YBa_(2)Cu_(3)O_(x) 초전도체 제작 및 특성 개선을 위한 첨가물 효과 연구
Effects of Additives for Improvement of YBa_(2)Cu_(3)O_(x) Superconductor Preparation and its Properties
초전도체 특성개선 첨가물;
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The improvement of critical temperature (Tc), critical magnetic field (Hc), and critical current density (Jc) of superconductor is important for practical applications. To improve those three critical properties, many researches have been performed vigorously and their results have been applied to the fabrication of various film or bulk type superconductors. In this dissertation, as an attempt to improve the preparation conditions of YBa2Cu3Ox superconducting powder or bulk samples, the properties of sintering temperature depending on additives have been investigated, and a study on the effects of additives to the density, grain alignment, and porosity of samples that affect the critical current density of superconductor has been performed. YBa2Cu3Ox powder was synthesized with citric acid and Barium nitric base by sol-gel method, showing a size distribution of 0.2~1 ㎛ and improved critical temperatures of 89~93 K. The additives added to promote grain growth and to decrease porosities and weak links between grain boundaries of YBa2Cu3Ox superconductors were such metal oxides or functional compound as Ag2O, PbO, both Ag2O and PbO, and BaPbO3. The melting point of pure YBa2Cu3Ox superconductor was 1015 ℃. When above mentioned additives were added it was shown that the melting points of samples with 15 wt% Ag2O, 20 wt% PbO, both 10 wt% Ag2O and 15 wt% PbO were decreased to 961℃, 959℃, and 920℃, respectively. In the measurement of critical current of pure YBa2Cu3Ox superconductor sample, sol-gel synthesized powder showed 36 % higher current density than solid state synthesized powder. In YBa2Cu3Ox superconductor sample with the additive of Ag2O, both samples prepared with sol-gel and solid state synthesized powder with 15 wt% Ag2O were found to improve the critical current by 6 % and 200 %, respectively. The samples prepared with sol-gel synthesized powder had better critical current property than those prepared with solid state synthesized powder. In the samples prepared by sol-gel synthesized powder with PbO addition of 5~20 wt%, the sample with 5 wt% PbO had the highest critical current of 4.15 A and the samples with both 10 wt% Ag2O and 10 wt% PbO showed the highest critical current of 4.35 A. In the samples prepared by sol-gel synthesized powder with 10, 20, and 30 wt% conductive BaPbO3 additives, the sample with 20 wt% BaPbO3 obtained the highest critical current of 4.74 A. Compared to conventional solid state powder synthesis, the sol-gel synthesis studied in this dissertation was found to reduce a powder preparation time and to get highly pure powder, and better superconducting properties. The preparation method of YBa2Cu3Ox superconductor by using sol-gel synthesized powder as well as applying additives can increase grain growth and interconnections of grain boundaries so that it could effectively applied to the process of large sized YBa2Cu3Ox superconductor. As the results of this dissertation were proved to produce the improved YBa2Cu3Ox superconductor, it is expected that the superconductor could be applied to various core fields of electric and electronic engineering technology such as superconductor fly wheel, permanent magnet, or magnet core etc..