Intergrain Effects Research Articles

A combined experimental and computational approach on La0.6Sr0.4MnO3 perovskite

We synthesize a high quality Sr-doped lanthanum manganite using solid state reaction route to investigate the various properties for device applications. The crystal structure of the synthesized perovskite was studied by X-ray diffraction (XRD) pattern and also compared with the crystallographic data obtained from the simulation calculations. The magnetization as a function of applied magnetic field and temperature of La0.6Sr0.4MnO3 exhibits ferromagnetic metal phase with the Curie temperature of 361 K. The electrical resistivity with temperature unexpectedly shows semiconducting behavior due to the intergrain effects. On the other hand, the energy dispersion studied by first principles calculations based on density functional theory (DFT) demonstrates metallic conduction in conformity with the available experimental results. No energy gap in the absorption spectrum done by UV–Visible spectrophotometer of this manganite also confirms the nature of identical conductivity. Finally, a quasi-harmonic Debye model was employed to calculate the thermal characteristics like Debye temperature, specific heat capacities, volume expansion coefficient, etc. in this LSMO perovskite.

Coupling behavior of Bi2Sr2−xInxCaCu2O8+d

The interplay between intrinsic grain and inter-grain effects at the superconducting state defines the behavior of bulk high-temperature superconductors. In this work, we use DC resistivity and AC magnetic susceptibility measurements to show how the intrinsic and inter-grain properties influence the intergrain phase coherence and flux dynamics of Bi2Sr2−xInxCaCu2O8+d. High In-doped samples, x ≥ 0.4, have broad resistive and diamagnetic superconducting transitions due to weak intergrain coupling caused by segregated impurities. Low doping levels, x ≤ 0.3, have sharp superconducting resistive and diamagnetic transitions, with higher transition temperature TC attributed to an increase superconducting pair density. This results to enhanced intergrain coupling and strong diamagnetic screening of intergrain void networks. An applied AC field of 0.77 mT amplitude deteriorates the intergrain diamagnetic screening at x = 0.2 through AC flux slip between grains.

Effects of Grain Boundaries on THz Conductivity in the Crystalline States of Ge2Sb2Te5 Phase‐Change Materials: Correlation with DC Loss

Herein, in situ temperature‐dependent THz and DC conductivity measurements in the crystalline states of Ge2Sb2Te5 (GST225) are performed at relatively high temperatures (>300 K). As observed in many nanomaterials, a non‐Drude type of THz conductivity is found in the crystalline state of phase‐change (PC) materials. Both the intra‐ and intergrain effects can be separated in the THz and DC conductivity. It is shown that grain boundaries significantly affect the THz and DC conductivities of the crystalline phase (distorted rock salt structure). The experimentally observed DC Hall mobility is different from the intragrain mobility extracted from the THz conductivity, suggesting that a series sequence of intra‐ and intergrain transport mechanisms controls the electronic transport in the crystalline state of GST225.

A Micromechanically Motivated Constitutive Model for Magnetostrictive Materials With Rate Effects

A micromechanically motivated uniaxial model with least number of state variables is presented in this paper to capture nonlinear hysteresis behavior of polycrystalline magnetostrictive materials, in particular Galfenol, with rate effects. This model is formulated based on the classical thermodynamic framework and relies on the domain rotation phenomenon occurring at the microscopic level. The macroscopically averaged domain rotation events are considered as the material state variables. The driving force required for the transformation of one state variable into another is extracted from the derived dissipation inequality equations. A special case, wherein two transformation systems activated simultaneously, is also taken into consideration in the model development. Back fields, accounting in a way the averaged inter-domain and inter-grain effects, are introduced in the proposed model. Rate-dependent effects are also set into the model in a straightforward way without compromising the core idea of this model development, i.e., a simplified approach, yet capturing most of the material responses. Finally, the proposed model, despite the reduced number of state variables, demonstrates its ability to collaborate well with many complex experimental observations of Galfenol reported in the literature.

Inter-grain effects on the magnetism of M-type strontium ferrite

By using films as model systems, a combined study based on experiments and micromagnetic simulations has been performed to get further insight into the magnetic behaviour of M-type SrFe12O19 (SrF) system in the nanosized range. Both nanocrystalline SrF quasi-isotropic and highly anisotropic films have been obtained by sputtering. Tuning of the preferred c-axis orientation, either normal or in the film plane, has been possible by varying the growth parameters and, importantly, with no need of any underlayer. Small changes in the oxygen ratio (up to 2%) and in-situ heating lead to different microstructure, which determine the strong variations of the magnetic behaviour observed, including both magnetization curves, easy axis directions, and their different coercive fields, which vary from 1 kOe to 5 kOe within the sample series. Micromagnetic simulations reproduce the experiments with a model based on a mixed population of randomly distributed SrF grains with in-plane and out-of-plane uniaxial anisotropies. The simulations show that intergranular interactions play the major role on the coercive field behaviour of the SrF system. A microstructure comprising largely uncoupled grains leads to improved magnetic performance of highly c-axis oriented SrF nanostructures.

Superconductive transition and the intergrain effects of mixture ceramic systems synthesized using Citrate pyrolysis precursor method

Superconductive characteristics of Pr2Ba4Cu7O15-δ (Pr247) ceramics with crystalline phase inhomogeneity for the stacking structures is examined experimentally, using reference observations for multi-phased ceramic sample consists simply of PrBa2Cu4O8 (Pr124) and PrBa2Cu3O7-δ (Pr123). After reduction treatment by vacuum-heating, the reference multiphased sample shows onset of abrupt electric resistivity dropping and also weak Meissner magnetization below ∼20 K. The results suggest that superconductivity at CuO double chains in Pr124 grains is caused by charge transfer from neighbouring Pr123 grains. Such a charge transfer effect is thought to occur also in Pr247 sample including phase inhomogeneity.

Study of intermetallic compounds isostructural to MgB/sub 2/

The small intergrain effect of MgB/sub 2/ on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature T/sub c/ is only 40 K. However, band-structure calculations predict that a higher T/sub c/ than that of MgB/sub 2/ is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on T/sub c/ observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise T/sub c/, but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa/sub 2/, AGa/sub 2-x/Si/sub x/, and AAl/sub 2-x/Si/sub x/, where A=Ca, Sr, or Ba, which are isostructural to MgB/sub 2/ and have greater lattice parameters than MgB/sub 2/. In spite of the greater lattice parameters, AGa/sub 2/ are not superconducting. However, all pseudobinary compounds AGa/sub 2-x/Si/sub x/ and AAl/sub 2-x/Si/sub x/ for 0.6<x<1.5 are superconducting. Superconductivity is reported for the first time in SrAlSi, with T/sub c/ of 5.1 K. Although T/sub c/ varies with x in a similar fashion for all members of the series, no specific correlation between T/sub c/ and lattice parameters or ionic mass is observed. The maximum T/sub c/ of these compound series with different A's varies between 5.5 and 7.8 K, much lower than that of MgB/sub 2/. The results strongly suggest the unique role of B in the superconductivity of C32 intermetallic compounds. They also demonstrate that factors additional to the lattice parameters and densities of states must play an important role, and that the rigid-band model is not sufficient to account for the observations.

X-ray Microbeam Diffraction Measurements in Polycrystalline Aluminum and Copper Thin Films

ABSTRACTThermally induced residual strains in polycrystalline Cu and Al films on single crystal Si and glass substrates, respectively, have been examined on a grain-by-grain basis by x-ray microbeam diffraction. The crystallographic orientation and the deviatoric strain tensor, εij*, are determined for each grain by white beam Laue diffraction. From grain orientation mapping and strain tensor measurements, information is obtained about the distributions of strains for similarly oriented grains, about strain variations within single grains, and about grain-to-grain correlations of strains. This type of information may be useful in developing and testing theories for intergrain effects in strain evolution in polycrystals.

Transport properties of pulsed laser deposited La 0.67Sr 0.33MnO 3 thin films

La 0.67Sr 0.33MnO 3 thin films have been prepared by pulsed laser deposition on (1 0 0) SrTiO 3, (1 0 0) Si and YSZ/CeO 2 buffered (1 0 0) Si substrates. Structural, electrical and magnetic characterisations were performed on each film, in order to put in evidence the grain boundaries effect in the magnetotransport properties. The films show a significant colossal magnetoresistance (CMR) effect near the Curie temperature. Textured and polycrystalline films show an additional magnetoresistance effect at low temperature originating on spin-polarised carriers tunnelling through grain boundaries or a non-ferromagnetic surface layer. The MR behaviour as a function of magnetic field and temperature shows that the suppression of magnetic fluctuations is the origin of the magnetoresistance in epitaxial films, while the intergrain effects dominate the MR in textured and polycrystalline films.

Conduction in perylene - , - and - complexes

Perylene (Per) was reacted with , and in solid-solution reactions to produce solvent-free , and . X-ray diffraction identified the unit cells of the compounds as being orthorhombic in the first two cases and tetragonal in the last case. All of the compounds exhibited an increase in resistance with decreasing temperature indicative of a semiconducting type of behaviour and had room temperature resistivities of , , and , respectively. The conduction process has been analysed by fitting the data with a number of parameters in order to account for a nonlinearity in the versus 1/T curves. The polaronic hopping model employed includes contributions of multiphonon processes, and intragrain and intergrain effects, and provides a good fit to the resistivity data as well as consistency with the results of Hall effect measurements.

Hydraulic characteristics of bioclastic deposits: new possibilities for environmental interpretation using settling velocity fractions

ABSTRACTThis paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions. Bioclastic deposits are divided into fractions by splitting hydraulically sorted samples. Compositional analysis of fractions shows that grains exhibit marked variation in composition, shape, size and density which control the hydraulic behaviour of bioclastic deposits. Despite the heterogeneous grain properties of fractions (analogous to sieve fractions) sedimentation analysis shows that they possess a narrow range of settling velocity values. Flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values. A threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20.5 to 65‐0cm s −1 for settling fractions with mean settling values of 5.5 chi (2‐2 cm s −1) to 1.4 chi (36.0 cm s −1), respectively. Heterogeneous settling fractions, due to intergrain effects, were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour. Thus, caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition.As settling velocity fractions reflect narrow hydraulic (settling and threshold) properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes.Comparison of settling and sieve‐size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes. The study indicates that examination of settling velocity fractions (for which hydraulic settling and threshold properties are known) coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit‐forming and energy processes in reef environments.

Transport study of Ba-deficient thin-film Y123 superconductors enriched with Na

Nominal Y0.3Ba0.6Na0.1CuO3- delta thin films were prepared with small grains having an orientation of the c axis perpendicular to the MgO substrate. The electrical resistivity and the thermoelectric power in the absence and in the presence of a magnetic field are reported. The magnetic field dependence of the irreversibility line is discussed. The role of the field for disentangling intragrain and intergrain effects in both coefficients is emphasized.

Dielectric breakdown in high-ε films for ULSI DRAMs: II. barium-strontium titanate ceramics

Abstract We present a study of breakdown fields in BaxSr1−xTiO3 ceramics as functions of thickness, temperature, applied voltage ramp rate, electrode material and cross-sectional area. The data show evidence of Zener-like breakdown mechanisms. The d.c. leakage currents at voltages below breakdown are dominated by tunneling injection and image forces at low voltages (≤3 V), Schottky behavior at intermediate voltages V a (3 V ≤ V a ≤ 6 V; 150 kV/cm ≤ E ≤ 300 kV/cm), and Fowler-Nordheim tunneling at high voltages; it is shown that both the Schottky emission regime and the ultimate breakdown arise from inter-grain effects rather than the BST/electrode interface, in accord with the theory of Neumann and Arlt and of Waser and Klee. In contrast to PbZrxTi1−xO3 (PZT), which displays avalanche breakdown and space-charge limited d.c. leakage currents at normal 5 V silicon IC voltages, BST exhibits Schottky-dominated leakage currents in this 5 V regime, and is dominated by Fowler-Nordheim tunneling in its breakdown ...

Intra- and intergrain effects in a.c. susceptibility of YBaCuO ceramics

The dependence of the initial magnetic susceptibility on the magnetic field, frequency and density has been investigated for several ceramic YBa 2Cu 3O 7 samples. The superconducting transitions show invariably two steps, associated with the grain and bulk (shielding) effects respectively. The width of transition decreases, whereas the lower critical field increases, with increasing density.