As humanity has made great progress in processing
elementary materials, they have become milestones that marked
the early stages of mankind development, such as Stone Age,
Bronze Age, Iron Age, etc. It was only the beginning of the
recent hundred years that materials became multifunctional
and required the optimization of different properties. With
the last evolution, the concept has been driving toward composite
materials where two or more distinct material phases are being
combined together to provide a better combination of properties.
Currently, the next evolutionary step is being contemplated
with the concept of smart materials.
Smart materials, similar to living beings, have
the ability to perform both sensing and actuating functions
and are capable of adapting to changes in the environment.
In other words, smart materials can change themselves in response
to an outside stimulus or respond to the stimulus by producing
a signal of some sort. By utilizing these materials, a complicated
part in a system consisting of individual structural, sensing,
and actuating components can now exist in a single component,
thereby reducing overall size and complexity of the system.
However, smart materials will never replace systems fully;
they usually are part of some smart systems. Examples include
smart medical systems treating diabetes with blood sugar sensors
and insulin delivery pumps, smart airplane wings achieving
greater fuel efficiency by altering their shape in response
to air pressure and flying speed, smart tennis rackets having
rapid internal adjustments for overhead smashes and delicate
drop shots, vibration-damping systems for large civil engineering
structures and automobile suspension systems, as well as smart
water purification systems for sensing and removing noxious
pollutants, to name a few. Nowadays, there are numerous research
activities at universities, companies, and government organizations
worldwide. Researchers are constantly finding combinations
of technologies to increase avenues for commercialization.
Smart materials can be conveniently subdivided
into passively and actively smart materials. A passively smart
material responds to an external change without thought or
signal processing while an actively smart material analyzes
the sensed signal, perhaps for its frequency components, and
then makes a choice as to what type of response to make. Virtually
all shape memory alloy materials fall into the first category
as they simply respond to temperature change around them by
changing shape without analyzing any signal. It is analogous
to the reflex responses of the human body, i.e. a knee jerk
reaction. An example of an actively smart material is the
vibration damping made from piezoelectric materials, which
utilize a feedback loop enabling them to both recognize the
change and initiate an appropriate response through an actuator
circuit. In the following sections, we will discuss briefly
the physical basis of these two groups of smart materials.
Piezoelectricity was discovered by Pierre and
Jacques Curie in the 1880s. The Curies noticed two things:
Firstly, applying mechanical stress on certain natural nonsymmetrical
crystals changes their internal electric polarization in proportion
to that stress. Secondly, the same crystals deform when they
are subjected to an electric field. These unique properties
are known as the direct and converse piezoelectric effects
respectively. The piezoelectric effect is practically linear
and direction-dependent. As shown in Fig. 1, compressive and
tensile stresses applied along the polarization direction
of the material will generate electric fields and hence voltages
of opposite polarity. The phenomenon is also reciprocal so
that upon application of electric fields of opposite polarity,
the material will expand and contract in accordance with the
applied fields. While these direct and converse effects are
mostly used in sensors/generators and actuators/motors respectively,
this dual functional ability enables piezoelectrics to be
employed as transducers, converting electrical energy into
mechanical energy and vice-versa.
Fig. 1. The piezoelectric effect.
The practical use of piezoelectric materials
was first demonstrated in quartz crystals as underwater sonar
transducers by Paul Langevin in 1916. Over time, more and
more single crystals, for instance Rochelle salt, tourmaline,
etc., became possible, but their usages were very limited
until the discovery of unusual piezoelectric properties in
barium titanate (  )
in 1941 (i.e. during World War II). Unlike single crystals,
is
a ceramic (oxide-based) material with polycrystalline structure.
It is generally more cost-effective to manufacture and less
limited in size. This success was followed by the discovery
of strong and stable effect in lead zirconate titanate (PZT)
ceramics in 1954, which was an important step as most commercially
available piezoelectric devices today are still PZT-based.
Nevertheless, there are several hundreds of known piezoelectric
materials. On the ceramic side, these include lead titanate
(PT) and lead magnesium niobate (PMN); on the polymer side,
these include polyvinylidence fluoride (PVDF) and its copolymers
such as polyvinylidence fluoride-trifluoroethylene [P(VDF-TrFE)];
and on the composite side, these include ceramic/polymer and
ceramic/glass composites.
For a material to exhibit the piezoelectric
effect, its structure should have no center of symmetry (noncentrosymmetric).
That is, the material has anisotropic characteristics, i.e.
its properties differ according to the direction of measurement.
Consequently, piezoelectricity is an anisotropic characteristic.
Fig. 2 shows the change in crystal structures of a PZT upon
temperature change. Above the Curie temperature (  ),
the ceramic has a cubic (centrosymmetric) structure with no
electric dipole moment within its unit cell. Below ,
however, the positively charged Ti/Zr ion shifted from its
central location along one of the several allowed directions,
thereby distorting the crystal lattice into perovskite (noncentrosymmetric)
structure and producing an electric dipole with a single axis
of symmetry. This implies that the piezoelectric performance
decreases steadily as the operational temperature is elevated
until no activity is noticed near or above .
In practice, it is recommended that the maximum operational
temperature should be kept at approximately half of the stated
of the material (e.g.  for
PZT-8 and  for
PVDF).
Fig. 2. Crystal structures of PZT ceramics
above and below their .
(PKI)
Shape memory alloys (SMAs) are the most common
group of metallic materials that can be deformed and revert
back to their original (undeformed) shapes when heated above
their transformation temperatures, just look like a memory.
The first developed SMA was an alloy of nickel and titanium
called Nitinol, which was discovered by scientists at the
U.S. Naval Ordnance Laboratory in 1965. This alloy can deform
up to 10 % and still regain its original form. Beyond this
limit, it deforms plastically and does not regain its original
shape. The newest types of SMAs are combinations of copper
and zinc that are allowed with other metals such as aluminium.
SMAs are being used in a variety of situations. One of these
is as joints/screws for underwater construction so that underwater
welding can be avoided. The other is for biomedical device
applications (e.g. surgical clamps) since Nitinol is biological
compatible.
The shape memory effect is illustrated in Fig.
3. Basically, shape recovery is due to solid-to-solid phase
transformation from martensite
to austenite.
SMAs are typically formed above their austenite finish temperatures
(i.e. 
for Nitinol), by which they exhibit a highly ordered austenitic
crystalline structure. Upon cooling below their martensite
finish temperatures (  ),
unstrained SMAs will have a twinned martensitic structure.
SMAs deform easily under stress due to twin boundaries propagating
throughout the structures in the stress direction. Austensite
crystalline structure is regained upon heating, retuning to
their original shapes.
- Shape Memory AlloyExperiment 1
- Shape Memory Alloy Experiment 2
SMAs can be operated in a one-way mechanism,
that is, upon heating above , they adapt to their desired
shapes as they originally had. However, it is also possible
to train them to operate fully reversible, which is known
as the two-way shape memory effect. Here the twinning during
each cycle takes place in exactly the same manner resulting
in a nearly identical microstructure after each deformation.
Generally, the one-way shape memory effect leads to higher
strains than the two-way effect.
Fig. 3. The shape memory effect. (AML,
UCLA)
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