Civil Engineering Reference
In-Depth Information
graphene, i.e., atomically thin layers of carbon atoms arranged in a honey-
comb lattice (Geim and Novoselov, 2007; Eda and Chhowalla, 2010); these
layers can be prepared via mechanical or chemical exfoliation of graphite
into individual sheets as well as by chemical vapour deposition. Successful
roll-to-roll production of graphene coatings was fi rst demonstrated during
2010 (Bae
et al.
, 2010, 2012).
Metal-based nanowire meshes is another possibility, and it has been
shown that silver nanowires with diameters of
∼
100 nm and lengths of
∼
m can be produced in large amounts by inexpensive reduction of
liquid silver nitrate (Hu
et al.
, 2010b, 2011). Suspensions of these nanowires
can be deposited, and the conduction between adjacent wires can be
improved by annealing. These coatings can have good electrical properties
but suffer from some diffuse scattering which limits their applications in
electrochromic glazings.
A fi nal example may be poly(3,4-ethylenedioxytiophene), known as
PEDOT, which cannot quite compete with regard to performance with
most of the other options mentioned above, but which nevertheless is inter-
esting since it can be prepared by printing at a very low cost (Elschner and
Lövenich, 2011).
10
μ
11.2.5 Flexible electrochromic foil
Tungsten oxide is the most extensively studied electrochromic material and
was discussed in some detail above. It has cathodic coloration and needs to
be combined with an appropriate anodic oxide to create an optimized
device. Iridium oxide works very well in tandem with tungsten oxide but it
is one of the rarest elements in the earth's crust and only found in abun-
dance in a few places. An alternative is hence needed for large-scale appli-
cations, and hydrous nickel oxide is such a material as discovered in the
1980s by Svensson and Granqvist (1986).
A number of studies of electrochromic devices based on tungsten oxide
and nickel oxide have been reported in the literature; they are either rigid
and based on depositions onto glass (Mathew
et al.
, 1997; Subrahmanyam
et al.
, 2007; Huang
et al.
, 2011) or fl exible and deposited onto PET foil
(Azens
et al.
, 2003b; Niklasson and Granqvist, 2007). Figure 11.2 illustrates
a specifi c device design that was mentioned in Section 11.2.2 above: one
PET foil is coated with ITO and tungsten oxide, another PET foil is coated
with ITO and nickel-based oxide, and the two electrochromic fi lms are
joined by an ion-conducting polymer adhesive. Figure 11.3 reports time-
dependent transmittance at a mid-luminous wavelength of 550 nm for ten
consecutive colouring/bleaching cycles. Colouring proceeds slower than
bleaching and the values shown do not indicate the darkest state that can
be reached. The highly repeatable properties should be noted, and the
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