X.G. Capdevila

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.

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.

Yield strength, Shear Stress and Toughness of YBCO samples textured by Bridgman technique

Mechanical properties of the orthorhombic phase of YBa2Cu3O7-δ (Y-123) at room temperature have been investigated at different applied loads using nanoindentation technique. The study was carried out for several monodomains on the (001) planes for textured Bridgman samples with dispersed Y2BaCuO5 (Y-211) particles as pinning centers. The yield strength (σys), shear stress (τm) and toughness (KIC) of Y123/Y211composite was determined at different applied loads. First and second mechanical properties have been calculated though the Hertz equations and the last one with Lawn et al. equations. Finally, the ultra-low imprints obtained by nanoindentation have been correlated with parameters obtained by Field Emission Scanning Electron Microscope (FE-SEM).

Solid Electrolyte Multisensor System for Detecting O2, CO, and NO2

A low-cost multisensor system for combustion control based on electrochemical gas sensors has been manufactured and studied. We followed simple technological steps to join three simple planar elements exhibiting complementary sensing properties. The system does not require a reference atmosphere and combines an amperometric module for O 2 measurement and two potentiometric elements for CO and NO 2 detection. The working temperature of 600°C was selected as a compromise between the electrochemical response signals and diffusion kinetics of the potentiometric CO- and NO 2 -sensing modules. We established the response and recovery times for CO, NO 2 , and oxygen pulses, together with their respective activation energies. For the whole range of gaseous compositions, the electrical measurements provided by the oxygen-sensing module permitted the correction of the oxygen cross-sensitivity for the two potentiometric elements. Principal component analysis has been used to show the feasibility of simultaneous detection of NO 2 , CO, and O 2 using this low-cost system, which reveals great promise for use in domestic gas boilers.

Non-Nernstian planar YSZ-based electrochemical sensors using semiconducting oxide electrodes

New gas sensing devices based on solid state electrochemistry with a planar geometry are presented. The complete devices combine an improved processing technology of yttria-stabilized zirconia (YSZ) foils with platinum and semiconducting oxide electrodes. The microstructural and catalytic properties of the electrodes defined the sensing response, which exhibited a nonNernstian electrochemical nature, as expected. Sensors with WO/sub 3/-based electrodes showed the best sensitivity to NO/sub x/ among other options, with the highest selectivities in respect to CO at the lowest temperatures. In the case of Al/sub 2/O/sub 3/:Pt sensing electrodes, a differential gas diffusion principle of operation was considered, detecting CO without much selectivity to NO/sub x/.