Search

The phase stability of the system Gd2O3-Nd2O3 up to 1350°C was studied. Mixed oxides were synthesized by coprecipitation of mixed oxalates. Depending on the thermal treatment, three possible structures of the oxides, namely the A (hexagonal, P63/mmm), B (monoclinic, C2/m) and C (cubic, Ia3) forms, were obtained. Below 1300°C the system Gd2O3-Nd2O3 shows three monophasic (A, B and C phase) and three biphasic regions (A+B, B+C and A+C phase). Below 550°C the C phase is stable over the whole composition range. A validation procedure for the existence of the C phase in pure Nd2O3 was also set up. [ABSTRACT FROM AUTHOR]

A thermal study was performed on the RuSr2GdCu2O8 (Ru-1212) magnetic superconductor phase to investigate the effect of the annealing treatments in flowing O2 on the stability limit of the phase and on its structural, magnetic and transport properties. DTA-TG measurements were utilized to determine the decomposition process and the dependence of the decomposition temperature on the annealing atmosphere. The decomposition of the Ru-1212 phase was found sensitive to the oxygen partial pressure and increases with PO2 and the annealing time. The annealing treatments exert a depressing effect on the strength of the magnetic interaction, an enhancement on the superconductive properties and the vanishing of the magnetostriction. A decomposition reaction of Ru-1212 phase was proposed and discussed. [ABSTRACT FROM AUTHOR]

The possibility to grow single crystals in the Ru-Sr-Gd-Cu-O system applying flux methods has been investigated. Several kinds of crystals were obtained: needle-like crystals of spurious phases (basically CuO and SRO), SrRuO[SUB3], and ruthenates. We also observed that Gd was generally depleted from the ruthenate crystals, and it was spread in the residual matrix on the wall of crucible. Some small ruthenocuprate single crystals were indeed observed. [ABSTRACT FROM AUTHOR]

The intermetallic perovskite MgCNi[SUB3], superconducting with a T[SUBc] of 8 K, is particularly interesting from several points of view: in spite of the large amount of nickel present the compound is not ferromagnetic; nevertheless band structure calculations put in evidence a Van Hove singularity just above the Fermi level suggesting that MgCNi[SUB3] is on the border of magnetic instability and that may be driven to ferromagnetism by partial substitution of Mg with a monovalent metal as Li or Na. The material is also interesting because of the presence of two kinds of pinning centres, grain boundaries and nanometer-scale precipitates inside the grains, combination which is not found in other metallic superconductors. We have synthesized polycrystalline samples with composition MgCNi3 by a solid state route. Both the resistive and the magnetic measurements show a double superconductive transition, the first one occurring at a temperature of 10 K, higher than that reported in literature (8 K) and a second one at 6 K. High temperature resistivity shows a metallic behaviour. [ABSTRACT FROM AUTHOR]

The synthesis and physical characterization of single phase polycrystalline samples with composition RuSr[SUB2]GdCu[SUB2]O[SUB8] has been performed. We report on the electrical and magnetic measurements carried out in order to investigate and relate the properties of this compound subjected to different annealing routes. We show that while intrinsic properties as T[SUBc] and structural properties as lattice parameters remain unchanged, the physical properties are strongly affected by the final annealing in flowing Oxygen. Coexistence of superconductivity and ferromagnetism is observed in all specimens studied, while Meissner effect visibility is induced by the annealing process. [ABSTRACT FROM AUTHOR]

CuNi, NiCr, NiV alloys and pure Ni have been processed to obtain metallic substrates for high critical current density YBCO film depositions. High purity alloys were prepared using plasma arc furnace and the material was cold rolled to low thickness tapes. For all the tapes the (100)[001] cube alignment was induced by a primary recrystallization heat treatment after heavy cold rolling (RABITS). Besides the cube texture a good substrate should be smooth, thin, strong, non magnetic, resistant to oxidation, cold workable and low cost. In this work mechanical, magnetic and oxidation resistance properties of the prepared textured substrates were measured and compared. [ABSTRACT FROM AUTHOR]

We prepared polycrystalline samples of YBa[sub 2]Cu[sub 2](CU[sub 1-x]M[sub x])O[sub 6+w], where M (= Sc, Cr, V, Fe, Al and Au) substitutes copper in the chain site, and measured their transport and magnetic properties. The substitution level χ was 0.05 and 0.10 for each substituent. We determined the solubility of the different metals and the dependence of T[sub c], irreversibility lines and critical current density j[sub c] on M and χ. An improvement of j[sub c] by a factor larger than 2 was obtained in the Fe-, Al-, Cr- and Vsubstituted materials. No improvement of the irreversibility line was obtained for any substituent. [ABSTRACT FROM AUTHOR]

In order to understand the difference between the effects induced by chemical substitutions and those induced by applying mechanical pressure, we synthesised and characterised samples of Y(Ba[sub 1-x]Sr[sub x])[sub 2]Cu[sub 3]O[sub 7-d]. The structural refinements based on powder neutron diffraction data were carried out as a function of x and, for x = 0.5, as a function of d. The electric and magnetic properties have also been measured and correlated with the structural parameters and oxygen content. It seems that the Ba cations play an important role in the high T[sub c] of YBCO. Its substitution with a smaller cation corresponds to the application of a chemical pressure, but the release of the associated strain seems to play a negative role on T[sub c]. [ABSTRACT FROM AUTHOR]

Bulk Bi Pb Sr Ca CuO superconductors with relative density in the range 88-90% were prepared by high cold pressing + pressureless sintering and subsequently hot forged to increase the extent of grain orientation. It has been found that the final density of the samples exceeds 95% and the orientation factor increases up to ≈80%. The introduction of a short high-temperature treatment (T=885 °C ) promotes the extrusion of secondary phases, leaving in the sample only the (2223) as a superconducting phase and islands of Ca-Sr-Cu-O and CuO. The annealing at 400 °C in oxygen was found essential to recover the oxygen content, lost during densification and texturing treatments, and to increase the T value up to 100-105 K. The employment of pressureless sintered samples (PLS) together with the step at 885 °C implies the release of the disorder near sub-grain boundaries usually found in hot-pressed samples. As a consequence of these features, textured PLS samples show a J value at 77 K one order of magnitude higher with respect to the simple PLS sample. The use of sol-gel powder improves the densification and texturing of the superconductor even for shorter treatment times. Since J of this sample is ≅700 A/cm , the addition to the treatment of the step at 885 °C is highly promising in the view to reach a further increase of critical current density. [ABSTRACT FROM AUTHOR]

The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures significantly lower than usually reported. The basic idea was to exploit the low vapuur pressure of HgO at low temperatures (≈600°C) and in oxygen atmosphere to favour the solid-state reaction between HgO and precursors. Recently, a novel alternative approach essentially based on the oxygen and mercury vapours reactivity for the synthesis of the superconducting phases has been successfully developed. Basically, solid precursors (powders, pellet or thick-film forms) are processed in a two-temperature zone reactor which allowed the independent control of precursors and HgO source temperatures, the partial pressure of HgO and the total pressure during the reaction. In the latter approach, a very low mercury vapour partial pressure (intrinsically below 1-2 atm) was demonstrated to be effective for synthesizing the superconducting phase. The respective merits of both these alternative techniques will be discussed in terms of structural, morphological and magnetic measurements. The low reaction pressure common to both the techniques simplifies largely and makes safer the preparation of these materials, especially in view of applications. [ABSTRACT FROM AUTHOR]

Examines the magnetic behavior and transport properties of mercury-1201 prepared by vapor-solid reaction. Operation of a vapor-solid reaction in two-temperature zone reactor at low pressure; Conduction of high sensitivity magnetization measurements to assess connectivity; Observation of hysteric behavior due to an intergranular critical state regime.