Introduction:


For centuries, materials have been known to react to the surrounding environment producing some form of response. For instance, in 1824 ,Rochelle salt was discovered to become electrically polarized by the application of heat. That was the first discovery of the effect known as pyroelectricity.

Since that time, numerous additional materials have been discovered having the inherent capability to convert one form of energy into another Sensors are materials that respond to a physical stimulus, such as a change in temperature, pressure, or illumination, and transmit a resulting signal for monitoring or operating a control. Actuators are materials that respond to a stimulus in the form of a mechanical property change such as a dimensional or a viscosity change.

In this table some of smart materials that used around the world:


Material Class

Stimulus

Response

Pyroelectrics

Temperature Change

Electric Polarization

Piezoelectrics

Mechanical Strain

Electric Polarization

Electrostrictors

Mechanical Strain

Electric Polarization

Magnetostrictors

Mechanical Strain

Change in Magnetic Field

Electroactive Polymers

Mechanical Strain

Electric Polarization

Electroluminescent materials

Electric Field

Light Emission

Electrochromic Materials

Incident Light

Light Emission

Photoluminescent Materials

Electric Field

Color Change

Piezoelectrics

Electric Current

Mechanical Strain

Electrostrictors

Electric Current

Mechanical Strain

Magnetostrictors

Magnetic/Electric Field

Mechanical Strain

Shape Memory Alloys

Temperature Change

Mechanical Strain

Electroactive Polymers

Electric Field/pH change

Mechanical Strain

Electrorheological Fluids

Electric Field

Viscosity Change

Now...

We'll discuss some of these materials:


1. shape memory alloys:


It's the materials that changes the properties when the temperature changes ,and retrieve its own shape when apply heat on it.
Arne Olander first observed these unusual properties in 1938 , but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), and CuZnAl.
and there are two types of shape memory alloys:
  • One-way memory effect

When a shape-memory alloy is in its cold state the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape, until deformed again.

  • Two-way memory effect

The two-way shape-memory effect is the that the material remembers two different shapes: one at low temperatures, and one at the high-temperature shape. A material that shows a shape-memory effect during both heating and cooling is called two-way shape memory.



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Applications:

Eyeglass Frames: In certain commercials, eyeglass companies demonstrate eyeglass frames that can be bent back and forth, and retain their shape. These frames are made from memory metals as well, and demonstrate super-elasticity.
Tubes, Wires, and Ribbons: For many applications that deal with a heated fluid flowing through tubes, or wire and ribbon applications where it is crucial for the alloys to maintain their shape in the midst of a heated environment, memory metals are ideal.
Robotics: There have also been limited studies on using these materials in robotics, for example the hobbyist robot Stiquito, as they make it possible to create very light robots. Weak points of the technology are energy inefficiency, slow response times, and large hysteresis.

and use also in spacecraft, aircraft, automobiles, electronics,and medicine.


2. Piezoelectric Materials:



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 link between electrostatics and mechanics.


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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.


Applications:

Generate green energy: In an airplane the engine and engine support are vibrating during a flight producing a loud noise, Piezoelectric Materials are able to convert the vibration and noise into electricity








Sensors: Piezoelectric sensor are devices using the piezoelectric effect to measure acceleration, pressure, strain or force and converting them to an electrical signal. Piezoelements are suitable for the detection of dynamic processes. In static applications the piezoelectric charges are too small, in order to be detected. An amplifier is used to convert the piezoelectric charges into a measurable electrical tension.




3.Magnetorheological fluid (MR) :



A magnetorheological fluid is a fascinating smart fluid with the ability to switch back and forth from a liquid to a near-solid under the influence of a magnetic field. It is usually used for applications in braking. The term "magnetorheological fluid" comes from a combination of magneto, meaning magnetic, and rheo, the prefix for the study of deformation of matter under applied stress. Magnetorheological fluids are not currently in wide use but are considered a futuristic type of material.

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How do they work?

Externally applied magnetic field in a direction normal to the fluid flow direction form dipoles in the iron particles, Magnetic poles start attracting

each others to the direction of the field, hence forming chains, The chains then form a skeleton within the fluid, which gains the fluid controllable yield stress.




magfluid.jpg




History:

Over fifty years ago both Rabinow and Winslow described basic MR fluid formulations that were every bit as strong as fluids today. A typical MR fluid used by Rabinow consisted of 9 parts by weight of carbonyl iron to one part of silicone oil, petroleum oil or kerosene.1 To this suspension he would optionally add grease or other thixotropic additive to improve settling stability. The strength of Rabinow’s MR fluid can be estimated from the result of a simple demonstration that he performed. Rabinow was able to suspend the weight of a young woman from a simple direct shear MR fluid device. He described the device as having a total shear area of 8 square inches and the weight of the woman as 117 pounds. For this demonstration to be successful it was thus necessary for the MR fluid to have yield strength of at least 100 KPa.

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Applications:


Human prosthesis:

Magnetorheological dampers are utilized in semi-active human prosthetic legs. Much like those used in military and commercial helicopters, a damper in the prosthetic leg decreases the shock delivered to the patients leg when jumping, for example. This results in an increased mobility and agility for the patient.









Optics:

Magnetorheological Finishing, a magnetorheological fluid-based optical polishing method, has proven to be highly precise. It was used in the construction of the Hubble Space Telescope's corrective lens.


Military and defense:

The U.S. Army Research Office is currently funding research into using MR fluid to enhance body armor. In 2003, researchers stated they were five to ten years away from making the fluid bullet resistant. In addition, Humvees, and various other all-terrain vehicles employ dynamic MR shock absorbers and/or dampers.



4. Auxetic Materials:



Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecules or a particular structure of macroscopic matter. Such materials are expected to have mechanical properties such as high energy absorption and fracture resistance.










No Natural Examples:

Auxetic materials are not natural, and no known biological examples exist. The first auxetics were foams with specifically engineered microstructures. Depending on the size of the air gaps in the microstructure, the auxetic effect in these materials can be more or less extreme. Most auxetic foams expand by a factor of about 30 percent or so before shredding because of the stretching force. With more advanced auxetics structured on the molecular level, more impressive expansion might be possible.


Potential Applications:

Proposals for the use of auxetics have been fairly wide in scope, although few implementations had actually been created as of 2011. Auxetics used in small medical probes could be used to dilate blood vessels. These materials expand so readily that they also would be ideal filters, capable of catching many foreign particles in their macrostructure. Unlike traditional filters, they could remain small and compact when not in use.

Threading auxetic fibers through composites could allow for strength improvements, with the tendency to expand under stretching stress helping keep the overall structure of the composite together. This is particularly true of composites consisting of materials that have a tendency to slide past each other. Many other potential applications for auxetics are yet to be developed, although the list is long and shows great promise in many fields.





References:

  1. wikipedia.org

  2. piezomaterials.com

  3. nixty.com

  4. wisegeek.com

  5. youtube.com

  6. Electrorheological Fluids and Magnetorheological Suspensions BY G. BOSSIS