Fabrication And Characterization Of Double-sided High-T_C Superconductor Tl₂Ba₂CaCu₂O_y Thin Films

Superconducting mechanism and application of high-temperature superconducting (HTS) materials have been widely studied since the discovery of it. The low microwave resistance makes superconductor films very promising for application in microwave devices such as resonators, antennae, filters, delay lines and so on. Extensive work has shown that microwave devices made of HTS films have many excellent microwave properties, for example: low insertion loss, , low power loss wide frequency, small size, high sensibility, low noise and high reliability etc. and commercial products are starting to penetrate into military and domestic mobile communicative market.Large area and double-sided films are needed for the microwave applications. The most commonly used HTS material is YBCO. Besides it, Tl2Ba2CaCu2Oy superconductor has also been used. The Tl-2212 has many advantages to YBCO. Because of its high TC0 value, low microwave surface resistance and chemical stability, especially against moisture, many groups in the world have been widely studied on fabrication of Tl-2212 thin films.This thesis presents an extensive study on preparation of Tl-2212 films using two-step technique. First, amorphous Ba5iCaCu2Ox precursor is deposited on (001) LaALOj substrate; by PLD or other methods. Then, thalliation is performed to form the crystalline superconducting material, which is more important than the first step. Due to the toxicky of Tl, it is desired to confine the Tl inside a closed space. In this paper, the thalliation is carried out in a completely closed steel-container or in aVIABSTRACTsemi-closed container, respectively. The high-quality T\l-2212 thin films, whose zero-resistance temperature is 108.8K, were obtained in both the containers. Usually, Tl-2212 film is performed in AbQj or Tl-2212 crucibles. So far as, there is no report about Tl-2212 thin films prepared in a steel-container. Due to its high-temperature (about 1200) resistance and other properties, the steel-container can be completely closed and easily shaped. So the steel-container is more safe and more economical than AlaOa crucible.A two-step method with long-time annealing was used to fabricate double-sided Tl-2212 thin films. The as-prepared films are composed of almost pure Tl-2212 phase and epitaxially grown with C-axis perpendicular to the substrate surface. Typical results obtained on a 15mmxl5mm double-sided film show an FWHM of 0.92and 1.21?for each side, respectively. The zero-resistance transition temperature is 108.8K and 106.7K for each side, respectively. The critical current density is about 106A/cm"2 for both sides. Scanning electron microscopy (SEM) shows that a plate-textured surface exists with a few bright particles dotted in the film's surface and the surface of the crystal grain is smooth. The growth mechanism of spirality and layer by layer is first discoveried.