Civil Engineering Reference
In-Depth Information
c
can also be infl uenced by several external param-
eters such as thickness, mechanical deformation, non-stoichiometry, small
crystallite size, etc.
Tungsten is known to be the most effi cient doping element for decreasing
same tendencies, but
τ
c
, i.e., it has the largest fall-off rate
R
as the amount of doping is increased.
This material has been discussed in detail in the literature. For bulk crystals
of W
x
V
1−
x
O
2
it has been reported that
R
τ
=
27
±
1°C/at%W (Goodenough,
1971; Hörlin
et al.
, 1972), while
R
21°C/at%W was stated in some subse-
quent work (Reyes
et al.
, 1976). It follows that as little as
=
∼
2 at% of tungsten
is able to bring
τ
c
to a comfort temperature. Thin fi lms of W
x
V
1−
x
O
2
also
have decreased values of
c
, but the quantitative magnitudes of
R
have been
found to lie between 7 and 26°C/at%W, i.e., differing by a factor of almost
four; detailed data have been given elsewhere (Li
et al.
, 2012). Tungsten
doping has only a very minor effect on the optical properties (Tazawa
et al.
,
1998).
There are two basic reasons why the data on
τ
τ
c
for thin fi lms are scattered,
one being that the electrical properties depend strongly on crystallinity,
grain size, and the conditions at the two interfaces of the fi lms; the other
reason is the diffi culty of evaluating unique 'critical' temperatures from
graded and hysteretic transitions of the resistance. Two experimental param-
eters of great signifi cance are the substrate temperature during deposition
and the post-deposition annealing temperature. Empirical data show that
either of these temperatures must exceed
450°C (Li
et al.
, 2012).
The fi lms of Mg
x
V
1−
x
O
2
, discussed in Section 11.3.3 above, also have
lowered values of their 'critical' temperatures, and
R
∼
3°C/at%Mg describes
the data (Mlyuka
et al.
, 2009c). This may be compared with measurements
on bulk crystals of Mg
x
V
1−
x
O
2−2
x
F
2
x
, which yielded
R
≈
≈
6°C/at%Mg (Akroune
et al.
, 1985).
11.4
Future trends in electrochromic and
thermochromic glazing
This chapter has provided an introduction to oxide-based electrochromics
and thermochromics and has discussed applications, device designs, and
critical materials issues with regard to eco-effi cient buildings. The discussion
is now widened and includes a number of perspectives.
Beginning with applications, there have been numerous claims over the
past decades that electrochromic glazings 'fi nally' are ready for implemen-
tation on a large scale. However, only prototypes and commercial products
delivered to select customers or for very specifi c applications or markets
have been presented so far. Is the situation going to change soon? The
answer is almost certainly 'yes', and the basic reason is the worldwide
awareness of the acute need for 'eco-effi cient' or 'green' technologies,
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