Silica-Resorcinol-Melamine-Formaldehyde Composite Aerogels as High-Performance Thermal Insulators

Here, we report the gelation and supercritical drying of ethanol-based silica-resorcinol-melamine-formaldehyde (RMF) composite aerogels with relative concentrations of initial reagents ranging from neat silica to neat RMF alcogels. The as-prepared materials are subsequently supercritically dried with carbon dioxide. Their properties include a thermal conductivity in the 15-20 mW·m^-1·K^-1 range even with a silica content as low as 20%_wt. The possible reasons behind this interesting insulation performance and the mechanisms leading to the underlying gel structure are discussed in depth. A focus is made on the different gelation modes happening between the RMF and silica phases, from a coating of silica surfaces with RMF species to discontinuous RMF particles within a silica backbone and a continuous RMF backbone with isolated silica particles. The implications in terms of mechanical properties and thermal conductivity are elaborated upon. The initial ratio of silica-RMF species in this ethanol-based synthesis affects the micro- and macrostructure of the composites, resulting in materials with drastically different pore structures and thus an interesting array of possibilities for a new class of silica-organic composite aerogels, based on a sol-gel process.