Preparation And Properties Of Superconducting Nano-materials Research

With the recent progress in nanotechnology, some nano-superconductors are successfully fabricated and many unique properties such as the quantum fluctuated phase slip (QPS) and paramagnetic Meissner effect are found, which has stimulated the study on nano-superconductor for both fundamental interest and potential application in cryoelectronics. However, as limited by the material fabrication and measurement, there are still many challenges in the study of nano-superconductivity, such as the QPS and lattice distortion effects on the high T_c superconductors at nanoscale; the morphology controllable synthesis of nano-superconductors and so on.This dissertation focuses on the above challenges: a method combined with sol-gel process and porous alumina template was developed to fabricate superconducting multi-component oxide nanowire; it was found that the superconductivity was suppressed by the lattice distortion in the as-obtained La_1.85Sr_0.15CuO₄ nanowires; a dual-active controlled solvo-thermal strategy was used for the morphology controllable synthesis of nano-superconductor, and coaxial Pb@PVA nanocables, Pb micro-hemispheres were successfully synthesized for the first time. The detailed experimental results and discussions are shown as follows:In chapter 1, the size and dimension effects on the properties of nano-superconducting materials; recent progresses on the fabrication of nano-superconductors through top-down strategy, bottom-up strategy and the other unique methods are reviewed. Some quantum information devices based on the superconducting nano-circuit are also provided as the promising application examples of the nano-superconductors. Some challenges in the study of nanoscale superconductivity are discussed at the end of this chapter.In chapter 2, a sol - gel method combined with porous alumina as morphology-directing hard template was developed for the fabrication of superconducting multi-component oxide nanowire arrays. The sol precursor derived from dual-complexation process can effectively reduce the formation temperature of these oxides, and the co-effect of vacuum and capillarity lead to a high filling ratio. Highly ordered superconducting La_1.85Sr_0.15CuO₄ nanowires with a diameter of about 30 nm have been successfully synthesized via this method. The magnetization measurements show that the annealed LSCO nanowires are superconducting, with T_c～30K, which is lower than that of the corresponding bulk LSCO sample. The suppression of superconductivity is caused by the weakening of in-plane hybridization in the nanowire system. This material can give insight into the study of fundamental properties of the complex cuprate superconductors with reduced size and dimension.In chapter 3, a new sol process using the mono-complex agent aqua ammonia was developed for the fabrication of La_1.85Sr_0.15CuO₄ nanowire arrays. It is found that there is a morphology crossover from nanorods to nanowires as the heating rate increases during the thermal decomposition of this new sol precursor. This method provides a slenderness ratio controllable strategy for the synthesis of multi-component oxide nanowire. The spindle-like LSCO nanorods with the diameter of about 20 nm were obtained by this strategy.In chapter 4, PVA was introduced to the PVP, Pb(CH₃COO)₂·3H₂O and C₂H₆O₂ solvo-thermal system for initiating the roll-up process of sheet-like precursors. Polymer tubes with the diameter of 100-150 nm were prepared firstly, which will act as both micro-reactor and template for the following growth of Pb. Then the superconducting coaxial Pb@PVA nanoca-bles were fabricated for the first time in this mild solution phase system. This fabrication also gives a simple one-step route to polymer sheathing active metal nanowires with better oxidation resistance, which will find more applications in nanodevice architectures.In chapter 5, well-defined superconducting Pb hemispheres have been obtained in a mixed surfactant, CTAB and PVP, introduced C₂H₆O₂ solvo-thermal system. A so-called dual-activity-controlled growth process was developed to explain this asymmetric fabrication, which proves the effectivity and simplicity of the combinatorial surfactants as the tailoring agents in fabricating functional materials with desired morphologies.