x射线磁性圆二色吸收谱分析Co膜厚度对原子磁矩和自旋磁矩的影响

利用软x射线磁性圆二色吸收谱(XMCD)研究了Si衬底上沉积的不同厚度的Co膜的轨道磁矩和 自旋磁矩.样品是磁控溅射方法制备的，膜的厚度分别是2nm,10nm和30nm，并在表面覆盖0.8 —1nm厚的金膜防止样品的氧化.根据XMCD求和定则计算得到的轨道磁矩和自旋磁矩分别是0. 249—0.195μ_B(玻尔磁子)和1.230—1.734μ_B.随着膜厚的减小，C o原子的轨道磁矩增加，而自旋磁矩下降.轨道磁矩与总磁矩的比值由0.101上升至0.168，即 2nm膜中Co原子的轨道磁矩对总磁矩的贡献比30nm膜中Co原子的大了83%. 关键词： x射线磁性圆二色 磁性薄膜 轨道磁矩和自旋磁矩 厚度效应     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.     

用耦合簇方法及相关一致基研究PH2(X2B1)自由基的解析势能函数

利用耦合簇方法和Dunning等提出的系列相关一致基对PH2自由基的基态结构进行优化,并使用优选出的cc-pV5Z基组对其进行频率计算.结果表明,平衡核间距RP-H=0.14185 nm,键角αHPH=91.8624°,离解能De(HP-H)=3.483 eV,对称伸缩振动频率ν1(a1)=2399.9781cm-1,弯曲振动频率ν2(a1)=1128.4213 cm-1,反对称伸缩振动频率ν3(b2)=2407.8374 cm-1.在此基础上采用多体项展式理论导出了PH2自由基的解析势能函数,其等值势能图准确再现了PH2自由基分子的平衡结构特征和动力学特征.     

无拉力弹性地基上矩形板屈曲/后屈曲问题的辛求解方法

无拉力弹性地基上矩形薄板的屈曲/后屈曲问题是板壳力学中一类重要课题,在工程中有着大量应用.因涉及接触非线性,目前主要采用数值方法对该类问题进行求解,发展具有重要基准价值的解析方法是当前面临的一项挑战.针对上述问题,本文将板划分为若干包含强制边界条件的板,形成子问题,在辛空间下利用分离变量与辛本征展开对子问题进行解析求解,通过子问题边界处的连续条件确定板与地基的接触状态;通过迭代求解上述过程,获得子问题划分的收敛结果,并得到最终屈曲载荷及模态.结果表明,无拉力弹性地基与Winkler地基上板的屈曲行为存在显著差异,且无拉力弹性地基的刚度对板的屈曲载荷与屈曲模态均有重要影响.在此基础上,结合Koiter摄动法与辛方法,对无拉力弹性地基上矩形板的后屈曲问题进行求解,获得板的后屈曲平衡路径.所得到的屈曲与后屈曲分析结果均与有限元计算结果吻合良好,确认了本文结果的正确性.由于本文方法数学推导严格,求解效率高,因此不仅为研究无拉力弹性地基上矩形薄板的屈曲/后屈曲行为提供了一种有价值的理论工具,更有望拓展至更多复杂板壳力学问题的求解.     

Predictions of pressure-induced structural transition, mechanical and thermodynamic properties of α-and β-Si3N4 ceramics: ab initio and quasi-harmonic Debye modeling

The plane-wave pseudo-potential method within the framework of ab initio technique is used to investigate the structural and elastic properties of α-and β-Si3N4.The ground-state parameters accord quite well with the experimental data.Our calculation reveals that α-Si3N4 can retain its stability to at least 40 GPa when compressed at 300 K.The α→β phase transformation would not occur in a pressure range of 0-40 GPa and a temperature range of 0-300 K.Actually,the α→β transition occurs at 1600 K and 7.98 GPa.For α-and β-Si3N4,the c axes are slightly more incompressible than the a axes.We conclude that β-Si3N4 is a hard material and ductile in nature.On the other hand,β-Si3N4 is also found to be an ionic material and can retain its mechanical stability in a pressure range of 0-10 GPa.Besides,the thermodynamic properties such as entropy,heat capacity,and Debye temperature of α-and β-Si3N4 are determined at various temperatures and pressures.Significant features in these properties are observed at high temperature.The calculated results are in good agreement with available experimental data and previous theoretical values.Many fundamental solid-state properties are reported at high pressure and high temperature.Therefore,our results may provide useful information for theoretical and experimental investigations of the Si3N4 polymorphs.     

Investigations on spectroscopic parameters, vibrational levels, classical turning points and inertial rotation and centrifugal distortion constants for the X1∑+g state of sodium dimer

The density functional theory (B3LYP, B3P86) and the quadratic configuration-interaction method including single and double substitutions (QCISD(T), QCISD) presented in Gaussian03 program package are employed to calculate the equilibrium internuclear distance $R_{\rm e}$, the dissociation energy $D_{\rm e }$ and the harmonic frequency $\omega _{\rm e}$ for the $X{}^{1}\Sigma^{ + }_{\rm g}$ state of sodium dimer in a number of basis sets. The conclusion is gained that the best $R_{\rm e}$, $D_{\rm e}$ and $\omega _{\rm e}$ results can be attained at the QCISD/6-311G(3df,3pd) level of theory. The potential energy curve at this level of theory for this state is obtained over a wide internuclear separation range from 0.16 to 2.0~nm and is fitted to the analytic Murrell--Sorbie function. The spectroscopic parameters $D_{\rm e}$, $D_{0}$, $R_{\rm e}$, $\omega _{\rm e}$, $\omega _{\rm e}\chi _{\rm e}$, $\alpha _{\rm e}$ and $B_{\rm e}$ are calculated to be 0.7219~eV, 0.7135~eV, 0.31813~nm, 151.63~cm$^{ - 1}$, 0.7288~cm$^{ - 1}$, 0.000729~cm$^{ - 1}$ and 0.1449~cm$^{ - 1}$, respectively, which are in good agreement with the measurements. With the potential obtained at the QCISD/6-311G(3df,3pd) level of theory, a total of 63 vibrational states is found when $J=0$ by solving the radial Schr\"{o}dinger equation of nuclear motion. The vibrational level, corresponding classical turning point and inertial rotation constant are computed for each vibrational state. The centrifugal distortion constants ($D_{\upsilon }\, H_{\upsilon }$, $L_{\upsilon }$, $M_{\upsilon }$, $N_{\upsilon }$ and $O_{\upsilon })$ are reported for the first time for the first 31 vibrational states when $J=0$.     

四边固支矩形弹性薄板的精确解析解

利用辛几何方法本文推导出了四边固支矩形弹性薄板弯曲问题的精确解析解.由于在求解过程中不需要事先人为的选取挠度函数,而是从弹性薄板的基本方程出发,首先将矩形薄板弯曲问题表示成Hamilton正则方程,然后利用分离变量和本征函数展开的方法求出可以完全满足四边固支边界条件的精确解析解.本文中所采用的方法突破了传统的半逆法的限制,使得问题的求解更加合理化.文中还给出了计算实例来证明推导结果的正确性.     

传导冷却单巴高功率半导体激光器热应力和smile研究

利用有限元模型分别研究了回流过程和工作过程中传导冷却高功率半导体激光器的正应力、切应力和形变,并借助理论公式分析了热应力和smile的产生原因和分布规律.分析表明,在回流过程中热膨胀系数不匹配造成的切应力是正应力和变形的根源,而在工作过程中,热膨胀系数不匹配和温度梯度共同影响着热应力和变形.在此基础上,将回流导致的剩余应力和变形作为初始条件施加在有限元模型上,对工作状态器件的热应力和smile进行模拟,以获得更精确的模拟结果.最后,通过有限元模型和实验手段研究了不同热沉温度对smile的影响.结果表明,工作过程会导致器件的smile增大,热沉温度的升高也会造成smile进一步增大.     

基于时变小世界模型的航空网优化评估

航空网的优化设计对于优化资源配置、改善航运效率、提高航空公司竞争力等具有重要的现实意义.而航线结构与航班计划密不可分.本文首先讨论了航空网的时变特征,揭示了航班频率与航线距离之间的时空耦合关联.通过构建时变空间小世界模型,揭示了时变条件下网络的最优结构指数与时空耦合强度的惟一约束关系.以运行总成本最小化为主要优化目标,提出了一种可以快速评估航线结构优化情况的方法.该方法能根据网络客流分布情况快速推算出航线网络的最优拓扑及相应的航班频率分布.并用2001—2010年中国航空网络数据对此方案进行实证研究,发现预测与实际数据基本符合,并逐渐趋于稳定.这一方法能把复杂问题简单化,对各个航空公司每年的航线航班调整是否合理,现有的航空网络是否在逐步优化做出动态评估.分析航空网络的发展趋势,从而对未来的优化提供建议.     

纳米WO3中空球的制备及其气敏性能研究

采用液相沉淀法在乙二醇/水混合体系中以钨酸钠、盐酸及水合肼为反应物制备了WO3纳米中空微球,通过XRD、FTIR、SEM等分析手段对产物的晶型结构、结构官能团和形貌进行了表征,探讨了WO3纳米中空球的形成机理,并对其气敏性能进行了研究.结果表明,制备的具有中空结构的纳米WO3为以单斜WO3和正交WO3为主的多晶相,晶粒度为25.93 nm,水合肼的添加为其中空结构的形成起到了重要作用,气敏性能研究表明,纳米WO3中空球对H2S具有很好的选择性,在工作电压为5.00V时,其灵敏度可达40.90.