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.     

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.     

Dielectric characterization of semiconducting ZnPc films sandwiched between Gold or Aluminum electrodes

The dependencies of complex dielectric functions (the dielectric constant, ε ₁, and the dielectric loss, ε ₂), on frequency and temperature of zinc phthalocyanine (ZnPc) thin films sandwiched between either gold or aluminum Ohmic-electrode contacts have been investigated in the temperature range of 93–470 K and frequency range 0.1–20 kHz. It is found that both values of ε ₁ and ε ₂ decrease with increasing frequency and increase with decreasing temperature. The rate of change depends greatly on the temperature and frequency ranges under consideration. Around room temperature, neither ε ₁ nor ε ₂ show any appreciable change through the whole range of frequencies. Thus, the dielectric dispersion is found to include of both dipolar and interfacial polarizations. The dependencies of both dielectric functions on frequency at different temperatures were found to follow a universal power law of the form ω ⁿ , where the index 0<n≤?1. This indicates that the correlated barrier hopping (CBH) model is a suitable mechanism to describe the dielectric behavior in ZnPc films. Furthermore, the results of the dielectric response indicate that polarization in these films could be in the form of non-Debye polarization. However, the Debye polarization can be traced below room temperature. The obtained results of the relaxation-time, τ, dependency on temperature have shown that a thermally-activated process may be dominated in ZnPc thin films conduction at high temperatures. Partial phase transition (from α- to β-phase) has been observed around 400 K in molecular relaxation-time, τ, and optical dielectric constant, ε _∞. Arrhenius behavior has been observed for all the dielectric loss and conductivity relaxation-times above room temperature and their activation energies are explained and reported. The optical dielectric constant ε _∞ was found to increase with temperature.     

Conduction mechanism and the dielectric relaxation process of a-Se75Te25−xGax (x=0, 5, 10 and 15 at wt%) chalcogenide glasses

Se₇₅Te_25−xGa_x (x=0, 5, 10 and 15 at wt%) chalcogenide compositions were prepared by the well known melt quenching technique. Thin films with different thicknesses in the range (185–630 nm) of the obtained compositions were deposited by thermal evaporation technique. X-ray diffraction patterns indicate that the amorphous nature of the obtained films. The ac conductivity and the dielectric properties of the studied films have been investigated in the frequency range (10²–10⁵ Hz) and in the temperature range (293–333 K). The ac conductivity was found to obey the power low ω^s where s≤1 independent of film thickness. The temperature dependence of both ac conductivity and the exponent s can be well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and dielectric loss ε₂ are frequency and temperature dependent. The maximum barrier height W_m calculated from the results of the dielectric loss according to the Guintini equation, and agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The density of localized state was estimated for the studied film compositions. The variation of the studied properties with Ga content was also investigated. The correlation between the ac conduction and the dielectric properties were verified.     

Investigation of dielectric and electrical behavior of Mn doped YCrO3 nanoparticles synthesized by the sol gel method

In the present work Mn doped YCrO₃ nanoparticles are synthesized by the sol–gel method. Samples have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis absorption spectroscopy. The optical band gap of Mn doped YCrO₃ nanoparticles increases with increase of doping concentration. The dc resistivity of the prepared samples decreases with increasing temperature. The variation of ac conductivity with frequency has been explained by the Correlated Barrier Hoping (CBH) conduction mechanism. Dielectric permittivity of the samples was studied and it 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 discussed in terms of electric modulus vector. Both activation energies due to dc resistance and dielectric response have been measured for the different samples and it is observed that it increases with the Mn content.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Effect of impurity (Sb and Ag) incorporation on the a.c. conductivity and dielectric properties of a-Se70Te30 glassy alloy

The effect of additives (Sb and Ag) on a.c. conductivity and dielectric properties of Se₇₀Te₃₀ glassy alloy at temperature range 300-350 K and frequency range 1 kHz-5 MHz has been studied. Experimental results indicate that a.c. conductivity and dielectric parameters depend on temperature, frequency and the impurity incorporated in Se-Te glassy system. The a.c. conductivity in the aforesaid frequency range is found to obey the ω^s law. A strong dependence of a.c. conductivity and exponent s in the entire temperature and frequency range contradicts quantum-mechanical tunneling (QMT) model and can be interpreted in terms of the correlated barrier hopping (CBH) model. The temperature and frequency dependence of the dielectric parameters are also studied and it is found that the results agrees by the theory of hopping of charge carriers over potential barrier as suggested by Elliott in chalcogenide glasses. The change in the dielectric parameters with the opposite influence of the replacement of Te by Sb on the one hand, and by Ag, on the other hand is being correlated by the nature of covalent character of the studied composition and with the change in density of defect states.     

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.     

Influence of frequency, temperature and composition on electrical properties of polycrystalline Co0.5CdxFe2.5−xO4 ferrites

Polycrystalline ferrites with general formula Co_0.5Cd_xFe_2.5−xO₄ (0.0?x?0.5) were prepared by sol-gel method. The dielectric properties ε′, ε″, loss tangent tan δ and ac conductivity σ_ac have been studied as a function of frequency, temperature and composition. The experimental results indicate that ε′, ε″, tan δ and σ_ac decrease as the frequency increases; whereas they increase as the temperature increases. These parameters are found to increase by increasing the concentration of Cd content up to x=0.2, after which they start to decrease with further increase in concentration of Cd ion. The dielectric properties and ac conductivity in studied samples have been explained on the basis of space charge polarization according to Maxwell and Wagner's two-layer model and the hoping between adjacent Fe²⁺ and Fe³⁺ as well as the hole hopping between Co³⁺and Co²⁺ ions at B-sites. The values of activation energies E_f for conduction process are determined from Arrhenius plots, and the variations in these activation energies as a function of Cd content are discussed. The complex impedance analysis is used to separate the grain and grain boundary of the system Co_0.5Cd_xFe_2.5−xO₄. The variations of both grain boundary and grain resistances with temperature and composition are evaluated in the frequency range 42 Hz-5 MHz.     

Electrical conduction and dielectric properties of vanadium phosphate glasses doped with lithium

AC conductivity and dielectric studies on vanadium phosphate glasses doped with lithium have been carried out in the frequency range 0.2-100 kHz and temperature range 290-493 K. The frequency dependence of the conductivity at higher frequencies in glasses obeys a power relationship, σ_ac=Aω^s. The obtained values of the power s lie in the range 0.5≤s≤1 for both undoped and doped with low lithium content which confirms the electron hopping between V⁴⁺ and V⁵⁺ ions. For doped glasses with high lithium content, the values of s≤0.5 which confirm the domination of ionic conductivity. The study of frequency dependence of both dielectric constant and dielectric loss showed a decrease with increasing frequency while they increase with increasing temperature. The results have been explained on the basis of frequency assistance of electron hopping besides the ionic polarization of the glasses. The bulk conductivity increases with increasing temperature whereas decreases with increasing lithium content which means a reduction of the V⁵⁺.     

Electron beam induced modifications in conductivity and dielectric property of polymer electrolyte film

This paper describes the effect of 8 MeV of electron beam (EB) energy irradiation on the electrical conductivity and dielectric properties of sodium fluoride NaF-doped polyethylene oxide (PEO) film. The structural and chemical characterizations were employed using X-ray diffractometry (XRD) and Fourier Transform Infrared (FTIR) techniques respectively before and after irradiation. The morphology study carried out using Scanning Electronic Microscopy (SEM) analysis. The DC electrical conductivity showed increases with dose and temperature and was consistent with Arrhenius behavior. The maximum conductivity of 1.1 × 10⁻⁵ S/cm and minimum activation energy of 0.25 eV were obtained at 25 kGy, 338 K; further increases in the dose resulted in a reduction in conductivity. The real (ε′) and imaginary (ε″) part of the dielectric constant suddenly decreased in a low frequency region (40–640 Hz), subsequently independent at higher frequency. The AC conductivity showed increases with frequency and temperature for all films. The dielectric constant and AC conductivity increased at the 25 kGy dose due to chain scission. Further increases in dose such as 50 and 75 kGy, resulted in a decrease in dielectric constant and AC conductivity due to cross-linking. The electric modulus approach was used to calculate the dielectric relaxation time (τ), which decreased at 25 kGy and then increased at 50 and 75 kGy doses. The modulus data were fitted using a non-exponential Kohlrausch–Williams–Watts (KWW) function ϕ (t), and the results indicate the existence of a non-Debye relaxation.     

High frequency study of electropolymerised conducting polypyrrole thin film

The polypyrrole (PPY) thin films were synthesized by electropolymerisation in potassium nitrate solution. The substrate used was stainless steel. DC conductivity, microwave reflection, microwave conductivity and microwave dielectric constant of the conducting PPY thin films are reported. DC conductivity was between 1.6 × 10⁻² S/cm and 42.3 × 10⁻² S/cm. Microwave conductivity was between 10 S/cm and 160 S/cm. The ?′ generally decreases as frequency increases similarly ?″ also decreases with increases in frequency. The measurements have been carried over the frequency range 8.2-12 GHz. These polypyrrole thin films were characterized by FTIR. The polypyrrole thin film increases the reflectivity of the stainless steel.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

A.c. conductivity and relaxation in LiCoVO4 ceramics

The LiCoVO₄ compound is synthesized by solution-based chemical method. X-ray diffraction analysis exhibits a single phase nature of the compound with cubic structure. The dielectric constant (ε_r), tangent loss (tanδ) and a.c. conductivity (σ_ac) have been studied as a function of frequency and temperature using complex impedance spectroscopy (CIS) technique. The variation of (ε_r and tanδ) with frequency at studied temperatures shows a dispersive behavior at low frequencies. Frequency dependence of σ_ac at different temperatures obeys Jonscher’s universal power law governed by the relation: σ_ac = σ_dc + Aωⁿ, where n is the frequency exponent in the range of 0 ⩽ n ⩽ 1 and A is a constant that depends upon temperature.     

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.     

Optical constants and dielectric function of DNA's nucleotides in UV range

We calculated frequency-dependent real n(ω) and imaginary k(ω) parts of the complex refractive index for adenine A, guanine G, cytosine C and thymine T in ultraviolet range using Kramers-Kroning analysis of their absorption spectra. The frequency-dependent real ε′(ω) and imaginary ε″(ω) parts of the dielectric function for the corresponding nucleotides were restored. We calculated also the dependence of the dielectric function on the imaginary frequency ε(iω) for the corresponding nucleotides.     

Rietveld refinement and impedance spectroscopy of calcium titanate

Single phase perovskite CaTiO₃ has been synthesized by conventional solid state reaction technique. The ceramic was characterized by XRD at room temperature and its Rietveld refinement inferred orthorhombic crystal structure with the space group Pbnm. The field dependence of dielectric relaxation and conductivity was measured over a wide frequency range from room temperature to 673 K. Analysis of Nyquist plots of CaTiO₃ revealed the contribution of many electrically active regions corresponding to bulk mechanism, distribution of grain boundaries and electrode processes. The dc conductivity depicted a semiconductor to metal type transition. Frequency dependence of dielectric constant (ε′) and tangent loss (tan δ) show a dispersive behavior at low frequencies and is explained on basis of Maxwell-Wagner model and Koop's theory. Both conductivity and electric modulus formalisms have been employed to study the relaxation dynamics of charge carriers. The variation of ac conductivity with frequency at different temperatures obeys the universal Jonscher's power law (σ_ac α ω^s). The values of exponent ‘s’ lie in the range 0.13 ≤ s ≤ 0.33, which in light of CBH model suggest a large polaron hopping type of conduction mechanism.     

Development of nanocrystalline Mn-Zn ferrites for high frequency transformer applications

Microwave-Hydrothermal (M-H) method has been successfully used for the synthesis of nanocrystalline Mn-Zn ferrites which are used for high-frequency applications. As synthesized powders were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The nanopowders were annealed at 600 °C/20 min using the microwave sintering method. The frequency dependence of dielectric constant (ε′) was measured in the range of 10 Hz-1.3 GHz and initial permeability (μ_i) was measured in the range of 10 Hz-1 MHz. The total power loss (P_t) was measured on the annealed samples at 100 kHz and 200 mT condition. Conductor-embedded-ferrite transformers were fabricated and output power (P_o), efficiency (η) and temperature rise (ΔT) were measured at sinusoidal voltage of 25 V at 1 MHz. The transformer efficiency (η) was found to be high and surface rise of temperature (ΔT) is very low.     

Dielectric dispersion in PbO-Sb2O3-As2O3:V2O5 glasses

Dielectric properties, viz. dielectric constant ε′, loss tan δ and a.c conductivity σ_ac (over a wide range of frequency and temperature) and dielectric breakdown strength of PbO-Sb₂O₃-As₂O₃ glasses doped with V₂O₅ (ranging from 0 to 0.5 mol%) are studied. Analysis of these results, based on optical absorption and ESR spectra, indicates that the insulating strength of the glasses is comparatively high when the concentration of V₂O₅ is about 0.3 mol% in the glass matrix.     

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.