Cyclic Deformation Of Nanocrystalline And Ultrafine-Grained Nickel

The cyclic deformation behavior Of ultrafine-grained (UFG) Ni samples synthesized by the electrodeposition method was studied. Different from those made by severely plastic deformation, the UFG samples used in this study are characterized by large-angle grain boundaries. Behaviors from nanocrystalline Ni and coarse-grained Ni samples were compared with that Of Ultrafine-grained Ni. With in situ neutron diffraction. unusual evolutions of residual lattice strains as well as cyclic hardening and softening behavior were demonstrated during the cyclic deformation. The microstructural changes investigated by TEM are discussed with respect to the unusual lattice strain and cyclic hardening/softening. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

相位恢复算法在量子关联衍射成像中的应用研究

随着研究工作的逐步深入,目前已经利用经典热光源实现了关联衍射成像,使得该技术有望在X射线以及中子衍射成像等方面得到广泛应用。在实验利用非相干光得到物体无透镜傅里叶变换频谱的基础上,采用误差消除与输入输出恢复算法,并结合过采样理论,实现了实验所用物体透射率函数的恢复。分别得到了纯振幅物体的振幅分布函数与纯相位物体的相位分布函数。此外,还讨论了实验所得傅里叶变换频谱的噪声等因素对图像恢复结果的影响。

Raman-forbidden mode and oxygen ordering in Bi2Sr2-xLaxCuO6+gamma single crystals annealed in oxygen

A Raman-forbidden phonon mode at about 840 cm(-1) is observed popularly on the surface of pun and La-doped Bi2Sr2-xLaxCuO6+y (0 less than or equal to x less than or equal to 0.8) single crystals annealed in oxygen. A remarkable excitation dependence of this additional line is found. Based on the properties of the structure of the Bi-O layer with excess oxygen atoms and the similarity in the appearance of the Raman-forbidden modes between RBa2Cu3Ox (R = Y, Nd, Gd, Pr) and Bi2Sr2-xLaxCuO6+y systems, we attribute the manifestation of this additional line to the ordering of the interstitial oxygen in the Bi-O layers. Our results provide Raman evidences for confirming that the ordering of the movable oxygen may exist universally in high-temperature superconductors.

THE MODULATION PRINCIPLE OF VACUUM ON THE SUPERCONDUCTIVITY OF YBCO

By neutron diffraction and other experiments, we have found that oxygen ions in YBCO can diffuse out of the sample in vacuo at room and low temperature, while the T(c) decreases greatly. We have also found that if the vacuum-deoxidation process lasts for several days there will be a damping oscillation of T(c) with time (t), and higher vacuum corresponds to a greater amplitude and a shorter period. We tentatively think that T(c) should satisfy the following function: T(c0) is-proportional-to T(c)e(-betat)cos (omegat + phi); it may be due to the diffusion of oxygen and the saturation of the valence state.

Effect of partial site occupation on the magnetic properties of Nd2Fe17-xSix by supercell calculation

The magnetic properties of the Nd2Fe17-xSix intermetallic compounds are studied by means of spin-polarized supercell calculations in which the selected sites of substitution are close to the situations in real samples. It is shown that the average Fe moment increases with x and saturates near x = 3. This correlates quite well with the experimental dependence of Te on x. The difference between supercell and unit cell calculations are pointed out and the influence of Si atoms on the density of states of the nearby Fe atoms is emphasized. (C) 1997 American Institute of Physics.

Investigation of Chemical Bond Characteristics, Thermal Expansion Coefficients and Bulk Moduli of alpha-R2MoO6 and R2Mo2O7 (R = rare earths) by Using a Dielectric Chemical Bond Method

Theoretical researches are performed on the alpha-R2MoO6 (R = Y, Gd, Tb Dy, Ho, Er, Tm and Yb) and pyrochlore-type R2Mo2O7 (R = Y, Nd, Sm, Gd, Tb and Dy) rare earth molybdates by using chemical bond theory of dielectric description. The chemical bonding characteristics and their relationship with thermal expansion property and compressibility are explored. The calculated values of linear thermal expansion coefficient (LTEC) and bulk modulus agree well with the available experimental values. The calculations reveal that the LTECs and the bulk moduli do have linear relationship with the ionic radii of the lanthanides: the LTEC decreases from 6.80 to 6.62 10(-6)/K and the bulk modulus increases from 141 to 154 GPa when R goes in the order Gd, Tb Dy, Ho, Er, Tm, and Yb in the alpha-R2MoO6 series; while in the R2Mo2O7 series, the LTEC ranges from 6.80 to 6.61 10(-6)/K and the bulk modulus ranges from 147 to 163 GPa when R varies in the order Nd, Sm, Gd, Tb and Dy.

Why [6,6]- and 1,2-Benzal-3-N-4-O-Cyclic Phenylimidate C-60 Undergo Electrochemically Induced Retro-Addition Reactions while 1,4-Dibenzyl-2,3-Cyclic Phenylimidate C-60 Does Not? C-H center dot center dot center dot X (X = N, O) Intramolecular Interactions in Organofullerenes

The electrochemical properties of a series of structurally related fullerooxazoles, [6,6] cyclic phenylimidate C-60 (1), 1,2-benzal-3-N-4-O-cyclic phenylimidate C-60 (2), and 1,4-dibenzyl-2,3-cyclic phenylimidate C-60 (3), are described, and the spectroscopic characterizations of their anionic species are reported. The results show that compounds I and 2 undergo retro-cycloaddition reactions that lead to the formation of C-60 and C61HPh, respectively, upon two-electron-transfer reduction. However, compound 3 demonstrates much more electrochemical stability as no retro-cycloaddition reaction occurs under similar conditions. Natural bond orbital (NBO) calculations on charge distribution show there is no significant difference among the dianions of 1, 2, and 3, indicating that the electrochemical stability of 3 is unlikely to be caused by the charge distribution difference of the dianions of three compounds. Examination on the crystal structure of compound 3 reveals close contacts of the C-H group with the heteroatoms (N and O) of cyclic phenylimidate, suggesting the existence of C-H center dot center dot center dot X (X = N, O) intramolecular hydrogen bonding among the addends, which is further confirmed by NBO analysis.

Synthesis, structural and electrical characterizations of Sr2Fe1-xMxNbO6 (M = Zn and Cu) with double perovskite structure

Sr2Fe1-xZnxNbO6-x/2 (0 <= x <= 0.5) and Sr2Fe1-xCuxNbO6-x/2 (0.01 <= x <= 0.05) with the double perovskite structure have been synthesized. The crystal structures at room temperature were determined from Rietveld refinements of X-ray powder diffraction data. The plots of the imaginary parts of the impedance spectrum, Z '', and the electric modulus, M '', versus log (frequency), possess maxima for both curves separated by less than a half decade in frequency with associated capacities of 2 nF. The enhancement of the overall conductivity Of Sr2Fe1-xMxNbO6-x/2 (M = Cu and Zn) is observed, as increases from 2.48 (3) x 10(-4) S/cm for Sr2FeNbO6 to 3.82 (5) x 10(-3) S/cm for Sr2Fe0.8Zn0.2NbO5.9 at 673 K. Sr2Fe0.8Zn0.2NbO5.9 is chemically stable under the oxygen partial pressure from 1 atm to 10(-22) atm at 873 K. The p and n-type electronic conductions are dominant under oxidizing and reducing conditions, respectively, suggesting a small-polaron hopping mechanism of electronic conduction.

Calculation of the thermal expansion coefficient for Bi2Sr2CaCu2O8

An estimation method of thermal expansion coefficient in term of lattice energy which was developed earlier for simple materials is extended to a complex material of Bi2Sr2CaCu2O8 (Bi-2212). The calculation of the chemical bond property and thermal expansion coefficient of Bi-2212 has been carried out and the theoretical values were in good agreement with the corresponding experimental results. The dependence of the thermal expansion coefficient on the different structures and on the flexible oxidation states of Bi and Cu are investigated. The results indicate that the thermal expansion coefficients of Bi-2212 are insensitive to the low lattice distortion of the average structure and the changes of formal valences of Bi and Cu ions.

Valences of Cu in Y1-xCaxBa2Cu3O6+y

The valences of Cu in Y1-xGaxBa2Cu3O6+y have been investigated by using complex chemical bond theory The results for the valences of Cu(1) and Cu(2) in the calculation suggest that the holes introduced by calcium substitution only reside in CuO2 planes, and there is a competition mechanism for the hole density in CuO2 planes between Ca doping and oxygen depletion. These conclusions are satisfactorily in agreement with experiments.

An Analytical Solution for Three-Dimensional Diffraction of Plane P-Waves by a Hemispherical Alluvial Valley with Saturated Soil Deposits

An analytical solution for the three-dimensional scattering and diffraction of plane P-waves by a hemispherical alluvial valley with saturated soil deposits is developed by employing Fourier-Bessel series expansion technique. Unlike previous studies, in which the saturated soil deposits were simulated with the single-phase elastic theory, in this paper, they are simulated with Biot's dynamic theory for saturated porous media, and the half space is assumed as a single-phase elastic medium. The effects of the dimensionless frequency, the incidence angle of P-wave and the porosity of soil deposits on the surface displacement magnifications of the hemispherical alluvial valley are investigated. Numerical results show that the existence of a saturated hemispherical alluvial valley has much influence on the surface displacement magnifications. It is more reasonable to simulate soil deposits with Biot's dynamic theory when evaluating the displacement responses of a hemispherical alluvial valley with an incidence of P-waves.

Shock wave diffraction by a square cavity filled with dusty gas

A simple two-dimensional square cavity model is used to study shock attenuating effects of dust suspension in air. The GRP scheme for compressible flows was extended to simulate the fluid dynamics of dilute dust suspensions, employing the conventional two-phase approximation. A planar shock of constant intensity propagated in pure air over Aat ground and diffracted into a square cavity filled with a dusty quiescent suspension. Shock intensities were M-s = 1.30 and M-s = 2.032, dust loading ratios were alpha = 1 and alpha = 5, and particle diameters were d = 1, 10 and 50 mum. It was found that the diffraction patterns in the cavity were decisively attenuated by the dust suspension, particularly for the higher loading ratio. The particle size has a pronounced effect on the flow and wave pattern developed inside the cavity. Wall pressure historics were recorded for each of the three cavity walls, showing a clear attenuating effect of the dust suspension.

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.

Shock wave diffraction and reflection around a dusty square cavity

The diffraction and reflection of planar shock wave around a dusty square cavity is investigated numerically, which is embedded in the net bottom surface of a two-dimensional channel, and the induced gas-particle two-phase now. The wave patterns at different times are obtained for three different values of the particle diameter. The computational results show that the existence of particles affects appreciably the shock wave diffraction and cavity flow.

Near-diffraction-limited, 35.4 W laser-diode end-pumped Nd:YVO4 slab laser operating at 1342 nm

A diode stack end-pumped Nd:YVO4 slab laser at 1342 nm with near-diffraction-limited beam quality by using a hybrid resonator was presented. At a pump power of 139.5 W, laser power of 35.4 W was obtained with a conversion efficiency of 25.4% of the laser diode to laser output. The beam quality M-2 factors were measured to be 1.2 in the unstable direction and 1.3 in the stable direction at the output power of 29 W. (C) 2009 Optical Society of America