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Preparation And Microstructure Characterization Of YBa2Cu3O7-δ And MgB2 Superconduting Films

Posted on:2007-05-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:T S WangFull Text:PDF
GTID:1101360182483105Subject:Materials science
Abstract/Summary:PDF Full Text Request
The purpose of this thesis is that providing a reference to fabrication ofYBCO films and ferroelectrics/YBCO bilayer films for their electronicsapplication and putting forward an in situ growth method of MgB2 filmsusing hot filament chemical vapor deposition.Miscut SrTiO3 (001) substrates were pretreated by ex situ annealing inflow oxygen and in situ annealing in vacuum. YBCO films were fabricated on015°-miscut SrTiO3 (001) single crystal substrates by pulsed laser ablating apowder sintered target. Ba0.1Sr0.9TiO3/YBCO bilayers were prepared on1.2°-miscut LaAlO3 (001) substrates by pulsed laser deposition (PLD). YBCOfilms have been deposited on SrTiO3 (001) substrates by pulsed laser ablatinga melt-textured target. MgB2 superconducting films were in situ grown onsapphire substrates by hot filament chemical vapor deposition. The effects ofmiscut angle of substrates on the surface morphologies, microstructure andsuperconductivity of films have been investigated. The basic microwaveproperties of the Ba0.1Sr0.9TiO3/YBCO bilayer were measured.The surface morphologies of pretreated substrates and films grown onthem were examined by atomic force microscopy, high-energy refectionelectron diffraction and scanning electron microscopy. The microstructures offilms were analyzed and characterized by X-ray diffractometry andtransmission electron microscopy. The superconducting critical transitiontemperatures (Tc) of films were determined by standard four-probe andmagnetization measurement.The research results show that c-axis oriented YBCO films can be grownin step flow mode on 015°-miscut SrTiO3 (001) substrates by PLD. As theincrease of miscut angle of substrates, the Tc value and crystallinity of filmsdecrease. If the miscut angle is more than 6°, large steps and holes or cracksnormal to step edge form on the surface of YBCO films causing the largesurface roughness. Ba0.1Sr0.9TiO3/YBCO bilayer heterostructure with highcrystallinity and epitaxy, and sharp-straight interface, was in situ grown on1.2°-miscut LaAlO3 (001) substrates by PLD. Tc value (88 K) of YBCOlayer is not affected by growing the ferroelectric film on it. Themeasurements of dielectric constant, loss tangent, tunability and figure ofmerit of the heterostructure show that the heterostructure can basically meetthe potential application in electrically tunable microwave devices operated atliquid nitrogen temperature.For YBCO films prepared by using melt-textured target, not only the Tcvalue is not depressed markedly but also particles is obviously suppressed toform on YBCO films surface. Nano-scaled rectangular Y2O3 inclusionparticles epitaxially and orientationally grow on substrate surface and insidethe YBCO films.Single-phase MgB2 superconducting films were in situ grown onsapphire substrates by using HFCVD. Increasing the growth temperature from400 to 600 °C, Tcs of the films increase from 18 to 36 K. The densificationand the crystallinityof the film is enlarged.
Keywords/Search Tags:high temperature superconducting film, microstructure, YBa2Cu3O7-δ, ferroelectric/superconductor heterostructure, MgB2, miscut substrate, pulsed laser deposition, hot filament chemical vapor deposition
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