Studies On The Synthesis And Physical Properties Of Superconducting Nanomaterials

In recent years, much attention has been paid to the superconductivity of nanomaterials due to the unique properties and potential applications. Such studies gave rise to new insight in the fundamental properties of superconductors. However, as limited by the material fabrications and measurements, there are few reports on the high transition temperature (Tc) superconductors and the nanoscale confining effects on these systems are still unknown.In this dissertation, we present a detailed study of La1.85Sr0.15CuO4 nanoparticle's superconductivity as a function of particle size; the controllable synthesises of NbSe3 and NbSe2 nanomaterials are also investigated. The detailed experimental results and discussions are shown as follows:In chapter 1, the recent progresses on superconducting nanomaterials, including the size and dimension effects on superconductivity and fabrication of superconducting nanomaterials, the antiferromagnetic fluctuations in the high-Tc superconductors and the ferromagnetism in non-superconducting nanoparticles are reviewed. Some challenges in the study of high-Tc superconducting nanomaterials are discussed at the end of the chapter.In chapter 2, a developed sol-gel method is used to fabricate La1.85Sr0.15CuO4 nanoparticles with the particle size between 66 and 800 nm. The structure analysis suggests that with decreasing particle size the a(b)-axis length increases and c-axis length decreases, which may lead to the suppression of high-Tc superconductivity. The experiment results indicate the nanoparticles have the stoichiometry oxygen contents.In chapter 3, magnetic and superconducting properties of La1.85Sr0.15CuO4 nanoparticles are systematically studied as a function of particle size. By systematically reducing the particle size, we observe that the suppression of the superconductivity is accompanied by the appearance of weak ferromagnetism. The Electron Spin Resonance spectra indicate that there are lots of uncompensated spins at the surface of nanoparticles. We propose that the origin of the weak ferromagnetism is due to the enhancement of Ruderman-Kittel-Kasuya-Yosida (RKKY) ferromagnetic coupling between uncompensated surface Cu2+ spins when the antiferromagnetic correlation is reduced by the finite size effect. It is believed that the suppression of superconductivity accompanied with the appearance of weak ferromagnetism is closely related to the destruction of antiferromagnetic correlation in the CuO2 plane.In chapter 4, the Vapor-phase synthesis of NbSe3 and NbSe2 nanomaterials are studied. With changing the sintering temperatures, the sintering times and cooling rates, NbSe3 nanowires with different morphologies are fabricated, which are used to convert to NbSe2 nanomaterials. The growth mechanism of the NbSe3 and NbSe2 nanomaterials are also discussed.