G. A. Costa

NIR Persistent Luminescence of Lanthanide Ion-Doped Rare-Earth Oxycarbonates: The Effect of Dopants

A series of luminescent rare-earth ion-doped hexagonal II-type Gd oxycarbonate phosphors Gd2-xRExO2CO3 (RE = Eu(3+), Yb(3+), Dy(3+)) have been successfully synthesized by thermal decomposition of the corresponding mixed oxalates. The Yb(3+) doped Gd-oxycarbonate has evidenced a high persistent luminescence in the NIR region, that is independent from the temperature and makes this materials particular attractive as optical probes for bioimaging.

Synthesis and characterisation of superconducting RuSr2GdCu2O8

Abstract We report on the structural, electrical and magnetic properties of RuSr2GdCu2O8 samples made by systematic synthesis work so that they differ from one another in number of annealings and sintering temperature. Our aim is to highlight the different behaviours as a consequence of the thermal treatments. In particular, we deal with the onset of superconductivity in relation to the homogenisation process, and its influence on resistivity, structural and magnetic ordering. Finally, the problems related to the magnetic measurements due to the superimposition of different magnetic states are examined.

High temperature structural study of Gd-doped ceria by synchrotron X-ray diffraction (673 K ≤ T ≤ 1073 K).

The crystallographic features of Gd-doped ceria were investigated at the operating temperature of solid oxides fuel cells, where these materials are used as solid electrolytes. (Ce(1-x)Gd(x))O(2-x/2) samples (x = 0.1, 0.3, 0.5, 0.7) were prepared by coprecipitation of mixed oxalates, treated at 1473 K in air, and analyzed by synchrotron X-ray diffraction in the temperature range 673 K ≤ T ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at x ∼ 0.2 between a CeO2-based solid solution (for x ≤ 0.2) and a structure where Gd2O3 microdomains grow within the CeO2 matrix, taking advantage of the similarity between Gd(3+) and Ce(4+) sizes; the existence of the boundary at x ∼ 0.2 was confirmed also by measurements of ionic conductivity performed by impedance spectroscopy. Similar to what observed at room temperature, the trend of the cell parameter shows the presence of a maximum; with increasing temperature, the composition corresponding to the maximum moves toward lower Gd content. This evidence can be explained by analyzing the behavior of the coefficient of thermal expansion as a function of composition.

Thermal analysis im the M-Ba-Cu-O systems (M = Y, La, Pr) in relation to high Tc superconductors

Abstract Metallographie, X-ray diffraction and differential thermal analyses were employed to scan suitable limited sections of M-Ba-Cu-O systems (M = Y, La, Pr) in order to determine the thermal stability of the occurring phases. Particularly in the above systems the thermal stability and structural data of the bounding oxide Y 2 O 3 - BaO, La 2 O 3 - Bao and Pro x - Cuo systems are reviewed. Significant results were obtained in determining the crystal structure of BaY 2 O 4 phase (CaFe 2 O 4 - type) and in the identification of a new phase in the BaO -La 2 O 3 system.

Synthesis and superconductive characterisation of RuSr2NdxGd1-xCu2O8 compounds (x = 0, 0.09, 0.18, 0.35)

Abstract We prepared various RuSr 2 Nd x Gd 1− x Cu 2 O 8 samples with systematically varying Nd amounts by x =0, 0.09, 0.18 and 0.35. The solid state route involved mixed oxides (Nd x Gd 1− x ) 2 O 3 obtained by coprecipitation. The XRD and microstructural analysis showed that a high phase purity was obtained. The Nd solubility limit can be placed between x =0.18 and x =0.35. The superconducting transition temperature dramatically decreased with increasing Nd content, confirming that Nd entered the lattice. The field dependence of superconducting transitions exhibited a polycrystalline behaviour.

Correlations between Structural and Electronic Properties in the Filled Skutterudite Smy(FexNi1–x)4Sb12

A structural study of the filled skutterudite Smy(FexNi1-x)4Sb12 was performed by means of X-ray powder diffraction and μ-Raman spectroscopy with the aim to unveil the correlations between structural and electronic properties of this material and to favor the improvement of its thermoelectric performance. Samples were prepared by direct reaction of the elements at 1223 K, followed by quenching and subsequent sintering at 873 K; microstructure and composition of the obtained products were determined by SEM-EDS. The position of the boundary separating regions that obey hole- and electron-based conduction mechanisms was found by X-ray diffraction at x ≈ 0.63 and y ≈ 0.30, confirmed by measurements of room-temperature Seebeck coefficient, and discussed on the basis of crystallographic data. The presence of a discontinuity is observed in several structural and spectroscopic parameters at the p/n crossover; it is interpreted as associated with the change in the conduction mechanism. The role of the rare earth filling fraction in driving the structural response of the material is investigated too. The advantage of using X-ray diffraction and μ-Raman spectroscopy as aids in the study of electronic properties of this material is highlighted, as well as the complementarity of the two techniques.

Topics on the preparation of HTSC via solid state reaction and melting process

The problems of the preparation of High Temperature Superconductors by sintering and melting are examined. Results of the thermal properties of the total substitution of Yttrium by a rare earth element are reported. The process of fabrication of HTSC after high temperature decomposition is indicated. The results of first characterisation are also reported.ZusammenfassungProbleme der Herstellung von Hochtemperatur-Supraleitern durch Sinter- und SchmelzvorgÄnge wurden untersucht.Es werden Resultate der thermischen Auswirkungen der vollstÄndigen Substitution von Yttrium durch Seltenerdelemente wiedergegeben. Der Prozess einer Herstellung von HT-Supraleitern nach thermischer Zersetzung im Bereich hoher Temperatur wird beschrieben, und es werden Resultate erster Charakterisierungen mitgeteilt.

Synthesis Effects on the Magnetic and Superconducting Properties of RuSr2GdCu2O8

A systematic study on the synthesis of the Ru-1212 compound by preparing a series of samples that were annealed at increasing temperatures and then quenched has been performed. It results that the optimal temperature for the annealing lies around 1060–1065°C; a further temperature increase worsens the phase formation. Structural order is very important and the subsequent grinding and annealing improves it. Even if from the structural point of view the samples appear substantially similar, the physical characterizations highlight great differences both in electrical and magnetic properties related to intrinsic properties of the phase as well as to the connection between the grains as inferred from the resistive and the Curie-Weiss behaviour at high temperature as well as in the visibility of field cooled and zero field cooled magnetic signals.

The influence of Ag and Ag(In) alloy on Y-Ba-Cu-O equilibrium diagram around the Y-123 superconducting phase

We studied the chemical compatibility of Ag, Ag(In)alloy, Y2BaCuO5 (the so-called Y-211 green phase) and YBa2Cu3O7–δ (Y-123) phase in order to check if better grain growth and alignment with minimum contamination were possible during the Y-123melt texturing. We demonstrate that the addition of silver to Y-123 always led to an enhanced texture. The typical microstructure of the composite Y-123 plus Y-211 (that is believed essential for high critical currents in these High Tc superconductors) was not disturbed by addition of silver up to 7.5 wt%, while higher concentrations of metal led to a degradation of the texture. DTA–TG analysis was used to investigate the influence of Ag and Ag(In) alloy on the Y–Ba–Cu–O equilibria around the Y-123 phase. We found a very unexpected thermal behaviour, similar to an eutectic equilibrium, when the silver concentration was increased to35 wt%. We believe that this effect was essentially due to the increasingly higher concentration of silver that could react with the secondary phases present in the melt.

Comparative study on the heat capacity of ABa2Cu3O7−x high TC superconducting compounds (A = Y, Eu, Ho).

Abstract We report heat capacity results obtained on single phase polycrystalline samples of high T c perovskite type superconductors YBa 2 Cu 3 O 7−x , EuBa 2 Cu 3 O 7−x , HoBa 2 Cu 3 O 7−x . The samples were obtained from the sintered product of well reacted A 2 O 3 , BaO 2 and Cuo powders. The calorimetric measurements were performed in a computer controlled continuous heating adiabatic calorimeter in the temperature range 77–300 K. The work is aimed to compare the thermal behaviour of compounds ABa 2 Cu 3 O 7−x in which elements with different internal magnetic field are present. Particularly a study at temperature above the superconducting transitions was compared with others, to investigate by calorimetric measurements thermal effects already observed in YBCO system by anelastic relaxation measurements at various temperatures.