handbook of modern sensors

512 17 Chemical Sensors
17.4.8 Elastomer Chemiresistors
Elastomer chemiresistors or polymer conductive composites (also polymer conductors
or simply PCs) are polymer films that adsorb chemical species and swell, increasing
resistance as a physical response to the presence of a chemical species. These can
be used as chemical detectors but do not truly employ a chemical reaction. The polymers
are designed and/or treated to attract subsets of chemicals providing a degree of
speciation or selectivity. PCs have become commercially viable [15] as the sensing
element inside of a more complex instrument. The PC sensors can respond to the
presence of simple hydrocarbons like isopropyl alcohol in only a couple of seconds,
whereas more complex oils may take 10–15 s. The PC element is not expected to
be tolerant of corrosives, but barring exposure to such, it should have a life span of
months in normal operation. The PC measurement strategy uses several differently
treated PC elements to produce an array, and then it samples the array to produce a
signature. The commercial instruments based on this technology can readily differentiate
between compounds such as acetone and acetic acid, but they are not designed
to be quantitative. These commercial instruments are complements to metal-oxide
sensors in that they are rather insensitive to fixed gases like O 2 ,Cl 2 ,H 2 , and NO that
are commonly detected using metal-oxide devices. Unlike metal-oxide-based sensors,
the PCs do not require the high controlled operating temperatures and consume
significantly less power.
To detect the presence of a liquid, a sensor usually must be specific to that particular
agent at a certain concentration; that is, it should be selective to the liquid’s
physical and/or chemical properties. An example of such a sensor is a resistive detector
of hydrocarbon fuel leaks (originally devised in Bell Communication Research
to protect buried telephone cables). A detector is made of silicone and carbon black
composite. The polymer matrix serves as the sensing element and the conductive
filler is used to achieve a relatively low volume resistivity, on the order of 10 cm in
the initial standby state. The composition is selectively sensitive to the presence of a
solvent with a large solvent–polymer interaction coefficient [16]. Because the sensor
is not susceptible to polar solvents such as water or alcohol, it is compatible with the
underground environment. The sensor is fabricated in the form of a thin film with a
very large surface/thickness ratio. Whenever the solvent is applied to the film sensor,
the polymer matrix swells, resulting in the separation between conductive particles.
This causes a conversion of the composite film from being a conductor to becoming
an isolator with a resistivity on the order 10 9 cm, or even higher. The response time
for a film sensor is less than 1 s. The sensor returns to its normally conductive state
when it is no longer in contact with the hydrocarbon fuel, making the device reusable.
17.5 Complex Sensors
Complex sensors involve chemical phenomena that change the state of an indicator
as a function of some chemical reaction. The indicator can be a temperature change,
an opacity change, an oscillation frequency change, and so forth. These indicators
require another transducer to convert the changing indicator to an electrical output.