nano-dielectric materials in electrical insulation application

NANO-DIELECTRIC MATERIALS IN ELECTRICAL INSULATION APPLICATION

Hulya Kirkici, Mert Serkan, and K. Koppisetty

Electrical and Computer Engineering Auburn University, Auburn, Alabama 36849, USA

ABSTRACT: High performance electrical insulation materials and structures must be free from unwanted and unpredictable dielectric breakdown through the insulator as well as surface flashover along the interface between the insulator and the ambient or vacuum. The last decade has witnessed significant developments in the area of nano-particulate and nano-dielectric materials, and significant effects of nano-scale fillers on electrical, thermal, and mechanical properties of polymeric materials have been observed. Surface flashover is one of the major issues in electrical insulation in power systems operating either in space (vacuum or partial vacuum) or atmospheric (earth-bound) environments. Thus, development of new and advanced materials to be used in power systems requires extensive studies on electrical insulation characteristics of these materials before they can be used in commercial systems. Most of the research in characterization of nano-dielectric materials has been for systems operating in atmospheric environment; there are limited or no data for systems using these materials in space environments. In this paper, we present experimental results of surface flashover characteristics of epoxy resin and epoxy resin cast with powdered Al2O3 in partial vacuum. The flashover studies were conducted separately using DC and AC signals. Surface flashover voltage and current waveforms of the samples and light emission waveforms and video images during the breakdown were recorded. Optical data collected by a video camera along with the PMT light emission data are analyzed and the results of the optical emission characteristics of surface flashover are presented along with the electrical data. The samples used in the experiments were produced by using either epoxy resin of known properties or by mixing known quantities of Al2O3 with epoxy. The electrode material placed over the dielectric samples was copper.

INTRODUCTION

Surface flashover across solid insulators in vacuum has been a research topic in the area of space power systems [1, 2]. One scenario where surface flashover in vacuum can be a problem is where a spacecraft operating in geosynchronous orbit (GEO) or low earth orbit (LEO) interacts with the local space

environment resulting in the differential charging of the spacecraft. This charging can sometimes reach to electrical potentials of several hundreds to a few thousand volts. When this charging reaches a critical value, surface flashover on the solid dielectric insulator may occur, resulting in loss of high voltage isolation and causing damage to the space power systems on board. Similarly, surface flashover can also occur across insulators in the utility power system in partial vacuum and/or atmospheric pressures as well. In general, random emission of a small number of electrons at the cathode results in positive charging of the dielectric surface at the triple junction, thus initiating surface flashover across the dielectric. Insulating materials play a significant role in the design and performance of high voltage power systems. They can be used for both insulation purposes and for charge storage (as on capacitive devices), in addition to other purposes like heat conduction (as in liquids in oil-transformers) and mechanical support (as in solids). Electric insulation in space power systems is relatively complicated because of the constraint faced in partial vacuum, space plasmas, and contamination in space [3]. Furthermore, development of high power electronic devices operating in high switching frequencies coupled with the desire to use higher voltages than the traditional 28-V system in space require further studies than currently available ones [4]. Despite the basic understanding of electrical breakdown of materials, electrical surface flashover phenomena, physical mechanisms responsible for the initiation of such unwanted electrical activities within an insulation system composed of such advanced materials must be investigated before they can be commercially available. The findings of such studies are essential for the development of nano-electric and other advanced materials and the techniques to predict the reliability of the advanced electrical systems which utilize these materials. Polymeric and ceramic composites consisting of dispersed nano-particles, cross-linked agents, specially functionalized molecules and other fillers can be used as advanced dielectric materials in ambient or space environments. Furthermore, there are significant observations onthe effects of the nano scale fillers on

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the thermal, mechanical, and electrical properties of polymeric materials [5]. Despite the significant developments in the area of nano-particulate and nanodielectric materials research, there are still some concerns regarding the use of these materials in space environment. It is also known that the nano-composite materials have lower electrical conductivity at elevated temperatures, and electrical breakdown strength might be higher compared to the nonfiller added polyimides [6-8]. These initial findings make nano-dielectric materials a prime candidate for electrical insulation for multi-stress and high repetition rate (high frequency operation) compact and/or high voltage/power electrical systems [9,10]. It is widely accepted that the surface flashover phenomenon is due to field emission of electrons at the cathode ‘triple-junction’ (electrode/insulator/vacuum interface) when the applied field exceeds a certain threshold value [11]. Random emission of a small number of electrons at the cathode results in positive charging of the dielectric surface at the triple junction. Consequently, this creates an electron avalanche across the surface towards the anode, which is followed by an electrical breakdown and plasma generation on the surface of the dielectric. There are other contributing factors, however, which may exacerbate breakdown, such as the type of dielectric material and its surface condition, the type of electrode material and its corresponding surface condition, and operating environment.

SAMPLE PREPARATION AND

EXPERIMENTAL SETUP

Figure 1: Al2O3 powder

The dielectric samples were prepared in-house using commercially available epoxy resin and Al2O3 powder. A thermally conductive and electrically insulating epoxy (EPO-TEK H65-175MP) specifically designed to comply with MIL STD 833D was purchased from Epoxy Technology. It is a single

component epoxy resin, and particularly designed for heat dissipation requirements. A 99.7+% pure Al2O3 fused insulating powder (Figure 1) purchased from AlfaAesar was used for preparing the epoxy/powder mixture. For the dielectric mixture, pure epoxy and Al2O3 in 10:1 weight ratio were mixed thoroughly and then cured under supplier recommended conditions on a Teflon substrate.

Figure 2: The Electrode Geometry

Figure 3: Experimental Setup.

The experimental setup consists of a high vacuum chamber, high voltage power system and a data acquisition system as shown in Figure 2. Helium and nitrogen are used separately as the operating gases in the experiments. The breakdown experiments were conducted over a pressure range of 0.2 torr to 1.2 torr, for DC and 20 kHz AC signals. The power supply used in the AC experiments is an in-house-built power source constructed from a DC and an AC source connected with a high frequency step-up transformer. The final AC signal generated is a DC-shifted sinusoid with no negative values. The applied voltage is gradually increased by simultaneously increasing the DC and the AC voltages, until a breakdown event is observed. Detailed experimental set-up is given in [9]. The electrode setup in the vacuum chamber consists of two tapered copper electrodes firmly attached on top of the sample as shown in Figure 3.

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Voltage and current waveforms, along with the light emission data from the surface flashover are collected. Optical data is collected by two means, i.e., video camera for optical images and PMT for time evolution of the light emission from the flashover. The video images are then converted into frame-by-frame still images. As the speed of a regular digital video camera is not sufficient, the acquired images may not display the complete event. Therefore, light emission waveforms acquired by PMT are used to complement the data. A Pearson coil and a HV-probe are used to acquire the current and the voltage waveforms.

RESULTS AND DISCUSSION DC flashover data as a function of pressure for pure epoxy sample for nitrogen and helium as operating gas is shown in Figure 4. However, a consistent DC breakdown voltage vs pressure graph could not be determined for cast epoxy resins for a variety of experimental conditions. Figures 5 and Figure 6 show the flashover voltage characteristics in N2 and He for AC. It was observed that the breakdown voltage characteristics under AC conditions also exhibit a pattern similar to the DC voltage curve but with a lower breakdown voltage. As seen in these figures, flashover voltage is a decreasing function of pressure at this frequency range. Further data and data analysis are needed to understand the physical characteristics of surface flashover across the composite dielectric material over a wider pressure range.

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Figure 4: Helium and Nitrogen, Pure Epoxy, DC

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Figure 5: Helium and Nitrogen, Pure Epoxy, AC

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Figure 6: Helium and Nitrogen, Cast Epoxy, AC

In general, a low intensity glow discharge is observed around both electrodes, but mainly seen at the cathode region at higher pressure, and at the anode region at lower pressures for both DC and AC signal cases. This can be observed in the frames shown in Figure 7. These frames belong to DC signal flashover events. Light emission for AC signal is relatively less intense than light emission for DC signal, thus we are not able to display them here. Because the video camera used in these experiments is not a low-light level camera, it is harder to observe the difference between high pressure low pressure glow discharges for AC signal events. It should also be noted that the images seen in Figure 7 are the afterglow discharge of the surface flashover across the dielectric sample. Because the camera is not a high speed camera but a regular video camera with 30 frames per second, we are not able to capture the flashover initiation images by this means. On the other hand the PMT data suggest that the light emission is initiated once the voltage collapse occurs across the electrodes.

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(a)

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Figure 7: a) Electrodes b) He DC cast epoxy 600 mtorr c) He DC cast epoxy 1200 mtorr d) N2 DC cast epoxy 600 mtorr e) N2 DC cast epoxy 1200 mtorr

Visual inspection of some of the acquired images may suggest that the breakdown has a lower intensity at lower pressures compared to the optical emission at higher pressures; however, this is not the case. Therefore, optical emission waveforms recorded by PMT is used for intensity comparison between low and high pressures. These waveforms show that there is higher emission rate at lower pressures compared to the ones in higher pressures as shown in Figure 8.

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Figure 8: a) N2 AC cast epoxy 600 mtorr b) N2 AC cast epoxy 1200 mtorr A representative voltage, current and the PMT waveforms as a function of time are shown in Figure 9 The voltage and current waveforms suggest that the flashover is a relatively short event occurring within a few microseconds. Once the breakdown is initiated, the discharge current follows the applied voltage signal. During this process, the light emission steadily increases, following a similar pattern as the current. Further observations concluded that the breakdown starts with a transient current and this current magnitude increases as the pressure increases [12]. The DC breakdown events were relatively faster than AC breakdown events.

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Figure 9: Voltage, Current and Optical Emission Waveforms for Helium at 200 millitorr AC a) Pure Epoxy b) Cast epoxy REFERENCES

[1] H. Kirkici, ‘‘Surface flashover characteristics of diamond-like carbon thin films in vacuum,’’ IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 4, pp:71 --- 78, 1997

[2] H. Kirkici, ‘‘Optical emission characteristics of polycrystalline diamond during surface flashover in vacuum,’’ Vol. 4, pp: 822 - 826, 1997

[3] Hulya Kirkici, High Frequency Breakdown and Electrical Insulation in Space Environment,’’ Proceedings of the 8th International Conference on Optimization of Electrical and Electronic Equipment --- OPTIM 2002, Brasov, Romania, May 20002

[4] G.B. Hillard and Hulya Kirkici, ‘‘progress in High Voltage Design in Space,’’ Proceedings of IEEE-Conference on Electrical Insulation and Dielectrics Phenomena, October 2001

[5] Joe Y. Zhou and Steven Boggs, ‘‘Dielectric Properties of Three Component Mixtures of Nonlinear, Resistive, and Non-conducting Grains’’, 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena.

[6] Yang Cao and Patricia C. Irwin, ‘‘The electrical conduction in polyimide nanocomposites’’, 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena.

[7] P. C. Irwin, Y. Cao, A. Bansal, and L. S. Schadler, ‘‘Thermal and mechanical properties of polyimide nanacomposites’’, 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, October 2003

[8] Masahiro Kozako, Norikazu Fuse, Kohei Shibata, Naoshi Hirai, Yoshimichi Ohki, Tatsuki Okamoto and Toshikatsu Tanaka, ‘‘Surface Change of Polyamide Nanocomposite Caused by Partial Discharges’’, 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena.

[9] P. Jain, and M. Tanju, ‘‘A 20 KHz Hybrid Resonant Power Source for the Space Station’’, IEEE Transactions on Aerospace and Electronic Systems, Vol. 25, No. 4, July 1989.

[10] M. R. Belmont, E. J. P. May and M. K. Horwood, ‘‘Generalized Frequency Response Applied to Dielectrics’’, Seventh International Conference on Dielectric Materials Measurements & Applications, 23-26 September 1996, Conference Publication No. 430, IEE 1996

[11] F. Hegeler, G. Masten, H. Krompholz, and L. Hatfield, “High Speed and High Resolution Diagnostics for the Early Phase of Dielectric Surface Flashover”, Conference Record of the 1992 IEEE International Symposium on Electrical Insulation, Baltimore, MD USA, June 7-10, 1992.

[12] Mert Serkan, Hulya Kirkici and Kalyan Koppisetty, ‘Surface Flashover Characteristics of Nano Particle Cast Epoxy Resin’, Fifteenth IEEE International Pulsed Power Conference, June 2005.

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nano-dielectric materials in electrical insulation application

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