Effect of ultrasound on molecular mobility in certain crystalline compounds

It is shown that the normal melting point ranges of ammonium alum dodecahydrate, and its potassium analogue, can be changed by ultrasound; under identical conditions the melting characteristics of hexamethylbenzene remain unchanged. NMR investigations suggest that the changes to the melting points of the alums results from acoustically induced motion of the water of crystallization in the crystal lattices.     

Effect of ultrasound on the characteristics of superplastic deformation

The deformation of aluminum–lithium alloy 1420 is investigated in the region of temperatures of its superplasticity T = 320–395°C under the simultaneous action of stretching load and axial ultrasonic vibrations. The estimates of the activation parameters of deformation show that the activation energy of deformation during stretching with ultrasonic vibrations and without them are close both at the stage of hardening and at the stage of softening. It is concluded that the ultrasonic vibrations facilitate intragranular deformation at the hardening stage and promote an increase in its contribution to the total deformation without changing the deformation mechanisms.     

Mathematical simulation of the plastic deformation of crystalline materials with a nanodispersed hardening phase

The effect of the scale characteristics of a nanodispersed hardening phase on the strain hardening and evolution of the defect subsystem in materials with an fcc matrix has been studied with the use of a mathematical model including differential balance equations for the components of the strain defective subsystem. The two-stage character of evolution of the strain defect subsystem and the possibility of plastic shearing in dispersion-hardened materials due to either the motion of individual dislocations or the generation of numerous dislocations in the shear band were taken into consideration. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 25–32, November, 2007.     

The influence of the shape of load stress pulses on the course of plastic deformation of crystalline materials

The present paper deals with the derivation of the relationship between the course of macroscopic deformation and the dynamic properties of dislocation in loading material with stress pulses of general shape, and duration in tens of microseconds. On the basis of the obtained relationships, the correctness of the definition of the yield point under these conditions of loading, suggested previously (Czech. J. Phys.B 20 (1970), 776) on the basis of the analysis of the phenomenon of delayed deformation at elevated loading rates, is demonstrated.In the calculation we proceed, on the one hand, from the relation for the dislocation rate proposed by J. J. Gilman= exp(-D/) and, on the other hand, the damping mechanism of a viscous type, where=b/B.In both cases the correctness of the suggested definition is shown. The results of the theoretical analysis agree with the hitherto known experimental results.     

Diffusion in crystalline materials

Recently nuclear scattering of synchrotron radiation proved to be a powerful new method to study the elementary diffusion jump in crystalline solids. The scattered radiation decays faster when atoms move on the time scale of the excited-state lifetime of a Mössbauer isotope because of a loss of coherence. The acceleration of the decay rate differs for different crystal orientations relative to the beam providing information not only about the rates but also about the directions of the elementary jumps. We discuss first applications of the method.

     

Effect of interface structure on deformation behavior of crystalline Cu/amorphous CuZr sandwich structures

The effect of interface types (namely, sharp interface and graded interface) and its thickness on the deformation behavior of crystalline/amorphous/crystalline sandwich structures (CACSSs) under tensile loading are studied using molecular dynamics simulation. Compared with the CACSSs with sharp interface, the CACSSs with gradient interface consistently exhibit good plasticity when the interface thickness is larger than 6 nm, due to the coupling effects among crystalline layer, amorphous layer and crystalline–amorphous interface. With the increase of interface thickness, the plastic deformation mechanism of CACSSs with gradient interface changes from the local plastic deformation in amorphous layer to the homogeneous plastic deformation.     

Structural factors in deformation of crystalline polymers

The multiple α absorption of bulk-crystallized polyethylene (PE) was separated into the α₁ and the α₂ absorptions on the assumption that this α₂ absorption is associated with shear deformation of lamellar crystals, i.e., has the same characteristics as in single crystal mats. The separated α₁ mechanism is related to the molecular motions in the intermosaic block region. The α₁ process is very sensitive to static and dynamic deformation, whereas the α₂ process is not affected. Plastic deformation of bulk crystallized PE was analyzed in terms of true stress and true strain. The temperature dependence of the critical yield stress below 60°C showed the same magnitude of activation energy (26 kcal/mole) as that of α₁. The leading mechanism of deformation at lower temperatures is the breakdown of lamellar crystals into mosaic blocks. Compressive deformation of solid-state extrudates along the molecular axis, giving rise to kink bands, was analyzed with X-ray goniometry and in terms of the strain-rate dependence of the yield stress. The deformation of the crystals in the kink bands occurred by superposition of intercrystallite slip (α₁) and uniform shear deformation (α₂). It was concluded that consideration of intermosaic slip mechanisms (α₁), in addition to the shear deformation (α₂) and the interlamellar deformation (β), is effective and helpful to understand the deformation process of crystalline polymers.     

Radiation stability of ionic crystalline materials

An analysis of the processes determining radiation stability of ionic compounds is performed. It is shown that the resistance of ionic crystalline compounds to the action of radiation is determined primarily by the degree of ionization of the bond, and the values of defect formation energy, effective dielectric constant, and primary defect mobility.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 85–90, August, 1977.     

Effect of ruptured cavitated bubble cluster on the extent of the cell deformation by ultrasound

In this paper, the bubble-cell model is presented. The effects of the spacing between the bubble population and the cell, the radius of the bubble and the bubble medium on the degree of cell deformation were investigated by solving the Helmholtz equation and the equilibrium of motion equation using COMSOL Multiphysis@ software. The ultrasonic transducer is applied in a round bottom flask with the bubble-cell model on the side of the ultrasonic transducer. When the distance between the bubble cluster and the cell gradually increases, the extent of deformation of the cell is reflected as first increasing and then decreasing, reaching the maximum deformation at D = 2. When the radius of the bubble is changed, there is a “constant frequency” at low frequency ultrasound in any distance case, at which the cell deformation will be violent. However, when the bubble medium is changed, there is no significant change in the degree of deformation of the cells. In other words, changes in the structure of the bubble-cell model affect the degree of cell deformation, but without structural changes, the degree of cell deformation changes very little.     

Influence of tensile deformation on the crystalline organization of ethylene copolymers

The influence of tensile deformation on the crystalline properties of ethylene copolymers (ethylene-vinyl acetate [EVA] copolymers) was investigated by differential scanning calorimetry (DSC). The consequence of drawing on the mobility of the amorphous phase also was investigated through the study of the glass transition temperature. The results indicate that more disorganized crystals, melting at a lower temperature, are present after the tensile deformation, reducing the mobility of the amorphous chains, as shown by an increase of the glass transition temperature. For the lowest molecular weight copolymer. less crystalline changes are observed after the tensile test, probably due to the fact that no stiffening appears during the drawing.     

Polarons in crystalline and non-crystalline materials

The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO 2 and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.     

Polarons in crystalline and non-crystalline materials

The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO₂ and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.     

单元素二维原子晶体材料研究进展

非碳元素构成的二维单质原子晶体材料,具有与石墨烯相似的单层结构和物理性质,对它们的研究有望发掘一些崭新的性能及相关的应用,是当前凝聚态物理及新材料等相关领域的重要研究方向之一。文章详细介绍了近几年发展起来的单一元素所形成的二维原子晶体材料,分别对其结构、物性、应用前景等方面的研究进展进行了综述介绍。     

Transition of deformation mechanisms in nanotwinned single crystalline SiC

ABSTRACT

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.     

Electronic properties of SnTe-class topological crystalline insulator materials

The rise of topological insulators in recent years has broken new ground both in the conceptual cognition of condensed matter physics and the promising revolution of the electronic devices.It also stimulates the explorations of more topological states of matter.Topological crystalline insulator is a new topological phase,which combines the electronic topology and crystal symmetry together.In this article,we review the recent progress in the studies of SnTe-class topological crystalline insulator materials.Starting from the topological identifications in the aspects of the bulk topology,surface states calculations,and experimental observations,we present the electronic properties of topological crystalline insulators under various perturbations,including native defect,chemical doping,strain,and thickness-dependent confinement effects,and then discuss their unique quantum transport properties,such as valley-selective filtering and helicity-resolved functionalities for Dirac fermions.The rich properties and high tunability make SnTe-class materials promising candidates for novel quantum devices.     

Femtosecond mechanisms of electronic excitations in crystalline materials

The lattice dynamics of crystals is investigated in the course of high-power electronic excitation. It is revealed that, at W _e < W _eo , atoms and ions are displaced from their regular sites for 100–300 fs. Subsequent relaxation of the crystal lattice in response to a strong local electric field induced by the collisional displacement of ions occurs for 10–50 ns in an oscillatory manner with a period of 0.5–1.5 ps.     

Mesomechanics of plastic deformation and fracture of partially crystalline polyethylene

From the standpoint of physical mesomechanics, we have investigated plastic deformation mechanisms and the mechanical properties of partially crystalline polyethylene. We show that from the very beginning, plastic deformation occurs at the mesoscopic level. Fracture is preceded by fragmentation of the material. The observed stages of the process of plastic deformation of polyethylene are qualitatively similar to the stages of this process for metallic materials. The effect of electron bombardment on the mechanical properties of polyethylene is explained by the size reduction in the mesoscopic substructure formed on plastic deformation. Tomsk Polytechnical University. Zhilin University, People’s Republic of China. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 48–53, January, 1997.     

Structural conditions of deformation and fracture of oriented crystalline polymers

Deformation of supermolecular structure elements of oriented crystalline polymers and nucleation of initial submicroscopic cracks induced by stress have been studied by the small-angle x-ray scattering technique. It is shown that the intrafibrillar amorphous interlayers have low strength and high deformability. The rupture of the weakest amorphous interlayers leads to nucleation of initial submicrocracks. The influence of submicrocracks on deformation around such cracks is revealed. The micromechanics of deformation and fracture of polymers is discussed.     

Air-coupled ultrasound inspection of various materials

Conventional ultrasound inspection, a standard non-destructive testing method, uses a coupling medium (e.g. water) because of impedance mismatch. This liquid contact is a drawback because it prevents inspection of many materials. There is a need, then, for air-coupled ultrasound testing, which is now feasible because of low impedance focused narrow band transducers and sensitive electronics, both of which improve the signal-to-noise ratio. We present results obtained on fibre-reinforced plastics, water sensitive materials (e.g. reinforced ceramics), and "shape adaptive" structures to reveal delaminations, impacts, and growth of internal defects. Actuators embedded in "adaptive" structures are used as transmitters while the receiver records the signals. Thus it is possible to image defect areas and non-linear behaviour of potential defects.