Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Phase transition in Pb4.9A0.1Ge2.7Si0.3O11 (A=Ca,Sr,Ba) ferroelectrics

Polycrystalline samples of modified lead germanate Pb_4.9A_0.1Ge_2.7Si_0.3O₁₁ (A=Ca,Sr,Ba) have been prepared by solid-state reaction technique. X-ray studies have been carried out to check the formation of single-phase composition and to obtain preliminary structural data. Measurements of dielectric permittivity () and loss (tan δ) have been studied both as a function of frequency (10²–10⁴ Hz) and temperature (room temperature to 180°C). Phase transition was observed in all three cases but with different T_c value. From the plots of conductivity vs. temperature using dielectric data, collected at 10 kHz activation energy was calculated to investigate the conduction mechanism in them.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

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.     

Relaxation and conductivity behaviour in the compounds: FeRGe2O7 (R=Pr,Tb)

AC measurements were performed on the thortveitite-like layered compounds, FeRGe₂O (R=Pr,Tb) in order to study their dielectric features, e.g. as a function of temperature. The main electrical response lies on impedance plots composed of two successive arcs with depressed centers. Bulk conductivity behavior is mostly Arrhenius for the measured temperatures. The associated bulk activation energies are close to 1 eV. Raw data were used to follow the temperature dependence of the dynamic parameters, ε′(ω) and σ′(ω). From logarithmic σ′(ω) vs. ω curves the dc component was obtained. ε′(ω) vs. log ω curves exhibit a dispersive behavior at low frequencies, reflecting blocking effects. E_dc and E_ac activation energies were also calculated, the last one obtained from σ′(ω) vs. 1000/T plots. Conductivity results suggest the occurrence of an extrinsic conducting mechanism. A structural instability was detected via the temperature dependence of permittivity, which has been ascribed to the presence of Ga-O-Ga bonds having associated angles different of 180°. Analyses of the results show that the interchange of Tb and Pr in the general formula FeRGe₂O₇ (R=Pr, Tb) involves only small differences in their global ac and dc behavior.     

Effect of Pb on the electrical properties of a-Ge20Se80 glasses

The present paper reports the effect of Pb impurity (low ∼2 at% and high ∼10 at%) on the ac conductivity (σ_ac) of a-Ge₂₀Se₈₀ glass. Frequency-dependent ac conductance and capacitance of the samples over a frequency range ∼100 Hz to 50 kHz have been taken in the temperature range ∼268 to 358 K. At frequency 2 kHz and temperature 298 K, the value of σ_ac increases at low as well as at higher concentration of Pb. σ_ac is proportional to ω^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases. The static permittivity (ε_s) increases at both Pb concentrations. These results have been explained on the basis of some structural changes at low and higher concentration of Pb impurity.     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Study of the structural, dielectric and magnetic properties of Bi2O3 and PbO addition on BiFeO3 ceramic matrix

In this paper we studied the effects of Bi₂O₃ and PbO addition on BiFeO₃ (BFO) ceramic matrix. The structural, dielectric and magnetic properties of fifteen BFO samples were discussed in view of possible applications in RF and microwave devices. The present work also reports the preparation of the samples. Polyvinyl alcohol (PVA) and tetraethyl orthosilicate (TEOS) were also added as a binder in the fabrication procedure. The samples have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and magnetic hysteresis measurements. Further, a study based on impedance spectroscopy also has been done. Dielectric permittivity (ε′) and dielectric loss (tan δ) were measured at room temperature in the frequency range 100 Hz-10 MHz, as well as a.c. conductivity. The -Im[Z(f)] versus Re[Z(f)] plot has been obtained. The samples were investigated in view of possible applications like miniaturized filters, diplexers and dielectric resonator antennas (DRA). In the RF and MW frequency region, the application of magneto-dielectric and multiferroic perovskite composite materials is desirable for the miniaturization of components.     

Manifestation of MnCl2 fillers incorporated into the polymer matrices in their dielectric properties

Polyethyl methacrylate (PEMA) films filled with different mass fractions of MnCl₂ were prepared using a casting method. The structural and electrical properties were studied. The filling content dependence of certain IR absorption bands was correlated with the obtained physical parameter characterizing the other properties. DC electrical resistivity (ρ) was measured in the temperature range 340-420 K for PEMA films filled with MnCl₂ fillers. An intrachain one-dimensional interpolaron hopping mechanism was assumed to interpret the electrical conduction. AC conductivity behavior of all the prepared samples was investigated over the frequency range (42-5M) Hz and under different isothermal stablilization in the temperature range 300-423 K. It suggested that the hopping mechanism might be playing an important role in the conduction process, in low temperature regime. The values of σ₀, A, and S satisfying the suitable fit of the conductivity data, as well as the corresponding (σ_DC).     

Thermodynamic and kinetic properties of Ho1−xTixCo2-hydrogen system

The hydrogen absorption behavior of Laves phase Ho_1−xTi_xCo₂ (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho_1−xTi_xCo₂-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed.     

Studies of dielectric and impedance properties of KCa2V5O15 ceramics

A polycrystalline sample, KCa₂V₅O₁₅, with tungsten bronze structure was prepared by a mixed-oxide method at low temperature (i.e., at 630 °C). A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Two dielectric anomalies at 131 and 275 °C were observed in the temperature dependency of dielectric response at various frequencies, which may be attributed to the ferroelastic-ferroelectric and ferroelectric-paraelectric transitions, respectively. The nature of variation of the electrical conductivity, and value of activation energy of different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies). The impedance plots showed only bulk contributions, and non-Debye type of relaxation process occurs in the material. A hopping mechanism of electrical transport processes in the system is evident from the modulus analysis. The activation energy of the compound (calculated both from loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers.     

Electrical behaviour of Li2O-ZnO-SiO2 glass and glass-ceramics system

Sealing quality lithium zinc silicate (LZS) glasses of compositions (wt.%) (a) LZSL- Li₂O: 12.65, ZnO: 1.85, SiO₂: 74.4, Al₂O₃: 3.8, K₂O: 2.95, P₂O₅: 3.15, B₂O₃: 1.2 (low ZnO), and (b) LZSH- Li₂O: 8.9, ZnO: 24.03, SiO₂: 53.7, Na₂O: 5.42, P₂O₅: 2.95, B₂O₃: 5 (high ZnO) were prepared by conventional melt-quench technique and converted to glass-ceramics by controlled crystallization process. The electrical properties of these samples were measured using ac impedance spectroscopy technique over a frequency range of 10 Hz-15 MHz at several temperatures in the range of 323-673 K. The ac conductivity, dc conductivity, dielectric constant and loss factor were obtained from these measurements. The dc conductivity (σ_dc) follows the Arrhenius behaviour with temperature. It is observed that σ_dc in LZSL glass is significantly higher than in the LSZH glass and the activation energies for σ_dc for LZSL and LZSH glasses are 0.59 and 1.08 eV, respectively. It further observed that the conductivity value decreases nearly one order of magnitude on conversion to glass-ceramics. The behaviour is explained on the basis of distributions and nature of alkali ions and network structures in these samples.     

Dielectric-spectroscopic and a.c. conductivity investigations on copper doped layered Na1.8K0.2Ti3O7 ceramics

Dielectric studies on copper doped derivatives of polycrystalline layered mixed alkali trititanate Na_1.8K_0.2Ti₃O₇ ceramics indicate that the losses are of mixed type and decrease on copper doping. However, the temperature dependent permittivity plots are characteristic of the diffuse nature of a possible ferroelectric phase transition and hence give indication of relaxor ferroelectric behaviour. From the EPR spectra, recorded at room temperature, it can be seen that the substitution of copper occurs at Ti⁴⁺ as preferred site with a divalent oxidation state (Cu²⁺) for all compositions. Also, copper doping enhances the transition temperature, which is indicative of the stabilization of the existing ferroelectric phase up higher temperatures. Besides bolstering electron hopping conduction, acceptor doping restrains the interlayer ionic conduction. Moreover, electron hopping (polaron) conduction is dominant over the lower temperature region, while interlayer ionic conduction prevails in the higher temperature region.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Dielectric properties of EuF2

Dielectric constant (?) and loss (tan δ) of EuF₂ have been measured for the first time as a function of frequency in the temperature range from liquid nitrogen to 300°C. The data has been used to obtain the values of conductivity (σ). The conductivity values are frequency independent in the high temperature region and yield 1.05 eV for the activation energy for conduction.     

Electrical conduction and magnetoelectric effect of (x) BaTiO3 + (1−x) Ni0.92Co0.03Cu0.05Fe2O4 composites in ferroelectric rich region

Particulate composites with composition (x)BaTiO₃+(1−x)Ni_0.92Co_0.03Cu_0.05Fe₂O₄ in which x varies as 1, 0.85, 0.70, 0.55 and 0 (in mol%) were prepared by the conventional double sintering ceramic technique. The presence of two phases viz. ferromagnetic (Ni_0.92Co_0.03Cu_0.05Fe₂O₄) and ferroelectric (BaTiO₃) was confirmed by X-ray diffraction analysis. The dc resistivity and thermo-emf measurements were carried out with variation of temperature. The ac conductivity (σ_ac) measurements investigated in the frequency range 100 Hz to 1 MHz conclude that the conduction in these composites is due to small polarons. The variation of dielectric constant and loss tangent with frequency (20 Hz to 1 MHz) was studied. The static magnetoelectric conversion factor, i.e. dc (dE/dH)_H was measured as a function of intensity of applied magnetic field. The changes were observed in electrical properties as well as in magnetoelectric voltage coefficient as the molar ratio of the constituent phases was varied. A maximum value of magnetoelectric conversion factor of 536.06 μV/cm Oe was observed for the composite with 70% BaTiO₃+30% Ni_0.92Co_0.03Cu_0.05Fe₂O₄ at a dc magnetic field of 2.3 K Oe. The maximum magnetoelectric conversion output has been explained in terms of ferrite-ferroelectric content, applied static magnetic field and resistivity.     

Crystal structure, electronic and transport properties of AgSbSe2 and AgSbTe2

Undoped and p- and n-doped AgSbX₂ (X=Se and Te) materials were synthesized by direct fusion technique. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured as a function of temperature in the range from 300 to 600 K.To enlighten electron transport behaviours observed in AgSbSe₂ and AgSbTe₂ compounds, electronic structure calculations have been performed by the Korringa-Kohn-Rostoker method as well as KKR with coherent potential approximation (KKR-CPA) for ordered (hypothetical AgX and SbX as well as AgSbX₂ approximates) and disordered systems (Ag_1−xSb_xX), respectively. The calculated density of states in the considered structural cases shows apparent tendencies to opening the energy gap near the Fermi level for the stoichiometric AgSbX₂ compositions, but a small overlap between valence and conduction bands is still present. Such electronic structure behaviour well agrees with the semimetallic properties of the analyzed samples.     

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.     

Study of (Bi2O3)(BaxMo1−xO3) polycrystalline ceramic as relaxor ferroelectric

Solid solutions of bismuth layered (Bi₂O₃)(Ba_xMo_1−xO₃) (0.2≤x≤0.8, x is in step of 0.2) ceramics were prepared by conventional solid-state reaction of the constitutive oxides at optimized temperatures with a view to study its electrical properties. Powder X-ray diffraction has been employed for physical characterization and an average grain size of ∼16 to 22 nm was obtained. XRD study reveals the single phase structure of the samples. Dielectric properties such as dielectric constant (ε′), dielectric loss (tanδ) and ac electrical conductivity (σ_ac) of the prepared ceramics sintered at various temperatures in the frequency range 10¹–10⁷ Hz have been studied. A strong dispersion observed in the dielectric properties shows the relaxor type behavior of the ceramic. The presence of maxima in the dielectric permittivity spectra indicates the ferroelectric behavior of the samples. Impedance plots (Cole–Cole plots) at different frequencies and temperatures were used to analyze the electric behavior. The value of grain resistance increases with the increase in Ba ion concentration. The conductivity mechanism shows a frequency dependence, which can be ascribed to the space charge mainly due to the oxygen vacancies. The relaxation observed for the M″ (ω) or Z″ (ω) curves is correlated to both localized and long range conduction. A single ‘master curve’ for the normalized plots of all the modulus isotherms observed for a given composition indicates that the conductivity relaxation is temperature independent.     

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.