Upper Critical Field and Specific Heat of Cuprates

A representative set of magnetotransport measurements in novel superconductors is analyzed. The resistive upper critical field, H _c2 (T) of many cuprates, of superconducting spin-ladders, and organic (TMTSF)2X systems has a universal nonlinear temperature dependence H _c2 (T_c – T)^3/2 in a wide temperature interval near T _c, while its low-temperature behavior depends on the chemical formula and sample quality. The unusual H _c2(T) is described as the Bose–Einstein condensation field of preformed pairs. Its universal temperature dependence follows from the scaling arguments. Controversy in the determination of H _c2 (T) from the resistivity and specific heat measurements is resolved in the framework of the charged Bose-gas model with the impurity scattering. It is shown that specific heat shows two anomalies. The high-temperature anomaly is strong and shows only weak shift with applied field. The low-temperature anomaly corresponds to resistive transition and is very weak in agreement with the experiments. Both anomalies coincide at H = 0.     

Quasi-One-Dimensional Superconductors: From Weak to Strong Magnetic Field

We discuss the possible existence of a superconducting phase at high magnetic field in organic quasi-one-dimensional conductors. We consider in particular (i) the formation of a Larkin–Ovchinnikov–Fulde–Ferrell state, (ii) the role of a temperature-induced dimensional crossover occurring when the transverse coherence length _z (T) becomes of the order of the lattice spacing, and (iii) the effect of a magnetic-field-induced dimensional crossover resulting from the localization of the wave functions at high magnetic field. In the case of singlet spin pairing, only the combination of (i) and (iii) yields a picture consistent with recent experiments in the Bechgaard salts showing the existence of a high-field superconducting phase. We point out that the vortex lattice is expected to exhibit unusual characteristics at high magnetic field.     

Stabilization of Superconductivity by Electromagnetic Field in Narrow Channel

The results on electromagnetic field effect upon the currents at which phase-slip centers appear in a superconducting channel are given. It is shown that in the presence of external microwave field, the resistive states of superconducting channels become more stable to direct current than in zero field and, consequently, stabilization of the resistive states of the channel due to the Josephson radiation from the phase-slip centers is possible.     

磁场中Bi-Mn合金中MnBi相定向排列组织的形成规律和机制

研究了Bi-Mn合金从固液两相区开始凝固，合金中MnBi相在磁场中形成定向排列组织的临界条件及其变化规律。实验研究表明，Bi-Mn合金中MnBi相在磁场中的定向排列能够迅速形成；MnBi相形成定向排列组织所需的临界温度随外磁场强度的增大而降低；恒温温度相同时，6％Mn合金形成定向排列组织所需的临界磁场强度高于3％Mn合金。在临界值以上，定向排列因子T随恒温温度的升高和磁场磁感应强度的增强而增大。理论分析结合实验研究表明，合金中MnBi晶体在磁场中取向的机制主要是旋转取向，晶体取向驱动力主要是晶体具有磁各向异性，在磁场中磁化过程中所受的磁力矩，阻力主要是晶体间碰撞阻碍。     

On the Wave Field Due to a Moving Body Performing Oscillations in the Vicinity of the Critical Frequency

A two-dimensional submerged body translating under a free surface with steady velocity U while performing small oscillations with frequency is considered. It has been known for a long time that for a single source the solution becomes unbounded at the critical frequency, which is given by =U/g=1/4 where g is the acceleration of gravity. It was therefore believed that also the motion due to an oscillating body was unbounded at this frequency. It has, however, in the last few years been shown that this motion is bounded for =1/4. In this paper previous results are discussed, and the strong variation of the forces with respect to close to =1/4 is examined. Recently, a mathematical argument was given that the motion at the critical frequency is bounded for bodies with nonzero cross-section area. It is proved that also the motion generated by a thin foil with zero cross-section area is bounded at =1/4.     

Exactly Aligned Magnetic Field and the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low-Dimensional Superconductors

In quasi-low-dimensional type II superconductors, the orbital pair-breaking effect is considerably reduced when the magnetic field is exactly aligned in a direction parallel to the highly conducting layer. Then, since the superconducting phase survives up to a very high magnetic field comparable to the Pauli-paramagnetic limit, we could expect some drastic phenomena, such as the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. In this paper, we discuss the condition of the tilt angle for the appearance of the FFLO state, and also discuss the stable spatial structure of the FFLO state. Further, we show an example of a quasi-two-dimensional system in which the FFLO state is remarkably enhanced, that is, a two-dimensional tight binding system of appropriate hole concentrations.     

稳态热度场对转子系统临界转速的影响

汽轮机转子系统在频繁启停时，将产生较大的热应力和热变形，使转子系统的振动特性发生变化。采用有限单元法，利用热-结构-动力学耦合理论，对稳态温度场对某汽轮机转子系统的临界转速的影响进行了探讨。结果表明，随着转子系统轴向温差和径向温差的增大，热变形程度逐渐增大，系统振动的临界转速逐渐降低。稳态温度场对低阶临界转速的影响较大，而对于高阶临界转速的影响较小，特别对第一阶临界转速具有较大的影响，但对各阶临界转速的振型影响很小。在对转子系统的振动研究中，不应当忽略温度的影响。     

二氧化钒薄膜材料相变临界场强调控方法研究

采用无机溶胶-凝胶法并结合真空退火工艺在Al_2O_3陶瓷基片上制备了二氧化钒及其他价态钒氧化物共存的薄膜材料。研究了退火时间对VO_2、V_2O_5、V_6O_(13)、V_6O_(11)等价态成分和含量的影响以及对薄膜的相变临界温度和相变临界电场强度的影响。实验采用的退火时间分别为10h、8h及6h,得到的薄膜的相变临界电场强度分别为1.8 MV/m、0.8 MV/m及0.4 MV/m,相变场强降低75%以上,且随着电场强度相变点的降低,薄膜材料相变点前后电阻变化倍数也降低,但相变临界温度没有明显变化。研究结果表明:通过控制真空退火时间能够实现对电场强度相变点的有效调控,利用该方法可以研制不同相变临界场强的薄膜材料,以适应不同电磁环境的防护应用要求。     

Field and temperature dependencies of critical current on industrial Nb3Sn strands

The increasing need for high field magnetic devices has focused attention on filamentary Nb₃Sn conductors, whose critical data are superior to NbTi conductors. To choose the suitable operating parameters and to determine the stability margin of magnet systems, it is very important to know the effect of temperature and magnetic field on the superconducting properties, especially on the critical current. Up to now, for design calculation, the so-called “Summers model” was assessed theoretically on experimental data obtained by Spencer et al., (The temperature and magnetic field dependence of superconducting critical current densities of multiinflammatory Nb₃Sn and NbTi composite wires. IEEE Trans Mag, Mag-15 (1979) 76) and Suenaga et al., Superconducting critical-current densities of commercial multifilamentary Nb₃Sn(Ti) wires made by the bronze process. Cryogenics (1985) 25, 123). Apart these very useful preliminary experimental data, very little has been done on the very different industrial strands which are now produced in the industry. Industrial Nb₃Sn strands are generally tested and checked only at 4.2 K and their operating design temperature is often very different, sometimes around 6 K. It is now urgent to validate the model and to confirm that the data taken up to now in the design calculations are conservative.     

Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

机械合金化合成高临界场Sm0.85Nd0.15FeAsO0.85F0.15超导体（英文）

通过采用机械合金化方法制备的高活性的粉体,可以高度可重复性地制备高质量的铁基超导材料Sm0.85Nd0.15FeAsO0.85F0.15.样品具有高临界温度Tc(约51 K)和高临界场Hc2(达到377 T).由WHH公式确定的Hc2显著高于常规固相方法制备样品的典型值(<200 T).高的临界磁场Hc2与样品微结构有很大关系.机械合金化处理的原始粉体包含大量的晶格畸变缺陷,在快速升温和低温退火制备的小晶粒陶瓷样品中这些缺陷会部分残留,因此形成有效的磁通钉扎,从而提高样品的临界场.     

Measurements of the vapor-liquid coexistence curve and the critical parameters for 1,1,1,2-tetrafluoroethane

Measurements of the vapor-liquid coexistence curve in the critical region for 1,1,1,2-tetrafluoroethane (R134a; CH₂FCF₃), which is currently considered as a prospective substitute for conventional refrigerant R12, have been performed by visual observation of the disappearance of the meniscus at the vapor-liquid interface within an optical cell. Twenty-seven saturated densities along the vapor-liquid coexistence curve between 208 and 999 kg·m^–3 have been obtained in the temperature range 343 K to the critical temperature. The experimental uncertainties in temperature and density measurements have been estimated to be within ±10mK and ±0.55%, respectively. On the basis of these measurements near the critical point, the critical temperature and the critical density for 1,1,1,2-tetrafluoroethane were determined in consideration of the meniscus disappearing level as well as the intensity of the critical opalescence. In addition, the critical exponent ß along the vapor-liquid coexistence curve has been determined in accord with the difference between the density of the saturated liquid and that of the saturated vapor.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

地基临塑荷载的分析

本文根据线弹性理论；分析讨论了地基的临望荷载和临界荷载，推导了较为精确的临塑荷载公式，并与目前教材中常用的临塑荷载公式和临界荷载公式进行了比较。     

Impact analysis of change in component reliabilities in safety‐critical systems

A safety‐critical system or life‐critical system is a system whose failure or malfunction may result in one (or more) of the following outcomes: death or serious injury to people and loss or severe damage to equipment/property. Such systems are very common in nuclear power plants and are composed of several components, performing different functions. The criticality of these components is ranked according to the criticality of the functions they perform. Therefore, the impact of component failure on system will be different for different components. It is essential to determine the impact of failure of any component on overall system to take preventive and corrective actions. This paper proposes a technique to determine the criticality of the components for their impact on the overall system using Bayesian approach. The theoretical basis and effectiveness of the proposed technique is shown and validated on a real case study of a nuclear power plant system.     

Theory of the universal and nonuniversal quantities of fluids at the critical point

We show that the hierarchical reference theory is an accurate global theory of fluids at least above the critical temperatureT _c. The hierarchy is truncated at the first equation, the one connecting the free energy to the pair correlation function, with an Ornstein-Zernike ansatz. In this approximation the theory can be considered as a sophisticated generalization of the optimized random phase approximation which has genuine nonclassical critical exponents and for which scaling is satisfied. We study the system of hard spheres plus the Lennard-Jones attractive well and find a good agreement with measuredPVT, specific heat, correlation length, and structure factor in rare gases. The accuracy of the theory remains very good up to freezing density.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Prediction of Critical Properties of Mixtures from the PRSV-2 Equation of State: A Correction for Predicted Critical Volumes

The predictive capability of the Peng–Robinson–Stryjek–Vera (PRSV-2) equation of state (1986) for critical properties of binary mixtures was investigated. The procedure adopted by Heidemann and Khalil (1980) and discussed by Abu-Eishah et al. (1998) was followed. An optimized value for the binary interaction parameter based on minimization of error between experimental and predicted critical temperatures was used. The standard and the average of the absolute relative deviations in critical properties are included. The predicted critical temperature and pressure for several nonpolar and polar systems agree well with experimental data and are always better than those predicted by the group-contribution method. A correction is introduced here to modify the predicted critical volume by the PRSV-2 equation of state, which makes the average deviations between predicted and experimental values very close to or even better than those predicted by the group-contribution method.     

流量测量临界流文丘里喷嘴的进展

简介颁布国际标准ISO9300《临界流文丘里喷嘴测量气体流量》(1990年)，十余年来国际上对该流量计的研究情况。主要在以下几方面有显著的进展：雷诺数使用范围的拓宽；流量方程中系数的研究；小型音速喷嘴的开发应用；小型音速喷嘴组合方法的研究；绝对测量法的探索等。国际标准化组织正在为修订ISO9300进行准备工作。     

一种平板状爆炸焊接用炸药

将硝酸铵、硝酸钠、尿素、水、机油和Span80等通过乳化技术制备出乳胶基质。将制备好的乳胶基质和泡沫树脂混合,得到一种平板状炸药。实验表明:该种炸药的密度可低至0.17g/cm³,爆轰的临界厚度为18 mm,在最低临界密度下的爆速为1 678 m/s;随着密度的增加,爆速增加。密度为0.23 g/cm³时,测得爆速为2 149 m/s。初步分析,该炸药可满足某些爆炸焊接工艺的要求。