Materials for Energy, Efficiency and Sustainability: TechConnect Briefs 2016Materials for Energy, Efficiency and Sustainability TechConnect Briefs 2016

Materials for Green Building Chapter 9

Utilization of Hollow Silica Nanospheres for thermal Insulation Purposes

S.A. Mofid, B.P. Jelle, T. Gao
NTNU, Norway

pp. 259 - 262

Keywords: nano insulation material, hollow silica nanospheres, thermal conductivity, NIM

Insulation materials play critical role in efforts made by governments to meet global, regional and national energy efficient targets. As a result of heavy energy cost and environmental concerns, insulation has once again become a top priority in homes, offices and public buildings to make significant savings. Buildings in general constitute a considerable part of world’s total energy use (~ 40%). Hence the attempts for development of new and smart materials that may be utilised to meet energy efficiency targets in the buildings. The traditional thermal insulation materials, like mineral wool, expanded polystyrene (EPS), extruded polystyrene (XPS) and polyurethane (PUR) foam have high thermal conductivity of around 30-40 mW/(mk) and their application have not been fully desirable for many reasons, e.g., architectural restrictions, safety issues and material consumption and cost. New and improved state-of-art thermal insulation materials or solutions such as vaccum insulation panels (VIPS) and silica aerogels are more favour- bale to use due to their lower thermal conductivity. 4- 7 mW/(mk) for VIPs and 10-20 mW/(mk) for silica aerogels, respectively (Beatens et al. 2011). However both VIP and silica aerogels as well have various weaknesses like fragility, perforation vulnerability, long-time performance and relatively high cost. We are interested in developing cheap and environmentally friendly high performance thermal insulating materials by employing nanoengineering; nano insulation materials has been studied and demonstrated with hollow silica nanospheres (HSNSs). HSNS has been synthesised through a coating process using polyacrylic acid (PAA) or polystyrene (PS) as sacrificial templates, where the templates PAA and PS are being removed by a washing or a heating process, respectively (the template materials diffusing and evaporating through the silica shell) (Gao et al. 2013). In conclusion, the removal of the templates results in the formation of silica shells around spherical voids, i.e. HSNS. Thermal conductivity has been measured for various powder samples of HSNS, where the conductivity values are typically in the range of 20 to 70 mW/(mK), but it is important to note that measurements might not be accurate as the uncertainties through Hot Disk apparatus can’t be addressed and has to be further clarified. We want to able to forsee the properties of HSNS that can exhibit much lower thermal conductivity by exploiting the Knudsen effect (Kaganer 1969). Accordingly the thermal conductivity can being attempted lowered by reducing the inner diameter size. Thus we try to find out if we use polystyrene (PS) or polyacrylic acid (PAA) template with smaller inner diameter size, e.g, 50 nm and vary the wall (shell) thickness, can it result in lower thermal conductivity and optimisation of the hollow silica sphere or there can be other major variables that can contribute to thermal conductivity of HSNS. It is also important to address the health, safety and environment impact of HSNS to it’s full extent.