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Smart Materials  Classification Smart Materials- Introduction Classification of Smart Materials Different Types of Smart Materials Their Applications Conclusion References   Materials are essential and primary thing to the construction of any engineering structure, from the smallest integrated circuit to the largest bridge.  In a new day every technology, the performance, reliability and cost are defined by the which materials is being used. As a conclusion, the drive to develop new materials and processes to improve existing properties of materials science and engineering is one of the most important and dynamic engineering disciplines. Materials science and engineering have strong relationships among the structures, properties, processing and performance of materials are very resulted to their function in engineering structures. Materials design engineers design materials for particular applications and develop improved processing techniques and structures. Materials encompasses in different types of areas, it is often divided according to types of materials like metals, ceramics, polymers, and semiconductors. Material has to many applications such as biomaterials, electronic materials, magnetic materials, structural materials. Smart materials are designed materials that have one or more properties that can be significantly changed in a controlled by external boost such as stress, temperature, moisture, pH, electric or magnetic fields. Smart materials have been around for many years and they have found a large number of applications. The use of the terms ‘smart’ and ‘intelligent’ to describe materials and systems came from the United States and started in the 1980’s despite the fact that some of these so-called smart materials had been around for decades. Many of the smart materials were developed by government agencies working on military and aerospace projects but in recent years their use has transferred into the mechanical and civil engineering sector for applications in the automotive, construction, transport, medical, leisure and domestic areas.  Smart material system which has intrinsic sensor, actuator and control mechanisms whereby it is capable of sensing a stimulus, responding it and reverting to its original state after the stimulus is removed. Smart materials are also called as functional materials. A material can be considered smart when an input stimulus of a variable changes the output of other variables not given as input. There are many groups of smart materials each exhibiting particular properties which can be used in a variety of high-tech and everyday applications. These include shape memory alloys, piezoelectric materials, magneto-rheological and electro-rheological materials, magneto strictive materials and halochromic materials, pH sensitive polymer, smart inorganic materials, temperature responsive polymers, ferrofluid, photomechanical materials, polycaprolactone, self-healing materials, die-electric elastomers, which change their color in reaction to various stimuli.  The distinction between a smart material and a smart structure should be emphasized. A smart structure incorporates some form of actuator and sensor which can be made from smart materials. It controls hardware and software to form a system which reacts to its environment. Like a structure of an aircraft wing which continuously alters its profile during flight to give the optimum shape for the operating conditions at the time.   Shape Memory Alloys  What is shape memory alloys? Shape memory alloys are one of the most well-known types of smart material.  The shape memory effect was seen in the gold-cadmium alloy in 1951. After ten years, in 1962 an equitoxic alloy of titanium and nickel was found to exhibit a significant shape memory effect and it is named as “Nitinol”, because it is made from nickel and titanium. Working of shape memory alloys Shape memory alloys display two distinct crystal structures. Temperature and internal stresses which play a role in part of super-elasticity. It determines the phase that the Shape memory alloys will be at Martensite exists at lower temperatures, and austenite exists at higher temperatures.  When a Shape memory alloys is in martensite form at lower temperatures, the metal can easily be deformed into any random shape.  When the alloy is heated, it goes through transformation from martensite to austenite state. In the austenite phase, the memory metal “remembers” the shape it had before it was deformed.  From the stress versus temperature graph as given below, we can see that at low stress and low temperature, martensite exists. At higher temperature and higher stress, austenite exists.    Shape memory alloys undergo temperature-dependent phase changes so it also changes its structure as given graph below:-    Uses of Shape Memory Alloys in Aerospace Engineering  Shape memory alloys have a large number of uses in aerospace, medicine and the leisure industry which is described as given below: It is uses in variable geometry chevron (VGC) and variable area fan nozzles (VAFN). It is uses as different type of actuators as given below: -Telescopic wing system   -Servo valve   -Retractable landing gear  -Reconfigurable Rotor Blade (RRB)   Uses of Shape Memory Alloys in Medical Engineering  Material nitinol is biocompatible, it can be used in the body without an adverse reaction, so it has found a number of medical uses. These include stents in which rings of SMA wire hold open a polymer tube to open up a blocked vein, blood filters, and bone plates which contract upon transformation to pull the two ends of the broken bone in to closer contact and encourage more rapid healing. It is also possible to use in dentist field for orthodontic braces which straighten teeth. It can be made in desired shape of the teeth.  Uses of Shape Memory Alloys in Domestic applications Shape memory alloys can be used to replace bimetallic strips in many domestic applications. It offers the advantage of giving a larger deflection and exerting a stronger force for a given change in temperature.  They can be used in cut out switches for kettles and other devices, security door locks, fire protection devices such as smoke alarms and cooking safety indicators.  Uses of Shape Memory Alloys in Manufacturing applications It can be used as couplings for connecting two tubes. Diameter of couplings is made slightly smaller than the tubes. The coupling is deformed such that it slips over the tube ends. The temperature changed to activate the memory. The coupling tube shrinks to hold the two ends together but can never fully transform so it exerts a constant force on the joined tubes.     Piezoelectric Materials   The piezoelectric effect was discovered in 1880 by Jaques and Pierre Curie who conducted a number of experiments using quartz crystals. It makes piezoelectric materials the oldest type of smart material. These materials, which are mainly ceramics, have since found a number of uses.   What is the piezoelectric effect?  The piezoelectric effect and electrostriction are opposite phenomena and both relate a shape change with voltage. As with shape memory alloys the shape change is associated with a change in the crystal structure of the material and piezoelectric materials also exhibit two crystalline forms. One form is ordered and this relates to the polarization of the molecules. The second state is non- polarized and this is disordered.  If a voltage is applied to the non-polarized material a shape change occurs as the molecules reorganize to align in the electrical field. This is known as electrostriction.  Conversely, an electrical field is generated if a mechanical force is applied to the material to change its shape. This is the piezoelectric effect.  The main advantage of these materials is the almost instantaneous change in the shape of the material or the generation of an electrical field.   What materials exhibit this effect?  The piezoelectric effect was first observed in quartz and various other crystals such as tourmaline. Barium titanite and cadmium sulphate have also been shown to demonstrate the effect but by far the most commonly used piezoelectric ceramic today is lead zirconium titanite (PZT). The physical properties of PZT can be controlled by changing the chemistry of the material and how it is processed. There are limitations associated with PZT; like all ceramics it is brittle giving rise to mechanical durability issues and there are also problems associated with joining it with other components in a system.   Uses of piezoelectric materials  The main use of piezoelectric ceramics is in actuators. An actuator can be described as a component or material which converts energy (in this case electrical) in to mechanical form. When an electric field is applied to the piezoelectric material it changes its shape very rapidly and very precisely in accordance with the magnitude of the field.  Applications exploiting the electro strictive effect of piezoelectric materials include actuators in the semiconductor industry in the systems used for handling silicon wafers, in the microbiology field in microscopic cell handling systems, in fiber optics and acoustics, in ink-jet printers where fine movement control is necessary for vibration damping.  The piezoelectric effect can also be used in sensors which generate an electrical field in response to a mechanical force. This is useful in damping systems and earthquake detection systems in buildings. Most well-known application is in the sensors which deploy car airbags. The material changes in shape with the impact thus generating a field which deploys the airbag.   Magneto and electro-rheological materials  There are some smart fluids which change their rheological properties in accordance with their environment.  What are smart fluids? There are two types of smart fluids which were both discovered in the 1940s. Electro-rheological (ER) materials change their properties with the application of an electrical field and consist of an insulating oil such as mineral oil containing a dispersion of solid particles. Magneto- rheological materials (MR) are again based on a mineral or silicone oil carrier but this time the solid dispersed within the fluid is a magnetically soft material such as iron and the properties of the fluid are altered by applying a magnetic field. In both cases the dispersed particles are of the order of microns in size.  Working of smart fluids  In both cases the smart fluid changes from a fluid to a solid with the application of the relevant field. The effect takes milliseconds to occur and is completely reversible by the removal of the field.  The small particles in the fluid align and are attracted to each other resulting in a dramatic change in viscosity as shown in below given figure.  Schematic diagram showing the structure of an electrorheological fluid between two electrodes.    Uses of smart fluids  Uses of these unusual materials in civil engineering, robotics and manufacturing is explored. Automotive and aerospace industries where the fluids are used in vibration damping and variable torque transmission. MR dampers are used to control the suspension in cars. Dampers are also used in prosthetic limbs to allow the patient to adapt to various movements.  Chromatic materials  Chromatic materials refer to those which change their color in response to a change in their environment, leading to the suffix chromic. A variety of chromic materials exist and they are described in terms of the stimuli which initiate a change as given below: Thermochromic materials change with temperature.  Photochromic materials change with the light level. Piezo chromic materials change with applied pressure.     In the case of electrochromic, solvate chromic and Carso chromic materials the stimulus is either an electrical potential, a liquid or an electron beam respectively.  Thermochromic, photochromic and piezo chromic materials are the most popular with the first two groups finding everyday applications.  Working of thermochromic materials There are two types of thermochromic systems as given below: Based on liquid crystals rearrangement Bases on molecular rearrangement  In either case at a given temperature a change in the structure of the material occurs giving rise to an apparent change in color. The change is reversible so as the material cools down it changes color back to its original state.  In liquid crystals the change from colored to transparent takes place over a small temperature range around 1 C and arises as the crystals in the material change their orientation as shown in figure.     Above figure shows liquid crystal contains needle shaped particles which are arranged randomly below the transformation temperature at top. Above this temperature the particles are aligned, changing how the material reflects light, thus showing a change in color bottom. However, liquid crystals are relatively very expensive and so where there is no need for the color change to take place in a very narrow temperature window molecular rearrangement material are employed.    Leucocytes change color by molecular rearrangement and the color and active temperature range of the dye can be controlled by changing the chemical groups on the corners and central site of the molecule. Leucocytes have a broader temperature range than liquid crystals and will usually become colorless over approximately 5 C.  In both cases the thermochromic material is encapsulated inside microscopic spherical particles to protect it. These encapsulating molecules must themselves be transparent and able to withstand the thermal cycling which the thermochromic artefact will undergo.   Uses of thermochromic materials Since thermochromic pigments can be used in a variety of ways they have found a varied range of applications.  The pigments can be incorporated in to dyes for fabric to produce clothing which changes color with temperature. Thermochromic inks can also be used for printing on to clothing and food packaging.  Thermochromic toothbrushes have been produced that change color as they are warmed in the hand. It takes roughly two minutes to warm the brush enough to see a change in the color and this is the length of time dentists recommend teeth should be brushed.    Thermochromic thermometers have been developed as they offer significant safety advantages over traditional glass or mercury thermometers. The plastic substrate consists of stripes of different colors representing the different temperatures. This is then coated with a layer of thermochromic dye of varying thickness. It is thinner at the cool end of the thermometer. It is higher at temperature end. As the thermometer is warmed by placing it on the forehead the thin layer of dye warms up and becomes transparent first. The higher the temperature the thicker the layer of dye which can be warmed sufficiently to change color.  This principle is also employed in the tester strips which appear on the sides of some batteries, but this time the heat is generated by the resistance heating effect of a small electrical current flowing across the battery.  Thermochromic materials have also found safety applications in kettles and baby spoons. The body of the kettle is actually made from pink plastic. However, this contains a small amount of a thermochromic dye which is blue at room temperature and becomes transparent when warm, thus showing the pink color. A series of photographs showing the change in color as the kettle is boiled are shown in figure as given below:  Figure shows gradual change in color as the thermochromic kettle boils.   Thermochromic pigments have also been employed in baby spoons which change color to warn if the food is too hot for feeding.     Babies generally like to eat their food no hotter than one degree above body temperature, so the spoons are designed to change color at 38 C. The room temperature and hot states of such a spoon are shown in figure as given below:  Figure shows that thermochromic baby spoon at room temperature (top) and after immersion in boiling water (bottom).  The food more rapidly and spoon changes from red to yellow. The bright yellow color has been achieved by immersing the spoon in boiling water.  Thermochromic dyes with a higher temperature resistance and higher transition temperature have also been produced and incorporated in to pans. These pans have a small colored circle in the bottom which changes color when the pan has reached the optimum temperature for cooking.       Polymers  A polymer is a large molecule or macromolecule, composed by many repetition subunits. Because of that range of properties, both synthetic and natural polymers play a very essential and ubiquitous role in everyday life.  Polymers range from familiar synthetic plastic such as synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function.  Polymers should be in both natural and synthetic form. That are created via polymerization of many small molecules which is known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, visco-elasticity, and a tendency to form glasses and semicrystalline structures rather than crystals.  Polymers has two types: Natural polymeric material such as shellac, amber, wool, silk and natural rubber. y of other Natural polymers exist such as cellulose which is the main constituent of wood and paper.  Synthetic polymers includes polyethylene, polypropylene, polystyrene, polyvinyl chloride, synthetic rubber,phenol formaldehyde resin or Bakelite, neoprene, nylon, polyacrylonitrile, PVB, silicone, and many more.  Polymerization process Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network.  During the polymerization process, some chemical groups may be lost from each monomer. For example, in the polymerization of PET polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2- CH2-OH) but the repeating unit is          -OC-C6H4-COO-CH2-CH2-O-, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are generally divided into two types as given below: Step-growth polymerization   Chain-growth polymerization  Polymer structure  An important microstructural feature of a polymer is its architecture and shape which is related to the way branch points lead to a deviation from a simple linear chain. Branched polymer molecule is composed of a main chain with one or more substituent side chains or branches.  Types of branched polymers include star polymers, comb polymers, brush polymers, dendronized polymers, ladder polymers, and dendrimers.  A polymer’s architecture affects many of its physical properties but not limited to such as solution viscosity, melt viscosity, solubility in various solvents, glass transition temperature and the size of individual polymer coils in solution.      Mechanical properties of polymers:   Tensile strength The tensile strength of a material quantifies how much elongating stress the material will endure before failure. Very important applications are polymer’s physical strength and durability. Tensile strength increases with polymer chain length and cross linking of polymer chains.  Young’s modulus of elasticity Young’s modulus quantifies the elasticity of the polymer. It is defined as for small strains as the ratio of rate of change of stress to strain.  Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers such as rubber bands. The modulus is strongly dependent on temperature. Visco elasticity describes a complex time-dependent elastic response, which will exhibit hysteresis in the stress-strain curve when the load is removed.  Transport properties Transport properties such as diffusivity relate to how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes.  Mixing behavior properties   Phase diagram of the typical mixing behavior of weakly interacting polymer solutions.                                                                  {1} Mohd Jani, Jaronie; Leary, Martin; Subic, Aleksandar; Gibson, Mark A. (April 2014). “A review of shape memory alloy research, applications and opportunities”. Materials & Design. 56: 1078–1113. doi:10.1016/j.matdes.2013.11.084.              

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