酸蒸气水热免洗法制备前驱物ZrMo2-xWxO7(OH,Cl)2·2H2O和立方ZrMo2-xWxO8热收缩化合物

采用酸蒸气水热免洗方法合成了立方ZrW2O8类型化合物的系列前驱物ZrMo2-xWxO7(OH, Cl)2·2H2O(x=0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8), 并由此制备了纯立方相ZrMo2-xWxO8(x=0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8). 提出了用X射线粉末衍射相对积分强度表征有序-无序介稳态晶体有序度的方法, 发现在制备条件下, 随着Mo/W摩尔比的减小, 立方相ZrMo2-xWxO8有序-无序介稳态的饱和有序度增加的规律. 讨论了原料选择以及中间产物的形貌对合成目标产物的影响, 探讨了以铵盐为原料制备前驱物反应的微观过程.     

Nanocrystalline undoped tetragonal and cubic zirconia synthesized using poly-acrylamide as gel and matrix

Nanocrystalline zirconia powders with tetragonal and cubic structure have been prepared from ZrO₂-polyacrylamide gel and precipitated zirconia in polyacrylamide matrix respectively. X-ray diffraction results of the samples annealed at high temperatures show that the tetragonal and cubic form, obtained from the gel, are fairly stable in air up to 1,173 K and partially stable in inert atmosphere, up to 1,273 K. The stability at such high temperatures is due to the presence of oxygen vacancies in zirconia sample, incorporated in the process of decomposition of polymer.     

Evolutionary computation for optimal knots allocation in smoothing splines

In this paper, a novel methodology is presented for optimal placement and selections of knots, for approximating or fitting curves to data, using smoothing splines. It is well-known that the placement of the knots in smoothing spline approximation has an important and considerable effect on the behavior of the final approximation [1]. However, as pointed out in [2], although spline for approximation is well understood, the knot placement problem has not been dealt with adequately. In the specialized bibliography, several methodologies have been presented for selection and optimization of parameters within B-spline, using techniques based on selecting knots called dominant points, adaptive knots placement, by data selection process, optimal control over the knots, and recently, by using paradigms from computational intelligent, and Bayesian model for automatically determining knot placement in spline modeling. However, a common two-step knot selection strategy, frequently used in the bibliography, is an homogeneous distribution of the knots or equally spaced approach [3].     

Small-angle X-ray scattering study of a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer gel under shear flow

 The structure and flow behaviour of a micellar “cubic” phase is studied, using small-angle X-ray scattering (SAXS) and constant stress rheometry on a poly(oxyphenylethylene)–poly(oxyethylene) diblock copolymer in water. The predominant structure is a face-centred cubic (fcc) array of spherical micelles, which under shear undergoes layer sliding to give a scattering pattern from stacked hexagonal close-packed layers. A detailed analysis of the SAXS data indicates the presence of a fraction of grains with a structure distorted from a fcc phase. The additional reflections that characterize this structure can be indexed to a rhombohedral unit cell, space group R3ˉm, with the same volume as the fcc unit cell. The rhombohedral unit cell corresponds to a cubic cell that has been “stretched” along a [111] direction, and it is suggested that such a structure results from the gradient in shear velocity in the Couette cell employed. Shearing at high shear rates leads to a “smearing out” of the reflections, but upon cessation of shear under these conditions a highly oriented SAXS pattern is obtained, which confirms the persistence of rhombohedral ordering. The shear-induced changes in orientation are correlated to a plateau observed in the stress plotted against shear rate, such a plateau being a sign of inhomogeneous flow. Received: 8 September 2000 Accepted: 29 November 2000     

Universality and asymptotics of graph counting problems in non-orientable surfaces

Bender-Canfield showed that a plethora of graph counting problems in orientable/non-orientable surfaces involve two constants tg and pg for the orientable and the non-orientable case, respectively. T.T.Q. Le and the authors recently discovered a hidden relation between the sequence tg and a formal power series solution u(z) of the Painlevé I equation which, among other things, allows to give exact asymptotic expansion of tg to all orders in 1/g for large g. The paper introduces a formal power series solution v(z) of a Riccati equation, gives a non-linear recursion for its coefficients and an exact asymptotic expansion to all orders in g for large g, using the theory of Borel transforms. In addition, we conjecture a precise relation between the sequence pg and v(z). Our conjecture is motivated by the enumerative aspects of a quartic matrix model for real symmetric matrices, and the analytic properties of its double scaling limit. In particular, the matrix model provides a computation of the number of rooted quadrangulations in the 2-dimensional projective plane. Our conjecture implies analyticity of the O(N)- and Sp(N)-types of free energy of an arbitrary closed 3-manifold in a neighborhood of zero. Finally, we give a matrix model calculation of the Stokes constants, pose several problems that can be answered by the Riemann-Hilbert approach, and provide ample numerical evidence for our results.     

Time series prediction of nonlinear ship structural responses in irregular seaways using a third-order Volterra model

To predict the nonlinear structural responses of a ship traveling through irregular waves, a third-order Volterra model was applied based on the given irregular data. A nonlinear wave–body interaction system was identified using the nonlinear autoregressive with exogenous input (NARX) technique, which is one of the most commonly used nonlinear system identification schemes. The harmonic probing method was applied to extract the first-, second- and third-order frequency response functions of the system. To achieve this, a given set of time history data of both the irregular wave excitation and the corresponding midship vertical bending moment for a certain sea state was fed into the three-layer perceptron neural network. The network parameters are determined based on the supervised training. Next, the harmonic probing method was applied to the identified system to extract the frequency response function of each order. While applying the harmonic probing method, the nonlinear activation function (i.e., the hyperbolic tangent function) was expanded into a Taylor series for harmonic component matching. After the frequency response functions were obtained, the structural responses of the ship under an arbitrary random wave excitation were easily calculated with rapidity using a third-order Volterra series. Additionally, the methodology was validated through the in-depth analysis of a nonlinear oscillator model for a weak quadratic and cubic stiffness term, whose analytic solutions are known. It was confirmed that the current method effectively predicts the nonlinear structural response of a large container carrier under arbitrary random wave excitation.     

二元团簇V$_{n}$C$_{3}$$^{0/-}$(${n}$=1$sim$6)的光电子谱和密度泛函理论计算研究

过渡金属碳化物良好的催化性能依赖于其组成和形貌结构. 研究碳化物的精确结构有利于理解它们的催化性能本质,但是极具挑战性,尤其是对于多组分多元体系. 本文利用负离子光电子谱结合密度泛函理论计算研究了二元团簇V$_n$C$_3$$^{0/-}$ ($n$=1$sim$6)及其对应中性团簇的几何结构与电子结构. 负离子光电子谱实验测得了该系列负离子团簇的绝热电子剥离能,即其对应中性团簇V$_n$C$_3$的电子亲和能. 理论计算表明,在小尺寸团簇V$_n$C$_3$$^{0/-}$ ($n$=1$sim$3) 中,碳原子与碳原子相互作用成键;随着团簇尺寸增加,主要是钒原子数目增加,碳原子与碳原子倾向于分离开,不再成键. 与此同时,团簇中形成了钒-碳和钒-钒键. 本文进一步研究了八原子团簇V$_n$C$_m$($n$+$m$=8) 的结构,并考察了团簇组成对该类八原子团簇的立方或类立方结构的影响.     

基于三次样条插值的IRFPA非均匀性校正算法*

针对红外成像制导跟踪系统工程应用的实际要求,对红外焦平面阵列工作在大动态范围条件下的非均匀性校正算法进行了深入研究,依据函数插值原理,导出了三次样条插值非均匀性校正算法.用模拟的非均匀性图像和实际的红外图像对算法进行了校验.结果表明该算法具有动态范围大、校正准确度高的优点,可对红外焦平面阵列实现非均匀性和非线性双重校正效果.