Search

In this study, the byproduct-driven conduction current in LDPE was investigated. We built an experimental fixture to measure bulk conductivity in LDPE soaked with byproducts. Ohmic conduction was observed below 10 kV/mm. At above 10 kV/mm, space charge limited current (SCLC) predicts observed conduction in LDPE soaked with byproducts. It is found that the addition of byproducts introduces charge traps with discrete energy levels which dominate conduction. Degassed samples, on the other hand, show slopes of greater than 2.5 in a current-voltage log-log plot which was attributed to the charge traps with continuous energy levels. By analyzing the thickness dependence of the current, we observed a transition from electrode- to bulk-limited current at above 20 kV/mm for acetophenone- and α-cumyl alcohol-soaked samples. The α-methylstyrene-soaked samples only show electrode-limited current.

The intrinsic diffusion of the acetophenone in low density polyethylene (LDPE) is characterized with infrared mapping and it is found to be nearly Fickean with decaying diffusion coefficient over time. The impact of temperature on the diffusion coefficient is examined and an Arrhenius relationship is derived for the temperature dependence. To study the influence of electric field on the acetophenone diffusion, field-test cells were designed and constructed. The infrared mapping revealed presence of a gradient in the acetophenone concentration. Using the COMSOL simulation, we observed the higher acetophenone concentration near high electric field gradient regions. This is attributed to the dielectrophoresis phenomenon that occurs due to higher permittivity of the acetophenone molecules compared to the surrounding medium. The findings of this study help to understand where the byproducts migrate in high voltage power cables.

The relaxor ferroelectric materials, because of their large and reversible electric field induced polarization, have been demonstrated to possess giant electrocaloric effect (ECE) over a broad temperature range, which are attractive for refrigeration with high energy efficiency and environmental friendliness. However, high electric fields are required to generate the giant ECE in these materials, posing challenge for these materials in practical cooling devices which also require high reliability and low cost. Here, a general approach is reported, for example, establishing an internal bias field in these relaxor ferroelectric polymers, to significantly improve ECE which can be induced at low electric fields. It is demonstrated that in a polymer blend (nanocomposite) with a properly controlled normal ferroelectric in nanophase dispersion in the relaxor polymer matrix, the charge neutrality in the blends can cause an internal biasing field, leading to more than 45% enhancement in the ECE at low electric field (≈50 MV m−1). This internal biasing approach provides a universal strategy to enhance other low field responses such as the electromechanical response in relaxor ferroelectrics.

As multifunctional electroactive materials, ferroelectric polymers are unique owing to their exceptionally high dielectric strength (>600 MV/m), high flexibility, and easy and low-temperature fabrication into required shapes. Although polyvinylidene difluoride (PVDF)-based ferroelectric polymers have been known for several decades, recent findings reveal the potential of this class of electroactive polymers (EAPs) to achieve giant electroactive responses by tuning the molecular, nano, and meso-structures. This paper presents these advances, including giant electrocaloric effect, giant electroactuation, and large, hysteresis-free polarization response. New developments in materials benefit applications, such as environmentally benign and potentially highly energy-efficient electrical field controlled solid-state refrigeration, artificial muscles, and high-energy and power density electric energy storage devices. The challenges in developing these materials to realize these applications, and strategies to further improve the responses of EAPs will be also discussed.

It is important to understand electronic and ionic transport in low-alkali boroaluminosilicate glass, which is a promising dielectric for high energy density capacitors. Electric field, thickness, and temperature dependence of the leakage currents for low-alkali boroaluminosilicate (Schott AF 45) were all investigated to elucidate potential conduction mechanisms under high electric field. The bulk conductivity ranged from 10−16 S/m at room temperature to 10−12 S/m at 200 °C. It was also found that conduction significantly increased with electric field and that a single conduction mechanism does not adequately describe leakage current data over wide electric field and temperature ranges. From the systematic analysis applied here we conclude that the conduction was governed by a combination of multiple conduction mechanisms. It is suggested that ion hopping conduction by the sodium ion migration dominated the conduction at the initial stage followed by space-charge-limited conduction from sodium ion depleted regions where electric field is enhanced.

The dielectric breakdown statistics of alkali-free glass was determined for various thicknesses with electrodes having controlled morphology and continuity. The characteristic electrical breakdown field strength increased from 400 to 1100 MV/m as the glass substrate thickness decreased from 58 to 5 μm, respectively. Surface roughness RMS values of as-drawn and etched glass substrates were in the 0.9–1.8 nm range, which is small in comparison to the glass substrate thickness. The glass etching, itself did not have significant effect on the dielectric breakdown strength. The dielectric breakdown strength was also independent of the sputtered-deposited electrode composition (Au,Pt); however, electrode thickness played an important role in controlling the breakdown process. The Au electrode morphology transitioned from a continuous sheet for thicker electrodes to discrete nano-scale islands for very thin electrodes (

Alkali-free flat panel display glass is produced in large quantity and has excellent electrical insulating properties at high temperature. Aluminum borosilicate glass with alkaline-earth modifier has low sodium content and low dielectric loss (tan δ

Impedance spectroscopy studies were conducted on amorphous tantalum oxide thin films prepared using pulsed-DC reactive sputtering, which were post-annealed to crystallize the films. X-ray diffraction results showed that crystallization to Ta2O5 β phase occurs for samples annealed above 650 °C, with a crystallite size of ∼40 nm. The film microstructure was studied by electron microscopy, and remnants of the columnar amorphous microstructure were found in the polycrystalline films. Complex impedance analyses revealed significant differences in dielectric behavior between the amorphous and crystalline films. Lumped circuit models were conducted on the films using resistors, capacitors, and constant phase elements. Amorphous films exhibited a single relaxation with Arrhenius activation energy of 1.1–1.3 eV. Crystallized films exhibited two relaxations with activation energies equal to 1.1 ± 0.08 and 0.6 ± 0.03 eV. The relative permittivity of the bulk crystalline grain in tantalum oxide films is close to the established permittivity of the β phase (er = 40) of Ta2O5.

Nonlinear ceramics that provide the basis for high-energy-density and high-temperature capacitors, as well as tunable microwave dielectrics, and their applications are discussed in this article.

Most high throughput studies focus on assessing the effect of composition within a single known fundamental structure type, such as perovskite. Here we demonstrate how high throughput synthesis and screening can be used to establish structure–property relations in the PbO–Nb2O5 system, for which eight distinct fundamental structure types are known to exist. PbNb4O11, PbNb2O6 and pyrochlore could be easily distinguished by X-ray diffraction (XRD). However, XRD was insensitive to distortions of the pyrochlore structure and instead Raman spectroscopy was utilized to determine changes in symmetry from cubic to rhombohedral as the PbO concentration increased. High throughput screening of the capacitance revealed permittivity (er) maxima in the PbNb4O11 (er = 700) and cubic pyrochlore phases (er = 450). The er of PbNb4O11 has not to date been reported but the value for cubic pyrochlore is higher than that reported for bulk ceramics (er = 270). Initial high electric field studies also revealed exceptionally high tunability (four times that reported for bismuth zinc niobate-based pyrochlores) of the capacitance in the pyrochlore phase.

We have studied the frequency and temperature dependences of the dielectric properties of a heterogeneous silica–titania (SiO2–TiO2) glass by ac impedance spectroscopy measurements in the temperature range 25–700°C and the frequency range 40–107 Hz. The heterogeneous glass was prepared by the controlled heat treatment of a homogenous SiO2–TiO2 glass, containing ∼7.5 wt% TiO2, in a reducing atmosphere to create a microstructure consisting of many discrete particles of the rutile/anatase TiO2 phase dispersed in a homogeneous SiO2-rich glassy matrix. Unlike the homogeneous SiO2–TiO2 glass, which exhibited only one electrical/dielectric dispersion, presumably due to ion hopping, the heterogeneous glass material exhibited two distinct electrical responses in the impedance formalism. We attributed these contributions to the two-phase microstructure and explained the dielectric behavior in terms of the Maxwell–Wagner relaxation arising at the interfaces between the TiO2 crystals and the SiO2-rich matrix phase.

The dielectric breakdown behavior of alkali-free glass was determined as a function of thickness and surface roughness. The thickness of commercially available glass (as-received thickness=50 μm) was reduced to a range of thicknesses between 47 and 5 μm by chemical etching. The RMS surface roughness of the as-received glass was in the range of 0.14–0.47 nm, and the surface roughness increased to up 10 nm after etching; it was also found that agitating the etching solution by ultrasound reduced the overall surface roughness. The evaluation of these samples revealed that the dielectric breakdown strength increased as the thickness decreased. However, the Weibull modulus representing the distribution of dielectric strengths showed a dependence on the surface roughness of the etched glass. A power law dependence, EB∝d−n, where d is the glass thickness and n=0.14 and 0.86, has been found to fit the data in the respective thickness ranges of 5–20 and 25–50 μm. Self-healing behavior, which allows the dielectric to continue to support a high electric field after breakdown, was found to be more likely as the dielectric layer thickness decreased. The susceptibility to self healing was correlated with the stored electrostatic energy and latent heat of vaporization for the gold electrode material.

Electrical conduction in silica-based glasses under a low electric field is dominated by high mobility ions such as sodium, and there is a transition from ionic transport to electronic transport as the electric field exceeds 108 V/m at low temperatures. Electrical conduction under a high electric field was investigated in thin low-alkali boroaluminosilicate glass samples, showing nonlinear conduction with the current density scaling approximately with E1/2, where E is the electric field. In addition, thermally stimulated depolarization current (TSDC) characterization was carried out on room-temperature electrically poled glass samples, and an anomalous discharging current flowing in the same direction as the charging current was observed. High electric field conduction and TSDC results led to the conclusion that Poole-Frenkel based electronic transport occurs in the mobile-cation-depleted region adjacent to the anode, and accounts for the observed anomalous current.

The electrical properties of the fresnoite BaO–TiO2–SiO2 system were studied over a range of amorphous and crystalline microstructures using as-quenched glass (BTS-gl), partially crystallized glass (BTS-pc, heat treated at 740°C for 100 min) and fully crystallized glass ceramic (BTS-gc, heat treated at 750°C for 180 min). Combination of impedance and moduli plots along with equivalent circuit models were utilized in an attempt to understand the contributions arising from different regions in the glass ceramic, i.e. the glassy matrix, crystalline phase, and crystal–glass interface region. Electrical conductivity of the samples exhibited frequency dispersions at different temperatures suggesting localized motion of ions. The activation energies calculated from dc resistivity–temperature plots (Edc) and relaxation frequency–temperature plots (Ef) seem to increase as the degree of crystallinity increases suggesting that ionic transport becomes more difficult in crystallized glasses.

We investigate nonlinear conduction in aromatic polyurea thin films and its influence on the dielectric applications over a broad temperature range. The experimental data indicate that several high field conduction mechanisms coexist in the polymer films and one conduction mechanism may dominate over the others depending on different temperatures. The results also reveal that owing to the non-linear increase of the conduction loss with field, the dielectric loss at high field can be many orders of magnitude larger than that at low field. Therefore, besides the electrical breakdown strength, the conduction loss at high field and their temperature dependence behavior are other key factors in developing polymer dielectric materials for high energy density dielectric applications over a broad temperature range.

Deposition of high-k tantalum oxide thin films on thin polymer substrates was investigated, using low-temperature (-100degC) pulsed-dc reactive sputtering. Degradation of two different polymers, polyethylene terephthalate (PET) and polypropylene (PP), were studied as a function of sputtering conditions. Two different deposition configurations have been explored for polymer films with aluminum electrode on one side. In one configuration, tantalum oxide was deposited on the nonelectroded side of the polymer, while in the other the deposition was on the electroded side of the polymer. The two fabricated structures have been characterized for dielectric permittivity, loss, and ac conductivity as a function of frequency and temperature. Sputtering tantalum oxide on the nonelectroded side of PET substrates results in a 37% higher permittivity for PET than the series model prediction of permittivity. Higher dielectric loss and ac conductivity accompany the higher permittivity. The alpha bulk relaxation in PET moves to slightly higher temperatures, indicating that there is an increase in the crystallinity of the bulk polymer. This observation is supported by the broader glass transition and an additional endothermic peak around 200degC in PET/Ta2O5 compared to neat PET. In addition, modifications of the space charge activation energy in PET from 1.35 eV to 1.82 eV and of dc conductivity in PET from 6 times10-15 S/m to 4 times10-14 S/m is observed. Sputtering Ta2O5 on the electroded side of the PET, under the same sputtering conditions, results in the formation of high-k tantalum oxide with dielectric permittivity, loss, and ac conductivity of 30

Capacitor miniaturization is directly related to improved energy density and power density, which are determined at the component and material levels for multilayer ceramic, electrolytic and polymer film classifications. The volumetric efficiency of a capacitor depends on capacitance value, operating voltage and equivalent series resistance. Permittivity and loss are the primary material metrics governing energy and power densities of dielectric materials. High temperature capacitor reliability must be improved to close this gap in terms of scientific understanding and engineering design of these components. Power density values range enormously from 108 W/cm3 at the material level through to the application level at 50 W/cm3 at the inverter level.

Semi-insulating silicon carbide (SiC) wafers are important as substrates for high frequency devices such as AlGaN-GaN HEMT’s. A nondestructive characterization technique has been developed to measure the dielectric properties of SiC wafers in the GHz frequency range where the devices will operate in order to validate wafers for high yield working devices. The dielectric loss is measured at approximately 16 GHz in a split microwave cavity. Initial results show a correlation where the dielectric loss decreases as the resistivity increases, where the resistivity was measured using a Contactless Resistivity Mapping system (COREMA). The uniformity of dielectric loss across SiC wafers was evaluated using a split post dielectric resonator cavity fixed at 5.5GHz to measure the dielectric loss at five points on a wafer. Dielectric loss as a function of temperature from room temperature to 400°C was also studied.

The inverse flexoelectric phenomenon describes the generation of a mechanical deformation in an insulating solid by an applied gradient of the electric field. Lead magnesium niobate-lead titanate (PMNT) ceramic permits study of the inverse flexoelectric behavior because of its large flexoelectric coefficient. Special conical samples were prepared for the generation of a high electric field gradient and cylindrical samples with uniform field. A strong deformation is caused by the electrostrictive behavior of PMNT ceramic. An asymmetric butterfly-shaped strain dependence on electric field is observed at wide temperature range on the conical samples. For evaluation of converse flexoelectric coefficient μ 33 (in matrix notation) it is necessary to know elastic properties of ceramic. The ultrasonic measurements on ceramic samples were performed for this purpose. In this letter we also report measurements of temperature dependence of flexoelectric coefficient μ 33 . The measurements confirmed a great enhancemen...

The purpose of this work was to characterize an influence of the electric bias field on the elastic stiffness constants of important and unusual piezoelectric material Lead Zinc Niobate-Lead Titanate (PZN-PT). The longitudinal and transverse mode of sound propagation in single crystals containing 4.5% PT were studied. The influence of electric field, which poles the sample, is investigated by the pulse-echo sound propagation technique. From these measurements, the values of complex elastic stiffness constants are evaluated at different poling conditions with different frequencies for longitudinal and sheer excitations. Crystal cuts [001], [110] and [111] served for measurements of the elastic stiffness constants under application of the electric field. Aspects of field-induced phase transitions in PZN-4.5%PT single crystals at room temperature are discussed. Hysteretic behavior has an influence on attenuation and velocity of ultrasound.

The hydrostatic piezoelectric coefficient d h of Pb(Zr x Ti1 − x)O3 ceramics (PZT), and of Pb(Zn1/3Nb2/ 3)O3-PbTiO3 and Pb(Yb1/2Nb1/2)O3-PbTiO3 (PZN-PT, PYN-PT, respectively) single crystals with compositions near to the morphotropic phase boundary (MPB) have been measured using a dynamic hydrostatic method. The effects of DC electric field and static component of hydrostatic stress on d h of PZT ceramics, PZN-PT and PYN-PT single crystals were studied. Changes of the piezoelectric hydrostatic coefficients d h caused by an electric field (DC bias) were observed along with pressure and temperature dependencies. The measurement of the hydrostatic piezoelectric coefficient d h seems to be promising for investigation of intrinsic (single domain) and extrinsic (domain-walls) contributions to piezoelectric behavior of single crystals and ceramic materials.

Polarization relaxation was studied in Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) single crystals that show fatigue anisotropy. To excite prepoled crystals, a modest dc voltage (

The effect of dc bias electric field on the hydrostatic piezoelectric coefficient dh in Lead Zinc Niobate-Lead Titanate (PZN:PT) single crystal solid solutions in the vicinity of the morphotropic phase boundary (MPB) is studied. The hydrostatic measurements can be very important to gain insight into these materials. In particular, the discrepancy between sum of piezoelectric coefficients d33+ 2 d31 vs. hydrostatic dh coefficient measured directly suggests the importance of extrinsic contributions. Limiting values of measured hydrostatic piezoelectric coefficients dh for high electric fields are compared with calculated theoretic values from the Landau-Devonshire-Ginzburg (LGD) phenomenological theory. The influence of complicated microstructure within the single domain region is studied. Such microstructure could possibly account for the inability of the LGD phenomenology to give correct electric field bias dependence of dielectric constant.

A two-parameter thermodynamic model is proposed for the ferroelectric solid solution system Pb(Zr1 − xTix)O3. The free energy comprises three sets of terms: two expansions of polarization, one representing each of the end-members, weighted linearly with respect to composition, and terms representing the coupling between the two polarization contributions. Employing previously published coefficients for PbTiO3 and values extrapolated from Zr-rich PZT compositions for PbZrO3, calculations predict the morphotropic phase boundary lies at x = 0.45 and is independent of the strength of coupling between the two order parameters. A monoclinic phase is predicted, the range of existence of which depends upon the strength of the parameter coupling, but in general, is broadest at low temperature. It is proposed that the two contributions to the polarization have their origins in the two types of cation shifts identified in PZT: the major, homogenous shifts parallel to the polar direction and randomly-oriented, or het...

The paper deals with phenomenological treatment of Lead Zinc Niobate—Lead Titanate (PZN:PT) crystals. Polarization and weak field permittivity as functions of temperature and bias electric field throughout the whole phase transitions range are of particular importance for the PZN-PT composition's behaviour close to the morphotropic phase boundary (MPB). The effect of DC bias field on phase transitions in PZN:PT solid solutions near the MPB was studied. A phase transition induced by electric field and associated changes in dielectric properties of PZN:PT were observed for different compositions of 4.5% and 8% PT. Relaxation behaviour was observed, too.

Polarization switching and dielectric temperature behavior have been investigated in 0.55Pb(Yb1/2Nb1/2)O3–0.45PbTiO3 (PYN–PT) single crystals. These compositions are of interest owing to their unusually high Curie temperature and high coercive fields. The dielectric permittivity was determined as function of temperature and the polarization was measured as a function of applied field for different drive frequencies. Analysis of the experimental results with a random-field model was found to show similar trends to recent reports for the Pb(Mg1/3Nb2/3)O3 relaxor materials. The frequency dispersion of the maximum in dielectric spectra, Tmax, was also analyzed with the Vogel–Fulcher relation yielding a high freezing temperature of 577 K. Polarization behavior was found to follow a stretched exponential time dependence and a fractal dimension of ∼2.2 was obtained. It is proposed that the polar cluster state in PYN-PT single crystals do not freeze in random orientations but rather arrange locally in preferred c...

Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70°C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12°C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

Among high dielectric constant materials being used for energy storage and tuning applications the dominant material for the last thirty years has been modified barium titanate. This is surprising since the requirements for tunable materials are very different from the energy storage ones. For the high energy storage dielectrics, it is required to extend high polarizability to as high field as possible while for tunable dielectrics as large a change in polarizability with field as possible are commonly desirable. In this presentation we will attempt to reconcile this apparent contradiction. We will also discuss alternative dielectrics for energy storage and tuning including composite materials. Interestingly, for the composites layered structures are very attractive for both applications but the preferred connectivity of phases for the energy storage is in series and for tunability is in parallel. We will present examples of tunable and energy storage materials based on our work in collaboration with TRS Technologies, Inc., University of Southhampton, and Sheffield University which were not presented at the prior pulsed power conferences.

A new type of high-displacement actuator called Rainbow (Reduced And INternally Biased Oxide Wafer) was recently developed, and it shows promising characteristics in a variety of potential applications. The fabrication and propertites of Rainbow actuators from PSZT antiferroelectric ceramic with compositions near the antiferroelectric-ferroelectric (AFE-FE) phase boundary were investigated.It was found that the chemical reduction reaction proceeded much more rapidly in PSZT than in PLZT ceramics. The optimum conditions for the processing of PSZT Rainbows were determined to be 850°C for 2–3 hours. Large axial displacements ranging from 102 to 273 μm were obtained from the PSZT Rainbow samples by application of electrif fields greater than the AFE-to-FE phase switching levels. The characteristics of the field-induced displacements of the Rainbow samples were dependent on the manner of applying mechanical load on the samples. At room temperature, the antiferroelectric PSZT Rainbows exhibited a conca...

Electro‐optic properties of thin‐film materials grown on opaque substrates are determined utilizing the principles of reflection differential ellipsometry. The scheme of the measurement involves the detection of the field‐induced phase shift in a probing light beam reflected from thin‐film samples. To quantitatively determine the field‐induced indices change or birefringence and the field‐induced strain it is essential to model the differential ellipsometric process in the stratified structure. The modeling reveals that the field‐induced changes of the ordinary and the extraordinary indices contribute to the measured phase shift with different incident‐angle dependences. The field‐induced strain gives yet another unique incident‐angle distribution. By means of an incident‐angle‐varying technique, therefore, the field‐induced changes of ordinary index and extraordinary index in a thin‐film material may be determined separately. Detailed descriptions of the measuring technique and the modeling work are presented.

In the last fifteen years considerable progress has been made in developing novel materials and devices for electromechanical actuator applications based on a variety of ferroelectric materials. Recently a novel type of high displacement actuator was developed. It is a monolithic, domed, stress-biased wafer consisting of oxide and reduced layers. These actuators are envisioned to be used in a variety of applications for which the knowledge of the resonance modes is essential. In this paper measurements of the resonant modes are compared to the predictions of the finite-element model. A number of low frequency modes were observed and identified as the bending modes. A higher frequency radial mode was determined to be less dependent on the sample dimensions than the bending modes. Effects of the boundary conditions on the resonant modes were modeled and investigated experimentally. For the modeling aspect of the study it was necessary to measure elastic constants, thermal expansion coefficients, and bulk de...

The reported work highlights that both polarizibility and quasi-DC conductivity, the two most important parameters for high energy density materials, can be influenced by the surface polarity of an oxide and interfacial surface tension between the oxide and polymer in a laminar composite. Surface modification of laminar composites, composed of thermally grown silica and vapor phase grown Parylene C thin films using organosilanes, was used to control silica surface polarity and interfacial surface tension. Surface polarity of thermally grown silica was found to control the interfacial dipolar energy states.

A discrete transverse electro-optic response associated with a field-induced antiferroelectric–ferroelectric phase transition has been observed to exist in lead zirconate thin films grown on Pt/Ti-coated silicon substrates. The magnitude of the birefringence jump from the antiferroelectric to the ferroelectric state is approximately 2.5 × 10−2. Quantitative correlation between the field-induced birefringence and the polarization was also experimentally studied. The discrete birefringent change in the thin films may be a desirable property for applications in optical switches or other integrated-optical devices.

High and reproducible dielectric breakdown strength is essential requirement for high energy pulse power capacitors. It is important to understand which features of a dielectric are instrumental in determining breakdown strength. Modeling of the dielectric breakdown complements experimental work in achieving deeper insights into dielectric breakdown and helps to identify parameters controlling breakdown strength for a given dielectric. We have performed Monte Carlo modeling of breakdown strength in ceramics, polymers, and layered organic/inorganic composites. In the case of ceramics, the effect of uniform and nonuniform porosity on the breakdown strength indicated that both the level and size of porosity influence dielectric strength. Furthermore, porosity has stronger influence on the breakdown strength as the pore size-to-dielectric thickness ratio becomes larger. It was also determined that the prediction of the model agrees well with experimentally observed area dependence of dielectric breakdown of commercial capacitors. In the case of semicrystalline polymers, the modeling work strongly suggests positive contribution to the breakdown strength from the interfaces between the crystalline and amorphous phases. This conclusion was reached through the comparison of Weibull modulus obtained from the experimental breakdown strength measurements of biaxially oriented polypropylene capacitors with that of Monte Carlo modeling of the same dielectrics. Only in the case of interfaces dominating the breakdown strength there was a good agreement between experimental and modeled Weibull modulus. The modeling work also indicate that unlike ceramics for which infinite Weibull modulus is theoretically achievable for the defect-free dielectric, in the case of semicrystalline dielectrics the Weibull modulus is finite in all cases, and is controlled by polymer microstructure and capacitor geometry. In the case of layered ceramic - polymer composites we focused on understanding tree propagation as a function of the dielectric contrast between the dielectrics and the placement of a single high K ceramic layer within the polymer. Both dielectric contrast between the layers and placement of the layer turned out to be important in determining the time required for sample destruction. Interestingly, these effects are observed in the electrostatic modeling without introduction of defects and space charges at the interfaces. A comparison of breakdown in heterogeneous ceramics, polymers, and composites indicates a great diversity and richness of observed behavior. In particular, both the beneficial and detrimental effects of heterogeneity were deduced from the modeling.

Article history: The interfacial interactions and their effects on electrical properties are reported for thermally grown SiO2-Parylene-C laminar composites. The segmental chain mobility of the interfacial Parylene-C was found to be enhanced in the composites. TSDC (thermally stimulated depolarization current) measure- ments showed the presence of dipolar interfacial energy states in these laminar composites. Function- alization of the oxide surface with 3-aminopropyltriethoxysilane (APTES) created dipolar energy states with larger activation energy at the interface between Parylene-C and SiO2. In addition to the modifica- tion of interfacial states, incorporation of APTES interfacial layer also increased polarizability (real part of permittivity) and reduced the quasi-DC conductivity of laminar composites. The systematic changes in interfacial characteristics with thickness of APTES layer were explained based on structural orientation and cross linking of silane molecules.

Laminar (2-2) composite is treated as a model system to better understand interfacial interactions between an oxide and a polymer in more complex particulate polymer composites. Performed impedance spectroscopy measurements implied the modification of the polymer in contact with the oxide. Thermally stimulated depolarization current measurements have shown that deeper energy states can be engineered at the oxide and polymer interface by incorporating interfacial polar charge withdrawing functional group.

Dielectrics with high electrostatic energy storage must have exceptionally high dielectric breakdown strength at elevated temperatures. Another important consideration in designing a high performance dielectric is understanding the thickness and temperature dependence of breakdown strengths. Here, we develop a numerical model which assumes a coupled ionic redistribution and electronic breakdown is applied to predict the breakdown strength of low-alkali glass. The ionic charge transport of three likely charge carriers (Na+, H+/H3O+, Ba2+) was used to calculate the ionic depletion width in low-alkali boroaluminosilicate which can further be used for the breakdown modeling. This model predicts the breakdown strengths in the 108–109 V/m range and also accounts for the experimentally observed two distinct thickness dependent regions for breakdown. Moreover, the model successfully predicts the temperature dependent breakdown strength for low-alkali glass from room temperature up to 150 °C. This model showed tha...

Amorphous zirconium oxide thin films were prepared by reactive magnetron sputtering. The dielectric films were characterized by impedance spectroscopy with temperature. The effect of annealing on capacitor performance was studied. Annealing gold electroded thin films at 250°C greatly reduced the losses with little changes in crystallinity. Space charge relaxation started to appear at 190°C. The activation energy for the relaxation was 0.84 eV with a very low relaxation frequency at room temperature (0.23μHz). Electrode effects dominated at very low frequencies at all temperatures. AC conductivity followed the universality behavior for the AC charge transport showing that the films are highly disordered. No DC conductivity regime was observed indicating that DC conductivity is very low. DC conductivity of the films was of the order of 10−13 S/m, which is lesser than the comparable thickness high quality gate oxides.

Poly(chloro-p- Xylene) or Parylene –C thin films are particularly attractive for dielectric as well as biomedical applications. In the current work the dielectric properties of Parylene-C thin films are investigated to form laminar composites with oxide thin films for high energy density pulsed power capacitors. Parylene-C thin films were synthesized by pyrolytic vapor decomposition polymerization of dichloro-di(p-Xylene) monomer. Annealing of films at 225°C has shown to enhance crystallinity of film. Conduction in Parylene-C thin films appears to be bulk-controlled with the hopping charges contributing to leakage current. The barrier height of 0.89eV and hopping distance of 2 - 2.5nm are physically plausible and similar to previously reported values in polymer literature.

This paper presents cryogenic actuators for high‐temperature superconducting (HTS) tunable filters. The designated piezo‐tuned‐HTS concept will enable a broad range of civilian and military applications that require precise band selectivity over a broad frequency range. In this paper, low profile flextensional single crystal actuators were developed with 150‐μm stroke at 77 K. This actuator was then used for Superconducting Technologies, Inc. (STI) HTS resonator tuning in liquid nitrogen. A fine tuning using the same actuator shows more than 3% tunability and less than 1‐dB insertion loss variation and the Q is about a few thousands.

A novel approach, utilizing piezoelectric actuators to control filters, resonators, and phase shifters with the minimum insertion loss, is described. Tunable, low-loss microwave devices can be fabricated using composite structures in which effective dielectric constant, /spl epsi//sub ef/, is controlled by piezoelectrically induced changes in the energy distribution within the devices. With this approach, a high degree of tunability is possible while utilizing high Q dielectrics. These devices usually consist of two dielectric parts with an air gap between them. This gap has to be arranged to provide the largest perturbation in the electromagnetic field. By means of a fast piezoelectric mini-actuator, the size of the air gap is changed and as a result, the /spl epsi//sub ef/ can be tuned. This tuning is used for device control. Highly tunable (more than 20%) dielectric and metal ring resonators, as well as wide-band, low-loss phase shifters are realized experimentally.

The objective of the Phase II SBIR project was to develop high Q, tunable microwave filters by using a cryogenic piezoelectric actuator to mechanically tune a high temperature superconducting (HTS) resonator. The concept, designated piezotun-HTS, will enable a broad range of civilian and military applications that require precise band selectivity over a broad frequency range. In Phase I of this project, a 25% frequency tuning range was successfully demonstrated between 2 and 8 GHz by using a piezoelectric bending element to adjust the width of an air gap between a microstrip resonator and a dielectric or metal tuning plate. In Phase II, the piezotune-HTS and strip-line resonator concepts are applied to achieve large tuning range and low insertion loss.

Various highly tunable low-loss microwave devices are elaborated by piezoelectrically induced mechanical changes in the microwave structures utilizing high Q dielectrics. In this connection, an effective dielectric constant, epsieff, of used composite structures is controlled by fast actuator that results in energy distribution within the devices. Typical device consist of dielectric and metallic parts with an air gap between them that provides the largest perturbation in the electromagnetic field. By mini-actuator, the size of the air gap is changed and, as a result, the epsieff can be tuned that is used for a device control. Highly tunable dielectric resonators as filters as well as wideband, low-loss phase shifters are realized experimentally

A thermodynamic model is proposed to account for the monoclinic phase observed in the solid solution system Pb(Zr/sub 1-x/Ti/sub x/)O/sub 3/. The free energy comprises three sets of terms: two expansions of polarization, one representing each of the end-members, weighted linearly with respect to composition, and terms representing the coupling between the two contributions. Employing previously published coefficients for PbTiO/sub 3/ and values extrapolated from Zr-rich PZT compositions for PbZrO/sub 3/, calculations predict the composition of the morphotropic phase boundary between the rhombohedral and tetragonal phases lies at x=0.45 and is virtually independent of temperature and the strength of coupling between the two order parameters. The range of existence of the monoclinic phase depends upon the strength of the coupling, but in general, is broadest at low temperature.

Lead lanthanum zirconium titanate (PLZT) 9.5/65/35 samples were electroded with BSCCO and YBCO superconductor materials. Extensive interaction was obtained between the PLZT and the YBCO, and very low interaction was observed between the BSCCO and the PLZT. The BSCCO superconductor on the PLZT substrate maintained its superconducting properties at liquid nitrogen temperature. The apparent dielectric constant of the PLZT was influenced by the electroding materials. Domain switching was found to be influenced by the electrode materials at low temperature. The BSCCO electroded samples showed the highest polarization switching rate among the samples electroded by different materials.

The effect of the impedance characteristics of an imbedded barrier and inclusions on tree growth and field distribution characteristics was explored through Monte Carlo and finite element method based simulations. A stochastic tree propagation computer model was developed for the laminate composites that were comprised of a high-permittivity barrier layer imbedded in the low permittivity matrix. The impedance of the high-permittivity barrier was altered by systematically changing the permittivity (er) of the barrier, resulting in a permittivity contrast ranging from 1 to 100. The model showed that there is a significant change in the various breakdown tree propagation characteristics such as number of breakdown steps, speed and shape with impedance contrast. A compilation of several simulation runs was statistically analyzed for each barrier and the number of breakdown steps increased significantly as the barrier permittivity approached 100. The results of this model can be generalized to impedance contrast between dielectric layers that include complex permittivity.

Activation energies (Ea) for ionic conduction in low-alkali boroaluminosilicate glasses due to alkaline-earth (Ba, Ca) and alkali (Na) ion transport have been estimated using thermally stimulated depolarization current (TSDC) and AC impedance spectroscopy techniques. The TSDC plot showed distinct relaxation peaks which shifted to higher temperatures with increasing ramp rates, and the dielectric dispersion plot showed individual low frequency relaxation peaks indicating space charge polarization due to transport of cations with different Ea (0.93, 1.83, and 3.5 eV for Na, Ba, and Ca, respectively). The higher value of Ea for Ca transport is attributed to mixed alkaline earth effect.