Feasibility of Designing Aerogel Process for Targeted Thermal Conductivity by Controlling Porosity

For decades, there have been global issues of environmental pollution and energy availability. Aerogels have been steadily suggested as alternative thermal insulation materials in various applications to aid in solutions to these issues. This effort has resulted in some aerogel products being commercialized and available on the market. The size of the aerogel market in the thermal insulation industry has been steadily increasing over time. This market for thermal insulation also continues to find new products and upgrade the aerogel products to satisfy the needs from both the suppliers and customers. In order for aerogel insulation to stay competitive, development of an aerogel production process that has high feasibility for controlling thermal conductivity is undoubtedly important. Successful development of the aerogel production process should also enable simple integration of the developed process with a conventional material process for hybrid aerogel products. In addition, the new process should contribute to reducing the production cost.
A good understanding of how thermal conduction occurs in aerogel is crucial to guide development of a new aerogel production process. In this research, the sensitivity of thermal conductivity in aerogel materials to the porosity is investigated. This investigation shows that higher porosity results in lower thermal conductivity for aerogels with porosities of 80% or greater. It also demonstrates that the production process of aerogel insulation should be continuously optimized even when the result of optimization only reduced thermal conductivity of the product by less than 1.0 mW/m·K.
Motivated by the investigation results, a new sol-gel process, called the direct-contact method (DCM, US Provisional Patent Application Serial No. 62/486,057 filed April 17, 2017) is developed and introduced in this research. This new process replaces the conventional pre-hydrolysis step in the two step sol-gel process. By employing this method, silica alkoxide is pre-hydrolyzed by a special combination of catalysts without solvent and heating. As a result, clear and pre-hydrolyzed silica sol is produced and directly available for further gelation steps within a few hours at ambient temperature. The DCM also yields a sol with higher concentration of silica than that obtained from a conventional method, so the density range of the final gel product is extended by approximately 3 times. This advantage increases feasibility of the entire sol-gel process with regards to combining with any existing material processes. The production costs of silica aerogel insulation can be remarkably reduced as the time and energy consumed by the DCM is much less compared to the conventional two step process. With these advantages, DCM still produces a pre-hydrolyzed sol comparable with a functional silane, which is commercially available at a much higher price than TEOS.
By taking advantage of the DCM, study on how porosity and the porous structure of silica aerogel is affected by the parameters of its production process is pursued. This study demonstrates successful integration of the DCM with the two step sol-gel route. The DCM enables simplification of the entire sol-gel process and still produces monolithic silica aerogels with over 90 % reproducibility in terms of gelation time, densities, and main pore size. It also shows high feasibility for tuning the synthetic conditions to obtain a desired target property of the produced silica aerogel including porosity.
In addition, studies on the effects of target density on porosity of yttria aerogels and organic aerogels are pursued and their results are introduced. Both types of aerogel monoliths were produced from modified sol-gel processes and characterized in order to outline how target density is related to apparent density and skeletal density. The study for yttria aerogel aids in recognizing the importance of controlling skeletal density of the produced yttria aerogel in order to obtain a desired porosity by strategizing the sol-gel process. Through the study of organic aerogel, it is recognized that the porosity has high sensitivity to the apparent density, as it rapidly increased when apparent density was decreased. These results aid in defining the range of target density for future research on organic aerogel in order to understand the porosity change as well as the porosity.
This research finally presents a newly developed experimental method for preparation of aerogel samples for thermal conductivity measurements using the established 3ɷ method. The new method requires installation of a wire set inside a monolithic aerogel so that it can play the role of the 4 connectors needed for the 3ɷ method. These experimental 3ɷ results show that thermal conductivities decreased with increased porosities, thus agreeing well with the discussion and model presented in this research. These results also demonstrate that the wire set implantation can be a reliable method that simplifies sample preparation for 3ɷ measurements in aerogel materials.