Ac Conductivity Studies Research Articles

Electric properties of carbon nano-onion/polyaniline composites: a combined electric modulus and ac conductivity study

The complex electric modulus and the ac conductivity of carbon nano-onion/polyaniline composites were studied from 1 mHz to 1 MHz at isothermal conditions ranging from 15 K to room temperature. The temperature dependence of the electric modulus and the dc conductivity analyses indicate a couple of hopping mechanisms. The distinction between thermally activated processes and the determination of cross-over temperature were achieved by exploring the temperature dependence of the fractional exponent of the dispersive ac conductivity and the bifurcation of the scaled ac conductivity isotherms. The results are analyzed by combining the granular metal model (inter-grain charge tunneling of extended electron states located within mesoscopic highly conducting polyaniline grains) and a 3D Mott variable range hopping model (phonon assisted tunneling within the carbon nano-onions and clusters).

Improved electrical characteristics of Pr-doped BiFeO3 ceramics prepared by sol–gel route

Ceramics of Bi1−xPrxFeO3 (x = 0–0.1) were fabricated using the nanocrystalline powders obtained via sol–gel route. X-ray powder diffraction studies confirmed that these belonged to rhombohedral perovskite structure associated with R3c space group. The dielectric properties of the ceramic samples as a function of frequency (100 Hz–10 MHz) and temperature (30 °C–250 °C) were studied. The dielectric constant increased while the loss decreased with the increase of Pr content. Dielectric dispersion in these samples was found to be poly dispersive Debye type relaxation as confirmed by invoking Cole–Cole relation. Impedance spectroscopy was employed to determine the electrical parameters associated with the grain and grain boundaries. Grain and grain boundary resistances were found to decrease with the increase of temperature for all the samples under study. The activation energies for the dielectric relaxation were evaluated by electric modulus spectra and these increase with the increase of Pr dopant level. The frequency dependent conductivity at various temperatures demonstrated the involvement of correlated barrier hopping conduction mechanism. The electrical conduction in these ceramics was ascribed to long and short range migration of oxygen ion vacancies as demonstrated by temperature dependent ac conductivity studies.

Dielectric relaxation and AC conductivity studies of Se90Cd10−xInx glassy alloys

Chalcogenide glassy alloys of Se90Cd10−xInx (x = 2, 4, 6, 8) are synthesized by melt quench technique. The prepared glassy alloys have been characterized by techniques such as differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX). Dielectric properties of Se90Cd10−xInx (x = 2, 4, 6, 8) chalcogenide glassy system have been studied using impedance spectroscopic technique in the frequency range 42 Hz to 5 MHz at room temperature. It is found that the dielectric constants ɛ′, dielectric loss factor ɛ″ and loss angle Tan δ depend on frequency. ɛ′, ɛ″ and loss angle Tan δ are found to be decreasing with the In content in Se90Cd10−xInx glassy system. AC conductivity of the prepared sample has also been studied. It is found that AC conductivity increases with frequency where as it has decreasing trend with increasing In content in Se–Cd matrix. The semicircles observed in the Cole–Cole plots indicate a single relaxation process.

Multiferroic and conduction characteristics of (Bi0.5Ba0.5) (Fe0.5Ti0.5) O3 solid solution

A solid solution of multiferroic BiFeO3 and ferroelectric BaTiO3 in equal molar ratio (i.e., (Bi0.5Ba0.5) (Fe0.5Ti0.5) O3 (BF–BT)) was synthesized by using solid state reaction route. By adding BaTiO3 in BiFeO3, the crystalline structure and ferroelectric properties of the system was improved. The significantly reduced leakage current results to get regular ferroelectric hysteresis loop of the parent compound. The dielectric relaxation observed at 365 °C may be related to the anti-ferromagnetism transition suggesting the coupling between the ferroelectric and magnetic orders essential for the multiferroic materials. Moreover, the remnant magnetization (2Mr) of 0.00014 emu/g and coercive field (2Hc) of 6.94 kOe were observed in the system. The ac conductivity study confirmed transport phenomena in the material are due to hopping of polarons.

Structural, dielectric, ac conductivity and electromagnetic shielding properties of polyaniline/Ni0.5Zn0.5Fe2O4 composites

A conducting polymer, polyaniline (PANI)/Ni0.5Zn0.5Fe2O4 composites with high dielectric absorbing properties and electromagnetic shielding effectiveness at low frequencies were successfully synthesized through a simple in situ emulsion polymerization. PANI was doped with hydrochloric acid to improve its electrical properties and interactions with ferrite particles. PANI/Ni0.5Zn0.5Fe2O4 composites were characterized by X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analysis. Frequency dependence of dielectric and ac conductivity (σac) studies have been undertaken on the PANI/Ni0.5Zn0.5Fe2O4 composites in the frequency range 50 Hz–5 MHz. The electrical conduction mechanism in the PANI/Ni0.5Zn0.5Fe2O4 is found to be in accordance with the electron hopping model. Further, frequency dependence of electromagnetic interference (EMI) shielding effectiveness (SE) is studied. The EMI shielding effectiveness is found to decrease with an increase in the frequency. The maximum value 55.14 dB of SE at 50 Hz was obtained at room temperature for PANI/Ni0.5Zn0.5Fe2O4 composites in the 50 Hz–5 MHz frequency range. PANI/Ni0.5Zn0.5Fe2O4 composites were demonstrated as a promising functional material for the absorbing of electromagnetic waves at low frequencies because of a large amount of dipole polarizations in the polymer backbone and at the interfaces of the Ni–Zn ferrite particles and PANI matrix.

Structural, dielectric and AC conductivity studies of Zn substituted nickel ferrites prepared by combustion technique

A series of zinc doped nickel ferrites having the chemical formula Ni1−xZnxFe2O4 [x = 0.0, 0.25, 0.50, 0.75, 1.0] were synthesized by combustion method. The powder X-ray diffraction results confirm the formation of single phase cubic spinel structure. The lattice parameter was found to increase from 8.348 to 8.443 A with increase in the Zn content. Average crystallite size calculated using Debye–Scherrer equation and Williamson–Hall plots confirms the nano crystalline nature of the samples. The EDAX analysis indicates the correct elemental composition of the synthesized powders. Fourier transform infrared spectra show the characteristic peaks of Ni–Zn spinel ferrites. The morphological features of the powder were examined by SEM and reveal the fine crystalline nature of the powder. A systematic study on the dielectric properties such as dielectric constant (ɛ′), dielectric loss tangent (tan δ) and AC conductivity (σac) were investigated as a function of Zn content and frequency in the range from 100 Hz to 1 MHz at room temperature. Complex impedance behavior was explained by means of the cole–cole plot.

Superlinear frequency dependence of AC conductivity and its scaling behavior in xAgI-(1 − x) AgPO3 glass superionic conductors

Low temperature AC conductivity studies on melt quenched xAgI-(1−x) AgPO3 glass electrolytes are presented. It is observed that doped glasses show Jonscher power law behavior at all temperatures, however, undoped AgPO3 glass also shows superlinear frequency-dependent conductivity at very low temperatures apart from the usual Jonscher power law behavior. This behavior in undoped AgPO3 glass is not yet reported in literature. The Summerfield scaling law is used to demonstrate that Jonscher power law behavior and superlinear power law behavior have different physical origins as scaling law is not followed in the superlinear regime. Moreover, scaling behavior of AC conductivity also indicates that conduction mechanisms are slightly different in doped and undoped glasses. However, scaling behavior provides only qualitative information about the conduction mechanisms. The conductivity spectra of doped and undoped glasses can be merged into a super-master conductivity curve except in the transition zone (where AC conductivity starts dominating over DC conductivity) where the conductivity master plots of doped and undoped glasses do not merge.

Structural, optical and conductivity properties of BaTi1−xNixO3

In this work, we studied the effects of Ni doping on the structural, optical, ac conductivity and dielectric loss properties of BaTi1−xNixO3 (0≤x≤0.1, in a step of 0.02) prepared through conventional solid-state reaction. Detailed analyses of XRD patterns and Raman spectra indicate that Ni dopants incorporate into the Ti sites of BaTiO3 host lattices, and the tetragonal-to-hexagonal transformation takes place at a threshold concentration of xc=0.06. Optical band gap value deduced from UV–Vis spectra decreases with the increase in Ni concentration, which confirms that Ni-doping into BaTiO3 causes the change in their electronic levels associated with lattice defects, creates new energy level to the forbidden gap, and, therefore, results in the reduction in the band gap. Study of ac conductivity indicates that conduction mechanism can be explained by the correlated barrier hopping (CBH) model.

Microstructure characterization and electrical transport of nanocrystalline Zn0.90Mn0.10O semiconductors synthesized by mechanical alloying

We report a comprehensive study of dc and ac conductivity, dielectric relaxation and capacitance–voltage characteristics of Mn doped ZnO nanocrystalline semiconductors prepared by mechanical alloying. Samples are characterized by HRTEM and XRD. Direct current conductivity increases with the increase in temperature and the thermal behavior of electrical dc conductivity for the investigated samples can be described by adiabatic polaronic hopping model. The frequency dependent conductivity obeys a power law σ′(f)∝fsTn. The temperature exponent n strongly depends on frequency. Analysis of the temperature dependence of frequency exponent s suggests a transformation from large polaron to small polaron conduction model with increase in temperature. Considering electric modulus the dielectric properties of the samples have been explained. Capacitance–voltage characteristic suggests the formation of Schottky diode between metallic electrode and semiconductor junction and diode parameter shows anomalous behavior with increasing milling time.

Defect chemistry and relaxation processes: effect of an amphoteric substituent in lead-free BCZT ceramics.

The effect of praseodymium (Pr), an amphoteric substituent, on phase transition, dielectric relaxation and electrical conductivity has been studied and analysed in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) ceramics synthesized by a solid state reaction method. Structural investigations showed co-existence of two phases - tetragonal (P4mm) and rhombohedral (R3m) - for compositions with x ≤ 0.05 wt% Pr. Temperature dependent dielectric studies revealed two phase transitions - rhombohedral (R) → tetragonal (T) and T → cubic (C) - that gradually evolved into one T → C transition for x > 0.05 wt% Pr in BCZT. A dielectric relaxation behaviour was observed in the temperature range of 275-500 °C that was attributed to the localized relaxation process (short-range hopping motion of oxygen vacancies) in the bulk of the material. Grain and grain boundary conductivity evaluated from the impedance data revealed that Pr acts as a donor dopant for x ≤ 0.05 wt% while it is an acceptor for higher concentration, in accordance with XRD observations. Defect chemistry analysis for better interpretation of the acquired data is presented. Frequency and temperature dependent ac conductivity studies were also performed and the obtained activation energy values were associated with possible conduction mechanisms.

Structural and AC conductivity study of CdTe nanomaterials

Cadmium telluride (CdTe) nanomaterials have been synthesized by soft chemical route using mercapto ethanol as a capping agent. Crystallization temperature of the sample is investigated using differential scanning calorimeter. X-ray diffraction and transmission electron microscope measurements show that the prepared sample belongs to cubic structure with the average particle size of 20nm. Impedance spectroscopy is applied to investigate the dielectric relaxation of the sample in a temperature range from 313 to 593K and in a frequency range from 42Hz to 1.1MHz. The complex impedance plane plot has been analyzed by an equivalent circuit consisting of two serially connected R-CPE units, each containing a resistance (R) and a constant phase element (CPE). Dielectric relaxation peaks are observed in the imaginary parts of the spectra. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole–Cole equation. The temperature dependence relaxation time is found to obey the Arrhenius law having activation energy ~0.704eV. The frequency dependent conductivity spectra are found to follow the power law. The frequency dependence ac conductivity is analyzed by power law.

Studies of structural, optical, dielectric relaxation and ac conductivity of different alkylbenzenesulfonic acids doped polypyrrole nanofibers

Polypyrrole (PPy) nanofibers doped with alkylbenzenesulfonic acids (ABSA) have been synthesized using interfacial polymerization method. HRTEM studies confirm the formation of PPy nanofibers with average diameter ranging from 13nm to 25nm. Broad X-ray diffraction peak in 2θ range 20–23.46° reveals amorphous structure of PPy nanofibers. The ordering or crystallinity of polymer chains increases, while their interplanar spacing (d) and interchain separation (R) decreases for short alkyl chain ABSA doped PPy nanofibers. FTIR studies reveal that short alkyl chain ABSA doped PPy nanofibers show higher value of “effective conjugation length”. PPy nanofibers doped with short alkyl chain ABSA dopant exhibit smaller optical band gap. TGA studies show enhanced thermal stability of short alkyl chain ABSA doped PPy nanofibers. Decrease in dielectric permittivity ε′(ω) with increasing frequency suggests presence of electrode polarization effects. Linear decrease in dielectric loss ε″(ω) with increasing frequency suggests dominant effect of dc conductivity process. Low value of non-exponential exponent β (<1) reveals non-Debye relaxation of charge carriers. Scaling of imaginary modulus (M″) reveals that the charge carriers follow the same relaxation mechanism. Moreover, the charge carriers in PPy nanofibers follow the correlated barrier hopping (CBH) transport mechanism.

Investigations on Microwave Assisted Synthesis and Characterization of Bi&lt;sub&gt;2&lt;/sub&gt;WO&lt;sub&gt;6&lt;/sub&gt; Nanoparticles

Bismuth tungstate (Bi2WO6) nanoparticles were synthesized by microwave assisted method. The prepared nanoparticles were investigated by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), UV-visible spectroscopy and dielectric studies. The formation of Bi2WO6nanoparticles was confirmed by X-ray diffraction (XRD). The morphology of Bi2WO6nanoparticle was characterized using scanning electron microscopy SEM. The optical properties were studied by the UV-Visible absorption spectrum. The dielectric properties of Bi2WO6nanoparticles were studied. The activation energy was calculated from AC conductivity studies.Key words: Bi2WO6nanoparticles, XRD, SEM, TEM, UV analysis, Dielectric studies and AC conductivity studies

INVESTIGATIONS ON STRUCTURAL AND ELECTRICAL PROPERTIES OF CADMIUM ZINC SULFIDE THIN FILMS

Nowadays, II – IV group semiconductor thin films have attracted considerable attention from the research community because of their wide range of application in the fabrication of solar cells and other opto-electronic devices. Cadmium zinc sulfide (Zn-CdS) thin films were grown by chemical bath deposition (CBD) technique. X-ray diffraction (XRD) is used to analyze the structure and crystallite size and scanning electron microscopy is used to study the morphology of Zn-CdS thin film. Optical studies have been carried out using UV-Visible-NIR transmission spectrum. The dielectric properties of Zn-CdS thin films have been studied in the different frequency at different temperatures. The AC conductivity study shows a normal dielectric behavior with frequency which reveals that the dispersion is due to the interfacial polarization.

Influence of polarization and iron content on the transport properties of praseodymium–barium manganite

Polarization and iron effects on the electrical properties of Pr0.67Ba0.33Mn1−xFexO3 have been studied using impedance measurements. When iron is introduced, the insulator–metal transition (MI), observed in free compound, disappears and destroying such transition needs an iron concentration less than 5%. We also found that electrical conductance decreases when increasing Fe content. Such results are attributed to the decrease of Mn3+/Mn4+ ratio. Also, they are ascribed to the high probability of encountering Fe3+–O–Fe3+ and Mn3+–O–Fe3+ interactions, which greatly weakens the influence of Mn3+–O–Mn4+ interactions. The AC conductivity studies indicate that different types of hopping are involved. The contribution of hopping mechanism is confirmed by the temperature dependence of the frequency exponent ‘s’. Conductivity analysis shows that small polaron hopping (SPH) and variable range hopping (VRH) models are present in the conduction process. For small iron concentrations (x<0.1), we found that activation energy (Ea) does not changes significantly. Such result is in good agreement with the literature. But, for high iron concentrations (x>0.1), we found that Ea depend strongly in Fe content. We also found in this work that DC-bias does not affect the conduction process but proves its thermal activation. The variation of the conductance with polarization is a proof of an electro-resistance effect.

Flux growth and grey colouration characteristics in KTiOPO4:Ln (Ln= Yb, Nd, Ho, Er, La)

Single crystals of KTiOPO4:Ln (Ln=Yb, Nd, Ho, Er and La) were successfully grown by the top seeded solution growth (TSSG) method using potassium polyphosphate as a flux. A series of rare earth metal (Yb, Nd, Ho, Er and La) doped KTP crystals was subjected to high electric field and hydrogen annealing treatment at 973K temperature. The vital role of alkali ionic conduction and Ti3+ in colouration of KTP has been studied by permittivity and ac conductivity studies. The hydrogen anneal- and UV bleach-induced grey colouration was studied by absorption and transmittance characteristics. The structural changes induced by grey tracking on KTP:Ln were studied by Raman shift analysis. The oxygen vacancies or Ti3+ centres were created by various treatments and the degree of colouration in the crystal was evaluated. The mechanisms of colouration in the crystal and resistance behaviour of grey colouration by lanthanide doping were studied.

Investigation of dielectric relaxation in BaTiO3 ceramics modified with BiYO3 by impedance spectroscopy

The polycrystalline (1 − x)BaTiO3 − xBiYO3 ((1 − x)BT − xBY) (x = 0.2, 0.3) ceramics were prepared by a solid state reaction technique. X-ray diffraction (XRD) analysis shows the information of single phase compound with a perovskite structure. Well grain growth is found in SEM range. The dielectric studies shows that the diffusive factor (γ) is close to 1.8, which confirms that (1 − x)BT − xBY exhibits a diffuse phase transition. Moreover, impedance and electric modulus spectroscopy analysis in the frequency of (20 Hz–2 MHz) within temperature domain (250–400 °C) show that two relaxation processes are involved which are contributed to bulk and grain boundary effects. The plots of Z″ and M″ are dispersed with increase in frequency at different temperatures, which proves the dielectric relaxation. The plots of Z′ and the ac conductivity studies show the NTCR character of the compounds. The experimental value of conduction activation is 0.98 eV, which is near to the conduction activated energy of the electron from second ionization of oxygen vacancies.

Impedance and AC conductivity study of nano crystalline, fine grained multiferroic bismuth ferrite (BiFeO3), synthesized by microwave sintering

In this paper, major reduction in sintering time,temperautre and significant improvement over final density of sitnered sample is reported for the microwave sintered nanocrystalline BiFeO3 (BFO) ceramic. Also, different sintering time and temperatures have been used to tailor the grain size and the final density of the resulting BFO ceramics synthesized from phase pure BFO nanoparticles (d̄ ≈10 nm). Microwave sintering resulted in reducing the sintering time substantially (by 1h), and has resulted in submicron sized grains and high resistivity ∼1.8 GΩ-cm. The AC conductivity is seen to follow the Jonscher’s power law behavior, suggesting correlated barrier hopping (CBH) mechanism in the sample. The role of oxygen vacancies at high temperature, due to volatility of bismuth, in dielectric and conductivity behavior is also discussed. Further, the sample displayed dielectric anomaly near magnetic transition temperature (∼180 °C) indicating bearing of magnetic moments on the dielectric properties. Using Impedance Spectroscopy (IS) we have established, the electrical heterogeneity of the ceramic BFO reavealing semiconducting nature of grains and insulating nature of grain boundary. This, formation of network of insulating grain boundaries and semiconducting grains could lead to formation of internal barrier layer capacitance (IBLC) leading to high dielectric constant in microwave sintered BFO.

Studies on role of grain-size on transport properties of Sm0.5Sr0.5CoO3−δ cathodic films

The transport properties of mixed conducting Sm0.5Sr0.5CoO3−δ (SSC) films featuring mean grain sizes from 44 nm to 4.42 μm are studied with regard to grain-size effects. A remarkable enhancement in total conductivity of about one order is found in nano-scaled films than that of micro-crystalline films. In micro-crystalline films, a change in the transport mechanism as a function of temperature is observed, which is attributed to a transition from bulk controlled (at high temperatures) to grain-boundary controlled (at low temperatures) charge transportation. The films having nano-grains facilitates the charge transportation via parallel grain-boundaries and attribute to enhance the total conductivity through grain-boundaries. Alongside, the AC conductivity study showed that the charge carriers in nano-scaled films follow a long-range hopping path in the entire temperature region. Hence, the nano-scaled Sm0.5Sr0.5CoO3−δ cathodic films will give higher and steady state performance in solid oxide fuel cell (SOFC) in the low temperature region.

Charge-carrier relaxation dynamics and ac conductivity scaling of poly(3,4-ethylenedioxythiophene) nanoparticles

An investigation of the dielectric characteristics of poly(3,4-ethylenedioxythiophene) nanoparticles has been conducted in the frequency range 20 Hz–2 MHz and the temperature range of 80–300 K through the studies of dielectric permittivity, modulus formalism and ac conductivity. High-resolution transmission electron micrographs confirm the formation of well-ordered spherical nanoparticles. Micro-Raman spectra confirm the conformational transition from benzenoid to quinoid structure with increasing dopant concentration. Linear variation of imaginary permittivity with frequency suggests that the dc conduction process is more significant than interfacial polarisation. The presence of single relaxation peak in the imaginary modulus formalism indicates the Debye-type relaxation. The decrease of frequency exponent with temperature confirms that the charge transport takes place through correlated barrier hopping mechanism. The scaling of ac conductivity spectra confirms that the relaxation dynamics of charge carriers in the poly(3,4-ethylenedioxythiophene) nanoparticles system is temperature independent.