Masahiro Kozako

Surface degradation of polyamide nanocomposites caused by partial discharges using IEC (b) electrodes

Partial discharge (PD) degradation of polyamide both without nanoscale fillers (nanofillers) and with 2,4 and 5 wt% additions of nanofillers was investigated. Such materials were subjected to PDs using the IEC (b) electrodes for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness is far smaller in specimens with nanofillers than those without nanofillers, and that the 2 wt% addition is sufficient for improvement of the surface roughness. Furthermore, it was elucidated that the difference of surface roughness of the degraded area due to PDs among the specimens originates from the difference in their crystalline structures. These results indicate that polyamide nanocomposite is more resistance to PDs than polyamide without nanofillers.

Effects of nano- and micro-filler mixture on electrical insulation properties of epoxy based composites

This paper focuses on the electrical insulation properties of a newly prepared composite material by nano- and micro-filler mixture. Nano- and micro-filler mixture composites were made by dispersing nano-scale layered silicate fillers and micro-scale silica fillers in epoxy resin. To investigate the effects of nano- and micro-filler mixture, the thermal expansion coefficient and insulation breakdown properties by a needle-plate electrode method were measured for the filler mixture composite and the conventional filled epoxy. The filler mixture composite had almost the same thermal expansion coefficient as the conventional filled epoxy. In a continuous voltage rising test, the filler mixture composite had 7% higher insulation breakdown strength than the conventional filled epoxy. Moreover, under constant AC voltage (10 kV at 1 kHz), the filler mixture composite had an insulation breakdown time of more than 20,000 minutes whereas the conventional filled epoxy had a breakdown time of 830 minutes. Electron microscope observation showed that the area surrounded by dispersed micro-scale silica fillers were also filled with the nano-scale layered silicate fillers. Furthermore, the estimate of spacing between the fillers and the filler/epoxy interface area showed a more densely-packed structure of the filler mixture composite than the conventional filled epoxy. The morphological feature of the filler mixture composite seems to improve its insulation breakdown strength and time.

Influence of temperature on mechanical and insulation properties of epoxy-layered silicate nanocomposite

The aim of this study is to investigate the influence of temperature on the mechanical and insulation properties of a newly developed epoxy-layered silicate nanocomposite. This nanocomposite has a higher thermal resistance with respect to mechanical properties than a base epoxy resin (epoxy resin without fillers). The volume resistivity of the nanocomposite gradually decreases with increasing temperature, and its relative permittivity gradually increases with increasing temperature. Its properties are more dependent on temperature than those of the base epoxy resin. Moreover, under a constant AC voltage, the insulation breakdown time of the nanocomposite was twice as long as that of the base epoxy resin at 20 /spl deg/C and six times as long at 80 /spl deg/C. In particular, at 145 /spl deg/C, the nanocomposite had a breakdown time of more than 20,000 minutes while the base epoxy resin had breakdown time of 280 minutes. This improvement in breakdown time resulted from electrical treeing shapes with many branches and smaller internal stress of the nanocomposite in comparison with the base epoxy resin.

Possible mechanisms of superior resistance of polyamide nanocomposites to partial discharges and plasmas

Degradation profiles induced by partial discharges and those induced by oxygen plasmas are compared for polyamide/mica nanocomposites. Both the resistances to partial discharges and to plasmas improve with an increase in nanofiller content. On the other hand, the partial discharge resistance is not improved if mum-sized glass fibers are added to polyamide. In order to investigate these phenomena, the superior resistance mechanism of nanocomposites is discussed, focusing on the effects of the nanofillers on the bulk and surface structures of the resin. It was revealed from X-ray diffraction and permittivity measurements that the nanofiller loading increases crystallinity of the resin and restricts the molecular motion. This should enhance the resistance to degradation. Furthermore, observation results by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction revealed that the nanofillers had piled up themselves to form a layered structure on the sample surface in an early stage of degradation. Such a structure acts as a barrier against impact of charged particles and diffusion of gases such as oxygen, which should contribute to the improvement of resistance to degradation as its direct effect and also as its indirect effect by suppressing the oxidation of resin. Moreover, it was also revealed from scanning electron microscopy that the nanofillers impede the growth of surface cavities by partial discharges drastically.

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.

Difference in surface degradation due to partial discharges between polyamide nanocomposite and microcomposite [electrical insulation applications]

Partial discharge (PD) degradation was investigated to compare polyamide nanocomposites with two kinds of polyamide microcomposites. Such materials were exposed to PDs under the IEC(b) electrode configuration for evaluation of PD resistance. Comparisons were made as to the surface roughness using a scanning electron microscope, an atomic force microscope, and a mechanical surface profilometer. It is concluded that the polyamide nanocomposite is more resistant to PDs than microcomposites, and that nano-effects would work against PD degradation. The nano-effects include filler-matrix bonding, inter-filler space, morphology, and mesoscopic interaction.

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.

Toward High Thermal Conductivity Nano Micro Epoxy Composites with Sufficient Endurance Voltage

We are aiming at the development of epoxy composite materials with both high thermal conductivity and sufficient voltage endurance. A target is set as 10 W/m/K with 5 kV withstand voltage for power electronics applications. In order to achieve high thermal conductivity for polymers such as epoxy, it is a common method to fill inorganic micro particles with high thermal conductivity, but such composites prepared in this way shows lower endurance voltage. To compensate such negative performance, it is considered to be a good way to add nano sized inorganic particles. After many trials were made, good results were obtained for a composite material with two modes of BN and alumina and surface treated nano silica. It has thermal conductivity larger than 5- 10 W/m/K with reasonable withstand voltage.

Surface change of polyamide nanocomposite caused by partial discharges

We have investigated partial discharge (PD) degradation for conventional polyamide-6. without nanoscale fillers (nanofillers) and polyamide-6 nanocomposites with 2 weight (wt) %, 4 wt% and 5 wt% addition. Such materials were subjected to partial discharge under the IEC (b) electrode configuration for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness was far smaller in specimens with nanofillers than those without nanofillers, and that the 2 wt% addition was sufficient for improvement. This result indicates that polyamide-6 nanocomposite is more resistive to PDs than polyamide-6 without nanofillers.

Possible mechanism of superior partial-discharge resistance of polyamide nanocomposites

The dielectric properties of polyamide nanocomposites were examined, focusing especially on their resistance against partial discharges. The paper compares the surface roughness caused by partial discharges and that caused by exposure to plasmas, between polyamide with and without inorganic nanofillers. From X-ray diffraction spectroscopy, it was confirmed that the nanofillers are rearranged in parallel with their surfaces with a mutual distance around 1 nm after the nanocomposite was subjected to partial discharges. Such ordered arrangement of nanofillers should contribute to the high durability against partial discharges in polyamide nanocomposites, in addition to the effects of the presence of mica, high crystallinity, and strong ionic interaction at mica/resin interfaces. Furthermore, the result of a preliminary attempt to detect the structural change through observation of photoluminescence spectra is also reported.