SMART MATERIALS

INTRDUCTION

Materials had been deeply involved with our culture since pre-civilization era. Historically, the advancement of societies was intimately tied with development of materials to fulfill the need of that era. That is why the civilizations have been named by the level of their material development e.g. stone age, bronze age, steel age, plastic age etc. were the common materials in historical days but scenario of present era is completely changed.

The advancement of any engineering discipline is not possible without the development of materials, their science, engineering and technology. Rapid advancement in electron-based computers or possible light-based computers in future; changes in electronic engineering from vacuum valves to very large scale microchips (VLSC); cement concrete (CC) to polymer reinforced concrete (PRC) in civil engineering; pure materials to duplex stainless steel (DSS) in mechanical engineering; wood to ferroelectrics, and ordinary steel to ferrite in electrical engineering are some illustrations which became possible due to developments in material science. The research is still going on with the development of advance materials like rolled armour steel for military tanks, maraging steel for motor casting of booster rockets, zircaloy tubes for nuclear reactors, etc. Some of the future materials like amorphous lanthanum-nickel based alloy for hydrogen storage, Ni-hydrogen battery, freon free air-conditioning; microalloyed steels for automobile structure; hetrojunction microelectronic multi-layered devices(each layer a couple of atoms thick) used for negative resistivity, and many other such materials are developed or will be developed in near future. One among them is smart materials.

SMART MATERIALS (OR INTELLIGENT MATERIALS)

Nature is full of magical materials which are to be discovered in form suitable to our needs such magical materials are known as smart materials or intelligent materials. Efforts are being directed toward the development of “smart,” or responsive, materials. Representing another attempt to mimic certain characteristics of living organisms, smart materials, with their built-in sensors and actuators, would react to their external environment by bringing on a desired response. This would be done by linking the mechanical, electrical, and magnetic properties of these materials. For example, piezoelectric materials generate an electrical current when they are bent; conversely, when an electrical current is passed through these materials, they stiffen. This property can be used to suppress vibration: the electrical current generated during vibration could be detected, amplified, and sent back, causing the material to stiffen and stop vibrating.

Smart materials can sense, process, stimulate and actuate a response. Their functioning is analogous to human brain, slow and fast muscles action. These materials are the wonder materials that can feel an action and suitably respond to it just like any living organism. Analogous to human immune system, the smart materials comprises three basic components which are

1. Sensors such as piezoelectric polymers (polyvinyldene), optical fibers
2. Processors such as conductive electroactive polymers, microchips
3. Actuators such as shape memory alloys (Ni Ti i.e. nitinol), chemically responding polymers (polypyrrole).

These components in the form of optical fibers or electro-rheological fluids are embedded or distributed in materials. They possess ability to change with environmental radiations, stress, temperature, pressure voltage etc.

Piezoelectric Ceramics

Many of the ferroelectric perovskite materials are piezoelectric; that is, they generate a voltage when stressed or, conversely, develop a strain when under an applied electromagnetic field. These effects result from relative displacements of the ions, rotations of the dipoles, and redistributions of electrons within the unit cell. Only certain crystal structures are piezoelectric. They are those which, like BaTiO3, lack what is known as an inversion centre, or centre of symmetry—that is, a centre point from which the structure is virtually identical in any two opposite directions. In the case of BaTiO3, the centre of symmetry is lost owing to the transition from a cubic to a tetragonal structure, which shifts the Ti4+ ion away from the central position that it occupies in the cube. Quartz is a naturally occurring crystal that lacks a centre of symmetry and whose piezoelectric properties are well known. Among the polycrystalline ceramics that display piezoelectricity, the most important are PZT (lead zirconate titanate, Pb[Zr,Ti]O2) and PMN (lead magnesium niobate, Pb[Mg1/3 Nb2/3 ]O3). These materials are processed in a similar manner to capacitor dielectrics except that they are subjected to poling, a technique of cooling the fired ceramic piece through the Curie point under the influence of an applied electric field in order to align the magnetic dipoles along a desired axis.

There are numerous uses of piezoelectric. For instance, plates cut from a single crystal can exhibit a specific natural resonance frequency (i.e., the frequency of an electromagnetic wave that causes it to vibrate mechanically at the same frequency); these can be used as a frequency standard in highly stable crystal-controlled clocks and in fixed-frequency communications devices. Other resonant applications include selective wave filters and transducers for sound generation, as in sonar. Broadband resonant devices (e.g., for ultrasonic cleaning and drilling) and nonresonant devices (e.g., accelerometers, pressure gauges, microphone pickups) are dominated by ceramic piezoelectrics. Precision positioners made from piezoelectric ceramics are utilized in the manufacture of integrated circuits and also in scanning tunneling microscopes, which obtain atomic-scale-resolution images of materials surfaces. Domestic uses of piezoelectrics include buzzers and manually operated gas igniters.

Capacitor dielectrics and piezoelectric devices are among many other applications of advanced electroceramics. The piezoelectric ceramics are even widely used in aircraft airfoils, identifying Braille alphabet (an aid for blind).

Viscoelastic (VE)

The German physicist Wilhelm Weber noticed in 1835 that a load applied to a silk thread produced not only an immediate extension but also a continuing elongation of the thread with time. This type of viscoelastic response is especially notable in polymeric solids but is present to some extent in all types of solids and often does not have a clear separation from what could be called viscoplastic, or creep, response. In general, if all of the strain is ultimately recovered when a load is removed from a body, the response is termed viscoelastic, but the term is also used in cases for which sustained loading leads to strains that are not fully recovered. The Austrian physicist Ludwig Boltzmann developed in 1874 the theory of linear viscoelastic stress-strain relations. In their most general form, these involve the notion that a step loading (a suddenly imposed stress that is subsequently maintained constant) causes an immediate strain followed by a time-dependent strain which, for different materials, either may have a finite limit at long time or may increase indefinitely with time. Within the assumption of linearity, the strain at time t in response to a general time-dependent stress history σ(t) can then be written as the sum (or integral) of terms that involve the step-loading strain response due to a step loading d t′dσ(t′)/d t′ at time t′. The theory of viscoelasticity is important for consideration of the attenuation of stress waves and the damping of vibrations.

Viscoelastic solids have molecules in which the load-deformation relationship is time-dependent. If a load is suddenly applied to such a material and then kept constant, the resulting deformation is not achieved immediately. Rather, the solid gradually deforms and attains its steady-state deformation only after a significant period of time. This behavior is called creep. Conversely, the sudden application of a fixed deformation to such a material produces initial stresses that can be very large; these stresses then slowly relax to a steady-state value as the material accommodates itself to the applied deformation. Such a procedure is known as a stress-relaxation test. The physical reasons for this behavior are too complex to be explained by any simple molecular model. Such behavior is characteristic of glass, rubber, many plastics, and some metals.

A material may be allocated to one category at room temperature and to another at higher temperatures. Time scales are also relevant to the categorization of materials. For example, rocks, which may be effectively characterized as elastic solids for normal engineering purposes, would have to be reclassified as viscoelastic solids in geologic studies in which the time scale may be millions of years.

Thus these materials act extraordinarily in different conditions. In general they relax any stress produced in it by external strain. It is mainly used in Damping in spacecrafts, earthquake prone structures, aircrafts, etc.

Electro-rheological (ER) Fluids

Theyare like suspended fine polaraizable particles which are to cohesive, and tend to coalesce. They form new chains even when old chains are broken.

The rheological properties of coatings (that is, their ability to flow) are of prime importance in their preparation, storage, and application, and in fluids such as coatings the key factor in rheology is the viscosity of the fluid. In some cases the viscous properties of the combination of the polymer, pigments, and solvent is sufficient to provide the correct viscosity for the coating. In other cases, however, specialty additives must be employed to achieve precise control of viscosity. These materials are often known as thickeners, and, as their name suggests, they are used to increase the viscosity of, or thicken, a coating when added in small amounts. Treated attapulgite clays, fine-particle-size silica aerogel-type pigments, and ultrahigh-molecular-weight polymers are used as thickeners in nonaqueous coatings, while modified cellulosic polymers, carrageenan (a natural polymer from seaweed), high-molecular-weight water-soluble polymers (e.g., polyacrylic acid), and the so-called associative thickeners are employed in aqueous systems. Polymers used as thickeners function by dissolving in and raising the viscosity of the solvent or carrier liquid portion of the coating. Pigmentary materials that are used specifically to raise viscosity act by forming interacting, connected networks or chains of particles in the solvent or carrier fluid. Another type of thickener, the associative thickeners, are relatively low-molecular-weight polymers that form networks in mainly aqueous systems based on their surfactant-like nature. These materials have enabled latex coatings for the retail market to provide flow and leveling during application in a manner heretofore available only in solvent-based coatings.

The examples of these types of materials are Zeolite in silicone oil, starch in corn oil, etc. These are mostly used in filling of graphite-epoxy beams to impart variable stiffness in them.

Shape Memory Alloys (SMA)

Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Arne Olander first observed these unusual properties in 1938 (Oksuta and Wayman 1998), 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), CuZnAl, and CuAlNi. The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea that comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite.

Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.

The unusual properties being applied to a wide variety of applications in a number of different fields. The buttons below are links to pages about some of the most promising applications of SMAs. Each page contains information about the application as well as videos and interactive applets which allow you to become more familiar with the behavior of SMAs.

Each individual type of smart material has a different property which can be significantly altered, such as viscosity, volume, and conductivity. The property that can be altered influences what types of applications the smart material can be used for.

Science and techenology have made amazing developments in the design of electronics and machinery using standard materials, which do not have particularly special properties (i.e. steel, aluminum, gold). Imagine the range of possibilities, which exist for special materials that have properties scientists can manipulate. Some such materials have the ability to change shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost instantly when near a magnet; these materials are called smart materials. Smart materials have one or more properties that can be dramatically altered. Most everyday materials have physical properties, which cannot be significantly altered; for example if oil is heated it will become a little thinner, whereas a smart material with variable viscosity may turn from a fluid which flows easily to a solid. A variety of smart materials already exist, and are being researched extensively. These include piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory alloys. Some everyday items are already incorporating smart materials (coffeepots, cars, the International Space Station, eyeglasses) and the number of applications for them is growing steadily.

CONCLUSION

Examples, salient characteristics and applications of smart material are given in table below: --

S.NO.	MATERIALS	CHARACTERSTICS	EXAMPLE	APPLICATIONS
1	Piezoelectric ceramics	Linear and shear deformations occur along longitudinal, transverse and thickness directions	Quartz, Pb Zr titanate	Aircraft airfoils, identifying Braille alphabet (an aid for blinds).
2	Viscoelastic (VE)	They relax any stress produced in it by any external strain	---------	Damping in spacecrafts, earthquake prone structures, aircrafts.
3	Electro-rheological (ER) Fluids	They are like suspended fine polaraizable particles which are to cohesive, and tend to coalesce. They form new chains even when old chains are broken.	Zeolite in silicone oil, starch in corn oil	Used in filling of graphite-epoxy beams to impart variable stiffness in them.
4	Shape Memory Alloys (SMA)	Below a critical temperature, they can deform plastically to their memorized shape.	Ni Ti (nitinol)	Fire alarm due to change of shape at transition temperature, valves, robotics