Fabrication and solar modulation of thermochromic hydrogel for smart window application

In the past half decades, hydrogels have received increasing attention, due to their exceptional wide range of promising applications such as drug delivery, sensing, and tissue engineering, etc. A large number of hydrogel products are sensitive to environmental stimuli, such as temperature, pH, etc. Due to the intrinsic properties, different thermos-responsive hydrogels have been fabricated and developed for energy saving smart windows. Because of the significantly increasing impact from urban heat island (UHI) and global warming, the rising demand for comfortable living and working environments result in serious impact on peak and total electricity demand from building electricity consumption sector. Electrochromic and thermochromic materials are the most promising material applied in smart windows. Compared with conventional thermochromic Vanadium dioxide (VO2), temperature sensitive hydrogel, one of the organic thermochromic materials has the advantages of low critical temperature (τc) (from 32-45 °C), and high luminous transmission (Tlum) below τc and large solar modulation (ΔTsol) which are highly pursued. However, there are several drawbacks of traditional hydrogel based thermochromic materials, such as large shrinkage during heating cross τc, poor water holding capacity, lack of active control and weak mechanical strength. To solve these problems as well as improve the thermochromic properties of smart windows, three approaches have been adopted to address those issues. Firstly, tungsten doped VO2 hybrided with cellulose microgel was synthesized, which can block both visible and infrared (IR) light to give a better thermochromic performance. Secondly, by combining with transparent heater, we reported the first flexible electro-thermochromic device, which responded to both temperature and electricity. Lastly, the first carbon and silica-based hydrogel (PNIPAm-Si-Al gel) were hybrided by dispersing poly-(N-isopropylacrylamide) (PNIPAm) microgels into silica-alumina based gel matrix, which gives negligible shrinkage across τc, high water-holding ability, and printability as it was able to produce complex structure through a 3D printer. Meanwhile, such hybrid hydrogel films enable fabrication of flexible electro-thermochromic devices with up to 75% of ΔTsol, depending on the film thickness and the applied voltage, which is among the highest reported results. A flexible smart thermo-sensitive waveguide switch device was also fabricated by such hybrid PNIPAm-Si-Al gel, which will act as temperature sensitive switch for light signal transfer. Doctor of Philosophy