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The Pennsylvania State University The Pennsylvania State University The Graduate School Department of Materials Science and Engineering TEMPERATURE DEPENDENCE OF DIELECTRIC BREAKDOWN IN POLYMERS A Thesis in Materials Science and Engineering by Cheolhong Min © 2008 Cheolhong Min Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science August 2008 ii The thesis of Cheolhong Min was reviewed and approved* by the following: Thomas R. Shrout Professor of Materials Science Thesis Co-Advisor Michael T. Lanagan Associate Professor of Engineering Science and Mechanics Thesis Co-Advisor Shujun Zhang Research Associate and Assistant Professor of Materials Science and Engineering Joan Redwing Professor of Materials Science and Engineering Chair, Intercollege Graduate Degree Program in Materials Science and Engineering *Signatures are on file in the Graduate School iii ABSTRACT Capacitors possess high-power densities and have the ability to deliver energy with short discharge times, which are in the micro-second to nano-second range. Both energy and power density are related to the dielectric materials used in the capacitor. One of the main challenges for capacitors is achieving high energy density as determined by the relative permittivity and dielectric breakdown strength of a material. Polymers are some of the most important dielectric materials for high-power capacitors because polymer films show high breakdown strength. A general trend in polymers is that the breakdown strength increases with decreasing temperature. An understanding of the temperature dependence relationships among electrical properties, polymer chemistry, and crystalline structure may lead to improved energy storage for polymer-based capacitors—this is the basis of the thesis. Various polymers including polypropylene (PP), polyimide (PI), polymethyl methacrylate (PMMA), poly(vinylidene fluoride-trifluoroethylene- chlorotrifluoroethylene) terpolymer (p(VDF-TrFE-CTFE)) were investigated in this study. Electrical properties of the polymers including low-field (<1X10-3MV/mm) permittivity (εr), dielectric loss (tanδ), high-field (>1MV/cm) permittivity, and dielectric breakdown strength will be discussed. Temperature dependence of low-field permittivity, dielectric loss, and dielectric breakdown in the polymers is also presented. The electrical properties of the polymers have been investigated over a temperature range from -196°C to 170°C. Breakdown strength (EBr) of all the polymers was found to decrease with increasing temperature. Maximum energy per unit volume (Evol) of the various polymers at maximum breakdown strength is also introduced using relative permittivity. iv TABLE OF CONTENTS LIST OF FIGURES .....................................................................................................vi LIST OF TABLES.......................................................................................................ix ACKNOWLEDGEMENTS.........................................................................................x Chapter 1 Introduction ................................................................................................1 Chapter 2 Dielectric Breakdown in Polymer Dielectrics............................................10 2.1 Introduction.....................................................................................................10 2.2 Electrical Breakdown Mechanisms ................................................................12 2.2.1 Electronic Breakdown ..........................................................................12 2.2.1.1 Intrinsic Breakdown ...................................................................12 2.2.1.2 Avalanche Breakdown ...............................................................15 2.2.2 Thermal Breakdown .............................................................................17 2.2.3 Electromechanical Breakdown.............................................................19 2.3 Structure of Polymer Dielectrics ....................................................................25 2.3.1 Glass-transition Temperature...............................................................25 2.3.2 Crystallinity..........................................................................................26 2.3.3 Cohesive Energy Density related to Dielectric Breakdown in Polymers.................................................................................................27 Chapter 3 Experimental Approach..............................................................................31 3.1 Introduction ....................................................................................................31 3.2 Materials.........................................................................................................33 3.2.1 Polypropylene (PP) ..............................................................................33 3.2.2 Polyimide (PI) ......................................................................................34 3.2.3 Polymethyl Methacrylate (PMMA) .....................................................35 3.2.4 P(VDF-TrFE-CTFE) Terpolymer ........................................................36 3.3 Film Preparation .............................................................................................38 3.3.1 Polymer Films ......................................................................................39 3.4 Relative Permittivity and Dielectric-Loss Measurements ..............................40 3.5 Dielectric-Displacement Measurements.........................................................41 3.6 Breakdown Measurements .............................................................................43 Chapter 4 Experimental Results and Discussion ........................................................49 4.1 Introduction ....................................................................................................49 4.2 Low Field Permittivity and Dielectric Loss....................................................49 4.2.1 Relative Permittivity and Dielectric Loss at Room Temperature ........50 v 4.2.2 Temperature Dependence of Relative Permittivity and Dielectric Loss ........................................................................................................51 4.3 High-field Permittivity ...................................................................................56 4.4 Dielectric Breakdown in Polymers.................................................................57 4.4.1 Dielectric Breakdown of Polymer Films at Room Temperature .........58 4.4.2 Temperature Dependence of the Breakdown Strength of Polymer Films.......................................................................................................60 4.5 Energy Density...............................................................................................66 Chapter 5 Conclusion and Future Work .....................................................................69 5.1 Conclusion ......................................................................................................69 5.2 Future Work ...................................................................................................71 vi LIST OF FIGURES Figure 1.1: Power density vs. energy density for various energy storage components. Capacitors possess high-power densities as a result of a short discharging time [1]..............................................................................................2 Figure 1.2: Frequency dependence of dielectric permittivity. Dielectric permittivity (εr') and loss (εr'') are affected by frequency [5] ...............................4 Figure 1.3: P-E of (a) a linear dielectric material and (b) a ferroelectric material that exhibits a hysteretic non-linear response.......................................................5 Figure 1.4: Comparison of energy density for various dielectric materials for high- power capacitors. Energy density is limited by dielectric permittivity and the electric field due to breakdown strength of a material. 1) biaxially oriented PP, 2) modified poly(vinylidene fluoride) polymer, 3) titanium oxide dielectric, 4) antiferroelectric/ferroelectric phase switch ceramic, 5) PP film capacitor, 6) PVDF film capacitor, 7, 8) ceramic capacitors, 9) commercial polymer film power capacitor [6] .......................................................................................6 Figure 1.5: Temperature dependence of electric strength in polymer dielectrics. The electric strength increased with decreasing temperature [7] .........................7 Figure 2.1: Mechanisms of time to breakdown and breakdown strength (EBr) when breakdown occurs [1] ...........................................................................................11 Figure 2.2: Partial discharge when an electric field is applied to (a) a void of a dielectric material, and (b) a crack between the dielectric material and a metallic electrode [1] ............................................................................................11 Figure 2.3: Fröhlich’s energy band model. It is assumed that a trap band is directly below a conduction band [2] ...................................................................14 Figure 2.4: Thickness-dependent DC breakdown strength in LDPE films at - 196°C [5]. .............................................................................................................17 ○: blank, ⊙: 1wt% pyrene doped, ●: 1wt% AS-1 doped.........................................17
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