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The bioactivity of 34 invert or conventional glasses in the P-free and P-containing Si-Ca-Na system was evaluated by examining the in vitro Hydroxy Carbonate Apatite (HCA) formation time. The silica content in the glasses varies between 39.48 and 55 mol %. The surfaces of glasses after soaking in simulated body fluid at 37 degrees C were analyzed by Fourier transform infraRed spectroscopy and scanning electron microscopy coupled with energy dispersive spectroscopy. To understand the HCA formation mechanism, the modification of the glass surface structure was studied. The aim was to understand, in the Si-Ca-Na-P system, the function of each constituent on glass reactivity. It was noted that the bioactivity of Si-Ca-Na-P glasses depends on the P ratio. One of the P-free glasses is as bioactive as the Hench's Bioglass noted 45S5, because it needs 12 h to develop HCA as for Hench's one. In the quaternary system, some of the P-rich and Na, Si-poor glasses are more bioactive than 45S5 (6-10 h to obtain HCA). Two phosphorus effects were observed: the repolymerization of the silicate network and the formation of phosphate-modifier cation complexes.

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Nanometer-sized BaTiO 3 powders have been synthesized hydrothermally from Ba(OH) 2 and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH) 2 prevents full incorporation of barium, and bariumdeficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K s (BaTiO 3 ,-25°C) = 7 × 10 −8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO 3 are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Roomtemperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.