Switchable glazing technology for eco-efficient construction - Nanotechnology in Eco-efficient Construction - page 248

monoclinic VO 2 (B) (Théobald et al. , 1976), paramontroseite VO 2 (Wu et al. ,
2008) and body centred cubic VO 2 (Wang et al. , 2008). The interesting ther-
mochromic properties ensue from VO 2 (R) and VO 2 (M) which exhibit a
reversible structural transformation and associated metal-insulator transi-
tion at a 'critical' temperature τ c in the neighbourhood of a comfort tem-

perature; VO 2 (M) is semiconducting and reasonably infrared transparent
at a temperature τ , so that τ < τ c , whereas VO 2 (R) is metallic and infrared

refl ecting for τ c .

The pioneering work on the thermochromism of VO 2 by Morin (1959)
was performed on bulk specimens. However, it was soon realized that reac-
tively sputter deposited and reactively evaporated thin fi lms could exhibit
a similar metal-insulator transition. Subsequently it has been found that
virtually any thin fi lm technology is capable of providing thermochromic
VO 2 . The reversibility of the metal-insulator transition can be excellent in
fi lms (Ko and Ramanathan, 2008), whereas bulk samples tend to deteriorate
upon repeated thermal cycling around

τ >

τ c . The possibilities to create energy-
effi cient fenestration by letting solar energy into a building when there is a
heating demand and rejecting solar energy when there is a cooling demand
were pointed out already in the 1980s (Greenberg, 1983; Jorgenson and Lee,
1986; Babulanam et al. , 1987), and various aspects of this technology have
been reviewed several times more recently (Granqvist, 2007; Parkin et al. ,
2008; Saeli et al. , 2010a,b; Smith and Granqvist, 2010).
Figure 11.6 introduces the characteristic features of the thermochromism
that can be seen in a thin VO 2 fi lm. Spectral normal transmittance T ( λ ) and

spectral near-normal refl ectance R ( λ ) were recorded at 22 and 100°C, i.e.,

at m-thick fi lm pro-

duced by reactive dc magnetron sputtering as reported elsewhere by Mlyuka
et al. (2009a). Similar optical data - usually of spectral transmittance - have
been reported many times in the scientifi c literature and are hence very
well established (see the paper by Li et al. (2012) for references). It is clear
from Fig. 11.6 that the short-wavelength optical properties are similar irre-
spective of the temperature, while the infrared refl ectance for wavelengths
beyond

τ < τ c and at τ > τ c . The data were obtained for a 0.05- μ

￿ ￿ ￿ ￿ ￿ ￿

τ c , thus giving proof for
the metal-insulator transition. The infrared transmittance at

∼ 1 μ m is higher for τ > τ c than it is for τ <

λ > 1 μ m shows

an analogous change.
Figure 11.6 also shows the spectral sensitivity of the light-adapted human
eye, denoted m wavelength range

(Wyszecki and Stiles, 2000) and the solar irradiance spectrum for air mass
1.5 (corresponding to the sun standing 37° above the horizon), denoted

ϕ lum and lying in the 0.4 < λ < 0.7 μ

ϕ sol

and extending across the 0.3 < λ < 3 μ m interval (ASTM, 2003). One

observes in particular that ϕ sol drops sharply towards long wavelengths.

Wavelength-integrated luminous and solar transmittance values are now
introduced by