Ewa Markiewicz

Dielectric relaxation in ferroelectric PZT–PVDF nanocomposites

Abstract Composites of electroactive ceramics and ferroelectric polymers are important for applications since they combine the most important features of the both phases and their properties can be easily tailored to various requirements. As the dielectric permittivity is involved in figures of merit of the quantities important for applications we studied dielectric response of Pb(Zr 0.5 Ti 0.5 )O 3 –polyvinylidene fluoride nanocomposites over a wide frequency and temperature range. It was found that though the permittivity value of the composites is high due to the high permittivity of the ceramics, the glass transition of the polymer and dielectric relaxation related to the wide-angle oscillation of polar groups attached to the main polymer chain determine the dielectric behaviour of the composites.

Dielectric and Pyroelectric Response of PLZT-P(VDF/TrFE) Nanocomposites

Dielectric response (100 Hz–1 MHz) and pyroelectric response (light modulation frequency range 10 Hz–1 kHz) of ferroelectric P(VDF/TrFE)(50/50) copolymer loaded with relaxor PLZT 9.5/65/35 ceramic powder of 10 nm crystallite size and ∼700 nm aggregates was studied. Though the dielectric permittivity of the composites increases due to high permittivity of the ceramic, their dielectric response is determined by dielectric relaxation processes of the polymer matrix. The dispersive dielectric anomalies of PLZT-0.9/65/35 relaxor ceramic affect mainly the dielectric absorption of the composites in the range of dipolar relaxation in the crystallite phase of the copolymer. The composites exhibit pyroelectric activity proper for application in operation frequency up to 1 kHz and temperature range 290–310 K.

The method of matching resonance frequencies in coupled transmitter PVDF/TRFE diaphragms

The influence of thermal aging on the resonance frequency and electric impedance of polyvinylidenefluoride (PVDF) type polymer films was studied with the aim to apply thermal treatment to modify and tune the parameters of polymer electromechanical transducers. Results of our study show that thermal treatment of PVDF/TRFE 75/25 copolymer in the temperature range 50 to 125/spl deg/C (10 min) can be applied for matching the resonance frequencies of coupled transducers. The resonance frequency of the copolymer was found to increase linearly with the rise of the annealing temperature by /spl sim/12%, whereas the related decrease in the electromechanical coupling coefficient does not exceed /spl sim/11%.

Effect of thermal treatment on magnetic and dielectric response of SrM hexaferrites obtained by hydrothermal synthesis

Electric, dielectric and magnetic properties of SrFe12O19 hexaferrite ceramics obtained from hydrothermally synthesized single-phase nanopowders were studied in wide temperature range. The effect of space charge polarization, related to highly conducting grains with poor conducting grain boundaries, was found to be apparent at high temperatures and at low frequencies. The activation energy of relaxation of the (Fe3+–Fe2+) dipoles in low conducting grain boundary regions was found to amount to 0.20 eV for non-annealed ceramics and to increase to 0.32 eV after thermal treatment. The temperature and frequency dependences of the dielectric permittivity for non-annealed and annealed SrFe12O19 ceramics were found to be correlated with respective dependences of the electric conductivity. We relate the observed increase in the saturation magnetization after annealing to an increase in coherent spin rotation in greater grains, which are however still below the critical single-domain size.

Simple electric equivalent circuit method to determine piezoelectric and elastic properties of piezoelectric polymer film

A simple method is proposed to determine the parameters of a piezoelectric resonator equivalent circuit in the frequency range from 1 MHz to 1 GHz. Traditional methods use an electronic setup which always involves significant frequency range limitations. The present method overcomes this limitation by simultaneous measurement of the reflectivity coefficient of two samples with HP4191A RF impedance analyzer. The measurements are carried out on films of polyvinylidenefluoride (PVDF) and its copolymer (PVDF/trifluoroethylene (TRFE))(0.75/0.25) which were corona charged as well as corona charged and then thermally aged. Piezoelectric and elastic constants are calculated using the parameters of the piezoelectric resonator equivalent circuit.

Impedance spectroscopy studies of SrMnO3, BaMnO3 and Ba0.5Sr0.5MnO3 ceramics

The impedance spectrum of hexagonal SrMnO3, rhombohedral BaMnO3 and orthorhombic Ba0.5Sr0.5MnO3 ceramics, synthesized by conventional high-temperature method, was studied in a wide temperature and frequency range. The complex impedance plots of Z″ versus Z′ pointed to two contributions originating from grains and grain boundaries. The parameters of electric equivalent circuit were calculated. The semicircles related to the grain boundary are located at the lower frequency side due to higher resistivity and capacity of the grain boundaries. The Ba0.5Sr0.5MnO3 ceramics is characterized by the lowest activation energy related to the grains. The conductivities σac of all the investigated samples show plateaus being a characteristic of the dc conductivity. The conductivity of Ba0.5Sr0.5MnO3 takes the values between those of BaMnO3 and SrMnO3.

BiFeO3 single crystal as resistive switching element for application in microelectronic devices

Local resistive switching was observed using AFM equipped with conducting tip addressing ∼15 unit cells in high-quality BiFeO3 single crystal grown from a non-stoichiometric melt. The switching appeared under high-vacuum conditions above 340 K and the studies at various temperatures yield the activation energy of 0.74 eV. The field-induced changes in the resistivity were ascribed to the presence of localized oxygen vacancies as a consequence of the mixed valence of Fe2+/Fe3+ ions.

Dielectric Relaxation in Confined Ferroelectric Polymer

The dynamics of segmental and chain motions in poly(vinylidene fluoride) confined to anodized aluminum oxide nanopores by melt-wetting was studied by broad-band dielectric spectroscopy. Molecular structure of the confined polymer was studied by means of wide-angle X-ray scattering in the transmission mode with ψ scanning. The confined polymer was found to form multilayered cylindrical mesostructure containing three polymorphs with the <020> direction of nonpolar polymorph II aligned along the pore channels. The relaxation rates of segmental motions in the amorphous phase of the polymer were found to be shorter in confined geometry than in the bulk, whereas the relaxation of the local mode was found to be broadened due to inhomogeneous relaxation times in the pores.

Pyroelectric Response of PZT-PVDF Nanocomposites of (0-3) Connectivity

Pyroelectric response of PVDF and P(VDF 0.5 /TrFE 0.5 )loaded with Pb(Zr 0.5 Ti 0.5 )O 3 ceramic powder of crystallite size ∼30 nm and aggregate size ∼100 nm was studied in light modulation frequency range 1 Hz - 1 kHz. As the ceramic crystallite size is comparable with intrinsic heterogeneity of the polymers we studied also dielectric dispersion and absorption of polymer and composite samples processed the same way. The effect of polymer heterogeneity is discussed and figures of merit of PZT-PVDF and PZT-P(VDF/TrFE) composites are given.

Properties of PVDF-MCM41 Nanocomposites Studied by Dielectric, Raman and NMR Spectroscopy

To obtain information on the polymer-inorganic filler interactions and on the confinement effect of polymer chains we studied the properties of solution-cast poly(vinylidene fluoride)-MCM41 molecular sieve nanocomposites. The dielectric response of the nanocomposites was found to be determined generally by the response of the polymer matrix, whereas the polymer-filler interactions resulted in an increase of the relaxation rates of the local mode and segmental motions in the amorphous phase of the poly(vinylidene fluoride) matrix. The effect has been related to a reduction of intermolecular correlations between the segments and was confirmed by the dispersion of 1H spin-lattice relaxation times in PVDF-MCM41 composites. Moreover, the relaxometry experiments have shown that the dynamics of poly(vinylidene fluoride) chains confined to MCM41 molecular sieve channels can be described by tube/reptation model proposed by De Gennes, Doi and Edwards.