Influence of the size of cellulose aerogel beads on thermal conductivity and mechanical deformation behavior

Abstract

This study performs the effect of cellulose bead size on the thermal conductivity and mechanical deformation behavior of aerogel materials. For this purpose, the cellulose aerogel beads were synthesized through a sol-gel process where a 7 wt% cellulose powder was dissolved in 7 wt% aqueous sodium hydroxide- urea solution, with 12 wt% urea added to the mixture. The cellulose solution was dropped in coagulation bath of 2M acetic acid using four different nozzle diameters. The beads subsequently subjected to solvent exchange was carried out with ethanol, followed by supercritical drying technique. The resulted cellulose beads have sizes between 2.1 mm to 3.3 mm. The spherical shape of the beads was chosen as a standardized form for all subsequent analysis. Various analytical techniques were employed including BET specific surface area, BJH pore volume, compressive test, thermal conductivity measurements. A two different scales were synthesized to ensure that a sufficient quantity of beads would be available for thermal conductivity measurement. Microstructure analysis of the cellulose beads was performed by N2 adsorption-desorption measurements, revealed BET specific surface area in range of 359 to 389 m²/g across the different sizes of cellulose beads. SEM imaging technique examined both the surface topography and internal structure of beads. It was found that a common finding included a randomly arranged 3D fibrillar network pore size range between meso- and macropores. It was found that thermal conductivity is not directly related to bead size, but is affected by other factors including the tapped porosity, BET surface area and BJH pore volume. The mechanical behavior was investigated under different compressive rates, showing elastic behavior up to ε 2 %, after which the pores began to deform. Around ε 40%, the pores are collapsed and compacted, resulting in a moderate loss of pore volume. At ε 80%, a reduction in physical properties was observed, including BET specific surface area, BJH pore volume, and the material retained an open porous structure with mesopores.