The projected increase in world energy consumption within the next 50 years, coupled with low emission requirements, has inspired an enormous effort towards the development of efficient, clean, and ...renewable energy sources. Efficient electrical energy storage solutions are keys to effective implementation of the electricity generated from these renewable sources. In step with the development of energy storage technology and the power electronics industry, dielectric materials with high energy density are in high demand. The dielectrics with a medium dielectric constant, high breakdown strength, and low polarization hysteresis are the most promising candidates for high-power energy storage applications. Inspiring energy densities have been achieved in current dielectrics, but challenges exist for practical applications, where the underlying mechanisms need to be understood for further enhancing their properties to meet future energy requirements. In this review, we summarize the principles of dielectric energy-storage applications, and recent developments on different types of dielectrics, namely linear dielectrics, paraelectrics, ferroelectrics, and antiferroelectrics, are surveyed, focusing on perovskite lead-free dielectrics. The new achievements of polymer-ceramic composites in energy-storage applications are also reviewed. The pros and cons of each type of dielectric, the existing challenges, and future perspectives are presented and discussed with respect to specific applications.
This paper presents results leading towards a superior understanding of the insulating properties of high vacuum by focusing mainly on the pre-breakdown and breakdown behavior of unbridged vacuum ...gaps under variable frequency alternating excitations. The supply frequency variation is from 30-300 Hz for a fixed inter-electrode spacing of 0.5 mm. The pre-breakdown (PBD) current magnitudes and the breakdown voltages are inversely proportional. The behavior is unpredictable over a specific range of frequencies. A pronounced breakdown voltage peak occurs at 130 Hz with stainless steel electrode pair, whereas this peak is less prominent with copper and aluminum electrodes. Fowler-Nordheim (F-N) emission, ion assisted field (I-F) emission and thermal interplay due to quasi-continuous shower of anode micro-particles predominantly neutral, lend support to the experimental observations. It is surmised that the apparent cause for time delay between voltage and current peaks is not only due to the presence of neutral molecules in the gap, but also principally due to their transit time.
High-performance dielectric ceramic capacitors are considered as one of the great hopeful apparatuses to solve the energy crisis and environmental pollution. However, the low energy storage density ...severely limits their further development. Herein, an ultrahigh recoverable energy density (Wrec) of 8.46 J/cm3 with excellent efficiency of 85.9% under 522 kV/cm is obtained in 0.90(Bi0.5Na0.5)0.65Sr0.35TiO3-0.10Bi(Mg0.5Zr0.5)O3 ceramic by the synergistic effect of delayed polarization saturation and improved breakdown field (Eb). The introducing of Bi(Mg0.5Zr0.5)O3 induces the quenched random field and increases the number of high dynamic polar nano-domains, which inhibits the forming of the long-range polar order and suppresses the early polarization saturation. Besides, the refined grains and compact structure reduce the oxygen vacancies concentration and enhance the activation energy, giving rise to higher Eb. Furthermore, the ceramic for x = 0.10 also displays outstanding thermal stability at 0–200 °C (with Wrec > 4.74 J/cm3 at 200 ℃ and the variation being only ~12.4%) due to the stable local structure and temperature-insensitive dielectric constant. Our work shows that the 0.90(Bi0.5Na0.5)0.65Sr0.35TiO3-0.10Bi(Mg0.5Zr0.5)O3 is a powerful competitor in next-generation energy storage materials and high temperature dielectric ceramic capacitors.
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•Ultrahigh Wrec of 8.46 J/cm3 and η of 85.9% are realized in BNST-0.1BMZ ceramic.•BNST-0.1BMZ ceramic exhibits excellent thermal stability from 0 °C to 200 °C.•The quenched random field hinders the early polarization saturation.•Refined grains reduce the oxygen vacancies and enhance the activation energy.
This paper presents an experimental investigation into the breakdown voltage fall time t f of lightning voltage breakdown of 2-4 centimeter vacuum gaps having a long breakdown delay time t d (>30 ...µs). The results show that the average 50% breakdown voltage fall time t 50 decreases with increasing vacuum gap. It is considered that the lightning impulse breakdown of the centimeter vacuum gaps with a long breakdown delay time may be initiated by the interaction between the cathode thermal processes and the micro-particles phenomena. The increase of the electrode diameter and the decrease of the electrode radius of curvature may contribute to the decrease of the breakdown voltage fall time.
In this paper, a novel modification method for biaxially oriented polypropylene (BOPP) membranes is proposed based on surface-grafting technology. The experimental results show that, compared with ...the pristine samples, the surface-grafted films show better breakdown strength under different voltage stress especially at higher temperatures (85 °C). The DC withstand strength of the surface-grafted PP membranes grows by 42.7% from 386.4 to 551.2 kV/mm. The breakdown strength climbs by 63.5% from 305.9 to 500.2 kV/mm under DC superimposed pulse voltage. Under DC superimposed harmonic stress, the breakdown strength is increased from 175.6 to 587.4 kV/mm by 234%. The influence of surface modification on charge transmission behavior is proposed based on the Density Functional Theory (DFT) simulation. This research provides a theoretical and methodological reference for performance optimization by surface modification for BOPP capacitors.
The insulation properties of transformer oil can be compromised by the presence of cellulose particles. These particles can gather in areas with high field strength and around the main windings due ...to the oil flow. Nonetheless, there is a need for additional exploration into the breakdown characteristics of contaminated flowing transformer oil. In this article, a circulating transformer oil flow device was constructed and breakdown tests were carried out. The results unveil that, in both ac and dc fields, the characteristic breakdown voltage (BDV) generally displays an initial decreasing trend (or exhibits slight fluctuations) first, then increases with the increase of flow velocity. At low flow velocities, the BDV with preload is consistently lower than that without preload, while the disparity becomes less pronounced at high flow velocities. The severity of cellulose bridge formation (in a bridge-like structure, influenced by oil flow and electric field) is found to be inversely proportional to the breakdown voltage, indicating that the creation of cellulose bridges plays a crucial role in initiating oil gap breakdown. This article contributes valuable insights into comprehending the breakdown mechanism of contaminated liquid dielectric in a flowing state.
The transient characteristics of groundings are decided by the impulse breakdown characteristics of soil, thus, the influence of soil ionization neighboring the grounding electrodes should be ...considered in the accurate lightning protection analysis. The breakdown delay is an important parameter of soil impulse characteristics, which are closely related to the soil properties. In this paper, the impulse breakdown delay of soils under varied temperatures and moistures were tested and the impacts of the amplitude and polarity of the applied impulse, the temperature and moisture of the tested soil samples on the breakdown delay were analyzed. A concept of critical breakdown delay of soil was proposed. From our tests, the soil breakdown under impulse is possibly an electrical but not a thermal process. According to the electrical breakdown mechanism of soil and the impulse breakdown mechanism of gas, the main conclusions were explained.
Partial discharge (PD) induced by bubble will reduce the insulation performance, lead to insulation failure and seriously affect the safe operation of power transformer. Studying the influence of PD ...in internal bubble on oil paper insulation and understanding the insulation failure mechanism can provide guidance for engineering practice. In this paper, a bubble PD defect model is designed to obtain the mechanism of PD degrading insulation. The inception and breakdown voltage of PD decrease seriously with the increase of bubble area and height. Combined with the surface state of breakdown paper observed by electron microscope, it is concluded that PD can induce the breakdown of oil paper insulation. Then, through the comparative analysis of bubble area and PD parameters, a positive feedback model for insulation degradation is obtained, representing that bubble defects will deteriorate rapidly in exponential form under the effect of PD. Finally, combined with the electron microscope and energy spectrum analysis, the deterioration process of insulating paper is divided into four stages, obtaining the micro deterioration and failure mechanism of insulating paper. This study contributes to understanding the mechanism of insulation failure by bubble, as well as provides a reference for risk assessment of transformers.
LIBS did not appear de novo in 1962, but was built upon accomplishments of the past. These started with very old concepts of indivisible units (atomos), chemical and physical experiments and ...theoretical advances that took place in the late 19th and early 20th centuries, the development of the laser, the discovery of gas breakdown, and the realization of the application to spectrochemistry. We sketch the historical developments and focus as well on the advances in LIBS methodology and instrumentation over the past 50years, culminating with a synopsis of the LIBS 2012 Conference in Luxor, Egypt.
Dielectric elastomers (DEs) used for energy conversion are often subjected to dc voltages or low-frequency varying voltages where the influence of space charge accumulation should be considered. The ...accumulation of space charge distorts the local electric field in the dielectric, which is closely related to critical dielectric performance, such as breakdown and electrical aging. Little research has been done on the effect of space charge in DE on the breakdown performance. In previous studies, it has been shown that adding nanosized silica may improve the breakdown strength, which is often ascribed to the traps created by the nanoparticles. Charge trapped in front of the electrode-dielectric interface is supposed to reduce the electric field and hence reduce charge injection and accumulation, resulting in a higher breakdown strength. In this article, to test this hypothesis, PDMS samples are prepared with different concentrations of nanosilica, as well as reference samples without filler. These specimens are characterized through space charge measurements. Conduction current measurements are performed to obtain additional information on the injection current. To relate space charge accumulation to a change in the breakdown characteristics, different types of breakdown tests are performed under ac and dc voltage. First, ac and dc ramp voltages are used, with the hypothesis that any accumulated space charge will have an effect only on dc breakdown because ac stress at power frequency does not allow enough time for space charge accumulation. In an additional step, step-up dc voltage tests are performed with the duration of each step long enough to allow space charge accumulation. The hypothesis in this case is that, due to the effect of space charge, the dc breakdown voltage would be lower than under dc ramp tests. The results of these experiments show that in the reference specimens, the breakdown strength is indeed affected by a strong build-up of space charge. Furthermore, the space charge can be mitigated by adding a small fraction of nanosilica. This mitigation of space charge accumulation is shown to lead to a significantly higher dc breakdown strength, whereas the ac breakdown strength is hardly affected.
