Boundary Contributions Research Articles

Electrical studies of Pb0.6Bi0.4Fe0.7Nb0.3O3 multiferroic

The multiferroic Pb0.6Bi0.4Fe0.7Nb0.3O3 was prepared using a single step solid state reaction technique. A comprehensive impedance spectroscopy technique was used to examine the dielectric, impedance, modulus and electrical conductivity of the sample. The dielectric constant of the sample was strongly affected by frequency and temperature. Temperature-dependent relaxation events in dielectric loss were seen in the material. With a non-Debye kind of relaxation process, the Nyquist plot shows the grains and grain boundary contribution to the conductivity. The bulk resistance reduces as the temperature rises and the temperature coefficient of resistance was found to be negative. The temperature-dependent relaxation processes with non-Debye type were shown in an electrical modulus study with increasing trend of AC conductivity with rise in temperature. Pb0.6Bi0.4Fe0.7Nb0.3O3 could be useful in multiferroic applications.

Probing the dielectric behavior of lead-free BCST ceramic composite: An impedance spectroscopic approach

Recently developedlead-free candidate, (Ba1-xCax) (Ti1-ySny)O3 (BCST) ceramics were synthesized using conventional sintering route. Extraordinarily frequency independent high dielectric propertiesexhibited by BCST composition with large BST proportion. Complex Impedance Spectroscopy technique establishes the relationship between microstructure of the system and the frequency dependence of various relaxation processes involved. Non-Debye type relaxation exists in all BCST samples except BCST3 (x = 0.12, y = 0.06), supported by Cole-Cole plots. Equivalent circuits were modeled containing minimum RC elements representing grain bulk and boundaries contribution to electrical response. Frequency-independent high dielectric constantwith low loss of BCST3 makes it suitable as RF dielectric substrate and dielectric filters.

Dielectric and electrical properties of hexagonal BaTi0.5Co0.5O3 ceramic with NTC effect

Barium Titanate doped with Cobalt is known for both its electric and magnetic properties. The synthesis and characterization of Cobalt doped barium titanate; BaTi1-xCoxO3(BTCO) x = 0.5was investigated with a view to understand its structural, magnetic and electrical properties. A finest possible sample of Co doped micro particles of BaTiO3 (BTO) with possible tetragonal structure via a solid-state route was prepared. Prepared samples of BaTi1-xCoxO3 (BTCO) were structural characterized by X-ray diffraction (XRD). The dielectric constant measurements of the samples were carried out at 1Hz to1 MHz Vibrating Sample Magnetometer (VSM) measurements revealed the magnetic nature of Cobalt doped BaTiO3. Ferroelectric hysteresis loop traced at the electric field in-between -15 to +15 (KV/cm). The relaxation phenomena that take place cane be attributable to the damping of dipole oscillator due to the application of external field. The impedance measurements were done up to 473 K in order to separate grain (bulk) and grain boundary contributions. The FESEM micrographs show proper grain growth and EDAX confirmed the presence of all the elements in samples. In the present study, various electrical properties of barium titanate based ceramics were explained and examples of the relevant applications were given. Copyright © 2017 VBRI Press.

Unveiling the effect of Gd–Co co-substitution in the enrichment of structural and dielectric properties of SrBaMn based magnetic oxide

Nano-crystalline Gd–Co substituted X-type hexagonal ferrites having formula SrBaMn2-xCoxGdyFe28-yO46 (where x = 0–1, y = −0.1) were synthesized employing sol-gel route. X-ray diffraction technique verified the formation of pure phase X-type ferrites for all samples. Crystallite size was reduced from 27.49 to 24.52 nm with Gd–Co concentration, which is favorable for attaining an appropriate signal-to-noise ratio in electronic devices. Lattice constants (a, c), bulk, and X-ray density slightly increased owing to the large ionic radius and density of Gd3+ ions than Fe3+ ions. Fourier transform infra-red spectroscopy displayed absorption bands in the lower wavenumber range (482-584 cm−1). The higher absorption bands (482-584 cm−1) are due to metal-oxygen vibration octahedral and tetrahedral sites. The influence of Gd–Co on dielectric properties such as dielectric constant, tangent loss, ac conductivity, and Q-value on frequency and composition were investigated in the frequency region of 1 MHz–3 GHz. The incorporation of Gd–Co resulted in the enhancement of dielectric properties of as-prepared X-type materials desired in electromagnetic devices operating at microwave frequencies. Electric modulus analyses indicated the contribution of grain boundary towards resistance.

Microstructural evolution, mechanical behaviors and strengthening mechanism of 300 M steel subjected to multi-pass laser shock peening

300 M steel was treated by multi-pass laser shock peening (LSP). The microstructures were characterized by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). In addition, the surface roughness, microhardness, residual stress, impact toughness and mechanical properties after LSP were measured. The results showed that high-density dislocations were generated after multi-pass LSP and the dislocation motion dominated the deformation process. The intensity of texture decreased while low-angle grain boundaries increased, and the grain refinement was not achieved after LSP. The residual stress transformed from tensile stress (+20 MPa) to compressive stress (−556 MPa) and the microhardness was 24.8% higher than the unpeened sample after LSP. Based on the linear superposition rule, a strengthening model was established to predict the contribution of dislocation and grain boundary strengthening on microhardness. Additionally, the fatigue life of peened samples increased by 165% compared with the unpeened ones. The improvement of high-cycle fatigue (HCF) properties was attributed to the rewarding compressive residual stress and microstructural evolution.

Comments on the Atiyah-Patodi-Singer index theorem, domain wall, and Berry phase

It is known that the Atiyah-Patodi-Singer index can be reformulated as the eta invariant of the Dirac operators with a domain wall mass which plays a key role in the anomaly inflow of the topological insulator with boundary. In this paper, we give a conjecture that the reformulated version of the Atiyah-Patodi-Singer index can be given simply from the Berry phase associated with domain wall Dirac operators when adiabatic approximation is valid. We explicitly confirm this conjecture for a special case in two dimensions where an analytic calculation is possible. The Berry phase is divided into the bulk and the boundary contributions, each of which gives the bulk integration of the Chern character and the eta-invariant.

Structure and properties of Ln6MoO12 (Ln = Nd, Sm, Gd, Dy) optoelectronic nanoceramics

Auto-igniting combustion technique is used for preparing Ln6MoO12 (Ln = Nd, Sm, Gd, Dy) nanoceramics. Crystallographic phase is ascertained by comparing the XRD patterns of prepared samples with ICDD data. X-ray diffraction line broadening is used to decide the average crystal size. Vibrational tools like Raman and FTIR spectroscopy are used to evaluate the metal oxide stretching frequencies. TEM analysis is used to investigate the particulate properties and SEM analysis is used to study the surface morphology of the sintered material. The ultraviolet–visible absorption spectra and photoluminescence spectra of the samples are noted and examined. The examination of ultraviolet absorption spectra discloses intense broad absorption in the UV-to-visible region with peaking at near UV region. Electrical properties of Nd6MoO12 are investigated using Impedance spectroscopy. The contributions of bulk and grain boundary to the overall electric response of the material are inferred by studying the electrical properties as a function of frequency (50 – 5 MHz) at various temperatures (400 – 850 °C).

On the Geometric Approach to the Boundary Problem in Supergravity

We review the geometric superspace approach to the boundary problem in supergravity, retracing the geometric construction of four-dimensional supergravity Lagrangians in the presence of a non-trivial boundary of spacetime. We first focus on pure N=1 and N=2 theories with negative cosmological constant. Here, the supersymmetry invariance of the action requires the addition of topological (boundary) contributions which generalize at the supersymmetric level the Euler-Gauss-Bonnet term. Moreover, one finds that the boundary values of the super field-strengths are dynamically fixed to constant values, corresponding to the vanishing of the OSp(N|4)-covariant supercurvatures at the boundary. We then consider the case of vanishing cosmological constant where, in the presence of a non-trivial boundary, the inclusion of boundary terms involving additional fields, which behave as auxiliary fields for the bulk theory, allows to restore supersymmetry. In all the cases listed above, the full, supersymmetric Lagrangian can be recast in a MacDowell-Mansouri(-like) form. We then report on the application of the results to specific problems regarding cases where the boundary is located asymptotically, relevant for a holographic analysis.

Studies of structural, dielectric and impedance spectroscopy of Sr/ Zr modified BiFeO3 ceramics

Bismuth ferrite (BiFeO3) is the most commonly used multiferroics for multifunctional applications. The polycrystalline samples of co-doped (Sr/Zr) bismuth ferrite (BiFeO3) of a general chemical composition, Bi1-xSrx)(Fe1-xZrx)O3, x = 0.05, 0.1, 0.15, 0.20, were prepared using a standard solid-state reaction method. The basic structural data of the above synthesized electro-ceramics were obtained by X-ray diffraction data and pattern. The surface morphology of the samples was recorded at room temperature by a scanning electron microscope. The scanning electron microscope images show the uniform distribution of grains throughout the surface of the samples. Studies of dielectric and impedance properties of the samples at varying frequency (1 kHz–1000 kHz) and temperature (250C–300 °C) have provided many important information about the materials. The existence of ferroelectricity in the materials was studied by the electric field dependent polarization. The contributions of both grains and grain boundaries towards the total resistance and capacitive behavior of the compounds have been studied by analyzing the frequency and temperature dependence of impedance data.. Study of frequency-temperature dependence of ac-conductivity has provided the information on the type of conduction mechanism. The J–E characteristics of the samples provide the Ohmic behavior with slope closer to 1.

Enhanced multiferroics properties of strontium substituted bismuth ferrite prepared by auto combustion method

A low cost and simple auto combustion method was used to prepare the Bi1-xSrxFeO3 (x=0.0, 0.04, 0.08, 0.12, 0.16) multiferroics samples. X-ray diffraction patterns confirms the integration of BFO pure phase nanoparticles with a perovskite structure of ABO3 type having space group R3c. Spherical structure with little agglomeration was observed during the morphological studies of the prepared samples. Sr2+ substituted at the A-site produces expansion in Raman modes and increased variations in cation site occupancy. Impedance analyzer was used to examine the frequency dependent dielectric parameters (ε´, tanδ), electric conductivity (σac), impedance (Z) and electric modulus (Mʹ and M″) of the samples in frequency between 20 Hz and 20 MHz). Maximum value of dielectric constant was observed at very low frequency for x=0.0 samples and it decreases by increasing concentration. Resonance peak was observed in tangent loss for high content of strontium. Maximum value of Impedance was observed at low frequency and it decreases by increasing frequency which is due to the grain and grain boundary contributions. It is observed that ac conductivity of the samples is found to be frequency dependent and it varies with doping of Sr in BFO. Improved polarization was examined in P-E loop of Sr doped BFO nanoparticles.

Quench-induced contributions of high angle grain boundary and low angle grain boundary to exfoliation corrosion propagation in an AlZnMgCu alloy

The exfoliation corrosion behavior of an AlZnMgCu alloy extruded plate subjected to different quenching rates has been investigated. With the decrease of quenching rate, the number of corroded low angle grain boundaries (LAGBs) gradually increases in addition to high angle grain boundaries (HAGBs), leading to rapid development of corrosion along the thickness direction. Consequently, the corrosion morphology changes from layered features to wavy features. The reason has been discussed based on the quench-induced change of precipitation features at HAGBs and LAGBs.

Study of the modified magnetic, dielectric, ferroelectric and optical properties in Ni substituted GdFe1−x Ni x O3 orthoferrites

Polycrystalline GdFe1−x Ni x O3 (x = 0.00, 0.02, 0.04) samples was synthesised using a glycine assisted sol–gel method to investigate the enhanced magnetic and electric properties of Ni substituted GdFeO3 systems. TG-DSC analysis of prepared samples confirms that GdFe1−x Ni x O3 have good thermal stability in high temperatures. The system has been stabilized in an orthorhombic structure with space group Pbnm. The elemental composition of GdFe1−x Ni x O3 has been estimated from EDAX spectrum. The results showed oxygen deficiency on increasing the Ni substitution and it has been supported by Rietveld refinement. FE-SEM images and Brunauer–Emmett–Teller analysis reveals that GdFe1−x Ni x O3 is a highly porous material and its porosity and specific area increases with Ni substitution. Magnetic measurements indicates that the system exhibited ferrimagnetic behaviour at low temperatures and canted antiferromagnetic behaviour at room temperature. For x = 0.04 Ni content, magnetization reversal for applied field of 25 Oe has been observed. Increased coercivity of GdFeO3 with Ni substitution has been attributed to the grain size effect. From electrical point of view, dielectric permittivity of GdFeO3 has been enhanced with Ni substitution. This enhancement has been attributed to the cumulative effects of hopping of Fe2+–Fe3+ ions, grain–grain boundary contribution, and space charge polarization. The role of grain–grain boundary contribution is evident from electric modulus spectrum. The space charge effect has been realized in both impedance spectrum and dielectric loss. Temperature-dependent dielectric studies were conducted to understand the mechanisms and various aspects that contribute to the dielectric enhancement. A highly lossy capacitive nature in the P–E loop also suggests space charge effects due to Ni substitution in Fe sites. Availability of free charge carrier concentration is correlated with the optical properties of GdFe1−x Ni x O3. The decrease of optical band gap (2.5–2.21 eV) on increasing Ni content suggests the increasing electronic contribution in the system.

Raman, impedance spectroscopy and ferroelectric studies of Sn4+ doped PbZr0.52Ti0.48O3 ceramics

Sn doped Pb(Zr0.52Ti0.48)1-xSnxO3 (x = 0. 0.05, 0.1) were prepared by conventional solid-state reaction method. Raman spectroscopic studies confirm the tetragonal structural of the material. Impedance spectroscopic studies were carried out to understand temperature and frequency dependent properties which shows the non-Debye kind of relaxation process. Cole-Cole plots reveal the contribution of grain and grain boundary to the conduction process. The values of activation energy suggests that conduction is ionic in nature. AC conductivity was found to obey the Johnscher’s power law. Ferroelectric studies were carried out and it was observed that the values of remnant polarization (Pr) increased with Sn substitution.

Superparamagnetic contributions, optical band gap tuning and dominant interfacial resistive mechanisms in ferrites nanostructures

Nanocrystalline Ni-Zn ferrite nanoparticles (NPs) NixZn1−xFe2O4 have been synthesized followed by detailed analysis of compositional effects on morphology, crystalline phase formation, lattice constant, crystallite size, magnetic, optical and dielectric properties describing the underlying physics as well. Cubic spinel structure with (311) preferred orientation and switching trend in crystallite size variation with gradual decrease in lattice parameter (from 8.377 to 8.323 Å) with the increase in nickel content has been observed through calculations. The magnetic behavior of NPs has been investigated at lower temperatures up to 5 K illustrating significantly improved magnetic parameters including Ms = 36.8 emu/g, Mr = 3.61 emu/g, SQ = Mr/Ms = 0.1028, Hc = 115.33 Oe, μB = 1.5664 emu, and K1 = 2122.18 erg/g. This enhancement has been found possibly due to less thermal fluctuations and significant superparamagnetic contributions from the very small fine nanoparticles being in blocking state owing to such a cool environment. Optical band gap for both direct (1.74–1.39 eV) and indirect (1.25–1.16 eV) transitions shows decreasing trend with the increase in Ni contents probably due to the redistribution of Ni or Zn contents from octahedral to tetrahedral site. Dielectric constant (real and imaginary) and tangent loss decreased with the increase of frequency, this refers to the Maxwell-Wagner type of polarization in the material in accordance with Koop’s theory, however, a switching is observed at the stoichiometric ratio of x = 0.25 and 0.75. The impedance (Z′) has maximum value 5539 kΩ at x = 0.75 and a minimum value 682 kΩ at x = 1.0. The Nyquist plot pointed out the contribution of grain boundaries, however the contribution varies with the stoichiometric ratio x. The Cole-Cole plot confirms the contribution of grains in the material for all value of x.

Grain-Scale Anisotropic Analysis of Steady-State Creep in Oligocrystalline SAC Solder Joints.

Heterogeneous integration is leading to unprecedented miniaturization of solder joints, often with thousands of joints within a single package. The thermomechanical behavior of such SAC solder joints is critically important to assembly performance and reliability, but can be difficult to predict due to the significant joint-to-joint variability caused by the stochastic variability of the arrangement of a few highly-anisotropic grains in each joint. This study relies on grain-scale testing to characterize the mechanical behavior of such oligocrystalline solder joints, while a grain-scale modeling approach has been developed to assess the effect of microstructure that lacks statistical homogeneity. The contribution of the grain boundaries is modeled with isotropic cohesive elements and identified by an inverse iterative method that extracts material properties by comparing simulation with experimental measurements. The properties are extracted from the results of one test and validated by verifying reasonable agreement with test results from a different specimen. Equivalent creep strain heterogeneity within the same specimen and between different specimens are compared to assess typical variability due to the variability of microstructure.

Studies of Structural, Electrical, and Magnetic Characteristics of Double Perovskite Ceramic: La2FeMnO6

Herein, detailed studies of structural, electrical, and magnetic characteristics of a double perovskite lanthanum iron manganate (La2FeMnO6) are reported. From the Rietveld analysis of X‐ray diffraction data and pattern, the orthorhombic structure of the compound is determined. The least‐squares refined lattice parameters of the compound are: a = 5.5370 Å, b = 7.7988 Å, and c = 5.5069 Å. The frequency–temperature dependence of the dielectric parameters is explained by the Maxwell–Wagner model to provide the polarization mechanism. Analysis of temperature dependence of impedance and modulus data of different frequency has shown the contributions of grains and grain boundaries in the electrical characteristics of the material. This analysis also shows the semiconductor characteristics, through a negative temperature coefficient of resistance nature. Based on both impedance and modulus spectroscopy, the dielectric relaxation process is found to be of a non‐Debye type. The study of scaling behavior of impedance and modulus data suggests that relaxation phenomena are nearly independent of temperature. Detailed analysis of the frequency dependence of conductivity data of different temperatures using the Jonscher's power law has shown the existence of different types of conduction mechanisms in the material. The nature of the current–voltage (I–V) curves suggests that the material behaves as a semiconductor diode type to be used for the rectifying purpose. The conduction mechanism follows the space charge‐limited conduction (SCLC) process which is obtained from leakage current analysis. The study of magnetic hysteresis of the material suggests that the material exhibits soft ferromagnetic ordering.

Influence of Isovalent ‘W’ Substitutions on the Structure and Electrical Properties of La2Mo2O9 Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

Lanthanum molybdenum oxide (La2Mo2O9, LAMOX)-based ion conductors have been used as potential electrolytes for solid oxide fuel cells. The parent compound La2Mo2O9 undergoes a structural phase transition from monoclinic (P21) to cubic (P213) at 580 °C, with an enhancement in oxide ion conductivity. The cubic phase is of interest because it is beneficial for oxide ion conduction. In search of alternative candidates with a similar structure that might have a stable cubic phase at lower temperatures, we have studied the variations of the crystal structure and ionic conductivity for 25, 50, 62.5 and 75 mol% W substitutions at the Mo site using high-temperature X-ray diffraction, dilatometry, and impedance spectroscopy. Highly dense ceramic samples have been synthesized by solid-state reaction in a two-step sintering process. Low-angle X-ray diffraction and Rietveld refinement confirm the stabilization of the cubic phase for all compounds in the entire temperature range considered. The substitutions of W at the Mo site produce a decrement in the lattice parameter. The thermal expansion coefficients in the high-temperature range of the W-substituted ceramics, as determined by dilatometry, are much higher than that of the unmodified sample. The impedance spectra have been modeled using a modified genetic algorithm within 300–600 °C. A distribution function of the relaxation times is obtained, and the contributions of ohmic drop, grains and grain boundaries to the conductivity have been identified. Overall, our investigation provides information about cationic substitution and insights into the understanding of oxide ion conductivity in LAMOX-based compounds for developing solid oxide fuel cells.

Synthesis, structural and impedance response of (Ba-Sr)-Co-O nanoparticles

Barium strontium cobaltite (BSC) (Ba0.5Sr0.5CoO3−δ) samples were successfully produced through modified chemical synthesis procedures. The methods used are hydrothermal, without water and surfactant (WOWS) sol-gel and co-precipitation technique. The effect of different synthesis procedures on structure, morphology and electrical conduction behaviour of BSC samples was studied. X-Ray diffraction showed the development of orthorhombic phase pure perovskite structure. FTIR spectroscopy confirmed the synthesis of perovskite structure by revealing its fingerprint peaks. Morphological studies by SEM displayed evenly dispersed, agglomerated and micro porous like morphology. Nyquist plots as a function of frequency displayed single semi-circular arcs. These arcs are an indication of grain boundary contribution in the electrical conduction mechanism. Modulus study displayed the Debye type conductivity relaxation. AC conductivity curves agreed well with Jonscher's power rule. Hydrothermally synthesized samples have the least impedances at higher frequencies. That form them possible candidate for cathode material in (IT-SOFCs) applications.

Effects of Sc and Zr Addition on Microstructure and Mechanical Properties of AA5182.

The effects of 0.1 wt.% Sc and 0.1 wt.% Zr addition in AA5182 on microstructure and mechanical properties were investigated. Results show that Al3(ScxZr1−x) dispersoids formed in AA5182. Observation of ingots microstructures showed that the grain size of 5182-Sc-Zr alloy was 56% lower than that of based AA5182. Isothermal annealing between 230 °C and 500 °C for 2 h was performed to study the recrystallization, tensile properties and dispersoid coarsening. The recrystallization was inhibited by the dispersoids, and the alloy microstructure remained deformed after annealing. Al3(ScxZr1−x) in AA5182 was stable when annealing below 400 °C, while parts of dispersoids coarsened significantly when heating at 500 °C. The addition of Sc and Zr allowed YS of 5182 alloy to achieve 247.8 MPa, which is 100 MPa higher than the corresponding AA5182. The contributions of Orowan strengthening and grain boundary strengthening were obtained by calculation.

Effects of frequency, temperature, and dc bias electric field on the dielectric properties of methylammonium lead iodide from the perspective of a relaxor-like ferroelectric

This work reports dielectric properties of methylammonium lead iodide as a function of frequency, temperature, and dc bias electric field studied in terms of grain and grain boundary contributions. These results were analyzed from the perspective of a relaxor-like ferroelectric nature. The temperature dependence of dielectric permittivity at different frequencies showed a wide dispersion in the vicinity of the tetragonal-cubic phase transition, suggesting a relaxor ferroelectric feature after excluding artifacts as Maxwell-Wagner effects and dc contributions. The results from plots of the Vogel-Fulcher and the modified Curie-Weiss laws indicate the existence of a freezing temperature of the dipoles responsible for the relaxor character of the material (Tf ∼ 270 K) and a diffuse phase transition at high temperatures (γ ∼ 1.52-1.74). Measurements on heating and cooling protocols under a dc bias electric field suggest field-induced polarized domains like relaxor ferroelectric. The observed signatures of transitions between non-ergodic and ergodic relaxors to ferroelectric states were attributed to polar nano regions dynamics, suggesting a critical electric field around 0.03 kV cm−1.