MgB2线带材工艺及性能研究进展

MgB_2线带材的加工经历了近15年的研究与发展,生产出的线带材可以在制冷机、较低磁场下使用,另外生产MgB_2线带材的原材料相对低廉,应用前景十分广泛。本文从MgB_2传统线带材加工和后续工艺方法的优化出发,概述了MgB_2线带材研究的进展,详细介绍了内部镁扩散法,即用中心镁棒代替传统镁粉,使用铜镍材料为包套,钽或铌为阻隔层并与镁之间填充硼粉,热处理后得到致密的MgB_2相。论述了未来MgB_2线带材加工的研究重点。     

Deformation processing of high-Tc superconducting oxides

Plastic deformation and texture development in polycrystalline YBa₂Cu₃O_{7? δ} has been studied to expedite the process development of high-critical-temperature (high-T_c) superconducting wires and tapes. It is anticipated that deformation texture will be a major processing consideration in terms of maximizing critical current density, assessing conductor-fabrication options in light of critical current density, and developing such mechanical properties as strength, toughness and thermal fatigue. The intrinsic texture development in YBa₂Cu₃O_{7? δ} deformation processing should be highly beneficial, insofar as the c axes of the crystals tend to become oriented along the compression axis. This means that conducting tapes and wires formed by rolling, extrusion and drawing can develop textures with the c axis in the transverse or radial direction, thus maximizing the flow of current along the length of the conductor.     

Advances in the processing and properties of YBa2Cu4O8

High-filament-count, silver-sheathed composite wires of YBa₂Cu₄O₈ were prepared by a metallic precursor route. The ductility of the metallic precursor enabled the manufacture of tapes containing up to 962,407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. The combination of thermal-mechanical treatment and fine filament dimensions resulted in significant biaxial crystallographic texture. Transport critical current densities in the oxide filaments of 69.5 kA/cm² at 4.2 Kin self-field with reduced weak-link behavior in an applied field were obtained. Critical current retention during bending was shown to increase as filament count increased, with the highest filament-count material showing close to one percent critical bending strain. These wires exhibited some of the best properties for a polycrystalline, sintered wire of YBCO in an applied magnetic field.     

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 bulk application off high-Tc superconductors in Japan

A great deal of progress in the bulk application of high-critical-temperature (high-T_c) superconductors has been achieved in Japan}, particularly with regard to the use of bismuth-based wires and tapes for current leads and magnets. Additional applications, such as power cables, are also being demonstrated. This article describes the status of high-T_cmaterials and applications in Japan, specifically in the areas of current leads, underground transmission cables, and magnets.     

The powder-in-tube processing and properties of Bi-2223

Substantial progress has been made in fabricating long lengths of flexible silver-clad high-critical-temperature superconductor tapes by the powder-in-tube method. Tapes with high critical current density (J_c) that are attractive for electrical power and high-field magnet applications have been produced. At liquid helium (4.2 K) and liquid neon (27 K) temperatures, tapes made by this improved processing method yielded J_c values greater than 100 kA/cm² at zero field; at liquid nitrogen temperatures (77 K), the J_c values exceeded 40 kA/cm². This article discusses the processing and properties of short-length tapes and of pancake coils made from long-length tapes via “wind and react.”     

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。     

Influence of High-Temperature Annealing of the Textured Metal Ni–W Substrate on the Structural Properties of Seed Layer in HTS 2G tapes

The surface reconstruction of the textured metal tapes at the temperatures typical for the formation of the seed buffer Y₂O₃ layer of HTS 2G tapes have been revealed and studied for the first time. The influence of a terrace structure of the substrate surface on the characteristics of the texture of the seed Y₂O₃ layer has been shown. This effect is critically important to the deposition of buffer layers on the moving tape and allows to expand the temperature range of growth, in which the full inheritance of substrate texture by the seed Y₂O₃ layer is occurred.     

Mechanical properties of superconducting MgB2 wire

This study aims at examining the mechanical properties of MgB₂ wires fabricated with PIT method by studying the effect of tensile and bending stresses on their current carrying capacity. Wires are mounted on a tensile machine and are subjected to different load increments within both the elastic and the plastic regions. The current carrying capacity is measured for each load and the behavior of I_c versus stress/strain is studied. Microstructures of MgB₂ core are studied for different loads by using SEM. For bending tests, two cases are examined. The first case is to anneal MgB₂ wires then wind them on mandrels with different diameters, while the second case is to wind un-annealed wires on the same mandrels with different diameters then anneal the winded wires. A comparison between both cases is made using SEM for all different diameters and measuring the corresponding I_c. The behavior of I_c versus bending strain is studied. This whole study aims at giving a clear picture of the optimum loading, bending and processing conditions at which MgB₂ wires will possess a high current carrying capacity for practical applications.     

Cube textured nickel alloy tapes as substrates for YBa2Cu3O7−δ-coated conductors

Suitable substrates for biaxially textured high temperature superconducting YBa₂Cu₃O_7−δ-thin films, which are necessary to carry high currents, are highly cube ({001}〈100〉) textured Ni-tapes that can be produced by heavy cold rolling and succeeding recrystallization annealing. At high temperatures which are necessary for the deposition of the YBa₂Cu₃O_7−δ-film such tapes often tend to abnormal grain growth destroying the cube texture. It was found that this can be avoided by microalloying Ni with 0.1 at.% of different elements each leading to a texture improvement with increasing annealing temperature. YBa₂Cu₃O_7−δ-films deposited on these microalloyed tapes reach critical current densities up to 0.5×10⁶ A/cm². Also non-magnetic Ni-alloys, namely Ni–13 at.%Cr and Ni–9 at.%V, were successfully processed to highly cube textured tapes. Tapes made of these alloys are not only an improvement because magnetic losses are avoided but also because of a three times enhanced yield strength which is due to solid solution hardening.     

The freeway model: New concepts in understanding supercurrent transport in Bi-2223 tapes

A variety of mechanisms for supercurrent transport in (Bi, Pb)₂ Sr₂ Ca₂ Cu₃ O_x (Bi-2223) tapes have been proposed, including the brick-wall and railway-switch models and the more recent freeway model. In this article, these models are compared to a growing library of data, including structural and transport studies on specific grain boundary types as well as the systematics of microstructure and transport in the polycrystalline tapes, including new transmission electron microscopy data on some of the highest-performing samples yet fabricated. Additionally, the freeway model is developed, with the concepts of rotary junction bottlenecks corresponding to edge colony boundaries, of lane changes corresponding to c-axis redistribution within colonies, and on-ramps corresponding to tilted and bent grains.     

C-MoS2-Fe2O3(Fe3O4)纳米润滑剂对无镀铜焊丝导电嘴磨损的影响

针对无镀铜实心焊丝在机器人自动焊接时导电嘴磨损问题,采用机械涂敷法在无镀铜实心焊丝表面制备了C-MoS₂-Fe₂O₃(Fe₃O₄)纳米复合润滑剂,研究了润滑剂配比对导电嘴磨损性能的影响. 结果表明,C-Fe₃O₄涂层的润滑性能优于C-Fe₂O₃涂层的润滑性能,随着涂层中纳米MoS₂含量的升高,导电嘴的抗磨性能增强. 纳米复合润滑剂在焊丝与导电嘴的摩擦界面发生摩擦化学反应形成了保护性的自修复膜,此膜主要由润滑性能优异的FeO,MoS₂,MoO₃组成,避免了焊丝与导电嘴内表面的直接接触,从而减少了导电嘴的磨损. 氧化磨损、磨粒磨损和电弧烧蚀是导电嘴磨损的主要机制.     

CaCuO2颗粒尺寸对双粉法制备Bi系高温超导带材性能的影响

本文通过基于共沉淀工艺的双粉法制备了Bi_1.76Pb_0.34Sr_1.93Ca_2.0Cu_3.06O_8+d (Bi-2223)前驱体粉末。在这一过程中,首先单独制备了Bi_1.76Pb_0.34Sr_1.93CaCu_2.06O_8+d (Bi-2212)和CaCuO₂(实际相组成为Ca₂CuO₃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CaCuO₂粉末,然后将Bi-2212与其按照相组成相组成为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CaCuO₂粉末的平均颗粒尺寸从1.1减小到0.75和0.60 mm。通过扫描电镜对不同尺寸CaCuO₂颗粒制备的Bi-2223生带、第一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CaCuO₂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,达到了12200 Acm^_-2。     

Fabricating Ag2O-stabilized superconductive wires

Fabrication of metal-shielded YBa₂Cu₃O_x superconductive wires provides a possible route to improve the poor mechanical properties and the low critical current densities associated with the ceramic nature of high-T_c superconductive oxide materials. However, results show that wires produced by the powder-in-tube method have low critical current densities and a wide superconducting transition range. A possible reason for these phenomena is the presence of a considerable amount of oxygen-deficient phase in the wire core due to insufficient oxygen diffusion through the metallic shield in the final annealing process. Silver oxide (Ag₂O) was introduced as an in-situ oxygen donor to stabilize the oxygen stoichiometry of the core material. Traditional powder and mechanical metallurgy methods have been used to fabricate these metal-shielded YBa₂Cu₃O_x superconductive wires for bulk electricity applications.     

Dispersion strengthened and highly cube textured Ni alloy tapes as possible substrates for coated conductors

Ni and Ni base alloys are good candidate substrate materials in the development of long lengths of coated superconductors due to the development of strong cube texture following heavy cold working and recrystallisation. Increasing the tape strength is an important issue in order to facilitate the production of very thin and long lengths of high current carrying superconductors. The present paper reports the development of strongly cube textured and thin tapes (40 and 20 μm thickness) of an Ni–1 at.% Al alloy and strengthening of these tapes through controlled internal oxidation. Precipitation of finely dispersed Al₂O₃ particles increased the yield strength by a factor of 3–5 compared to pure Ni tapes without losing the strong cube texture. The problem of surface degradation due to precipitation of Al₂O₃ particles during internal oxidation was sought to overcome by the design of an Ni–Ni–1.5 at.% Al composite tape.     

Phase relations and alignment in bismuth-based high-Tc wires

This article concerns a small region of the immense variable space involved in processing Bi₂Sr₂CaCuO_y (referred to as 2212) wires. Progress toward fabricating wires has been rapid, and the science that underlies and supports the wire technology in this complicated system is becoming known. Over the next few years we can expect much longer lengths of wire with higher critical current density and the emergence of a viable magnet technology using these wires.     

Transport properties of sealed MgB2/Fe/Ni multifilamentary wires heat treated in air

Ni sheathed multifilamentary MgB₂ wires with Fe barrier and Cu stabilizer were prepared by the in situ Powder-In-Tube (PIT) method. After rolling, the ends of the wires were sealed by a simple capping technique and the wires were directly heat treated in air, without vacuum or any inert atmosphere. The quality of the wires was assessed by analysing the phase assemblage and measurement of superconducting properties such as R-T, J_C-T and J_C-H. Phase analysis revealed that only traces of MgO was formed in the superconductor core. Typical multifilamentary wires prepared by this method showed a T_C ≈ 38.5 K and ΔT_C ≈ 1 K and J_C of the order of 10⁵ A/cm² at 6 K (0 T) and 10⁴ A/cm² at 4.2 K (6 T) respectively. These values are quite comparable with the values obtained for wires heat treated in inert atmosphere.     

Microstructure development during pack aluminization of nickel and nickel–chromium wires

Pack aluminization – a chemical vapor deposition process widely used to form protective coatings on Ni-based superalloy components – was used to form shells of Ni₂Al₃, NiAl and/or γ′-Ni₃Al on the surface of γ-Ni wires with diameters of 127 μm. The growth kinetics of these Al-rich intermetallic shells are studied as a function of aluminization time and pack activity at 1000 °C. Similar kinetics but additional phases (Cr/Ni two-phase shell, Cr silicide particles and Al-rich particles distributed in Ni₂Al₃) are found in the shells of pack-aluminized Ni–20 wt.% Cr wires with similar diameters. Fully homogenous Ni–Al and Ni–Cr–Al wires are achieved by interdiffusion at 1200 °C between the deposited Al-rich intermetallic shells and the Ni-rich core of both types of wires. Upon subsequent aging at 900 °C, wires with γ/γ′ structure and high hardness indicative of precipitation strengthening are obtained.     

Corrugated glass–ceramics from LZSA cast tapes

The aim of this work was to fabricate corrugated lightweight structures from green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass–ceramics. The green tapes were cast, thermopressed, corrugated, and sintered at 800 °C for 1 h, yielding low porous and homogenous microstructures confirmed by electron microscopy analysis. Dilatometric analysis demonstrated three linear regions, each one exhibiting a specific thermal expansion coefficient (TEC) in the 25–800 °C temperature range. It was also concluded that 600 °C was the critical temperature above which the LZSA glass–ceramic structure could not be assembled. The fabricated bodies were also characterized macroscopically and geometrically to verify the integrity and the morphology of the structures.