Melt-textured Composites Research Articles

Addition of Ca-compounds nanoparticles in melt-textured Bi:2212

Superconducting BSCCO samples made by melt-texturing process were prepared with the addition of calcium zirconate and calcium silicate nanoparticles. Bi:2212 melt-textured composites prepared with 1 wt.% of either addition showed different behavior for the critical current density as a function of the applied field, indicating that for each additional compound the improvement can be associated to different enhancement mechanisms, such as the creation of pinning centers and the increase on the connectivity of the grains. The estimated pinning forces indicated higher values for the calcium compound containing samples.

Subboundary mesostructures and variable dislocation networks in single domain melt textured YBa 2Cu 3O 7––implications on critical currents

The microstructure of subboundaries (SBs) occurring in single domain YBa 2Cu 3O 7 (Y123) melt textured composites is studied by transmission electron microscopy. The form of SB dislocation networks is controlled by the properties of constituting dislocations. For arbitrary SB configurations, it can be parametrised using the generalised Frank's formula, allowing the calculation of dislocation densities, and an estimation of the SB critical current. Besides the underlying dislocation networks, SBs may develop mesostructures like faceting and stepped interfaces accommodating the deviation from low index planes. Implications of the observed microstructures on the bulk critical currents are addressed.

Melt growth and microstructure development of high critical current REBa2Cu3O7 superconductors with a natural mixture of rare earths

A natural mixture of heavy rare earth oxides has been used to prepare high critical current bulk superconducting ceramics which may be used in electrotechnical devices. Melt processing of a starting mixture of REBa2Cu3O7 and Y2BaCuO5 powders (RE denotes natural mixture of rare earth elements), leads to a final ceramic composite REBa2Cu3O7/RE2′BaCuO5, where RE and RE′ are different mixtures of heavy rare earths and yttrium. It is found that the superconducting matrix and the core of the large RE2′BaCuO5 particles are yttrium rich while the small RE2′BaCuO5 particles are rich in heavy RE ions. The differential solubility of the rare earths in the high-temperature semisolid state leads to a strong inversion of the rare earth composition between the superconducting matrix and the insulating precipitates which is originated during the melt growth process. The superconducting properties of the new compound having natural RE mixtures are found to be very similar to those of pure YBa2Cu3O7/Y2BaCuO5 melt textured composites.

Subgrain boundary structure in melt-texturedRBa2Cu3O7(R=Y,Nd):Limitation of critical currents versus flux pinning

The microstructure of subgrain boundaries occurring in single-domain $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ $(R=\mathrm{Y}$ and Nd) melt-textured composites has been studied by transmission electron microscopy. It is found that subgrain boundaries (SGB's) have a strong tendency to develop parallel to the (100), (010), and {110} planes, while the form of dislocation networks is controlled by the properties of constituting dislocations. In ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ presenting one unique glide plane, (001), boundaries stabilized on {110} planes are accommodated by dislocations with lines running along the c axis. The occurrence of such an unusual line direction is discussed in terms of the line energy anisotropy on the (001) plane. On the other hand, in ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ with (100), (010), and {110} glide planes in addition to (001), c-axis oriented dislocations are also found stabilized on (100)/(010)-faced boundaries. For arbitrary SGB configurations, the form of dislocation networks can be parametrized using the generalized Frank's formula, allowing the calculation of dislocation densities. Besides the underlying dislocation networks, SGB's may develop mesostructures such as faceting and stepped interfaces accommodating the deviation from low-index planes. The way these defects may affect the transport critical currents is discussed on the basis of a simple geometrical model. Since the form of dislocation networks is governed by intrinsic materials parameters, the present results can be extended to other large-scale materials for which a strong incidence of low-angle grain boundary microstructure is anticipated.

Critical current enhancement in YBCO–Ag melt-textured composites: influence of microcrack density

A strong increase of the critical current density of YBa 2Cu 3O 7 (YBCO) melt-textured composites has been observed by adding Ag 2O particles in the precursor mixture while the concentration of Y 2BaCuO 5 (Y211) precipitates remains constant. Critical current density enhancement factors as high as 200% at 5 K ( J c ab (5 K, 0 T)∼2.5×10 6 A/cm 2) and 1000% at 77 K and 2 T ( J c ab (77 K, 2 T)∼2.2×10 4 A/cm 2) have been reached in samples with 20 wt.% of Ag 2O. Critical current measurements at 77 K for H∥ ab also confirm the increase of the J c enhancement factor with the magnetic field (∼100% at 0 T and 140% at 2 T). The improvement of the critical currents in the YBCO–Ag melt-textured composites is correlated with the reduction of the density of microcracks parallel to the ab plane, as observed by scanning electron microscopy (SEM). Results are interpreted in terms of a release of the current limiting factors and an improvement of flux pinning properties by preexisting defects.

Hybrid superconducting fault current limiter based on bulk melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ ceramic composites

A hybrid fault current limiter (FCL) based on single domain bars of Y123/211 melt textured composites has been designed to operate at a 1 kV/400 A line. This consists of a conventional Cu primary coil inductively coupled to a secondary coil formed by the paralleling of single Cu turns each one shorted by a superconducting bar. The superconducting material has been prepared by the solidification techniques of Bridgman and top seeding in air which has allowed us to obtain bars with transport critical current densities of 20000 A/cm/sup 2/ at 77 K. Preliminary tests on the quench current, recovery time and AC-losses in normal operation have been performed. The differences and advantages of a hybrid FCL in comparison with a resistive and an inductive FCL is discussed.

Tailoring of microstructure and critical currents in directionally solidified

Directionally solidified composites display a complex microstructure with several defects coexisting in the matrix and hence the identification of the flux-pinning mechanisms requires a detailed analysis of the critical currents. In this work we review the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with the refining strategies of (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic. The influence of these defects and post-processing treatments on the critical currents is then analysed focusing on their relative importance as flux-pinning centres at different temperatures and magnetic fields up to 20 T. A complete magnetic phase diagram is finally proposed as a basis for a systematic classification of the observed irreversible superconducting behaviour in single-grain melt-textured composites.

Physicochemical causes for the microstructure of melt-textured composites

We report semiquantitative results for a physicochemical model intended to describe the multigrain growth of (123/211) composites under isothermal undercooling. This model takes into account various ingredients supposed to be controlling the growth of such superconducting compounds, i.e. the presence of secondary phases. Moreover, a possible dynamical interaction between the growing 123 grains and the 211 solid particles is also included. The numerical investigations were performed for a model bivariate-like 211 particle distribution. The complete chemical dissolution of the small particles is allowed but the large ones are only partially dissolved in the first step which can be followed when chemically possible by a complete dissolution. The model is restricted to a two-dimensional square lattice. Various microstructural morphologies are obtained as a function of the initial composition of the melt and the initial size distributions of the 211 particles. Results are in quite good agreement with experimental observations. The quantity of 123 phase is predicted depending on the initial conditions. The optimal situation when a minimum fraction of liquid phase segregates at the grain boundaries is found to occur for 20% excess of 211 phase in the initial melt.

Fabrication of highly aligned YBa2Cu3O7− δ- Ag melt-textured composites

Liquid phase processing techniques with slow cooling through the peritectic transformation have been utilized to fabricate bulk aligned 123-Ag composites. Coarse (∼30–40 μm) and fine (∼5–10 μm) particles of silver are distributed between and within the 123 grains. Although the processing temperatures employed are much higher than the melting point of silver, particles of silver are retained behind the advancing solid-liquid interface. The peritectic reaction occurs at the solid-liquid interface and the rate of the reaction is limited by diffusion between the two solid phases, hence the transformation invariably results in some unreacted phases. Ellipsoidal, Y 2BaCuO itx particles varying in size from 10–25 μm are trapped within the 123 phase. In addition, regions of CuO and BaCuO 2 are present in the sample as separate phases. Silver particles are also dispersed in these insulating regions of the microstructure. The 123-Ag composites have superior mechanical properties compared to melt-textured stoichiometric 123 samples. A detailed study describing the effects of processing on the microstructureof these aligned composites is reported.