Promising Bialkali Bismuthides Cs(Na, K)2Bi for High-Performance Nanoscale Electromechanical Devices: Prediction of Mechanical and Anisotropic Elastic Properties under Hydrostatic Tension and Compression and Tunable Auxetic Properties

Using first-principles calculations, we predict highly stable cubic bialkali bismuthides Cs(Na, K)2Bi with several technologically important mechanical and anisotropic elastic properties. We investigate the mechanical and anisotropic elastic properties under hydrostatic tension and compression. At zero pressure, CsK2Bi is characterized by elastic anisotropy with maximum and minimum stiffness along the directions of [111] and [100], respectively. Unlike CsK2Bi, CsNa2Bi exhibits almost isotropic elastic behavior at zero pressure. We found that hydrostatic tension and compression change the isotropic and anisotropic mechanical responses of these compounds. Moreover, the auxetic nature of the CsK2Bi compound is tunable under pressure. This compound transforms into a material with a positive Poisson’s ratio under hydrostatic compression, while it holds a large negative Poisson’s ratio of about −0.45 along the [111] direction under hydrostatic tension. An auxetic nature is not observed in CsNa2Bi, and Poisson’s ratio shows completely isotropic behavior under hydrostatic compression. A directional elastic wave velocity analysis shows that hydrostatic pressure effectively changes the propagation pattern of the elastic waves of both compounds and switches the directions of propagation. Cohesive energy, phonon dispersion, and Born–Huang conditions show that these compounds are thermodynamically, mechanically, and dynamically stable, confirming the practical feasibility of their synthesis. The identified mechanisms for controlling the auxetic and anisotropic elastic behavior of these compounds offer a vital feature for designing and developing high-performance nanoscale electromechanical devices.

A dislocation-based solution for stress introduced by arbitrary volume expansion in cylinders

A cylindrical structure undergoing volume expansion and contraction is common in engineering practice. For example, the charging (discharging) process of axisymmetrical batteries will give rise to volume expansion (shrinkage). The nitriding process of axles for better fatigue performance also introduces volume expansion. Here, by taking the equivalence of volume expansion (or shrinkage) as continuous insertion (or distraction) of infinitesimal dislocations, we supply a framework to solve the stress field of a cylinder with arbitrary insertion (distraction) profile of materials along the radial direction. Under the assumptions that the volume expansion profile along the axis of a cylinder is uniform and the deformation is small so that the current configuration is regarded as the original, we supply analytical solutions of stress fields to several typical volume expansion or shrinkage profiles. Our analysis shows that different volume variation gives rise to either high tensile stress in the surface or hydrostatic tension in the core, and supplies distinct failure mechanisms in cylindrical batteries.

Cavitation in inhomogeneous soft solids

When a large hydrostatic tension is applied to an inhomogeneous soft solid, cavitation instability can be induced in a way that is different from the homogeneous case.

Property control by elastic strain engineering: Application to graphene

Various carbon nanostructures, including graphene, are of great interest nowadays for many applications. It has been shown that graphene has unique physical and mechanical properties and its properties can be controlled by the applied strain. The objective of the present paper is to describe several physical properties of graphene that can be controlled by means of elastic strain engineering. The space of in-plane elastic strain components is divided into regions with different structural configurations and physical properties of graphene. It is shown that a gap in the phonon density of states is observed when graphene is strained close to the appearance of ripples. Sound velocities of unstrained graphene do not depend on the propagation direction but application of strain, apart hydrostatic tension, makes graphene elastically anisotropic. The orientation, amplitude and wavelength of unidirectional ripples in graphene can be controlled by a change in the components of the applied strain.

Cavitation to fracture transition in a soft solid

When large hydrostatic tension is applied onto a soft solid, crack extension can be induced during the cavitating process.

On Cavitation in Rubberlike Materials

Microscopic voids can irreversibly grow into the macroscopic ones under hydrostatic tension. To explain this phenomenon, it was suggested in the literature to use the asymptotic value of the hydrostatic tension in the plateau yieldlike region on the stress–stretch curve obtained for the neo-Hookean model. Such an explanation has two limitations: (a) it relies on analysis of only one material model and (b) the hyperelasticity theory is used for the explanation of the failure phenomenon. In view of the mentioned limitations, the objective of the present note is twofold. First, we simulate the cavity expansion in rubber by using various experimentally calibrated hyperelastic models in order to check whether the stress–stretch curves have the plateau yieldlike regions independently of the constitutive law. Second, we repeat simulations via these same models enhanced with a failure description. We find (and that was not reported in the literature) that the process of cavity expansion simulated via hyperelastic constitutive models exhibiting stiffening, due to unfolding of long molecules, is completely stable and there are no plateau yieldlike regions on the stress–stretch curves associated with cavitation. In addition, we find that the instability in the form of yielding observed in experiments does appear in all simulations when the constitutive laws incorporate failure description with energy limiters.