Epitaxial Growth of CeO2/YSZ/CeO2 Buffer Layers on Textured Ni Substrates for YBCO Conductors

CeO₂/YSZ/CeO₂ buffer layers were deposited on textured Ni substrates by in situ pulsed laser deposition. The out-of-plane texture and in-plane texture of the buffer layers were characterized by X-ray diffraction ω-scans and ϕ-scans. Using this CeO₂/YSZ/CeO₂ architecture as the buffer layers, high quality YBCO films with a zero-resistance T _c about 90 K and a self-field critical current densities J _c above 10⁶ A/cm² at 77 K can be obtained on Ni substrates.     

Epitaxial Growth of CeO<Subscript>2</Subscript>/YSZ/CeO<Subscript>2</Subscript> Buffer Layers on Textured Ni Substrates for YBCO Conductors

CeO₂/YSZ/CeO₂ buffer layers were deposited on textured Ni substrates byin situ pulsed laser deposition. The out-of-plane texture and in-plane texture of the buffer layers were characterized by X-ray diffraction ω-scans and ϕ-scans. Using this CeO₂/YSZ/CeO₂ architecture as the buffer layers, high quality YBCO films with a zero-resistanceT _c about 90 K and a self-field critical current densitiesJ _c above 10⁶ A/cm² at 77 K can be obtained on Ni substrates.     

Studies of solution deposited cerium oxide thin films on textured Ni-alloy substrates for YBCO superconductor

Cerium oxide (CeO₂) buffer layers play an important role for the development of YBa₂Cu₃O_7−x (YBCO) based superconducting tapes using the rolling assisted biaxially textured substrates (RABiTS) approach. The chemical solution deposition (CSD) approach has been used to grow epitaxial CeO₂ films on textured Ni–3 at.% W alloy substrates with various starting precursors of ceria. Precursors such as cerium acetate, cerium acetylacetonate, cerium 2-ethylhexanoate, cerium nitrate, and cerium trifluoroacetate were prepared in suitable solvents. The optimum growth conditions for these cerium precursors were Ar–4% H₂ gas processing atmosphere, solution concentration levels of 0.2–0.5 M, a dwell time of 15 min, and a process temperature range of 1050–1150 °C. X-ray diffraction, AFM, SEM, and optical microscopy were used to characterize the CeO₂ films. Highly textured CeO₂ layers were obtained on Ni–W substrates with both cerium acetate and cerium acetylacetonate as starting precursors. YBCO films with a J_c of 1.5 MA/cm² were obtained on cerium acetylacetonate-based CeO₂ films with sputtered YSZ and CeO₂ cap layers.     

Single cerium zirconate buffer layer on biaxially textured metal substrates for high performance coated conductors

Textured cerium zirconate (Ce _x Zr_1−x O₂) films were deposited on biaxially textured Ni-5at.%W substrate by direct-current (dc) reactive magnetron sputtering for low cost production of high performance YBa₂Cu₃O_7−δ (YBCO) coated conductors. Film composition was controlled by modulating dc power applied to the Ce metal target. X-ray diffraction analysis shows that all the samples exhibit epitaxial growth, with c-axis perpendicular to the substrate surface. The YBCO film deposited directly on the Ce_0.32Zr_0.68O₂ layer for optimized lattice matching shows a transition temperature T _c and critical current density J _c (75.5 K, self field) of 90.4 K and 1.3 MA/cm². The in-field dependence of J _c is similar to the standard CeO₂/YSZ/CeO₂ buffered samples. These results demonstrate that a single Ce _x Zr_1−x O₂ buffer layer, instead of CeO₂/YSZ/CeO₂ multi-buffer layers for the fabrication of YBCO coated conductors, provides advantages such as simplified architecture and potentially reduced cost due to the reduced fabrication steps.     

Continuous Deposition of YBCO Films for Coated Conductor Using PLD Method

A reel-to-reel PLD system was set up for studying YBCO coated conductor. YBCO films could be continuously grown on the CeO₂/YSZ/Y₂O₃ buffered Ni-5W tape. Some deposition parameters were investigated. XRD θ–2θ scans were employed to characterize the c-axis orientation and in-plane texture of YBCO films deposited with different laser repetition rates and tape moving speeds. We investigated the dependence of critical current I _c on laser repetition rates and tape moving speeds for YBCO films. It had been found that a-axis oriented grains appeared as YBCO layer thickness increased, which prevented the values of I _c improved.     

High-Quality YBa2Cu3O7?x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7–x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7–x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from -scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

Influence of CeO2-Cap Layer on the Texture and Critical Current Density of YBCO Film

YBa₂Cu₃O₇ (YBCO) thin films have been fabricated on different textured CeO₂-cap layers by pulsed laser deposition (PLD). The texture and critical current density J _c of YBCO thin films have been systematically investigated. Both in-plane and out-of-plane textures of YBCO films and CeO₂-cap films were characterized by X-ray diffraction (XRD). And the critical currents of YBCO films were measured by the conventional four-probe method. It was found that the texture and J _c of YBCO films were largely dependent on the texture of CeO₂-cap layers under the optimized deposition conditions. With increasing the degree of in-plane and out-of-plane texture of CeO₂-cap layers, J _c of YBCO films decreased from 4.23×10⁶ A/cm² to 0.47×10⁶ A/cm². The FWHM values of the omega scan rocking curves of YBCO films decreased from 3.71 to 1.84° and the phi scan rocking curves from 6.68 to 4.91° with improvement of CeO₂-cap layer texture. Our results indicated that the fabrication of high texture quality of CeO₂-cap layer was necessary for the epitaxial growth of high-J _c YBCO films. The high-quality YBCO films which are comparable with those grown on single crystal substrates could be achieved on high textured CeO₂-cap layers buffered metal substrates.     

High-Quality YBa2Cu3O7−x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7?x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7?x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

Effect of Firing Temperature on the Structure and Properties of YBa2Cu3O7−x Films by TFA-MOD Method

YBa₂Cu₃O_7−x (YBCO) films were fabricated on LaAlO₃ (LAO) substrate under various firing temperatures (760–870 °C) in the crystallization process by metalorganic deposition (MOD) method using trifluoroacetates. The effect of firing temperature on the structure and properties of YBCO films was systematically investigated. According to the XRD and SEM images, the films fired at low temperature (760–800 °C) showed poor electrical performance due to rough surfaces and impurity phases. However, the films fired at 850 °C showed the highest critical temperature of 90 K and the highest J _c of 3.1 MA/cm² which attribute to the formation of a purer YBCO phase, fewer pores, and stronger biaxial texture.     

Fabrication and Properties of Coated Conductors with All Fluorine-Free CSD Route

A fluorine-free chemical solution deposition (CSD) method has been developed to fabricate YBCO epitaxial films and SmBiO₃ or Sm-doped CeO₂ (SCO) buffer layers on crystal substrates and textured Ni5%W metal substrates. The critical current density, J _c , is over 4 MA/cm² for YBCO deposited on crystal substrates, over 3 MA/cm² on SmBiO₃-buffered single crystal substrates, and it reaches 1 MA/cm² on a (Sm,Ce)O₂-buffered metal substrate, demonstrating the feasibility of an all-CSD method for fabricating high quality coated conductors.     

Effect of Firing Temperature on the Structure and Superconducting Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films

YBa₂Cu₃O_7−x (YBCO) films were prepared on LaAlO₃ single crystal substrate under various firing temperatures (750–800 °C) in the crystallization process by metalorganic deposition (MOD) method. The coating solution was made by mixing the fluorine-free precursor solution containing Y and Cu with Ba–fluorine precursor solution (Ba-TFA). The effect of firing temperature on the structure and superconducting properties of YBCO films was systematically investigated. The results indicated that YBCO-films were smooth, crack-free, exhibited good textures and retain high oxygen content according to the XRD and SEM images. Sample of YBCO-film fired at 780 °C showed highest superconducting properties including high critical transition temperature T _c=89 K, sharp transition temperature ΔT _c<1 K, and critical current density J _c=2.8 MA cm⁻², which are attributable to excellent in-plane textures and dense microstructures with good connectivity between the grains.     

Buffer layers for high-Tc thin films on sapphire

Buffer layers of various oxides including CeO₂ and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa₂Cu₃O_7– (YBCO) thin films. An ion beam channeling minimum yield of 3% was obtained in the CeO₂ layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO₂ was found to be the best one for YBCO thin films onR-plane sapphire. HighT _c andJ _c were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were 4 m at 77 K and 25 GHz.     

Influence of Heat Treatment on Structure and Superconducting Properties of YBCO Film by Chemical Solution Deposition Method

YBCO films have been fabricated on a (00l) LaAlO₃ single-crystal substrate via self-developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method. The influence of heat treatment on texture, microstructure and superconducting properties of YBCO films has been investigated. After a pyrolysis process ranging from 145 °C to 500 °C with different heating rates, the samples were fired at 760–780 °C in Ar and O₂ mixture gas followed by annealing at 450 °C in pure O₂. The results indicate the film fired at 770 °C after decomposition at the rate of 0.5 °C/min showed the highest T _c of 90.4 K and J _c (77 K, 0 T) over 2 MA/cm². According to the XRD patterns, phi-scan and omega-scan curves as well as SEM images, the good properties may be attributed to better biaxial texture and purer YBCO phase as well as better grain connectivity.     

Influence of BaZrO<Subscript>3</Subscript> Amount on Microstructure and Properties in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films Prepared by TFA-MOD Process

Epitaxial YBa₂Cu₃O_7−x /BaZrO₃ films were prepared by complex metal–organic solutions including Y, Ba and Cu anhydrous trifluoroacetate precursors and Zr acetylacetonate precursors. The influence of the amount of BaZrO₃ (BZO) on the structure and properties of YBa₂Cu₃O_7−x (YBCO) films was systematically investigated. The YBCO films having 7 mol % BZO have a maximum critical current density (J _c) value (77 K, 0 T) of 6.0 MA/cm². The enhanced J _c derives from a high density of BZO nanoparticles existing in the YBCO matrix. With increasing BZO amount, J _c of the YBCO films begins to decrease due to larger particles.     

Effect of Melt Processing Programme on Microstructure and Properties of YBCO Thick Films

The screen printing technique was used in the fabrication of YBa₂Cu₃O_7−σ (YBCO) superconducting thick films on yttria stabilised zirconia (YSZ) substrates. Different slow cooling rates were used in the preparation of YBCO thick films after fast cooling from the melt processing temperature. The effects of the melt processing programme on texturing, microstructure and superconducting properties of the melt processed YBCO films were studied. Slow cooling rates between 1005 and 990 °C were effective in increasing the interaction of viscous molten with reduced film/substrate, and hence a relatively large grain size has been obtained. Moreover, different c-axis texturing ratios and grain morphologies were observed.     

Buffer layers for high-T c thin films on sapphire

Buffer layers of various oxides including CeO₂ and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa₂Cu₃O_7?δ (YBCO) thin films. An ion beam channeling minimum yield of ～3% was obtained in the CeO₂ layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO₂ was found to be the best one for YBCO thin films onR-plane sapphire. HighT _c andJ _c were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were ～4 mω at 77 K and 25 GHz.     

Nanostructural characterization of YBCO films on metal tape with textured buffer layer fabricated by pulsed-laser deposition

Thick YBa₂Cu₃O_7-x (YBCO) films with high critical current density (J_c) values were deposited by pulsed-laser deposition (PLD) on Hastelloy with a textured CeO₂/Gd₂Zr₂O₇ buffer layer. Both cross-sectional and plan-view TEM specimens of the YBCO films were prepared, and then the nanostructural characterization of the films was performed by transmission electron microscopy (TEM). The YBCO films less than 1 μm thick were predominantly composed of c-axis-oriented grains, however, many a-axis-oriented grains, which grew larger with the increase of the thickness of the YBCO film, were formed beyond about 1 μm from the CeO₂ interface. We found Y₂O₃ and copper oxides between a- and c-axes-oriented grains. In particular, Y₂O₃ grains were formed between the {001} plane of an a-axis-oriented grain and the {100} or {010} plane of a c-axis-oriented grain. The orientation relationships between Y₂O₃ and YBCO are found to be; (001)YBCO//(001)Y₂O₃ and (100)YBCO//(110)Y₂O₃. In addition, we also found gaps between YBCO grains. Since a-axis-oriented grain growth and the formation of Y₂O₃, copper oxides and the gaps are considered to reduce the J_c values of the YBCO film, it is important to determine the optimum process conditions to suppress the nucleation of a-axis-oriented grains, impurity oxides and gaps.     

YBa2Cu3O7 − δ thin films with enhanced film properties grown on sapphire using Y2O3/CeO2 bi-layer buffer

Highly epitaxial, microcrack-free thin films of YBa₂Cu₃O_7 − δ (YBCO) have been grown by pulsed laser deposition on sapphire substrates with a double buffer of Y₂O₃/CeO₂. The Y₂O₃ layer, which has excellent compatibility with CeO₂ and YBCO, has been found to be beneficial in reducing the surface porosity of YBCO films as well as inhibiting a-axis-oriented epitaxial growth. The reduction in porosity is attributed to the presence of the Y₂O₃ layer which acts as a suitable barrier for the chemical reaction occurring at high deposition temperatures between YBCO and CeO₂. Due to the improvement in film quality and surface morphology, enhancement of the self-field critical current density (J_c, 77.3 K) by as high as 30% was obtained for a 650-nm thick YBCO film deposited on Y₂O₃/CeO₂ bi-layer buffer compared to simultaneously-deposited YBCO film on CeO₂ single-layer buffer only.     

Influence of deposition and annealing parameters on some properties of YBCO films prepared by spray pyrolysis

Superconducting YBCO films were prepared by chemical deposition of aerosol generated from nitrate solutions by pneumatic and ultrasonic atomizers. Single crystalline YSZ and MgO substrates were used. Two different substrate temperatures were applied, with the chemical composition of starting solutions being adjusted accordingly. Deposited precursor films were thermally processed under two basically different conditions, i.e., either in atmospheric O₂ atT900°C, or in atT600°C. The influence of these different deposition and annealing parameters onT _c andJ _c values, microstructure, and surface morphology is reported. By optimizing our technological procedure, a real possibility exists to prepare (by means of aerosol deposition) YBCO films withJ _c values of at least 10⁴–10⁵ A/cm², at processing temperatures (both deposition and annealing as well) not exceeding 600–700°C.     

Fabrication of SmBa2Cu3O7 Films on CeO2 Buffered Sapphire

SmBa₂Cu₃O_7−δ (SmBCO) thin films and CeO₂ buffer layers were deposited on γ-cut sapphire by pulsed laser deposition (PLD) and characterized with X-ray diffraction (XRD) and atomic force microscope (AFM). The θ–2θ XRD scans of the SmBCO/CeO₂/sapphire structures revealed that the CeO₂ and SmBCO films were grown with c-axis perpendicular to the substrate. In Φ-scan XRD patterns, four (103) peaks of the SmBCO film were observed and the peak positions were shifted by 45° from (202) peaks of the CeO₂ films. From the peak shifts we could conclude that the [110]_SmBCO crystal axis is parallel to the [100]_CeO2 crystal axis. Moreover, pole figure also confirmed that SmBCO films were grown on the substrates epitaxially along in-plane direction. The SmBCO films show very flat surfaces with root mean square (RMS) about 5 nm. In agreement with this crystalline perfection, SmBCO thin films present excellent superconducting properties: T _c0 > 90 K, transition width 0.4 K, and J _c(77 K) > 2 MA/cm².