Kai Shi

A rapid process of YBa2Cu3O7−δ thin film fabrication using trifluoroacetate metal–organic deposition with polyethylene glycol additive

Trifluoroacetate metal–organic deposition (TFA-MOD) is a promising technique to fabricate YBa2Cu3O7−δ (YBCO) superconducting films. However, its slow pyrolysis process, which usually takes more than 10 h, constitutes a barrier for industrial production. In this study, polyethylene glycol (PEG) was utilized to reduce the stress generation inside the coated films when the strong pyrolysis reactions happen. With the addition of 30 wt% PEG2000 to the precursor solution, a smooth film surface could be obtained through a rapid pyrolysis process of 15 min. After the optimizations of the crystallization and oxygenation processes, mass percentage and molecular weight of PEG additive, YBCO thin films with Jc of about 4.5 MA cm−2 (77 K, self-field) could be routinely fabricated using (20–30) wt% PEG(1000–2000) additive with a total treatment time of about 2 h including the 15 min pyrolysis process time. The effects of PEG additive were discussed using one of the mechanisms of buckling formation. The reduction of compressive stress by PEG additive was suggested to be the reason for preventing buckling.

A low-fluorine solution with a 2:1 F/Ba mole ratio for the fabrication of YBCO films

In previously reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. Further lowering the F/Ba ratio would bring benefits for the environment, thick film deposition, and an understanding of the heat treatment process. On the other hand, the F/Ba ratio must be at least 2 for full conversion of the Ba-precursor to BaF2 to avoid the formation of BaCO3, which is detrimental to the superconducting performance of YBCO films. In this study, a solution with a 2:1 F/Ba mole ratio was developed, and the fluorine content of this solution was approximately only 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transform-infrared spectra (ATR-FT-IR) revealed that BaCO3 was remarkably suppressed in the as-pyrolyzed film?and eliminated at 700??C. Thus, YBCO films with a critical current density (Jc) of over 5?MA?cm?2 (77?K, 0?T, 200?nm thickness) could be obtained on lanthanum aluminate single-crystal substrates. In?situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all F atoms might remain in the film as fluorides. X-ray diffraction ?/2? scans showed the presence of BaF2?but not of Y F3 or CuF2?in films quenched at 400?800??C. The formation priority of BaF2 over Y F3 and CuF2 was interpreted by examining the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of precursor solutions for MOD-YBCO fabrication, and serve as a foundation for continuously and systematically studying the influence of fluorine content in the precursor solutions.

Resputtering effect during MgO buffer layer deposition by magnetron sputtering for superconducting coated conductors

In this study, MgO thin films were deposited by radio-frequency magnetron sputtering. The film thickness in the deposition area directly facing the target center obviously decreased compared with that in other areas. This reduction in thickness could be attributed to the resputtering effect resulting from bombardment by energetic particles mainly comprising oxygen atoms and negative oxygen ions. The influences of deposition position and sputtering pressure on the deposition rate were investigated. Resputtering altered the orientation of the MgO film from (111) to (001) when the film was deposited on a single crystal yttria-stabilized zirconia substrate. The density distribution of energetic particles was calculated on the basis of the measured thicknesses of the MgO films deposited at different positions. The divergence angle of the energetic particle flux was estimated to be approximately 15°. The energetic particle flux might be similar to the assisting ion flux in the ion beam assisted deposition process and could affect the orientation of the MgO film growth.

A water-free metal organic deposition method for YBa2Cu3O7?? thin film fabrication

During the fabrication of YBa2Cu3O7?? (YBCO) thin films via the metal organic deposition (MOD) method, the water content of the precursor solution should be reduced to prevent precipitation and other detriment. In most of the reported MOD-YBCO studies, water was used as a solvent, and vacuum distillation was utilized to reduce the water content of the precursor solution. In this study, we propose a water-free MOD method using propionic anhydride and methanol as the solvents. The crystal water of raw precursor salts was removed by heating, and the water content in the final precursor solution was as low as 0.2?wt%. Using this solution with low fluorine content and the optimized maximum temperature of pyrolysis, the final YBCO thin films about 250?nm thick had the critical current density (Jc) of 3.8?MA?cm?2 (77?K, self-field). The water content of the precursor solution was further reduced to less than 10?2?wt% by vacuum distillation. According to the Jc performance, thermal analysis, x-ray diffraction investigations, and morphology observations, no significant improvement was found for the final YBCO thin films fabricated by the distilled solution, indicating that vacuum distillation is not necessary in the water-free MOD method.

A low-fluorine solution with a 2:1 F/Ba mole ratio for the fabrication of YBCO films: Paper

In previously reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. Further lowering the F/Ba ratio would bring benefits for the environment, thick film deposition, and an understanding of the heat treatment process. On the other hand, the F/Ba ratio must be at least 2 for full conversion of the Ba-precursor to BaF2 to avoid the formation of BaCO3, which is detrimental to the superconducting performance of YBCO films. In this study, a solution with a 2:1 F/Ba mole ratio was developed, and the fluorine content of this solution was approximately only 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transform-infrared spectra (ATR-FT-IR) revealed that BaCO3 was remarkably suppressed in the as-pyrolyzed film?and eliminated at 700??C. Thus, YBCO films with a critical current density (Jc) of over 5?MA?cm?2 (77?K, 0?T, 200?nm thickness) could be obtained on lanthanum aluminate single-crystal substrates. In?situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all F atoms might remain in the film as fluorides. X-ray diffraction ?/2? scans showed the presence of BaF2?but not of Y F3 or CuF2?in films quenched at 400?800??C. The formation priority of BaF2 over Y F3 and CuF2 was interpreted by examining the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of precursor solutions for MOD-YBCO fabrication, and serve as a foundation for continuously and systematically studying the influence of fluorine content in the precursor solutions.

A Low-Fluorine Solution with the F/Ba Mole Ratio of 2 for the Fabrication of YBCO Films

In previously reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of the precursor solution was 4.5. Further lowering the F/Ba ratio would bring benefits for the environment, thick film deposition, and an understanding of the heat treatment process. On the other hand, the F/Ba ratio must be at least 2 for full conversion of the Ba-precursor to BaF2 to avoid the formation of BaCO3, which is detrimental to the superconducting performance of YBCO films. In this study, a solution with a 2:1 F/Ba mole ratio was developed, and the fluorine content of this solution was approximately only 10.3% of that used in the conventional TFA-MOD method. Attenuated total reflectance-Fourier transform-infrared spectra (ATR-FT-IR) revealed that BaCO3 was remarkably suppressed in the as-pyrolyzed film?and eliminated at 700??C. Thus, YBCO films with a critical current density (Jc) of over 5?MA?cm?2 (77?K, 0?T, 200?nm thickness) could be obtained on lanthanum aluminate single-crystal substrates. In?situ FT-IR spectra showed that no obvious fluorinated gaseous by-products were detected in the pyrolysis step, which indicated that all F atoms might remain in the film as fluorides. X-ray diffraction ?/2? scans showed the presence of BaF2?but not of Y F3 or CuF2?in films quenched at 400?800??C. The formation priority of BaF2 over Y F3 and CuF2 was interpreted by examining the chemical equilibrium of the potential reactions. Our study could enlarge the synthesis window of precursor solutions for MOD-YBCO fabrication, and serve as a foundation for continuously and systematically studying the influence of fluorine content in the precursor solutions.

A Review of the Ion Beam Assisted Deposition Researches towards Industrialization for the Second Generation High Temperature Superconducting Wire Fabrication

on beam assisted deposition (IBAD) is an important technique to fabricate the second generation high temperature superconducting (2G HTS) wires. Among the fabrication routes of 2G HTS long wires, IBAD route achieved the best performance in recent years. IBAD was adopted in this field in 1991 to obtain biaxially textured buffer layers, which helped to deposit high quality YBCO superconducting films on metallic substrates for the first time. Series of experimental and industrial researches on IBAD were carried out by many groups worldwide. And in the researches lasting for over two decades, the focused material for IBAD was changed from Yttria-Stabilized Zirconia (YSZ), Gd2Zr2O7 (GZO) to MgO. In this paper, the research progresses and the main achievements were briefly reviewed.