新型超导材料的研究进展

综合评述了铜氧化合物高温超导材料研究进展,详细介绍了近年来发展的铜氧高温超导材料改性的各种方法,阐述了硼化镁、有机超导体研究现状,特别是有关硼化镁结构及性质的研究结果;简要介绍了最近开展的钴氧化物新型超导材料研究等.     

Superconductor Stability Against Quench and Its Correlation with Current Propagation and Limiting

This paper presents a numerical (finite element) analysis of superconductor stability and current propagation under random variations of critical superconductor parameters. Instead of using singular (homogeneous) values, random variations potentially are appropriate to take into account any conductor inhomogeneity that can be considered as an obstacle to current propagation. Traditional assumptions like homogeneous current distribution, critical temperature, critical current density and critical magnetic fields are not justified in general; a local disturbance (for example, release of mechanical stress energy), if not immediately distributed by solid conduction, would generate a transient increase of local conductor temperature. Local critical current density and magnetic field then will be reduced, and current distribution will change. Disturbances may arise also from transport currents that locally exceed the critical current of the superconductor. Disturbances of all kinds may increase the conductor temperature above its critical value. A local analysis of all superconductor states thus is mandatory to safely avoid a quench. As an extension of standard stability models, also flux flow resistive states are taken into account. We will try to find a possibly existing correlation between current propagation and superconductor stability. Fault current limiting is discussed as a special case of current propagation. The analysis is applied to a bundle of high-temperature superconductor (HTSC) filaments. As will be shown, temperature profiles in a superconductor do not allow a clear distinction between Ohmic resistive or flux flow resistive fault current limiting. Though frequently made in the literature, this separation is highly questionable, because Ohmic resistive and flux flow resistive states may locally coexist, side by side, but are not very stable in the superconductor volume.     

Influence of oxide film defects generated in filling on mechanical strength of aluminium alloy castings

Abstract

The influence of oxide film defects generated from the filling process on the mechanical strength of aluminium alloy castings has been investigated. Using numerical simulation and experimental validation, the investigation aims to reveal the relationships among the liquid aluminium flow behaviour in the filling, the likely oxide film defect distribution caused by surface turbulence and the final mechanical strength of the castings. CFD modelling was used to investigate the liquid metal flow behaviour and the likely oxide film defect distribution in the filling at different ingate velocities. In particular, a numerical algorithm - Oxide Film Entrainment Tracking (OFET, 2-D) has been proposed and developed for predicting such oxide film defect distribution in the liquid aluminium throughout the filling. Also, light microscopic and SEM techniques were used to identify the microstructures of oxide film casting defects. The Weibull statistics method was employed to quantify the effects of oxide film distributions on the mechanical strength and reliability of the acquired aluminium alloy castings for different runner systems. It was found from the numerical simulations that the ingate velocities acquired using different runner systems have significant influence on the distribution of oxide film defects generated by surface turbulence in the filling process, which results in the disparities of the final mechanical strength of the castings. The results of the mechanical property test and the SEM micro-structural analysis of the castings are consistent with the numerical simulations.     

氧化石墨烯改性玄武岩纤维及其增强环氧树脂复合材料性能

为了改善玄武岩纤维/环氧树脂复合材料的界面性能,通过偶联剂对氧化石墨烯进行改性,并将改性后的氧化石墨烯引入到上浆剂中对玄武岩纤维进行表面涂覆改性,同时制备了氧化石墨烯-玄武岩纤维/环氧树脂复合材料.采用FTIR表征了氧化石墨烯的改性效果;运用SEM分析了改性上浆剂处理对玄武岩纤维表面及复合材料断口形貌的影响和作用机制.结果表明:偶联剂成功接枝到氧化石墨烯表面;玄武岩纤维经氧化石墨烯改性的上浆剂处理后,表面粗糙度及活性官能团含量增加,氧化石墨烯-玄武岩纤维/环氧树脂界面处的机械齿合作用及化学键合作用增强,界面黏结强度得到改善,玄武岩纤维的断裂强力提高了30.8%,氧化石墨烯-玄武岩纤维/环氧树脂复合材料的层间剪切强度提高了10.6%.     

Josephson supercurrent through a topological insulator surface state

The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, direct evidence for Josephson supercurrents in superconductor (Nb)-topological insulator (Bi(2)Te(3))-superconductor electron-beam fabricated junctions is provided by the observation of clear Shapiro steps under microwave irradiation, and a Fraunhofer-type dependence of the critical current on magnetic field. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. This surface state is attributed to mediate the ballistic Josephson current despite the fact that the normal state transport is dominated by diffusive bulk conductivity. The lateral Nb-Bi(2)Te(3)-Nb junctions hence provide prospects for the realization of devices supporting Majorana fermions.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

纳米二氧化钛对质子辐照下MQ增强硅橡胶力学性能的影响

采用空间综合辐照模拟设备研究了纳米二氧化钛对100 keV和150 keV能量质子辐照下MQ增强加成型硅橡胶力学性能的影响。试验结果表明,质子辐照后,未添加纳米粒子的硅橡胶表面颜色明显加深,同时出现不同程度的老化龟裂裂纹,裂纹数量和深度随辐照能量和剂量的增加而增加;硅橡胶的硬度和拉伸强度随辐照剂量的增加先增加而后降低。添加纳米二氧化钛的硅橡胶与未改性硅橡胶相比,经过相同能量、剂量的质子辐照后,表面颜色加深和表面裂纹损伤的程度均减小,硬度和力学性能的损伤程度下降,表现出明显的抗质子辐照性能。     

表面改性对PVC/稻糠发泡复合材料的影响

研究了纳米CaCO3、硅烷偶联剂、甲基丙烯酸甲酯(MMA)改性稻糠对PVC/稻糠发泡复合材料性能的影响.结果表明:纳米CaCO3能改善稻糠的耐热性,用硅烷偶联剂处理稻糠的发泡复合材料冲击性能好于用MMA微波处理稻糠发泡复合材料;而用MMA微波处理稻糠的发泡复合材料在拉伸性能、弯弹性模量及加工性能优于硅烷偶联剂处理的稻糠发泡复合材料.     

表面改性对纳米蒙脱土/聚酰胺6-66复合材料性能的影响

选取三种不同的纳米蒙脱土（nMMT）（分别为钠基蒙脱土（Na-MMT）、氨基酸改性nMMT（OMMT-A）和CH₃（CH₂）₁₇N（CH₃）[（CH₂CH₂OH）₂]+改性nMMT（OMMT-B））,通过熔融共混法制备了不同改性的纳米蒙脱土/聚酰胺6-66（nMMT/PA6-66）复合材料,研究了不同表面改性对nMMT/PA6-66复合材料的结晶、流变和力学等性能的影响。结果表明,nMMT的加入促进了nMMT/PA6-66复合材料中γ晶的形成,提高了复合材料的结晶温度,但加入OMMT-B后OMMT-B/PA6-66复合材料的异相成核作用效率有一定程度减弱;同时,OMMT-B能更好地改善PA6-66的储能模量,提高PA6-66的流动性。力学性能测试表明,nMMT提高了nMMT/PA6-66复合材料的强度,降低了复合材料的韧性,但效果不同。其中,加入OMMT-B后OMMT-B/PA6-66复合材料的韧性几乎保持不变,拉伸强度和弯曲强度相对于纯PA6-66分别提高了26%和28%,表现出最佳的综合力学性能。综合研究结果表明,不同表面改性nMMT对PA6-66性能的影响主要取决于改性剂和PA6-66分子链之间相互作用的强弱。      

辐照改性LLDPE及增容增韧作用

采用FT-IR、XRD、SEM和力学性能测试等研究了紫外辐照对线性低密度聚乙烯(LLDPE)的结构与性能的影响,以及辐照改性LLDPE的增容作用。结果表明,通过紫外辐照,在LLDPE分子链上引入了C=O和C-O等基团。辐照改性LLDPE的晶面间距没有发生变化,但是其拉伸强度和断裂伸长率下降。制备了尼龙66/辐照改性LLDPE(90/10)复合材料。辐照改性LLDPE与尼龙66之间的相容性好,辐照改性LLDPE对尼龙66具有较好的增容增韧作用。     

拟薄水铝石包覆氧化石墨烯改性环氧树脂-氰酸酯复合材料的微观表征及力学性能

以物理法石墨烯为原料,采用改进的Hummers法制备了氧化石墨烯(GO),通过溶胶-凝胶法在氧化石墨烯表面生长拟薄水铝石(AlOOH)纳米颗粒,得到拟薄水铝石包覆氧化石墨烯(GO@AlOOH),以环氧树脂(E51)和双酚A型氰酸酯(BCE)为原料,GO@AlOOH作为增强体,制备GO@AlOOH/E51-BCE复合材料。采用红外光谱和X射线表征GO@AlOOH,结果表明:AlOOH和GO存在着相互作用,且其表面含有羟基活性基团。复合材料的SEM结果显示:GO@AlOOH的加入使得树脂断裂面明显改变,性能得到提升。当掺杂量为0.6wt%时,复合材料的弯曲强度、弯曲模量以及冲击强度达到最大,分别为158.23 MPa、2.37 GPa和46.96 kJ/m2。该复合材料具有优异的力学性能,具有一定的发展潜力。      

Effect of metal oxides addition on the superconducting properties of YBaCuO

The superconducting properties of YBCO superconductor with ZrO2 addition prepared by different contents were prepared by a unique method so called thermal pyrolysis process to study the effect of the zirconium oxide on the electromagnetic properties and superconducting mechanism of the superconductor. The critical temperature of YBCO superconductor was not predominately dependent upon the zirconium oxide. The maximum magnetism was observed by adding 2% zirconium oxide. The addition of zirconium oxide forming a pinning center of magnetic flux due to ZrO2 phase which was related to the change of electromagnetic properties of the YBCO superconductor. The result indicates that magnetization is proportional to the number of magnetic flux lines passing through the sample. The added ZrO2 appear to contribute the increasing Vickers hardness.     

Low energy O2 and N2O ion beam modification of polyimide for improving adhesive strength of Cu/PI in roll-to-roll process

To enhance the weak mechanical durability of directly deposited copper layers on polyimide (PI) film due to their poor adhesive strength, a continuous roll-to-roll process involving surface modification using a reactive ion beam irradiation and in-situ deposition process is studied. The polyimide film is modified by an ion source with a linear stationary plasma thruster (LSPT) in the vacuum roll-to-roll process. An O₂ ion beam, with beam energy of 214 eV and beam current density of 0.78 mA/cm², and N₂O ion beam, with 220 eV and 0.69 mA/cm², irradiate PI film in winding speed of 0.5 m/min. The surface energy increases from 38 mN/m for the pristine PI film to 80 mN/m after beam irradiation at an ion fluence of 3.5 × 10¹⁶ ions/s. After beam irradiation, a 10 nm thick tie layer and 200 nm thick copper layer are successively deposited by in-situ DC magnetron web coating. The peel strength of the copper layer on the PI film is enhanced from 0.4 kgf/cm without ion beam treatment to 0.71 kgf/cm after O₂ beam treatment and to 0.75 kgf/cm after N₂O beam treatment. This enhancement is closely related to the increase in the polar force originating from the formation of hydrophilic CO (carbonyl) groups on the modified PI surface.     

Isochoric magnet system tests

Aniobium-titanum solenoid has been operated under isochoric, ie sealed constant volume, conditions. Pressure were allowed to rise to over 400 kPa and the operation was continued beyond the critical J-H-T surface of the superconductor, which was found to be in good agreement with published data. Demountable current leads of a modified Efferson type, and a mechanical semi-persistent switch, were used with the coil. The results support the design proposals for isochoric operation of the superconducting magnets of the Canadian Maglev system, but the test system required a more sophisticated thermodynamic analysis to account for the large neck effects in the test dewar.     

Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites

A novel method of biomimetic surface modification was used for aramid fibres aiming to enhance the interface properties between epoxy resin and the modified aramid fibre. Inspired by the composition of adhesive proteins in mussels, a thin layer of poly(dopamine) (PDA) was self-polymerized onto the surface of the aramid fibre. The graphene oxide (GO) was then grafted on the surface of PDA-coated aramid fibres. The microstructure and chemical characteristics of the pristine and modified fibres were characterised using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), indicating successful grafting of GO on the PDA-coated aramid fibres. Single fibre tensile test and microbond test were carried out to evaluate the mechanical properties of the modified fibres. It was found that the fibre surface modification improved the interfacial shear strength by 210% and the fibre tensile strength was protected by GO-PDA coating.     

Optimization of cryostabilized superconducting generator windings cooled by the thermosiphon method

The problem of optimum parameter choice for the superconducting field winding when using the thermosophon cooling method has been discussed.As an optimum criterion, the magnetic flow within the superconducting field winding at a given outer radius of rim cylinder is chosen. The restrictions it imposes on the superconducting field winding cryostabilization and mechanical strength of its supporting elements are taken into consideration. The parameters to be optimized are: the inner and outer radii of the superconducting field winding, superconducting field winding current, ratio of the superconductor area to the matrix area in the multifilamentary composite conductor, cooling channel dimensions, and dimensions of the superconducting field winding supporting elements. The optimum magnetic flows for the stabilized and non-stabilized windings and for windings made of normal metal are compared.     

氧化石墨烯可见光催化能力的影响因素

为了验证氧化石墨烯的光催化能力,研究能增强其光催化效果的最佳条件,用改进的Hummer法制备了氧化石墨烯及分散液,通过扫描电镜、透射电镜、红外图谱、拉曼光谱以及荧光共聚焦光谱,对氧化石墨烯的微观形貌、成分、结构进行表征研究;在光照条件下降解亚甲基蓝验证其光催化能力,通过改变催化剂浓度、搅拌条件、曝气氧气浓度和外加电流,研究了四种条件对光催化能力的影响.在亚甲基蓝浓度为40 mg/L,氧化石墨烯浓度为20 mg/L,曝气量为2 L/min,氧气浓度为50%时,2 h可见光照射后亚甲基蓝降解率为62.9%,总去除率为89.9%;当不进行曝气处理,外加电流密度为1 m A/cm~2时,2 h可见光照射后亚甲基蓝降解率为53%,总去除率为87.9%.氧化石墨烯可以通过光催化产生羟基自由基降解污染物,通过改变条件来增加自由基含量可以提升催化效果.     

Mechanical,flow and electrical properties of thermoplastic polyurethane/fullerene composites: Effect of surface modification of fullerene

Thermoplastic polyurethane (TPU) composites with fullerene loadings varying from 0.5 to 2 weight% were prepared by melt-mixing method. Nitric acid oxidation and silanization were applied to fullerene surface to improve interfacial interactions with TPU matrix. The influence of surface modifications of fullerene on mechanical, melt flow and electrical properties of TPU based composites were investigated. Incorporation of fullerene leads to nearly twofold increase in tensile strength and Young's modulus of the composites in addition to enhancing the flexibility. The best results are obtained in nitric acid and silane modified fullerene containing composites at the lowest concentration (0.5%). Higher MFI values were observed for composites loaded with surface treated fullerenes compared to pristine fullerene because of their better dispersion in TPU. Electrical properties of TPU also improved by the addition of surface modified fullerene particles. Surface oxidation and silanization gave rise to dispersion homogeneity which may be the reason of both tensile strength and strain improvements at the same time.     

Applied current density in the section of a type-II superconductor

A thermodynamic treatment suggests that the applied current density is located in the surface sheet in order to create a null magnetic local field inside the bulk superconductor according to the London penetration law, until there is no dissipation in the sample. A direct investigation of this evidence is performed by measuring the field created by the current near the surface of square- or rectangular-section samples. This field is known by the measurement of the flux variation in small pickup coils while the current is turned on and off. The main experimental result shows that the critical current intensity does not flow in a uniform distribution inside the sample section and so denies a physical meaning to the notion of critical current density.Laboratoire associé au C.N.R.S., 24 rue Lhomond, 75231 Paris Cedex 05.     

Magnetization and pinning effect of a Josephson junction

The magnetization and the pinning effect of a Josephson junction have been analyzed by using a mechanical analog. The influences of an applied current on the magnetization and the critical current have been studied. The peak and the training effects have been observed when inhomogeneities are introduced into the junction. The behavior of the junction is qualitatively analogous to that of a type II superconductor. The mechanical model is a useful analog computer for studying precisely and visually the vortex behavior of the junction and is also helpful in understanding qualitatively that of a type II superconductor.