Electrical/thermal properties of nanodielectrics
21世纪以来, 我国经济社会持续快速发展, 电力和电气化需求进入了高速增长期, 这前所未有地使介电材料成为世界舞台上的焦点. 介电材料是电容器、电力电子、微电子、嵌入式集成电路、电缆和发电机等的关键组成部分. 其中, 纳米电介质属于一类“具有纳米结构的多组分电介质材料”,自 1994年 Lewis 教授首次提出纳米电介质理论以来, 在近三十年的时间里发展迅猛且被誉为“未来的绝缘材料”. 纳米介电工程的出现, 推动了电工绝缘与微 (光)电子器件绝缘材料的创新性发展. 具有先进介电功能的纳米电介质材料的开发是驱动国家能源战略实施、维护电网系统运行及国家安全的重中之重.
当下, 器件的微型化、集成化极大地带动了以集成电路为代表的微电子制造业的发展. 但是, 器件的高集成度必然伴随着高的功耗和发热, 导致电介质材料出现严重变形、老化、损伤、碳化, 极大地破坏电工装备及封装等的稳定性和可靠性. 此外, 高场下绝缘系统中空间电荷的积聚会引起局部电场畸变, 加快绝缘老化甚至造成电击穿, 产生严重的电气事故. 而纳米电介质的介于宏观和微观之间界面区域的电介质行为, 具有独特的时空多层次结构和优良的性质, 在提高设备效能、缩小部件尺寸、节约能源和材料等方面效果显著. 对于一些特定的领域, 如特高压输电、高性能电机中的主绝缘材料、薄膜电容器中的储能材料、微电子行业中的低介电材料、电子设备中的导热材料, 必须利用纳米电介质才能达到所需的性能指标. 因此, 为满足微电子、电力电气行业快速发展的需求, 研究在多物理场下具有优异电-热输运特性的纳米电介质材料具有重要意义.
近年来, 基于量子/尺寸效应、密度泛函、有限元模拟等理论方法, 辅以新材料设计理念, 科研人员通过研究纳米电介质的微观特殊结构、独特的表面化学和宏观界面工程逐步实现了对纳米电介质的电老化性能、短期击穿增强、介电、储能、导热等电-热特性的协同优化和提高. 然而, 目前关于纳米电介质材料电热特性的综合调控仍处于开发的初级阶段, 纳米电介质的智能化转型与多功能化发展速度也相对迟缓.
为进一步提升纳米电介质研究领域的影响力, 推动本领域专家学者的沟通与交流, 《物理学报》组织出版“纳米电介质电-热特性”专题, 重点关注纳米电介质电-热特性调控, 从理论分析、材料设计及计算学等多角度介绍本领域的最新研究进展和未来发展趋势. 鉴于纳米电介质电-热特性涉及物理、材料、电气等多学科的交叉, 研究方向丰富, 本专题只能重点介绍其中的部分研究成果, 包括:纳米电介质物化特性, 纳米电介质界面与表面, 组成、结构与性能, 纳米电介质智能化与多功能化(高/低介电、导热绝缘等), 电介质测试、计算与模拟, 纳米电介质材料应用等, 与读者和同行共享.
2024, 73 (2): 027702.
doi: 10.7498/aps.73.20230614
Abstract +
2024, 73 (2): 027301.
doi: 10.7498/aps.73.20230556
Abstract +
With the development of science and technology, polymer dielectric capacitors are widely used in energy, electronics, transportation, aerospace, and many other areas. For polymer dielectric energy storage capacitors to remain effective in practical applications, excellent charge and discharge performance is essential. However, the performance of the common polymer dielectric capacitors will deteriorate rapidly at high temperature, which makes them fail to work efficiently under worse working conditions. Dielectric trap energy levels and trap densities increase when nanoparticles are incorporated into the dielectric. The change in trap parameters will affect carrier transport. Therefore, the high temperature energy storage performance of polymer nanocomposite dielectric can be improved by changing the trap parameters to regulate the carrier transport process. However, the quantitative relationship between trap energy level and trap density and the energy storage properties of nanocomposite dielectric need further studying. In this paper, the energy storage and release model for exponentially distributed trapped charge jump transport in linear polymer nanocomposite dielectrics is constructed and simulated. The volume resistivity and electric displacement-electric field loops of pure polyetherimide are simulated at 150 ℃, and the simulation results match the experimental results, which demonstrates the validity of the model. Following that, under different temperatures and electric fields, the current density, electric displacement-electric field loops, discharge energy density and charge-discharge efficiency of polyetherimide nanocomposite dielectric are simulated by using different trap parameters. The results show that increasing the maximum trap energy level and the total trap density can effectively reduce the carrier mobility, current density and conductivity loss, and enhance the discharge energy density and the charge-discharge efficiency of the nanocomposite dielectric. On condition that temperature is 150 ℃ and applied electric field is 550 kV/mm, the polyetherimide nanocomposite dielectric with a maximum trap energy level of 1.0 eV and a total trap density of 1×1027 m–3, has 4.26 J·cm–3 of discharge energy density and 98.93% of energy efficiency. Compared with pure polyetherimide, the rate of improvement is 91.09% and 227.58%, respectively. The energy storage performance under high temperature and high electric field is obviously improved. It provides theoretical and model support for the research and development of capacitors with high temperature resistance and energy storage performance.
2024, 73 (2): 027703.
doi: 10.7498/aps.73.20230708
Abstract +
Adding nanofillers into epoxy resin matrices is a common method to achieve their multi-function. Boron nitride nanotubes (BNNTs) with one-dimensional nanostructures have attracted much attention because of their ultra-high thermal conductivity, wide energy level band gap, high aspect ratio and mechanical strength. Yet, the strong π-π non-covalent bonding and lip-lip interactions make BNNTs prone to agglomeration in the epoxy resin matrix. Moreover, the different physicochemical properties of BNNTs and epoxy resins as well as the chemical inertness of BNNTs surface lead to the lack of effective interfacial interaction between BNNTs and epoxy resin matrix. Therefore, the performance of the epoxy composite dielectric is not enhanced by simple blending solely, but will even have the opposite effect. To address the problems of BNNTs, in this study, the surface structure of BNNTs is constructed from the perspective of interface modulation by using sol-gel method to coat mesoporous silica (mSiO2) on BNNTs’ surface and further introducing silane coupling agent (KH560). The results indicate that the surface structure of BNNTs can optimize the level of interfacial interaction between BNNTs and epoxy resin matrix, which leads to stronger interfacial connection and elimination of internal pore phenomenon. The dielectric constant and loss of the composite dielectric prepared in this way are further reduced, reaching 4.1 and 0.005 respectively at power frequency, which is significantly lower than that of pure epoxy resin. At the same time, the mechanical toughness (3.01 MJ/m3) and thermal conductivity (0.34 W/(m⋅K)) are greatly improved compared with the counterparts of pure epoxy resin. In addition, the unique nano-mesoporous structure of mSiO2 endows the composite dielectric with a large number of deep traps, which effectively hinders the migration of electrons, thereby improving the electrical strength of the composite dielectric, and the breakdown field strength reaches 95.42 kV/mm. Furthermore, the interfacial mechanism of BNNTs’ surface structure on dielectric relaxation and trap distribution of composite dielectrics is systematically studied by Tanaka multinuclear model. The above results indicate that the good interfacial interaction between BNNTs and epoxy resin matrix is crucial in establishing the micro-interface structure and improving the macroscopic properties of composite dielectrics. This study presents a novel idea for the multifunctionalities of epoxy resin, and also provides some experimental data support for revealing the correlation among surface properties of nano-fillers, microstructure and macroscopic properties of composite dielectric.
2024, 73 (7): 078801.
doi: 10.7498/aps.73.20240201
Abstract +
Cross-linked polyethylene (XLPE) has been widely used in the field of power cables due to its excellent mechanical properties and insulating properties. However, during the manufacturing of high voltage cables, XLPE will inevitably be affected by electrical aging, thermal aging and electro-thermal combined aging, which makes the resistance and life of the material decline. Therefore, it is necessary to enhance the aging resistance of XLPE without affecting its mechanical properties and insulating properties, so as to extend its service life. In this work, the structural characteristics and cross-linking mechanism of XLPE are introduced, the aging process and influencing mechanism are systematically analyzed, and the life decay problems of XLPE due to aging are explored by using methods such as the temperature Arrhenius equation and the inverse power law of voltage. The improvement strategies such as grafting, blending, and nanoparticle modification can be used to enhance the thermal stability, antioxidant properties, and thermal aging resistance of XLPE, thereby extending its service life. Finally, the strategies of adjusting and controlling the service life of XLPE cable insulation materials in the future are discussed, which provide theoretical guidance for further improving long-term stable operation of XLPE cable insulation materials.
2024, 73 (7): 070701.
doi: 10.7498/aps.73.20231869
Abstract +
During the long-term operation of a cable, the electrical field, high temperature, and interface stress may age or deteriorate the silicon rubber (SIR) insulation of the cable accessories, affecting the combined electrical-thermal-force performance of the accessories, and easily causing discharge faults. In this work, the electrical-thermal-force properties of silicone rubber for cable accessories under thermal aging and combined force-thermal aging are studied experimentally and numerically. The changes and mechanisms of physical and chemical properties, electrical properties, thermal properties and mechanical properties of silicone rubber are tested and compared before and after aging. The changes of electric, thermal and force field of cable accessories, caused by the change of SIR material parameters under different aging time and aging form, are further simulated. The experimental results show that the crosslinking degree and molecular motion system of SIR will change with the deepening of the aging degree, which will change the electrical-thermal-force properties of the material to different degree. After aging, large agglomeration protrudes and small cavities appear in SIR section, and the damage is more serious under force-thermal aging. The relative dielectric constant first decreases and then increases with the aging time increasing. The volume resistivity, breakdown strength and flashover voltage all first increase and then decrease. The thermal conductivity first increases and then decreases with aging time increasing. In addition, with the increase of aging time, the tensile strength and elongation at break decrease gradually. Considering the change of properties after aging, the destruction of SIR material by force-thermal aging is more serious. The simulation results show that under the two aging modes, the maximum electric field strength at the stress cone root of the cable accessories first increases and then decreases with the increase of time. The electric field strength at the stress cone root of the cable accessories, caused by the force-thermal aging, changes little, maintaining about 2.2 kV/mm. The difference in temperature between the inside and the outside of the insulation layer is obvious under different aging degree, and the temperature difference shows a first decreasing and then increasing trend under both aging modes, and the maximum temperature gradient is 9.15 ℃. The interface stress at the stress cone root decreases from 0.263 to 0.230 MPa, which is about 12.5% lower. This work has guiding significance in evaluating the insulation performance and analyzing the fault of distribution cable accessories.
2024, 73 (7): 070702.
doi: 10.7498/aps.73.20240150
Abstract +
Triboelectric nanogenerator (TENG), as a micro-nano power source or self-powered sensor, has shown great prospects in various industries in recent years. The TENG output performance is closely related to the contact electrification characteristics of the triboelectric dielectric material. Herein, we first introduce the relevant fundamental theory and models of TENG and tribo-dielectrics. Then, we introduce the material selection, modification method (including surface modification and bulk modification) and structural design strategy of TENG dielectric material. Surface and bulk modification mainly involve surface roughness control, surface functional group regulation, and optimization of dielectric parameters. In terms of dielectric structural design, the principle of charge transport, trapping, and blocking layers as well as typical techniques to improve the dielectric properties of TENGs through multi-layer structures are highlighted. Finally, challenges and directions for future research are discussed, which is conducive to the fabricating of high-performance TENG dielectric materials.
2024, 73 (12): 127701.
doi: 10.7498/aps.73.20240208
Abstract +
With the surge in electrical loads and increasing voltage levels, the mechanical performance and thermal stability of insulating paper are facing severe challenges. However, due to the lack of direct scientific theories or simulation guidance, traditional inefficient “trial-and-error” experiments are difficult to effectively develop new types of cellulose composite insulating papers. For solving this problem, in this work we are to enhance the effects of nanoscale zinc oxide (nano-ZnO) on the mechanical and thermal properties of cellulose through molecular dynamics simulations. Initially, we model the nano-ZnO/cellulose composite material , then carry out a microscopic analysis of the mechanical performance and thermal stability of modified cellulose with varying nano-ZnO content, thus determining the optimal ratio of nano-ZnO to cellulose. The results indicate that compared with the outcomes from the unmodified model, the mechanical performance, cohesive energy density, glass transition temperature, and thermal conductivity of the nano-ZnO-modified cellulose model are all improved, with the highest increase in elastic modulus reaching 45.31% and the highest increase in thermal conductivity attaining 41.49%. The addition of nano-ZnO effectively fills the gaps in the fiber network and enhances the interactions between cellulose chains and thermal conduction channels, thereby improving the thermodynamic performance of cellulose. This work provides valuable theoretical references for rapidly preparing modified cellulose insulating papers with excellent thermodynamic performance.
2024, 73 (12): 127702.
doi: 10.7498/aps.73.20232041
Abstract +
To investigate the effect of the interface electronic structure of core-shell quantum dots on the conductivity and space charge characteristics of polyethylene insulation, nanocomposite insulations, namely CdSe@ZnS/LDPE and ZnSe@ZnS/LDPE, are synthesized. The study focuses on elucidating the evolution patterns of DC conductivity and space charge in the nanocomposite insulation, and analyzing the effect of the interfacial electronic structure of core-shell quantum dots on the distribution of charge traps. Comparative analysis reveals that in contrast to LDPE insulation, ZnSe@ZnS/LDPE nanocomposite insulation demonstrates a substantial reduction in DC conductivity by 47.2% and a decrease in space charge accumulation by 40.3% under the conditions of elevated temperature and strong electric field. The increase of trap energy level means an enhanced trap effect on charger carriers. According to density functional theory, the band structure characteristics of core-shell quantum dots integrated with polyethylene are computationally assessed. The findings underscore that the band misalignment at the core-shell interface and the shell-insulation interface induces shifts in the conduction band bottom and at the valence band top, respectively. These shifts impose a confinement effect on electrons and holes, with the extent of this effect escalating with the augment of the difference in band gap between the core layer and the shell layer. Consequently, this phenomenon curtails carrier migration, thereby inhibiting space charge accumulation under the conditions of elevated temperature and strong electric fields.
2024, 73 (12): 127703.
doi: 10.7498/aps.73.20240215
Abstract +
2023, 72 (10): 107701.
doi: 10.7498/aps.72.20230253
Abstract +
The dielectric relaxation characteristic and mechanism of thermochromic microcapsule-epoxy insulating material are investigated. The results show that thermochromic microcapsule-epoxy insulating material exhibits non-monotonic dielectric relaxation characteristic, namely the dielectric relaxation time gradually increases with the temperature rising in a range of about 58–66 ℃, which cannot be depicted by the conventional Arrhenius equation or Vogel-Fulcher-Tammann equation. It is proposed that the non-monotonic dielectric relaxation characteristic is derived from the free volume variation induced by the confined phase transition in microcapsule. With the increase of temperature, the solid-liquid phase transition occurs in the limited space of microcapsule, reducing the free volume inside the microcapsule, which could restrict the reorientation of dipole with the external electric field and lead to the increase of dielectric relaxation time. The non-monotonic dielectric relaxation characteristic of thermochromic epoxy specimen is fitted based on the confined dielectric relaxation model, obtaining the activation energy of dielectric relaxation. The relaxation activation energy values of thermochromic epoxy insulating materials with different microcapsule content are of the same order of magnitude, indicating that the non-monotonic dielectric relaxations occur inside the thermochromic microcapsule, verifying the role of confined phase transition in the non-monotonic dielectric relaxation characteristic.
