Bozena Hilczer

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 response of polymer relaxors

Dielectric response of vinylidene fluoride type ferroelectric polymers is dominated by that of segmental motions in the amorphous phase in temperature range 200–300 K and contributions related to the local mode and ferroelectric–paraelectric transition in the crystalline phase of the polymer at higher temperatures. Diffuse and frequency-dependent dielectric anomaly observed in fast electron irradiated polyvinylidene fluoride-trifluoroethylene P(VDF/TrFE) has been related to relaxor-like behavior induced in the semicrystalline ferroelectric copolymers. As random field and the response of polar nanosize clusters determine the relaxor behavior the effects of disorder and fast electron irradiation (below and above TC) on the three contributions to the dielectric response of PVDF, P(VDF/TrFE)(75/25) and P(VDF/TrFE)(50/50) are shown. The processes involved in radiation-induced functionalization of PVDF-type polymers are discussed on the basis of results of ESR, IR and Raman spectroscopy studies.

Peculiarities of dielectric response of 1:1 family relaxors

A mixed ferroglass phase with coexistence of long and short range order was proposed as the model for quantitative description of the 1:1 family relaxors. Calculations of dielectric response of these relaxors were performed in the random field theory framework. Four and three groups of maxima were obtained in the dielectric permittivity temperature dependence at several frequencies for less and more ordered samples respectively. The frequency dependence of maxima at the lowest temperatures was shown to obey the Vogel-Fulcher (V-F) law, whereas other maxima the Arrhenius law. The peculiar feature of more ordered relaxors was shown to be the proximity of freezing temperature Tg to the temperature of maximum position Tm. Measurement of dielectric response of Pb(Sc0.5Nb0.2Ta0.3)O3 (PSNT) single crystals revealed two groups of maxima, one at T310 K and another at T>330 K. The frequency dependences of low temperature maxima and high temperature ones were shown to obey the V-F law with Tg = 292.5 K and the Arrhenius law with activation energy 0.49 eV. The comparison of obtained experimental data for PSNT as well as those for two samples of PbSc1/2Ta1/2O3 exhibiting different levels of order with the theory have shown that a mixed ferroglass model with different ratio of long and short range order describes fairly well the main peculiarities of the dielectric response of the 1:1 family relaxors.

FIR studies of α and β polymorphs of LiNH4SO4 single crystals

Abstract LiNH 4 SO 4 crystals occur in two polymorphous modifications. The α polymorph (space group P ca2 1 ) undergoes an irreversible transformation to the β modification (space group P c2 1 n) exhibiting a ferroelastic—paraelastic phase transition at about 284 K and a ferroelectric—paraelectric transition at T c ≈ 460 K. Using FIR spectroscopy we studied the differences in the extenal vibrations of the two polymorphs, and the α → β polymorphous transformation.

Theory of radiation induced relaxor behavior of poly(vinylidene fluoride-trifluoroethylene) copolymers

We propose a theory of the dynamic dielectric response in the organic ferroelectrics polyvinylidene fluoride (PVDF), poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] copolymer before and after irradiation by fast electrons. For this purpose, we use our random field formalism. In the adopted model we consider the earlier polymers as the disordered ferroelectrics in a mixed ferroglass phase (FG) with a coexistence of polar short- and long-range order regions. Short-range order regions correspond to amorphous layers, while long-range order regions appears in the crystalline phase of all-trans conformation. In our formalism, we account for the change of VDF content as well as for the influence of the irradiation, by variation of the content of electric dipoles and other random field sources. The downward shift of the paraferroelectric phase transition temperature and gradual transformation of the mixed FG phase into a dipole glass state with the increase of the irradiation dose was shown to be relate...

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.

Structure, hydrogen bond network and proton conductivity of new benzimidazole compounds with dicarboxylic acids

Dicarboxylic acids are interesting for crystal engineering due to their ability of hydrogen bond formation. To find a relationship between the molecular structure and the properties of proton conducting materials, we synthesized two compounds of benzimidazole with dicarboxylic acids of different chain length: glutaric and pimelic acids. The structure of the compounds was determined by X-ray diffraction and compared with molecular arrangement studied by 13C CP/MAS NMR spectroscopy supported by Density Functional Theory computations for benzimidazole. Benzimidazole was found to form salt with glutaric acid in a 1 : 1 ratio and monoclinic layer-type structure with P21/n space group. Flat layers, parallel to (−102) plane, are built of glutaric acid molecules linked into rectangular-type chains by O–H⋯O bonds and benzimidazole molecules attached to the chains by N–H⋯O bonds. The molecule of the benzimidazole compound with pimelic acid contains two benzimidazole rings and one pimelic acid chain. The structure with alternating layers, built of two groups of benzimidazole dimers and layers of pimelic acid chains, belongs to P21/n space group. The acid layers are built of pimelic acid molecules linked by O–H⋯O hydrogen bonds into chains parallel to [30−1] direction and the benzimidazole dimers are linked to the acid layers by N–H⋯O bonds. Impedance spectroscopy studies in wide frequency and temperature range of pellets made of powdered compounds enabled to separate the contributions of crystalline and grain boundary parts to the electric conductivity. The conductivity (averaged over all directions) of the crystalline compounds of benzimidazole with glutaric and pimelic acid is characterized by activation energy of 2.5 eV and 1.6 eV, respectively, which is in an agreement with the hydrogen bond network in the materials.

Raman Scattering Studies of Lead Free (1-x)K0.5Na0.5NbO3-xSrTiO3 Relaxors

We present the first results of FT NIR Raman studies of (K 0.5 Na 0.5 NbO 3 ) 1 − x (SrTiO 3 ) x (x = 0.25, 0.20, 0.15) relaxors made in wide temperature range. Although the crystal structure of the investigated relaxors exhibits a pseudocubic symmetry Raman spectra are observed. The Raman spectra of (K 0.5 Na 0.5 NbO 3 ) 1 − x (SrTiO 3 ) solid solution resemble those observed for orthorhombic K 0.5 Na 0.5 NbO 3 . The results are discussed in the frame of molecular dynamics of NbO 6 octahedral network. It was found that, the changes in dynamics of NbO 6 molecular groups appear at T ≈ 196 K for x = 0.25, T ≈ 254 K for x = 0.20 and T ≈ 336 K for x = 0.15.

Spiral patterns on GASH

Abstract On the center region of cleavage surface of GASH (guanidinium aluminum sulfate hexahydrate) crystals grown at high temperatures, peculiar spiral domains have been observed by a powder deposition technique. Triangular etch pits found at the center of the spirals are direct evidence for the screw dislocation mechanism of crystal growth. Dislocations of high density are associated with the defects at the sites of crystallization water introduced during crystal growth at high temperatures. The formation model of the spiral domains are discussed on the basis of the spiral growth theory of crystal.

Superionic phase transition in CsHSeO4 and CsDSeO4single crystal

Abstract Deuterization of CsHSeO4 does not affect of the superionic phase transition but increases the activation energy in the superionic phase.