Thick Transparent Nanoparticle-Based Mesoporous Silica Monolithic Slabs for Thermally Insulating Window Materials

Abstract

This paper presents a novel template-free water-based sol–gel method to synthesize thick transparent and thermally insulating mesoporous silica monolithic slabs by gelation and drying of a colloidal suspension of silica nanoparticles under ambient conditions. For the first time, mesoporous silica slabs were synthesized on perfluorocarbon liquid substrates to reduce adhesion and enable the gels to shrink freely during aging and drying without incurring significant stress that could cause fracture. The free-standing nanoparticle-based mesoporous silica slabs were disks or squares, with thickness between 1 and 6 mm and porosity around 50%. The slabs had high transmittance and low haze in the visible spectrum due to small nanoparticles (6–12 nm) and pore size (<10 nm), narrow pore size distribution, and optically smooth surfaces (roughness <15 nm). The slabs’ effective thermal conductivity of 104–160 mW m–1 K–1 at room temperature was smaller than that of other mesoporous silicas with similar or even larger porosity reported in the literature. This was attributed to the slabs fractal structure and high mass fractal dimension. The mechanical properties were similar to those of common polymers. The simple synthesis is readily scalable and offers promising materials for window solutions and solar–thermal energy conversion, for example.

Engineering mesoporous silica for superior optical and thermal properties

“…7,30 Nanoparticle synthesis approaches seem to produce transparent monoliths with controlled pore size (<5 nm), but with porosities of ∼50%, low thermal conductivity is not expected. 31 Recently, our group reported the synthesis of mesoporous silica monoliths, thicker than 1 mm, from an aqueous suspension of silica nanoparticles. 31 These materials had excellent optical transmission (>90%) across the visible spectrum.…”

“…31 Recently, our group reported the synthesis of mesoporous silica monoliths, thicker than 1 mm, from an aqueous suspension of silica nanoparticles. 31 These materials had excellent optical transmission (>90%) across the visible spectrum. However, their porosity of ∼50% resulted in a thermal conductivity above 0.10 W/(m K).…”

“…(2) and 3, increasing porosity is not the only method in which to reduce the effective thermal conductivity of a porous solid. 31 The variations in porosity and pore size of the ambiently dried samples can lead to variations in branching and tortuosity of the silica backbone and, therefore, affect the ambigel fractal architecture. In order to gain insight into this effect, the effective thermal conductivity of the ambigel samples synthesized was measured either by the time-domain thermoreflectance (TDTR) method at room temperature under vacuum or by the guarded hot place method at room temperature in ambient air.…”

Artificial neural network-based smart aerogel glazing in low-energy buildings: A state-of-the-art review

“…Depending on the application components in buildings, aerogels are mainly applied in windows and envelopes. In terms of window, a nanoparticle-based mesoporous silica monolithic slab was produced for thermally insulated windows, with thermal conductivity of 0.104–0.160 W/(m·K) ( Marszewski et al., 2019 ). With respect to the building envelope, foam concrete reinforced silica aerogel was applied in building envelopes with the thermal conductivity at 0.049 W/(m·K) ( Liu et al., 2018 ).…”

Porous Silica Nanotube Thin Films as Thermally Insulating Barrier Coatings

“…Aerogels are lightweight, synthetic materials that can be fabricated from a variety of chemical compounds [including silica (SiO 2 )], but they generally lack structural integrity and do not have a clear path for MEMS integration. Recent explorations into using nanostructures to create spectrally selective surfaces for radiative cooling indicate promise in tailoring thermal and structural properties. , Silica-based, porous materials constructed from nanoparticles have also appeared in recent literature, demonstrating low thermal conductivities of 0.10–0.16 W m –1 K –1 using sol–gel methods to obtain free-standing silica monoliths …”