Pressure-induced structural phase transition and elastic properties of rare earth Pr chalcogenides and pnictides

Pressure-induced structural aspects and elastic properties of NaCl-type (B1) to CsCl-type (B2) structure in praseodymium chalcogenides and pnictides are presented. Ground-state properties are numerically computed by considering long-range Coulomb interactions, Hafemeister and Flygare type short-range overlap repulsion, and van der Waals interaction in the interionic potential. From the elastic constants, Poisson's ratio ν, the ratio R_G/B of G (shear modulus) over B (bulk modulus), anisotropy parameter, shear and Young's moduli, Lamé's constant, Kleinman parameter, elastic wave velocity and thermodynamical property such as Debye temperature are calculated. Poisson's ratio ν and the ratio R_G/B indicate that PrX and PrY are brittle in B1 phase and ductile in B2 phase. To our knowledge, this is the first quantitative theoretical prediction of the ductile (brittle) nature of praseodymium chalcogenides and pnictides and still awaits experimental confirmation.     

Giant Auxetic Behaviour in Engineered Graphene

Graphene monolayers engineered to adopt a corrugated conformation are found to exhibit very pronounced lateral expansion characteristics upon uniaxial stretching in specific directions, i.e. giant negative Poisson's ratio (auxeticity). Such anomalous properties are manifested in‐plane and may be controlled through the shape and amplitude of the wave‐like pattern of the corrugation, which in the case of graphene may be controlled through the introduction of patterned ‘defects’. This confirms that auxeticity via corrugation can be achieved even at the nanoscale, as demonstrated here for graphene with patterned ‘defects’, suggesting that this mechanism should be operational at different scales of structures, thus providing a new blueprint for the design or manufacture of auxetic materials and metamaterials which can have tailor‐made giant negative Poisson's ratio properties.     

Elastic band structures of two-dimensional solid phononic crystal with negative Poisson's ratios

In this paper, the elastic band structures of two-dimensional solid phononic crystals (PCs) with both negative and positive Poisson's ratios are investigated based on the finite difference domain method. Systems with different combinations of mass density ratio and shear modulus ratio, filling fractions and lattices are considered. The numerical results show that for the PCs with both large mass density ratio and shear modulus ratio, the first bandgap becomes narrower with its upper edge becoming lower as Poisson's ratio of the scatterers decreases from −0.1 to −0.9. Generally, introducing the material with a negative Poisson's ratio for scatterers will make this bandgap lower and narrower. For the PCs with large mass density ratio and small shear modulus ratio, the first bandgap becomes wider with Poisson's ratio of the scatterers decreasing and that of the host increasing. It is easy to obtain a wide low-frequency bandgap by embedding scatterers with a negative Poisson's ratio into the host with a positive Poisson's ratio. The PCs with large filling fractions are more sensitive to the variations of Poisson's ratios. Use of negative Poisson's ratio provides us a way of tuning bandgaps.     

Temperature Fall and Rise of Plastics During Uniaxial Stretching

Dumbbell specimens of common plastics—polyethylene, polypropylene, and polyamide—were uniaxially stretched and the surface temperature was measured by thermography. The surface temperature decreased at small strain below the yield point and then increased at larger strain. The endothermic deformation in the elastic regime was unexpected. It might be a characteristic of polymer material, which is possessed of free volume. The endotherm is interpreted by the volume increment with stretching as represented by Poisson's ratio. The temperature rise at larger strain is not surprising for plastic deformation over the yield point. The exotherm is interpreted in terms of the melting of crystallites and re-crystallization induced by large deformation.     

Partial auxeticity induced by nanoslits in the Yukawa crystal

Monte Carlo simulations in the isobaric–isothermal ensemble with variable shape of the periodic box were used to investigate Poisson's ratios of the hard‐core repulsive Yukawa crystals with periodically distributed nanoslits. Each nanoslit, oriented perpendicularly to the crystallographic direction [010], was filled by a monolayer of hard spheres. It is shown that presence of the nanoslits leads to negative Poisson's ratio, as low as –0.57(2), in the [100][001]‐direction. This direction is not auxetic in the pure Yukawa crystal, i.e. shows positive Poisson's ratio, equal to 0.43(1).     

Coordinate transformation and construction of finite element mesh in a diverted tokamak geometry

A coordinate transformation technique between straight magnetic field line coordinate system (Ψ, θ) and Cartesian coordinate system (R, Z) is presented employing a Solov'ev solution of the Grad-Shafranov equation. Employing the equilibrium solution, the poloidal magnetic flux Ψ(R, Z) of a diverted tokamak, magnetic field line equation is solved computationally to find curves of constant poloidal angle θ, which provides us with explicit relations R = R(Ψ, θ) and Z = Z(Ψ, θ). Correspondingly, conversion from one coordinate to the other along particle trajectories in the vicinity of separatrix is demonstrated. Based on the magnetic structure, a finite element mesh is generated in a diverted tokamak geometry to solve Poisson's equation.     

Three-dimensional static Green's function for a half space over mixed PEC and dielectric wedges

A pseudo-analytical solution technique is proposed to determine the three-dimensional static Green's function for a half space over mixed perfect electric conductor-dielectric wedges. The governing Poisson's equation is solved, using the Kontorovich–Lebedev and Fourier transforms. The solution, expressed in terms of image contributions, consists of an excitation point source, a set of point images in physical space, and two line images located in complex space for each region of the problem geometry. The validity of the proposed technique is confirmed by comparing our results with the existing analytical solutions and those obtained numerically using the using a finite integration code.     

Negative Linear Compressibility and Auxeticity in Boron Arsenate

A novel mechanism is identified leading to negative linear compressibility (NLC) in boron arsenate which is shown to arise from deformations in the framework tetrahedra rather than more conspicuous tetrahedral rotations. Instead, such rotations, which manifest as “rotating squares” when viewed down the c direction, are found to result in negative Poisson's ratio (NPR) in the (001) plane, which in turn augments the compressibility, a phenomenon that should be applicable to other auxetic materials. It is hypothesized that the generic NLC and NPR mechanisms identified here, as well as the augmented compressibility due to auxeticity should also feature in materials and metamaterials with similar characteristics.     

具有负泊松比效应的2D碳纳米结构设计及其性能的理论计算研究

本文利用第一性原理PBE密度泛函理论计算的方法设计了一种由炔基链、吡啶环及少量氢原子组成的具有内凹六边形结构单元的新型理想二维碳纳米结构,并对其平面内负泊松比效应等力学性能和光学性能与电子结构进行了预测.计算表明,该2D材料具有较好的结构稳定性和特殊的力学性能.当将该2D结构在面内(bc面)沿c方向压缩时,其在b方向收缩;当沿c方向拉伸时,其在b方向伸长,即该2D结构同样具有期望的负泊松比效应.材料的泊松比为-3.26;将该2D结构沿b方向拉伸时,c方向将随之伸长;沿b方向压缩时,c方向将随之收缩.沿b方向拉伸或压缩时,泊松比约为-1.951.即该2D材料在面内具有非常显著的负泊松比效应.此外,该2D材料表现出半导体材料的电子结构特征和良好的光反射和折射性能.希望本工作能为具有本征负泊松比效应和优良电子与光学功能的理想二维碳纳米材料的开发提供一种理想的结构设计策略.     

Ultrasonic,FTIR and thermal investigations of SiO2–Na2O–CaO–P2O5 glasses doped with CeO2

The longitudinal and shear ultrasonic wave velocities for different compositions of SiO₂–Na₂O–CaO–P₂O₅ glasses were measured at room temperature (305 K) using a pulse-echo method at a frequency of 4 MHz. The elastic moduli, Poisson's ratio, microhardness, Debye temperature and other ultrasonic parameters were obtained from experimental data and analyzed using bond compression theory. By calculating the number of network bonds per unit volume, the average stretching constant, and the average ring size, information about the structure of the glass can be deduced. Structural changes after doping with CeO₂ were investigated by FTIR spectroscopy, and by measurements of the thermal expansion coefficient, glass transition and softening temperature to throw more light on the characterization of these glasses.     

Evaluation of the mechanical properties of PMC interface using slice compression test — Analysis of transfer mechanism of interfacial shear stress

Properties of the fiber/matrix interface in SiO₂/epoxy and SiC/epoxy composite are investigated using the slice compression test (SCT) for the single fiber, where the specimen is loaded and unloaded between a plate which has different mechanical properties. It is found that the interfacial debonding proceeds from the polished surface at a soft plate side and that the fiber protrusion occurs after unloading. The fiber-protrusion length is directly measured at each applied stress level using a scanning electron microscope. Interfacial shear-sliding stress is obtained based on the constant shear-sliding stress analysis employing the obtained protrusion length. It is demonstrated that the value of interfacial shear-sliding stress shows good agreement with that obtained from another technique, the push-out test, on the same system. The relation between the fiber-protrusion length and applied stress is proportional to a certain extent. From this result, it is analytically pointed out that the applied stress has a limiting value in this SCT because of Poisson's effect. Also, two interfacial debonding criteria, which are determined analytically for the PMC, are discussed.     

Ab initio study of anisotropic mechanical and electronic properties of strained carbon-nitride nanosheet with interlayer bonding

Due to the noticeable structural similarity and being neighborhood in periodic table of group-IV and-V elemental monolayers, whether the combination of group-IV and-V elements could have stable nanosheet structures with optimistic properties has attracted great research interest. In this work, we performed first-principles simulations to investigate the elastic, vibrational and electronic properties of the carbon nitride (CN) nanosheet in the puckered honeycomb structure with covalent interlayer bonding. It has been demonstrated that the structural stability of CN nanosheet is essentially maintained by the strong interlayer σ bonding between adjacent carbon atoms in the opposite atomic layers. A negative Poisson’s ratio in the out-of-plane direction under biaxial deformation, and the extreme in-plane stiffness of CN nanosheet, only slightly inferior to the monolayer graphene, are revealed. Moreover, the highly anisotropic mechanical and electronic response of CN nanosheet to tensile strain have been explored.     

Comments on a paper by B. Schulz about Bell's inequalities

Schulz claims to have constructed an actively local stochastic theory which violates Bell's inequality. This is false.     

Free electron gas spectrum parameters of noble gases in dc electric field

Free electron gas is present in every gas, whether it is of atomic or molecular structure. Since the Maxwell spectrum type is the consequence of only thermal motion of constitutive gas particles; therefore, the presence of electric field leads to change the spectrum of charged particles due to their directed motion. However, it has been shown that in the case of occurrence only of elastic interactions between electrons and neutral gas particles (a condition that has been met in the case of weakly ionized noble gases of a relatively huge volume) the deviation of the gas spectrum of free electrons in the electric field from the Maxwell type is negligible. In such a case, the gas spectrum of free electrons is either of Maxwell type (if the frequency collision value is energy-independent) or of Druyvesteyn type (if the mean free electron path value is energy-independent). The Maxwell and Druyvesteyn distribution types are very similar. The only noticeable difference is that the tail of the Maxwell distribution decreases with the energy exponent to the first degree of energy, and the tail of Druyvesteyn distribution with the energy exponent to the second degree of energy. The aim of this paper is to determine whether the gas spectrum of free electrons in weakly ionized noble gases at small values of the product pd (pressure and inter-electrode distance) follows either the Maxwell's or Druyvesteyn's type, as well as to determine the dependence of spectrum parameters on the product pd. It has been established that better results are obtained on the assumption that the mean value of collision frequency is energy-independent.