基于离散元方法的颗粒材料缓冲性能及影响因素分析

在冲击荷载作用下, 颗粒材料通过颗粒间的摩擦及非弹性碰撞可有效进行能量耗散实现缓冲作用. 本文采用离散元方法对冲击载荷下颗粒材料的缓冲过程进行数值分析, 研究不同厚度下颗粒材料的缓冲性能. 计算结果表明: 颗粒层厚度H是影响颗粒材料缓冲性能的关键因素, 并存在一个临界厚度H_c. 当H < H_c时, 冲击力随H的增加而降低; 当H > H_c时, 冲击力对H的变化不敏感并趋于稳定值. 此外, 在不同颗粒摩擦系数和初始密集度下对缓冲过程的离散元分析表明, 光滑和疏松颗粒材料具有更好的缓冲性能. 最后, 对颗粒材料在冲击过程中的力链结构和底板的压力分布进行了讨论, 以揭示颗粒材料缓冲性能的内在机理.     

Pt插层对铁磁/反铁磁界面交换耦合的影响

在铁磁层(FM)/反铁磁层(FeMn)耦合体系中插入Pt 插层或对靠近FM/FeMn界面处的FeMn掺杂Pt元素,研究了体系的交换偏置场 H_ex及矫顽力H_c随Pt插层深度 d_Pt与Pt掺杂层厚度t_PtFeMn的变化关系. 实验结果表明,引入Pt插层后NiFe/FeMn(d_Pt)/Pt/FeMn体系的未补偿磁矩(UCS)的数量得到很大的提高,从而对H_ex与H_c 起到增强的作用; 同时, 从实验结果可以推测FeMn层内部UCS的分布深度约为1.3 nm. 另外,对靠近FM/FeMn界面处的FeMn掺杂Pt元素,发现掺入Pt元素后体系的H_ex 得到有效增强, 这是因为掺入Pt元素后体系UCS的数量也得到很大的提高.     

Y对Fe-Si-B 合金非晶形成能力及软磁性能的影响

本文通过铜模吸铸法和单辊甩带法分别制备出一系列楔形试样和非晶条带试样, 系统研究了稀土金属Y对Fe₇₈Si₉B₁₃合金非晶形成能力及其软磁性能的影响. 结果表明, 少量Y取代 Fe-Si-B 非晶合金中的Fe 可大大提高该合金的非晶形成能力并促进过冷液相区的产生. 当Y含量为3 at.%时, 合金具有最大的非晶形成能力, 其临界厚度为313 μm, 相应的非晶过冷液相区宽度达到65 K. 该系列非晶合金具有优良的软磁性能, 其矫顽力(H_c)均低于200 A/m, Y含量为1 at.%时, 饱和磁感应强度(B_s) 达到最大值1.67 T.     

基于超二次曲面的颗粒材料缓冲性能离散元分析

自然界或工业中普遍是由非球形颗粒组成的复杂体系,与球形颗粒相比,非球形颗粒间的高离散和咬合互锁可使冲击载荷引起的能量有效衰减实现缓冲作用.基于连续函数包络的超二次曲面单元能准确地描述非球形颗粒的几何形态,并可精确地计算单元间的接触碰撞作用.本文采用离散元方法对冲击载荷作用下非球形颗粒物质的缓冲性能进行数值分析,并与圆柱体冲击的理论结果和球体冲击的实验结果进行对比验证.在此基础之上,进一步研究了筒底作用力在不同颗粒层厚度和形状等因素影响下的变化规律.计算结果表明:不同颗粒形状都存在一个临界厚度H_c.当HH_c时,缓冲率随H的增加而增加;当HH_c时,缓冲率的变化不再显著并趋于稳定值.此外,减小颗粒表面尖锐度和增加或减小圆柱形和长方形颗粒的长宽比都会提高颗粒材料的缓冲效果.     

四能级量子制冷循环

本文提出以两个qubit量子纠缠系统为工质的四能级制冷循环模型, 基于量子热力学第一定律和热纠缠概念, 分析了在循环中系统与外界交换的热量、输入功、制冷系数等热力学参数与量子纠缠之间的关系, 结果表明: 制冷系数等高线图是环状曲线, 随纠缠比r增加而非单调变化; 当相互作用常数J比较小时, 量子制冷机运行区间在c₁>c₂, 当增加J值时, 制冷机运行区间在c₁>c₂和 c₁<c₂两个区域; 最大制冷系数ε_max随J值增大而增加.     

声悬浮条件下黏性液滴的扇谐振荡规律研究

采用单轴式声悬浮方法研究了黏度μ =0.94-75.65 mPa·s的甘油-水溶液液滴的扇谐振荡规律. 发现一定阶数的振荡模式存在一定的临界黏度μ_c, 只有当μ < μ_c时, 该阶扇谐振荡才能被激发. 实验测定了声场调制幅度η = 0.23 时, l =2-9 阶扇谐振荡的临界黏度, 发现ln μ_c与l近似呈线性递减关系. 采用参数共振理论分析了黏性液滴的扇谐振荡过程, 发现激发扇谐振荡的液滴赤道半径扰动阈值h_c正比于液滴黏度μ, 并随l增大而增大, 因此扇谐振荡难以在高黏度和高阶模式下发生. 实验还发现, 各阶扇谐振荡的振幅和共振频率宽度随液滴黏度增大而减小, 黏度对液滴本征频率无明显影响.     

相干和非相干合成光束对金属瑞利粒子的光学俘获

研究了两光束的合成方式(相干和非相干合成)对俘获金属瑞利粒子的辐射力和稳定性的影响,着重研究了辐射力与合成方式、离轴距离、相干参数和粒子半径的关系.结果表明,不同合成方式下,离轴距离和相干参数都分别存在临界值d_c和α_c,在0d_c或α>α_c时,焦面处光强呈中心凹陷分布,此时横向梯度力不能作为回复力俘获金属瑞利粒子.在0<d≤d_c时,与非相干合成光束比较,相干合成光束在焦面处光强、辐射力、俘获刚性和纵向俘获范围更大.因此,适当选择合成方式,较小离轴距离和较低相干参数可有利于合成光束对金属瑞利粒子的俘获.     

多层膜外退火方法制备MgB2超导薄膜

报道了利用电子束蒸发的Mg/B多层膜作为前驱体，然后退火制备MgB₂薄膜的工作. 实验中发现，采用翻转膜面的退火处理方式可以有效地避免降温过程中Mg蒸气在薄膜表面形成的颗粒凝结，由此稳定地实现了面积为10 mm×10 mm，均匀、平整的超导薄膜的制备，T_c达35 K，转变宽度为0.8 K，在5 μm×5 μm的区域内薄膜的平均粗糙度小于10 nm. 为了便于后续器件制作过程中的微加工工艺，研究了膜厚小于1000 ?时薄膜的成相规律，发现当样品厚度减薄后，T_c会有明显降低. 通过调整前驱薄膜中的不同分层厚度，仍可实现转变温度达30 K以上、厚度约600 ?的MgB₂薄膜，在20 K时的临界电流密度为2.4×10⁶ A/cm².     

添加Nb在快淬NdFeB永磁体中的作用研究

系统研究了Nb元素的添加对快淬(Nd,Dy)_11.5Fe_82.4-nNb_nB_6.1(n=0,0.5,1,1.5,2)永磁体磁性能、温度特性及显微组织的影响. 结果表明：少量Nb元素的添加可以在不显著影响剩磁的情况下较大幅度提高磁体的内禀矫顽力，降低磁通不可逆损失. Nb在NdFeB磁体中的作用是使晶粒细小化、均匀化、规则化，提高交换耦合钉扎场H_p，减小材料内部的散磁场，显著降低磁通不可逆损失，改善NdFeB磁体在高温下的使用性能.     

自旋为1/2的XY模型亚铁磁棱型链的物性和有序-无序竞争

利用不变本征算符法, 计算低温下自旋为1/2的XY模型一维亚铁磁棱型链系统的元激发谱, 讨论在此系统中不同的特殊情形下的元激发能量, 从而给出体系的三个临界磁场强度的解析解H_C1, H_C2, H_peak. 分析不同外磁场下 体系的磁化强度随温度的变化规律, 发现三个临界磁场强度的解析解H_C1, H_C2, H_peak是正确的, 并从三个元激发对磁化强度的贡献进行了说明. 低温下磁化强度随外磁场的变化呈现1/3磁化平台. 体系的磁化率随温度或者外磁场的变化都出现了双峰现象. 这说明双峰源于二聚体分子内电子自旋平行排列的铁磁交换作 用能和二聚体与单基体分子间电子自旋反平行排列的反铁磁交换作用能, 热无序能, 外磁场强度相关的自旋磁矩势能之间的竞争.     

Effect of thickness on the microstructure and soft magnetic properties of CoFeHfO thin films

The thickness effects on the microstructure and soft magnetic properties of CoFeHfO thin films have been investigated in the range of 100–600 nm. There was a significant change in the coercivity (H_c) and the anisotropy (H_k) value with increasing film thickness, but the saturation induction and the resistivity almost remain unchanged. H_c and H_k reached a minimum value of 0.19 Oe and a maximum value of 50 Oe, respectively at 200 nm film thickness. The high saturation magnetic induction is 21 kG and resistivity is 500 μΩ cm. The origin of the changing H_c and H_k values is discussed in detail based on change of microstructure along with film thickness.     

Triplet superfluidity in neutron matter with Skyrme forces at subnuclear and supranuclear densities in a strong magnetic field

Within a generalized non-relativistic Fermi-liquid approach we have found general analytical formulae for phase-transition temperatures T _c,1(n, H) and T _c,2(n, H) (which are nonlinear functions of density, n, and linear of magnetic field, H) for phase transitions in spatially uniform, dense, pure neutron matter from normal to superfluid states with spin-triplet p-wave pairing (similar to anisotropic superfluid phases ³He - A₁ and ³He - A₂) in steady and homogeneous sufficiently strong magnetic field (but |μ _n |H ≪ E _c < ɛ _F (n), where μ _n is the magnetic dipole moment of a neutron, E _c is the cutoff energy and ɛ _F (n)is the Fermi energy in neutron matter). General formulae for T _c,1,2(n,H) are valid for arbitrary parameterization of the effective Skyrme forces in neutron matter. We have used for definiteness the so-called SLy2, Gs and RATP parameterizations of the Skyrme forces with different exponents in their power dependence on density n (at sub- and supranuclear densities) from the interval 0.7 n ₀ ≲ n < n _c (Skyrme)< 2 n ₀, where n ₀ =0.17 fm⁻³ is the nuclear density and n _c (Skyrme)is the the critical density of the ferromagnetic instability in superfluid neutron matter. These phase transitions might exist in the liquid outer core of magnetized neutron stars.     

Effect of the variation of the exchange energy on the superconducting critical temperature of S/F/S trilayers

The effect of the exchange energy variation in weakly ferromagnetic alloys on the superconducting resistive transition of superconductor/ferromagnet/superconductor (S/F/S) trilayers is studied. Critical temperature, T _c , and resistive transitions versus the F-layer thickness, d _F , have been analyzed in Nb/Cu_0.41Ni_0.59/Nb and Nb/Pd_0.81Ni_0.19/Nb trilayers. We show that T _c (d _F ) dependence is sensitive to magnetic inhomogeneities in the F-layer for values of d _F corresponding to thickness range where the π-superconducting state is established.     

Hydrodynamic vacuum sources of dark matter self-generation in an accelerating universe without a Big Bang

We have obtained a generalization of the hydrodynamic theory of vacuum in the context of general relativity. While retaining the Lagrangian character of general relativity, the new theory provides a natural alternative to the view that the singularity is inevitable in general relativity and the theory of a hot Universe. We show that the macroscopic source-sink motion as a whole of ordinary (dark) matter that emerges during the production of particles out of the vacuum can be a new source of gravitational vacuum polarization (determining the variability of the cosmological term in general relativity). We have removed the well-known problems of the cosmological constant by refining the physical nature of dark energy associated precisely with this hydrodynamically initiated variability of the vacuum energy density. A new exact solution of the modified general relativity equations that contains no free (fitting) parameter additional to those available in general relativity has been obtained. It corresponds to the continuous and metric-affecting production of ultralight dark matter particles (with mass m ₀ = (ħ/c ²) $ \sqrt {12\rho _0 k} $ \sqrt {12\rho _0 k} ≈ 3 × 10⁻⁶⁶ g, k is the gravitational constant) out of the vacuum, with its density ρ₀, constant during the exponential expansion of a spatially flat Universe, being retained. This solution is shown to be stable in the regime of cosmological expansion in the time interval −∞ < t < t _max, when t = 0 corresponds to the present epoch and t _max= 2/3H ₀ cΩ_0m ≈ 38 × 10⁹ yr at Ω_0m = ρ₀/ρ_c ≈ 0.28 (H ₀ is the Hubble constant, ρ_c is the critical density). For t > t _max, the solution becomes exponentially unstable and characterizes the inverse process of dark matter particle absorption by the vacuum in the regime of contraction of the Universe. We consider the admissibility of the fact that scalar massive photon pairs can be these dark matter particles. Good quantitative agreement of this exact solution with the cosmological observations of SnIa, SDSS-BAO, and the decrease in the acceleration of the expansion of the Universe has been obtained.     

Phase behavior of a suspension of hard spherocylinders plus ideal polymer chains

We study isotropic-isotropic and isotropic-nematic phase transitions of fluid mixtures containing hard spherocylinders (HSC) and added non-adsorbing ideal polymer chains using scaled particle theory (SPT). First, we investigate isotropic-nematic (I -N phase coexistence using SPT in the absence of polymer. We compare the results obtained using a Gaussian form of the orientational distribution function (ODF) to minimize the free energy versus minimizing numerically. We find that formal numerical minimization gives results that are much closer to computer simulation results. In order to describe mixtures of HSC plus ideal chains we studied the depletion of ideal chains around a HSC. We analyze the density profiles of ideal chains near a hard cylinder and find the depletion thickness δ is a function of the ratio of the polymer's radius of gyration R_g and the cylinder radius R_c. Our results are compared with a common approximation in which the depletion thickness is taken equal to the radius of gyration of the polymer chain. We incorporate the correct depletion thickness into SPT and find that for R _g/R _c < 1.56 using ideal chains gives phase transitions at smaller polymer concentrations, whereas for R _g/R _c > 1.56 , which is a common experimental situation, the phase transitions are found at larger polymer concentrations with respect to δ = R _g . The differences are significant, especially for R _g ≫ R _c , so we can conclude it is essential to take into account the properties of ideal polymer chains and the resulting depletion near a cylinder. Finally, we present phase diagrams for rod-polymer mixtures which could be realized under experimental conditions.     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

Classical trajectory calculations for state-resolved Penning ionisation reactions of polycyclic aromatic hydrocarbon C10H8 in collision with He*(23S)

ABSTRACT

The reaction dynamics of Penning ionisation of a polycyclic aromatic hydrocarbon (PAH), naphthalene C₁₀H₈, in collision with the metastable He*(2³S) atom is studied by classical trajectory calculations using an approximate interaction potential energy surface between He* and the molecule, which is constructed based on ab initio calculations for the isovalent Li?+?C₁₀H₈ system. The ionisation width (rate) around the molecular surface are obtained from overlap integrals of the He 1s orbital and the molecular orbital. The calculated collision energy dependences of partial Penning ionisation cross sections (CEDPICS) in the range 50–500?meV at 300?K have reproduced the experimental results semi-quantitatively. The opacity functions, which represent the reaction probability with respect to the impact parameter b, are discussed in connection with collision energy, interaction with He* and the exterior electron density of molecular orbitals. They indicate that the collisional ionisations of C₁₀H₈ can be classified into three types: π electron ionisations with negative collision energy dependences which are predominantly determined by attractive interaction with He*; σ orbitals ionisations of the hardcore type; σ orbital ionisations which reflect interaction potentials around CH bonds. The critical impact parameters b_c become larger with increasing collision energy due to the centrifugal barrier.