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C: Energy Conversion and Storage; Energy and Charge Transport
Multistep and Multiscale Electron Trapping for High-Efficiency Modulation of Electrical Degradation in Polymer Dielectrics Jingang Su, Boxue Du, Tao Han, Zhonglei Li, Meng Xiao, and Jin Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00349 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019
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The Journal of Physical Chemistry
Multistep and Multiscale Electron Trapping for High-Efficiency Modulation of Electrical Degradation in Polymer Dielectrics Jingang Su, Boxue Du*, Tao Han, Zhonglei Li*, Meng Xiao and Jin Li* Key Laboratory of Smart Grid of Education Ministry, School of Electrical and Information Engineering, Tianjin University, Tianjin, 300072, China ABSTRACT: Modulating the electron or hole trapping process is crucial for enhancing electrical degradation resistance in polymer dielectrics in modern electrical and electronic equipment. Using density functional theory (DFT), we demonstrate for the first time that synergistic antioxidants can introduce multistep and multiscale charge trapping sites to highly efficiently scatter the electron energy and can act as voltage stabilizers for polymer dielectrics to enhance the electrical degradation durability. Important parameters, such as the energy level, density of states (DOS) and 3D electrostatic potential, are calculated to identify the charge modulating mechanisms. Moreover, it is demonstrated that the electronic structures of synergistic antioxidant by-products can facilitate the realization of long-term charge modulation. Synergistic antioxidants with sought-after properties, which act as high-efficiency and high-versatility voltage stabilizers, offer a novel and important reference for designing polymer dielectrics that have excellent electrical degradation resistance.
1. INTRODUCTION Enhancing electrical degradation resistance for polymer dielectrics is an urgent requirement for raising the upper voltage limit in current harsh-environment electrical and electronic applications. The electrical degradation of polymer dielectrics is dramatically affected by the electronic states (or electronic structures), which are localized states or trap states between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of polymer materials. 1 The localizations and lifetimes of electronic states could influence charge transportation processes and the chemical reactivity by re-distributing molecular charge and regulating electron and hole migration. 2, 3 In polymer nanocomposites, the electronic states can enhance the electrical properties by reducing the injected (excess) electron mobility, which is caused by inelastic scattering or charge trapping of interfacial regions between the polymers and nanoparticles. 4 The trapped charge releases energy in radiated or non-radiated pattern to generate photons or hot electrons, which are less destructive to molecular chains. 5 However, the dispersion of inorganic additives far from satisfies the insulation requirements in electrical and electronic equipment, 6 which prevents large-scale application. Antioxidants have demonstrated high potential in a range of application areas, such as solvents, adhesives, plastics, rubbers and fabrics, and in polymer dielectrics that are used in dielectric capacitors, electric wires, composite insulators, and high-voltage direct current (HVDC) and high-voltage alternating current (HVAC) cables in advanced energy storage, power electronics and power transmission. 7-11 Another valuable property of antioxidants is their ability to modulate charge transportation via trapping electrons or holes that are injected from the electrode or excited by a strong electric field. As antioxidants can inhibit the long-term electrical aging of polymers with electric field, especially to enhance electrical
tree degradation resistance, they are employed as voltage stabilizers. Various organic additives, such as phenolic, sulfurcontaining and phosphorus-containing antioxidants, contain polycyclic aromatic or benzophenone-like structures. They have lower barriers for accepting electrons into their lowest unoccupied molecular orbital than polyolefin chain compounds, which endows these conjugated benzene ring molecules with a stronger electron trapping capability. 12 To trap electrons and decrease the average free path over which the charges were accelerated, organic additives with lower LUMO energy levels were introduced into the matrix material. 13 It was reported that organic additives substantially enhanced the dielectric strength of the polymers. The organic additives with short (methyl) side chains that are linked to the benzyl core via an ester or tertiary amine group showed the highest improvement in dielectric strength. 14 [6, 6]-phenyl-C61butyric acid methyl ester (PCBM) with a solubilizing side chain substantially enhanced the processability from common organic solvents, which could scavenge high-energy electrons to degrade polyethylene chains. 15 The synergistic antioxidants were more effective in improving the thermooxidative resistance, as one antioxidant was effectively regenerated by another antioxidant or the mechanisms of the antioxidants differed. 16, 17 Phenolic antioxidants could serve as voltage stabilizers by donating a hydrogen to ROO•, modifying ROO• to ROOH and interrupting the oxidation chain reaction. 18 The sulfurcontaining or phosphorus-containing antioxidant had a peroxide decomposition function, by modifying ROOH into chemically stable ROH. The development of synergistic antioxidants is attracting increasing attention because of their
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potential use in electrical and electronic systems to increase the electrical degradation durability. 19, 20 However, the commercial viability of using synergistic antioxidants to restrain electrical degradation in electrical or electronic engineering has yet to be demonstrated. The current use of synergistic antioxidants in the polymer dielectrics of electrical or electronic equipment is a niche application at most. Therefore, for a complete description of the role of synergistic antioxidants in polymer dielectrics, it is important to understand the localizations and lifetimes of their electronic states and how the electronic states could be identified to realize long-term charge modulation. In this paper, we investigated the electrical degradation of polymer dielectrics by observing the electrical treeing process. 21, 22 The charge trap distribution behaviors were characterized via the surface potential decay (SPD) technique. 23, 24 To examine the charge-trapping process in the electrical degradation of polymer dielectrics, the synergistic antioxidants’ electronic structures were calculated via density functional theory (DFT). 12 For the first time, it was demonstrated that the synergistic antioxidants could act as efficient voltage stabilizers with multistep and multiscale charge trapping to highly efficiently modulate electrical degradation in polymer dielectrics.
2. EXPERIMENTS AND COMPUTATIONS 2.1 Materials. In this paper, pentaerythritol tetrakis(3-(3,5di-tert-butyl-4-hydroxyphenyl)propionate) and tris (2,4-ditertbutylphenyl) phosphite (AOA and AOD) were employed as the antioxidants; in combined application, they were synergistic antioxidants (Figure S1, Supporting Information). EPDM, which is widely applied as an insulating material in electric wires, composite insulators, and HVDC and HVAC cables, was employed as the polymer matrix. 21 In a twin-roll mill, the raw EPDM materials were mixed uniformly with antioxidants before the cross-linking agent was added. Then, the mixed compounds were hot-pressed in a stainless-steel mold to prepare the specimens. Details on the procedures for preparing the EPDM/antioxidant composite materials are provided in Supporting Information. 2.2 Electrical degradation characterization. Electrical treeing, which is an electrical degradation phenomenon in polymers, was accompanied by charge trapping and detrapping. 22 It was a complex process that included a physical structure change and a chemical reaction, which depended highly on the electronic states of the polymer dielectrics and additives. In this paper, electrical treeing was observed by stressing the samples on a needle-plane electrode system. Details on the electrical tree experiment are provided in Supporting Information. 2.3 SPD methods. Charge trapping and detrapping processes are closely related to the trap distribution in polymer dielectrics. 23, 24 Studying the charge dynamics was a convenient approach to investigate the trap characteristics. Via the SPD method, which was based on isothermal relaxation current (IRC) theory and was formulated by Simmons and Tam, 25 the trap energy level and trap density were obtained.
Details on the SPD experiment are provided in Supporting Information. 2.4 DFT calculations. Quantum chemical methods are considered useful approaches for the prediction of the electronic energy level, density of states (DOS) and 3D electrostatic potential distribution in molecules.26 DFT is suitable for application to large molecules in terms of accuracy and computation time and is based on the first-principles calculation and the solution of the basic Schrodinger equation to determine the wave functions.27 Molecular orbitals (MOs) are constructed by combining the atomic and hybrid orbitals from each atom of the molecule, which can be obtained via the self-consistent field (SCF) method. In this research, the B3LYP hybrid functional with the 6-31G (d) basis is selected, which has been successfully employed to study the electronic structure, DOS and 3D electrostatic potential in molecules.26, 28
3. RESULTS AND DISCUSSION 3.1 Electrical degradation behaviors. In a first set of experiments, the electrical degradation behaviors of polymer dielectrics are investigated via electrical treeing experiments in which a single antioxidant or synergistic antioxidants are used. Electrical treeing, which is an electrical degradation phenomenon in polymers, was accompanied by charge trapping and detrapping. 22 It was a complex process that included a physical structure change and a chemical reaction, which depended highly on the electronic states of the polymer dielectrics and additives. In EPDM, the electron affinity is negative; 29 hence, EPDM is electrophobic. With a negative voltage, the electrons are injected into polymers via the Schottky effect, and affected by Poole-Frenkel effect during their transportation process in polymers. 30 The excess electrons prefer to move to the low-density intermolecular regions with lower energy in the polymer matrix. The injected electrons are channeled into the free volume and submicrovoid spaces 31 and behave as quasi-free in directions that are parallel to the chains or along contours that skirt the polymer surfaces. The injected electrons that have insufficient energy can be quickly trapped after a few scatterings. The energy that evolves from a trapping event may be transferred to another electron and cause it to become a hot electron. The hot electron can continue being accelerated in the free volume until sufficient energy has been obtained for dissociating a molecule into free radicals or trapping in new traps. 32 The absorbed energy of hot electrons from a single trapping event depends on the obtained energy of previous electrons in the free volume. With positive voltage, the electrons are extracted from the bulk of polymers and leave holes in the samples. In contrast to electron transportation, hole transportation can take place only via electron vacancies that are in or closely associated with the valence bands of the polymer molecules, which are confined to the intrachain of the polymers. Long-range transport requires interchain hole transformation, which must overcome the hopping barrier between the closely adjacent polymer chains. In the hole hopping process, the electrons are extracted from one polymer chain and transported to another chain in the free volume. Based on the electron and hole transport processes, it DOI: ×××× ×××× J. Phys. Chem. C×××× ××××
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is proposed that regulating these processes with charge trapping sites by introducing organic additives can improve the electrical degradation resistance of polymer dielectrics. Figure 1 shows electrical degradation resistance with the use of a single antioxidant or synergistic antioxidants in EPDM specimens, as measured via the electrical tree test. Figure 1a shows electrical tree morphologies in EPDM/antioxidants with positive voltage, which can directly reflect the electrical degradation of the specimens. The high voltage (HV) electrode is shown in Figure 1a1, the electrical tree channels are generated from the electrode tip. Compared with the electrical tree in pure EPDM, the destruction region is severely reduced by antioxidants, especially in the EPDM/AOA&AOD specimen. Figure 1b shows electrical tree morphologies in EPDM/antioxidants with negative voltage, which exhibit a similar tendency to those in Figure 1a. The electrical tree inception probability is referred to the proportion of samples with tree structures in 5 minutes, 20 samples are tested in each group. To investigate the effects of using a single antioxidant or synergistic antioxidants on the
inception process of electrical degradation, the electrical tree inception probability is measured with positive and negative voltages, as shown in Figure 1c. With positive voltage, the electrical tree inception probabilities of the specimens are ordered as follows: EPDM > EPDM/AOD > EPDM/AOA > EPDM/AOA&AOD. With negative voltage, the tendency of the inception probability is similar to that in the case of positive voltage. The results demonstrate the synergistic antioxidants outperform a single antioxidant in enhancing the electrical degradation resistance by retarding electrical tree inception. The tree length is referred to the distance between the needle tip and the end of the longest tree branch along the electric field direction as shown in Figure 1a1. As the tree channels propagate to bridge the needle and ground electrodes, polymer dielectric breakdown occurs, which is regarded as a parameter that reflects the electrical degradation resistance. The electrical tree length is measured with positive and negative voltages, as shown in Figure 1d. With positive voltage,
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Figure 1. Electrical tree characteristics. a) The electrical tree morphologies after 20 minutes; a1~a4 were obtained with a pulse voltage of +25 kV. b) The electrical tree morphologies after 20 minutes; b1~b4 were obtained with a pulse voltage of -25 kV. c) The electrical tree inception probabilities in EPDM/antioxidants with ± 17 kV. d) The electrical tree lengths in EPDM/antioxidants with ± 25 kV after 20 minutes.
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The Journal of Physical Chemistry C the electrical tree lengths of the specimens are ordered as follows: EPDM > EPDM/AOD > EPDM/AOA > EPDM/AOA&AOD. With negative voltage, the tendency of the tree length is similar to that in the case of positive voltage. In EPDM/AOA&AOD, the tree length is the shortest within the same treeing time; hence, synergistic antioxidants can further enhance the electrical degradation resistance by decelerating electrical tree propagation compared to a single antioxidant. 3.2 Charge trap distribution behaviors. The electrical degradation depends on the charge transportation, which is
influenced by the trapping and de-trapping properties of the polymer dielectrics. To fully investigate the electrical degradation process, it is necessary to characterize the trap distributions of polymer dielectrics, which are closely related to their microscopic structures and chemical groups. EPDM is an amorphous polymer in which there are broken bonds, end groups, and crosslinking by-products in the amorphous region, which can generate “trapping sites” for capturing the charges. The shallow traps are originated from physical imperfections, while the deep traps are originated from organic or inorganic additives, etc. 33
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Figure 2. Trap distribution behaviors. a) The electron trap distribution behaviors of EPDM/antioxidants. EPDM/antioxidants were measured via the SPD method with DC constant voltages of -10 kV and -5 kV applied to the needle electrode and the mesh grid, respectively. b) The electron trap distribution behaviors of EPDM/antioxidants. EPDM/antioxidants were measured via the SPD method with DC constant voltages of -12 kV and -7 kV applied to the needle electrode and the mesh grid, respectively. c) The hole trap distribution behaviors of EPDM/antioxidants. EPDM/antioxidants were measured via the SPD method with DC constant voltages of +10 kV and +5 kV applied to the needle electrode and the mesh grid, respectively. d) The hole trap distribution behaviors of EPDM/antioxidants. EPDM/antioxidants were measured via the SPD method with DC constant voltages of +12 kV and +7 kV applied to the needle electrode and the mesh grid, respectively. e, f) Band diagrams in polymer dielectrics with shallow or deep traps. EVL is the vacuum energy level, HOMO is the highest occupied molecular orbital, and LUMO is the lowest unoccupied molecular orbital.
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The SPD method is a powerful tool in investigating charge carrier transport processes in polymer dielectrics. The trapped charges are jumped out traps based on the self-built electric field and their own thermal motion. 34 The shallow and deep traps simultaneously control the surface potential decay processes, two peaks that represents the maximum value of shallow trapped charge density and the deep one are measured via the SPD method as shown in Figure 2a. The peak that is located at an energy level of less than 0.90 eV is the shallow trapping center and the time that is taken by trapped charges to de-trap is much shorter, while the peak that is located at an energy level of larger than 0.90 eV is the deep trapping center and the trapped charges can remain there for several minutes or even days. When AOA and AOD are filled into EPDM, the deep trapping centers move toward higher values. The energy levels of the deep trapping centers are ordered as follows: EPDM < EPDM/AOD < EPDM/AOA < EPDM/AOA&AOD. The charge transportation is modulated in the specimens as the deep traps are introduced by antioxidants to capture the charges. In the EPDM/AOA&AOD specimen, the energy level of the deep trapping center is the highest among all the specimens; hence, the synergistic antioxidants exhibit synergistic effects on modulating electron transportation. In polymers, based on the residence time ( /s) of the charges in traps that is estimated via the two-potential model, is expressed as: 35 1 = (1) = 0exp [ ] exp ( ) (2) where Ei is the barrier between the two-well minima and corresponds to the trap energy level, E is the electric field, AB represents the hopping probability from well A to well B, k= 1.38×10-23 J/K is the Boltzmann’s constant, T =293 K is the temperature, v0 =1.336×1014 s-1 is the attempt frequency, and trap=14.77 Å is the average distance between traps. As the higher electric field is applied to the specimens, is reduced substantially. Thus, the charges require less time to de-trap at a higher electric field when they are trapped in the same energy levels; the higher electron trap energy level and larger trap density are shown in Figure 2b. Figure 2c and Figure 2d show the hole trap distribution behaviors with a single antioxidant or synergistic antioxidants in EPDM specimens, as measured via the SPD method. The hole deep trapping centers are smaller than the electron ones. This is because electrons migrate to the conduction band and the holes move to the valence band, which have different electron and hole trap energy levels in polymer dielectrics.31 The energy levels of the deep hole trapping centers are ordered as EPDM
