Multi-functional Adaptive Windows
Thermochromic coatings are based on materials that change their solar radiation properties with varying temperature due to chemical or physical structure changes within the material. This effect can be exploited for a variety of different applications ranging from thermochromic inks for smart packaging in food industries or functional textiles and clothing to smart windows systems. Vanadium dioxide (VO2) is of particular interest for spectrally selective coatings on building glass modulating the solar energy transmitted into the interior of the building. The transition metal oxide switches above a critical temperature TT from an infrared-transparent cold state to a hot state which is reflective to near infrared radiation (IR) and at the same time maintains its transparency for visible light. This modulation of the optical properties is triggered by a reversible first order phase transition between two polymorphs of VO2, a semiconducting monoclinic phase (M phase, space group P21/c) and a metallic rutile structure (R phase, space group P42/mnm). In order to be successfully implemented as a solar-responsive coating in modern smart window systems it is necessary to adjust some of its optical and chemical properties. For example, pristine VO2 show a semiconductor-to-metal transition around 68 °C which is far above the temperatures simulations showed to be optimal for its application as energy-efficient coating. Moreover, the unpleasant brown colour of the vanadium dioxide often requires to compromise between a sufficient visible light transmittance Tvis and a high solar modulation efficiency ΔTsol. Further key challenges that need to be addressed include the hysteresis ΔTT during thermal cycling between the cool state and hot state, pure adhesion of the thermochromic coating to the glass surface and the susceptibility of the metal oxide to chemical attack. In our group we are working on complementary methods to tackle all of those problems. We are aiming to fine tune and control the key performance parameters of VO2 coatings by chemically modifying the material properties and developing new window designs to enhance its thermochromic and optical properties.
A bioinspired solution: VO2-coated moth-eye surface nanostructures
In our labs we are developing a new class of moth-eye thermochromic VO2 based smart-window that employs extreme surface roughness (inspired by natural anti-reflection structures) to both improve the optical properties of the window and induce desirable wettability properties. In contrast to popular implementations of VO2 based smart-windows which utilize multilayer thin-films of VO2 interspersed with dielectric material to form resonant cavities the moth-eye smart-window is polarization insensitive and operates efficiently over a wider range of angles. State-of-art nanofabrication techniques of moth-eye nanostructures are investigated by using at least one of the following methods; colloidal lithography, block-copolymer lithography, nanoimprint lithography, laser interference lithography and roll-to-roll processes. One of the key challenges is the conformal coating of the textured surface with a highly crystalline, thin layer (up to 40 nm) of VO2. Currently we are working on different approaches including chemical vapour deposition and atomic layer deposition of VO2 films as these techniques are not line-of-sight processes.
VO2 nanoparticles-polymer composites
Nanothermochromic composites comprising VO2 nanoparticles in a dielectric host are another promising approach as they offer a number of specific advantages over coatings based on VO2 films including a higher luminous transmittance combined with a significantly enhanced solar energy transmittance modulation ΔTsol.
Our group uses a combination of simulation methods to link the microscopic properties of the nanoparticles to the macroscopic properties of nanocomposite films, allowing us to efficiently design films with desired visible transmission and shading properties, whilst accurately quantifying transmission haze.
We are currently working on the development of a scalable synthesis method for VO2 nanoparticles. In general, the synthesis of monoclinic VO2 (M) requires long reaction times and/or high reaction temperatures as the thermodynamically most stable phases tend not to be formed initially (Ostwald rule of stages). Syntheses that using lower temperatures or less harsh conditions often result in the isolation of the metastable polymorphs VO2 (A) or VO2(B) or product mixtures. Within our group, we are aiming at developing new synthetic protocols for pure, highly crystalline VO2(M) nanoparticles which allow a precise control over particle morphology, size and composition.