Classification of materials

Classification of materials

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  1 CLASSIFICATION OF MATERIALS by ProfA. BALASUBRAMANIAN CENTRE FOR ADVANCED STUDIES IN EARTH SCIENCE UNIVERSITY OF MYSORE MYSORE-6 (Image: NOVA)
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  2 Introduction: Earth's Materials havefound to have played a dominant role in the development of civilization that we associate with them in all ages. In the origin of human life on Earth, the Stone Age, people used only natural materials, like stone, clay, skins, and wood. When people found copper and how to make it harder by alloying, the Bronze Age started about 3000 BC. The use of iron and steel, a stronger material that gave advantage in wars started at about 1200 BC. Understanding of how materials behave like they do, and why they differ in properties was only possible with the atomistic understanding allowed by quantum mechanics, that first explained atoms and then solids starting in the 1930s. The combination of physics, chemistry, and the focus on the relationship between the properties of a material and its microstructure is the domain of Materials Science. The development of this science allowed designing materials and provided a knowledge base for the engineering applications (Materials Engineering). This report highlights the classification of the engineering materials and their processing techniques. The engineering materials can broadly be classified as: a) Ferrous Metals b) Non-ferrous Metals (aluminum, magnesium, copper, nickel, titanium) c) Plastics (thermoplastics, thermosets) d) Ceramics and Diamond e) Composite Materials & f) Nano-materials. The engineering materials are often primarily selected based on their mechanical, physical, chemical and manufacturing properties. The secondary factors to be considered are the cost and availability, appearance, service life and recyclability. Metals : Metals are usually lustrous, ductile, malleable, and good conductors of electricity They are divided into 2 categories: A) FERROUS: the group which contains mainly iron (Fe). Iron is the most important metal in industrialized countries B) NON-FERROUS: other metallic materials containing no iron likecopper (Cu) or aluminium (Al)  Ferrous metals and alloys (irons, carbon steels, alloy steels, stainless steels, tool and die steels)  Nonferrous metals and alloys (aluminum, copper, magnesium, nickel, titanium, precious metals, refractory metals, super-alloys) All the elements are broadly divided into metals and non-metals according to their properties. Metals are element substances which readily give up electrons to form metallic bonds and conduct electricity. Some of the important basic properties of metals are: (a) metals are usually good electrical and thermal conductors, (b) at ordinary temperature metals are usually solid, (c) to some extent metals are malleable and ductile, (d) the freshly cut surfaces of metals are lustrous, (e) when struck metal produce typical sound, and (f) most of the metals form alloys. When two or more pure metals are melted together to form a new metal whose properties are quite different from those of original metals, it is called an alloy.
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  3 Metallic materials possessspecific properties like plasticity and strength. Few favourable characteristics of metallic materials are high lustre, hardness, resistance to corrosion, good thermal and electrical conductivity, malleability, stiffness, the property of magnetism, etc. Metals may be magnetic, non-magnetic in nature. These properties of metallic materials are due to: (i) the atoms of which these metallic materials are composed and (ii) the way in which these atoms are arranged in the space lattice. Metallic materials are typically classified according to their use in engineering as under: Pure Metals and alloys. Pure Metals: Generally it is very difficult to obtain pure metal. Usually, they are obtained by refining the ore. Mostly, pure metals are not of any use to the engineers. Materials in this group are composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for example, carbon, nitrogen, and oxygen) in relatively small amounts. Atoms in metals and their alloys are arranged in a very orderly manner and in comparison to the ceramics and polymers, are relatively dense. With regard to mechanical characteristics, these materials are relatively stiff, and strong yet are ductile (i.e., capable of large amounts of deformation without fracture), and are resistant to fracture , which accounts for their widespread use in structural applications. Metallic materials have large numbers of nonlocalized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely good conductors of electricity and heat, and are not transparent to visible light; a polished metal surface has a lustrous appearance. In addition, some of the metals (viz., Fe, Co, and Ni) have desirable magnetic properties. Metals: Normally metallic materials are combinations of metallic elements. Metallic materials have large number of nonlocalized electrons, i.e. electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. All metals are characterized by metallic properties, e.g. luster, opacity, malleability, ductility and electrical conductivity. Although metals compose about three fourth of the known elements but few find service in their pure form. The desired properties for engineering purposes are often found in alloys. Typical examples of metallic materials are iron, aluminium, copper, zinc, etc. and their alloys. They can be used either in bulk or powder form. Metals are extremely good conductors of electricity and heat are not transparent to visible light; a polished metal surface has a lustrous appearance. Moreover, metals are quite strong, yet deformable, which accounts for their extensive use in structural applications. Metallic materials are always crystalline in nature. Scientists have developed amorphous (non-crystalline) alloys by very rapid cooling of a melt or by very high-energy mechanical milling. Recently, scientists have developed materials through rapid solidification called as quasicrystals. These are neither crystalline nor amorphous, but form an ordered structure
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  4 somewhere between twoknown structures. These materials are expected to exhibit far reaching electrical properties. Inorganic Materials: These materials include metals, clays, sand rocks, gravels, minerals and ceramics and have mineral origin. These materials are formed due to natural growth and development of living organisms and are not biological materials. Rocks are the units which form the crust of the earth. The three major groups of rocks are: (i) Igneous Rocks: These rocks are formed by the consolidation of semi-liquid of liquid material (magma). These are called as Plutonic if their consolidation takes place deep within the earth and volcanic if lava or magma solidifies on the earth’s surface. Basalt is igneous volcanic where as granite is igneous plutonic. (ii) Sedimentary Rocks: When broken down remains of existing rocks are consolidated under pressure, then the rocks are named as sedimentary rocks, e.g., shale and sandstone rocks. The required pressure for the formation of sedimentary rocks is supplied by the overlying rocky material. (iii) Metamorphic Rocks: These rocks are basically sedimentary rocks which are changed into new rocks by intense heat and pressure, e.g., marble and slates. The structure of these rocks is in between igneous rocks and sedimentary rocks. Rock materials are widely used for the construction of buildings, houses, bridges, monuments, arches, tombs, etc. The slate, which has got great hardness is still used as roofing material. Basalt, dolerite and rhyolite are crushed into stones and used as concrete aggregate and road construction material. Another type of materials, i.e. Pozzolanics, are of particular interest to engineers because they are naturally occurring or synthetic silicious materials which hydrate to form cement. Volcanic ash, blast furnace slag, some shales and fly ash are examples of pozzolanic materials. When the cement contains 10- 20% ground blast furnace slag, then it is called pozzolans-portland cement, which sets more slowly than ordinary portland cement and has greater resistance to sulphate solutions and sea water. Rocks, stone, wood, copper, silver, gold etc. are the naturally occurring materials exist in nature in the form in which they are to be used. However, naturally occurring materials are not many in number. Nowadays, most of the materials are manufactured as per requirements. Obviously, the study of engineering materials is also related with the manufacturing process by which the materials are produced to acquire the properties as per requirement.
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  5 Copper, silver, gold,etc. metals, which occur in nature, in their free state are mostly chemically inert and highly malleable and ductile as well as extremely corrosion resistant. Alloys of these metals are harder than the basic metals. Carbonates, sulphates and sulphide ores are more reactive metals. Biological Materials Leather, limestone, bone, horn, wax, wood etc. are biological materials. Wood is fibrous composition of hydrocarbon, cellulose and lignin and is used for many purposes. Apart from these components a small amount of gum, starch, resins, wax and organic acids are also present in wood. One can classify wood as soft wood and hard wood. Fresh wood contains high percentage of water and to dry out it, seasoning is done. If proper seasoning is not done, defects such as cracks, twist, wrap etc. may occur. Leather is obtained from the skin of animals after cleaning and tanning operations. Nowadays, it is used for making belts, boxes, shoes, purses etc. To preserve the leather, tanning is used. Following two tanning techniques are widely used: (a) Vegetable Tanning: It consist of soaking the skin in tanning liquor for several days and then dried to optimum conditions of leather. (b) Chrome Tanning: This technique involves pickling the skin in acid solution and then revolving in a drum which contains chromium salt solution. After that the leather is dried and rolled. Limestone is an important material which is not organic but has biological origin. It mainly consist of calcium carbonate and limestone. It is widely used to manufacture cement. In Iron and Steel Industries, limestone in pure form is used as flux. In early days bones of animals were used to make tools and weapons. Nowadays bones are used for the manufacture of glue, gelatin etc. Bones are laminate of organic substances and phosphates and carbonates of calcium. These are stronger in compression as compared to tension. Organic Materials Organic materials are carbon compounds and their derivatives. They are solids composed of long molecular chains. The study of organic compounds is very important because all biological systems are composed of carbon compounds. There are also some materials of biological origin which do not possess organic composition, e.g., limestone. These materials are carbon compounds in which carbon is chemically bonded with hydrogen, oxygen and other non-metallic substances. The structure of these compounds is complex. Common organic materials are plastics and synthetic rubbers which are termed as organic polymers. Other examples of organic materials are wood, many types of waxes and petroleum derivatives. Organic polymers are prepared by polymerisation reactions, in which simple molecules are chemically combined into long chain molecules or three-dimensional structures.
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  6 Organic polymers aresolids composed of long molecular chains. These materials have low specific gravity and good strength. The two important classes of organic polymers are: (a) Thermoplastics: On heating, these materials become soft and hardened again upon cooling, e.g., nylon,polythene, etc. (b) Thermosetting plastics: These materials cannot be resoftened after polymerisation, e.g., urea-formaldehyde, phenol formaldehyde, etc. Due to cross-linking, these materials are hard, tough, non-swelling and brittle. These materials are ideal for moulding and casting into components. They have good corrosion resistance. The excellent resistance to corrosion, ease of fabrication into desired shape and size, fine lusture, light weight, strength, rigidity have established the polymeric materials and these materials are fast replacing many metallic components. PVC (Polyvinyl Chloride) and polycarbonate polymers are widely used for glazing, roofing and cladding of buildings. Plastics are also used for reducing weight of mobile objects, e.g., cars, aircrafts and rockets. Polypropylenes and polyethylene are used in pipes and manufacturing of tanks. Thermo-plastic films are widely used as lining to avoid seepage of water in canals and lagoons. To protect metal structure from corrosion, plastics are used as surface coatings. Plastics are also used as main ingredients of adhesives. The lower hardness of plastic materials compared with other materials makes them subjective to attack by insects and rodents. Because of the presence of carbon, plastics are combustible. The maximum service temperature is of the order of 100°C. These materials are used as thermal insulators because of lower thermal conductivity. Plastic materials have low modulus of rigidity, which can be improved by addition of filters, e.g., glass fibres. Natural rubber, which is an organic material of biological origin, is an thermoplastic material. It is prepared from a fluid, provided by the rubber trees. Rubber materials are widely used for tyres of automobiles, insulation of metal components, toys and other rubber products. Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (viz.O,N, and Si). Furthermore, they have very large molecular structures, often chain-like in nature that have a backbone of carbon atoms. Some of the common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities , whereas their mechanical characteristics are generally dissimilar to the metallic and ceramic materials—they are not as stiff nor as strong as these other material types.
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  7 However, on thebasis of their low densities,many times their stiffnesses and strengths on a per mass basis are comparable to the metals and ceramics. In addition, many of the polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. In general, they are relatively inert chemically and unreactive in a large number of environments. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. Furthermore, they have low electrical conductivities and are nonmagnetic. Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbreviated “GFRP.” Many of these are organic substances and derivatives of carbon and hydrogen. Polymers include the familiar plastic and rubber materials. Usually polymers are classified into three categories: thermoplastic polymers, thermosetting polymers and elastomers, better called as rubbers. Polymers have very large molecular structures. Most plastic polymers are light in weight and are soft in comparison to metals. Polymer materials have typically low densities and may be extremely flexible and widely used as insulators, both thermal and electrical. Typical examples of polymers are polyesters, phenolics, polyethylene, nylon and rubber. The overriding consideration of the selection of a given polymer is whether or not the material can be processed into the required article easily and economically.  Crude oil supplies the majority of the raw material for the production of polymers, also called plastics  Polymers can be divided into 3 categories: Thermoplastics: usually soft and easy to be recycled Thermosetting plastics: usually stiff and not easy to be recycled. Elastomers: flexible (rubbers) Ceramic materials The word ceramic is derived from the Greek word keramikos. The term covers inorganic non -metallic materials whose formation is due to the action of heat Clays, bricks, cements, glass are the most important ones. These are crystalline compounds between metallic and non-metallic elements. They are most frequently oxides, nitrides and carbides. Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, some of the common ceramic materials include aluminum oxide (or alumina,Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC)& silicon nitride (Si3N4). In addition, some are referred to as the traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as cement, and glass. With regard to mechanical behavior, ceramic materials are relatively stiff and strong— stiffnesses and strengths are comparable to those of the metals. In addition, ceramics are typically very hard.
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  8 On the otherhand, they are extremely brittle (lack ductility), and are highly susceptible to fracture. These materials are typically insulative to the passage of heat and electricity (i.e., have low electrical conductivities, and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque, and some of the oxide ceramics (e.g., Fe O ) exhibit magnetic behavior. Nowadays graphite is also categorized in ceramics. The wide range of materials which falls within this classification include ceramics that are composed of clay minerals, cement and glass. Glass is grouped with this class because it has similar properties but most glasses are amorphous. Ceramics are characterised by high hardness, abrasion resistance, brittleness and chemical inertness. Ceramics are typically insulative to the passage of electricity and heat, and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to mechanical behaviour, these materials are hard but very brittle. These materials are widely categorized into oxide and non-oxide ceramics. Examples Ceramics: Glasses; Glass ceramics; Graphite; Diamond. Composites A composite is a composition of two or more materials in the first three categories, e.g. metals, ceramics and polymers, that has properties from its constituents. Large number of composite materials have been engineered. Few typical examples of composite materials are wood, clad metals, fibre glass, reinforced plastics, cemented carbides, etc. Fibre glass is a most familiar composite material, in which glass fibres are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials. Fibre glass acquires strength from the glass and the flexibility from the polymer. A composite is composed of two (or more) individual materials, which come from the categories discussed above—viz., metals, ceramics, and polymers. The design goal of a composite is to achieve a combination of properties that is not displayed by any single material, and also to incorporate the best characteristics of each of the component materials. A large number of composite types exist that are represented by different combinations of metals, ceramics, and polymers. Furthermore, some naturally-occurring materials are also considered to be composites—for example, wood and bone. However, most of those we consider in our discussions are synthetic (or man-made) composites. One of the most common and familiar composites is fiber-glass, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester).
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  9 The glass fibersare relatively strong and stiff (but also brittle), whereas the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is relatively stiff, strong, flexible, and ductile. In addition, it has a low density. Another of these technologically important materials is the “carbon fiber-reinforced polymer” (or “CFRP”) composite—carbon fibers that are embedded within a polymer. These materials are stiffer and stronger than the glass fiber-reinforced materials, yet they are more expensive. The CFRP composites are used in some aircraft and aerospace applications, as well as high- tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, and skis/snowboards). A true composite structure should show matrix material completely surrounding its reinforcing material in which the two phases act together to exhibit desired characteristics. These materials as a class of engineering material provide almost an unlimited potential for higher strength, stiffness, and corrosion resistance over the ‘pure’ material systems of metals, ceramics and polymers. Many of the recent developments of materials have involved composite materials. Probably, the composites will be the steels of this century. Nowadays, the rapidly expanding field of nano composites is generating many exciting new materials with novel properties. The general class of nano composite organic or inorganic material is a fast growing field of research. Significant efforts are going on to obtain control of nano composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics. The lamellar class of intercalated organic/ inorganic nano composites and namely those systems that exhibit electronic properties in at least one of the composites offers the possibility of obtaining well ordered systems some of which may lead to unusual electrical and mechanical properties. Polymer-based nano composites are also being developed for electronic applications such as thin-film capacitors in integrated circuits and solid polymer electrolytes for batteries. No doubt, the field of nano composites is of broad scientific interest with extremely impressive technological promise. Other materials coming under this group are: Reinforced plastics; Metal-matrix composites; Ceramic-matrix composites; Sandwich structures; and Concrete. Advanced Materials Materials that are utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. By high technology we mean a device or product that operates or functions using relatively intricate and sophisticated principles; examples include electronic equipment (camcorders, CD/DVD players, etc.), computers, fiber-optic systems, spacecraft, aircraft, and military rocketry.
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  10 These advanced materialsare typically traditional materials whose properties have been enhanced, and, also newly developed, high-performance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers), and are normally expensive. Advanced materials include semiconductors, biomaterials, and what we may term “materials of the future” (that is, smart materials and nanoengineered materials), which we discuss below. The properties and applications of a number of these advanced materials -for example, materials that are used for lasers, integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber optics. These are new engineering materials which exhibit high strength, great hardness, and superior thermal, electrical, optical and chemical properties. Advanced materials have dramatically altered communication technologies, reshaped data analysis, restructured medical devices, advanced space travel and transformed industrial production process. These materials are often synthesized from the biproducts of conventional commodity materials and often possess following characteristics: • These materials are created for specific purposes, • These materials are highly processed and possess a high value-to weight ratio, • These materials are developed and replaced with high frequency, and • These materials are frequently combined into new composites. Nowadays, there is considerable interest in making advanced materials that are usually graded by chemical composition, density or coefficient of thermal expansion of material or based on microstructural features, e.g. a particular arrangement of second-phase particles or fibres in a matrix. Such materials are referred as functionally graded materials. Instead of having a step function, one may strive to achieve a gradual change. Such gradual change will reduce the chances of mechanical and thermal stresses, generally present otherwise. We may note that the concept of a functionally graded material is applicable to any material metal, polymer or ceramic. Semiconductors: These materials have electrical properties that are intermediate between electrical conductors and insulators. Moreover, the electrical characteristics of semiconducting materials are extremely sensitive to the presence of minute concentrations of impurity atoms; these concentrations may be controlled over very small spatial regions. Silicon, Germanium and some more compounds form the vast majority of semiconducting crystals. Semiconductors have electrical properties that are intermediate between the electrical conductors (viz. metals and metal alloys) and insulators (viz. ceramics and polymers). Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial regions.
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  11 Semiconductors have madepossible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries (not to mention our lives) over the past three decades. These semiconducting materials are used in a number of solid state devices, e.g. diodes, transistors, photoelectric devices, solar batteries, radiation detectors, thermistors and lasers. The semiconductors have made possible the advent of integrated circuitary that has completely revolutionized the electronics and computer industries. Biomaterials: Biomaterials are employed in components implanted into the human body for replacement of diseased or damaged body parts.These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the above materials—metals, ceramics, polymers, composites, and semiconductors— may be used as biomaterials. Some biomaterials that are utilized in artificial hip replacements. Materials of the Future- Smart Materials: Smart (or intelligent) materials are a group of new and state-of-the-art materials now being developed that will have a significant influence on many of our technologies. The adjective “smart” implies that these materials are able to sense changes in their environments and then respond to these changes in predetermined manners traits that are also found in living organisms. In addition, this “smart” concept is being extended to rather sophisticated systems that consist of both smart and traditional materials. function). Actuators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and/or magnetic fields. Four types of materials are commonly used for actuators: shape memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/ magnetorheological fluids. Shape memory alloys are metals that, after having been deformed, revert back to their original shapes when temperature is changed. Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered. The behavior of magnetostrictive materials is analogous to that of the piezoelectrics, except that they are responsive to magnetic fields. Also, electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscosity upon the application of electric and magnetic fields, respectively. Materials/devices employed as sensors include optical fibers, piezoelectric materials (including some polymers), and microelectromechanical devices. For example, one type of smart system is used in helicopters to reduce aerodynamic cockpit noise that is created by the rotating rotor blades.
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  12 Piezoelectric sensors insertedinto the blades monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer-controlled adaptive device, which generates noise-canceling antinoise. Smart or intelligent materials form a group of new and state of art materials now being developed that will have a significant influence on many of present-day technologies. The adjective ‘smart’ implies that these materials are able to sense changes in their environments and then respond to these changes in predetermined manners—traits that are also found in living organisms. In addition, the concept of smart materials is being extended to rather sophisticated systems that consist of both smart and traditional materials. The field of smart materials attempts to combine the sensor (that detects an input signal), actuator (that performs a responsive and adaptive function) and the control circuit or as one integrated unit. Acutators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and or magnetic fields. Usually, four types of materials are commonly used for actuators: shape memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological / magnetorheological fluids. Shape memory alloys are metals that, after having been deformed, revert back to their original shapes when temperature is changed. Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely these materials also generate an electric field when their dimensions are altered. The behaviour of magnetostrictive materials is analogous to that of the piezoelectric ceramic materials, except that they are responsive to magnetic fields. Also, electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscocity upon application of electric and magnetic fields, respectively. The combined system of sensor, actuator and control circuit or as one IC unit, emulates a biological system. These are known as smart sensors, Micro System Technology (MST) or Micro Electro Mechanical Systems (MEMS). Materials / devices employed as sensors include optical fibres, piezoelectric materials (including some polymers) and MEMS. For example, one type of smart system is used in helicopters to reduce aero-dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric sensors inserted into the blades, monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer controlled adaptive device, which generates noise cancelling antidose. MEMS devices are small in size, light weight, low cost, reliable with large batch fabrication technology. They generally consist of sensors that gather environmental information such as pressure, temperature, acceleration, etc., integrated electronics to process the data collected and actuators to influence and control the environment in the desired manner. The MEMS technology involves a large number of materials. Silicon forms the backbone of these systems also due to its excellent mechanical properties as well as mature micro-fabrication technology including lithography, etching, and bonding.
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  13 Other materials havingpiezoelectric, piezoresistive, ferroelectric and other properties are widely used for sensing and actuating functions in conjunction with silicon. Nano Materials Nano-structured (NS) materials are defined as solids having microstructural features in the range of 1–100 nm (= (1–100) × 10−9 m) in at least in one dimension. These materials have outstanding mechanical and physical properties due to their extremely fine grain size and high grain boundary volume fraction. Usually, the clusters of atoms consisting of typically hundreds to thousands on the nanometer scale are called as nanoclusters. These small group of atoms, in general, go by different names such as nano particles, nanocrystals, quantum dots and quantum boxes. Significant work in being carried out in the domain of nano-structured materials and nano tubes since they were found to have potential for high technology engineering applications. Nano-structured materials exhibit properties which are quite different from their bulk properties. These materials contain a controlled morphology with atleast one nano scale dimension. Nano crystals, nano wires and nano tubes of a large number of inorganic materials have been synthesized and characterized in the last few years. Some of the nano materials exhibit properties of potential technological value. This is particularly true for nano-structures of semiconducting materials such as metal chalcogenides and nitrides. The mixing of nano-particles with polymers to form composite materials has been practiced for decades. For example, the clay reinforced resin known as Bakelite is the first mass-produced polymer- nanoparticle composites and fundamentally transformed the nature of practical household materials. Even before bakelite, nano composites were finding applications in the form of nano particle-toughened automobile tires prepared by blending carbon black, zinc oxide, and/or magnesium sulfate particles with vulcanized rubber. Despite these early successes, the broad scientific community was not galvanized by nano composites until the early 1990s, when reports revealed that adding mica to nylon produced a five-fold increase in the yield and tensile strength of the material. Subsequent developments have further contributed to the surging interest in polymer–nano particle composites. Nanoengineered Materials Until very recent times the general procedure utilized by scientists to understand the chemistry and physics of materials has been to begin by studying large and complex structures, and then to investigate the fundamental building blocks of these structures that are smaller and simpler. This approach is sometimes termed “top-down” science. However, with the advent of scanning probe microscopes, which permit observation of individual atoms and molecules, it has become possible to manipulate and move atoms and molecules to form new structures and, thus, design new materials that are built from simple atomic-level constituents (i.e., “materials by design”).
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  14 This ability tocarefully arrange atoms provides opportunities to develop mechanical, electrical, magnetic, and other properties that are not otherwise possible. We call this the “bottom-up” approach, and the study of the properties of these materials is termed “nanotechnology”; the “nano” prefix denotes that the dimensions of these structural entities are on the order of a nanometer (10^-9 m)—as a rule, less than 100 nanometers (equivalent to approximately 500 atom diameters). Materials produced out of nanoparticles have some special features, e.g. (i) very high ductility (ii) very high hardness ~4 to 5 times more than usual conventional materials (iii) transparent ceramics achievable (iv) manipulation of colour (v) extremely high coercivity magnets (vi) developing conducting inks and polymers. Material science has expanded from the traditional metallurgy and ceramics into new areas such as electronic polymers, complex fluids, intelligent materials, organic composites, structural composites, biomedical materials (for implants and other applications), biomimetics, artificial tissues, biocompatible materials, “auxetic” materials (which grow fatter when stretched), elastomers, dielectric ceramics (which yield thinner dielectric layers for more compact electronics), ferroelectric films (for non-volatile memories), more efficient photovoltaic converters, ceramic superconductors, improved battery technologies, self-assembling materials, fuel cell materials, optoelectronics, artificial diamonds, improved sensors (based on metal oxides, or conducting polymers), grated light values, ceramic coatings in air (by plasma deposition), electrostrictive polymers, chemical—mechanical polishing, alkali-metal thermoelectric converters, luminescent silicon, planar optical displays without phosphors, MEMS, and super molecular materials. However, there still remain technological challenges, including the development of even more sophisticated and specialized materials, as well as consideration of the environmental impact of materials production.
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  15 Material scientists areinterested in green approaches, by entering the field of environmental—biological science, by developing environmentally friendly processing techniques and by inventing more recyclable materials. The following table shows the properties of materials to be considered for different applications: Manufacturing processes Functional requirements Cost considerations Operating parameters Plasticity Strength Raw material Pressure Malleability Hardness Processing Temperature Ductility Rigidity Storage Flow Machinability Toughness Manpower Type of material Casting properties Thermal conductivity Special treatment Corrosion requirements Weldability Fatigue Inspection Environment Heat Electrical treatment Packaging properties Protection from fire Tooling Creep Inventory Weathering Surface finish Aesthetic look Taxes and custom duty Biological effects Materials science: The combination of physics, chemistry, and the focus on the relationship between the properties of a material and its microstructure is the domain of Materials Science. The development of this science allowed designing materials and provided a knowledge base for the engineering applications (Materials Engineering). Conclusion: Materials play a central role in society. Beyond the physical and chemical properties of materials, their cultural properties have often been overlooked in anthropological studies: finished products have been perceived as 'social' yet the materials which comprise them are considered 'raw' or natural'. Human societies have always worked with materials. However, the customs and traditions surrounding this differ according to the place, the time and the material itself. Materials are rapidly emerging as one of the most important and exciting research fields globally and across disciplines.