09 Jul 2003
Two researchers in Australia have come up with a simple way to make glass that lets in sunlight but blocks most of its heat.
By doping a laminate sheet with nanoparticles and sandwiching it between two clear glass panes, researchers in Australia say they have made glazing that transmits sunlight while blocking out most its heat. (Applied Physics Letters 82 4346).
Solar control glazing that transmits visible light but blocks infrared light would benefit those in warmer climates who wish to keep the interior of buildings cool. One established approach is to place thin silver films between layers of glazing.
As an alternative to the thin silver films, Stefan Schelm and Geoff Smith from the University of Technology in Sydney doped a polyvinyl butyral laminate sheet with rare-earth lanthanum hexaboride (LaB6) nanoparticles.
“The optimum material is one which absorbs in a narrow band, so as not to effect the visible transmission excessively while blocking most of the near infrared solar energy,” say the duo.
According to the inventors, LaB6 is a good material to use as it has a resonance peak near 1000 nm and absorbs the crucial heating wavelengths between 750 to 1300 without absorbing too much visible light.
The researchers used nanoparticles ranging between 20 and 200 nm in size, but say that the optimal size is 80 nm. “The only difference the actual size makes is the scattering contribution in the visible range resulting in unwanted haze, hence the upper limit of 80 nm,” say the authors. “For diffuse (non-viewing) glazing, larger particles are possible.”
The duo has been experimenting with nanoparticle concentrations as low as 0.01 wt%. “At 0.03 wt%, the reduction in total visible light from the sun is 41%, while the reduction in total solar heat transmitted is 71%,” they say.
The key to the research though is striking a balance between the concentration of the nanoparticles and the overall transparency of the glass. According to the researchers, a higher concentration yields higher absorption of infrared light but also increases the coloring effect and reduces visibility through the glazing. At present using the low concentrations, the windows look transparent but have a greenish tinge and appear to have a blueish haze at oblique angles.
The researchers are now looking at nanoparticles that are easier to mass-produce and also block ultraviolet light.
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.