D. Bourgault

Large inverse magnetocaloric effect in Ni45Co5Mn37.5In12.5 single crystal above 300 K

A large inverse magnetocaloric effect has been observed in a Ni45Co5Mn37.5In12.5 single crystal at room temperature. Magnetothermal measurements performed at different magnetic fields reveal a nonmagnetic to ferromagnetic transition correlated with the austenite-martensite phase transformation. The Heusler single crystal shows a large entropy change of 30 J/Kg K at an applied magnetic field of 7 T during the first-order magnetostructural transition at 355 K. It leads to a net refrigerant capacity of 267 J/Kg at 7 T, which is very encouraging for magnetic refrigeration applications.

Bulk textured BiPbSrCaCuO (2223) ceramics by solidification in a magnetic field

Abstract Bulk textured BiPbSrCaCuO (2223) ceramics have been prepared by solidification in a magnetic field. We have determined the optimal temperature required in the melt in order to maximize the effect of the magnetic field. X-ray pole figures and scanning electron microscopy have been used to characterize the nature and degree of texture. Magnetic and electrical transport measurements in the superconducting state are also reported. Transport critical current densities as high as 1450 A/cm 2 at 77 K and 0 tesla have been reached. This value is significantly greater than that, 900 A/cm 2 , obtained following the same heat treatment in the absence of an applied magnetic field.

Processing of large Y1Ba2Cu3O7-x single domains for current-limiting applications

Bulk-textured YBa2Cu3O7-x single domains could be used for current-limiting applications by cutting and assembling long meanders, which would be submitted to network electric fields before using a breaker to interrupt a fault current. For that purpose, large YBaCuO single domains up to 93 mm in diameter can be isothermally grown by using a standard melt texturing growth (MTG) process with a SmBaCuO seed. The essential parameters that have to be controlled in order to reach this size are the temperature growth window, the substrate reactivity and the temperature homogeneity in the sample. Standard 45 mm diameter single domains show excellent superconducting properties, such as Jc above 105 A cm-2 and a homogeneous superconducting-to-normal transition at 91.8 K for 20 cm long conductors cut in these pellets. These measurements demonstrate the long -range homogeneity of single domains regarding Tc and Jc. Nevertheless the high Jc values lead to a too large a dissipation in the normal state at T = 77 K. Different methods to reduce the critical current density are described in order to fulfill the conditions for a safe recovery of the material after undergoing a magnetothermal transition.

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

Abstract The unidirectional solidification method was used to prepare well textured YBa 2 Cu 3 O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G ) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a – b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2 ) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R >2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.

Effect of adding MgO to bulk Bi2212 melt textured in a high magnetic field

Bulk samples of Bi2212 pure and doped with MgO or were prepared by melt processing in a high magnetic field. The structure of the samples was analysed using XRD and SEM and their transport properties were studied. The results show that when a magnetic field is applied during melt processing the bulk Bi2212 is textured with the c-axis parallel to the field direction. The addition of MgO induces an improvement in the material properties. This is not observed using . A significant improvement is observed at 77 K in the critical current density in self-field and in an external magnetic field. This improvement can be related to the better texturation obtained when MgO is added: it maintains high viscosity during melt processing and thus the texturation induced by the magnetic field is not weakened by liquid-phase spreading. The addition of MgO also induces a decrease in volume of residual phases. The improvement in properties in an external magnetic field can be attributed to a better indirect vortex pinning due to submicron MgO-rich inclusions in the superconducting matrix.

Quench in bulk HTS materials : application to the fault current limiter

We report theoretical and experimental studies on the use of bulk Bi or Y materials for fault current limiters. The materials are compared in terms of required volume, recuperation time, .... The problem of the unavoidable hot spots in high Jc materials is discussed. A moderate Jc and an operation near Tc make the quench effective and non-destructive for YBCO single domain bars or meanders. Experiments carried out above 90 K are reported. YBCO doping with MgO shows good opportunities to operate at 77 K for example.

High critical transport currents of melt textured YBCO up to 6000 A

Abstract A melting zone processing has been developed to texture long bars of YBCO (10 cm length and 5 mm diameter). The misorientation of ( a , b ) planes with the sample long axis is weak. The transport current is produced by a capacitor discharge with period varying between 1.25 and 20 ms. Pulsed transport currents reaching 3000 A (15000 A/cm 2 ) and 6000 A (30000 A/cm 2 ) could cross resistanceless samples at 77 K over several centimeters length. These results show that YBCO can transport high nominal currents even up to its critical current thanks to high quality of contancts. Above the critical current, the material has been protected against the development of the magnetothermal transition.

Optimization of texturing process by hot pressing of Bi-Pb-Sr-Ca-Cu-O (2223) ceramics

Abstract We have optimized a texturing process of Bi-Pb-Sr-Ca-Cu-O (2223) superconducting ceramics by hot pressing. The Hoechst precursor powder has been first pressurized at 1 GPa and then heated under uniaxial stress up to 35 MPa, just below the melting temperature between 850°C and 857°C. We have obtained large textured samples having critical transport current densities J c as large as 2500 A/cm 2 . In this work, we have optimized the stress value as well as the temperature treatment on the basis of characterizations such as magnetization and electrical-transport measurements.

Magnetically melt textured YBa2Cu3O7−δ: Disagreement between Jc determined by magnetic and direct transport measurements

Abstract Textured YBa 2 Cu 3 O 7−δ (Y-123) ceramic material has been prepared from a mixture of Y 2 BaCuO 3 , BaCuO 2 and CuO, by solidification in a magnetic field of 7 T. The application of the magnetic field during solidification allows one to control the orientation of the crystals within the material. An annealing temperature above 1040°C is necessary to obtain a well textured ceramic. The composition of the starting material has an influence on the form of the sample: an excess of 25% by weight of Y 2 BaCuO 3 allows the initial form of the sample to be preserved. Electrical transport and magnetization measurements are reported. Transport critical current densities of 7200 A/cm 2 in a magnetic field of 2 T and 4300 A/cm 2 in 6 T in the case H ∥ ab planes have been observed at 77 K. A transport critical current density of 1600 A/cm 2 in 6 T in the case H ∥ c -axis has been measured, in spite of the fact that critical currents deduced from magnetic measurements using the Bean model are negligible for fields in excess of 5 T.

High performance in a bulk melt-textured YBa2Cu3O7- delta ceramic

In the present work a YBa2Cu3O7- delta melt-textured growth (MTG) sample was used to achieve grain orientation in a thermal gradient over a large domain using 211 bars as support. Starting from a YBa2Cu3O7- delta precursor sintered at 950 degrees C, the authors were able to obtain unidirectional orientation over a large area, 22*8*7 mm, using a 20 degrees C cm-1 gradient and a very slow cooling rate, 1 degrees C h-1. The so-obtained microstructure has been characterized. The study of cleaved surfaces using diffraction and Laue patterns shows widespread development of a-b planes parallel to the thermal gradient and to the bar axis. Moreover, microstructural observations point out that the texturization may extend to the 211 support. Polarized optical microscopy observations show large twinning domains all over the sample which confirm orthorhombicity and thus good reoxygenation of the sample. Magnetic and electrical characteristic were investigated. Large magnetic hystereses were obtained, even at 77 K, revealing the presence of strong pinning centres. Transport critical current density higher than 3.5*104 A cm-2 at 77 K in zero field and 2.6*104 A cm-2 at 77 K in a 7 T magnetic field were measured.