Switchable glazing technology for eco-efficient construction - Nanotechnology in Eco-efficient Construction - page 247

80

70

60

50

40

30

After 750 cycles
Bleaching time 40 s
Colouring time 120 s 1400 cycles

20

0.00 0.05 0.10 0.15 0.20 0.25

Time (s) × 10 6

11.5 Initial data on transmittance modulation of luminous radiation in
an electrochromic foil device made by roll-to-roll manufacturing and
continuous lamination.

more than 50 years (Morin, 1959). However, vanadium dioxide is not
directly applicable to glazings, and three particular challenges can be identi-
fi ed as elaborated in Section 11.3.1 below. In particular, the modulation of
the solar energy throughput must be suffi ciently large in order to have a
signifi cant impact on the buildings' energy expenditure, and this leads to
the recently introduced concept of 'nanothermochromics' (Li et al. , 2010),
which also is advantageous for yielding a large luminous transmittance as
discussed in Section 11.3.2. The luminous transmittance can be further
enhanced by doping of the VO 2 , especially by magnesium, as discussed in
Section 11.3.3. Finally, the modulation of the solar energy throughput must
occur in the vicinity of a comfort temperature of about 25°C, which again
can be accomplished by doping - in this case most expediently by tungsten
- as elaborated in Section 11.3.4.

￿ ￿ ￿ ￿ ￿ ￿

11.3.1 Vanadium dioxide-based thin fi lms: three challenges

Vanadium dioxide is an interesting and complex material with at least seven
different polymorphs among which rutile VO 2 (R), monoclinic VO 2 (M)
(Morin, 1959), and triclinic VO 2 (T) (Mitsuishi, 1967) phases are similar
in structure, and there are also tetragonal VO 2 (A) (Oka et al. , 1990),