Low Temperature RF-Sputtered VO
2
Thermochromic Films for Smart Window
Applications
E. Gagaoudakis
1, 2, *
, G. Michail
2
, E. Aperathitis
2
, V. Binas
2
, M. Eleftheriou
3
, E. Poulakis
3
,
G. Kiriakidis
1,2
1
Physics Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece
2
Institute of Electronic Structure & Laser (IESL), Foundation for Research and Technology
(FORTH) Hellas, P.O. Box 1385, Heraklion 70013, Crete, Greece
3
Lyceum of Tzermiadon, Crete, Greece
* Corresponding author
:
E. Gagaoudakis,
Vanadium dioxide (VO
2
) is a well-known thermochromic material that undergoes a
Semiconductor to Metal Transition (SMT) at critical transition temperature of T
c
= 68
o
C.
Below this temperature it behaves as an insulator, having a monoclinic structure and high IR
transmittance, while above Tc it turns to metallic state with a tetragonal rutile structure and
high IR reflectance. The fact that this variation of IR transmittance happens at T
c
close to
room temperature, makes VO
2
an appropriate candidate as a coating for smart windows.
In this work, thermochromic VO
2
thermochromic films were produced by RF Sputtering
technique, using a V metallic target. Total pressure during deposition was kept at 5 mTorr,
while sputtering power was 400 W. All films were deposited on, 1 inch x 1 inch x 4 mm
thick, Pilkington K-Glass (SnO
2
/Glass, low-E) substrates at 300
o
C. Oxygen content in Ar/O
2
plasma and deposition time was varied in order to optimize thermochromic properties of the
produced films. Film structure was examined by X-Ray Diffraction technique, while
thermochromic properties were identified by transmittance measurements at temperature well
below (25
o
C) and well above (80
o
C) T
c
.
All films were polycrystalline having a preferred orientation of (011) at 2θ = 27.8
o
. The
critical transition temperature found to be T
c
= 55
o
C, while the width of transmittance
hysteresis loop was about ΔT = 8
o
C. Moreover, the variation of transmittance at λ = 2000 nm
between 25
o
C and 80
o
C was about ΔTr
IR
= 15%, while visible transmittance at λ = 600 nm
was Tr
vis
= 45%.
Finally, nine VO
2
-coated K-Glass substrates and nine bare K-Glass substrates were placed
as windows in two homemade demo houses, respectively, in order to be compared, against
heat exposure, as shown in figure 1. The temperature was monitored by appropriate thermal
sensor and it was found that the temperature in the house having the thermochromic windows
was 4.3
o
C less than the temperature in the house having the low-E windows.
Figure 1:
Demo houses for thermochromic windows
O 9
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