Reynald Passerini

Direct observation of the Bi,Pb(2223) phase formation inside Ag-sheathed tapes and quantitative secondary phase analysis by means of in situ high-temperature neutron diffraction

Abstract Reactions occurring in Ag-sheathed Bi,Pb(2223) monofilamentary tapes have been studied in situ using high-temperature neutron diffraction. A neutron compatible furnace enabled us to anneal tapes under the same processing conditions as those used for standard high-performance monofilamentary tapes. Scattering data were collected all the time during the thermal treatment and different cooling procedures were compared. The data were analyzed using a full-pattern profile refinement: seven phases were simultaneously refined so that it was possible to carry out a full quantitative analysis of secondary phases during the reaction thermal treatment. The steady conversion of Bi,Pb(2212) into Bi,Pb(2223) was quantified, and the structural transformations occurring in the Bi(2212) prior to its conversion in to Bi(2223) were carefully analyzed. The role of different Pb-rich phases, such as Ca 2 PbO 4 and Pb 3 (Sr,Bi) 3 Ca 2 CuO y , at this stage of the reaction is discussed. Formation of Sr 8 Ca 6 Cu 24 O 41 and Bi(2201) was observed at high temperature and quantitatively analyzed. During slow cooling, we observed a re-growth of Bi(2212) which did not correspond to any decomposition of Bi,Pb(2223). At the same time, the decomposition of Sr 8 Ca 6 Cu 24 O 41 and Bi(2201) was observed and this phenomenon was likely to be related to the increase of the Bi(2212) amount on cooling. Thanks to the neutron diffraction technique, which allows a direct measurement of the absolute amount of crystalline matter inside the sample, it was possible to estimate for the first time the amount of the amorphous matter present. It was found to vary with time and temperature: its non-monotonic evaluation provides a proof of melting and recrystallization occurring over a wide time window at the early stages of the reaction.

The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

Abstract The behaviour of the critical current in longitudinally strained Bi,Pb(2223) tapes shows a strain-insensitive plateau up to an irreversible strain limit e irr . For higher strains, the formation of cracks induces an abrupt decrease of the critical current. We investigate the relationship between precompression and irreversible strain limit with a set of tapes made with different filling factors as well as an in situ Inconel 600-reinforced tape. I c ( e ) curves were measured in a longitudinal strain apparatus at 77 K. The precompression at the measurement temperature is numerically estimated for each sample as well as the evolution of precompression during the cool-down. These calculated values are compared to the fracture susceptibility of extracted filaments, which gives an empirical estimate of the precompression. The main hypothesis of the “Irreversible I c Reduction Model” is confirmed, i.e. the irreversible strain limit essentially depends on precompression. However, we also found that the regime where I c remains constant contains a tensile component: the plateau extends beyond the external tensile strain needed to relieve the thermal precompression and includes a regime where the ceramic is further elongated non-destructively. This non-destructive deformation can be understood as a “connected-grains” behaviour, and extends the strain-insensitive plateau ∼0.1% beyond the precompression strain. This value is confirmed with a three points bending experiment performed on single filaments which gives a similar value for the bending failure strain. A comparable regime was found to exist also under compressive strain. These non-destructive regimes are of great importance for practical applications since up to a certain level the precompression can develop without any I c degradation.

Preparation and characterization of (100) cube textured Ag substrates for in-plane oriented HTS tapes

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Mechanical properties of Bi,Pb(2223) single filaments and Ic(ε) behaviour in longitudinally strained tapes

The Youngs modulus and fracture stress of isolated Bi,Pb(2223) filaments were deduced from three-point bending tests performed at different stages of the tapes preparation. These results were introduced in the model describing the evolution of critical current of tapes submitted to a longitudinal strain in view to predict their irreversible strain limit eirr. These calculated irreversible strain limits were compared to measured values, taken from a set of tapes made with different filling factors and composite matrices. This experiment shows that the predicted irreversible strain limits correspond to the measured ones. Presenting the Ic behaviour of highly stressed tapes in a magnetic field, we discuss the evolution of the ratio Istrongc0/Ic0 versus strain. This value, representative of the fraction of the critical current attributed to strongly connected grains, increases significantly during the crack formation regime at e > eirr. This indicates that mechanically weak links correspond to electromagnetically weak ones. This result is further confirmed by comparing the modulus of rupture obtained in single filaments extracted from tapes with different Ic values.

Reversible melting and equilibrium phase formation of (Bi, Pb)2Sr2Ca2Cu3O10

The decomposition and the re-formation of the (Bi, Pb)2Sr2Ca2Cu3O10+δ (Bi, Pb(2223)) phase have been investigated in situ by means of high-temperature neutron diffraction, both in sintered bulk samples and in Ag-sheathed monofilamentary tapes. Several decomposition experiments were performed at various temperatures and under various annealing atmospheres, under flowing gas as well as in sealed tubes, in order to study the appropriate conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was found to melt incongruently into (Ca, Sr)2CuO3, (Sr, Ca)14Cu24O41 and a Pb,Bi-rich liquid phase. Phase re-formation after melting was successfully obtained both in bulk samples and in Ag-sheathed tapes. The possibility of crystallizing the Bi,Pb(2223) phase from the melt was found to be extremely sensitive to the temperature and strongly dependent on the Pb losses. The study of the mass losses due to Pb evaporation was complemented by thermogravimetric analysis which proved that Pb losses are responsible for moving away from equilibrium and therefore hinder the re-formation of the Bi,Pb(2223) phase from the melt. Thanks to the full pattern profile refinement, a quantitative phase analysis was carried out as a function of time and temperature and the role of the secondary phases was investigated. Lattice distortions and/or transitions were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212), (Ca, Sr)2CuO3 and (Sr, Ca)14Cu24O41, due to cation diffusion and stoichiometry changes. The results indicate that it is possible to form the Bi,Pb(2223) phase from a liquid close to equilibrium conditions, such as Bi(2212) and Bi(2201), and open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape processing.The decomposition and the reformation of the (Bi,Pb)2Sr2Ca2Cu3O10+d (?Bi,Pb(2223)?) phase have been investigated in-situ by means of High-Temperature Neutron Diffraction, both in sintered bulk samples and in Ag-sheathed monofilamentary tapes. Several decomposition experiments were performed at various temperatures and under various annealing atmospheres, under flowing gas as well as in sealed tubes, in order to study the appropriate conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was found to melt incongruently into (Ca,Sr)2CuO3, (Sr,Ca)14Cu24O41 and a Pb,Bi-rich liquid phase. Phase reformation after melting was successfully obtained both in bulk samples and Ag-sheathed tapes. The possibility of crystallising the Bi,Pb(2223) phase from the melt was found to be extremely sensitive to the temperature and strongly dependent on the Pb losses. The study of the mass losses due to Pb evaporation was complemented by thermogravimetric analysis which proved that Pb losses are responsible for moving away from equilibrium and therefore hinder the reformation of the Bi,Pb(2223) phase from the melt. Thanks to the full pattern profile refinement, a quantitative phase analysis was carried out as a function of time and temperature and the role of the secondary phases was investigated. Lattice distortions and/or transitions were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212), (Ca,Sr)2CuO3 and (Sr,Ca)14Cu24O41, due to cation diffusion and stoichiometry changes. The results indicate that it is possible to form the Bi,Pb(2223) phase from a liquid close to equilibrium conditions, like Bi(2212) and Bi(2201), and open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape processing.

ac Losses in Bi, Pb(2223) barrier tapes

Abstract For many applications of Bi,Pb(2223) tapes involving alternative currents and/or fields, their ac losses are still too high. Several methods have been tried for reducing these losses and at present the most successful one is the introduction of a resistive barrier between filaments combined with filament twisting. At the University of Geneva, we tested an extensive series of different materials for their suitability as barrier materials. In this paper, we start by treating the different criteria these materials should meet for their use as barrier: no reactivity with Bi,Pb(2223) precursor, good deformability and low cost. We show why we used first BaZrO 3 and later SrZrO 3 as barriers and explain the differences between these two materials. We next discuss which modifications of deformation process are needed in order to have tapes with smooth and continuous barrier layers and to avoid too high a degradation of the critical current density. Finally, we show the effect of barriers on the ac losses and the effective matrix resistivity.

Hot isostatic pressure reaction treatment of Ag-sheathed Bi,Pb(2223) tapes

Abstract One of the parameters of power in tube (PIT) processed Bi,Pb(2223) tapes which still leaves room for improvement is the density of the filaments, presently reported to be around 80–90%. To some extent this can be addressed with optimised intermediate deformation methods, such as periodic pressing, but this solution is limited by the necessity to heal the damage after deformation. The synthesis of the Bi,Pb(2223) phase whilst applying a hot isostatic pressure (HIP) has been found to be an attractive alternative route to obtain higher ceramic density. In this work we compare the properties of tapes processed under various isostatic pressure up to 1000 bar. High pressure was found to fasten the kinetics of phase formation. SEM observation showed an increased density of the ceramic core in HIP processed tapes. 20 h annealing under 100 bar improved the critical current density by 30% compared to the standard first heat treatment in PIT process.

Bi,Pb(2223) equilibrium and decomposition: in situ high-temperature neutron diffraction study

Abstract The non-equilibrium reaction path which is commonly used to form the Bi,Pb(2223) phase inside Ag-clad tapes involves a small amount of a local transient liquid. New high-temperature reaction paths involving a larger amount of a stable liquid that are closer to equilibrium are needed to improve the Bi,Pb(2223)–Ag tape performance and achieve their high expected potential. In order to understand the reactions involved at high temperature during the Bi,Pb(2223) decomposition, an in situ neutron diffraction study has been carried out. Both Bi,Pb(2223) pellets and Ag-clad tapes have been melted and slowly cooled at different rates under various oxygen partial pressures. A phase analysis from the full pattern profile refinement has been performed. Bi,Pb(2223) decomposes into a Bi,Pb-rich liquid and (Ca,Sr)2CuO3 which is found to be the main phase at high temperature regardless of the oxygen partial pressure. Direct crystallisation of Bi,Pb(2223) from the melt has been observed in Ag-sheathed tapes and the critical current can be partially recovered after melting and slow cooling.

Observation of the Bi,Pb(2223) reaction mechanism and alternative ways of producing tapes with new filament configurations

Investigations on the Bi,Pb(2223) phase formation confirm a mechanism based on nucleation and growth. The same mechanism was found to hold under various external conditions: a) in pressed samples, b) in Ag sheathed tapes, c) in air or in reduced oxygen partial pressure and d) with or without the presence of Pb. A high temperature neutron diffraction investigation on monofilamentary Bi,Pb(2223) tapes shows that this phase remains stable during the cooling process after reaction. On cooling, an enhancement of the Bi(2212) phase is observed, which occurs at the expense of the other phases, (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ and Bi(2201). New tape configurations are presented, in view of a) the reduction of anisotropy (on both, tapes or wires) and b) the reduction of AC losses. These configurations require the use of nonconventional techniques, e.g. two-axis rolling and/or periodic pressing. Using periodic pressing, j/sub c/(77K,0T) values of 35,000 A/cm/sup 2/ have been obtained in Bi,Pb(2223) multifilamentary tapes of lengths >2 m.

Improved J/sub c/ of multifilamentary Bi,Pb(2223)/Ag tapes by periodic pressing

Critical current densities of multifilamentary Ag-sheathed Bi(2223) tapes up to about 35 000 A/cm/sup 2/ have been achieved at 77 K and self field for lengths of several meters using an improved route: periodic pressing. This corresponds to an increase by 30-40% compared to the values obtained for conventionally rolled tapes starting from the same powders. Several pressing steps have been introduced during the anneal instead of the only standard rolling step (based on previous studies performed on both mono- and multifilamentary tapes). In contrast to earlier attempts by pressing techniques, periodic pressing is a practical and scaleable process for the fabrication of long lengths of Bi(2223) conductor as the standard intermediate rolling step. Engineering critical current densities of 8 000 A/cm/sup 2/ have successfully been obtained for tapes with high filling factor (35%).