Page 50 - An Introduction to Microelectromechanical Systems Engineering

Important Material Properties and Physical Effects 29

where ε is the dielectric permittivity of the material. In this case, d units of C/N are
used. The reversibility between strain and voltage makes piezoelectric materials
ideal for both sensing and actuation. Further detailed reading on piezoelectricity
may be found in [23, 24].
Quartz is a widely used stand-alone piezoelectric material, but there are no
available methods to deposit crystalline quartz as a thin film over silicon substrates
(see Table 2.5). Piezoelectric ceramics are also common. Lithium niobate (LiNbO )
3
and barium titanate (BaTiO ) are two well-known examples, but they are also diffi-
3
cult to deposit as thin films. Piezoelectric materials that can be deposited as thin film
with relative ease are lead zirconate titanate (PZT)—a ceramic based on solid solu-
tions of lead zirconate (PbZrO ) and lead titanate (PbTiO )—ZnO, and PVDF. Zinc
3 3
oxide is typically sputtered and PZT can be either sputtered or deposited in a sol-gel
process (Chapter 3 describes the deposition processes in more detail). PVDF is a
polymer that can be spun on. All of these deposited films must be poled (i.e., polar-
ized by heating above the Curie temperature, then cooling with a large electric field
across them) in order to exhibit piezoelectric behavior.

Thermoelectricity
Interactions between electricity and temperature are common and were the subject
of extensive studies in the nineteenth century, though the underlying theory was not
put in place until early in the twentieth century by Boltzmann. In the absence of a
magnetic field, there are three distinct thermoelectric effects: the Seebeck, the Pel-
tier, and the Thomson effects [25]. The Seebeck effect is the most frequently used
(e.g., in thermocouples for the measurement of temperature differences). The Peltier
effect is used to make thermoelectric coolers (TECs) and refrigerators. The Thom-
son effect is less known and uncommon in daily applications.
In the Peltier effect, current flow across a junction of two dissimilar materials
causes a heat flux, thus cooling one side and heating the other. Mobile wet bars with
Peltier refrigerators were touted in 1950s as the newest innovation in home appli-
ances, but their economic viability was quickly jeopardized by the poor energy con-
version efficiency. Today, Peltier devices are made of n-type and p-type bismuth
telluride elements and are used to cool high-performance microprocessors, laser
diodes, and infrared sensors. Peltier devices have proven to be difficult to implement
as micromachined thin-film structures.

Table 2.5 Piezoelectric Coefficients and Other Relevant Properties for a Selected List of Piezoelectric
Materials
Material Piezoelectric Relative Density Young’s Acoustic
3
Constant (d ) Permittivity (g/cm ) Modulus Impedance
ijj
2
6
(10 −12 C/N) (ε ) (GPa) (10 kg/m ⋅s)
rr
Quartz d = 2.31 4.5 2.65 107 15
33
Polyvinylidene-fluoride d = 23 12 1.78 3 2.7
31
(PVDF) d =−33
33
LiNbO 3 d =−4, d = 23 28 4.6 245 34
31
33
BaTiO3 d = 78, d = 190 1,700 5.7 30
33
31
PZT d =−171 d = 370 1,700 7.7 53 30
31
33
zinc oxide (ZnO) d = 5.2, d = 246 1,400 5.7 123 33
31
33