A sustainable aqueous-based route is reported for the synthesis of low density mesoporous silica/chitosan nanocomposite aerogels by cogelation of a chitosan biopolymer dissolved in silicic acid. The random “cluster–cluster” aggregate silica structure intertwined at the molecular level with chitosan yields a three-dimensional semi-interpenetrating network with greatly improved mechanical properties when compared to a silica aerogel of similar density. The physical properties of the resulting aerogels depend significantly on the gelation pH. A silica aerogel reference material synthesized at a low pH of 3 features very low density and high porosity resulting in a highly elastic behavior but comparatively weak skeletal structure (final strength <1 MPa). By compounding the acid catalyzed gel with a chitosan coprecursor, an inorganic–organic nanocomposite aerogel is formed that retains high mechanical flexibility (strain at rupture >80%) but with greatly increased yield strength (>7 MPa). Importantly, the reinforcement does not significantly increase density or thermal conductivity. The volume fraction of the biopolymer coprecursor, which has abundant amino and hydroxyl functional groups, can be adjusted to tune the bulk properties of the composite aerogel, enabling the design of nanoscale inorganic/organic biocomposite materials for a wide range of thermal insulation, sorption, catalysis, and other applications where structural integrity is indispensable.

With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol-gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump applications. The engineering of such aerogel membranes, which implies pore structure control and chemical surface modification, requires both chemical processing know-how and a detailed understanding of the influence of the material properties on the spatial flow rate density. Optimal pumping performance was found for devices with integrated membranes with a density of 0.062 g cm(-3) and an average pore size of 142.0 nm. Benchmarking of such low-density hydrophobic active aerogel membranes gave an air flow rate density of 3.85 sccm·cm(-2) at an operating temperature of 400 °C. Such a silica aerogel membrane based system has shown more than 50% higher pumping performance when compared to conventional transpiration pump membrane materials as well as the ability to withstand higher operating temperatures (up to 440 °C). This study highlights new perspectives for the development of miniaturized thermal transpiration air pumps while offering insights into the fundamentals of molecular pumping in three-dimensional open-mesoporous materials.

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