Thermal Conversion Process of Tetraethyl Orthosilicate-Based Silica Sol and Perhydropolysilazane into Inorganic Silica Films

As the source for inorganic silica films in the solution process, perhydropolysilazane (PHPS) and silica sol derived from sol–gel chemistry have been widely adopted. During the conversion from solution precursors to inorganic silica, high-temperature treatment is crucial for the inorganic material’s final composition and densification. However, the conversion processes of PHPS and silica sol into inorganic silica differ significantly, which has not been thoroughly investigated. Therefore, this study systematically examines the thermal conversion process of PHPS and tetraethyl orthosilicate (TEOS)-derived silica sols. Results reveal that the conversion of PHPS into inorganic silica involves hydrolysis, oxidation, and condensation reactions of Si–H, Si–N, and N–H bonds. In contrast, TEOS hydrolysis produces silica sol with −OH groups, which undergoes dehydration condensation and sintering during high-temperature thermal treatment. After heat treatment at 500 °C, the chemical structure of TEOS-derived films converts to resemble inorganic amorphous silica, while complete conversion of PHPS-derived films requires 600 °C, with densification processes occurring for both films as the temperature rises to 800 °C. Moreover, these distinct conversion processes may lead to differences in stress generation, resulting in higher density, mechanical strength, and critical cracking thickness for PHPS-derived compared to TEOS-derived films. These findings provide guidelines for the application of two kinds of solution precursor for inorganic silica films.