Electro-optical characteristics of photoinduced isomerization in 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline]-azo-dye-doped nematic liquid crystals

The formation of photoinduced gratings was realized and dielectric properties were investigated in a nematic liquid crystal (LC) doped by 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline] azo dye (AD). The enhancement of the optical and electrical properties of the system was studied by dielectric spectroscopy and diffraction grating measurements. We report the electro-optical properties of LC cells containing pure LC (E7) and its doped form with 1% (wt/wt) 2,2′-azobis[5,6-bis(dodecyloxy)quinoxaline] AD. Diffraction efficiencies, η, of 441 nm pump and 632 nm probe beams were measured in two-wave-mixing experiment. Diffraction efficiency has a noteworthy increase in AD-doped nematic liquid crytals. A maximum diffraction efficiency of 14% was found for cell doped with AD, while cells without AD had a maximum efficiency of 1%. Photoinduced refractive index change Δn and nonlinear index coefficient n₂ were also determined by the two-wave-mixing method. The dielectric parameters of AD-doped E7 and pure E7 LC have been investigated in the frequency range 1 kHz-1 MHz by the dielectric-spectroscopy method. Dielectric anisotropy (Δε) property of the LCs changes from the positive type to negative type and dielectric anisotropy values decrease with AD.     

Dielectric relaxation and magnetic field dependent alternating current conductivity of nanocrystalline cadmium-zinc ferrite below room temperature

The dielectric permittivity of nanocrystalline cadmium-zinc ferrite prepared by the ball milling method has been investigated within a temperature range 77≤T≤300 K in presence of a magnetic field up to 1 T and in the frequency range 20 Hz-1 MHz. The dielectric permittivity follows the power law ε^/(f)∝Tⁿ where the temperature exponent ‘n’ is found to be frequency dependent. The dielectric properties of the samples have been analyzed in terms of electric modulus vector. The dielectric relaxation has been explained by interfacial polarization. The variation of the relaxation time with temperature indicates the presence of two different activation energies. The ac magnetoconductivity is positive for the milled sample and becomes negative for the unmilled sample. This behavior can be explained in terms of grain and grain boundary contribution to impedance of the samples.     

Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal

In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency (η) and refractive index modulation (Δn) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz-10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye.     

Dielectric relaxation study of dipropylsulfoxide/water mixtures

The complex dielectric spectra of dipropylsulfoxide (DPSO)/water mixtures in the whole concentration range have been measured as a function of frequency between 100 MHz and 20 GHz at four temperatures between 298.15 K and 328.15 K. The dielectric parameters, static dielectric constant (ε_s), relaxation time (τ) and relaxation strength (Δε) have been obtained by the least squares fit method. The relaxation in these mixtures can be described by two Debye functions, whereas for pure DPSO Cole-Davidson type is valid. The relaxation times of the mixtures show a maximum at about x(DPSO) ≈ 0.3. In the concentration range where a maximum appears, the interaction of DPSO with water is presumably the result of hydrogen bonding between water and the sulfonyl group of the sulfoxide molecule. The concentration and temperature dependent excess dielectric constant and effective Kirkwood correlation factor of the binary mixtures have been determined. The excess permittivity is found to be negative for all concentrations.     

Dielectric and optical behaviors in relaxor ferroelectric Pb(In1/2Nb1/2)1−xTixO3 crystal

Dielectric permittivities (ε′,ε″) have been measured as functions of temperature (140-535 K) and frequency (500 Hz-2.0 MHz) in a (001)-cut Pb(In_1/2Nb_1/2)_0.7Ti_0.3O₃ (PINT30%) single crystal grown by the modified Bridgman method with Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMNT29%) seed crystal. A diffused phase transition was observed in the temperature region of ∼430-460 K with strong frequency dispersion. Above the Burns temperature T_B≅510 K, the dielectric permittivity was found to follow the Curie-Weiss behavior, ε′=C/(T−T_C), with parameters C=3.9×10⁵ and T_C=472 K. Below T_B≅510 K, polar nanoclusters are considered to appear and are responsible for the diffused dielectric anomaly. Optical transmission, refractive indices, and the Cauchy equations were obtained as a function of wavelength at room temperature. The unpoled crystal shows almost no birefringence, indicating that the average structural symmetry is optically isotropic. The crystal exhibits a broad transparency in the wavelength range of ∼0.4-6.0 μm.     

The effect of filler on the temperature coefficient of the relative permittivity of PTFE/ceramic composites

High permittivity and low-loss ceramic fillers have been prepared by means of the solid state ceramic route. Ceramic-filled composites were prepared by the Sigma Mixing, Extrusion, Calendering, which was followed by the Hot pressing (SMECH) process. The microwave dielectric properties of the composites were studied using X-band waveguide cavity perturbation technique. The temperature coefficient of the relative permittivity of the composites was investigated in the 0-100 °C temperature range using a hot and cold chamber coupled with an impedance analyzer. The temperature coefficient of the relative permittivity of the composites showed strong dependence on the temperature coefficient of the relative permittivity of the filler material. In the present study, a high-permittivity polymer/ceramic composite, having τ_εr ∼63 ppm/K, has been realized. This composite is suitable for outdoor wireless applications.     

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).     

Conformation transitions of blood proteins under influence of physical factors on microwave dielectric method

In this article, the influence of γ-irradiation and temperature on albumin and fibrinogen conformation and dielectric properties of protein solutions have been studied by the microwave dielectric method. Both the values of the real part ε′ (dielectric permittivity) and the imaginary part ε″ (dielectric losses) of the complex dielectric permittivity of the aqueous solution of bovine serum albumin and human fibrinogen as functions of temperature and γ-irradiation dose have been obtained. The time of dielectric relaxation of water molecules in the protein solutions was calculated. The hydration of the albumin and fibrinogen molecules was determined. The temperature dependencies of hydration are non-monotonous and have a number of characteristic features at the temperatures 30-34 and 44-47 °C for serum albumin, and 24 and 32 °C for fibrinogen.     

Understanding the molecular interaction and relaxation findings in amphiphilics on solution state using TDR

The dielectric behavior of stearic acid in 1,4-dioxane medium at various temperature 303 K–288 K and frequency regime (10 MHz–30 GHz) determined from the complex dielectric permittivity spectra obtained by Time Domain Reflectometry(TDR). In this frequency range, the dielectric study gives the electrostatic interaction as well as orientational polarizability of complex mixtures of long chain molecules significantly. Dielectric parameters were calculated from the complex spectra of the binary mixture by non linear least square fit method. The excess permittivity (ε^E), correlation factors were calculated for the binary system. Thermal parameters(ΔH-enthalpy, ΔS-entropy and ΔG-Gibbs free energy) were calculated and the direction of reaction is determined. The FTIR spectrum of the binary system recorded and the assignments are discussed. The FTIR spectral assignments confirm the molecular interactions.     

Ferroelectric relaxor behavior in hafnium doped barium-titanate ceramic

Temperature and frequency dependence of the real (ε′) and imaginary (ε″) parts of the dielectric permitivity of cubic Ba(Ti_0.7Hf_0.3)O₃ ceramic has been studied in the temperature range of 100 K to 350 K at the frequencies 0.1 kHz, 1 kHz, 10 kHz, 100 kHz for the first time. Diffuse phase transition and frequency dispersion is observed in the permittivity-vs-temperature plots. This has been attributed to the occurrence of relaxor ferroelectric behavior. The observed relaxor behavior has been quantitatively characterized based on phenomenological parameters. A comparison with the Zr doped BaTiO₃ has also been presented. For Hf doped samples transmission electron microscopy (TEM) characterization do show the presence of highly disordered microstructure at length scales of few tens of nano-meters.     

Spectroscopic investigations and electrical properties of PVA/PVP blend filled with different concentrations of nickel chloride

Films of PVA/PVP blend (50/50) filled with different concentrations of NiCl₂ were prepared by casting method. The prepared films were investigated by different techniques. XRD scans demonstrated that the peak intensity at 2θ≈20° decreased and the band width increased with increase in the concentrations of NiCl₂ content, which implied decrease in the degree of crystallization and hence causes increase in the amorphous region. UV-vis analysis revealed that the values of the optical band gap are affected with increase in NiCl₂ content. This indicates the formation of charge transfer complexes between the polymer blend and the filler. The rise of conductivity is significant with increased concentration of NiCl₂ filler; this reveals an increase in degree of amorphosity. AC conductivity (σ_ac) behavior of all the prepared films was investigated over the frequency range 42 Hz-5 MHz and under different isothermal stabilization in the temperature range 313-393 K. It suggests that the hopping mechanism might be playing an important role in the conduction process in high frequency region. The dielectric behavior was analyzed using dielectric permittivity (ε´, ε″) dielectric loss tangent (tan δ) and electric modulus (M″). The decrease in dielectric permittivity was observed with increase in the concentration of NiCl₂ filler. This suggests the role of NiCl₂ as filler to improve the electrical conductivity of PVA/PVP blend.     

BiFeO3 ceramic matrix with Bi2O3 or PbO added: Mössbauer, Raman and dielectric spectroscopy studies

In this paper Mössbauer, Raman and dielectric spectroscopy studies of BiFeO₃ (BFO) ceramic matrix with 3 or 10 wt% of Bi₂O₃ or PbO added, obtained through a new procedure based on the solid-state method, are presented. Mössbauer spectroscopy shows the presence of a single magnetically ordered phase with a hyperfine magnetic field of 50 T. Raman spectra of BFO over the frequency range of 100-900 cm⁻¹ have been investigated, at room temperature, under the excitation of 632.8 nm wavelength in order to evaluate the effect of additives on the structure of the ceramic matrix. Detailed studies of the dielectric properties of BiFeO₃ ceramic matrix like capacitance (C), dielectric permittivity (ε) and dielectric loss (tan δ), were investigated in a wide frequency range (1 Hz-1 MHz), and in a temperature range (303-373 K). The complex impedance spectroscopy (CIS) technique, showed that these properties are strongly dependent on frequency, temperature and on the added level of impurity. The temperature coefficient of capacitance (TCC) of the samples was also evaluated. The study of the imaginary impedance (−Z″) and imaginary electric modulus (M″) as functions of frequency and temperature leads to the measurement of the activation energy (E_ac), which is directly linked to the relaxation process associated with the interfacial polarization effect in these samples.     

Dielectric properties of Cu0.5Tl0.5Ba2Ca3Cu4O12−δ bulk superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12−δ superconductor samples were studied at 79 and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 kHz to 10 MHz. A negative capacitance (NC) phenomenon has been observed, which is most likely arising due to higher Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge located at the multiple insulator–superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 79 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

AC conductivity and dielectric properties of bulk tungsten trioxide (WO3)

AC conductivity and dielectric properties of tungsten trioxide (WO₃) in a pellet form were studied in the frequency range from 42 Hz to 5 MHz with a variation of temperature in the range from 303 K to 463 K. AC conductivity, σ_ac(ω) was found to be a function of ω^s where ω is the angular frequency and s is the frequency exponent. The values of s were found to be less than unity and decrease with increasing temperature, which supports the correlated barrier hopping mechanism (CBH) as the dominant mechanism for the conduction in WO₃. The dielectric constant (ε′) and dielectric loss (ε″) were measured. The Cole–Cole diagram determined complex impedance for different temperatures.     

Dipole relaxation and proton transport in polycrystalline γ-cyclodextrin hydrate: A dielectric spectroscopy study

The polycrystalline γ-cyclodextrin hydrate (γ-CD·12.2H₂O) has been investigated via dielectric spectroscopy over a frequency range of 0-100 kHz and the temperature ranges of 108.0-298.5 K (cooling) and 109.0-433.0 K (heating). At T < 250.0 K, the electrical properties of the sample accept a great contribution from the flip-flop proton orientational disorder and a much lesser one from the positional fluctuations of the water molecules. Moreover, a strong synergy is observed between the stability of the γ-CD molecules and the dynamic disorder of the infinite flip-flop chains. This type of disorder disappears upon cooling (T_trans = 186.7 K) and reappears upon heating (T_trans = 194.5 K). At T > 250.0 K, the dielectric permittivity ε′ and loss ε″ increase abruptly due to the proton dc-conductance of γ-CD·12.2H₂O which has been interpreted in terms of a theoretical model (Pnevmatikos, 1988) being consistent with the generation of ionic defects and their combination with the dipole reorientations in a collective motion of soliton-type. The influence of the simultaneous dehydration process on this charge transport mechanism relies on the very sensitive balance between the diffusive motion of water molecules (exchange between symmetry related positions) and their removal from the crystal lattice. The Arrhenius semiconductive behavior of the ac-conductivity in the ranges of 257.1-313.2 K (E_a = 0.42 eV) and 331.2-385.1 K (E_a = 0.39 eV) implies the dominance of water diffusion which conserves the structural integrity of the endless hydrogen-bonded chains and the proton transfer along them. The limited decrease of the ac-conductivity from 313.2 to 331.2 K along with its rapid decrease above 385.1 K, indicates that the removal of the water molecules rules out their diffusive motion. The Cole-Cole diagrams (ε″ vs. ε′) make clear that during the heating process the grain boundary polarization gradually becomes more significant than the grain interior one. In the range of 348.0-385.1 K, the constrictive grain boundary resistances are totally eliminated allowing the extensive proton transport through the grains of the polycrystalline specimen.     

Role of Cu concentration on the structure and transport properties of Cr-Zn ferrites

The influence of Cu concentration on the transport and microstructure characteristics of Cu_yZn_1−yCr_0.8Fe_1.2O₄ with 0.2≤y≤1 ferrite was studied. X-ray, energy dispersive X- ray (EDAX) and infrared spectra (IR) were carried out to assure the formation of the sample in the proper form. The dielectric constant (ε′) and ac conductivity were measured at different frequencies ranging from 600 kHz to 5 MHz from room temperature up to 800 K. The obtained data reveals that, a single phase cubic spinel structure for all the concentrations. From the results of IR spectra, mainly two bands were observed. The dielectric constant and the dielectric loss tangent decrease with increasing frequency and Cu concentration. The dielectric constant shows a dispersion peak (ε′_max) which shifts to higher frequency with increasing the temperature. The results are explained as due to the fact that the dielectric polarization process is similar to that of conduction. The appearance of the dispersion peak is related to the contribution of two types of charge carriers.     

AC conductivity and dielectric relaxation in Ba(Sm1/2Nb1/2)O3 ceramic

The complex perovskite oxide a barium samarium niobate (BSN) synthesized by solid-state reaction technique has single phase with cubic structure. The scanning electron micrograph of the sample shows the average grain size of BSN∼1.22 μm. The field dependence of dielectric response and loss tangent were measured in the temperature range from 323 to 463 K and in the frequency range from 50 Hz to 1 MHz. The complex plane impedance plots show the grain boundary contribution for higher value of dielectric constant in the low frequency region. An analysis of the dielectric constant (ε′) and loss tangent (tan δ) with frequency was performed assuming a distribution of relaxation times as confirmed by the scaling behaviour of electric modulus spectra. The low frequency dielectric dispersion corresponds to DC conductivity. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with an activation energy of 0.71 eV. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary part of electric modulus M″ and dielectric loss spectra suggest that the relaxation describes the same mechanism at various temperatures in BSN. All the observations indicate the polydispersive relaxation in BSN.     

Dielectric relaxation study of poly(ethylene glycols) using TDR technique

Temperature dependent dielectric relaxation and thermodynamic properties of polyethylene glycols HO[CH₂CH₂O)_nH with number average molecular weight 200 (n = 4), 300 (n = 7), 400 (n = 9) and 600 (n = 14) g mol^− 1 have been studied using Time Domain Reflectometry (TDR) in the frequency range 10 MHz to 20 GHz. The frequency dependence of the complex dielectric permittivity is analyzed by the Havriliak-Negami expression. The static permittivity ε₀, high frequency limiting static permittivity ε_∞, average relaxation time τ₀ and thermodynamic energy parameters such as free energy, enthalpy of activation and entropy of activation have been determined. The average free energy of activation ΔF_τ for PEG molecules was found to be in the range 4-5 kcal mol^− 1. The values of entropy ΔS_τ for PEG-200, PEG-400 and PEG-600 molecules were found to be positive while entropy ΔS_τ for PEG-300 molecules was found negative, which confirms that the configuration of PEG-300 involved in the dipolar orientation has an activated state, which is more ordered than the normal state compared to PEG-200, PEG-400 and PEG-600 molecules.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.