A. Calleja

Enhanced stability of charge-order in underdoped Bi0.75Sr0.25MnO3

Abstract We report that the charge-ordered phase of Bi0.75Sr0.25MnO3 presents a transition temperature T CO ≃600 K , well above the ordering temperature in half-doped Bi0.50Sr0.50MnO3 (T CO ≃525 K ). Magnetization, calorimetry, and neutron diffraction measurements evidence that, in underdoped Bi0.75Sr0.25MnO3, an off-stoichiometry corresponding to a Mn3+/Mn4+ ratio of 3 increases TCO by ∼75 K with respect to half-doping. This unusual feature confirms the active role of Bi in this family, the only known manganites presenting charge order well above room temperature.

Synthesizing the Y-123//Y-211 composite by the PVA method

A polymerized organic–inorganic synthesis of a mixture containing 69% Y-123, 30% Y-211, 1% CeO2 in weight has been studied with the aid of polyvinylic alcohol (PVA). The advantages of this method are (a) avoidance of extensive and repetitive firing steps and (b) no need for mechanical mixing of components. By heating the solution of stoichiometric quantities of the nitrate salts, a viscous gel is obtained which turns into an aerogel by subsequent heating. The resulting aerogel was calcined in a single step and the powder was used to prepare Y-123/Y-211 composite monodomain rods by the vertical Bridgman solidification technique. Characterization of both powders and grown monodomains have been performed. Fine and dispersed Y-211 phase distribution was successfully achieved. Finally, magnetic measurements were performed by SQUID magnetometry.

Structural and magnetic properties of ZnO:TM (TM: Co, Mn) nanopowders

Abstract We report on the structural and magnetic characterization of Co 0.1 Zn 0.9 O and Mn 0.1 Zn 0.9 O nanopowders obtained by a soft chemistry route. We show that those samples fired at low temperatures display a ferromagnetic interaction that cannot be attributed to the presence of impurities. A magnetic aging mechanism is observed, reflecting the key role played by defects in the stabilization of ferromagnetism in this kind of diluted magnetic semiconductors.

Exploring the polyvinyl alcohol method for preparing cuprates and manganites

The polyvinyl alcohol route is investigated in the formation of cuprate and manganite ceramics. By evaporation of the nitrate solution containing the stoichiometric quantities of the cations and the PVA, a spongy mass is produced, which upon a single calcination converts into the desired phases. We chose YBa2Cu3Oy, a high temperature superconductor, and La2/3Ca1/3MnO3, a perovskite manganite showing colossal magnetoresistance as case studies for the preparation of technological interesting ceramic powders. XRD is employed to monitor the phase transformation leading to the single phases. Moreover, carbon content in the final powders is measured to assure complete elimination of carbonaceous species. This preliminar study shows that the PVA solution can be a potentially interesting technique to prepare these advanced ceramic powders as compared to classical ceramic methods. # 2002 Elsevier Science Ltd. All rights reserved.

High temperature transformation of electrospun BaZrO3 nanotubes into nanoparticle chains

BaZrO3 nanofibers were electrospun from a 0.25 M metal cation solution containing barium acetate and zirconium 2,4-pentadionate dissolved in glacial acetic acid. Polyvinylpyrrolidone was added to increase the solution viscosity and allow formation of hollow fibers during electrospinning. Microstructural evolution during thermal annealing at temperatures between 600 °C and 1400 °C was studied revealing crystal growth evolution. Strong grain coarsening is observed above 1200 °C, leading to linear and branched nanoparticle chains originating from the finer initial hollow nanofibers. Faceting of the nanoparticles occurs in the low-energy surfaces. On the other hand, grain boundaries are planar and follow the same facet planes as the nanoparticles. High resolution transmission electron microscopy reveals no general epitaxial relationship at the interface with the exception of a minor amount of low angle grain boundaries.

Solution design for low-fluorine trifluoroacetate route to YBa2Cu3O7 films

We present our work in the preparation of metallorganic precursor solutions with reduced fluorine content, able to fulfil the requirements for high-performance superconducting YBCO epitaxial layers as a promising approach to low cost and scalable coated conductors. Six different solutions using different quantities of fluorine and non-fluorine carboxylate precursors with a total amount of fluorine from 10 to 50% that of standard trifluoroacetic acid (TFA) solutions. For stabilization purposes different coordinating agents have been used and the solution rheology has been modified for proper substrate wettability. Thermal decomposition analysis and infrared spectroscopy performed directly in films, have revealed that the decomposition takes place in two consecutive stages around 265 and 310 °C respectively, and nuclear magnetic resonance (NMR) analysis could unveil the chemical reactions taking place in the solution. Using the solutions with 20% fluorine and upon optimization of the growth process parameters, YBCO layers with T c and J c(77 K) of 90 K and 4 MA cm−2 are obtained.

Low AC Loss Inkjet-Printed Multifilamentary YBCO Coated Conductors

Impressive performance has been achieved in (RE)Ba2Cu3O7-δ (REBCO) coated conductors, but for many applications, the high cost and ac losses remain prohibitive. Inkjet printing methods combine scalability and low equipment cost with high-resolution patterning, potentially addressing both issues by enabling the production of multifilamentary coated conductors without subtractive processing. The successful production of multifilamentary superconducting YBa2Cu3O7-δ (YBCO) structures by inkjet printing of a low-fluorine YBCO precursor solution on SS/ABAD-YSZ/CZO substrates is reported. Two approaches have been developed. In the first method, YBCO filaments were directly deposited on the buffered substrate by piezoelectric printing; and in the second approach, CeO2 tracks were first printed to pattern a subsequently overprinted YBCO film, creating a multifilamentary structure by an inverse technique. Scanning Hall probe measurements have been used to compare the filamentary structures and critical currents achieved by both methods, and a Jc of up to 3 MA cm-2 has been obtained at 77 K. For the inverse printing approach, the ac hysteresis losses have been also measured and compared with theoretical models.

Inkjet printing of multifilamentary YBCO for low AC loss coated conductors

Considerable progress has been made with the development of REBCO coated conductors in recent years, and high performance conductors are available commercially. For many applications, however, the cost remains prohibitive, and AC losses discourage their selection for higher frequency applications. Chemical solution deposition (CSD) methods are attractive for low-cost, scalable preparation of buffer and superconductor layers, and in many respects inkjet printing is the method of choice, permitting non-contact deposition with minimal materials wastage and excellent control of coating thickness. Highly textured coatings of YBCO and Gd-doped CeO2 have previously been reported on buffered metal substrates. Inkjet printing also introduces the possibility of patterning - directly depositing two and three dimensional structures without subtractive processing - offering a low-cost route to coated conductors with reduced AC losses. In this contribution, the inkjet deposition of superconducting YBCO tracks is reported on industrially relevant buffered metal substrates both by direct printing and an inverse patterning approach. In the latter approach, ceria tracks were printed reported, which are a candidate both for resistive filament spacers and buffer layers. TFA-based precursor solutions have been printed on SS/ABAD-YSZ/CeO2 and Ni-W/LZO/CeO2 RABiTS substrates, and the resulting multifilamentary samples characterised by microscopy and scanning Hall probe measurements. The prospects for future inkjet-printed low AC loss coated conductors are discussed, including control of interfilamentary resistivity and bridging, transposed filamentary structures and stabilisation material.

Epitaxial BaZrO3 tracks by electrospinning of metalorganic fibers on single crystals

Electrospinning was used to produce fibers of barium zirconate (BZO) by metalorganic decomposition. Using short deposition times, a low density of randomly distributed fibers were deposited on lanthanum aluminate single crystals with (100) orientation. Then the films were subjected to thermal treatment at 600 °C and 800 °C to decompose the organic matter and crystallize the perovskite phase. X-ray diffraction, field-emission electron and atomic force microscopies were used as characterization tools. Continuous BZO tracks are obtained after removal of organics at 600 °C. At 800 °C, cube-on-cube heteroepitaxy of barium zirconate with respect to the lanthanum aluminate template was detected by synchrotron X-ray diffraction, although a fraction of misoriented BaZrO3 crystals were also detected by grazing incidence X-ray diffraction (GIXRD). For precursor fibers closely attached to the single crystal surface, BaZrO3 grows as epitaxial nanoparticles, which then coarsen and self-organize along the fiber tracks, showing oscillations in the interparticle separation distance. Growth of the coarsened low-aspect ratio crystals follows an anisotropic atomic diffusion mechanism, in which mass transport mainly occurs in the direction of the initially deposited fiber. The misoriented nanoparticles observed by GIXRD are attributed to the fibers deposited on top of those arriving first to the substrate and to the loss of the interfacial epitaxial growth at large crystal thickness. The distribution and morphology of the particles have been further analyzed by means of atomic force microscopy.

Thickness–concentration–viscosity relationships in spin-coated metalorganic ceria films containing polyvinylpyrrolidone

AbstractnZr-doped ceria thin films were prepared by spin-coating metalorganic solutions containing polyvinylpyrrolidone (PVP), which serves as a relaxing agent to relieve the strong mechanical stresses developed during the thermal decomposition step. The precursor solutions were deposited on silicon substrates and subsequently heated in air at 500xa0°C. The complete decomposition of organics was checked by infrared spectroscopy with attenuated total reflectance. The molar ratio between PVP and total metal ions was varied between zero and one. Furthermore, the effect of using two different PVP molecular weights was also investigated. For the high molecular weight of PVP, crack-free films as high as 150xa0nm could be obtained compared to 30xa0nm when no PVP is added. Thickness after spin-coating and after thermal decomposition were determined by profilometry and correlated with the polymer concentration in the precursor solution, showing a linear dependence with PVP concentration in both cases. The main controlling parameter of the final thickness is the viscosity, with similar power law dependencies both before and after thermal treatment, which would indicate that the porosity fraction remains essentially constant in the final films. Furthermore, AFM analysis was used to investigate the flatness and surface porosity of the films after the thermal treatment.n