'Smart materials\r'

'The primary winding lineament of a â€Å" skilful veridical” is that it has the ability to respond to remote stimuli in a technic eithery reusable and technic everyy witnessled delegacy. The haggle â€Å"technic exclusivelyy multipurpose” and â€Å"technic ally controlled” atomic get on 18 emphasized since all cloths respond to outside(a) stimuli of s diminishly sort or early(a) (as a simple ex international amperele, all textiles respond to temperature by changing their volume), however, to be catched a â€Å" sassy sensible” the chemic reaction must(prenominal)iness be unmatchable that is phthisisful in an employ science natural covering. and so, each discussion of wise to(p) genuines must let in a stipulation of the performance of these worldlys. Animals and plants could be considered as lie downing of a large-scale numeral of brightness textiles, however, the scope of this oblige pull up stakes be confine to in radical and organic strongs that ar utilize in a to a great extent traditional take aiming sense. ) The term stylishness material a lot in any case has a historical context, only creation employ to comparatively new materials. For example, consider the simple bi coatlic strip.Bi admixturelic elementlic strips throw out-of-door been nigh for centuries and lie in of two metals Joined so that the difference in the coefficient of caloric expansion causes the strip to twist in resolution to a turn in temperature. This disregard be employ, eg, to open or close a echanical valve or galvanical circuit. The stimuli may either be provided by the inborn environment or engineered into a twist that the material is touch off of. However, bimetallic strips atomic material body 18 a great learn not thought of as clean materials because they fox been virtually and apply for a bulky time.Smart materials ar to a fault a good deal characterized by the fac t that they swop push un littleton from angiotensin converting enzyme fashion to some an separate(prenominal)(prenominal), eg, from galvanizingal aptitude to mechanised qualification. Smart materials be besides very much incorpo appreciated in so-called Smart bodily structures, which argon structures that, as well as being the geomorphological complement of a building or vehicle, similarly birth a further function. For example, a stretch out-bearing(prenominal) structure that also measures the load that it is carrying is an example of a chichi structure. 2.Classification Schemes How a material is considered or course of studyified dep mop ups to some intent on the scientific or technical discipline that is considering the material. For example, a material scientist is touched in the inner(a) structure and how this may permute downstairs the influence of an remote stimulus, whereas an application engineer may be more concerned with the function of the material and what it fundament be utilize for. 2. 1 . Classification concord to Function. Smart materials rump be sort out according to the mean primary function, eg, as a demodulator, actuator, or goose egg generator.piezo galvanizing automobile material is an example of a class of hurt materials that foot be use in all unity-third of these functions. When subjected to distort these materials turn in an galvanising voltage that great deal be utilize as a response savetocks and also to succumb coiffe. Alternatively, when subjected to a voltage these materials generate a mechanically skillful stress. The inter heightenable spirit of stimulus and response is a familiar, but not universal, singularity ot 1 Kirk-Othmer Encyclopedia of chemic Technology. Copyright John Wiley & Sons, Inc. All rights reserved. vol. O giftedness materials. reals that evidence this inter transposeable disposition argon often superb rousedidates for smart structures with minimal foreign support demandments: one smart material some(prenominal) catching and generating power and also resourceful of providing a mechanical or other response. energetic signal spotting Materials. The primary carry through of these materials is to shift one contour line of energy to other body-build of energy for, in most cases, a further widget or material utilize to in that respectfore generate a response. Active sensing materials may often be utilize to generate energy.For example, the voltage developed by electric polarization of a material washstand be used to accumulate bash, and the solar cell some(prenominal) detects the mien of calamity ignition and generates a signifi thunder mugt voltage and incumbent. dormant Sensing Materials. These materials bewilder a passive response to impertinent stimuli, ie, do not convert an energy source to another energy. Thus these devices need an foreign source of power to conk out. They be not reall y â€Å"smart”, but be entangle here for f arness since they do pop out as demodulators in bigger smart schemas.An example of a passive response is a pitch in galvanizing conductivity collectable o, eg, a change in pH of a solution. Actuating Materials. The primary function is to transform one form of energy to another form as a response to some external stimuli and to perform an action. 2. 2. Classification correspond to Smartness. Another possible motley is according to the degree of liberty of operation that the smart material needs to operate in a smart structure. In this case, materials that require the least external support (eg, external power or processing capability) being considered smarter.A common example of a totally autonomous smart material is that used in photochromic sunglasses. In this case, the energy unavoidable to drive the response, darkening of the sunglasses is provided directly by the stimulus, the incident light. 2. 3. Classification According to Material Properties. A further management to look at smart materials is to consider the primary corporal- chemical substance plaza of the material that is used in smart applications. This is particularly useful in studying and brain the operation of smart materials. crystallizing Structure Modification. A design of classes of smart materials gravel crystal structural properties that depose be used directly for smart activate or sensing applications. These accommodate materials that undergo a crystallographic configuration change (eg, play remembering debasements), materials that yield an electrical polarization natural to the crystal structure that responds to external stress (eg, piezoelectric materials) and materials that undergo re alinement ot internal publics, much(prenominal) as electro- and mag lollyostrictive materials.Energy Absorption-Emission. This accommodates materials that absorb incident energy by an internal excitation process (eg, onw ard motion of electrons, on absorption of a photon, to a full(prenominal)er energy luck). These often feature a de-excitation process that involves the emission of energy. Examples admit semiconductor devices as light sensors and energy glean devices. macroscopic Ordering. Includes ( mostly) mixtures of materials where the large scale club of the particles in the mixture induces a change in properties of the mixture.Examples intromit electro- and magnetorheological fluids, 3 where an employ line of products induces particles in the fluid to align resulting in a change in viscousness. The Liquid Crystal Display finish also be considered in this category. chemic Interaction. Includes materials with sensitized surfaces that interact with hind end chemical or biological species. Examples entangle conducting polymers with functional groups that stick with to chemical species and change the resistivity of the polymer, and antigen coatings on microbalance rush detectors. forefinger chemicals that change color with, eg, pH, argon another form of chemical sensor. . Smart Materials Over get a line Table 1 is a summary ofa number of materials that can be used in smart applications, ‘e, classified as to function. The fol minoring component parts consider a number of these materials in more detail. Some sections deal with a particular lineament of smart material, whereas others deal with a class of material types. Inevitably, thither argon verlaps amidst these two, however, trusty will of smart materials in this way is beneficial since, as discussed supra, the view of the subject is colored by the point of view of the discussion.Smart materials accommodate such a ample concatenation of materials and possibilities it is worthwhile debunk the subject from a number of angles. The fol downhearteding is not an gross(a) list of smart materials, but serves as examples of the more common smart materials. Many of the materials in Table 1 can be d escribed as â€Å"responsive” rather than actually smart, ie, these materials can form one part of a smart arranging, either the sensor or ctuator, but require another responsive material for the actuator or sensor, respectively, to make up a empty smart transcription.Only a few materials, eg, the photochromic sunglasses mentioned in the section classification schemes, atomic number 18 both sensor and actuator. even out materials that can be used as an actuator and sensor often cannot be engineered to perform both functions loadively at the comparable time. Thus, most smart materials argon used in combinations to form smart systems. An example of a possible system is an anti cycle device that uses a piezoelectric sensor to detect the chill and a magnetorheological fluid as the damper.The reterence section overwhelms a number ot profit web sites tor turtner practical breeding and as examples of the use and current availability of some smart materials. A number of th ese references atomic number 18 to commercial web sites as useful sites regarding the application and maturity of some of the technologies, but this in no way is an implied visage of the particular companies. 4. Piezoelectric and cogitate Phenomena Piezoelectric materials ar materials that exhibit a linear kindred surrounded by electric and mechanical versatiles.The electric polarization is relative to the mechanical stress. Piezoelectric materials atomic number 18 â€Å" bipartisan; (a) they convert mechanical stress into an electric champaign (Fig. la), and this notion is employed in piezoelectric sensors; (b) application of an electric correction produces a mechanical 4 stress (Fig. 1b, c), and this behavior is utilised in actuator and power generator- harvesting devices. Only materials with an electrically poled, aeolotropic crystal stucture can form piezoelectrics; ‘e, there must be an intrinsic electric knit stitch maintained in a particular direction thr oughout the material.Thus the material must be pyroelectric. A feature of a pyroelectric material is the slice of this pontaneous electric line of business supra the curie point temperature. Piezoelectric devices ar make by raising the material to a juicy ge ber(prenominal) place the Curie point temperature and thus cooling in the social chance uponment of a strong electric field. The common term for this action is to say that the material is â€Å"poled” in the front end of the electric field. This results in partial or complete alignment of the extemporaneous electric field in spite of appearance the material. A related material office is the ferroelectric effect.All ferroelectric materials be necessarily both pyroelectric and piezoelectric. The ferroelectric effect is the electric olarization brought about by the complete or partial realignment of the spontaneous polarization resulting in a static electric field at the surfaces of the materials. (But note, a net field is not normally detected because the surface charge is speedyly neutralized by ambient charged particles. ) at that place ar two leash types of materials that can function as piezoelectrics: the ceramics and polymers. 1 .Ceramics: The most astray used materials are the piezoceramics establish on the consume zirconate titanate (PZT) formations, conflate sodium and potassium niobates, lithium niobate, and quartz. The gains of these piezoceramics are that they have a high piezoelectric performance and they can be put on in some(prenominal) contrasting devises. 2. Polymers: Poly(vinylidene fluoride), PVDF, is the most full(a) used polymer piezoelectric material. It has the receiptss over ceramic devices of flexibleness, formability and can be patently cut to shape. Polymer piezoelectric materials have miserableer authority (force and displacement) than ceramic devices.The PVDF structure is a good example of the spontaneous electric field seen in these types of material (Fig. A limitation of piezoelectric materials is that they exhibit small dimensional changes but with high utilize force). Composite structures consisting of a distance of piezoelectric bonded to a nonpiezoelectric substratum can be used to convert the lateral change in dimension of the piezoelectric to a bending force. In this way, larger actuation cam strokes can be achieved. Piezoelectric materials are used in many contrasting types of sensing and touch off devices and also for power harvesting.Examples include Sensors: vibration, sound, accelerometers, pressure, ultrasonics, line, power generation. Actuators: print heads, vibration suppression, speakers and buzzers, spark generators, ultrasonic ransducers, micropositioning and translation. galvanising components: filters and resonators. 5 5. unalikeiate keeping Alloys and Polymers Metal shape retrospect intermixtures (SMAs) exhibit the properties of pseudo-elasticity and the shape storage effect. These vitiates undergo thermomechanical changes passing from a martensitic phase at low temperature to an austenitic phase at higher temperature (Fig. a). 5. 1 . Shape store Effect. In the martensite phase, the devalue is softer and easily manipulated through large strains with a little change in stress, ie, it can be easily deformed. As the temperature of the alloy is increased above the vituperative innovation) temperature, it changes into the austentic phase. In the austentic phase, the alloy regains its high strength and high modulus and also reverts spine to its original shape. Thus a SMA can be formed into shape above the transformation temperature, cooled below the transformation temperature, and formed into divers(prenominal) shape.On heating, the SMA will revert to the shape that it was formed into above the transformation temperature (Fig. 3b). 5. 2. Pseudo-Elasticity. This occurs when the alloy is completely composed of austenite (‘e, the temperature is above the transformation temperature). If the temperature is kept perpetual and the material is loaded, thusly at some point there will be a transformation to the martensite phase only if due to loading. The load is intent by deformation of the softer martensite phase, but upon unloading the martensite starts to transform back into austenite and the materials springs back into its original shape (Fig. c). Shape retrospection alloys (SMAs) can be divided into three functional groups: one-way SMAs, two-way SMAs, and magnetically controlled SMAs. The magnetically controlled SMAS draw great potential as actuator materials for smart tructures because they could provide rapid strokes with large amplitudes under hairsplitting control. The most extensively used conventional shape depot alloys are the nickel-titanium, copper-zinc-aluminium, and copper- aluminium-nickel alloys. Due to their low cost, iron- found shape memory alloys are becoming more prevalent in smart structure applications.I ron-manganese- ti steels alloyed with chromium, nickel, and conscientious objector, and iron-manganese- silicon steels alloyed with nitrogen all fit into this category. As previously mentioned, the nickel-titanium alloys have been the most widely used shape memory alloys. This family of nickel-titanium alloys is cognize as Nitinol (Nickel si Naval Ordnance laboratory in honor of the place where this material behavior was maiden observed). Nitinol can be used in robotics actuators and micromanipulators that simulate military man muscle motion.The ability of Nitinol to work a smooth, controlled force when aroused is an advantage of this material family. SMAS have been used for military, medical, safety, and robotics applications. Specific usages include hydraulic lines, medical tweezers, anchors for attaching tendons to bones, eyeglass frames, control of hot plate windows, underwire brassieres, and ntiscalding valves used in pee taps, and registerer heads. 5. 3. Shape Mem ory Polymers. Shape memory polymers (SMP) are polymers (polyurethane based thermo elastics) that can be heated (above the glass regeneration temperature), deformed, and cooled to retain the deformed 6 shape.Upon heating above the transition temperature, the material softens and returns to the shape that it had prior(prenominal) to deformation. Advantages of SMPs over metallic SMAS include light weight, high recoverable strains (up to 400%), injection boundary line (to form complex shapes), low cost and SMPs have shape recovery temperatures selectable between A30 and 708C. The SMAs, however, have superior force characteristics and can operate at higher temperatures. 6. Electrostrictive Materials Electrostrictive materials are materials that exhibit a quadratic relationship between mechanical stress and an applied electric polarization (Fig. ). Electrostriction can occur in any material. Whenever an electric field is applied, the generate charges in the material take out each ot her resulting in a compressive force. This attraction is independent of the sign of the electric field. The strain in the material lies on the axis of the induced polarization, which is instead the direction of the applied lectric field. Electrostriction is a small effect and, in contrast to piezoelectric materials, electrostrictive materials show a large effect near the Curie temperature, curiously for ferroelectric sum of moneys, such as members of the perovskite family.Typical electrostrictive materials include such compounds as hightail it manganese niobate:lead titanate (PMN:PT) and lead lanthanium zirconate titanate (PLZT). Electrostriction is used in actuators for close and fine positioning. Electrostrictive translators are less stable than piezoelectric devices with greater sensitivity to temperature. The one advantage they offer is lower hysteresis than piezoelectric materials at temperatures exemplaryly >1 7. Magnetostrictive Materials The same type of material r esponse as that seen in electrostrictive materials discussed above can be observed when the stimulus is a magnetic field.Shape changes are the largest in ferromagnetic and ferrimagnetic consentaneouss. The repositioning of domain walls that occur when these solids are fixed in magnetic field leads to hysteresis between magnetization and an applied magnetic field. All of these effectuate disappear when the ferromagnetic material is heated above its Curie temperature. Ferrimagnetic materials have macroscopic properties confusable to ferromagnetics; however, their microscopic properties are different. The magnetic dipoles of a ferromagnetic solid are aligned match to each other; whereas in a ferrimagnetic the alignment can be either couple or in other directions.Materials that have shown a response to a magnetic stimuli are primarily inorganic: alloys of iron, nickel, and cobalt doped with rare earths. TERFENOLD, an alloy of terbium, dysprosium, and iron, TbxDy1 …xFey with x between 0. 27 and 0. 30 and y between 1. 90 and 1. 95, is the most effective magnetostrictive material and has been escribed as the â€Å" two hundred lb” gorilla of magnetostrictive materials. The name TERFENOL is an acronym for two of the elements present in the alloy and NOL refers to the Naval jurisprudence Laboratory where this type 7 of material behavior was developed.Magnetostriction occurs at its fullest potential in lucid materials. Cost still appears to be one of the hindrances to magnetostrictive materials becoming commercially important. Piezoelectric materials are generally more compact and require less energy to operate than magnetostrictive materials. Magnetostrictive materials (‘e, TERFENOL-D) are useful where high force, igh power, and a long stroke are needed or where the high drive voltages emblematic of the piezoelectric materials cannot be tolerated. 8.Electro- and Magnetorheological Materials Electrorheological and magnetorheological materia ls are fluids that exhibit a hammy change in viscosity with an applied electric or magnetic field. The fluid can change from a heavy oily consistency to closely a solid substance within a millisecond. there are a wide assortment of electrorheological and magnetorheological fluids, which are usually a uniform airing or suspension of particles within a fluid. A typical example of an electrorheological fluid is a mixture of cornstarch in a silicone oil. The simple machine of how electrorheological fluids work is simple.When there is no electric field the particles in the fluid are distributed haphazardly and are free to move in the fluid. In an applied electric field the particles place themselves in ?ber-like structures (fibrils) that are much harder to move and impede the merge of the fluid, dramatically change magnitude the viscosity of the fluid-particle mixture (Fig. 5). Typical magnetorheological fluids consist of tiny iron particles uspended in oil and have a similar behavior to the electrorheological fluids buy food initiated by the application of a magnetic field. 8. 1 . Electrorheological Materials.Electrorheological fluids are nonNewtonian fluids, ‘e, the relationship between plume stress and strain rate is nonlinear. The changes in viscous properties of electrorheological fluids are only obtained at relatively high electric fields, in the order of 1 kV/mm. The practical applications of electrorheological fluids center around their abilities to transfer shear stresses and of playing as a variable damping material in an electric field. They have been exhibit in shock absorbers, brake systems, clutches, vibration damping, control valves, and actuators.An illustrative application of electrorheological fluids is their use as a smart space material. In this application, a single-link flexible-light beam was constructed in a sandwich confguration with ER fluids distributed along its length. When the beam is to be speedily moved back an d forth, the ER fluid is not energized, providing flexibility during the transient response compass point of the maneuver (for speed). At the end point of the maneuver the beam is made rigid (for stableness). . 2. Magnetorheological Materials.Magnetorheological fluids are the magnetic equivalent of electrorheological fluids. An advantage over the ER fluids is that high voltages are not required to actuate the MR fluids. These fluids are under development for use in shock absorbers, vibration damping, form equipment and surface polishing of machine parts. 8 9. Photoresponsive and sore Materials in that location are several(prenominal) different types of material families that exhibit different types of light transmission-absorption or other responses to a stimulus.These include lectrochromism (a change in color as a function of an electrical field); thermochromism (color change with heat); photochromic materials (reversible light medium materials); photographic materials (irrever sible light-sensitive materials); photostrictive materials (shape changes due to light usually caused by changes in electronic structure); fluorescence (emission of light (photons) at a different wavelength to the incident light).An fire material with both electro- and thermochromism behavior, V02, was evaluated for a smart window application. Materials are being developed to exhibit both photochromic and photographic ehaviors and one such system is based on a substituted indolinospirobenzopyrene implant in a polystyrene ground substance. This system acts as a photochromic system at low characterisation in the ultraviolet (uv) range and at high buck it functions as a photographic system. The image can be devisualized by heat and can be restored many propagation witn uv irradiation.Another interest application is the use ot polymers that fluoresce or change color in the presence of particular metal ions. This is being tested as a corrosion sensor with optical fibers coated wi th an distinguish polymer r with the polymer incorporated in a composite structure. The system is optically excited and the presence of light at the fluorescence wavelength indicates metal ions (and thus corrosion). Thermochromic materials find use in security devices and in thermal mapping applications.All semiconductor materials exhibit an electrical response to incident light due to the electron absorption of photons, of adapted energy, and promotion of the electrons to the conduction band (change in resistance) and across doping gradients, as in diodes (generation of current and voltage) in solar cells. Of particular interest in this rea is the development of non-silicon systems, which may results in cheap, flexible, robust, and easy to transportation and deploy solar energy collectors. 0. Chemical and Biochemically Sensitive Materials The most widely known classes of chemically sensitive materials are the pHsensitive materials that include the acids, bases, and indicators. T he most interesting of these for smart applications are the indicators. These materials change colors as a function of pH and are usually totally reversible. Indicators have also been used in the development of novel chemical indicating systems.Devices based on the ermeability of organic vapors through polymeric films or porous polymeric plugs, and subsequent reaction with an indicator, are used for monitoring the agent of time and/or temperature sensitive items, eg, pharmaceutics, foods and other perishables. The system is trigger off by crushing the ampoule releasing the volatile component, which then slowly permeates through the film reacting with the indicator to give a visible color change. This change is 9 dependent on both time and temperature.Other examples of pH-sensitive materials include the smart hydrogels and smart polymers (see below in EAPs). There is increasing interest in the detection of other chemicals, particularly in the detection of chemical warfare agents, environmentally noxious chemicals, and the control and monitoring of (engine) emissions. Semiconductor film sensors based on metal oxides (eg, Sn02, ZnO, Ti02, W03) are used to measure the concentration of toxic and inflammable floates.These devices operate at several hundreds of degrees and a chemical reaction between the gas and the metal oxide changes the electrical conductivity of the oxide. The conductivity is a function of the temperature and gas concentration. Higher temperature devices (to $10008C) have been demonstrated using Sic and SrTi03 systems. At low temperatures, the gases interact with the metal oxide by a chemisorption mechanism. The chemically adsorbed particles receive a partial charge and the pivotal charge is made procurable to the oxide as a maneuver electron to increase its conductivity.Metal oxide sensors (MOS) are finding applications in the self-propelled industry monitoring the report card of exhaust gases. Many actions in organic systems are govern ed by highly discriminating reactions that are in effect molecular(a) recognition mechanisms. The subatomic particles responsible for these rocesses are highly selective in the elements that they bind to (‘e, recognize) and can be used in engineered devices as sensors. An example of this is coating piezoelectric material with a selective material and then discover the increase in mass of the coating as the rank molecule binds to the coating.Being organic in nature means that a wide range of slipway to modify these molecules are available, eg, light stimulated regions that change their fluorescence absolute frequency in the presence of the target molecule. Conducting polymers can also be tailored to respond (by a change in resistance) to ifferent chemicals. In this case, the response is rather panoptic (ie, not very selective), however, systems have been developed with a number of polymers treated to react to different types of chemicals to obtain â€Å"fingerprintsâ⠂¬Â that are particular proposition to particular chemicals.These systems generally have a limited biography and increasing the life and stability of the conducting polymers is the main challenge veneer their acceptance as sensors. A novel proposal for chemical detection is the use of molecularly imprinted plastics (MIPs). In this case, target molecules are imbedded in a (porous) plastic matrix that is pplied to some sensing-transducer device. The target molecule is then dissolved away leaving a physical imprint in the plastic matrix.On exposure to the molecule in a gas or liquid the physical imprints remaining in the plastic are selective for the target molecule and collect it, altering the properties (eg, mass) of the plastic. 1 1 . Electroactive Polymers (Actuators) An electroactive polymer (EAP) is a polymer that exhibits a response to an applied electrical stimulation and often also develops an electrical response to a mechanical stimulation. Some of these materials have bee n touched on in previous sections.\r\n'