Y. Dai

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

In addition to x-ray diffraction, Raman spectrum measurements provide direct evidence of the tetragonal and orthorhombic phases coexistence in lead-free ceramics (1−x)K0.5Na0.5NbO3 (KNN)–xLiTaO3 when x equals 5mol%. This is caused by the phase transition temperature between tetragonal and orthorhombic decreasing to around room temperature due to the Li and Ta doping in KNN, and not by constituting a region of morphotropic phase boundary as presented by most published papers. The results indicate that although this kind of ceramic displays good properties, it needs further study to verify if it is suitable to be used in a varying temperature environment.

Morphotropic phase boundary and electrical properties of K1−xNaxNbO3 lead-free ceramics

Lead-free K{sub 1-x}Na{sub x}NbO{sub 3} ceramics with x=0.48-0.54 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on phase structures and electrical properties. The results suggest that a typical morphotropic phase boundary exists at x=0.52-0.525, separating the monoclinic and orthorhombic phases. The sample with the composition near x=0.52 shows the maximum values of the piezoelectric constant (d{sub 33}=160 pC/N) and the electromechanical coupling coefficient (k{sub t}=47%). The results provide a helpful guidance to consider the optimal ratio of K to Na for designing and developing new (K,Na)NbO{sub 3}-based ceramics.Lead-free K1−xNaxNbO3 ceramics with x=0.48–0.54 were prepared by a conventional solid-state reaction method to investigate the influence of the K/Na ratio on phase structures and electrical properties. The results suggest that a typical morphotropic phase boundary exists at x=0.52–0.525, separating the monoclinic and orthorhombic phases. The sample with the composition near x=0.52 shows the maximum values of the piezoelectric constant (d33=160 pC/N) and the electromechanical coupling coefficient (kt=47%). The results provide a helpful guidance to consider the optimal ratio of K to Na for designing and developing new (K,Na)NbO3-based ceramics.

Formation and structure of Al-Zr metallic glasses studied by Monte Carlo simulations

Based on the recently constructed n-body potential, both molecular dynamics and Monte Carlo simulations revealed that the Al-Zr amorphous alloy or metallic glass can be obtained within the composition range of 24–66 at. % Zr. The revealed composition range could be considered the intrinsic glass-forming range and it quantitatively indicates the glass-forming ability of the Al-Zr system. The underlying physics of the finding is that, within the composition range, the amorphous alloys are energetically favored to form. In addition, it is proposed that the energy difference between a solid solution and the amorphous phase could serve as the driving force of the crystalline to amorphous transition and the driving force should be sufficiently large for amorphization to take place. The minimum driving forces for fcc Al-based and hcp Zr-based Al-Zr solid solutions to amorphize are calculated to be about −0.05 and −0.03 eV/atom, respectively, whereas the maximum driving force is found to be −0.23 eV/atom at the a...

Long-range empirical potential model: extension to hexagonal close-packed metals

An n-body potential is developed and satisfactorily applied to hcp metals, Co, Hf, Mg, Re, Ti, and Zr, in the form of long-range empirical potential. The potential can well reproduce the lattice constants, c/a ratios, cohesive energies, and the bulk modulus for their stable structures (hcp) and metastable structures (bcc or fcc). Meanwhile, the potential can correctly predict the order of structural stability and distinguish the energy differences between their stable hcp structure and other structures. The energies and forces derived by the potential can smoothly go to zero at cutoff radius, thus completely avoiding the unphysical behaviors in the simulations. The developed potential is applied to study the vacancy, surface fault, stacking fault and self-interstitial atom in the hcp metals. The calculated formation energies of vacancy and divacancy and activation energies of self-diffusion by vacancies are in good agreement with the values in experiments and in other works. The calculated surface energies and stacking fault energies are also consistent with the experimental data and those obtained in other theoretical works. The calculated formation energies generally agree with the results in other works, although the stable configurations of self-interstitial atoms predicted in this work somewhat contrast with those predicted by other methods. The proposed potential is shown to be relevant for describing the interaction of bcc, fcc and hcp metal systems, bringing great convenience for researchers in constructing potentials for metal systems constituted by any combination of bcc, fcc and hcp metals.

Ferroelectric polarization and domain walls in orthorhombic (K1−xNax)NbO3 lead-free ferroelectric ceramics

Combining aberration-corrected high-resolution transmission electron microscopy with first-principles calculations, we have investigated the ferroelectric polarization and the atomic structures of 60°/120° domain walls in orthorhombic (K0.46Na0.54)NbO3 lead-free ferroelectric ceramics. The projections of cation-oxygen dipoles across the 60°/120° domain walls were determined using the recently developed negative spherical-aberration imaging technique. The measured ferroelectric distortion matched well with that obtained from first-principles calculations. The width across the wall was measured to be ∼1.1 nm.

Interatomic potential to calculate the driving force, optimized composition, and atomic structure of the Cu-Hf-Al metallic glasses

An interatomic potential is proposed for the Cu-Hf-Al system and applied in molecular dynamics/statics simulations. Simulations predict a hexagonal composition region for the Cu-Hf-Al metallic glass formation. Kinetically, a local maximum driving force, defined by energy difference between the solid solution and disordered state, is predicted to be at Cu48Hf41Al11, close to the experimentally measured optimized composition. Moreover, Voronoi tessellation analysis shows that though the icosahedron and icosidihedron are dominant configurations, the fractions of both icosihexahedron and icosioctahedron decrease with increasing Al content, correlating closely with the atomic radii and the heat of mixing of the component metals.

Magnetic properties of some metastable Co–Ru alloys studied by ion beam mixing and ab initio calculation

Magnetic fcc-Co75Ru25 (L12) phase and nonmagnetic fcc-Co25Ru75 (L12) phase are obtained by ion beam mixing in the respective Co–Ru multilayered films. Interestingly, in the Co50Ru50 multilayered films, a magnetic dual-fcc phase, identified to be a mixture consisting of a magnetic fcc-Co75Ru25 and a nonmagnetic fcc-Co25Ru75 phases, is observed at an irradiation dose of 3×1015 Xe+/cm2 and, upon further irradiation to a dose of 7×1015 Xe+/cm2, transformed into a nonmagnetic single-fcc phase with an alloy composition back to be Co50Ru50.

Ab initio calculations to determine the formation enthalpy of Cu3Au phases

Ab initio calculations ascertain that the difference of formation enthalpy between order and disordered Cu3Au phases is at least 0.044 eV per atom. The calculations also suggest that the tetrahedral clustering configuration and its spatial distribution have drastic effect on the formation enthalpy of the disordered structure and that segregation of the Au atoms significantly lowers the formation enthalpy of the disordered phases.

Microchemical inhomogeneity to characterize atomic configurations in the heating and quenching of a CuHf2 alloy

Based on the constructed Cu-Hf interatomic potential, Monte Carlo simulations were conducted to reveal the atomic configurations in heating and quenching of a CuHf(2) alloy through scrutinizing the evolution of microchemical inhomogeneity. Simulations show that the CuHf(2) crystalline structure becomes more homogeneous during heating but an obvious drop in microchemical inhomogeneity appears when reaching its melting point. During the quenching process, the CuHf(2) melt becomes increasingly inhomogeneous and shows a change in the slope in the microchemical inhomogeneity around glass transition temperature. Simulation results were evidenced by the atomic packing analysis through the Voronoi tessellation method. The implications of our study suggest that the glass transition could be visualized as a process involving increase of microchemical inhomogeneity from the liquid to glassy state.