Thermochromic Materials

Smart materials for thermal management

Figure 1 - The principle of the operation of thermochromic windows

Space cooling is a fundamental means to attain people’s good health and well-being due to its direct correlation with human comfort, increased productivity levels and concomitant health benefits. Nonetheless, cooling is one of the most energy intensive and highly polluting processes associated with human activity. The statistics are staggering; the energy consumed by air-conditioners and electric fans account for ~20% of the total electricity used in buildings around the world today and contribute over 1.2GT of CO₂ equivalent emissions per year. An efficient air conditioning system requires good control of the heat transfer mechanisms between a building and its surroundings, namely convection, conduction and radiation. The former two have historically been the main focus of research, giving room for the development of more mature technologies, such as double-pane windows, and advanced thermal insulation panels. Manipulating the spectral properties of materials opens up new opportunities to accurately control the vast amount of solar radiation falling onto buildings as well as the numerous thermal emissions. Technologies such as Low-E coating and cool-roof technologies, are good examples of construction materials designed for efficient control of radiative heat transfer. However, these technologies are static, in other words, they are unable to adapt to the evolving environmental conditions.

In Pi-Lab we develop smart technologies to improve the energy efficiency in the built environment and reduce the need for indoor climate control systems such as air conditioners. Our research aims to design self-adaptive materials able to tune their optical properties with changes in temperature. We particularly focus on smart windows coatings, that modulate the near-IR solar radiation while maintaining high transparency in the visible spectrum. This is achieved through the use of thermochromics materials, i.e., materials that change their solar radiation properties with varying temperature due to chemical, physical and electronic structure changes within the material. 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. This transition metal oxide switches above a critical temperature from an infrared-transparent cold state to a hot state which is reflective to near-IR radiation and at the same time maintains its transparency for visible light.

In order to successfully implement vanadium dioxide as a solar-responsive coating in modern smart materials, 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 optimal temperatures for its application as energy-efficient coating. In the particular case of smart windows, the unpleasant brown color of the vanadium dioxide often requires to compromise between a sufficient visible light transmittance and a high modulation of near-IR transmittance. Further key challenges that need to be addressed include the hysteresis 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.

Image showing highly efficient thermochromic window achieved with a multilayer Bragg structure. For details refer to our paper C. Sol, et al., "High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching

Our smart materials are based on multilayered thin films, nanostructured surfaces and nanocomposites. We use a combination of computer simulation methods to link the microscopic properties of the particles together with the macroscopic properties of the composite, allowing us to efficiently design films with desired visible transmission and shading properties, whilst accurately quantifying transmission haze.We work on 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. 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 followed by the surface modification and embedding into the polymer matrix.

Figure 2 - Image showing highly efficient thermochromic window achieved with a multilayer Bragg structure. For details refer to our paper C. Sol, et al., "High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching," ACS Appl. Mater. Interfaces 12, 8140–8145 (2020).

Image showing several thermochromic nanoantennas fabricated in pi-lab. These antennas can be integrated into a polymer matrix to create a composite polymer thermochromic foil.

Figure 3 - Image showing several thermochromic nanoantennas fabricated in pi-lab. These antennas can be integrated into a polymer matrix to create a composite polymer thermochromic foil.

Representative publications

1. M. Arnesano et al., "Optimization of the thermochromic glazing design for curtain wall buildings based on experimental measurements and dynamic simulation," Sol. Energy 216, 14–25 (2021).

2. K. L. Gurunatha, et al. "Combined Effect of Temperature Induced Strain and Oxygen Vacancy on Metal-Insulator Transition of VO2 Colloidal Particles," Adv. Funct. Mater. 30, 2005311 (2020).

3. C. Sol, et al., "High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching," ACS Appl. Mater. Interfaces 12, 8140–8145 (2020).

4. J. Schläefer, et al. "Thermochromic VO2−SiO2 nanocomposite smart window coatings with narrow phase transition hysteresis and transition gradient width," Sol. Energy Mater. Sol. Cells 200, 109944 (2019).

5. V. P. Prasadam, et al., "Atomic layer deposition of vanadium oxides: process and application review," Mater. Today Chem. 12, 396–423 (2019).

6. C. Sol, et al., "Mitigation of hysteresis due to a pseudo-photochromic effect in thermochromic smart window coatings," Sci. Rep. 8, 1–6 (2018).