A Novel Method to Significantly Improve the Mechanical Properties of n-Type Bi(1−x)Sbx Thermoelectrics Due to Plastic Deformation

A unique method was developed to significantly improve the strength of Bi(1−x)Sbx single crystals, the most effective thermoelectric (TE) materials in the temperature range from 100 to 200 K due to their plastic deformation by extrusion. After plastic deformation at room temperature under all-round hydrostatic compression in a liquid medium, n-type Bi–Sb polycrystalline solid solutions show a significant increase in mechanical strength compared to Bi–Sb single crystals in the temperature range from 300 to 80 K. The significantly higher strength of extruded alloys in comparison with Bi–Sb single crystals is associated with the development of numerous grains with a high boundary surface as well as structural defects, such as dislocations, that accumulate at grain boundaries. Significant stability of the structure of extruded samples is achieved due to the uniformity of crystal plastic deformation under all-round hydrostatic compression and the formation of the polycrystalline structure consisting of grains with the orientation of the main crystallographic directions close to the original single crystal. The strengthening of Bi–Sb single crystals after plastic deformation allows for the first time to create workable TE devices that cannot be created on the basis of single crystals that have excellent TE properties, but low strength.

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.

High-pressure reversibility in a plastically flexible coordination polymer crystal

Single crystals which exhibit mechanical flexibility are promising materials for advanced technological applications. Before such materials can be used, a detailed understanding of the mechanisms of bending is needed. Using single crystal X-ray diffraction and microfocus Raman spectroscopy, we study in atomic detail the high-pressure response of the plastically flexible coordination polymer [Zn(μ-Cl)2(3,5-dichloropyridine)2]n (1). Contradictory to three-point bending, quasi-hydrostatic compression of (1) is completely reversible, even following compression to over 9 GPa. A structural phase transition is observed at ca. 5 GPa. DFT calculations show this transition to result from the pressure-induced softening of low-frequency vibrations. This phase transition is not observed during three-point-bending. Microfocus synchrotron X-ray diffraction revealed that bending yields significant mosaicity, as opposed to compression. Hence, our studies indicate of overall disparate mechanical responses of bulk flexibility and quasi-hydrostatic compression within the same crystal lattice. We suspect this to be a general feature of plastically bendable materials.

Influence of Temperature on Void Collapse in Single Crystal Nickel under Hydrostatic Compression

Employing atomistic simulations, we investigated the void collapse mechanisms in single crystal Ni during hydrostatic compression and explored how the atomistic mechanisms of void collapse are influenced by temperature. Our results suggest that the emission and associated mutual interactions of dislocation loops around the void is the primary mechanism of void collapse, irrespective of the temperature. The rate of void collapse is almost insensitive to the temperature, and the process is not thermally activated until a high temperature (∼1200–1500 K) is reached. Our simulations reveal that, at elevated temperatures, dislocation motion is assisted by vacancy diffusion and consequently the void is observed to collapse continuously without showing appreciable strain hardening around it. In contrast, at low and ambient temperatures (1 and 300 K), void collapse is delayed after an initial stage of closure due to significant strain hardening around the void. Furthermore, we observe that the dislocation network produced during void collapse remains the sample even after complete void collapse, as was observed in a recent experiment of nickel-base superalloy after hot isostatic pressing.

Analytical study on stability of microspheres in syntactic foams structure under hydrostatic compression

This paper analyses prediction methods for failure pressure of microspheres in the structure of syntactic foams, discussing several micromechanical models. The solutions were obtained as per linear elasticity theory and finite-element method. It was investigated how geometric and physical & mechanical characteristic of syntactic foams depend on its components. Pressure failure is much lower if the boundary of the medium is close to the inclusions. This paper suggests an analytical model that yielded conservative estimate of critical pressure for microspheres.