ENGINEERING TECHNOLOGY

“Insulation” and “thermal conductivity”, breaking through the bottleneck of the development of cutting-edge electronic equipment


Polymers are an important class of electrical insulation materials, but the thermal conductivity of polymer materials is generally poor, and improving the thermal conductivity of polymers is often at the expense of insulation performance. The “contradiction between insulation and thermal conduction” is one of the bottlenecks restricting the application of polymer materials in cutting-edge electrical and electronic equipment.

On March 2, Nature published the latest research results of Professor Huang Xingyi’s team and collaborators at the School of Chemistry and Chemical Engineering of Shanghai Jiao Tong University. The researchers constructed the arrayed nanoregion by layered arrangement of isotactic segments, and introduced electrophilic trap groups in the arrayed nanoregion, which greatly improved the thermal conductivity of flexible polymer dielectric films by an order of magnitude and solved the contradiction between thermal conductivity and insulation of polymer materials. This polymer dielectric film has stable performance and good breakdown self-healing, so it will have broad application prospects in electromagnetic energy equipment, new energy vehicles, power electronics and other fields.

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Molecular structure and self-assembly morphology of double-stranded polymer dielectric films Courtesy of Shanghai Jiaotong University

Contradiction between thermal conductivity and insulation

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. Therefore, the ability of the dielectric film to withstand the electric field increases, and the charge storage density increases rapidly. However, polymer films are dominated by electronic conductance under high electric fields, which no longer conform to Ohm’s law, and the conductance current increases exponentially with the increase of electric field strength, which will generate a large amount of heat.

The thermal conductivity of traditional polymer dielectrics is generally low, and the heat dissipation efficiency is also very low, which will cause the temperature of the medium to rise rapidly, which will cause 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 nanoaddition, this often comes at the expense of electrical strength, and more importantly, nanoaddition also poses 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.

Design of double-stranded structure copolymers

To solve such problems, Huang’s team designed a double-stranded structure copolymer (PSBNP-co-PTN). The copolymer self-assembles 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.

“This shows that the ordered array is parallel to the surface and, therefore, the dielectric film exhibits high thermal conductivity in the vertical plane.” Huang Xingyi said.

Through density functional theory analysis and thermal stimulation current experiments, the research team found that there was a charge trap with a depth of 1.51 eV between the chain structure segments of this copolymer, and the charge trap depth further increased with the increase of the external electric field strength. By introducing a certain amount of PTNI molecules into the PSBNP ordered array, the copolymer can exhibit the best electrical insulation and the highest electrical breakdown strength. Polarized energy storage tests show that its maximum discharge energy density is much better than that of existing polymers and their composite dielectric films.

Break through the bottleneck of electronic equipment development

Heat generation during continuous charge-discharge cycles of common polymers and polyetherimides (PEI, the best known commercial high-temperature polymer dielectric films) did not occur in this highly thermally conductive copolymer dielectric film, and researchers did not even observe local heat accumulation. Experiments show that the continuous charge-discharge cycle life of this copolymer dielectric film is 6 times that of PEI film.

It is worth mentioning that the carbon content of the film is relatively low, which gives it excellent self-healing, and the electron microscopy image clearly shows that the aluminum metal electrode around the electrical 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 the continuous charge-discharge cycle can still be carried out.

“The intrinsic thermal conductivity of this copolymer dielectric film thickness direction is 1.96 ± 0.06 W/(mK), which is the highest value of the intrinsic thermal conductivity of insulating polymers currently reported.” Chen Jie, co-first author and assistant researcher of the paper, said, “The copolymer dielectric film is still stable after 50,000 charge-discharge cycles, and has good breakdown self-healing. ”

“This research is a deep cross-integration of electrical engineering, chemistry, materials, engineering thermophysics and other disciplines.” Huang Xingyi said that 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 all participated in the study.

At present, the relevant technology has been authorized for invention patents, and related products will be widely used in electromagnetic energy equipment, new energy vehicles, power electronics and other fields. (Source: China Science News, Zhang Shuanghu, Li Chenyang)

Related paper information:https://doi.org/10.1038/s41586-022-05671-4



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