Hollow silica nanospheres as thermal insulation materials for construction: Impact of their morphologies as a function of synthesis pathways and starting materials

Abstract

Hollow silica nanospheres (HSNS) show a promising potential to become good thermal insulators with low thermal conductivity values for construction purposes. The thermal conductivity of HSNSs is dependent on their structural features such as sizes (inner diameter and shell thickness) and shell structures (porous or dense), which are affected by the synthetic methods and procedures including reaction medium, polystyrene template, and silica precursor. . Formation of thermally insulating HSNS was in general favoured by alkaline reaction, whereby highly porous silica shells were formed, promoting less silica per volume of material, thus a lower solid state thermal conductivity. The Knudsen effect is in general reducing the gas thermal conductivity including the gas and pore wall interaction for materials with pore diameters in the nanometer range, which is also valid for our HSNS reported here.Further decreasing the pore sizes would invoke a higher impact from the Knudsen effect. The additional insulating effect of the inter-silica voids (median diameter D50 ≈ 15 nm) within the shell coating contributed also to the insulating properties of HSNS. The synthesis route with tetraethyl orthosilicate 2 (TEOS) was more robust and produced more porous silica shells than the one with water glass (Na2SiO3, WG), although the latter might represent a greener synthetic method.

TEOS and Na2SiO3 as silica sources: study of synthesis and characterization of hollow silica nanospheres as nano thermal insulation materials

“…HSN are currently mainly prepared by hard template method (e.g. polystyrene, Polymethylmethacrylate, metal nanoparticles, and C spheres) (Schneider et al 2017;Ng et al 2018;Xu et al 2012) and soft template method (e.g. emulsion droplets, micelles, vesicles) (Tao et al 2016;Li et al 2018), and have been recognized as an excellent alternative to silica aerogels because of its simple preparation techniques (Liu et al 2016).…”

“…HSN has been successfully prepared with particle size of ~ 150 nm and shell thickness of 10-15 nm (Gao et al 2013). Thermally insulating materials of successfully synthesized HSN have low thermal conductivity value (20-141 mW m −1 k −1 ) (Ng et al 2018;Ruckdeschel et al 2015Ruckdeschel et al , 2017. However, the ethanol consumption in the coating process accounts for the majority of the total emissions, and TEOS as an organic silica source, exhibits hazardous and relatively high carbon footprint.…”

“…Yoo et al 2014 converted directly Na 2 SiO 3 hollow microspheres into hollow SiO 2 microspheres with a mean diameter of 10-50 μm. Ng et al 2018 proposed a new method to prepare HSN using water glass as a silica source to achieve a greener, more sustainable and more cost-benefit synthetic route.…”

Polystyrene nanofibers for nonwoven porous building insulation materials

“…Highly porous materials are currently of great interest in thermal insulation applications . According to the study of Spagnol et al, the main contribution to the thermal insulation efficiency of nanomaterials is gas conduction (62%), contrary to solid conduction (24%), and radiation (14%).…”

“…Highly porous materials are currently of great interest in thermal insulation applications. [19][20][21][22] According to the study of Spagnol et al, 23 the main contribution to the thermal insulation efficiency of nanomaterials is gas conduction (62%), contrary to solid conduction (24%), and radiation (14%). For heat insulation in air, pores of diameters smaller than the mean free path of the gas molecules lead to the Knudsen effect, 24 which further reduces the thermal conductivity due to pores of very small size.…”