The Effect Of Polymer On Microstucture And Properties Of Superconductoprs

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.