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Overview of Smart Materials Technology
Dr. Siu Wing OR
Department of Applied Physics, The Hong Kong Polytechnic University
Why Smart Materials?
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.
Classification of Smart Materials
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.
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Piezoelectric 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 directiondependent. 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 ( BaTiO 3 ) in 1941 (i.e. during World War II). Unlike single crystals,
BaTiO 3 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,
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piezoelectricity is an anisotropic characteristic. Fig. 2 shows the change in crystal
structures of a PZT upon temperature change. Above the Curie temperature ( Tc ), the
ceramic has a cubic (centrosymmetric) structure with no electric dipole moment
within its unit cell. Below Tc , 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 Tc . In practice, it is recommended that the maximum
operational temperature should be kept at approximately half of the stated Tc of the
material (e.g. Tc  360C for PZT-8 and Tc  110C for PVDF).
Fig. 2. Crystal structures of PZT ceramics above and below their
. (PKI)
Shape Memory Alloy Smart Materials
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. TAF  500  700C for Nitinol),
by which they exhibit a highly ordered austenitic crystalline structure. Upon cooling
below their martensite finish temperatures ( TAF ), unstrained SMAs will have a
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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.
SMAs can be operated in a one-way mechanism, that is, upon heating above TAF ,
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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[About the authors: Dr. Siu Wing OR received his B.Sc.(1st Hons.), M.Phil., and
Ph.D. degrees from The Hong Kong Polytechnic University in 1995, 1997, and 2001,
respectively. He worked at various R&D positions in both Hong Kong and Singapore
hi-tech industries from 1995 to 2001. He then worked in the Mechanical and
Aerospace Engineering Department at University of California, Los Angeles (UCLA)
for 1.5 years as a Postdoctoral Research Fellow before he returned to Hong Kong and
joined the Department of Applied Physics at PolyU as a Lecturer in September 2002.
Dr. Or is a member of IEEE and ASME. His research interests include smart materials
and structures, MEMS, and microelectronic packaging technology.]
Keywords:
Smart materials, Sensors, actuators, and transducers, Piezoelectrics, Shape memory
alloys, Metals, ceramics, polymers, and composites
Related Topics in the Syllabus:
Electricity, Mechanics, Heat
Extensions (from the syllabus):
Bring out ideas: Structures, properties, and applications of materials, Conversion and
utilization of energy
Warm-up discussion:
 Most plastics can change its size to some extent, how about metal?
 Is such kind of metal suitable for making cars if it can be changed to some extent
under some conditions?
 Sensors are important devices and scientists do much research to develop
sensors. Let's think wildly. What kinds of sensors would you like to see
developed? Why? Describe their possible applications.
Points for further discussion:
 Is there any difference between the terms “smart materials”, “intelligent
materials”, and “active materials”?
 What is the difference between the terms “smart materials”, “smart devices”,
“smart systems”, and “smart structures”?
 Try to find out other groups of smart materials and to understand their physical
behaviours and potential applications.
 Give a qualitative comparison of different groups of smart materials. Which group
is the most useful?
 What is an electric dipole moment? Consider the structures in Fig.2 of the article,
why is there an electric dipole moment at temperatures below Tc?
 Discuss the sciences involved in developing a useful material. Take the material
PZT mentioned in the article as an example, discuss the role played by different
branches of science in the research and development of a useful material.
Activities:
 Debate: Will Shape Memory Alloys be used instead of common metals in the
future?
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

Appreciation of materials science: Students are asked to find out the
applications of piezoelectric materials, and to find out how these materials are
being used in industry and appliances.
Experiment/demo: Shape memory alloys are readily available. Teachers may
look up websites and buy SMA wires. The shape memory effect can be easily
demonstrated by controlling the temperature simply by using a hair-dryer.
Related web sites:
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Encyclopedia of Smart Materials
This site contains many good materials and news on smart materials.
http://www.mrw.interscience.wiley.com/esm/
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Smart Materials and Structures Laboratory
This site introduces the work and research of Automation and
Computer-aided Engineering Department on the field of smart
materials.
http://www2.acae.cuhk.edu.hk/~sms/intro_frame.html
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Physics World---Metal has memory??
This page provides the principle of shape memory alloy.
http://www.hkphy.org/iq/memory_alloy/memory_alloy_e.htm
o
Physics World---What is piezoelectricity?
This page introduces what is piezoelectricity and one application in our
daily life.
http://www.hk-phy.org/iq/piezo/piezo_e.html
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