introduction to smart materials

1. SMART MATERIALS and MEMS

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Smart Materials
Smart materials are designed materials that have one or more properties that can be
significantly changed in a controlled fashion by external stimuli, such as stress, temperature,
moisture, pH, electric or magnetic fields.
Smart or intelligent materials are materials that have the intrinsic and extrinsic
capabilities, first, to respond to stimuli and environmental changes and, second, to activate
their functions according to these changes. The stimuli could originate internally or
externally.

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Types of smart materials
Piezoelectric Materials: When subjected to an electric charge or a variation in voltage, piezoelectric
material will undergo some mechanical change, and vice versa. These events are called the direct and
converse effects.
Electrostrictive Materials: This material has the same properties as piezoelectric material, but the
mechanical change is proportional to the square of the electric field. This characteristic will always
produce displacementsin the same direction.
Magnetostrictive Materials: When subjected to a magnetic field, and vice versa (direct and converse
effects), this material will undergo an induced mechanical strain. Consequently, it can be used as sensors
and/or actuators. (Example: Terfenol-D.)

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Shape Memory Alloys: When subjected to a thermal field, this material will undergo phase
transformations which will produce shape changes. It deforms to its ‘martensitic’ condition with
low temperature, and regains its original shape in its ‘austenite’ condition when heated (high
temperature). (Example: NitiNOL TiNi.)
Halochromic Materials: These are commonly used materials that change their colour as a
result of changing acidity. One suggested application is for paints that can change colour to
indicate corrosion in the metal underneath them.
Optical Fibres: Fibres that use intensity, phase, frequency or polarization of modulation to
measure strain, temperature, electrical/magnetic fields, pressure and other measurable
quantities. They are excellent sensors.

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Characteristics of smart materials:
Immediacy – they respond in real‐time.
Transiency – they respond to more than one environmental state.
Self‐actuation – intelligence is internal to rather than external to the ‘material’.
Selectivity – their response is discrete and predictable.
Directness – the response is local to the ‘activating’ event.
Property change
– undergo a change in a property or properties
– chemical, thermal, mechanical, magnetic, optical or electrical – in response to a change in the conditions of the environment of the
material
– thermochromics, electrochromics, photochromics
Energy change
– change an input energy into another form to produce output energy in accordance with the First Law of Thermodynamics
– piezoelectrics, pyroelectrics, photovoltaics, ……
Reversibility / directionality
Size / location

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Shape Memory Alloys:
Nickel-Titanium-Naval-Ordnance-Laboratories (NiTiNOL)
Nickel-Titanium-Copper (NiTiCu)
Copper-Zinc-Aluminum-Nickel
Applications of Shape MemoryEffect:
Self-expandable cardiovascular stent (a small support that is put in the side of a blood vessel tube)
Blood clot filters
Engines
Actuators for smart systems
Flaps that change direction of airflow depending upon temperature (for air conditioners)
Couplings
Springs
Fire Alarms

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The key properties of NiTiNOL include
• Large forces that can be generated due to the shape memory effect.
• Excellent damping properties below the transition temperature
• Excellent corrosion resistance
• Nonmagnetic
• High fatigue strength
• Moderate impact resistance
• Moderate heat resistance
• Biocompatible

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The applications of NiTiNOL are
• Aerospace and naval applications - coupling have are being used in military
aircraft and naval craft.
• Medical Applications - Tweezers for removing foreign objects via small
incisions, anchors for tendon fixation and stents for cardiovascular applications.
• Dentistry - Orthodontic wires, which do not need to be retightened and adjusted
• Safety devices - Safety valves/actuators to control water temperature and fire
sprinklers
• Fasteners, seals, connectors and clamps
• Safety devices – Safety valves/actuators to control water temperature and fire
sprinklers.

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Magnetic Materials:
The materials that can be either attracted or repelled when placed in an external
magnetic field and can be magnetized themselves.
Example: Iron or its alloys which are used in various electrical appliances like
generators, televisions, cassette recorders, magnetic core computer memories, etc.
On the basis of orientation, the magnetic materials are classified into five categories.
1) Diamagnetic
2) Paramagnetic
3) Ferromagnetic
4) Anti Ferromagnetic
5) Ferrimagnetic

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Diamagnetic materials have a weak, negative susceptibility to magnetic fields. Diamagnetic
materials are slightly repelled by a magnetic field and the material does not retain the magnetic
properties when the external field is removed. Most elements in the periodic table, including
copper, silver, and gold, are diamagnetic.
Paramagnetic materials have a small, positive susceptibility to magnetic fields. These
materials are slightly attracted by a magnetic field and the material does not retain the
magnetic properties when the external field is removed. Paramagnetic materials include
magnesium, molybdenum, lithium, and tantalum.
Ferromagnetic materials have a large, positive susceptibility to an external magnetic field.
They exhibit a strong attraction to magnetic fields and are able to retain their magnetic
properties after the external field has been removed. Iron, nickel, and cobalt are examples of
ferromagnetic materials.
Ferrimagnetic materials are similar to ferromagnetic materials but weaker. The difference
between ferromagnetic and ferrimagnetic materials is related to their microscopic structure.
Magnetite is the best example of ferrimagnetic materials.

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Characteristicsof electromagneticmaterials
High permeability
Saturationflux density
High electrical resistivity
Small hysteresis loop
Small corrosive field
Characteristicsof magnetic materials
High permeability
Strong magnetic reluctance
 High value of BH product
Large hysteresis loop
High corrosive field