Civil Engineering Reference
In-Depth Information
These linutations indicate three challenges that must be met for practical
thermochromic glazings, and VO
2
has to be modifi ed in order to achieve
Δ
25°C.
A moderate boost of
T
lum
and/or
T
sol
can be obtained by straightforward
optical design using high-refractive-index dielectric coatings, and data are
available for a number of two-, three- and fi ve-layer coatings with vanadium
dioxide fi lms between fi lms of, for example, titanium dioxide and zirconium
dioxide. The best properties were reached for TiO
2
/VO
2
/TiO
2
/VO
2
/TiO
2
(Mlyuka
et al.
, 2009a,b). However, these improvements are not suffi cient
for making thermochromic glazings of general practical interest. Fluorina-
tion is an alternative route to improve
T
lum
for VO
2
fi lms to some extent
(Khan
et al.
, 1988; Khan and Granqvist, 1989).
T
sol
>>
10%,
T
lum
>>
40%, and
τ
c
≈
11.3.2 VO
2
nanoparticle composites with enhanced solar
energy modulation and luminous transmittance
'Nanothermochromics' is a new concept that was introduced recently by
the author of this chapter and his coworkers (Li
et al.
, 2010). It deals with
VO
2
nanoparticles dispersed in a dielectric host and implies that the com-
posite can be represented as an 'effective medium' with properties inter-
mediate between those of the nanoparticles and their matrix. The particles
are small enough not to cause optical scattering. The 'effective' dielectric
function
MG
is (Smith and Granqvist, 2010; Granqvist and Hunderi, 1977,
ε
1978):
2
3
1
+
f
α
ε
MG
=
ε
,
[11.2]
m
1
3
1
−
f
α
where
m
accounts for the matrix and
f
is the 'fi lling factor', i.e., the volume
fraction occupied by the particles. The calculations to be presented below
employed
f
ε
=
0.01 and a thickness of 5
μ
m (so that the VO
2
mass thickness
was 0.05
m, i.e., the same as for the VO
2
fi lm reported on in Fig. 11.6).
Equation [11.2] is appropriate for the Maxwell-Garnett (
MG
) theory
(Maxwell-Garnett, 1904), which pertains to nanoparticles in a continuous
matrix (Niklasson
et al.
, 1981). There are many effective medium formula-
tions - applicable to a multitude of nanotopologies - all of which coincide
in the limit of a small fi lling factor; this implies that Eq. [11.2] can be used
in the present case without any loss of generality.
The parameter
μ
α
in Eq. [11.2] is given by
εε
−
p
m
α
=
,
[11.3]
(
)
ε
+
L
ε
−
ε
m
p
m
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