Engeering Horizon

SMART MATERIALS

SMART MATERIALS

SMART MATERIALS
August 08
10:50 2014

By: Dr. Muhammad Muneeb Asim, Syed Khalid Mehmood Shah, Sabah Zaman, Institute of Industrial Control Systems, Rawalpindi

Any material that responds to a specified environment or condition in a sensible manner is termed as a “smart material.” These materials are tailored to receive, transmit or process a stimulus and respond by producing a useful effect, which could be further exploited to prompt a system. Some of the stimuli that are utilized to trigger these materials are strain, stress, temperature, chemicals, electric field, magnetic field, hydrostatic pressure and different types of radiation. The effect can be caused by absorption of a proton, a chemical reaction, integration of a series of events, translation or rotation of segments within the molecular structure, creation and motion of crystallographic defects or other localized conformations, alteration of local­ized stress and strain fields, and others. The effects produced can be a color change, a change in index of refraction, a change in the distribution of stresses and strains, or a volume change. Smart materials are normally integrated into smart structures to enhance the output features.

 

Few examples of smart materials and structures are composite materials embedded with fiber optics, actuators, sensors, micro-electromechanical systems (MEMSs), vibration control, sound control, shape control, product health or lifetime monitoring, cure monitoring, intelligent processing, active and passive controls, self-repair (healing), artificial organs, designed magnets, damping aero-elastic stability and stress distri­butions. Smart structures are now being commonly used in automobiles, space systems, naval vessels, civil structures, machine tools, recreation, and medical devices. Smart materials encompass all fields of science and engineering; for that reason only a few classes of smart materials are described here.

 

  1. PIEZOELECTRIC MATERIALS

These materials generate an electric field on pressing or squeezing or vice versa. This effect, named piezoelectric effect, exists in a number of naturally occurring crystals, such as quartz, tourmaline, and sodium potassium tartrate. Piezoelectric crystal must not have a center of symmetry. The effect is approximately linear. The polarization is directly related to the applied stress and is direction dependent. Thus, compressive and tensile forces will generate electric fields and voltages of opposite polarity. The effect is also reciprocal; thus, when the crystal is exposed to an electric field, it undergoes an elastic strain changing its length based upon the field polarity.

 

Manmade piezoelectric materials are polycrystalline ferroelectric ceramics with a perovskite structure. The crystal structure is tetragonal/ rhombohedral with a close proximity to cubic in nature. Their general formula is A2+B4+O32-, where A represents a large divalent metal ion such as barium or lead and B is one or more tetravalent metal ions such as titanium, zirconium, or manganese. These ceram­ics are prepared by powders or gels that finally get crystal form at the Curie temperature (Tc). Above Tc, these have a simple cubic sym­metry and below Tc tetragonal symmetry; which lacks a center of symmetry with the positive and negative charge sites. Thus each unit cell forms an electric dipole. At this stage, the material is considered to be ferroelectric and the samples do not show any dominant piezoelectric effect due to randomness of unit cells. In order to induce permanent and appreciable piezoelectric feature into the ceramic, a large electric field is applied at an elevated temperature, named poling, to generate an internal remnant polarization.

 

  1.  ELECTROSTRICTIVE MATERIALS

Electrostrictive effect is somewhat similar to piezoelectric behavior but differ in certain aspects. These materials undergo strain when electric field is applied but vice versa is not true. The sign of strain is independent of the sense of the electric field. This effect occurs in all materials and is very small. However in high permittivity materials this effect is large enough, especially in the vicinity of its Curie temperature, for exploitation in useful applications. These materials return to original dimensions immediately as compared to piezoelectric materials, as these do not contain domains and do not age.

 

Some of the examples of electrostrictive materials include lead manganese niobate-lead titanate (PMN-PT) and lead lanthanum zirconate titanate (PLZT). An interesting application of electrostrictive materials is in active optical applications. During the Cold War, the satellites that flew over the Soviet Union used active optical systems to eliminate atmospheric turbulence effects. Electrostrictive materials have the ad­vantage over piezoelectric materials of being able to adjust the position of optical components due to the reduced hysteresis associated with the motion. Similar multilayer actuators were used to correct the position of the optical elements in the Hubble telescope. Another example is of commonly used supermarket bar code readers and scanners in which actuators and flexible mirrors made up of these materials are fitted.

 

  1. MAGNETOSTRICTIVE MATERIALS

Magnetostrictive materials show mechanical deformation when stimulated by a magnetic field. This effect is largest in ferro­magnetic and ferrimagnetic materials. Spin-orbit lattice coupling is the main source of this effect. Changes in spin directions result in changes in the orientation of the coupled orbits which leads to slightly alter the lattice dimensions, as orbits are restrained to lattices. The crystal lattice inside each domain is spontaneously deformed in the direction of domain magnetization and its strain axis rotates with a rotation of the domain magnetization, thus resulting in a deformation of the specimen as a whole. Such effects disappear when these materials are heated above their Curie temperature. Magnetostrictive materials are usually al­loys of iron and nickel, doped with rare earths. One of the effective magnetostrictive materials is an alloy of terbium, dysprosium, and iron. Over the past few decades new giant magneto-strictive materials, colossal magnetostrictive materials has occurred. Ratios of these new magneto-strictive materials are in excess of 100,000%. Some of the new materials are epitaxial lanthanum-calcium-manganese-oxygen thin films, polycrystalline lanthanum-yttrium-calcium-manganese-oxygen, and polycrystalline lanthanum-barium-manganese-oxygen.

 

  1. SHAPE MEMORY ALLOYS

Shape memory alloys have the ability to remember their shape, even after several severe deformations. There are two types of alloys well known for commercial applications, namely Ni-Ti alloys and Cu-based alloys. The former has provided the best combination of materials properties for medical applications by to date. The crystalline structure of such materials, such as nickel—titanium alloys, enters into the martensitic phase as the alloy is cooled below a critical temperature. In this stage the material is easily manipulated through large strains with a little change in stress. As the temperature of the material is increased above the critical temperature, it transforms into the austenitic phase. In this phase the material regains its high strength and high modulus and behaves normally. The material shrinks during the change from martensitic to aus­tenitic phase.

Some of the medical applications include filters to trap blood clots, anchors and plates for bones, jaw plates, spacers for spine, various implants, self expandable stent, etc. Generally, shape memory alloys are ideally suited for fasteners, seals, connectors and clamps in variety of applications.

 

  1. ELECTRO AND MAGNETO RHEOLOGICAL MATERIALS

Rheological materials comprise an exciting group of smart materials. Electro and magneto rheological materials can change their rheological properties instantly through the application of an electric or a magnetic field. Electro-rheological materials (fluids) have been known for several centuries. The rheological or viscous properties of these fluids, which are usually uniform dispersions or suspensions of particles within a fluid, are changed with the application of an electric field. In an applied electric field the particles orient themselves in fiber-like structures (fibrils). When the electric field is switched off, the fibrils disorient themselves and clog the fluid flow. Another way to imag­ine this behavior is to consider logs in a river. If the logs are aligned, they flow down the river. If they are disordered, they will cause a log jam, clogging up the river. A typical example of an electro-rheological fluid is a mixture of corn starch in silicone oil.

Another feature of electro-rheological systems is that their damping characteristics can be changed from flexible to rigid and vice versa. Electro-rheological fluids were evaluated using a single-link flexible-beam test bed. The beam was a sandwich configuration with electro-­rheological fluids distributed along its length. When the beam was rapidly moved back and forth, the electro-rheological fluid was used to provide flexibility during the transient response period of the maneuver for speed and made rigid at the end point of the maneuver for stability. A practical way of viewing this behavior is to compare it with fly fishing. The use of rheological fluids in the construction of fishing rods and golf clubs are also in future considerations.

 

Magneto-rheological materials consist of ferromagnetic or ferrimagnetic particles dispersed in some oil and stimulus is a magnetic field. A simple example is of iron powder dispersed in motor oil. An interesting exploitation of magneto-strictive fluids is a series of elastomeric matrix composites embedded with iron particles. During the thermal cure of the elastomer, a strong magnetic field is applied to align the iron particles into chains. These chains of iron particles were locked into place within the composite through a cross-linked network of the cured elastomer. When a compressive force stimulated the composite, it was 60% more resistant to deformation in a magnetic field. Similarly, when the composite was subjected to a shear force, its magnetic-field-induced modulus was an order of magnitude higher than its modulus in a zero magnetic field.

 

  1. THERMO-RESPONSIVE MATERIALS

Materials that response to temperature variation fall in this category. Amorphous and semi-crystalline thermoplastic polymeric materials are unique due to the presence of a glass transition temperature at which their specific volume and rate of change occur. A variety of indicating devices can be developed by exploiting their temperature-specific volume behavior.

 

An interesting example of such materials is of cottons, polyesters, and polyamide/polyurethanes modified by poly (ethylene glycol) which respond smartly to changes in temperature and moisture; serve as smart pressure bandages.. When exposed to an aqueous medium such as blood, these fabrics contract and apply pressure. Once the fabric dries, it releases the pressure. Similarly, polymers based upon vinyl methyl ether shrinks when heated to about 40oC, this unique property can be utilized to devise a device which would grasp the objects mimicking human hand.

 

  1. LIGHT SENSITIVE MATERIALS

There are several families of materials that exhibit different behavior to a light stimulus. Electrochromism is a change in color as a function of an electrical field. Thermo-chromism change colour with heat, photo-­chromism change colour with light, and photo-strictism changes shape caused by changes in electronic configuration due to light.

An interesting example is of smart windows that act as blinds, from transparent to opaque when voltage is varied. Intensive research has been carried out on such projects for last few dec­ades. Such switchable window consists of an electro-chromic layer, an ion storage layer, and between these two layers an ion conductor. There have been over 1800 patents issued worldwide for optical switching devices, with the bulk issued in Japan. Similar materials have been developed to exhibit both photo-chromic and photographic behaviors. One such system contains photosensitive material embedded in a polystyrene matrix that produce image by UV irradiation and de-visualized by heat.

 

  1. SMART POLYMERS

Polymers that show a sharp, predictive, and large change in properties upon small changes in the external environment like temperature, pH, light, ionic strength, radiation, enzymes, bacteria, virus, presence of certain metabolic chemicals, change in electrical or magnetic field etc. They can adapt their property profile to the environment and thereby gain in functionality and complexity. The property modulation can affect for example shape, size, polymer solubility, colour, conductivity, polarizability, adhesion, density etc. The uniqueness of these materials lies not only in the fast microscopic changes occurring in their structure but also these transitions being reversible.

Gels offer a potential to smart polymers. They consist of cross linked polymeric networks which may be inflated with a change in their solvent such as water. The liable nature of the solvent enables the rapid and reversible swelling or shrinkage in response to a small change in their environment. The most common gel forming polymers are polyvinlyalchol, polyacrylic acid and polyacrylonitrile. Micro sized gel fibers may contract in milliseconds, while thick polymers may require much longer to react. It has been suggested that these gels can potentially deliver a stress equivalent to that of a human muscle of about equivalent size.

The use of smart polymers in living systems may increase patient compliance, maintain stability of drug, and maintain the drug level in therapeutic window. The pharmaceutical uses also include targeted drug delivery system, bioseparation, micro fluidic processes, tissue engineering, gene carrier, biosensors, reversible biocatalysts, as actuators in protein folding and many other major applications. Smart polymers are becoming increasingly more common, as scientists learn about the chemistry and triggers that include conformational changes in polymer structure and create ways to take advantage of them and eventually control them. The stimuli triggered property modulation can be very useful for many different applications, for example, in biomedical field for drug delivery or other metabolic control mechanisms, antibacterial characteristics, biodegradation, cell adhesion, in optical applications for transparency control, adjustable reflectivity, thermochromism, in mechanical devices for artificial muscles, self-propelled movement, strengthening or softening systems, self-healing and shape memory systems. Recent work has revealed the major applications of smart polymers in the field of chemistry which included hydro-gels, plasters, biodegradable plastic bags, non stick chewing gum and even biological applications like detecting blood glucose levels and triggering the release of insulin.

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Engineering Horizons

Engineering Horizons

“Engineering Horizons” is the first & leading technical magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics field under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.

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