Commercial buildings in the United States account for nearly 40% of the total energy consumption. Among them, electricity is the largest energy source for buildings. Therefore, the design of new energy-efficient materials and technologies is crucial to meet goals such as the Net-Zero Energy Commercial Building Initiative (CBI) put forward by the U.S. Department of Energy (DOE). Approaches to block or reflect intense sunlight when needed, and to capture the Sun’s light and heat when desirable, remain complex and expensive to fabricate.
A low-cost, durable, composite film was developed as a window “shade” in which the transparency can be tailored to adjust heat and light. The film could also be mounted between two panes of glass and motor-controlled. The composite film consists of a thin layer of a quasi-amorphous array of silica nanoparticles (NPs) embedded in a bulk elastomeric polymer film.
The film is highly transparent (>90% transmittance in the visible wavelength) in the initial state. Upon mechanical stretching, the transmittance is dramatically reduced to 30%, and displays angle-independent structural color at a strain >30%. The displayed reflective color is a structural color that could be tuned by the silica NP size; the color displayed is invariant with the applied strain.
The switch between transparency and colored states has been reversibly cycled more than 1,000 times without losing the film’s structural and optical integrity. The initial state is transparent vs. technologies such as polymer-dispersed liquid crystals and electrochromic displays.
A “smart window” using the film can be mass-produced using low-cost methods from abundant materials. In addition to light-blocking smart windows, it can also be used for display or security applications.