A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

LAMMPS - a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales

Structural materials for fission & fusion energy

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution.

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Atomistic spin model simulations of magnetic nanomaterials.

combined application of the Langevin spin dynamics and the fluctuation-dissipation theorem. We investigate several applications of the method, performing microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls, and isothermal-isobaric ensemble dynamical simulations of thermally equilibrated homogeneous systems at various temperatures. The predicted isothermal magnetization curve agrees well with the experimental data for a broad range of temperatures. The equilibrium as well as time-correlation functions of spin orientations exhibit the presence of short-range magnetic order above the Curie temperature. Furthermore, short-range order spin fluctuations are shown to contribute to the thermal expansion of the material. Our analysis illustrates the significant part played by the spin degrees of freedom in the dynamics of motion of atoms in magnetic iron and iron-based alloys. It also shows that the spin-lattice dynamics algorithm developed in this paper offers a viable way of performing large-scale dynamical atomistic simulations of magnetic materials.

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Large-scale simulation of the spin-lattice dynamics in ferromagnetic iron

We systematically investigated the structure, electronic properties, zone-center phonon modes, and structure instability of four cubic perovskite $\mathrm{Bi}M{\mathrm{O}}_{3}$ compounds, with three of the $M$ ions being IIIB metals (Al, Ga, and In) and one IIIA transition-metal Sc, using first-principles density-functional calculations. Optimized lattice parameters, bulk moduli, band structures, densities of states, as well as charge density distributions are calculated and compared with the available theoretical data. Our results are in good agreement with those previously reported in the literature. All the $\mathrm{Bi}M{\mathrm{O}}_{3}$ oxides considered in the present work are semiconductors with an indirect band gap between the occupied $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ and unoccupied $\mathrm{Bi}\phantom{\rule{0.2em}{0ex}}6p$ states varying between 0.17 and $1.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Their electronic properties are determined mainly by Bi--O bonding, which, in turn, depends on the $M--\mathrm{O}$ bonding. Ferroelectric properties of these oxides come from the $6{s}^{2}$ lone pair on the $A$-site Bi ion and is similarly affected by the $M$ ions through their influence on the Bi--O bonding, as suggested by our calculations of density of state, Born effective charge, and soft modes. The existence of soft modes and eight [111] minima suggests that the phase transition in $\mathrm{Bi}\mathrm{Al}{\mathrm{O}}_{3}$ has a mixed displacive and order-disorder character. There is evidence that ferroelectricity is absent in $\mathrm{Bi}\mathrm{Ga}{\mathrm{O}}_{3}$. Our investigation suggests that the $\mathrm{Bi}M{\mathrm{O}}_{3}$ oxides or their modified versions are promising ferroelectric, piezoelectric, multiferroic, and photocatalytic materials.

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First-principles study of the cubic perovskites Bi M O 3 ( M = Al , Ga, In, and Sc)

The Schwoebel-Ehrlich barrier—the additional barrier for an adatom to diffuse down a surface step—dictates the growth modes of thin films. The conventional concept of this barrier is two dimensional (2D), with the surface step being one monolayer. We propose the concept of a three-dimensional (3D) Schwoebel-Ehrlich barrier, and identify the 2D to 3D transition, taking aluminum as a prototype and using the molecular statics method. Our results show that: (1) substantial differences exist between the 2D and 3D barriers; (2) the transition completes in four monolayers; and (3) there is a major disparity in the 3D barriers between two facets; further, alteration of this disparity using surfactants can lead to the dominance of surface facet against thermodynamics.

/pdf/schwoebel-ehrlich-barrier-from-two-to-three-dimensions-18ptf3h450.pdf

Schwoebel-Ehrlich barrier: from two to three dimensions

We develop a dynamic spin-lattice-electron model for simulating the time-dependent evolution of coupled spin, atomic, and electronic degrees of freedom in a magnetic material. Using the model, we relate the dissipative parameters entering the Langevin equations for the lattice and spin degrees of freedom to the heat transfer coefficients of a phenomenological spin-lattice-electron three-temperature model. We apply spin-lattice-electron dynamics simulations to the interpretation of experiments on laser-induced demagnetization of iron thin films, and estimate the rates of heat transfer between the spins and electrons, and between atoms and electrons. To model the dynamics of energy dissipation in a magnetic material undergoing plastic deformation, we develop an algorithm that separates the local collective modes of motion of atoms from their random thermal motion. Using this approach, we simulate the propagation of compressive shock waves through magnetic iron. We also explore the microscopic dynamics of dissipative coupling between the spin and lattice subsystems, and show that the rate of spin-lattice heat transfer is proportional to the integral of the four-spin time-dependent correlation function.

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Spin-lattice-electron dynamics simulations of magnetic materials

In molecular dynamics simulations, temperature is evaluated, via the equipartition principle, by computing the mean kinetic energy of atoms. There is no similar recipe yet for evaluating temperature of a dynamic system of interacting spins. By solving semiclassical Langevin spin-dynamics equations, and applying the fluctuation-dissipation theorem, we derive an equation for the temperature of a spin ensemble, expressed in terms of dynamic spin variables. The fact that definitions for the kinetic and spin temperatures are fully consistent is illustrated using large-scale spin dynamics and spin-lattice dynamics simulations.

http://ira.lib.polyu.edu.hk/bitstream/10397/5383/1/Ma_Temperature_Dynamic_Spin.pdf

Chung-ho Woo

Papers

Large-scale simulation of the spin-lattice dynamics in ferromagnetic iron

First-principles study of the cubic perovskites Bi M O 3 ( M = Al , Ga, In, and Sc)

Schwoebel-Ehrlich barrier: from two to three dimensions

Spin-lattice-electron dynamics simulations of magnetic materials

Temperature for a dynamic spin ensemble