Optically
transparent wood has emerged as a promising glazing material.
Thanks to the high optical transmittance, strong mechanical properties,
and excellent thermal insulation capability of transparent wood, it
offers a potential alternative to glass for window applications. Recently,
thermo-, electro-, and photochromic transparent woods that dynamically
modulate light transmittance have been investigated to improve building
energy efficiency. However, it remains challenging to widely replace
windows with transparent wood because of its poor weather resistance.
In this study, an environment-friendly thermochromic transparent wood
film (TTWF) with thermal switching of transmittance is proposed and
demonstrated. To achieve thermochromism, the bleached wood is impregnated
with the vanadium dioxide (VO2)/polyvinyl alcohol composite.
Due to the self-densification of cellulose microfibrils during the
evaporation of solvents, the transparent wood is in the form of thin
films, which can be attached on the inner face of a window to protect
it from severe weather conditions, making the installation convenient
and low-cost. Furthermore, the surface of VO2-TTWF is modified
by octadecyltrichlorosilane to enhance the waterproof ability and
achieve self-cleaning and antidust functions. The proposed VO2-TTWF shows great potential for application in energy-efficient
buildings using sustainable materials with advanced optical properties
(i.e., T
lum = 50.5%,
ΔT
sol = 3.4%, and haze = 70%) that
are mechanically robust (i.e., σ = 130.6 MPa
along the wood growth direction), have low-thermal conductivity (i.e., K = 0.29 W m–1 K–1 along the perpendicular direction to the wood fibers),
and demonstrate hydrophobic self-cleaning and antidust functions (i.e., contact angle: 121.9°). An experiment, using
a model house, showed that the VO2-TTWF attached on the
inner face of the window could significantly reduce the indoor air
temperature by 33.9 °C compared with a bare glass panel, proving
that VO2-TTWF has potential to be applied as a new-generation
energy-efficient material for smart windows.
Recently, organic hybrid halide perovskites have been found to show thermochromism with good optical performance, which can be applied in smart windows to reduce building energy consumption. However, these perovskites have shortcomings regarding their thermochromic performance, namely long transition time, high transition temperature, and large transition hysteresis width. In this study, a hydrated MAPbI3−xClx thermochromic perovskite smart window (H‐MAPbI3−xClx TPSW) is proposed, which undergoes a reversible transition between a transparent state and a dark reddish‐brown tinted state with a high solar modulation ability of 23.7%. Most importantly, the H‐MAPbI3−xClx TPSW possesses a tunable low transition temperature of 29.4 to 51.4 °C, a controllable and narrow transition hysteresis width (7.7–13.2 °C) and a short transition time (1–4 min). Additionally, a mathematical model is developed to predict the transition temperature of the H‐MAPbI3−xClx TPSW. A field test is also conducted, demonstrating that the H‐MAPbI3−xClx TPSW fitted to a model house can reduce the indoor air temperature by 3.5 °C compared to using a quartz glass window. Overall, the H‐MAPbI3−xClx TPSW can yield excellent optical properties, while simultaneously providing remarkable transition properties, making it potentially useful for a wide range of applications in energy‐efficient buildings.
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