Smart Materials

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Smart Materials Concepts & Applications
Submitted by:

SAMSON T.
DEPARTMENT OF MECHANICAL ENGINEERING
COLLEGE OF ENGINEERING TRIVANDRUM
August 2010

ABSTRACT


`Smart' materials have the ability to perform both sensing and actuating
functions. Passively smart materials respond to external change in a useful
manner without assistance, while actively smart materials have a feedback loop
which allows them to both recognize the change and initiate an appropriate
response through an actuator circuit. One of the techniques used to impart
intelligence into materials is `Biomimetics', the imitation of biological functions
in engineering materials. Composite ferroelectrics fashioned after the lateral line
and swim bladders of fish are used to illustrate this idea. `Very smart' materials,
in addition to sensing and actuating, have the ability to `learn' by altering their
property coefficients in response to the environment. Field-induced changes in
the nonlinear properties of relaxor ferroelectrics and soft rubber are utilized to
construct tunable transducers. Integration of these different technologies into
compact, multifunction packages is the ultimate goal of research in the area of
smart materials

New 'smart' materials are being developed which can alter their properties
automatically in response to changes in the environment. Effects such as colour
changes can be produced from stimuli such as temperature. Other 'smart'
materials can adapt to their environments in a similar way to plants, with
actuators and sensors integrated into structural materials such as concrete and
steel, allowing the materials to monitor themselves. Designers are in a position to
identify needs and develop the best specifications for new materials. Truly
intelligent materials could be available in early 21st century, with technology for
manipulating molecular order in materials rapidly advancing.

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VII 1.0 INTRODUCTION

`Smart' materials have the ability to perform both sensing and actuating
functions. Passively smart materials respond to external change in a useful
manner without assistance, while actively smart materials have a feedback
loop which allows them to both recognize the change and initiate an
appropriate response through an actuator circuit. One of the techniques used
to impart intelligence into materials is `Biomimetics', the imitation of
biological functions in engineering materials. Composite ferroelectrics
fashioned after the lateral line and swim bladders of fish are used to illustrate
this idea. `Very smart' materials, in addition to sensing and actuating, have
the ability to `learn' by altering their property coefficients in response to the
environment. Field-induced changes in the nonlinear properties of relaxor
ferroelectrics and soft rubber are utilized to construct tunable transducers.
Integration of these different technologies into compact, multifunction
packages is the ultimate goal of research in the area of smart materials

Materials that have one or more properties that can be significantly
changed in a controlled fashion by external stimuli, such as

¾ stress
¾ temperature
¾ electric or magnetic fields.

1The main advantages of the smart materials are the following:
¾ No moving parts
¾ High reliability
¾ Low power requirements, etc
¾ High energy density (compared to pneumatic and hydraulic
actuators)
¾ Excellent bandwidth

Classification of Smart Materials
¾ Actuating Materials
o Electrorheological Fluids (ER Fluids)
o Shape Memory Alloys (SMA)
¾ Sensing Materials
o Fiber Optic (F.O.) sensors
¾ Dual-Purpose Materials (Actuating & Sensing)
o Magnetostrictive Materials
¾ Piezoelectric Materials

2 2.0 CLASSIFICATION


Material Type
¾ Shape memory alloys
¾ Piezoelectric
¾ Electrostrictive
¾ Magnetostrictive materials
¾ Fiber-optic sensor systems
¾ Conductive polymers
¾ Chromogenic materials and systems:
¾ Thermo chromic
¾ Electro chromic
¾ Controllable fluids:
¾ Electro rheological
¾ Magneto rheological
¾ Biomimetic polymers and gels
¾ Fullerenes and carbon nanotubes


33.0 PIEZOELECTRIC MATERIALS
Fig: 1 – Piezoelectric Principle
The piezoelectric effect describes the
relation between a mechanical stress and an
electrical voltage in solids.
It is reversbile: an applied mechanical stress
will generate a voltage and an applied
voltage will change the shape of the solid by
a small amount (up to a 4% change in
volume). In physics, the piezoelectric effect
can be described as the the page link between electrostatics and mechanics.

History
The piezoelectric effect was discovered in 1880 by the Jacques and Pierre
Curie brothers. They found out that when a mechanical stress was applied on
crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane
sugar, electrical charges appeared, and this voltage was proportional to the
stress. First applications were piezoelectric ultrasonic transducers and soon
swinging quartz for standards of frequency (quartz clocks). An everyday life
application example is your car's airbag sensor. The material detects the
intensity of the shock and sends an electricla signal which triggers the airbag.

Piezoelectric materials
The piezoelectric effect occurs only in non conductive materials. Piezoelectric
materials can be divided in 2 main groups: crystals and cermaics. The most
well-known piezoelectric material is quartz (SiO2).
4 4.0 ELECTROSTRICTIVE MATERIALS

These materials can also change their dimensions on the application of
an electric field. Although the changes thus obtained are not linear in either
direction, these materials have widespread application in medical and
engineering fields.
Electrostriction is a property of all dielectric materials, and is caused by
the presence of randomly-aligned electrical domains within the material.
When an electric field is applied to the dielectric, the opposite sides of the
domains become differently charged and attract each other, reducing
material thickness in the direction of the applied field (and increasing
thickness in the orthogonal directions due to Poisson's ratio). The resulting
strain (ratio of deformation to the original dimension) is proportional to the
square of the polarization. Reversal of the electric field does not reverse the
direction of the deformation.
Electrostrictive materials, such as the ceramic PMN/PT/La
(0.9/0.1/1%), operating above Tmax with a DC bias field behave as a
piezoelectric ceramic material with C<is proportional to> symmetry. The
effective piezoelectric and electromechanical coupling coefficients are found
to be linear as a function of the DC bias field up to about 0.5MV/m, while the
elastic constant (at constant field) and the dielectric constant (at constant
stress) are found to have a quadratic dependence on the DC bias field. Above
0.5 MV/m the piezoelectric and the electromechanical coupling constants
begin to saturate due to higher 4th order electrostriction (S <is proportional
to> kE4, k negative) In essence these materials behave as tunable
piezoelectric materials with the piezoelectric coefficient being directly
proportional to the electrostrictive coefficient and the DC bias field (d3 =
2Q33EDC) up to saturation. The properties of DC biased resonators of this
5material are derived from a nonlinear theory based on the Taylor's series
expansion of the thermodynamic potentials to 3rd and higher order terms in
field and stress. The resonance equations for the DC biased length
extensional (LE) resonator are presented and it is shown that DC biased
resonance techniques can be used to measure the electrostrictive and other
higher order coefficients at frequencies of interest to tbe ultrasonics
community. The experimental apparatus used to measure these properties
will be described and the limitations with regards to isolation of the
measurement signal and the DC bias signal will be discussed. We will show
that these materials, in conjunction with standard piezoelectric ceramics,
offer the transducer design engineer an extra degree of freedom and the
feasibility of unique transducer designs that will allow, for example, multiple
beam patterns from the same circular/linear array using an adjustable DC
bias profile on the array or tbe possible use of the field dependence of the
compliance to fabricate electrically active backing materials. In conclusion
we discuss how a better understanding of the macroscopic theory of
piezoelectric and electrostrictive materials can benefit the transducer
designer.
65.0 MAGNETOSTRICTIVE MATERIAL

These are quite similar to Electrostrictive materials, except for the fact
that they respond to magnetic fields. The most widely used Magnetostrictive
material is TERFENOL-D, which is made from the rarest of the rare earth
elements, i.e. Terbium. This material is highly non-linear and has the
capability to produce large strains.
Magnetostrictive (MS) technology and Magneto-Rheological Fluid
(MRF) technology are old “newcomers” coming to the market at high speed.
Various industries including the automotive industry are full of potential MS
and MRF applications. Magnetostrictive technology and Magneto-
Rheological Fluid technology have been successfully employed already in
various low and high volume applications. A structure based on MS might be
the next generation in design for products where power density, accuracy
and dynamic performance are the key features. Since the introduction of
active (MS) materials such as Terfenol-D, with stable characteristics over a
wide range of temperatures and high magnetoelastic properties, interest in
MS technology has been growing. Additionally, for products where there is a
need to control fluid motion by varying the viscosity, a structure based on
MRF might be an improvement in functionality and costs.
Two aspects of this technology, direct shear mode (used in brakes and
clutches) and valve mode (used in dampers) have been studied thoroughly
and several applications are already present on the market. Excellent
features like fast response, simple interface between electrical power input
and mechanical power output, and precise controllability make MRF
technology attractive for many applications.
7Magnetostriction effects

Crystals of ferromagnetic materials change their shape when they are
placed in a magnetic field.. This phenomenon is called magnetostriction. It is
related to various other physical effects. Magnetostriction is, in general, a
reversible exchange of energy between the mechanical form and the
magnetic form. The ability to convert an amount of energy from one form
into another allows the use of magnetostrictive materials in actuator and
sensor applications.
8 6.0 RHEOLOGICAL MATERIALS

These fluids may find applications in brakes, shock absorbers and
dampers for vehicle seats. These can be fitted to buildings and bridges to
suppress the damaging effects, for example, high winds or earthquakes.


Fig. 2 Rheological Materials


Rheology is the study of the flow of matter: primarily in the liquid
state, but also as 'soft solids' or solids under conditions in which they
respond with plastic flow rather than deforming elastically in response to an
applied force. It applies to substances which have a complex molecular
structure, such as muds, sludges, suspensions, polymers and other glass
formers (e.g. silicates), as well as many foods and additives, bodily fluids (e.g.
blood) and other biological materials.
The flow of these substances cannot be characterized by a single value of
viscosity (at a fixed temperature). While the viscosity of liquids normally
9varies with temperature, it is variations with other factors which are studied
in rheology. For example, ketchup can have its viscosity reduced by shaking
(or other forms of mechanical agitation) but water cannot. Since Sir Isaac
Newton originated the concept of viscosity, the study of variable viscosity
liquids is also often called Non-Newtonian fluid mechanics.

An electro-rheological fluid is a material in which a particulate
solid is suspended in an electrically non-conducting fluid such as oil. On
the application of an electric field, the viscosity and other material
properties undergo dramatic and significant changes. In this paper, the
particulate imbedded fluid is considered as a homogeneous continuum.
It is assumed that the Cauchy stress depends on the velocity gradient
and the electric field vector. A representation for the constitutive
equation is developed using standard methods of continuum mechanics.
The stress components are calculated for a shear flow in which the
electric field vector is normal to the velocity vector. The model predicts
(i) a viscosity which depends on the shear rate and electric field and
(ii) normal stresses due to the interaction between the shear flow and
the electric field. These expressions are used to study several
fundamental shear flows: the flow between parallel plates, Couette flow,
and flow in an eccentric rotating disc device. Detailed solutions are
presented when the shear response is that of a Bingham fluid whose
yield stress and viscosity depends on the electric field.
107.0 THERMORESPONSIVE MATERIALS
Thermoresponsive materials are sometimes also known as shape
memory alloys or shape memory polymers. These materials alter their shape
under the influence of the ambient temperature.

Magnetic Shape Memory Alloys
Like thermoresponsive materials that alter their shape under the
influence of the ambient temperature, magnetic shape memory alloys change
shape due to changes in magnetic fields.

Polychromic, Chromogenic or Halochromic Materials
Polychromic, chromogenic and halochromic materials all change
colour due to external influences. These external influences can be
alterations in pH, temperature, light or electricity. Materials that change
colour due to temperature are normally known as thermochromic materials
and those that alter due to light are photochromic materials.

Applications of Smart Nanomaterials
Smart nanomaterials are expected to make their presence strongly felt
in areas like:
¾ Healthcare, with smart materials that respond to injuries by delivering
drugs and antibiotics or by hardening to produce a cast on a broken
limb.
¾ Implants and prostheses made from materials that modify surfaces and
biofunctionality to increase biocompatibility
¾ Energy generation and conservation with highly efficient batteries and
energy generating materials.
¾ Security and Terrorism Defence with smart materials that can detect
11toxins and either render them neutral, warn people nearby or protect
them from it.
¾ Smart textiles that can change colour, such as camouflage materials
that change colour and pattern depending upon the appearance of the
surrounding environment. These materials may even project an image
of what is behind the person in order to render them invisible.
¾ Surveillance using “Smartdust” and “Smartdust Motes” that are
nanosized machines housing a range of sensors and wireless
communication devices. Individually they can float undetected in a
room with other dust particles. By combining the information gathered
from hundreds, thousands or millions of these tiny specs can give a full
report on what is occurring with the area including sound and images.

Fig. 3 Shape Memory Alloys
128.0 ELECTROCHROMIC MATERIALS
Electrochromism is the phenomenon displayed by some materials of
reversibly changing color when a burst of charge is applied. Various types of
materials and structures can be used to construct electrochromic devices,
depending on the specific applications.
One good example of an electrochromic material is polyaniline which
can be formed either by the electrochemical or chemical oxidation of aniline.
If an electrode is immersed in hydrochloric acid which contains a small
concentration of aniline, then a film of polyaniline can be grown on the
electrode. Depending on the oxidation state, polyaniline can either be pale
yellow or dark green/black. Other electrochromic materials that have found
technological application include the viologens and polyoxotungstates. Other
electrochromic materials include tungsten oxide (WO3), which is the main
chemical used in the production of electrochromic windows or smart glass.
Polymer-based solutions have recently been developed by John Reynolds
and colleagues at the University of Florida. These promise to provide flexible
and cheap electrochromics in a variety of colours, going all the way up to
black.
Fig. 4 Electrochromic Materials
As the color change is persistent
and energy need only be applied to
effect a change, electrochromic
materials are used to control the
amount of light and heat allowed to pass
through windows ("smart windows"),
and has also been applied in the
automobile industry to automatically
tint rear-view mirrors in various lighting
13conditions. Viologen is used in conjunction with titanium dioxide (TiO2) in
the creation of small digital displays. It is hoped that these will replace liquid
crystal displays as the viologen, which is typically dark blue, has a high
contrast compared to the bright white of the titania, thereby providing the
display high visibility.
ICE 3 high speed trains use electrochromatic glass panels between the
passenger compartment and the driver's cabin. The standard mode is
clear/lucent and can be switched by the driver to frosted/translucent mainly
to keep passengers off "unwanted sights" for example in case of (human)
obstacles.





Fig. 10 Electrochromic
Materials


14 9.0 APPLICATION
¾ Fast response valves
¾ High power density hydraulic pumps
¾ Active bearings for reduction of machinery noise
¾ Footwear
¾ Sports equipment
¾ Precision machining
¾ Vibration and acoustic sensors
¾ Dampers, etc.
¾ Healthcare, with smart materials that respond to injuries by delivering
drugs and antibiotics or by hardening to produce a cast on a broken
limb.
¾ Energy generation and conservation with highly efficient batteries and
energy generating materials.
¾ Security and Terrorism Defence with smart materials that can detect
toxins and either render them neutral, warn people nearby or protect
them from it.
¾ Smart textiles that can change colour, such as camouflage materials
that change colour and pattern depending upon the appearance of the
surrounding environment. These materials may even project an image
of what is behind the person in order to render them invisible.
¾ Surveillance using “Smartdust” and “Smartdust Motes” that are
nanosized machines housing a range of sensors and wireless
communication devices. Individually they can float undetected in a
room with other dust particles. By combining the information gathered
from hundreds, thousands or millions of these tiny specs can give a full
report on what is occurring with the area including sound and images.
1510.0 CONCLUSION

Optical-fibre crack-monitoring sensors are capable of detecting crack-
width up to 5µm. The technologies using smart materials are useful for both
existing and emerging technologies.
Smart Materials described here need further research to evolve the
design guidelines of systems.
The technologies using smart materials are useful for both new and
existing constructions. Of the many emerging technologies available the few
described here need further research to evolve the design guidelines of
systems. Codes, standards and practices are of crucial importance for the
further development.
Piezoelectrics and magnetostrictives are most effective for high
frequency control applications. Piezoelectrics for applications were size of the
element is of concern. Magnetostrictives are good when size is of no concern.
Shape memory alloys are very effective for low frequency vibration or shape
control. Electrorheological fluids are still being explored. Sandwich beams
are generally the only structural application
Fiber optic sensors are very effective in all application
1611.0 REFERENCE


→ Erin B. Murphy, Fred Wudl, “The world of smart healable materials”, Review
Article Progress in Polymer Science, Volume 35, Issues 1-2, January-February
2010, Pages 223-251
→ Frank Stajano a,*, Neil Hoult b,1, Ian Wassell a, Peter Bennett b, Campbell
Middleton b, Kenichi Soga b, Smart bridges, smart tunnels: Transforming wireless
sensor networks from research prototypes into robust engineering infrastructure,
Technical report, Federal Highway Administration, Turner- Fairbank Highway
Research Center, January 2008.
→ Giola B. Santoni, Lingyu Yu, Buli Xu, Victor Giurgiutiu, “Lamb Wave-Mode Tuning
of Piezoelectric Wafer Active Sensors for Structural Health Monitoring”,
Transactions of the ASME Vol. 129, DECEMBER 2007, PP. 752-762.
→ Victor Giurgiutiu, Andrei Zagrai, JingJing Bao, “Embedded Active Sensors for In-
Situ Structural Health Monitoring of Thin-Wall Structures”, Journal of Pressure
Vessel Technology, AUGUST 2002, Vol. 124, pp. 293-302.
→ K F Hale, “An optical-fibre fatigue crack-detection and monitoring system”, Smart
Mater. Struct. 1 (1992) 156-161.