The influence of L-threonine admixture on the dielectric properties of triglycine sulphate (TGS) crystals was studied. With increasing concentration of the admixture in the solution from which the crystals were grown, their maximum permittivity and spontaneous polarization were found to decrease, their Curie point was found not much different than that for pure TGS crystals (322 K), the coercive field and bias field increased. The domain structure of the obtained crystals was also studied by the liquid crystal method.
Microwave measurement techniques attract attention due to its practical solutions in breast cancer imaging. Early detection of cancer is the purpose of these imaging studies. For these imaging processes, phantoms which reflect the properties of the area to be imaged are produced. The produced homogeneous phantoms are imaged by stepper frequency of radar technology, as the narrow bands are used for heterogeneous mixtures. Phantoms are used since 70's until today. Different methods are experimented for producing phantoms of tissue, muscle, fat, skin, etc. Iron powder, polymer materials, various oils and gelatins used today are the materials used for producing the phantoms. In this study, phantom breast imaging is aimed. Phantoms breast and breast tumor samples are created. For phantoms and tumor samples, reflection and transmittion measurements are done with Network Analyzer in X-Band (8.2-12.4 GHz). Complex permittivity is calculated with Agilent 85071 Material Measurement Software-Fast Method (NIST-iterative method). It was considered breast phantom with tumor model for 3D imagine at 2-3.5 GHz frequency range.
Dielectric properties of a D-serine admixtured triglycine sulphate (TGS) crystal were studied on samples cut out from different sites in the crystal. The value of spontaneous polarization P_{s} measured on b-cuts was found to be independent of the site. Whereas the coercive field E_{c} values were higher for the samples cut out closer to the seed than for those cut out at the ends of the crystal along b axis. After rejuvenation the values of the bias field E_{b} increased in contrary to the effect observed in the pure TGS crystals. An exponentially decreasing dependence of the maximum permittivity ε_{max} on the site of the samples (b-plates) cut out along b axis from -b to +b positions, was obtained.
Single crystal of erbium doped La_3Ga_{5.5}Ta_{0.5}O_{14} grown by the Czochralski method have been investigated by electron paramagnetic resonance and dielectric spectroscopy methods. Dielectric permittivity ε measurements performed in 90-440 K temperature range have shown negligible dispersion for 1 kHz - 1 MHz frequencies and a Curie-Weiss type behaviour with C=47700 K andθ=-340 K. Electron paramagnetic resonance studies have revealed the presence of two different paramagnetic, monoclinic centres. The calculated g factor values are: g_1=1.449, g_2=11. 534, g_3=4.24 for the main M_1 centre and g_1=1.98, g_2=4.169, g_3=4.25 for the weaker M_2 centre. The temperature dependence of EPR line intensity for centre M_1 and M_2 is quite different - while lines attributed to M_1 could only be observed at low temperatures, below 20 K, lines of M_2 centre persisted up to 200 K. The M_1 centre is connected with Er^{3+} ion entering substitutionally into La^{3+} site, while M_2 is probably connected with 3d ions at the same site, unintentionally introduced into the material as an admixture.
On the basis of the optical and dielectric investigations of [(CH_3)_2NH_2]_5Cd_2CuCl_{11} solid solutions the existence of phase transitions at T_1=175 K and T_2=117.5 K was confirmed. Both phase transitions were found to be shifted toward lower temperatures with respect to the corresponding transitions in the "host" [(CH_3)_2NH_2]_5Cd_3Cl_{11} crystals. It was found that the proton conductivity in [(CH_3)_2NH_2 ]_5Cd_2CuCl_{11} crystals is realized through the Grotthus mechanism. The investigations of the birefringent properties confirmed existence of the structural changes at T_0=320 K connected with the complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn-Teller distortion with temperature.
Dielectric relaxation study of nematogenic 4-n-alkyl-4'-cyanobiphenyls (nCB, n = 5, 6), 4-(trans-4'-n-alkylcyclohexyl)isothiocyanatobenzenes (nCHBT, n = 6, 8), 4-cyanophenyl-4'-n-alkylbenzoates (nCPB, n = 6, 8), 4-cyano-3-fluorophenyl-4'-n-octyloxybenzoate (8OCFPB), and 4-cyanophenyl-4'-n-octyloxy-3'-fluorobenzoate (8OCPFB) was performed in the frequency range from 50 kHz to 100 MHz in the nematic and isotropic phases. The static permittivity and the relaxation process related to the rotation of molecules around their short axis was analyzed. For some of these liquid crystals anomalous temperature dependence of static permittivity in the pretransitional region of the isotropic phase was observed. Based on the Meier-Saupe-Martin model of molecular diffusion in nematics, the orientational order parameter 〈P_2〉 was determined from dielectric relaxation times and retardation factor. The values of 〈P_2〉 calculated from the dielectric relaxation data were compared with the results obtained from measurements of polarized electronic absorption. Correlations between the magnitude of the dielectric pretransitional effect and the orientational order in the nematic phase were discussed.
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.
The static dielectric constants and the optical birefringence for four members of the homologous series of bicyclo[2,2,2]octane derivatives with -NCS terminal group (pentyl to octyl) have been measured as a function of temperature. From the dielectric anisotropy, the optical birefringence as well as the polarized absorption spectra of the dichroic dye dissolved in the liquid crystalline host the order parameter has been determined. The results obtained by using various experimental methods have been compared with those calculated from the Maier-Saupe mean field theory. It has been found that the order parameter for the successive homologues shows alternation similar to that exhibited for the nematic-isotropic transition temperature.
The AC conductivity σ (ω) and the complex dielectric permittivity ε*(ω) were studied as function of temperature 300 K < T < 600 K and at some selected frequencies (1-20 kHz) for polycrystalline sample of KClO_4. The differential thermal analysis (DTA) thermograph was also performed. The combined data support each other and indicate the existence of a structural phase transition at ≈ 575 K. Moreover, the temperature dependence of the ac conductivity behaves in accordance with Arrhenius relation, whereas the frequency dependent conductivity obeys the power law σ(ω) = Aω^{s(T)}. The behavior of s with temperature suggests that the hopping over the barrier model prevails. No evidence for the existence of a ferroelectric phase transition at the transition temperature.
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.
Until now, relaxor ferroelectrics are considered as a class of disordered materials possessing peculiar structures and properties which are not yet generalized into a universal model explaining the significant amount of experimental data available. In this work, we demonstrate that one feature of relaxor ferroelectrics - the extraordinary dielectric response - is well-suited for application in electrocaloric refrigerators. We consider the electrocaloric effect with special attention to relaxor ferroelectrics, the dielectric response in the temperature region of interest, the efficiency and the figure of merit of relaxor ferroelectrics for electrocaloric application.
Ceramic composites (100 - n)Bi_{4}V_{2}O_{11-z}-nCe_{0.9}Gd_{0.1}O_{1.9} with n=0÷25 wt% were prepared and studied by the X-ray diffraction, dielectric spectroscopy, and impedance methods. Slight increase in the unit cell volume accompanied by monotonous decrease in temperatures and broadening of the α-β and β-γ phase transitions with increasing fluorite content was observed in the composites studied. Increase in melting temperatures of composites with n ≥ 10 with the retention of their high ionic conductivity was also proved.
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.
The dielectric behaviour of a magnetic fluid with magnetite particles dispersed in kerosene was analyzed. Therefore, the frequency (f) and temperature (T) dependences of the complex dielectric permittivity, ε (f, T), over the ranges 4 kHz to 2 MHz and 25C to 90C were measured. Based on the experimental results of ε (f, T) and using the Clausius-Mossotti equation, we have determined the temperature dependence of the real part of the total polarizability α', of a magnetic fluid. The computations have taken into account that the magnetic fluid consists of three components, namely magnetite particles, surfactant, and carrier liquid. The results show that at a given frequency, α' increases with temperature in the low frequency range (4 kHz to 100 kHz) and decreases with temperature above 100 kHz. This behaviour demonstrates that in low frequency range the polarization mechanism related to the deformation of the counter ions atmosphere around each particle is predominant and above 100 kHz the orientation of the dipole moments is the main polarization mechanism of the magnetic fluid. These measurements enabled the evaluation of the effective dipole moment of the magnetic fluid, in order of 1.21 × 10^{-30} C m.
This contribution is devoted to study of dielectric and magnetodielectric properties in transformer oil based magnetic fluid. The dielectric permittivity and their anisotropy at various mutual orientation electric (50 Hz) and magnetic field were shown at the different volume concentrations of nanoparticles at room temperature. The linear increase of dielectric constant with volume concentration was confirmed which is in good agreement with the theoretical Maxwell prediction. The dielectric anisotropy factor g(B, ω) is very close to g=1. The values of permittivity increased with the increase of volume fraction of magnetite nanoparticles. The highest value of anisotropy characterized by deviation of permittivity Δε=ε_{∥} - ε_{⊥} was found for the highest volume concentration which could mean that no aggregation of magnetite nanoparticles had appeared.
Phase transition in polycrystalline fullerene C_{60} from fcc phase to a simple cubic phase sc induced by hydrostatic pressure up to 0.5GPa was studied by dielectric method. Pressure coefficient dT_{c}/dp=144±8 K/GPa was determined. This value is consistent with pressure coefficients obtained with other experimental techniques.
This is a review of our recent developments in the physics of lead telluride nanostructures. PbTe is a IV-VI narrow gap paraelectric semiconductor, characterized by the huge static dielectric constant ε >1000 at helium temperatures. We nanostructurized this material by means of e-beam lithography and wet chemical etching of modulation doped PbTe/Pb_{1-x}Eu_xTe quantum wells. Magnetoresistance measurements performed on the nano-structures revealed a number of magnetosize effects, confirming a ballistic motion of the carriers. The most important observation is that the conductance of narrow constrictions shows a precise zero-field quantization in 2e^2/h units, despite a significant amount of charged defects in the vicinity of the conducting channel. This unusual result is a consequence of a strong suppression of the Coulomb potential fluctuations in PbTe, an effect confirmed by numerical simulations. Furthermore, the orbital degeneracy of electron waveguide modes can be controlled by the width of PbTe/Pb_{1-x}Eu_xTe quantum wells, so that unusual sequences of plateau conductance can be observed. Finally, conductance measurements in a nonlinear regime allowed for an estimation of the energy spacing between the one-dimensional subbands.
Cadmium pyroniobiate (CNO) is the ferroic material with unusual diffuse phase diagram which includes both relaxor and nanocluster domains. We present the data for CNO crystals with several admixtures of paramagnetic ions studied on X and S microwave bands at zero-external magnetic field and classical EPR conditions. Our results lead to an assertion of a ferroelectric resonance effect due to "electronic ferroelectricity". The data fit well to the Falicov-Kimball theoretical model applicable to strongly-correlated-electron or mixed-valence systems.
Broadband dielectric spectroscopy measurements of Nb₆VSb₃O₂₅ showed that in both the real (ε') and imaginary (ε'') components of permittivity there is visible relaxation process strongly obscured by dc conductivity. Application of the electric modulus representation of the data enables to study temperature evolution of this relaxation together with conductivity relaxation. It was showed that the activation energies for both processes are close. Low-frequency loss tangent increases strongly with temperature, suggesting that in the compound under study additional energy losses are associated with the conduction of electric current, as determined by the Joule-Lenz law.
In this paper, we report on the temperature dependent broadband dielectric response of a ferrofluid based on transformer oil and magnetite nanoparticles covered with oleic acid molecules. For that purpose the method of dielectric spectroscopy has been chosen in the frequency range from 20 Hz up to 100 kHz. The experiments were carried out on thin film ferrofluid samples confined in a glass plate capacitor containing indium tin oxide (ITO) plate electrodes. The obtained complex permittivity spectrum shows a pronounced dielectric dispersion in the low frequency range. Taking into account the ferrofluid composition we associate this relaxation with ion impurity polarization at the nanoparticle-oil interface. The strong temperature dependence of the relaxation process has been found when conducting the experiments in the temperature range from 298 K to 358 K. The relaxation time of the revealed process exhibits a typical Arrhenius behavior. Based on the conducted experiments and analysis, we propose some reasonable practical applications of the studied ferrofluid in the field of electrical engineering.
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