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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities 6 Capacitor Technology INTRODUCTION A capacitor is defined as a device that stores energy in an electrostatic field. In the early days of electrotechnology, energy storage technologies could be categorized unambiguously as follows: Batteries, which store energy in a chemical reaction; Capacitors, which store energy in an electrostatic field; and Inductors, which store energy in a magnetostatic field. Recent developments in electrotechnology have blurred these distinctions, especially between batteries and capacitors. There is now something of a continuum between a film capacitor, which truly stores energy in an electrostatic field and can absorb and release energy extremely rapidly, and a true battery (e.g., lead acid), which stores energy in a chemical reaction. For example, a double layer “supercapacitor” might be considered 75 percent capacitor and 25 percent battery, while an Li-ion battery might be considered 25 percent capacitor and 75 percent battery. The present discussion focuses primarily on capacitors that store energy in an electrostatic field. In the context of combat hybrid power systems (CHPS), where a hybrid electric drive is likely to have substantial stored energy in a high-discharge battery technology such as Li-ion, many of the moderate pulsed loads (e.g., in the 1-kA range to drive diode-pumped lasers) can be supplied directly from an Li-ion battery, assuming that frequent shallow discharges and charges do not impair battery life. Loads such as active armor, electrochemical guns, and so on, which require currents in the 100-kA range, will require high energy density capacitors. This chapter describes the types of capacitors most relevant to CHPS, their current status, and prospects for improvement of their properties. Promising research avenues for the future are also identified. This chapter does not address inertial energy storage and power production using flywheels (kilowatts to a few megawatts) or pulsed alternators (tens to thousands of megawatts). These inductive energy storage technologies are likely to be a more distant option for pulsed power for some applications.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities CAPACITOR TYPES AND CHARACTERISTICS Polymer Film Capacitors Organic polymers have a dominant position as the dielectric of choice for high-voltage, microsecond-discharge, high energy density capacitors. Typical organic polymers such as polyethylene (PE) have theoretical band gaps in the range of 8 to 10 eV. However, their effective band gaps are in the range of 1 eV as a result of impurity states near the conduction band. The nature of these impurity states is not clear. Some of them reside at the crystalline-amorphous interface in semicrystalline materials such as PE, but others are associated with impurities introduced during manufacture that can range from oxidation products (e.g., carbonyl groups) to catalyst residues. Thus, the useful band gap of such materials typically ranges from 0.5 eV to 1.5 eV. The conductivity of polymers is generally greater along the polymer backbone than between polymer chains. Thus, alignment of the backbones (in crystallites) in the plane of the film, as is likely during thin film formation, would result in an anisotropic conductivity with the low conductivity through the film and a higher conductivity in the plane of the film. This very likely explains why capacitor films such as biaxially oriented polypropylene (BOPP) can operate at substantially greater fields than would be suggested by the typical bulk properties. Polymer film capacitors are often constructed by metallizing the film with evaporated Al, and external connections are made by plasma metal spraying, usually based on a Zn alloy. Electrical connections are soldered to the plasma-sprayed end connections. Metallized film capacitors have the major advantage of being self-healing, that is, if the film punctures, the nanometer-thick metallization of the film is evaporated in the region of the breakdown, leaving a “clearing spot” that isolates the breakdown region electrically from the remainder of the capacitor. Metallized film capacitors can be designed to operate near the dielectric limits of the film. Clearing failures during operation simply result in gradual loss of capacitance. For energy storage applications, as opposed to tuned circuits, such a gradual loss of capacitance is often an acceptable trade-off for increased energy density. Ceramic Capacitors The electronic polarizability of most solids results in a relative dielectric constant of about 2. A few materials, such as ferroelectric and antiferroelectric oxides, have dielectric constants in the range of 104; however, this often comes at the expense of substantial temperature and field dependence of the dielectric properties. Advances in materials processing technology have focused on smaller ceramic grain sizes and reduced porosity in the dielectric, which has increased the operating fields to 10 kV/mm. Effective dielectric constants in the range of 1000 with low loss can be achieved in
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities (Ba,Sr)TiO3 based capacitors with curie temperatures below room temperature. Barium titanate is the most popular dielectric material used for multilayer ceramic capacitors (MLC), but the dielectric constant drops as a function of electric field. Antiferroelectrics show an increase in the dielectric constant with increase in the bias field due to switching from the antiferroelectric (AFE) to the ferroelectric (FE) phase. The application of an electric field greater than the switching field (Es) causes a phase transition from the orthorhombic to the tetragonal crystal structure. The AFE/FE property of rare earth doped lead zirconate titanate (PZT) compositions has been exploited to fabricate high energy density capacitors for power electronic inverter applications. Dielectric constants >5000 at a bias field of 5 kV/mm have been observed for proprietary lead lanthanum zirconate stannate titanate (PLZST) compositions. Ceramic capacitors are made from layers of ceramic material with metallic electrodes applied to the surfaces. Such structures can be arranged to place multiple layers either in series or parallel to increase the voltage withstand or capacitance, respectively. High-voltage capacitors in the range of tens of kVdc are also made from bulk ceramic of substantial thickness and are used in pulsed lasers, and other applications. HIGH ENERGY DENSITY CAPACITOR STATE OF THE ART Film/foil capacitors can operate at very high ripple current, but have low energy densities. The best commercial high-voltage capacitors achieve an energy density in the range of 0.6 J/cm3. This has apparently been extended to about 1 J/cm3 using modified forms of common capacitors materials such as metallized BOPP and PET film. Other technologies approaching this energy density include soggy foil capacitors with a high dielectric constant polymer coating applied to the nonmetallized side of the foil. Thus, the present state of the art in high-voltage film capacitors appears to be in the range of 1 J/cm3. Ceramic capacitors can be efficient at lower voltages, where they can be designed to required voltages below the efficient voltage range of film capacitors. Ceramic capacitors can also operate at very high ripple currents, where metalized film capacitors are limited by the end connections. Greater energy density is the main advantage of nonlinear dielectrics, typically 8 to 15 J/cm3 for modified PZT as compared to 1 to 2 J/cm3 for common dielectrics. However, for multilayer ceramic capacitors, there is substantial loss in energy density due to significant electric field derating and packaging. The energy density for a fully packaged AFE/FE capacitor, manufactured by Medronics, is in the 2 J/cm3 range. In addition, since ceramic capacitors cannot recover from a breakdown in the capacitor structure, they must be designed relatively conservatively. Double layer “supercapacitors” achieve the largest energy densities presently available in capacitors. However, the operating voltage of such capacitors is limited to a
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities few volts, they do not necessarily operate gracefully in series, and they degrade rapidly at elevated temperatures. Operation at the upper end of the military temperature range is also problematic. Nevertheless, double layer capacitors have a potential role to play as intermediate storage, as they can be charged and discharged much more rapidly than a battery. Also, progress in electrode materials and design is likely to reduce high-temperature degradation and lead to extended life at elevated temperature. POTENTIAL FOR HIGHER ENERGY DENSITIES High Dielectric Constant Polymer Film Capacitors As noted above, self-clearing is essential for high energy density polymer film capacitors, as it allows operation very near the breakdown field of the film. Self-clearing characteristics are a strong function of interfacial pressure within the winding, which is much more uniform in a round winding than in an oval winding. On the other hand, a round winding is volumetrically less efficient when many elements must be combined in series and parallel. Assuming that round windings are required for the reason mentioned above, the volumetric packing efficiency can be no better than π/4. When end connections and edge margins are included, the packing efficiency is unlikely to be better than 50 percent. Thus, to achieve a packaged energy density of 2 J/cm3, for example, the winding energy density must be at least 4 J/cm3. For a film relative dielectric constant of 2.2, this implies an operating field of 640 MV/m, which is roughly the inherent breakdown field of BOPP film. The highest field at which BOPP film can be operated is in the range of 450 MV/m, which would result in a packaged energy density of about 1 J/cm3, which is roughly the present state of the art. The only option is some combination of increasing the operating field or increasing the dielectric constant while maintaining the operating field. Data in the literature suggest that the dielectric withstand of BOPP can be increased from the range of 600 MV/m to over 800 MV/m through impregnation with oil. However, such impregnation requires an unreasonable amount of time and would probably make the film impossible to process. More rapid impregnation with a small molecule followed by polymerization or cross linking may be a possibility. If the operating field could be increased by 25 percent, the energy density would increase by about 50 percent, from about 1 J/cm3 to about 1.5 J/cm3. If the impregnant were highly polar and raised the dielectric constant, the increase in energy density might be somewhat greater, perhaps as much as 1.75 to 2 J/cm3. The other alternative is a high dielectric constant polymer film. PET ("mylar") is widely used as a capacitor film; however, it typically has a lower breakdown and operating field than BOPP. This may be the result of a less advantageous polymer morphology than BOPP, which has a high crystallinity with laminar crystallites aligned in the plane of the film. This puts the electric field across the polymer chains, in the direction of low conductivity and probably accounts for the outstanding dielectric
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities strength of BOPP. If the dielectric strength of PET could be increased to that of BOPP, the energy density would be about 50 percent greater as a result of the larger dielectric constant, which could bring the packaged energy density to the range of 1.5 J/cm3. Reaching an energy density of 2.5 J/cm3 and above will require new materials. One problem with polymer films is that they are expensive to produce in small quantities. Thus, most capacitor films have been byproducts of other, much larger applications. Probably the only film that is now manufactured solely for capacitor use (i.e., where the entire output of a polymer film facility is dedicated to the manufacture of capacitors) is BOPP. As a result, even if a high dielectric constant film with dielectric withstand similar to BOPP were developed, getting it manufactured could be problematic. However, if the relative dielectric constant of capacitor film can be increased to 8, in the range that can be achieved through asymmetric fluorine substitution into appropriate polymers, the required operating stress for an energy density of 2.5 J/cm3 at 50 percent packing efficiency would be about 375 kV/mm, which is very reasonable for a metallized film capacitor. Dielectric constants in the range of 8 are probably feasible, and if such a film could be developed with the dielectric strength of BOPP, a packaged energy density of about 3.5 J/cm3 should be possible. Thus, research into such films is worthwhile. Enough is understood of the morphology required for high dielectric strength and the molecular structure required for high dielectric constant that such development would not be purely trial and error. Films at Higher Operating Fields As noted above, increasing the operating field brings much more rapid benefits than increasing the dielectric constant, as the energy density is proportional to the dielectric constant but to the square of the field. However, the operating field of polymers is unlikely to be increased substantially beyond that of (possibly impregnated) BOPP, which suggests that metallized film capacitors have the short to medium term potential for packaged energy densities up to about 5 J/cm3 but probably not much greater. Still, this represents a factor of 5 increase from the present state of the art. The electrical conductivity of all insulators increases at high fields, and the increase nearly always starts in the range of 10 kV/mm. Materials that can withstand very high fields typically do so as a result of two phenomena. One is an immunity to oxidation, so that high field-induced damage is not cumulative. The second is strong interaction between hot electrons and optical phonons, which drains energy from the high energy tail of the electron energy distribution and deposits that energy as heat in the material lattice. Thin films of SiO2 are an excellent example of a material with both of these advantages. SiO2 is already oxidized, and optical phonons in SiO2 interact strongly with hot electrons. The result is a breakdown field in the range of 1500 kV/mm, much higher than that of polymer films. One problem with polymer films is that, being amorphous or semicrystalline, they may not have the medium to long-range order required to support optical phonons.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Nanocomposites One option for addressing this problem is nanocomposites of such inorganic, high field materials as SiO2 and polymers. Mixing nanoparticles into a liquid is normally very difficult as a result of surface energy considerations that tend to cause the nanoparticles to form strings which increase the viscosity greatly. This can be avoided by growing polymer off the surface of the nanoparticles prior to incorporation through surface-initiated polymerization. Basically, polymer chains initiate at an oxygen at the SiO2 surface and grow from it. When such compatibilized particles are incorporated into polymers, they look like polymer and do not cause large increases in viscosity, which facilitates mixing. The hope would be that the overlap of the electron wave function of hot electrons in the polymer with the SiO2 would result in cooling of the electrons as occurs in bulk SiO2, resulting in a flexible composite with higher dielectric withstand. Past work has indicated than when filler particles become smaller than about 1 μm and are incorporated at a substantial level, the dielectric properties of the composite become similar to those of the filler. This could lead to substantial improvements in dielectric withstand of capacitor films based on presently available polymers. Surface-initiated polymerization from inorganic nanoparticles requires catalysts, which can be difficult to remove after polymerization. Catalyst residue generally degrades electrical properties, such as conductivity, as has been a problem with metalocine catalysts. Thus, the development of high-performance nanocomposites must be considered high risk. Diamond-like Coatings Increasing the operating field of a capacitor dielectric requires increasing the effective band gap of the dielectric and controlling impurity-induced conductivity. Promising technologies include plasma deposited diamond-like coatings. These coatings must be deposited on a very smooth substrate, such as 2-μm metallized capacitor film. The volume of the diamond-like coatings in which the field resides would only be in the range of 25 to 50 percent of the total volume of the structure, compared to polymer films where the field resides in essentially 100 percent of the volume, as the metallization thickness is negligible compared to the polymer thickness. The other issue is that the deposition rate of such films is presently glacial; however, this can always be overcome with enough capital. Still, such coatings have the potential for higher operating fields, although issues of capacitor structure, self-clearing characteristics, and the operating field actually achievable over large film areas make this technology, at its present early stage of development, a high-risk proposition. Ceramic Capacitors Ceramic capacitors are relatively mature. However, as in the case of polymer film capacitors, substantial progress has been made over the last decade. Ceramic capacitors tend to have very high dielectric constant but relatively low dielectric strength as a result of substantial porosity, which is typical of sintered materials. One approach to reducing the free volume associated with sintering is to suspend the high dielectric constant particles in a polymer. This will require good compatibilization of the polymer with the ceramic particles, as can be achieved through surface-initiated polymerization, as discussed above.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities Another area in which progress is inevitable is the further development of high dielectric constant, high-temperature, low-loss ceramic capacitor dielectrics. Obviously, a spectrum of such materials with trade-offs among these key parameters is highly desirable. Ceramic capacitor technology would undoubtedly benefit from funding for investigations of high-temperature, high-stress failure mechanisms. These may be the result of ion migration and other processes but are not yet well understood. Understanding of high-temperature, high-stress failure mechanisms would clearly benefit, or perhaps should be a prerequisite to the development of, new materials. Double Layer Capacitors Double layer capacitors are energy storage devices that convert chemical into electrical energy and are particularly suited to providing the energy to power electrical devices. These capacitors have two electrodes separated by a separator having an ionically conductive electrolyte, but their energy can be composed of double layer capacitance. They are rechargeable devices designed to have a very high cycle life. Like conventional capacitors, double layer capacitors have higher power and lower energy than rechargeable batteries. Double layer capacitors are at a much earlier stage of development than are film capacitors. As a result, progress is more easily predictable. At present, aqueous electrolytes provide voltage drops per electrode a little over 1 V with relatively low equivalent series resistance (ESR). Organic electrolytes can operate at over 3 V, but have much greater ESR. Clearly, a combination of greater voltage per electrode combined with low ESR is highly desirable, and work to define the fundamental limits of electrolyte technology in this context would probably contribute toward progress. Consistency is another issue. Double layer capacitors have relatively poor high-temperature aging characteristics, which may be related to impurities in the carbon electrode and/or temperature-dependent leakage current. The nature of impurities that are detrimental to long-term, high-temperature operation needs to be determined, as does any basic relationship between leakage current and capacitor life. Double layer capacitors have complex dielectric time constants related to the pore structure of the carbon electrode. Present pore structure reduces the effective electrode surface area to substantially below the theoretical value. Thus, research into the carbon electrode is likely to result in progress on a large number of fronts, including high-temperature aging, volumetric efficiency, and dielectric performance. For double layer capacitors to reach their full promise, capacitors must be manufactured with well-defined, stable operating characteristics, in terms of dielectric time constants, long-term and high-temperature degradation, and so on, so that large
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities numbers of double layer capacitors can be operated in series over long periods of time through a wide range of charge-discharge cycles without the need for complex voltage balancing circuitry. This will require substantial improvements in materials and uniformity of materials. At this point in time, the relevant materials parameters are not understood, let alone the means by which the quality of these parameters might be controlled. Thus, the first step is to undertake research to quantify the relevant materials parameters. The improved understanding resulting from such work is likely to have payoffs well beyond the short-term need to improve product quality and uniformity. SUMMARY The present state of the art in microsecond discharge, high energy density capacitors appears to be about 1 J/cm3 (packaged) based on improved polypropylene dielectric. From the preceding discussion, and acknowledging that technology forecasting is extremely risky, the limits to microsecond polymer film capacitor technology appear to be in the range of 5 J/cm3 based on the use of yet-to-be-developed high dielectric constant films at operating fields typical of BOPP. Moving beyond 5 J/cm3 would require operating at substantially increased fields and would require a fundamental improvement in dielectric technology, such as a nanocomposite or diamond-like coating dielectric, which has yet to be developed. Thus, short to medium term, high energy density can be pursued through the development of improved high dielectric constant polymer films and the technology to manufacture them in modest quantities, at least on a prototype basis, such as vacuum polymer deposition. For the longer term, dielectrics with the potential for increased operating field should be pursued, such as diamond-like coatings and nanocomposites. Ceramic capacitor technology will almost certainly be important for power converters with a hybrid electric vehicle, for which filters make a significant contribution to mass and volume. As well, the upper operating temperature of such capacitors will be very important. Thus, research to improve the energy density and operating temperature range of ceramic capacitors should have a definite payoff to CHPS. The relevance of double layer capacitors to CHPS is less clear and will depend on the capability of the battery to deliver large, short-term currents, undergo frequent, shallow charge-discharge cycles, and so on. Technical constraints may favor the inclusion of a double layer capacitor bank as a buffer that is more capable of rapid charge–discharge cycles. A summary of the technical challenges and opportunities for improvement of high energy density capacitors is given in Table 6-1.
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities TABLE 6-1 Technical Challenges, Performance Metrics, and Research Priorities Associated with High Energy Density Capacitors Component/System Technical Challenges Performance Metrics R&D Priorities Polymer film capacitors Films with improved dielectric properties Dielectric constant Dielectric withstand New polymer films with increased dielectric constant and dielectric withstand similar to biaxially oriented polypropylene Filled polymer films: either inorganic filler to improve dielectric strength, high dielectric constant filler to increase dielectric constant, or high dielectric polymer filler to reduce volume within the film, resulting in a combination of increased operating field and increased dielectric constant Ceramic capacitors Lack of understanding of aging/failure mechanisms Research on aging/failure mechanisms under high-temperature, high-field conditions Dielectrics with improved properties Dielectric constant Dielectric withstand Research to improve high energy density, high-temperature ceramic dielectrics Improved operating electric field Operating field Ceramic-polymer composites or other technologies that reduce the free volume within the ceramic Double layer capacitors Lack of understanding of aging and degradation processes at high temperature Investigate role of impurities in the carbon electrodes and interactions among the electrodes, electrolyte, and separator Improvement of properties of electrolytes, increase in cell voltage, and reduction of equivalent series resistance Cell voltage equivalent series resistance Predictability of performance over time Stability of properties Materials and processes that achieve reproducible cell characteristics that are stable
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Combat Hybrid Power System Component Technologies: Technical Challenges and Research Priorities over time, or age uniformly over time Reduction of current densities Effective electrode surface area Research into materials and manufacturing processes that increase the effective surface area of electrodes
Representative terms from entire chapter: