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Hesham Mohsen Hussein – 1093233
Advanced Materials Engineering
Introduction to Smart Materials
Abstract
Smart materials engineering is a relatively new and promising branch in modern
materials engineering. Smart materials are materials which can produce various effects
and results, when subject to various external effects, such as stress or heat. Such effects
produced, mostly in the form of electrical charge, are manipulated to produce various
practical applications, such as using piezoelectric materials to as motion sensors, for
example. This essay is going to discuss the different types of smart materials, the
manufacturing process of smart materials (specifically piezoelectric materials) and the
different applications, and finally, the application of energy harvesting is discussed.
History
The pyroelectric effect, by which a material generates an electric potential in
response to a temperature change, was studied by Carl Linnaeus and Franz Aepinus in the
mid-18th century. Drawing on this knowledge, both René Just Haüy and Antoine César
Becquerel posited a relationship between mechanical stress and electric charge; however,
experiments by both proved inconclusive.
The first demonstration of the direct piezoelectric effect was in 1880 by the
brothers Pierre Curie and Jacques Curie. They combined their knowledge of
pyroelectricity with their understanding of the underlying crystal structures that gave rise
to pyroelectricity to predict crystal behavior, and demonstrated the effect using crystals
of tourmaline, quartz, topaz, cane sugar, and Rochelle salt (sodium potassium tartrate
tetrahydrate).
For the next few decades, piezoelectricity remained something of a laboratory
curiosity. More work was done to explore and define the crystal structures that exhibited
piezoelectricity. This culminated in 1910 with the publication of Woldemar
Voigt's Lehrbuch der Kristallphysik (Textbook on Crystal Physics), which described the
20 natural crystal classes capable of piezoelectricity, and rigorously defined the
piezoelectric constants using tensor analysis.
The first practical application for piezoelectric devices was sonar, first developed
during World War I. In France in 1917, Paul Langevin and his coworkers developed
an ultrasonicsubmarine detector. The detector consisted of a transducer, made of thin
quartz crystals carefully glued between two steel plates, and a hydrophone to detect the
returned echo. By emitting a high-frequency chirp from the transducer, and measuring the
amount of time it takes to hear an echo from the sound waves bouncing off an object, one
can calculate the distance to that object.
The use of piezoelectricity in sonar, and the success of that project, has created
intense interest in piezoelectric devices. Over the next few decades, new piezoelectric
materials and new applications for those materials were explored and developed.
During World War II, independent research groups in the United States, Russia,
and Japan discovered a new class of synthetic materials, called ferroelectrics, which
exhibited piezoelectric constants many times higher than natural materials. This led to
intense research to develop barium titanate and later lead zirconate titanate materials with
specific properties for particular applications.
Types
1- Piezoelectric materials: Materials that give an electronic charge when stress is
applied on them.
2- Shape memory alloys: Materials in which large deformation can be induced and
recovered through temperature changes or stress changes
3- Magnetostrictive: Materials which show change in shape under the influence of
magnetic field, and also a change in their magnetization under the influence of
mechanical stress.
4- pH sensitive polymers: Materials that change in volume when the pH of their
medium changes.
5- Temp-responsive polymers: Materials which change when subjected to a change
in temperature.
6- Photomechanical materials: Materials which change shape when exposed to
light.
7- Self-healing materials
8- Dielectric elastomers: Materials which produce large strains when subjected to
an external electric field
9- Macrofibre composites:
The Macro Fiber Composite (MFC) is the leading low-profile
actuator and sensor offering high performance, flexibility and
reliability in a cost competitive device.
The MFC consists of rectangular piezo ceramic rods sandwiched
between layers of adhesive, electrodes and polyimide film. This
assembly enables in-plane poling, actuation and sensing in a sealed and durable, ready to
use package. As a thin, surface conformable sheet it can be applied (normally bonded) to
various types of structures or embedded in a composite structure. If voltage is applied it
will bend or distort materials, counteract vibrations or generate vibrations. If no voltage is
applied it can work as a very sensitive strain gauge, sensing deformations, noise and
vibrations. The MFC is also an excellent device to harvest energy from vibrations.
10- 1-3 Composites:
1-3 Piezo Composites have become the material of choice for many high performance
ultrasound transducer since it was invented by R.E. Newnham and L.E. Cross in the late
1970's.
A variety of piezocomposite materials can be made by combining piezo ceramic
elements with a passive polymer such as epoxy or active polymer. Piezo-composites are
classified according to their connectivity (such as 2-2, 1-3, 0-3 etc.,). Connectivity is
defined as the number of dimensions through which the material is continuous. It is
conventional for the first digit to refer to the piezoelectrically active phase.
Today the most piezo composites on the market are with the 1-3 and 2-2
connectivity used in ultrasound transducers, actuators and sensors.
The biggest single market for the 1-3 piezo composite is the medical diagnostic
ultrasound market which is using more 1-3 piezo composite than the other markets
combined. Today's medical ultrasound imaging systems would be not possible without
the advancements in 1-3 piezo composites.
Manufacturing of Piezoelectric Materials
Bulk Process
Piezoceramic bulk elements are manufactured from
spray-dried granular material by mechanical hydraulic
presses. The compacts are either manufactured true to
size, taking into account the sintering contraction, or with
machining excesses which are then reworked to achieve
the required precision. The bulk process starts with
mixing and ball milling of the raw materials. Next, the
mixture is heated to 75% of the sintering temperature to
accelerate reaction of the components. The
polycrystalline, calcinated powder is ball milled again to
increase its reactivity. Granulation with the binder is next
to improve processing properties. After shaping and
pressing the (green) ceramics is heated to 750 to burn
out the binder.
The next phase is sintering at temperatures between
1250° C and 1350° C. The sintered ceramic material is
hard and can be sawn cut, ground, polished, lapped, etc.,
to the desired shape and tolerance if required. Screen
printing is used to metallize the piezoelements and
sputtering processes (PVD) are employed for thin
metallizing layers (electrodes are applied). The sintered
elements are then polarized. The last step is the poling
process which takes place in a heated oil bath at electrical fields up to several kV/mm.
Thin Film Multilayer Process
Multilayer Piezo actuators require a different
manufacturing process. After milling a slurry is
prepared. A foil casting process allows layer thickness
down to 20 µm. Next, the sheets are screen printed and
laminated. A compacting process increases density of
the "green" ceramics and removes air trapped between
the layers. The final steps are the binder burnout,
sintering (co-firing) at temperatures below 1100 C, end
termination and poling.
All processes, especially the heating and sintering cycles
must be controlled to very tight tolerances. The smallest
change affects quality and properties of the Piezo
material. 100% final testing of the piezo material and
components at PI Ceramic guarantees the highest
product quality.
Applications of Piezoelectric Materials
Energy Harvesting
Energy harvesting of piezoelectric materials is a promising branch of research.
Anything that induces mechanical stress can be used to apply stress to a piezoelectric
material, causing it to give electric charge. Although the electricity produced from such
stresses is fairly small (usually measured in milliwatts), but it has lots of applications, like
charging electronic devices such as cellphones. Some applications have had successful
integration of piezoelectric materials in clothing; most commonly shoes.
Sensors
A piezoelectric sensor is a device that uses the piezoelectric effect to
measure pressure, acceleration, strain or force by converting them to
an electrical charge. Piezoelectric sensors have proven to be versatile tools for the
measurement of various processes. They are used for quality assurance, process
control and for research and development in many different industries. Although the
piezoelectric effect was discovered by Pierre Curie in 1880, it was only in the 1950s that
the piezoelectric effect started to be used for industrial sensing applications. Since then,
this measuring principle has been increasingly used and can be regarded as a mature
technology with an outstanding inherent reliability. It has been successfully used in
various applications, such as in medical, aerospace, nuclear instrumentation, and as a
pressure sensor in the touch pads of mobile phones. In the automotive industry,
piezoelectric elements are used to monitor combustion when developing internal
combustion engines. The sensors are either directly mounted into additional holes into the
cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric
sensor.
The rise of piezoelectric technology is directly related to a set of inherent
advantages. Even though piezoelectric sensors are electromechanical systems that react
to compression, the sensing elements show almost zero deflection. This is the reason why
piezoelectric sensors are so rugged, have an extremely high natural frequency and an
excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is
insensitive to electromagnetic fields and radiation, enabling measurements under harsh
conditions. Some materials used (especially gallium phosphate or tourmaline) have an
extreme stability even at high temperature, enabling sensors to have a working range of
up to 1000 °C. Tourmaline shows pyroelectricity in addition to the piezoelectric effect;
this is the ability to generate an electrical signal when the temperature of the crystal
changes.
Actuators
As very high electric fields correspond to only tiny changes in the width of the
crystal, this width can be changed with better-than-µm precision, making piezo crystals
the most important tool for positioning objects with extreme accuracy — thus their use
in actuators. Multilayer ceramics, using layers thinner than 100 µm, allow reaching high
electric fields with voltage lower than 150 V. These ceramics are used within two kinds
of actuators: direct piezo actuators and Amplified piezoelectric actuators. While direct
actuator's stroke is generally lower than 100 µm, amplified piezo actuators can reach
millimeter strokes.
Piezoelectric Motors
Motors are made in both linear and rotary types. Of these, one drive technique is
to use piezoelectric ceramics to push a stator. These piezoelectric motors use three groups
of crystals: two of which are Locking and one Motive, permanently connected to either
the motor's casing or stator (not both) and sandwiched between the other two, which
provides the motion. These piezoelectric motors are fundamentally stepping motors, with
each step comprising either two or three actions, based on the locking type. Another
mechanism employs the use of surface acoustic waves (SAW) to generate linear or
rotational motion.
A second drive technique is illustrated by the Squiggle motor, in which
piezoelectric elements are bonded orthogonally to a nut and their ultrasonic vibrations
rotate and translate a central lead screw. This is a direct drive mechanism.
References
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Gautschi, G. (2002). Piezoelectric sensorics. Springer Berlin, Heidelberg, New
York. p. 3
http://www.physikinstrumente.com/tutorial/4_16.html
Katzir, S. (2012-06-20). "Who knew piezoelectricity?
Rutherford and Langevin on submarine detection and the invention of sonar".
http://www.smart-material.com/MFC-product-main.html
http://www.smart-material.com/13CompOverview.html
http://www.slideshare.net/SlavaAlexey/piezomotors
Srinivasan (2001). Smart Structures- Analysis and Design.
Wikipedia
Youtube