P. Barboux

Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol‐gel processes

The synthesis of high‐temperature superconducting oxides using solution chemistry has been investigated and physical properties are compared to ceramics obtained by conventional solid‐state reactions. We report on controlled precipitation and sol‐gel processes, both of which produce materials with particle sizes smaller than 5 μm. We find that the superconducting properties of the high Tc ceramics are affected by their manner of preparation, such that the transitions are slightly lower in temperature, but sharper, for samples made by solution rather than solid‐state chemistry. The ability to prepare stable viscous gels provides an opportunity for obtaining large areas of superconducting coatings. For thick films on alumina or silicon substrates, contamination from the substrate is shown to be a problem. Finally, we observe that the sol‐gel process lowers the synthesis temperature by 100 °C.

New non-superconducting layered Bi-oxide phases of formula Bi2M3Co2Oy containing Co instead of Cu

Abstract We report the cationic substitution of Cu by Co within the 85K Bi based high Tc oxides to produce new phases of general formula Bi 2 M 3 Co 2 O y with M = Ca, Sr and Ba. Single crystals of these phases, grown using Co 3 O 4 (excess) flux, were investigated for their structural, magnetic and electrical properties. The subcell appears similar to that of Bi 2 Sr 2 CaCu 2 O y phase but the superstructure is more complicated due to the presence of a more complex incommensurate structural modulation. The substitution of Cu by Co is accompanied by an uptake of additional oxygen. With M = Sr or Ca, the compounds are semiconductors, whereas when M = Ba the compound is metallic above 100K, and semiconducting below 100K. The magnetic susceptibility varies little with temperature, and shows no magnetic transitions. Photoemission measurements show that Co is in a + 3 oxidation state with a low spin configuration.

Chemical Doping and Physical Properties of the New High Temperature Superconducting Perovskites

The effects of chemical doping on the structural, magnetic, transport and superconducting properties of the new high Tc superconducting oxides are reviewed. The substitution of Sr or rare earths for La in La2CuO4−y and the replacement for Y by rare earths and for Ba by Sr in the YBa2Cu3O7−y system have been investigated, as well as the substitution of Ni for Cu in both systems. In contrast to the rare earth substitutions, a Ni substitution dramatically affects Tc. The importance of the oxygen content (y) in these oxides, which strongly depend on their processing conditions (heat treating time, temperature and ambient), is emphasized and a detailed study of changes in the physical properties as a function of y is reported. The superconducting properties of these perovskites can be destroyed by decreasing their oxygen content. However, full superconductivity can be recovered by reannealing the sample under oxygen or by a low temperature (80°C) plasma oxidation. This later method may be very useful for the technological application of these materials.

Thick films of Bi‐Sr‐Ca‐Cu‐O and Tl‐Ba‐Ca‐Cu‐O by solution processes

We have prepared superconducting thick films of the Bi‐based and the Tl‐based cuprates via the decomposition of aqueous‐glycerol solutions containing the salts of the elements. Preliminary results are presented in this work. The substrates are coated prior to heat treating, either by dipping or by spraying on various substrates heated at 200 °C. Short firing times are required in order to minimize the loss of the constituent Bi (Tl). We find that nitrates of the constituents dissolved in a water‐glycerol solution increase the reaction rate in comparison to pure nitrate aqueous solutions. They also help to produce the correct superconducting phase before some reaction with the substrate occurs or too much of the constituent Bi (Tl) is lost during heating to form the superconducting phase. However, the thallium phases cannot be obtained if the films are not fired in the presence of a high pressure of thallium in a sealed capsule. The films are composed of platelets, a few microns large, that are on average ...

On synthesis of high Tc superconducting perovskites

Abstract The synthesis procedures (solid state and solution) used to produced YBa 2 Cu 3 O 7−x superconducting material in bulk or thick film form are briefly reviewed. Our results indicate that a solution technique (sol-gel process) permits lower reaction temperatures and yields dense (90%) and homogeneous ceramics with conducting properties which, in the normal states, tend toward those obtained on single crystals. However, independent of the synthesis technique used, the critical currents I c remain low probably because of the presence of carbon or carbonate at the grain boundaries. Stable, viscous gels were used to prepare superconducting thick films on various substrates and problems specifically associated with thick film processing (i.e. film adhesion and reaction with the substrate) are discussed. Solid state reactions have been used to perform doping studies and to produced 3d-metal-substituted perovskites YBa 2 Cu 3−x M x O y−y in which the oxygen content has been found to depend upon the nature and concentration of the dopant (i.e. M).

Preparation, structure and properties of the high Tc Bi-based and Y-based cuprates

Abstract The structure and some properties of the new bismuth-based high T c superconductor are discussed and compared to those of the 40K and 90K materials. While the superconducting behavior of the 90K is very sensitive to the oxygen composition and therefore to the annealing procedure, the Bi-based phase is dramatically less affected. But it is more sensitive to the solid state reaction temperature and starting composition. Thus a better control of the chemistry using a solution technique should enhance the performances of this material. The Y-based 90K superconducting oxide has been synthesized using a novel solution technique (Sol-Gel process). Physical properties, including resistivity and critical current, are compared to those obtained by solid state reaction in which the starting materials include carbonates, acetates and nitrates. The solution technique produces YBa 2 Cu 3 O 7−y powders at lower reaction temperatures and shorter times. The smaller particle size obtained by this method (1μm) yields denser compacts upon sintering. The solution derived materials are made from inexpensive reagent-grade (98% pure) starting materials and they superconduct at 91K with narrow transition widths (0.6K). Although the material is homogeneous, the critical currents do not significantly exceed those reported for solid-state reacted materials and we attribute this to the presence of carbon at the grain boundaries. Finally, the ability to produce stable viscous gels allows us to prepare large-area thick films on various substrates. Best results are obtained on SrTiO 3 or MgO substrates, and problems encountered in making thick films on substrates of technological interest are discussed.

Optical properties related to local structures in CuO superconductors

Abstract Several optical features in the visible-near-UV spectra of the cuprite superconductors are associated with localized structures within the complex unit cells. Here, we emphasize our study of features, associated with the Cu O planes, that depend on doping. In addition, we present results from oriented samples that demonstrate the importance of the long-range modulation in Bi 2 Sr 2 CaCu 2 O 8 to the electronic properties of the Bi O plnes.

Charge Transfer and Bond Lengths in YBa 2 Cu 3−x M x O 6+y

We discuss the effects of doping on the Cu chain sites in YBa 2 Cu 3−x M x O 6+y . The relationship between bond lengths obtained from neutron scattering and charge transfer is evaluated in terms of bond valence. In particular, it is concluded that removing an oxygen from the chains transfers one electron to the planes.

3d-Metal Doping (Fe,Co,Ni,Zn) Of The High Tc Perovskite YBa2Cu3O7-y

The structural, magnetic and superconducting properties of the mixed compounds YBa 2 Cu 3−x M x O 7-y (M = Ni,Zn,Fe and Co) are reported. Values of y, determined by titration, are found to be dependent on the nature and amount of the doping. The range of solubility is greater for the Fe and Co compounds (x = 1) than for those with Ni or Zn (x = 0.3). The undoped material is orthorhombic and remains orthorhombic after substitution for Cu by Ni or Zn, whereas a tetragonal phase is observed when Fe, Co are substituted for Cu. DC resistance and AC susceptibility measurements show that T c is depressed from 90K (x = 0) to 45K (x = 0.2) for both the Ni and Zn doped compounds and T c is destroyed in the Fe and Co doped compounds when x reaches 0.4. We suggest that a valence of 2 be assigned to the Ni and Zn and 3 to the Fe and Co ions.

Tunneling attempts in single-phase Bi4Sr3Ca3Cu4O16+y and chemical doping on the Cu-sites in 90 and 40K superconductors

Planar tunneling junctions on Bi 4 Sr 3 Ca 3 Cu 4 O 16+y (4334) which clearly exhibit the gap of the top electrode (In or Pb) have been prepared. Junction resistances range from 1Ω to 6kΩ over a 1 – 2mm 2 area, the lower resistances being obtained only after plasma oxidation of the surface. We observe a linear conductance at varying biases above the gap region. No evidence of a gap in the 4334 material and no Josephson current has been observed even in the lowest resistance junctions. A short summary of the Bellcore work on the Cu-doped YBa 2 Cu 3 O 7−y and La 1.85 Sr .15 CuO 4−y is also given.