Effects of nano-filler addition on partial discharge resistance and dielectric breakdown strength of Micro-Al2O3Epoxy composite

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1070-9878/10/$25.00 2010 IEEE Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown Strength of Micro-Al2O3/Epoxy Composite Zhe Li IPS Graduate School, Waseda University, 2-7, Hibikino, Wakamatsu-ku, Kitakyushu-shi, Japan Kenji Okamoto Product Technology Laboratory, Fuji Electric Advanced Technology, 1, Fuji-machi, Hino-shi, Tokyo, Japan Yoshimichi Ohki Department of Electrical Engineering and Bioscience, Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo, Japan and Toshikatsu Tanaka IPS Research Center, Waseda University, 2-2, Hibikino, Wakamatsu-ku, Kitakyushu-shi, Japan ABSTRACT It is often observed that the insulation structure for an insulated gate bipolar transistor (IGBT) suffers from dielectric failure, when the insulation is made of epoxy resin to which micro fillers with a high thermal conductivity were added. In order to reveal the above phenomena and to clarify the breakdown (BD) mechanism, we have carried out experiments using an MB-PWB (metal-base printed wiring board) insulation simulated structure. As a result, it was clarified that the IGBT insulation breaks down after successive partial discharges (PDs). It was also elucidated that BD strength becomes lower, when epoxy resin was loaded with high content of micro-fillers. A trial was made to raise the once-lowered BD strength by adding nano-Al2O3 fillers. Three kinds of experiments were carried out, i.e. an MB-PWB insulation simulated structure for dielectric failure, a rod-to-plane electrode for PD erosion resistance, and a sphere-to-sphere electrode for BD strength for four kinds of insulation samples, i.e. neat epoxy, 5-wt% nano- Al2O3/epoxy composite, 60-wt% micro-Al2O3/epoxy composite, and combined 2-wt% nano- and 60-wt% micro-Al2O3/epoxy composite. It was clarified that the nano-micro-composite is higher in both BD strength and PD resistance than the micro-composite. It should be noted that the addition of nano-fillers would provide an excellent approach that can increase the dielectric BD strength and time of micro-filled epoxy composites. Index Terms Nano-composite, epoxy, nano-micro-composite, nano-Al2O3, surface discharge, PD resistance, erosion, breakdown strength. 1 INTRODUCTION Insulation for electronics devices has attracted much more attention than ever, because high density integration of such devices including PCB (Printed Circuit Board) is required, and because replacement of ceramics by epoxy resins is expected for power electronics modules. In the case of PCBs, ion migration is one of the limiting factors as long term characteristics for proper design, and dielectric breakdown over inter-wire insulation based on air or gel is critical against repetitive transient surge impulses coming from outside inverter-fed modules, devices or motors. Some technical measures should be taken to reduce ion migration and inter-wire distance. MB-PWB (metal-base printed wiring board) insulation for power electronics devices such as IGBT (Insulated Gate Bipolar Transistor) will need new insulation instead of ceramics. Ceramics own sufficiently high thermal conductivity and heat resistance, but are brittle and expensive. They can be replaced by epoxy resins that are considered to be mechanically tough and rather inexpensive. Epoxy resins should be mixed with appropriate micrometer sized fillers or micro fillers to make micro composites in order to obtain high thermal conductivity and high heat resistance, but such micro-filled epoxy resins will suffer lower breakdown strength. Addition of nano-fillers to micro composites must be useful to raise the once Manuscript received on 4 June 2009, in final form 23 October 2009. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 3; June 2010 653lowered breakdown strength [1-4], and in other words, nano- structured micro composites must take back their intrinsic characteristics of breakdown strength. This paper focuses on this aspect of the problems related to electrical insulation for power electronics devices. It was already clarified that the addition of nano-fillers such as layered silicate and silica would increase the electric strength of micro silica filled epoxy composite [5]. It was also found that the dielectric strength of EVA and PP increased with the increase in the content of organophilic nano-silicate [6]. Some researchers reported that dielectric and insulation characteristics of material improved by blending low content of nano-fillers [7-8]. Surface erosion due to partial discharge (PD) was also intensively investigated resulting in the fact that the addition of small amount of nano fillers would increase PD resistance in insulation such as epoxy and polyethylene enormously [9-22]. Based on the experimental results and previous research, it was found that thermal conductivity of composite increases with the content of micro-filler [23]. The thermal conductivity of ceramic is more than 20 W/(mK), which implies that it is a purpose to preparing 90 wt% micro alumina/ epoxy composite for this thermal conductivity. However, degas and curing becomes difficulty due to the high viscosity in high content micro composite. The 60 wt% concentration micro composite is prepared successfully in our laboratory, and the mixing technology will be improved for higher than 60 wt% concentration micro composite in the future. 2 EXPERIMENTS In this paper, four kinds of material were prepared for investigation, which were neat epoxy, 5 wt% nano alumina (Al2O3)/epoxy composite (NC), 60 wt% micro alumina/ epoxy composite (MC) and 2 wt% nano- 60 wt% micro alumina/epoxy composite (NMC). Three kinds of experiments were carried out, which are investigation of partial discharge erosion and followed breakdown processes in an MB-PWB insulation simulated structure, PD erosion tests in the rod-sample-plane structure and breakdown strength tests in the sphere to sphere electrode structure with a flat sample inserted. Table 1. Materials used to prepare composites. Materials: Manufacturers Epoxy Resin 816B Japan Epoxy Resin Co. Micro-Al2O3 (AO-809, spherical, OD:10 m) Admatechs Co. Nano- Al2O3 (TM-300, spherical, OD: 7 nm) Taimei Chemicals Co. Coupling Agent KBM-403 ShinEtsu Chemical Co Hardener 113 ShinEtsu Chemical Co Table 2. Content of fillers in 4 kinds of samples. Nano-Al2O3 wt % Micro-Al2O3 wt % Coupling Agent wt Neat epoxy 0 0 0 Nano alumina /epoxy composite 5 0 7.5 Micro alumina /epoxy composite 0 60 0 NanoMicro alumina /epoxy composite 2.03 59.2 3 2.1 PREPARATION OF SAMPLES The materials used to prepare samples were shown in Table 1. Nano-Al2O3 fillers and coupling agents were put into a solution of epoxy. In order to disperse the nano fillers in epoxy homogeneously, the solution was mixed by the Triple Roll Mill Noritake Co., Ltdand then passed through the Nanomizer System (YSNM-1500: YOSHIDA Co.) to make an intermixture. Hardener was then blended into the intermixture, and then mixed via the mixer of ARE-250 (THINKY Co.). Then, the intermixture was degassed in a vacuum drying oven. Thus obtained solution without any air bubbles was decanted into a cast mold composed of a 2002000.2 mm space. A resulted plate sample was then cured in an oven at 70 C for 3 h, post-cured at 120 C for 3h, and finally cooled down gradually to room temperature at 1 C/ min to obtain a final shape of the sample. Four kinds of material were prepared for investigation, which were neat epoxy, 5 wt% NC, 60 wt% MC and 2 wt% & 60 wt% NMC shown in Table 2. In order to improve the interface between nano-fillers and epoxy, coupling agent was added. In order to simulate MB-PWB insulation structure, an electrode-sample structure was prepared before curing the epoxy. A sheet of 30300.003 mm copper foil was mounted on a 30300.2 mm sample. A top electrode 10100.003 mm was formed via an etching method. The sides and corners of the top electrode were observed via an optical microscope, which looked smooth as shown in Figure 1. SEM (electron scanning microscope) images of nano-Al2O3/epoxy composite and nano-micro-Al2O3/epoxy composite shown in Figure 2 indicate that good filler dispersion was obtained. Figure 1. Etching method used and an image in the corner observed with the optical microscope. a) nano-Al2O3/epoxy composite b) nano-micro-Al2O3/epoxy composite Figure 2. SEM images of nano-composite and nano-micro-composite samples. Curing Etching 654 Z. Li et al.: Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown Strength2.2 PARTIAL DISCHARGE AND BREAKDOWN TIME MEASUREMENTS IN AN MB-PWB INSULATION SIMULATED STRUCTURE The 600 Hz voltages of 3, 3.5 and 4 kVrms were applied to a metal-base PWB. In order to evaluate the discharge intensity at different voltages, surface discharge images captured by the CCD camera (Hamamatsu Photonics K.K., Model 9164) as shown in Figure 3. Figure 3. An MB-PWB insulation simulated structure for partial discharge observation by CCD camera. The light intensity analysis function of the CCD camera was used to analyze the light intensities emitted from discharges at the sides and corners of the electrode. Surface erosion was comparatively investigated for epoxy and composite samples of neat epoxy, nano-Al2O3 /epoxy composite, micro-Al2O3/epoxy composite and nano-micro- Al2O3 /epoxy composite. A voltage of 600Hz 4.8 kVrms was applied to the above samples for surface discharge exposure experiments for 2 h to observe the surface erosion traces caused by PDs. In order to evaluate the PD and BD resistance of materials, PD time was extended more than 2 h until breakdown occurs and the breakdown time was recorded. 2.3 TEST OF EROSION DEPTH CAUSED BY PARTIAL DISCHARGES Depth of erosion caused by partial discharges was investigated to evaluate PD resistance of several kinds of samples in a rod to plane electrode system as shown in Figure 4. It is composed of a tungsten rod electrode of 1 mm in diameter facing a plate dielectric sample contacted with a brass plate grounding electrode with a air gap separation of 0.2 mm. Voltage of 600 Hz 2kVrms was applied to the rod electrode to age the four kinds of samples for 60 Hz equivalent time of 120, 240 and 445 h. The erosion depth is measured by the violet laser microscope (KEYENCE Corporation VK-9510). Figure 4. A rod to plane electrode system used for PD erosion experiments. 2.4 BREAKDOWN STRENGTHS TEST IN A SPHERE-PLATE SAMPLE-SPHERE ELECTRODE SYSTEM Dielectric breakdown experiments were carried out in such an electrode system that a dielectric plate sample of 0.2 mm in thickness was inserted between a pair of sphere steel balls of 20 mm in diameter as shown in Figure 5, and a whole electrode system was immersed in silicone oil. High voltage of 60 Hz was applied via a transformer and boosted by the rate of 0.5 kVrms/s. Figure 5. The structure of sphere-plate sample-sphere electrode system 3 EXPERIMENTAL RESULTS 3.1 PARTIAL DISCHARGE AND BREAKDOWN TIME PD erosion tests were carried out in an electrode-sample structure shown in Fig. 3 and sub-Figure 6a. When voltage is increased, discharge light becomes visible slightly at 3 kVrms, as seen in sub-Figure 6b. Such discharge light tends to clearly increase in their strength with the increase of voltage as shown in sub-Figures c and d. A surface discharge phenomenon in the initial stage of 4 kVrms voltage application is shown in sub-Figure 6d. Series of photos d and e indicate that light intensity emitted from surface discharges tends to increase with time under 4 kVrms voltage application. That is, discharges must grow with time from their initial stage at least up to 2 h. (a) (b) (c) (d) (e) (a) Electrode structure, (b) 3 kVrms, (c) 3.5 kVrms, (d) 4 kVrms, and (e) 4 kVrms for 2 h. Figure 6. Electrode structure and surface discharge images of epoxy sample captured with a CCD camera in the same exposure time of 1.67 ms. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 3; June 2010 655camera As shown in Figure 6, the light intensity emitted from surface discharges in the four corners of electrode is the highest, and the initial discharges begin from the corners. As voltage is increased, discharges occur at the sides of the electrode. Even if etching was carried out at the process of preparing samples, the smoothness of each edge is not necessarily guaranteed as well as the surface roughness of samples. It must cause irregular surface discharge behaviors at the edge of the electrode. It was found that the light intensity at the corners is about two times that at the sides in initial stage under 4 kVrms voltage application, while it becomes more than three times after 2 h shown in the photos d and e. It can be recognized that the stronger light was emitted at 2 h after 4 kVrms voltage application. Surface discharge can erode the insulation materials, and long-time discharge tends to produce erosion pits to cause treeing channels possibly leading even to breakdown. Figure 7 shows some of the resulted photos of surface conditions around the corner and side of clad foil electrodes. As shown in sub-Figure 7a, the side and corner parts of the clad electrode in the original samples were smooth. However, they became rough, when voltage was applied to cause surface discharges for some time. Change of surface conditions was recognized, after the voltage had been continuously applied for less than 2 h. As shown in sub-figures 7b, 7c, 7d and 7e, the neat epoxy, the nano alumina/epoxy composite, and the nano-micro alumina /epoxy composite withstood the applied voltage for 2 h, but on the other hand, the micro alumina/epoxy composite suffered from breakdown in 1.5 h. Figure 7. Erosion processes for four types of samples subjected to surface discharges at 4.8 kVrms for the time duration up to 2 h. In order to evaluate the failure strength of the epoxy and composite samples prepared, voltage application time was extended to longer than 2h until dielectric failure or breakdown occurs. As shown in Table 3, neat epoxy, nano Al2O3/epoxy composite and nano-micro-Al2O3 /epoxy composite broke down at 186 min, 307 min and 275 min respectively. Based on the evaluation by the MB-PWB insulation, it was elucidated that failure or BD time increased by 65 % and 192 %, if neat epoxy and micro-composite had been nano-structured, respectively. It can be concluded that the order of failure withstand level is nano-composite> nano-micro-composite > neat epoxy >micro-composite as shown in Figure 8. Table 3. Breakdown time of 4 kinds of materials. Breakdown time (min) Neat epoxy 186 Nano-composite 307 Micro-composite 94 Nano-micro-composite 275 18630794275050100150200250300350Epoxy NC wi t h CA MC NMC wi t h CABreakdown Time (min) Figure 8. Comparison of dielectric failure time for 4 kinds of samples. Causes of the failure may include PD and BD processes. 3.2 EROSION DEPTH CAUSED BY PARTIAL DISCHARGES Figure 9 shows the temporal dependence of the depth of PD caused erosion for the four kinds of samples. It can be seen that the erosion depth increases with time but tends to saturate for a long run for nano composite and nano micro composite samples, while it increases with time linearly for neat epoxy samples. Figure 10 demonstrates the depths of erosion obtained at 455 h after voltage application. According to Figure 10, nano-composite and nano- micro-composite samples exhibit much more reduction in erosion depth in the long aging time range of 455 h than neat epoxy and micro composite samples. It can be concluded that nano-structured epoxy composite samples show superior performances in PD resistance to neat epoxy and micro composite samples in long PD exposure time over 60 Hz equivalent 455 h. a) Original epoxy samples clad with a copper foil electrode b) Epoxy samples subjected to 4.8 kVrms for 2 h c) Epoxy nano-composite samples subjected to 4.8 kVrms for 2 h d) Epoxy micro-composite samples subjected to 4.8 kVrms for 1.5 h 656 Z. Li et al.: Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown Strengthe) Epoxy nano-micro-composite samples subjected to 4.8 kVrms for 2 h 100 150 200 250 300 350 400 450-20020406080100120140160180200220Erosion depth (m)60 Hz equivalent Time (h) epoxy MC NC NMC Figure 9. Depth of PD caused erosion for the 4 kinds of samples versus aging time. NC: Nano-composite, MC: Micro-composite, NMC: Nano-micro-composite. 020406080100120140160180200Epoxy MC NC NMCErosion depth (m) Figure 10. Bar graph of erosion depth of the 4 kinds of samples at the time 455 h. 3.2 BREAKDOWN STRENGTH Micro composites are considered to show lower BD strength than their neat polymers [1-4], but they are expected to exhibit some increase, when they are nano- structured, or actually epoxy parts are nano-structured. Figure 11 shows experimental results of breakdown strengths represented by the Weibull distribution for four kinds of samples, i.e. neat epoxy, micro-composite, nano-composite, and nano-micro-composite. 567891010203040506070809060 70 80 90 100 200 30063.2E kV/mmPercentage [%] Epoxy 816B NC without CA NC with CA MC NMC without CA NMC with CA Figure 11. Weibull distribution of breakdown strength evaluated for 4 kinds of samples by the sphere-flat sample-sphere electrode system. The breakdown strengths at the percentage are 63.2 % are shown in Figure 12. It was elucidated that micro-composite has a low breakdown strength as compared to neat epoxy. BD strength decreases down by 55 % from 202.8 to 90.3 kVrms /mm. It actually increases by 4.3 % from 94.3 to 90.3 kVrms /mm when the micro-composite is loaded by nano-fillers. Coupling agent has no significant effect on BD strength in this case as shown in Figure 11. 202.8 21290.3 94.2050100150200250Epoxy NC with CA MC NMC with CAE kV/mm Figure 12. The breakdown strengths (percentage is 63.2%) of the four kinds of samples using the sphere-flat sample-sphere electrode system. 4. DISCUSSION Micro-Al2O3 fillers can increase the thermal conductivity of epoxy composite as advantage. But they will cause some reduction in dielectric breakdown strength as disadvantage. This becomes a big issue to solve in power electronics field. From our experiences in nano-composites, it would be worthwhile to add nano fillers into a micro-composite to help compensate such reduction to a certain degree. Based on the experiments of PD resistance of sheet samples through the evaluation of erosion caused by partial discharges and surface discharges in metal-base PWB samples, nano-structured composites actually exhibit better performances than the unstructured samples. Breakdown of MB-PWB insulation consists of two processes, i.e. PD erosion and followed breakdown processes. The two processes have been explored in this paper, it is now sufficient that we assume breakdown paths or treeing paths tend to be blocked by nano-fillers. In the case of micro fillers, it should be taken into consideration that they can be defects resulting in lower breakdown field. The experiments in 3 kinds of electrode-sample structures were investigated, and different erosion and breakdown processes would be concluded. Three kinds of electrode-sample structures can be defined: a) PD erosion experiments; b) breakdown strength experiments and c) dielectric failure experiments in MB-PWB insulation simulated structure. Different rankings of 4 kinds of materials were obtain for evaluation, which were nano-micro-composite > nano- composite > micro-composite > neat epoxy in the case of a) as shown in Figure 10; nano-composite > neat epoxy >> nano-micro-composite > micro-composite in the case of b) as shown in Figure 12; IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 3; June 2010 657nano-composite> nano-micro-composite > neat epoxy > micro-composite in the case of c) as shown in Figure 8. Large Circles: Micro fillers, Small Circles: Nano fillers, and Yellow rod: tungsten rod electrode Figure 13. Partial discharge erosion behaviors. In the case of a) PD erosion experiment, the tungsten electrode didnt touch the sample as shown in Figure 13. The materials were eroded by corona discharge, and this erosion propagates covering an arc area. Electrical trees hardly formed, so the defect introduced by micro-fillers didnt play an important role in this erosion process, while micro-filler play a role to resistant the erosion caused by corona discharge, as well as nano-filler. A nano- or micro- composite is eroded mainly in its polymer regions. The permittivity of inorganic filler materials is higher than that of organic polymers, so it is subjected more to PD than the latter. This phenomenon will help increasing PD resistance. As a whole, it can be said that nanometer scale segmentation is more favorable than micrometer scale segmentation as for PD and BD resistance [9]. Depending on this opinion, it is easily understood that micro-Al2O3/epoxy composite and nano- Al2O3/epoxy composite have a better PD resistance than neat epoxy; furthermore, nano-micro- Al2O3/epoxy composite has the best PD resistance. Large Circles: Micro fillers, Small Circles: Nano fillers,Yellow half ball: sphere electrode Figure 14. Breakdown process caused by electrical tree propagation. In the case of b) breakdown strength experiments, as compared to case a) PD erosion experiments, there is a osculant structure between electrode and sample in the sphere-sample-sphere structure. An electrical tree forms easily under the condition of applying the voltage between both electrodes. As boosting the voltage continuously, the tree will penetrate into bulk of the sample, and propagates rapidly as shown in Figure 14. The tree will accelerate to grow when its tip touches the defect between micro-particle and polymer. That is why the breakdown strength of micro- Al2O3/epoxy composite decrease 55 % that of neat epoxy heavily. In this case, defects introduced by addition of micro-filler play an important role in reducing the breakdown strength of composite. However, such defect doesnt exist in nano filled epoxy, so nano-particles could be considered playing a role resistant to tree channel. That was reported previously [8-11]. In the case of c) dielectric failed experiments in MB-PWB insulation simulated structure, which electrode structure consists of cooper foil electrode, sample and brass board electrode. As shown in Figure 15, surface discharge erosion and electrical tree occurs at the same time. In the case of micro-Al2O3/epoxy composite, surface discharge eroded the surface of the material, and an electrical tree propagates until breakdown occurs. Defects are devastating to materials as electrical tree propagates, so the addition of micro-fillers reduced the breakdown time of epoxy. The depth of surface discharge erosion is decreased and the tree channel is blocked by nano-particles, as nano-fillers are added. It is concluded that nano-filler improves the PD resistance greatly and plays an important role to block the electrical tree channel. Large Circles: Micro fillers, Small Circles: Nano fillers, and Yellow rectangle: copper foil electrode and brass board electrode Figure 15. Partial discharge erosion behaviors and the breakdown process caused by electrical tree propagation. In addition, there is a different breakdown process between the case b) sphere-sample-sphere structure in Figure 14 and the case c) MB-PWB insulation simulated structure in Figure 15. As boosting the voltage continuously, the electrical tree propagates rapidly, and even bypasses the particles if it is blocked. That is why nano-Al2O3 just increases about 8% breakdown strength of neat epoxy and micro-Al2O3/epoxy composite in the system of sphere-sample-sphere structure, i.e. the effect 658 Z. Li et al.: Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown Strengthof nano-fillers is not prominent due to electrical tree propagates rapidly caused by boosting voltage continuously in the sample. In the case of the experiments of MB-PWB insulation simulated structure, voltage is fixed at 4.8 kV, hence the tree penetrates into the bulk of polymer and propagates slowly. In this condition, breakdown is a long-time process, and nano-filler has an obvious effect to resist the electrical tree as BD resistance. As mentioned above, nano-structured epoxy and micro-composite increased 65 % and 192 % BD time of neat epoxy and micro-composite. Surfaces of nano-particles were treated by coupling agent for the experiment of breakdown strength test in sphere-sample-sphere structure. The effect of coupling agent seems not prominent as shown in Figure 11. It is possible that insufficient surface treatment was carried out. Good surface treatment will be done in the future, and some treatment method has been proved effective to increase the dielectric properties of nano-composite [24-27]. Careful examination is needed of the improving breakdown strength on MB-PWB insulation by addition of nano-fillers. Breakdown is determined by either PD resistance while PDs are taking place, or no occurrence of PDs due to some reason. If PDs tends to diminish under a certain condition, PD erosion must not be anticipated. This problem has been often encountered in PDs in a closed void, which is not understood properly in the study of PD resistance. This is also out of the scope in this paper, but this should be clarified in the future to develop such MB-PWB insulation systems. Results of 3 kinds of experiment indicate that nano-structured composites exhibited better PD and BD resistance than unstructured epoxy and micro-composites. With these findings in mind, several models are considered to explain breakdown caused by the propagation of tree as well as PD and surface discharge resistance in epoxy composites. However, the effect of nano-fillers at our present level of sample preparation is not tremendous. Improvement is expected probably from more homogeneous dispersion and more proper surface treatment of nano-fillers. 5. CONCLUSIONS It is concluded that neat epoxy resins and micro-filled epoxy resins can be improved in such dielectric performances as short-time BD (breakdown) strength, PD (partial discharge) resistance, and BD by failure processes to be encountered in an MB-PWB(metal-base printed wiring board) insulation simulated structure, if both of them are nano-structured. This conclusion was derived from the following experiments. The four kinds of insulation samples, i.e. neat epoxy, (NC) 5 wt% nano-Al2O3/epoxy composite, (MC) 60 wt% micro-Al2O3/epoxy composite and (NMC) 2 wt% nano-60 wt% micro-Al2O3/epoxy composite were prepared in our laboratory to evaluate the dielectric performances of epoxy/alumina composites. Three kinds of electrode systems, i.e. an MB-PWB insulation simulated structure for dielectric failure, a rod-to-plane electrode for PD erosion resistance, and a sphere-to-sphere electrode for short time BD strength were constructed to evaluate the prepared insulation samples. A summary of the experimental results is as follows: 1) MB-PWB insulation is considered to often suffer from dielectric failure, when the insulation is made of epoxy resins with high thermal conductivity micro fillers. This performance was experimentally confirmed by using an MB-PWB insulation simulation structure to reveal that the insulation failed due to PD aging and followed BD. 2) It was obtained for MB-PWB samples that failure or BD time increased by 65 % and 192 %, if neat epoxy and micro-composite had been nano-structured, respectively. Ranking in this case is NC > NMC > neat epoxy > MC. 3) It was clarified that nano-micro-Al2O3/epoxy composite is higher in PD resistance than micro-Al2O3/ epoxy composite. The depth of PD caused erosion increases with time but tends to saturate for a long run for nano composite and nano micro composite samples, while it increases with time linearly for neat epoxy samples. Ranking in PD resistance is NMC > NC > MC > neat epoxy. 4) It was elucidated for BD strength of flat samples evaluated by a sphere to sphere electrode system that BD strength becomes lowered, when epoxy resins were loaded with high content of micro-Al2O3 fillers. BD strength decreases down by 55 % from 202.8 to 90.3 kVrms /mm. It actually increases by 4.3 % from 90.3 to 94.2 kVrms /mm when the micro-composite is loaded by nano-fillers. Coupling agent seems to have no significant effect on BD strength in our present case. Ranking in this case is NC > neat epoxy >> NMC > MC. 5) From the comparative analysis of the rankings for failure modes of MB-PWB insulation simulated structure, PD resistance and short time BD strength, the failure mode of MB-PWB insulation is considered to be determined by a PD initiated BD process. If PD is dominant in this process, the dielectric performance of this insulation system can be much improved via nano-structured performance. 6) It should be noted that the addition of nano-fillers would provide a good approach enable to increase the dielectric breakdown strength of micro-filled epoxy composites, too. 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Tanaka, Insulation Properties of Nan0- and Micro-Filler Mixture Composite, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp.171-174, 2005. [17] T. Tanaka, S. Kuge, M. Kozako, T. Imai, T. Ozaki and T. Shimizu, Nano Effects on PD Endurance of Epoxy Nanocomposites, Intern. Conf. Engineering Education (ICEE), No. ME1-01, pp.4, 2006. [18] T. Tanaka, A. Nose, Y. Ohki and Y. Murata, PD Resistance Evaluation of LDPE/MgO Nanocomposite by a Rod-to-Plane Electrode System, IEEE Intern. Conf. Properties and Applications of Dielectr. Materials (ICPADM), pp.319-322, 2006. [19] T. Tanaka, Y. Ohki, T. Shimizu and S. Okabe, Superiority in Partial Discharge Resistance of Several Polymer Nanocomposites, CIGRE Paper D1-303, pp.8, 2006. [20] T. Tanaka, T. Yazawa, Y. Ohki, M. Ochi, M. Hara and T. Imai, Frequency Accelerated Partial Discharge Resistance of Epoxy/Clay Nanocomposite Prepared by Newly Developed Organic Modification and Solubilization Methods, IEEE Intern. Conf. Solid Dielectr. (ICSD), pp. 337-340, 2007. [21] T. Tanaka, Y. Ohki, M. Ochi, M. Hara and T. Imai, Enhanced Partial Discharge Resistance of Epoxy/Clay Nanocomposite Prepared by Newly Developed Organic Modification and Solubilization Methods, IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 81-89, 2008. [22] T. Tanaka, Interpretation of Several Key Phenomena Peculiar to Nano Dielectrics in terms of a Multi-core Model, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 298-301, 2006. [23] H. Ishida and S. Rimdusit, Very High Thermal Conductivity Obtained by Boron Nitride-filled Polybenzoxazine Thermochim, ACTA, Vol. 320, pp. 177-186, 1998. [24] X. Y. Huang, P. K. Jiang and C. U. Kim, Electrical Properties of Polyethylene/Aluminum Nanocomposites, J. Appl. Phys., Vol. 102, 124103-1-124103-8, 2007. [25] X. Y. Huang, C. U. Kim, P.K. Jiang, Y. Yin and Z. Li, Influence of Aluminum Nanoparticle Surface Treatment on the Electrical Properties of Polyethylene Composites, J. Appl. Phys., Vol. 105, 014105-1-014105-10, 2009. [26] P. Maity, S. V. Kasisomayajula, V. Parameswaran, S. Basu and N. Gupta, Improvement in surface degradation properties of polymer composites due to pre-processed nanometric alumina fillers, IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp.63-72, 2008. [27] D. Ma, R. W. Siegel, J. Hong, L. S. Schadler, E. Mrtensson and C. nneby, Influence of Nanoparticle Surfaces on the Electrical Breakdown Strength of Nanoparticle-filled Low-density Polyethylene, Materials Research Society, USA, Vol. 19, pp. 857-863, 2004. Zhe Li was born in Guangxi Province, China in 1978. He obtained the B.Sc., M.E. and Ph.D. degrees in high voltage and insulation technology at Shanghai Jiaotong University, China in 2000, 2005 and 2007, respectively. He has been a faculty in the Department of Electrical Engineering, Shanghai Jiaotong University, and he was invited to Waseda University, Japan as a visiting researcher from April, 2008 to Mar. 2010. His interest is dielectric properties of polymer nano-composite and electrical insulation. Kenji Okamoto received the Ph.D. degree in electrical engineering from Tokai University, Kanagawa, Japan, in 2002. In 1982, he joined Fuji Electric Corporate Research and Development, Ltd., Tokyo, Japan, and since then he has been involved in solid insulation technology of epoxy mould, etc. and its application to products. Primarily, he has conducted research on electrical deterioration with regard to partial discharge and voltage-time characteristics of epoxy resin. For some years, he has been engaged in studying the application of metal-base printed circuit board to a power circuit for general purpose inverters, power modules and so on. In 2004, he joined Fuji Electric Advanced Technology Co., Ltd. as the Chief Research Engineer in the Material and Science Laboratory. His research interests include the mechanism of copper ionic migration in a metal-base printed wiring board. He is a member of the IEE of Japan and the Japan Institute of Electronics Packaging. 660 Z. Li et al.: Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown StrengthYoshimichi Ohki (SM98-F00) received the B.Eng., M.Eng., and Dr. Eng. degrees in 1973, 1975, and 1978, respectively, all from Waseda University, Japan. He joined the teaching staff of the Department of EE, Waseda University in 1976 and is presently a Professor. He was a Visiting Scientist at the Massachusetts Institute of Technology from 1982 to 1984. He is a recipient of IEEE-DEIS Forster Award and Whitehead Award, two Best Paper Awards from IEE Japan, and other awards. Toshikatsu Tanaka (SM95-F00). He is Fellow at Waseda University IPS Research center. He worked as Professor at Waseda University for 7 years until 2008. He obtained the D. Eng. Degree from Osaka University. He worked for CRIEPI for 38 years, and temporarily for Salford University in UK (1970-72), Rennselaer Polytechnic Institute and General Electric in the USA (1975-76), and Kyushu University in Japan (1993-1998). He is a recipient of the Ministry of Science and Technology Prize in 2000, IEEJ Technology Progress Award in 1988, IEEE Whitehead Memorial Lecture Award in 2000 and IEEE Dakin Award in 2002. He is active in IEEJ (Fellow) and CIGRE (Senior). 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