Interaction of shear waves and propagating cracks

Shear waves generated from an ultrasonic transducer are used to twist dynamically growing crack fronts; the response of crack front to such external perturbations is examined in order to investigate the primary cause of surface roughening in brittle materials. The response of the crack front is found to be linear in amplitude and frequency of the perturbing wave and without persistence. The response to random perturbations, e.g., by localized material inhomogeneities at the free surface, is also discussed.     

Simultaneous three-photon-excited violet upconversion luminescence of Ce3+:Lu2Si2O7 single crystals by femtosecond laser irradiation

We found that Ce3+:Lu2Si2O7 single crystals could be excited at 800 nm by using a femtosecond Ti:sapphire laser. The emission spectra of Ce3+:Lu2Si2O7 crystals were the same for one-photon excitation at 267 nm as for excitation at 800 nm. The emission intensity of Ce3+:Lu2Si2O7 crystals was found to depend on the cube of the laser power at 800 nm, consistent with simultaneous absorption of three 800 nm photons. The measured value of the three-photon absorption cross section is sigma'3=2.44x10(-77) cm6 s2.     

Thermal insulating behavior in crystals at high frequencies

When solving heat-conduction problems with periodic temperature perturbations, the thermal conductivity is assumed to remain frequency independent. We, however, show by using the molecular dynamics technique and the fluctuation-dissipation theorem a decrease of the effective thermal conductivity of 2 orders of magnitude when the excitation frequency approaches or exceeds the reverse of the phonon mean relaxation time. Most of the dielectric and semiconductor materials have to be considered as strongly insulating in those conditions. The comparison between molecular-dynamics simulations performed in Si crystals and theoretical predictions reveals a clear agreement.     

Diffusion of Au atoms and (5×2) phase formation on the Si(111) (7×7) surface

In this article we investigate the complex 1D mesoscopic model of adatom diffusion and the evolution of an ordered phase on the substrate surface. The analysis of the theoretical model is compared with the experimental results of the spreading of Au adatoms on Si(111)-(7×7) surface. The steady state solutions and their stability conditions are determined within the concept of the traveling-wave solution. It is shown that the formation of the ordered phase (5×2) and the difference in the diffusion of Au on (7×7) and on (5×2) structure results in a sharp edge of diffusion front which corresponds to the coverage of a saturated (5×2) phase. This edge moves linearly in time and α can be determined by experiment. The system of model equations enables the damped waves solution or temporary evolution of two steps.     

Periodic cracks and temperature-dependent stress in Mo/Si multilayers on Si substrates

This work examines formation of the peculiar periodic crack patterns observed in the thermally loaded Mo/Si multilayers. Using the substrate curvature measurements, the macroscopic film stress evolution during thermal cycling was investigated. Then high-speed microscopic observation of crack propagation in the annealed Mo/Si multilayers was presented providing experimental evidence of the mechanism underlying formation of the periodic crack patterns. The origin of the peculiar periodic crack patterns was determined. They were observed to form by the slow crack propagation under quasi-static conditions as a result of the interaction between the channelling crack propagation and the advance of the delamination front.     

Atomistic configurations and energetics of crack extension in silicon

We report the first atomistic determination of the minimum energy path for a series of bond ruptures to advance a crack front. Saddle-point configurations on (111) cleavage planes in Si reveal a steplike distribution of atomic displacements, implying a kink mechanism which is known to control dislocation mobility. Manifestations of lattice trapping and directional cleavage anisotropy are further elucidated.     

Restless rays, steady wave fronts

Observations of underwater acoustic fields with vertical line arrays and numerical simulations of long-range sound propagation in an ocean perturbed by internal gravity waves indicate that acoustic wave fronts are much more stable than the rays comprising these wave fronts. This paper provides a theoretical explanation of the phenomenon of wave front stability in a medium with weak sound-speed perturbations. It is shown analytically that at propagation ranges that are large compared to the correlation length of the sound-speed perturbations but smaller than ranges at which ray chaos develops, end points of rays launched from a point source and having a given travel time are scattered primarily along the wave front corresponding to the same travel time in the unperturbed environment. The ratio of root mean square displacements of the ray end points along and across the unperturbed wave front increases with range as the ratio of ray length to correlation length of environmental perturbations. An intuitive physical explanation of the theoretical results is proposed. The relative stability of wave fronts compared to rays is shown to follow from Fermat's principle and dimensional considerations.     

On the neutral stability of a shock wave in real media

The results of the theoretical analysis and computer simulation of the behavior of neutrally stable shock waves with real (van der Waals gas, magnesium) equations of state are presented. An approach is developed in which the region of the neutral stability of a shock wave for each pressure value in front of the wave is determined from the analysis of the equation of state. A simple algorithm is developed to determine the cause of acoustic perturbations (a shock front or an external source) immediately from the flow pattern. In contrast to the predictions of the linear theory, the amplitude of the perturbations of the neutrally stable shock wave decreases with time, although this process is noticeably slower than in the case of an absolutely stable shock wave.     

Measurement of mass attenuation coefficients for holmium doped and undoped layered semiconductors InSe at different energies and the validity of mixture rule for crystals around the absorption edge

The mass attenuation coefficients of InSe and InSe having different holmium concentrations were measured in the energy region 15.746-40.930 keV using a Si(Li) detector. InSe and InSe:holmium(0.0025), InSe:holmium(0.0050), InSe:holmium(0.025) and InSe:holmium(0.05) crystals were grown by the Bridgman-Stocbarger method. The measured values are compared with the theoretical ones obtained using WinXcom which is a Windows version of XCOM. The measurement of mass attenuation coefficients of ternary semiconductors is very important because of its use in technology.     

Fracture patterns generated by diffusion controlled volume changing reactions

A simple two-dimensional model was developed for the growth of fractures in a chemically decomposing solid. Simulations were carried out under rapid chemical decomposition conditions for which the kinetics of fracture growth is controlled by diffusion of the volatile reaction product or the kinetics of evaporation. The growth of the fracture pattern is self-sustaining due to the volume reduction associated with the decomposition process. Consistent with the theoretical analysis of Yakobson [Phys. Rev. Lett. 67, 1590 (1991)10.1103/PhysRevLett.67.1590], the fracture front propagates with a constant velocity v approximately=k2/3(Dl0)1/3 under evaporation controlled conditions and v approximately=D/l0 under diffusion controlled conditions, where k is the evaporation rate constant, D is the diffusion constant for the volatile reaction product in the solid, and l0 is the critical stable crack length. Under diffusion controlled conditions, the front width w scales as w approximately=(kl0/)D.     

Energy Band Structure of Be–(C,Si, Ge,Sn)–N2 Crystals

Russian Physics Journal - For the BeMN2 (M = C, Si, Ge, Sn) crystals with the chalcopyrite lattice, model calculations of the electronic structure are performed, equilibrium parameters of the...     

Atomistic simulations to study crack tip behaviour in single crystal of bcc niobium and hcp zirconium

In this article, molecular dynamics based simulations were carried out to study the fracture toughness of single crystals of niobium (Nb) and zirconium (Zr). Separate set of simulations were performed with different orientations of crack plane in Nb and Zr. In each case, efforts were made to align the crack front with principal planes of corresponding crystal structure, that was bcc for Nb and hcp for Zr. Deformation in single crystal of Nb and Zr was governed either by twinning or emergence of dislocations from the crack tip and surfaces. The deformation mechanism in single crystal of Nb and Zr also helps in governing the overall toughness of the material, while deforming via twinning leads to higher change in the crack length, whereas dislocations emerging from the crack tip blunts the opening and improves the toughness.     

Morphological instabilities of polymer crystals

We present experimental observations at comparatively low supercooling of morphology transitions from dendritic to faceted structures in polymer crystals growing in thin films of a poly-2-vinylpyridineblock-polyethyleneoxid copolymer. Our results are compared with theoretical concepts describing morphological instabilities of single crystals. Although these concepts originally were not developed for polymers, they allow to describe and interpret our experimental results quite well. In particular, the measured temperature dependence of the width W and frequency of dendritic side branches and the radius of curvature ρ of the growth tips of the crystals follow these concepts. We present preliminary evidence for the influence of polymer attachment kinetics and reorganisation processes behind the growth front. Polymer thin films provide valuable model systems for studying general concepts of crystallisation and allow to distinguish at which point the connectivity of the crystallising units within chain-like molecules starts to play a measurable role.     

A non-viscous-featured fractograph in metallic glasses

A fractograph of non-viscous feature but pure shear-offsets was found in three-point bending samples of a ductile Pd–Cu–Si metallic glass. A sustainable shear band multiplication with large plasticity during notch propagation was observed. Such non-viscous-featured fractograph was formed by a crack propagation manner of continual multiple shear bands formation in front of the crack-tip, instead of the conventional rapid fracture along shear bands. With a 2D model of crack propagation by multiple shear bands, we showed that such fracture process was achieved by a faster stress relaxation than shear-softening effect in the sample. This study confirmed that the viscous fracture along shear bands could be not a necessary process in ductile metallic glasses fracture, and could provide new ways to understand the plasticity in the shear-softened metallic glasses.     

Local waiting time fluctuations along a randomly pinned crack front

The propagation of an interfacial crack along a heterogeneous weak plane of a transparent Plexiglas block is followed using a high resolution fast camera. We show that the fracture front dynamics is governed by local and irregular avalanches with very large size and velocity fluctuations. We characterize the intermittent dynamics observed, i.e., the local pinnings and depinnings of the crack front by measuring the local waiting time fluctuations along the crack front during its propagation. The deduced local front line velocity distribution exhibits a power law behavior, P(v) alpha v-eta with eta=2.55+/-0.15, for velocities v larger than the average front speed . The burst size distribution is also a power law, P(S) alpha S-gamma with gamma=1.7+/-0.1. Above a characteristic length scale of disorder Ld approximately 15 microm, the avalanche clusters become anisotropic providing an estimate of the roughness exponent of the crack front line, H=0.66.     

新型硫族化合物在中红外非线性光学晶体方面研究进展

利用红外非线性光学晶体材料对已有的成熟的激光光源进行频率转换是获得3~5μm和8~12μm"大气窗口"红外激光的主要方法。传统的红外非线性光学晶体存在缺陷,不能满足应用要求,需要探索新型的红外材料。综述了以硫族化合物为研究对象,寻找新型中红外非线性光学晶体材料的研究进展。介绍了Li MQ2(M=Ga,In;Q=S,Se,Te),BaGa4S7,BaGa4Se7,BaGa2MQ6(M=Si,Ge;Q=S,Se)及Li2In2MQ6(M=Si,Ge;Q=S,Se)的研究进展。     

Low temperature thermal annealing-induced α-FeSi2 derived phase in an amorphous Si matrix

Thermal annealing-induced recrystallisation in Fe ion-implanted Si was investigated by transmission electron microscopy. Single crystals of Si(111) were implanted with 120 keV Fe ions to a fluence of 1.0×10¹⁷ cm^-2 at cryogenic temperature. A buried amorphous Fe-Si layer in an amorphous Si matrix was formed in the as-implanted sample. Nanobeam electron diffraction revealed that metastable α-FeSi₂ precipitates embedded in the amorphous Si matrix were formed after annealing at 350 °C for 8 h. The formation of this α-FeSi₂-derived phase was unusual, because it has been observed only in epitaxially grown thin films. Based on the Fe_1-xSi (0<x<0.5) phase with the CsCl structure, which is another metastable phase in the Fe-Si binary system, we discuss the formation process of the metastable α-FeSi₂ in the amorphous matrix. PACS 61.43.Dq; 61.14.Lj; 61.80.Jh     

X-ray equipment for laboratory and synchrotron studies of thin-film crystalline structures

The capabilities of X-ray equipment produced by Radicon Ltd. for the high-resolution structural investigation of single crystals and thin films using different methods including high-resolution methods, are demonstrated. A double-crystal DSO-1T X-ray diffractometer with a large sample-to-detector distance and a narrow slit in front of the detector allows one to perform high-quality mapping of the GaAs/GaP/Si heteroepitaxial structure in reciprocal space. A new double-crystal PDP console for the DRON-3 diffractometer provides the mapping of large wafers through the use of rocking curves. A new specialized X-ray DSO-2-01 diffractometer equipped with a motorized console for linear displacement of detector and rotary slit changer in front of the detector ensures automatic crystal orientation and precise measurements of the crystal lattice. All X-ray diffractometers manufactured by Radicon have rather flexible software including a custom macros editor.     

Dynamics of interface in three-dimensional anisotropic bistable reaction-diffusion system

This paper presents a theoretical investigation of dynamics of interface (wave front) in three-dimensional (3D) reaction-diffusion (RD) system for bistable media with anisotropy constructed by means of anisotropic surface tension. An equation of motion for the wave front is derived to carry out stability analysis of transverse perturbations, which discloses mechanism of pattern formation such as labyrinthine in 3D bistable media. Particularly, the effects of anisotropy on wave propagation are studied. It was found that, sufficiently strong anisotropy can induce dynamical instabilities and lead to breakup of the wave front. With the fast-inhibitor limit, the bistable system can further be described by a variational dynamics so that the boundary integral method is adopted to study the dynamics of wave fronts.     

Light induced adsorption of Si nano-composites in LiF crystals at 157 nm

Si nano-composites were precipitated on LiF crystals following ablation from Si targets with laser light at 157 nm. The LiF/Si interface was analyzed with scanning electron microscopy, atomic force microscopy and energy dispersive X-ray microanalysis. It was found that Si composites were strongly attached to LiF ionic sites to form inhomogeneous structures consisted of small isotropic crystals 0.1-1 μm long, rich in Si and fluorine, which eventually further agglomerate to form larger structures. The thickness of the LiF/Si interface was increased from 50 nm to 2 μm following laser irradiation at 157 nm, due to accelerated adsorption of Si in the LiF interface by VUV light.