Effect of cooling rate on evolution of superconducting phases during decomposition and recrystallization of (Bi,Pb)-2223 core in Ag-sheathed tape

The reformation of (Bi,Pb)-2223 from the liquid or melt is very important for a melting process of (Bi,Pb)-2223 tape. By combination of quenching experiment with X-ray diffraction (XRD) analysis, the effect of cooling rate on the evolution of three superconducting phases in the (Bi,Pb)-2223 core of Ag-sheathed tape was investigated. The results show that (Bi,Pb)-2223 reformation from the melt seems to experience different routes during slowly cooling at different rates. One is that (Bi,Pb)-2223 phase reformed directly from the melt, and no Bi-2212 participate in this process. The other is that (Bi,Pb)-2223 is converted from the intermediate product, Bi-2212, which formed from the melt during the first cooling stage. Due to the inherent sluggish formation kinetics of (Bi,Pb)-2223 from Bi-2212, only partial (Bi,Pb)-2223 can finally be reformed with the second route.

Optimization of Rolling Process for Bi(2223)/Ag Superconducting Tapes by a Statistical Method

Ag-sheathed (Bi,Pb)(2)SoCa(2)Cu(3)O(x) tapes were prepared by the powder-in-tube method. The influences of rolling parameters on superconducting characteristics of Bi(2223)/Ag tapes were analyzed qualitatively with a statistical method. The results demonstrate that roll diameter and reduction per pass significantly influence the properties of Bi(2223)/Ag superconducting tapes while roll speed does less and working friction the least. An optimized rolling process was therefore achieved according to the above results.

INFLUENTIAL FACTORS ON CRITICAL CURRENT DENSITY OF Ag-SHEATHED Bi-2223 TAPES

The silver-sheathed Bi-Pb-Sr-Ca-Cu-O(Bi-2223) superconducting tapes have been fabricated by the powder-in-tube method. In the process,we investigated the effects of the particle size, rolling and pressing on Jc and found that the Jc value can be a ffected greatly by the three factors.

Fabrication of non-superconducting and superconducting joints of Ag/Bi-2223 tapes by diffusion bonding

Diffusion bonding of multifilamentary Ag/Bi-2223 （ Bi2 Sr2 Ca2 Cu3Ox ） superconducting tapes has been performed successfally. Three types of joints, namely, with sih,er interlayer （type Ⅰ）, bonded directly （type Ⅱ）, with Bi-2212 （BizSr2Ca1 Cu2Ox ） powder interlayer or with Bi-2223 powder interlayer （type Ⅲ） were researched, respectively. The critical currents（ Ic ） of these joints were measured ond microstructures of the joints were evaluated by scanning electron microscope （SEM）. The result shows that type Ⅰ is non-superconducting joint with very low resistance, and type Ⅱ joint and type Ⅲ （with Bi-2212 or Bi-2223 ） joints are both of superconducting property. By comparing, type Ⅱ joint possesses most exceUent superconductivity, type Ⅲ joint with Bi-2223 powder interlayer takes the second place, and type ,任家烈 joint with Bi-2212 powder interlayer is least.

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.

TEM Study on the Ag-BSCCO Interface of Silver-sheathed Bi-2223 Superconducting Tape

The Ag/Bi Sr Ca Cu O (BSCCO) interfaces of silver sheathed Bi 2223 superconducting tapes were studied by using transmission electron microscopy (TEM). Most of the interfaces are even and well bonded. The (001) planes of 2223 phase are parallel to the interfaces. Some step like interfaces were observed. There is occasionally a layer of one half unit cell of 2201 phase at the interface. No orientation relationship between the superconductor and the silver exists in the tapes.

Phase Evolution Study and Optimization of the Heat Treatment Process for High Current Capacity Bi-2223 Tapes

Powder in tube process（PIT） was adopted for the fabrication of single filament Bi-2223 tapes, and a heat treatment process including the first heat treatment（HT1）, intermediate rolling（IR）, and second heat treatment（HT2） was performed. The phase evolution mechanism and microstructure changes during these heat treatment processes were systematically discussed. The influences of HT1 parameters on the phase evolution process of Bi-2223 tapes were discussed. With the optimized HT1 process, a proper Bi-2223 content of about 90% was achieved. HT2 process was also optimized by adding a post annealing process. An obvious increase of current capacity was obtained due to the enhancement of intergrain connections. Single filament Bi-2223 tapes with the critical current of Ic-90 A were fabricated with the optimized sintering process.

Effects of Deformation and Heat－treatment Process on Texture of Bi－2223／Ag Su perconducting Tapes

The effects of deformation and heat-treatment on texture of BSCCO-2223/Ag tapes were investigatedby using single orientation quantitative texture analytical methods. The results show that when tapes weresintered at 835℃ for 24～48 h, the texture developed quickly, and further prolonging sintering tirne hadno significant influence on it. With the increase of deformation rate, the volume fraction of c-axis texturewas improved in linear form at first and then changed slowly. The highest volume fraction of c-axis texture(vol%)in these experiments is 33 . 49 % .

Influence of Precursor Powder Fabrication Methods on the Superconducting Properties of Bi-2223 Tapes

Bi-2223 precursor powders are prepared by both oxalate co-precipitation(CP) and spray pyrolysis(SP) methods.The influence of fabrication methods on the superconducting properties of Bi-2223 tapes are systematically studied. Compared to the CP method, SP powder exhibits spherical particle before calcination and smaller particle size after calcinations with more uniform chemical composition, which leads to a lower reaction temperature during calcination process for Bi-2223 tapes. Meanwhile, the non-superconducting phases in SP powder are more uniformly distributed with smaller particle sizes. These features result in finer homogeneity of critical current in large-length of Bi-2223 tape, higher density of filaments and better texture after heat treatment. Therefore,the SP method could be considered as a better route to prepare precursor powder for large-length Bi-2223 tape fabrication.

Physical Effects of Mechanical Processes on Bi-2223 Tapes

The effects of intermediate mechanical deformation （IMD） and bending processing on Bi-2223 tapes were studied. Bi-2223 tapes were manufactured by powder-in-tube process with an IMD. Normal rolling （NR）, pressing （P） and sandwich rolling （SR） with different reduction rate were used in the IMD. And there were an optimum reduction rate existing for the three MID techniques, at which critical current reached maximum. Critical current densities Jc of Bi-2223 crystals were measured with an applied magnetic field B respectively parallel to ab face and c axis. Relations of Jc dependences of reduction rate and superconducting materials density D were respectively studied and showed a Gaussian distribution law. Maximum pinning force density Fmax and irreversible magnetic field Birr were introduced to describe the effects of mechanical processing. Analysis of experimental results showed that Jcs Fmax and Birr were linear dependence on D. Obviously, increasing D was a vital way to enhance Jc Bending experiments were performed for SR tapes sheathed by Ag and Ag/Sb and Ag/Mg alloy, respectively. Silver alloy sheathed tapes showed better bending properties than pure silver sheathed one. Therefore, silver alloy sheathed, optimum reduction rate of IMD, and increasing D for Bi-2223 tapes＇ applications were important technical strategies to enhance their mechanical, electrical, and magnetic properties.

(Bi,Pb)-2223相分解-再形成过程中超导相的演变

研究了低氧分压气氛中，不同熔化处理温度下单芯银包套带材（Bi，Pb）-2223（以下称Bi-2223）芯部熔化分解及随后的缓慢冷却阶段，淬火试样中Bi-2223，Bi-2201和Bi-2212相之间的演变关系。结果显示，在Tp=855℃-865℃范围内，随着熔化处理温度（Tp）的升高，淬火带材中Bi-2223相相对含量逐渐减少，而Bi-2201相含量则明显增加。Tp=855℃-860℃时，Bi-2223相发生部分分解，其淬火后的主要产物为Bi-2201，（Sr,Ca）2CuO3（2：1AEC）和（Sr,Ca）14Cu24O41（14：24AEC）。经过随后的缓慢冷却，部分分解的Bi-2223相可以直接从液相中再形成。在所有熔化试样及部分熔化后慢冷所得的试样中均未发现Bi-2212相。当Tp=865℃时，Bi-2223相则全部分解，在同样冷却条件下，其回复过程经历了两个不同的途径，一是少量Bi-2223相直接从液相中生成，二是先从液相中析出Bi-2212相，在随后继续冷却过程中再转变为Bi-2223相。由于Bi-2212转变为Bi-2223所固有的迟缓特性，以致在本实验的冷却条件下，完全分解的Bi-2223相只能部分得到回复。

高温超导带材Bi-2223/Ag的焊接接头电阻研究

针对高温超导带材Bi-2223／Ag的焊接问题进行了研究。利用ANSYS建立焊接接头电阻特性的有限元计算模型，仿真和计算分析了焊料材料、搭接长度和焊锡层厚度对接头电阻值的影响，并进行了实验验证。实验表明：可通过减少焊料厚度，增加焊接搭接长度来减少接头电阻，当焊接长度在2—5cm，焊料厚度为0．5mm时，Bi-2223／Ag带材间的接头电阻在液氮温度下可以控制在10^-7～10^-8Ω之间，可以满足超导装置稳定运行的要求。

磁体用多芯Bi-2223/Ag超导带材的制备和性能

研究了用粉末装管技术制备磁体用多芯Bi－2223／Ag超导带材的工艺和性能。结果表明，得到的氧化物芯丝均匀的多芯Bi－2223／Ag超导带材，其Bi－2223单相性较好巨晶粒具有c轴取向。在77K，OT下，短带的临界电流密度Jc达2．2×104A／cm2（判据1μV／cm）。用此带制做的小型超导磁体（外径为85mm，内径10mm，高60mm），在4．2K和77K下，磁体通过的；临界电流Ic分别为135A和17A（判据1μV／cm），对应此电流，该磁体产生的磁场分别为1．04T和0．13T。

拉伸应变对Bi-2223银及银合金包套带材临界电流的影响

研究了Bi-2223带材的应力-应变特征,比较了包套材料、芯丝数目和Ag/超比例对Bi-2223带材不可逆拉伸应变的影响。实验发现,采用高强度的AgMn合金包套材料可提高芯丝的残余压应变,并通过提高Ag/超界面的平整性,减少因为界面不均匀而产生的应力集中,芯丝中微裂纹的出现和扩展将被延迟到更高的应变状态,因此可显著提高带材的不可逆拉应变。同时,通过增加芯丝数量和Ag/超比例也可增强带材抗拉伸应变能力。

Bi2223/Ag和YBCO高温超导带材在交流过电流冲击下的失超及恢复特性研究

采用交流冲击实验装置测量了在不同交流过电流、持续交流工作电流及冲击时间情况下,不同散热条件下Bi2223/Ag和YBCO高温超导带材的电压、电流和温度随时间变化曲线,分析了冲击期间和冲击后带材各参数变化及失超恢复时间与散热条件和冲击参数的关系。研究结果表明:带材在交流过电流冲击下的失超和恢复过程主要取决于冲击能量和冲击过后带材内发热与散热的竞争效应。

磁场、温度对Bi-2223/Ag多芯带Jc的影响

介绍了一套液氮温区入（Ｂ，Ｔ）测量系统，该系统用增大液氮的饱和蒸气压来达到提高液氮温度的目的。用该系统研究了磁场和温度（测量范围分别为０．１Ｔ～０．８Ｔ和７７Ｋ～９０Ｋ）对Ｂｉ－２２２３／Ａｇ多芯带材（１９芯和３７芯）临界电流的影响。

Bi系2223高温超导带材力学性质

对4种B i系2223高温超导带材77 K下的力学性质进行了实验研究。通过选择合适的合金包套替代传统的银包套,带材的机械性能能有所提高,但电学性质有所降低,尤其是内包套采用合金、外包套采用银,其机械性能反而低于纯银包套带材。通过X射线衍射图谱发现,当包套采用合金材料,尤其是内包套采用合金、外包套仍采用纯银,会大大降低样品中2223相的体积含量。相比较临界电流密度,n值对材料的破坏更为敏感。利用X光同步辐射技术初步探讨了银包套带材在室温和77 K下的损伤破坏过程。

影响Bi(2223)/Ag超导复合带J_c及J_c-B特性的因素

研究了Bi(2223)/Ag超导复合带的微观结构变化及J_c随磁场的依赖关系,实验结果表明:中间压制及烧结工艺有利于杂相的减少及高T_c单相的形成,是提高J_c非常有效的手段;杂相的弥散分布有利于弱连接特性的改善及J_c的提高;随零场下J_c的提高,J_c在磁场下的性能大大改善,但在磁场平行和垂直带面时,影响J_c-B特性的磁通钉扎机制是不同的。我们制备的超导带材的最佳J_c为3.46×10~4A/cm^2(77K,OT)、6.8×10~3A/cm^2(77K,1T)。

Bi2223/Ag带HTS双饼线圈中的接头焊接

降低双饼线圈间的搭接电阻 ,可有效提高 HTS磁体的稳定性并降低制冷费用。文中在分析影响搭接电阻因素的基础上 ,对双饼线圈间的焊接工艺和接头性能进行了实验研究 。