R. Tournier

Structure of the 100 K Superconductor Ba2YCu3O7 between (5 ÷ 300) K by Neutron Powder Diffraction

Recently Siegrist et al. proposed a structure for a high-Tc superconductor Ba2YCu3O(9-∂) based on an orthorhombic (a = ap, b = ap, c = 3ap) perovskitelike model containing Ba and Y cations ordered over the A-sites of the ABO3 structure. This ordering is responsible for the tripling of the c-axis. Half of the oxygen vacancies (at z = 1/2) are ordered, while the other half (at z = 0) are disordered over two sites. Using profile refinement of high-resolution neutron powder data at six different temperatures between 5 K and 300 K, we have refined the structure of a pure and well-characterised powder sample with onset of superconductivity at 100 K. At all temperatures we confirmed the previous model except that in our structure, all oxygen vacancies are ordered. Two-thirds of the copper cations have a pyramidal coordination and one-third has a square coordination. In our structure all squares are parallel to the (b, c)-plane, while in the one reported by Siegrist et al. the squares are disorderedly parallel to either the (a, c)- or (b, c)-plane. The difference between the two models is probably due to the fact that the single crystal used in the earlier work was highly twinned. Empirical calculations of the copper valences show that the Cu+++ cations are almost equally distributed over the two sites. No structural change has been detected at the transition.

Thallium cuprates: The critical temperature is mainly governed by the oxygen nonstoichiometry

Abstract The superconducting properties of the thallium cuprates, and especially their critical temperatures, are strongly affected by annealings in various gaseous atmospheres (nitrogen, oxygen, argon and hydrogen). These annealing effects have been studied for Tl 2 Ba 2 CaCu 2 O 8 , TlBa 2 CaCu 2 O 7 , TlBa 2 Ca 2 Cu 3 O 9 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 as well as for the substituted oxides TlBa 2 Ca 1− x Ln x Cu 2 O 7 and Tl 2 Ba 2 Ca 1− x Ln x Cu 2 O 8 (Ln=Y, Nd). It is demonstrated that a drastic increase of T c can be obtained, due to a very small oxygen deficiency. The most spectacular effect is obtained with a gas mixture H 2 /Ar, which can be performed at a temperature as low as 300°C, leading for example for the 2212 phase to an increase of T c from 97 K to 118 K (to be compared to 125 K for the 2223 phase). Moreover it is shown by electron microscopy that at this annealing temperature the crystal remains unchanged. However, thermal treatments at 400°C or beyond alter the crystals in connection with a slight volatilization of thallium oxide. This dramatic effect of oxygen nonstoichiometry upon T c decreses as the number m of copper layers in the perovskite slabs of the superconductors TlBa 2 Ca m −1 Cu m O 2 m +3 and Tl 2 Ba 2 Ca m −1 Cu m O 2 m +4 increses. These results show that the important factor which governs T c in the thallium cuprates is the hole number per mole of copper in the structure and that this factor seems to be predominant with respect to the number m of copper layers.

Orientation by solidification in a magnetic field A new process to texture SmCo compounds used as permanent magnets

Abstract The solidification of molten alloys in a static magnetic field is proposed as a new way of orienting polycrystalline materials. A high degree of orientation is obtained with samarium-cobalt compounds solidified in a static magnetic field. Whatever the cooling condition used from the liquid state, a magnetic field of several tesla induces crystallographic orientation in the solid. The easy magnetization axis of the polycrystal lies along the direction of the field applied during solidification. This texturing process is applied to the elaboration of Sm 2 Co 17 permanent magnets. Anisotropic bulk magnets with a coercive field up to 2250 kA/m and energy product above 160 kJ/m 3 are obtained. This process provides an alternative to the currently used industrial technology which is based on powder metallurgy. The paramagnetic susceptibility of the substituted Sm 2 Co 17 compounds is measured at high temperatures from which the susceptibility anisotropy at solidification temperature is determined. The orientation of the sample, solidified in a cold induction crucible, is analysed as a function of the applied magnetic field. Assuming a model in which particles are free to orient before complete solidification takes place, a critical size of these particles is deduced.

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...

Superconductivity of La2CuxO4 − y

Abstract Bulk superconductivity is present in La 2 Cu x O 4 − y for 0.98 ⩽ x ⩽ 1.04 and for an y value which is strongly reduced by annealing under high pressure of oxygen. Two superconducting temperatures are observed T 2 and T c ; the first one, T 2 = 37 K, corresponds to the resistivity fall; the second one, T c , corresponds to the appearance of diamagnetism. T 2 and the critical fields of samples stay unchanged whatever the normal state resistivity is, even if it varies by a factor 40000 from a semi conducting to a metallic state. Then a conventional mechanism of pairing is unconceivable. The existence of a bump in the resistivity at T = 220 K of samples which undergo a superconducting transition suggest a possible correlation between coherence effects among copper spins and superconductivity.

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.

Dynamics of magnetically levitated droplets

Liquid droplets are steadily levitated without any contact by the magnetic force exerted by a superconducting magnet. When a single levitated droplet is set in a particular vibration mode, the surface tension of the liquid can be derived from the measurement of the frequency. Magnetic forces may modify the equilibrium shape and the frequency, and numerical calculations are performed to evaluate this effect. When two droplets are injected into the levitation zone, the radial magnetic forces drive them together to the field axis. Frequent bounces are observed and first qualitative results from test experiments are presented to characterize the mechanism.

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.