by Dimitra Papadaki (NKUA)
The growing field of nanotechnology can be found in healthcare, electronics, cosmetics, information technology, and environmental protection, but also in the building sector. It involves the production of materials and structures (nanomaterials) at a scale of 100 nanometres or less.
Some multifunctional materials, for example cementitious or calcareous materials, show enhanced mechanical performance and durability when reinforced with nanomaterials. Examples of enhanced properties are hydrophobicity/ water resistance, self-cleaning, or self-healing, crack control and increased strength. Widely used nanomaterials found in building materials’ matrices include: Titanium dioxide (TiO2), zinc oxide (ZnO), Carbon nanotubes (CNTs), grapheme, nanoclay, Nanosilica (SiO2), Alumina (Al2O3), ZrO2, Carbon nanofibers (CNFs) etc. Some of the main applications of nanomaterials in the building sector include smart windows applications, third generation photovoltaics, insulation, self-healing and photocatalytic materials.
Smart Window Technologies
Vanadium dioxide (VO2) is a material that undergoes a metal to insulator (MIT) transition when heated over the critical temperature of 68°C. VO2 belongs to the family of “photochromic” materials and is considered a promising oxide for smart windows applications. When applied to windows, it allows regulating the internal temperature of the building via the variation of solar IR transmittance, thus contributing to energy efficient buildings and indoor thermal comfort. This coating is referred to as thermochromic.
An example of a thermochromic smart window (Source: https://futurism.com/)
Quantum Dot third generation photovoltaics
Traditional photovoltaic cells are made of silicon crystals. When light hits the crystal, it produces electrons, working as a conductor. These moving electrons are responsible for the creation of the electric current. A nanocrystal therefore is a crystal with dimensions smaller than 100 nm. Its small size is responsible for different behaviour when compared to larger crystals. For example a typical solar cell generates only one electron per photon of incoming sunlight. Silicon nanocrystals can produce two or three electrons per photon of high-energy sunlight. The small size nanocrystals, also called quantum dots, lead to novel quantum-mechanical effects that convert the energy of incoming sunlight, into electrons. Quantum dots theoretically convert more than 40% of the incoming light into electrical power. Therefore nanocrystals have attracted interest in the field of solar photo conversion devices.
The concept behind the use of quantisation effects in semiconductors in order to increase solar photo conversion efficiency is now the basis of these third generation PVs, making them good candidates for energy generation in the building sector.
Quantum dots for solar cells application (Source: http://nanocluster.mit.edu)
Phase Change Materials (PCM)
The high costs and associated environmental impact of fossil fuel consumption for space heating and cooling have raised awareness on thermal energy storage. Focus has been given on building materials that can successfully store sufficient energy during warm periods, while releasing excess heat during periods of lower temperatures. PCMs have large thermal capacity and melting temperatures close to human comfort. The function of a PCM is as follows: during warm periods it undergoes a phase transition, as it can absorb part of the ambient heat through a melting process. During cooler periods this heat is released by a solidification process, resulting in a lower heat flow from outdoors to indoors and vice versa.
Vacuum Insulation panels
Using these novel insulation materials, Vacuum Insulations Panels (VIPs), the thermal inertia of a building increases, while the energy efficiency of the building is improved. VIPs have insulation performance of five to eight times better than conventional insulation. The VIPs enable thin, highly insulating constructions for walls, roofs and floors, making them highly effective and therefore attractive to the construction industry.
Self healing materials
In the nano-concrete industry, a current trend is the self-healing process of materials. These materials are mimicking nature and have a built-in capability to repair early stage damage. The mechanisms include microcapsules containing repair agent or bacteria that precipitate calcite, also vascular flow networks that circulate a healing agent, or even shape memory polymers that shrink when triggered.
The recovery via this self-healing mechanism results in a higher life span of the concrete.
Some nanomaterials like Titanium dioxide, are used for self-cleaning and photocatalytic applications. The photocatalytic activity of nanomaterials is triggered both under UV and/or a Visible light source. The photocatalyst can decompose both organic (bacteria, fungi, nicotine) and inorganic pollutants into carbon dioxide. Therefore these applications are essential for indoor air quality, since 90% of the air we inhale is indoor air.
Nanomaterials can be used in the building sector in order to enhance the properties of conventional building materials. Some nanomaterials can be used for repairing mortar or other self-healing applications. Others may be used for air purification or self-cleaning surfaces. Materials synthesised as water or germ repellents, anti –mosses, -mould, -graffiti can be selected for their properties by the building industry. Nanomaterials can be incorporated in many construction materials and products such as cement, mortar, concrete, insulation materials, glass, paints and coatings. Their use reduces weight of building components, enhances safety and fire resistance, energy efficiency, thermal stability and increases the lifespan of the building materials.
The use of nanomaterials in the construction industry will be a further step in creating greener, more efficient and smarter buildings. Future trends are undoubtedly leading towards minimising poor health risks while creating stronger, cleaner and more energy-efficient nanomaterials to be used in the building sector.