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This work is devoted to study the comparison of the conduction mechanism in bare barium hexaferrite (BaFe12O19) nanoparticles and tetraethyl orthosilicate-coated (SiO2)x/BaFe12O19 (x = 5, 10, 15 and 20 wt. %) nanoparticles. The modified sol–gel method was adopted for the synthesis of these nanoparticles. The structures, morphologies, elemental composition, vibrational modes, and dielectric properties of these nanoparticles were studied by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy, and dielectric measurements, respectively. The crystallite size and particle size of BaFe12O19 nanoparticles decreased with SiO2 coatings, which can be attributed to the confinement of crystal growth of these nanoparticles with the coating. It was found that these (SiO2)x/BaFe12O19 nanoparticles have a uniform spherical shape, are non-agglomerated, and have a narrow particle size distribution. At room temperature, these (SiO2)x/BaFe12O19 nanoparticles showed distinctive behavior with varying frequency and SiO2 coatings. The experimental results in this work may provide fundamental support to the research and development of materials for energy storage devices. [ABSTRACT FROM AUTHOR]

In order to tailor the multiferroic as well as dielectric properties, we have synthesized BiFeO3 (BFO) and Ni-doped BFO nanoparticles of composition BiFe1-xNixO3 (0 ≤ x ≤ 0.07) via the sol-gel method. The influence of Ni-doping at the Fe site of BiFeO3 on the structural, dielectric, and multiferroic properties was investigated via various characterization techniques such as Raman spectroscopy, vibrating sample magnetometer, polarization-electric field (P-E) loop tracer, and LCR meter. The Raman spectra reveal the rhombohedral distorted perovskite structure of the samples in the R3c space group. The frequency dependent dielectric measurements at room temperature show that the dielectric constant for the pristine sample is an order of magnitude lower than the 1% Ni-doped BFO sample. It is also evident that the ac conductivity increases with the increase in frequency as well as Ni content. Further, the universal dielectric response (UDR) phenomenon is responsible for the dielectric behavior of all the samples in the whole frequency range except for the BiFeO3 and BiFe0.97Ni0.03O3 samples. These samples exhibit deviation from UDR phenomenon at higher frequencies. The room temperature hysteresis (M-H) loops exhibit the saturation in magnetization at higher magnetic fields. The result demonstrates a significant influence on the saturation magnetization with the Ni doping in BFO, prompting the ferromagnetic nature of the samples. The saturation magnetization of the 7% Ni-doped sample is approximately six times higher than the pristine one. The saturated P-E hysteresis loops establish the ferroelectric nature of the synthesized samples that are enhanced significantly on Ni doping. Thus, the doping of Ni in BiFeO3 improves the multiferroic properties of the matrix. [ABSTRACT FROM AUTHOR]

Polycrystalline Pr0.8Ca0.2MnO3 has been synthesized through solid state reaction route and phase purity is analyzed using synchrotron XRD. Magnetization M(T) showed Mn spin alignment due to ferromagnetic ordering around 125 K (TC), whereas M(H) showed an onset of non-linear behavior from 200 K and with temperature re-orientation of magnetic moment is discussed. The modulation of relaxation processes revealed different formations of impedance plane plots with temperatures. A change in equivalent circuit models from (R1C1)(R2Q2) to (R1C1)(R2Q2)(R3Q3) at 100 K is being reported. The analysis of the impedance data is carried out by calculating impedance of grains and grain boundaries. Fitted parameters derived from these equivalent circuit parameters showed a change in conduction mechanism from small polaronic hopping model (SPH) to Mott's variable range hopping (MVRH) model, around 125 K (TC). Carriers hop to larger distance with multiple activation energies and are described by MVRH below TC. Above this temperature, different trap centers start facilitating these carriers through SPH. Dielectric relaxation shows dispersion around 125 K and the origin of this polarization lies close to the relaxation derived from the grains and their interfaces. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic and electrical response of the sol-gel synthesized NiFe2O4 nanoparticles. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of the material. Thermally activated hopping carriers between Fe3+-Fe2+ and Ni2+-Ni3+ ions have been determined for a decrease in the resistance of the sample and a change in the conduction mechanism around 318 K. The mixed spinel structure and broken exchange bonds due to small size effects are due to the canted spin structure at the surface of the nanoparticles. The magnetization is found to be influenced by the surface spin canting and anisotropy. We have established the semiconducting to metallic transition (SMT) temperature to be around 358 K in terms of localized and delocalized eg electrons along with a transition from less conductive [Fe3+-O2--Fe3+] and [Ni2+-O2--Ni2+] linkage to more conductive [Fe3+-Fe2+] and [Ni2+-Ni3+] linkage at the octahedral B site. A decrease in the dielectric constant with temperature has been discussed in terms of the depletion of space charge layers due to the repulsion of delocalized eg electrons from the grain boundary planes. The anomalies in tangent loss and conductivity data around 358 K are discussed in the context of the SMT. [ABSTRACT FROM AUTHOR]

In this work, we address the issue of microstructural changes in the electrical properties of charge ordered polycrystalline La0.50Ca0.50MnO3+δ sample during thermal cycling through impedance spectroscopy. The modulations of relaxation time with temperatures and different thermal cycling shows different magnitudes of impedance plane plots. Thermal cycling induces varying mechanical and electrical stresses in the grains and in the grain boundaries, changing the resistance of the grains (R1) and the grain boundaries (R2) by changing the boundary potentials and trap state scattering cross sections. At metal-insulator transition temperature (MIT), R1 increases with polynomial first order and R2 increases with polynomial second order. The values of R1 and R2 show different orders of variations along with reversible and irreversible behaviors around and below MIT. We report a method by virtue of which charge ordering temperature TCO has been determined with the change in the magnitude of grain boundary resistance by thermal cycling. Localization of charge carriers in the traps with thermal cycling around MIT and delocalization of charge carriers with different frequencies have been discussed. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy has been employed to investigate the microscopic nature of the charge/orbital ordering (COO) in polycrystalline La0.50Ca0.50MnO3+δ sample at different temperatures. The modulations of relaxation time with temperatures show different behaviors of Z′ and Z″. An equivalent circuit model (R1C)(R2Q) has been employed to investigate the electrical parameters of La0.50Ca0.50MnO3+δ at different temperatures. The resistance of grain and grain boundaries shows maxima at metal-insulator (MI) transition temperature (≈153 K). Three temperature zones have been identified. In the first temperature zone which has been from 85 to 140 K, below MI transition temperature, melting of COO with strong Jahn–Teller (JT) distortions has been reported. In the second zone, around MI transition temperature, melting and collapse of COO with weak JT distortion result in colossal drop in Z′. In the third temperature zone, above MI transition temperature, charge ordering (CO) around 203 K and onset of short range CO at 253 K have been determined. [ABSTRACT FROM AUTHOR]

The linear stability of the ion-temperature-gradient modes (ITG) in the electrostatic limit is examined in the short wavelength region by using a two fluid reactive model in fully three-dimensional Wendelstein 7-X (W7-X) stellarator [G. Grieger et al., Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 525] geometry. The spectrum of stable and unstable modes and their real frequencies and eigenfunctions are calculated. The effects of density gradients, temperature gradients, temperature ratios, wavevector, ballooning angle, curvature and local magnetic shear on the ITG mode are also investigated. The frequency and growth rate of the most unstable ITG mode is calculated and visualized for a specific magnetic flux surface. For the equilibrium under investigation both localized and extended eigenmodes are found. The effect of small and large temperature ratios, small and large density gradients as well as large local magnetic shear are all found to be stabilizing. The highest growth rates are found at the outer part of the surface where the local magnetic shear is small and normal curvature is unfavorable. [ABSTRACT FROM AUTHOR]

The influence of plasma geometry on the linear stability of electrostatic ion-temperature-gradient driven drift modes (ITG or η[sub i]=L[sub n]/L[sub Ti] modes) is investigated. An advanced fluid model is used for the ions together with Boltzmann distributed electrons. The derived eigenvalue equation is solved numerically. A comparison is made between an H–1NF [Fusion Technol. 17, 123 (1990)] like stellarator equilibrium, a numerical tokamak equilibrium and the analytical s⁁-α equilibrium. The numerical and the analytical tokamak are found to be in good agreement in the low inverse aspect ratio limit. The growth rates of the tokamak and stellarator are comparable whereas the modulus of the real frequency is substantially larger in the stellarator. The threshold in η[sub i] for the stellarator is found to be somewhat larger. In addition, a stronger stabilization of the ITG mode growth is found for large ε[sub n](=L[sub n]/R) in the stellarator case. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polycrystalline Pr0.5-xLaxCa0.5MnO3 (x = 0.0-0.40) ceramics are synthesized by conventional solid state reaction method, and phase purity is confirmed by employing X-ray diffraction. Temperature dependent ac impedance spectroscopic measurements enable us to determine an increasing trend in resistance values of these samples with the decrease in temperatures. However, a decreasing trend in resistance values with increase in the La-doping at Pr-site is observed. A metal to insulator transition (MIT) is reported for x ≥ 0.2, which is shifted to higher temperature values with further increase of x doping. Two equivalent circuit models, i.e., (ReQe)(RgbQgb) and (ReQe)(RgbQgb)(RgQg) are employed to explain the impedance data with and without MIT, respectively. Mott variable range hopping model is found to be an appropriate model for defining the conduction mechanism of charge carriers in the semiconducting region. The decrease in the impedance with x doping is explained in terms of increase in the localization length obtained from the fitting of Rgb. Using tanδ results, thermally activated relaxation behavior is discussed for x = 0.0 and 0.1; whereas for x = 0.2, a temperature independent relaxation behavior is conferred due to the change in the hopping process of charge carriers. [ABSTRACT FROM AUTHOR]

We have investigated the electrical response on the pellet of ZnO nanopowder by impedance spectroscopy. Different trends of impedance plane plots have been discussed due to the modulations of relaxation time with temperatures. Thermal cycling induces irreversible changes in the resistance of the material which has been explained by incorporating the role of extended defects like oxidation of Zn interstitials at surfaces and intrinsic/extrinsic grain boundaries. Metal-insulator transition has been reported and discussed in term of combined effects of Zn interstitials and oxygen vacancies. Irreversible increase in the values of dielectric constant have been conferred with thermal cycling’s. [ABSTRACT FROM AUTHOR]

A general local dispersion relation is derived for the circulating electron-temperature-gradient-driven drift waves in an inhomogeneous magnetized electron–ion plasma. By taking specific limits, two new instabilities are shown to exist. The resulting quasilinear electron energy and particle fluxes are also obtained. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy has been employed to investigate the dielectric and electric transport phenomena in sol-gel synthesized CuO nanograins. Semiconducting features of the grains and grain boundaries have been endorsed to the thermal activation of the localized charge carriers. On cooling below 303 K, a transition from Jahn-Teller polaron hopping mechanism to the Mott's variable range hopping mechanism has been observed owing to random potential fluctuations among localized sites. Activation energies for conduction and relaxation processes at grain boundaries provide strong signatures for the involvement of Jahn-Teller adiabatic small polarons as a charge transport mechanism in CuO nanograins. [ABSTRACT FROM AUTHOR]

A study of the effect of local magnetic shear on the drift wave stability is presented. The eigenvalue problem for the drift wave equation is solved numerically in fully three-dimensional stellarator plasma using the ballooning mode formalism. It is found that negative local magnetic shear has a stabilizing effect on the drift wave instability and positive local shear is destabilizing. This is in agreement with the effect of negative global magnetic shear in tokamaks and also agrees with the simple estimates. As a consequence the highly unstable modes found on a specific magnetic surface are localized in the region of positive local magnetic shear. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The semiconductor/insulator blends for organic field-effect transistors are a potential solution to improve the charge transport in the active layer by inducing phase separation in the blends. However, the technique is less investigated for long-chain conducting polymers such as Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)] (DPPDTT), and lateral phase separation is generally reported due to the instability during solvent evaporation, which results in degraded device performance. Herein, we report how to tailor the dominant mechanism of phase separation in such blends and the molecular assembly of the polymer. For DPPDTT/PMMA blends, we found that for higher DPPDTT concentrations (more than 75%) where the vertical phase separation mechanism is dominant, PMMA assisted in the self-assembly of DPPDTT to form nanowires and micro-transport channels on top of PMMA. The formation of nanowires yielded 13 times higher mobility as compared to pristine devices. For blend ratios with DPPDTT ≤ 50%, both the competing mechanisms, vertical and lateral phase separation, are taking place. It resulted in somewhat lower charge carrier mobilities. Hence, our results show that by systematic tuning of the blend ratio, PMMA can act as an excellent binding material in long-chain polymers such as DPPDTT and produce vertically stratified and aligned structures to ensure high mobility devices., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)