Research Of IBAD Buffer Templates For YBCO Coated Conductors

Due to their excellent electrical performances,YBa2Cu3O7-x（YBCO）coated conductors（CCs）have broad application prospects in transmission cable,magnet motors,current limiters and so on.In order to achieve high performance YBCO superconductor wires,it is necessary to obtain high quality buffer layers for YBCO.Right to now,there still exist some challenges limiting the practical use of buffer layers,such as biaxial texture with high quality,production cost and uniformity along the tape.Thus,this dissertation focuses on the low cost,high quality,highly uniformity of buffer layers for YBCO,and the details are as the following:A home-designed solution deposition planarization（SDP）system has been developed.Amorphous Y₂O₃ films were deposited on bare Hastelloy C276 tapes by SDP method.The influence of heat-treated temperature and number of coatings on the surface morphology and crystallization of Y₂O₃ films was studied.The optimized root-mean-square（RMS）roughness value of Y₂O₃ layer on a 5 μm′5 μm area was 0.2 nm.However,it was possible that Y₂O₃ would be crystallized during SDP process.Moreover,Y₂O₃ films would be re-crystallined during further LaMn O3（LMO）process.Therefore,Al₂O₃ was dopted into Y₂O₃ films to increase the crystallization temperature,In contrast to Y₂O₃ film（500 °C）,Y₂O₃-Al₂O₃（YAlO）films has shown higher thermal stability and wider temperature window（480⁵40 °C）.Additionly,YAlO films would not be re-crystallined during LMO process.Biaxially textured MgO films were grown on SDP-buffered templates by ion-beam-assisted deposition（IBAD）technology.The best biaxial texture of IBAD-MgO films has shown out-of-plane texture（（35）w）and in-plane texture（（35）（37））yield full width at half maximum（FWHM）values of ².4° and ³.7° on short sample.Texture development was characterized in real time by reflection high-energy electron diffraction（RHEED）in situ monitoring.The biaxial texture was developed during the nucleation stage（².2 nm）.Brewer et al.theorized that texture development was caused by solid phase crystallization during nucleation.Based on the mechanism,a two-step method was applied to accelerate the development of in-plane texture.During the process,the biaxial texture was developed with IBAD-MgO thickness of 0.5 nm.To improve the biaxial texture and surface morphology of IBAD-MgO,additional MgO films（Epi-MgO）were coated on IBAD-MgO films.We has successfully prepared a 50 m-long single-sided Epi-MgO/IBAD-MgO tape.The out-of-plane and in-plane textures along this tape yield full-width at half-maximum（FWHM）values of ³° and ⁶°.A novel double-sided IBAD-MgO buffer template was firstly proposed and designed,where YBCO functional layer could be deposited on both sides of the template for doubling the critical current and yielding higher cost-effectiveness.A 50 m-long double-sided MgO tape has been prepared by overcoming some technical problems.The ω-scans of（002）MgO and（37）-scans of（202）MgO yield FWHM values of 2.8³.2° and 5.3⁶.7° for one side,respectively,as well as 2.6³.3° and 4.2⁶° for the other side.A promising mid-frequency（MF）alternating current magnetron sputtering technique was developed to decrease the procuction cost and improve the manufacture efficiency of LMO cap layer.In this system,a new heating method that a direct-current source was utilized to generate electric current through the tape has been developed.This heating method could improve the uniformity of temperature field and yield high energy efficiency.The highest moving speed of MF-LMO has reached 67 m/h and the biaxial textures on both sides of a 20 m-long MF-LMO tape have shown high uniformity along the tape.The ω-scans of（002）LMO and（37）-scans of（202）LMO yield FWHM values of 2° and 4⁵° for one side,respectively,as well as 2° and 5⁶° for the other side.Finally,the double-sided YBCO films deposited on the double-sided MgO buffer template achieved a high superconducting transition temperature of 90 K and an overall critical current of 290 A/cm-width at self-field（110 A/cm-width and 180A/cm-width for each side）,exhibiting the feasibility of double-sided structure.To simplify the process of buffer template,LMO cap layer was directly deposited on IBAD-MgO layer without Epi-MgO layer.Due to its high lattice-mismatch induced strain energy between LMO and IBAD-MgO,XRD q-2q scans revealed that LMO films deposited on IBAD-MgO layer require a higher growth temperature to develop to fully（002）orientation than that on Epi-MgO layer.By optimizing LMO deposition parameter,biaxially textured LMO films could be epitaxially grown on IBAD-MgO layer with FWHM values of 3.5° out-of-plane texture and 7.2° in-plane mosaic,respectively.In contrast to the commercialized buffer template（6 processes）,this buffer strcture could extensively reduce the process complexity（3 processes）and decrease the production cost.Some novel IBAD texturing materials（NaF,NaCl）has been explored.Comparing with IBAD-MgO,the advantages of IBAD-NaF and IBAD-NaCl were as the following: First,well textured IBAD-NaF and IBAD-NaCl films could be obtained on 4 Y₂O₃ coatings（5 μm′5μm,RMS=8 nm）.The biaxial textures yield FWHM values of 2° out-of-plane texture and 7.5° in-plane mosaic for Epi-NaF/IBAD-NaF,2.1° out-of-plane texture and 8° in-plane mosaic for Epi-NaCl/IBAD-NaCl,respectively.However,IBAD-MgO demand very smooth surface（5 μm′5μm,RMS=2 nm）to achieve well-aligned texture（at least 16 Y₂O₃ coatings）.Second,the process windows of deposition rate for Na F and NaCl were 0.08² nm/s and 0.06³.5 nm/s with ion beam flux of 25 mA,respectively,while the process window for MgO was 0.06⁰.12 nm/s。The advantages above demonstrated that IBAD-NaF and IBAD-NaCl had broad application prospects for biaxially textured templates with low cost and high production efficiency.