دانلود رایگان مقاله آئروژل سلولز اسپری با خواص عایقسازی بهینه حرارتی

abstract

Nanofibrillated cellulose (NFC) aerogels were prepared by spray freeze-drying (SFD). Their structural, mechanical and thermal insulation properties were compared to those of NFC aerogels prepared by conventional freeze-drying (CFD). The purpose of this investigation is to develop superinsulating bioaerogels by reducing their pore size. Severe reduction of the aerogel pore size and skeleton architecture were observed by SEM, aerogels prepared by SFD method show a fibril skeleton morphology, which defines a mesoporous structure. BET analyses confirm the appearance of a new organization structure with pores of nanometric sizes. As a consequence, the thermal insulation properties were significantly improved for SFD materials compared to CFD aerogel, reaching values of thermal conductivity as low as 0.018W/(m K). Moreover, NFC aerogels have a thermal conductivity below that of air in ambient conditions, making them one of the best cellulose based thermal superinsulating material.

4. Conclusions

In this article, bioaerogels were prepared from NFC suspensions by osmotic concentration followed by drying step done by CFD and SFD methods. Monolithic and homogeneous aerogels were successfully obtained. All bioaerogels have a very low density (0.012–0.033 g/cm3) and a high porosity (98–99%). SEM characterization highlighted microstructure differences between both aerogels. When bioaerogels prepared by CFD exhibit a 2D-sheetlike morphology with macropores, bioaerogels made by SFD yield a 3D-fibril nanostructured skeleton morphology with pore size going from few tens of nanometers to few microns. Specific surface area of SFD aerogels was determined using nitrogen adsorptiondesorption and the BET method. A value around 100 m2/g was obtained. The average pore size of the SFD aerogels was estimated from the nitrogen desorption isotherm according to the BJH method. Results confirm the presence of mesoporous domains as it was found in SEM images. The mechanical properties of bioaerogels were measured by uniaxial compression. CFD aerogel structure offers superior resistance to compression than SFD structure allowing achieving higher compressive modulus. The most important result obtained in this study is that SFD aerogels have very low thermal conductivity. The best value obtained was 0.018W/(m K). These are the first bioaerogels, prepared by another technique than supercritical drying, having thermal superinsulating properties. Therefore, the present study provides a new and effective method to prepare NFC aerogels with thermal superinsulating properties.