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High Temperature Polymer Dielectrics
Overview on how to achieve polymer dielectrics at high temperatures, with emphasis on diverse applications in various electrical insulation fields
High Temperature Polymer Dielectrics: Fundamentals and Applications in Power Equipment systematically describes the latest research progress surrounding high-temperature polymer dielectric (HTPD) materials and their applications in electrical insulation fields such as high-temperature energy storage capacitors, motors, packaging, printed circuit board, new energy power equipment, and aerospace electrical equipment. The comprehensive text provides a description of the market demand and theoretical research value of HTPDs in electrical equipment and enables readers to improve the performance and design of existing HTPD materials, and to develop efficient new high temperature polymer dielectric materials in general.
Specific sample topics covered in High Temperature Polymer Dielectrics include:
- Thermal and electrical properties of high-temperature polymers, and the excellent thermal stability, mechanical properties, and long service life of polymer dielectrics
- Why fluorinated polymers are more thermally stable than their corresponding hydrogen-substituted polymers
- Static Thermomechanical Analysis (TMA), a technique for measuring the functional relationship between the deformation of the materials and the temperature and time under different actions
- Polyetheretherketone (PEEK), a semi-crystalline polymer material with ether bonds and ketone carbonyl groups in molecular chains
Providing a complete overview of the state-of-the-art high temperature polymer dielectrics, with a focus on fundamental background and recent advances, High Temperature Polymer Dielectrics is an essential resource for materials scientists, electrical engineers, polymer chemists, physicists, and professionals working in the chemical industry as a whole.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
1 Overview of High‐Temperature Polymers
1.1 Introduction
1.2 Development of High‐Temperature Polymers
1.3 Parameters of Polymers with High Temperature Resistance
1.4 Thermal Analysis Technology
1.5 High‐Temperature Polymer Materials
1.6 Summary and Outlook
References
2 Basic Principles of Dielectrics
2.1 Introduction
2.2 Definition of Dielectrics
2.3 Dipole Moment and Types of Dielectric Materials
2.4 Polarization and Dielectric Permittivity
2.5 Polarization Under Static Electric Field
2.6 Polarization Under Time Varying Electric Field
2.7 Conduction Phenomena in Dielectrics
2.8 Active Dielectrics
2.9 Polymers as Dielectric Materials
2.10 Thermal Properties of Dielectrics
2.11 Concluding Remarks
Acknowledgements
References
3 High‐Temperature Energy Storage Polymer Dielectrics for Capacitors
3.1 Introduction
3.2 Basic Parameters of High‐Temperature Capacitor Materials
3.3 Randomly Dispersed Polymer/Inorganic Nanofiller Composites
3.4 Core@Shell‐Structured Nanofillers for Polymer Composites
3.5 Layered Polymer Composites
3.6 Novel Polymers and All‐Organic Polymer Composites
3.7 Conclusion and Perspective
References
4 Review on High‐Temperature Polymers for Cable Insulation: State‐of‐the‐Art and Future Developments
4.1 Brief History of Cables Development and Insulating Materials
4.2 Technologies of Modern Power Cables
4.3 Review of the Most Relevant Electrical Characteristics of High Temperature Insulating Materials
4.4 Trends and Outlooks
4.4 Author's Note
References
5 High‐Temperature Polymer‐Based Dielectrics for Advanced Electronic Packaging
5.1 Introduction
5.2 High‐Temperature Polymer and Polymer‐Based Dielectrics
5.3 Summary and Perspectives
References
6 High‐Temperature Polymer Dielectrics for Printed Circuit Board
6.1 Epoxy Resin Used for PCB
6.2 Phenolic Resins Used for PCB
6.3 Polyimide Used for PCB
6.4 Polymer Materials Used for PCB at High Frequency
References
7 High‐Temperature Polymer Dielectrics for New Energy Power Equipment
7.1 Introduction
7.2 High‐frequency Power Transformer and Dry‐type Bushing
7.3 Modification of Polyimide
7.4 High‐temperature Resistant Dielectric Material for Capacitor
7.5 High‐temperature Resistant Dielectric Material for IGBT
7.6 Concluding Remarks
References
8 High‐Temperature Polymer Dielectrics for Aerospace Electrical Equipment
8.1 Introduction
8.2 Challenges of Insulating Materials Under High Temperatures
8.3 High Temperature Resistant and Strong DC Insulating Polymer Dielectrics
8.4 High‐temperature‐Resistant Polymer Dielectrics with Strong Nonlinear Conductivity
8.5 High‐Temperature‐Resistant Polymer Dielectrics Under the Coupling of Electron Irradiation and High Voltage
8.6 High Temperature Resistant and High‐Frequency Strong Insulating Polymer Dielectrics
References
9 Smart Polymer Dielectrics
9.1 Introduction
9.2 Self‐Adaptive Dielectrics
9.3 Self‐Reporting Dielectrics
9.4 Self‐Healing Dielectrics
9.5 Outlook
References
10 The Future Development of High‐temperature Polymer Dielectrics
10.1 Introduction
10.2 Present Development and Challenges
10.3 Future Perspectives and Trends
10.4 Summary
Acknowledgments
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Thermal and electrical properties of high‐temperature polymers [4,...
Table 1.2 Cleavage energy of representative chemical bonds.
Table 1.3 Thermal conductivity of high‐temperature polymers and their compos...
Table 1.4 Imidization degree and thermal property of the PI films [31].
Chapter 3
Table 3.1 Summary of key dielectric and energy storage parameters of commerc...
Chapter 4
Table 4.1 Examples of power wrapped cables used in aeronautic and aerospace ...
Table 4.2 Most relevant standards for measuring different properties of insu...
Table 4.3 Activation energy of different high‐temperature materials under 10...
Chapter 5
Table 5.1 Thermal conductivity of common polymers.
Chapter 6
Table 6.1 Compositions of FR‐4 and FR‐5 glue solutions (unit: mol).
Table 6.2 Main application areas of high‐frequency CCL.
Table 6.3 Basic properties of LCP materials.
Table 6.4 Liquid crystal units commonly used in liquid crystal polymers.
Chapter 7
Table 7.1 Thermal properties of PI‐1 to PI‐9.
Table 7.2 Common high‐temperature resistant polymers and their heat resistan...
Chapter 8
Table 8.1 Service Conditions of Satellite and All‐Electric Aircraft [12, 13]...
Chapter 9
Table 9.1 The threshold electric field
E
b
(V mm
−1
) and nonlinearity co...
List of Illustrations
Chapter 1
Figure 1.1 (a) Structure formula and (b) DSC of PEEK polymers [29]. Source: ...
Figure 1.2
E
′ and tan
δ
as a function of temperature for PI, PEIs, and ...
Figure 1.3 (a) TGA curves of the PI films with different IDs. (b) Three step...
Figure 1.4 (a) Structure formula, (b) DSC, and (c) TMA of CPI and PAI films ...
Figure 1.5 (a) Schematic diagram of the structure of PBP composite film. (b)...
Figure 1.6 Temperature‐dependent (a)
ε
r
and (b) tan
δ
of PI and PE...
Figure 1.7 The structural formula for several common high‐temperature polyme...
Figure 1.8 (a) Schematic of the synthesis process for POFNB. (b) The bandgap...
Figure 1.9 (a) Schematic of the synthesis process for c‐BCB/BNNS. (b) Weibul...
Chapter 2
Figure 2.1 Schematic configuration of an electric dipole. (a) In the absence...
Figure 2.2 Schematic representation of the types of polarization. (a) Electr...
Figure 2.3 (a) Dielectric with spherical cavity according to the Clausius an...
Figure 2.4 Spherical cavity and field's lines in a dielectric medium accordi...
Figure 2.5 Typical dispersion of ac conductivity with frequency, at various ...
Chapter 3
Figure 3.1 Schemes of (a) dielectric capacitor structure and (b) internal en...
Figure 3.2 Schematic of (a) electric displacement–electric field (
D
–
E
) loop ...
Figure 3.3 Schematic of (a) electronic breakdown, (b) electromechanical brea...
Figure 3.4 Schematic of charge injection processes under
E
a
including (a) Sc...
Figure 3.5 Schematic of successful and unsuccessful self‐clearing processes....
Figure 3.6 (a) Schematic of in situ preparation of the PI‐based polymer nano...
Figure 3.7 (a) Schematic of the preparation of APTES‐MgO and PP‐mah‐MgO nano...
Figure 3.8 (a) Cross‐sectional SEM images of SSN‐25. (b)
U
e
and
η
of SS...
Figure 3.9 (a) Schematic of the synthesis process for the ODPA‐MPD films wit...
Figure 3.10 (a) Schematic of chemical structures of polyimide and polyetheri...
Figure 3.11 (a) Schematic diagram of the KPFM testing. The circuit is used f...
Chapter 4
Figure 4.1 Historical milestones in the development of high‐temperature insu...
Figure 4.2 Examples of wrapping machines: (a) paper taping machine for under...
Figure 4.3 Air voids between insulating layers in different wrapped KT cable...
Figure 4.4 Scheme of cable extrusion line [49]. Source: Reproduced with perm...
Figure 4.5 Different images of a triple extrusion line production of an aero...
Figure 4.6 Cross‐sections of different extruded cables: (a) One‐layer PFA in...
Figure 4.7 Illustration of the micro‐multilayer multifunctional electrical i...
Figure 4.8 Pyramid of most‐used insulating polymers in power cables accordin...
Figure 4.9 Stressors and conditions leading to the degradation of cable insu...
Figure 4.10 Failure modes of cable systems in aircraft according to Navy Saf...
Figure 4.11 Scheme of an experimental setup used for temperature measurement...
Figure 4.12 (a) Illustration of the large‐scale fire test by IEC 60332‐1‐3, ...
Figure 4.13 CERN Classification of several insulating polymers according to ...
Figure 4.14 Dielectric constant as a function of temperature and electric fi...
Figure 4.15 AC conductivity as function of temperature and electric field: (...
Figure 4.16 Some potential locations of PD in cables according to the pressu...
Figure 4.17 (a) Example of a microscopy cross‐section of a wrapped (KT) aero...
Figure 4.18 (a) Example of a cable model with triple extrusion, (b) electric...
Figure 4.19 Charge density and electric field distributions for different hi...
Figure 4.20 DC conductivity of different high‐temperature materials versus t...
Figure 4.21 Changes in FTIR spectra of different materials aged under differ...
Chapter 5
Figure 5.1 Diagram of integrated 5G/satellite 6G communication. Source: Ikra...
Figure 5.2 Historical progression of integration technologies from 2D to 3D....
Figure 5.3 Examples of 3D packaging (without TSV). Source: Lau [17]/IEEE. (a...
Figure 5.4 Examples on 3‐D IC integration. (a) HBM with μbumps and TSVs. (b)...
Figure 5.5 (a) Cross section of the heterogeneous integration package. (b) T...
Figure 5.6 (a) Advanced Electronic Packaging ranking according to their dens...
Figure 5.7 (a) More Moore and More Than Moore. (b) GPU Module Package: GPU c...
Figure 5.8 Change in coefficient of thermal expansion with temperature.
Figure 5.9 Schematic of resin shrinkage after curing. The
x
‐axis represents ...
Figure 5.10 Comparison of the dielectric constants of different materials. S...
Figure 5.11 Structure of fluorine‐containing monomers [40].
Figure 5.12 Dielectric properties of fluorinated PI. Source: Li et al. [43]/...
Figure 5.13 Structure of diamines containing nonplanar polybiphenyl. Source:...
Figure 5.14 Structure of some alicyclic monomers and dielectric constant of ...
Figure 5.15 Schematic illustration of the different microstructures of liqui...
Figure 5.16 Examples of various types of benzocyclobutene monomers. Source: ...
Figure 5.17 Preparation of main‐chain benzoxazine copolymer oligomers [64]....
Figure 5.18 “Porous” PI film fabricated by increasing the steric hindrance o...
Figure 5.19 Thermally conductive epoxy/boron nitride composites with high gl...
Figure 5.20 (a) Representative 3D structure and (b) chemical structure of PO...
Figure 5.21 (a) Schematic diagram of the preparation of epoxy composites; (b...
Figure 5.22 Schematic diagram for preparation of the EP/ZIF‐8 composites. So...
Chapter 6
Figure 6.1 Chemical structure formulas of different kinds of epoxy resins.
Figure 6.2 The chemical structure formula of bisphenol‐A epoxy resins.
Figure 6.3 Reaction Schematics for Synthesis of Epoxy Resins.
Figure 6.4 Flow chart for the preparation process of FR‐4 CCL.
Figure 6.5 Chemical structure of partial epoxy resins.
Figure 6.6 Molecular structure of thermoplastic phenolic resins.
Figure 6.7 Molecular structure of thermosetting phenolic resins.
Figure 6.8 Reaction process of thermoplastic phenolic resins.
Figure 6.9 Ideal molecular structure of the thermoplastic resin.
Figure 6.10 Curing process of phenolic resins.
Figure 6.11 Initial reaction of phenol with formaldehyde.
Figure 6.12 Reaction process of thermosetting phenolic resins.
Figure 6.13 Reaction process of nitrogenous phenolic resins.
Figure 6.14 Synthetic route of PBZ.
Figure 6.15 Cyclic imide structure and phthalimide structure.
Figure 6.16 Two‐step synthesis of PI.
Figure 6.17 Synthesis flow chart of non‐fusible PI.
Figure 6.18 Synthesis of Bismaleimide.
Figure 6.19 Picture of FCCL.
Figure 6.20 Structures and molding techniques of FCCL.
Figure 6.21 Molecular structure of PTFE.
Figure 6.22 Common structure diagram of PTFE CCL.
Figure 6.23 Different states of matter.
Figure 6.24 (a) Curing mechanism of CE; (b) Mechanism of the reaction betwee...
Figure 6.25 (a) DSC curves of CE and cPES/CE prepolymers. Source: Zhao et al...
Figure 6.26 The chemical structure of PPO.
Figure 6.27 Mechanism of oxidative coupling of DMP.
Figure 6.28 Allylation route of PPO.
Chapter 7
Figure 7.1 (a) Chemical structures and molecular simulation results of TGDDM...
Figure 7.2 TG/DTG curves of PI/silica hybrid films: (a), (b) API system unde...
Figure 7.3 (a) Schematic diagram of the synthesis of PI and crosslinking rea...
Figure 7.4 (a) Schematic of the preparation of c‐BCB/BNNS films; (b, c) Tran...
Figure 7.5 (a) The schematic illustration for the fabrication of c‐PEI. The ...
Chapter 8
Figure 8.1 Typical power and electronic equipment and commonly used high‐tem...
Figure 8.2 (a) Potential barrier diagram of hopping conductance model, (b) c...
Figure 8.3 Schematic diagram of hopping polarization (a) and electrode polar...
Figure 8.4 Schematic diagram of bipolar charge transport model and space cha...
Figure 8.5 Vacuum surface flashover of insulating materials under high‐energ...
Figure 8.6 (a) The relative permittivity and dielectric loss factor of PI PN...
Figure 8.7 (a) The relationship between volume resistivity and filler concen...
Figure 8.8 Charge density and electric field distributions at the interfaces...
Figure 8.9 Microstructure formation process and breakdown strength measureme...
Figure 8.10 Internal electric field distributions under external voltages ap...
Figure 8.11 (a) Schematic diagram of deep dielectric charging in SADA struct...
Figure 8.12 Electrical conductivity versus externally applied electric field...
Figure 8.13 Electrical conductivity as a function of the electric field of e...
Figure 8.14 The PI vacuum DC surface discharge voltage changed with electron...
Figure 8.15 Variation of surface discharge voltage with electron beam kineti...
Figure 8.16 (a) Dimensions and shapes of electrodes. Surface discharge volta...
Figure 8.17 Comparisons of the surface discharge voltage of the single‐layer...
Figure 8.18 Variation of growth time (a), fractal dimension (b), and initiat...
Figure 8.19 Space charge behavior of pure EP and MN samples under an electri...
Figure 8.20 Electric field distribution characteristics in different EP samp...
Figure 8.21 Corona resistance lifetime as a function of temperature (a), mea...
Figure 8.22 Variation of dielectric loss tangent (a) and relative permittivi...
Chapter 9
Figure 9.1 Life cycle control of biomimetic smart materials for next‐generat...
Figure 9.2 Application mechanism of self‐adaptive dielectrics. (a) Typical n...
Figure 9.3 Examples of power apparatus and devices based on SADs. (a) Schema...
Figure 9.4 Calculated nonlinear conductivity curves of single grain boundari...
Figure 9.5 Schematic diagram of conduction paths within the compound as a fu...
Figure 9.6 Nonlinear
J
(
E
) characteristics of several ZnO/SR samples before a...
Figure 9.7 Photochromism and acidochromism of spiropyran. (a) Reversible tra...
Figure 9.8 Multicolor mechanochromism of a polymer/silica composite with dua...
Figure 9.9 Thermochromic artificial nacre based on montmorillonite. (a) Illu...
Figure 9.10 Robust damage‐reporting strategy enabled by dual‐compartment mic...
Figure 9.11 A schematic of healing mechanical damage by incorporating magnet...
Figure 9.12 Defect‐targeted self‐healing dielectric polymers with superparam...
Figure 9.13 Thermoplastic‐thermoset composite self‐healing dielectrics by in...
Figure 9.14 Microcapsule‐based dielectric polymers with the self‐healing cap...
Figure 9.15 Microcapsule‐based dielectric polymers with self‐healing capabil...
Figure 9.16 Microcapsule‐based self‐healing dielectric polymers triggered by...
Figure 9.17 Inspiration and mechanisms of self‐healing by reversible crossli...
Figure 9.18 Self‐healing high‐temperature dielectric polymer by disulfide bo...
Guide
Cover Page
Title Page
Copyright
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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High Temperature Polymer Dielectrics
Fundamentals and Applications in Power Equipment
Edited by Jun-Wei Zha and Zhi-Min Dang
The Editors
Prof. Jun‐Wei Zha
University of Science and Technology Beijing
No. 30 Xueyuan Road
Haidian
Beijing
China
100083
Prof. Zhi‐Min Dang
Tsinghua University
Zhongguancun North Street
Beijing
China
100084
Cover Image: © leamsign studio/Getty Images
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Preface
In recent decades, the research on high‐temperature polymer dielectrics (HTPDs) has made great progress, which has laid the foundation for their application in the field of electrical insulation. HTPDs have excellent high temperature stability and environmental stability. With the continuous development of electrical insulation technology in the direction of high temperature, the demand for HTPDs is becoming increasingly urgent. The HTPDs are of great significance for achieving miniaturization, lightweight, and high capacity of electrical equipment. Currently, HTPD and its composites have played an important role in the field of national defense and civil high technology. All countries have carried out a lot of basic and applied research on HTPDs and classified them as strategic materials.
HTPD can generally be divided into intrinsic HTPD and composite HTPD. Common intrinsic HTPDs, such as polyimides, polysulfones, and polyphenylimidazoles, have typical thermally stable structures. Composite HTPDs typically include organic‐inorganic composites and polymer‐metal composites. There are many varieties of organic‐inorganic composites with high temperature resistance and dielectric properties. High‐temperature polymers compounded with silicon, boron, etc., have more remarkable comprehensive properties. This type of polymer as a transition species of organic materials to inorganic materials greatly enriched the polymer molecular structure composition. As a result, HTPD has been widely noticed and studied by numerous researchers and engineering experts in the field of modern insulation.
However, there are no existing specific books on HTPDs for electrical equipment. Based on the theoretical research value of HTPD in electrical equipment. First, this book outlines the preparation process and properties of representative HTPDs. Subsequently, focusing on the relationship between the structure and properties of HTPDs, this book highlights the applications of HTPDs in the field of electrical insulation, such as high‐temperature energy storage capacitors, electric motors, encapsulation, printed circuit boards, new energy power equipment, aerospace electrical equipment, and other electrical insulation fields. Overall, the monomer synthesis, material preparation, characterization methods, properties, and applications of various HTPDs are comprehensively presented.
This book covers a wide range of topics rather than merely a single introduction to HTPDs, and it will provide an excellent guide to improving the performance of existing HTPDs, as well as the design and development of novel HTPDs. Furthermore, this book is intended for graduate students and researchers at universities and research institutes, along with engineers at related companies. It will work across academia, industry, education, and public service to its matched audience.
1Overview of High‐Temperature Polymers
Xue‐Jie Liu, Mengyu Xiao, Wenjie Huang, Xing Yang, and Jun‐Wei Zha
University of Science and Technology Beijing, Beijing Advanced Innovation Center for Materials Genome Engineering, School of Chemistry and Biological Engineering, 30 Xueyuan Road, Haidian District, Beijing, 100083, P. R. China
1.1 Introduction
In recent years, the wide utilization of new energy sources has made it necessary for power equipment to operate in harsh environments such as high voltage, high power, and high temperature [1, 2]. Therefore, as an essential component of power equipment, dielectric materials have attracted more attention. Solid dielectrics used in power equipment can be divided into polymer dielectrics and inorganic dielectrics. Inorganic dielectric has high temperature stability but also has the disadvantages of low breakdown strength (Eb) and poor flexibility, which cause difficulties in large‐scale preparation that cannot be ignored. Different from inorganic dielectrics, polymer dielectrics have the advantages of being lightweight, having good flexibility, and being easy to process [3]. Meanwhile, excellent dielectric properties (high Eb, low dielectric loss [tan δ]) make it widely used in power equipment. With the continued miniaturization and increased power output of electronic and power systems, many fields require polymer dielectrics for reliable operation in harsh environments. For example, control and sensing electronics near the shells of rockets and space shuttles require high‐temperature dielectric materials to operate above 250 °C. In underground oil and gas exploration, operating temperatures exceed 200 °C [4]. Unfortunately, the poor thermal stability of traditional polymer dielectrics seriously threatens the reliable operation of power equipment and significantly shortens its life cycle. Therefore, secondary cooling equipment has been used to reduce the working temperature in high‐temperature applications. However, secondary cooling is difficult to achieve in view of the extreme temperatures experienced by large facilities such as underground exploration and space stations. Thus, a more attractive strategy is to develop high‐temperature‐resistant polymer dielectrics that carry out long‐term work at high temperatures. This strategy can improve system reliability, reduce costs, and eliminate the need for large cooling systems and the wiring and interconnections required for remote placement of electronics [5, 6].
Currently, polymers such as polyethylene (PE) and polypropylene (PP) are commonly used, but their operating temperatures are less than 105 °C [7]. This fact can be attributed to the tendency of molecular chains to move or even to break under heating conditions. Polymers with high temperature resistance have been developed such as polyimide (PI), polyetherimide (PEI), and polyetheretherketone (PEEK) [8–14]. The favorable high temperature resistance is primarily caused by the ultra‐strong molecular structures derived from the conjugated bonds (multiple bonds or aromatic rings) on the polymer backbone. Furthermore, the hydrogen bonds and cross‐linked structures lead to strong interactions within and between the polymer chains, which is directly reflected in the elevated high temperature resistance [15]. In addition, the introduction of an inorganic filler can further optimize the high‐temperature polymer [16]. Based on the above strategies, excellent polymer dielectrics are prepared through molecular structure modification and composite structure design to meet the urgent requirement of high temperatures for rapidly developing power equipment.
This chapter briefly introduces the history of high‐temperature polymers. Then, the influence of parameters such as glass transition temperature (Tg), melting point (Tm), and thermal conductivity (λ) on the high temperature resistance of polymers is analyzed, and the method of determination of the above parameters is displayed. Subsequently, the research progress of high‐temperature polymers and their composite materials is discussed. Finally, the future challenges of high‐temperature polymer materials are presented. This chapter will instruct on the development of high‐temperature polymers in power equipment.
1.2 Development of High‐Temperature Polymers
Since the early twentieth century, the rapid development of commercially available synthetic polymers has led to their widespread usage in military and civilian applications. Research on polymers with high temperature resistance attained its peak in the 1950s. During the development of novel products, functional groups and segments were introduced into polymers for the improvement of high temperature resistance. It leads to poor processability and high costs, which hinder large‐scale production. A few high‐temperature polymers have been developed and used nowadays, such as PI, PEI, PEEK, and fluorene polyester (FPE). It has been found that they are characterized by aromatic and heterocyclic structures. Table 1.1 summarizes the thermal and electrical properties of high‐temperature polymers, suggesting that the common characteristics of high‐temperature polymers are high Tg and relatively low dielectric permittivity (εr) and tan δ [4, 17]. With the booming development of smart grids, aerospace, electronic packaging, and other technical fields, polymer dielectrics are expected to have excellent thermal stability, mechanical properties, dielectric properties, and long service lives. It is also developing toward being lightweight and low‐cost to meet various special applications. For example, Fan et al. [18] introduced alumina (Al2O3) nanoparticles into the PEI matrix, and the Td5% of the prepared composite films was increased from 479 to 525 °C. Additionally, the Eb also exhibited an increase from 422 to 503 MV m−1 at 150 °C. The wide bandgap of Al2O3 nanoparticles is the primary factor for the significant increase in Eb. Li et al. [19] incorporated polysulfone (PSF) with a high εr into PI matrix. While maintaining high‐temperature stability (Tg = 285 °C), a dramatic improvement in the εr was obtained. When the mass fraction of PSF reached 40%, the composite film exhibited the highest εr of 6.4 at 1 kHz, versus 3.25 for pure PI.
Table 1.1 Thermal and electrical properties of high‐temperature polymers [4, 17].
Polymer
T
g
(°C)
Volume resistivity (Ω cm
−1
)
E
b
(MV m
−1
)
ε
r
tan
δ
(1 kHz)
Kapton (PI)
360–410
2.3 × 10
17
154–303 (7.6–127 μm)
2.7–3.5
0.13–0.26
Upilex (PI)
285–500
10
16
–10
17
147–320 (12.5–125 μm)
3.2–3.5
0.13–0.7
Ultem (PEI)
217–247
10
17
200 (25 μm)
3.15
0.12
Ketaspire (PEEK)
150
2.6 × 10
16
150 (50 μm)
3.1
0.3
Kepstan (PEKK)
162
10
16
84 (100 μm)
2.6
0.7
PTFE
327 (
T
m
)
>10
18
23.6 (2 mm)
2.1
0.03
PET
245–265 (
T
m
)
10
15
∼500
3.3
0.3
Isaryl (FPE)
330
10
15
–10
17
220–320 (100–120 μm)
3.2–3.5
0.31–0.7
Cyclotene (BCB)
>350
10
18
300 (10 μm)
2.75
0.12
1.3 Parameters of Polymers with High Temperature Resistance
The high temperature resistance of polymers determines their practical application. Tg is a vital factor affecting the high temperature resistance of a polymer. Tg is the critical temperature at which the chain segments of the noncrystalline phase of polymers transition from freezing to motion and depends on the difficulty of polymer chain rotation. Chain rotation relies on the types of chemical bonds (such as covalent bonds and hydrogen bonds) and polar groups, as well as the shape of the molecular chains [20]. Firstly, the strength of the covalent bonds plays a major role in the thermal stability of polymers. Table 1.2 lists various chemical bonds in polymers and their fracture energies, which indicates that the fracture energy of double bonds is much higher than that of single bonds [21]. High‐temperature polymers are usually composed of aromatic ring structures because they provide more stable covalent bonds. Then, the C—F bond has bond energy of up to 126 kcal mol−1, significantly higher than the C—H bond. Therefore, fluorinated polymers are more thermally stable than their corresponding hydrogen‐substituted polymers. Furthermore, the presence of non‐covalent bonds such as hydrogen bonds and van der Waals forces makes the polymer chains more attractive. This has a greater impact on the parameters such as Tg and modulus, so the polymer is more resistant to high temperature. In addition, flexible polymer chains are known to lower the Tg, therefore, polymers containing rigid groups (such as sulfone and cyano groups) have higher Tg than those containing ether bonds. And in the design of polymer molecular structure, the presence of Si, P, B, and F atoms in the main chains of polymers is beneficial to improving the high temperature resistance of polymers.
Table 1.2 Cleavage energy of representative chemical bonds.
Chemical bond
C
aryl
—C
aliph
C
aryl
—C
aryl
C=C
C—N
C=N
C=O
Cleavage energy (kcal mol
−1
)
93
100
145
70
147
84
Chemical bond
C—H
C—F
C—S
Si—O
Ti—O
B—O
Cleavage energy (kcal mol
−1
)
99
123
62
88
180
185
Source: Cottrell [21]/Elsevier.
For crystalline polymers, the melting point (Tm) is more practical than Tg. Tm is the critical temperature at which crystalline polymers change from the crystalline state to the molten state [1, 4]. The molecular chains of crystalline polymers are tightly aligned during the crystallization process. It still has good mechanical properties when the temperature is much higher than Tg. The actual operating temperature is significantly elevated above Tg. Therefore, the crystallinity and regularity of the molecular structure are also one of the reasons for high temperature resistance of the polymer. For example, the oriented PI films show an aligned structure that facilitates their mechanical and thermal properties. Cross‐linked polymers typically have superior resistance to high temperatures than linear polymers. The disruption of the molecular chains of cross‐linked polymers is only completed when more than two chemical bonds are broken, which effectively suppresses the degradation process.
High Tg and Tm are not enough to protect the polymers from a series of failures caused by heat for a long time. The enhanced λ allows more rapid conduction of heat away from the polymer, reducing the risk of high temperatures. For most polymers, the main mechanism of heat conduction is generated by lattice vibration through phonons. The λ of the polymer can be obtained from the Debye equation, as shown below [22].
where Cp is the specific heat capacity per unit volume, ν is the phonon velocity, and l represents the phonon mean free path. Due to scattering with other phonons, defects, and grain boundaries, the l value of most polymers is quite low, essentially less than 0.5 W m−1 K−1. Due to the shortcomings of the complex synthesis process and low preparation efficiency of intrinsic thermally conductive polymers, researchers have gradually focused on the improvement of λ of polymer‐based composite materials in recent years [23, 24]. Guo et al. [23] prepared PI composites containing highly aligned hybrid fillers with high thermal conductivity (1.49 W m−1 K−1), which is a multifunctional dielectric material with excellent thermal management capabilities. Table 1.3 lists the λ values of polymers and their composites [23–28], where the λ of the polymers is between 0.2 and 0.5 W m−1 K−1, as previously described. The λ of polymer‐based composite materials has been significantly improved.
Table 1.3 Thermal conductivity of high‐temperature polymers and their composites [23–28].
Polymers
λ
(W m
−1
K
−1
)
Composite materials
λ
(W m
−1
K
−1
)
Polycarbonate (PC)
0.23
3 wt% Ag + 12 wt% rGO/PI
2.12
PEI
0.21
4.0 wt%MWCNTs @ PPD/PI
0.43
PI
0.27
30 wt%mBN/PI
0.696
PEEK
0.25
rGO‐PI/BNNS‐PI (total filler loading constant at 3.8 wt%)
1.49
1.4 Thermal Analysis Technology
Due to the diversity of polymer characteristics, there are different methods and conditions for measuring thermal properties. A variety of thermal property analysis techniques have been used to evaluate the damage to polymers at high temperatures. Several common analysis techniques for polymers are described below.
1.4.1 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) is a commonly used technique for measuring the Tg and Tm of materials. Huang et al. [29] characterized the Tg values of PEEKs containing different side chains by DSC tests, as shown in Figure 1.1a. Figure 1.1b depicts that carboxylated polymer (PEEK‐COOH) achieved a maximum Tg of 213 °C. Due to the longer –SO2 side chains and –CN side chains, the Tg of sulfonylated polymer (PEEK‐SO2) and cyanylated polymer (PEEK‐CN) decreased to 156 and 155 °C, respectively.
1.4.2 Dynamic Thermomechanical Analysis (DMA)
Dynamic thermomechanical analysis (DMA) characterizes the mechanical properties of viscoelastic materials as a function of time, temperature, or frequency. On the one hand, DMA can be used to evaluate the mechanical behavior of materials by giving mechanical properties over a wide range of temperatures and frequencies. On the other hand, the Tg and secondary relaxation processes of polymers can be detected, which are closely related to the chain structure and condensed structure of polymers. Niu et al. [30] characterized the energy storage modulus (E′) and tan δ of PI, PEIs, and sandwich PEIs/PI/PEIs films by DMA tests. As shown in Figure 1.2, the Tg values of PI and PEI and sandwich PEIs/PI/PEIs reached 336, 249, and 241 °C, respectively, indicating excellent high temperature resistance.
Figure 1.1 (a) Structure formula and (b) DSC of PEEK polymers [29]. Source: Huang et al. [29]. Reproduced with permission of Wiley‐VCH.
Figure 1.2E′ and tan δ as a function of temperature for PI, PEIs, and three‐layer PEIs/PI/PEIs films [30]. Source: Niu et al. [30]. Reproduced with permission of Elsevier.
1.4.3 Thermogravimetric Analysis (TGA)
TGA is used to characterize the variation of the sample mass in relation to the programming temperature; thus, it is used to analyze the thermal degradation temperature (Td) and composition of the materials. Liu et al. [31] calculated the imidization degree (ID) of PI from TGA data. As shown in Figure 1.3a,b, the ID was obtained by Eq. (1.2) based on the weight loss generated in the second step (cyclized dehydration of the PAA chain), and the results of Td and ID are shown in Table 1.4.
Figure 1.3 (a) TGA curves of the PI films with different IDs. (b) Three steps of the thermal weight loss process for the PI film were obtained from the TGA curve [31]. Source: Liu et al. [31]. Reproduced with permission of the Royal Society of Chemistry.
Table 1.4 Imidization degree and thermal property of the PI films [31].
ODPA‐MPD
1
2
3
4
5
6
7
Source: Liu et al. [31]. Reproduced with permission of the Royal Society of Chemistry.
1.4.4 Static Thermomechanical Analysis (TMA)
Thermomechanical analysis (TMA) refers to a technique for measuring the functional relationship between the deformation of the materials and the temperature and time under the action of a programmed temperature and non‐vibration load, which can be used to measure the coefficient of expansion and phase transition temperature of materials. Figure 1.4c shows the coefficient of thermal expansion (CTE) of the colorless polyimide (CPI) and polyamideimide (PAI) films measured by TMA [32]. The CTE value of the CPI film is relatively high due to the bulky trifluoromethyl side groups. PAI films are obtained by the polymerization of diacids with diamines Figure 1.4a. Figure 1.4b demonstrates that intermolecular interactions between the amide units restrict the motion of the PAI chain segments. As the temperature approaches Tg, the films start to swell due to the increased mobility of the molecular chains. The PAI‐1 (CTE = 15 ppm °C−1) and PAI‐2 (CTE = 21 ppm °C−1) films have higher dimensional stability compared with CPI films due to the rigid‐rod amide units that increase the linearity and rigidity of the polymer chains.
Figure 1.4 (a) Structure formula, (b) DSC, and (c) TMA of CPI and PAI films [32]. Source: Zuo et al. [32]. Reproduced with permission of the Royal Society of Chemistry.
1.4.5 Thermal Conductivity
Thermal conductivity reflects the ability of materials to conduct heat. Polymer materials with high thermal conductivity are conducive to emitting waste heat generated during operation, so they are beneficial to the optimization of electrical properties, life cycle, and reliability of power equipment. Zhou et al. [33] introduced a sandwich‐structured PAI composite film as shown in Figure 1.5. The design of the layered structure and highly oriented functionalized BN bring about high in‐plane thermal conductivity (λ//). The composite film achieved a high λ// of 45.7 W m−1 K−1 with 23 wt% BN loading.
Figure 1.5 (a) Schematic diagram of the structure of PBP composite film. (b) λ// of pure PAI and PBP composite films with different numbers of filler layers [33]. Source: Zhou et al. [33]. Reproduced with permission of Elsevier.
Figure 1.6 Temperature‐dependent (a) εr and (b) tan δ of PI and PEI measured at 10 kHz [34]. Source: Li et al. [34]. Reproduced with permission of Nature Publishing Group.
1.4.6 Dynamic Dielectric Analysis (DEA)
Dynamic dielectric analysis (DEA) is a technique for analyzing the εr and tan δ of dielectric materials as a function of temperature and frequency under electric fields. As shown in Figure 1.6, the εr of PI steadily decreases from 3.3 to 2.8 and remains low tan δ (<0.3%) as the temperature increases from 25 to 350 °C [34]. Although the εr of PEI remains stable between 25 and 200 °C, its relatively low Tg causes the εr and tan δ of PEI to change dramatically with increasing temperatures above 200 °C. Furthermore, a significant relaxation peak in tan δ is observed at about 175 °C.
1.5 High‐Temperature Polymer Materials
Compared with metal and ceramic materials, high‐temperature polymers have the advantages of being lightweight, easy to process, and having tunable properties. This section describes high‐temperature polymers and their composite materials that are widely used in electronic power systems.
1.5.1 Commercial High‐Temperature Polymer
Figure 1.7 shows the structural formulas for several high‐temperature polymers. PI is a thermosetting polymer containing imide rings, which is one of the most high‐temperature‐resistant polymer materials available in practice. PI is usually prepared through a traditional two‐step process. Firstly, the dianhydride and diamine monomers form PAA through polycondensation. Then the PAA is converted to PI by the imide reaction. PI with different properties can be synthesized by selecting different types of diamine and dianhydride monomers. One of the most commercially valuable PI films is the Kapton film, made by DuPont. Kapton is synthesized from pyromellitic dianhydride (PMDA) and 4,4'‐oxydianiline (ODA) monomers. Marketed since the mid‐1960s, it has been widely used for high‐temperature wire and cable insulation on aircraft with continuous operating temperatures of 300–350 °C [35, 36]. Aromatic polyimides have strong rigidity, which brings difficulties for processing and manufacturing. In order to improve the processing properties of PI, researchers have introduced flexible groups (such as ether bonds and alkyl groups) into the PI molecular chains. For example, PEI is obtained by introducing flexible groups (such as ether bonds) onto the PI skeleton. The classic PEI is ULTEM, produced by SABIC using the disodium salt of bisphenol A and 1,3‐bis(4‐nitrophthalimido) benzene [37]. The existence of the flexible bond makes PEI transform into a thermoplastic polymer and obtain a lower Tg = 217 °C. Thermoplastic PEI has excellent machining properties, including extrudable, injectable, and compressible molding, and is soluble in a variety of organic solvents, which provides conditions for application in different equipment [38].
Figure 1.7 The structural formula for several common high‐temperature polymers.
PEEK is a semi‐crystalline polymer material with ether bonds and ketone carbonyl groups in molecular chains, which is formed by the polymerization of 4,4′‐difluorophenone and dibenzoate. PEEK not only has good high temperature resistance (Tg ≈ 150 °C), but also excellent mechanical strength, flame retardant, radiation resistance, insulation, and other advantages [5]. The semi‐crystalline characteristics of PEEK make it have hot melting characteristics, which can be directly wounded with copper wire at high temperatures, and then the corresponding insulation products are prepared. The thermal properties of polyketone materials can be adjusted by changing the ratio of ether bonds and ketone carbonyl groups. For example, when the ratio of ether bond and ketone carbonyl group is changed from 2 : 1 in PEEK to 1 : 2, poly(ether ketone ketone) (PEKK) with a similar structure to PEEK is obtained. PEKK is generally polymerized from diphenyl ethers and benzene dicarboxylic acid halides [39]. The commercial PEKK named Kepstan, manufactured by Arkema, has a high Tg of 162 °C [40].
PTFE (polytetrafluoroethylene) can be prepared from tetrafluoroethylene monomers by polymerization. PTFE was accidentally discovered by Plunkett in 1938 [41]. As a semi‐crystalline polymer, PTFE also has a high Tm of 327 °C, which gives it excellent thermal stability at 260 °C [42]. In addition, PTFE is widely used in the aerospace field due to its corrosion resistance, electrical insulation, and good anti‐aging durability. Polyphenylene sulfide (PPS) is a thermoplastic crystalline resin composed of aromatic rings connected with sulfur. Although the Tg of PPS is only 120 °C, it can work in the range of 200–240 °C for a long time, and the maximum temperature can reach 260 °C for a short time. Moreover, the Td of PPS in the air is higher than 500 °C. In addition to excellent thermal stability, PPS also has good corrosion resistance (second only to PTFE), electrical performance, and flame retardancy [43]. However, its high crystallinity (75%) and rigid molecular structure lead to its high brittleness and low toughness.
Although some widely used high‐temperature polymers have been discussed above, they still have different shortcomings in practical application. Thus, it is necessary to develop novel high‐temperature polymers. The recent research progress in high‐temperature polymers and composite materials from the perspectives of molecular structure design and composite structure optimization is presented as shown below.
1.5.2 Molecular Structure Modification of High‐Temperature Polymer
Homogeneous high‐temperature polymer films can be obtained based on molecular structure modification strategies. There are extremely few defects in the film‐forming process, which is conducive to large‐scale preparation. Suppressing the motion of molecular chains is an effective strategy to make polymers with excellent high temperature resistance. This can be achieved by enhancing intermolecular forces, such as by building rigid and cross‐linked structures. Researchers have made a lot of valuable progress in this field.
Most high‐temperature‐resistant polymers have rigid structures, and their molecular chains should contain aromatic rings or heterocyclic rings. In addition, the conjugate structure can be further designed on the main chains to improve the rigidity of polymers. Based on the above design concept, Pan et al. [44] incorporated rigid and asymmetric phenyl phthalazinone ether ketone molecules into the polymer skeleton to obtain poly(phthalazinone ether ketone) (PPEK). The Tg of PPEK was 250 °C, and Td5% reached 490 °C. Although the presence of conjugated structures improves the high temperature resistance of polymers, the stacking of π–π structures leads to a reduction in bandgap, which deteriorates electrical properties. Figure 1.8a depicts that Wu et al. [45] synthesized polyoxafluoronorbornene (POFNB) through ring‐opening metathesis polymerization reaction using Grubbs generation 2 catalyst. The aromatic structure was replaced by a saturated, fused bicyclic structure in the polymer. This nonplanar and non‐conjugate rigid structure allows the polymer to achieve high temperature resistance while avoiding bandgap reduction due to π–π stacking between benzene rings. As shown in Figure 1.8b, POFNB achieved a high Tg of 186 °C and a wide bandgap of 4.9 eV. The wide bandgap enables the conduction current of POFNB to be effectively restrained at high temperatures and in high fields, thus obtaining excellent insulation performance at 150 °C.
Figure 1.8 (a) Schematic of the synthesis process for POFNB. (b) The bandgap versus Tg for POFNB and high‐temperature polymers [45]. Source: Wu et al. [45]. Reproduced with permission of Wiley‐VCH. (c) Fabrication of the cross‐linked PEN. (d) DMA of the PEN treated at different temperatures [46]. Source: You et al. [46]. Reproduced with permission of the American Chemical Society.
Cross‐linking is a process in which polymers are linked by covalent bonds to form networks. There are substantial intermolecular forces in cross‐linked polymers that significantly restrict the movement of molecular chains. For example, You et al. [46] synthesized self‐cross‐linking poly(arylene ether nitriles) (PEN) at different temperatures, and the high temperature resistance of PEN was significantly improved. As shown in Figure 1.8c, the phthalocyanine rings were formed by the cross‐linking of phthalonitriles at the end of PEN at 320 °C, while the –CN groups on the side chains of PEN participate in the reaction to form triazine rings at 350 °C. Figure 1.8d illustrates that the 350 °C cross‐linked PEN had a high Tg of 370 °C, and its operating temperature reached 300 °C.
Strong intermolecular interactions resulting from the introduction of polar groups in polymer chains have also been used to suppress the motion of molecular chains and improve high temperature resistance. Zhu et al. [47] synthesized a new type of diamine monomer with two ortho‐position nitrile groups in the benzene ring through chemical reactions and obtained a series of PI after reacting with different dianhydrides. It was found that Tg was improved by the introduction of nitrile groups. Moreover, the strong interaction between nitrile groups enhances the mechanical properties of PI. Tang et al. [48] achieved the improvement of Tg and Young's modulus of polystyrene (PS) by introducing nitro groups. It is worth noting that the thermal decomposition of nitro groups results in a slight decrease in Td of the polymer.
1.5.3 High‐Temperature Polymer‐Based Composite Materials
Polymers have great designability due to their diverse chemical structures. Moreover, it is also an effective way to enhance the high‐temperature properties of traditional polymers by introducing fillers to prepare polymer‐based composites.
Inorganic fillers have the advantages of excellent thermal stability and electrical properties, which can be combined with the advantages of machinability and high Eb of the polymers through the two‐phase composite method. The thermal stability of inorganic/polymer composite materials is generally higher than that of pure polymers, which is more obvious at high temperatures. For example, Zahra et al. [49] added 5 wt% sepiolite nanowires into PEI matrix to prepare sepiolite/PEI composite materials. Compared with pure PEI, the Tg of composite material increased from 215 to 223 °C, and the Td5% increased from 478 to 487 °C. At high temperatures, the polymers will be softened. It was found that the addition of carbon nanotubes (CNTs) enhanced the mechanical properties of polymers at high temperatures. Mamedov et al. [50] fabricated CNTs/PEI composites using a layer‐by‐layer deposition method. Compared with pure PEI, the modulus of the CNT/PEI composites increases from 0.3 to ∼11 GPa, and strength increases from ∼9 to 325 MPa. As shown in Figure 1.9a,b, Li et al. [34] thermally cross‐linked divinyltetramethyldisiloxane‐bis (benzocyclobutene) (BCB) in the presence of boron nitride nanosheets (BNNSs) to obtain c‐BCB/BNNS nanocomposites (Tg > 350 °C). Due to the high thermal conductivity and wide bandgap of BNNS, the excellent thermal conductivity of 1.8 W m−1 K−1 and high‐temperature insulation performance (403 MV m−1 at 250 °C) of c‐BCB/BNNS were achieved.
Figure 1.9 (a) Schematic of the synthesis process for c‐BCB/BNNS. (b) Weibull breakdown strength of c‐BCB and c‐BCB/BNNS as a function of temperature [34]. Source: Li et al. [34]. Reproduced with permission of Nature Publishing Group.
The incorporation of inorganic fillers has improved the high temperature and electrical properties of the polymers. However, the introduction of a large number of inorganic fillers leads to poor interfacial compatibility between the two phases, which destroys the flexibility of nanocomposite materials. Compared to polymer‐based nanocomposite materials, all‐organic composite materials ensure great high‐temperature properties while maintaining the inherent flexibility of polymers, which enhances the possibilities of high‐temperature polymer applications. Polyacrylonitrile is further dehydrogenated to form a trapezoidal conjugated structure (PcLS) at 400–600 °C. Liao et al. [51] prepared PcLS/PI all‐organic composite films by in‐situ polymerization, which had an excellent T5% of 600 °C. Zhang et al. [52] chose PEI and PI to obtain composite materials with excellent high‐temperature insulation performance by co‐blending method. The strong electrostatic interaction between positive and negative delocalized electrons was used to improve the packing density of molecular chains, thereby reducing voids and lowering the free volume. The composite material of 50 wt% PEI and 50 wt% PI not only had the Tg of 246 °C but also obtained excellent insulation performance (Eb of 550 MV m−1) at 200 °C. Zhang et al. [53] designed a novel 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTCDA)/PEI composite film. As a molecular semiconductor, NTCDA has high electron affinity, which significantly inhibits carrier migration, thus improving the high‐temperature insulation performance of PEI. Compared with PEI film (Eb = 440 MV m−1), the Eb of NTCDA/PEI composite film was 510 MV m−1 at 200 °C.
1.6 Summary and Outlook
In summary, researchers have conducted a lot of exploratory experiments on the structure design, preparation process, performance optimization, and application scenarios of high‐temperature polymer materials. It is remarkable that many high‐temperature polymers have been commercialized and applied to power equipment. The development of high‐temperature polymers makes it possible to break through the bottleneck of their poor temperature resistance. However, as the power equipment in various fields needs to cope with a more severe working environment, higher requirements are put forward for high‐temperature polymers. The growth of high‐temperature polymer materials needs to pay attention to the following issues:
Based on the design of molecular structure, it is crucial to optimize the morphology of molecular chains through the preparation process for intrinsic high‐temperature polymers. In order to ensure the uniform dispersion and reasonable distribution of the fillers in the polymer matrix, the surface modification of the fillers and the design of the multiple structures should be paid attention to in polymer‐based composite materials.
It is crucial to explore and understand the inherent mechanisms of high‐temperature polymers with the participation of various simulation models. On the one hand, the molecular structure of the initial screen can be obtained by molecular simulation of the intrinsic polymers, which reduces the cost of the traditional “trial and error” method. On the other hand, simulation models are often used to understand the influence of two‐phase interfacial compatibility on the properties of high‐temperature polymer‐based composite materials. In addition, the screening conditions for high‐temperature polymers should not be limited to their high thermal stability and electrical properties. Film forming and mechanical properties should be considered together to greatly improve the possibility of future applications.
In the early development stage of high‐temperature polymers, excessive attention is paid to their thermal stability while their processability is ignored. For example, the introduction of a large number of aromatic rings increases the thermal stability but makes the polymer insoluble and difficult to reprocess. The cooperative regulation mechanisms of processability and high temperature resistance should be established. In the long term, successful production of high‐temperature polymers requires innovative structural design and preparation processes to balance the relationship between the above two parameters.
On the basis of the third issue, excellent electrical properties such as high insulation, corona resistance, and low loss should be maintained at high temperatures to meet the different requirements of each electrical field. It is necessary to strengthen the coordination between high‐temperature polymers and application scenarios to provide targeted performance optimization.
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