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91.
92.
BOWLER Matthew W. CLIFF Matthew J. WALTHO Jonathan P. BLACKBURN G. Michael 《中国科学:化学》2010,(7):927-939
磷酸单酯和双酯被漫长的进化过程选择为生命体的核心.磷酸酯功能的多样性需要磷酰基转移酶不断地进化来实现.现今磷酰基转移酶的催化机制已经得到了广泛研究.通过高分辨X射线晶体衍射技术(X-ray)与19FNMR的联用,来研究磷酰基转移酶的金属氟化物过渡态类似物,可以从原子层面上阐述磷酰基转移的机理.该催化机理充分地解释了磷酸酯的一对相互"矛盾"的性质,即磷酸酯结构的极其稳定性与磷酰基在信号传导过程中的灵活可调性.通过对元素周期表中其他元素的含氧酸根及其酯的性质的研究,目前还无法找出一个更加合适的元素来替代磷的这些重要功能,因而这些证据都支持了Todd的说法:"哪里有生命,哪里就一定有磷". 相似文献
93.
Scatter observed in the fatigue response of a nickel-based superalloy, U720, is linked to the variability in the microstructure. Our approach is to model the energy of a persistent slip band (PSB) structure and use its stability with respect to dislocation motion as our failure criterion for fatigue crack initiation. The components that contribute to the energy of the PSB are identified, namely, the stress field resulting from the applied external forces, dislocation pile-ups, and work-hardening of the material is calculated at the continuum scale. Further, energies for dislocations creating slip in the matrix/precipitates, interacting with the GBs, and nucleating/agglomerating within the PSB are computed via molecular dynamics simulations. Through this methodology, fatigue life is predicted based on the energy of the PSB, which inherently accounts for the microstructure of the material. The present approach circumvents the introduction of uncertainty principles in material properties. It builds a framework based on mechanics of microstructure, and from this framework, we construct simulated microstructures based on the measured distributions of grain size, orientation, neighbor information, and grain boundary character, which allows us to calculate fatigue scatter using a deterministic approach. The uniqueness of the approach is that it avoids the large number of parameters prevalent in previous fatigue models. The predicted lives are in excellent agreement with the experimental data validating the model capabilities. 相似文献
94.
In this paper we study a family of stochastic orders of random variables defined via the comparison of their percentile residual life functions. Some interpretations of these stochastic orders are given, and various properties of them are derived. The relationships to other stochastic orders are also studied. Finally, some applications in reliability theory and finance are described. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
95.
The effect of a geometrical relationship between a grain boundary (GB) plane and a tensile axis on intergranular fatigue cracking along 3(1 1 2) twin boundaries has been investigated in Fe-30%Cr alloy crystals. Fatigue experiments were carried out on the three kinds of the specimens containing the 3(1 1 2) twin boundary. It was found that the fatigue cracking behavior was sensitive to the geometry of the GB plane. In a specimen where both the GB plane and a slip vector lying in the GB plane in adjacent grains are inclined to the tensile axis at 45°, the fatigue cracks were nucleated preferentially along the twin boundary at a stress amplitude of 170 MPa. The specimen with the GB plane normal to the tensile axis showed that the fatigue crack was initiated from a slip band formed within a constituent grain at a stress amplitude of 300 MPa. When the GB plane was inclined to the tensile axis but the slip vector lying in the GB plane was normal to the tensile axis, development of additional slips formed perpendicular to the GB plane were observed at a specific site of the GB. Initiation of intergranular fatigue cracks at the site was recognized at a stress amplitude of 250 MPa. It can be suggested that the GB plane normal to the tensile axis provides the highest fatigue performance among them. The difference in the cracking property among these specimens could be understood in terms of the effective Schmid factor derived from elastically incompatible stress. 相似文献
96.
Juan?Jose?Del Coz DiazEmail author Paulino?Jose?Garcia Nieto Francisco?Jose?Suarez Dominguez 《Journal of Mathematical Modelling and Algorithms》2005,4(3):275-287
The aim of this work is to study the behaviour of a carbon/epoxi post housed in a canine tooth after endodontic treatment
in order to support the typical loads present during mastication. The three-D basic design of the dental piece consisting
of tooth + post was carried out with a three-dimensional parametric design program. We study the stresses and displacements
of the different elements of the dental piece under normal load conditions, and present the results and conclusions. 相似文献
97.
V.?G.?KlyuevEmail author T.?L.?Maiorova A.?V.?Naumov V.?N.?Semenov 《Journal of Applied Spectroscopy》2005,72(4):552-557
CdS films, both pure and doped with the alkali metals Na and K, with Cl and without it, obtained by the aerosol pyrolysis
method from thiourea coordination compounds at a temperature of 250 to 500°C are investigated. The photoluminescence spectra
and the dependence of the luminescence intensity on the duration of exposure of samples to a high-power UV radiation have
been obtained. The doping of samples results in a tenfold increase in the intensity of the luminescence band in the visible
spectrum range (650–850 nm) as compared to pure samples of CdS. Upon irradiation of samples by high-power UV radiation for
1 h, no noticeable change in the intensity of luminescence of CdS films was observed. This is indicative of the stability
of the films under study against the action of UV radiation. The luminescent properties of the films, obtained pyrolytically
using thiourea coordination complexes, are determined by the synthesis conditions as well as by the chemical nature of the
original substances.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 509–513, July–August, 2005. 相似文献
98.
Wave cycles, i.e. pairs of local maxima and minima, play an important role in many engineering fields. Many cycle definitions are used for specific purposes, such as crest–trough cycles in wave studies in ocean engineering and rainflow cycles for fatigue life predicition in mechanical engineering. The simplest cycle, that of a pair of local maximum and the following local minimum is also of interest as a basis for the study of more complicated cycles. This paper presents and illustrates modern computational tools for the analysis of different cycle distributions for stationary Gaussian processes with general spectrum. It is shown that numerically exact but slow methods will produce distributions in almost complete agreement with simulated data, but also that approximate and quick methods work well in most cases. Of special interest is the dependence relation between the cycle average and the cycle range for the simple maximum–minimum cycle and its implication for the range distribution. It is observed that for a Gaussian process with rectangular box spectrum, these quantities are almost independent and that the range is not far from a Rayleigh distribution. It will also be shown that had there been a Gaussian process where exact independence hold then the range would have had an exact Rayleigh distribution. Unfortunately no such Gaussian process exists.This revised version was published online in March 2005 with corrections to the cover date. 相似文献
99.
We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities,
sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the
gel network. Healing can be attributed to a frictional instability occurring at the slip velocity V = V
c. For V > V
c, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order
the (nanometric) mesh size, containing a solution of polymer chain ends hanging from the network. In spite of its high degree
of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the
static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing
is compatible with the existence of a velocity-weakening regime at velocities smaller than V
c, hence with the existence of the healing instability.
Received: 7 March 2003 / Accepted: 2 May 2003 / Published online: 11 June 2003
RID="b"
ID="b"e-mail: ronsin@gps.jussieu.fr 相似文献
100.
铁电体独特的自发电极化双稳性质和非线性光学性质使其在光电子器件中得到广泛应用.为了实现器件的小型化和与微电子、光电子工艺兼容,铁电薄膜已成为一个研究热点.自发电极化的大小和取向以及外电场、缺陷和铁电薄膜/电极界面与自发电极化的交互作用决定了铁电薄膜的性质和服役行为.文章以铁电存储器和光电子器件应用为背景,选择了具有重大应用前景的Bi4-xLaxTi3O12(BLT)、SrBi2Ta2O9(SBT)、PbZrxTi1-xO3(PET)和LiNbO3(LN)铁电薄膜以及相关的La(Sr,Co)O3(LSCO)和LaNiO3(LNO)等电极材料为研究对象,研究了缺陷电荷和电畴的交互作用和它们在交变外电场中的动力学行为,探明了铁电薄膜疲劳现象的物理本质;从晶格结构与缺陷的观察研究入手,探索了材料铁电性质的起源和优化材料铁电性质的途径;从铁电薄膜/电极界面结构与性质的研究入手,寻找更有效、更稳定的电极材料与结构,从而为器件应用打下了基础;在研究外电场对铁电薄膜生长机制影响的基础上,找到了利用外电场调控铁电薄膜结构的新途径,发展了新的、与半导体器件和光电子器件工艺兼容的制膜方法. 相似文献