Na掺杂对Bi-2212高温超导体性能影响研究

本文采用放电等离子烧结(SPS)工艺制备了Na掺杂的Bi_2.1Sr_1.96Ca_1-xNa_xCu_2.0O_8+d超导材料。Bi-2212前驱体粉末采用改进的共沉淀工艺制备,并在烧结前将前驱体粉末与NaOH混合。本文系统研究了Na离子掺杂对Bi-2212体系晶格结构、相组成、微结构和相关超导性能的影响。Na离子掺杂导致Bi-2212体系相转变温度的显著降低。因此在SPS过程中,烧结温度相应发生了变化。但Na掺杂仍然导致大量Bi₂Sr₂CuO₆ (Bi-2201)相生成,进而导致了超导相含量的降低和织构度的破坏,所以给体系的超导电性带来了负面的影响。另一方面,Na掺杂相Bi-2212中掺入了大量的点缺陷,这些点缺陷可以作为有效钉扎中心。因此,通过优化Na掺杂Bi-2212的烧结工艺,有可能在高场条件下获得体系性能的改善。     

CaCuO_2颗粒尺寸对双粉法制备Bi系高温超导带材性能的影响（英文）

通过基于共沉淀工艺的双粉法制备了Bi_(1.76)Pb_(0.34)Sr_(1.93)Ca_(2.0)Cu_(3.06)O_(8+δ)(Bi-2223)前驱体粉末。在这一过程中,首先单独制备了Bi_(1.76)Pb_(0.34)Sr_(1.93)CaCu_(2.06)O_(8+δ)(Bi-2212)和CaCuO_2 (实际相组成为Ca_2CuO_3和CuO)粉末,并分别进行了烧结。通过调节共沉淀工艺过程中的pH值,获得了颗粒尺寸不同的CaCuO_2粉末,然后将Bi-2212与CaCuO_2粉末按照相组成1:1进行混合,并装入Ag包套中,通过一系列的旋锻、拉拔和轧制工艺,获得设计尺寸的Bi-2223带材。比表面积测试表明随着pH值从3.0增加到5.0和6.5,获得CaCuO_2粉末的平均颗粒尺寸从1.10mm减小到0.75和0.60mm。通过扫描电镜对不同尺寸CaCuO_2颗粒制备的Bi-2223生带、第1次热处理和后处理之后带材的相组成和分布进行了表征。结果表明,适当尺寸的CaCuO_2颗粒可以避免团聚现象的出现,因此有利于高载流性能带材的获得。最终通过进一步调节带材的尺寸,1#带材的性能最高,电流密度达到了约12 200 A·cm~(-2)。     

喷雾热解与共沉淀法制备高性能线材用Bi-2212粉末的中间相演变对比研究

采用喷雾热解和共沉淀工艺分别制备了Bi₂Sr₂CaCu₂O_8+δ (Bi-2212)前驱粉末,研究了Bi-2212前驱粉的中间相演化过程。与共沉淀工艺相比较,喷雾热解工艺提高了前驱粉末中Bi-2212晶粒的相转变效率;而且喷雾热解后粉末中残留的硝酸盐对Bi-2212晶粒的生长影响较小,共沉淀后粉末中残留碳酸盐会阻碍Bi-2212晶粒的形成。采用喷雾热解法制备的Bi-2212线材的性能与采用传统共沉淀工艺法制备的线材性能相近,但喷雾热解工艺的制备效率更高。因此,采用喷雾热解制备前驱粉末有助于实现Bi-2212线材的大规模生产。     

喷雾热分解制备Bi-2212前驱粉末的相演变和超导性能

采用喷雾热分解技术制备高温超导Bi₂Sr₂CaCu₂O_x（Bi-2212）前驱粉末,并研究了粉末热处理过程中的相演变过程及线材的超导性能。结果表明,喷雾热分解制备的粉末平均粒度为3.03 μm,颗粒为球状并呈弥散分布。粉末在热处理过程中的相演变包含4个过程：粉末在527 ℃下主要进行硝酸盐的分解和组元之间的初步反应;由于喷雾粉末具有很高的活性,在588 ℃时生成Bi₂Sr₂CuO_x（Bi-2201）相;喷雾粉末在780 ℃发生Bi-2212相的成相反应;在834 ℃时粉末完全融化。Bi-2212/Ag超导线材的热处理温度窗口很窄,最高热处理温度T_max变化±2 ℃时,临界电流I_c的降幅达到了31 A。Bi-2212/Ag超导线材的最佳T_max为885 ℃,在该温度条件下制得的线材临界电流I_c（4.2 K, 0 T）达到最高值,约为486 A。前驱粉末的热处理气氛为氧气时,线材的临界电流可以进一步提高到712 A。     

热处理温度对Bi-2212喷雾热分解超导粉末成相和性能的影响

采用喷雾热分解法制备了Bi₂Sr₂CaCu₂O_x(Bi-2212)初级粉末,系统地研究了热处理温度对粉末成相和性能的影响。使用TG-DSC分析初级粉末的失重和相变情况,采用XRD分析粉末的相组成,通过SEM观察粉末的微观形貌,采用磁化法对样品的超导物理性能进行判定。结果发现,535~570℃,喷雾热分解初级粉末中的硝酸盐残留物逐渐分解;605~640℃,Bi-2201相形成;温度达到775℃,Bi-2212相开始形成,随热处理温度的提高,粉末结晶度持续增加,Bi-2212相含量先增加后减少,860℃是最佳的成相烧结温度。另外,通过对DSC曲线和M-T曲线的深入分析发现,先进行640℃的低温预分解,再进行860℃的高温成相烧结,可进一步改善最终粉末的质量。     

增强芯丝密度对Bi-2212高温超导带材性能的影响

采用共沉淀工艺制备Bi-2212前驱体粉末,并通过粉末装管法(PIT)制备了Bi-2212单芯带材。通过对装管粉末颗粒尺寸的控制,分别获得了具有不同芯丝密度的带材,并对其所引起的超导带材的微结构和超导载流性能的变化进行了系统的分析。SEM照片显示,前驱体粉末是由3~5微米的初级颗粒组成的,但是由于烧结过程中发生硬团聚,有大量的大尺寸团簇颗粒产生,颗粒尺寸由10微米到180微米不等。通过过筛将前驱体粉末按照颗粒尺寸分为四组,分别进行装管。结果表明,随着平均颗粒尺寸的减小,在冷加工和热处理过程中带材的芯丝密度均有明显增加。因此最终带材的晶间连接性较强,从而获得了带材载流性能较高。本研究可以作为装管工艺优化的一个重要理论依据,为Bi-2212超导线材性能的进一步提高奠定基础。     

增强芯丝密度对Bi-2212高温超导带材性能的影响（英文）

采用共沉淀工艺制备Bi-2212前驱体粉末,并通过粉末装管法(PIT)制备了Bi-2212单芯带材。通过对装管粉末颗粒尺寸的控制,分别获得了具有不同芯丝密度的带材,并对其所引起的超导带材的微结构和超导载流性能的变化进行了系统的分析。SEM照片显示,前驱体粉末是由3~5μm的初级颗粒组成的,但是由于烧结过程中发生硬团聚,有大量的大尺寸团簇颗粒产生,颗粒尺寸由10到180μm不等。通过过筛将前驱体粉末按照颗粒尺寸分为4组,分别进行装管。结果表明,随着平均颗粒尺寸的减小,在冷加工和热处理过程中带材的芯丝密度均有明显增加。因此最终带材的晶间连接性较强,从而获得带材载流性能较高。本研究可以作为装管工艺优化的一个重要理论依据,为Bi-2212超导线材性能的进一步提高奠定基础。     

降温速率对Bi-2223/AgAu带材微观结构和传输性能的影响

采用PIT工艺制备了单芯Bi-2223/AgAu带材,系统地研究了第二次热处理阶段(HT2)降温速率对带材相组成、微观结构和传输性能的影响。结果表明：随着降温速率的减小,富铅相3321不断增加,CuO颗粒尺寸逐渐增大。当冷却速率从600°C/h减小到1°C/h时,临界电流密度Jc从7 kA/cm₂增加到11.5 kA/cm₂,增加了64%。由于晶间连接性能和磁通钉扎性能的提高,在较低的降温速率下,Bi-2223/AgAu带材在磁场下的临界电流密度也得到了提高。     

FABRICATION AND CRYSTALLINITY OF Bi2Sr2CaCu2O8+δ THIN FILMS BY MOLECULAR BEAM EPITAXY

在BiO--(Sr+Ca)O--CuO相图上的Bi₂Sr₂CaCu₂O_8+δ(Bi--2212) 相附近选择不同成分, 用分子束外延法制备成薄膜, 利用XRD, EDS,SEM和AFM研究了成分、衬底温度和臭氧分压对Bi--2212相薄膜成相的影响, 分析了生长速率和错配度对Bi--2212相薄膜质量的影响. 结果表明, Bi--2212相薄膜单相生成的成分范围 (原子分数) 分别为Bi 26.3%---32.4%, (Sr+Ca)37.4%---46.5%, Cu 24.8%---32.6%; 当衬底温度为720℃且臭氧分压为1.3×10^-3 Pa时, 在MgO(100) 衬底上生长出质量较高的c轴外延Bi--2212相薄膜; 通过调整生长速率、更换衬底和插入不同厚度的Bi₂Sr₂CuO_6+δ过渡层的方法, 可以改善Bi--2212相薄膜的结晶质量、表面形貌和导电特性.     

热处理气氛对(Bi,Pb)2Sr2Ca2Cu3Oy超导带材晶粒尺寸及相组成的影响

采用粉末装管法(PIT)制备(Bi,Pb)2Sr2Ca2Cu3Oy(Bi-2223)超导带材,通过X射线衍射仪、扫描电镜研究热处理气氛对Bi-2223晶粒尺寸、结晶度及相组成的影响.研究结果表明:采用空气热处理有利于Bi-2223晶粒的长大;采用7.5%O2-Ar气氛热处理带材中存在较少的非超导相,且Bi-2223的结晶度较高;通过一种复合气氛热处理(热处理过程中使用两种气氛),带材Bi-2223晶粒尺寸较大且非超导相较少.工艺优化后最终带材的临界电流密度超过20000A/cm2(自场,77K).     

Critical current enhancement in Bi-2223/Ag superconducting tapes by controlling its first sintering process

Microstructural evolution of the Bi-2223 phase, liquid phase and secondary phase in the first sintering process has been studied by means of XRD and SEM/EDS. Experiments show that the first sintering temperature and time have a great influence on the Bi-2223 phase formation. The cooling rate after the first sintering process determines the type and grain size of the secondary phases, which in turn decides the critical current of the fully reacted tapes. The fast cooled sample contains a few secondary phase particles with very small size, whereas the slow cooled sample produces large CuO particles. The (Ca,Sr)₂CuO₃ and (Ca,Sr)₁₄Cu₂₄O₄₁ phases are easy to deform, while CuO phase particle has a high hardness and is difficult to deform during uniaxial press. The critical current is largely improved by controlling the cooling rate of the first sintering process.     

Processing high critical current density Bi-2223 wires and tapes

Considerable progress has been made in fabricating (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ in high-T_c superconductor wires or tapes with high critical current densities that are attractive for electric power and high-field magnet applications. The powder-in-tube technique appears to be useful for making silver-clad Bi-2223 composites. This article discusses the processing and the excellent superconducting properties of the resulting wires and tapes.     

铋系高温超导粉体的制备

首先采用喷雾热分解法制备了铋系超导粉体，然后对得到的粉体进行焙烧处理，焙烧时采用不同的工艺参数，包括温度、保护气体的氧分压等。利用XRD，SEM等分析手段，研究了焙烧后粉体的粒度分布，形貌，相组成及其均匀性等。最后，采用粉末套管法将焙烧后的超导粉体制备成单芯超导带材，在形变热处理后，带材样品获得了超过30A的临界电流。     

Models for long-range current flow in bulk oxide superconductors

Mitigating the weak-link behavior at grain boundaries, which limits critical current density (J_c) in polycrystalline high-transition-temperature (high-T_c) superconducting materials, is required for all applications in magnetic fields. Although in general oxide superconductors in polycrystalline form have very poor in-field J_c, conductors containing the bismuth-based compounds Bi₂Sr₂CaCu₂O₈ (Bi-2212)andBi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) are very important exceptions. Long-range, strongly linked conduction has been demonstrated in just two additional polycrystalline materials: TlBa₂Ca₂Cu₃O₁₀ (Tl-1223) deposits prepared by spray pyrolysis, and YBa₂Cu₄O₈ (Y-124) conductor made by the oxidation of metallic precursor method. Determining the characteristics of these materials that permit strongly linked conduction is of critical importance to the development of better conductors. Models of strongly linked polycrystalline conductors are reviewed. Recent studies reveal that small-angle boundaries are present in much higher numbers than expected from calculations based on macroscopic texture, indicating a strong influence of grain boundary energy.It is proposed that strongly linked current flows through a connected network of low energy grain boundaries. Evidence has been obtained to support this model in Bi-2223, Y-124 and Tl-1223.     

Microstructure and properties of Bi-2223/Ag/Ni tapes

To reduce the production cost of Bi-2223 superconducting tapes, Bi-2223/Ag/Ni tapes were fabricated by the powder-in-tube process. The effect of heat treatment on the microstructure and critical current of the Bi-2223/Ag/Ni tapes were studied. The phase compositions of the samples were characterized using XRD. The microstructure was observed using SEM. Experimental results indicate that higher temperature is more conducive to the formation of Bi-2223 phase at an atmosphere of 8.5% O 2 . After the two-step heat treatment, the critical current of samples is about 67.5 A.     

Critical issues in the OPIT processing of high-Jc BSCCO superconductors

The oxide-powder-in-tube (OPIT) method has produced superconducting wires or tapes that exhibit high critical current densities in high magnetic fields when the superconducting oxide is the two-layer compound Bi₂Sr₂Ca₁Cu₂O_y or the three-layer compound Bi_2?xPb_xSr₂Ca₂Cu₃O_y. For good properties to be obtained, however, numerous critical processing issues must be addressed.     

THE INFLUENCE OF PRECURSOR VARIABLES ON THE PHASE FORMATION,MICROSTRUCTURE AND TRANSPORT PROPERTY OF Bi—2223／Ag TAPES

The Bi-2223/Ag tapes were prepared with spray-died powders, which are of different particle sizes and phase assemblages by varying the annealing time in pure oxygen. Longer time annealing degraded the reactivity of precursor powder, which in turn resulted in an incomplete conversion from precursors to Bi-2223, porosity core and misaligned grains in fully processed tapes. The best Jc in short pressed samples varied from 29.7 to 47kA/cm2 for the tapes made from different powders.