Wave equations in linear viscoelastic materials

Conditions for writing wave equations in linear viscoelastic materials are investigated. The study is restricted to the infinitesimal theory and an application is suggested in modeling ultrasound propagation in soft biological tissues. First, a general wave equation is obtained for the displacement field in an inhomogeneous medium. Second, the propagation of "the mean principal stress" (i.e., minus the arithmetical mean of the principal stresses) is examined. That quantity is particularly relevant when the force per unit area is detected at the surface of a nondissipative coupling medium. If the material is homogeneous, a wave equation is always obtained for the mean principal stress. Otherwise, supplementary conditions have to be assumed on the material and possibly on the motion. Results are illustrated by examples which present linearly elastic perfect fluids and linearly elastic Newtonian viscous fluids as particular viscoelastic materials.     

溶液样品温度对ICP光源发射强度的影响

通过光电检测法研究了溶液样品温度在20,40和60 ℃条件下,对电感耦合等离子体(ICP)光源发射强度的影响。实验结果表明,随着溶液温度的提高,元素钡、铜和锌的谱线强度有明显的增强。观察了在不同溶液温度下,谱线强度随光源观察高度和载气压力的变化规律, 发现温度的提高使最佳观测高度位置降低,而最佳载气压力升高。     

外场下压力及屏蔽对无限深量子阱中施主结合能的影响

考虑外加磁场、压力及屏蔽效应,利用变分方法数值计算GaN/AlxGa1-xN无限深量子阱系统中的杂质态结合能。给出结合能随磁场和阱宽的变化关系,同时讨论了有无屏蔽时的区别。结果表明:在磁场和压力作用下,结合能随阱宽的增大而减小;阱宽和压力一定时,结合能随磁场的增大而增大。屏蔽效应使得有效库仑吸引作用减弱而导致杂质态结合能显著下降。屏蔽效应对结合能的影响随压力增大而增强,随磁场强度增大而减弱。     

Miura transformation for the “good” Boussinesq equation

It is well known that each solution of the modified Korteveg–de Vries (mKdV) equation gives rise, via the Miura transformation, to a solution of the Korteveg–de Vries (KdV) equation. In this work, we show that a similar Miura-type transformation exists also for the “good” Boussinesq equation. This transformation maps solutions of a second-order equation to solutions of the fourth-order Boussinesq equation. Just like in the case of mKdV and KdV, the correspondence exists also at the level of the underlying Riemann–Hilbert problems and this is in fact how we construct the new transformation.     

N_2和O_2纯转动拉曼光谱(PRRS)的温度特性

计算了N2 和O2 的PRRS ,讨论了其温度特性。提出通过测量N2 和O2 的PRRS计算大气温度的方法。     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

流动注射在线分离富集-火焰原子吸收光谱法测定环境水样中Cr(Ⅲ)和Cr(Ⅵ)的形态

采用单阀阴离子和阳离子交换树脂微柱并联 ,两柱交替采样逆向洗脱流动注射在线分离富集环境水样中Cr(Ⅲ )和Cr(Ⅵ ) ,分别用 15 %HNO3和 8%NH4 NO3洗脱 ,火焰原子吸收光谱法直接检测。富集 1min时Cr(Ⅲ )和Cr(Ⅵ )的特征浓度分别为 :1 5 0 μg·L- 1 和 1 39μg·L- 1 ,Cr(Ⅲ )和Cr(Ⅵ )检出限 (3σ)分别为 1 0 3μg·L- 1 和 0 5 4 μg·L- 1 ;相对标准偏差 (10 μg·L- 1 )分别为 :3 4 1%和 1 80 % ,分析样品加标回收率在 93 5 %～ 10 7 5 %之间。