Polymer dielectrics have been widely used in electrical and electronic systems for capacitive energy storage and electrical insulation. However, emerging applications such as electric vehicles and hybrid electric aircraft demand improved polymer dielectrics for operation not only under high electric fields and high temperatures, but also extreme conditions, for example, low pressures at high altitudes, with largely increased likelihood of electrical partial discharges. To meet these stringent requirements of grand electrifications for payload efficiency, polymers with enhanced discharge resistance are highly desired. Here, we present a surface-engineering approach for Kapton® coated with self-assembled two-dimensional montmorillonite nanosheets. By suppressing the magnitude of the high-field partial discharges, this nanocoating endows polymers with improved discharge resistance, with satisfactory discharge endurance life of 200 hours at a high electric field of 46 kV mm −1 while maintaining the surface morphology of the polymer. Moreover, the MMT nanocoating can also improve the thermal stability of Kapton®, with significantly suppressed temperature coefficients for both the dielectric constant and dielectric loss over a wide temperature range from 25 to 205 °C. This work provides a practical method of surface nanocoating to explore high-discharge-resistant polymers for harsh condition electrification.
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Study of EPR-based nanodielectrics under operational conditions for DC cable insulation
A model DC material based on ethylene propylene rubber (EPR) including the pure EPR and the EPR-based nanodielectrics incorporated with two different nanoclays, Kaoline and Talc, under operational conditions was investigated. The operational conditions include a 20 kV/mm electric field at 25 °C, a 20 kV/mm electric field at 50 °C with a thermal gradient, and a 40 kV/mm electric field at 50 °C with a thermal gradient and polarity reversal. Space charge distribution, surface potential, and electrical conductivity were measured to characterize the model DC material and interpret the discrete charge dynamics in the bulk and at the interface of the three samples. The experimental results revealed that the electrical conductivity of Talc-filled nanodielectric has the least dependency on electric field and temperature, which reduces the conductivity gradient across the dielectric. Moreover, the successful suppression of space charge and the lower dielectric time constant in the Talc-filled nanodielectric result in a tuning electric field in the bulk not only under normal operating conditions but also more importantly under polarity reversal conditions. The maximum of absolute charge density decreases from 10.6 C/m 3 for EPR to 2.9 C/m 3 for the Talc-filled nanodielectric under 40 kV/mm with polarity reversal and at 50 °C with the thermal gradient. The maximum of local electric field enhancement for the mentioned condition reduces significantly from 97 kV/mm, 142% enhancement, for EPR to 45 kV/mm, only 12.5% enhancement, for the Talc-filled nanodielectric.
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- Award ID(s):
- 1650544
- NSF-PAR ID:
- 10401945
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 132
- Issue:
- 8
- ISSN:
- 0021-8979
- Page Range / eLocation ID:
- 084101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The electric field distribution in the ionization waves propagating over a microchannel array dielectric surface, with the channels either empty or filled with distilled water, is measured by ps Electric Field Induced Second Harmonic (EFISH) generation. The surface ionization wave is initiated by the atmospheric pressure N2-Ar plasma jet impinging on the surface vertically and powered by ns pulse discharge bursts. The results show that the electric field inside the microchannels, specifically its horizontal component, is enhanced by up to a factor of 2. The field enhancement region is localized within the channels. The vertical electric field inside the channels lags in time compared to the field measured at the ridges, indicating the transient reversal of the ionization wave propagation direction across the channels (toward the jet). This is consistent with the phase-locked plasma emission images and confirmed by the kinetic modeling predictions, which show that the ionization wave “jumps” over the empty channels and propagates into the channels only after the jump between the adjacent ridges. When the channels are filled with water, the wave speed increases by up to 50%, due to the higher effective dielectric constant of the surface. No evidence of a significant electric field enhancement near the dielectric surface (ceramic or water) has been detected, within the spatial resolution of the present diagnostic, ~100 μm.
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null (Ed.)Thermal anisotropy/isotropy is one of the fundamental thermal transport properties of materials and plays a critical role in a wide range of practical applications. Manipulation of anisotropic to isotropic thermal transport or vice versa is in increasing demand. However, almost all the existing approaches for tuning anisotropy or isotropy focus on structure engineering or materials processing, which is time and cost consuming and irreversible, while little progress has been made with an intact, robust, and reversible method. Motivated by the inherent relationship between interatomic interaction mediated phonon transport and electronic charges, we comprehensively investigate the effect of external electric field on thermal transport in two-dimensional (2D) borophene by performing first-principles calculations along with the phonon Boltzmann transport equation. Under external electric field, the lattice thermal conductivity of borophene in both in-plane directions first increases significantly to peak values with the maximum augmentation factor of 2.82, and the intrinsic anisotropy (the ratio of thermal conductivity along two in-plane directions) is boosted to the highest value of 2.13. After that, thermal conductivities drop down steeply and anisotropy exhibits oscillating decay. With the electric field increasing to 0.4 V Å −1 , the thermal conductivity is dramatically suppressed to 1/40 of the original value at no electric field. More interestingly, the anisotropy of the thermal conductivity decreases to the minimum value of 1.25, showing almost isotropic thermal transport. Such abnormal anisotropic to isotropic thermal transport transition stems from the large enhancement and suppression of phonon lifetime at moderate and high strength of electric field, respectively, and acts as an amplifying or reducing factor to the thermal conductivity. We further explain the tunability of phonon lifetime of the dominant acoustic mode by an electron localization function. By comparing the electric field-modulated thermal conductivity of borophene with the dielectric constant, it is found that the screened potential resulting from the redistributed charge density leads to phonon renormalization and the modulation of phonon anharmonicity and anisotropy through electric field. Our study paves the way for robust tuning of anisotropy of phonon transport in materials by applying intact, robust, and reversible external electric field without altering their atomic structure and would have a significant impact on emerging applications, such as thermal management of nanoelectronics and thermoelectric energy conversion.more » « less
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A vast majority of research on (U)WBG power modules has been going on to implement nonlinear resistive field grading material on metal-brazed substrates in reducing the electric field that is maximum at triple points (TPs). However, nearly all investigations have been conducted under either DC or 50/60 HZ sinusoidal AC voltages, even though the actual operation of envisioned (U)WBG power modules involves high-frequency square voltages with high slew rates. It has been validated by several studies that fast rise times of square voltages rapidly degrade the breakdown strength of insulation materials, leading to premature failure. Therefore, this paper introduces a nonlinear resistive field grading material or field-dependent conductivity (FDC) layer around the TP and metal edges to evaluate the electric field mitigation under a high frequency and high slew rate square voltage. The modeling and simulation of both coated and uncoated (U)WBG substrates were performed in COMSOL Multiphysics to assess the electric field reduction with the nonlinear FDC layer. The improvement of reduction in electric field under 100 kHz high slew rate square voltage is compared with that of 60 Hz. The results reveal a significant decrease in field stress at the TP, even under square voltages with fast rise times and high frequencies, when applying a nonlinear FDC coating, as opposed to the uncoated substrate. The influence of switching field (Eb) and nonlinearity coefficient (α) of nonlinear FDC layer is studied under 100 kHz square voltage, and it is concluded that α and Eb should be more than 10 and less than 8 kV/mm, respectively to achieve effective performance of resistive field grading material.more » « less
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Abstract Coupon specimens of poled and depoled lead zirconate titanate (PZT) are examined under combined stress wave and electric loading conditions. Mode‐I crack initiation and fracture behavior is examined using ultrahigh‐speed imaging and two‐dimensional digital image correlation. The dynamic critical stress intensity factor (
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0091930
