X.Q. Ma

Lattice, elastic, polarization, and electrostrictive properties of BaTiO3 from first-principles

Predicting the domain structures and properties in both bulk single crystal and thin film ferroelectrics using the phase-field approach requires the knowledge of fundamental mechanical, electrical, and electromechanical coupling properties of a single-domain state. In this work, the elastic properties and structural parameters of cubic single crystals as well as tetragonal, orthorhombic, and rhombohedral BaTiO3 single domain states are obtained using first-principles calculations under the local density approximation. The calculated lattice constants, bulk modulus, and elastic constants are in good agreement with experiments for both the cubic paraelectric phase and the low-temperature ferroelectric phases. Spontaneous polarizations for all three ferroelectric phases and the electrostrictive coefficients of cubic BaTiO3 are also computed using the Berry’s phase approach, and the results agree well with existing experimentally measured values.

Oxidation behavior of TiNi shape memory alloy at 450–750 °C

The isothermal oxidation behavior of a commercial TiNi shape memory alloy (SMA) in pure oxygen over the temperature range of 450–750 ◦ C was studied. The oxidation products were identified by X-ray diffraction (XRD). Characterization of the specimens after oxidation was conducted using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation kinetics at different temperature was studied, according to parabolic and logarithmic laws. The mechanism and activation energy of oxidation were discussed based on the oxidation kinetics. The spallation of the multi-layer scale formed during oxidation was analyzed by growth induced thermal stresses.

Temperature-pressure phase diagram and ferroelectric properties of BaTiO3 single crystal based on a modified Landau potential

A modified eighth-order Landau potential was proposed for the BaTiO3 single crystal by taking account into the quantum mechanical effects at low temperature. While all existing thermodynamic potentials for BaTiO3 fail to accurately describe the pressure dependence of ferroelectric transition temperatures, the temperature and hydrostatic pressure phase diagram constructed using the modified potential shows excellent agreement with experimental measurements by Ishidate, Abe, Takahashi, and Mori [Phys. Rev. Lett. 78, 2397 (1997)]. On the basis of the new proposed Landau potential, we calculated the dielectric coefficients, spontaneous polarizations, temperature-electric field phase diagram, and piezoelectric coefficients, all in good agreement well with existing experimental data.

Phase-field simulations of stress-strain behavior in ferromagnetic shape memory alloy Ni2MnGa

The evolution of strain, magnetic domain structure, and martensite microstructure during compressive stress loading and unloading of Ni2MnGa was studied using a phase-field model at several selected magnetic fields. We observed a typical quasiplastic behavior at zero field and a pseudoelastic behavior at 300 kA/m. At an intermediate field, 150 kA/m, the stress-strain relation is partially pseudoelastic. It was demonstrated that the magnetic domain structure has little influence on the recovered strain while the demagnetization factor impacts the strain reversal.

Effect of applied load on nucleation and growth of γ-hydrides in zirconium

c-hydride precipitation and growth in zirconium were investigated by using a phase-field kinetic model. The orientation difference of hydride is represented by non-conservative structural field variables, whereas the concentration difference of hydrogen in precipitates and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg–Landau equations for the structural variables and the Cahn–Hilliard diffusion equation for the concentration variable. It is demonstrated that a certain load level is required to completely re-orient hydride precipitates and it is most effective to apply loads during the initial nucleation stage for producing anisotropic precipitate alignment. � 2002 Elsevier Science B.V. All rights reserved.

Micromagnetic simulation of spin-transfer switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar

We investigated the spin-transfer switching in a full-Heusler Co2FeAl0.5Si0.5 alloy spin-valve nanopillar through micromagnetic simulation. A two-step switching hysteresis loop due to the fourfold in-plane magnetocrystalline anisotropy of Co2FeAl0.5Si0.5 layers was obtained. The simulation explains the experimental result of the resistance versus current hysteresis loop and yields good agreement with the measured critical current. Furthermore, the magnetization trajectory and magnetization distribution were shown and analyzed to elucidate the different characters of two-step switching.

Atom-Thin SnS2–xSex with Adjustable Compositions by Direct Liquid Exfoliation from Single Crystals

Two-dimensional (2D) chalcogenide materials are fundamentally and technologically fascinating for their suitable band gap energy and carrier type relevant to their adjustable composition, structure, and dimensionality. Here, we demonstrate the exfoliation of single-crystal SnS2-xSex (SSS) with S/Se vacancies into an atom-thin layer by simple sonication in ethanol without additive. The introduction of vacancies at the S/Se site, the conflicting atomic radius of sulfur in selenium layers, and easy incorporation with an ethanol molecule lead to high ion accessibility; therefore, atom-thin SSS flakes can be effectively prepared by exfoliating the single crystal via sonication. The in situ pyrolysis of such materials can further adjust their compositions, representing tunable activation energy, band gap, and also tunable response to analytes of such materials. As the most basic and crucial step of the 2D material field, the successful synthesis of an uncontaminated and atom-thin sample will further push ahead the large-scale applications of 2D materials, including, but not limited to, electronics, sensing, catalysis, and energy storage fields.

Simulation of γ-hydride precipitation in bi-crystalline zirconium under uniformly applied load

The morphology evolution of -hydride precipitation and growth in a zirconium bi-crystal was simulated using a phase field kinetic model. The effects of grain boundary and uniformly applied load were studied. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg–Landau equations for the structural variables and the Cahn–Hilliard diffusion equation for the concentration variable. It is demonstrated that nucleation density of the hydride at the grain boundary increases as compared to the bulk and favorable hydride precipitation at the grain boundary become weaker when an external load is applied. The result also showed that hydrides will grow in those habit planes that are near the perpendicular direction of the applied tensile load.

Phase-field model of multiferroic composites: Domain structures of ferroelectric particles embedded in a ferromagnetic matrix

The ferroelectric domain structures in 1-3 type bulk magnetoelectric composites were studied using the phase-field method. The ferroelectric polarization distributions in triangle, square, circle, and other polygons-shaped rods embedded in a ferromagnetic matrix were obtained. The influences of ferroelectric particle size and symmetry and the surrounding magnetic phase medium on the stability of double vortex structures are discussed.

Phase-field simulations of magnetic field-induced strain in Ni2MnGa ferromagnetic shape memory alloy

The magnetization and magnetic field-induced strain behavior of the ferromagnetic shape memory alloy, Ni2MnGa, under constant compressive stress were studied using the phase-field method. Based on the evolving magnetic domain and martensitic structures, we analyzed the cycling effect, magnetization hysteresis, strain recoveries, and coupling between the domain wall and martensite twin boundaries. We compared the switching behavior of single variant and multivariant martensite structures. We observed three types of magnetic field-induced strain mechanisms, depending on the magnitude of the applied compressive stress. The study revealed that the martensite microstructure of the magnetic shape memory alloy plays an important role in magnetization and strain evolution during loading and unloading of an external magnetic field under different stress conditions. The results are compared with existing experimental observations.