大块非晶临界冷却速率的非等温转变计算模型

在非等温转变理论以及非稳态形核理论基础上提出了计算大块非晶合金连续冷却转变曲线和 临界冷却速率的新模型,以用于评估合金的非晶形成能力. 依据此模型对Zr基和Pd基8种合金 进行了计算，计算结果与实验值符合较好. 计算结果表明，影响临界冷却速率的主要因素为 黏度、临界形核功和临界结晶分数. 随着黏度增大，临界冷却速率降低；随临界形核功增大 ，临界冷却速率急剧降低；随临界结晶分数增大，临界冷却速率起初降低较快，达到一定程 度后下降速率趋于缓慢. 关键词： 临界冷却速率 非晶形成能力 大块非晶 连续冷却转变曲线     

H2-O2(N2)混气着火过程非线性特性研究

本文采用基元反应模拟H2-O2(N2)混合气体的着火过程,得到了不同散热和不同燃料-氧化剂初始浓度比条件下着火临界曲线。结果表明: H2/O2摩尔比相同时,不同散热条件下的着火临界曲线非常相似,可近似看成同一曲线在第二区的“延伸线”上滑移。临界曲线第二区的P—T关系符合2k1=ks[Ms]。散热对着火极限的影响和着火延迟时间有密切关系,在临界曲线第二区延迟时间最小,导致散热对该区的影响最弱,从而使着火临界曲线非常相似。     

Bi2Sr2—xLaxCuO6薄膜的热激活磁通耗散行为

利用不同磁场下的Ｒ－Ｔ曲线，得到了Ｂｉ２Ｓｒ２－ｘＬａｘＣｕＯ６薄膜的上临界场Ｈｃ２，讨论了热激活磁通蠕动对上临界场Ｈｃ２的影响。用热激活磁通蠕动理论解释了磁场下Ｒ－Ｔ曲线的展宽效应，得到激活能对温度的线性变化关系，对磁场的指数变化关系。     

湍流火焰自相似临界微尺度分析

自相似临界尺度表征了湍流火焰内保持自相似特性的最大尺度，是确定湍流各相同性的特征尺度．本文利用光学显微技术和计算机图像处理技术，提取并分析了湍流火焰微尺度结构的曲线度和线性回归均方差在不同放大倍率下的变化规律，提出了利用相似度偏差和临界放大倍率来确定自相似临界尺度的思路，并借助于Ｋｏｌｍｏｇｏｒｏｖ微尺度η的定义，推导出临界尺度η＇的数学关联式．     

临界厚度d_0和纵场临界电流曲线

本简报对理想第二类超导平板的纵场临界电流曲线进行的分析表明了,当平板厚度d大于临界厚度d_0时,曲线呈倒U字形;当d小于d_0时,曲线呈现出峰值效应。     

Bi—2223带材在拉伸应力下的临界电流

研究了不锈钢加强的Bi-2223/Ag高温超导带材在不同的拉伸应力下临界电流的变化情况。采用改造后的拉伸设备,在温度77K下对超导带材施加拉伸应力的同时,利用四引线法测量带材的临界电流,得到了超导带材的应力—应变曲线以及临界电流—应变曲线。利用理论公式计算了应力对超导带材临界电流的影响,与实验结果进行对比,并对两者的差别进行了分析。     

临界厚度d0和纵场临界电流曲线

本简报对理想第二类超导平板的纵场临界电流曲线进行的分析表明了,当平板厚度d大于临界厚度d₀时,曲线呈倒U字形;当d小于d₀时,曲线呈现出峰值效应。 关键词：     

BiSrCaCuO体系中的临界电流特性

我们在低磁场下测量了 BiSrCaCuO 样品的临界电流和磁化曲线,发现 BiSrCaCuO 多相样品的临界电流比 YBaCuO 小一至二个数量级.从磁化曲线估计的晶粒区域的临界电流为10~3A/cm~2(77K),说明 BiSrCaCuO 体系中钉扎力相当小,有些样品的磁化曲线接近理想的第二类超导体.     

粉末套管BPSCCO／Ag复合带载流各向异性特征的研究

本工作采用直流传输电流和直流磁化行为的测量研究了经恰当热－机制处理后的ＢＰＳＣＣＯ／Ａｇ复合带的临界电流特性，表明：由于ＢＰＳＣＣＯ晶粒的强ｃ轴择优取向，传输和磁化临界电流密度呈现大的各向异性，磁场在ｃ轴的分量起有主要作用，因传输和磁人监蚧电流密度相应于各向异性临界电流密度的不同主分量，故它们随角度变化的行为不同，传输电流的Ｉ－Ｖ曲线入磁驰豫Ｍ－ｔ曲线都呈现幂指数关系，即分别为Ｖ∝ＡＩ和Ｍ∝Ｂｔ，     

c轴取向MOCVD-YBCO膜临界电流密度的角度效应

本工作测量了低压快速生长的 c 轴取向的 MOCVD-YBCO 膜(ab)面上临界电流密度随外磁场与(ab)面间夹角θ的变化.结果表明:在77.3K,临界电流密度对角度变化极为敏感,随磁场强度增加,临界电流密度的这种各向异性越为明显.在 V-I 曲线的低 V 区.V-I 曲线可用 V=AI~n 描述,n 值随角度θ的变化和临界电流密度的变化有一定对应性.     

Theory of finite-temperature magnetism of NiMn. III

The magnetic properties of NiMn alloys at finite temperatures are investigated on the basis of an improved theory of the local environment effect (LEE). Magnetization vs. concentration curves, magnetization-temperature curves, phase diagram, high-field susceptibility and atomic short-range order dependence of the magnetization vs. temperature curves are calculated. The asymmetric behaviour of the high-field susceptibility around the critical concentration of ferromagnetism is explained by the LEE, as well as the itinerant character of the amplitude of the Ni local moment, for the first time. Two self-consistent solutions, corresponding to the high magnetization state and the low magnetization state, are found for a certain intermediate region of atomic order in Ni₃Mn. Therefore, the existence of metamagnetism is suggested in that region. The anomalous double-stage magnetization-temperature curves in Ni₃Mn alloys are explained by the transition from the high magnetization state to the low magnetization state.     

Phase transition,lattice dynamical and thermodynamic properties of yttrium antimonide with the rock salt structure under high pressure: first-principles investigations

We investigate the structural, electronic, first-order pressure-induced phase transition, lattice dynamical, and thermodynamic properties of yttrium antimonide (YSb) with the rock salt structure at high pressures and high temperatures using the projector-augmented wave method based on the density-functional theory. By the usual condition of equal enthalpy, we find that the rock salt-structured YSb is stable up to 31.10 GPa, and then transforms to the CsCl-type structure, this is consistent with the experiment result which begins transform from 26 GPa then ends at 36 GPa. The phonon dispersion curves of the rock salt-structured YSb are calculated under high pressure for the first time using a linear-response approach to density-functional perturbation theory successfully. Within the calculated phonon density of state and the quasi-harmonic approximation, we predict further the thermal physical properties of YSb under high temperature and high pressure systematically.     

高压高温处理对YNi_2B_2C晶界效应的影响

在测量经压力处理的氩弧熔炼的YNi2 B2 C的金属间化合物超导体中场冷磁化率时 ,观察到升温磁化率曲线在临界温度附近有一个负的磁化率峰。通过研究高压高温处理对粉末样品和块状样品的场冷和零场冷却磁化率曲线的影响 ,认为场冷时反常负磁化率峰是由于样品中晶界的存在 ,从而导致晶粒间的弱连接所引起的 ,而不是由磁相变或超导和磁性共存现象所引起的。并认为有可能利用高压高温技术制备这类化合物和生长单晶。     

Numerische Berechnung von Spin-Korrelationsfunktionen und Magnetisierungskurven von Ferromagnetica

The static correlation functions and the magnetization curves of finite (125–1000 spins) Ising- and Heisenberg-ferromagnets are calculated using a Monte-Carlomethod for all temperatures. We assumeS=∞, a simple cubic lattice and nearest neighbours interactions. Various theoretical assumptions concerning the correlation functions and the magnetization may be examined by means of the calculated results. Magnetization curves for superparamagnetic systems were computed also.     

Modelling the effect of pressure on the critical shear stress of MgO single crystals

A hierarchical multi-scale model was used to study the effect of high pressure on the critical shear stresses of MgO. The two main slip systems, ½?110?{110} and ½?110?{100}, were considered. Based on a generalised Peierls–Nabarro model, it is shown that the core structure of ½?110? screw dislocations is strongly sensitive to pressure. Mostly planar and spread in {110} at ambient pressure, the core of screw dislocations tends to spread in {100} with increasing pressure. Subsequently, an inversion of the easiest slip systems is observed between 30 and 60?GPa. At high pressure, the plasticity of MgO single crystals is expected to be controlled by ½?110?{100} slip systems, except at high temperature where both slip systems become active. Pressure is also found to increase the critical resolved shear stresses and to shift the athermal temperature toward higher temperatures. Under high pressure, MgO is thus characterised by a significant lattice friction on both slip systems.     

Magnetic Properties of XXZ Heisenberg Antiferromagnetic and Ferrimagnetic Nanotubes

The spin-1/2 antiferromagnetic and spin-(1/2, 1) ferrimagnetic single-walled nanotubes are described by XXZ Heisenberg model. The sublattice magnetization and the critical temperature of the system are calculated by using the double-time spin Green's function method. At zero temperature, with the increase of the exchange interaction in the circumferential direction, a maximum value appears in the sublattice magnetization curves of antiferromagnetic and ferrimagnetic systems. As the diameter of the tube increases, the spin quantum fluctuations and thermal fluctuations are suppressed. In addition, the spin quantum fluctuation of the spin-1/2 antiferromagnetic system is greater than that of the spin-(1/2, 1) ferrimagnetic system. The critical temperature of the system increases firstly and then tends to a constant with the increase of the diameter of tube, and it decreases to zero as the exchange anisotropy of the system disappears.     

Coulombic Phase Transitions in Dense Plasmas

We give a survey on the predictions of Coulombic phase transitions in dense plasmas (PPT) and derive several new results on the properties of these transitions. In particular we discuss several types of the critical point and the spinodal curves of quantum Coulombic systems. We construct a simple theoretical model which shows (in dependence on the parameter values) either one alkali-type transition (Coulombic and van der Waals forces determine the critical point) or one Coulombic transition and another van der Waals transition. We investigate the conditions to find separate Van der Waals and Coulomb transitions in one system (typical for hydrogen and noble gas-type plasmas). The separated Coulombic transitions which are strongly influenced by quantum effects are the hypothetical PPT, they are in full analogy to the known Coulombic transitions in classical ionic systems. Finally we give a discussion of several numerical and experimental results referring to the PPT in high pressure plasmas.     

Theoretical study of melting curves on Ta, Mo, and W at high pressures

The melting curves of tantalum (Ta), molybdenum (Mo), and tungsten (W) are calculated using a dislocation-mediated melting model. The calculated melting curves are in good agreement with shock-wave data, and partially in agreement with wire explosion and piston-cylinder data, but show large discrepancies with diamond-anvil cell (DAC) data. We propose that the melting mechanism caused by shock-wave and laser-heated DAC techniques are probably different, and that a systematic difference exists in the two melting processes.