Thermal insulation behavior of functionally graded aerogel: The role of novolac molecular-weight

Abstract In this research, a novel aerogel with a functionally-graded ( FG ) structure as a thermal insulator has been fabricated without interfaces through sol-gel polymerization. The effects of molecular-weight ( M w ) and initial-solvent concentration as decisive parameters in the microstructure of phenol-formaldehyde aerogels are systematically studied. The results show that a broad spectrum of M w created via the precise control of the cross-linking ( CL ) process of the resin. Moreover, the sol-size have been remarkably augmented when increasing the M w . The porousness structure assessment illustrated that the porosity decreases with enhancing the amount of solvent, but increasing M w has no significant effect on the porosity of specimens. In FG-aerogel microstructure, more homogeneous distribution in the sol-size and significant decrement in pore size is observed while the M w and solution concentration are simultaneously increased throughout the structure. Therefore, FG -aerogel structure contains a wide range of porosity, thermal conductivity (0.029 - 0.422 ( W⁄(m.K ))) and pore size (200-2000 nm) that leads to unique structural features of aerogels.