Switchable glazing technology for eco-efficient construction - Nanotechnology in Eco-efficient Construction - page 240

refl ectance and can operate in conjunction with electrochromic thin fi lms
in devices; constructions with fi lms based on nickel-magnesium hydride
have been investigated in depth (Tajima et al. , 2010). Today's devices of this
type tend to suffer from limitations in longevity, modulation span and high-
temperature stability (Tajima et al. , 2011).
There are also several device designs that are distinctly different from
the 'battery' type. One of these is the suspended particle device (often
referred to as an 'SPD') which is rooted in the pioneering work on 'light
valves' done by Land already in the 1930s (Marks, 1969). Essentially, a
suspension of rod-like molecules, for example of herapathite (Kahr et al. ,
2009), are aligned under an ac voltage of 100 V (i.e., some two orders larger

than for the electrochromic device illustrated in Fig. 11.1) and are randomly
oriented in the absence of this fi eld; the optical transmittance through suit-
ably confi ned suspensions is then changed. The modulation can be strong
for luminous radiation, but not for infrared light, and the devices exhibit
some haze (Vergaz et al. , 2008). Alternatively, liquid crystals can be used in
several different ways to create variable transmittance; the most common
construction with regard to glazings uses polymer-dispersed liquid crystals
(known as 'PDLCs') and suffers from signifi cant haze (Cupelli et al. , 2009;
Gardiner et al. , 2009). Another possibility is offered by some organic com-
pounds, which can display optical absorption when a small current is drawn
through them, and this phenomenon has been used very successfully for
some two decades in 'self-dimming' rear view mirrors for automobiles. As
a fi nal possibility one may point at reversible electroplating, which in prin-
ciple is able to give variable refl ectance between very widely separated
limits. This option has been investigated intensely (Ziegler, 1999; Laik
et al. , 2001) but has not yet led to practically useful glazings.

∼

11.2.3 Electrochromic thin fi lms

￿ ￿ ￿ ￿ ￿ ￿

There are two types of electrochromic metal oxides, which are referred to
as 'cathodic' (colouring under ion insertion) and 'anodic' (colouring under
ion extraction); they are discussed in detail in Granqvist (1995). The stan-
dard electrochromic device, such as the one shown in Fig. 11.1, embodies
two electrochromic thin fi lms and it is clearly advantageous to combine one
'cathodic' oxide (e.g., based on W, Mo, or Nb) and another 'anodic' oxide
(e.g., based on Ni or Ir). Shuttling ions between the two electrochromic fi lms
one way makes both of these fi lms colour, whereas shuttling ions the other
way makes them both bleach; this is sometimes referred to as a 'rocking
chair' operation. Coincidentally there are 'cathodic' and 'anodic' oxides
which can work in tandem and jointly yield electrochromic glazings with a
rather neutral visual appearance that is appropriate for general applications
in architecture.