Zengbin Wang

Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength partI - sample preparations and thermal conductivity

The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m·K, 3.95 W/m·K and 10.1 W/m·K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m·K, 5.94 W/m·K and 12.3 W/m·K, respectively, by adding extra smaller A1N (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of c-BN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m·K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m·K was obtained by adding extra AIN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.

Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength part II-breakdown strength

The aim of this research is to find a way to achieve the epoxy composites with high thermal conductivity and acceptable dielectric breakdown (BD) strength. A value 12.3 W/m·K is the highest thermal conductivity obtained for epoxy composite in Part I. Dielectric breakdown performances such as short-time dielectric breakdown strength (BD strength), partial discharge (PD) resistance and BD time for composites were investigated in the Part II. In general, micro filler inclusion will increase thermal conductivity and decrease dielectric breakdown performance. Influencing factors are considered to be the orientation of filler, the content of void space, the content ratio in the case of co-mixing, the addition of nano filler, and filler surface modification. Twenty six kinds of composites were prepared in consideration of the above influencing factors. There are two options for most appropriate ones among the composites evaluated in the research. One is an epoxy/ conglomerated h-BN composite with co-loaded nano SiO2 and micro AIN filler. It has 12.3 W/m·K in thermal conductivity, 75.1 kVpeak/mm in BD strength and 260 % of BD time for neat epoxy. It is most suitable when low BD strength and high thermal conductivity is needed. The other one is an epoxy/ h-BN composite with co-loaded nano silica and AIN filler for requirement of very high BD strength but lower thermal conductivity. Optimum thermal conductivity is obtained if flaky h-BN filler is oriented in parallel to heat flow. Since it is difficult to realize full orientation, the use of conglomerated h-BN filler is a suitable option. Optimum BD performance is obtained if void space is reduced by certain methods such as co-dispersion of different size fillers and addition of nano filler.

High thermal conductivity epoxy/BN composites with sufficient dielectric breakdown strength

Various types of epoxy/BN composites were prepared to optimize material conditions for high thermal conductivity and high breakdown strength. The best composite was found to be epoxy/conglomerated h-BN/ nano silica nano micro composite with thermal conductivity 12 W/m•K and BD strength 15 kVpeak/0.2 mm. One of the most important factors to obtain high values of the two performances is to reduce void content. Surface treatment of fillers and nano filler addition are also useful.

Pulsed Vacuum Surface Flashover Characteristics of

In this paper, micro (mean size is 1 μm) and nano (mean size is 10 nm) TiO2 (Titania) were loaded into epoxy resin at the same time. Eleven kinds of samples with different filler contents were prepared. Flashover characteristics of the epoxy composites were investigated under 50 ns (rise time)/1.5 μs (half-height width time) unipolar pulse and 5 × 10-3 Pa vacuum degree. It was found that the nano TiO2/epoxy composites are of higher flashover voltage than neat epoxy, and the addition of micro TiO2 in appropriate amount was able to further improve the flashover strength of the nano TiO2 /epoxy composite. In order to research the mechanism of micro and nano fillers, the permittivity and thermally stimulated depolarization current of the samples were carried out. It was clarified that, when micro filler is added into nano composite, deeplike traps can be increased remarkably, whereas shallowlike traps increase much less and even decrease in some samples. Considering the result of flashover voltage, it was confirmed that deeplike traps will increase flashover voltage.

\hbox{TiO}_{2}/\hbox{Epoxy}

The research on vacuum surface flashover mechanism of insulating material under steep high voltage impulse is important for the design and manufacture of high power pulse generator. This report studies the vacuum surface flashover characteristic of epoxy composite material with different fillers is studied. Generally, various kinds of micron-sized inorganic fillers are utilized to promote different properties (such as reinforcement, arc resistance and fire retardant etc.) of epoxy composite. In this report, comparison study is carried out on the epoxy composites with different kinds of micron-fillers, such as Al2O3, Al2O3·3H2O (ATH), TiO2, BaTiO3 and the oxide films of Al2O3 and TiO2. By using two single-pulse generators (rise time and the half-height pulse width time are 50ns/600ns and 65ns/600ns, and 10¿4Pa ultimate vacuum level), the flashover characteristics of these samples are measured, and the effects of different weight proportion of ATH powder and different kinds of micro-fillers are also analyzed.

Nano–Micro Composites

The pulsed flashover strength of epoxy composite material in vacuum is related to the vacuum level, the electrode material, the relative dielectric constant, and the surface charging quantity of the sample. By using a high power single pulse generator (65ns rise time, 600ns half-height pulse width time and 10¿4Pa ultimate vacuum level), the vacuum surface flashover characteristics of epoxy and epoxy/Al2O3·3H2O composite material are studied by using the fingertype electrode with different materials (brass, duralumin and stainless steel). In the case of stainless steel-stainless steel fingertype electrode system, the flashover experiments under different vacuum level are investigated. Based on the ANSOFT Maxwell 3D software, E-field distribution of the electrode system, the effect of the relative dielectric constant of the sample on the maximum value of E-field magnitude and the role of surface charging of the sample on the maximum value of E-field magnitude are simulated and discussed.

Study on the Vacuum Surface Flashover Characteristics of Epoxy Composites with Different Fillers under Steep High-Voltage Impulse

Surface flashover in vacuum is a primary limitation to pulsed-power devices. Recently, there are more studies on the pulsed vacuum surface flashover focused on the flashover mechanism. In this paper, the UV illumination is used to study the mechanism of vacuum surface flashover. The effects of different illumination positions, different illumination durations and different illumination intensities are investigated. The results show that the pulsed vacuum surface flashover characteristic is affected by the UV illumination significantly. When the UV light illuminates the cathode triple-point junction, the flashover voltage is reduced; this reduction is almost unaffected by the illumination duration, but significantly increased with the enhancement of illumination intensity. When the UV light illuminates the sample surface, the flashover voltage reduction is increased both by the increases of illumination duration and illumination intensity. Moreover, UV illumination can also change the surface potential and thus influence the surface flashover characteristic.

Experiments and Simulations on Influencing Factors of Pulsed Surface Flashover in Vacuum

Temperature gradient through the cable thickness occurs due to Joule heat arising from the electric current in the high voltage cable conductor, and affects the injection and migration of charge, leading to the insulation failure while a sudden power off or polarity reversal takes place due to the charge accumulation. In this article, based on the PEA (pulsed electroacoustic) method, the space charge accumulation and maximum transient electric field distortion during polarity reversal were studied under the different temperature gradient and a direct current stress of 50kV/mm. The experimental results show that the maximum steady-state electric field was measured near the low temperature electrode, but the maximum transient electric field could appear near the high temperature electrode.

Effect of UV illumination on surface flashover characteristics of epoxy resin under pulse

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.

Effects of temperature gradient on space charge accumulation at applied voltage reversal in LDPE film

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.