【作者】 张子立；

【导师】 索红莉； Judith MacManus-Driscoll；

【作者基本信息】 北京工业大学 ， 材料学， 2013， 博士

【摘要】 超导体由于其零电阻性和完全抗磁性，在输电、军工、器件等方面有着广泛的应用。而在超导材料的制备过程中，常常会用到聚合物作为一部分初始原料。比如在MgB2超导体的合成中，加入适量的聚合物可以有效地提高其临界电流密度（Jc）；而对于YBa2Cu3O7-δ（YBCO）超导体，在金属有机盐沉积或金属有机盐化学气相沉积以及生物矿化法等路线中，聚合物则常常作为辅助合成YBCO的重要原料之一。然而由于这些超导材料的合成与聚合物的分解和反应都发生在复杂的热处理过程中，目前聚合物与超导材料的反应机理以及相互作用机制尚不清晰。该论文正是针对上述问题进行了相关研究，并取得了以下创新性成果。首先，采用化学方法合成了高分子金属络合物－聚丙烯酸锌（PZA），并研究了其掺杂对MgB₂超导块材性能的影响。经过PZA掺杂的MgB2块材在5K、7T下，Jc值为1.67×104A/cm²，超过同等条件下未掺杂样品的十倍以上，并且随外磁场的增大，掺杂样品的Jc值下降的非常缓慢。进一步采用PZA和SiC对MgB2块材进行了双重掺杂，其Jc值在5K、7T下达到了3.3×104A/cm2，高于PZA和SiC各自单独掺杂的值，这表明用高分子金属络合物单掺以及聚合物和SiC双重掺杂都是一条在现有掺杂基础上进一步改进MgB2超导性能的可行路线。通过对掺杂机理的研究发现，高分子金属络合物以及SiC分解释放出来的第二相粒子和可以进入MgB2晶格的碳是提其高场性能的主要原因。其次，本文开发了一条以高分子聚合物作为辅助采用生物矿化法合成具有平片状结构YBCO粉末的新路线。通过本方法制备的YBCO粉末具有很高的相纯度，在粉末内所有YBCO晶粒均呈现平片状，而每一个平片状晶粒的典型尺寸约为6μm6μm1μm。这些平片状晶粒会在某些区域自发聚集生长成有相似生长方向的晶粒簇。在具有独特微观结构的同时，生物矿化法制备的YBCO粉末还具有优异的超导性能，在77、1T下，YBCO粉末的Jc值为0.024MA/cm2，是同等条件下商业粉末的20倍，且高于熔融织构法制备YBCO块材的Jc值。通过分析首次发现，在用生物矿化法制备的YBCO粉末内部存在有自发超导连接现象，其超导连接区域大致包含3⁵个平片状晶粒。在以上研究基础上，重点构建了生物矿化法制备具有平片状晶粒YBCO粉末的反应模型，结果发现作为有机物框架的右旋糖酐是YBCO平片状晶粒聚集成具有相似生长方向晶粒簇的关键因素。在反应初期右旋糖酐会将前驱溶液锁于自身构成的牢笼结构内部并发生生物矿化过程，从而产生了大量纳米级别的YBCO种子。同时作为掺杂物的NaCl则是使YBCO晶粒呈现平片状的主要原因，在烧结温度达到880°C时，NaCl会溶解前驱溶液分解产生的氧化物，并在后续保温冷却过程中，为以纳米级别的YBCO种子缓慢结晶长大提供了液相环境。通过进一步比较右旋糖酐和壳聚糖这两种具有不同结构的聚合物对生物矿化法生成YBCO种子形状与分布的影响，发现由于官能团位置的不同使YBCO种子优先形核的晶面存在差异，从而决定了YBCO种子的形态。而聚合物自身结构重复性的不同也会导致YBCO种子在分布上有很大差别。YBCO种子形状和分布的这些差异最终决定了生成的YBCO平片状晶粒的形状和排列方式。这一结果揭示了我们可以预先设计好无机物的微观结构，并通过选取合适的高分子有机物，并采用生物矿化法来合成我们所设计的材料。以生物矿化法制备的平片状YBCO粉末为初始原料，本论文开发了一条全新的制备YBCO导体的路线。在将具有平片状晶粒的粉末研磨分散之后直接撒在基板之上，经过热处理烧结制备了YBCO导体。得到的YBCO导体厚度达到了300μm，具有很锐利的超导转变，其零电阻时Tc值为87K。导体可以实际承载超导传输电流（Ic），在65K和77K时，Ic值分别为20mA和1.5mA。这种全新的YBCO导体制备路线对基带和过渡层没有织构以及表面粗糙度的相关要求，并且可以有效地制备具有一定厚度的导体。综上，本论文致力于研究高分子聚合物对超导粉末及块材微观结构及性能的影响。首先采用高分子金属络合物对MgB2超导体进行掺杂，有效地提高了其超导性能；其次采用高分子聚合物为辅助通过生物矿化法制备了YBCO粉末，在这种粉末中，所有晶粒都呈现出平片状，并自发聚集成有相似生长方向的晶粒簇，且首次发现在这种晶粒簇内部存在有超导连接；此外系统阐明了聚合物在生物矿化法制备这种平片状粉末中的作用，并构建了其反应模型；最后用生物矿化法制备的平片状YBCO粉末为初始原料，直接制备了YBCO导体，这揭示了一条全新的低成本、简单的制备YBCO导体的路线，有望解决现有YBCO导体制备中存在的相关问题。更多还原

【Abstract】 Superconducting materials have wide applications in the areas of electricaltransmission, the defense industry and devices due to their zero dc electrical resistanceand perfect diamagnetism. Polymers are often used as additives in the synthesis ofsuperconductors. In the MgB2superconductor, polymers have been shown to be oneof the most effective dopants able to enhance the critical current density （Jc）, while inthe case of YBa2Cu3O7-δ（YBCO）, polymers are also used to assist in the synthesis bymeans of metal organic deposition （MOD）, metal organic chemical vapor deposition（MOCVD） and the biomimetic method. Since the decomposition and reaction of thepolymer ocurs during the synthesis of the superconductor, so the reaction process andthe acting mechanism of these polymer additives are still not clear. In order to resolvethese questions, a series of studies have been performed.Firstly, poly zinc acrylate （PZA） was used as a representative polymer metalliccomplex dopant and its effect on MgB2bulks was studied. Compared to morecommon dopants, the polymer metallic complex provides carbon and metallicparticles simultaneously, thereby achieving multiple doping using only a single dopantmaterial. The Jcof the MgB2bulks with PZA doping reached1.67×104A/cm2at5Kand7T, over one order of magnitude higher than for undoped samples. Co-dopingwith PZA and SiC was also studied, in which case the Jcreached3.3×104A/cm2at5Kand7T, higher than achieved with either PZA or SiC alone. This indicates thatco-doping with SiC and a polymer is an effective way to further improve thesuperconducting properties of MgB₂.Secondly, a biomimetic synthesis method using a biopolymer to assist crystalgrowth is demonstrated, which is able to produce uniformly anisotropic, plate-likeYBCO crystallites. The as-synthesized biomimetic YBCO powder comprises almostphase-pure YBCO platelets with typical dimension6μm6μm1μm. Thehierarchical structure of these crystallites, revealed by transmission electronmicroscopy, is responsible for an unprecedented bulk intragranular critical currentdensity reaching0.024MA/cm2at77K and1T—more than an order of magnitudehigher than commercially available powder material. Furthermore, the naturally-occurring close crystallographic alignment of adjacent plate-like grains canlead to an electrical connectivity that allows an intergranular supercurrent to flowacross3-5grains.Based on the above research, the mechanism of plate-like grain formation duringthe biomimetic process is investigated, and aninterpretation of the mechanism ispresented. The key to the production of plate-like YBCO grains is the addition of bothdextran and NaCl to the precursor mixture, the role of the dextran being to produce avery fine encapsulation of the precursors allowing them to react to form finelydispersed nanoscale seed crystallites of YBCO. These YBCO seed crystallites thenacts as nuclei for the growth of aligned clusters of YBCO platelet crystals after theprecursor dissolves in the presence of NaCl.The heterogeneous nucleation of the YBCO crystallites on seeds formed by usingdextran and chitosan through the biomimetic method is also studied. The differentpossibilities for positional arrangement on the functional groups in dextran andchitosan impose constraints on both the size and shape of the individual YBCOcrystallites and their macroscopic arrangement. This provides the prospect ofcontrolling the shape and arrangement of the seed crystals and the ultimatemorphology of the synthesis product, by choosing a polymer with an appropriatefunctional geometry.Having engineered plate-like YBCO crystallites with high Jc, a novel method ofYBCO conductor fabrication from this powder can be established by depositingindividual plate-like crystallites on the substrate. The final YBCO conductor preparedfrom the plate-like powder had a thickness of300μm. It showed a zero resistancecritical transition temperature （Tc） of87K and its critical current （Ic） at65K and77Kwas20mA and1.5mA, respectively. There are two benefits of this new method:firstly there is no requirement for crystallographic texture and smoothness of thesubstrate and buffer layers, and secondly it is easy to fabricate conductors of largethickness.In summary, this thesis focused on the effect of polymer additives on themicrostructure and superconductivity of various superconducting materials. MgB2bulk samples were doped by a polymer metallic complex, which strongly improvedthe Jcvalue, especially at high fields. The bio polymer-mediated growth of large,strongly anisotropic, plate-like crystallites of YBCO is demonstrated. The plate-like YBCO crystallites self-assemble into regions of closely aligned crystallographicorientation allowing unimpeded intergranular supercurrent flow across several grains.The reaction mechanism of the biomimetic method and the function of the biopolymerare also outlined. Finally, an YBCO conductor is fabricated based on the biomimeticpowder, demonstrating macroscopic supercurrent flow. This work provides animportant starting point for an alternative route to the synthesis of high-current,cost-effective YBCO conductors that has the potential to overcome many of thelimitations of existing thin film based approaches.更多还原