Effect of Zr on the Microstructure and High-Temperature Phase Separation Evolution of SiOC Aerogels

SiZrOC aerogels were synthesized through the pyrolysis of the zirconium source-doped SiOC system using zirconyl chloride octahydrate (ZrOCl₂·8H₂O) at temperatures ranging from 900 to 1300 °C. This study investigates the microstructure evolution and phase separation of SiOC and SiZrOC aerogels during the pyrolysis process. Upon pyrolysis, both aerogels exhibited a Si-O-C structure with a high thermal stability. The introduction of zirconium elements significantly enhanced the pore volume (3.20 cm³/g) and porosity (96.0%) and reduced the thermal conductivity (0.023 W·m^-1·K^-1) of the organic-inorganic precursor aerogel. Moreover, the three-dimensional pore structure was retained even under high-temperature pyrolysis conditions. SiZrOC-1100 displayed a high specific surface area of 273.52 m²/g, a high pore volume of 1.70 cm³/g, and a low thermal conductivity of 0.033 W·m^-1·K^-1. At high temperatures, the SiZrOC phase transformation produces tetragonal ZrO₂, which inhibits the graphitization process of free carbon and the growth of SiC grains. Furthermore, the phase separation process of the SiOC matrix structure generated oxygen-rich SiOC_4- units, while carbon-rich SiOC_4- units were negligible below a pyrolysis temperature of 1200 °C. Between 900 and 1200 °C, SiZrOC is composed of amorphous SiOC, amorphous ZrO₂, microcrystalline t-ZrO₂, and free carbon phase. These findings provide valuable insights into the preparation of high-performance SiOC aerogels.