Page 245 - Mechanics of Microelectromechanical Systems

232 Chapter 4

where the induction vector {B} replaces the dielectric displacement vector
{D}, the magnetic permeability matrix substitutes the electrical
permittivity matrix (both calculated for constant stress), and the magnetic
field H is used instead of the electric field E. The changes mentioned here in
Eqs. (4.120) and (4.121) are also valid for the two problems solved that
studied the piezoelectric effect. The remark has to be made that the coupling
factor is defined here as:

where the piezomagnetic energy is:

and the magnetic energy is:

Piezomagnetic materials, such as Terfenol-D, can be deposited in thin or
thick layers on various substrates in order to create composite
microcantilevers that can be used for MEMS actuation purposes especially,
as will be shown in the sections presenting the bimorphs and the
multimorphs, later in this chapter.

7 SHAPE MEMORY ALLOY (SMA)
TRANSDUCTION

The shape memory alloys, in their bulk (macroscopic) form, are utilized
in many applications, particularly in the medical industry and are mainly
noted for two properties: the shape memory effect (SME) and the
superelasticity (SE). Shape memory alloy thin films are shown to preserve the
important advantages of SMAs in macro-scale designs, namely the large
levels of actuation force and deformation, while substantially improving
(reducing) the response time (which is a deficiency of macro-scale SMA
designs) due to higher surface-to-volume ratios. Medical applications include
arch wires for orthodontic correction, dental implants (teeth-root prostheses)
and the attachments for partial dentures, orthopedics where SMA plates are
used as prosthetic joints to attach broken bones, the spinal bent calibration bar
(the Harrington bar), actuators in artificial organs such as heart or kidney,
active endoscopes and guidewires. Other SMA applications are free and