X. Chaud

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.

Magnetic alignment of carbon nanofibers in polymer composites and anisotropy of mechanical properties

Engineering applications of carbon nanofibers and nanotubes require their alignment in specific directions. Single-walled carbon nanotubes can be aligned in a magnetic field due to the presence of small amounts of catalyst elements, such as Ni and Co. However, for carbon nanofibers, their extremely low magnetic susceptibility is not sufficient for magnetically induced alignment. We present a method of solution-coating of NiO and CoO onto the surface of the carbon nanofibers. Due to the NiO- and CoO-coating, these nanofibers can be well aligned in the polymer composites under moderate magnetic field (3 T). Both transmission electron microscopy and scanning electron microscopy results show the well-aligned nanofibers in a polymer matrix. Mechanical testing shows a pronounced anisotropy in tensile strength in directions normal (12.1 MPa) and parallel (22 MPa) to the applied field, resulting from the well-aligned nanofibers in the polymer matrix. The mechanism of magnetic alignment due to coating of NiO and C...

Current limitation with bulk Y-Ba-Cu-O

The fault current limiter is a very attractive device for electric networks. Meander pattern conductors cut from bulk melt textured YBCO were studied for this application. The meanders are put in series and/or in parallel to match the required current and voltage. The YBCO materials are attractive because they show a very effective limitation with a relative low volume (high engineering current density and normal state resistivity). However, they are sensitive to hot spots. To avoid these destructive hot spots the operating temperature is chosen very close to the critical temperature (above 90 K). This temperature range is reached using a pressurised liquid nitrogen bath. Working close to Tc has two major advantages. The first is reduced values of Jc which limit the power dissipation. The Jc can be matched by changing the pressure on the nitrogen bath. The second is that the proximity of the normal state is favourable for homogeneous quenches along the whole meander as it can be experimentally recorded. Results obtained on single meanders and on the whole assembly are reported under steady state operation as well as during current limitation. Forty three meanders in series limited the current to 740 A (11 000 A unlimited value) under 1 kV.

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.

Cracking and oxygenation of YBaCuO bulk superconductors: application to c-axis elements for current limitation

Abstract A process to elaborate crack free, oxygenated c -axis samples in YBa 2 Cu 3 O x bulk superconductors is reported. This includes texturing of crack free pellets and development of an adequate oxygen annealing treatment to avoid cracking during oxygen uptake. For current limitation in c -direction such a process is needed as c -axis elements are particularly susceptible to ab -plane cracking during oxygenation. Mechanical properties have been compared for different annealing treatments by flexural strength measurements. With the load applied in ab -direction the mean flexural strength is 58 MPa in as-textured c -axis bars and 48 MPa in c -axis bars which were oxygenated using a customized annealing treatment. The conventional oxygen annealing treatments lead to macroscopic oxygenation cracks which reduced flexural strength to 10–20 MPa. Bars cut in ab -direction do not show significant variation in flexural strength (load applied in c -direction) as a function of the annealing condition with an average value of 145 MPa.

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.

Development in processing of MgB2 cryo-magnet superconductors

Spark Plasma Sintering is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain MgB2-based ceramics with tailored microstructures. Commercial MgB2 powders were spark plasma sintered with an applied mechanical pressure, leading to MgB2 pellets with 99% relative density. The obtained samples show a sharp superconducting transition with an onset at 38.5 K. The critical current density was measured equal to 5.6 × 104 A/cm2 at 20 K under 1 T applied field. Otherwise, the trapped field was also investigated. A 5 mm thick pellet with a 20 mm diameter prepared with optimized processing temperature demonstrated a trapped field of 1.2 T at 15 K and up to 2.5 T at 10 K.

Isothermal growth of large YBa2Cu3O7-x single domains up to 93 mm

Abstract The growth of YBa 2 Cu 3 O 7−x single domains larger than 30–40 mm by a classical Top Seeding Melt Texturing (TSMT) method appears to be difficult. Because of the very narrow solidification range between the onset of the nucleation from the seed and the nucleation of grains in the liquid, reaction with the substrate as well as uncontrolled thermal gradients introduce severe limitations to the growth. After suppressing nucleation from the substrate and improving the temperature homogeneity around the sample, the growth rate measured in situ was used to carefully adjust the process. A YBa 2 Cu 3 O 7−x single domain has been grown up to 93 mm without applied thermal gradient nor large overheating. The texture is confirmed on a large volume by neutron scattering and an homogeneous 211 size distribution and content are found across the sample. This suggests that steady growth conditions have been maintained and that even larger samples could be grown.

Growth of YBaCuO single-domains up to 7 cm

Abstract The growth of large top seeded YBaCuO single-domain has been studied by means of in situ video recording during the process. The growth for samples (O ≤ 4 cm), textured under a thermal gradient, follows a model based on yttrium diffusion through the melt. The growth appears to be limited (3 × 3 cm 2 ) for an isothermal texturation whereas single-domains up to 7 cm are grown using whether a radial thermal gradient or a overheating process.

HTS Magnets: Opportunities and Issues for SMES

SMES (superconducting magnetic energy storage) devices are an attractive solution for pulsed current sources. However, the specific stored energy is moderate and has to be improved. High temperature superconductor (HTS) materials offer opportunities in terms of current carrying capabilities as well as mechanical strength. We report the optimization of solenoids with regards to mechanical stress and current density constraints. 20 kJ/kg requires current densities above 150 MA/m2 and stresses of about 400 MPa for energies in the range of 5 to 50 MJ. The optimization code makes possible the study of the influence of different parameters such as the maximum allowable magnetic field. A method is proposed to discriminate space of solution. The best configuration is achieved keeping in mind the peculiar properties of HTS tape (which impact protection of SMES), and the specifications of the application. Parallel to these theoretical approaches, we are developing the magnet technology by first realizing small magnets we test under very high fields (up to 18 T) to apply the large mechanical stresses.