Phonon dispersion curves of bcc Ba

Nonlocal Damage Mechanics for Quantification of Health for Piezoelectric Sensor

In this paper, a novel method to quantify the incubation of damage on piezoelectric crystal is presented. An intrinsic length scale parameter obtained from nonlocal field theory is used as a novel measure for quantification of damage precursor. Features such as amplitude decay, attenuation, frequency shifts and higher harmonics of guided waves are commonly-used damage features. Quantification of the precursors to damage by considering the mentioned features in a single framework is a difficult proposition. Therefore, a nonlocal field theory is formulated and a nonlocal damage index is proposed. The underlying idea of the paper is that inception of the damage at the micro scale manifests the evolution of damage at the macro scale. In this paper, we proposed a nonlocal field theory, which can efficiently quantify the inception of damage on piezoelectric crystals. The strength of the method is demonstrated by employing the surface acoustic waves (SAWs) and longitudinal bulk waves in Lithium Niobate (LiNbO3) single crystal. A control damage was introduced and its manifestation was expressed using the intrinsic dominant length scale. The SAWs were excited and detected using interdigital transducers (IDT) for healthy and damage state. The acoustic imaging of microscale damage in piezoelectric crystal was conducted using scanning acoustic microscopy (SAM). The intrinsic damage state was then quantified by overlaying changes in time of flight (TOF) and frequency shift on the angular dispersion relationship.

Orbital free ab initio simulations of liquid alkaline earth metals: from pseudopotential construction to structural and dynamic properties

We have performed a comprehensive study of the properties of liquid Be, Ca and Ba, through the use of orbital free ab initio simulations. To this end we have developed a force-matching method to construct the necessary local pseudopotentials from standard ab initio calculations. The structural magnitudes are analyzed, including the average and local structures and the dynamic properties are studied. We find several common features, like an asymmetric second peak in the structure factor, a large amount of local structures with five-fold symmetry, a quasi-universal behaviour of the single-particle dynamic properties and a large degree of positive dispersion in the propagation of collective density fluctuations, whose damping is dictated by slow thermal relaxations and fast viscoelastic ones. Some peculiarities in the dynamic properties are however observed, like a very high sound velocity and a large violation of the Stokes-Einstein relation for Be, or an extremely high positive dispersion and a large slope in the dispersion relation of shear waves at the onset of the wavevector region where they are supported for Ba.

Melting of elements

The aim is to identify and evaluate the most important factors that determine the melting temperature T m of the elements. Experimental data are analysed in terms of statistical mechanical theories of the energy and entropy of crystal and liquid phases. The elements are divided into two groups, according to their value of the entropy of fusion at constant density, ∆ S : the normal elements have ∆ S close to 0.79 k per atom,and the anomalous elements have ∆ S much larger. For the normal elements, a melting rule is constructed in term s of two factors, the lattice dynamics characteristic temperature for entropy, and the liquid correlation entropy. For the anomalous elements, the large ∆ S is attributed to a change in electronic groundstate upon melting, and the melting rule depends also on the corresponding electronic energy change. The melting rules give T m to an accuracy of around 20%, and at this level, anharmonicity and crystal symmetry effects are negligible. Two conclusions regarding the relative motion of ions in crystal and liquid phases are: ( a ) liquid higher order correlations lower T m because they lead to a more ordered liquid, and ( b ) the loss of long-range order in melting corresponds to ∆ S ≈ 0.79 k per atom for normal and anomalous elements alike.