Basic Study On Preparation Of YBaCuO Superconductor Tapes

Due to the higher cost (approximately $300/kA m) of first-generation (1G) high-temperature superconductor (HTS) km-length tapes based on BSCCO materials using the oxide-powder-in-tube (OPIT) process, researchers worldwide shifted their efforts toward the development of second-generation (2G) YBaCuO tapes. 2G tapes possess some important advantages over 1G tapes including better in-field electrical performance at higher temperatures, potentially lower processing costs, and low ac losses.YBaCuO tapes are mainly obtained by depositing YBaCuO film on flexible metal substrates. However, the complex deposition techniques of multilayer and vacuum condition make the actual price of 2G tapes much higher than the predicted. Ag alloy substrates do not diffuse toward Y123 films, which make it possible to deposit superconductor films directly on them. It is more economical to some extent since the complex deposition processes on Ni substrates are abandoned. Electrophoretic deposition method has the advantages of simple equipments, easy to operate and fast deposition speed. Hence it can largely reduce the cost of YBaCuO tapes prepared by electrophoretic depositing YBaCuO thick films on Ag substratesOne of the main obstacles to the manufacture of YBaCuO tape has been the phenomenon of weak links between grain boundaries. Melting process has been proved to be the only way to remove weak links. But Ag substrate would also melt during the melting process. There are two approaches to resolve this problem. One is to increase the melting point of silver substrate, and the other is to decrease the melting temperature of YBaCuO thick films.Doping is one of the effective methods to explore high temperature superconductivity mechanism and new superconductors. In this paper dopingmethod is adopted to decrease the melting temperature of YBa2Cu3O7-^. The experimental results prove that either Ag or PbO dopant can lower the melting temperature of YBa2Cu3O7-,j. 15wt% of Ag dopant makes the melting temperature of YBa2Cu307-a down to 970°C, and 10wt%PbO dopant to 980°C. But PbO tends to react with YBa2Cu3O7-<$ at high temperature which decreases the superconducting phase and weaken the superconductivities of samples. The product BaPbO3 has outstanding electrical conductivity. It does not react with YBa2Cu3O7-,5 and has little influence on superconductivity of YBa2Cu3O7- s. Doping 5wt%Na2CO3 in YBa2Cu3O7-<$ will decrease the meting temperature to 965°C and 10wt% dopant to 960°C. The XRD result indicates that Na atoms displace Y locations but not Cu(2), which means Na2CO3 dopant has little effect on the superconductivity of YBa2Cu3O7-<5. Even a small quantity of L12CO3 dopant badly destroys the superconductivity of YBa2Cu3O7-<$ because Li displaces Cu(2) located on the CuO2 plane which reduces current carrier density. Electrophoretic deposition method and doping Ag are combined by co-elect-rophoretic depositing Ag-YBa2Cu3O7-,j thick films. About co-electrophoretic deposition there are some experimental formula and models for reference 0-nly but does not exist unified explanation. In this paper the behavior of A-g particles during co-electrophoretic deposition is studied. It is found that Ag particles can also absorb protons and migrate to cathode. The contents of deposited Ag increase with those in original materials. The relationship of quantity of electricity carried by YBa2Cu3O7-,5 and Ag is described asQybco=-0. 16380^+0.1519FDensity of electrical field and electric charge on particles are key factors for co-electrophoretic deposition of Ag and YBa2Cu3O7-<5 in acetone. According to the phase diagram of Ag alloys, there are only few noble elements such as Au, Pt, Pd can increase the melting point of Ag by solid solution which is not economical. In this paper aluminium oxide but not metal is selected as addition to improve the properties of Ag by powder metallurgy method. The results indicate that 3wt% of AI2O3 doped in Ag matrix increase the melting pointof Ag up to 990°C. When 2wt% AI2O3 is doped, the linear expansion factor of A^Cb-Ag sample is near to that of YBa2Cu3O7-<5. Thus the two properties related to temperature satisfy the requirement of substrates for superconductor tapes. The hardness of A^C^-Ag composites shows irregular variation due to the existence of a great deal of hard AI2O3 and holes. There is no linear relationship between tensile strength and hardness of A^Ch-Ag composites. It may be caused by the limitation of hardness measurement method. The elongation percentages of AbC^-Ag composites decrease with the increasing content of AI2O3. When YBa2Cu3O7-<5 is heated above peritectic reaction temperature of 1010 °C, it will decompose to liquid rich in Cu and Y2BaCuOs(Y211), and give off oxygen:2YBa2Cu307-^----->Y2BaCu05 (s) + 3BaCuO2 (1) + 2CuO (1) +(\-S)O2 (g)According to Clausius-Clapeyron equation, low oxygen partial pressure will decrease the peritectic reaction temperature. It has been proved by Nd system superconductor. Here melting texture growth process under low oxygen partial pressure is studied. The result shows that pure YBa2Cu3O7-<5 samples will melt at 940 °C with flowing argon, and the melting temperature of samples with 10wt% Ag doped are decreased to 930 °C. The critical current density values of melting textured samples are still very low, the results can be improved by adding seed grain and prolonging oxygenation time. Textures in the cross section of samples are observed under scanning electron microscope. Low oxygen partial pressure will depress the proceeding of peritectic reaction. Flowing argon will minish temperature gradient of annealing furnace which is unfavorable for texture growth of YBa2Cu3C>7-<5 samples.The progress of OPIT for YBaCuO/Ag tapes gets little development for it concerns high temperature treatment. MTG progress under low oxygen partial pressure can avoid Ag melting and sticking to furnace by lowering treatment temperature. Particle density in Ag tube has certain effects on critical current density of tapes. Tapes with YBa2Cu3O7-^ powder whose particle size is less than 40um by solid reaction method can obtain higher Jc value. Longer oxygenation also favor to better superconductivity, and the Jc value of the melting textured...