Grain Boundary Contributions Research Articles

Functional Properties of Donor (Al) and Acceptor (Cu) Codoped High Dielectric Constant ZnO Nanoparticles

In this work, we have synthesized donor‐acceptor (Al‐Cu) codoped ZnO nanoparticles with a doping concentration of 0%, 0.25%, 0.5%, and 0.75% by coprecipitation method. The synthesized samples were then annealed at 350°C and 600°C. All the samples showed wurtzite structure of ZnO with no secondary phase. The increase in doping concentration led to deterioration of crystalline quality, while improved crystallinity was observed at higher annealing temperature. The morphological study of these samples showed good grain‐to‐grain contact with less isolated pores. These samples were further characterized by impedance spectroscopy for analyzing dielectric properties. The values of the real part of dielectric constant and tangent loss showed decreasing trend with frequency. The appearance of semicircular arcs in the impedance complex plane plots indicates contribution of grains and grain boundaries and presence of different relaxation processes. 0.5% Al and Cu codoped ZnO showed the best dielectric response with a high value of dielectric constant and low tangent loss.

Dielectric, impedance spectroscopy and hydroelectric phenomena in lanthanum doped bismuth ferrite (Bi0.85La0.15FeO3)

Multiferroic materials are fascinating to discover due to presence of more than two ferroic orderings in the same phase. The lanthanum doped bismuth ferrite (Bi0.85La0.15FeO3) multiferroic nanoparticles are synthesized by sol–gel technique. This 15 % substitution of La in BiFeO3 is chosen due to its improved structural and electrical properties. The structural and impedance properties of Bi0.85La0.15FeO3 has been reported successfully. X-ray diffraction studies and EDX analysis are done to investigate the structural properties. X-ray powder diffraction patterns confirm the formation of single-phase perovskite structure of Bi0.85La0.15FeO3 (BLFO). SEM-EDAX analysis indicates the circular grains in Bi0.85La0.15FeO3 with extensive agglomeration. The dielectric studies display conventional Maxwell-Wagner Polarization in the nanoparticles. Impedance spectroscopy confirms high resistance of BLFO as well as contribution of grains and grain boundaries in BLFO. The hydroelectric cell fabricated using BLFO nanoparticles display improved potential. These properties are owing to improved structural stability and suggest their use in hydroelectric cell application.

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.

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).

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.

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.

Synthesis and study of the electrical conductive properties of Cs2Fe2TiO16 in various gas atmospheres

In this work, titanate Cs2Fe2Ti6O16 with a hollandite-type structure was synthesized by solid-phase synthesis. The electrical conductivity of the obtained ceramics was investigated in the temperature range from 25 to 800 °C in air and in the presence of an argon-hydrogen mixture (3% H2). It was found that the electrical conductivity of the compound under consideration depends on the composition of the atmosphere — the activation energy of conduction changes (0.57 eV in air and 0.91 eV in the presence of hydrogen). The volumetric and grain-boundary contributions to the total resistance of the sample in the gases under consideration are separated by impedance spectroscopy at a temperature of 800 °C.

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.

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.

Crystal-structure analysis, Raman spectroscopy, dielectric measurements, magnetic and optical properties of Cr3+–Ni2+-substituted Co2Y-type barium hexaferrites

The Co2Y-type barium hexaferrite Ba2−2xCrxNixCo2Fe12O22 (x = 0.0, 0.1, 0.15) was synthesized using the sol–gel auto-combustion technique. The formation phase of the Y-type hexaferrites was observed using X-ray diffraction (XRD) analysis. Fourier-transform infrared (FTIR) analysis confirms the formation of Y-type hexaferrites. Field-emission scanning electron microscopy (FESEM) reveals particles with hexagonal plate-like morphologies. The optical band gap of the prepared samples lies between 0.74 and 2.16 eV. The coercivity values of the prepared samples demonstrate their soft-magnetic nature. Dielectric analyses show the influence of the dopant ions on the dielectric parameters of the prepared samples. Impedance spectroscopy using a Nyquist plot shows that most of the dielectric response results from grain-boundary contributions.

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.

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.

Impedance and Electric Modulus Spectroscopy of Polycrystalline La0.5Sr0.5Bi0.2Co0.4Fe0.4O3–d Cathode Ceramic for Intermediate Temperature SOFCs

In the present research work, La 0.5 Sr 0.5 Bi 0.2 Co 0 . 4 Fe 0.4 O 3– d cathode ceramic powder is synthesized through cost-effective flash pyrolysis process and further conventional heat treatment for sintering for IT-SOFCs. The crystal structure, unit cell parameters and bond length are determined by using the Rietveld refinement program. The XRD result indicates existence of a pure single phase with R C space group symmetry detected from the sample which has been sintered at 700 ℃. SEM images of fracture surface of sample sintered at 700 ℃ showed a high porosity and nano grain sizes (50-100 nm). Impedance and electric modulus spectroscopic methods are used to identify the relaxation phenomena in La 0.5 Sr 0.5 Bi 0.2 Co 0 . 4 Fe 0.4 O 3– d ceramic over a broad range of temperature and frequency. A single relaxation peak is observed in the imaginary part of impedance and electric modulus spectra, which could be due to the contribution of grain boundary of La 0.5 Sr 0.5 Bi 0.2 Co 0 . 4 Fe 0.4 O 3– d ceramic. The imaginary part of modulus ( ) spectra is analyzed with help of non-exponential decay function or Kohlrausch–Williams–Watts (KWW) parameter ( β ). In the combined plot of the imaginary part of impedance and electric modulus spectra at 210 ℃, only a single peak of and is observed at the same frequency which indicates that the conduction process is a long-range motion of the charge carriers. The frequency-dependent conductivity is followed by the Jonscher’s double power law in the temperature range 30-210 ℃.

Co-ferrite nanostructures prepared by solvothermal route; new ultra-low k dielectrics

Co spinel ferrite nanoparticles were successfully prepared using a solvothermal route. Their structure, morphology and cationic distribution were studied by X-ray diffraction, Transmission Electron Microscopy and 57Fe Mössbauer Spectrometry. Though, their electric conductivity and dielectric relaxation were studied on nanostructured pellets resulting from the compaction of nanoparticles, by complex impedance spectroscopy in the frequency range 1–106 Hz from 25 up to 250 °C. The dielectric properties are investigated in detail. The presence of two thermally activated relaxation peaks in the modulus loss spectra confirms the contribution of grains and grain boundaries to the electrical behavior of the material. The main dielectric relaxation comportment and the conduction process are governed by the grain boundaries. Ac-conductivity, which deviates from the usual Universal Dielectric Response, can be described by a double power-law evidenced by a Nearly Constant Loss trend followed by a Super-Linear Power Law that decreases with increasing temperature. The change in the cationic distribution in CoFe2O4, deduced from the 57Fe Mössbauer spectra, may explain the observed trend in conductivity. The classical dielectric relaxation behavior is observed and very low values of permittivity are estimated. This compound can therefore be classified as an ultra-low-k dielectric.

Study of Charge Transfer Mechanism and Dielectric Relaxation of CsCuCl3 Perovskite Nanoparticles

Non-toxic inorganic metal halide perovskites have commercially established dominance over all the other optoelectronic devices. In this paper, a non-toxic CsCuCl3 metal halide is successfully synthesized through the slow evaporation solution growth technique. The hexagonal phase of the material is checked using the X-ray diffraction measurement. The morphological study indicates the spherical shape of nanoparticles with about 20 nm size. CsCuCl3 demonstrates a semiconducting property with a direct band gap value of approximately 2.18 eV. Over the 10−1–107 Hz frequency range, the dielectric constant, the loss factor, the electric modulus and also the electrical conductivity of CsCuCl3 show strong temperature dependence. The Nyquist plot confirms the various contributions of grains and grain boundaries to the total impedance. In the high-frequency region, the dielectric constant tends to increase with temperature. The modified Cole–Cole plot asserts that while the relaxation time decreases with the rise in temperature, the space charge and free charge conductivity increase the moment the temperature climbs. In accordance with the modified Kohlrausch-Williams-Watts (KWW) equation, an asymmetrical nature corresponding to the non-Debye type of the perovskite is noticed in the electric modulus spectra at different temperatures. Moreover, the imaginary part of the electric modulus spectra is found to shift from the non-Debye toward the Debye type with the increase in temperature despite not getting the exact Debye response and emerging as a semi-conductor material. The alternating current (AC) conductivity of CsCuCl3 is shown based on the non-overlapping small-polaron tunneling (NSPT) mechanism. The fact that the direct current (DC) conductivity matches well with the activation energy which is based on the two modulus spectra indicates that the relaxation behavior and the conduction mechanism are remarkably similar. Additionally, CsCuCl3 is likely to be an energy-harvesting device since it has a high dielectric constant related to its low dielectric loss.

Effect of annealing temperature on dielectric properties of iron oxide prepared by sol–gel auto combustion method

In this study, different phases of iron oxide have been synthesized by sol–gel auto combustion route followed by annealing. Temperature and frequency-dependent dielectric properties have been studied in detail which provide the information on the contribution of grain and grain boundary in charge transport. Conductivity and electric modulus spectra have also been analyzed. The scaled coordination of impedance spectrum merges in a single master curve which signifies that the distribution of relaxation time is temperature independent. Complex impedance spectra have been used to evaluate activation energy (Ea ) by using Arrhenius relation. The activation energy calculated for α-Fe2O3 is 1.36 eV.

A unified mechanistic model for Hall–Petch and inverse Hall–Petch relations of nanocrystalline metals based on intragranular dislocation storage

Nanocrystalline (NC) metals often show transition from Hall–Petch (HP) strengthening to inverse HP softening as the average grain size decreases below a critical value. Compared to the HP behavior whose mechanism is well understood as the dislocation pile-up at grain boundaries (GBs), several hypotheses have been proposed to explain the veiled inverse HP behavior and no consensus has been reached yet. In this work, we propose a size-dependent model considering the influence of grain size on the intragranular dislocation storage ability and unify the HP and inverse HP relations for NC metals. The reduction of the intragranular dislocation storage ability with decreasing grain size is revealed as the underlying mechanism of the breakdown of the HP behavior in NC. Prediction of the critical grain size of 26.9nm for the HP-inverse HP transition of NC copper agrees well with experimental results. Numerical results suggest that the harmonized deformation of GB and grain interior (GI) dominates the remarkable ductility enhancement of NC metals in the inverse HP region. Moreover, our results suggest that the formation of dimple structures spanning several grains at fracture surfaces of NC metals is attributed to the coalescence of inter- and intra-granular microcracks and microvoids in clustered grains with Goss texture in local shear bands. As the GB strength increases, NC metals show enhancement in yield strength and delay in occurrence of the inverse HP behavior. Our studies give new insight into the contribution of GB sliding to the plastic behaviors of NC metals, and provide valuable guidance for the rational design of NC metals with high ductility and high strength.

Structural and Electrical Performance of (Bi1/2Ba1/4Sr1/4)(Ti1/2Fe1/2)O3Relaxor

The polycrystalline sample of a ternary system of 0.5BF-0.25ST-0.25BT (i.e., Bi0.5Ba0.25Sr0.25)(Ti0.5Fe0.5)O3 was prepared via a conventional high-temperature solid-state reaction technique. The preliminary structural analysis using room temperature X-ray diffraction (XRD) data shows the tetragonal as well as pseudo-cubic nature of the compound. Analysis of temperature and frequency-dependent dielectric parameters, obtained in a wide range of temperature and frequency, exhibits diffused phase transition with relaxor-ferroelectric nature of the material. This observation has been confirmed through temperature and frequency evolved P-E loops. The sample is free from impurities and is structurally stable. The Vogel-Fulcher plot was used to confirm the relaxor type of ferroelectric behavior of the material. The dielectric relaxation, grain, grain-boundary contribution, and semiconductor (negative temperature coefficient of resistance) nature of the compound have been analysed through impedance analysis. The current-voltage (I-V) and leakage characteristics confirm the bulk limited (SCLC) and interface limited (Schottky, Poole-Frankel emitted and Fowler-Nordheim type) conduction mechanisms. The energy bandgap, calculated from the I-V-spectra, is found to be 0.93 eV in the particular (high) temperature range indicating the semiconducting nature of the material.