Field Dielectric Properties Research Articles

Processing and characterisation of hot rolling pressed PVDF films with enhanced field-induced polarisation

Poly (vinylidene fluoride) (PVDF) based polymers are good candidates for high-power energy storage in dielectric capacitors. So far, most studies reporting PVDF with high energy density are based on solution casting, a method ill-suited for large-scale production. In this work, we develop PVDF films with high crystallinity (47%) and ultrahigh β-phase (∼97%) using Hot Rolling Press (HRP). During HRP, the crystallinity, fraction of β-phase, internal stress, and the piezoelectric coefficient d33 increased with rolling temperature. The anisotropic stress imposed by two rollers led to a highly preferred orientation of the HRP film along the rolling direction. Notably, the HRP film exhibits an enhanced field-induced polarisation Pin-max value (0.16 C m−2) compared to conventional PVDF films (0.10 C m−2), with an energy density of 32.6 J cm−3 at 871 kV mm-1. The good low field dielectric (εr ∼19 @ 1 kHz) and piezoelectric (d33 ∼30 pC N-1) properties with high-temperature stability (up to 100 °C), and its suitability for in-line continuous production, make HRP PVDF films promising for low field dielectric and piezoelectric applications.

Non‐uniform voltage distribution characteristics of metallized film capacitors in flexible HVDC projects

AbstractThe problem of non‐uniform voltage distribution in the internal element of metallized film capacitors in flexible HVDC transmission systems exists, the influence of electrical parameters of dielectric and metal copper row are not considered in the existing metallized film capacitor equivalent circuit model. In this paper, the conductance of biaxially oriented polypropylene film (BOPP) for metalized film capacitors is measured at different electric field intensity and dielectric properties at different frequencies. The current density distribution and voltammetric properties of the metalized film capacitor elements under AC and DC superimposed voltages are calculated by the finite element method, and the voltammetric properties of the copper row considering the skin effect are derived. The equivalent circuit model of the metalized film capacitor under the AC‐DC superposition voltage is proposed. The simulation results of the capacitor equivalent circuit model show that the voltage distribution of the internal elements of the capacitor is uniform at low frequency AC voltages (<102 Hz), and the voltage amplitude and phase difference between the elements of the capacitor gradually increases with increasing AC voltage frequency. At an AC voltage frequency of 105 Hz, the difference in magnitude between the two capacitor elements on the same copper row was up to 2.01% and the phase difference was 13.5%, while the difference in magnitude between capacitors at the same horizontal position on different copper rows was up to 56.8% and the phase difference was 10.44%. The results of this paper provide some guidance for the modeling of the internal components of metalized film capacitors, the calculation of voltage distribution and the structural optimization design.

Paraelectric Doping Simultaneously Improves the Field Frequency Adaptability and Dielectric Properties of Ferroelectric Materials

Paraelectric Doping Simultaneously Improves the Field Frequency Adaptability and Dielectric Properties of Ferroelectric Materials

Piezoelectric biosensor for smart cardiovascular grafts based on NaNbO3 fibers/PDMS structured composite

An artificial graft inserted in the vascular system can cause thrombosis, which may lead to catastrophic consequences. New intelligent vascular prosthetics that can recognize the presence of clots are urgently needed. For this purpose, a novel biocompatible piezocomposite is designed for self-monitoring smart vascular grafts. The new insight of this work involves the fabrication of a composite with low filler content (5 %v) and high piezoelectric sensitivity (g33 ∼ 130 mV m N−1) suitable for biosensors. The composite is produced by structuring fibers (NaNbO3 fibers) in elastomeric matrix via dielectrophoresis. The matrix with low elastic modulus offers great stretchability to prevent graft performance deterioration. Numerical simulation together with experimental characterizations confirm the improvements in terms of electric field distribution, dielectric and piezoelectric properties. The new composite, integrated on a graft and placed into a motorized cardiovascular simulator performs successfully online monitoring of the blood pressure, identifying anomalies related to a thrombus. The composite coupled with an inductive coil forms a resonant sensor, whose resonance frequency shifts when an occlusion develops. In the future, this system could be implemented using radio frequency identification tags (RFID-tag) to wirelessly supervise the condition of the graft, paving the way to a next generation of self-monitoring grafts.

Enhancing high field dielectric properties of polymer films by wrapping a thin layer of self-assembled boron nitride film

Polymer dielectrics play an important role in modern electric power systems because of their ultrahigh power density, low cost, and lightweight. Current high-k polymers represented by poly(vinylidene fluoride) (PVDF) based ferroelectric polymers suffer from the high energy loss as well as the low temperature level. To overcome the conduction induced high loss, in present work, a BN thin layer fabricated from an oil-water self-assembly method is transferred on poly(vinylidene fluoride-chlorotrifluoroethyelen) (P(VDF-CTFE)) films. The BN layer with high bandgap may significantly depress the dielectric loss of polymer films, and reduce the charge injection in the films at both elevated temperature and electric field. That is responsible for the dramatically increased breakdown strength, energy density, discharge efficiency and even temperature level of P(VDF-CTFE). The optimized sample (c.a. BP-3) possesses a high energy density of 12.95 J/cm3 at 525 MV/m at ambient temperature. At 80 °C, BP-3 sample has an energy density of 6.38 J/cm3, which is nearly 8 times that of pristine P(VDF-CTFE) (0.78 J/cm3). This work offers a promising strategy to improve the breakdown strength and the dielectric performance of polymeric dielectrics.

High field dielectric properties of clay filled silicone rubber composites

The present research work was aimed at improving the high field dielectric properties of silicone rubber (SR) composites used for power cable insulation. Liquid silicone rubber composites were developed using two types of clays as filler materials; organically modified montmorillonite (ommt) and calcined, surface treated kaolin with the commercial name Translink 37 (T37). The results revealed that the surface chemistry of the clay had a distinct effect on both dispersion as well as electrical performance of silicone rubber-clay composites. We observed that surface treated kaolin (T37) were well dispersed in the SR matrix, but had adverse impact on the crosslinking kinetics of liquid silicone rubber resulting in higher dielectric loss at low frequencies. On the other hand, organically modified clay showed poor dispersion in the silicone rubber matrix while exhibiting much lower dc conductivity characteristics at high temperature. The agglomerated ommt macroparticles introduced the interfacial traps in SR matrix and charge carriers migration was limited within the matrix.

Magnetically influenced dielectric and electrical transport of inorganic–organic polymer-based hybrid nanocomposites

Polymer-based hybrid nanocomposites of [Formula: see text]-([Formula: see text]) poly(vinylidene fluoride) (PVDF) ([Formula: see text], 0.6 and 0.7) have been prepared through low-cost chemical reaction process. The improvement of dielectric properties in polymer-based hybrid [Formula: see text]–PVDF nanocomposites can be utilized to make a suitable memory storage device. Surface modified [Formula: see text] (LFO) nanoparticles are well distributed inside the polymer resulting in improved dielectric constant and reduced dielectric loss. The study of complex and modulus impedance spectroscopy (IS) shows some interesting results for an influence of grain and grain boundary effects of electrical conductivity attributing the non-Debye type phenomena. The change of relaxation frequency with an applied magnetic field is ascribing the spin-dependent mechanism of electrical transport at grain boundaries (GBs). The fitting of Nyquist plot attributes the magnetic domain wall containing GB’s pinning center in the system. The higher concentration of filler LFO nanoparticles shows maximum values of dielectric constant assigning the Maxwell–Wagner–Sillars (MWS) polarization. The current study is focused towards the query to learn the complete study of magnetic field dependence electrical and dielectric properties in a system holding an excessive potential for a capable candidate for industrial applications.

Dielectric and Breakdown Properties of MWCNT- and OMMT-Reinforced Epoxy Composites

Development of the power industry, especially in the field of high-power electricity, is limited by the insulating materials. The main problem of power generation equipment, especially ultra-high-voltage and high-power equipment, is the ability of insulating materials to withstanding voltage. Epoxy resin (EP) is widely used in the power industry. Therefore, it is necessary to improve the performance of EP and demonstrate an energy model of material electrical breakdown. EP has excellent insulation properties, yet poor toughness. By doping with nanomaterials, the toughness of the matrix can be improved on the premise of having little influence on insulation performance. In this paper, two kinds of nanomaterials were added into the matrix, namely multi-walled carbon nanotubes (MWCNTs) and organically modified montmorillonite (OMMT). MWCNTs are conductive, and the insulation properties of EP will be adversely affected if doped in large quantities. OMMT is a nanomaterial with lamellar structure. The toughness of EP can be improved by being doped with OMMT without adverse effect on the insulation performance. Various factors affecting the electric field strength and dielectric properties are analyzed, and models of electric and thermal breakdown are established. In addition, by observing the phenomena of material after breakdown, the energy relation equation of breakdown is re-established.

High field dielectric property and piezoelectric response in PMS-PZT piezoelectric ceramics modified with BiFeO3

ABSTRACTThe piezoelectric ceramic Pb0.98Sr0.02(Mn1/3Sb2/3)0.05Zr0.48Ti0.47O3+0.25 wt%CeO2 modified with BiFeO3 was synthesized by conventional ceramics technology and the high field dielectric property was investigated. The XRD patterns show that BiFeO3 can be commendably dissolved into the PMS-PZT solid solution. Bi3+ replacement for A-site Pb2+ and Fe3+ replacement for B-site Ti4+ or Zr4+ cause BiFeO3 to act as “soft” and “hard” dopant. Therefore, an appropriate amount of BiFeO3 enhances the piezoelectric properties and high field dielectric properties simultaneously. The ceramic modified with 0.2 mol% BiFeO3 exhibited the following optimal properties: d33 = 359pC/N, kp = 0.622, Qm = 1519, ϵr = 1760, tanδ = 0.46% (1 kHz, 1V/mm), and tanδ = 3.26% (1 kHz, 500V/mm).

Structured Polyethylene Nanocomposites: Effects of Crystal Orientation and Nanofiller Alignment on High Field Dielectric Properties

ABSTRACTIn previous work we have shown that aligned high aspect-ratio (pseudo-2D) nanofillers can yield large dielectric breakdown strength (EBD) improvements for a nanocomposite with a low-crystallinity polyethylene matrix. Here, we report a systematic study which delineates the contributions of the aligned inorganic fillers and of the aligned polymer crystallites in the overall EBD improvement achieved in the nanocomposites. Specifically, extrusion blown-molded polyethylene/montmorillonite nanocomposite films were cold-stretched to various strains, to further align the nanoparticles parallel to the film surface; this filler alignment is accompanied by a commensurate alignment of the polymer crystallites, especially those heterogeneously nucleated by the fillers. A systematic series of films are studied, with increased extent of alignment of the fillers and of the crystalline lamellae (quantified through Hermans orientation order parameters from 2D X-ray diffraction studies) and the aligned structure is correlated to the electric field breakdown strength (quantified through Weibull failure studies). It is shown that aligned pseudo-2D inorganic nanofillers provide additional strong improvements in EBD, improvements that are beyond, and added in excess of, any EBD increases due to polymer-crystal orientation.

Temperature dependence of dielectric and ferroelectric properties for (1−x)Bi0.5(Na0.8K0.2)0.5TiO3-xBaZn0.5W0.5O3 lead-free piezoelectric ceramics

ABSTRACTThe ferroelectric and piezoelectric properties of (1−x)Bi0.5(Na0.8-K0.2)0.5TiO3-xBaZn0.5W0.5O3 ((1−x)BNKT-xBZW) ceramics have been studied to clarify the temperature dependence of the polarization-electric field (P-E) hysteresis and dielectric properties. Based on the temperature dependence of the dielectric constant, it was found that the depolarization temperature (Td) shifted to lower temperatures, depending on the composition x. The crystal structure refinement of (1−x)BNKT-xBZW ceramics with rhombohedral and tetragonal phases revealed that the fraction of tetragonal phase to rhombohedral phase increased in the composition range of 0-0.05, which implies a formation of a morphotropic phase boundary (MPB). The piezoelectric constant (d33) of the (1−x)BNKT-xBZW ceramics increased with the composition x up to x = 0.05 and the maximum value was 168 pC/N. Such an increase in d33 is attributed to the increase in the dielectric constant at room temperature which is due to the shift of Td; a pinched P-E hysteresis loop was obtained at x = 0.05. The temperature dependence of the P-E hysteresis loops indicated that the shape of the hysteresis loop is closely related to the coexistence of ferroelectric and antiferroelectric phases.

Local Field Factors and Dielectric Properties of Liquid Benzene.

Local electric field factors are calculated for liquid benzene by combining molecular dynamic simulations with a subsequent force-field model based on a combined charge-transfer and point-dipole interaction model for the local field factor. The local field factor is obtained as a linear response of the local field to an external electric field, and the response is calculated at frequencies through the first absorption maximum. It is found that the largest static local field factor is around 2.4, while it is around 6.4 at the absorption frequency. The linear susceptibility, the dielectric constant, and the first absorption maximum of liquid benzene are also studied. The electronic contribution to the dielectric constant is around 2.3 at zero frequency, in good agreement with the experimental value around 2.2, while it increases to 6.3 at the absorption frequency. The π → π* excitation energy is around 6.0 eV, as compared to the gas-phase value of around 6.3 eV, while the experimental values are 6.5 and 6.9 eV for the liquid and gas phase, respectively, demonstrating that the gas-to-liquid shift is well-described.

Study of the role of porosity on the functional properties of (Ba,Sr)TiO3 ceramics

The role of porosity on the low and high field dielectric properties was studied in Ba0.70Sr0.30TiO3 ceramics with various porosity levels obtained by using lamellar graphite as sacrificial template. The permittivity decreases with increasing porosity, from around ε∼7690 (dense ceramic) down to 380 (ceramic with 29% porosity), while preserving the Curie temperature at about 35°C. The effective permittivity was discussed by using Effective Medium Approximation and Finite Element Method approaches. The role of porosity and specific microstructural characteristics induced by the pore forming addition, from closed porosity (0–3) towards a combined (0–3, 2–2) with lamellar-type of microstructures for the most porous ceramics was taken into consideration. All the investigated ceramics preserve a high level of tunability as in the dense material, irrespective of the porosity level, while the zero field permittivity was decreased to a few hundreds.

Numerical Simulation of High Frequency Electromagnetic Wave in Microwave Cavity for Soot Oxidation

Soot oxidation temperature by high frequency electromagnetic energy was proposed using numerical simulation by combining electromagnetic with transient thermal analyses. Equation of electric field distribution in a microwave cavity with perfect electric conductor surfaces and TE10 mode is formulated from Helmholtz equation. The dissipated heat distribution is calculated from the electric field distribution. Six study cases for electric field and dissipated heat distributions were implemented by using ANSYS software based on finite element method. The impact of dielectric sample properties, position, size and shape inside the microwave cavity were predicted. The results from the simulation of electric field and dissipated heat were compared with available data in literature and showed the validity of the analysis. It was found that the electric field forming hot spots at penetration depth and front corners of the soot sample and penetration depth is equal to 12mm but equal to 0 for samples with dimensions less than penetration depth. Dissipated heat pattern depend on electric field pattern and dielectric properties.

Endurance of Thin Insulation Polyimide Films for High-Temperature Power Module Applications

This paper deals with thermal endurance tests carried out on thin polyimide (PI) films widely used in high-temperature power electronics applications. Specimens deposited on stainless steel supports are aged in an oven in air at four different temperatures up to the glass transition temperature (between 250 $^{\circ}{\rm C}$ and 360 $^{\circ}{\rm C}$ ). Dielectric strength is measured at different aging times as an indicator of PI films endurance. In this paper, in addition to this property, other properties are measured during aging, such as dc conductivity at low field, film thickness, and film morphology. The evolution of these parameters is correlated to the breakdown voltage of the films to establish more accurate endpoint criteria. The results show that the breakdown voltage decreased during aging but not likewise over the whole aging period, because of the effect of the initial surface degradation. This might lead to an underestimation of the lifetime of the films. The thickness degradation rate is constant over the rest of the aging period, making it a suitable endpoint criterion. However, the low field dielectric properties show an improvement during the aging making them an unsuitable aging marker.

Study of field emission and dielectric properties of AlN films prepared by DC sputtering technique at different substrate temperatures

Nanocrystalline AlN thin films were prepared via DC sputtering technique at different substrate temperature. The crystal orientation and particle size of aluminum nitride thin films were investigated by XRD analysis. Study indicated that the sample contained pure phase hexagonal AlN nanoparticles with a single peak corresponding to the (100) planes. The peak at 665 cm−1 in the FTIR spectrum of film was assigned to the LO phonon of hexagonal AlN. The particle size of the film, prepared at substrate temperature 200°C was about 9.5 nm, as investigated by atomic force microscope. Field emission study indicated that it can be used as a good field emitter. Turn-on field (Eto) of 15.02 V/μm was observed for the AlN films synthesized at substrate temperature 200°C. Dielectric constant of the AlN film was found nearly independent of frequencies in the measured frequency range 1 KHz to 1 MHz, i.e. in the audio frequency range. The values of dielectric constant (e) were 10.07, 9.46 and 8.65 for the film prepared at 70°C, 150°C and 200°C, respectively, at frequency 1 KHz.

Reverse dynamic calorimetry of a viscous ionic liquid

We compare the time scale of thermal relaxation with that of the electric modulus in the deeply supercooled regime of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Thermal relaxation refers to the process of configurational temperatures of the slow degrees of freedom equilibrating toward the vibrational temperature, which is a reliable indicator for the time scale of structural relaxation. Energy is supplied to the sample by absorption from a sinusoidal electric field with amplitude as high as 387 kV/cm and frequencies in the 0.2 Hz-56 kHz range, analogous to microwave heating. The time resolved response of configurational temperature as well as the low field dielectric properties are measured in a single high field impedance setup. Near T(g), we find that the macroscopic field (or modulus M) relaxes considerably faster than the structure (in terms of thermal relaxation, solvation dynamics, and probe rotation), although the liquid is entirely composed of mobile ions.

Electrostrictive forces on vesicles with compartmentalized permittivity and conductivity conditions

Electrostrictive forces on the plasma membrane of a lipid bilayer vesicle that result as a consequence of an applied electric field and differential dielectric material properties can be calculated via the Maxwell stress tensor. In this situation, the plasma membrane is proposed as a barrier that separates compartments of a system with different conductivity and relative permittivity values. A numerical model of this case is presented. Model force calculations compare with analytical equation results and were used to validate published experimental work. The model also was used to study electrostatic forces in a simple vesicle system contrasting such forces to frequency dependent deformations. Model results for vesicles in variable conductivity and relative permittivity environments are analyzed to build a framework with the potential to become a tool to study more complex problems with multiple compartments such as cells and tissues. Impedance spectroscopy is also explored as a potential experimental method to predict cell and tissue system behavior in the presence of electric fields.

Influence of substrate microstructure on the high field dielectric properties of BaTiO3 films

The temperature dependence of the electrical leakage current density of chemical solution deposited BaTiO3 films on high purity Ni foils was investigated as function of the underlying Ni microstructure. Depending on the Ni heat treatment prior to BaTiO3 deposition, it was found that pores in the dielectric followed the profiles of the underlying Ni grain boundary grooves. The electrical properties were then characterized on capacitors with and without the presence of Ni grain boundaries. When a Ni grain boundary from the substrate was present in the capacitor used during the electrical measurements, the loss tangent of the capacitor rose rapidly for dc biases exceeding ∼25 kV/cm. The critical bias increases to ∼100 kV/cm when no substrate grain boundaries are included in the capacitor. In addition, the capacitance-voltage curves are much more symmetric when grain boundaries are absent. This disparity in the electrical behavior was analyzed in terms of the mechanisms of charge conduction across the Ni-dielectric interface. While a reverse biased Schottky emission mechanism dominates the current in areas free of Ni grain boundaries, the barrier at the cathode is ineffective when Ni grain boundaries are present in the substrate. This, in turn, leads to a larger leakage current dominated by the forward biased Schottky barrier at the anode. The results are important to both embedded and surface mount capacitors.

Templated Grain Growth of Textured Piezoelectric Ceramics

ABSTRACT Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6, and (Na1/2Bi1/2)TiO3-BaTiO3. Highly oriented Bi4Ti3O12, Sr2Nb2O7, CaBi4Ti4O15, Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6 and (Na1/2Bi1/2)TiO3-BaTiO3 ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d 33) of ∼1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.