•Self-healing dielectrics are demonstrated to autonomously repair electrical damage•Healing is triggered by in situ electroluminescence generated by electrical treeing•UV-shielding shell of the microcapsules can protect the healing agent before healing•Microcapsules with high dielectric constant can attract electrical tree trajectory The transition from fossil to carbon-dioxide-neutral energy source (solar, wind, hydro)-based electric power necessitates efficient long-distance transmission of electric power with minimum losses to highly populated regions. This requires power transmission technologies that involve very high voltages and can only be achieved through highly insulating polymers. Electrical treeing—which is typically regarded as an irreversible damaging process leading to structure degradation and electrical conduction, and, ultimately, catastrophic failure of devices—is the most common electrical breakdown phenomenon in polymer insulation. We report self-healing polymers capable of completely restoring the dielectric properties in response to electrical treeing. The self-healing approach offers new opportunities to develop an entirely different class of smart polymer dielectrics with a prolonged lifetime and improved reliability for advanced energy and electronic applications. Dielectric polymers are ubiquitous as electrical insulation in electrical power systems and electronic devices. Electrical treeing is typically regarded as an irreversible degradation process of dielectric polymers subjected to high electric stresses, which significantly limits their lifespan and endangers the reliability of electronics and electrical power systems. Here, we report self-healing polymers consisting of microcapsules that are capable of completely restoring the dielectric properties as a direct response to electrical tree degradation. We design the microcapsules with a relatively higher dielectric constant than polymer matrix to guide the propagation of electrical trees. The electroluminescence generated in situ during electrical treeing of polymers triggers crosslinking of the healing agents released from the capsules to repair the electrically damaged areas under ambient conditions. Notably, the breakdown strength of the mended region is largely enhanced upon healing, which was initiated at different locations. The integration of autonomous healing capability into dielectric materials opens new avenues to smart polymers for electronics and energy applications. Dielectric polymers are ubiquitous as electrical insulation in electrical power systems and electronic devices. Electrical treeing is typically regarded as an irreversible degradation process of dielectric polymers subjected to high electric stresses, which significantly limits their lifespan and endangers the reliability of electronics and electrical power systems. Here, we report self-healing polymers consisting of microcapsules that are capable of completely restoring the dielectric properties as a direct response to electrical tree degradation. We design the microcapsules with a relatively higher dielectric constant than polymer matrix to guide the propagation of electrical trees. The electroluminescence generated in situ during electrical treeing of polymers triggers crosslinking of the healing agents released from the capsules to repair the electrically damaged areas under ambient conditions. Notably, the breakdown strength of the mended region is largely enhanced upon healing, which was initiated at different locations. The integration of autonomous healing capability into dielectric materials opens new avenues to smart polymers for electronics and energy applications. Polymers are universally utilized as dielectric materials in electronic devices and electrical power systems because of high breakdown strength and low dielectric loss in addition to their inherent processing, weight, and cost advantages.1Dissado L.A. Fothergill J.C. Electrical Degradation and Breakdown in Polymers. IET, 1992Crossref Google Scholar,2Tanaka T. Okamoto T. Nakanishi K. Miyamoto T. Aging and related phenomena in modern electric-power systems.IEEE Trans. Dielectr. Electr. Insul. 1993; 28: 826-844Crossref Scopus (37) Google Scholar Electrical treeing is recognized as one of the main failure mechanisms for polymer insulation under high electric fields.3Eichhorn R.M. Treeing in solid extruded electrical insulation.IEEE Trans. Dielectr. Electr. Insul. 1977; 1: 2-18Crossref Scopus (180) Google Scholar, 4Garton A. Bamji S. 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Insul. 1997; 4: 259-279Crossref Scopus (112) Google Scholar Driven by continuous partial discharges, the resulting dendritic microchannels grow progressively in the polymer and yield electron avalanches and impulse thermal or impulse mechanical breakdown depending on the physical and chemical nature of the polymer.8Zakrevskii V.A. Sudar N.T. Zaopo A. Dubitsky Y.A. Mechanism of electrical degradation and breakdown of insulating polymers.J. Appl. Phys. 2003; 93: 2135Crossref Scopus (95) Google Scholar, 9Lv Z. Rowland S.M. Chen S. Zheng H. Iddrissu I. Evolution of partial discharges during early tree propagation in epoxy resin.IEEE Trans. Dielect. Electr. Insul. 2017; 24: 2995-3003Crossref Scopus (45) Google Scholar, 10Chen X. Xu Y. Cao X. Dodd S.J. Dissado L.A. Effect of tree channel conductivity on electrical tree shape and breakdown in XLPE cable insulation samples.IEEE Trans. Dielectr. Electr. Insul. 2011; 18: 847-860Crossref Scopus (127) Google Scholar, 11Pallon L.K.H. Nilsson F. Yu S. Liu D. Diaz A. Holler M. Chen X.R. Gubanski S. Hedenqvist M.S. Olsson R.T. et al.Three-dimensional nanometer features of direct current electrical trees in low-density polyethylene.Nano Lett. 2017; 17: 1402-1408Crossref PubMed Scopus (34) Google Scholar It is generally accepted that the maximum electrical field that polymers can withstand is correlated with the onset of electrical treeing. While there has been extensive research and development in impeding electrical treeing by using a variety of organic and inorganic additives, including improvement of tree initiation voltage by voltage stabilizers,12Jarvid M. Johansson A. Kroon R. Bjuggren J.M. Wutzel H. Englund V. Gubanski S. Andersson M.R. Müller C. A new application area for fullerenes: voltage stabilizers for power cable insulation.Adv. Mater. 2015; 27: 897-902Crossref PubMed Scopus (85) Google Scholar, 13Wutzel H. Jarvid M. Bjuggren J.M. Johansson A. Englund V. Gubanski S. Andersson M.R. Thioxanthone derivatives as stabilizers against electrical breakdown in cross-linked polyethylene for high voltage cable applications.Polym. Degrad. Stab. 2015; 112: 63-69Crossref Scopus (45) Google Scholar, 14Jarvid M. Johansson A. Englund V. Lundin A. Gubanski S. Müller C. Andersson M.R. High electron affinity: a guiding criterion for voltage stabilizer design.J. Mater. Chem. A. 2015; 3: 7273-7286Crossref Google Scholar obstruction of tree path by nanofillers,15Danikas M. Tanaka T. Nanocomposites—a review of electrical treeing and breakdown.IEEE Electr. Insul. Mag. 2009; 25: 19-25Crossref Scopus (242) Google Scholar and inhibition of tree propagation by chemical absorption of degradation products,16Yang Y. Hu J. He J. Mesoporous nano-silica serves as the degradation inhibitor in polymer dielectrics.Sci. Rep. 2016; 6: 28749Crossref PubMed Scopus (18) Google Scholar very few studies have investigated the repair of damaged areas after the formation of electrical trees. For example, the introduction of a large amount of liquids into silicon gels was found to facilitate the disappearance of the tree structures.17Salvatierra L.M. Kovalevski L.I. Dammig Quina P.L. Irurzun I.M. Mola E.E. Dodd S.J. Dissado L.A. Self-healing during electrical treeing: a feature of the two-phase liquid-solid nature of silicone gels.IEEE Trans. Dielectr. Electr. Insul. 2016; 23: 757-767Crossref Scopus (17) Google Scholar Silica nanoparticles have been surface-functionalized with the crosslinkable groups with the aim to repair voids in the polymer exposed to corona discharge.18Nelson, J.K., Benicewicz, B., Rungta, A. and Schadler, L.S. (2014). US Patent 8 796 372.Google Scholar Research focusing on the performance recovery of ultrathin film (of nanometer-scale thickness) after breakdown has been reported.19Huang W. Besar K. Zhang Y. Yang S. Wiedman G. Liu Y. Guo W. Song J. Hemker K. Hristova K. et al.A high-capacitance salt-free dielectric for self-healable, printable, and flexible organic field effect transistors and chemical sensor.Adv. Funct. Mater. 2015; 25: 3745-3755Crossref PubMed Scopus (103) Google Scholar, 20Dumas C. El Zein R. Dallaporta H. Charrier A.M. Autonomic self-healing lipid monolayer: a new class of ultrathin dielectric.Langmuir. 2011; 27: 13643-13647Crossref PubMed Scopus (21) Google Scholar, 21Lu C.-C. Lin Y.-C. Yeh C.-H. Huang J.-C. Chiu P.-W. High mobility flexible graphene field-effect transistors with self-healing gate dielectrics.ACS Nano. 2012; 6: 4469-4474Crossref PubMed Scopus (161) Google Scholar However, none of existing studies have restored the electrical insulating performance (e.g., dielectric strength and electrical resistance) of bulk (microscale and larger) thermoset dielectrics after electrical tree damage. The latest breakthrough involves the utilization of the superparamagnetic nanoparticles to fix the electrical tree channels and restore the insulating properties of a thermoplastic polymer by using magnetic heating.22Yang Y. He J. Li Q. Gao L. Hu J. Zeng R. Qin J. Wang S.X. Wang Q. Self-healing of electrical damage in polymers using superparamagnetic nanoparticles.Nat. Nanotechnol. 2019; 14: 151-155Crossref PubMed Scopus (128) Google Scholar In this work, we employ the microcapsule-based approach23White S.R. Sottos N.R. Geubelle P.H. Moore J.S. Kessler M.R. Sriram S.R. Brown E.N. Viswanathan S. Autonomic healing of polymer composites.Nature. 2001; 409: 794-797Crossref PubMed Scopus (3618) Google Scholar, 24Cho S.H. Andersson H.M. White S.R. Sottos N.R. Braun P.V. Polydimethylsiloxane-based self-healing materials.Adv. Mater. 2006; 18: 997-1000Crossref Scopus (446) Google Scholar, 25Caruso M.M. Blaiszik B.J. Jin H. Schelkopf S.R. Stradley D.S. Sottos N.R. White S.R. Moore J.S. Robust, double-walled microcapsules for self-healing polymeric materials.ACS Appl. Mater. Interfaces. 2010; 2: 1195-1199Crossref PubMed Scopus (192) Google Scholar, 26Kang S. Baginska M. White S.R. Sottos N.R. Core-shell polymeric microcapsules with superior thermal and solvent stability.ACS Appl. Mater. Interfaces. 2015; 7: 10952-10956Crossref PubMed Scopus (77) Google Scholar, 27Chevalier Y. Bolzinger M.-A. Emulsions stabilized with solid nanoparticles: Pickering emulsions.Colloids Surf. A Physicochem. Eng. Asp. 2013; 439: 23-34Crossref Scopus (1149) Google Scholar, 28Chen T. Colver P.J. Bon S.A.F. Organic-inorganic hybrid hollow spheres prepared from TiO2-stabilized pickering emulsion polymerization.Adv. Mater. 2007; 19: 2286-2289Crossref Scopus (360) Google Scholar and capitalize on the electroluminescence29Shimizu N. Katsukawa H. Miyauchi M. Kosaki M. Horii K. The space charge behavior and luminescence phenomena in polymers at 77 K.IEEE Trans. Dielectr. Electr. Insul. 1979; EI-14: 256-263Crossref Scopus (85) Google Scholar, 30Bamji S.S. Bulinski A.T. Densley R.J. Degradation of polymeric insulation due to photoemission caused by high electric fields.IEEE Trans. Dielectr. Electr. Insul. 1989; 24: 91-98Crossref Scopus (90) Google Scholar, 31Teyssedre G. Laurent C. Montanari G.C. Palmieri F. See A. Dissado L.A. Fothergill J.C. Charge distribution and electroluminescence in cross-linked polyethylene under DC field.J. Phys. D. Appl. Phys. 2001; 34: 2830-2844Crossref Scopus (88) Google Scholar generated in situ during electrical treeing for the curing of healing agents released by microcapsules. As a result, the healing of the electrical tree damage is accomplished under ambient conditions and does not require any external stimuli such as heating or irradiation to initiate the healing process. Moreover, by designing the polymer composite with dielectric inhomogeneity, the propagation of electrical trees is guided toward the microcapsules, which enables the use of a relatively small concentration of the microcapsules. This design minimizes the detrimental impact of liquid curing agents on the electrical insulating properties of the polymer matrix. The initiation, growth, and healing of the electrical trees have been monitored by using electron microscopy, three-dimensional computed tomography (3D CT), and optical microscopy. The recovery of the dielectric and electrical properties of the polymer upon healing and repeatable healing ability of the strategy have been systematically studied by using a combined simulation and experimental approach. Figure 1A illustrates the healing of electrical trees using the microcapsules in the polymer. After electrical tree inception, the propagation of the electrical tree is attracted by the embedded microcapsules (the mechanism is discussed below), and the microcapsule nearest to the tree initiation site is hit and ruptured by the forefront branch of trees. The encapsulated healing agents are liberated into the hollow channels and fill up the void volume created by treeing. The healing agents are then solidified by the electroluminescence to mend the channels. Accordingly, the dielectric strength and electrical resistivity of the polymer are regained. The key to this design is the utilization of the luminescence yielded in situ during electrical treeing as a stimulus for the self-healing process, which permits autonomic healing under ambient conditions without any external energy. The UV light is thought to arise from the recombination between charge carriers injected from the electrodes and the impact excitation of polymer molecules by accelerated electrons.6Shimizu N. Laurent C. Electrical tree initiation.IEEE Trans. Dielectr. Electr. Insul. 1998; 5: 651-659Crossref Scopus (216) Google Scholar Figure S2A shows the electroluminescence spectrum of epoxy, a typical dielectric polymer, under the applied electric fields, which confirms the emission of UV light centered at 334, 351, and 374 nm during the tree propagation. Figure 1B outlines the preparation of the poly(urea-formaldehyde) (PFU)/TiO2 hybrid core-shell microcapsules via one-step Pickering emulsion polymerization27Chevalier Y. Bolzinger M.-A. Emulsions stabilized with solid nanoparticles: Pickering emulsions.Colloids Surf. A Physicochem. Eng. Asp. 2013; 439: 23-34Crossref Scopus (1149) Google Scholar,28Chen T. Colver P.J. Bon S.A.F. Organic-inorganic hybrid hollow spheres prepared from TiO2-stabilized pickering emulsion polymerization.Adv. Mater. 2007; 19: 2286-2289Crossref Scopus (360) Google Scholar,32Binks B.P. Lumsdon S.O. Influence of particle wettability on the type and stability of surfactant-free emulsions.Langmuir. 2000; 16: 8622-8631Crossref Scopus (1154) Google Scholar using the surface-modified TiO2 nanoparticles as stabilizers (Figures S3–S5). As epoxy resin (with a glass transition temperature of around 62°C) is used in this work to be repaired, bisphenol A epoxy acrylate is chosen as the main component of the photoresponsive healing agents to ensure compatibility and low shrinkage with the matrix when healed. In addition, a reactive monomer is added to the healing agents to reduce viscosity and promote the filling of the tree channels with the healing agents. Among various liquid monomers that have been tested, such as trimethylolhexane triacylate (TMPTA), 1,6-hexanediol diacrylate (HDDA), and tripropylene glycol diacrylate (TPGDA), TMPTA is selected because it gives rise to the highest breakdown strength and the lowest electrical conductivity after crosslinking, which are even superior to the epoxy matrix (Figure S8). 1-Hydroxycyclohexyl phenyl ketone is included as the photocatalyst in the healing agents. The light-induced crosslinking reaction of the healing agent is shown in Figure 2A. TiO2 is incorporated in the shell of the microspheres to protect the encapsulated healing agents from electroluminescence discharged during treeing, thus allowing the healing agents to remain as liquid in the microcapsules until they are released from the ruptured microcapsules. As shown in the UV-visible (UV-vis) absorption spectrum (Figure 2B), PUF microcapsules (Figure 2C) with 20 wt % TiO2 exhibit a strong absorption band from 200 to 400 nm with the highest absorbance of 0.97, indicating that most of light in this range is absorbed by the composite shell. Indeed, as demonstrated in the reference experiments in which the microcapsules are directly irradiated with a UV lamp (1.2 W), the solidification time of the healing agents protected by the hybrid shells has been increased to more than five times that of the agents encompassed by pure PUF shells (Figure S3). The chemical structures of the prepared hybrid microcapsules have been verified by Fourier transform infrared and X-ray fluorescence spectroscopy (Figures S4 and S5). Scanning electron microscopy (SEM) imaging of the microcapsules reveals their core-shell structure (Figures 2D and 2E). It is found that the prepared microcapsules possess a diameter of 190 ± 70 μm (Figure S7A) with a shell thickness of 4.9 ± 1.4 μm (Figure S7B). The TiO2 nanoparticles characterized by scanning transmission electron microscopy (STEM) are shown in Figure 2F. Considering the 20 wt % content of TiO2 in shells as confirmed by thermogravimetric analysis (Figure S5B), the density and diameter of titania, and shell thickness, the TiO2 density on the surface of the microcapsules can be calculated as 0.8–1.5 × 10−12 g/μm3. Both the simulation and experimental results reveal that the deterioration of the dielectric breakdown strength and the electrical resistivity of the epoxy with the incorporation of the microcapsules can be minimized when the microcapsule content is ≤5 vol % (see Supplemental Experimental Procedures and Figures S9–S12). Furthermore, the SiO2 nanoparticles with a diameter of 20 ± 10 nm are introduced to the healing agents to lessen the negative impact of the microcapsule on the electrical insulating properties of epoxy (Figures S14–S16). It is found that the dielectric breakdown strength analyzed by Weibull statistics, which corresponds to a 63.2% probability of failure, is reduced from 41.5 kV/mm of pristine epoxy to 35.6 kV/mm of the epoxy composite containing 5 vol % of the microcapsules without SiO2 nanoparticles. The addition of 2.5 vol % SiO2 nanoparticles in the microcapsules increases the Weibull breakdown strength of the epoxy composite to 37.4 kV/mm (Figure S14A). Concurrently, the electrical resistivity of the composites is increased from 1.90 × 1013 to 2.61 × 1013 Ω∙m (Figure S14B). The resistivity of pure epoxy without any microcapsules is 5.10 × 1013 Ω∙m. The electrical degradation experiments are carried out by using a typical needle-plate electrode configuration (Figure S17), whereby the steel needle electrode with tip radius of 5 μm is embedded in the sample to simulate the material defect. The insulation distance between the needle tip and the plate electrode is 2.0 mm. A voltage is applied on the electrode and ramped up until an electrical tree is initiated at the needle tip. As shown in Figure S18, the characteristic tree inception voltage evaluated by the Weibull distribution function is 11.56 kV for the polymer composite, which is very close to that of pristine epoxy (i.e., 11.60 kV). Unlike the dielectric breakdown strength of the bulk materials, which is sensitive to the full-volume electrical field distortion induced by the microcapsules containing liquid components, the tree inception voltage of the samples is determined by the local material properties of the needle-tip region. The incorporated microcapsules are usually hundreds of micrometers away from the needle tip and thus have negligible influence on the local electrical field or the tree inception voltage of the tip region. After tree inception, the samples are subjected to electrical treeing whereby periodically altered voltages are applied to form electrical treeing cycles. In our accelerated electrical treeing tests, we deliberately set the transient overvoltage (V2) at 10 kV, which is almost one and a half times that of a normal voltage (V1) applied on the dielectric materials. For each cycle, V2 and V1 are applied for 2 min and 2 h on the samples, respectively (Figure S19). It is known that electrical transients in the form of voltage surges always exist in electrical transmission and distribution systems. Although voltage spikes normally last less than 20 ms, overvoltage significantly accelerates the electrical treeing of dielectric materials, especially in the presence of electrical trees, and triggers electrical breakdown of power apparatus and electronic devices. A variety of techniques including electron microscopy, 3D CT, and optical microscopy have been used to monitor the formation, propagation, and healing of the electrical trees. The holes (with the typical size of 0.5–1 μm) punctured by electrical trees on the microcapsule shells are clearly seen in colored SEM (Figure 3A). It is understood that the gases generated by degradation of the polymer during the treeing process are released into the microcapsules and squeeze the healing agents into the hollow tree channels, leading to healing of the tree branches (Figure 3B). With the addition of coumarin-6 as a fluorescent dye in the healing agent, the filling of the electrical tree channels by the healing agent released from the ruptured microcapsules is clearly evidenced in the fluorescence microscopic image (Figure 3C). The repairing of the tree damage is also substantiated in the optical microscopic images (Figure 3D and Video S1) and 3D CT results (Figures 3E and 3F; Videos S2 and S3). As estimated from 3D CT (Figures 3E and 3F), more than 95% of the tree channels have been filled with the healing agents and subsequently mended by the photoinitiated crosslinking reaction. The evidence of the crosslinking reaction is provided by a substantial decrease in the relative intensity at 334 nm, the characteristic absorption peak of the healing agent during curing, in the UV-vis spectrum of the composite (Figure S20). eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiJmOTVmM2M2OTY2MGNiNzgzMjc5OTU0MjBkODI2ZTkyNCIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjc5MDMzMTQ5fQ.nc1qHvQKREWMylcJBuK4w9vq3WZ32cl_MQsyD6oVLUx6jbX1Qu7ymQKqu7wVMyLXGXj8R5pgMF11fYgXu_khuwoENLforaA6KiWiF47F9W3usoWXbtHELShKU8jdbkj4wfpEeTGgB_DcOuTDKcFkVgBrLq8YlomJyUF4imJVRjtg1jByLMXXjSv23-henAFLjd0XCeP-f8YIwKCnys-mMsyThHsAS1CWAYPX3Ei-Otq9NC50p4CiTSY0h4Qv3X9GPtXOq67VnkTwJvNrKLscfvFVX0iVZNB6-QClL9phpNAWzYmkcY0kaY19fDCIx2KWvds8_trsZ3roB_eObEh2WQ Download .mp4 (5.46 MB) Help with .mp4 files Video S1. Healing Process Characterized by Optical Microscopy eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiJlN2FjYmNkMzlhMTdlZTNhY2JmN2IyYWI1MjUzZjExMiIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjc5MDMzMTQ5fQ.geYHP37yoKuopOG0stPXEWEaGGvHl6cfufRf2pLD0lGtlTTvVpmOTYNJBv3Z_qiiSR2YdMzAbxpdRdEHUcO-M3byWfhK-9UWWyUMlsUNgkjmueuq02tIJlzTWJjX60RVW5mI7SPwxkhLjImtm85Z3US8InD3c0X2OWWh9Xhu-8uddu4PenRtCGGrcCYXS9deX8-DHagvOylfd5c3svys-9WgRD4-q-SP50xVgbAkkBtxiG_3pOahnxUhLp5Pqvj0q2XyOKpWtuIv-p0GFnRFImeDqh8gNLociTZU3IRDsYMdsGYGKpbdjVCcm8LKZcopgUT5gDYs5DN-bFttOfmB2A Download .mp4 (1.26 MB) Help with .mp4 files Video S2. Pristine Tree Channels Characterized by Micro-CT eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiIyMDhkMGU4OTdiNzBiNWIzNzAzNmQwN2M5Y2VjMDA0OCIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjc5MDMzMTUwfQ.q2hNhX1IHvqZbN3Zss3RxUqEF-cQkQwNXFs5oWpnd_J3j_XPs5frQQ8KbTI-r5XFO7Z2RP4nYIIvb4AFBTYxhs3gohWgh1dRsu7ImZp39JgZGFa8eGj6EBPiWGT4Upjq6jSDRIbNPYORvfMQOTOFK9TkysAtJlk-49N_eTVQXeUkwTYV2IXOg88O-G9C9wOWPh4xqIQvbZgW8brtOqt_qiGoioBSCYA-3iaAxhnW-EQXYMvrFZrC5QVr3CqCUkJ8Ga4YLmRCtIyupvPkIgbdKRolsV4QG-KAsuSxVOZKL-shUiwn8UBoYXzNREFd5S8TFiPOrGY7Nm9cVTPnGchEIw Download .mp4 (0.44 MB) Help with .mp4 files Video S3. Healed Tree Cracks Characterized by Micro-CT Figure 4A plots the direct-current electrical resistance of the epoxy resin over the electrical treeing cycles (Figure S21), accompanied by images recorded simultaneously by an optical microscope with a CCD camera. While electrical trees continuously propagate with the cycle number, a substantial decrease in electrical resistance (i.e., about 80% that of the pristine epoxy) is detected when the lengths of the trees are more than 500 μm. Once the length of tree branches reaches about 1,000 μm, the electrical resistance decreases rapidly and leads to catastrophic breakdown of epoxy (Figure 4A). On the other hand, as the electrical treeing cycles increase, the electrical resistance of the composite decreases initially, owing to the growth of electrical trees, then gradually increases to a stable value of 2 × 1013 Ω, which is comparable with that of the composite before the inception of treeing (i.e., 2.61 × 1013 Ω) and a strong indication of restored insulation properties. The optical images of the dielectric composites shown in Figure 4B taken during the electrical treeing cycles corroborate the efficient healing of the damaged area. The electrical resistivity of the composites can be recovered to a stable value close to the pristine samples by another eight electrical treeing cycles after triggering of the healing process, indicating that the curing time of the healing agent is about 16 h. To further evaluate the electrical insulating performance upon healing, we studied the re-treeing inception voltage of the healed polymer, i.e., the voltage required to initialize the second electrical tree at a healed defect location. It is found that the re-treeing inception voltage of the healed polymer is about 30% greater than the voltage of the first tree inception, i.e., 15 versus 11.56 kV (Figure 4C), which agrees well with the simulation results calculated from a modified Wiesmann-Zeller model33Wiesmann H.J. Zeller H.R. A fractal model of dielectric breakdown and prebreakdown in solid dielectrics.J. Appl. Phys. 1986; 60: 1770Crossref Scopus (238) Google Scholar,34Niemeyer L. Pietronero L. Wiesmann H.J. Fractal dimension of dielectric breakdown.Phys. Rev. Lett. 1984; 52: 1033-1036Crossref Scopus (1251) Google Scholar (Figure S22). The improved re-treeing inception voltage in the healed sample is attributed to superior intrinsic breakdown strength of the healing agent (Figure S8). As shown in Figure 4D, the Weibull breakdown voltages of the healed and the pristine polymers characterized using the needle-plate electrode configuration are 35.2 and 33.5 kV, respectively. Comparatively, the initial treeing in neat epoxy leads to a large decrease in the Weibull breakdown strength to 32.0 kV. Because of the improved local insulating properties of the healed region, the electrical trees produced from re-treeing tend to develop new trajectories rather than grow along the original tree paths, as evident in the optical images (Figures 4E and 4F) and further verified by the computational simulations (Figure S25). There commonly exist many defects in practical dielectric materials, which are capable of initiating the formation of multiple electrical trees with different inception voltages (Figure S27). As shown in Figures S28A–S28B, we simulated the polymer composite comprising multiple microcapsules and microdefects (Video S4). Efficient healing is observed even when two electrical trees are consecutively initiated at different locations with varied inception voltages (Figure S29), which is also ascribed to the enhanced local insulating properties upon healing. eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiI2NDdkY2M0MTUxNzA2MWQwZDFhMmQ2MGQ4NjQ0MjA1OSIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjc5MDMzMTUwfQ.Ra580eOrxP29OWnu9tbvrAgZ3Y4209n0DBTBAZnUbHa7LoYv7p8mUSUcNU9YtOkrLNXpiJKUbRHdXx06saTzZLrMS459rHRHjoxW8PEKxY__lsgThXgCfO-FaIQCjmiezd3dkoFMP8RrwGLdYK-exItsA-zsegObf7nf-gJ1RJ6mSmgGV4ch2E4EF4O2BhVWzEmZVHcruArN7gmPFYaSlG711ho7e-AIGjhmApdTQkLLSE0Z6oDHHco_V_pQwny6V6YwosqFk9Em8N_fEG311S5NRaosP2lLmWqQOqCfbXTo6pSBde6MAOm2XimqlWTBzh6HvaLTMd_4xqKUDv-Phg Download .mp4 (3.17 MB) Help with .mp4 files Video S4. Simulation of Tree Healing with Multiple Defects Another crucial design of this self-healing approach is to introduce the microcapsules with a higher dielectric constant than the epoxy matrix (i.e., 10.6 versus 4, Table S2). Accordingly, the propagation of the electrical tree is guided by the nearby microcapsule to rupture the microcapsule in the shortest possible pathway. This design permits the use of a much smaller concentration of the microcapsules than those typically used in the healing of mechanical damage in polymers (i.e., 10–30 vol %).23White S.R. Sottos N.R. Geubelle P.H. Moore J.S. Kessler M.R. Sriram S.R. Brown E.N. Viswanathan S. Autonomic healing of polymer composites.Nature. 2001; 409: 794-797Crossref PubMed Scopus (3618) Google Scholar,35Cho S.H. White S.R. Braun P.V. Self-healing polymer coatings.Adv. Mater. 2009; 21: 645-649Crossref Scopus (621) Google Scholar, 36Jin H. Mangun C.L. Griffin A.S. Moore J.S. Sottos N.R. White S.R. Thermally stable autonomic healing in epoxy using a dual-microcapsule system.Adv. Mater. 2014; 26: 282-287Crossref PubMed Scopus (174) Google Scholar, 37Odom S.A. Tyler T.P. Caruso M.M. Ritchey J.A. Schulmerich M.V. Robinson S.J. Bhargava R. Sottos N.R. White S.R. Hersam M.C. et al.Autonomic restoration of electrical conductivity using polymer-stabilized carbon nanotube and graphene microcapsules.Appl. Phys. Lett. 2012; 101: 43106Crossref Scopus (50) Google Scholar, 38Patrick J.F. Robb M.J. Sottos N.R. Moore J.S. White S.R. Polymers with autonomous life-cycle control.Nature. 2016; 540: 363-370Crossref PubMed Scopus (258) Google Scholar, 39Blaiszik B.J. Kramer S.L.B. Olugebefola S.C. Moore J.S. Sottos N.R. White S.R. Self-healing polymers and composites.Annu. Rev. Mater. Res. 2010; 40: 179-211Crossref Scopus (1121) Google Scholar, 40Motornov M. Roiter Y. Tokarev I. Minko S. Stimuli-responsive nanoparticles, nanogels and capsules for integrated multifunctional intelligent systems.Prog. Polym. Sci. 2010; 35: 174-211Crossref Scopus (655) Google Scholar Indeed, the growth of the trees toward the microcapsules is experimentally evidenced in Figure 3D and Video S1. An improved planar simulation algorithm based on the Wiesmann-Zeller model has been developed to confirm the attraction of electrical tree trajectory by the microcapsules with a larger dielectric permittivity, as illustrated in Figure S25 and Video S5 (see Supplemental Experimental Procedures). eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiIyY2RiMDIwYTAyOWY0OWMwZjU3ODFiYjQwZTMyNDdhOSIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjc5MDMzMTUwfQ.R6zySUnVKQniWYWd4FqijtUDQ_l8BM2VffDSjlatPy8YneWOmJs8AOzCppVC666u3HdU1iy8RJ-4z69GVD3vxbsL9PIkyQS7vUp8---IxkC_brkp4fxt1u8J6fCwty4-C6koXw5VcwRIhHdKO-w1tCr1XDwZX1xSvDNbQktqOx075rbaZmHR98pvm1WQTgYZxSxPS_O9P1v3x4R5X5Df3cZKCyIMc4_guHvKr3ctu7_3w034cGLoW-RSDnfdgGJT4yMUFOUXP9V6rXD1OL_j1G1Hb0WhZdslfny9ha4r-3VEqsFNcKggqu-FAeACCzEnI7QIfB1hoJbpLYIUHH187Q Download .mp4 (1.75 MB) Help with .mp4 files Video S5. Simulation of Tree Trajectory Attracted by the Microcapsule with High Dielectric Permittivity The contrast in the dielectric constants of the microcapsules and epoxy leads to the accumulation of polarization charge at their interfaces, which alters the distribution of local electric fields and results in a higher local electric field between the needle and the capsules. Since the probability of developing new tree branches is proportional to the magnitude of the local electric field, the inclusion with a higher dielectric constant exhibits the attraction effect. The inclusions with a higher electrical conductivity also attract the tree trajectories owing to the accumulation of conduction charges at the interfaces. On the other hand, the breakdown strength only has short-ranged influence on the tree trajectories because it does not affect the distribution of local electric fields. The inclusions with high breakdown strength exclude the tree trajectories when they are in direct contact. Therefore, the long-distance trajectory of electrical trees is influenced by polarization charges (dielectric constant) and conduction charges (electrical conductivity) around the microcapsules. While the healing capsules with low breakdown strength, high conductivity, and high dielectric constant are beneficial in attracting the propagation of the electrical tree, the opposite properties are needed to minimize the negative impact of the inclusions on the electrical properties of pristine polymer and warrant the desirable electrical insulating properties after healing. Hence, these contradictory requirements must be carefully balanced in the design of the self-healing polymers via optimization of the content of the included microcapsules. With the assumption of homogeneous dispersion of the microcapsules in the matrix, the distribution of the local electric field in the composite is simulated by COMSOL Multiphysics 5.2 (see Supplemental Experimental Procedures). Since the inclusions having a higher dielectric constant and a larger electrical conductivity (i.e., 7.7 × 106 versus 1.0 × 1013 Ω∙m of the matrix), the local electrical field around the microcapsule is enhanced (Figure S9), leading to attraction of electrical tree propagation. This effect is substantiated by the simulations based on the well-known Wiesmann-Zeller model (see Supplemental Experimental Procedures). As shown in Figure S10, the distortion of the local electric fields, which gives rise to accelerated electrical treeing, increases radically when the content of the microcapsule is above 5 vol %, which is consistent with the experimental results summarized in Figure S14. It is estimated that the tree channels are able to find the microcapsules before the lengths of the trees reach ∼370 μm for the polymer containing 5 vol % microcapsules (see Supplemental Experimental Procedures). Figure S12 presents the simulated relationship between the microcapsule content and tree lengths at various treeing angles in the composites. When the tree size exceeds a certain threshold, i.e., 500 ± 100 μm as obtained based on the experimental observations using the experimental parameters reported in this study, the electrical insulating properties of the dielectric materials are unable to be completely recovered owing to the formation of macro-sized conducting carbonized trees caused by partial discharge. This is because the healing agent cannot remove the conductive degradation by-products inside the tree channel even though the hollow channels can be fully filled. For the applications in glassy dielectrics under low electric fields, more carbonization may occur,10Chen X. Xu Y. Cao X. Dodd S.J. Dissado L.A. Effect of tree channel conductivity on electrical tree shape and breakdown in XLPE cable insulation samples.IEEE Trans. Dielectr. Electr. Insul. 2011; 18: 847-860Crossref Scopus (127) Google Scholar,41Champion J.V. Dodd S.J. Simulation of partial discharges in conducting and non-conducting electrical tree structures.J. Phys. D. Appl. Phys. 2001; 34: 1235-1242Crossref Scopus (71) Google Scholar and the maximum tree length that can be healed using this method may vary. The size and concentration of microcapsules need to be redesigned based on the calculation method in Supplemental Experimental Procedures so that the tree channels are able to find the microcapsules before serious carbonization occurs. The restoration and even enhancement of electrical insulating properties by autonomous self-healing of electrical trees would yield a prolonged lifetime of polymer dielectrics. The lifetimes of pristine epoxy and self-healing composites were evaluated through an electrical treeing and healing process under various applied voltages until failure, and the time to failure was recorded (Figure S31A). Based on the fluctuation model,42Hill R.M. Dissado L.A. Theoretical basis for the statistics of dielectric-breakdown.J. Phys. C. 1983; 16: 2145Crossref Scopus (68) Google Scholar the cumulative probability of breakdown with tree defects in the dielectric isPF(Ea,t)=1−exp{−νNt(β)1/m[(Ea−E1)/E0]1/m},(Equation 1) where the frequency ν is an intrinsic factor, N is the number of clusters at risk (equal to the system volume divided by the cluster volume), t is the time, β is a geometrical field enhancement factor of order unity, E1 and E0 are experimentally determinable parameters, Ea is the applied field, and m is an index measuring the degree of connection between the motions in different clusters (see Supplemental Experimental Procedures for details). If we assume that breakdown occurs at a constant value of cumulative probability, then−νNt(β)1/m(Ea−E1E0)1m=constant(Equation 2) andlnt+kln(Ea−E1)=constant,(Equation 3) where k is an experimentally determinable parameter. The lifetime of the self-healing composite and pristine epoxy is fitted as shown in Figures S31B and S31C. The electrical fields are replaced by voltage, since the former cannot be easily obtained. It is evident from Figures S31B and S31C that the experimental results correspond well with the model. The self-healing polymer with the included microcapsules displays a higher ability to withstand electrical trees, since it has a higher threshold voltage U1 than the pristine polymer (7.5 versus 5.8 kV). The threshold voltage U1 can be considered as a voltage below which the trees fail to be developed. Furthermore, the larger value of parameter k of the self-healing composite is indicative of its excellent insulating performance and suggests a greater lifetime of the dielectrics. In conclusion, the microcapsule approach has been successfully utilized to mend the electrical trees in polymer dielectrics under ambient conditions. Different from the current approaches using microcapsules to repair mechanical cracks in polymers, the initiation of the healing chemistry reported herein does not require any external irradiation or elevated temperatures but rather is based on the electroluminescence generated in situ during electrical treeing. This leads to autonomous self-healing properties of the polymer triggered by electrical treeing degradation. In particular, the microcapsules are engineered with a higher dielectric constant relative to the polymer to direct the propagation of the electrical trees. The attraction of electrical treeing by the microcapsules impedes the development of tree channels with large open volumes and ensures the self-healing ability of the polymer with a relatively low content of the microcapsules. Remarkably, it is found experimentally and confirmed by simulations that the dielectric properties of the repaired area are greatly improved, which endow the polymer with excellent repeatable healing capability. As the microcapsule-based self-healing approach has proved effective in repairing mechanical damage in polymers, and electroluminescence is a universal phenomenon in typical dielectric polymers, we trust that this approach will be adopted as a general approach to healing of electrical damage in a variety of polymers. Note that to utilize this approach in thermoplastic polymers, e.g., polypropylene, polyethylene and other thermoplastic polyolefins that are usually processed by melt extrusion, the structures of the microcapsules need to be specifically designed so that the microcapsules can survive under conditions of high temperature and strong shear force during polymer processing. We expect this work to offer new opportunities to develop an entirely different class of polymer dielectrics with prolonged lifetime and improved reliability for electrical power systems and advanced electronics.