Electrical Conductivity of Biaxially Oriented Polypropylene Under High Field at Elevated Temperature

Transcription

1 Electrical Conductivity of Biaxially Oriented Polypropylene Under High Field at Elevated Temperature by Janet Ho and Richard Jow ARL-TR-5720 September 2011 Approved for public release; distribution unlimited.

2 NOTICES Disclaimers The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. Citation of manufacturer s or trade names does not constitute an official endorsement or approval of the use thereof. Destroy this report when it is no longer needed. Do not return it to the originator.

3 Army Research Laboratory Adelphi, MD ARL-TR-5720 September 2011 Electrical Conductivity of Biaxially Oriented Polypropylene Under High Field at Elevated Temperature Janet Ho and Richard Jow Sensors and Electron Devices Directorate, ARL Approved for public release; distribution unlimited

4 REPORT DOCUMENTATION PAGE Form Approved OMB No Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports ( ), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) September REPORT TYPE Final 4. TITLE AND SUBTITLE Electrical Conductivity of Biaxially Oriented Polypropylene Under High Field at Elevated Temperature 3. DATES COVERED (From - To) 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Janet Ho and Richard Jow 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army Research Laboratory ATTN: RDRL-SED-C 2800 Powder Mill Road Adelphi MD PERFORMING ORGANIZATION REPORT NUMBER ARL-TR SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR S ACRONYM(S) 11. SPONSOR/MONITOR'S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited. 13. SUPPLEMENTARY NOTES 14. ABSTRACT Although biaxially oriented polypropylene thin films are a common dielectric for many high voltage pulsed power capacitor applications, the electrical conductivity under high fields at elevated temperatures is mostly unknown. Such knowledge is valuable for improving the understanding of the origin of the charge species and transport mechanisms at high fields. Results suggested that conduction is by hopping with the hopping distance increasing from 1.4 nm at 35 C to 3.2 nm at 100 C. The thermal activation energy was determined to be 0.75 ev and field-independent. Such a finding allows the use of an Arrhenius term and a field-dependent term to describe the field-dependent conductivity up to breakdown. 15. SUBJECT TERMS Polypropylene, capacitor, electrical conductivity, breakdown, thermal activation energy 16. SECURITY CLASSIFICATION OF: a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT UU ii 18. NUMBER OF PAGES 18 19a. NAME OF RESPONSIBLE PERSON Janet Ho 19b. TELEPHONE NUMBER (Include area code) (301) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18

5 Contents List of Figures iv 1. Introduction 1 2. Experimental Details 2 3. Data and Analysis Conduction Mechanism Activation Energy Conclusions 8 5. References 9 Distribution List 11 iii

6 List of Figures Figure 1. Time dependence of the charging current for 7- m BOPP at 50 C with various applied fields....3 Figure 2. Charging current as a function of time for 7- m BOPP at 57 MV/m at various temperatures....3 Figure 3. Electrical conductivity for 7- m BOPP as a function of field at various temperatures....4 Figure 4. Field dependence of the conduction current density for 7- m BOPP at various temperatures. The solid lines represent the hyperbolic sine curve fitting....5 Figure 5. An Arrhenius plot of electrical conductivity as a function of temperature to compute the activation energy at various electric fields....6 Figure 6. A 3-D representation of equation 2 to the electrical conductivity data as a function of temperature at various fields above 100 MV/m....7 iv

7 1. Introduction Biaxially oriented polypropylene (BOPP) is the present state-of-the-art capacitor dielectric for most high voltage, pulsed power applications because of its high breakdown strength (~600 MV/m at capacitor level), self-clearing capability, and low dissipation factor (~10 4 ), which is independent of frequency. As no dielectric is a perfect insulator, trace electrical conduction is always present, especially at high electric fields and/or elevated temperatures. At low electric fields, conduction may arise from ionic impurities like residual catalysts in polymer resins. At higher fields (>10 MV/m), conduction is generally attributed to electron and hole mobility, which is promoted by impurity states in the wide bandgap of the insulator. These impurity states, which are caused by the physical disorder inherent in the amorphous region (~0.5 ev from the conduction band minimum) and chemical impurities such as carbonyl, vinyl, and double bonds (0.8 to 3 ev from the valence and conduction band edges), reduce the effective bandgap to ~1 ev in common polymeric insulators such as polyethylene (1 4). Although electrical conduction in polypropylene at various temperatures and electric fields has been studied previously (5 12), the range of fields investigated was below 100 MV/m. Knowledge of electrical conductivity of capacitor-grade polypropylene thin films at higher electric fields is technologically important but unavailable, even though in terms of a capacitor, conduction through the film might be negligible compared to surface leakage across the unmetallized margin to the end sprayed connection. Nonetheless, such knowledge is valuable both academically and industrially, as it provides a basis for improved understanding of the origin of the charge species and transport mechanisms. To this end, the objective of this work is to study the electrical conductivity of BOPP capacitor film manufactured by the tenter process as a function of temperature and electric field over the practical range of fields and temperatures applicable, from which various mechanisms of charge transport were investigated. Analysis of the conductivity data indicates that the thermal activation energy associated with conduction is about 0.75 ev and is independent of electric field. Both two- (2-D) and threedimensional (3-D) curve fitting to the conductivity data as a function of temperature and electric field conform to a formula for conductivity that can be expressed as exp( be ) in field and an Arrhenius term in temperature, i.e., a product of field-dependent and temperature-dependent terms. 1

8 2. Experimental Details Samples cut from rolls of commercially available 7- m capacitor-grade BOPP film were used as-received. Gold layers of 100 nm thickness were evaporated on both sides of the samples to ensure good electrical contact with the stainless steel electrodes. The effective electrode area was about 3 cm 2. A guard ring was also evaporated on the low voltage side to minimize contributions from surface current. Prior to measurement, samples were conditioned for 24 h by short-circuiting the gold electrodes at about 70 C, which improved the consistency of the data (3). The preconditioning method probably removed the electrical memory in the samples. The apparatus used to measure the conduction current consisted of a ±3-kV DC power supply (Keithley 247), an electrometer (Keithley 6514), and a set of stainless steel electrodes, which consist of a guarded measuring electrode and a counter electrode. The current as a function of time was recorded to a personal computer using the analog output from the electrometer. Prior to applying an electric field, the sample was equilibrated for about an hour at the test temperature, which was controlled to ±1 C. A thermocouple was affixed on the stainless steel electrode near the sample for monitoring the sample temperature. The charging current was monitored from 1 s after the application of the electric field until the current reached steady state, after which the applied field was removed and the sample was allowed to relax overnight. The same sample was used for the next higher field until the sample broke down. All measurements were conducted in air. 3. Data and Analysis Figure 1 shows a typical charging current as a function of time at various fields for 7- m BOPP at 50 C. The time for the charging current to reach steady state decreased as the applied field was increased. This sample broke down at 426 MV/m shortly after the steady-state current was reached. The time dependence of the charging current at 57 MV/m at various temperatures is shown in figure 2. Both figures show that the current reached steady state after 10 4 s, from which the electrical conductivity was determined. Figure 3 summarizes the electrical conductivity of BOPP as a function of field at various temperatures. As shown in figure 3, the electrical conductivity appears to be field-independent below 100 MV/m but increases substantially at higher fields. The decrease in electrical conductivity with increasing field at 35 C for fields below 100 MV/m was most likely due to poor signal-to-noise ratio, as indicated in figure 2. 2

9 Figure 1. Time dependence of the charging current for 7- m BOPP at 50 C with various applied fields. Figure 2. Charging current as a function of time for 7- m BOPP at 57 MV/m at various temperatures. 3

10 Figure 3. Electrical conductivity for 7- m BOPP as a function of field at various temperatures. 3.1 Conduction Mechanism The field dependence of the conductivity can be explained by examining the various postulated conduction mechanisms, such as Poole-Frenkel, Schottky, space charge limited conduction (SCLC), and hopping conduction. Both the Poole-Frenkel and Schottky mechanisms predict that ln(j) or ln( ) E ½, where J,, and E are the current density, volume conductivity, and applied field, respectively, and the slope of the line is related to the dielectric constant of the sample. Although data from the present work fall approximately on a straight line for such a plot, the dielectric constants calculated from the slope were 16 and 9 in the case of 50 C for Poole- Frenkel and Schottky, respectively, which is too high for polypropylene, the measured dielectric constant of which is 2.25 from 10 mhz to 1 MHz. Because of the incorrect dielectric constants calculated from these two mechanisms, both the Poole-Frenkel and Schottky mechanisms are excluded as the dominant conduction mechanism. SCLC was also examined. The lack of a square law relationship between the current and the applied field suggests that SCLC can be eliminated. Another plausible mechanism is hopping conduction, in which the current density, J, is given as ew ee J 2ne exp sinh k BT 2k BT A sinh BE, (1) where n is the carrier concentration, e is the electric charge of the carriers, is the hopping distance, is the attempt-to-escape frequency, W is the activation energy in ev, k B is Boltzmann s constant, T is the absolute temperature, and E is the applied electric field. 4

11 Figure 4 shows the curve fit of equation 1 to the conduction current density as a function of applied field at various temperatures. Equation 1 predicts the observed current quite well, and therefore, hopping conduction appears to be the dominant mechanism. The hopping distance, can be calculated from the fit parameter B. As indicated in figure 4, the hopping distance increased from 1.4 nm to 3.2 nm as the temperature increased from 35 to 100 C. The values of the hopping distance seem rather small compared to the work by Ikezaki et al. (8), which reported a range of 4.5 to 10 nm at 72 C depending on the crystallinity. The density of traps inferred from the hopping distance in our work is to m 3, which seems rather high. Nonetheless, such values have been reported by Lawson (13) for polyethylene and McCubbin (14) for octacosane. Figure 4. Field dependence of the conduction current density for 7- m BOPP at various temperatures. The solid lines represent the hyperbolic sine curve fitting. The data at 35 C are somewhat scattered as a result of the low current density at this temperature. Therefore, the discussion will be focused on the data for 50 and 100 C as they have less scatter and the fits to the data are better. The energy gained from the electric field between traps is given by = ee. At low electric fields, this energy is below the thermal energy (0.026, 0.028, and ev at 35, 50, and 100 C, respectively), and the conduction is ohmic. At fields in the range of 30 MV/m, the energy becomes comparable to the thermal energy, and the energy gained between traps contributes appreciably to detrapping, as a result of which, the conductivity becomes field-dependent (15). Based on the computed separation between traps, the field at which the conductivity becomes nonlinear should be in the range of 35 and 25 MV/m at 50 and 100 C, respectively. Examination of figure 3 suggests that the field at which the conductivity becomes field-dependent is closer to 100 MV/m. 5

12 When the field is sufficient that the energy gained between traps becomes comparable to the trap depth, which can be taken as the thermal activation energy of conductivity, charge carriers are no longer localized by the traps, carrier mobility becomes high, and very rapid high field aging occurs. The rapid increase in the carrier mobility (conductivity) with the electric field results in the creation of a space charge limited field when the charge can redistribute as necessary to limit the field (16, 17). 3.2 Activation Energy The electrical conductivity measured as a function of temperature at various fields has been analyzed through 2-D curve fitting to compute the activation energy for different fields as shown in figure 5. The data indicate that the activation energy at fields from 57 to 300 MV/m is constant within experimental error, with an average value of 0.75 ev. Similar activation energy has been reported by Das Gupta et al. (7) and Ikezaki et al. (8). Figure 5. An Arrhenius plot of electrical conductivity as a function of temperature to compute the activation energy at various electric fields. The activation energy can also be computed through a 3-D curve fit to the electrical conductivity data using an equation of the form ew E, T a exp exp be, k BT (2) where is the electrical conductivity, a and b are constants, and the remaining symbols are the same as described in equation 1. Many other functional relationships for the field-dependent term were examined including exp( b E ), exp( b E / T ), and exp( be / T ). Exp( b E) and 6

13 equation 2 provide similar fits to the data. Given that the form exp( be ) also corresponds to the hopping conduction as shown in figure 4, equation 2 was used to fit to the data above 100 MV/m. As seen in figure 6, the resulting activation energy is 0.75 ev, which is consistent with the 2-D curve fitting at the various fields (figure 5). Figure 6. A 3-D representation of equation 2 to the electrical conductivity data as a function of temperature at various fields above 100 MV/m. Density functional theory-based computations by Stournara and Ramprasad (18) of the impurity states in the bandgap of bulk isotactic polypropylene caused by carbonyl and double bonds indicate that carbonyl causes a hole trap about 0.8 ev above the valence band, and double bonds cause electron traps about 0.7 ev below the conduction band, one or both of which may cause the measured 0.75 ev activation energy. Their work, however, does not provide an analysis of the spatial distribution of the impurity state wave functions. Such analysis would give better insight into the likelihood of these states promoting interchain charge transfer, which is the major impediment to high field conduction in typical polymers, as carrier mobility along the polymer chain is many orders of magnitude greater than that between polymer backbones (15). 7

14 4. Conclusions Results of electrical conductivity measurement of BOPP suggest that the conduction is through hopping mechanism with the hopping distance increased from 1.4 nm at 35 C to 3.2 nm at 100 C. The thermal activation energy was determined to be 0.75 ev and independent of the electric field. This finding allows the use of an Arrhenius term together with a field-dependent term to describe the field-dependent conductivity. 8

15 5. References 1. Teyssedre, G.; Laurent, C. Charge Transport Modeling in Insulating Polymers: From Molecular to Macroscopic Scale. IEEE Trans. Dielectr. Electr. Insul. 2005, 12, Huzayyin, A.; Boggs, S.; Ramprasad, R. Density Functional Analysis of Chemical Impurities in Dielectric Polyethylene. Trans. Dielectr. Electr. Insul. 2010, 17, Huzayyin, A.; Boggs, S.; Ramprasad, R. Quantum Mechanical Studies of Carbonyl Impurities in Dielectric Polyehtylene. Trans. Dielectr. Electr. Insul. 2010, 17, Huzayyin, A.; Boggs, S.; Ramprasad, R. Density Functional Theory Analysis of the Effect of Iodine in Polyethylene. Trans. Dielectr. Electr. Insul. 2011, 18, Foss, R. A.; Dannhauser, W. Electrical Conductivity of Polypropylene. J. Appl. Poly. Sci. 1963, 7, Mehendru, P. C.; Pathak, N. L.; Singh, Satbir; Mehendru, P. Electrical Conduction in Polypropylene Thin Films. Phys. Stat. Sol. (a) 1976, 38, Das Gupta, D. K.; Joyner, K. A Study of Absorption Currents in Polypropylene. J. Phys. D: Appl. Phys. 1976, 9, Ikezaki, K.; Kaneko, T.; Sakakibara, T. Effect of Crystallinity on Electrical Conduction in Polypropylene. Jap. J. Appl. Phys. 1981, 20, (3), Singh, H. P.; Gupta, D. Conductivity Variation of Polypropylene with Electrode Materials. Ind. J. Pure & Appl. Phys. 1985, 23, Mizutani, T.; Ikeda, S.; Ieda, M. Influence of Chemical Structure on Electrical Conduction in Insulating Polymers. Jap. J. Appl. Phys. 1985, 24, (9), Mittal, A.; Jain, V.; Mittal, J. Transient Charging and Discharging Current Studies on Unstretched and Stretched Polypropylene Films. J. Mat. Sci. Let. 2001, 20, Guadagno, L.; Raimondo, M.; Vittoria, V.; Do Bartolomeo, A.; De Vito, B.; Lamberti, P.; Tucci, V. Dependence of Electrical Properties of Polypropylene Isomers on Morphology and Chain Conformation. J. Phys. D: Appl. Phys. 2009, 42, Lawson, W. G. High-field Conduction and Breakdown in Polythene. Brit. J. Appl. Phys. 1965, 16, McCubbin, W. L. Electronic Processes in Paraffinic Hydrocarbons. Trans. Faraday Soc. 1962, 58,

16 15. Dissado, L. A.; Fothergill, J. C. Electrical Degradation and Breakdown in Polymers, Peter Peregrinus Ltd, England, Hibma, T.; Zeller, H. R. Direct Measurement of Space-charge Injection from a Needle Electrode into Dielectrics. J. Appl. Phys. 1986, 59, Boggs, S. A. Very High Field Phenomena in Dielectrics. IEEE Trans. Dielectr. Electr. Insul. 2005, 12, Stournara, M. E.; Ramprasad, R. A First Principles Investigation of Isotactic Polypropylene. J. Mater. Sci. 2010, 45,

17 NO. OF COPIES ORGANIZATION 1 ADMNSTR DEFNS TECHL INFO CTR ATTN DTIC OCP 8725 JOHN J KINGMAN RD STE 0944 FT BELVOIR VA US ARMY RSRCH LAB ATTN IMNE ALC HRR MAIL & RECORDS MGMT ATTN RDRL CIO LL TECHL LIB ATTN RDRL CIO MT TECHL PUB ATTN RDRL SED C JANET HO RICHARD JOW ADELPHI MD

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