A comparative approach to predicting effective dielectric, piezoelectric and elastic properties of PZT/PVDF composites

The present study addresses the problem of quantitative prediction of effective relative permittivity, dielectric loss factor, piezoelectric charge coefficient, and Young's modulus of PZT/PVDF diphasic ceramic-polymer composite as a function of volume fraction of PZT in the different compositions. Theoretical results for effective relative permittivity derived from several dielectric mixture equations like those of Knott, Rother-Lichtenecker, Bruggeman, Maxwell-Wagner-Webmann-Skipetrov or Dias-Dasgupta, Furukawa, Lewin, Wiener, Jayasundere-Smith, Modified Cule-Torquato, Taylor, Poon-Shin and Rao et al. were fitted to the experimental data taken from previous works of Yamada et al. Similarly, the results for effective piezoelectric coefficient and Young's modulus, derived from different appropriate equations were fitted to the corresponding experimental data taken from the literature. The study revealed that only a few equations like modified Rother-Lichtenecker equation, Dias-Dasgupta equation and Rao equation for dielectric and piezoelectric properties while the four new equations developed in the present study of elastic property (Young's modulus) well fitted the corresponding experimental results. Further, the acceptable data put to various regression analyses showed that in most of the cases the third order polynomial regression analysis provided more acceptable fits.     

Optimized AC conductivity correlated to structure,morphology and thermal properties of PVDF/PVA/Nafion composites

Polyvinylidene fluoride (PVDF) and polyvinyl alcohol (PVA) composites were prepared by controlled loading of Nafion (5 to 15 wt%) by solution casting using water and dimethylformamide (DMF) as a solvent. The surface morphology of composite analyzed by atomic force microscopy (AFM) reveals the presence of Nafion ionomers. The increase in interlayer spacing of modified PVDF/PVA polymer system as a function of Nafion was detected by X-ray diffraction (XRD). The major change in Fourier transform infrared (FTIR) spectroscopy confirms the chemical bond C=O stretching around 1,700 cm^?1 due to Nafion. Differential scanning calorimetry (DSC) demonstrates the thermal stability of polymer composites and the decrease in melting temperature (T _m). The optimized AC conductivity (σ) of the prepared composite was evaluated by using an impedance analyzer as a function of temperature (40 to 150 °C) at constant 30-MHz frequency. The highest conductivity of 1.3?×?10^?2 S m^?1 was observed at 80 °C for 10 wt% of Nafion and correlated with structure, morphology and thermal properties of modified PVDF/PVA/Nafion composites. The experimental results may be useful for sensors, fuel cells and battery application domains.     

Effects of CNT inclusions on structure and dielectric properties of PVDF/CNT nanocomposites

ABSTRACT

The effects of multiwall carbon nanotube (CNT) inclusions on the crystalline structure of poly(vinylidene fluoride) (PVDF), and on the dielectric properties of PVDF/CNT nanocomposites (NCs), prepared by melt mixing, were investigated by employing X-ray diffraction, differential scanning calorimetry, and dielectric spectroscopy techniques. Our results imply that, in the NCs, the formation of β-phase crystals depends on specific compression treatment in the melt and fast cooling. Dielectric measurements on NCs, with CNT concentrations below the electrical percolation threshold, reveal that the dielectric strength of the two relaxation processes in the amorphous phase and dielectric permittivity, ?′, measured within the broad temperature range from ?150 °C to 60 °C, increase strongly with increasing CNT concentration. This enhancement of amorphous PVDF polarizability has been attributed to the increase of the local electric field, due to local polarization generated at the surface of conductive inclusions/CNT clusters.     

Study on dielectric properties of oil/water random composites

Considering the electric double layers between oil and water, a new “complex model” of dielectric constant of oil/water composites was built up. Starting from the Maxwell–Garnett theory and Bruggeman theory, the effective dielectric constant of oil/water random composites is presented. The nonlinearity of the theory is obvious. The model is especially suited to study the dielectric properties of oil/water composites of different nature. The model is also suited to study the dielectric properties of two-phase random composites with an interfacial shell. The theoretical results on dielectric properties of different kinds of oil/water composites are in good agreement with experimental data.     

Effect of nickel nanofillers on the dielectric and magnetic properties of composites based on rubber in the X-band

Nickel–rubber nanocomposites were synthesized by incorporating ferromagnetic nickel nanoparticles in a natural rubber as well as neoprene rubber matrix. Complex dielectric permittivity and magnetic permeability of these composites were evaluated in the X-band microwave frequencies at room temperature using cavity perturbation technique. The dielectric loss in natural rubber is smaller compared to neoprene rubber. A steady increase in the dielectric permittivity is observed with increase in the content of nickel in both the composites. The magnetic permeability exhibits a steady decrease with increase in frequency and magnetic loss shows a relaxation at 8 GHz. The suitability of these composites as microwave absorbers is modeled based on the reflection loss which is dependant on the real and imaginary components of the complex dielectric permittivity and magnetic permeability.     

Frequency dependences of dielectric properties of metal-insulator composites

Based on the fractal model of an inhomogeneous medium with a chaotic structure and the iteration method of averaging, frequency dependences of the dielectric properties of metal-insulator composites were determined. In the low-frequency limit, the considered methods of the investigation of two-component media were shown to permit one to obtain detailed information on the metal-insulator transition.     

Microwave dielectric properties of composites modified by carbon nanostructures

The dielectric properties of an epoxyamine composite modified by carbon nanostructures up to 2 wt % are studied at a frequency of 2.73 GHz, and its permittivity is shown to behave nonmonotonically (anomalously) as a function of the filler concentration. Possible causes of this anomalous behavior of the dielectric properties are discussed.     

Microwave dielectric behavior of PVDF and VDF-TrFE copolymer

Measurements of the temperature dependent microwave dielectric permittivity of PVDF and 60%–40% VDF-TrFE copolymer are presented. A cavity perturbation method is used. At room temperature a surprisingly high absorption of microwave power is observed, which decreases with decreasing temperature. This behavior seems to be connected with the ferroelectric-to-para-electric phase transition. With the copolymers a dependence of the complex permittivity on annealing temperature is found. A model considering paraelectric fluctuations is proposed.     

Effective dielectric functions of alkali halide composites and their spectral representation

In the electrostatic approximation inhomogeneous samples with characteristic scale of inhomogeneities much smaller than the wavelength of light can be characterized by an effective dielectric function _eff. This paper analyzes in which cases simple mixing formulas can be used to calculate _eff from the dielectric functions of the consituents. These considerations are compared with reflectance measurements of alkali halide composites in the far infrared. In many cases it turns out that it is essential to employ the general ansatz of the Bergman spectral representation in order to describe all effects due to the geometrical arrangement of the phases in a proper manner. It is necessary to adjust Bergman's spectral density to measurements. This can be done by use of a Monte-Carlolike algorithm. Further on we try to answer the question how to extract the dielectric function of one of the constituents from measurements of the effective dielectric function.     

Plasma crystallization of polymer-ferroelectric/piezoelectric ceramic composites and their piezoelectric properties

Physics of the Solid State - The crystallization of “polymer-ferroelectric/piezoelectric ceramic” composites under the action of an electric discharge plasma and temperature is...     

Optimizing mechanical and thermal expansion properties of carbon/carbon composites by controlling textures

The present study explored the effect of medium texture (MT) content on flexural properties and thermal expansion coefficients (CTE_S) of carbon/carbon (C/C) composites with multilayered pyrolytic carbon. The specimen with 39% MT exhibited maximum flexural strength of 221.55 MPa, increasing by 52% compared with pure high texture. While the flexural strength decreased when the MT content exceeded 39%. The excellent strength can be attributed to crack deflection between multilayered texture and the strong interface bonding between fibers and matrix. Moreover, the four specimens expressed a similar trend of CTE_S in the direction of XY and Z. In the direction of XY, the specimen with 39% MT had the lowest CTE_S from 800 °C to 2100 °C. Therefore, the C/C composites with 39% MT have the best mechanical and thermal expansion properties, which means that the properties of C/C composites can be optimized by controlling the texture.     

Photoacoustic study of thermal properties of biological tissues detected by PVDF film transducer

A PVDF piezoelectric film transducer is used in a photoacoustic piezoelectric technique for studying thermal diffusivities of biological tissues. The experimental precision may be improved by using PVDF films because the acoustic impedance of PVDF films is close to that of biological tissues. Thermal diffusivities of several fresh porcine tissues in vitro are studied and the results are compared with previously obtained ones.     

各向异性堆叠结构环氧树脂复合材料的热防护性能

基于导热-隔热原理,通过在环氧树脂(Epon)中添加质量分数为5%,15%,25%的六方氮化硼(h-BN)作为填料制备环氧基散热层,质量分数为1%的膨胀蛭石(E-ver)作为填料制备环氧基隔热层,设计了宏观交替堆叠的环氧复合材料,并进行了热防护性能的研究。研究结果表明：具有各向异性结构的复合材料,顶部中心温度较传统材料的温度下降13～16 ℃,热延迟时间大大提升,并随着h-BN含量的增加,热性能得到明显改善。理论分析了该堆叠结构下复合材料“横向散热、纵向抑热”的机理。     

Optical properties and thermal stability of LaYbO3 ternary oxide for high-k dielectric application

A new ternary rare oxide dielectric LaYbO₃ film had been prepared on silicon wafers and quartz substrates by reactive sputtering method using a La-Yb metal target. A range of analysis techniques was performed to determine the optical band gap, thermal stability, and electrical property of the deposited samples. It was found the band gap of LaYbO₃ film was about 5.8 eV. And the crystallization temperature for rapid thermal annealing (20 s) was between 900 and 950 °C. X-ray photoelectron spectroscopy results indicate the formation of the SiO₂ and silicate in the interface between silicon wafer and LaYbO₃ film. The dielectric constant is about 23 from the calculation of capacitance-voltage curve, which is comparable higher than previously reported La₂O₃ or Yb₂O₃ film.     

Nonlinear properties of composites based on dielectric layers containing copper and silver nanoparticles

Nonlinear optical characteristics of copper and silver nanoparticles in glass host matrices are studied by the Z-scan method at the wavelength of a Nd:YAG laser (λ=1064 nm) in a field of picosecond pulses. It is found that the third-order nonlinear susceptibility is more pronounced in glasses with copper nanoparticles than in glasses with silver nanoparticles. On the basis of experimental data obtained for samples with copper nanoparticles synthesized by ion implantation, it is shown for the first time that the nonlinear absorption of laser radiation with a wavelength lying out of the plasmon resonance region can be caused by a two-photon effect in metal particles. The character of the optical limiting process in the samples with copper nanoparticles when two-photon absorption is involved is discussed.     

Unique dielectric properties in polyaniline/poly(vinylidene fluoride) composites induced by temperature variation

To explore an effect of temperature on the dielectric properties in polyaniline/poly(vinylidene fluoride) (PANI/PVDF) composites, the dielectric properties of these composites with different volumetric fractions of PANI (?_PANI) were studied in a wide temperature range. An increase in the effective conductivity (σ_eff) and dielectric permittivity (?_eff) was observed with increasing temperature in all PANI/PVDF composites. Particularly, for the composite with ?_PANI = 0.01, less than the percolation threshold (?_C = 0.045), the increase in σ_eff and ?_eff was most significant. A tunneling effect could be responsible for the unique dielectric properties. The results provided us useful information related to the microstructure of composites, which was not reported previously. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile and rapid synthesis of nickel nanowires and their magnetic properties

The present work reports a facile and rapid microwave-assisted route to synthesize nickel nanowires with a necklace-like morphology and lengths up to several hundreds of microns. The wires consist of many crystallites with an average size of 25 ± 2 nm. The synthesis does not use templates or magnetic fields and needs only 6 min, which is more than 480 times faster than that needed for Ni wires prepared at 180 °C using conventional heating. Nickel nanostructures with various morphologies including spheres, chains and irregular particles with porous surfaces can also be obtained by adjusting reaction parameters. Polyvinylpyrrolidone (PVP) is found to be vital for the formation of the one-dimensional chains and a high concentration of PVP smoothes their surfaces to result in the appearance of wires. This rapid one-pot procedure combines the formation of nanoparticles, their oriented assembly into chains, and the subsequent shaping of wires. The Ni nanostructures show variable magnetic properties. The prepared nickel wires have a high mechanical stability and exhibit much higher coercivity than bulk nickel, Ni nanoparticles and their aggregations, which promise potential applications in micromechanical sensors, memory devices and other fields.