Biomedical Engineering Reference
In-Depth Information
were reported by Quang
. [260]. They studied the effect of the
ammonia gas detection on the electrical properties of the SWCNTs
bundles, deposited by screen printing onto alumina substrates. When
temperature increases, an arrangement in the structure takes place,
leading to a change in the values (
et al
), i.e., a change in the electrical
properties of nanotubes from semiconducting to metallic behavior. In
other terms, the metallic part in the bundles increases with increasing
temperature, thus most of tubes become metallic, suppressing the
semiconducting tubes in the bundles. In the contrast, during the
cooling process, the structural arrangement occurs in the opposite
tendency. However, experimental evidence that the resistance of
sensor increases by both heating (metallic response) and cooling
(semiconducting response) when the temperature exceeded 300°C,
provides a support for transition, induced by thermal treatment,
from metallic to a semiconducting response of the SWCNT bundles.
These SWCNT-bundle-based sensors were able to detect NH
n
,
m
as low
3
as concentration of 5 ppm, at room temperature.
. [150] demonstrated impedance spectroscopy
used to study the gas sensing characteristics of both capacitance
and resistance of sensors employing MWCNTs as active gas-sensing
element. These studies revealed the chemisorption of the tested
reducing gases upon the surface of MWCNTs layer. Increasing
sensor impedance was observed with increasing humidity or
partial pressures of NH
Varghese
et al
, CO, and CO
. The impedance changes
3
2
were attributed to
-type conductivity in the semiconducting
MWCNTs, and the formation of Schottky barriers between metallic
and semiconducting nanotubes in the carbon bundles. Reversible
responses were recorded for MWCNT sensors exposed toward
relative humidity, CO and CO
p
; while they were strongly responsive
2
toward NH
behaving as dosimeters.
3
. [263] fabricated a gas sensor by growing
SWCNT thin film directly onto a conventional sensor substrate.
NO
Wongwiriyapan
et al
gases were detected down to 50 ppb level under room
temperature operation with a fast response. Using an electrical
breakdown technique, gas response was improved by an order
of magnitude. The relationship between gas concentration and
sensor response was fitted with a Langmuir adsorption isotherm.
A detection limit was calculated of 8 ppb NO
and Cl
2
2
. This sensor was
2
insensitive toward CO
, alkane, aromatic hydrocarbon, alcohol,
ketone, and carboxylic acid groups. A merit of the device was the its
simplicity in fabrication and feasible application.
, H
2
2
