Enhancement of the power factor of [Bi1.68Ca2O4](RS)[CoO2](1.69) - Ag composites prepared by the spray-drying method

[Bi1.68Ca2O4](RS)[CoO2](1.69) (BCCO) sample and Ag-BCCO composites (with 10, 20 or 30 wt% Ag) have been prepared by the spray-drying technique and uniaxially/isostatically packed. Scanning electron microscopy reveals that the Ag particles are well distributed in the BCCO cobaltite matrix at low Ag contents. The Ag particles have an important effect on densification and grain orientation of the samples, with a direct impact on their electrical conductivity. The electrical conductivity is higher for the uniaxial samples and increases with the Ag content up to 20% in weight, while the Seebeck coefficient is hardly affected. These features induce an improvement of the power factor, reaching a maximum value of 2.2 mu W K-2 cm(-1) at similar to 1050 K for the uniaxial sample with 20 wt% Ag. Our results suggest that the spray-drying technique is a promising method to obtain composites with a well-dispersed secondary phase. (C) 2010 Elsevier Masson SAS. All rights reserved

Analyse de PZT morphotropique par diffraction X haute température : influence du procédé d'élaboration sur la dispersion chimique

La solution solide PbZrxTi1-xO3, (PZT) présente à l'ambiante une zone morphotropique pour x voisin de 0,54 et pour laquelle coexistent deux phases de même composition chimique mais de structure différente (quadratique et rhomboédrique). Ces solutions sont classiquement obtenues par chamottage, réaction de synthèse souvent incomplète. Les deux phases constitutives du PZT présentent des variations de composition chimique autour de la composition moyenne et qui modifient de façon complexe le diffractogramme, empêchant son interprétation aisée. Nous présentons dans cet article une étude par diffraction X en température sur des PZT chamottés de façon différente. Les diffractogrammes sont enregistrés à 550°C, température pour laquelle il n'existe qu'une seule phase cubique. Les pics sont considérés comme résultant d'une solution cubique de composition moyenne déterminée par les positions des maxima. De la mesure des largeurs de raies, est calculée une fluctuation de paramètre de maille, puis une fluctuation chimique à l'aide d'une courbe d'étalonnage réalisée à partir d'un lot de PZT de compositions déterminées par fluorescence X, technique ayant fait l'objet d'une optimisation particulière.By means of temperature x-ray diffraction, we estimate the compositional fluctuation in morphotropic PbZrxTi1-xO3 (PZT) solid solutions. In these systems, near the morphotropic phase boundary (x≈ 0.54), the tetragonal and rhomboedral phases coexist at room temperature, with respective constant lattice parameters and with relative amounts according to the average law. Because of the overlapping and the broadening of the diffraction lines, the morphotropic PZT x-ray pattern is complex. A convenient method is to measure diagrams at 550°C, temperature above Curie temperature (490°C) for which the PZT had cubic symmetry. From the observed line broadening, a fluctuation of lattice spacing is calculated. The compositional fluctuation is then deduced from a calibration curve made out with some PZT samples whose compositions are determined by x-ray fluoresence

Spray drying: An alternative synthesis method for polycationic oxide compounds

Synthesis of polycationic compounds by the spray-drying technique is an interesting alternative in the domain of aqueous precursor synthesis methods. Spray drying yields high quality samples with good reproducibility. The possibility of scaling up for production of large quantities with fast processing time is well established by the commercial availability of powders of various compositions. In this paper, we have discussed the advantages and limitations of this method and demonstrated its interest by synthesizing a few polycationic compounds selected for their attractive properties of thermoelectricity [Bi1.68Ca2Co1.69O 8, La0.95A0.05CoO3 (A=Ca, Sr, Ba)] or magnetoresistance [La0.70A0.30MnO3 (A=Sr, Ba)]. We have confirmed the quality of these samples by reporting their structure, magnetic and transport properties. © 2010 Elsevier Ltd All rights reserved