Magnetotransport properties of a single grain boundary in a bulk La-Ca-Mn-O material

peer reviewedBesides the "intrinsic" colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an "extrinsic" magnetoresistance related to the presence of grain boundaries. We report electrical transport and magnetic measurements carried out on a bigrain sample extracted from a floating zone method-grown rod of calcium doped lanthanum manganate. Electrical resistance was measured both within a grain and across the grain boundary, between 20 and 300 K and from 0 to 8 T. Magnetoresistance values up to 99% are reached within the grain. The temperature dependence of the resistance across the grain boundary displays a "foot-like" feature towards the bottom of the transition. Low field and high field magnetoresistance effects are examined. We compare our results for a "bulk" grain boundary to those obtained by other authors for bicrystal thin films and bulk polycrystalline materials. (C) 2001 American Institute of Physics

Cu doping as a tool for understanding CMR

Doping at the Mn-site in CMR manganate-based perovskites has been shown to affect strongly the physical properties of those compounds. We study here the effect of copper substitution in the low doping range on the electrical transport properties of La0.7Ca0.3MnO3. It turns out that the transition temperature decrease observed in doped samples can be drastically reduced by addition of silicon dioxide SiO2. It is shown that copper is trapped in a secondary phase composed of La,Ca,Si,Cu and O. The resultant Mn-site vacancies appear to be less detrimental to the electronic conduction than the likely antiferromagnetic clusters induced by the copper ions in the Mn-O network

Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique

peer reviewedLa0.7Ca0.3MnO3/Mn3O4 composites can be synthesized in one step by thermal treatment of a spray-dried precursor, instead of mixing pre-synthesized powders. Another advantage of this composite system is that a long sintering step can be used without leading to significant modification of the manganite composition. The percolation threshold is reached at ~ 20 vol% of manganite phase. The 77 K low field magnetoresistance is enhanced to ~ 11 % at 0.15 T when the composition is close to the percolation threshold. (c) 2007 Elsevier Ltd. All rights reserved

Poly(ionic liquid)-derived N-doped carbons with hierarchical porosity for lithium and sodium ion batteries

The performance of lithium and sodium ion batteries relies notably on the accessibility to carbon electrodes of controllable porous structure and chemical composition. This work reports a facile synthesis of well-defined porous N-doped carbons (NPCs) using a poly(ionic liquid) (PIL) as precursor, and graphene oxide (GO)-stabilized poly(methyl methacrylate) (PMMA) nanoparticles as sacrificial template. The GO-stabilized PMMA nanoparticles were first prepared and then decorated by a thin PIL coating before carbonization. The resulting NPCs reached a satisfactory specific surface area of up to 561 m2/g and a hierarchically meso- and macroporous structure while keeping a nitrogen content of 2.6 wt %. Such NPCs delivered a high reversible charge/discharge capacity of 1013 mA h/g over 200 cycles at 0.4 A/g for lithium ion batteries (LIBs), and showed a good capacity of 204 mA h/g over 100 cycles at 0.1 A/g for sodium ion batteries (SIBs).Comment: 14 pages, 9 figure

Structural and electrical properties of tellurovanadate glasses containing Li2O

Glassy materials are promising intercalation compounds, due to their open network structure and absence of grain boundaries. Some glasses containing alkali ions and a high concentration of transition metal ions can present mixed ionic-electronic conductivity and are therefore potential candidates for application as cathode material in Li-ion batteries. The present work is devoted to the ternary system xLi(2)O-(1-x)[0.3V(2)O(5)-0.7TeO(2)] with 0 <= x <= 0.4. These compounds were prepared by heat treatment in air at 800 degrees C followed by traditional quenching. Raman spectroscopy and V-51 nuclear magnetic resonance measurements were performed in order to highlight the structural short range order modifications induced by the introduction of the Li2O network modifier. These structural effects can be related to the electrical behaviour, as studied by complex impedance spectroscopy measurements. (c) 2006 Elsevier B.V. All rights reserved

Mullite coatings on ceramic substrates: Stabilisation of Al2O3-SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying

The present work deals with the preparation of stable alumina + silica suspensions with high solid loading for the production of spray-dried composite powders. These composite powders are to be used for reactive plasma spraying whereby the formation of mullite and the coating on a ceramic substrate are achieved in a single step process. Electrostatic stabilisation of alumina and silica suspensions has been studied as a function of pH. Silica suspensions are most stable at basic pH whereas alumina suspensions are stable at acidic pH. The addition of ammonium polymethacrylate (APMA) makes it possible to stabilise alumina and prepare a stable 50 wt% alumina + silica suspension at pH 10. The optimum amounts of dispersant and binder have been determined by zeta potential, viscosity and sedimentation measurements. Spray drying of the suspension yields composite powders whose morphology, size distribution and flowability have been characterized before realizing reactive plasma spraying tests. (C) 2009 Elsevier Ltd. All fights reserved

Low-field magnetoresistance in La0.7Ca0.3MnO3 manganite compounds prepared by the spray drying technique

peer reviewedCalcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method - whose main advantages are versatility, high reproducibility and scalability - yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with. (C) 2005 Springer Science + Business Media, Inc

Electrochromic Nanomaterials Based on Hybrid Molybdenum-Tungsten Oxide: Synthesis, Wet Coating and Structural / Optoelectronic Characterizations

Hybrid Molybdenum-Tungsten oxides nanomaterials are obtained by solvothermal synthesis. In comparaison with undoped WO3-x, a boost in the optical signature has been observed, which can be linked to a concentration increase of reduced species and oxygen vacancies in the material. The « MoWOx » powder is then wet-processed as thin film by spin coating and succesfully used as plasmonic EC material, dispalying a selective and independent modulation of both visible and NIR wavelengths as a function of the applied potential.PLASMON_E

Microstructure and properties of SLM AlSi10Mg: Understanding the influence of the local thermal history

peer reviewedSelective Laser Melting (SLM) is an Additive Manufacturing technique that is widely used to produce AlSi10Mg parts with a good strength-to-weight ratio. Indeed, strongly refined microstructures are obtained due to the ultra-fast cooling rates reached in this process, conferring high strength to the parts, even in the as-built state. However, microstructural heterogeneities at the scale of the melt pool may exert a detrimental influence on the mechanical properties e.g. by causing a loss in ductility. This study thus aims at a better understanding of the influence of the local thermal history on local variations of microstructure and mechanical properties. Microscopy (i.e. SEM+EDS) and nanoindentation have been combined to reach a detailed knowledge of the local microstructure and properties. In particular, the solute Si content in the -Al matrix, the volume fraction and the size of Si precipitates have been quantified by microscopy analysis. These local microstructural parameters are correlated with the matrix hardness as revealed by nanoindentation. Finally, the results of this detailed characterization are linked with the local thermal history that is approached in two different ways i.e. (i) an analytical description of thermal gradients inside the melt pool based on Rosenthal’s and Matyja’s equations and (ii) a simple Finite Element model for the deposition of a few layers in the SLM process