Hardening of fcc hard-sphere crystals by introducing nanochannels: Auxetic aspects.

Tailoring the materials for a given task by modifying their elastic properties is attractive to material scientists. However, recent studies of purely geometrical atomic models with structural modifications showed that designing a particular change to achieve the desired elastic properties is complex. This work concerns the impact of nanochannel inclusions in fcc hard sphere crystal on its elastic properties, especially auxetic ones. The models containing six nanochannel arrays of spheres of another diameter, oriented along the [110]-direction and its symmetric equivalents, have been studied by Monte Carlo simulations in the isothermal-isobaric (NpT) ensemble using the Parinello-Rahman approach. The inclusions have been designed such that they do not affect the cubic symmetry of the crystal. The elastic properties of three different models containing inclusions of various sizes are investigated under four thermodynamic conditions. We find that six nanochannels filled with hard spheres of larger diameter increase system stiffness compared with the fcc crystal without nanoinclusions. The current finding contrasts the recently reported results [J.W. Narojczyk et al. Phys. Status Solidi B 259, 2200464 (2022)0370-197210.1002/pssb.202200464], where the fcc hard sphere crystal with four nanochannels shows reduced stiffness compared to the system without nanoinclusions. Moreover, the six nanochannel models preserve auxetic properties in contrast to the fcc hard sphere crystal with four nanochannel arrays, which loses auxeticity.

Removing Auxetic Properties in f.c.c. Hard Sphere Crystals by Orthogonal Nanochannels with Hard Spheres of Another Diameter.

Negative Poisson's ratio materials (called auxetics) reshape our centuries-long understanding of the elastic properties of materials. Their vast set of potential applications drives us to search for auxetic properties in real systems and to create new materials with those properties. One of the ways to achieve the latter is to modify the elastic properties of existing materials. Studying the impact of inclusions in a crystalline lattice on macroscopic elastic properties is one of such possibilities. This article presents computer studies of elastic properties of f.c.c. hard sphere crystals with structural modifications. The studies were performed with numerical methods, using Monte Carlo simulations. Inclusions take the form of periodic arrays of nanochannels filled by hard spheres of another diameter. The resulting system is made up of two types of particles that differ in size. Two different layouts of mutually orthogonal nanochannels are considered. It is shown that with careful choice of inclusions, not only can one impact elastic properties by eliminating auxetic properties while maintaining the effective cubic symmetry, but also one can control the anisotropy of the cubic system.

Cancellation of Auxetic Properties in F.C.C. Hard Sphere Crystals by Hybrid Layer-Channel Nanoinclusions Filled by Hard Spheres of Another Diameter.

The elastic properties of f.c.c. hard sphere crystals with periodic arrays of nanoinclusions filled by hard spheres of another diameter are the subject of this paper. It has been shown that a simple modification of the model structure is sufficient to cause very significant changes in its elastic properties. The use of inclusions in the form of joined (mutually orthogonal) layers and channels showed that the resulting tetragonal system exhibited a complete lack of auxetic properties when the inclusion spheres reached sufficiently large diameter. Moreover, it was very surprising that this hybrid inclusion, which can completely eliminate auxeticity, was composed of components that, alone, in these conditions, enhanced the auxeticity either slightly (layer) or strongly (channel). The study was performed with computer simulations using the Monte Carlo method in the isothermal-isobaric (NpT) ensemble with a variable box shape.

Wear Resistance Improvement of Cemented Tungsten Carbide Deep-Hole Drills after Ion Implantation.

The paper is dedicated to the lifetime prolongation of the tools designed for deep-hole drilling. Among available methods, an ion implantation process was used to improve the durability of tungsten carbide (WC)-Co guide pads. Nitrogen fluencies of 3 × 1017 cm-2, 4 × 1017 cm-2 and 5 × 1017 cm-2 were applied, and scanning electron microscope (SEM) observations, energy dispersive spectroscopy (EDS) analyses, X-ray photoelectron spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) measurements were performed for both nonimplanted and implanted tools. The durability tests of nonimplanted and the modified tools were performed in industrial conditions. The durability of implanted guide pads was above 2.5 times more than nonimplanted ones in the best case, presumably due to the presence of a carbon-rich layer and extremely hard tungsten nitrides. The achieved effect may be attributed to the dissociation of tungsten carbide phase and to the lubrication effect. The latter was due to the presence of pure carbon layer with a thickness of a few dozen nanometers. Notably, this layer was formed at a temperature of 200 °C, much smaller than in previously reported research, which makes the findings even more valuable from economic and environmental perspectives.

Poisson's Ratio of the f.c.c. Hard Sphere Crystals with Periodically Stacked (001)-Nanolayers of Hard Spheres of Another Diameter.

The results of studies on the influence of periodically stacked nanolayer inclusions, introduced into the face-centered cubic (f.c.c.) hard sphere crystal, on Poisson's ratio of the obtained nanocomposite system are presented. The monolayers are orthogonal to the [ 001 ] -direction. They are formed by hard spheres with diameter different from the spheres forming the matrix of the system. The Monte Carlo computer simulations show that in such a case the symmetry of the system changes from the cubic to tetragonal one. When the diameter of the inclusion spheres increases at certain range, a decrease of the negative Poisson's ratio in the [ 101 ] [ 1 ¯ 01 ] -directions is observed, i.e., the system enhances its partial auxeticity. The dependence of the maximal, average, and negative parts of the minimal Poisson's ratio on the direction of the applied load are shown in a form of surfaces in spherical coordinates, plotted for selected values of nanolayer particle diameters. The most negative value of the Poisson's ratio found among all studied systems was - 0.11 (at pressure p * = 100 , which is about ten times higher than the melting pressure) what is almost twice more negative than in the f.c.c. crystal of identical hard spheres. The observed effect weakens along with the decrease of pressure and becomes hardly noticeable near melting. This study indicates that modifying only the size of the inclusion particles one can change Poisson's ratio of nanocomposites at high pressures.

High Partial Auxeticity Induced by Nanochannels in [111]-Direction in a Simple Model with Yukawa Interactions.

Computer simulations using Monte Carlo method in the isobaric-isothermal ensemble were used to investigate the impact of nanoinclusions in the form of very narrow channels in the [ 111 ] -direction on elastic properties of crystals, whose particles interact via Yukawa potential. The studies were performed for several selected values of Debye screening length ( ( κ σ ) - 1 ). It has been observed that introduction of the nanoinclusions into the system reduces the negative value of Poisson's ratio towards [ 110 ] [ 1 1 ¯ 0 ] , maintaining practically constant values of Poisson's ratio in the directions [ 100 ] and [ 111 ] . These studies also show that concentration of particles forming the nanoinclusions in the system has a significant effect on the value of Poisson's ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction. A strong (more than fourfold) decrease of Poisson's ratio in this direction was observed, from - 0.147 ( 3 ) (system without inclusions) to - 0.614 ( 14 ) (system with nanoinclusions) at κ σ = 10 when the inclusion particles constituted about 10 percent of all particles. The research also showed an increase in the degree of auxeticity in the system with increasing concentration of nanoinclusion particles for all the screening lengths considered.

Lévy-like movement patterns of metastatic cancer cells revealed in microfabricated systems and implicated in vivo.

Metastatic cancer cells differ from their non-metastatic counterparts not only in terms of molecular composition and genetics, but also by the very strategy they employ for locomotion. Here, we analyzed large-scale statistics for cells migrating on linear microtracks to show that metastatic cancer cells follow a qualitatively different movement strategy than their non-invasive counterparts. The trajectories of metastatic cells display clusters of small steps that are interspersed with long "flights". Such movements are characterized by heavy-tailed, truncated power law distributions of persistence times and are consistent with the Lévy walks that are also often employed by animal predators searching for scarce prey or food sources. In contrast, non-metastatic cancerous cells perform simple diffusive movements. These findings are supported by preliminary experiments with cancer cells migrating away from primary tumors in vivo. The use of chemical inhibitors targeting actin-binding proteins allows for "reprogramming" the Lévy walks into either diffusive or ballistic movements.

Pressure-Volume Work for Metastable Liquid and Solid at Zero Pressure.

Unlike with gases, for liquids and solids the pressure of a system can be not only positive, but also negative, or even zero. Upon isobaric heat exchange (heating or cooling) at p = 0, the volume work (p-V) should be zero, assuming the general validity of traditional δW = dWp = -pdV equality. This means that at zero pressure, a special process can be realized; a macroscopic change of volume achieved by isobaric heating/cooling without any work done by the system on its surroundings or by the surroundings on the system. A neologism is proposed for these dWp = 0 (and in general, also for non-trivial δW = 0 and W = 0) processes: "aergiatic" (from Greek: Ἀεργία, "inactivity"). In this way, two phenomenologically similar processes-adiabatic without any heat exchange, and aergiatic without any work-would have matching, but well-distinguishable terms.

Auxeticity of Yukawa Systems with Nanolayers in the (111) Crystallographic Plane.

Elastic properties of model crystalline systems, in which the particles interact via the hard potential (infinite when any particles overlap and zero otherwise) and the hard-core repulsive Yukawa interaction, were determined by Monte Carlo simulations. The influence of structural modifications, in the form of periodic nanolayers being perpendicular to the crystallographic axis [111], on auxetic properties of the crystal was investigated. It has been shown that the hard sphere nanolayers introduced into Yukawa crystals allow one to control the elastic properties of the system. It has been also found that the introduction of the Yukawa monolayers to the hard sphere crystal induces auxeticity in the [ 11 1 ¯ ] [ 112 ] -direction, while maintaining the negative Poisson's ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction, thus expanding the partial auxeticity of the system to an additional important crystallographic direction.

Auxeticity enhancement due to size polydispersity in fcc crystals of hard-core repulsive Yukawa particles.

The Poisson's ratio of the fcc hard-core repulsive Yukawa crystals with size polydispersity was determined by Monte Carlo simulations in the isothermal-isobaric ensemble. The effect of size polydispersity on the auxetic properties of Yukawa crystals has been studied. It has been found that an increase of particle size polydispersity causes a decrease of the Poisson's ratio in auxetic directions as well as appearance of a negative Poisson's ratio in formerly non-auxetic directions. A measure of auxeticity was introduced to estimate quantitatively an enhancement of auxetic properties in polydisperse Yukawa crystals. The proposed measure of auxeticity can be applied to appraise the auxeticity of any studied system.