Paul Poix

Synthesis of glass “precursor” BaO,TiO2,nH2O by gel processing

Abstract Sol-gel methods are currently applied for binary systems such as BaOTiO 2 using the alkoxide hydrolysis method. The control of hydrolysis kinetics with the use of weak concentration and low temperature leads to monolithic gels. The drying process stage leads to a gel-glass ceramic displaying small egg-shaped crystals of BaTiO 3 in a glass matrix at 725°C. The TGA, DTA and dilatometric curves of this glass are given with SEM studies.

Structure et proprietes magnetiques de la perovskite Ba2TaCoO6

Abstract A new oxide Ba 2 TaCoO 6 has been prepared. The crystallographic properties are determined, this compound is of the perovskite type, its space group is Pm3m and the parameter a = 4.077 A°. The variation of the magnetic susceptibility suggests the appearance of cobalt in the low spin state (Co III ) between 4 and 125 K.

Crystallographic study of BaTiO3-BaM1/3N2/3O3 solid solutions (M=Co or Mg and N=Nb or Ta)

In order to understand the effect of niobium, tantalum, cobalt and magnesium cations on the ferroelectric properties of barium titanate we study solid solutions between BaTiO3 and BaM1/3N2/3O3 perovskite compounds where M=Co2+ or Mg2+ and N=Nb5+ or Ta5+. These solid solutions become cubic at room temperature for very weak additions. This result cannot be explained by geometrical consideration and suggests that an intermediate solid solution is made for weak additions.

Preparations and characterization of lanthana catalysts: study of their activity in CO/H2 reactions

Two original methods of preparation of lanthana with hexagonal structure as well as the stabilization of its cubic structure by addition of yttria were investigated. The physical properties and the catalytic behaviour of these samples are compared with classical preparations. The influence of the preparation techniques and of the pretreatment on the specific area, morphology and chemisorption of probe molecules were tested, the latter using mainly Fourier transform infrared spectroscopy. Catalytic tests, operated at high CO-H2 pressure and high temperature, led to the classical isosynthesis product distribution. The prepared lanthana samples are active in the hydrogenation of carbonate to formate species which can be involved in the formation of formaldehyde in the first step of the isosynthesis reaction. The evolution of the oxide catalysts to hydrates and carbonates during the reaction was studied.

Dielectric characterization of BaTiO3-BaM1/3N2/3 O3 systems (with M=Co, Mg and N=Nb or Ta)

The dielectric behaviour of (1−x)BaTiO3−x BaM1/3N2/3O3 systems, in which M=Co, Mg and N=Nb, Ta is described whenx varies from 0 to 0.1. The evolution of the Curie temperature and of the dielectric constant, either in the ferroelectric state or in the paraelectric state are correlated to the structure and the ceramic microstructure. An observed diffuse phase transition is discussed.

Oxidative Coupling of Methane Over LnLiO2 Catalysts. (Ln = Sm, Nd, La). Promoting Effect of MgO and CaO.

The catalytic activity in methane coupling oxidation on definite LnLiO2 (Ln = Sm, Nd, La) compounds where the alkali is enclosed in the crystal lattice is reported. LnLiO2 structures have the advantage to be a host structure for promoting cations (Mg, Ca). The doped systems provide significant increases in activity and selectivity towards C2 hydrocarbons.

Preparation of perovskite type catalysts containing cobalt for post combustion reactions

Abstract To evaluate differences in the reactivity of a series of cobalt containingperovskite type catalysts for the CO oxidation reaction, a “single path” preparation route is desirable. Two such routes were tested using three precursors of BaCoO 3 , LaCoO 3 and Ba 2 CoWO 6 : namely co-precipitation and sol-gel methods.

Methane oxidative coupling by definite compounds (e.g. perovskite, cubic or monoclinic structure,…) obtained by low temperature processes

Abstract We investigate the catalytic properties of several rare earth definite compounds (e.g. perovskite, cubic or monoclinic structure,…). We notice that the nature of the rare earth oxygen environment is in relation with the selectivity. Coulombian energy computations show clearly these relationships.

Sintering of Nd2O3 and Ceramic Stability to Hydration

Study of the sintering of Nd2O3 made possible to determine the best conditions for its high densification. Unfortunately, the compacts show a very poor resistance to humidity. This is mainly due to the A hexagonal structure. Stabilization against hydration was attempted by coating the Nd2O3 grains, to avoid the existence of an interface oxide-OH-. Sintering of samples doped with TiO2 leads to the hexagonal oxide which appears as Nd2O3 particles surrounded by an intergranular phase. The later was identified as Nd2TiO5 by use of microprobe analysis. The resistance to hydration of A-Nd2O3 was then four times improved.