Lattice positions of Sn in kesterite Cu2ZnSnS4 and Cu2SnS3 nanoparticles and thin films were investigated by XANES (x-ray absorption near edge structure) analysis at the S K-edge. XANES spectra were analyzed by comparison with simulations taking into account anti-site defects and vacancies. Annealing of Cu2ZnSnS4 nanoparticle thin films led to a decrease of Sn at its native and defect sites. The results show that XANES analysis at the S K-edge is a sensitive tool for the investigation of defect sites, being critical in kesterite thin film solar cells.

The (1 -x)BiFeO 3 -xBaTiO 3 (with x = 0.1, 0.2, 0.3, and 0.4) ceramics were fabricated successfully by solid-state reaction method. Single-phase perovskite was obtained in all ceramics, as confirmed by XRD technique. It was observed that 0.7BiFeO 3 -0.3BaTiO 3 was the morphotropic phase boundary (MPB) between rhombohedral and cubic phases, as also revealed from ferroelectric and magnetic properties. The simulated and experimental X-Ray Absorption Spectroscopy (XAS) study revealed that BT in 0.75BF-0.25BT is possibly taken a rhombohedral structure. Furthermore, the rounded ferroelectric hysteresis loops observed for 0.9BiFeO 3 -0.1BaTiO 3 and 0.8BiFeO 3 -0.2BaTiO 3 compositions could be attributed to their microstructure and surface charge effects and electron transfer between Fe 3+ and Fe 2+ ions. It was also found that high dielectric constant of 0.9BiFeO 3 -0.1BaTiO 3 composition was a result of grain and grain-boundary effects, as observed in SEM micrographs. In addition, a strong signature of dielectric relaxation behavior was observed in this ceramic system with the activation energy 0.467 eV obtained from the Arrhenius' law. Finally, the local structure investigation with XAS technique provided additional information to better understand the electric and magnetic properties in the BF-BT ceramic system.

No abstract

No abstract

Effects of compressive stress on the dielectric properties of complex perovskite PZBT-PMNT ceramic were investigated. The dielectric properties measured under stress-free condition showed a composite nature with two distinct temperatures of dielectric maximum associated with PZBT and PMNT end members. The dielectric properties under the compressive stress were observed at stress levels up to 230 MPa using a home-built compressometer. The results clearly showed that the compression load significantly reduced both the dielectric constant and the dielectric loss tangent in every measuring frequency. The change of the dielectric constant with stress was attributed to competing influences of the intrinsic contribution of non-polar matrix and the extrinsic contributions of re-polarization and growth of micro-polar regions, while the clamping of the domain walls contributed to the stress-dependent changes of the dielectric loss tangent. Finally, a large drop of the dielectric constant after a stress cycle was likely caused by the stress induced decrease in switchable part of spontaneous polarization.

In this work, the Preisach technique was employed to characterize temperature dependent hysteresis loops extracted from the Ising model considered under the mean-field (MF) framework. In constructing the Preisach densities for these MF loops, a stochastic process was used to optimize the hysterons' weight. Then, from the obtained Preisach densities, series of the MF hysteresis loops were accurately re-constructed, and the temperature independence of densities was observed. In addition, an increase of reversible polarization and a decline of irreversible polarization with increasing temperature were evident. These phenomena were examined and found to be caused by the enhancement of thermal fluctuation at higher temperature, which alleviates the spin flipping viscosity and lessens the phase-lag in hysteresis characteristic.

Iron oxide nanoparticles were synthesized by employing an ultrasonic homogenizer in conjunction to the co-precipitation of Fe (NO3)3·9H2O, FeCl2·4H2O and NaOH. Synchrotron X-ray absorption near edge structure (XANES) spectra could be fitted with the contributions from hematite (α-Fe2O3) and, to a lesser extent, metallic Fe. The increase in reagent concentrations increased the Fe fraction which partly explained the change in magnetization of superparamagnetic particles. Nevertheless, the α-Fe2O3 composition was increased by using a longer reaction time.

This work performed Preisach modeling on hysteresis loops, where the contributions of Preisach density characteristics on hysteresis reversals were investigated. Specifically, the three-dimensional Gaussian-distribution function was used to construct the Preisach densities for extracting the associated hysteresis loops. In particular, the influences of three key Gaussian Preisach density characteristics (i.e., density sharpness, density center, and splitting densities) on hysteresis behavior were examined. It was found that sharper density induces more harmonized domain switching, so polarization derivative is enhanced at the coercivity, while the density center indicates the range of external field at which most switching occurs. Moreover, the splitting of the density was found to represent pinching in hysteresis loops, where material tends to actively respond in two different ranges of external field. Consequently, based on these results, significant hysteresis behavior can be revealed using minimal parameters via appropriate mathematical function; that is, another step enhances the fundamental understanding in the hysteresis topic using the Preisach framework.