Abstract:

Reverse microemulsions consisting of aqueous cerium nitrate solution as the internal phase and polycarbosilane dissolved in heptane as the continuous phase were used as a precursor for the controlled synthesis of dispersed cerium oxide particles inside a porous SiC matrix. According to dynamic light scattering experiments, the effective diameter of the cerium hydroxide particles obtained after ammonia addition is effectively controlled in a range of 2–10 nm with the molar water/surfactant ratio (R_w = 6–16). Pyrolysis at 1200 to 1500 °C produces materials with specific surface areas up to 240 m² g⁻¹. Whereas crystallization of the matrix is achieved only at higher temperature, cerium oxide particles form agglomerates composed of smaller nanoparticles that tend to dissolve into the ceramic matrix at 1500 °C leaving macropores behind. The high specific surface area is attributed to the presence of mesopores with a broad size distribution. Excess carbon present after pyrolysis is removed by oxidation at 900 °C causing a significant decrease of the surface area for high surface area materials, whereas intermediate surface area materials (50–100 m² g⁻¹) show a high textural stability.