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排序方式: 共有184条查询结果,搜索用时 31 毫秒
1.
2.
Ray Leslie Withers Carlos Otero-Diaz Adrian Gómez-Herrero Albert Prodan Lasse Norén 《Journal of solid state chemistry》2005,178(10):3159-3168
Ternary derivatives of 1T-TaS2 have been synthesized and the variation in the highly structured diffuse intensity distributions characteristic of such materials carefully monitored to investigate the effect that such substitution has upon the band structures and Fermi surfaces (FSs) of the materials. Removal of d electrons via the replacement of Ta ions with lower valent transition metal ions leads to a systematic increase in the radii of the characteristic structured diffuse intensity distribution. Extended Hückel tight binding calculations of the FSs of the doped samples are carried out and used to predict possible nesting wave-vectors. The results are in reasonably good agreement with the radii of the experimentally observed diffuse intensity distributions. 相似文献
3.
The perovskite-based rare earth cobaltates (Ln0.33Sr0.67CoO3−δ) (Ln=Y3+, Ho3+ and Dy3+) have been synthesized at 1100°C under 1 atm oxygen. A thermogravimetric study has determined the overall oxygen content in each case while a combined electron diffraction (ED) and synchrotron X-ray diffraction study has revealed the presence of a complex, previously unreported, perovskite-related superstructure phase. ED gave a resultant C1c1 but most probably Cmcm (, , ) perovskite-related superstructure, describable as a modulated I4/mmm intermediate parent structure. Synchrotron X-ray data has been used to refine the intermediate parent structures of all three compounds. Coupled Ln/Sr and O/vacancy ordering and associated structural relaxation is shown to be responsible for the observed superstructure. 相似文献
4.
R.K.B GoverN.D. Withers S. AllenR.L. Withers J.S.O. Evans 《Journal of solid state chemistry》2002,166(1):42-48
SnP2O7 is a member of the ZrP2O7 family of materials, several of which show unusual thermal expansion behavior over certain temperature ranges and which show a number of displacive phase transitions on cooling from high temperature. Here we describe the structural properties of SnP2O7 from 100 to 1243 K as determined by X-ray and neutron powder diffraction. These studies reveal that SnP2O7 shows two phase transitions in this temperature range. At room temperature the material has a pseudo-cubic 3×3×3× superstructure. Electron diffraction studies show that the symmetry of this structure is P213 or lower. On warming to ∼560 K it undergoes a phase transition to a structure in which the subcell reflections show a triclinic distortion; above 830 K the subcell reflections show a rhombohedral distortion. Significant hysteresis in cell parameters is observed between heating and cooling. The structure of SnP2O7 is discussed with references to other members of the AM2O7 family of materials. 相似文献
5.
Varrot A Tarling CA Macdonald JM Stick RV Zechel DL Withers SG Davies GJ 《Journal of the American Chemical Society》2003,125(25):7496-7497
Glycosidases are some of the most ubiquitous enzyme in nature. Their biological significance, coupled to their enormous catalytic prowess derived from tight binding of the transition state, is reflected in their importance as therapeutic targets. Many glycosidase inhibitors are known. Imino sugars are often potent inhibitors, yet many facets of their mode of action, such as their degree, if any, of transition-state "mimicry" and their protonation state when bound to the target glycosidase remain unclear. Atomic resolution analysis of the endoglucanase, Cel5A, in complex with a cellobio-derived isofagomine in conjunction with the pH dependence of Ki and kcat/KM reveals that this compound binds as a protonated sugar. Surprisingly, both the enzymatic nucleophile and the acid/base are unprotonated in the complex. 相似文献
6.
Kabin JA Withers ST Grant CS Carbonell RG Sáez AE 《Journal of colloid and interface science》2000,228(2):344-358
Measurements have been made of the rate of removal of a solid organic film (phenanthrene) from the surface of a rotating disk using emulsions containing water, the nonionic surfactant Tween 20, and d-limonene as the organic phase. The results show that phenanthrene removal initially occurs by the uptake of phenanthrene into the emulsion drops as small aggregates. Simultaneously, the organic phase penetrates into the phenanthrene film, diminishing the adhesive force between the film and the substrate. After sufficient time, the phenanthrene film detaches from the rotating disk surface as a solid. This detachment mechanism accounts for the vast majority of the phenanthrene removal ( approximately 90%). Initial solubilization rates were analyzed using two solubilization models. Both models assume that phenanthrene removal occurs via a mass transfer limited removal of phenanthrene-laden emulsion drops from the phenanthrene film surface into the bulk solution. One model treats the emulsion as homogeneous while the other accounts for the finite size of the emulsion droplets. The latter model was also used to relate the flux of organic phase impacting the phenanthrene film to the detachment times. Copyright 2000 Academic Press. 相似文献
7.
C. S. Withers 《Annals of the Institute of Statistical Mathematics》1991,43(1):147-156
Based on a sample of size n, we investigate a class of estimators of the mean of a p-variate normal distribution with independent components having unknown covariance. This class includes the James-Stein estimator and Lindley's estimator as special cases and was proposed by Stein. The mean squares error improves on that of the sample mean for p3. Simple approximations imations for this improvement are given for large n or p. Lindley's estimator improves on that of James and Stein if either n is large, and the coefficient of variation of is less than a certain increasing function of p, or if p is large. An adaptive estimator is given which for large samples always performs at least as well as these two estimators. 相似文献
8.
Single phase perovskite-based rare earth cobaltates (Ln1−xSrxCoO3−δ) (Ln=La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, Y3+, Ho3+, Er3+, Tm3+ and Yb3+; 0.67?x?0.9) have been synthesized at 1100°C under 1 atmosphere of oxygen. X-ray diffraction of phases containing the larger rare earth ions La3+, Pr3+ and Nd3+ reveals simple cubic structures; however electron diffraction shows orientational twinning of a local, tetragonal (I4/mmm; ap×ap×2ap) superstructure phase. Orientational twinning is also present for Ln1−xSrxCoO3−δ compounds containing rare earth ions smaller than Nd3+. These compounds show a modulated intermediate parent with a tetragonal superstructure (I4/mmm; 2ap×2ap×4ap). Thermogravimetric measurements have determined the overall oxygen content, and these phases show mixed valence (3+/4+) cobalt oxidation states with up to 50% Co(IV). X-ray diffraction data and Rietveld techniques have been used to refine the structures of each of these tetragonal superstructure phases (Ln=Sm3+-Yb3+). Coupled Ln/Sr and oxygen/vacancy ordering and associated structural relaxation are shown to be responsible for the observed superstructure. 相似文献
9.
A full-field, multi-axial computation technique is described for determining residual stresses using the hole-drilling method with DIC. The computational method takes advantage of the large quantity of data available from full-field images to ameliorate the effect of modest deformation sensitivity of DIC measurements. It also provides uniform residual stress sensitivity in all in-plane directions and accounts for artifacts that commonly occur within experimental measurements. These artifacts include image shift, stretch and shear. The calculation method uses a large fraction of the pixels available within the measured images and requires minimal human guidance in its operation. The method is demonstrated using measurements where residual stresses are made on a microscopic scale with hole drilling done using a Focused Ion Beam – Scanning Electron Microscope (FIB-SEM). This is a very challenging application because SEM images are subject to fluctuations that can introduce large artifacts when using DIC. Several series of measurements are described to illustrate the operation and effectiveness of the proposed residual stress computation technique. 相似文献
10.
A method of ring artefact suppression in X-ray computerised tomography (CT) reconstructions is proposed. The method is based on the assumption that a sinogram is a smooth function along the horizontal spatial coordinate. Methods based on the theory of ill-posed problems are applied to find a regularised solution. An analytical formula for the solution is proposed allowing fast ring artefact suppression. Its performance is demonstrated for parallel beam synchrotron X-ray tomography. 相似文献