DIELECTRIC CHARACTERIZATION OF POLYMER-CERAMIC NANOCOMPOSITES∗

K. A. O’ConnorξCenter for Physical and Power Electronics, University of Missouri-Columbia, 349 Engineering Bldg. West

, J. Smith, and R. D. Curry

Columbia, Missouri 65211, USA

∗ Work supported by the Office of Naval Research under contract N00014-08-1-0267 ξ email: OConnorKA@missouri.edu

Abstract

Polymer-ceramic nanocomposites for high power applications are being developed at the University of Missouri-Columbia. Several polymers and epoxies have been investigated as candidate matrix materials. Two of the candidate matrix materials were used to create composites with several loading factors of nanoparticles. The matrix materials and composites are characterized through measurements of the dielectric constant and loss over a wide range of frequencies, dielectric strength in pulsed high voltage conditions, and scanning electron microscopy.

Two test stands have been implemented for measurement of the complex dielectric permittivity in two frequency ranges of interest. Measurements from 100 kHz to 30 MHz are performed using parallel-plate methods and a precision LCR meter. Measurements from 200 MHz up to 4.5 GHz are performed by utilizing network analysis.

The nanocomposites were also characterized for dielectric breakdown. A test stand was built to characterize the nanocomposites under pulsed conditions. Dielectric strength measurements were conducted with a pulse generator capable of up to nearly 160 kV. The 10%-90% voltage risetime in pulsed dielectric strength measurements was typically 60 ns.

A summary of the composite materials, diagnostic methods, and preliminary results are reported. The dielectric constant, dielectric loss, and dielectric strength are reported for several matrix material candidates. Preliminary results of the first nanocomposite materials are also presented.

I. INTRODUCTION

Polymer-ceramic nanocomposites with high dielectric constants provide the combined advantages of high electrical energy storage density and improved mechanical and processing properties. A common technique for achieving high dielectric constant materials involves introducing filler materials with a high dielectric constant into a matrix material with a relatively low dielectric constant but good mechanical and processing properties [1, 2]. The filler material is often a ceramic

with a perovskite structure, such as barium titanate, strontium titanate, or a solid solution of barium strontium titanate [3]. The matrix material is typically a polymer or epoxy.

Despite the number of previous investigations of high dielectric constant ceramic-polymer nanocomposites, relatively few researchers have focused on high voltage systems [4]. Many researchers have focused on the development of these materials for embedded capacitor printed circuit board applications [5]. However, embedded capacitors often have very different requirements than high power pulsed components. High voltage systems often require much thicker bulk samples with a high dielectric strength, which presents additional constraints for material homogeneity and uniformity of properties throughout the sample.

The Center for Physical and Power Electronics at the University of Missouri-Columbia is investigating bulk composites formed with nanoceramics and polymers for high voltage pulsed applications. Due to their application in high voltage systems, the samples have been processed in thicknesses adequate for high power components. Experimental test stands have been constructed for the measurement of permittivity at two frequency ranges of interest and for the determination of the pulsed dielectric strength. The subsequent sections describe the test methods for permittivity and dielectric strength followed by preliminary results of the nanocomposite materials.

II. PERMITTIVITY TEST METHODS

The complex permittivity was measured in two frequency ranges of interest. The lower frequency range was between 100 kHz and 30 MHz. The measurements were performed with the 16451B dielectric test stand and a 4285A precision LCR meter from Agilent Technologies. The low frequency measurements utilized a parallel plate arrangement as shown in Figure 1. To limit errors from edge capacitance, the geometry utilizes three electrodes in which one of the electrodes of the parallel plate is encircled by a guard electrode. Another potential source of error is an air gap between the electrodes and any imperfect sample surface. The error produced from an air gap can be eliminated by applying thin film electrodes to the sample or by utilizing the air gap measurement

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method. Figure 1 defines the sample thickness, ta, and electrode spacing, tg, used in the air gap method.

Figure 1. Cross-sectional view of the low frequency permittivity measurement fixture. Adapted from [6].

Two permittivity measurements are taken at each frequency. One measurement includes the sample under test, and the other measurement is simply of air. The dielectric constant of the sample, εr, is then calculated with equation (1) [6].The symbols CS1 and CS2 represent the measured capacitance without the sample and with the sample in the fixture, respectively.

(1)

The dissipation factor of the sample, D, can be calculated using the air gap method with an equation developed by Endicott and McGowan [6, 7]. The symbols D1 and D2 are the measured dissipation factor without and with the sample in the fixture, respectively. (2)

A higher frequency range can be measured between 200 MHz and 4.5 GHz. The 85070E dielectric measurement fixture is implemented with the E5071C network analyzer and software from Agilent Technologies. The high frequency measurements are controlled and recorded by a computer. The high frequency range can be measured on liquids, soft solids, and select solid materials.

Figure 2. Suite of permittivity measurement equipment

III. DIELECTRIC STRENGTH TEST STAND

The dielectric strength of the nanocomposites is

characterized under pulsed conditions. A PA-80 pulse generator from L-3 Communications Pulse Sciences is the high voltage source of the dielectric strength test stand

[8]. Figure 3 shows the circuit of the dielectric strength test stand.

Figure 3. Circuit schematic of pulsed dielectric strength test stand

A 75 nF capacitance, CS, is resistively charged to -80 kV from a DC supply. After charging, the switch, S, is triggered to close, discharging the capacitor into the transmission line, T. Since one of the capacitor terminals is grounded through switch S, the polarity of the pulse on the transmission line is positive. The transmission line is a 50 Ω cable of RG-218/U. The transmission line is terminated into a high impedance load consisting of a parallel arrangement of the dielectric test cell, a capacitance, Cload, of 0.54 nF, and a high resistance, R1 + R2. Since Cload is much smaller than CS and the resistance of R1 + R2 is very high, the voltage across the nanocomposite sample can reach a peak up to nearly 160 kV if no breakdown in the sample occurs. The 10% to 90% risetime has been measured as approximately 60 ns.

The voltage diagnostics consist of resistive voltage dividers, which monitor the voltage on each side of the test cell. A current monitor identifies the point at which electrical breakdown of the sample occurs. The voltage immediately before current is observed through the test sample is recorded as the dielectric breakdown voltage. The dielectric strength of the sample is calculated with this breakdown voltage and the measured thickness of the sample. Twenty measurements of the dielectric strength are taken for each sample.

Figure 4 shows example waveforms recorded during dielectric strength testing. The voltage waveforms were measured from both sides of the dielectric sample.

Figure 4. Example voltage waveforms from both sides of test cell

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Figure 5 displays the voltage across the dielectric sample. In this example, a peak voltage of approximately 20 kV was applied across the sample before dielectric breakdown.

Figure 5. Example voltage across dielectric sample

The test cell in which the voltage pulse is applied across the samples consists of an acrylic housing in which mineral oil, transformer oil, pressurized gas, or vacuum can be used as the high voltage background insulator. The electrodes have a taper that ends in circular flat faces with a diameter of 6.35 mm. Elastic bands around one electrode ensure the sample is suspended between the electrode faces with adequate compressive force. Figure 6 shows a detailed view of the dielectric strength test cell. Figure 7 shows the PA-80, transmission line load, and diagnostic arrangement.

Figure 6. Dielectric strength test cell

Figure 7. PA-80 pulse generator in background with test cell and diagnostics in foreground

IV. RESULTS A. Matrix Materials

Several plastics commonly used as insulators were studied for their potential application as matrix materials in the nanocomposites. The plastics were evaluated based upon their dielectric constant, dissipation factor, and dielectric strength through the previously-described methods. Figure 8 is a graph of the dielectric constant of PVDF, nylon, Mylar, polyethylene, and Teflon. The dielectric constant was measured at 100 kHz, 1 MHz, and multiples of 5MHz up to 30 MHz.

Figure 8. Dielectric constant of potential matrix materials

PVDF exhibited a significantly higher dielectric constant at low frequencies. However, PVDF also showed substantial dispersion as the dielectric constant dropped by approximately 50% between 100 kHz and 30 MHz. Nylon, Mylar, polyethylene, and Teflon all exhibited much lower dielectric constants generally between two and four with little dispersion.

The dielectric losses of the plastic materials were also evaluated between 100 kHz and 30 MHz, and the calculated dissipation factors are shown in Figure 9. PVDF exhibited a significantly higher dissipation factor than all the other materials above 100 kHz. The next highest dissipation factor was observed for nylon, which was measured to be slightly more lossy than Mylar. Teflon and polyethylene showed the lowest losses.

Figure 9. Dissipation factor of potential matrix materials

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Figure 10 displays the cumulative probability of breakdown for the selection of potential matrix materials. Although PVDF had the highest dielectric constant values, it also exhibited the lowest breakdown field strength. Alternatively, although Teflon had a relatively low dielectric constant of about 2.1, Teflon had the highest breakdown field strength. Based upon the advantageous properties of PVDF and Teflon for dielectric constant and dielectric strength, respectively, they were used as matrix materials in the first nanocomposites samples.

Figure 10. Cumulative probability of breakdown for potential matrix materials

B. Nanocomposite Materials Nanocomposites were produced in cooperation with a

polymer manufacturer with PVDF and Teflon as matrix materials. Barium strontium titanate (BST) was used as the filler material. The ratio of barium to strontium in the solid solution was approximately 1.5:1, and the primary size of the nanoparticles was 50 nm. The nanocomposites were produced in bulk sizes that may be required for high voltage systems. The first samples produced were approximately 1.25 cm thick and had a 10.16 cm diameter. The disks were cut by a lapidary saw into strips of 508 µm for dielectric strength tests. For permittivity measurements, sections of the disks were milled to a thickness of 1.52 mm. Figure 11 displays the nanocomposite disk as originally produced, cut into strips for dielectric strength testing, and milled for permittivity testing.

Figure 11. Nanocomposite materials processed for testing

Figure 12 shows the dielectric constant of nanocomposites with various loadings of BST by weight in PVDF. The highest dielectric constant, greater than 33

at 100 kHz, was measured for an 80% loading by weight of BST in PVDF. The dielectric constant of the PVDF-based composites increased with higher loading percentages of BST. Additionally, the decrease in the dielectric constant with increasing frequency is much less severe than the decrease observed in pure PVDF. Nanocomposites based on Teflon exhibited much lower dielectric constants than those incorporating PVDF. Improving the distribution of nanoparticles, reducing agglomeration of the nanoparticles, and modifying the interaction of the nanoparticles and matrix material can further increase the dielectric constants of the nanocomposites.

Figure 12. Dielectric constant of first nanocomposites

The dissipation factors of the nanocomposites based on PVDF are shown in Figure 13. The dissipation factors of the nanocomposites were significantly lower than that of pure PVDF. At the highest frequencies, the dissipation factor was generally lower for nanocomposites with higher percentages of BST. The dissipation factor of a composite with 70% BST by weight in Teflon exhibited a much lower dissipation factor than any of the PVDF samples.

Figure 13. Dissipation factor of first nanocomposites

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Figure 14 shows the cumulative probability of breakdown for two nanocomposites produced with Teflon as the matrix material. One composite incorporated the BST nanopowder previously described while the other used a strontium titanate (ST) nanopowder. The ST composite exhibited a much higher dielectric strength than that incorporating BST. However, compared with the 50% probability of breakdown of pure Teflon of approximately 3.25 kV/mil, both nanocomposites exhibited a significantly reduced dielectric strength. As with the dielectric constant, the dielectric strength can be significantly improved through improved processing of the nanopowders and nanocomposites.

Figure 14. Cumulative probability of electrical breakdown vs. electric field for Teflon-based nanocomposites

Figure 15 shows the cumulative probability of breakdown for five nanocomposites with filler loadings of 60% to 80% BST by weight in PVDF. There is a clear correspondence between the dielectric strength and the filler loading level as the dielectric strength is lower for increased levels of filler loading. Compared with a 50% probability of breakdown of approximately 1.9 kV/mil for pure PVDF, the dielectric strength of the nanocomposites was reduced by a factor of 1.5 – 2.

Figure 15. Cumulative probability of breakdown vs. electric field in PVDF-based nanocomposites

V. SUMMARY

The Center for Physical and Power Electronics is developing advanced dielectrics for pulsed power applications. The dielectrics incorporate nanopowders of ceramics with a high dielectric constant into matrix materials with lower dielectric constants but advantageous mechanical properties. A suite of diagnostics has been developed to test the dielectric constant, dissipation factor, and dielectric strength. The dielectric constant and dissipation factor can be measured from 100 kHz to 30 MHz and from 200 MHz to 4.5 GHz. The dielectric strength test stand is capable of applying high voltage pulses up to nearly 160 kV across the samples under test. Pulsed characterization allows the materials to be evaluated for applications in compact high power components.

Various plastics were evaluated as potential matrix materials. Of the materials tested, PVDF had the highest dielectric constant, and Teflon had the highest dielectric strength. The first nanocomposites produced and evaluated for this study incorporated barium strontium titanate and strontium titanate nanopowders in Teflon and PVDF matrices. Nanocomposites utilizing barium strontium titanate and PVDF had the highest dielectric constants of this first batch of samples, reaching greater than 33. Compared with the 50% decrease of the dielectric constant of pure PVDF from 100 kHz to 30 MHz, the dispersion of the nanocomposites with PVDF was dramatically reduced. The dielectric constant increased with increased loading of BST in PVDF, and the dielectric losses were generally lower at high frequencies with higher loading percentages of BST in PVDF. Nanocomposites with Teflon had significantly reduced dielectric strengths, and the dielectric strength of nanocomposites with PVDF was reduced by a factor of 1.5 – 2.

The dielectric properties of the nanocomposites can be further improved through processing of the nanopowders and matrix materials. Extensive research has continued since these first samples were characterized to reduce agglomerations of the nanoparticles, improve particle dispersion, and control the interaction between the nanoparticles and the matrix material. The results of the most recent nanocomposites will be reported in subsequent publications.

VI. ACKNOWLEDGMENTS

This work was supported through the Office of Naval Research under contract N00014-08-1-0267. The authors wish to express their appreciation to Lee Mastroianni for his work in program management.

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VII. REFERENCES [1] Y. Kobayashi, T. Tanase, T. Tabata, T. Miwa, and M. Konno, "Fabrication and dielectric properties of the barium titanate - polymer nano-composite thin films," Journal of the European Chemical Society, vol. 28, pp. 117-122, 2008. [2] C. K. Chiang and R. Popielarz, "Polymer composites with high dielectric constant," Ferroelectrics, vol. 275, pp. 1-9, 2002. [3] "Ceramic Materials for Electronics: Processing, Properties, and Applications," R. C. Buchanan, Ed. New York: Marcel Dekker, Inc., 1986. [4] Y. Cao, P. C. Irwin, and K. Younsi, "The future of nanodielectrics in the electrical power industry," IEEE Trans. on Dielectrics and Electrical Insulation, vol. 11, pp. 797-807, 2004.

[5] J. Xu, K.-S. Moon, P. Pramanik, S. Bhattacharya, and C. P. Wong, "Optimization of Epoxy-Barium Titanate Nanocomposites for High Performance Embedded Capacitor Components," IEEE Trans. on Components and Packaging Technologies, vol. 30, pp. 248-253, 2007. [6] "Agilent 16451B Dielectric Test Fixture Operation and Service Manual," Agilent Technologies Japan, Ltd., Hyogo, Japan 2000. [7] H. S. Endicott and E. J. McGowan, "Measurement of permittivity and dissipation factor without attached electrodes," in Annual Report Conference on Electrical Insulation Washington, D.C.: NAS-NRC, 1960, pp. 19-30. [8] "PA-80 and PA-100 Trigger Generators," L-3 Communications Pulse Sciences, San Leandro, CA.

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