Environmentally benign three-dimensional aerogels for air filtration and the mechanisms

• Main Author: Ma, Daphne Xiu Yun
• Other Authors: Lu Xuehong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/146137
• Institution: Nanyang Technological University

In this PhD study, three-dimensional (3D), robust and highly-porous aerogels were employed as air filters because their large internal volume could promote higher removal efficiency of particulate matter (PM), especially for ultrafine particles, while their ultrahigh porosity could facilitate relatively low pressure drops (Ps). This synergistic behavior of aerogel filters could effectively address the efficiency- P trade-off of conventional fibrous filters. The aerogels can be categorized as fibrous and non-fibrous, and they can be fabricated via two separate different methods: (1) freeze-drying in a mold or (2) ice-crystals-induced templating method. Apart from the intrinsic structures of the starting materials, the fabrication method adopted would determine the final microstructure of the aerogels. In short, the aerogels prepared in this work would possess either isotropic or aligned porous architectures. Nevertheless, a potential drawback of highly porous 3D aerogels is their relatively poor structural stabilities, which hinders their practical applications as air filters due to the constant compressive force exerted by the airflow in the filtration process. This problem may be overcome by employing chemically- or physically-induced cross-linking to reinforce the aerogel structure. Hence, the structures and morphologies of the aerogels would greatly affect the filtration performance of these aerogel filters. Thus, the aim of this thesis is to prepare highly robust 3D aerogel air filters and investigate the effects of the microstructures on the filtration performance as well as the respective underlying filtration mechanisms of the different as-prepared aerogels. Three aerogel systems were adopted in this thesis study; (1) PLLA/PDLA blend stereocomplex nanofibers, (2) biomass-based lignin with graphene as reinforcement agent (3) lignin reinforcement agent and cellulose nanofibers. We report that highlyrobust aerogels, both fibrous and non-fibrous, with either isotropic or preferably aligned pores are successfully fabricated via different facile cross-linking approaches; spontaneous inter-fiber stereocomplexation-induced cross-linking for Abstract ii the PLLA/PDLA blend system while thermal treatment was adopted for the other remaining systems. For each system, it is demonstrated that the extent of crosslinking can be easily controlled by adjusting the microstructures and macroscopic shapes of the aerogels, which could influence the filtration performance of the various aerogels. Furthermore, as compared to conventional compact fibrous filters, all the as-prepared aerogel filters demonstrated higher overall filtration efficiency accompanied with a significantly lower ΔP. Through this work, it is believed that the understandings derived can serve as a guideline to facilely fabricate highly-robust and efficient filters for various air filtration applications.