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)     

Effect of the ceramic grain size and concentration on?the?dynamical mechanical and dielectric behavior of?poly(vinilidene fluoride) Pb(Zr0.53Ti0.47)O3 composites

In this work, poly(vinilidene fluoride)/Pb(Zr_0.53Ti_0.47)O₃([PVDF]_1−x /[PZT] _x ) composites of volumetric fractions x and (0–3) type connectivity were prepared in the form of thin films. PZT powder of crystallite size of 0.84, 1.68, and 2.35 μm in different amounts of PZT (10, 20, 30, and 40%) was mixed with the polymeric matrix. The crystalline phase of the polymeric matrix was the nonpolar α-phase and the polar β-phase. Dielectric and dynamic mechanical (DMA) measurements were performed to these composites in order to evaluate the influence of particle size and the amount of PZT filler with respect to the PVDF matrix. The inclusion of ceramic particles in the PVDF polymer matrix increases the complex dielectric constant and dynamical mechanical response of the composites. A similar behavior is observed for the α- or β-phase of the polymeric matrix indicating that the PVDF polymer matrix is not particularly relevant for the composite behavior. On the other hand, ceramic size and especially content play the major role in the increase of the dielectric response and the room temperature storage modulus. In particular, the storage modulus increases with increasing PZT concentration, but this increase is more pronounced, in terms of maximum value, for the sample with 2.35 μm particle size; DMA reveals two main relaxations in the analyzed samples. A low-temperature process maximum at ca. −40°C, usually labeled by β or α _a associated to the T _g of the polymer and the α-relaxation at temperatures above 30°C. The β-relaxation is also observed in the dielectric measurements. The models used to asses the dielectric behavior of the samples with increasing PZT concentration indicate that the particle–matrix interaction plays a relevant role, as well as the particle asymmetry and relative orientation, being the Yamada model the most appropriate to describe the composite behavior. An erratum to this article can be found at     

Relaxor Behaviour and Ferroelectric Properties of （Li0.12Na0.88） （Nb0.9-xWa0.10Sbx）O3 Lead-Free Ceramics

New lead-free ceramics （Lio.12Na0.88） （Nbo.9-x Ta0.10 Sbx） 03 （0.01 × 0.06） are synthesized by solid-state reaction method. The dielectric, piezoelectric and ferroelectric properties of the ceramics are studied. The dielectric constant dependence with temperature and frequency of the ceramic specimen with x = 0.04 shows typical characteristics of relaxor ferroelectrics, and the Vogel-Fulcher relationship is fulfilled. The dielectric behaviour and its relation to the phase transition phenomena are discussed. The polarization hysteresis loops at room temperature are also measured.     

Dependence of excess bismuth content in precursor sols on ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films fabricated by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were fabricated on Pt/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD), and the dependence of ferroelectric and dielectric properties of the as-deposited BLT thin films on excess Bi content in precursor sols was studied. It is found that the prepared BLT thin film shows the best polarization-electric field, capacitance-voltage and dielectric constant (?_r)-frequency characteristics, when the value of excess Bi content in precursor sols is 10%. In detail, its remnant polarization (2P_r) value is 40 μC/cm², the capacitance tunability is 21% measured at room temperature under conditions of an applied voltage of 8 V and measurement frequency of 10 kHz, and the ?_r is 696 at 100 kHz frequency.     

Particle Size Dependence of the Dielectric Properties of Polyvinyledene Fluoride/Silver Composites

The dependence of the dielectric properties of micro- (m-) and nano- (n-) silver (Ag)/poly(vinylidene fluoride) (PVDF) composites on the Ag particle size was determined. The magnitude of dielectric constant and conductivity for the PVDF/n-Ag composites was much higher than that of the PVDF/m-Ag composites at the same Ag volume loading. Our results suggest that the percolative behaviors were quite different for the m- and n-systems owing to the Ag particle size effect. The dielectric property depends on the synergistic effects of interfacial area, interparticle distance, and interfacial adhesion, all of which are highly dependent on the Ag particle size. The increased interfacial area, reduced interparticle distance, and improved interfacial adhesion contributed to the better dielectric properties of the PVDF/n-Ag composites.     

Finely-reconciled high dielectric constant and low dielectric loss in ternary polymer/Cr2C3/montmorillonite composite films by filler-synergy strategy

Polymer/conductive ceramic composites with high dielectric constant have become research hotspot of dielectric capacitor materials. However, the conductivity and dielectric loss increase when high dielectric constant is achieved. In order to reconcile high dielectric constant and low dielectric loss, in this study, poly (vinylidene fluoride) (PVDF)/chromium carbide (Cr₂C₃)/montmorillonite (MMT) ternary composite films were prepared by solution cast. Dielectric response based on interfacial polarization was improved and dielectric constant of composites was increased. MMT ceramic was used to suppress interface leakage current. Compared with PVDF/Cr₂C₃ composites, the conductivity and dielectric loss of ternary composites were reduced.     

Dielectric characteristics and polarization response of lead-free ferroelectric (Bi0.5Na0.5)0.94Ba0.06TiO3-P(VDF-TrFE) 0-3 composites

Bismuth sodium barium titanate/poly(vinylidene fluoride-trifluoroethylene) 70/30 [(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-P(VDF-TrFE)] 0-3 composites were prepared by a hot-press method for different volume fractions of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramic powder in a P(VDF-TrFE) 70/30 copolymer matrix. The relative permittivity and dielectric loss of the composites increased with increase in the volume fraction of the ceramics, which well follows the Bruggeman model. The polarization responses of the composites were strongly dependent on the ceramic volume fraction. The composites with a higher ceramic volume fraction showed an increase in remanent polarization. At room temperature, a 0.3(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-0.7P(VDF-TrFE) composite showed a relative permittivity ε_r=30, remanent polarization and coercive field      

Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation

The phase transformation from the non-polar α-phase to the polar electroactive β-phase of polyvinylidene fluoride (PVDF) has been investigated using the fluorescence from Nile red. Films of α-PVDF doped with Nile red were stretched at controlled rates at a temperature of 80 °C to produce the α- to β-phase transition. The thermo/mechanical dependent changes in the crystalline structure are related to the physical rotation of the polar (CH₂-CF₂) group, which can be monitored by steady state fluorescence techniques. The degree of phase transformation is related to variation in the fluorescence, which in turn is linked to local dielectric constant of the polymer. The variation of the refractive index is more associated to the alignment of the polymeric chains than to the phase transformation. Thus, fluorescence is a suitable technique to monitor phase transitions coupled to a variation in the polarity of the dielectric medium.     

Structural and electrical properties of polycrystalline PbZr0.5Ti0.5O3 films deposited on La0.5Sr0.5CoO3 coated silicon by sol-gel process

La_0.5Sr_0.5CoO₃ (LSCO) films have been grown on Si (100) by a metalorganic chemical liquid deposition (MOCLD) technique using lanthanum acetate, strontium acetate and cobalt acetate as the starting materials. Subsequent PbZr_0.5Ti_0.5O₃ (PZT) films were deposited onto LSCO films by a modified sol-gel method. Field-emission scanning electron microscopy and X-ray diffraction analysis show that PZT and LSCO films are polycrystalline and entirely perovskite phase. At an applied electric field of 250 kV/cm, the Pt/PZT/LSCO capacitor shows no polarization fatigue after 3×10⁹ switching cycles and an internal electric field; the remnant polarization P_r and the coercive field E_c are about 22 μC/cm² and 73 kV/cm, respectively. The dielectric constant of PZT films is 650 at a frequency of 1 kHz. Received: 20 February 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

The permittivities of PVDF/PMMA blends at hydrostatic pressure

The complex permittivities of poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends have been measured under variation of temperature T(20°-60°C), frequency v(5 Hz-300 kHz), and hydrostatic pressure p (0-260 MPa). The values can be represented by a master curve with the shift factors △p/△ log (v/Hz) = ?140 MPa at room temperature and △ (1 /T) /△ log (v/Hz) = ?2. 10^?4 K^?1 at atmospheric pressure. The dependence of the activation energy △E _A on pressure p is then given by △E _A=(100 + 0.02 MPa^?1 p) kJ/mol. Furthermore, the results indicate that the β-relaxation of PVDF is due to motions in the crystal-amorphous interphase. The interactions between the two polymers, which are miscible at all compositions, disturb the correlations between the PVDF monomer units at that location as well as the mobility of the PMMA side group.     

Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

Time-domain terahertz and infrared spectroscopy of BaTiO3–PVDF nanocomposites

Several composites of BaTiO₃ nanoparticles obtained by mechanosynthesis embedded into a PVDF matrix were studied using time-domain THz and FTIR spectroscopies and microwave measurements with the split-post-resonator technique. No indication of the ferroelectric phase transition could be seen in the temperature dependence of the effective dielectric response, in agreement with expectations based on various effective medium models. The effective soft-mode frequency is strongly shifted up and smeared. No conclusion on the size effect of ferroelectricity can be drawn from the effective dielectric response.     

Properties of Sr- and Sb-doped PZT–Portland cement composites

Lead zirconate titanate (PZT) ceramic-cement-based composites have increasingly been recognized as an attractive new composite material for use as a sensor in structural applications. In this work, PZT was doped with Sr and Sb (PSZT) to give it greater dielectric constant (ε _r) and higher piezoelectric coefficient (d ₃₃) values than normal PZT and is the first time that it is mixed with normal Portland cement to produce a 0–3 connectivity PSZT–Portland cement composite using PSZT contents of 50% and 70% by volume. Scanning electron micrographs show PSZT ceramic particles closely surrounded by the hydrated cement matrix where a dense microstructure can be observed in the interfacial zone. Both the ε _r and d ₃₃ values were found to increase with PSZT content and the values are amongst the highest so far for these types of composites, where the ε _r and d ₃₃ values reached 590 and 48 pC/N, respectively.     

High-Temperature Dielectric Response and Multiscale Mechanism of SiO2/Si3N4 Nanocomposites

The high-temperature dielectric properties of SiO₂/Si₃N₄ nanocomposites are investigated theoretically and experimentally. Its permittivities and loss tangents at the temperature ranging from room temperature to 1300°C at 9.0GHz are measured by the resonant cavity method. The SiO_2/Si₃N₄ nanocomposites show complex dielectric behaviour at elevated temperature, and a multi-scale model is proposed to describe the dependence of the dielectric properties in the SiO₂/Si₃N₄ on its compositional variations. Such a theory is needed so that the available property measurements could be extrapolated to other operating frequencies and temperatures.     

KNN Based Lead-Free Piezoceramics with Improved Thermal Stability

Lead-free piezoelectric ceramics （1 - x） （Na0.53K0.404 Li0.066）Nb0.92 Sb0.08 03 ＋xZrTiO3 are fabricated by conventional solid-state sintering method, and their dielectric and piezoelectric characteristics are investigated. With the addition of SrTiO3, the growth of the grain size is restrained, meanwhile the phase transition temperature of orthorhombic-tetragonal is shifted below room temperature. It is found that the ceramics with x = 0.010 exhibit excellent piezoelectric properties （d33 = 220 pC/N, kp = 41%, kt = 39%） and improved thermal stability around room temperature. The results indicate that these materials are promising lead-free piezoceramics for practical operations.     

Fabrications and dielectric performances of novel composites: Calcium copper titanate / Polyvinylidene fluoride

CCTO (Calcium copper titanate) powder as inorganic filler was synthesized by the Sol-gel method firstly. CCTO/PVDF (Polyvinylidene fluoride) composite was fabricated by solution mixing based on high temperature resistance of PVDF and insulated property of CCTO. The composite of CCTO/PVDF were characterized by X-ray diffraction, Fourier Transform infrared spectroscopy, Scanning Electron Microscope and impedance analysis. The results showed that the addition of CCTO and increasing in its content did not affect the phase- and micro-structure of the composites,but the increase of CCTO content can induce the generation of C–F new bonds. PVDF/CCTO composites were enhanced in performance of thermal and frequency-depended stability with increasing in the fraction of CCTO. The dielectric constant of CCTO/PVDF composite materials with 50% CCTO achieved to a maximum value of 50 almost, which is 5 times higher the pure PVDF. The conductivity felled into 10⁻⁸ to 10⁻¹ S m⁻¹ during the frequency of 10²–10⁸ Hz. The composite material would be expected to be applied in the field of integrated circuit.     

Radio-frequency (RF) studies of the magneto-dielectric composites: Cr0.75Fe1.25O3 (CRFO)-Fe0.5Cu0.75Ti0.75O3 (FCTO)

This paper concerns a study about the electrical properties of Cr_0.75Fe_1.25O₃ (CRFO)/Fe_0.5Cu_0.75Ti_0.75O₃ (FCTO) magneto-dielectric composites. These compounds were prepared by the conventional solid-state reaction synthesis. The samples synthesized, as well their two-phase composites [Cr_0.75Fe_1.25O₃]_Z-[Fe_0.5Cu_0.75Ti_0.75O₃]_100−Z (Z=17, 34, 50, 66, 83), were characterized by X-ray powder diffraction technique (XRD). Rietveld's method was employed to verify the quantitative phase abundances in the composites’ and their theoretical densities, which were compared with the experimental densities (pycnometer method).To predict the effect of the phases in the composites effective dielectric function (κ), traditional dielectric mixing models such as parallel, series, and Lichtenecker's model were observed. An alternative approach, a sigmoidal fitting function based on the Boltzmann equation, was proposed to fit the experimental data.     

Dispersion characteristics for low-frequency magnetoelectric coefficients in bulk ferrite-piezoelectric composites

Ferrite-piezoelectric composites are magnetoelectric (ME) due to the interaction between magnetic and electrical subsystems through mechanical forces. A theory for the low-frequency Maxwell-Wagner relaxation in ME coefficients is discussed for bulk composites of nickel or cobalt ferrite and lead zirconate titanate (PZT). ME coefficients versus frequency spectra show two types of relaxation, over 0.1-100 μHz and 1-1000 Hz. The relaxation frequencies and the magnitude of the ME coefficients are dependent on the electrical and composite parameters and volume fraction for the two phases. The ME coefficient α_E is in the range 10⁻¹-10⁴ mV/cm Oe, higher in cobalt ferrite-PZT than for nickel ferrite-PZT, and is strongly dependent on PZT volume fraction v. Estimates of α_E and relaxation frequencies versus v provided here are useful for engineering composites with maximum ME effects for specific frequency bands.     

Preparation and characterisation of compositionally graded BaxSr1-xTiO3 thin films

Ba_xSr_1-xTiO₃ thin films with a compositional gradient of x=0.3 to 1 (in 0.1 mole fraction increments) were fabricated on Pt/Ti/SiO₂/Si substrates using a modified sol–gel technique. The graded film crystallised in a perovskite structure and consists of a uniform microstructure with comparatively larger grains. The room-temperature relative dielectric constant (ε_r) and dielectric loss (cosδ) at 100 kHz were found to be 305 and 0.03 respectively. Dielectric peaks were not observed in the temperature range from -20 °C to 120 °C. The dielectric constant and dielectric loss were almost independent of temperature. Polarisation–electric field measurements at room temperature revealed a saturated but slim hysteresis loop with a remanent polarisation (P_r) of 0.6 μC/cm² and a coercive field (E_c) of 2.4 kV/mm. The graded film behaves as a stack of Ba_xSr_1-xTiO₃ capacitors connected in series and hence the dielectric Curie peaks are removed. Received: 4 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Novel Ag–BaTiO3/PVDF three-component nanocomposites with high energy density and the influence of nano-Ag on the dielectric properties

Novel Ag–BaTiO₃/PVDF (polyvinylidene fluoride) three-component nanocomposites and traditional BaTiO₃/PVDF two-component nanocomposites were prepared by the same procedures. The dielectric properties of these two kinds of composites were compared. The results showed that the kind of three-component nanocomposites had better dielectric properties. The energy density of such kind of composites could reach nearly 10 J/cm³, which indicated that these nanocomposites could be used as the dielectric layers of pulse-power capacitors. The Coulomb blockade effect was used to explain the dielectric breakdown properties and the resistivities under high electric field of such new kind of nanocomposites.