Dielectric Breakdown of Polycrystalline Alumina: A Weakest‐Link Failure Analysis

The effects of varying electrode geometry (ball and ring) and size (radius), dielectric liquid (castor oil and Diala^® oil), and specimen thickness on the dielectric breakdown of a commercial‐grade alumina were investigated. The breakdown strength was expressed in terms of the maximum electric field in the ceramic calculated by finite element analysis (FEA) at the breakdown voltage. The breakdown strength decreased systematically with increasing electrode radius and specimen thickness, and the strength was higher in the Diala^® oil (dielectric constant, ε_r = 2.3 ± 0.06) as compared to the castor oil (ε_r = 4.6 ± 0.13). These effects of the electrode geometry, specimen thickness, and of the dielectric liquid on the breakdown strength of the alumina were analyzed with a weakest‐link failure model employing Laplace and Weibull distributions for a population of defects in the material. The measured size or scaling effects of the electrodes, specimen thickness, and of the dielectric liquid on breakdown strength were in better agreement with the Laplace distribution for a population of surface defects. The dielectric breakdown is likely initiated at surface pits produced by grain pullout. The measured area concentration of surface pits agreed with the defect density analyzed from the weakest‐link failure theory. FEA of specimens containing surface and subsurface cavities revealed that electric field concentrations were always greater for surface pits as compared to subsurface cavities. There is, in fact, no electric field concentration at a subsurface cavity located more than about 100–800 μm below the surface depending on the top electrode size.     

Nanocomposites of Surface‐Modified BaTiO3 Nanoparticles Filled Ferroelectric Polymer with Enhanced Energy Density

We report nanocomposites of increased dielectric permittivity, enhanced electric breakdown strength and high‐energy density based on surface‐modified BaTiO₃ (BT) nanoparticles filled poly(vinylidene fluoride) polymer. Polyvinylprrolidone (PVP) is used as the surface modification agent and homogeneous nanocomposite films have been prepared by solution casting processing. The dielectric permittivity of the nanocomposite with treated BT is higher than those with untreated BT and reaches the maximum value of 77 (1 kHz) at BT concentration of 55 vol%. The electric breakdown strength of the nanocomposite is greatly enhanced to 336 MV/m at BT concentration of 10 vol% and the calculated energy density is 6.8 J/cm³. The results indicate that using PVP as surface modification agent can greatly enhance the dielectric permittivity and electric breakdown strength of the ceramic–polymer nanocomposite and achieve high‐energy density for energy storage and power capacitor applications.     

Dynamic fracture behavior of piezoelectric ceramics under impact: Force-electric response and electrical breakdown

The brittle fracture may occur in the application of piezoelectric ceramics, but the traditional research is still limited to the static fracture of the materials. Based on the improved Hopkinson pressure bar loading system and high-speed photography technology, the experimental study on the fracture behavior of piezoelectric ceramics under impact loading was carried out. The dynamic mechanical and electrical response of lead zirconate titanate (PZT) and the possible electric breakdown phenomenon were analyzed. The experimental results show that the output voltage is stable and the maximum output voltage is 889 V when the impact load does not cause the material to fracture. When the material breaks, its macroscopic output voltage fluctuates due to electric breakdown. Combined with the finite element simulation of the impact fracture process, the distribution characteristics of the stress field and electric field near the crack during the fracture process were analyzed. The results show that the sliding between grains formed the crack cavity parallel to the electric field during the impact process. Furthermore, based on the theory of dielectric breakdown, the possibility of electric breakdown in the initial defect and the elliptical cavity formed by the impact is analyzed.     

Electrical ac and dc behavior of epoxy nanocomposites containing graphene oxide

This article deals with the investigation of electrical properties of epoxy‐based nanocomposites containing graphene oxide nanofillers dispersed in the polymer matrix through two‐phase extraction. Broadband dielectric spectroscopy and dc electrical conductivity as a function of electric field have been evaluated in specimens containing up to 0.5 wt % of nanofiller. Nanocomposites containing pristine graphene oxide do not show significant changes of electrical properties. On the contrary, the same materials after a proper thermal treatment at 135°C, able to provoke the in situ reduction of graphene oxide, exhibit higher permittivity and electrical conductivity, without showing large decrease of breakdown voltage. Moreover, a nonlinear behavior of the electrical conductivity is observed in the range of electric fields investigated, i.e. 2–30 kV mm^?1. A new relaxation phenomenon with a very low temperature dependence is also evidenced at high frequency in reduced graphene oxide composites, likely associated to induced polarization of electrically conductive nanoparticles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41923.     

Statistical study on short‐term dielectric breakdown of poly(vinyl chloride)

This paper reports the breakdown voltage of plasticized poly(vinyl chloride) under AC and DC electric fields. The effect of ramp rate on breakdown voltage was investigated. A statistical analysis of breakdown data was carried out by using the two‐parameter Weibull distribution. The best linear fit was determined by an estimation based on the method of maximum likelihood. The 90% confidence intervals of Weibull plots were calculated, and the model was validated by the χ² test. The study showed that the shape parameter varied with ramp rate. This result could be explained by the evolution of defect number and size. Breakdown voltage increased with ramp speed. This phenomenon was attributed to the time necessary for the accumulation of defects and charges to form a breakdown path. Breakdown voltage was higher under a DC field. This increase was due to the difference in dissipated energy under the two types of field and to the fatigue produced by AC stress. The DC breakdown voltage was greater under negative polarity. This finding was ascribed to the ratio difference of space charge accumulated under the two polarities. J. VINYL ADDIT. TECHNOL., 19:177‐182, 2013. © 2013 Society of Plastics Engineers     

Electro-mechanical deformation of amorphous and semi-crystalline polymeric films

In our previous study, electrically induced mechanical stress was produced on monolithic polycarbonate (PC) films under a DC voltage using a needle-plane electrode setup. This study investigated other materials with various structures and dielectric constants, in order to further understand the deformation mechanism. It was found that the elastic behavior occurred at electric fields intensities below that initiating measurable surface deformation. The amorphous materials, PS, and the semi-crystalline materials, HDPE and PP, having dielectric constants all around 2.5, exhibited a similar observable deformation onset electric field at 200 MV/m. While PVDF, having a dielectric constant of 10.0–12.0, showed an onset at only 30 MV/m. The data was also compared to our previous study on PC. The depth and diameter of the deformation for all materials increased relative to the applied electric field up to film breakdown. Thermal annealing of the deformed films revealed a recoverable “delayed elastic” component and an irreversible “plastic” component. A three-stage electrically induced mechanical deformation mechanism was proposed for amorphous materials, while a two-stage mechanism was proposed for the semi-crystalline materials. The difference on the energy loss versus deformed volume for amorphous and semi-crystalline polymers was also determined and discussed.     

Grain‐size–dependent dielectric properties in nanograin ferroelectrics

A series of ferroelectric ceramic models with grain and grain‐boundary structures of different sizes are established via Voronoi tessellations. A phase‐field model is introduced to study the dielectric breakdown strength of these ferroelectric ceramics. Afterward, the relation between the electric displacement and electric field and the hysteresis loop are calculated using a finite element method based on a classical and modified hyperbolic tangent model. The results indicate that as the grain size decreases, the dielectric strength is enhanced, but the dielectric permittivity is reduced. The discharge energy density and energy storage efficiency of these ferroelectric ceramics extracted from the as‐calculated hysteresis both increase along with a decrease in their grain size at their breakdown points. However, under the same applied electric field, the ferroelectric ceramic with a smaller grain size possesses a lower discharge energy density but a higher energy storage efficiency. The results suggest that ferroelectric ceramics with smaller grain sizes possess advantages for applications in energy storage devices.     

Design and optimization of planar mesa termination for diamond Schottky barrier diodes

In this paper, we present planar mesa termination structure with high k dielectric Al₂O₃ for high-voltage diamond Schottky barrier diode. Analysis, design, and optimization are carried out by simulations using finite element technology computer-aided design (TCAD) Sentaurus Device software. The performances of planar mesa termination structure are compared to those of conventional field plate termination structure. It is found that optimum geometry of planar mesa terminated diode requires shorter metal plate extension (1/3 of the field plate terminated diode). Consequently, planar mesa terminated diode can be designed with bigger Schottky contact to increase its current carrying capability. Breakdown performance of field plate termination structure is limited at 1480 V due to peak electric field at the corner of Schottky contact (no oxide breakdown occurs). In contrast, peak electric field in planar mesa termination structure only occurs in the field oxide such that its breakdown performance is highly dependent on the oxide material. Due to Al₂O₃ breakdown, planar mesa termination structure suffers premature breakdown at 1440 V. Considering no oxide breakdown occurs, planar mesa termination structure can realize higher breakdown voltage of 1751 V. Therefore, to fully realize the potential of planar mesa terminated diode, it is important to choose suitable high k dielectric material with sufficient breakdown electric field for the field oxide.     

A novel method for investigating electrical breakdown enhancement by nm-sized features

Electrical transport studies across nm-thick dielectric films can be complicated, and datasets compromised, by local electrical breakdown enhanced by nm-sized features. To avoid this problem we need to know the minimal voltage that causes the enhanced electrical breakdown, a task that usually requires numerous measurements and simulation of which is not trivial. Here we describe and use a model system, using a "floating" gold pad to contact Au nanoparticles, NPs, to simultaneously measure numerous junctions with high aspect ratio NP contacts, with a dielectric film, thus revealing the lowest electrical breakdown voltage of a specific dielectric-nanocontact combination. For a 48 ± 1.5 ? SiO(2) layer and a ～7 ? monolayer of organic molecules (to link the Au NPs) we show how the breakdown voltage decreases from 4.5 ± 0.4 V for a flat contact, to 2.4 ± 0.4 V if 5 nm Au NPs are introduced on the surface. The fact that larger Au NPs on the surface do not necessarily result in significantly higher breakdown voltages illustrates the need for combining experiments with model calculations. This combination shows two opposite effects of increasing the particle size, i.e., increase in defect density in the insulator and decrease in electric field strength. Understanding the process then explains why these systems are vulnerable to electrical breakdown as a result of spikes in regular electrical grids. Finally we use XPS-based chemically resolved electrical measurements to confirm that breakdown occurs indeed right below the nm-sized features.     

High energy density nanocomposites based on poly(vinylidene fluoride‐chlorotrifluoroethylene) and barium titanate

Three series of poly (vinylidene fluoride‐chlorotrifluoroethylene)/barium titanate (BT) nanocomposites with varied compositions were fabricated via solution cast process followed by thermally treated in different ways. Quenching the composite samples at lower temperature could effectively enhance their dielectric constant, breakdown strength as well as the energy density. The highest energy density (13.6 J/cm³) is observed in the sample quenched from 200°C to ?94°C with 5 vol% BT, which is much higher than nanocomposites reported in the current literature. The addition of ceramic particles leads to the improvement of dielectric permittivity and energy density measured under the same electric field. However, the dielectric breakdown strength and the energy density measured at breakdown strength of the resultant composites are reduced as a function of BT content. The fixed maximum electric displacement and reduction of saturation electric field suggest that the addition of ceramic particles with high dielectric constant may help increase the energy density of composites under low electric field but not for high electric field. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

On the size dependence of the dielectric breakdown strength of solid insulators at room temperature

The dielectric breakdown strength of solid insulators is usually dependent on the dimensions of the sample. In particular, for “thick” insulating sheets, the dielectric breakdown strength is often found to be roughly proportional to the inverse of the square root of the sample thickness. In this contribution, the hypothesis is examined that the latter type of size dependence is related to electric field inhomogeneities associated with the geometrical electrode configuration. In order to test the hypothesis, DC dielectric breakdown experiments have been performed for alumina ceramic sheets of different thicknesses ranging from 0.25 mm to 2 mm for both polarities and voltage ramp rates in the range between 0.1 and 5.8 kV/s. An electrode configuration has been used which allows for an analysis of the local electric field enhancement at the electrode edge in terms of fracture mechanics concepts. It is concluded from a comparison of experimental and theoretical results that the mentioned dependence of the dielectric breakdown strength on sample thickness is likely related to electrode edge effects.     

Enhancement of Thermal Conductivity with CuO for Nanofluids

The enhancement of the thermal conductivity of ethylene glycol in the presence of copper oxide (CuO) is investigated. CuO nanofluids are prepared in a two‐step method. No surfactant is employed as a dispersant. The volume fraction of CuO nanoparticles suspended in ethylene glycol liquid is below 5 vol.‐%. The crystalline phases of the CuO powders are measured with x‐ray diffraction patterns (XRD). CuO nanoparticles are examined using scanning electron microscopy (SEM) to determine their microstructure. The thermal conductivities of the CuO suspensions are measured by a modified transient hot wire method. The viscosity was measured with a viscosity instrument. The results show that CuO nanofluids with low concentrations of nanoparticles have considerably higher thermal conductivities than the identical ethylene glycol base liquids without solid nanoparticles. The thermal conductivity ratio improvement for CuO nanofluids is approximately linear with the volume fraction of nanoparticles. For CuO nanoparticles at a volume fraction of 0.05 (5 vol‐.%) thermal conductivity was enhanced by up to 22.4 %. CuO nanofluids thus have good potential for effective heat transfer applications.     

Enhanced energy storage performance of nanocomposites filled with paraelectric ceramic nanoparticles by weakening the electric field distortion

Polymer-based dielectric nanocomposites, which combines the high dielectric constant of ceramic materials and the high breakdown strength of polymer materials, has emerged as one of the most effective progress for the advanced dielectric energy storage materials. To improve energy storage performance, the core-shell structured SiO₂@SrTiO₃ paraelectric nanoparticles are used as fillers in constructing the polymer-based nanocomposites. Hence, this paper systematically investigates the impacts of filler content on energy storage performance and breakdown strength, and provides insight into the polarization behavior of different composites filled with paraelectric and ferroelectric nanoparticles (SiO₂@BaTiO₃), respectively. Combined finite element simulations, it is shown that the dielectric constant of the paraelectric ceramic is more similar to the polymer matrix, resulting in weakening the electric field distortion in the dielectric. Furthermore, due to the paraelectric characteristics of SrTiO₃ nanoparticles and the diminution of interface polarization, the remnant polarization of the nanocomposites can be significantly reduced. The polymer-based dielectric nanocomposites exhibit more impressive energy storage, of 11.42 J/cm³ at 350 MV/m with 2.5 vol% paraelectric SiO₂@SrTiO₃ nanoparticles, which is superior to the composite filled with ferroelectric nanoparticles. Overall, this finding not only establishes a new direction for the structural design of fillers but also provides insight into an underlying mechanism to control interface polarization in the dielectric composites.     

Electrical boundary condition at the crack surface in ferroelectrics

Electrical boundary condition (EBC) at the crack surface has significant influence on the fracture behavior of ferroelectrics. Here, we characterized the electrical displacement (ED) response of ferroelectrics specimens with through cracks under cyclic electric field. The physical processes occurred between the crack surfaces are in situ observed. It is found that when the distance between the crack surfaces was smaller than a critical value, significant ED was measured, which cannot be rationalized by the previous EBCs. The appearance of bubbles and arcing between the crack surfaces demonstrates that dielectric breakdown occurs during electric field loading. These results imply that the EBC of the crack surface is permeable, and the dielectric breakdown process should be taken into consideration.     

Polarization Response and Thermally Stimulated Depolarization Current of BaTiO3‐based Y5V Ceramic Multilayer Capacitors

Polarization response and thermally stimulated depolarization current (TSDC) of BaTiO₃‐based ceramic multilayer capacitors with Y5V specification were studied. The temperature dependence of dielectric behavior shows that as the dc electric field increases, the polarization response in the whole measurement range (from ?125°C to +350°C) is suppressed. As the temperature rises to about 250°C, dielectric loss significantly increases and has a dependence on dc electric field, due to the leakage behavior at high temperature. According to the hysteresis loops, the calculated electrostatic energy density and energy efficiency are also closely related to polarization‐electric field. Utilizing a fixed measuring polarization condition, two TSDC relaxation peaks are observed and both are associated with oxygen vacancies. It is demonstrated that the weak peak originates from the in‐grain migration of oxygen vacancies and the strong peak with high relaxation temperature is caused by the across grain‐boundary oxygen vacancies. The activation energy estimated for the relaxation of oxygen vacancies across grain boundaries is about 0.78 eV. The main contribution for the leakage behavior is from the across grain‐boundary relaxation of oxygen vacancies. With increasing of temperature and electric field stress, the extrinsic oxygen vacancy defects show more fluent migration, which eventually leads to the resistance degradation and breakdown.     

Polyethylene/aluminum nanocomposites: Improvement of dielectric strength by nanoparticle surface modification

The effect of the surface modification with a silane coupling agent (octyl‐trimethoxysilane) of aluminum (Al) nanoparticles on the dielectric breakdown behaviors of polyethylene (PE)/Al nanocomposites was investigated in comparison of the influence of the improvement of the interfacial adhesion between Al nanoparticles and PE using a compatibilizer (maleic anhydride grafted polyethylene). It was found that when compared with the other modification approaches, the surface‐treated Al nanofiller with the silane coupling agent makes it possible for the PE/Al nanocomposites to still keep the relatively higher breakdown strength even in the higher Al loading level above 14 vol %, which can be understood in terms of the better interfacial adhesion between the surface‐treated particle dispersion and the matrix. The combined effects of the Al nanoparticles on the different factors which influence the dielectric breakdown processes in polymer matrix such as microstructure, conductivity, and crystallinity of the nanocomposites were discussed in detail. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermal and dielectric properties of the aluminum particle/epoxy resin composites

Microsized aluminum/epoxy resin composites were prepared, and the thermal and dielectric properties of the composites were investigated in terms of composition, aluminum particle sizes, frequency, and temperature. The results showed that the introduction of aluminum particles to the composites hardly influenced the thermal stability behavior, and decreased T_g of the epoxy resin; moreover, the size, concentration, and surface modification of aluminum particles had an effect on their thermal conductivity and dielectric properties. The dielectric permittivity increased smoothly with a rise of aluminum particle content, as well as with a decrease in frequency at high loading with aluminum particles. While the dissipation factor value increased slightly with an increase in frequency, it still remained at a low level. The dielectric permittivity and loss increased with temperature, owing to the segmental mobility of the polymer molecules. We found that the aluminum/epoxy composite containing 48 vol % aluminum‐particle content possessed a high thermal conductivity and a high dielectric permittivity, but a low loss factor, a low electric conductivity, and a higher breakdown voltage. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

High Dielectric Breakdown Strength Nanoplatelet-Based Multilayer Thin Films

Dielectric materials that can withstand high voltages are of great interest due to the growing need for high-performance insulation systems in electronics. Polymer nanocomposites have gained popularity as electrical insulators due to their processability, high operating voltage, and tortuous paths for current flow created by the nanoparticles in the polymer matrix. The dielectric breakdown strength of a relatively thick multilayer thin film containing polyethylenimine (PEI) and vermiculite clay (VMT), thickened with tris(hydroxymethyl)aminomethane (tris), is evaluated as a function of bilayers (BL) deposited. The resulting nanobrick wall structure of this clay-based assembly is ideal for protective insulation. An 8 BL PEI+tris/VMT film achieves a dielectric breakdown strength of 245 kV mm⁻¹, with a thickness of 5 µm. With increasing bilayers, the breakdown strength gradually decreases, but 20 BL of PEI+tris/VMT achieves a breakdown voltage of 2.36 kV. This nanoplatelet-based system is the first “thick growing” layer-by-layer deposited film to be used as an insulating layer. Its unusually high breakdown strength can be useful for the protection of various high voltage electronics.     

Optimal drift region for diamond power devices

In power devices such as Schottky Barrier Diodes or Field Effect Transistors, the breakdown voltage is linked to the design of the drift layer but also to the physical properties of the material used. Diamond, with its high critical electric field due to its large band gap, opens the way to power components able to withstand very high voltage with outstanding figures of merit. Nevertheless, a particular attention has to be paid to the design of the drift layer to take benefit of these outstanding properties. Indeed, the drift region thickness, doping level and consequently the punch through or non-punch through designs must be well designed to reach the desired breakdown value and to minimize the ON state resistance at the same time. Here, a focus on the optimization of the specific ON state resistance as function of the breakdown voltage figure of merit has been carried out, while optimizing the drift layer and calculating the specific ON state resistance of unipolar high voltage diamond power devices. Based on the ionization integral calculation with impact ionization coefficients adapted to diamond, we performed an accurate analysis to find the best punch through design of the drift layer offering the lowest ON state resistance at a given breakdown voltage value. This theoretical study has been first applied in a one dimensional approach of the breakdown voltage. An additional 2D cylindrical coordinate analysis was performed to quantify the radius effect on the breakdown voltage value, and to compare the 2D breakdown voltage with the 1D breakdown voltage, for different drift region designs. These results offer preliminary design rules to fabricate more efficient unipolar diamond power devices. At the material level, this analysis also points out that thicknesses and doping levels required to achieve such structures are quite challenging for crystal growth in the context of high voltage power devices.     

Evaluation of dielectric breakdown of BaTiO3 by novel indentation method

Succinct evaluation of dielectric breakdown strength of dielectric materials has been hampered by extrinsic parameters like size/thickness dependence, electrode configuration, and defect concentration which all mask the materials’ intrinsic value. A novel indentation method to prepare arch-shaped indents on the surface of BaTiO₃ green stacks has been developed to overcome this problem. Utilizing this method enables simple and reliable sample preparation for investigation of dielectric breakdown behavior. Theoretical and experimental investigations reveal vanishing electrode edge effects governed by the improved distribution of electric field due to the arch-shaped indents. Thus, the novel indentation method bears advantages compared to measuring DBS of disk-shaped samples and allows for narrowing influences of extrinsic parameters. By obtaining breakdown channels close to the center of the indent, this technique additionally features the possibility of localizing the breakdown event.