Biomedical Engineering Reference
In-Depth Information
out that solid state sensors measure the change in the electrical resistance of an oxide film as a
function of different gas concentrations. This is the functionality of these types of devices.
Pokhrel et al. (2007)
used sol-gel-derived polycrystalline Cr
1.8
Ti
0.2
O
3
thick films for alcohol
sensing applications. These authors report that titanium-substituted chromium oxide,
Cr
1.8
Ti
0.2
O
3
(CTO) is fabricated easily, demonstrates chemical stability to operating tempera-
ture, exhibits a resistance change that may be measured, and has a good gas response.
Pokhrel et al. (2007)
obtained a powdered sample of CTO by the sol-gel method. The thick
films were deposited (developed) on 4 mm length ceramic tubes. These ceramic tubes com-
prised of two Au-electrodes. An eight-layer film was prepared by mixing CTO with glass
powder and
a
-terpinol.
Figure 14.5
shows the binding and dissociation (change in resistance) of ethanol at 40% RH
to the TFM-850 [sintered at 850
C; titanium substituted chromium oxide, CTO thick
film] sensor. A dual-fractal analysis is required to adequately describe the binding kinetics.
A single-fractal analysis is adequate to describe the dissociation kinetics. It is of interest to
note that as the fractal dimension increases by a factor of 1.61 from a value of
D
f1
equal
to 1.828 to
D
f2
equal to 2.948 the binding rate coefficient increases by a factor of 5.21 from
a value of
k
1
equal to 119.11 to
k
2
equal to 620.27. Once again, changes in the degree of
heterogeneity or the fractal dimension on the bentonite-vanadium (V) sol-gel surface and
in the binding rate coefficient are in the same direction.
Pokhrel et al. (2007)
analyzed the binding and dissociation kinetics of alcohol vapors to a
TFE-850 ethanol sensor.
Figure 14.6a
shows the binding and dissociation kinetics observed
when the TFE-850 ethanol sensor was exposed for 36 h to alcohol vapors. A dual-fractal
800
600
400
200
0
0
20
40
Time (s)
60
80
100
Figure 14.5
Binding and dissociation of ethanol vapors in 40% RH (relative humidity) to a CTO (powdered
sample of Cr
1.8
Ti
0.2
O
3
; titanium substituted chromium oxide) thick film in a sol-gel-derived
polycrystalline biosensor (
Pokhrel et al., 2007
).
