Smart Windows

Smart Windows

Principal investigator: Shu Yang

Background

Switchable optical materials, which possess reversible light transmission properties in response to external stimuli, offer many interesting possibilities. Potential applications include energy-efficient windows, roofings, and skylights that can transmit or block light. Many groups have demonstrated the tuning of optical properties between two states: between color and transparency, between color and whiteness, and between whiteness and transparency, for example. However, few have demonstrated reversible tuning between all three states.

Previously, the Yang Lab has created tilted pillar arrays on wrinkled elastomeric silicone as a reversibly switchable optical window, which can achieve the dramatic and reversible visual effect between colorful, white, and transparent states. However, the initial state is opaque and it cannot achieve high transparency (> 90% transmittance) since the surface roughness cannot be completely eliminated. Likewise, in literature (including nanoparticle assembly, liquid crystals, chromogenic materials or micro-/nanostructured materials), the initial state is typically opaque or colored. Furthermore, the displayed color from highly ordered structures is angle-dependent. Therefore, it will be highly desirable to create materials that in the initial state are transparent and can be switched to angle-independent colors or opaqueness.

Smart Window FabricationA Different Smart Window

In this technology, we have created a composite film consisting of a thin layer of silica nanoparticles (NPs) embedded in silicone. The film is completely transparent in the initial state due to a refractive index match between the silica NPs and the silicone. Upon mechanical stretching, the transmittance was dramatically reduced to 30% and displayed angle-independent structural color at a strain >40%. The color could be tuned by the silica NP size.

Compared to the smart windows reported in literature, ours have several unique characteristics:

  1. The initial state is truly transparent
  2. The change of transmittance in the vis-NIR region is very large, ~60%
  3. The sprayed NPs are much more robust against peeling and stretching in comparison to highly ordered colloidal crystals. They offer angle-independent color display upon stretching whereas most stretchable smart windows display angle-dependent colors.
  4. The displayed color is independent of stretching strain, and instead dependent on NP size
  5. The film is highly robust in repeated stretching and releasing (at least 1000 cycles) since the majority of the film under strain is the bulk silicone layer

The demonstrated smart windows can also be used in applications such as displays, camouflages, and security, as well as heat/solar gain control.

Materials Cost

At current rates paid by the laboratory, the elastomer film costs $96.30/kg, and the silica nanoparticles cost $12/g. This equates to approximately $0.0283 per square centimeter of smart window film. At an industrial scale, this cost can be reduced anywhere from 1/2 to as much as 1/10th of the original cost, depending on the volume purchased.

_Smart-Window-illustration-1Future Work

The mechanical robustness of the film in practical daily usage is yet to be established. The required stretch strain to achieve the optical changes is relatively large. Ideally, we’d like to see dramatic change of transmittance occur at 10% strain or smaller. In some applications (e.g. for airplane windows or a car sunroof), it’s preferable to have < 10% transmittance in the visible wavelength in the “off” state.

Other interests include a window which is transparent in the visible wavelength but IR- or UV-reflective in the “off” state.

We believe our technology concept can be applied to other functional material systems to design highly responsive yet mechanically robust, nano-/microstructured materials that are sensitive to heat, light, and moisture.

References

Smart Windows article

Read more in the Penn Current.

  1. Ge, D., Lee, E., Yang, L., Cho, Y. Li, M., Gianola, D. S., and Yang, S., “A robust smart window: reversibly switching from high transparency to angle-independent structural color display“, Advanced Materials. 2015, 27 (15), 2489–2495.
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About the Lab

Shu Yang

Shu Yang

The Yang lab is broadly interested in synthesis and engineering of well-defined polymers, gels, colloidal particles, biomaterials, and organic-inorganic hybrids with controlled size, shape, and morphology over multiple length scales. By extending the obtained knowledge, her group seeks to direct patterning and assembly of nano- and micro-objects in solutions and on patterned surfaces to create hierarchical structures. In turn, they explore surface (non)wettability, optical responses, and mechanical properties, and their dynamic tuning.