Self-Strengthening Dielectric Elastomer of Triblock Copolymer with Significantly Improved Electromechanical Performance under Low Voltage

Dielectric elastomer actuators (DEAs) are promising soft electromechanical transducers for soft robotics. Fabricating a high-performance DEA actuated by sub-kV voltage remains challenging. Here, a facile method not only to fabricate ultrathin dielectric elastomer films of triblock copolymers but also to enhance the dielectric breakdown strength and thus enhance the electromechanical performance is reported. A thick thermoplastic elastomer film of poly(styrene-b-butyl acrylate-b-styrene) from solution blading is symmetrically pre-stretched and relaxed at 120 °C to fabricate a freestanding ultrathin DE film. Compared with the pristine DE film of the same thickness (12 µm), the thermally-relaxed DE film with equally biaxial pre-stretch ratio 3.5 × 3.5 exhibits increased electrical breakdown strength by a factor of 1.9 (from 43 to 82 V µm⁻¹), maximum actuation area strain by a factor of 1.9 (from 11.7% to 22.4%), and highest energy density by a factor of 5.7 (from 4.5 to 25.8 kJ m⁻³). The enhancement may be ascribed to the self-reinforcement of the dielectric breakdown strength due to the morphology change of polystyrene nanodomains from spheres to oblate spheroids. Thanks to the ultra-thinness, the high electromechanical performance is achieved within sub-kV driving voltage in all cases.     

Maximum actuation strain for dissipative dielectric elastomers with simultaneous effect of prestretch and temperature

A dielectric elastomer can generate giant deformation by the voltage actuation, but the deformation is often hindered by the electromechanical instability and “snap‐through deformation,” which may lead to electrical breakdown. In this study, for the first time, the mathematical model is established for dissipative dielectric elastomers in the dynamic model with simultaneous effect of prestretch and temperature in order to achieve maximum actuation strain. The deformation of the dissipative dielectric elastomer: VHB 4905/4910 is investigated for the two simple actuation methods: constant and ramping voltage actuation, respectively. The best combined conditions of voltage and prestretch to obtain a large deformation at different operating temperatures are studied in detail. Under the best combined conditions, the influences of three factors: voltage, prestretch, and temperature on the maximum actuation strain are analyzed. This study should offer a great help in the design of dielectric elastomer actuators, and give the guidance to the accomplishment of the large deformation of dissipative dielectric elastomer actuators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45850.     

Electrical properties of conventional polyimide films: Effects of chemical structure and water uptake

A series of conventional polyimide (CPI) films, based on pyromellitic dianhydride (PMDA) and benzophenonetetracarboxylicdianhydride (BTDA), were prepared by a two step process, and their dielectrical constant, dielectrical loss, and DC conduction behaviors were studied at different frequencies and voltages. Their dielectrical breakdown voltage, water uptake, and solubility properties were also investigated. The effects of chemical structure and water uptake on the electrical properties of the films are discussed in detail. The dielectric constants of the CPI films vary between 2.93 and 3.72 at 1 MHz frequency and they are in the following decreasing order: BTDA‐DDS > BTDA‐DDE > PMDA‐DDS > PMDA‐DDE. The structure and thermal and oxidative stability of films were analyzed by FTIR‐ATR and TGA, respectively. The results showed that all CPI films have good insulating properties, such as high dielectric breakdown voltage, low dielectric constant with stability for long period of frequency, and low leakage density. Our results concerning electrical properties also suggest that electron hopping is responsible for AC conduction and Poole‐Frenkel mechanism is predominant for DC conduction of all CPI films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 810–818, 2006     

Enhancement of electromechanical properties of natural rubber by adding barium titanate filler: An electro-mechanical study

Natural rubber is one of the most potential electro-active polymers for sensors, actuators, and energy harvesting applications. Enhancing the characteristic properties of polymers by reinforcing with fillers that possess multifunctional attributes have attracted considerable attention. In the present study, barium titanate reinforced natural rubber composite is prepared by using two-roll mill mixing. Afterwards, mechanical, electrical, and electromechanical properties of the composites are extensively analyzed by reinforcing different amounts of barium titanate into the matrix of natural rubber. The fabricated dielectric composite shows excellent properties such as high dielectric constant, low dielectric losses, high dielectric breakdown strength, and extreme stretchability. It is observed that as the filler loading reaches the value of 11 parts per hundred rubber (phr), maximum agglomeration of the particles occurs. Maximum stretchability and highest ratio of dielectric constant to elastic modulus are obtained at 8 phr of barium titanate fillers and at the loading, a maximum actuation strain of 11.24% is achieved. This study provides a simple, economical, and effective method for preparing enhanced mechanical, electrical, and electromechanical properties of natural rubber composites, facilitating the wide applications of dielectric materials as actuators and generators.     

Silicone elastomers with high dielectric permittivity and high dielectric breakdown strength based on dipolar copolymers

Dielectric elastomers (DEs) are a promising new transducer technology, but high driving voltages limit their current commercial potential. One method used to lower driving voltage is to increase dielectric permittivity of the elastomer. A novel silicone elastomer system with high dielectric permittivity was prepared through the synthesis of siloxane copolymers, thereby allowing for the attachment of high dielectric permittivity molecules through copper-catalysed azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The copolymers have a high degree of chemical freedom, as the dimethylsiloxane spacer units between the functional groups, as well as the degree of functionalisation, can be varied. Thus, the best overall properties were obtained for an elastomer prepared with a copolymer with a 1200 g mol⁻¹ dimethylsiloxane spacer unit and 5.6 wt% of the high dielectric permittivity molecule 1-ethynyl-4-nitrobenzene. Here, a high increase in dielectric permittivity (∼70%) was obtained without compromising other favourable DE properties such as elastic modulus, gel fraction, dielectric loss and electrical breakdown strength.     

Electromechanical instability in silicone‐ and acrylate‐based dielectric elastomers

Electromechanical instability (EMI) is regarded as a significant factor in preventing dielectric elastomers (DEs) from achieving large voltage‐induced deformations. In this study, the strain‐stiffening effect was used to control the occurrence of EMI in DEs. The results show that the stretching ratio required to provide a feasible strain‐stiffening effect in silicone rubber (SR) was smaller than that needed for a commercial DE material, VHB 4910. The experimental data were compared with currently used models for the simulation of EMI in DEs. We found that EMI could be eliminated in the deformation of these elastomers when prestretching was used. Through the application of a prestretching ratio of above 2.0, EMI was suppressed in both the VHB 4910 and SR samples. The findings of this research are of great significance in the maximization of the electromechanical performance of DE materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45733.     

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.     

Material characterization of a high‐dielectric‐constant polymer–ceramic composite for embedded capacitor for RF applications

Embedded capacitor technology can improve electrical performance and reduce assembly cost compared with traditional discrete capacitor technology. Polymer–ceramic composites have been of great interest as embedded capacitor materials because they combine the processability of polymers with the desired electrical properties of ceramics. We have developed a novel nanostructure polymer–ceramic composite with a very high dielectric constant (ε_r ≈ 150, a new record for the highest reported ε_r value of a nanocomposite) in a previous work. RF applications of embedded capacitors require that the insulating material have a high ε_r at a high frequency (in the gigahertz range), low leakage current, high breakdown voltage, and high reliability. A set of electrical tests were conducted in this study to characterize the electrical properties of the novel high‐ε_r polymer–ceramic nanocomposite developed in‐ house. The results show that this material had a fairly high ε_r in the RF range, low electrical leakage, and high breakdown voltage. An 85°C/85% thermal humidity aging test was been performed, and it showed that this novel high‐K material had good reliability. An embedded capacitor prototype with a capacitance density of 35 nF/cm² was manufactured with this nanocomposite with spin‐coating technology. This novel nanocomposite can be used for the integral capacitors for RF applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2228–2231, 2004     

The effect of inorganic and organic nucleating agents on the electrical breakdown strength of polyethylene

Altering the morphology of polyethylene affects physical and electrical properties with reduced spherulite size correlating with higher electrical breakdown strength. Nucleating agents in polyethylene influence the final crystal morphology by increasing the number of spherulites and reducing spherulite size. Few studies are available that relate the nucleating activity to improved electrical breakdown strength. Although nanosilica is known to improve electrical breakdown strength of polyethylene in addition to serving as a nucleating agent, previous studies have not fully addressed the relationship between the improved breakdown strength and nucleating activity. In this article, direct current electrical breakdown strength and nucleation effects on morphology are assessed on a single set of controlled polyethylene compositions containing two types of surface treated nanosilica particles. The results are compared to composites with two types of organic nucleating agents 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol or calcium 1,2‐cyclohexanedicarboxylate (CDA). CDA was the most effective organic nucleating agent and the hexamethyldisilizane treated nanosilica was the most effective inorganic nucleating agent in reducing spherulite sizes in low density polyethylene (LDPE). Reduced spherulite sizes in nucleated samples correlated with increased breakdown strength and lower conduction current compared to the neat LDPE. The LDPE sample with CDA also had the highest increase in crystallization temperature indicating stronger nucleating agent performance than the nanosilica and 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol composite samples. The addition of these inorganic and organic nucleating agents all resulted in improvements in electrical breakdown strength. The results show that nucleation deserves more attention as a potential cause for improved breakdown strength observed with silica and organic nucleating agents. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46325.     

Octakis(phenyl)-T8-silsesquioxane-filled silicone elastomers with enhanced electromechanical capability

The first dielectric elastomer actuators based on electroactive nanocomposites with octakis(phenyl)-T8-silsesquioxane (phenyl-T8), obtained ex-situ, used as voltage stabilizer filler for silicone elastomers are reported. The incorporation and homogeneous dispersion of crystalline phenyl-T8 in percentages of 2.5, 3.5, 5, and 10 into the amorphous matrix consisting in a polydimethylsiloxane-α,ω-diol with Mn = 240,000 g/mol was successfully achieved by solution mixing and crosslinking. For the sample with the best actuation performance (that containing 3.5 wt.% filler), an optimized filled elastomer was obtained by dispersing 3.6 wt. % phenyl-T8 in the matrix using a suitable surfactant (Pluronic L81), thus gaining an increased electrical breakdown of 30% compared with the pristine sample. Beside dielectric strength, the matured films were characterized in terms of morphology, mechanical, dielectric and actuation tests. In spite of structural incompatibility between the filler and the matrix, the obtained materials are soft elastomers showing high strain (~800%) and low Young's modulus of 50–100 kPa. The use of phenyl-T8 in a silicone matrix lead to electroactive films with slightly increased lateral actuation strain and electric breakdown strength.     

变压器油质量的主要影响因素及改进探讨

评价变压器油电气性能的指标主要是介质损耗因素（90℃）、击穿电压。而变压器油介质损耗因素、击穿电压的影响因素主要是杂质和水分。润滑油茂名分公司通过改造抽真空过滤系统,增强脱水效果,有效降低介质损耗因素和含水量,提高击穿电压,确保变压器油的质量。     

Electro-Thermal model of thermal breakdown in multilayered dielectric elastomers

Energy transduction of dielectric elastomers involves minute electrical and mechanical losses, both of which potentially increase the temperature within the elastomer. Thermal breakdown of dielectric elastomers occur when heat generated therein cannot be balanced by heat loss on the surface, which is more likely to occur in stacked dielectric elastomers. In this article an electro-thermal model of a multilayered dielectric elastomer able to predict the possible number of layers in a stack before thermal breakdown occurs is presented. Simulation results show that point of breakdown is greatly affected by an increase in surrounding temperature and applied electric field. Furthermore, if the stack diameter is large, thermal insulation of the cylindrical surface is a valid approximation. Two different expressions for the electrical conductivity are used, and it is concluded that the Frank-Kamenetskii expression is more conservative in prediction of point of breakdown than the Arrhenius expression, except at high surrounding temperature. © 2018 American Institute of Chemical Engineers AIChE J, 65: 859–864, 2019     

Control of electromechanical properties of multilayer ceramic capacitors for vibration reduction

A multilayer ceramic capacitor (MLCC) contains layers of ceramics as the dielectric materials. It has been known that Class 2 MLCCs, made of ferroelectric ceramics such as barium titanate, tend to suffer from electromechanical coupling hence vibration, which leads to the generation of acoustic humming noise, a source of annoyance in many modern electronic devices. In this article, a repoling method to control the electromechanical properties and the resulting vibration of MLCCs is presented. The repoling protocol hinges on the understanding that two independent mechanisms are responsible for the electromechanical coupling in MLCCs: piezoelectricity and electrostriction of the ceramic layers. The vibration due to piezoelectricity is linearly proportional to the input voltage, whereas the vibration due to electrostriction shows a quadratic dependence. Given the DC bias and the AC input voltage under normal operating conditions, the vibration is composed of the fundamental component at the frequency of the AC input and the second harmonic component spawned by the quadratic nonlinearity of electrostriction. It is demonstrated that by engineering the coefficients of piezoelectricity and electrostriction of the ceramic layers through a carefully designed repoling treatment, vibration reduction can be achieved for both the fundamental and second harmonic components. Especially, the fundamental component of vibration can be reduced significantly, as the piezoelectric effect is made to offset the electrostrictive effect.     

Copper phthalocyanine oligomer grafted acrylic elastomer nanocomposites with high dielectric constants

For many applications of dielectric elastomer (DE) actuators, it is desirable to endow the DE with a high dielectric constant (ε), high breakdown field, and good flexibility. In this study, a high‐ε nanocomposite acrylic elastomer (ACM)‐g‐copper phthalocyanine (CuPc) was fabricated, in which the CuPc oligomer was grafted onto the backbone of ACM. This grafted composite exhibited several benefits over the physically blended one. Transmission electron microscopy micrographs indicated that the size of the grafted CuPc was in the range 15–30 nm, which was more than 25 times smaller than that of the simply blended one. At room temperature, ε of ACM‐g‐CuPc (with 15 wt % CuPc) reached 303 at 100 Hz. The remarkable enhancement in the dielectric response could be attributed to the greatly strengthened exchange coupling effect and the Maxwell–Wagner–Sillars polarization mechanism. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39975.     

Contribution to the understanding of the relationship between mechanical and dielectric strengths of Alumina

The experimental evolutions of the Alumina dielectric strength versus thickness (127 μm to 2.54 mm), purity (92%, 96% and 99.5%) and crystallography (single or polycrystal) have been investigated. In order to find crucial information about the mechanism responsible for the dielectric breakdown, optical and scanning electron micrograph observations have also been performed. Each breakdown channel was found to be terminated by a crater from which matter has been extracted during the breakdown process. Investigations have been focused on the breakdown path, on the evolution of the crater size versus sample thickness and on the location of molten matter after breakdown. The results tend to confirm that the dielectric breakdown of Alumina is probably originated from a mechanical failure induced by electromechanical forces acting during the voltage application.     

Nanofiltration modeling: the role of dielectric exclusion in membrane characterization

A general model is presented, called Donnan steric pore model & dielectric exclusion (DSPM&DE) to describe mass transfer of electrolytes and neutral solutes through nanofiltration membranes. The transport equations of ions through the membrane are based on the extended Nernst-Planck equation, accounting for ionic diffusion, electromigration and convection in the membrane pores; the hindered nature of diffusion and convection of the species inside the membrane is considered. Ionic partitioning at the interfaces between the membrane and the external phases takes into account of three separation mechanisms: steric hindrance, Donnan equilibrium and dielectric exclusion. The role of the difference existing between the dielectric constant of the aqueous solution in the pores and the dielectric constant of the membrane material is assumed dominant in determining the rejection mechanism related to the dielectric effects. The membrane is characterized through the use of adjustable parameters such as the average pore radius, the effective membrane thickness and the volumetric charge density.A general assessment is presented for membrane characterization. A new procedure is introduced for the membrane parameters calculation, based on a simple analytical relationship developed to describe rejection of single symmetric electrolytes.Two versions of the DSPM&DE model are presented, in which the “integral” model is a weak simplification of the general “differential” model. The validity of the models presented is fully demonstrated in the case of negatively charged membranes, through the comparison with experimental results performed in a wide range of operative conditions. The separation effect related to the dielectric exclusion is relevant with respect to Donnan equilibrium in determining bivalent counter-ions rejection, such as CaCl₂ as well as MgSO₄, whereas dielectric effects are not so remarkable in the case of mixtures containing various co-ions, such as NaCl+Na₂SO₄.     

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.     

Natural rubber/ethylene propylene diene blends for high insulation iron crossarms

This research studied the composition and behavior of natural rubber (NR) and ethylene propylene diene monomer (EPDM) blends at various carbon black concentrations (0–30 phr) in terms of electrical resistivity, dielectric breakdown voltage testing, and physical properties. The blends having electrical properties suitable for application in high‐insulation iron crossarms were selected for investigation of compatibility and increased physical properties. The effect of the homogenizing agent concentration on improvement of compatibility of blends was studied by scanning electron microscopy, pulsed nuclear magnetic resonance spectroscopy, and rheology techniques. We also examined mechanical properties such as tensile strength, tear strength, elongation at break, and hardness. The NR/EPDM blends filled with a fixed concentration of silica were investigated for ozone resistance. A carbon black content as high as 10 phr is still suitable for the insulation coating material, which can withstand electrical voltage at 10 kVac. Addition of the homogenizing agent at 5 phr can improve the mechanical compatibility of blends, as evidenced by the positive deviation of shear viscosity of the rubber blend, that is, the calculated shear viscosity being higher than that of experimental data. Moreover, the pulsed NMR results indicated that the spin‐spin relaxation (T₂) of all three components of the rubber blend was compressed upon the addition of the homogenizing agent. The ratio of NR/EPDM in the blend to best resist the ozone gas is 80/20 with the addition of silica of 30 phr into the blend. Also, the NR/EPDM filled with silica had a decreased change in thermal and mechanical properties of blends after thermal aging. The synergistic effect of silica content and high NR content (80) in 20 phr EPDM could improve antioxidation by ozone in the absence of a normal antioxidant for natural rubber. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3401–3416, 2004     

Inhomogeneous Conductivity in Varistor Ceramics: Methods of Investigation

The grain boundaries in zinc oxide ceramics exhibit different electrical behaviors. This results in separate paths of current flux through the microstructure and also in different breakdown voltages of each path. The paper describes some new methods for characterization of these paths. A galvanic determination is able to show the number of paths and their distribution across the varistor surface. The differences in breakdown voltage are visible using a line scan method. Current images in SEM can detect the paths of current along a varistor surface. Possible reasons for inhomogeneous current flux are inhomogeneous distribution of dopants, insufficient binder burnout, and pressing faults.     

Structural,dielectric and piezoelectric properties of xBiFeO3–(1−x)BaTi0.9Zr0.1O3 ceramics

Effect of BiFeO₃ (BFO) content on the microstructure and electrical properties of BaTi_0.9Zr_0.1O₃ (BTZ) ceramics prepared by the solid-state reaction technique was investigated. X-ray diffraction analyses show that BFO diffused into the lattice of BTZ to form a solid solution with perovskite structure. The relative density of the BTZ ceramics is increased by the introduction of BFO. The dielectric study reveals that the dielectric constant and the average dielectric loss of the solid solution decreased simultaneously with an increase in BFO content. The materials undergo a diffuse type ferroelectric phase transition. The diffusivity increases with increase in BFO contents in the studied composition range. On the other hand, the piezoelectric coefficient and electromechanical coupling coefficient decrease simultaneously with increasing the BFO content, whereas the mechanical quality factor increases gradually. The structure–property relationship and the mechanism associated with the change of the electrical properties are discussed intensively.