A process combining the pyrolysis of a lignocellulosic structure and reactive gas treatments has been developed to prepare porous TiC-SiC ceramics for solar receivers. The natural micro-porosity of balsa was complemented by a high open macro-porosity by laser cutting a periodical arrangement of parallel channels. The lignocellulosic structure was first pyrolysed into carbon. This reactive carbon material was then converted into TiC by Reactive Chemical Vapor Deposition (RCVD) using TiCl4/H2. After controlling the absence of cracks due to volume changes, the TiC structure was finally infiltrated by the Chemical Vapor Infiltration (CVI) of SiC using CH3SiCl3/H2. The density, porous structure, elemental and phase compositions, oxidation behavior and crushing strength were assessed after pyrolysis, RCVD and CVI. The SiC CVI coating significantly improves the compressive strength, the oxidation resistance and the thermal properties. The SiC layer is no longer fully protective at high temperature but the mechanical properties remain reasonably high.

International audienceThe nitridation enhancement of a TiSi2 powder by nickel addition was examined. The quantities of the added metal were comprised between 2.5 and 25mol.%, the balance being TiSi2. The isothermal heat treatments were made at 1100°C for durations up to 40 hours under normal pressure and continuous flow of pure nitrogen. Comparing to pure TiSi2 powder, an improvement of the conversion in TiN and Si3N4 is obtained for compositions containing 10.0; 12.5 and 15.0 mol.% of nickel. According to the quaternary Ti-Si-Ni-N phase diagram, nickel was found to form Ni4Ti4Si7 compound. Thermogravimetric analysis of these three compositions showed the existence of three successive stages during the conversion. First, an initiate reaction occurred with a very weak weight gain. Then, the second stage exhibited an acceleration of the weight gain. Those two stages are both controlled by nucleation and growth, as represented by the kinetic equation [-ln(1-)] 1/2 = k.t. Finally, the reaction is limited by three-dimension diffusion, as represented by the kinetic equation: [1-(1-) 1/3 ] 2 = k.t. From this behavior, an activation mechanism is proposed