Large-area, uniform, high critical current density (Jc) YBa2Cu3O7−x (YBCO) superconductor films are now routinely obtained by metalorganic deposition using trifluoroacetates (TFA-MOD) This method does not require any expensive vacuum apparatus at any time during the whole process Thus, TFA-MOD is regarded as one of the most suitable candidates for fabricating a YBCO tape for many high-power applications This method originated from an electron beam process using BaF2 developed by Mankiewich et al Afterwards, Gupta et al reported using TFA-MOD to prepare a similar precursor film These two ex situ processes used fluorides instead of BaCO3 to avoid the fatal deterioration in Jc, which is caused in the resulting films through metal carboxylic groups Fluorides not only avoid such deterioration but also lead to perfectly c-axis-oriented epitaxial crystal growth In conventional metalorganic deposition, nucleation in the precursor film causes random orientation in the resulting film However, in TFA-MOD, nanocrystallites in the precursor film never cause such disorder Furthermore, during the firing process of TFA-MOD, water and HF gas diffuse quickly between the film surface and growth front of the YBCO layer This diffusion never limits the growth rate of YBCO What distinguishes TFA-MOD from conventional metalorganic deposition? What happens during heat treatment? In this paper, we discuss all the TFA-MOD processes and the peculiar growth scheme of the YBCO layer in TFA-MOD using the model of a quasi-liquid network In addition, we review the history of TFA-MOD and recent results and discuss the prospects of future applications

https://iopscience.iop.org/article/10.1088/0953-2048/16/11/R01/pdf

Review of a chemical approach to YBa2Cu3O7−x-coated superconductors—metalorganic deposition using trifluoroacetates

Enhanced hardening in Cu/330 stainless steel multilayers by nanoscale twinning

Growth of the 2223 phase has been studied in the leaded Bi-Sr-Ca-Cu-O system. Differential thermal analysis showed that the enhanced formation of the 2223 phase occurs in the temperature range between 835°C and 869°C where partial melting occurs. The density of the pellet was found to increase by 30% at one hour and then to decrease by 40% from the highest density attained. The former is attributed to the growth of the 2223 phase with the aid of the partially melted liquid phase and the latter to the flaky shape of the resultant 2223 crystals. The effect of Pb addition for enhanced 2223 phase formation will be discussed.

Growth of the 2223 Phase in Leaded Bi-Sr-Ca-Cu-O System : Electrical Properties of Condensed Matter

This paper reviews the usual techniques for producing YBCO-type single-domains and the microstructure of the as-obtained samples. The problems of seed dissolution and parasite nucleations are discussed in detail. The formation of microstructural defects, such as pores and cracks, is examined. An important part of this review is devoted to the study of the influence of RE-211 particles (RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5) for the microstructure and properties of RE?Ba?Cu?O single-domains. Pushing/trapping theory is described in order to explain the spatial distribution of RE-211 particles in the RE-123 ((RE)Ba2Cu3O7??) monoliths. The formation of RE-211-free regions is discussed. Different ways to limit the RE-211 coarsening are reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211 particles are presented. It is also shown that RE-211 particles play a significant role in the mechanical properties of single-domain samples. We finish this review by discussing the infiltration and growth process as a good technique to control the microstructure.

From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains

The effects of (NEG-211), Pt and additions on OCMG-processed (`NEG') composites have been investigated in detail using a combination of scanning electron microscopy and measurements of critical current densities, . Microstructural observations clearly indicated that the platinum addition was effective in refining the NEG-211 secondary-phase particles, which led to a dramatic increase in in the low-field as well as in the high-field region. TEM-EDX analyses revealed that the small secondary-phase particles (diameter ) consist mainly of Gd in the rare earth site, while the large particles contain Nd, Eu and Gd in the rare earth site with an even ratio, which is identical to the nominal composition of the precursor powder. For an NEG sample with 40 mol% NEG-211 additions, a large critical current density of 60 000 A at 77 K was observed in a magnetic field of 3 T applied parallel to the c-axis of the sample. The critical current density measurements showed that the zero field and peak values are similar in samples with 30 and 40 mol% of NEG-211 added. In contrast to this, the combination of and Pt addition was not as effective in refining the NEG-211 particles, and also the values were found to decrease in the entire field region.

Microstructure and pinning in high- and large- (Nd, Eu, Gd)-123 superconductors prepared by OCMG process

High quality Bi-2212 powders were prepared using three different synthetic methods (solid state, sol–gel and polymer matrix), showing that the polymer matrix method is very promising from the point of view of the synthesis time and grain size The Bi-2212 sintered pellets were then submitted to sinter-forging process to obtain textured ceramic discs The discs were characterized by XRD, pole figures and SEM to determine the phase purity, the texture degree and the microstructure, respectively Following characterization, the discs were cut into bars and annealed for 12 h at different temperatures ranging between 845 and 873°C to determine the best thermal treatment, whereafter they were characterized using resistivity and transport Jc measurements It has been found that most of the bars had a Tc around 92 K and that transport properties can be correlated to the samples' microstructure

Relationship among synthesis, microstructure and properties in sinter-forged Bi-2212 ceramics

The present work is devoted to investigating the effect of , and additives on the morphology and the melt processing of YBCO. The samples were synthesized by the melt texture growth (MTG) process in order to observe the morphology in the liquid phase at high temperature, in the matrix resulting from the peritectic reaction, and the resulting microstructure of . It is concluded that these investigated dopants exhibit similar effects concerning the limitation of coarsening in the melt. The evolution of the aspect ratio (l/w) versus the additive content in the melt demonstrates the change of the morphology of the grains. The magnetization measurements were carried out on cleaved samples using a SQUID magnetometer at 77 K with axis. The best compromise between an homogeneous and submicrometre distribution through the textured area and a high critical current density corresponds to either `' or `'. The distinct effects of cerium and platinum dopings on the superconducting properties are revealed by the critical current density measurements. Indeed, the cerium doped sample exhibits a in zero field with a relatively important field dependence behaviour: . In contrast the platinum doped sample strongly reduces this field degradation of with .

https://iopscience.iop.org/article/10.1088/0953-2048/9/7/005/pdf

Influence of , or additions on the microstructure and the critical current density of melt processed YBCO samples

The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.

https://iopscience.iop.org/article/10.1088/0953-2048/10/3/005/pdf

Top-seeding melt texture growth of single-domain superconducting pellets

The thermal texture cycle for the top seeding of Ce doped YBCO pellets has been studied and optimised. Several quenches have proved the importance of a presintering step to have a homogeneous distribution of the Y2BaCuO5 particles in the melt. The temperature window, in which a heterogeneous nucleated grain can grow, without being hindered by homogeneous nucleated grains, has been determined. By adjusting the process rate to the crystal growth rate in this window, large well textured single domains of YBa2Cu3O7−δ have been obtained, with a high degree of reproducibility. Microstructure and texture homogeneity, as well as superconducting properties have been investigated as a function of the position in the bulk pellet. The results have shown the high quality of the textured sample, and its high homogeneity.

Optimization of the texture formation and characterization of large size top-seeded-melt-grown YBCO pellets

Most of the potential applications of the high-temperature superconductors require large critical currents, much higher than those obtained using a classical sintering process. This paper reviews different processes and corresponding recent results in the field of texturing high-Tc superconductors in order to align and develop the planes where the current flows easily. Different melt-textured growth methods are discussed from both theoretical and practical viewpoints, and the limitations associated with the diffusion and interface phenomena are introduced. Influence of the process parameters, as well as the microstructure of the precursors and the use of additives in order to enlarge the single domain size and to optimise the superconducting characteristics is discussed. Mechanical texturing and texturing under a magnetic field, as well as combination of both processes, are also considered. Recent results, which appear to be an important breakthrough for the industrial development because of the large crystallization rates are also mentioned.

https://iopscience.iop.org/article/10.1088/0953-2048/12/7/202/pdf

G Desgardin

Papers

Relationship among synthesis, microstructure and properties in sinter-forged Bi-2212 ceramics

Influence of , or additions on the microstructure and the critical current density of melt processed YBCO samples

Top-seeding melt texture growth of single-domain superconducting pellets

Optimization of the texture formation and characterization of large size top-seeded-melt-grown YBCO pellets

Texturing of high-Tc superconductors