Nanotechnology-Enabled Sensors

48 Chapter 2: Sensor Characteristics and Physical Effects
sensors, which are fabricated from pyroelectric materials such as lithium
tantalate and PZT, generate electric charges with small temperature
changes caused by irradiation of the surface of the crystal. 69
Most of the measurement standards for radiometry are based on thermal
detectors. These devices employ some form of thermal-absorber coating
such as carbon-based paint or diffuse metals such as gold. 70 Potentially,
coatings with nanomaterials present an alternative to these technologies
which provides higher sensitivity for measurements. 70
It is possible to increase the pyroelectric coefficient of polymers by adding
nano-particles. Zhang et al 71 showed that nanocrystalline calcium and
lanthanum powder incorporated into a polyvinylidene fluoride-trifluoroethylene
[P(VDF-TrFE)] copolymer matrix have a higher pyroelectric
coefficient (by ~35%) than those of the P(VDF-TrFE) film of a similar
thickness. It is also possible to indirectly use nanomaterials to enhance the
performance of pyroelectric sensors. For instance, Liang et al 72 uatlized
porous SiO2 film as a thermal-insulation layer to block the diffusion of
heat flow from the pyroelectric layer to the silicon substrate in multilayer
pyroelectric thin film IR detector. This improves the energy confinement
within the pyroelectric sensing layer, resulting in an enhanced performance
of the sensor.
2.3.15 Magneto-Mechanical Effect (Magnetostriction)
Magnetostriction, also called the magneto-mechanical effect, is the
change in a material’s dimensions when subjected to an applied magnetic
field, or alternatively it is a change in a material’s magnetic properties under
the influence of stress and strain. It was first identified in 1842 by
James Joule while examining a sample of nickel. Its name originates from
the Greek word, magnet and the Latin word strictus (meaning compressed,
pressured, tense).
The mechanism that occurs in magnetostriction is illustrated in Fig.
2.22. As can be seen, the domains arrange themselves randomly in a non-
magnetized material. Upon magnetizing, the material’s domains orient with
their axes which changes the length of the structure.
Magnetostrictive materials convert magnetic energy into kinetic energy,
and vice versa. Therefore, they are regularly used for sensing and actuation.
Interestingly, this effect causes the familiar humming sound that is
heard in electrical transformers.
Magnetostriction is defined by the magnetostrictive coefficient, Λ. It is
defined as the fractional change in length as the magnetization of the material
increases from zero to the saturation value. The coefficient, which is