Transparent Inorganic Materials with Near Infrared Absorbing or Reflecting Properties
Materials with specific optical properties have diverse but very important applications that range from simple absorption properties in the visible region, leading to use in pigments, to specialist fluorescent, phosphorescent and interference effects. Recently, transparent materials with infrared reflecting or absorbing properties have been attracting more and more interest (eg. Solarflair for coating or Lazerflair for laser welding purposes, both developed by Merck). Indeed, if a material is able to absorb or reflect the near-infrared energy incoming from the sun, it can be used to filter the heat while allowing all visible light to be transmitted thus keeping the transparency intact (Figure 1). In this field, most of the research is carried on organic compounds due to relative ease of development. Inorganic materials however have greater photo-, thermo- and chemo- stability, which make them perfect candidates for long life applications.
1) The electromagnetic spectrum adjacent to the visible region showing the human eye response and solar spectrum.
To create a material that absorbs near infrared and stays clear, there are few possible routes. The first typically uses a tinting material in glass or polymer substrate that absorbs to some extent across the whole electromagnetic spectrum but this approach generally produces a material that has to be quite strongly coloured in order to produce a strong enough absorption in the near-IR. The second possibility is to use a material with a narrow absorption band in the near infrared, and none in the visible region producing perfect transparency though this is difficult to achieve in practice.
This second possibility is the one investigated in this project, using three approaches:
The first approach involves tuning the band gap of inorganic oxides by introducing defects in order to absorb the desired wavelengths (Figure 2).
The second approach is based on electronic transition; selecting the right elements the aim is to use charge transfer mechanism or d-d transition mechanism to select and absorb a narrow range of wavelengths (Figure 3).
The third approach intends to use the overtone from strong infrared absorbing groups to obtain the desired property.
2) Narrow semiconductor bands and dopant to control the optical absorption
3) Octahedral and Tetrahedral Splitting
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