Switchable glazing technology for eco-efficient construction - Nanotechnology in Eco-efficient Construction - page 258

particularly for the built environment as discussed by Smith and Granqvist
(2010). There are several interesting electrochromic technologies with dif-
ferent pros and cons and a notion that one technology takes it all is most
likely going to be false. One may compare this with the case of photovoltaics
where a vast number of different device types and materials currently
compete fi ercely for market share.
What device types and materials will be dominating for electrochromic
glazings? Again there is no obvious answer, but mass fabrication speaks in
favour of low-cost roll-to-roll coating and continuous lamination, which
defi nitely are applicable to electrochromics. Combinations of cathodic and
anodic electrochromic oxides can be used to boost the coloration effi ciency
and obtain suitable luminous properties. These oxides are unlikely to be
based on a single metal. Possibly the required transparent electrical conduc-
tors are getting ready for a revolution, and current work on metal-based
and carbon-based materials may lead to radical cost cuts for electrochromic
glazings.
Added functionality is of interest for electrochromic glazings devices. For
example, should the infrared part of the solar radiation - with about half
of the energy - be admitted or not when the devices are in their bright
state? Near-infrared absorption can be accomplished by adding non-
scattering nanoparticles to the polymer electrolyte; this was demonstrated
for the case of ITO (Bayrak Pehlivan et al. , 2012), and LaB 6 (Schelm et al. ,
2005) as well as Cs x WO 3 (Guo et al. , 2011), which are other alternative
nano-pigments. However, except under very cold climate conditions there
is sometimes a need for heating, so that electrochromic glazing should
transmit in the infrared, while at other times there is a need for cooling so
that the infrared transmittance should be kept to a minimum. This function-
ality clearly calls for thermochromism.
Thermochromic glazings have a long history of unfulfi lled promises
regarding applications in buildings and automobiles. The fi rst problem, that
was obvious from the very beginning, was that the thermochromic switching
has to take place around a comfort temperature of about 25°C. However,
this was readily accomplished by tungsten doping of vanadium dioxide
fi lms. The fi lm must be suffi ciently crystalline which requires deposition or
annealing temperatures of at least 400-500°C. Such temperatures are not
any problem for coatings on fl oat glass, but they are not compatible with
deposition onto plastic web in a straightforward way.
Recent work has taken some steps towards thermochromic fenestration
of practical interest. The fi rst of these steps was the discovery (Mlyuka
et al. , 2009c) of a doping element that was capable of widening the
fundamental band gap of VO 2 so that the luminous transmittance was
boosted. Another very signifi cant step was the realization that VO 2 -based
nanoparticles could give a strong, broad and temperature-dependent plasma

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