This paper shows that CaCu_3Ti_4O_{12} (CCTO) can be synthesized through the high-energy ball milling of CaO, CuO and TiO_2 powders. The dielectric characterization of CCTO obtained mechanochemically as well as, for comparison, by a high-temperature method is presented. Moreover, it is illustrated that zirconium oxide in contrast to a metallic iron generated during milling processes improves the properties of CCTO in terms of ceramic capacitors.
The complex dielectric permittivity ε*(ν,T) = ε'(ν,T) - iε''(ν,T) of n-p-(ethoxybenzylidene) p'-propylaniline has been measured in the frequency range from 1 Hz to 13 MHz and temperature range from 360 K to 180 K on heating and on cooling the sample. Dielectric relaxations have been found in nematic and solid phases. A detailed analysis of the observed processes has been performed and the parameters describing molecular dynamics have been evaluated. The comparison of results with those obtained for the other Schiff bases is discussed.
This study reports on the spontaneous polarization and molecular dynamics of two diastereomeric ferroelectric liquid crystals with a phenyl core ring but with different types of diastereomeric structures. The magnitudes of spontaneous polarization, measured via an Automated LC Property Tester, were similar for the two ferroelectric liquid crystals. The complex dielectric spectra, measured via an impedance analyzer, indicated the existence of the Goldstone mode in the ferroelectric (SmC*) phase for the two ferroelectric liquid crystals. Moreover, a novel ferrielectric-like (SmCγ*-like) phase was observed at certain temperatures. The complex dielectric spectra of the two ferroelectric liquid crystals were optimally analyzed by a theoretical model composed of the Debye model and the Cole-Cole model. The physical parameters such as relaxation frequency and dielectric strength were determined by a dielectric relaxation study, and the obtained parameters varied in accordance with phase transition sequence. The experimental results of dielectric spectra and spontaneous polarization are explained in the discussion of the mesomorphic properties related to the ferroelectric liquid crystal molecular structure.
For 4-cyano-3-fluorophenyl 4-butylbenzoate, nematic glassformer at ambient pressure, dielectric relaxation studies were performed under elevated pressure. In the isobaric experiment, on cooling the nematic phase two superarrhenius α-relaxations, ascribed to the reorientations of molecules around short axes and precession of long molecular axes were found and the Arrhenius β-relaxation related to intramolecular motions. Complexity of dynamics at elevated and ambient pressure is similar. Shift towards lower frequencies was found for both α-relaxations.
The dielectric relaxation processes observed in solid phases of MBBA, OH-MBBA, and EBBA belonging to the homologous series of the Schiff bases are discussed. The data are compared with the results completed by the present authors for EBPA compound. The uniform interpretation of the temperature changes of relaxation for all mentioned compounds is proposed.
A composite of polystyrene (PS)/ferroelectric barium stannate titanate (BST) with different BST concentration was prepared. The dielectric properties were measured using HP low impedance frequency analyzer. It is found that samples behaved as an RC network. Composites become more capacitive than resistive with increasing frequency while resistance decreases with increasing BST ceramic concentration. Temperature effects were monitored throughout the annealing process. It is found that the capacity and the ac conductivity increased after annealing while the bulk resistance decreased. Our results suggest that, the obtained material is expected to be a promising candidate for electronic ceramics. The study provides a better understanding of the relationship among these parameters.
The changes of dielectric parameters in oil-based ferrofluid have been measured in an external magnetic field. The frequency dependent real permittivity and the dissipation factor were measured within the frequency ranges from 1 mHz to 2 MHz by a capacitance method. These parameters have been studied in combined electric and magnetic field, when fields were parallel and perpendicular. The Cole-Cole model has been used to analyze measured data. When a magnetic field was applied, the interaction between the magnetic field and magnetic moments of nanoparticles led to the aggregation of magnetic nanoparticles to new structures - thick chains which had influence on the value of dielectric permittivity. At constant magnetic field the dependence of real permittivity and tanδ on angle between the electric and magnetic field (anisotropy) were measured, too. The various influences of magnetic field development on the investigated liquid are discussed.
Dielectric relaxation study was carried out for the nematic and isotropic phases of 4-(trans-4'-n- heptylcyclohexyl)isothiocyanatobenzene in the frequency range from 100 kHz to 1 GHz. In the nematic phase two relaxation processes were recorded for the electric permittivity component measured parallel to the molecular orientation (director n) and three relaxation processes - for the perpendicular permittivity component. The strength of the nematic potential and the nematic order parameter were estimated.
Derivatographic, dielectric and complex impedance studies of Rb_{4}LiH_{3}(SeO_{4})_{4} crystals revealed a superionic phase transition at T_{s} ≈ 448 K. The bulk conductivity rises from 10^{-6} to 10^{-4} (Ωm)^{-1} at T_{s}, and the activation energy of conductivity in the direction of crystal axes a and b (where a = b for tetragonal symmetry) W_{a} = W_{b} decreases from 0.94 eV in the low temperature phase to the value of 0.27 eV at T > T_{s}. The analysis of the results of X-ray studies at room temperature and at 450 K indicates that the fast ion transport, characterized by low activation energy, is related to the delocalization of the H2 protons in the O(12)-H2-O(24) bonds linking the Se(1)O_{4} and Se(2)O_{4} tetrahedra. The "cogwheel" mechanism is suggested to be involved in the fast proton transport.
The complex dielectric permittivity of 4-(2-hexyloxy-ethyloxy) 4'-cyano-biphenyl measured in the frequency range from 0.03 Hz to 10^9 Hz is presented. Monotropic sequence of thermodynamic phases was found with two stable phases (isotropic and crystal) and three metastable states (supercooled isotropic phase, its glass and a smectic phase observed only on heating the sample first cooled rapidly to the liquid nitrogen temperature). In isotropic and supercooled isotropic phases the temperature dependence of the dielectric relaxation time was described by the same Vogel-Fulcher-Tammann formula while in a smectic phase it was of the Arrhenius type. In the transition from a supercooled isotropic phase to a smectic phase the relaxation rate of molecular motions around the short axes increases by two decades. Comparison of the relaxation behaviour and polymorphism of 4-(2-hexyloxy-ethyloxy) 4'-cyano-biphenyl of linear molecules with two chiral cyanobiphenyls is discussed.
Piezomagnetic materials in composition Ni_{0.284}Zn_{0.549}Cu_{0.183}Fe_{1.984}O₄ were prepared by mixed oxide method at 1100°C. Powder X-ray diffraction studies confirm the crystalline nature of the synthesized Ni_{0.284}Zn_{0.549}Cu_{0.183}Fe_{1.984}O₄ piezomagnetic material. The crystallite size is calculated to be 19.332 μ m using the Debye-Scherrer formula. The surface morphology and particle size of the samples has been studied by scanning electron microscopy. The thermal stability and decomposition behaviour of Ni_{0.284}Zn_{0.549}Cu_{0.183}Fe_{1.984}O₄ piezomagnetic material have been studied by thermogravimetric analysis at a heating rate of 15°C/min. The effective activation energy of the prepared composite was calculated using single heating rate methods: Broido's and Coats-Redfern methods. Dielectric properties of Ni_{0.284}Zn_{0.549}Cu_{0.183}Fe_{1.984}O₄ piezomagnetic material have been studied in a wide range of frequencies and temperatures. The magnetic behavior of Ni_{0.284} Zn_{0.549} Cu_{0.183} Fe_{1.984}O₄ piezomagnetic material at room temperature has been confirmed by vibrational sample magnetometer studies.
The dielectric study of Sr_{0.73}Ba_{0.27}Nb_2O_6:Ce (SBN) crystals along [010] crystallographic axis was performed in the temperature region of 310-360 K and frequency range from 25 Hz up to 1 MHz. The thermal dipole relaxation of quasi-Debye-type for this orientation of sample was observed in both investigated structural phases. The phase transition was most clearly seen from the temperature dependence of the relaxation time at T_{c} = 320 K. The relaxation processes were related to the collective oscillations of the Nb-O bonds in the two different type corner-sharing NbO_6 octahedra aligned along c-axis.
The paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity, and the splay and bend elastic constants for 4-n-hexyloxy-4' -cyanobiphenyl (C_6H_{13}O-Ph-Ph-C≡N). On the basis of the static values of the nematic principal permittivities, ε_ǁ (T) and ε_(T), the angle between the dipole moment vector and the long axis of 4-n-hexyloxy-4'-cyanobiphenyl molecule, the apparent dipole moment and the nematic order parameter were determined. From the temperature dependences of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of 4-n-hexyloxy-4'-cyanobiphenyl molecule (in isotropic phase) were estimated. The K_{11} and K_{33} elastic constants were determined from the voltage dependence of the capacitance of the planar nematic cell with the method proposed by Gruler et al. and Uchida et al.
In the work tests of the influence of the technological conditions on the basic applied properties of the Pb(Fe_{0.5}Nb_{0.5})O_3 ceramics were carried out. The Pb(Fe_{0.5}Nb_{0.5})O_3 specimens were obtained by a two-stage method of synthesizing (the niobite method), a technique of the powder mixture calcinations, changing a temperature range of the iron-niobate (FeNbO_{4}), whereas compacting was conducted by free sintering.
In the paper technological conditions were looked for to obtain the Pb(Fe_{1/2}Nb_{1/2})O_{3} (PFN) ceramics and the PFN ceramics with the lithium admixture with the highest electric conduction and conditions appropriate for the PTC-R effect formation. Measurements of dc electrical resistivity, dielectric permittivity and dielectric loss were made as functions of temperature from room temperature to 190°C, at 1 kHz. The best set of values of dielectric loss and dielectric constant, from the ferroelectricity point of view, were obtained when the precursor with orthorhombic structure was employed. By creating appropriate technological conditions in the PFN ceramics the PTC-R effect can occur above the Curie temperature. Their electric properties are determined by presence of a basic phase of the potential barrier on the grain boundaries.
In this work an attempt was made to obtain three compositions of the solid solution (1-x)Pb(Fe_{0.5}Nb_{0.5})O_{3}-(x)BiFeO_{3} for x=0.8, 0.7 and 0.6. The obtained specimens were subjected to microstructure and dielectric examinations and temperature dependences of the internal friction Q^{-1}(T) and Young's modulus E(T).
The ordered PbSc_{1/2}Nb_{1/2}O_3 ceramics were studied by THz transmission spectroscopy in the temperature range of 80-300 K. Below ferroelectric phase transition temperature the strength of central mode gradually decreases and gives evidence for a mixed displacive and order-disorder character of the transitions. Ferroelectric phase transition is connected with an abrupt freezing and rise of polar nanoregions into ferroelectric domains.
The paper presents influence of external factors on results of bushing dissipation factor tanδ and capacity measurements performed on not energized transformer as well as with operation voltage using on-line method. There are discussed methods and real cases of measurements, with emphasized differences in results interpretation. Conclusions and literature are summarized.
This paper presents the results of investigations of the temperature dependence of heat capacity and dielectric dispersion in the vicinity of ferroelectric-ferroelastic phase transition of dimethylammonium metal sulphate hexahydrate crystals DMAAl_{1-x}Cr_{x}S. In particular, it is shown that the isomorphous substitution of metal ion noticeably changes the temperature of phase transition and parameters of the fundamental ferroelectric dispersion observed around T_{c1}. These changes are explained in terms of clusters sizes and dynamics in the framework of order-disorder type phase transition mechanism.
Ceramic solid solutions (Bi_{1-y}La_{y})_{4}(V_{1-x}Zr_{x})_{2}O_{11-z} with x = 0-0.05, y = 0-0.16 have been prepared by the solid state reaction method. The samples were studied by differential thermal analysis, X-ray diffraction, dielectric spectroscopy, and impedance methods. The concentration and temperature stabilization regions of the polymorphous α-, β-, γ'-, γ-modifications have been determined. The effects observed in dielectric properties, conductivity, and impedance data confirmed the influence both of intrinsic oxygen vacancies and those "pinned" at ferroelectric domain boundaries on the temperature hysteresis of α-β phase transition and their contribution to mechanism of oxygen ion transport.
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