The application of superinsulation materials (SIM) reaching thermal conductivities far below 20 mW/(mK) allows the construction of relatively thin building envelopes while still maintaining a high thermal resistance, which also increases the architectural design possibilities for both new buildings and refurbishment of existing ones. To accomplish such a task without applying vacuum solutions and their inherit weaknesses may be possible from theoretical principles by utilizing the Knudsen effect for reduced thermal gas conductance in nanopores.
This study presents the attempts to develop nano insulation materials (NIM) through the synthesis of hollow silica nanospheres (HSNS), indicating that HSNS may represent a promising candidate or stepping-stone for achieving SIM. Furthermore, initial experiments with aerogel-incorporated concrete and the conceptual work concerning NanoCon are presented.
Electrochromic materials (ECM) and windows (ECW) are able to regulate the solar radiation throughput by application of an external electrical voltage. Thus, ECWs may decrease heating, cooling, lighting and electricity loads in buildings by admitting the optimum level of solar energy and daylight at any given time, e.g. cold winter climate versus warm summer climate demands. It is crucial to be able to compare the dynamic solar radiation control for different ECWs and hence require specific ECW properties. The solar radiation control for ECWs may readily be characterized by several solar radiation glazing factors, where a comparison for various ECW configurations enables one to select the most appropriate ones for specific smart window applications in energy-efficient buildings. As an example a particular ECW based on the ECMs polyaniline, prussian blue and tungsten oxide is presented, being able to regulate as much as 60 % of the visible and
59 % of the total solar radiation.
Sodium tungstate (Na-WO3) nanorods with typical diameters of 10-200 nm and lengths of several microns were prepared via hydrothermal synthesis. X-ray diffraction showed that the material crystallized in a hexagonal phase (space group
P6/mmm) with unit cell dimensions of a = 7.3166(8) Å and c = 3.8990(8) Å. The as-prepared Na-WO3 nanorods showed a distinctive visible-light-driven photochromism related to a proton-electron double injection process. The involved local
structural evolutions were monitored by Fourier transform infrared (FTIR) and Raman scattering spectroscopy. One diagnostic FTIR absorption at 585 cm-1 and one Raman band at 813 cm-1 were identified and assigned to the O-W-O stretching vibration. These two modes were strongly affected by the proton and electron insertion, showing promises for studying the chromogenic properties of hexagonal WO3 materials.
The potential of silver (Ag) nanoparticles as low emissivity (low-e) coating materials for window glazing applications has been discussed. Ag nanoparticles were prepared via a wet chemical method and applied on the surface of flat glass through spin coating. A mild heat treatment at 200°C was employed to achieve the low-e effect, which results in a total surface emissivity of about 0.015, compared to about 0.837 of the plain glass substrate. By applying such low-e coatings, the heat loss through a single-glazed window pane could be reduced by about 45% (U-value from 5.75 to 3.18 W/(m2K)).
The path toward energy-efficient buildings with a low or zero carbon footprint, e.g. zero energy and zero emission buildings, involves the development of high-performance thermal insulation, aiming at reaching thermal conductivities far below 20 mW/(mK). Applying such superinsulation will allow the construction of relatively thin building envelopes yet maintaining a high thermal resistance, thus also increasing the architectural design possibilities. A vacuum insulation panel (VIP) represents a stateof-the-art thermal insulation solution with a thermal conductivity of typical 4 mW/(mK) in the pristine and non-aged condition. However, the VIPs have issues with fragility, perforation vulnerability, increasing thermal conductivity during time and lack of building site adaption by cutting as four cardinal weaknesses, in addition to heat bridge effects and relatively high costs. Therefore, the VIPs of today do not represent a robust solution. Hence, our aim is from theoretical principles, utilizing the Knudsen effect for reduced thermal gas conductance in nanopores, to develop experimentally a high-performance nano insulation material (NIM). This work presents the current status of the development of NIM as hollow silica nanospheres (HSNS) in our laboratories, from the experimental synthesis to the material characterization by e.g. thermal conductivity measurements. One attempted approach for tailor-making HSNS is the sacrificial template method and optimization of the sphere diameter and shell thickness with respect to low thermal conductivity. The results so far indicate that HSNS represent a promising candidate for achieving the high-performance thermal superinsulation for application in the buildings of tomorrow.
This chapter reports an approach to enhance the mechanical strength of silica aerogels via densification. Although the loss of porosity and consequently the increase of thermal conductivity of silica aerogels represent drawbacks related to the densification process, a combination of enhanced mechanical performance and optical transparency indicates that the densificated silica aerogels may be used as new glass material for window glazing application. Preliminary experimental results indicate lightweight (density 1.8 g/cm3, compared to 2.5 g/cm3 for float glass) and thermal insulating (thermal conductivity k ≈ 0.18 W/(mK), compared to about 0.92 W/(mK) for float glass) aerogel glass materials with high visible transparency (Tvis ≈ 95.4% at 500 nm, compared to 92.0% for float glass) can be achieved by annealing an acid-catalyzed silica aerogel precursor at 700 °C. Typical elastic modulus Er of the obtained aerogel glass materials is about 6.42 GPa, which can be further enhanced by, e.g., increasing the annealing temperatures.
Silica aerogels are a nanoporous material with extremely high porosity (up to ~99.8 %), low density (as low as ~0.005 g/cm3), and low thermal conductivity (~0.010–0.020 W/(mK)). Aerogels can also be made with a translucent or transparent state. These structural and functional features make aerogels a multifunctional material for many important applications. In this work, we discuss the perspective of aerogels as super insulation materials and window glazings in the building and construction sector. It shows that different research and development (R&D) strategies of aerogels shall be considered when aiming for different applications; reducing the manufacture cost, improving the service durability, and minimizing the environmental impacts of aerogels are important factors to be addressed. We show also the R&D potentials of developing aerogel-like materials with improved structural or functional performance for building related applications.