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We have developed a long Y-123 coated conductors by ion-beam-assisted deposition (IBAD) and pulsed-laser-deposition (PLD) method. Now, we can routinely obtained 100 m class Y-123 tapes with over 100 A at 77 K. For power applications using Y-123 conductors, coiling and cooling techniques are important elements. From 2004, we have developed and demonstrated a solenoid type coil that is the most suitable for practical application because it has advantage such as (1) uniformity of magnetic field (2) no joint. In this paper, we describe the development of conduction-cooled HTS test coil that has 14 turn × 22 layers using 100 m Y-123 conductors. In order to use the conduction cooling system, the Y-123 conductors were stabilized for 0.1 mm thick Cu tapes and spacers between the layers consisted of aluminum nitride in the coils. In the demonstration of this coil at 30 K, 40 K, 50 K, 60 K and 77 K by cryo-cooling system, the good cooling and superconducting performances were observed. The central magnetic fields of over 1 T were successfully obtained with operating currents of over 400 A and under 40 K. Furthermore, this coil was operated on additional magnetic fields of 3 T by combination of LTS magnets. The central magnetic fields of 0.5 T were generated from the cooled HTS coils with operating current of 190 A on additional magnetic fields of 3 T by LTS magnets at 30 K. Total magnetic fields were exceeded 3.5 T at 30 K.

We have developed long tape processing for coated conductors by the advanced TFA-MOF method. In order to obtain uniform and high superconducting performance, a 25-m long tape with dummy tapes, which were connected at the both end of the 25-m long tape, was fabricated by the reel-to-reel (RTR) system with a relatively high gas flow rate. As a result, a uniform I c value over 100 A and a high end-to-end I c value of 100 A were obtained in the tape. It revealed the I c · L product of 2500 A m. For higher I c · L products in long tapes, the key process factors were investigated.

We have investigated the effects of a heating rate in the calcination process on microstructures of YBa 2 Cu 3 O 7− y (YBCO) precursor and final films. As a result, it was found CuO segregation in the YBCO precursor film, when the precursor was prepared at a rapid heating rate in the calcination. In the case of heating this precursor film in the crystallization process, many large pores and misoriented YBCO crystals tend to remain in the YBCO final film and its I C value deteriorates compared with the case of crystallizing the precursor, which is prepared at the slow heating rate in the calcination. It is important to control the heating rate in the calcination, in order to attain higher J C of the YBCO final films.

Recent progress in the research and development of the TFA-MOD processing for the coated conductors was reviewed. For the higher I c performance, crack generation in the thick films was a problem, which limits increasing I c values. It was found that both lower heating rate and low P H 2 O in the crystallization step are effective to improve the critical thickness for obtaining crack-free films. However, the high P H 2 O is preferable for a high production rate and high J c performance. Then, the new heat treatment profile, which could suppress crack formation even in thick films under the high P H 2 O , was developed. Consequently, the high I c value of 470 A at 77 K was realized in the film with 3 μm in thick. Concerning the long-tape processing, the key factors for high I c performance by TFA-MOD process were investigated. The highly textured substrate for high crystallinity of the YBCO layer, the thick film with maintaining reasonable high J c and full conversion reaction to form the YBCO layer are important to obtain high superconducting performance in long tapes. At present, a 25 m long tape with a reasonable high I c value of 100 A, and a high I c value of 250 A in a 4 m long tape were achieved. In order to realize a high production rate, the processes for both calcination and crystallization steps have to be developed. In the calcination step, the multi-turning continuous system was developed to maintain an effective travelling rate even in the multi-coating method. On the other hand, in the crystallization step, it was confirmed that optimization of the processing parameters such as a high water vapour partial pressure, a low total pressure, and a high gas flow rate could make the YBCO growth rate increase. The combination effect of these parameters is experimentally confirmed and the 5 times higher production rate was achieved in the crystallization step. Additionally, the equipment design of the multi-turning system with a new gas flow concept, which is a vertical gas flow system, was investigated by means of both numerically and experimentally, and the concept was confirmed at least. Consequently, a high production rate over 5 m/h could be expected by combining the above mentioned effects.

High quality YBa 2 Cu 3 O 7− δ coated conductors with a length of several hundred meters are available now. For practical applications of long tapes, non-destructive and efficient techniques for characterization are necessary. For that purpose, we installed a commercial machine. The measuring principle is that a tape is introduced into a space of an applied magnetic field and the induced current within the superconducting tape is evaluated by measuring the magnetic field distribution just above the tape surface using a Hall sensor array. The advantages of the technique are that the measurement speed is rather high, 15 mm/s at maximum, and tapes are absolutely safe from damages by excessive heating. On the other hand, the estimation of the critical current is not straightforward because the issue of the calibration is not trivial. A 2D computer simulation code was developed to calculate the distribution of the induced current within the tape. In the simulation, it was assumed that the current–voltage relation of the superconductor obeys a power law. It is shown that the critical current deduced from the measurement is strongly dependent on the n -value.

We have investigated influences of the H2O gas inlet temperature in the calcination process on both microstructures and superconducting properties of YBCO films fabricated by the advanced TFA-MOD process, from the viewpoint of the process control in the reel-to-reel system. Low Ic values were recognized in the films which were prepared by introducing the H2O gas from the lower temperature than 200 °C. XRD measurements and cross-sectional TEM observations suggest that the YBCO films calcined with the H2O inlet temperature below 200 °C revealed a random orientation without epitaxial growth. Consequently, it has been understood that control to keep low partial pressure of the H2O gas in a lower temperature region is important for fabrication of long-length YBCO tapes with high performance by the reel-to-reel system.

We fabricated Y 1− x Sm x Ba 2 Cu 3 O y and YSm x Ba 2 Cu 3 O y (YSmBCO) films on SrTiO 3 (STO) single crystalline substrates and IBAD buffered metallic tapes (PLD-CeO 2 /IBAD-GZO/Hastelloy) by the advanced TFA-MOD process by mixing TFA salts of Y, Sm, Ba and Cu naphthenate, and tried to improve the superconductivity properties compared with those of the pure YBa 2 Cu 3 O 7− δ (YBCO). As a result, J c values of Y 1− x Sm x Ba 2 Cu 3 O y films with x = 0.05 or 0.3, which were fabricated on STO substrates, were improved under the high magnetic fields compared with those of the YBCO without substitution. However, Sm segregation was detected near the STO substrate which was obtained by the TEM–EDS (transmission electron microscopy–energy-dispersive X-ray spectroscopy) analysis. On the other hand, Y 1− x Sm x Ba 2 Cu 3 O y films with x = 0.05 and YSm x Ba 2 Cu 3 O y films with x = 0.3, which were fabricated on IBAD buffered metallic substrates, also improved I c value under the high magnetic fields compared with those of the YBCO without substitution. Moreover, from the TEM-EDS analysis, Y or Sm segregation was not found. The difference in the Sm segregation behavior between the film on STO and CeO 2 buffered substrates could be explained by the difference in the ratios of the misfit parameters between YBCO/substrate and SmBCO/substrate. Additionally, it was considered that the above phenomenon might be unique in the MOD system and could not be observed in the PLD system, because of the difference in the crystal growth mechanism. In the film of YSm 0.3 Ba 2 Cu 3 O y , we observed the grains of (Y, Sm) 2 O 3 and (Y, Sm) 2 Cu 2 O 5 by TEM observation. We will further investigate the origin of pinning centers.

To realize a higher production rate is one of the important issues in the development of the long-tape processing for the YBCO coated conductors by the TFA-MOD process using the reel-to-reel system. Not only scale-up of the furnace and decrease of YBCO thickness with the higher Jc value, but higher YBCO growth rates could be effective to achieve a higher traveling rate in this system. It was confirmed that optimization of the process parameters such as high water vapor partial pressure, high diffusion constants due to a low applied total pressure, and a high gas flow rate affect the YBCO growth rate. Consequently, these higher growth rate conditions were simultaneously applied to the reel-to-reel system. As a result, the traveling rate of 1.2 m/h was achieved for fabricating YBCO films in the reel-to-reel system, which is about five times as fast as the previous traveling rate with maintaining high Ic value of higher than 200 A/cm-width by controlling above conditions.

We have investigated the influence of the heating rate in the calcination process on the superconducting properties in order to optimize an advanced TFA-MOD process. The increase of the heating rate makes J c values lower, and the maximum values of HF gas generation rate higher. From TEM observation of the precursor film, the large pores and segregation of Cu elements were observed in the films at high heating rates. Consequently, it is clear that the increase of the heating rate during calcination process caused abrupt HF gas generation leading to degradation of the surface morphologies, formation of large pores and segregation of Cu elements. These morphological and microstructual differences could be one of the reasons for a reduction in J c values of the YBCO films.

We have prepared Y123 quenched and fully crystallized films by the conventional and the advanced TFA-MOD processes. Then, we have investigated microstructures of the films by means of transmission electron microscopy, in order to understand the initial stage of the Y123 growth. As a result, it is found that Y–Ba–O–F grains firstly nucleate on LaAlO 3 single crystals as well as on CeO 2 buffered metal substrates both in the TFA-MOD processes. The Y–Ba–O–F is grown with epitaxially orientated relationship to the substrate. A BaCeO 3 reaction product layer is formed due to Ba diffusion from Y123 to CeO 2 mainly through the grain boundaries. At the crystallization temperature of 760 °C, the diffusion rate of Ba is slow. Therefore, it was found that the thickness of the BaCeO 3 layer does not increase much further with increasing the holding time until about 1000 min.

We have developed a reel-to-reel reaction furnace equipped with seven transverse flow modules, for the continuous TFA-MOD process. Experimental results have revealed that as the wire length increases, the processing parameters need to be adjusted in order to obtain YBCO films with high critical current density ( J c ) than 1 MA/cm 2 at 77 K and self-field. As a result of fabrication under the appropriate process conditions, we have successfully produced uniform tapes with a high performance in a short sample. In the recent progress, heat-treatment conditions have been optimized and the higher gas flow rate was applied to the long tape fabrication, and the I c value of higher than 100 A as the end-to-end one of a 8.5 m long tape was attained.

The recent progress in coated conductor development by the TFA-MOD process was reviewed. Much progress was recognized in the R&D on TFA-MOD processing for satisfying several requirements, such as high superconducting performance, long tape production, high production rate, etc., considering the real application. For high superconducting performance, the high I c value of 413 A was achieved in a short sample with an architecture of MOD-YBCO/IBAD-GZO/IBAD-GZO/Hastelloy. It was realized by improving the in-plane crystal grain alignment of the CeO 2 buffer layer and optimizing conditions of the heat treatments in the calcination and crystallization processes. In the calcination process, it was found that lower heating rate was effective to improve the morphology of the multi-layered precursor by preventing from pore generation and segregation of Cu elements. In the crystallization process, it was found that high humid gas partial pressure was effective to improve the J c values by reducing the amounts and size of pores in the YBCO layers. Furthermore, in the case of thicker firm, it was found that the low heating rate in the crystallization process was effective to prevent from the crack formation. On the other hand, in order to obtain the long tape with uniform I c values, in the crystallization process, the gas flow condition in a large scale equipment for the continuous long tape process was investigated using a computer simulation technique. Consequently, the 8.6 m YBCO on PLD-CeO 2 /IBAD-Gd 2 Zr 2 O 7 /Hastelloy tape which shows high I c performance of 119 A as an end-to-end value at 77.3 K in self fields, was successfully obtained. As a result, I c × L (length) value of 1024 A m was achieved.

YBCO films were fabricated on PLD-CeO 2 /IBAD-Gd 2 Zr 2 O 7 /Hastelloy substrates using the TFA-MOD process. In order to obtain higher I c performance, the thicker films maintaining high- J c values are required. J c depended strongly on the P H 2 O during the crystallization in the process. The pore size was smaller in the high- J c films crystallized under the medium P H 2 O condition and became larger in the low- J c films under both lower and higher P H 2 O for the YBCO films with a constant thickness. Furthermore, crack generation was observed in thick films crystallized at the high P H 2 O condition. Consequently, both pore and crack formations limit the J c properties since large pore causes local reduction of current paths and additionally results in local concentration of electric fields. J c also depended strongly on the YBCO crystal grain alignment which is affected by in-plane grain alignment of the buffered substrate. Then, the effect of in-plane grain alignment of the CeO 2 cap layer on J c was investigated for thicker YBCO films. As a result, the J c value increased with improvement of the crystal alignment of the CeO 2 cap layers. Finally, a YBCO film with a high- I c value of 413 A at 77 K in self-field was achieved in a film fabricated on Δ ϕ = 4° of the CeO 2 layer using the appropriate crystallization conditions for crack-free film.

We proposed a multi-dimensional numerical model of the YBCO growth in the TFA-MOD process, which was established in consideration of the basic concept of the one-dimensional analytical growth model, multi-component diffusion and convective gas flow. It was confirmed that the calculated growth rate values were in good agreement with experimental ones. Furthermore, this numerical method revealed an effective tool to determine a suitable configuration of the gas flow in the reaction tube for a high production rate in long YBCO coated conductors.