applied to enhance the performance of the organic composite material. In this article, the experimental results of vaccum surface flashover characteristics of epoxy resin and epoxy resin cast with 1μm powdered TO samples are presented. In addition, their dielectric spectrum and thermally stimulated depolarization current (TSDC) are measured. Considering the influence of the dielectric constant, the conductivity, the trap energy and the trap density, associated with the interface theory, the article analyzes the influence mechanism of micro-scale TO fillers on the vacuum flashover voltage of epoxy composite material.

Demonstration of field emission driven microscale gas breakdown for pulsed voltages using in-situ optical imaging

While multiple studies have explored the mechanism for DC and AC microscale gas breakdown, few have assessed the mechanism for pulsed voltage gas breakdown at the microscale. This study experimentally and analytically investigates gas breakdown for gap widths from 1 μm to 25 μm. Using an electrical-optical measurement system with a spatial resolution of 1 μm and a temporal resolution of 2 ns, we measure the breakdown voltages and determine breakdown morphology as a function of the gap width. An empirical fit shows that the breakdown voltage varies linearly with the gap distance at smaller gaps, agreeing with an analytical theory for DC microscale gas breakdown coupling field emission and Townsend avalanche that shows that the slope is a function of field emission properties. Furthermore, the curved breakdown paths captured between 5 μm and 10 μm demonstrate a similar effective length (∼11.7 μm) independent of the gap width, which is consistent with a “plateau” in breakdown voltage. This indicates that Townsend avalanche alone is insufficient to drive breakdown for these gaps and that ion enhanced field emission must contribute, in agreement with theory. The overall agreement of measured breakdown voltage with theoretical predictions from 1 μm to 25 μm indicates the applicability of DC microscale gas breakdown theory to pulsed breakdown, demonstrating that pulsed voltages induce a similar transition from Townsend avalanche to field emission as DC and AC voltages at the microscale.While multiple studies have explored the mechanism for DC and AC microscale gas breakdown, few have assessed the mechanism for pulsed voltage gas breakdown at the microscale. This study experimentally and analytically investigates gas breakdown for gap widths from 1 μm to 25 μm. Using an electrical-optical measurement system with a spatial resolution of 1 μm and a temporal resolution of 2 ns, we measure the breakdown voltages and determine breakdown morphology as a function of the gap width. An empirical fit shows that the breakdown voltage varies linearly with the gap distance at smaller gaps, agreeing with an analytical theory for DC microscale gas breakdown coupling field emission and Townsend avalanche that shows that the slope is a function of field emission properties. Furthermore, the curved breakdown paths captured between 5 μm and 10 μm demonstrate a similar effective length (∼11.7 μm) independent of the gap width, which is consistent with a “plateau” in breakdown voltage. This indicates that Tow...

Breakdown characteristics of PCB paralleled traces injected by rectangular pulse

In the paper the breakdown characteristics of Printed Circuit Board (PCB) paralleled traces is investigated. Rectangular pulse with single pulse mode and repetition frequency pulse mode is injected directly to paralleled traces structure on PCB. To obtain desired pulse waveforms, a pulsed power generator is designed and set up which can output rectangular pulses with different pulse widths, pulse repetition frequency (PRF) and amplitudes. Experimental results show that the breakdown behavior between paralleled traces can be considered as “solid-gas” composite dielectric breakdown, and the breakdown characteristics are related to spacing of paralleled traces, pulse width, PRF and pulse duration, etc. Generally the breakdown voltage (Ubd) rises as trace spacing or pulse width increases and decreases as PRF or pulse duration increases. Overall, the research of susceptibility of PCB trace with conducted pulse injection could provide certain suggestions for PCB EMC design.

Surface flashover of micro TiO 2 epoxy composite dielectric under nanosecond pulse in transformer oil

Micro TiO2/epoxy composite samples with different filler weight percentage were prepared. Their flashover voltages under the 50ns (rise time)/2.5μm (half-height width time) pulse in transformer oil were measured. The results of surface flashover voltages as the number of flashovers, weight percentage of the filler, fluid pressure and the steepness of the pulse were presented. It is demonstrated that the flashover strength of epoxy composite can be enhanced by filling with a huge mount of micro TiO2. Moreover, the results also show that the flashover characteristic of epoxy is significantly affected by the number of flashovers, fluid pressure and the steepness of the pulse.

Pulsed vacuum flashover properties of micro and nano Al 2 O 3 ·3H 2 O/epoxy composites

Micro and nano Al<inf>2</inf>O<inf>3</inf>·3H<inf>2</inf>O fillers are loaded into the epoxy resin by mixing different weight ratio. The flashover studies were conducted using three kinds of nanosecond rise pulse under 5×10⁻³Pa vacuum. The surface flashover properties of nano and micro Al<inf>2</inf>O<inf>3</inf>·3H<inf>2</inf>O/epoxy composites are presented. The influence of filler size and content on the interface structure, interface layer distribution and interface volume proportion was discussed. The relationship between particle size influnence and flashover voltage was analyzed. A deep and shallow trap model based on the multi-layer model was proposed. Considering the size influnence of fillers, the flashover characteristic was able to be explained.

Insulation performance of atomic hexogonal boron nitride film under ultra-high DC electric stress

Hexagonal boron nitride (h-BN) is an ideal twodimensional dielectric material, with excellent physical, chemical, thermal, mechanical and dielectric properties, which makes it especially attractive for logic device applications. For the h-BN film used as a dielectric layer in graphene-based electronic devices, the insulation performance evaluation under ultra-high DC electric stress is a key issue. Nevertheless, only a few works can be found on the insulation performance investigation of atomic h-BN film so far. In the present work, we report on the experimental investigation of current-voltage (I-V) properties and electrical breakdown limit of the multilayer h-BN film (9nm–20nm) using electrical probing system. The I-V properties are measured, and the dielectric strength dependence on the film thickness is also measured. Results show that when the DC electric stress is applied onto the h-BN films, the I-V curves demonstrate three different stages which is independent of the thickness of the films. The breakdown thresholds are various from 0.67V/nm for 9nm to 0.96V/nm for 19nm, which exhibit high dielectric strength. Consequently, the insulation performance evaluation of atomic h-BN film could pave the way towards two-dimensional electronic device applications.