Review of Recent Activities on Dielectric Films for Capacitor Applications

Transcription

1 Journal of International Council on Electrical Engineering Vol. 4, No. 1, pp.1~6, Review of Recent Activities on Dielectric Films for Capacitor Applications Lejun Qi, Linnea Petersson** and Tieliang Liu* Abstract Polypropylene (PP) film has been used in capacitors since 1970s. The high breakdown strength, low dielectric losses, and high availability make PP well suitable for use as capacitor dielectric. At the moment, the typical energy density achieved with PP film at room temperature is about 1.2 J/cm 3. Recent research on PP films focus on improving the permittivity, breakdown strength, thermal withstandability, and thermal conductivity, by producing copolymers, blending with other polymers, or adding inorganic particles. Some results claim energy density higher than double that of commercial biaxially oriented PP capacitor film. Detailed research approaches and dielectric properties improvements are reviewed in this paper. The major focus is often placed on permittivity and breakdown strength. Unfortunately, little research shows positive results for maintaining the low dielectric losses. Keywords: Capacitor, Dielectric film, Polypropylene, Polyester, Polycarbonate, Poly(vinylidene fluoride) 1. Introduction Capacitors are important passive components for electrical networks and apparatus. They can either improve the efficiency of HVAC power networks, or act as energy storage and stabilize the voltage level in HVDC systems. Dielectric materials in HV capacitors play a key role for charge control and energy storage. Before the 1970s, impregnated kraft paper was the main capacitor dielectric and it was used in combination with mineral oil, polychlorinated diphenol (PCB), as impregnated liquid [1]. However, due to the low dissipation factor, high insulation resistance, good stability, and high availability, the polymer films gradually replaced the kraft paper in capacitors. A switch from paper to polymer film also shortened the production process for capacitors, by reducing the drying time needed before impregnation. In general, the energy density of a dielectric material can be expressed as: U e =1/2ε r ε 0 E 2, Where ε 0 is the vacuum permittivity (= F/m), ε r is a relative dielectric permittivity of linear dielectric materials and E is applied electric field. Although polymers exhibit high breakdown strength, their low dielectric Corresponding Author: ABB China Corporate Research Center, Beijing, China ([email protected]) * ABB China Corporate Research Center, Beijing, China ** ABB AB, Corporate Research, Vasteras, Sweden Received: November 19, 2013; Accepted: January 6, 2014 permittivity (most, <10) still limits the energy density [2]. As a result, an improvement of the intrinsic electrical properties of the most commonly used polymers is highly requested. It will have a high impact of the size of power capacitors. Table 1 shows the main features of the most common dielectric polymer films used today in different capacitor applications. The main characteristics of these polymer films will be reviewed in the following sections of the paper. Table 1. General characteristic of common dielectric polymer films [3] Polymer film Polypropylene (PP) ε r Maximum temperature ( ) Breakdown strength (MV/m) Energy Density (J/cm 3 ) Dissipation Factor % 1kHz <0.02 Polyester (PET) <0.5 Polycarbonate (PC) Polyphenylene sulfide (PPS) Polyvinylidenefluoried (PVDF) < < < Polypropylene films The most important polymer film used in commercial capacitors is bi-axially oriented isotactic polypropylene. The beneficial properties of PP come from the polymers chain structure which does not contain any polar groups. 1

2 Review of Recent Activities on Dielectric Films for Capacitor Applications PP has the highest breakdown strength among common polymer films used in capacitor applications. PP also has a low dielectric loss and excellent self-healing capability, which allows operation at high electric field. On the other hand, PP has relatively low permittivity, resulting in a typical energy density of 1.2 J/cm 3 at room temperature [3]. Much work has been dedicated towards improving the dielectric properties of PP film. Examples trying to increase the permittivity and breakdown strength are quite common. Researchers have tried to blend PP with other polymers and nanoparticles, or to synthesize PP-based copolymers. 2.1 Copolymers after an applied electric field of E=600 MV/m, without showing any significant increase of energy loss. A family of thermally cross-linkable isotactic poly (propylene-co-p-(3-butenyl)styrene) (PP-BSt) copolymers have been synthesized [4,5]. The thin films made of these copolymers have also been prepared. Upon cross-linking, the x-pp thin film dielectric having 3.65 mol % BSt units (x-pp-3 as shown in Fig. 1), exhibits a ε~3, which is independent of temperature ( 20 C C) and frequency (100 Hz - 1 MHz). The cross-linked PP also shows high breakdown strength (E=645 MV/m), narrow breakdown distribution (β=42), and reliable energy storage capacity >5 J/cm 3 (double that of state-of-the-art BOPP capacitors) [6]. 2.2 Blends Fig. 2. Temperature dependence of the dielectric permittivity of PVDF/PP blend composite in a 1/1 volume fraction at selected frequencies as indicated. [8]. Fig. 1. (a) Dielectric constants (vs frequency and temperature) and (b) Weibull distribution for breakdown strength of PP, x-pp-1, x-pp-2, and x-pp-3 thin film dielectrics, which respectively have 0, 0.64%, 2.97%, 3.65% BSt units. [6]. In order to increase the permittivity, poly(propylene-coundecen-11-ol) copolymer has been synthesized, containing flexible OH polar groups [4]. This polar group structure shows a significant increase in the dielectric constant to about 4.6. A linear reversible charge storage behavior was also observed with a high releasing energy density (7 J/cm 3 ) Besides synthesizing copolymers, many efforts have been made to blend other polymers directly with PP base. For instance, low density polyethylene (LDPE), poly-vinyl chloride (PVC), ethylene propylene rubber (EPR), and ethylene vinyl acetate (EVA) are easy to process and are excellent electrical insulators in electrical dielectric designs [7]. Furthermore, poly(vinylidene fluoride) (PVDF) / polypropylene (PP) blend composites [8] and polypropylene (PP) / acrylonitrile-butadiene-styrene (ABS) blends [9] have been investigated. Dang et al. prepared PVDF/PP blends using a Haake mixer close to the melting temperature [8]. The blends had high permittivity values over wide frequency ranges. The permittivity increases with the temperature and the amount of PVDF added into the blend. The dielectric permittivity was about 5.4 for a composite with 50 vol % PVDF at room temperature. It was 2

3 Lejun Qi, Linnea Petersson and Tieliang Liu also revealed that PP-g-MAH could improve the interface interaction between two polymer matrices [10]. 2.3 Nanofillers There are many investigations on dielectric and electrical properties of composites with inorganic fillers in isotactic (ipp) or syndiotactic (spp) polypropylene matrices, especially nanofillers [11], such as multiwalled carbon nanotube [12], layered silicate [13, 14, 15], Fe 2 O 3, ZrO 2 and Al 2 O 3 as core in shell nanoparticles (NPs) [16], carbon nanofiber [17], carbon black [18], and glass beads [19]. Increased permittivity and loss factor were often seen compared with unfilled ipp [20-23], It has been shown that trapping properties are highly modified by the presence of nanofillers [24]. The composites also exhibit improved thermal stability, and higher crystallinity due to that the nanofillers hinders the mobility of the polymer chains [10]. A novel two-phase composite system using polypropylene (PP) as matrix and semiconductor bismuth sulfide (Bi 2 S 3 ) as filler has been reported [25]. The results reveal a percolative behavior of the Bi 2 S 3 /PP composites at the threshold of 8 vol%. The composite demonstrates high thermal stability and breakdown strength, which show their potential in film capacitors applications. 3.1 Laminates 3. Other dielectric films The aim of producing laminates is to ensure both high energy density and high electrical breakdown strength. Multilayer structures are produced by combing two (or more) materials that are commercially available and used in capacitor applications. The multilayer structure can be produced either directly co-extruded during film manufacturing, or by melting together films that are already prepared. Baer et al. [26-29] produced multilayer dielectric films through the process of multilayer coextrusion. Layermultiplying coextrusion is an advanced polymer processing technique for combing two or more polymers in a layered configuration with controlled architecture. It is a continuous processing technique, capable of economically producing films with up to thousands of layers with individual layer thickness from micro- to nano-meter scale. Fig. 3 shows that the highest measured energy density, U d ~14 J/cm 3, for the 50/50 layered material was ~60% greater than that measured for pure PVDF and was nearly double that of pure PC. The 32-layered films showed comparable results for high PC concentrations but much lower U d at high PVDF concentrations. Fig. 3. Energy density of 32-layer (solid circles) and 256- layer (open circle) PC/PVDF films [29]. 3.2 Polyethylene terephthalate (PET) Next to PP, PET is the second most commonly used dielectric polymer in high-performance capacitor applications. PET offers slightly lower breakdown strength than PP films, a reasonable dielectric constant (ε=3.3), and a high operating temperature (125 C) [30]. When only dielectric constant counts, the energy density of a capacitor is 50% higher if PET films are used to replace PP films. The disadvantage of PET is the relatively high dissipation factor, which increases with temperature and frequency with a minimum at around 50 Hz. In case of a high repetition rate, polyester is not suitable for high pulse power applications [31]. Hybrid coatings, based on poly (ethylene oxide) (PEO) or polycaprolactone (PCL), and silica (SiO 2 ) have been used atop the PET films [32]. The electrical strength of the coated films increases up to 10-15% of the uncoated ones regardless of polymer type (PEO/PCL) and amount of inorganic phase, as far as the thickness of the coating is below 5 µm. However, the loss factor together with the permittivity also increases particularly at low frequencies (< 10 Hz). To reduce the dielectric loss of PET film, acrylate/barium titanate nanocomposites have been prepared and coated on PET [33]. The composite film had higher permittivity, higher dielectric strength and lower dielectric loss. This was a new method of producing high stored energy density capacitor with low cost industrially. 3

4 Review of Recent Activities on Dielectric Films for Capacitor Applications 3.3 Polycarbonate (PC) When polycarbonate (PC) film was first introduced as capacitor dielectric in the 1960s, many electronic systems relied on PC film capacitors for operation at temperatures not exceeding 125 C. The use of PC in capacitors was successful for decades [34]. However, because of the large shrinkage, small Young's modulus, and easy formation of defects and weak spots, the amount of PC used for capacitors has been significantly reduced. It is today only available in small quantities. Today, other commercially available plastic dielectric films are used to replace PC dielectrics, such as polyimide (PI, Kapton as trademark name by Dupont) and polyphenylene sulfide (PPS). Because the Kapton TM is only available in thick films, it is impractical for capacitors. PPS is a good candidate for PC replacement. PPS is a crystalline polymer, a high quality engineering plastics, with outstanding chemical resistance, low water absorption (about 0.02%), excellent thermal stability (heat deflection temperature of greater than 260 C), high elastic modulus, and it is flame-retardant. The comparison by Mark A. Crater of constructed capacitors showed that PPS exhibits lower dissipation factor and equivalent series resistance versus frequency than PC [35]. PPS films have gradually been employed in both AC and DC capacitors. Aromatic polyurea thin films fabricated through vapor deposition polymerization also possesses a high thermal stability, above 200 C [36]. This high quality polymer film exhibits a high breakdown field of 800 MV/m and a highenergy density of 12J/cm 3 with ε = 4.2 at room temperature, which make it attractive for high-energy density capacitors, especially under high temperatures. However, the production routine of such film does not allow for large scale applications. 3.4 Polyvinylidene fluoride (PVDF) PVDF and its copolymer P(VDF-TrFE) seem to be the best known ferroelectric polymer with a very high dielectric constant. They appeared to be promising in a variety of applications.. However, they also show high dissipation, due to the polar vinylidene fluoride group, leading to high losses [37]. It has been found that P(VDF-TrFE) can be changed from a normal ferroelectrics to a relaxor ferroelectrics (RFE) as small defect concentration introduced by highenergy electron irradiation [38]. The RFE developed here exhibits a high electrostrictive strain which is greatly attractive for many actuator and transducer applications. Meanwhile, REF also has a high dielectric constant, which is attractive for development of high-density energy storage capacitors and electronic packaging. As shown in Table 2, PVDF and P(VDF-TrFE) could also be converted into relaxor ferroelectrics, e.g. P(VDF-CTFE)[10], P(VDF- HFP)Tomer, and P(VDF-TrFE-CFE)[11], by importing bulky groups CTFE, HFP and CFE as defects in molecular structures to obtain high electric energy density. The novel Ag-BaTiO 3 /PVDF (polyvinylidene fluoride) three-component nanocomposites have been prepared, which exhibit high energy density of nearly 10 J/cm 3 with ε=28 at room temperature[39]. Table 2. Comparison of some dielectric polymers based on PVDF and P(VDF-TrFE) [39] Polymer film ε r Breakdown strength (MV/m) Energy Density (J/cm 3 ) P(VDF-CTFE) P(VDF-HFP) P(VDF-TrFE- CFE) Ag-BaTiO 3 /PVDF Conclusion The most commonly used dielectric material in power capacitors today is isotactic polypropylene due to its high breakdown strength, low losses, good self-healing properties, and its excellent ageing behavior. Capacitors are becoming a larger part of many types of power system applications and as a result it is becoming increasingly important to reduce the size of capacitor. The development of new dielectric materials with high breakdown strength and high energy density is of high focus for many research groups. Improvements using different methods have been reviewed in this paper. Some improvements in dielectric strength, permittivity, as well as energy density, have been claimed. However, the losses are difficult to maintain at the low level of pure ipp. References [1] Lowe, S.K., Development, testing and performance of HV power capacitors. Power Engineering Journal, (4). [2] Bestaoui-Spurr, N., et al., Polymer nanocomposites for High energy storage capacitors. Proceedings of the Asme Conference on Smart Materials, Adaptive Structures and Intelligent Systems, 2010, Vol

5 Lejun Qi, Linnea Petersson and Tieliang Liu [3] Rabuffi, M. and G. Picci, Status quo and future prospects for metallized polypropylene energy storage capacitors. IEEE Transactions on Plasma Science, (5): p [4] Yuan, X., Y. Matsuyama, and T.C. Chung, Synthesis of functionalized isotactic polypropylene dielectrics for electric energy storage applications, Macromolecules, 2010, 43: p [5] Langston, J., R.Colby, F.Shimizu, T.Suzuki, M.Aoki, and T.C.Chung, One-pot synthesis of long chain branch PP (LCBPP) Using Ziegler-Natta catalyst and branching reagents, Macromolecule, 2007, 40, p: [6] Yuan, X. and T.C.M. Chung, Cross-linking effect on dielectric properties of polypropylene thin films and applications in electric energy storage. Applied Physics Letters, (6). [7] Hosier, I.L., A.S. Vaughan, and S.G. Swingler, An investigation of the potential of polypropylene and its blends for use in recyclable high voltage cable insulation systems. Journal of Materials Science, (11): p [8] Dang, Z.M., W.T. Yan, and H.P. Xu, Novel highdielectric-permittivity poly(vinylidene fluoride)/ polypropylene blend composites: The influence of the poly(vinylidene fluoride) concentration and compatibilizer. Journal of Applied Polymer Science, (6): p [9] Khare, R.A., et al., Influence of Multiwall Carbon Nanotubes on Morphology and Electrical Conductivity of PP/ABS Blends. Journal of Polymer Science Part B-Polymer Physics, (21): p [10] Fuse, N., Y. Ohki, and T. Tanaka, Comparison of Nano-structuration Effects in Polypropylene among Four Typical Dielectric Properties. IEEE Transactions on Dielectrics and Electrical Insulation, (3): p [11] Alig, I., et al., Conductivity spectroscopy on melt processed polypropylene-multiwalled carbon nanotube composites: Recovery after shear and crystallization. Polymer, (4): p [12] Izci, E., N. Bowler, Dielectric Properties of Isotactic Polypropylene and Montmorillonite Nanocomposites, in Proceedings of the 2010 IEEE International Conference on Solid Dielectrics [13] Motori, A., et al., Electrical conductivity and polarization processes in nanocomposites based on isotactic polypropylene and modified synthetic clay. Journal of Polymer Science Part B-Polymer Physics, (6): p [14] Li, Z., et al., In Situ Catalytic Encapsulation of Core- Shell Nanoparticles Having Variable Shell Thickness: Dielectric and Energy Storage Properties of High- Permittivity Metal Oxide Nanocomposites. Chemistry of Materials, (18): p [15] Yu, C.R., et al., Electrical and dielectric properties of polypropylene nanocomposites based on carbon nanotubes and barium titanate nanoparticles. Composites Science and Technology, (15): p [16] Takala, M., et al., Dielectric Properties and Partial Discharge Endurance of Polypropylene-Silica Nanocomposite. IEEE Transactions on Dielectrics and Electrical Insulation, (4): p [17] Xu, H.P., et al., Enhanced dielectric properties and positive temperature coefficient effect in the binary polymer composites with surface modified carbon black. Journal of Materials Chemistry, (2): p [18] Li, Y., et al., Poly(propylene)/Graphene Nanoplatelet Nanocomposites: Melt Rheological Behavior and Thermal, Electrical, and Electronic Properties. Macromolecular Chemistry and Physics, (18): p [19] Sui, G., et al., Dielectric properties and conductivity of carbon nanofiber/semi-crystalline polymer composites. Acta Materialia, (10): p [20] Koskinen, J., et al, Polymer nanocomposites development for electronic industry needs. Solid State Phenomena, 2009, 151: p. 3-9 [21] Takala, M. et al, Dielectric properties of nanostructured polypropylene-polyhedral oligomeric silsesquioxane compounds, IEEE Transactions on Dielectrics and Electrical Insulation, 2008, 15 (1): p [22] Kanapitsas, A., et al., Study of electrical /dielectric and thermomechanical properties of polymer - carbon nanotubes nanocomposites. Proceedings of the 1st Wseas International Conference on Recent Advances in Nanotechnology, Landgrebe [23] Fuse, N., et al., Evaluation of Dielectric Properties in Polypropylene/clay Nanocomposites, 2009 Annual Report Conference on Electrical Insulation and Dielectric Phenomena p [24] Moucka, R., et al., Electromagnetic absorption efficiency of polypropylene / montmorillonite / polypyrrole nanocomposites. Materials & Design, (4): p

6 Review of Recent Activities on Dielectric Films for Capacitor Applications [25] Dang, Y., et al., Enhanced dielectric properties of polypropylene based composite using Bi(2)S(3) nanorod filler. Progress in Natural Science-Materials International, (3): p [26] Baer, E. et al, US Patent, US 2010/ A1, Jul 8, [27] Mackey, M. et al. Enhanced breakdown strength of multlayered films fabricated by forced assembly microlayer coextrusion. J.Phys.D: Appl.Phys. 2009, 42: [28] Ponting, et al, Polymer nanostructures by forced assembly: process, structure and properties. Macromol. Symp. 2010, 294-1: [29] Wolak, M. A et al., Dielectric response of structured multilayered polymer films fabricated by forced assembly. Applied Physics Letters, 2008, 92: [30] Neagu, E., et al., Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films. Journal of Physics D-Applied Physics, (11): p [31] Laihonen, S.J., et al., Influence of electrode area on dielectric breakdown strength of thin poly(ethylene terephthalate) films Annual Report Conference on Electrical Insulation and Dielectric Phenomena [32] Saccani, A., et al., Electrical behavior of PET films coated with nanostructured organic-inorganic hybrids. Journal of Applied Polymer Science, (5): p [33] Shi, Z.Q., et al., Study of acrylate / barium titanate nanocomposite. Electronic Components & Materials, (5). [34] Carter, M.A., Is there a substitute for polycarbonate film capacitors? Power Electronics Technology, (4). [35] Wang, Y., et al., High-energy density in aromatic polyurea thin films. Applied Physics Letters, (20). [36] Cheng, Z.Y., et al., Evolution of property and microstructure of P(VDF-TrFE) copolymers modified by irradiation introduced defects, in Polymer/Metal Interfaces and Defect Mediated Phenomena in Ordered Polymers, E.M.G.G. Manias, Editor p [37] Chu, B., et al., A dielectric polymer with high electric energy density and fast discharge speed. Science, (5785): p [38] Zhang, S.H., et al., Normal ferroelectric to ferroelectric relaxor conversion in fluorinated polymers and the relaxor dynamics. Journal of Materials Science, (1): p [39] Wang, Y., et al., Recent Development of High Energy Density Polymers for Dielectric Capacitors. IEEE Transactions on Dielectrics and Electrical Insulation, (4): p Lejun Qi is a Scientist at the ABB Corporate Research Center in Beijing, China. He holds a Ph.D. degree in material science and engineering from the University of Minnesota, USA. Prior to joining ABB, he worked for Formulated Systems R&D of Dow Chemical, developing epoxy formulations for electrical insulation and fiberreinforced composites. He has been involved in research and development projects for insulation materials for outdoor applications and dielectric films for power capacitor applications. Linnea Petersson is a Principal Scientist at the ABB Corporate Research Center in Västerås, Sweden. She holds a Ph.D. in material science and engineering from the Norwegian University of Science and Technology, Norway. Prior to joining ABB she worked as Materials R&D Engineer at Zarlink Semiconductor. At ABB she has been leading research and development projects on insulation materials for high voltage applications, with special focus on metalized film power capacitors. 6