用探测棒研究颗粒堆中的最大静摩擦力

实验研究了筒仓内颗粒堆中部的竖直探测棒被提拉时受到的最大静摩擦力F.研究发现：当筒仓半径R小于0.040　m时， F随R的增加而减少，当R大于0.045　m时，F几乎不受R的影响. F随颗粒堆高度h变化的实验数据可以用公式F=A［h+B(e^-h/B-1)］拟合，拟合参数A与探测棒横截面形状及备制颗粒堆的流量有关，拟合参数B与备制颗粒堆的流量有关. 关键词： 颗粒物质 静摩擦力 数据拟合     

颗粒尺寸分散度对颗粒体系力学和几何结构特性的影响

利用颗粒离散元方法, 研究了由2048个光滑颗粒组成的体系在各向同性压缩条件下, 颗粒尺寸分散度s对颗粒体系力学和几何结构特性的影响. 结果表明: 配位数、剪切模量、静态结构因子以及取向序关联函数等都随分散度的变化而变化, 而力的累积分布不受分散度的影响. 其中, 单分散体系(亦即s=0)的静态结构因子在低波数区域(亦即低k)遵从幂律标度S(k)∝0.2k^-4/3, 各分散度下取向序关联函数峰值符合e指数变化规律g₆(r)∝ae^-r/ξ₆, 其中序关联长度ξ₆随分散度s增大而减小. 关键词： 颗粒物质 尺寸分散度 结构因子 取向序关联     

二维颗粒流从稀疏态到密集态的临界转变

研究了斜槽中的二维颗粒流由稀疏到密集转变的临界现象.在二维颗粒槽的入口流量Q₀和出口尺寸d固定的条件下，记录并统计了稀疏流转变为密集流所经历的时间.研究发现，在统计时间内转变不发生的概率C(t)随时间指数衰减，其衰减的特征时间尺度α^-1(d)可以很好地由幂律函数a(d_c-d)^-γ来拟合，其中d_c为临界开口尺寸.此临界尺寸的存在确定了稀疏流到密集流转变的临界现象. 关键词： 颗粒物质 颗粒流 非平衡态相变 几何相变     

电压控制不连续导电模式DC-DC变换器的熵特性研究

结合DC-DC变换器非线性特性随反馈增益k变化的关系,以及熵能够反映序列总体统计特征的特点,提出一种基于熵估计DC-DC变换器非线性行为的新方法. 以一阶电压反馈不连续导电模式 DCM Buck和Boost变换器为例,详细分析了不同反馈增益k和初值x₀形成的数值序列及熵的分布情况. 研究结果表明：DC-DC变换器的熵由反馈增益k决定,与初值x₀无关,最终小于理论极大值log₂N （N为统计区间个数）;熵能够准确区分DC-DC变换器的倍周期分岔和混沌行为,从而得到一种新的可量化的DC-DC变换器非线性动力学行为指标;完善了该类变换器非线性动力学分析的理论和方法. 关键词： DC-DC变换器 熵 混沌 k')" href="#">反馈增益k     

基态MgB2分子的结构与分析势能函数

应用群论及原子分子反应静力学方法推导MgB₂分子的电子状态及其离解极限,采用密度泛函B3LYP和从头计算QCISD方法在6-311++G**基组水平上,对MgB₂分子可能的状态进行优化计算,得出MgB₂的三重态能量最低,其稳定构型为C_2v,平衡核间距R_e=2.2977,键角α_BMgB=41.5521°,能量为-248.9645a.u.同时还计算了基态的简正振动频率:对称伸缩振动频率ν（B₂）=315.4430 cm^-1,反对称伸缩振动频率ν（A₁）=418.1883 cm^-1和弯曲振动频率ν（A₁）=968.9672 cm^-1.在此基础上,使用多体项展式理论方法,导出了基态MgB₂分子的解析势能函数,其等势面准确再现了基态MgB₂平衡结构和离解能,并由此讨论了B+MgB和Mg+BB分子反应的势能面静态特征. 关键词： 2')" href="#">MgB₂ 多体项展式理论 解析势能函数     

虚阴极产生微波的一些基本特性

用Vlasov-Poisson方程对相对论电子束在单板、双板间的传播过程进行了数值模拟,给出了单板模型空间电荷积累最大的位置,不同位置上的电流J、电子数密度n、电场E的振荡频率随入射电子数密度n₀、入射速度v₀的变化关系,双板模型空间电荷积累最大的位置,J、n、E的振荡频率随入射流J₀及两板间距离的变化关系。虚阴极位置的数值结果与稳态理论给出的结果相近,它的振荡频率符合经验公式(1~√2π)ω_peb。单板时入射电子数密度按速度服从高斯分布,能散△E_n/E_n < 10%时的数值结果给出与单能情况基本相同的结论。     

非对称量子点中磁极化子性质的磁场和温度依赖性

采用Tokuda线性组合算符法和Lee-Low-Pines变换法,研究了温度和磁场对非对称抛物量子点中强耦合磁极化子性质的影响,简捷地得到了作为量子点的横向受限强度ω₁、纵向受限强度ω₂、电子-声子耦合强度α、外磁场的回旋频率ω_c和温度参数γ的函数的磁极化子的振动频率λ、基态能量E₀和有效质量m 关键词： 非对称量子点 强耦合磁极化子 磁场和温度依赖性     

最短轨道凝聚模型的相变

应用重整化群计算最短轨道模型的生长几率分布{P_α,i}及其构型权重C_α(2×2原胞和3×3原胞),从而得出多分形热力学的配分函数Z(q,L),自由能F(q,L),能量E(q,L),比热c(q,L)和广义维数D_q,结果表明该模型在q=q_c≈0处发生相变,即当q < q_c时,生长几率分布{P_α,i}不具有多分形性质。     

非线性柱形涂层复合介质有效的直流-交流电响应

利用摄动展开方法, 研究了由圆柱形带涂层杂质随机嵌入基质所形成的非线性复合介质在外加 的带有不同频率和振幅的混合电场E₀ + E₁ sinωt+E₃ sin3ωt作用下有效的直流-交流电响应, 分别推导了复合介质在杂质核、涂层及基质区域的电势分布, 并在低杂质浓度下给出了复合介质有效的非线性响应及它们之间的关系. 关键词： 非线性柱形涂层复合介质 有效的非线性响应 外加交直流电场     

抛物量子线中束缚磁极化子的性质

研究抛物量子线中束缚磁极化子的性质,采用线性组合算符和幺正变换方法导出了强、弱耦合两种情况下的基态能量、振动频率和光学声子平均数.结果表明,无论是强耦合还是弱耦合情况,抛物量子线中束缚磁极化子的振动频率λ、基态能量E₀和光学声子平均数N都随约束强度ω₀的增大而迅速增大.     

Hysteresis from dynamically pinned sliding states

We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F_ext. A critical threshold value F_c of the adiabatically applied force F_ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of “dynamic” stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F_ext. Hysteretic behavior is also observed as a function of the external driving velocity.     

Friction between α-Al2O3(0 0 0 1) surfaces and the effects of surface hydroxylation

Molecular dynamics simulations were performed to study the friction between hydroxylated α-Al₂O₃(0 0 0 1) surfaces at the temperature of 300 K. Effects of the degree of surface hydroxylation and sliding velocity have been discussed. Results indicate that the friction coefficient decreases with increased degrees of hydroxylation. For all degrees of surface hydroxylation, the friction law crosses over from thermal activation to viscous damping at sliding velocity of 80 m/s.     

The tribology properties of alumina/silica composite nanoparticles as lubricant additives

The as-prepared alumina/silica (Al₂O₃/SiO₂) composite nanoparticles were synthesized with a hydrothermal method and modified by silane coupling agent. The tribological properties of the modified Al₂O₃/SiO₂ composite nanoparticles as lubricating oil additives were investigated by four-ball and thrust-ring tests in terms of wear scar diameter, friction coefficient, and the morphology of thrust-ring. It is found that their anti-wear and anti-friction performances are better than those of pure Al₂O₃ or SiO₂ nanoparticles. When the optimized concentration of nanoparticle additive is 0.5 wt.%, the diameters of wear scar and friction coefficients are both smallest. Such modified composite nanoparticles can adsorb onto the friction surfaces, which results in rolling friction. Therefore, the friction coefficient is reduced.     

Effect of an edge field on the acceptance of a quadrupole mass filter operating at the bottom vertex of the stability rectangle

A quadrupole mass filter (QMF) can operate with a large acceptance and high transmission at the bottom vertex S (a=2.5210, q=2.8153) of the stability quadrilateral. The combined acceptance at a level of 50% transmission and a resolving power of 100 equals 2.0×10⁻³ r ₀ ⁴ f ², which is comparable to the acceptance (5.1×10⁻³ r ₀ ⁴ f ²) of the standard operating regime of a QMF in the first stability region under the same computational conditions and optimal on-axis ion velocity. The acceptance is approximately three times higher in the presence of edge fields than in their absence. The optimal on-axis ion energy equals 1.15r ₀ f, where r ₀ is the radius of the field (the radius of the inscribed circle between the vertices of the electrodes) and f is the working frequency. In the gas-analysis regime a sensitivity of 10⁻⁵ A/Pa is achieved on a mass filter with rod length and rod diameter of 15 cm and 8 mm, respectively, frequency f=1 MHz, and field radius r ₀=0.35 cm. Zh. Tekh. Fiz. 67, 121–124 (October 1997)     

0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3压电薄膜的摩擦、磨损性能

具有准同型相界组分的0.5Ba(Ti_0.8Zr_0.2)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT)陶瓷, 表现出优异的铁电、压电性能, 作为一种具有潜在应用前景的无铅压电材料得到广泛关注. 本文采用溶胶-凝胶方法在Si(100)基底上制备了BZT-0.5BCT压电薄膜. 使用原子力显微镜和扫描电子显微镜测量得到样品的形貌图, 形貌图表明该方法制备的无铅压电薄膜表面光滑, 晶粒大小均匀、呈半球形, 直径为80–100 nm, 厚度为1.7 μm, 膜的内部有气孔.摩擦力实验表明, 压电薄膜样品与硅针尖之间存在静电力的作用, 导致其摩擦力远大于硅针尖与SiO₂之间的摩擦力, 但是两者的摩擦系数基本相同.划痕实验表明, BZT-0.5BC薄膜具有很强的法向承载能力, 但是切向抗磨损能力差, 样品的平均弹性模量为23.64 GPa± 5 GPa, 其硬度为2.7–4 GPa, 两者均略低于压电陶瓷Pb(Zr, Ti)O₃材料的体态值. 关键词： BZT-BCT薄膜 纳米摩擦力 纳米压痕 纳米划痕     

Self-healing slip pulses and the friction of gelatin gels

We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities, sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the gel network. Healing can be attributed to a frictional instability occurring at the slip velocity V = V _c. For V > V _c, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order the (nanometric) mesh size, containing a solution of polymer chain ends hanging from the network. In spite of its high degree of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing is compatible with the existence of a velocity-weakening regime at velocities smaller than V _c, hence with the existence of the healing instability. Received: 7 March 2003 / Accepted: 2 May 2003 / Published online: 11 June 2003 RID="b" ID="b"e-mail: ronsin@gps.jussieu.fr     

Effect of Resonant Photon Emission in Radiative Heat Transfer and Generation

The effect of resonances in the photon emission rate in the radiative heat generation and transfer and the Casimir friction at the sliding of two polar dielectric plates has been studied. The resonances have different origin in the frequency ranges of the normal and anomalous Doppler effects. In the frequency range of the normal Doppler effect, the resonances are due to resonant photon tunneling between surface phonon/plas-mon polaritons of plates. Such resonances exist only at a relative sliding velocity v = 0 for two identical plates. However, the resonances may occur at v ≠ 0 for different plates. In the frequency range of the anomalous Doppler effect, the resonances are due to the excitation generation in both plates. While the resonances are finite in the frequency range of the normal Doppler effect, singular resonances are possible even in the presence of dissipation in the system in the frequency range of the anomalous Doppler effect. The resonances for identical and different sliding plates have been considered.

     

Dependence of |V ub/Vcb| on Fermi momentump F in ACCMM model

The Gaussian width of Fermi momentum,p _F, is the most important parameter of the ACCMM model, and its value is essential in the determination of |V _ub/V_cb| because the experimental analysis is allowed only at the end-point region of inclusive semileptonicB-decay spectrum. We extract the value of |V _ub/V_cb| as a function ofp _F. We also calculate the parameterp _F in the relativistic quark model using the variational method, and obtainp _F=0.54 GeV which is much larger than the commonly used value, ∼0.3 GeV, in experimental analyses. When we usep _F=0.5 GeV instead of 0.3 GeV, the value of |V _ub/V_cb| from ACCMM model is increased by a factor 1.81, and can give a good agreement with Isgur et al. model.     

Can information be transferred faster than light? II. The relativistic Doppler effect on electromagnetic wave packets with suboptic and superoptic group velocities

It is shown that (a) both the dispersion relations between the mean frequency ₀ and the mean wave number k ₀ are invariant under the Lorentz transformation; and (b) the relativistic Doppler effects on ₀ and k ₀ differ. In the suboptic packet there is anomalous red shift in the mean wave number k' ₀ received from a source receding with speed v: k ₀ changes sign through zero as v goes through the value v_g, the mean group velocity in the packet. In the superoptic packet the anomalous red shift is in the mean frequency ₀ which reverses sign through zero as v goes through the value v_p, the mean phase velocity in the packet. This last finding indicates a blackout of the superoptic signal when propagated toward a receiver moving away from the source at a speed greater than v_p. There is no violation of causality involved, and the final conclusion of the paper is that it is not a fundamental axiom of special relativity, as usually believed, to deny that information can be transmitted at speeds greater than c.