Investigation of the reactivity of AlCl3 and CoCl2 toward molten alkali-metal nitrates in order to synthesize CoAl2O4

Cobalt aluminate CoAl2O4 powder, constituted of nano-sized crystallites, is prepared, involving the reactivity of AlCl3 and CoCl2 with molten alkali-metal nitrates. The reaction at 450 °C for 2 h leads to a mixture of spinel oxide Co3O4 and amorphous γ-Al2O3. It is transformed into the spinel oxide CoAl2O4 by heating at 1000 °C. The powders are mainly characterized by XRD, FTIR, ICP, electron microscopy and diffraction, X-EDS and diffuse reflection. Their properties are compared to those of powders obtained by solid state reactions of a mechanical mixture of chlorides or oxides submitted to the same thermal treatment

Frittage et évolution microstructurale de céramiques de type oxyde

The sinterability and the microstructural development of various zirconia powders stabilized with yttrium oxide and alumina are studied. The zirconia powders of average specific surface area obtained industrially by vapor phase hydrolysis of zirconium tetrachloride, sintered at an optimum temperature (1300-1350 ° C.), lead to dense me terials containing very little monoclinic phase. The poor sinterability of the zirconia powders, produced by pyrolysis of mixed acetates of zirconium and yttrium, is related to their strong agglomeration. The reaction zirconia synthesis in molten salt media produces very fine and densifiable powders. Fine particle size, low impurity content, high green density and the addition of magnesium oxide favor the densification of alumina powders ex-alum. These parameters also contribute to obtaining a sintered material having a fine and homogeneous microstructure. At the end of the sintering of ex-alum aluminas, the characteristics (density, microstructure) of the samples do not depend significantly on the content of transition phases (gamma alumina) of the powders. In the absence of exaggerated grain magnification, the median value of the number distribution of grain size of a sintered alumina is correlated with the density of the sample. During the final stage of the sintering of the Bayer aluminas, the exaggerated magnification of the grains develops. For the finest powders, this magnification is accompanied by a drop in density.La frittabilité et le développement microstructural de différentes poudres de zircone stabilisée à l'oxyde d'yttrium et d'alumine sont etudiés. Les poudres de zircone de surface spécifique moyenne obtenues industriellement par hydrolyse en phase vapeur du tétrachlorure de zirconium, frittées à une température optimale (1300-1350°C), conduisent a des méteriaux denses contenant très peu de phase monoclinique. La mauvaise frittabilité des poudres de zircone, élaborées par pyrolyse d'acétates mixtes de zirconium et d'yttrium, est reliée a leur forte agglomération. La synthèse de zircone par réaction dans les milieux sels fondus produit des poudres très fines et densifiables. Une granulométrie fine, une faible teneur en impuretés, une densité à vert élevée et l'addition d'oxyde de magnésium favorisent la densification des poudres d'alumine ex-alun. Ces paramètres contribuent aussi a l'obtention d'un matériau fritté ayant une microstructure fine et homogène. En fin de frittage des alumines ex-alun, les caractéristiques (densité, microstructure) des échantillons ne dépendent pas de façon significative de la teneur en phases de transition (alumine gamma) des poudres. En l'absence de grossissement exagéré des grains, la valeur médiane de la distribution en nombre de la taille des grains d'une alumine frittée est corrélée à la densité de l'échantillon. Au cours du stade final du frittage des alumines bayer, le grossissement éxagéré des grains se développe. Pour les poudres les plus fines, ce grossissement est accompagné d'une chute de densité

Quantitative microstructure evaluation, correlation with processing in alumina

International audienc

Sinterability of gamma phase containing alumina powders

International audienc

Alumina microstructural behaviour under pressureless sintering and hot-pressing

International audienceThe aim of this study is to observe the microstructural evolution of a Bayer alumina as a function of different parameters under pressureless sintering and hot-pressing: soaking temperature, soaking time and applied pressure. Experiments indicate a preponderant contribution of the temperature on the microstructure control whatever the sintering mode may be. A semi-manual technique of digitization of the microstructure using a digital tablet allows quantification of the size, the shape and the orientation of the grains. For both sintering modes and beyond a grain size threshold, increase of temperature leads to anisotropic grain growth of the coarser fraction at the expense of the finer fraction. The pressure has an effect on the mobility of grain boundaries and contributes to an elongation that is similar for all the grains. Nevertheless the microstructures under high pressure produce a distribution of strongly elongated grains with an orientation perpendicular to the pressure axis

Sintering of gamma phase containing alumina powders

International audienc

Frittage et evolution microstructurale de ceramiques du type oxyde. Application a l'alumine et a la zircone yttriee

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 82902 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Sintering behavior of 3 mole % yttrium doped chemically synthesized zirconia

International audienc

Influence of production conditions on the densification of zirconia powders obtained from acetates

International audienceTetragonal yttrium-stabilized zirconia samples were obtained by pyrolysis of amorphous acetates containing both metals, at 950 °C in air. The dilatometric behaviour of samples indicates a dedensification when pyrolysis is carried out with a non-linear heating rate causing an absence of fine open porosity. A destabilization when cooling is observed for samples exhibiting a wide granulometry with agglomerate sizes reaching 100 μm. Even when neither one, nor the other phenomenon is noticed, an important aggregation state of the starting powders limits the densification and leads to poor sintered densities