Rafael Ibáñez

Polymer solution processing of (Bi, Pb)SrCaCuO

Abstract A simple polymer processing route to the 110 K phase (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor has been developed. The influence of the polymer to metal starting ratio, as well as the sintering temperature and time on the quality of the resulting superconducting powders has been studied by XRD, a.c. susceptibility and resistivity measurements. Microstructure of ceramic compacts has been studied by SEM, and qualitatively analysed by EDS for compositional homogeneity. The results indicate that the polymer synthesis route described here may offer a good alternative to the conventional solid state preparation methods towards attaining homogeneous 110 K superconductor powder within a reasonable processing time.

Soil emissivity and reflectance spectra measurements.

We present an analysis of the laboratory reflectance and emissivity spectra of 11 soil samples collected on different field campaigns carried out over a diverse suite of test sites in Europe, North Africa, and South America from 2002 to 2008. Hemispherical reflectance spectra were measured from 2.0 to 14 microm with a Fourier transform infrared spectrometer, and x-ray diffraction analysis (XRD) was used to determine the mineralogical phases of the soil samples. Emissivity spectra were obtained from the hemispherical reflectance measurements using Kirchhoffs law and compared with in situ radiance measurements obtained with a CIMEL Electronique CE312-2 thermal radiometer and converted to emissivity using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) temperature and emissivity separation algorithm. The CIMEL has five narrow bands at approximately the same positions as the ASTER. Results show a root mean square error typically below 0.015 between laboratory emissivity measurements and emissivity measurements derived from the field radiometer.

(Bi,Pb)2Sr2Ca2Cu3O10+δ superconductor composites: Ceramics vs. fibers

Abstract Well characterized (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor powder has been used to prepare superconductor-glass, -metal and -alloy composites through solid state reaction method. A recently developed Laser Floating Zone (LFZ) apparatus has been used to transform the ceramic precursors into oriented fibers. The diamagnetic properties have been studied by a.c. susceptibility. The microstructure of fibers has been studied by SEM and compared with that of the original ceramic precursors. XRD has been used to study phase composition on representative composite samples and fibers. The results indicate some potential for the 2223-Ag composite, which displays improved diamagnetic properties and grain orientation (fiber).

SYNTHETIC PATHWAYS TO VANADYL PHOSPHATES

Abstract A general scheme concerning the synthesis and reactivity of oxovanadium phosphates is presented. In particular, the isolation of new materials of composition NH 4 (VO 2 )(HPO 4 ), [M(en) 2 ] x VOPO 4 ·(2−2 x )H 2 O (M=Cu or Ni; 0.03⩽ x ⩽0.20; en=ethylendiamine) and Na x VOPO 4 is described. The structures of α-NH 4 (VO 2 ) (HPO 4 ), β-A(VO 2 ) (HPO 4 ) (A=NH + 4 , k + , Rb + , Cs + ) and VOHPO 4 ·2H 2 O are reported. New spectroscopic (IR, EPR, NMR) results are furnished. The magnetic properties of vanadium (IV)-containing derivatives are discussed on the basis of the structural results.

Low-temperature synthesis, structure and magnetoresistance ofsubmicrometric La1-xKxMnO3+δperovskites

The use of a precursor-based synthetic method, namely, the freeze-drying of acetic acid solutions, allows the preparation, at temperatures as low as 600 °C, of single phase perovskites in the solid solution series La 1-x K x MnO 3+δ . This soft treatment of the precursors yields products constituted of homogeneous nanometric particles (30–50 nm). Samples prepared at 1000 °C remain single phase forx=0.00, 0.05, 0.10 and 0.15, and are constituted by submicrometer homogeneous particles, without deviation from nominal stoichiometries upon heating. Their crystal structures were refined by the Rietveld method from X-ray powder diffraction data. Ac susceptibility, magnetization, resistivity and magnetoresistivity measurements have allowed us to establish, for the first time, relevant points of the electronic phase diagram of an alkali-metal-doped lanthanide manganate system.

Procedures for synthesis of single-phase 2212 bismuth material☆

Abstract Benefiting from the features of the Bi2O3-CuO binary system, it is possible to isolate a single-phase Bi2Sr2CaCu2O8 high-Tc superconducting material starting from Bi2CuO4. The existence of (BiO2)n chains in the channel-containing open structure of this cuprate, which melts at relatively low temperature, makes it a suitable solid state precursor of the 2212 material.

Crystal fibers of BiSrCaCuO materials grown by the laser floating zone method

The technological importance of good-quality fibers of the new high-Tc superconducting oxide materials warrants their investigation through a variety of preparatory techniques. In particular, the laser floating zone (LFZ) growth method is very versatile for rapidly growing fibers of specific length, diameter and orientation. This report describes a study of the preparation, morphology and electrical properties of dense, oriented, LFZ-grown polycrystalline fibers derived from ceramic precursors in the BiCaSrCuO system. The features of the resulting materials are discussed on the basis of the procedural variables and chemical history of the polycrystalline precursors.

New vanadyl hydrogenphosphate hydrates. Electronic spectra of the VO2+ ion in the VO(HxPO4)x•yH2O system

Abstract A general synthetic procedure to obtain oxovanadium(IV) derivatives with the general stoichiometry VO(H x PO 4 x ·yH 2 O is presented. This method has enabled us to isolate for the first time four new members of this series (corresponding to x=1 and y=1,2( α ), 2(β) and 3). New structural types are evidenced. A ligand-field analysis employing the angular overlap model of the electronic d-d spectra of these materials is performed.

A new improved synthesis of the 110 K bismuth superconducting phase: freeze-drying of acetic solutions

Abstract Metastability has greatly hindered the separated synthesis of the high-temperature superconducting phases represented as (Bi1−x, Pbx)2Sr2Can−1CunO4 + 2n (n = 2, 2-2-1-2, Tc≈80 K, and n = 3, 2-2-2-3, Tc≈110 K). By careful control of the synthetic variables, it becomes possible to obtain the 110 K phase as the only superconducting one through processing of freeze-dried acetic solutions. This technique leads to homogeneously sized (5–10 μm) micaceous platelets of the superconducting material.

X-RAY DIFFRACTION LINE-BROADENING STUDY ON TWO VIBRATING, DRY-MILLING PROCEDURES IN KAOLINITES

Due to the great technological importance of the microstructure of kaolinite, characterizing its evolution during dry milling of kaolin and analyzing the microstructural information obtained from different methods were the main aims of this work. The microstructural alteration of kaolinite is evaluated by X-ray diffraction and electron microscopy methods, comparing the results obtained and analyzing the correlations between them. The Warren-Averbach and Voigt-function methods of X-ray diffraction microstructural analysis have been applied successfully to the study of the effects of two different, vibrating-cup dry-milling configurations in the microstructure of kaolinite from the reflections corresponding to (001) diffracting planes. The crystallite-size estimates obtained from the two methods correlate well. Field emission scanning electron microscopy measurements of kaolinite particle thickness are in good agreement with the crystallite size estimated by the two methods. The Warren-Averbach method also provided estimates of the contribution to line broadening. Vibrating-cup milling has been proved to be a more efficient method of strain comminution of kaolinites than other milling techniques, reaching extensive microstructural degradation within seconds.