Biomedical Engineering Reference
In-Depth Information
this layered SAW sensor providing frequency shifts of 41.5 and
122.5 kHz respectively due to the mass loading. The schematic of the
fabricated layered SAW gas sensor based on ZnO/LiTaO
structure
3
is shown in Fig. 9.38. The magnitude and phase of S
transmission
21
response of a layered SAW ZnO/LiTaO
device coated with 10 LB
monolayers of SWCNT-based 75 wt.% nanocomposite film were
measured. The insertion loss of SAW device was less than 19 dB
at operating frequency of 69.3 MHz. The transient responses and
related calibration curves of this SAW 69 MHz Love wave dual sensor
toward NO
3
(2-10 ppm) and NH
(166-1000 ppm) are shown in
2
3
Fig. 9.39.
In addition, the SWCNT-CdA LB film nanocomposites have
been used by Penza group at ENEA laboratories as highly gas-
sensitive layer other than SAW devices [113, 179, 186, 291], as
shortly mentioned, also for various piezoelectric transducers using
principles of operation such as quartz crystal microbalance (QCM)
[37, 152, 153, 294] and thin film bulk acoustic resonator (TFBAR)
devices [292].
. [293] demonstrated a SAW 156 MHz quartz
device coated by a spray layer of MWCNTs, both CVD as-grown and
purified with functional COOH- groups, for ethanol detection at room
temperature. They demonstrated that the SAW sensor with modified
MWCNTs layer exhibited higher sensitivity compared to unmodified
CNT layer due to better gas adsorption, and enhanced response time
and recovery time due to improved physisorption capability of the
ethanol molecules into modified CNT layer.
Conventional 10 MHz QCM devices consisting of a circularly
shaped AT-cut quartz crystal with a diameter of about 10 mm and
a thickness of 165
Also, Hsu
et al
m are usually used as robust transducers for
gas sensing applications due to easy to fabricate, intrinsic high
sensitivity, double sensing area on both sides of crystal, sensor
array, and mass production capability. The metal electrodes (e.g., Al,
Au, Ag, Pt) on both sides of the piezoelectric substrate are typically
0.10
µ
µ
m thick and 3-6 mm in diameter. The operating frequency
of resonant crystal ranging from 5 to 30 MHz at the fundamental
frequency depends inversely on the thickness of quartz. This means
that higher frequencies of 30 MHz are forbidden for QCM devices,
otherwise the quartz disk is very thin and thus mechanically fragile.
The sensitive area in the QCM device can be covered by a sensitive
material for gas adsorption, and consequently the frequency shifts
proportionally to the adsorbed molecules mass with a relationship
