Using three-dimensional fractal dimension to analyze the complexity of fetal cortical surface from magnetic resonance images

This study aims to investigate the complexity of the developing fetal cortical surface based on the notion of fractal dimension (FD). Forty-four fetal MR images were selected at 22–36 weeks of gestational age (GA) and distributed between two groups: 32 normal fetal brains (excluding twins) and 12 abnormal fetal brains, including twins, mild ventricular dilatation, Cornelia de Lange syndrome (small brain), and cortical dysplasia (developmental delay). We adopted the commonly used box-counting (BC) method to estimate the FD of the developing fetal cortical surface. Results from normal fetal brains show that the increase of cortical complexity is highly correlated with fetal developing weeks of GA. In addition, after 28 weeks of GA, the value of FD increases more rapidly because of the faster development of convolved folds. In comparison with results from the normal fetal group, the abnormal fetal brains were examined and the results show that: (1) mild ventricular dilatation has no significant developing difference compared with normal fetal brains; (2) twins had lower FD than that of normal fetal brains, which may be a delay of 2–3 weeks; (3) the case of cortical dysplasia also had low FD, indicating that developing delay may mean less cortical complexity. The results of the normal group are in good agreement with fetal brain development and demonstrate the effectiveness of FD as a promising means for the quantification of complexity of the fetal cortical surface.     

Early cure behavior of a liquid dicyanate ester resin

The early cure behavior of 4,4‐dicyanato 1,1‐diphenolethane resin with and without incorporating Cr(acac)₃, Co(acac)₃, and Cu(acac)₂, respectively, as catalysts was investigated by gel permeation chromatography. The curing intermediates were separated by the column elution method and characterized by Fourier transform infrared, ¹H, and ³C NMR spectroscopies. The results indicated that the formed dimer in the early cure stage is a straight chain containing a primary amino group. The formed triazine ring in the trimer has a strong catalytic effect on the remaining cyanate groups so that the reactivity of the trimers was significantly increased. The reactivities of the curing intermediates decreased with molecular size until 7‐mer was reached. The initial monomer consumption is described by second‐order‐rate kinetics. In the presence of metal acetylacetonates, the curing reactions may be accelerated, but they did not change the reaction path and preceding sequence of reactivities. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3085–3092, 2001     

全光汤姆孙散射

随着激光和加速器技术的发展,激光场强度和粒子能量也有所提升,在高场强和高电子能量的条件下,电子与光子的汤姆孙散射过程将达到高度非线性状态,在这种状态下会发生多光子效应,即单个电子同时与多个光子相互作用并辐射一个高能光子,此过程通常称为多光子汤姆孙散射.当场强和粒子能量变得更高时,需要引入量子电动力学理论来解决极端光场物理中的动理学过程.近期,全球多台数拍瓦激光装置逐渐投入使用,激光等离子体相互作用中的此类效应会变得极其显著.而全光汤姆孙散射成为目前研究极端光场物理最佳的实验方案,因此,系统地研究全光多光子汤姆孙散射是本领域未来十年极其重要的方向.本文对近年来全光汤姆孙散射实验从单光子、低阶多光子到高阶多光子的研究进展进行了综述,并对其未来的发展方向进行了展望.另外,伴随着散射过程产生的准直高亮X/伽马射线,有望发展成为具有重要应用价值的紧凑型超亮高能光源.     

极强激光场驱动超亮伽马辐射和正负电子对产生的研究进展

高功率超短超强激光脉冲的诞生开启了相对论非线性光学、高强场物理、新型激光聚变、实验室天体物理等前沿领域.近年来,随着数拍瓦级乃至更高峰值功率激光装置的建成,超强激光与等离子体相互作用进入到一个全新的高强场范畴.这种极强激光场与等离子体相互作用蕴含着丰富的物理过程,除了经典的波与粒子作用、相对论效应、有质动力效应等非线性物理过程外,量子电动力学(QED)效应变得格外重要,例如辐射阻尼效应、正负电子对产生、强伽马射线辐射、QED级联、真空极化等.本文主要介绍我们近年来在极端强激光场与等离子体相互作用中激发的QED效应以及伴随的超亮强伽马射线辐射和稠密正负电子对产生等方面的研究进展.     

Whirling of a forced cantilevered beam with static deflection. III: Passage through resonance

Nonstationary excitations of slender, elastic, cantilevered beams with equal principal moments of inertia are considered. The excitation frequency is slowly increased or decreased through a resonance of the first mode at a constant rate. Three resonances are investigated: primary resonance, superharmonic resonance of order two and subharmonic resonance of order two. After application of Galerkin's method with three modes, the nonlinear, nonstationary response of the first mode of the beam is determined by two methods: integration of the modulation equations obtained from the method of multiple scales, and direct numerical integration of the temporal equations of motion. Time histories are presented and the effects of excitation amplitude, rate of acceleration or deceleration through resonance, damping and initial conditions of the disturbance on the maximum response are studied. The effect of a persistent random disturbance is also examined. Although the excitation acts in the vertical plane, whirling occurs if the beam is subjected to out-of-plane disturbances.     

Theoretical investigations on the chemical bonding, electronic structure, and optical properties of the metal-organic framework MOF-5

The chemical bonding, electronic structure, and optical properties of metal-organic framework-5 (MOF-5) were systematically investigated using ab initio density functional calculations. The unit cell volume and atomic positions were optimized with the Perdew-Burke-Ernzerhof (PBE) functional leading to a good agreement between the experimental and the theoretical equilibrium structural parameters. The calculated bulk modulus indicates that MOF-5 is a soft material. The estimated band gap from a density of state (DOS) calculation for MOF-5 is about 3.4 eV, indicating a nonmetallic character. As MOFs are considered as potential materials for photocatalysts, active components in hybrid solar cells, and electroluminescence cells, the optical properties of this material were investigated. The detailed analysis of chemical bonding in MOF-5 reveals the nature of the Zn-O, O-C, H-C, and C-C bonds, that is, Zn-O having mainly ionic interaction whereas O-C, H-C, and C-C exhibit mainly covalent interactions. The findings in this paper may contribute to a comprehensive understanding about this kind of material and shed insight into the synthesis and application of novel and stable MOFs.     

高效液相色谱的4种商品手性柱对38种手性化合物的拆分研究

采用纤维素-三(3,5-二甲基苯基氨基甲酸酯)涂敷型手性固定相(Chiralcel OD柱)、直链淀粉-三(3,5-二甲基苯基氨基甲酸酯)涂敷型手性固定相(Chiralpak AD柱)、直链淀粉-三(3,5-二甲基苯基氨基甲酸酯)键合型手性固定相(Chiralpak IA柱)和Pirkle型的(S,S)-Whelk-01手性固定相对38种外消旋体化合物进行手性拆分。实验结果表明,4种固定相的手性识别能力为:OD>AD>IA>(S,S)-Whelk-01,OD固定相的手性识别率达到60%,并且它们之间的手性识别性能还具有一定的互补性。本研究对4种常用手性固定相的拆分能力进行了对比,为拆分手性化合物时有的放矢地选择手性固定相提供了参考。     

ICP-AES测定高含量锌、镉和铜

采用ICP-AES测定高含量锌、镉和铜,选择了最佳仪器工作条件和样品处理方法。在实验优选条件下,对铜镉渣中铜的含量为8%—12%;镉的含量为35%—38%;锌的含量为21%—23%进行了测定。回收率分别为铜99.8%—100.0%,镉98.3%—99.9%,锌99.3%—99.5%。     

Theoretical investigation of new type of ternary magnesium alloys AMgNi4 (A=Y,La, Ce,Pr and Nd)

New ternary magnesium alloys AMgNi₄ (A=Y, La, Ce, Pr and Nd) have been studied by First-Principles calculations within the generalized gradient approximation. The optimized structural parameters were in good agreement with the available experimental data. The calculated cohesive energies and formation enthalpies showed that these alloys had strong structural stability. Then the elastic constants C_ij of these AMgNi₄ alloys were calculated, and the bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν and anisotropy value A of polycrystalline materials were derived from the elastic constants, the related mechanical properties were further discussed. The electronic structures were also calculated to reveal the underlying mechanism for the structural stability and the elastic property.     

Influence of anisotropic dipole matrix element on optical response of AB-stacked graphene superlattice

When the polarization direction of the laser beam ê lies on the graphene plane, the absorption spectrum A(ω)is isotropic and includes one sharp peak and some shoulders. As for ê along the stacking direction, A(ω) is much weaker, and shows only one broadened peak. Because of the dipole matrix element M(cv), the optical excitations do not fully reflect the features of electronic structures [or the joint density of states (JDOS)]. M(cv) plays an important role in the relationship between A(ω) and JDOS. It is strongly dependent on ê, showing an anisotropic property.