临界点附近CO2物性畸变特性的分子动力学研究

通过分子动力学模拟方法,从微观角度研究了临界点附近CO₂物性畸变特性。分子动力学模拟表明COMPASS力场在远离临界点时具有较高精度,在临界点附近精度较低但可反映密度畸变现象。通过划分模拟空间,获得体系密度涨落特性,分析表明,临界点附近存在很大的密度涨落,且临界点前后密度涨落值呈现不对称特性;在CO₂分子上建立局部坐标,并定义了二聚体构型三参数描述方法,该描述方法能够全面描述二聚体构型分布,模拟结果显示在平行构型、T构型及十字构型中,T构型出现概率最高,3种构型的转变与物性畸变或存在紧密关联。      

同步辐射EXAFS研究FeCl3溶液中Fe离子的区域结构

EXAFS研究FeCl3溶液中Fe^3 的区域环境结构随溶液浓度的变化，Fe^3 的径向结构函数表明，随着FeCl3溶液的浓度降低，第一配位峰的距离逐渐缩短。当溶液浓度从3.0mol/L降到0.2mol/L时，配位距离减小0.014nm，但振幅峰强度以0.8mol/L浓度为最小，其强度比浓的或稀的FeCl3浓度的低约30％。结构参数拟合结果表明，0.2mol/L FeCl3稀溶液中Fe^3 的近邻配位为6个H2O的O原子形成八面体配位；3.0mol/L FeCl3溶液中Fe^3 的近邻配位为2个Cl^-和4个H2O中的O原子。0.8mol/L FeCl3溶液中Fe^3 的近邻配位有1个Cl^-和5个O原子。其结构无序相对较大，是Cl^-与O组成的扭曲八面体配位。     

超临界流体物性畸变特性的多尺度研究

为填补超临界流体物性畸变特性的理论空白,本文分别从微观尺度的分子动力学模拟、介观尺度的平均场理论和标度理论及宏观尺度的物性测量实验等方面展开研究。应用分子动力学模拟方法得到了跨越拟临界点时二氧化碳体系的微观结构特性,通过分析双体分布函数等结构参数的变化规律得到了物性畸变的分子动力学机理。基于朗道二级相变理论和标度理论,提出了一种考虑外场的气液相变模型,定量预测了超临界流体在拟临界区附近和趋于临界点时密度、定压比热等热力学量的畸变规律。在宏观尺度方面,本文开展了超临界二氧化碳密度、定压比热、黏度的高精度测量实验,指出了现有物性计算模型的潜在缺陷。研究结果表明,对于不同的超临界流体,临界点和拟临界区附近的物性畸变特性存在普适规律。在相图上,超临界流体区可进一步划分为4个区域：临界点邻域、类气区、类液区和拟临界区。不同区域内,超临界流体特性由不同尺度下的影响机制所主导。     

高温原位XAFS研究熔态Sb的局域结构

利用高温原位XAFS技术研究了半金属Sb在固态和熔态时的局域结构特点。结果表明在893K高温时固态Sb的局域结构与常温(298K)时晶态Sb的相似,但其热无序度较大。随着温度再升高20K到熔化后的913K时,熔态Sb的主配位峰形状有较大变化,其强度仅为熔化前的70％左右。这一结果表明熔化导致Sb的结构无序显著增加,我们认为这是熔态Sb中Sb原子的第一近邻配位的大部分共价键断裂造成的。模型无关的Reverse Monte Carlo方法拟合计算结果也表明熔化引起Sb样品的第一近邻配位数由固态的6增加到熔态的9。当温度升高至1058K时,Sb样品的Sb原子的径向结构函数曲线的形状与913K的Sb样品的相似,说明在熔化后的100至200K的温度范围,熔态Sb中Sb原子的局域结构差别不大。     

机械合金化Fe100—xCux体系的XAFS研究

本利用XAFS方法研究机械合金化方法制备的Fe100-xCux(x=0,10,20,40,60,70,80,100,x为原子百分比）合金中Fe、Cu原子的局域环境结构随组成的变化。对于Fe100-xCux二元混合物，当x≥40时，Fe原子的近邻配位结构从bcc转变为fcc，但Cu原子的近邻结构保持其fcc不变；与之相反，当x≤20时，Fe原子的近邻配位保持bcc结构而Cu原子的近邻配位结构从fcc转变为bcc结构。XAFS结果还表明fcc结构的Fe100-xCux中Fe的无序因子σ（0．009A）比bcc结构的Fe100-xCux中的σ（0．081A）大得多；并且在同一机械合金化Fe100-xCux（x≥40）样品中Fe原子的σ（0．099A）比Cu原子的σ（0．089A）大。这表明机械合金化的Fe100-xCux样品中Fe和Cu原子可以有相同的局域结构环境但不是均匀的过饱和固溶体，而是fcc或bcc合金相同时存在Fe富集区和Cu富集区。     

高铁硅酸盐玻璃体系的穆斯堡尔谱学研究

玻璃是一种近程有序、远程无序的无定形态物质,因而X射线衍射技术不能用于研究玻璃态。近年来,发现高铁玻璃具有某些特殊功用,例如能有效地吸收太阳能并蓄热、产生电开关特性,能用于制造磁头部件和处理核废料等,因此日益受到重视。在这些研究中,了解玻璃中铁的结构特点是十分重要的。 穆斯堡尔谱学适用于研究高铁硅酸盐玻璃体系。本文通过对Na_2O-Fe_2O_3-SiO_2及Na_2O-Fe_2O_3-SiO_2-B_2O_3体     

小角散射研究PAMAM树形大分子及其复合物

正利用小角中子和X射线散射方法,研究温度对溶液中第5代PAMAM树形大分子分子间结构的影响。随着温度的升高,结构因子散射峰逐渐宽化。这是由于分子间库仑相互作用引起的分子间类液态有序排列,受热扰动而趋向于无序化。测量了20、70、90℃3个不同温度下第5代PAMAM树形大分子水溶液小角散射,结果示于图1。实验样品为加入一定量盐酸的浓度为20mg/mL的第5代PAMAM树形大分子水溶液,盐酸的量保持与PAMAM末端氨基基团数目一致。盐酸使得溶液中的PAMAM大分子末端     

磷脂体系的同步辐射SXAS研究

用同步辐射小角X射线散射(SAXS)技术检测正己烷中磷脂形成的反相胶束在氧化不同时间后结构的变化.含亚油酸链和磷脂酰乙醇胺的磷脂(PLPE)体系的SAXS数据表明PLPE在这个体系中高度分散;PLPE形成的散射粒子比其它磷脂体系的直径大,有较长的椭球形结构.磷脂样品氧化3 d后,散射粒子存在两种不同粒径分布;经过一定沉降过程,氧化6 d后的散射粒子最大颗粒的粒径分布曲线更窄,表明3 d的氧化时间是氧化过程的一个临界点.     

机械合金化Fe0.6Ni0.4体系的结构研究

利用XAFS(X射线吸收精细结构)和XRD(X射线衍射)方法研究了机械合金化Fe0．6Ni0.4系统在制备过程中的晶体结构和Fe原子的局域配位环境。结果表明，球磨5h，样品中Fe、Ni原子开始产生合金化，但仍然为bcc和fcc混合的两种物相；球磨10h后，bcc结构Fe的特征配位峰消失，转化为fcc结构，随着球磨时间的增加，配位无序度逐渐增大；球磨40h后，Fe原子的近邻结构又有新的变化，是由多种物态组成的混合物。     

含单空位纯α-Zr晶体的结构、电子和能量性质的第一性原理研究

采用第一性原理方法,使用Win2k软件对含不同单空位浓度纯α-Zr体系的结构、电子和能量性质开展了研究,获得了含不同单空位浓度的纯α-Zr晶格常数的变化,并计算了其电子密度、态密度和能带结构。结果表明,随着单空位浓度的升高,纯α-Zr的晶格常数减小;当空位作为纯α-Zr晶格中Zr原子的第一近邻原子时,其电子密度在相邻Zr原子的方向上延伸,而不含空位的Zr原子的电子密度呈对称分布;随着体系空位浓度的降低,总态密度主峰升高,能带结构和费米面变得更加复杂。     

Atomic-scale structure of irradiated GaN compared to amorphised GaP and GaAs

We have compared the atomic-scale structure of ion irradiated GaN to that of amorphised GaP and GaAs. While continuous and homogenous amorphised layers were easily achieved in GaP and GaAs, ion irradiation of GaN yielded both structural and chemical inhomogeneities. Transmission electron microscopy revealed GaN crystallites and N₂ bubbles were interspersed within an amorphous GaN matrix. The crystallite orientation was random relative to the unirradiated epitaxial structure, suggesting their formation was irradiation-induced, while the crystallite fraction was approximately constant for all ion fluences beyond the amorphisation threshold, consistent with a balance between amorphisation and recrystallisation processes. Extended X-ray absorption fine structure measurements at the Ga K-edge showed short-range order was retained in the amorphous phase for all three binary compounds. For ion irradiated GaN, the stoichiometric imbalance due to N₂ bubble formation was not accommodated by Ga–Ga bonding in the amorphous phase or precipitation of metallic Ga but instead by a greater reduction in Ga coordination number.     

方形封闭旁支管流致声共振实验研究

流致声共振是发生在特定结构中的流场与声场相互作用的现象。针对方形封闭旁支管结构的声共振特征开展实验,研究了主管道流速和旁支管长度对封闭旁支管声共振现象的影响,获得了声共振发生区域和压力脉动特征。实验研究的3种旁支管长度分别为L/d=5、6和7,实验的雷诺数Re=2.74×10⁴~2.429×10⁵,马赫数Ma=0.025~0.218。研究结果表明,声共振频率随流速的增加表现出频率锁定特征,随旁支管长度的增大,共振工况声压幅值降低,声共振频率降低。在本实验结构中,声共振现象发生在斯特鲁哈数为0.3~0.6和0.7~1.0的区域内。     

圆形封闭旁支管流致声共振实验研究

旁支管流致声共振是由特定结构下流场和声场相互作用的结果。本文针对圆形封闭旁支管结构的声共振特征展开实验，研究了旁支管高度和主管道流速对旁支管声共振现象的影响，获得了压力脉动特征和声共振发生区域。实验研究的5种旁支管高度（H/d）为6、8、10、12和14，实验雷诺数Re=3.24×10⁴～2.89×10⁵，马赫数Ma=0.029～0.26。研究结果表明：随着流速的增加，声共振频率会出现频率锁定现象；随着旁支管高度的增加，同阶声模态对应的主控频率降低，共振工况声压幅值降低，声模态转变点对应的流速减小，实验流速范围内声模态数增加。在本实验结构中，声共振主要发生在斯特鲁哈尔数St=0.2～0.55区域内。     

Ordered water monolayer on ionic model substrates studied by molecular dynamics simulations

The molecular behaviors of interfacial water molecules at the solid/liquid interface are of a fundamental significance in a diverse set of technical and scientific contexts,thus have drawn extensive attentions.On certain surfaces,the water monolayer may exhibit an ordered feature,which may result in the novel wetting phenomenon.In this article,based on the molecular dynamics simulations,we make a detailed structure analysis of the ordered water monolayer on ionic model surface with graphene-like hexagonal lattices under various charges and unit cell sizes.We carefully analyze the water density profiles and potential of mean force,which are the origin of the special hexagonal ordered water structures near the solid surface.The number of hydrogen bonds of the ordered water monolayer near the solid surface is carefully investigated.     

Thermal conductivity of non-stoichiometric americium oxide: A molecular dynamics study

The aim of this paper is to investigate the influence of multi-valency of americium in its oxide for the lowering of the thermal conductivity and the uncertainty in measurement. In the present study, thermal conductivity of non-stoichiometric americium oxide was evaluated up to 2000 K by the non-equilibrium molecular dynamics calculations using the Born-Mayer-Huggins interatomic potential with the partially ionic model. The oxygen-to-americium ratio (O/Am) was varied from 1.6 to 1.9, which corresponded to the variation of the ratio of Am³⁺/Am⁴⁺. So, we prepared potential parameters for both Am³⁺ and Am⁴⁺. The calculated thermal conductivity of non-stoichiometric americium oxide decreased with an increase of temperature, and the degree of the temperature dependence became smaller with a decrease of the O/Am ratio. This was mainly caused by the phonon-scattering due to oxygen vacancies induced with Am³⁺ ions. Comparing two supercells in which (1) short-range ordered Am³⁺ clusters were contained and (2) Am³⁺ ions were randomly distributed, the thermal conductivity of the former seemed to be somewhat larger than that of the latter.     

Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO₂ by ion implantation and thermal annealing. Annealing at two different temperatures of 950 °C and 650 °C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5 nm, respectively. Subsequently the NCs were exposed to 5.0 MeV Sn³⁺ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5 nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1 nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.     

Dynamic analysis of a nuclear reactor with fluid–structure interaction: Part II: Shock loading, influence of fluid compressibility

The present paper is related to the dynamic (shock) analysis of a naval propulsion (on-board) reactor with fluid–structure interaction modelling. In a previous study, low frequency analysis has been performed; the present study deals with high frequency analysis, i.e. taking into account compressibility effects in the fluid medium. Elasto-acoustic coupling effects are studied and described in the industrial case. The coupled problem is formulated using the so-called (u, p, φ) formulation which yields symmetric matrices. A modal analysis is first performed on the fluid problem alone, then for the coupled fluid–structure problem in the following cases: (i) with incompressible fluid; (ii) with compressible fluid at standard pressure and temperature conditions; (iii) with compressible fluid at the operating pressure and temperature conditions. Elasto-coupling effects are then highlighted, in particular through the calculation of an elastic energy ratio. As a general conclusion, compressibility effects are proved significant in the dynamic response of the reactor in the high frequency range.     

Discussion on heat transfer correlation in turbulent channel flow imposed wall-normal magnetic field

Heat transfer degradation in high Prandtl number fluid was evaluated via direct numerical simulation (DNS). Target flow fields were fully developed turbulent channel flows imposed a wall-normal magnetic field in the high and low Prandtl number conditions (Pr = 5.25 and 0.025, respectively). Values of the bulk Reynolds number (Re_b = 14,000) and the Hartmann number (Ha = 0-32) were set to be equivalent to those of the previous experimental study by Yokomine et al. The numerical results of the Nusselt number for the high Prandtl number fluid were in good agreement with the experimental results by Yokomine et al. However, the magneto-hydrodynamics (MHD) effect on the heat transfer degradation was considerably larger than the empirical correlation proposed by Blum, particularly in the large interaction parameter range. On the other hand, the DNS results for the low Prandtl number fluid were consistent with the empirical correlation proposed by Blum and the experimental results by Gardner and Lykoudis.Therefore, we proposed a new correlation of the MHD heat transfer in high Prandtl number fluid (Pr = 5.25), and suggested that the empirical correlation proposed by Blum could be recommended for low Prandtl number fluid in the large interaction parameter range.     

Natural orbital description of the halo nucleus ~6He

Ab initio calculations of nuclei face the challenge of simultaneously describing the strong short-range internucleon correlations and the long-range properties of weakly bound halo nucleons. Natural orbitals, which diagonalize the one-body density matrix, provide a basis which is better matched to the physical structure of the many-body wave function. We demonstrate that the use of natural orbitals significantly improves convergence for ab initio no-core configuration interaction calculations of the neutron halo nucleus ~6He, relative to the traditional oscillator basis.     

Multiple Reflection Effects Observed for S-1, S-11, and S-20 Photocathodes

A simple optical model has been used to describe the influence of light trapping on the performance of photocathodes. The model is based on the assumption that light trapping occurs as a result of total internal reflection in the faceplate-photocathode structure. The analysis based on the model predicts that successive light photocathode interactions are related through a geometric progression, and the ratios of light intensities and photoelectric yields associated with successive light photocathode interactions are equal to the common ratio of the progression. Experiments on S-1, S-11, and S-20 type photocathodes tend to confirm three predictions based on the analysis: that the logarithm of the photocurrent for a specific interaction is a linear function of the number denoting the interaction; that successive photocurrents and light intensities have the same ratio denoted K, and that the ratio of the total photocurrent to that of the first interaction approaches 1/1-K.