这是用户在 2024-10-22 10:54 为 https://www.annualreviews.org/content/journals/10.1146/annurev-matsci-070317-124435 保存的双语快照页面,由 沉浸式翻译 提供双语支持。了解如何保存?
1932
  • Access provided by: Xi'an Jiaotong University

Abstract

The demand for high-temperature dielectric materials arises from numerous emerging applications such as electric vehicles, wind generators, solar converters, aerospace power conditioning, and downhole oil and gas explorations, in which the power systems and electronic devices have to operate at elevated temperatures. This article presents an overview of recent progress in the field of nanostructured dielectric materials targeted for high-temperature capacitive energy storage applications. Polymers, polymer nanocomposites, and bulk ceramics and thin films are the focus of the materials reviewed. Both commercial products and the latest research results are covered. While general design considerations are briefly discussed, emphasis is placed on material specifications oriented toward the intended high-temperature applications, such as dielectric properties, temperature stability, energy density, and charge-discharge efficiency. The advantages and shortcomings of the existing dielectric materials are identified. Challenges along with future research opportunities are highlighted at the end of this review.
高温介电材料的需求源于众多新兴应用,如电动汽车、风力发电机、太阳能转换器、航空航天功率调节和井下油气勘探,在这些应用中,电力系统和电子设备必须在高温下运行。本文概述了针对高温电容器储能应用领域纳米结构介电材料近期取得的进展。所审查的材料重点是聚合物、聚合物纳米复合材料、块状陶瓷和薄膜。涵盖了商业产品和最新的研究成果。虽然简要讨论了通用设计考虑因素,但重点放在了面向预期高温应用的材料规格上,例如介电性能、温度稳定性、能量密度和充放电效率。确定了现有介电材料的优缺点。在本文的结尾,突出了挑战以及未来的研究机会。

Keyword(s): capacitorsdielectric polymerselectroactive ceramicsenergy storagehigh temperature
关键词:电容器、介电聚合物、电活性陶瓷、储能、高温
Loading

Article metrics loading...
文章指标加载中...

/content/journals/10.1146/annurev-matsci-070317-124435
2018-07-01
2024-10-22

High-Temperature Dielectric Materials for Electrical Energy Storage

    ; ; ; ; ;
    1State Key Laboratory of Control and Simulation of Power System and Generation Equipments, Department of Electrical Engineering, Tsinghua University, Beijing, 100084, China 2Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA; email: wang@matse.psu.edu 3State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China 4Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China; email: wangh6@sustc.edu.cn
 

The demand for high-temperature dielectric materials arises from numerous emerging applications such as electric vehicles, wind generators, solar converters, aerospace power conditioning, and downhole oil and gas explorations, in which the power systems and electronic devices have to operate at elevated temperatures. This article presents an overview of recent progress in the field of nanostructured dielectric materials targeted for high-temperature capacitive energy storage applications. Polymers, polymer nanocomposites, and bulk ceramics and thin films are the focus of the materials reviewed. Both commercial products and the latest research results are covered. While general design considerations are briefly discussed, emphasis is placed on material specifications oriented toward the intended high-temperature applications, such as dielectric properties, temperature stability, energy density, and charge-discharge efficiency. The advantages and shortcomings of the existing dielectric materials are identified. Challenges along with future research opportunities are highlighted at the end of this review.
高温介电材料的需求源于众多新兴应用,如电动汽车、风力发电机、太阳能转换器、航空航天功率调节和井下油气勘探,在这些应用中,电力系统和电子设备必须在高温下运行。本文概述了针对高温电容器储能应用领域纳米结构介电材料近期取得的进展。所审查的材料重点是聚合物、聚合物纳米复合材料、块状陶瓷和薄膜。涵盖了商业产品和最新的研究成果。虽然简要讨论了通用设计考虑因素,但重点放在了面向预期高温应用的材料规格上,例如介电性能、温度稳定性、能量密度和充放电效率。确定了现有介电材料的优缺点。在本文的结尾,突出了挑战以及未来的研究机会。

Keywords 关键词
 

Dielectric materials form the basis of one of the most fundamental passive components known as dielectric capacitors, which are present in almost every sort of electronic circuit (16). The function of dielectric capacitors is to store electric energy by holding opposite charges on electrodes separated by an insulator, the dielectric material. Dielectric capacitors are special among the various electrical energy storage devices, e.g., batteries, because they can release the stored energy in an extremely short period of time (on a microsecond scale) to create intense power pulses (710). This capacity enables many pulsed power applications such as medical defibrillators, transversely excited atmospheric lasers, and advanced electromagnetic systems, in which capacitors convert a low-power, long-time input into a high-power, short-time output. Lately, emerging products related to renewable energy, such as hybrid electric vehicles (HEVs), grid-tied photovoltaics, and wind turbine generators, have created a great demand for dielectric capacitors, as they are vital electronic elements for the conversion of collected/stored direct current (dc) to alternating current (ac) energy (11, 12).
介电材料是被称为介电电容器的一种最基本的无源元件的基础,这种电容器几乎存在于所有类型的电子电路中(1-6)。介电电容器的功能是通过在由绝缘材料隔开的电极上保持相反的电荷来储存电能。在各种电能存储设备中,如电池,介电电容器是特殊的,因为它们可以在极短的时间内(微秒级)释放储存的电能,以产生强烈的功率脉冲(7-10)。这种能力使得许多脉冲功率应用成为可能,例如医疗除颤器、横向激发大气激光器和先进的电磁系统,在这些系统中,电容器将低功率、长时间输入转换为高功率、短时间输出。 最近,与可再生能源相关的产品,如混合动力汽车(HEV)、并网光伏和风力涡轮发电机,对介电电容器产生了巨大需求,因为它们是将收集/储存的直流(dc)转换为交流(ac)能源的关键电子元件(11,12)。

The urgent need for the high-temperature capability of dielectric capacitors comes from the recent boom in, among many other areas (13), the avionic and automotive industries (14, 15), underground oil and gas explorations (16), and advanced propulsion systems (17), in which high power, high current, and elevated-temperature conditions are present ( ). In HEVs, the underhood temperature can be more than 140°C. Power inverters represent one of the most important electronic components of HEVs and are used to convert dc electricity supplied by batteries to the ac power needed to drive the traction motor. Capacitors constitute the major component of power inverters; e.g., in Toyota Prius® HEVs, capacitors account for ∼40% of the cost and ∼35 vol% and ∼23 wt% of the inverter. The mainstream capacitors currently employed in power inverters use biaxially oriented polypropylenes (BOPP) as the dielectric. The mismatch between ambient temperature (140°C) and the maximum operating temperature of BOPP [105°C (18)] requires either active cooling systems to be involved or all the electronic devices and circuitries to be redesigned and remanufactured. The present strategy of manufacturers is to introduce a secondary cooling system set at ∼65°C in addition to the engine radiator for stable operation of power inverters (19). This auxiliary cooling loop brings in extra weight, volume, and complexity in the design of power systems, and such factors are unfavorable for both the manufacturing cost and performance of HEVs (2023).
对介电电容器高温性能的迫切需求源于近期在众多领域(13)的繁荣,包括航空和汽车工业(14、15)、地下油气勘探(16)和先进推进系统(17),在这些领域中存在高功率、高电流和高温条件(图 1)。在混合动力汽车(HEV)中,发动机舱温度可能超过 140°C。功率逆变器是 HEV 中最重要的电子元件之一,用于将电池提供的直流电转换为驱动牵引电机所需的交流电。电容器是功率逆变器的主体组成部分;例如,在丰田普锐斯 ® 混合动力汽车中,电容器占成本的约 40%和逆变器的约 35 体积百分比和约 23 质量百分比。目前用于功率逆变器的主流电容器使用双向拉伸聚丙烯(BOPP)作为介电材料。环境温度(140°C)与 BOPP 最大工作温度[∼105°C(18)]之间的不匹配要求要么涉及主动冷却系统,要么重新设计和重新制造所有电子设备和电路。 当前制造商的策略是在发动机散热器的基础上引入一个设置在约 65°C 的二级冷却系统,以确保功率逆变器的稳定运行(19)。这个辅助冷却循环给电力系统的设计带来了额外的重量、体积和复杂性,这些因素对混合动力汽车(HEVs)的制造成本和性能都不利(20-23)。

Emerging applications demanding high-temperature dielectric capacitors, including hybrid electric vehicles, wind turbine generators, avionic industries, underground oil and gas explorations, and advanced propulsion systems.
 

In other applications, the local temperature of electronic devices is also higher than the maximum operating temperature of BOPP, while creation of the physical space could be costly or even impossible. One example is the development of a more electric aircraft (15). The need for high-temperature dielectric materials (up to 250°C) arises from the proximity of the power electronics to the heat sources involving turbine engines, generators, and motors, as well as control and sensing electronics placed near the outer shells of rockets and space shuttles (13, 16). Under some circumstances, cooling may not be practical. In underground oil and gas explorations, temperatures can exceed 200°C, and heating is not confined locally to electronics in the drill (16). Under such harsh environmental conditions, cooling is ineffective, and electronic systems have to operate at high temperatures.

To meet these demanding requirements, there has been considerable research interest in high-temperature capabilities of dielectric materials (13, 24, 25). Yet major challenges and limitations exist. This review provides an overview of the currently available high-temperature dielectric materials (>150°C rated), describing their advantages and potential, together with some of the fundamental and technical issues to be addressed. We conclude by highlighting some challenges and opportunities for future developments. In particular, focus is on dielectric polymers, polymer nanocomposites, and ceramics, as they are currently the most important and promising classes of dielectric materials in the areas of high-temperature power systems and electronic component technologies.

 

There have been many review articles on advancing capacitor dielectrics (2631), but these articles focus mainly on room temperature applications. The design of high-temperature dielectric materials, however, is different and should therefore be reconsidered to fulfill a stringent set of requirements in high-temperature capacitor applications.

2.1.   Thermal Stability

Thermal stability is a prerequisite for reliable insulating properties under high-temperature conditions because this quality determines the ability of materials to resist collapse of physical integrity under thermal stress. In the temperature span covering most of the intended applications (i.e., room temperature to 350°C), ceramics are the most thermally stable, and dielectric polymers are, by contrast, very susceptible to thermal stress. For most dielectric polymers, the glass transition temperature ( ) is less than 100°C (32). Amorphous polymers lose their stiffness and exhibit an abrupt change in physical properties as temperature increases to above . For instance, thermal excursions through dramatically increase the free volume in a polymer material, i.e., the space not occupied by polymer molecules, and hence lead to significantly fluctuating dielectric properties. In addition, a high free volume and high mobility of polymer chains would allow for considerable electronic and ionic conduction due to the ease of propagation of charge carriers across the material. These issues are closely related to the stability, reliability, and service lifetime of polymer dielectrics at elevated temperatures, and therefore is usually the basic criterion in the assessment of high-temperature polymer dielectric materials. For polymers with very high crystallinities, where the crystalline phase plays a dominant role, the melting point ( ) is also used to evaluate temperature capability.

As both and are controlled mainly by main-chain stiffness, they are usually coupled in homopolymers. To obtain higher values of and , a very common method is to incorporate rigid structural units into the main chains of polymers to impede rotation and improve stiffness of the main chains. Alternatively, the presence of bulky side groups is also very effective in securing high and values, as such side groups physically restrict bond rotation. Moreover, when the side groups are polar, they restrict the rotation further as a result of polar interactions. A third approach is to create intermolecular chemical cross-links that strongly restrict molecular motion. For some highly cross-linked polymers, the glass transition may not even occur (32).

2.2.   Dielectric Breakdown Models

Dielectric breakdown strength ( ) is the electric field at which a rapid reduction in the resistance of an electrical insulator is observed when that field is applied on the dielectric material. is one of the governing parameters for the energy storage density of capacitors; e.g., the energy density of a linear dielectric material scales with the square of electric field according to

1. 

where is the energy density, is the permittivity of vacuum, is the relative permittivity, and is the electric field. Among the many breakdown mechanisms of polymer dielectrics, free volume breakdown, electromechanical breakdown, and thermal breakdown have the greatest chance of occurring under high-temperature conditions (3335) and thus need particular consideration.

Free volume breakdown is a process in which electrons are accelerated in the free volume of polymers to energies sufficient to create free electron–hole pairs via collisions with bound electrons and to eventually cause the failure of dielectrics (33). Polymer materials have a large increase in free volume approximately at , again necessitating high of polymer dielectrics for high-temperature applications. Cross-linking is commonly accepted as an efficient route to reduce and restrict the free volume and is therefore favorable for improving the breakdown strength of polymer dielectrics (8, 36).

The electromechanical breakdown model concerns the mechanical collapse associated with the Maxwell stress exerted by the applied electric field (34). This type of breakdown is usually observed in soft polymer dielectrics such as polyethylene because the critical field is dependent on the mechanical strength of the material, given by

2. 

where is the Young's modulus, is the permittivity of vacuum, and is the relative permittivity. Since the elastic modulus of a polymer material usually decreases with rising temperature, and especially when the temperature increases beyond or , this specific breakdown mechanism becomes applicable to many polymers in the high-temperature region.

Thermal breakdown describes the catastrophic thermal runaway of dielectric materials as the rate of heat generation within the material due to dissipation in an applied electric field exceeds the rate of heat loss to the external medium (35). This exceedingly complicated and nonlinear process, which is poorly understood at the microscopic level, can be expressed macroscopically as

3. 

Here and are the density and heat capacity, respectively; is the electrical conductivity; is the thermal conductivity; and denotes the applied electrical field (37). At elevated temperatures, the conduction loss of polymer dielectrics becomes much more significant compared with that at room temperature owing to the many temperature-dependent conduction mechanisms (some of which are discussed in Section 2.3), which leads to the production of a vast amount of Joule heat.

As polymer materials have relatively low thermal conductivities, i.e., within the range of 0.2–0.5 W/(m·K), heat cannot be conducted away fast enough and so accumulates, causing the temperature to rise eventually. Under continuous operation, the device may fail due to thermal breakdown of dielectric materials. The underlying mechanism of thermal breakdown implies that a high value of or is not adequate to protect polymer dielectrics against this type of failure under high-temperature conditions, because heat conduction occurs at a much lower rate than does heat generation. Under continuous operation of capacitors without an active cooling system, the internal temperature of a device can exceed the rated value provided by the present level of thermal stability of polymer materials. More reasonable ways to tackle this issue are () suppression of conduction losses to mitigate the internal heating and () improvements in the efficiency of heat transfer to keep the material relatively cool.

The breakdown strength of inorganic dielectrics is closely associated with the intrinsic bandgap of the materials and can be significantly influenced by extrinsic factors, such as defect chemistry, sample thickness, grain size, and structural configuration. A thermochemical model describes the fundamental relationship between dielectric breakdown strength and dielectric constant, where breakdown strength is approximately inversely proportional to the square root of the dielectric constant (38). The high local electric fields in high-dielectric-constant materials may make the polar molecular bonds vulnerable by standard Boltzmann processes and/or by hole capture. Consequently, the vulnerability of the polar molecular bonds lowers breakdown strength.

2.3.   Conduction Mechanisms

Under high electric fields and elevated temperatures, significant leakage current occurs in many polymer dielectrics due to emerging conduction events (3942), including those occurring in the material bulk and at the electrode-dielectric interface. Leakage current can lead to continuous energy loss and undesirable temperature rise inside the dielectrics. This energy loss–induced temperature rise is highly detrimental in the context of high-temperature capacitor applications, given that thermal runaway is one of the main factors limiting the temperature ratings and voltage ratings of film capacitors.

Almost all conduction mechanisms in polymers—such as ohmic conduction, ionic conduction, hopping conduction, Poole-Frenkel emission, Schottky emission, and thermionic-field emission—increase with temperature, usually exponentially (39, 41). Poole-Frenkel emission and Schottky emission are among the most often seen conduction mechanisms in the high-temperature region. Poole-Frenkel emission belongs to the bulk-limited type of conduction mechanisms related to the trap energy level in polymer dielectrics. When charge carriers constrained in a trapping center obtain enough energy to overcome the potential barrier, they are excited to the conduction band, and electric current is observed. In particular, if the energy is from thermal excitation, the conduction falls within the category of Poole-Frenkel emission. Schottky emission, sometimes referred to as thermionic emission, denotes a phenomenon in which charge carriers in a metal electrode can overcome the energy barrier at the metal-dielectric interface to be injected into the dielectric when they obtain enough energy from thermal activation. Schottky emission is an electrode-limited conduction mechanism, as it is governed by the barrier height at the metal-dielectric interface.

The above conduction models predict that electric conduction in polymer dielectrics is much more significant at elevated temperatures than at room temperature, suggesting that the rising temperature dramatically increases the probability of thermal runaway. The environmental temperature can have an even greater impact under high electric fields due to field-dependent conduction mechanisms; i.e., the leakage current increases exponentially with increasing electric fields beyond the critical point at which conduction is no longer ohmic (39, 41, 42). Similar to the scenario in polymer dielectrics, the above conduction mechanisms are applicable to inorganic dielectric films as well. However, for inorganic bulk dielectrics, ohmic conduction is valid at low applied fields, and space charge–limited conduction dominates close to the breakdown field (43).

2.4.   Relative Permittivity and Dissipation Factor

The relative permittivity ( , also commonly known as dielectric constant) of a dielectric material is the ratio of () the capacitance of a capacitor adopting that dielectric to () the capacitance of a similar capacitor using vacuum as its dielectric. As Equation 1 shows, the energy storage density of a dielectric material is directly proportional to , which implies that the specific energy density of a dielectric capacitor can be improved by using a dielectric material with high values or that, in turn, the physical size of the capacitor can be reduced to meet a given energy specification. The miniaturized capacitors would help to reduce space and weight in HEVs and aerospace applications. It would reduce the overall cost of printed circuit boards, which is typically calculated on the basis of area. With inadequate energy densities, dielectric capacitors have to operate at a high repetition rate of charge-discharge cycles, which may cause deleterious effects such as accelerated heating and fast aging (10, 19).

Dissipation factor, or dielectric loss tangent, is a measure of the rate of energy loss during polarization and depolarization of dielectric materials. It occurs when the relaxation time and the frequency of the applied field are similar. This form of energy loss not only deteriorates energy storage capability but also builds up equivalent series resistance (ESR) coupled with the capacitor (19). In high-current circuitries, ESR is harmful, as it contributes to continuous heating. It is therefore desirable to maximize while maintaining a low dissipation factor of dielectrics in capacitor applications. Since the polarization and relaxation of electrical charges and dipoles are dependent on both temperature and time, values (and hence the capacitance) and dissipation factor are usually variable with respect to temperature and electric field frequency. The relevant standard of Electronic Industries Alliance (Arlington, VA) defines commercial X7R capacitors as having less than ±15% deviation of capacitance from the room temperature value over the temperature range of −55 to 125°C, while ceramic chip-on-glass capacitors possess temperature coefficients of capacitance of 0±30 ppm/°C. This standard provides the guideline for some high-temperature dielectrics.

 
3.1.   High- Polymers

The advantages of polymer dielectrics for capacitor applications have been well documented and include high breakdown strength, low mass density, inexpensive production, flexibility, and ease of processing (4, 7, 18). In this section, the promise of state-of-the-art high-temperature polymer dielectric films from both commercial sources (with basic information compiled in and chemical structures summarized in ) and noncommercial sources for capacitor applications are discussed, and relevant studies concerning structural modifications of these polymers are reviewed.

Overview of high-temperature polymer dielectric films from commercial sources

Toggle display: Table 1     Open Table 1  fullscreen
Kapton (PI)360–4102.7-3.50.13-0.26154–303 (7.6–127 μm)2.3×1017
UPILEX (PI)285–5003.2-3.50.13-0.7147–320 (12.5–125 μm)1016–1017
ULTEM (PEI)217–2473.150.12200 (25 μm)1.0×1017
ISARYL (FPE)3303.2-3.50.31-0.7220–320 (100–120 μm)1015–1017
Cyclotene (BCB)>3502.750.12300 (10 μm)1.0×1018
TORELINA (PPS)1183.0<0.1490 (9 μm)1.0×1016
KetaSpire (PEEK)1503.10.3150 (50 μm)2.6×1016
Kepstan (PEKK)1622.60.784 (100 μm)1.0×1016

Abbreviations: BCB, divinyltetramethyldisiloxane-bis(benzocyclobutene); FPE, fluorene polyester; PEEK, poly(ether ether ketone); PEI, poly(ether imide); PEKK, poly(ether ketone ketone); PI, polyimide; PPS, poly(phenylene sulfide).

Chemical structures of various high-temperature polymer dielectrics.
 

3.1.1.   Polyimide and poly(ether imide).

Polyimide (PI) is a thermoset polymer synthesized from dianhydride and diamine (or diisocyanate) monomers through a condensation reaction followed by chemical imidization (44, 45). PIs are characterized by exceptional resistance to heat and chemicals and decent mechanical strength; these characteristics stem from the imide structure in the main chain (46), which, in conjunction with the aromatic structure, results in very high values up to 500°C. PIs also have excellent insulating properties, including low dissipation factor, high breakdown strength, and high volume resistivity, in addition to an that is 50% higher than that of polypropylene (PP). The good thermal stability and outstanding dielectric performance make PIs a good candidate for high-temperature film capacitor applications.

Kapton® is a typical example of a PI and was developed by DuPont in the late 1960s. The low dissipation factor and high breakdown strength of Kapton are well retained up to 200°C (24, 36, 47). But under high electric fields, the high-temperature dielectric performance is much inferior to that at room temperature (37). For instance, under an electric field of 200 MV/m and at 150°C, the conduction loss of Kapton is as high as 24%; i.e., 24% of stored energy is dissipated in the form of Joule heat, resulting in a 76% charge-discharge efficiency () and a relatively low dischargeable energy density of ∼0.44 J/cm3 ( ). With further increases in temperature, the conduction loss is even larger. At 250°C, the conduction loss of Kapton is almost 100%, suggesting that the upper bound of the high-field operating temperature of Kapton is much below its value. It is argued that the presence of a diphenylether moiety in Kapton has an adverse effect on the thermal-oxidative resistance (48). In addition, the imide ring structure in the main chain endows PIs with unfavorable moisture sensitivity and high water uptake. For film capacitor applications, these issues can be addressed by impregnating the windings with an insulating fluid and by hermetically sealing the interior.

(,) Electric field–dependent discharged energy density and (,) charge-discharge efficiency of high-temperature polymer-based dielectrics measured at 150°C and 200°C. Abbreviations: -BCB/BNNS, cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene)/boron nitride nanosheet; FPE, fluorene polyester; PC, polycarbonate; PEEK, poly(ether ether ketone); PEI, poly(ether imide). Adapted with permission from Reference 37. Copyright © 2015, Nature Publishing Group.
 

Poly(ether imides) (PEIs), a modified version of PI, are amorphous dielectric polymers with improved processability at the expense of compromised thermal stability with respect to PIs. The improved processability and reduced thermal stability are due to the presence of flexible ether linkages incorporated into the backbone of PEIs through the nucleophilic aromatic substitution of leaving groups from phthalic anhydride by bisphenol A. The most important commercial PEI product is ULTEM® by SABIC, with ranging from 217°C to 247°C. The and the dissipation factor of PEIs are similar to those of PIs. Unexpectedly, the energy storage properties of PEIs are better than those of PIs up to 200°C. For example, the dischargeable energy density and of PEIs are 0.5 J/cm3 and 90%, respectively, under an electric field of 200 MV/m and at 150°C (36) ( ). Even at 200°C, a temperature very close to the value, the of PEIs still approaches 80%. Chemically modified PEIs have a greater ; e.g., cyano-PEIs are reported to offer an of ∼4.6 (49).

One of the major drawbacks of PIs with regard to high-temperature film capacitor applications may arise from the high temperature coefficient of [also known as the temperature coefficient of capacitance (TCC)]. TCC is the measure of variation relative to that at room temperature in a given temperature range. For PIs, the TCC can be as high as −15% from room temperature to 300°C, where the minus sign indicates that the of PI decreases with increasing temperature. Variations in dielectric constant are undesirable for most capacitor applications, especially for power conditioning, filtering, timing, and tuning circuitries. PEIs also show a considerable TCC of ∼6% from room temperature to 200°C.

3.1.2.   Fluorene polyester and cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene).

Fluorene polyesters (FPEs) and cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutenes) (-BCBs), with a similar grade of thermal stability as PIs, offer superior dielectric stability versus temperature. FPEs ( = 330°C) represent a class of amorphous polyarylates prepared through the reaction of fluorene bisphenol with phthalic chlorides. Pristine FPE shows a TCC exceeding 8% (37), but for chemically modified FPE, referred to as FDAPE (50), the TCC is less than 1% in the temperature range of 25–350°C, and the dissipation factor of FDAPE is constantly lower than 0.4% in the same temperature span. This makes FDAPE an excellent dielectric for high-temperature power conditioning applications, in which a high dielectric stability with temperature is required. Moreover, at 150°C, the maximum dischargeable energy density of FPE is ∼1.2 J/cm3 (achieved at 350 MV/m), which is twice as high as that of Kapton (37) ( ).

As thermoset dielectric polymers, -BCBs are thermally cross-linked or photo-cross-linked from the BCB monomer through a 4+2 Diels–Alder reaction, i.e., an intermolecular reaction between the alkene unit and the -quinodimethane intermediate from ring opening of the benzocyclobutene (51). These materials are available in both monomer and -staged (partially polymerized) forms from commercial sources such as cyclotene resins by Dow Chemicals. Fully cured -BCBs possess an of ∼2.7 and a dissipation factor of ∼0.15% at room temperature and 1 kHz, and such values remain almost constant up to 300°C and 2 MHz (37). The TCC of -BCBs is approximately −2.4% in the temperature range of 25–350°C (37). While -BCBs undergo no glass transition before thermally decomposing (>350°C) (37), they are sensitive to oxygen at elevated temperatures because of the oxidation of benzylic CH groups to anhydride and/or carbonyl species (52). This event in turn leads to weight loss over time at temperatures above 300°C. Modified formulations of -BCBs have been successfully developed to reduce the sensitivity to oxidation (53).

3.1.3.   Polycarbonate and poly(phenylene sulfide).

High-quality PIs, FPEs, and -BCBs have excellent thermal resistance but are expensive. For lower requirements of temperature capability, polymer dielectrics with compromised thermal stability but lower price are of higher interest for practical applications. Polycarbonate (PC) is a dielectric polymer synthesized from carbonic acids and dihydric alcohols. The temperature rating of PCs is limited to approximately 150°C by its . The advantage of PCs for high-temperature film capacitor applications is their dielectric stability over temperature, i.e., a low TCC value of approximately −3% (37). After the discontinuation of capacitor PC films by the main supplier, Bayer AG, at the end of 2000, most manufacturers ceased their production of PC film capacitors (54). Poly(phenylene sulfides) (PPSs), consisting of aromatic rings linked with sulfides, are considered an ideal replacement for PC films in high-temperature capacitor applications on account of their very similar dielectric properties, including a low dissipation factor and decent dielectric stability with temperature. Although PPSs have a low (∼120°C) relative to other high-temperature polymer dielectrics, they can be rated at 150°C or even 200°C for film capacitor applications (55). TORELINA® is a trade name given to a commercialized PPS film manufactured by Toray Industries. The TCC of TORELINA films is only approximately 1.5% within the projected operating temperature range, and the dissipation factor stays at an extremely low level (below 0.1% at 1 kHz) until the temperature increases to above 100°C (still below 0.5% up to 150°C). However, reports on high-temperature energy storage performance of PPSs are rare.

3.1.4.   Polyketone.

Polyketones are a series of polymers containing ketone groups in the backbone. Poly(ether ether ketone) (PEEK) is one of the most important members of the polyketone family. PEEKs can be readily polymerized from the dialkylation of bisphenolate salts. Commercialized PEEKs are available from multiple sources such as Solvay and Victrex. For example, Ketaspire® PEEK film by Solvay is characterized by a of 150°C. The dissipation factor of PEEKs increases rapidly above 150°C. At 150°C, PEEK delivers an energy density of ∼0.5 J/cm3 at a low efficiency of 55% at 200 MV/m (37) ( ). The physical properties of polyketone materials are tunable by varying the ether:ketone group ratio, which results in materials analogous to PEEKs. For instance, poly(ether ketone ketone) (PEKK) has a chemical structure similar to that of PEEKs; i.e., the mole ratio of ether group to ketone group in PEKKs is 1:2, while that in PEEKs is 2:1. PEKKs are synthesized via a reaction between diphenyl ether and a mixture of benzene dicarboxylic acid halides (56). PEKKs are usually a copolymer of terephthalic (T) moieties and isophthalic (I) moieties, and the T:I ratio determines the main physical properties of the resultant PEKKs. The and of PEKKs vary between 305°C and 360°C and between 160°C and 165°C, respectively, with changing T:I ratio. Kepstan® by Arkema is a commercialized PEKK copolymer with a of 162°C. This PEKK is able to discharge an energy density of ∼0.6 J/cm3 with an efficiency of 90% under an electric field of 200 MV/m and at 160°C (57). This performance is among the best in all the high-temperature polymer dielectrics under comparable conditions. The incorporation of the rigid asymmetric phenyl phthalazinone moiety into the polyketone backbone yields poly(phthalazinone ether ketone) (PPEK) with a of 250°C. PPEK displays excellent stability in dielectric properties over a broad frequency and temperature range. Little change in the breakdown field (∼440 MV/m) and discharge time has been observed in PPEK with an increase of temperature up to 190°C (58).

3.2.   Polymer Nanocomposites

The polymer composite approach to dielectric materials combines the unique thermal, mechanical, and electrical properties of inorganic inclusions with the facile processability and high breakdown strength of polymeric matrices for improved dielectric properties and capacitive energy storage performance (27, 29, 5961).

3.2.1.   Single-layer composites.

Incorporation of inorganic dopants creates interfacial interaction and physical confinement on the polymer dielectric and thus drives (62, 63) toward higher values. For example, PEIs filled with 5 wt% of sepiolite [i.e., hydrated magnesium silicate with the half-unit-cell formula of MgSiO(OH)·12HO] needles have an improved (i.e., 223°C) relative to the bare polymer, which has a of 215°C (64). Moreover, high-aspect-ratio dopants commonly reinforce the mechanical properties of polymer materials (6568). The reinforcement of mechanical properties could, in principle, result in much improved dielectric breakdown strength in polymer nanocomposites since the electromechanical breakdown model is dominant for low-modulus materials (69). However, invariably particles lead to a decreased Weibull modulus of the breakdown field. This effect is even more pronounced under elevated-temperature conditions because polymers usually soften with increasing temperature; e.g., -BCBs filled with 10 vol% of boron nitride nanosheets (BNNSs) show an ∼50% increase in Weibull breakdown strength at 250°C (37).

Aside from improving the mechanical properties of polymers, there is the possibility of using suitably oriented high-aspect-ratio fillers to enhance the insulating properties of polymer dielectrics. The enhancement in insulating properties is usually correlated with one of the following three phenomena but remains to be established. First, the introduction of a second phase augments the path length of carriers responsible for electrical conduction (70). Second, phase-field modeling indicates that high-aspect-ratio fillers are favorable for mitigating local field concentration (71, 72). Third, high-aspect-ratio nanofillers can provide resistance to electric treeing inception (73). Li et al. (37) reported that the leakage current density of BNNS-filled -BCB is more than an order of magnitude less than that of the pristine -BCB. As such, -BCB/BNNS outperforms all previous high-temperature polymer dielectrics in terms of high-temperature energy storage capability ( ). For example, under an electric field of 200 MV/m and at 150°C, the conduction loss of -BCB/BNNS is merely 3%.

Some high-aspect-ratio fillers, e.g., boron nitride, have both excellent insulating properties and high thermal conductivity. Among the many allotropes of boron nitride, hexagonal BNNSs and boron nitride nanotubes are the most attractive, given their high efficiency in heat transport in polymer matrix (74). Due to the significantly improved thermal conductivity and much decreased leakage current of polymer dielectrics, their thermal runaway under elevated-temperature conditions is suppressed (37). Furthermore, inorganic fillers with high are doped in polymer dielectric to increase the overall permittivity and hence the capacitance and energy storage densities (2729, 7577). For example, PIs loaded with 30 vol% of barium titanate fibers exhibit an as high as 27—almost an order of magnitude higher than that of the pristine polymer—in addition to a relatively low dissipation factor of 1.5% (77).

3.2.2.   Sandwich-structured composites.

The single-layered composite approach to high generally results in dramatically reduced breakdown strength owing to the local field distortion (29). To bypass this issue, sandwich-structured composites have been developed (7880). The main idea is to create spatially organized dielectric hard (low- ) and dielectric soft (high- ) layers to rearrange the distribution of the electric field. The effective and strong barrier interfaces that exist between adjacent layers in the sandwich structure protect the composite films from total breakdown and suppress the formation of conductive paths in the hard layer, while high polarization induced by the barium titanate (BT) nanoparticles in soft layers guarantees a high dielectric maximum polarization. Further improvements in the breakdown strength and energy density have been realized for a group of three-tiered ferroelectric poly(vinylidene fluoride) films with an increase in BT content in a layer-by-layer gradient (81).

For high-temperature applications, however, to simply mitigate the field concentration does not solve all the problems, as the main issue is the significant conduction loss associated with thermally activated charge carriers. In this regard, a sandwich-structured polymer nanocomposite with BNNSs spreading throughout the outer polymeric layers and high- nanoparticles in the interior layer has been rationally designed and experimentally demonstrated (10, 82) ( ). BNNSs serve not only as scattering centers to the migration of charge carriers but also as barriers against the thermionic charge injection that is a main source of conduction loss at elevated temperatures. With -BCB as the polymeric matrix, this configuration has led to the highest value of energy density at 150°C (i.e., 4 J/cm3) in polymer-based dielectrics reported thus far (10) ( ). More recently, chemical-vapor-deposited hexagonal boron nitride (-BN) with controlled thickness was successfully transferred from a copper foil to the surface of PEI films (83) ( ). The -BN-coated PEI films are capable of operating with >90% efficiencies and delivering high energy densities, i.e., 1.2 J/cm3, even at a temperature close to the of polymer (i.e., 217°C), at which pristine PEI almost fails. Outstanding cyclability and dielectric stability over 55,000 continuous charge-discharge cycles have been demonstrated in -BN-coated PEI at high temperatures.

() Cross-sectional SEM image and () energy storage performance of sandwich-structured nanocomposites measured at 150°C. Reprinted with permission from Reference 10. Copyright © 2016, National Academy of Sciences of the United States of America.
 

() Maximum discharged energy density at above 90% charge-discharge efficiency of polymers and polymer nanocomposites. () Cyclic performance of PEI film coated with boron nitride at 150°C. Abbreviations: BOPP, biaxially oriented polypropylene; -BCB/BNNS, cross-linked divinyltetramethyldisiloxane-bis(benzocyclobutene)/boron nitride nanosheet; FPE, fluorene polyester; -BN, hexagonal boron nitride; PEI, poly(ether imide); PI, polyimide. Reprinted with permission from Reference 83. Copyright © 2017, Wiley-VCH.
 

3.3.   High-Temperature Inorganic Dielectrics

In parallel with the advancements in high-temperature polymer dielectrics, significant developments have been made in inorganic dielectric materials, which can be categorized into four groups, namely inorganic film, ceramic, glass, and ceramic-glass composites. Although the ambient figures of merit of inorganic dielectrics have been reviewed (84, 85), recent progress in high-temperature inorganic dielectrics remains to be summarized and is addressed in the following sections, with an emphasis on inorganic films and ceramics.

3.3.1.   Main material systems.

There are four classical dielectrics for energy storage: linear dielectric, ferroelectric, relaxor ferroelectric, and antiferroelectric materials. Linear dielectric ceramics, which have relatively low dielectric constant, low dielectric loss, and high dielectric breakdown strength, are of great potential for high-temperature energy storage applications. The representative high-temperature linear dielectrics are perovskite CaTiO- and CaZrO-based solid solutions. Incipient ferroelectric CaTiO has a relatively high permittivity of approximately 171, while CaZrO is an interesting material with a large bandgap of 4.0–5.5 eV. Therefore, solid solutions of CaTiO and CaZrO are promising materials for high-temperature applications. Ca(ZrTi)O ceramics are reported to have a permittivity of 43 at room temperature and a low TCC of −0.03% over the temperature range from 25°C to 250°C. Ca(ZrTi)O capacitors show a high energy density of 2.49 J/cm3 and 4.19 J/cm3 at 25°C and 250°C, respectively (86). However, the low permittivity of CaZrO limits improvements in energy storage performance. Subsequently, high-permittivity CaTiO modified by CaHfO, which has a higher bandgap (∼6.4 eV) than does CaZrO, has been investigated. Ca(TiHf)O (CHT) capacitors yield an ambient energy density as high as 9.0 J/cm3 but show a drastic decrease in energy density above 100°C. Mn doping could effectively mitigate high temperature and high field losses of CHT capacitors, resulting in an energy storage density that is similar to that of ambient values (∼9.5 J/cm3) up to 200°C and that remains high (6.5 J/cm3) even at 300°C (87). The design rationale here lies in developing materials with large bandgap, linear or weakly nonlinear permittivity, and high breakdown strength. Thus, there is enormous experimental design space for high-temperature linear dielectric materials such as BaZrO-CaTiO and SrZrO-CaTiO (88).

In contrast to linear dielectrics, ferroelectrics exhibit nonlinear hysteresis, moderate electric breakdown strength, and high saturated polarization but low energy storage density and efficiency as a result of high remnant polarization and hysteresis. Fortunately, relaxor ferroelectrics inherit the advantages of typical ferroelectrics such as high saturated polarization but have dramatically reduced remnant polarization and hysteresis relative to other ferroelectrics, which make relaxor ferroelectrics good candidates in the search for high-temperature inorganic dielectrics for energy storage. Lead-based relaxor ferroelectric films have recently attracted increasing attention, particularly since a high energy storage density of 85 J/cm3 was reported for (Pb,La)(Zr,Ti)O (PLZT) relaxor ferroelectric films (89). Tong et al. (90) reported the temperature dependence of energy storage properties of polycrystalline PLZT relaxor ferroelectric films deposited on nickel buffered by a lanthanum nickel oxide buffer layer. The energy density and efficiency are almost temperature independent and remain constant, at 25 J/cm3 and 70%, respectively, in the temperature range of 20°C to 200°C (measured at 200 MV/m). Beyond the PLZT system, high-temperature piezoelectric materials are screened as potential alternatives for energy storage, as exemplified by Bi(NaHf)O-PbTiO (BNH-PT) solid solutions, which have the inherent advantages of high Curie temperature and strong relaxor behavior (91).

Driven by the ever-increasing concerns regarding environmental sustainability, recent advances in high-temperature dielectrics have also occurred for lead-free relaxor ferroelectrics, primarily bismuth-based compounds, such as (Bi,Na)TiO and Bi(Me3+)O (Me can be either a single trivalent cation or two cations with an average of +3 valence that occupy the octahedral sites) and their derivatives (92101). (BiNa)LaBa(TiZr)O epitaxial lead-free relaxor thin films with coexisting ferroelectric and antiferroelectric phases can withstand a unipolar electric field up to 350 MV/m, yielding a high energy density of 154 J/cm3 with an efficiency of 97%, and the energy density fluctuates slightly from room temperature to 250°C (94). Solid solutions of bismuth-based compounds with BaTiO are of great interest because the phase transition of complex perovskites can be compositionally manipulated to engineer the temperature dependence of dielectric properties. Recent work by Kwon & Lee (102) unveiled the weakly coupled relaxor behavior of BaTiO-Bi(Mg,Ti)O thin films. These films present a nearly linear polarization response, with a high permittivity exceeding 900 and a breakdown strength of 208 MV/m, contributing to a high energy density of 37 J/cm3. Of particular significance is the fact that the dielectric permittivity and energy storage properties can be maintained at high temperatures up to 200°C. BaTiO/SrTiO-substituted BiFeO, a multiferroelectric with high intrinsic polarization and high Curie temperature, also exhibits strong relaxor behavior and temperature-insensitive permittivity over a broad temperature range, showing potential for high-temperature dielectric capacitors (95, 103, 104). For example, a high energy density of 51 J/cm3 at a field of 350 MV/m was experimentally reported for Mn-doped 0.4BiFeO-0.6SrTiO relaxor ferroelectric thin films, accompanied by decent resistance against thermal stimulation and electrical cycling (95). Most research on (Bi,Na)TiO-based lead-free ceramics focuses on exploring the transition between ergodic and nonergodic relaxor phases by incorporating a second or even a third component to optimize energy storage performance. For instance, BiNaTiO ceramics comodified by BaTiO and KNaNbO show temperature-insensitive permittivity up to 300°C (normalized permittivity / varies no more than ±10% from 43°C to 319°C), although at the expense of a small reduction in permittivity (96). Ceramics with the composition 0.76BiNaTiO-0.19SrTiO-0.05NaNbO are endowed with a nearly temperature-invariant recoverable energy density of 0.6 J/cm3 from 25°C to 160°C within the ergodic region (97).

Antiferroelectrics characterized by double hysteresis are considered to be promising candidates for electrostatic energy storage; PbZrO-based antiferroelectrics in particular have attracted much attention. Similar to other O-type perovskites, PbZrO has been extensively modified with isovalent additives (such as Sr or Ba in sites and Ti or Sn in sites) and/or off-valence additives (such as La in sites) to strengthen its performance (105108). For instance, the synergistic effects of composition optimization and strain engineering endow textured PLZT antiferroelectric films with excellent thermal stability, where a high recoverable energy storage density of 20 J/cm3 and an efficiency above 60% are nearly independent of temperature up to 280°C (at 160 MV/m), as shown in (108). Researchers retain a keen interest in exploring new lead-free systems for high-temperature electrostatic capacitors. HfO-based thin films emerge as promising candidates. HfO with various dopants, such as Si, Al, and Zr, was reported to be a ferroelectric or an antiferroelectric material. In particular, Zr-doped HfO has a much higher bandgap (5.5 eV) than does Pb(Zr,Ti)O (3.0 eV), which ensures a higher breakdown strength. Experiments have confirmed that the breakdown field and energy storage density of HfZrO antiferroelectric thin films prepared using atomic layer deposition are as high as 435 MV/m and 45 J/cm3, respectively (109). Equally important features are robust thermal stability up to 175°C and fatigue resistance up to 109 times of electrical cycling (109). Lead-free antiferroelectric ceramics, primarily the niobates, have also attracted great attention for high-temperature energy storage applications. Despite the debates on the ferroelectric or antiferroelectric characteristics of NaNbO, lowering the tolerance factor could credibly stabilize the antiferroelectricity in NaNbO through chemical modification (110112). As an analog of NaNbO, lead-free AgNbO antiferroelectric ceramics show a peak recoverable energy density of 1.6 J/cm3 at 14 MV/m (113). Tantalum-modified AgNbO has enhanced antiferroelectricity and increased dielectric breakdown strength, due to the reduced polarizability of -site cations and increased bulk density, respectively, yielding significantly enhanced performance, with an energy density of 4.2 J/cm3 and high thermal stability of energy density (minimal variation of <±5%) over a broad temperature range (114).

() Representative hysteresis at different temperatures and () temperature dependence of energy storage density ( ) and energy storage efficiency () for (Pb,La)(Zr,Ti)O 2/95/5 thick films. Abbreviations: , electric field; , polarization; , temperature. Reprinted with permission from Reference 108. Copyright © 2016, AIP Publishing LLC.
 

3.3.2.   Synthetic approach.

The emerging advanced approaches to material synthesis benefit the development of high-end inorganic dielectrics. PLZT-based antiferroelectric ceramics prepared through the conventional solid-state sintering (CS) method possess () low energy storage density (<1.5 J/cm3) because of defect-induced low dielectric breakdown strength and () poor thermal stability originating from the destabilized antiferroelectricity at high temperatures approaching the Curie point (115117). To address these issues, spark plasma sintering (SPS), an advanced sintering technique, was employed to deliver high-performance antiferroelectric composite ceramics (PbBaLaY)(ZrSnTi)O-(PbLa)(ZrSnTi)O (PBLYZST-PLZST) (118, 119). In contrast to the CS process, SPS—with the advantages of low sintering temperature, short soaking durance, and a stress-assisted densification process—could strongly suppress diffusion between tetragonal antiferroelectric PBLYZST and orthorhombic antiferroelectric PLZST. In consequence, both a considerably high ferroelectric-to-antiferroelectric phase transition field of 16.2 MV/m and improved temperature stability of orthorhombic antiferroelectricity contributed by the tetragonal PBLYZST phase were obtained. PBLYZST-PLZST composite ceramics that underwent SPS showed a high recoverable energy density of 6.4 J/cm3 and excellent thermal stability of energy storage density of 1.16×10−2 J/(°C·cm3); this energy density and its thermal stability were much superior to those of samples that underwent CS (118, 119). Nonetheless, the SPS technique may be more suitable for the laboratory scale rather than for mass production, given the lower degrees of freedom and the higher cost for SPS processing relative to CS processing.

3.3.3.   Defect engineering and microstructure control.

The influence of defects, such as oxygen vacancies and grain boundaries, cannot be overlooked in inorganic dielectrics. Defects are double edged, as they can be harmful to the electrical properties of materials but can also be manipulated to boost performance. Thus, there are two options for achieving superior energy storage properties: lowering the concentration of defects or making the best of them. For example, Hu et al. (120) prepared high-quality epitaxial PLZT relaxor ferroelectric thin films by using pulsed laser deposition. In contrast to the polycrystalline counterparts, epitaxial PLZT films have a reduced number of defects and grain boundaries, showing a significantly enhanced breakdown strength of 227 MV/m and an energy storage density of 31 J/cm3. Moreover, the high energy storage density is negligibly temperature dependent over a wide temperature range from 20°C to 180°C. In contrast, an off-valence additive (Mn) was deliberately incorporated into PLZT relaxor ferroelectric thin films to form a defect complex between Mn and oxygen vacancies (121). The defect complex can be oriented along the direction of the external field, contributing to improved dielectric and ferroelectric properties. Moreover, the concentration of oxygen vacancies can be reduced, and such reduction is advantageous for higher breakdown strength. Accordingly, Mn-doped PLZT relaxor ferroelectric films have superior energy storage behavior. Consistent results also occur in Mn-incorporated 0.4BiFeO-0.6SrTiO relaxor ferroelectric thin films (95) and AgNbO lead-free antiferroelectric ceramics (113).

3.3.4.   Multilayer structure and interface effect.

Rational configuration design can be an effective approach to explore dielectrics with desirable energy storage performance, such as layered structures (122124) and superlattices (125), given the significance of interfaces in ferroelectric/dielectric multilayers or superlattices. Zhao et al. (123) found that compositionally graded multilayer PLZT antiferroelectric thick films exhibit significantly enhanced dielectric properties and energy storage performance, particularly for upgraded films, and such improvements were attributed to the strain and gradient of polarization near the interfaces of adjacent layers. The energy density varies slightly from 19.2 to 17.9 J/cm3, and an efficiency of approximately 76% is maintained over the temperature range of 20°C to 150°C. Sun et al. (124) devised (BaCaTiO/BaZrTiO) ( = 2, 4, 8) multilayer structures by the magnetron sputtering technique ( ). The interfaces effectively impede the development of electric trees ( ), leading to a remarkable increase in breakdown strength ( ). Multilayers with = 8 possess an extraordinary breakdown strength of 470 MV/m. Hence, a superior energy storage density of 52.4 J/cm3 with a high efficiency of 72.3% was recorded at 450 MV/m ( ). Most importantly, the = 4 multilayers present excellent thermal stability with an energy density of 34.8 J/cm3 and an efficiency of 75.1% over a wide temperature range from 25°C to 140°C ( ), representing the benchmark in lead-free thin films for high-temperature dielectric capacitors.

() Cross-sectional STEM images and () simulated development of electric trees in BaCaTiO/BaZrTiO (BCT/BZT) multilayers with = 2, 4, 8. () Weibull distribution for the dielectric breakdown strength . is the shape parameter indicating the dispersion of the experimental data. (,) Dependence of energy density and efficiency on () electric field and () temperature for the BCT/BZT multilayers. Reprinted with permission from Reference 124. Copyright © 2017, Wiley-VCH.
 

3.3.5.   Prototyped devices.

For dielectric ceramics, making complex devices by using these materials in the form of multilayers is essential. Single-layer capacitors were prototyped to confirm the feasibility of 0.3BiScO-0.7BaTiO ceramics, which have a high and temperature-stable dielectric constant (∼1,000 up to 300°C) coupled with a high electrical resistivity (∼1012 Ω · m at 250°C), for applications in high-energy density capacitors operating at elevated temperature (99). The ambient energy density of the thin dielectric layer capacitors is as high as 6.1 J/cm3 at a field of 73 MV/m. Another attractive aspect is that these capacitors exhibit decent thermal stability up to 300°C with a relatively high energy density value of approximately 3.0 J/cm3 at 37 MV/m (99). The comprehensive performance of 0.3BiScO-0.7BaTiO capacitors is comparable to or even surpasses that of commercial capacitors, including NP0, base metal electrode X7R, and precious metal electrode X7R (98). Correia et al. (126) demonstrated that low self-heating (reduced losses), high energy density (2.7 J/cm3 at 32 MV/m), and good thermal stability (with energy density varying by less than 18% over the temperature range 20–200°C) allow 0.2BiFeO-0.8SrTiO multilayer capacitors to be viable high-temperature pulsed power devices.

 

The availability of high-temperature dielectrics is key to the development of advanced electronic and electrical power systems operating under extreme environment conditions. There have been many exciting developments over the past several years. The currently available material candidates and approaches, however, still fall short of the desired specifications, and there is plenty of room for further improvement, as several issues remain to be addressed. Future development of high-temperature dielectrics should combine, for example, an improved fundamental understanding, rational design of materials, and scalable synthesis and processing approaches. In this context, challenges and opportunities may include, but are not limited to, the following aspects.

To attain high thermal resistance and hence good dielectric stability at elevated temperatures, most high-temperature polymer dielectrics contain aromatic or heteroaromatic molecular units in the backbone. But these electronically conjugated structures may have adverse effects on the insulating properties of polymer dielectrics in that they introduce impurity states into the energy bandgap (127). Another issue associated with the presence of aromatic or heteroaromatic molecular units is the possible impact on the self-clearing capability of metallized film capacitors. In some cases, e.g., in metallized PIs and PPSs, traces of carbon may remain in the area when the defect site is burned out (when clearing occurs), which results in poor electrical properties. As such, new molecular design of the skeleton structure of dielectric polymers should be performed to balance the dimensional and thermal stability and to perfect insulating properties. A systematic study of polymer structure versus high-temperature high-field dielectric performance has not yet been carried out.

Under both high temperatures and high electric fields, the conduction loss of polymer materials is significant. For instance, while the weak-field dissipation factor of Kapton is below 0.1% at 150°C, under an electric field of 200 MV/m, the conduction loss is as high as 24%. The dissipated energy manifests as Joule heat and may cause thermal runaway of the device. The energy dissipation is associated with various temperature- and field-dependent conduction events such as thermionic and thermionic-field charge injections, Poole-Frenkel emission, hopping, and tunneling. To suppress these conduction events, a high-energy barrier at the electrode-dielectric interface and deep energy traps in the material bulk may need to be developed. To this end, surface functionalization as well as molecular engineering in polymer dielectric films should be considered. Deep traps can be generated via incorporation of inorganic units into polymers to form organic-inorganic hybrid and composite structures (128).

The current high-temperature polymer dielectrics possess relatively low values, i.e., below 4, which severely limit the energy storage density and leave a large footprint of film capacitors in electronic devices and power systems. The various forms of loss associated with conventional approaches for permittivity lifting are unacceptably high for high-temperature film capacitor applications. To address this issue, topological structure engineering of composite systems and judicious molecular modification have been carried out, and both show promise for high-permittivity and low-loss materials, especially at elevated temperatures and high fields. But this specific area of study is still in its infancy, and large-scale production of these new materials remains a challenge. Much research still remains to understand the complex structure-property relationships, e.g., the polarity of functional groups versus dielectric permittivity/loss under conditions of elevated temperatures and high applied electric fields.

Thermal conductivity is also a limiting factor for high-temperature polymer dielectric materials because the operating temperature is determined by the ability to conduct out heat. However, polymers have relatively low thermal conductivities, with very few exceptions. A strategy capitalizing on engineered interchain interactions very recently resulted in amorphous polymer blends with high thermal conductivities up to 1.5 W/(m·K) (129). This strategy is expected to be extended to high-temperature polymer dielectric materials. Moreover, composite approaches are well known for being able to improve the thermal conductivity of polymer dielectrics, given the existence of various types of nanofillers with high thermal conductivities and decent insulating properties. The realization of this strategy depends on the nanomorphology of the fillers as well as on the spatial organization of the nanofillers in the polymer matrix.

For inorganic dielectrics, intrinsically, the synergistic effects of large bandgap and high polarization are advantageous; extrinsically, multiscale structures—ranging from atomic-scale defects, to nanoscale structures such as polar nanoregions in relaxor ferroelectrics, to micro/macrostructure design such as that involving multilayers—play a crucial role in tailoring the energy storage properties. Thus, enormous experimental design space still remains for high-temperature inorganic dielectric materials.

Wafer-scale inorganic films prepared by facile approaches with homogeneous composition and microstructure and decent comprehensive energy storage properties are desired for practical applications. As our ability to produce high-quality materials and integrate them into novel structures improves, production of free-standing inorganic dielectric films opens the way to multifunctional devices. For dielectric ceramics, mass production of multilayered ceramic capacitors is demanded for practical uses. Effort should be paid to rational device design strategies, such as structure design, to optimize the performance of capacitors. The compatibility of ceramics with internal electrodes, particularly low-cost metals (e.g., nickel), should also be considered.

In conclusion, we are witnessing groundbreaking developments in materials science. It is to be expected that these developments, coupled with recent major developments in high-performance polymers and electroactive ceramics and ongoing fundamental research on dielectric phenomena, should lead to the development of scalable, high-performance dielectric materials that will revolutionize energy storage devices built for harsh environments.

 

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

 

The work was supported by the US Office of Naval Research (QW), by National Natural Science Foundation of China (grant 51777101) and self-determined research funds of the State Key Laboratory of Control and Simulation of Power System and Generation Equipments (SKLD17M07) (QL), and by the National Key Research Program of China (2015CB654603 and 2017YFB0406303) (HW). F-ZY acknowledges the China Postdoctoral Science Foundation for support (2016M602812).

 
  1. 1.  Sarjeant WJ, Zirnheld J, MacDougall FW 1998. Capacitors. IEEE Trans. Plasma Sci. 26:1368–92
    [Google Scholar]
  2. 2.  Irvine JTS, Sinclair DC, West AR 1990. Electroceramics: characterization by impedance spectroscopy. Adv. Mater. 2:132–38
    [Google Scholar]
  3. 3.  Sarjeant WJ, Clelland IW, Price RA 2001. Capacitive components for power electronics. Proc. IEEE 89:846–55
    [Google Scholar]
  4. 4.  Tan Q, Irwin P, Cao Y 2006. Advanced dielectrics for capacitors. IEEJ Trans. Fundam. Mater. 126:1152–59
    [Google Scholar]
  5. 5.  Reaney IM, Iddles D 2006. Microwave dielectric ceramics for resonators and filters in mobile phone networks. J. Am. Ceram. Soc. 89:2063–72
    [Google Scholar]
  6. 6.  Bell AJ 2008. Ferroelectrics: the role of ceramic science and engineering. J. Eur. Ceram. Soc. 28:1307–17
    [Google Scholar]
  7. 7.  Chu BJ, Zhou X, Ren K, Neese B, Lin M et al. 2006. A dielectric polymer with high electric energy density and fast discharge speed. Science 313:334–36
    [Google Scholar]
  8. 8.  Khanchaitit P, Han K, Gadinski MR, Li Q, Wang Q 2013. Ferroelectric polymer networks with high energy density and improved discharged efficiency for dielectric energy storage. Nat. Commun. 4:2845
    [Google Scholar]
  9. 9.  Li Q, Han K, Gadinski MR, Zhang G, Wang Q 2014. High energy and power density capacitors from solution-processed ternary ferroelectric polymer nanocomposites. Adv. Mater. 26:6244–49
    [Google Scholar]
  10. 10.  Li Q, Liu F, Yang T, Gadinski MR, Zhang G et al. 2016. Sandwich-structured polymer nanocomposites with high energy density and great charge-discharge efficiency at elevated temperatures. PNAS 113:9995–10000
    [Google Scholar]
  11. 11.  Montanari D, Saarinen K, Scagliarini F, Zeidler D, Niskala M, Nender D 2009. Film capacitors for automotive and industrial applications. Proc. CARTS Jacksonville, FL: Apr 23–38
    [Google Scholar]
  12. 12.  Bower D 2000. Inverters-critical photovoltaic balance-of-system components: status, issues, and new-millennium opportunities. Prog. Photovolt. Res. Appl. 8:113–26
    [Google Scholar]
  13. 13.  Tan D, Zhang L, Chen Q, Irwin P 2014. High-temperature capacitor polymer films. J. Electron. Mater. 43:4569–75
    [Google Scholar]
  14. 14.  Johnson RW, Evans JL, Jacobsen P, Thompson JR, Christopher M 2004. The changing automotive environment: high-temperature electronics. IEEE Trans. Electron. Packag. Manuf. 27:164–76
    [Google Scholar]
  15. 15.  Weimer JA 1993. Electrical power technology for the more electric aircraft. Proc. AIAA/IEEE Digit. Avion. Syst. Conf., 12th445–50
    [Google Scholar]
  16. 16.  Watson J, Castro G 2012. High-temperature electronics pose design and reliability challenges. Analog Dialogue 46:1–7
    [Google Scholar]
  17. 17.  Barshaw EJ, White J, Chait MJ, Cornette JB, Bustamante J et al. 2007. High energy density (HED) biaxially-oriented poly-propylene (BOPP) capacitors for pulse power applications. IEEE Trans. Magn. 43:223–25
    [Google Scholar]
  18. 18.  Rabuffi M, Picci G 2002. Status quo and future prospects for metallized polypropylene energy storage capacitors. IEEE. Trans. Plasma Sci. 30:1939–42
    [Google Scholar]
  19. 19.  Zhang S, Zou C, Kushner DI, Zhou X, Orchard RJ Jr. et al. 2012. Semicrystalline polymers with high dielectric constant, melting temperature, and charge-discharge efficiency. IEEE Trans. Dielectr. Electr. Insul. 19:1158–66
    [Google Scholar]
  20. 20.  Burress TA, Coomer CL, Campbell SL, Wereszczak AA, Cunningham JP et al. 2008. Evaluation of the 2008 Lexus LS 600H hybrid synergy drive system Tech. Rep. ORNL/TM-2008/185, Oak Ridge Natl. Lab.
    [Google Scholar]
  21. 21.  Burress TA, Coomer CL, Campbell SL, Seiber LE, Marlino LD et al. 2007. Evaluation of the 2007 Toyota Camry hybrid synergy drive system Tech. Rep. ORNL/TM-2007/190, Oak Ridge Natl. Lab.
    [Google Scholar]
  22. 22.  Hsu J, Staunton R, Starke M 2006. Barriers to the application of high-temperature coolants in hybrid electric vehicles Tech. Rep. ORNL/TM-2006/514, Oak Ridge Natl. Lab.
    [Google Scholar]
  23. 23.  Bennion K, Thornton M 2010. Integrated vehicle thermal management for advanced vehicle propulsion technologies Presented at SAE World Cong., Detroit, MI, SAE Tech. Pap. 2010-01-0836
    [Google Scholar]
  24. 24.  Zhang X, Liu J, Yang S 2016. A review on recent progress of R&D for high-temperature resistant polymer dielectrics and their applications in electrical and electronic insulation. Rev. Adv. Mater. Sci. 46:22–38
    [Google Scholar]
  25. 25.  Randall CA, Ogihara H, Kim JR, Yang GY, Stringer CS et al. 2009. High temperature and high energy density dielectric materials Presented at IEEE Pulsed Power Conf Washington, DC:
    [Google Scholar]
  26. 26.  Zhu L, Wang Q 2012. Novel ferroelectric polymers for high energy density and low loss dielectrics. Macromolecules 45:2937–54
    [Google Scholar]
  27. 27.  Wang Q, Zhu L 2011. Polymer nanocomposites for electrical energy storage. J. Polym. Sci. B Polym. Phys. 49:1421–29
    [Google Scholar]
  28. 28.  Nan CW, Shen Y, Ma J 2010. Physical properties of composites near percolation. Annu. Rev. Mater. Res. 40:131–51
    [Google Scholar]
  29. 29.  Dang ZM, Yuan JK, Yao SH, Liao RJ 2013. Flexible nanodielectric materials with high permittivity for power energy storage. Adv. Mater. 25:6334–65
    [Google Scholar]
  30. 30.  Zhu L 2014. Exploring strategies for high dielectric constant and low loss polymer dielectrics. J. Phys. Chem. Lett. 5:3677–87
    [Google Scholar]
  31. 31.  Li Q, Wang Q 2016. Ferroelectric polymers and their energy-related applications. Macromol. Chem. Phys. 217:1228–44
    [Google Scholar]
  32. 32.  Young RJ, Lovell PA 1991. Introduction to Polymers London: Chapman & Hall
    [Google Scholar]
  33. 33.  Ieda M 1980. Dielectric breakdown process of polymers. IEEE Trans. Dielectr. Electr. Insul. EI-15:206–24
    [Google Scholar]
  34. 34.  Stark KH, Garton CG 1955. Electric strength of irradiated polythene. Nature 176:1225–26
    [Google Scholar]
  35. 35.  Zebouchi N, Bendaoud M, Essolbi R, Malec D, Ai B, Giam H 1996. Electrical breakdown theories applied to polyethylene terephthalate films under the combined effects of pressure and temperature. J. Appl. Phys. 79:2497–501
    [Google Scholar]
  36. 36.  Hanley TL, Burford RP, Fleming RJ, Barber KW 2003. A general review of polymeric insulation for use in HVDC cables. IEEE Electr. Insul. Mag. 19:13–24
    [Google Scholar]
  37. 37.  Li Q, Chen L, Gadinski MR, Zhang S, Zhang G et al. 2015. Flexible high-temperature dielectric materials from polymer nanocomposites. Nature 523:576–80
    [Google Scholar]
  38. 38.  McPherson J, Kim JY, Shanware A, Mogul H 2003. Thermochemical description of dielectric breakdown in high dielectric constant materials. Appl. Phys. Lett. 82:2121
    [Google Scholar]
  39. 39.  Chiu FC 2014. A review on conduction mechanisms in dielectric films. Adv. Mater. Sci. Eng. 2014:578168
    [Google Scholar]
  40. 40.  Ho J, Jow TR 2012. High field conduction in biaxially oriented polypropylene at elevated temperature. IEEE Trans. Dielectr. Electr. Insul. 19:990–95
    [Google Scholar]
  41. 41.  Ieda M 1984. Electrical conduction and carrier traps in polymeric materials. IEEE Trans. Electr. Insul. 19:162–78
    [Google Scholar]
  42. 42.  Ambegaokar V, Halperin BI, Langer JS 1971. Hopping conductivity in disordered systems. Phys. Rev. B 4:2612
    [Google Scholar]
  43. 43.  Neusel C, Jelitto H, Schneider GA 2015. Electrical conduction mechanism in bulk ceramic insulators at high voltages until dielectric breakdown. J. Appl. Phys. 117:154902
    [Google Scholar]
  44. 44.  Kirby AJ 1992. Polyimides: Materials Processing and Applications Oxford, UK: Pergamon Press
    [Google Scholar]
  45. 45.  Cassidy PE, Fawcett NC 1979. Polyimides. Encyclopedia of Chemical Technology 18 HF Mark, A Standen 704–19 New York: John Wiley & Sons
    [Google Scholar]
  46. 46.  Vanherck K, Koeckelberghs G, Vankelecom IFJ 2013. Crosslinking polyimides for membrane applications: a review. Prog. Polym. Sci. 38:874–96
    [Google Scholar]
  47. 47.  Diaham S, Zelmat S, Locatelli ML, Dinculescu S, Decup M, Lebey T 2010. Dielectric breakdown of polyimide films: area, thickness and temperature dependence. IEEE Trans. Electr. Insul. 17:18–27
    [Google Scholar]
  48. 48.  Tsukiji M, Bitoh W, Enomoto J 1990. Thermal degradation and endurance of polyimide films. Electrical Insulation (Conf. Rec. 1990 IEEE Int. Symp.)88–91
    [Google Scholar]
  49. 49.  Tan D, Zhang L, Chen Q, Irwin P 2014. High-temperature capacitor polymer films. J. Electron. Mater. 43:4569–75
    [Google Scholar]
  50. 50.  Venkat N, Dang TD, Bai Z, McNier VK, DeCerbo JN et al. 2010. High temperature polymer film dielectrics for aerospace power conditioning capacitor applications. Mater. Sci. Eng. B 168:16–21
    [Google Scholar]
  51. 51.  Sadana AK, Saini RK, Billups WE 2003. Cyclobutarenes and related compounds. Chem. Rev. 103:1539–602
    [Google Scholar]
  52. 52.  Schwödiauer R, Neugschwandtner GS, Bauer-Gogonea S, Bauer S, Wirges W 1999. Low-dielectric-constant cross-linking polymers: film electrets with excellent charge stability. Appl. Phys. Lett. 75:3998–4000
    [Google Scholar]
  53. 53.  Heistand R II, DeVellis R, Garrou P, Burdeaux D, Stokich T et al. 1992. Cyclotene 3022 (BCB) for non-hermetic packaging. Proc. ISHM 1992 San Francisco, Oct. 19–21 584–90
    [Google Scholar]
  54. 54. WIMA. Substitution of obsolete polycarbonate (PC) capacitors http://www.wimausa.com/EN/polycarbonate.htm
    [Google Scholar]
  55. 55.  Ho J, Jow TR 2009. Characterization of high temperature polymer thin films for power conditioning capacitors Tech. Rep. ARL-TR-4880, Army Res. Lab.
    [Google Scholar]
  56. 56.  Cheng SZD, Ho RM, Hsiao BS, Gardner KH 1996. Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s. Macromol. Chem. Phys. 197:185–213
    [Google Scholar]
  57. 57.  Pan J, Li K, Li J, Hsu T, Wang Q 2009. Dielectric characteristics of poly(ether ketone ketone) for high temperature capacitive energy storage. Appl. Phys. Lett. 95:022902
    [Google Scholar]
  58. 58.  Pan J, Li K, Chuayprakong S, Hsu T, Wang Q 2010. High-temperature poly(phthalazinone ether ketone) thin films for dielectric energy storage. ACS Appl. Mater. Interface 2:1286–89
    [Google Scholar]
  59. 59.  Zhang X, Shen Y, Zhang Q, Gu L, Hu Y et al. 2015. Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering. Adv. Mater. 27:819–24
    [Google Scholar]
  60. 60.  Huang X, Jiang P 2015. Core-shell structured high-k polymer nanocomposites for energy storage and dielectric applications. Adv. Mater. 27:546–54
    [Google Scholar]
  61. 61.  Li Q, Zhang G, Zhang X, Jiang S, Zeng Y, Wang Q 2015. Relaxor ferroelectric-based electrocaloric polymer nanocomposites with a broad operating temperature range and high cooling energy. Adv. Mater. 27:2236–41
    [Google Scholar]
  62. 62.  Yim A, Chahal RS, St. Pierre LE 1973. The effect of polymer-filler interaction energy on the Tg of filled polymers. J. Colloid Interface Sci. 43:583–90
    [Google Scholar]
  63. 63.  Fragiadakis D, Pissis P, Bokobza L 2005. Glass transition and molecular dynamics in poly(dimethylsiloxane)/silica nanocomposites. Polymer 46:6001–8
    [Google Scholar]
  64. 64.  Tabatabaei-Yazdi Z, Mehdipour-Ataei S 2015. Poly(ether-imide) and related sepiolite nanocomposites: investigation of physical, thermal, and mechanical properties. Polym. Adv. Technol. 26:308–14
    [Google Scholar]
  65. 65.  Coleman JN, Khan U, Gun'ko YK 2006. Mechanical reinforcement of polymers using carbon nanotubes. Adv. Mater. 18:689–706
    [Google Scholar]
  66. 66.  Zhou SJ, Ma CY, Meng YY, Su HF, Zhu Z et al. 2012. Activation of boron nitride nanotubes and their polymer composites for improving mechanical performance. Nanotechnology 23:055708
    [Google Scholar]
  67. 67.  Zhi C, Bando Y, Tang C, Kuwahara H, Golberg D 2009. Large-scale fabrication of boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties. Adv. Mater. 21:2889–93
    [Google Scholar]
  68. 68.  Esfandiari A, Nazokdast H, Rashidi A-S, Yazdanshenas M-E 2008. Review of polymer-organoclay nanocomposites. J. Appl. Sci. 8:545–61
    [Google Scholar]
  69. 69.  Yu J, Mo H, Jiang P 2015. Polymer/boron nitride nanosheet composite with high thermal conductivity and sufficient dielectric strength. Polym. Adv. Technol. 26:514–20
    [Google Scholar]
  70. 70.  Fujita F, Ruike M, Baba M 1996. Treeing breakdown voltage and TSC of alumina filled epoxy resin. IEEE Intern. Symp. Electr. Insul. 2:738–41
    [Google Scholar]
  71. 71.  Wang YU, Tan DQ 2011. Computational study of filler microstructure and effective property relations in dielectric composites. J. Appl. Phys. 109:104102
    [Google Scholar]
  72. 72.  Zhang G, Zhang X, Yang T, Li Q, Chen LQ et al. 2015. Colossal room-temperature electrocaloric effect in ferroelectric polymer nanocomposites using nanostructured barium strontium titanates. ACS Nano 9:7164–74
    [Google Scholar]
  73. 73.  Tomer V, Polizos G, Randall CA, Manias E 2011. Polyethylene nanocomposite dielectrics: implications of nanofiller orientation on high field properties and energy storage. J. Appl. Phys. 109:074113
    [Google Scholar]
  74. 74.  Golberg D, Bando Y, Huang Y, Terao T, Mitome M et al. 2010. Boron nitride nanotubes and nanosheets. ACS Nano 4:2979–93
    [Google Scholar]
  75. 75.  Levy O, Stroud D 1997. Maxwell Garnett theory for mixtures of anisotropic inclusions: application to conducting polymers. Phys. Rev. B 56:8035–46
    [Google Scholar]
  76. 76.  Rao Y, Qu J, Marinis T, Wong CP 2000. A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory. IEEE Trans. Compon. Packag. Technol. 23:680–83
    [Google Scholar]
  77. 77.  Wu YH, Zha JW, Yao ZQ, Sun F, Li RKY, Dang ZM 2015. Thermally stable polyimide nanocomposite films from electrospun BaTiO3 fibers for high-density energy storage capacitors. RSC Adv 5:44749–55
    [Google Scholar]
  78. 78.  Hu