Marc Onillon’s research while affiliated with Université Bordeaux-I and other places

The densification kinetics of 0.99 SnO2-0.01 CuO molar mixtures have been studied between 850 and 1150 C. Both experimental analyses and theoretical modelling show the role of a liquid phase for sintering temperatures T s 940 C: sintering is controlled by a liquid phase after a fast shrinkage due to grain rearrangement. The analytical formulation of the shrinkage behaviour suggests that dissolution at the solid-liquid interface is the limiting process when T s 1000 C and diffusion is the limiting step at higher temperatures.

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in -Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of -Fe2O3 and limit the mobility of n type carriers.

The sintering of a 0·99SnO20·01CuO molar mixture was studied at 1150°C in air. A fast and high densification was observed: the compactness obtained can reach 98·7%. After grain rearrangement, the simultaneous changes in shrinkage and grain size observed versus time were in agreement with a liquid phase sintering mechanism.It was shown that copper dissolves into the SnO2 rutile type structure. The electrical behaviour of the material obtained was in agreement with an interstitial position for copper ions.ZusammenfassungDas Sinterverhalten einer 0·99SnO20·01CuO Mischung wurde bei 1150°C in Luft untersucht. Dabei wurde eine hohe Sintergeschwindigkeit mit einer hohen Enddichte beobachtet: es können Enddichten bis zu 98·7% erreicht werden. Nach einer anfänglichen Umlagerung der Körner konnte mittels paralleler Verfolgung der Schwindung und der Korngröße als Funktion der Versuchszeit auf einen Flüssigphasensintermechanismus geschlossen werden. Es konnte nachgewiesen werden, daß sich das Kupfer in der Rutil-Struktur des SnO2 löst. Auch die elektrischen Eigenschaften des gewonnenen Werkstoffs lassen auf eine interstitielle Position des Kupferions schließen.RésuméLe frittage à l'air à 1150° d'un mélange de composition molaire 0·99SnO20·01CuO conduit à une densification rapide résultant de l'apparition d'un liquide riche en cuivre à T > 1092°C.Au début du frittage un réarrangement des grains se produit. Les phénomènes observés ensuite sont paramètrés en utilisant le modéle de frittage en phase liquide. Par ailleurs, pendant le traitement thermique le cuivre se dissout dans le réseau rutile de SnO2. Le comportement électrique suggère l'insertion du cuivre en site interstitiel.

The equilibria between (Cs1−x)3Sb and alkali vapours have been studied experimentally at 200 °C. The phases synthesized by gas-solid reaction have been compared to those obtained by solid state reaction. The results have been interpreted thermodynamically. They can provide guidance for the manufacture of photoemissive layers.

Electrical properties of grain boundaries in α Fe2O3. α Fe2O3 ceramics processed from calcined iron ll oxihydroxides exhibit non-linear properties from 20 up to 250°C. Such a behaviour is likely to originate from capacitance effects between adjacent grains. The height of the potential barriers located at the grain boundaries depends on the n or p character of the predominant carriers within the grains. In any case however the intergranular charge transfer is driven by a Poole-Frenkel type emission. According to our study, the potential barriers would build up after preferential annealing of structural defect in the outer core of the grains, which would occur during the sintering process. Also the donor states concentration would depend on the amount of defects in the grains boundaries whereas the acceptor states concentration would be governed by oxido-reduction phenomena.

Fine powders of α-Fe2O3 were obtained from precipitation of Fe3+ ions in aqueous solution, drying and calcination. After characterization of the intermediate and final products, it appeared that their properties depend strongly on the ageing of the initial precipitate in the solution.For example, all the powders obtained from fresh precipitates are constituted of agglomerated fine particles, whereas those obtained after ageing of the precipitates show large particle sizes, little agglomeration and microstrains. On sintering, the powders obtained from aged precipitates are those with the highest ability to densify.We believe that these differences in morphology and properties originate in the fixation of variable amounts of crystallization water on the hydrous oxides, followed by the formation of micropores, and a subsequent increase in the specific area, when this water is released upon calcination.

The electrical properties of α-Fe2O3 ceramics depend strongly upon the nature of the precursor and on the sintering conditions. The ceramics obtained from fresh precursors are characterized by the presence of extended defects, which tend to order as the sintering goes on. The p-type conductivity of those ceramics is mainly due to the presence of cationic impurities. The appearance of localized defects occurs during the sintering period of aged precursors. As these defects are associated to donor states, a predominant n-type conductivity is observed in the corresponding ceramics.