内应力对铁磁性形状记忆合金Ni-Mn-Ga马氏体相变路径的影响

在单晶样品Ni52Mn245Ga235中观察到了单纯由温度诱发的完全的热弹性中间马氏体相变,测定了母相和中间马氏体相的晶体结构和晶格参数.通过对研磨成不同晶粒度大小的单晶样品的研究,发现晶粒度小于50μm时,由于机械研磨引入的内应力可以使中间马氏体相变消失.但这种引入的内应力并不引起马氏体相变温度的明显改变.计算了不同晶粒度大小的样品由于机械研磨引起的微观应变和引入的微观内应力.分析指出,马氏体相变路径的选取与机械研磨引入的内应力大小密切相关. 关键词： Ni52Mn245Ga235 中间马氏体相变     

Increasing hydrodynamic efficiency by reducing cross-beam energy transfer in direct-drive-implosion experiments

A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by ～35%, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by 20%. Over this range, the absorption is measured to increase by 15%, resulting in a 17% increase in the implosion velocity and a 10% earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed 8 μm and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.     

Atomistic calculation of internal stress in nanoscale polycrystalline materials

Internal stress in polycrystalline materials is an intrinsic attribute of the microstructure that affects a broad range of material properties. It is usually acquired through experiment in conjunction with continuum mechanics modelling, but its determination at nanometre and submicron scales is extremely difficult. Here, we report a bottom-up approach using atomistic calculation. We obtain the internal stress in polycrystalline copper with nanosized grains by first computing the stress associated with each atom and then sorting the stress into those associated with different self-equilibrating length scales, i.e. sample scale and grain cell, which gives type I, II and III residual stresses, respectively. The result shows highly non-uniform internal stress distribution; the internal stress depends sensitively on grain size and the grain shape anisotropy. Statistical distributions of the internal stresses, along with the means and variance, are calculated as a function of the mean grain size and temperature. The implementation of this work in assisting the interpretation of experimental results and predicting material properties is discussed.     

Phase-velocity spectra of background internal friction in polycrystalline solids

The phase-velocity spectrum of background internal friction has been shown to be a material property. The shape of such spectra seems to be different, in particular, for monophasic (e.g. pure metals) and multiphase materials (typically composites and rocks). An explanation for such difference, as yet not known, is proposed in the present work. All spectra are interpreted to be a combination of two or more simple relaxation sources, each emanating from point defect diffusion with a specific diffusion scale length and inducing dissipation into one another. The single dissipation sources are assumed to combine with the laws of linear circuits in parallel. In the case of monophasic metals a model with two sources of dissipation, one in the microstructure and the other in the lattice, fits the experimental spectra qualitatively well. For multiphase materials, including natural rocks and artificial conglomerates, which exhibit flat spectra, a continuous-source model is suggested.     

Tensor of internal stresses in polycrystalline grains with complex bending

Processes that occur upon the straining of a polycrystal are studied. Components of the internal stress tensor were determined. Changes in the bending and torsion stresses in the crystal lattice of a strained polycrystal are analyzed. A correlation between changes in the internal stress tensor components and the fraction of material with complex bending is established.     

High transmission through waveguide bends by use of polycrystalline photonic-crystal structures

A hybrid photonic-crystal structure is presented as a candidate for enhancing transmission through sharp photonic-crystal waveguide bends built on a perforated dielectric slab. This structure, which we refer to as a polycrystalline structure, combines two photonic-crystal lattices. Polycrystalline photonic-crystal structures offer the ability to minimize reflections as well as mismatches that a propagating wave might encounter while undergoing a sharp corner or a discontinuity between different waveguide sections. The availability of polycrystalline structures in photonic crystals opens a broad range of possibilities for the development of optical devices. Numerical experiments are performed with two- and three-dimensional finite-difference time domain methods.     

Generating porosity in metal oxides thin films through introduction of polymer micelles

The generation of porosity in Nb₂O₅ sol–gel films through introduction of polymer micelles of commercially available copolymer Pluronic PE6800 [poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)] was studied using direct observation of film morphology by Transmission Electron Microscopy and indirect calculations of free volume fraction by optical means. Mesoporous Nb₂O₅ films were deposited by the sol–gel and spin-coating methods and templated with aqueous solutions of Pluronic PE6800 with concentrations below and above critical micelle concentration determined by the modified dye solubilization method. The influence of the duration of the postdeposition annealing on total pore volumes was investigated by reflectance measurements and consequent calculation of optical properties and free volume fraction by Bruggeman effective medium theory. The possible application of the films in optical sensing of vapors has been demonstrated through reflectance measurements prior to and after acetone vapors exposure.     

Internal stress dependent structural transition in ferromagnetic Ni-Mn-Ga nanoparticles prepared by ball milling

Ni-Mn-Ga nanoparticles were prepared by ball milling technique. X-ray diffraction pattern of the milled powders has a broad peak near the location of the prominent peak for the Heusler phase of Ni₂MnGa, indicating very disordered structures with small particle sizes. Structural properties of milled Ni-Mn-Ga particles recover to those of the bulk state after appropriate annealing temperature. It is worth noting that particles with size above 50 nm exhibit martensitic transformation. The average internal stress was calculated to be 2.83-1.13 MPa stored in the distorted lattice. Saturation magnetization of the milled sample increases with annealing temperature due to re-crystallization and grain growth.     

Understanding guest and pressure‐induced porosity through structural transition in flexible interpenetrated MOF by Raman spectroscopy

Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non‐porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4′‐Bipy)_0.5 · DMF(H₂O)_0.5} was selected for study. We have investigated structural transformation in MOF‐508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF‐508 framework is driven by ligand conformation change and inter‐linker interactions. The presence of Raman signatures of adsorbed CO₂ and its librational mode at 149 cm⁻¹ suggests cooperative adsorption of CO₂ in the MOF‐508 framework, which is also confirmed from DFT calculations that give a binding energy of 34 kJ/mol. Copyright © 2015 John Wiley & Sons, Ltd.     

Radiation produced by electrons making multiple passes through thin internal targets in the Tomsk Synchrotron

It is shown experimentally and theoretically that the use of thin (approximately 10^–3 r.l.) internal targets in a synchrotron results in electrons making multiple passes through them during each shot. The number of passes the electrons make is determined by their energy, the synchrotron parameters, and the thickness, structure, and material of the target, and gives rise to an increase in the effective thickness of the target. Using a scraper (a thick movable target at a given azimuth of the synchrotron) allows the electrons to make a single pass, which permits the spectral and angular properties of the -radiation from the electrons to be studied in connection with thin single crystals in which the effect of dechanneling can be ignored.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 81–87, June, 1991.     

Variation of internal stresses as a function of deformation and distance from sources in Cu–Al polycrystalline alloys

Internal stress fields in deformed Cu–Al polycrystalline alloys are studied via TEM. The sources of such stresses are determined. Internal stress fields are measured as a function of distance from different sources. The effect grain size has on the formation of internal stresses is determined.     

Enhanced triplet formation in polycrystalline tetracene films by femtosecond optical-pulse shaping

Polycrystalline tetracene films have been explored using weak ～ 30 fs visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay. Adaptive pulse shaping identifies multipulse optimal fields which maximize this absorption feature by ～ 20%. These are comprised of subpulses separated by time delays well correlated with the period of lattice vibrations suggesting such modes control the yield of SF photochemistry.     

Lattice imperfection studies in polycrystalline materials by x-ray diffraction line-profile analysis

This review concerns our recent investigations with a series of binary fcc Ag- and Cu-base alloys (viz Ag-Ga, Ag-Ge, Ag-Al and Cu-Ga, Cu-Ge) from detailed analyses of x-ray diffraction line profiles, the importance of which has been briefly summarized. The theoretical formulations of the Warren-Averbach’s method of Fourier analysis of peak-shapes along with the methods of peak-shift and peak-asymmetry have been outlined. A preview on the significant studies carried out earlier with Ag- and Cu-base (fcc) binary alloys has been made in short. A detailed analyses on the recorded profiles in the present considerations revealed, in general, quantitative estimates of several microstructural parameters characterising the deformed state of the materials namely, propensity of stacking faults (intrinsic, extrinsic and twin faults), rms microstrains, coherent domain sizes, long-range residual stresses, lattice parameter changes, dislocation density and stacking fault energy. The results indicate a general trend of increase in the concentrations of stacking faults, primarily, of intrinsic character, with increase in solute concentrations; which are solely responsible for the observed peak-shifts as well as domain size broadening. Small asymmetry in the profiles is due to the presence of extrinsic stacking faults, relatively less in magnitude compared to the intrinsic ones while the deformation twin faults are almost absent—an observation with significance. The dislocation density, quite appreciable in magnitude, has been evaluated from the anisotropic values of the coherent domain sizes and rms microstrains. The stacking fault energies of pure Ag and Cu, an important parameter have also been estimated and compared with those obtained from electron microscopy. Annealing experiments with a Ag-5·8% Al alloy, aluminium being a precipitating solute, do not reveal any detectable evidence of solute segregation at the stacking faults. The occurrence of stacking faults in the alloy systems has been correlated with a number of physical factors involving solvent-solute types.     

Rotational magnetic anisotropy in polycrystalline FePt films fabricated by solid-state synthesis

Physics of the Solid State - It is shown that annealing 550°C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al2O3 substrates in an atomic ratio of Fe:...     

Stress in polycrystalline GaN films prepared by r.f sputtering

Undoped, Be-doped and Si-doped polycrystalline GaN films were deposited by R.F. sputtering onto fused silica substrates. The films were deposited at various deposition temperatures ranging from 300 K to 623 K and characterized by optical measurements while the microstructural information was obtained from SEM and XRD studies. The compositional study for the GaN film was carried out using SIMS. Residual stresses in these films were evaluated from the band tail of the absorption spectra as well as from direct measurements of hardness by commercially available depth sensing indentometer. It was observed that undoped GaN films had the highest hardness followed by that for Be-doped and Si-doped films. The values of hardness obtained form the above optical measurement tallied quite well with those obtained from direct indentation measurement.     

Studies of porosity and diffusion coefficient in porous matrices by computer simulations

We present a series of molecular dynamics simulations to study the porosity on different matrix configurations. The matrices were prepared using two different processes. In the fist method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at different densities and at different matrix particle interactions. The results showed that the matrix structure and the matrix porosity were affected by the way the porous matrices were prepared. Finally, we also investigated the diffusion of a fluid inside the matrices. The diffusion coefficient was measured by mean square displacements of the particles in the fluid. It was observed that this quantity was also affected by the kind of porous matrix employed. The calculations were performed for several fluids at different densities in the different porous matrices. From these studies we observed that the highest porosity and diffusion coefficient were found in matrices prepared with attractive particle interactions and without any template.     

Increasing Preservation Efficiency and Product Quality through Control of Temperature Distributions in High Pressure Applications

The effectiveness of HP sterilisation is a function of both temperature and pressure. As during pressurisation the product temperature increases, heat transfer to the colder HPP vessel wall occurs and the product fraction near the vessel wall will be colder than the product in the middle of the vessel. The effect of the temperature distribution in the vessel on the inactivation of Bacillus stearothermophilus has been examined. A mathematical model has been built, in which both thermodynamics and inactivation kinetics are integrated. Heat transfer is based on a Finite Element simulation, inactivation kinetics are based on first order kinetics. Based on this model and experiments the effect of an homogeneous temperature distribution on inactivation is demonstrated.     

Analysis of point defects in polycrystalline BaTiO3 by electron paramagnetic resonance

Several paramagnetic point defects in BaTiO₃ polycrystals were detected and identified using the electron paramagnetic resonance technique. Polycrystalline samples sintered in a reducing atmosphere showed a broad signal with a giromagnetic constant of 1.932. This signal, observed only in conducting ceramics, was assigned to Ti³⁺ and its related complexes. V_Ba and V_Ti defects were found at giromagetic constants of 1.974 and 2.004, respectively. The EPR intensity of these defects increases after oxidising at T>1000 °C. The appearance of the V_Ba and V_Ti signals in EPR spectra of oxidized ceramics correlates with the onset of the PTCR effect. At donor dopant levels ≥0.3 at.%, the concentration of V_Ba and V_Ti shows good agreement with the defect compensation mechanism of donor doped BaTiO₃. However, small amounts of V_Ba and V_Ti were also detected in undoped BaTiO₃, contrary to the generally accepted defect model.