Bowan Tao

A novel process for CeO2 single buffer layer on biaxially textured metal substrates in YBCO coated conductors

We report a newly developed method, namely the in situ postannealing texture (IPAT) technique, based on a two-step principle to fabricate CeO2 single buffer layer on biaxially textured Ni–5 at.%W substrate by a reactive direct-current sputtering method. The annealing temperature and time dependence of the grain alignment in CeO2 buffer layers was studied. High-quality CeO2 layers were achieved at annealing temperatures from 750 to 850 °C, with annealing times from 10 to 20 min. Atomic force microscopy revealed, via the novel IPAT method, a continuous, dense, and crack-free surface morphology for CeO2 thin films with a thickness of up to 300 nm, which act as a sufficiently good single buffer for YBa2Cu3O7−δ (YBCO) coated conductors (CCs). The preparation of subsequent YBCO CCs grown on the IPAT-prepared CeO2 single buffer layers was also investigated.

YBCO melt-textured growth seeded by superheating YBCO/MgO thin film

Using YBCO/MgO thin films, for the first time, we report a success in the melt-textured growth of YBCO bulks by the cold-seeding method. The grown bulk has a good orientation relation of . In this process, the YBCO film withstood heating over its peritectic temperature (up to 1045 °C) for 1.5 h and acted as a seed for the YBCO bulk growth, indicating an intrinsic superheating property, which was reported in other work. The unique superheating mechanism in this study was clarified. It was found that the a–b alignment of YBCO film plays a key role in controlling the single grain growth of YBCO bulk.

Properties Optimization for Perovskite Oxide Thin Films by Formation of Desired Microstructure

Perovskite oxide materials are very important for the electronics industry, because they exhibit promising properties. With an interest in the obvious applications, significant effort has been invested in the growth of highly crystalline epitaxial perovskite oxide thin films in our laboratory. And the desired structure of films was formed to achieve excellent properties. Y₁Ba₂Cu₃O 7-x (YBCO) superconducting thin films were simultaneously deposited on both sides of 3 inch wafer by inverted cylindrical sputtering. Values of microwave surface resistance R s (75 K, 145 ㎓, 0 T) smaller than 100 mΩ were reached over the whole area of YBCO thin films by pre-seeded a self-template layer. For implementation of voltage tunable high-quality varactor, A tri-layer structured SrTiO₃ (STO) thin films with different tetragonal distortion degree was prepared in order to simultaneously achieve a large relative capacitance change and a small dielectric loss. Highly a-axis textured Ba 0.65 Sr 0.35 TiO₃ (BST65/35) thin films was grown on Pt/Ti/SiO₂/Si substrate for monolithic bolometers by introducing Ba 0.65 Sr 0.35 RuO₃ (BSR65/35) thin films as buffer layer. With the buffer layer, the leakage current density of BST65/35 thin films were greatly reduced, and the pyroelectric coefficient of 7.6×10?? C ㎝?² K?¹ was achieved at 6 V/㎛ bias and room temperature.

Fabrication of long-length ion beam-assisted deposited MgO templates for YBCO-coated conductors

The biaxially textured ion beam-assisted deposited (IBAD)-MgO templates were successfully prepared on polycrystalline Hastelloy metal substrate with reel-to-reel system for YB2Cu3O7−δ (YBCO)-coated conductors. By the solution deposition planarization technique, amorphous Y2O3 films were coated on untreated Hastelloy substrate as the bed layer to obtain smooth, dense, and crack-free surface for subsequent IBAD-MgO deposition. The 50 m long IBAD-MgO and homo-epitaxial (epi)-MgO buffer layers deposited on Y2O3 films exhibit excellent crystallographic consistency along the scope with full width half maximum (FWHM) values of (110) ΔΦ and (200) Δω in the range of 5.5°–6.0° and 3.0°–3.5°, respectively. To match the lattice constant of YBCO material, LaMnO3 cap layer was fabricated on IBAD-MgO templates by radio frequency (rf) magnetron sputtering with the FWHM values of in-plane and out-of-plane of 6.8° and 3.7°, respectively, indicating excellent biaxial texture.

Intrinsic structure and thermal stability of YBCO thin film

The melting behaviours of two kinds of c-axis oriented YBa2Cu3Ox (YBCO) films with different four-symmetry in-plane orientations were observed in situ by using high temperature optical microscopy. The in-plane orientation was detected by pole figures and the atomic configuration in the vicinity of the interface of one sample was evaluated by electron diffraction and high-resolution transmission electron microscopy. It is found that the melting mode and the thermal stability of the YBCO thin films with different in-plane orientations differ completely from each other. Such a phenomenon is considerably influenced by the geometrical lattice matching between the YBCO thin films and the MgO substrate, as well as the terminal plane of the YBCO thin films.

A biaxial rotation for depositing homogeneous large-area double-sided YBa2Cu3O7−X thin films

To deposit homogeneous large-area double-sided YBa2Cu3O7–X thin films simultaneously, a single inverted cylindrical target sputtering system with a simple rotation apparatus and a tubelike heater has been developed. The substrate (2 in. LaAlO3) rotated continuously in the heater so that films can be deposited on both sides simultaneously. Three different rotation modes, which are vertical in-plane rotation, out-of-plane rotation, and biaxial rotation (filed as a Chinese patent), have been tried out. The thickness distributions have been measured and compared with theoretical investigation results. While deposited with the subtle biaxial rotation mode, the film shows a much better thickness homogeneity than that with the other two single-axial rotation modes. The measurement result of critical transition temperature (TC) gives the demonstration that the biaxial rotation technique effectively improves the homogeneity of large-area thin films deposited by inverted cylindrical sputtering. Microwave surface re...

Double-sided reel-to-reel metal-organic chemical vapor deposition system of YBa2Cu3O7-δ thin films

Two-micrometer thick YBa2Cu3O7-δ (YBCO) films have been successfully deposited on both sides of LaAlO3 single crystalline substrates by using a home-made reel-to-reel metal-organic chemical vapor deposition (MOCVD) system, which has two opposite symmetrical shower heads and a special-designed heater. This technique can simultaneously fabricate double-sided films with high deposition rate up to 500 nm/min, and lead to doubling current carrying capability of YBCO, especially for coated conductors (CCs). X-ray diffraction analysis showed that YBCO films were well crystallized and highly epitaxial with the full width at half maximum values of 0.2° ∼ 0.3° for the rocking curves of (005) YBCO and 1.0° for Φ-scans of (103) YBCO. Scanning electron microscope revealed dense, crack-free, slightly rough surface with Ba-Cu-O precipitates. The films showed critical current density (Jc, 77 K, 0 T) of about 1 MA/cm2, and overall critical current of 400 A/cm, ascribed to the double-sided structure. Our results also demon...

Some calculations for the thickness distribution of large-area high-temperature superconducting film deposited by inverted cylindrical sputtering

The relative thickness distribution of large-area films deposited from an inverted cylindrical dc sputter gun with high gas pressure is calculated according to Feck’s diffusion law, based on a source-transformation method. The effects of the off-axis displacement rotation, tilted substrate rotation, and a biaxial rotation are compared. The calculated results show that homogeneous large-area films can be deposited with a small target-to-substrate distance by in-plane rotation with a 45° tilting angle or combined tilting together with an off-axis displacement. The biaxial rotation can be used to deposit also homogeneous large-area films. It is suitable for double-sided films. Some experiment data have been presented and show good consistence with the theoretical results. Both tilted in-plane rotation and biaxial rotation have the potential to deposit films with a high deposition rate and to increase the produce efficiency and the utilized ratio of the target. When deposited with these methods, the growth ra...

Thickness Effect on the Structural and Electrical Properties of Sputtered YBCO Coated Conductors

In this paper we studied the structural and electrical properties variations with the thickness of YBa2Cu3O7-X (YBCO) film. In our experiments, the critical current density (Jc) has a maximum at 1.2 micron rather than monotonically decreased with increasing film thickness. The in-plane and out-plane alignments of the films, measured with X-ray Diffraction Omega-scan and Phi-scan, were poor at the beginning stage of the deposition and then became better as the film thickness increased.The surfaces of different samples also change a lot while films less than 700 nm in thickness. Under optimized deposition conditions, biaxial textured YBCO films have been obtained, with the full width at half maximum being 2.1° and 4.5° for out-of-plane and in-plane orientations respectively, Jc greater than 1.2 MA/cm2 at 77 K, self field.

Microstructure modification of La2Zr2O7 buffer films for coated conductors by metal organic decomposition

La2Zr2O7 (LZO) film directly deposited on Ni-5 at.%W by a chemical solution technique, metal organic decomposition (MOD), indicated a poor texture characteristic, which would result in high-angle grain boundaries in subsequent YBa2Cu3O7−δ (YBCO) associated with weak-link behavior. Different ultrathin MOD-LZO, Y2O3, and CeO2 seed layers (~several nanometers) with various annealing temperature were inserted to improve the crystallographic alignment. The relation between the texture and annealing temperature was systematically investigated. A CeO2 seed layer allows us to grow high quality LZO epitaxial films with values of full width at half-maximum around 5.61° and 5.13° for the Φ-scan of (222) and rocking curve of (400) LZO, respectively, which is comparable to the microstructure of films grown using physical vapor deposited Y2O3 as a seed layer. This buffer template, serving for YBCO coated conductors, could potentially decrease the overall fabrication cost.