Day 1

Summary:
1. Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, e.g. in baby bottles (changes to a different color when cool enough to drink) or kettles (changes when water is at or near boiling point). Thermochromism is one of several types of chromism.

The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light passing the crystal undergoes Bragg diffraction on these layers, and the wavelength with the greatest constructive interference is reflected back, which is perceived as a spectral color. As the crystal undergoes changes in temperature, thermal expansion occurs, resulting in change of spacing between the layers, and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from black through the spectral colors to black again, depending on the temperature.

2. A VO2 film can respond to the environmental temperature to intelligently regulate infrared transmittance while maintaining visible transparency, and this special optical property makes it an attractive material for thermochromic smart windows

3. Vanadium dioxide has been investigated for use as a "spectrally-selective" window coating to block infrared transmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductor at lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungsten lowers the transition temperature to 29 °C