High-temperature resistant, intrinsically high thermal conductivity polymer dielectric film

On March 2, 2023, Beijing time, the team of Professor Huang Xingyi of Shanghai Jiao Tong University and the team of Professor Wang Qing of Pennsylvania State University made a major breakthrough in the field of polymer electrical insulation materials, and the relevant results were published in Nature under the title “Ladderphane copolymers for high temperature capacitive energy storage”.

Polymer is an important class of electrical insulation materials, but the thermal conductivity of polymer materials is generally poor, improving the thermal conductivity of polymers is often at the expense of insulation performance, “insulation and thermal conductivity contradictory” is one of the bottleneck problems restricting the development of polymer materials in the advanced electrical and electronic equipment. The arrayed nanoregion is constructed by the layered arrangement of isotactic segments, and the electrophilic trap group is introduced in the arrayed nanoregion, which greatly improves the thermal conductivity of flexible polymer dielectric films by an order of magnitude and solves the contradiction between thermal conductivity and insulation. The intrinsic thermal conductivity of polymer dielectric film thickness is 1.96 ± 0.06 W/(mK), which is the highest value of intrinsic thermal conductivity of insulating polymers reported so far. The discharge energy density of the polymer dielectric film at 200 °C and 90% efficiency is 5.34 J/cm3, and the energy storage is still stable after 50000 charge-discharge cycles, and has good breakdown self-healing properties, which has great application prospects in electromagnetic energy equipment, new energy vehicles, power electronics and other fields.

Professor Huang Xingyi and Professor Wang Qing are the corresponding authors, Assistant Professor Chen Jie, Dr. Zhou Yao and Professor Huang Xingyi are the co-first authors, and Shanghai Jiao Tong University is the first completion unit of the paper. The relevant invention patents applied by Professor Huang Xingyi and Dr. Chen Jie have been authorized.

Polymer dielectric film capacitors have extremely high energy conversion rates and play a crucial role in electromagnetic energy equipment, power electronics and new energy equipment. With the development of equipment and devices in the direction of compactness, lightweight, and extreme working environment, the requirements for the energy storage density and high temperature resistance of polymer dielectric films are getting higher and higher. The charge storage density is proportional to the square of the electric field strength, so the charge storage density increases rapidly as the electric field subjected to the dielectric film increases. However, polymer films are dominated by electron conductance under high electric fields, which no longer conforms to Ohm’s law, and the conductance current increases exponentially with the increase of electric field strength, which will produce a large amount of Joule heat. The thermal conductivity of traditional polymer dielectrics is generally low (< 0.2 W/(mK)), and the heat dissipation efficiency is very low, which will cause the temperature of the medium to rise rapidly, which in turn causes chain reactions such as an increase in conductivity index and a rapid decrease in electrical strength, resulting in serious problems such as device and equipment failure. Although the thermal conductivity of polymer dielectrics can be increased by introducing nano addition, this often comes at the expense of electrical strength, and more importantly, nano addition brings great challenges to the thin film manufacturing process. Therefore, the development of polymer dielectric films with high temperature resistance and intrinsic high thermal conductivity is the best choice.

Huang’s team designed a fluorine-defective double-stranded structure copolymer PSBNP-co-PTNI, which self-assembled into a highly ordered array by π-π stacking. Through polarization Raman spectroscopy, it is found that the polarization signal of the copolymer film is isotropic in the plane and anisotropic in the fracture plane, indicating that the ordered array is parallel to the surface, so the dielectric film exhibits a high thermal conductivity of 1.96 ± 0.06 W/(mK) in the vertical plane direction.

Figure 1: Molecular structure and self-assembly morphology of a double-stranded polymer dielectric film

Density functional theory analysis and thermal stimulation current experiments show that there is a charge trap with a depth of 1.51 eV between the PSBNP and PTNI blocks, and the charge trap depth further increases with the increase of the external electric field strength. By introducing 2 molni molecules into the PSBNP ordered array, the copolymer PSBNP-co-PTNI0.02 exhibits optimal electrical insulation and highest electrical breakdown strength.

Figure 2: Conductivity and breakdown strength of double-stranded polymer dielectrics

The polarization energy storage test showed that the maximum discharge energy density of PSBNP-co-PTNI0.02 at 150 °C and 200 °C was 10.42 J/cm3 and 8.37 J/cm3, respectively, and the discharge energy density at 90% efficiency was 6.18 J/cm3 and 5.34 J/cm3, respectively, which was far better than the existing polymer and its composite dielectric film.

Figure 3: Electrostatic energy storage of double-stranded polymer dielectrics

THE HEAT GENERATION PHENOMENON DURING THE CONTINUOUS CHARGE-DISCHARGE CYCLE OF PSBNP-CO-PTNI0.02 and polyetherimide (PEI, the best known commercial high-temperature resistant polymer dielectric film) at 200 °C and 300 MV/m was visually studied by infrared camera, and no local thermal accumulation was observed in the high thermal conductivity PSBNP-co-PTNI0.02 film, while the low thermal conductivity PEI film showed obvious local thermal accumulation. The research team simulated the thermal field distribution of the dielectric film capacitor core and found that the core temperature of the PSBNP-co-PTNI0.02 film capacitor core was much lower than that of the PEI film capacitor core, and the charge-discharge cycle was more stable, and the experiment also proved that the PSBNP-co-PTNI0.02 film continuous charge-discharge cycle life was 6 times that of the PEI film.

It is worth mentioning that the carbon content of PSBNP-co-PTNI0.02 is relatively low, which gives it excellent self-healing, and the electron microscopy image clearly shows that the aluminum metal electrode around the electric breakdown area is removed by evaporation, and the carbonization channel is isolated from the metal electrode, so that the metallized polymer film after breakdown still maintains high insulation overall. After self-healing, the energy storage performance has not deteriorated significantly, and 10,000 continuous charge-discharge cycles can still be carried out.

Figure 4: Cycling stability and self-healing of double-stranded polymer dielectrics

The research is a deep cross-integration of electrical engineering, chemistry, materials, engineering thermophysics and other disciplines. Professor Jiang Pingkai, Professor Zhu Xinyuan, Associate Professor Yu Chunyang, Professor Qian Xiaoshi, Professor Bao Hua, Professor Li Shengtao of Xi’an Jiaotong University and Professor Wu Guangning of Southwest Jiaotong University also participated in the study. His research has been supported by the National Natural Science Foundation of China (51877132, 52003153, U19A20105, 51522703, 52103303), Shanghai Excellent Academic Leader Program (21XD1401600), and the State Key Laboratory of Electrical Insulation of Power Equipment (EIPE20203, EIPE21206). (Source: Science Network)

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