Boya Zhang

Universal behavior of giant electroresistance in epitaxial La0.67Ca0.33MnO3 thin films

We report a giant resistance drop induced by dc electrical currents in La0.67Ca0.33MnO3 epitaxial thin films. Resistance of the patterned thin films decreases exponentially with increasing current and a maximum drop shows at the temperature of resistance peak Tp. Variation of resistance with current densities can be scaled below and above Tp, respectively. This work can be useful for the future applications of electroresistance.

Suppression of surface charge accumulation on Al2O3-filled epoxy resin insulator under dc voltage by direct fluorination

Surface charge accumulation on insulators under high dc voltage is a major factor that may lead to the reduction of insulation levels in gas insulated devices. In this paper, disc insulators made of Al2O3-filled epoxy resin were surface fluorinated using a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for different durations of 15 min, 30 min and 60 min. A dc voltage was applied to the insulator for 30 min and the charge density on its surface was measured by an electrostatic probe. The results revealed significant lower surface charge densities on the fluorinated insulators in comparison with the original one. Surface conductivity measurements indicated a higher surface conductivity by over three orders of magnitude after fluorination, which would allow the charges to transfer along the surface and thus may suppress their accumulation. Further, attenuated total reflection infrared analysis and surface morphology observations of the samples revealed that the introduction of fluoride groups altered the sur...

Fluorine gas treatment improves surface degradation inhibiting property of alumina-filled epoxy composite

Epoxy resins blended with micro-sized alumina show more compact surface condition and exhibit lower light emission after modified in fluorinated atmosphere. Surface morphology reflected by atomic force microscope shows that the fluorinated surface layer can overlap alumina particles inlayed in the original surface and a smoother and more compact surface condition is generated. Thermally stimulated current test exhibits that surface fluorination is able to bring more shallow traps to surface states, and the trap amount increases with the extension of fluorinating time. The photon counting technique illustrates that the fluorinated surface has lower light emission which indicates better aging inhibition properties. Therefore, we believe that the surface fluorination technique may have potential significance in modifying epoxy based insulators used in high voltage applications.

Experimental study on the emission spectra of microwave plasma at atmospheric pressure

An experimental study on microwave plasma at atmospheric pressure was conducted by employing optical emission spectroscopy. Based on a microwave plasma generation device developed for nanoparticle synthesis, we studied the influence of input microwave power and gas flow rate on the optical emission behaviors and electron temperature of plasma using Ar, He, and N2 as working gas, respectively. The physics behind these behaviors was discussed. The results are useful in characterizing microwave plasma at atmospheric pressure and can be used for improving nanoparticle synthesis system for commercial use in the future.

Charge accumulation patterns on spacer surface in HVDC gas-insulated system: Dominant uniform charging and random charge speckles

Charge accumulation on a solid insulator surface is one of the critical factors for the development of dc gas-insulated equipment since it will lead to the overstress of polymeric insulation due to local field distortion and enhancement. Therefore, it is important to study the charge accumulation phenomenon on spacer surface under dc field. For decades, researchers have made tremendous progress on this subject by measurement and simulation. However, measurement results are quite different by different researchers due to various electrode configurations and experimental conditions. Further, most researchers use potential to represent charge density, which is not rigorous in that many charge density distribution details are hidden behind the potential. As for pure numerical simulation, reports are rather academic and sometimes cannot accord with the real fact. In this paper, attempts are made to characterize the charge accumulation patterns on spacer surface in HVDC gas-insulated system. Surface charge distributions on a model GIL spacer in 0.1 MPa air under DC voltage are obtained by an advanced measurement method, from which the dominant uniform charging pattern and random charge speckles are separated. Mechanism responsible for the dominant uniform charging pattern is discussed with the aid of a simulation model. Results indicate that, in a well-cleaned system, the electric current through the spacer bulk is the principal factor, but gas conduction is not negligible due to some inevitable ion sources. Highly localized pockets of charge are also observed, which are referred to as speckles. They may originate from micro discharges due to tiny metal particles on the spacer surface or microscopic protrusions on the electrodes.

Dynamic observation of dc surface charge dissipation for epoxy-resin/alumina composite

The low surface DC flashover voltage for the cone-type insulator has restrained the industrial application of high voltage apparatus especially for gas-insulated substation and gas-insulated transmission lines. However, the surface charge under DC, which produces uniform electric fields resulting in surface flashover under low direct voltage, are still not investigated clearly. In this paper, the dissipation characteristics of the surface charges are studied and the mechanism of the dissipation for surface voltage potential is discussed. The experiment sample is from a 550kV cone-type insulator of GIS with the surface shined by 220# sandpaper, and cut into the size of 140mm×40mm×4mm. 30kV was applied by a cylinder electrode for 10 minutes in atmosphere. To gain the dissipation characteristics of the surface voltage potential, several consecutive points along the insulator surface were chosen, and the voltage potential was detected continuously with an electrostatic probe after the voltage being removed. In addition, the overall surface potential was detected by sliding the probe along the surface of the insulator. The results showed that only negative voltage potential appeared along the surface when the voltage was applied. After the voltage was removed, the surface potential suffered great changes and positive charges were found “flooding” suddenly from both electrodes and moving towards the middle of the insulator surface with time. Only positive voltage potential was detected on the surface after more than 6 hours.

Thermal gradient effects on surface charge of HVDC spacer in gas insulated system

HVDC transmission might increasingly be realized in the form of GIS and GIL. And surface charge accumulation on spacers is one of the main factors that lead to the reduction of insulation level in dc gas insulated system. Due to dc, the electric fields are not capacitively but resistively determined. Further, the electrical conductivity of polymeric materials strongly depends on the temperature. Since actual dc operation is connected with ohmic heating of the conductor, inhomogeneous temperature distribution is inside the spacer. Therefore, the thermal gradient effects on surface charge must be considered. In this paper, surface charge distributions on a model GIL spacer in 0.1 MPa SF6 under DC voltage were obtained by the electrostatic probe method. With the help of a simulation model, the observed results with and without thermal gradient are compared and discussed. It shows that bulk conduction dominates the surface charge accumulation and thermal gradient facilitates the charge injection from the bulk. Surface charge density is higher under thermal gradient and follows an exponential distribution, which coincides with the thermal-dependent conductivity distribution.

Using indium tin oxide material to implement the imaging of microwave plasma ignition process

In this paper, a method is introduced to get global observation of microwave plasma ignition process at high pressure. A microwave resonator was designed with an indium tin oxide coated glass at bottom. Microwave plasma ignition was implemented in methane and air mixture at 10 bars by a 2 ms-3 kW-2.45 GHz microwave pulse, and the high speed images of the ignition process were obtained. The images visually proved that microwave plasma ignition could lead to a multi-point ignition. The system may also be applied to obtain Schlieren images, which is commonly used to observe the development of flame kernel in an ignition process.

Inversion Algorithm to Calculate Charge Density on Solid Dielectric Surface Based on Surface Potential Measurement

Charge accumulation on a solid dielectric surface is one of the critical concerns for the design and optimization of the insulation system in a high-voltage power equipment, since it will lead to the overstress of electrical insulation. Therefore, it is important to obtain the charge density distribution on a solid dielectric surface with high accuracy. The acquisition of surface charge for insulators requires multipoint potential measurements to establish the inverse calculation for the determination of an unknown charge distribution. Up to now, extensive studies have been conducted on this subject; nevertheless, the methods are either too complicated and time consuming, or only applicable for specific arrangements, or with poor accuracy. In this paper, the problem is divided into two categories, i.e., shift-variant system and shift-invariant system, and the basic principle of an improved inversion algorithm is interpreted to solve the problem. The 2-D Fourier transform and Wiener filter techniques are employed in the algorithm for shift-invariant system thus the relationship between potential and charge density can be processed in spatial frequency domain, which tremendously simplifies the conventional procedure. The accuracy and resolution of the algorithm are discussed in detail with the aid of numerical examples. In the end, experiments are conducted and the effectiveness of the algorithm is verified.

Decomposition of P-Xylene by microwave plasma generated at atmospheric pressure

This is an experimental study on effectively decomposing volatile organic waste by atmospheric-pressure microwave plasma. P-Xylene (PX) was used as a sample agent in the experiment because it is the main volatile organic compound (VOC) exhaust waste produced in many chemical industries and difficult to eliminate completely. We developed a microwave plasma apparatus which was designed for research on characteristic of atmospheric plasma and material treatment. The microwave plasma could be easily generated and be stably existing at the microwave frequency of 2.45GHz, and the power of 2-2.5 kW in the air. 2-4 lpm nitrogen as the carrier gas conducted p-Xylene into the air plasma zone to react. The GC analysis presented that the microwave plasma could eliminate p-Xylene efficiently at the destruction rate over 98%. And optical emission spectroscopy was also studied for research of reaction process. NO2, the main by-product which was produced by recombination of N2 and O2 at a high temperature, could be eliminated completely by passing the Exhaust gas into the alkaline solution. Due to its high efficiency, microwave plasma could be an advantageous option in VOC decomposing in PX industry.