Year

2012

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Yttrium barium copper oxide, a ceramic with the formula YBa2Cu3O7-δ (YBCO), is the first material into the superconducting state above 77 K, which means the low-cost liquid nitrogen can be used as coolant. Because YBCO not only has high critical transition temperature (Tc), but also has high irreversibility field (Hirr) and high critical current density (Jc), it has attracted enormous interest and efforts to make it realize real applications since being discovered in 1987. For most applications, such as the superconducting microwave filter, YBCO is usually required to be in an epitaxial film form, since biaxial texturing technology can minimize the weak-link effect at the grain boundaries to achieve high performance on current transport and related properties. In this work, a cost-competitive, easily scalable preparation route, the trifluoroacetate metal-organic deposition (TFA-MOD), was employed to fabricate undoped and doped YBCO thin films. There are two main focuses for the author’s own work in this thesis: I. Improvements of processing conditions to fabricate buffer layers and pure YBCO thin films on the selected single crystal substrates; II. Introduction of second phase into YBCO matrix, and research on the pining mechanisms of selected doped samples.

The deposition of YBCO film on Al2O3 substrate can produce a promising device for microwave application; however, an appropriate buffer layer has to be inserted due to the diffusion of aluminum during heat treatment and the relatively large lattice mismatch between YBCO and Al2O3. In this work, pure (h00)-textured CeO2 buffer layers were deposited on single crystal r-plane Al2O3 (1-102) substrate by a hybrid process, which was combined with magnetron sputtering for the seed layer and metal-organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO2 buffered r-cut Al2O3 (1-102) substrates by means of the TFA-MOD method. Atomic force microscopy and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current density of the YBCO films exceeds 1.6 MA/cm2 at 77 K, with a superconducting transition temperature of 90 K. The surface resistivity is as low as 14 µΩ at 1 GHz frequency. The flux pinning type of the derived film has been investigated by Dew-Hughes model. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the advantages of sputtering for preparing highly c-axis oriented CeO2 buffer layers and the advantages of metal-organic deposition with rapid processing, low cost, and easy preparation of large-area YBCO films.

The next generation of wireless communication demands microwave filters with high bandwidth and low noise. Superconducting films epitaxially grown on some single crystal substrates are the most promising substitutes to replace the conventional copper filters. YBCO thin film and Al2O3 substrate make a good combination for fabricating large-area high-temperature superconducting (HTS) microwave filters. YBCO is hard to deposit on Al2O3, however, without a buffer layer due to diffusion of aluminum and the relatively large lattice mismatch. In this work, highly (h00)-oriented CeO2 buffer layers were prepared by means of an all-chemical solution deposition route, which has never been achieved before on r-plane Al2O3 substrate. Nevertheless, it is disappointing that (00l)-oriented YBCO thin film was not successfully formed on the buffer layer due to poor in-plane alignment. YBCO superconducting films can be used for microwave applications due to their low surface resistance. Both LaAlO3 (LAO) and Al2O3 can be considered as suitable substrates due to their excellent properties for the preparation of state-of-the-art YBCO films and for the design of microwave elements. In order to optimize the annealing temperature, which has a direct impact on the quality of thin films, the microstructure and properties were investigated for YBCO/LAO and YBCO/CeO2/r-plane Al2O3 films prepared by TFA-MOD. The results indicate that the annealing temperature window is broader for YBCO films on LAO substrate; whereas, the window is relatively narrower for YBCO films on CeO2/Al2O3 substrate. The reasons are mainly the different lattice mismatch and the employment of a buffer layer. The surface resistivity (Rs) values were measured for the samples showing the highest Jc at self-field. The results provide useful information about processing effects on TFA-MOD derived YBCO films used for microwave applications.

For realizing commercial applications, the superconducting properties of high temperature superconducting films can be dramatically improved by introducing artificial pinning centers (APCs). According to many previous works, adding second phase particles using nanotechnology is one of the most effective and easily scalable methods to promote the Jc of YBCO films. In this work, some metal elements: tin (Sn), titanium (Ti), and zirconium (Zr), which all can form stable barium based oxides, are introduced into YBCO films by means of the chemical solution deposition approach. Compared the Jc results (77 K) of optimum samples with the pure samples as reference, Sn doped YBCO only shows moderate improvement of in-field Jc due to the employment of an inorganic precursor chemical; Zr doping dramatically enhances Jc beyond the single flux regime; the introduction of Ti promotes Jc significantly, especially at self-field; and dual doping with Zr and Ti combines the advantages of each individual doping. According to the tendency towards power-law decay regime of the Jc(H) curve, the pinning mechanism of Ti doped YBCO is very different from those of Sn and Zr doped YBCO, which can be explained by the structural characterizations. Overall, the factors impacting on the effects of second phase doping are various, including molar ratio to YBCO of appropriate chemicals, crystal form and orientation, precursor chemicals of doped element, etc.; however, viewed in a different way, adjusting the Jc and pinning behavior of YBCO samples becomes flexible and easy to engineer.

Magnetic flux acts as both angel and devil in type-II superconductors because its existence creates bulk current, but its motion degrades current density. Thus, to trap magnetic flux is a unique approach to resolve the contradiction. Second phase doping has been proved to promote Jc by introducing effective flux pinning centers. Both BaTiO3 (BTO) and BaZrO3 (BZO) doped YBCO thin films have achieved quite high Jc results, but reveal different pinning mechanisms. In this work, BTO or BZO second phase is introduced into YBCO thin film with optimal contents of precursor chemicals. The composition and microstructure of doped films were determined by X-ray diffraction and field-emission scanning electron microscopy. Then, the pinning behavior and mechanisms were investigated by means of Jc(H) plots, Jc(Angular) plots, the Griessen model, and the Dew-Hughes model. Because these data or models possess limitations to a greater or lesser extent, the relationships between them have been established to analyze various types of pinning more intuitively, accurately, and comprehensively. According to the relationships, schematic diagrams have been drawn to describe the increased pinning centers due to Ti or Zr doping, based on the pinning ‘background’ of a pure YBCO sample.

FoR codes (2008)

020404 Electronic and Magnetic Properties of Condensed Matter; Superconductivity, 091201 Ceramics, 091205 Functional Materials

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.