SiC掺杂对MgB2/Fe超导线材临界电流密度和显微结构的影响

利用原位粉末套管法制备出SiC微粉掺杂的MgB2-x(SiC)x2／Fe(z=0．00、0．05、0．10、0．20)超导线材。在750℃,流通高纯氩气的条件下热处理1h后,大部分SiC没有参与取代B位的反应。随着x的增大,线材中非超导相SiC和Mg的含量增加,MgB2的平均晶粒尺寸变小,从而使可作为磁通钉扎中心的晶界的面积相应增加。在外加磁场中,MgB2超导线材的临界电流密度(Jc)随x增大逐步升高,至x=0．10时Jc性能最好,其在6K,5T时的Jc达到了8480A／cm^2,比未掺杂线材的Jc高出约70％。但是,当x=0．20时,Jc却有所下降。Jc的这种变化规律与SiC掺杂引起的MgB2晶粒变小,以及非超导相物质含量之间的相互平衡有关,其中MgB2晶粒变小是Jc提高的主要原因。     

分步反应法制备SiC掺杂MgB2超导线材的研究

以干燥的B粉和Mg粉为原料,采用分步反应法制备了SiC(40～45μm)掺杂的MgB2/Nb/Cu超导线材.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)研究了在650℃热处理温度下不同掺杂量对所制备样品的相组成、微观结构的影响.采用标准四引线法测定了样品的电阻温度曲线,结果表明,当该工艺第二步热处理温度为650℃时,反应不完全,应适当升高温度;同时高SiC掺杂量有利于在MgB2线材中引入更多B位C掺杂,表现为Tc略有降低,从而提高了其磁通钉扎特性.     

烧结方法对MgB2超导体结构和性能的影响

以Mg粉和B粉为主要原料,采用两种烧结方法制备出MgB2超导体.利用x射线衍射仪、扫描电子显微镜、阿基米德法和标准的直流四引线法等,分别研究了样品的物相组成、显微结构、密度和临界电流密度等.研究发现,MgB2块材的密度随Mg粉粒度的增加而降低.与常规烧结相比,快速烧结时MgB2块材的密度较低且含有更多的非超导相,如Mg和MgBx(x=4,6或12).快速烧结MgB2线材的临界电流密度低于常规烧结线材的.结果表明,快速烧结不利于获得高性能的MgB2超导体.     

Campbell法研究MgB2临界电流密度

PIT(powder in tube)法制备了MgB2超导材料;用Campbell法测量了样品的外加交流磁场bac与磁通渗透深度λ′的关系,并计算得到样品在外磁场中的临界电流密度;讨论了bac-λ′,曲线误差形成的原因和选取曲线平稳部分为计算有效段原则;Campbell法测量考虑了样品的整体结构影响电流的特性,得到的结果更切合超导材料的实际临界电流密度。     

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.     

Bi-2212超导线材研究进展

Bi₂Sr₂CaCu₂O_8+x（Bi-2212）超导体有很高的临界转变温度和极高的上临界磁场,以及在高场下具有优异的载流性能,这使得Bi-2212成为制备高场超导磁体的绝佳材料。并且,Bi-2212是唯一可以制备成各向同性圆线的高温超导材料,极大地简化了Bi-2212超导电缆和线圈的制备工艺。所以Bi-2212在强磁场领域,如核磁共振、磁悬浮列车和未来可控核聚变装置等表现出极大的应用潜力。简述了Bi-2212多芯线材的制备工艺,包括粉末装管工艺以及相应的前驱体粉末的制备方法、线材加工技术和部分熔融热处理工艺等,总结了影响Bi-2212超导线材临界电流密度的主要因素以及提高Bi-2212超导线材临界电流密度的方法。     

二硼化镁超导线带材及磁体应用研究进展

由于具有超导转变温度（39K）较高,晶体结构简单,原材料成本低廉以及长线制备容易等一系列特点,金属间化合物二硼化镁（ MgB2）超导体自2001年被日本科学家发现以来,引起人们广泛的关注,被认为是目前最有可能首先实现大规模工业应用的超导材料。尤其在制冷机工作温度（15～20 K）、较低磁场（1～2 T）条件下的医疗核磁共振成像仪（ MRI）超导磁体应用上有着广泛的前景。本文主要围绕实用化 MgB2超导长线（带）制备研究而展开,重点回顾了近年来粉末套管法、连续粉末装管成型法及中心镁扩散法等MgB2超导线（带）材制备及加工方面的最新研究进展;同时综述了在 MgB2超导线带材工程临界电流密度性能改进方面的最新研究工作;最后,对近几年来 MgB2超导磁体及线圈等应用研究进展进行了回顾。     

TiC和SiC掺杂MgB2超导体的对比研究

分别制备了8％（质量分数）的TiC和5％（质量分数）的SiC掺杂的MgB2超导块材，并对比分析了这两种掺杂物对MgB2超导块材的性能影响。所有样品均在流动的Ar气保护下在不同退火温度下退火，研究发现TiC掺杂和SiC掺杂的MgB2样品的最优化工艺参数分别是900℃保温1h和720℃保温1h。随后采用XRD和SEM分别对样品进行了相成份和微观结构的分析，并采用PPMS测试了样品的磁滞回线并由Bean模型计算出了样品的临界电流密度。在4．2K，0T下TiC掺杂的五值为1．0×10^5A／cm^2，而10K，0T下SiC掺杂样品的五值为4×100A／cm^2。而且随着外加磁场的增加，SiC掺杂MgB2样品的五值下降得比TiC掺杂的MgB2样品要缓慢很多，这表明了SiC掺杂比TiC掺杂更有利于改善MgB2在高场下的超导电性能。     

新型超导体MgB2及其掺杂研究

MgB2是在2001年新发现的超导材料.它作为一种新型超导体材料备受关注且发展迅速,但其较低的不可逆场和较小的高磁场临界电流密度已成为阻碍其发展的关键.化学掺杂由于具有方便快速、能均匀改性等优点而成为当前提高MgB2超导性能的研究热点.重点评述了有关MgB2的元素或化合物掺杂研究的国内外最新进展,并从基础和应用的角度提出了MgB2超导材料需要深入研究的问题.     

新型铁基超导线带材的研究进展

新型铁基超导材料具有较高的临界转变温度、超高的上临界场和非常小的各向异性等优点,在高场磁体领域应用前景广阔。目前粉末装管法已广泛应用于铁基超导线带材的制备,临界电流密度在4.2 K和10 T下高达1.7×104 A/cm2,接近实用化水平。首先介绍了铁基超导材料的典型结构与基本超导特性,详细评述了铁基超导粉末的合成和粉末装管法的工艺流程,主要包括元素配比、热处理工艺以及粉末装管原位法和先位法的对比分析,重点探讨了影响线带材临界电流密度的因素及其关键制备技术。通过包套材料的选择、掺杂改性、热等静压和轧制织构等方法来解决线带材中包套与超导芯易反应、杂相多、致密度低以及晶界弱连接等难题。此外还简要介绍了多芯线带材的最新研究进展。最后,对铁基超导线带材的发展趋势做了展望。     

Fabrication and Properties of Powder-in-Tube Processed MgB2 Tapes and Wires

MgB₂ tapes were fabricated with MgB₂ powder and several sheath materials such as Cu, Cu-Ni, Fe, carbon steel (Fe-C) and stainless steel. High-density MgB₂ cores were obtained for these sheath materials. J _c of the as-cold rolled (non heat treated) tape significantly increased with increasing the cross sectional area reduction by the cold working. Hard sheath materials (Fe-C and stainless steel) are effective to enhance J _c values. These results can be explained by the densification of MgB₂ core. Non heat treated MgB₂/(stainless steel) and MgB₂/(Fe-C) tapes showed extrapolated J _c values of 300–450 kA/cm² at 4.2 K and zero field. MgB₂ tapes show anisotropy in J _c with respect to field orientation. This anisotropy can be explained by the MgB₂ grain orientation. Heat treatment after the cold rolling is effective to enhance J _c values. An order of magnitude higher J _c values were obtained for Fe-C and stainless steel sheathed tapes by the heat treatment. J _c values extrapolated to zero field of MgB₂/(SUS 316) and MgB₂/(Fe-C) tapes reached 1,000 kA/cm² at 4.2 K.     

Experimentally Determined Magnetization ac Losses of Mono and Multifilamentary MgB2 Wires

Magnetization AC loss measurements have been performed by the calibration free method on mono, 19 filaments un-twisted and 19 filaments twisted MgB₂ superconducting wires with magnetic fields perpendicular to the wire axis at various frequencies and temperatures between 24 and 40 K. AC losses were also calculated based on critical state model for mono-filamentary Nb sheathed MgB₂ wire with conditions corresponding to experiments. Good agreement is observed between model and experimental data. AC losses go through a peak when the wire was cooled below T _c. In terms of AC magnetization losses, the twisted wire has the minimum losses in order of 10^?5 J/m at 30 K in AC magnetic field with 20 mT amplitude.     

Fabrication and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB₂ coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB₂ coatings were produced by using pre-synthesized MgB₂ powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB₂ by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.     

Effects of Mg-Coating and Boron Type on Electromechanical Properties of IMD MgB2/Fe Wires

Journal of Superconductivity and Novel Magnetism - In this study, mechanical and electrical characterization measurements were performed on non-bent and gradually bent IMD MgB2/Fe superconducting...     

Fabrication and Transport Properties of MgB2 Thin Films and Tunnel Junctions

We report on fabrication and characterization of MgB₂ thin films and tunnel junction structures. The MgB₂ films were prepared on Al₂O₃, Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-μm-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB₂ polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T _c of up to 39 K and the current density j _c at 4.2 K of about 10⁷ A/cm².     

ITER项目用NbTi/Cu复合超导线的工艺研究

为实现高均匀性、高性能的NbTi/Cu多芯复合线的批量生产,需要优化多芯复合线的传统工艺.对复合线材均匀性和超导性能的研究结果表明:经优化后的拉拔-扒皮工艺完全可以取代传统的拉拔-轧制工艺且能改善NbTi/Cu超导线材的Jc值,为低温超导线材的推广使用打下了坚实基础.     

Investigation on MgB2 Wires Doped by ZrH2

ZrH₂-doped MgB₂ wires, with the component of Mg:ZrH₂:B= (1-x):x:2(x = 0, 5, 8, and 10%), were fabricated through thein situ powder-in-tube method by using low-carbon steel tube as the sheath material Samples were sintered at 750^{^}C for 1h in a flow of high-purity argon.The lattice parameters a and c of MgB₂exhibit a little decrease with the doping of ZrH₂.J_c(B)property of MgB₂wires are improved gradually with the increase of x, reaches the best whenx = 8%, but decreases significantlywhen x = 10%. The decrease of MgB₂grain size, the excellent grain connection, and the substitution of Zr⁴⁺for Mg²⁺which is caused by the doping of ZrH₂, are responsiblefor the enhancement of J_c(B)property forthe doped MgB₂wires. PACS: 74.70.Ad; 74.62.Dh; 74.62.Bf; 74.25.Sv     

纳米Al2O3p/Cu复合材料的制备及其性能研究

采用高能球磨法制备了纳米Al2O3p/Cu复合材料粉体,复合粉体经过压制、烧结和挤压后成为铜复合材料,采用X射线衍射(XRD)、扫描电镜(SEM)、能谱仪(EDS)、电导仪等测试方法,研究了球磨后的复合粉体和复合材料显微结构、电导率和硬度.实验证明,粉体经过2h以上球磨后,Al2O3相逐渐消失,复合粉体为纳米晶结构,晶粒直径66～87nm.随着Al2O3粉体含量增加,铜复合材料的电导率显著下降.烧结后挤压有利于复合材料导电性能的提高.当Al2O3粉体含量1wt%、球磨6h时,烧结后重新挤压的复合材料试样电导率IACS 76%、硬度HB 83.8.     

Effect of Ti Doping on the Superconductivities of MgB2/Fe Wires

Mg_1–x Ti _x B₂/Fe (x=0, 0.1 and 0.2) wires were made by Powder-In-Tube method. XRD patterns indicate the main phase is MgB₂ and there are Ti particles in the Ti doped wires and no other Ti based impurities in the cores. SEM images show that the MgB₂ particles are distributed evenly in porous core. The J_c of the wire (Mg_1–x Ti _x B₂/Fe, x=0.1) measured magnetically is higher than that of those wires (Mg_1–x Ti _x B₂/Fe, x=0 and 0.2), while the H_irr, are almost the same for three wires.