2. Classification of Smart Materials
2.1. Piezoelectric materials
2.2. Electro-rheological and magneto-rheological materials
2.3. pH sensitive Polymer
2.4. Shape Memory Alloys
2.5. Optical Fibres
2.6. Temperature responsive Polymers
3. Governing Equations
4. Significance of Smart Materials and Systems
5. Applications of Smart Materials
5.2. Reducing waste
5.4. Smart solution to water pollution
6. Smart Materials: Future Prospects
Smart materials and structures are those that can sense external stimuli such as temperature, pressure, electric or magnetic field, pH etc. via internal sensing or /and actuation, and respond with active control to those stimuli in real or near real time.
Smart systems respond and adapt to changes in condition or environment by integrating the function of sense, logic, action, and control usually in repetitive manner. Or in other words, Materials that can significantly change their mechanical properties (such as shape, stiffness, and viscosity), or their thermal, optical, or electromagnetic properties, in a predictable or controllable manner in response to their environment are called smart materials. Smart materials react to a variety of stimuli from the environment, internal or external, and can adapt accordingly. These materials find many applications including sensors, actuators, and nano-prossessors. Smart materials modify their physical characteristics, shape, color, viscosity, etc. as a function of the encountered signals. The range of smart materials is broad and potential applications extensive. Some recent examples include: self-healing materials, phase change materials, a liquid changing color in a magnetic field, an electric cable visualizing the energy consumption, packages controlling the oxygen intake, a bioactive paper to detect pathogenic agents, an insulating self-cleaning window generating energy etc. Smart materials are becoming a need for the world as they provide more reliable, accurate and smart services.
2. CLASSIFICATION OF SMART MATERIALS
Various types of smart materials are classified as follows: Piezoelectric materials, Electro rheological and magneto rheological material, pH sensitive polymers, Shape memory alloys and Optical fibres.
2.1.Piezoelectric materials- Piezoelectric materials have two unique properties which are interrelated. When a piezoelectric material is deformed, it gives off a small but measurable electrical discharge. Alternately, when an electrical current is passed through a piezoelectric material it experiences a significant increase in size (up to a 4% change in volume). Piezoelectric materials are most widely used as sensors in different environments. They are often used to measure fluid compositions, fluid density, fluid viscosity, or the force of an impact. An example of a piezoelectric material in everyday life is the airbag sensor in cars, where the piezoelectric material senses the force of an impact on the car and sends an electric charge, there by triggering airbag inflation.
2.2.Electro-rheological and Magneto-rheological material- Electro-rheological (ER) and magneto-rheological (MR) materials are fluids, which can experience a dramatic change in their viscosity. These fluids can change from a thick fluid (similar to motor oil) to nearly a solid substance within the span of a millisecond when exposed to a magnetic or electric field; the effect can be completely reversed just as quickly when the field is removed. MR fluids experience a viscosity change when exposed to a magnetic field, while ER fluids experience similar changes in an electric field. The composition of each type of smart fluid varies widely. The most common form of MR fluid consists of tiny iron particles suspended in oil, while ER fluids can be as simple as milk chocolate or cornstarch and oil. MR fluids are being developed for use in car shocks, damping washing machine vibration, prosthetic limbs, exercise equipment, and surface polishing of machine parts. ER fluids have mainly been developed for use in clutches and valves, as well as engine mounts designed to reduce noise and vibration in vehicles.
2.3.pH sensitive polymer- pH sensitive or pH responsive polymers are materials which will respond to the changes in the pH of the surrounding medium by varying their dimensions. Such materials swell or collapse depending on the pH of their environment. This behaviour is exhibited due to the presence of certain functional groups in the polymer chain. There are two kinds of pH sensitive materials: one which have acidic group (-COOH, -SO 3 H) and swell in basic pH, and others which have basic groups (-NH 2 ) and swell in acidic pH. Polyacrylic acid is an example of the former and Chitosan is an example of the latter. The mechanism of response is same for both, just the stimuli vary. The response is triggered due to the presence of ionisable functional groups (like -COOH, -NH2 ) which get ionized and acquire a charge (+/-) in a certain pH. The polymer chains now have many similarly charged groups which cause repulsion and hence the material expands in dimensions. The opposite happens when pH changes and the functional groups lose their charge hence the repulsion is gone and the material collapses back.
2.4. Shape memory alloys- Shape memory alloys are materials that can be plastically deformed at some relatively low temperature and, upon exposure to some higher temperature, will return to their shape prior to the deformation. When SMA is cold or below its transformation temperature it has a very low yield strength and can be deformed quite easily into any new shape, which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to original shape. The most common shape memory material is an alloy of Nickel and Titanium called Nitinol .This particular alloy has very good electrical and mechanical properties, long fatigue life and corrosion resistance.
· Nitinol- Current techniques of producing nickel-titanium alloys include vacuum melting techniques such as electron-beam melting, vacuum arc melting or vacuum induction melting. The cast ingot is press-forged and/or rotary forged prior to rod and wire rolling. Hot working to this point is done at temperatures between 700 °C and 900 °C.Properties- The physical properties of Nitinol include a melting point around 1240 ° C to 1310 ° C, and a density of around 6.5 g/cmł. Mechanical properties tested include hardness, impact toughness, fatigue strength, and machinability.
Nitinol has two solid phases. The lower temperature phase is soft bendable. If you heat it above a certain temperature, it will transition into a higher-temperature phase and bounce back into its original shape. This can be used to move things, or work as a switch to turn something on or off. Another useful property of Nitinol is that it is superelastic. The higher-temperature phase is stiff. But if you try to bend a Nitinol wire or rod in this phase, the pressure you exert on the metal by bending it will cause it to temporarily change back into the softer phase. Once you release your push on the Nitinol metal, it turns back into austenite again. This makes it incredibly springy.
Applications of nitinol-
§ Safety shower head- A nitinol spring can stop hot water flow when water becomes sufficiently hot for body.
§ Cell phone antennae- These are flexible and kink resistance.
§ Orthodontic archwires- Using memory metal causes less pain for patients and increase the time between needed visits.
§ Stir spoons- These spoons twist when coffee or tea is stirred.
§ Bone staples- Nitinol hold blood broken bones together until they have healed.
2.5.Optical fibre- An optical fibre is a thin, flexible, transparent fibre that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fibre. The field of applied science and engineering concerned with the design and application of optical fibres is known as fibre optics. Optical fibres are widely used in fibre-optics communication, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibres are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. Fibres are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibres are used for a variety of other applications, including sensors and fibre lasers.
Optical fibre typically consists of a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fibre to act as a waveguide. Fibres which support many propagation paths or transverse modes are called multi-mode fibres (MMF), while those which can only support a single mode are called single-mode fibres (SMF). Multi-mode fibres generally have a larger core diameter, and are used for short-distance communication distance communication links and for applications where high power must be transmitted. Single-mode fibres are used for most communication links longer than 1,050 meters (3,440 ft).
Joining lengths of optical fibre is more complex than joining electrical wire or cable. The ends of the fibres must be carefully cleaved, and then spliced together, and then spliced together either mechanically or by fusing them together with heat. Special optical fibre connectors are used to make removable connections.
2.6.Temperature responsive polymer- A temperature-responsive polymer is a polymer which undergoes a physical change when external thermal stimuli are present. The ability to undergo such changes under easily controlled conditions makes this class of polymers fall into the category of smart materials. These physical changes can be exploited for many analytical techniques, especially for separation chemistry. After numerous investigations of poly(N-isopropylacrylamide) (poly-NIPAAm), there was a sparked interest in the applications of this and many other stimuli-responsive polymers. There has been extensive research in the applications of intelligent polymers for use as stationary phases, extraction compounds, surface modifiers, drug delivery, and gene delivery.
3. GOVERNING EQUATIONS
Piezoelectricity and piezoelectric materials
The word ‘piezo’ is derived from a Greek word meaning pressure. The phenomenon of piezoelectricity was discovered in 1880 by Pierre and Paul-Jacques Curie. It occurs in non-centro symmetric crystals, such as quartz (SiO2), Lithium Niobate (LiNbO3), PZT [Pb(Zr1-xTix)O3)] and PLZT [(Pb1-xLax)(Zr1-yTiy)O3)], in which electric dipoles (and hence surface charges) are generated when the crystals are loaded with mechanical deformations. The same crystals also exhibit the converse effect; that is, they undergo mechanical deformations when subjected to electric fields.
[IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image001.gif[/IMG]In centro-symmetric crystals, the act of deformation does not induce any dipole moment, as shown in Fig. 3. However, in non-centro symmetric crystals, this
Fig. 3 Centro-symmetric crystals: the act of stretching does not cause any dipole moment (μ = Dipole moment).
[IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image003.gif[/IMG][IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image004.gif[/IMG] µ=0 µ≠0
Fig. 4 Noncentro-symmetric crystals: the act of stretching causes dipole moment in the crystal (μ = Dipole moment).
leads to a net dipole moment, as illustrated in Fig. 4. Similarly, the act of applying an electric field induces mechanical strains in the non-centro symmetric crystals.
The constitutive relations for piezoelectric materials, under small field condition are (IEEE standard, 1987)
[IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image005.gif[/IMG]Di = εij E j + dimTm
Sk = d jk E j + skmTm
Eq. (1) represents the so called direct effect (that is stress induced electrical charge) whereas Eq. (2) represents the converse effect (that is electric field induced mechanical strain). Sensor applications are based on the direct effect, and actuator applications are based on the converse effect. When the sensor is exposed to a stress field, it generates proportional charge in response, which can be measured. On the other hand, the actuator is bonded to the structure and an external field is applied to it, which results in an induced strain field. In more general terms, Eqs.(2.1) and (2.2) can be rewritten in the tensor form as (Sirohi and Chopra, 2000b)
where [D] (3x1) (C/m2) is the electric displacement vector, [S](3x3) the second order strain tensor, [E](3x1) (V/m) the applied external electric field vector and [T][IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image006.gif[/IMG][IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image007.gif[/IMG](3x3) (N/m2) the stress tensor. Accordingly, [ ε T ] (F/m) is the second order dielelectric permittivity tensor under constant stress, [dd] (C/N) and [dc] (m/V) the third order piezoelectric strain coefficient tensors, and [ s E ] (m2/N) the fourth order elastic compliance tensor under constant electric field.
[IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image008.gif[/IMG]If static electric field is applied under the boundary condition that the crystal is free to deform, no mechanical stresses will develop. Similarly, if the stress is applied under the condition that the electrodes are short-circuited, no electric field(or surface charges) will develop. For a sheet of piezoelectric material, as shown in Fig.5, the poling direction is usually along the thickness and is denoted as 3-axis. The 1- axis and 2-axis are in the plane of the sheet.
[IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image009.gif[/IMG]Fig. 5 A piezoelectric material sheet with conventional 1, 2 and 3 axes.
The matrix [dc] depends on crystal structure. For example, it is different for PZT and quartz, as given by (Zhu, 2003)
where the coefficients d31, d32 and d33 relate the normal strain in the 1, 2 and 3 directions respectively to an electric field along the poling direction 3. For PZT crystals, the coefficient d15 relates the shear strain in the 1-3 plane to the field E1 and d24 relates the shear strain in the 2-3 plane to the electric field E2. It is not possible to produce shear in the 1-2 plane purely by the application of an electric field, since all terms in the last row of the matrix [dc] are zero. Similarly, shear stress in the 1-2 plane does not generate any electric response. In all poled piezoelectric materials, d31 is negative and d33 is positive. For a good sensor, the algebraic sum of d31 and d33 should be the maximum and at the same time, ε33 and
the mechanical loss factor should be minimum (Kumar, 1991).
The compliance matrix has the form
From energy considerations, the compliance matrix is symmetric, which leaves only 21 independent coefficients. Further, for isotropic materials, there are only two independent coefficients, as expressed below (remaining terms are zero)
where Y E is the complex Young’s modulus of elasticity (at constant electric field), [IMG]file:///C:/Users/nisha/AppData/Local/Temp/msohtmlclip1/01/clip_image010.gif[/IMG]G E the complex shear modulus (at constant electric field) and ν the Poisson’s ratio. It may be noted that the static moduli, YE and GE, are related by
4. SIGNIFICANCE OF SMART MATERIALS AND SYSTEMS
Smart materials and systems open up new possibilities, such as clothes that can interact with a mobile phone or structures that can repair themselves. They also allow existing technology to be improved. Using a smart material instead of conventional mechanisms to sense and respond, can simplify devices, reducing weight and the chance of failure. The commercial importance of smart materials is beginning to be recognised. In 2003, smart materials were the subject of a report produced by the Foresight Materials Panel - a government funded project that brings experts together to provide technological visions of the future. It predicted that “the key to 21st century competitive advantage will be the development of products with increasing levels of functionality. Smart materials will play a critical role in this development.
5. APPLICATIONS OF SMART MATERIALS
Smart materials and systems have a wide range of applications. Investment in research and development is driven by factors such as legislation, reducing waste and demand for higher quality of life.
Structures such as buildings, bridges, pipelines, ships and aircraft must be robustly designed and regularly inspected to prevent ‘wear and tear’ damage from causing catastrophic failures. Inspection is expensive and time consuming, while designing to prevent damage can compromise performance. With some modern materials, damage can be internally serious but leave very little surface evidence. Researchers at institutes such as the Universities of Bristol and Sheffield are working on systems that can diagnose and repair this type of damage automatically in both defence and civil applications.
• Structural health monitoring - Embedding sensors within structures to monitor stress and damage can reduce maintenance costs and increase lifespan. This is already used in over forty bridges worldwide.
· Self-repair - One method in development involves embedding thin tubes containing uncured resin into materials. When damage occurs, these tubes break, exposing the resin which fills any damage and sets. Self-repair could be important in inaccessible environments such as underwater or in space. The European Space Agency is collaborating on work in this area.
Legislation is forcing producers to consider the entire life of a product at the design stage and customers are increasingly demanding more environmentally sensitive products. Innovative use of smart materials has the potential to reduce waste and to simplify recycling.
· Electronic waste- Electronic waste is the fastest growing component of domestic waste. The EU Directive on waste electronic and electrical equipment (WEEE) requires that it be processed before disposal to remove hazardous and recyclable materials. Manual disassembly is expensive and time consuming but the use of smart materials could help to automate the process.
Research in this new area of ‘active disassembly’ has been carried out by UK company, Active Disassembly Research Ltd. One example uses fasteners constructed from shape memory materials that can self release on heating. Once the fasteners have been released, components can be separated simply by shaking the product. By using fasteners that react to different temperatures, products could be disassembled hierarchically so that materials can be sorted automatically. The company has collaborated with Nokia and believe that this technology could be in use in the next two years.
· Reducing food waste- Food makes up approximately one fifth of the world’s waste. One third of food grown for consumption is thrown away, much of which is food that has reached its best before date without being eaten. These dates are conservative estimates and actual product life may be longer. Manufacturers are now looking for ways to extend product life with packaging, often using smart materials.
As food becomes less fresh, chemical reactions take place within the packaging and bacteria build up. Smart labels have been developed that change colour to indicate the presence of an increased level of a chemical or bacteria. A ripeness sensor for pears is currently beingtrialled by Tesco.
Storage temperature has a much greater effect than time on the degradation of most products. Some companies have developed ‘time-temperature indicators’ that change colour over time at a speed dependent on temperature, such as the Onvu™ from Ciba Speciality Chemicals and TRACEO® by Cryolog. French supermarket Monoprix has been using time-temperature indicators for many years, but they are not yet sufficiently accurate or convenient for more widespread introduction.
Companies like Smith & Nephew and DePuy are developing smart orthopaedic implants such as fracture plates that can sense whether bones are healing and communicate data to the surgeon. Small scale clinical trials of such implants have been successful and they could be available within the next five years. Other possible devices include replacement joints that communicate when they become loose or if there is an infection.
Current technology limits the response of these devices to transmitting data but in the future, they could respond directly by self-tightening or releasing antibiotics. This could reduce the need for invasive surgery.
Biosensors made from smart materials can be used to monitor blood sugar levels in diabetics and communicate with a pump that administers insulin as required. However, the human body is a hostile environment and sensors are easily damaged. Researchers at Queen Mary,
University of London are working on barrier materials to protect sensors.
· The ageing population- There are now more people aged over 60 than there are children, creating a new market for products that make life easier for the elderly. Many of these could use smart materials and systems to include added functionality. For example, shape memory materials could be used in food packaging that automatically opens on heating for people with arthritis. Researchers at the University of Bath have developed a smart home for people with dementia that uses sensors to monitor behaviour and to ensure that the resident is safe. This technology is already in use in a joint project with Bristol City Council.
· Vaccines- Time and temperature cause degradation in vaccines. This is a particular challenge in developing countries where vaccines must often withstand high temperatures, poor refrigeration facilities and travel over large distances.
Time-temperature indicators are now used worldwide to ensure the quality of vaccines. For example, every vial of oral polio vaccine supplied through UNICEF has a time-temperature indicator.
5.4.Smart solution to water pollution
Water purification for human and agricultural use is a global challenge and a high impact area for innovation using functional/smart materials. Smart Material can both detect and eliminate water pollutant. Removing toxic pollutants from water typically requires a number of steps. The aberrant molecules must first be identified, then destroyed or extracted. Finally, the water is tested to ensure its purity zinc oxide (ZnO), could degrade organic contaminants in water. Typically, ZnO emits visible radiation. When the material was exposed to water contaminated with a type of chlorinated phenol, however, the amount of light emitted dropped drastically. The researchers further found that this response was measurable for pollution concentrations as low as one part per million and occurred in under a minute. Moreover, once the ZnO detects the offending organic molecules, it can also help eliminate them. When exposed to UV light, ZnO aids in the breakdown of the contaminants without being destroyed itself. After the pollutants have been converted into harmless by products, the ZnO film begins to glow more brightly, signaling that its job is done. Though commercial applications are not yet available, the researchers suggest that ZnO film-based nansensorbased nanosensors should be useful in applications such as checking the quality of drinking water, or assessing the quality of drinking water, or assessing the contamination in underground water.
Smart sponge technology
One treatment technology, which has recently been receiving much attention in the United States, is the Smart Sponge, a polymer-based filtration material resembling popcorn, which absorbs hydrocarbons and other contaminants within its porous structure and has the capacity to destroy bacteria. The Smart Sponge addresses a variety of water quality issues. The technology is widely used in catch basin filters to treat polluted storm water runoff; in storm water outfalls to destroy bacteria near beaches and recreation areas; in vault applications to capture contaminants found in industry or construction wastewater; in airports to clean up oil spills and to reduce hydrocarbons from entering nearby water supplies; and in numerous other water treatment applications.
The Smart Sponge is a relatively simple system based on synthetic polymers. Its unique molecular structure is chemically selective to hydrocarbons and also has the capacity to destroy bacteria. The antimicrobial agent is permanently bound to the Smart Sponge polymer surface, so that it doesn’t leak, avoiding any toxicity issues downstream.
6. SMART MATERIALS: FUTURE APPLICATIONS
Seasoned researchers often share visionary ideas about the future of smart materials in conferences and seminars. According to Prof. Rogers (Rogers, 1990), following advancements could be possible in the field of smart materials and structures.
• Materials which can restrain the propagation of cracks by automatically producing compressive stresses around them (Damage arrest).
• Materials, which can discriminate whether the loading is static or shock and can generate a large force against shock stresses (Shock absorbers).
• Materials possessing self-repairing capabilities, which can heal damages in due course of time (Self-healing materials).
• Materials which are usable up to ultra-high temperatures (such as those encountered by space shuttles when they re-enter the earth’s atmosphere from outer space), by suitably changing composition through transformation (thermal mitigation).