On December 31, 2018, Prof. He Jinliang and Associate Prof. Li Qi, Department of Electrical Engineering (EEA), Tsinghua University, and their co-authors published online a research paper entitled "Self-healing of Electrical Damage in Polymers Using Superparamagnetic Nanoparticles" in the journal Nature Nanotechnology (Impact factor 37.49). In this paper, a method of self-healing of electrical damage in solid insulating materials is proposed. It is the first time that the self-healing of electrical tree channel and self-recovery of insulating property after insulating materials are destroyed by electrical tree are realized, while the basic electrical properties of materials are not affected. The self-healing strategy is widely applied to thermoplastic polymer insulating materials such as polyolefins, which provides a new method for greatly improving the service life and reliability of power equipment (such as power cables) and electronic equipment.

In recent years, the voltage level of power grid is gradually improved, and the scale of power grid is expanding with the rapid development of global energy Internet and UHV transmission technology. In order to maintain the stable operation of transmission network with increasing load, it is necessary to continuously improve the reliability and service life of electrical equipment under extreme working conditions. The service life of electrical equipment, especially high-voltage power equipment, often depends on the service life of insulating components. The main reason for insulation damage is the electrical tree defect formed in the long-term operation of insulating media. For a long time, electrical tree defects of solid insulating materials have been considered as irreversible permanent damage. The research on electrical tree aging is mainly to delay the development of electrical tree by adding voltage stabilizers and electrical tree blockers. However, the electrical tree aging of insulating materials is unavoidable. Once the electrical tree defects are formed, the insulation life will be greatly reduced, and even the permanent damage of equipment will occur.

In order to obtain insulating materials with both electrical damage healing and high dielectric strength, the research team used polyolefin cable insulating materials as the base material, and achieved the target repetitive healing of electrical tree damage of thermoplastic insulating materials by the entropy dissipation and migration behavior of nanoparticles in polymers based on the magnetocaloric effect of superparamagnetic nanoparticles. The team also verified migration and diffusion behaviors of nanoparticles in the process of electrical tree damage healing by molecular dynamics simulation and microscopic experimental characterization based on the Gauss chain model. Leakage current and partial discharge tests show that the self-healing method can completely heal the electrical insulation properties of polyolefin insulating materials which cause electrical tree damage, and maintain the same level as pure polyolefin insulation in many healings. The defect healing mechanism can be realized by using very low superparamagnetic nanoparticles (below 0.1 Vol.%). Therefore, the electrical breakdown strength of self-healing insulating materials can be maintained at more than 94% of the base materials (e.g. 490 kV/mm), which can meet the application requirements of ultra-high voltage cable transmission and other power energy fields. In addition, for power electronic devices, electric vehicle wireless charging devices and other electrical equipment, this method is also expected to achieve live self-healing and online maintenance of insulation damage in these areas.

The first author of this paper is Yang Yang, a doctoral student of 2014 in the EEA, Tsinghua University. The corresponding authors are Prof. He Jinliang and Associate Prof. Li Qi of the EEA, and Prof. Wang Qing of Pennsylvania State University. The co-authors include Dr. Gao Lei, Associate Prof. Hu Jun and Prof. Zeng Rong of the EEA, and Assistant Prof. Qin Jian and Prof. Wang Shanxiang of Stanford University. The research was funded by the National Key Basic Research Development Program (2014 CB239500). Prof. He Jinliang is the chief scientist of the project.