Civil Engineering Reference
In-Depth Information
(Cairns et al. , 2000; Lan et al. , 2010). All of the mentioned oxides are trans-
parent for most of the spectrum characterizing solar radiation.
ITO fi lms can be expensive as a result of the high price of indium (despite
the fact that this element is not uncommon in the earth's crust) and require
careful process control for deposition; zinc oxide-based fi lms can have
similar optical and electrical properties but usually demand even more
stringent process control; and good FTO apparently can only be made on
very hot glass. Thus each of the transparent conducting oxide fi lms has
particular challenges and there is today no 'best' alternative for applications
in electrochromic glazings. Health issues for the manufacturing of transpar-
ent conductors have been brought to attention recently, and it has been
reported that the production of indium-containing oxides may lead to pul-
monary disorders sometimes referred to as 'indium lung' (Taguchi and
Chonan, 2006), which clearly can be an important concern for large-scale
manufacturing.
Metal fi lms can serve as excellent alternatives to the oxide-based trans-
parent conductors. The coinage metals (Cu, Ag and Au) have conductivities
that are some two orders of magnitude higher than for the best transparent
conducting oxides so that comparative electrical properties can be achieved
at about a hundredth of the fi lm thickness; the luminous absorptance of the
metal fi lms can be of the order of 10%. The metal fi lms are stretchable to
a much larger degree than the oxide-based fi lms (Graz et al. , 2009).
The relevant metal fi lm thicknesses are extremely small, which means
that details of the thin fi lm growth are important. Continued deposition
onto a dielectric substrate such as glass or PET causes the deposited metal
to go through a number of distinct growth stages: tiny metallic nuclei are
formed initially; they grow and create increasingly irregular 'islands'; these
'islands' interconnect and form a contiguous meandering network at a
thickness corresponding to 'large-scale coalescence'; the network then
transforms into a 'holey' fi lm; and fi nally a well defi ned metallic fi lm can be
formed (Smith et al. , 1986; Lansåker et al. , 2009). The most interesting fi lms
have thicknesses only slightly above that for 'large-scale coalescence', in
practice around 10 nm (Hövel et al. , 2010). Refl ectance at the two interfaces
of the metal fi lm limits the luminous transmittance to
￿ ￿ ￿ ￿ ￿ ￿
50%, but the trans-
mittance can be very signifi cantly enhanced if the coinage metal fi lms are
positioned between high-refractive-index transparent layers that serve as
antirefl ection coatings for the metal fi lms.
There are several alternatives to the oxide-based and metal-based trans-
parent conductors that are explored today (Hecht and Kaner 2011), and
carbon-based transparent conductors may be of particular importance. Thus
meshes of carbon nanotubes can combine high transmittance for luminous
and solar radiation with good electrical conductivity (Hu et al. , 2010a; Niu,
2011). Another alternative - which currently enjoys intense interest - is
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