Dimensional and Structural Control of Silica Aerogel Membranes for Miniaturized Motionless Gas Pumps

Abstract

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.

A Novel Environmental Route to Ambient Pressure Dried Thermal Insulating Silica Aerogel via Recycled Coal Gangue

“…A detailed examination of the aerogel materials under TEM (Figure 7) verifies a hierarchical porous structure with a typical pearl necklace structure with most of the primary colloidal particles in a range of 5-8 nm and 1-2 nm interparticle pores, which is in agreement with the reported typical waterglass-based silica aerogel structures [28]. The hierarchical structure of the aerogel materials observed from the microscopic analysis is mainly attributed to the different size of "building blocks" of the gel particles; the TEM shows around 5 nm primary SiO 2 particles; the primary particles were agglomerated to build the secondary clusters and shown as the blackberry structure in the SEM images.…”

“…The BJH pore size analysis is mostly valid for the pore size in the range of ∼ 1 to 150 nm (mesopores and part of macroporous regime); therefore, some of the big pores built by the secondary clusters (shown in SEM, Figure 6) are not detectable. But the pore size and surface area of the coal gangue derived aerogel are comparable to the typical aerogels prepared from TEOS and waterglass [28]; in this case, it could be assumed that the large macropores are the minority species and indicate that the aerogel materials prepared from coal gangue present similar structures as typical silica aerogel [29] and should display identical performance, that is, thermal insulation, to such material.…”

“…The in-house measurement device was calibrated by the standard material (Expanded Polystyrene Board) with known thermal conductivity and measurements were repeated 3 times with the deviation of 3.9%. A slight higher value (typical silica aerogel thermal conductance of 12-18 × 10 −3 W m −1 K −1 ) [28] is probably due to a high bulk density of the gel and loosely packed aerogel granule during measurement [30].…”

“…The N 2 adsorption-desorption isotherms and relevant pore size distribution of the aerogel prepared from CSi-4-1 are shown in Figure 8. The BET measurement could be influenced equilibrium intervals of the gas adsorption [28]. Therefore, in this paper, we tried to repeat the measurements twice with different equilibration times, and the deviations of the main parameters (e.g., surface area, pore size, and pore volume) were around 5%.…”

Effect of aging on silica aerogel properties

“…Silica aerogels are thermal superinsulators with thermal conductivities as low as 12 mW/(m$K), i.e., less than half the value of conventional insulation, because of their small pore sizes in the order of the mean free path of air, limiting the gas phase conduction [3,4]. Aerogels also find use in other applications, such as waste management (gas absorption, radioactive waste confinement), laser experiments, sensors (ultrasonic and gas) [5], nuclear particle detection (Cherenkov), optics (lightguides) [6], Knudsen pump [7], electronic devices, capacitors, high explosive research and catalysts [8].…”