Continuum mechanics

Laws Conservation of mass Conservation of momentum Conservation of energy Entropy inequality Solid mechanics Solids · Stress · Deformation · Finite strain theory · Infinitesimal strain theory · Elasticity · Linear elasticity · Plasticity · Viscoelasticity · Hooke's law · Rheology Fluid mechanics Fluids · Fluid statics Fluid dynamics · Viscosity · Newtonian fluids Non-Newtonian fluids Surface tension Scientists Newton · Stokes · Navier · Cauchy· Hooke · Bernoulli

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Matter is generally found in three different forms: solid, liquid, and gas (or vapor). The solid state of matter is characterized by a distinct structural rigidity and resistance to deformation (that is changes of shape and/or volume). Most solids have high values both of Young's modulus and of the shear modulus of elasticity. This contrasts with liquids or fluids, which have zero static shear modulus and exhibit the capacity for macroscopic viscous flow.

The branch of physics that deals with solids is called solid-state physics, and is the main branch of condensed matter physics which also includes liquids. The emerging field of materials science is primarily concerned with the physical and chemical properties of solids and their applications as high performance materials in modern technology. Solid-state chemistry is especially concerned with the synthesis of novel materials, as well as the science of identification and chemical composition.

Because solids have thermal energy or heat capacity, their atoms vibrate about fixed mean positions within the ordered (or disordered) lattice. Shown here are the one-dimensional normal modes of vibration in a crystalline solid. The amplitude of the motion has been exaggerated, and is actually much smaller than the lattice parameter. The entire spectrum of lattice vibrations in a crystalline or glassy network plays a key role in the kinetic theory of solids.

[edit] Crystalline vs. glassy state

A crystalline solid: atomic resolution image of strontium titanate. Brighter atoms are Sr and darker ones are Ti.

Schematic representation of a random-network glassy form (top) and ordered crystalline lattice (bottom) of identical chemical composition.

In crystalline solids, the atoms or molecules that compose the solid are packed closely together. These constituent elements have fixed positions in space relative to each other. This accounts for the solid's structural rigidity. In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A specific symmetry or crystal structure is composed of a Bravais lattice which is typically represented by a single unit cell. The unit cell is periodically repeated in three dimensions on a lattice. The spacings between unit cells in various directions are called lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its physical properties, such as cleavage, electronic band structure, and optical properties.

Non-crystalline or glassy solids are often referred to as supercooled liquids, but possess the mechanical properties of both a solid and a liquid, depending on the time scale under consideration. In their molecular structure, their molecules do not exhibit the long-range order exhibited by crystalline substances. In addition, while a glassy solid does exhibit some viscous flow and plastic deformation, this only occurs on geological timescales. Thus, it behaves mechanically as a solid for all practical intents and purposes—and most experimental timescales.

Generally speaking, the atomic or molecular structure of glass exists in a metastable state with respect to its crystalline form. Glass would convert into a more stable crystalline form, but the rate of this conversion is slow. This essentially reflects the basic physics of glass, the glass transition, and its formation from a non-equilibrium supercooled liquid state.^[1][2][3]

Much work has been done to elucidate the primary microstructural features of glass forming substances (e.g. silicates) on both small (microscopic) and large (macroscopic) scales. One emerging school of thought is that a glass is simply the "limiting case" of a polycrystalline solid at small crystal size. Within this framework, domains, exhibiting various degrees of short-range order, become the building blocks of both metals and alloys, as well as glasses and ceramics. The microstructural defects of both within and between these domains provide the natural sites for atomic diffusion and the occurrence of viscous flow and plastic deformation in solids.^[4]

• Note: Because solids have thermal energy, their atoms vibrate about fixed mean positions within the ordered (or disordered) lattice. The spectrum of lattice vibrations in a crystalline or glassy network provides the foundation for the kinetic theory of solids. This motion occurs at the atomic level, and thus cannot be observed or detected without highly specialized equipment, such as that used in spectroscopy.

[edit] Classes of solids

Metals occupy the bulk of the periodic table, while non-metallic elements can only be found on the right-hand-side of the periodic table of the elements. A diagonal line drawn from boron (B) to polonium (Po) separates the metals from the non-metals. Most elements on this line are metalloids, or more commonly referred to as semiconductors. This is due to the fact that these elements exhibit electronic properties common to both conductors and insulators. Elements to the lower left of this division line are called metals, while elements to the upper right are called non-metals.

[edit] Metals

The study of metallic elements and their alloys makes up a significant portion of the fields of solid-state chemistry, physics, materials science and engineering. Generally speaking, metals have delocalized electrons and an electronic band structure containing partially filled bands. The resulting large number of free electrons (often referred to as a "sea of electrons") gives metals their high values of electrical and thermal conductivity.

When considering the electronic band structure and binding energy of a metal, it is necessary to take into account the positive potential caused by the specific arrangement of the ion cores, which is periodic in crystals. The most important consequence of the periodic potential is the formation of a small band gap at the boundary of the Brillouin zone. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly free electron model.

Mechanical properties of metals include their ductility, which is largely due to their inherent capacity for plastic deformation. Thus, elasticity in metals can be described by Hooke's Law for restoring forces, where the stress is linearly proportional to the strain. Larger forces in excess of the elastic limit may cause a permanent (irreversible) deformation of the object. This is what is known in the literature as plastic deformation -- or plasticity. This irreversible change in atomic arrangement may occur as a result of either (or both) of the following factors:

• The action of an applied force (or work)
• A change in temperature (or heat).

In the former case, the applied force may be tensile (pulling) force, compressive (pushing) force, shear, bending or torsion (twisting) forces. In the latter case, the most significant factor which is determined by the temperature is the mobility of the structural defects such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally activated, and thus limited by the rate of atomic diffusion.

Viscous flow near grain boundaries, for example, can give rise to internal slip, creep, fatigue in metals. It can also contribute to significant changes in the microstructure like grain growth and localized densification due to the elimination of intergranular porosity. Screw dislocations may slip in the direction of any lattice plane containing the dislocation, while the principal driving force for "dislocation climb" is the movement or diffusion of vacancies through a crystal lattice.

Optically speaking, metals are opaque, shiny and lustrous. This is due to the fact that visible lightwaves are not readily transmitted through the bulk of their microstructure. The large number of free electrons in any typical metallic solid (element or alloy) is responsible for the fact that they can never be categorized as transparent materials.

Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion both by quantity and commercial value. Iron alloyed with various proportions of carbon gives low, mid and high carbon steels, with increasing carbon levels reducing ductility and toughness. The addition of silicon will produce cast irons, while the addition of chromium, nickel and molybdenum to carbon steels (more than 10%) results in stainless steels. Other significant metallic alloys are those of aluminium, titanium, copper and magnesium. Copper alloys have been known since the Bronze Age, and have many applications today, most importantly in electrical wiring. while the alloys of the other three metals have been developed relatively recently - chemical reactivity of these metals, requires modern electrolytic extraction processes. The alloys of aluminium, titanium and magnesium are also known and valued for their high strength-to-weight ratios and, in the case of magnesium, for the ability to provide electromagnetic shielding. These materials are ideal for situations where high strength-to-weight ratios are more important than bulk cost, such as in aerospace and in certain automotive applications.

[edit] Polymers

Other than metals, polymers and ceramics are also an important part of materials science. Polymers are the raw materials (the resins) used to make what we commonly call plastics. Plastics are the final product, created after one or more polymers or additives have been added to a resin during processing, which is then shaped into a final form. Polymers which have been around, and which are in current widespread use, include polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylons, polyesters, acrylics, polyurethane, and polycarbonates. Plastics are generally classified as "commodity", "specialty" and "engineering" plastics.

The dividing line between the various types of plastics is not based on materials but rather on their properties and applications. For instance, polyethylene (PE) is a cheap, slippery polymer commonly used to make disposable shopping bags and trash bags, and is considered a commodity plastic, whereas medium density polyethylene (MDPE) is used for underground gas and water pipes, and another variety called ultra high molecular weight polyethylene (UHMWPE) is an engineering plastic which is used extensively as the glide rails for industrial equipment.

Polyvinyl chloride (PVC) is a commodity plastic; it is widely used, inexpensive, and annual production quantities are huge. It lends itself to an incredible array of applications including artificial leather, electrical insulation, packaging and vessels. Its fabrication and processing are simple and well established. The versatility of PVC is due to the wide range of additives that it accepts. The term "additives" in polymer science refers to the chemicals and compounds added to the polymer base to modify its material properties.

Polycarbonate would normally be considered an engineering plastic (other examples include PEEK and ABS). Engineering plastics are valued for their superior strengths and other special material properties. They are usually not used for disposable applications, unlike commodity plastics. Specialty plastics are materials with unique characteristics, such as ultra-high strength, electrical conductivity, electroluminescence, high thermal stability, etc.

Rubber is an elastic hydrocarbon polymer, which occurs as a milky emulsion (known as latex) in the sap of several varieties of plants. Synthetic rubber is made through the polymerization of a variety of monomers. The major commercial source of natural sap used to create rubber is the Para rubber tree. This is largely because it responds to wounding by producing more latex. The chemical process of vulcanization is a type of cross-linking and it changes the property of rubber to the hard, durable material we associate with tires.

The only element other than carbon, that can produce polymers, is silicon. The silicones, however, show one major difference from carbon-based polymers. Unlike the direct C-C bonds of those based on carbon in silicones, the Si atoms are joined indirectly through oxygen links (e.g. silica SiO₂ — beach sand, quartz or window glass). Silicones, also called polysiloxanes, are inorganic-organic polymers with the chemical formula [R₂SiO]_n, where R = organic groups such as methyl, ethyl, and phenyl. These materials consist of an inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side groups attached to the silicon atoms, which are four-coordinated. In some cases, organic side-groups can link two or more of these -Si-O- backbones. By varying the -Si-O- chain lengths, functional side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions.

[edit] Ceramics

A ceramic material may be defined as any inorganic polycrystalline solid or mineral. Mechanically speaking, ceramic materials are brittle, hard, strong in compression, weak in shearing and tension. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness indicates how much energy a material can absorb before mechanical failure, while fracture toughness (denoted K_Ic ) describes the ability of a material with inherent microstructural flaws to resist fracture via crack growth and propagation. If a material has a large value of fracture toughness, the basic principles of fracture mechanics suggest that it will most likely undergo ductile fracture. Brittle fracture is very characteristic of most ceramic and glass-ceramic materials which typically exhibit low (and inconsistent) values of K_Ic.

Ceramic solids are chemically inert (or stable), and often are capable of withstanding chemical erosion that occurs in an acidic or caustic environment. Ceramics generally can withstand high temperatures ranging from 1,000 °C to 1,600 °C (1,800 °F to 3,000 °F). Exceptions include non-oxide inorganic materials, such as nitrides, borides and carbides.

Ceramic engineering is the science and technology of creating solid-state devices from inorganic, non-metallic materials. This is done either by the action of heat, or, at lower temperatures, using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components, and the study of their structure, composition and properties. Ceramic materials may have a crystalline or partly crystalline structure, with long-range order on a molecular scale.

Glass ceramics may have an amorphous or glassy structure, with limited or short-range molecular order. They are typically formed from a molten mass that solidifies on cooling, or formed and matured by the action of heat. Glass by definition is not a ceramic because, although it may be identical in chemical composition (e.g. glassy SiO₂ vs. crystalline quartz) it is an amorphous solid.

Traditional ceramic raw materials include clay minerals such as kaolinite, more recent materials include aluminium oxide (alumina). The modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in such applications as the wear plates of crushing equipment in mining operations. Advanced ceramics are also used in the medicine, electrical and electronics industries.

Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque materials, as opposed to transparent materials. Recent nanoscale (e.g. sol-gel) technology has, however, made possible the production of polycrystalline transparent ceramics such as transparent alumina and alumina compounds for such applications as high-power lasers.

[edit] Composites

Another application of material science in industry is the making of composite materials. Composite materials are structured materials composed of two or more macroscopic phases. While there is considerable interest in composites with one or more non-ceramic constituents, the greatest attention is on composites in which all constituents are ceramic. These typically comprise two ceramic constituents: a continuous matrix, and a dispersed phase of ceramic particles, whiskers, or short (chopped) or continuous ceramic fibers.

The challenge, as in wet chemical processing, is to obtain a uniform distribution of the dispersed particle or fiber phase. Applications range from structural elements such as steel-reinforced concrete, to the thermally insulative tiles used to protect the surface of NASA Space Shuttles from the heat of re-entry into the Earth's atmosphere. Domestic examples can be seen in the "plastic" casings of television sets, cell-phones and so on. These plastic casings are usually a composite made up of a thermoplastic matrix such as acrylonitrile butadiene styrene (ABS) in which calcium carbonate chalk, talc, glass fibers or carbon fibers have been added for strength, bulk, or electro-static dispersion. These additions may be referred to as reinforcing fibers, or dispersants, depending on their purpose.

[edit] Biomaterials

Most natural (or biological) materials are complex composites whose mechanical properties are often outstanding, considering the weak constituents from which they are assembled. These complex structures, which have risen from hundreds of million years of evolution, are inspiring materials scientists in the design of novel materials. Their defining characteristics include structural hierarchy, multifunctionality and self-healing capability. Self-organization is also a fundamental feature of many biological materials and the manner by which the structures are assembled from the molecular level up.

The basic building blocks often begin with the 20 amino acids, and proceed to polypeptides, polysaccharides, and polypeptides–saccharides. These compose the basic proteins, which are the primary constituents of ‘soft tissues’ and are also present in most biominerals. There are over 1000 proteins, including collagen, chitin, keratin, and elastin. The ‘hard’ phases of biomaterials are primarily strengthened by minerals, which nucleate and grow in a biomediated environment that determines the size, shape and distribution of individual crystals. The most important mineral phases hydroxyapatite, silica, and aragonite.

Thus, the principal mechanical characteristics and structures of biological ceramics, polymer composites, elastomers, and cellular materials are being investigated. Molecular self-assembly is found widely in biological organisms and provides the basis of a wide variety of biological structures. For example, the crystallization of inorganic materials in nature generally occurs at ambient temperature and pressure. Yet the vital organisms through which these inorganic materials form are able to create extremely precise and complex structures. Understanding the process in which living organisms control the growth of inorganic materials could lead to significant advances in materials science, opening the door to novel synthesis techniques for nanoscale composite materials.

One system which has been under intense scientific scrutiny by several major research groups is the microstructure of the mother-of-pearl (or nacre) portion of the abalone shell. This natural material exhibits the highest mechanical strength and fracture toughness of any non-metallic substance known. Electron microscopy has revealed neatly stacked (or ordered) mineral tiles separated by thin organic sheets along with a macrostructure of larger periodic growth bands which collectively form what scientists are currently referring to as a hierarchical composite structure. (The term hierarchy simply implies that there is a range of structural features which exist over a wide range of length scales). Early work showed that the overall nacre composite consists of only 5 wt.% organic material. Yet the work necessary to fracture the body was increased by up to 3000 times over inorganic CaCO₃ crystals as a result of the intricate hierarchy of structural organization.^[5][6]

Self-assembly is also emerging as a new strategy in chemical synthesis, nanotechnology and biotechnology. Technical ceramics are in a very dynamic stage of development because of the increasingly diverse nature of ceramic needs and opportunities. This introduces an increasing need for improved properties, greater uniformity, reproducibility and reliability. This is coupled with the need for larger scale, more efficient production. All of these demands can benefit from further development in both basic science and the engineering aspects of the field.

[edit] Semiconductors

Semiconductors are materials that have an electrical resistivity (and conductivity) between that of metallic conductors and non-metallic insulators. They can be found in the periodic table moving diagonally downward right from boron. They separate the electrical conductors (or metals, to the left) from the insulators (to the right).

Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, etc. Semiconductor devices include the transistor, solar cells, diodes and integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy.

In a metallic conductor, current is carried by the flow of a "sea of electrons". In semiconductors, current can be carried either by the flow of electrons or by the flow of positively charged "holes" in the electronic band structure of the material. Silicon is used to create most semiconductors. Other semiconductor materials of commercial interest include germanium (Ge) and gallium arsenide (GaAs).

[edit] Chemical analysis

Now that the basic components of matter have been defined, the proper selection of analytical techniques that will allow to identify or compare matter can best be understood by classifying all substances into one of two very broad chemical groups: organics and inorganics. Organic substances are based on the element carbon, and comprise all forms of matter and energy which live and breathe. Inorganic substances encompass all other known chemical substances. Each of these two broad categories has both chemical properties and physical properties that are quite distinctive and characteristic.

Organic compounds include carbon atoms, hydrogen atoms, and functional groups. The valence of carbon is 4, and hydrogen is 1, functional groups are generally 1. From the number of carbon atoms and hydrogen atoms in a molecule the degree of unsaturation can be obtained. Many, but not all structures can be envisioned by the simple valence rule that there will be one bond for each valence number. The knowledge of the chemical formula for an organic compound is not sufficient information because many isomers (different structural forms with the same chemical formula) can exist.

Organic compounds often exist as mixtures. Because many organic compounds have relatively low boiling points and/or dissolve easily in organic solvents, there exist many methods for separating mixtures into pure constituents that are specific to organic chemistry such as distillation, crystallization and chromatography techniques. There are many methods for deducing the structure and/or chemical composition of a compound. These include infrared (IR) and Raman spectroscopy, atomic spectroscopy, mass spectrometry, nuclear magnetic resonance (NMR), X-ray diffraction, UV/visible spectrophotometry, and chromatography.

X-ray diffraction is the study of long-range atomic structure in solid crystalline materials. As the X-rays penetrate the crystal, a portion of the beam is reflected by each of the atomic planes. As the reflected beams leave the crystal’s planes, they combine with one another to form a series of light and dark bands known as a diffraction pattern. Every mineral compound has its own unique diffraction pattern, depending on the atomic sizes, the interatomic spacings, and the type of long-range crystal structure.

[edit] Chemistry of solids

[edit] Inorganic

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. Major branches of inorganic groups include minerals from the Earth's crust (e.g. SiO₂, MgO, Al₂O₃) and other compounds containing non-metallic elements like silicon, phosphorus, sulfur, chlorine and oxygen (e.g. water). Also important are compounds of elements of groups I, II with group VII elements to form ionically bonded salts (e.g. NaCl, table salt). Also included are simple carbon compounds which do not contain C-C bonds (e.g. oxides, acids, salts, carbides, and minerals) as well as metal alloys and hydrated metal complexes. Many inorganic species exist in living organisms and are essential to life. Examples include sodium, potassium, and chloride ions as well as the phosphate and nitrate ions. The distinction between what constitutes an organic compound and what constitutes an inorganic compound is far from absolute. Overlap exists most notably in the field of organometallic chemistry.

[edit] Organic

Organic chemistry is a specific discipline within the field of chemistry. It is the scientific study of the structure, properties, composition, reactions, and preparation by synthesis (or other means) of chemical compounds of carbon and hydrogen, which may contain any number of other elements. A list of such elements includes nitrogen, oxygen, phosphorus, sulfur and the halogens (fluorine, chlorine, bromine, iodine). Because of their unique properties, multi-carbon hydrocarbon compounds exhibit extremely large variety and the range of application of organic compounds is enormous. They form the chemical basis of many products (e.g. paints, plastics, explosives, pharmaceuticals, fossil fuels, petrochemicals) and they form the basis of all life processes.

One important property of carbon in organic chemistry is that it can form certain compounds, the individual molecules of which are capable of attaching themselves to one another, thereby forming a chain or a network. The process is called polymerization and the chains or networks polymers, while the source compound is a monomer. Two main groups of polymers exist: those artificially manufactured are referred to as industrial polymers or synthetic polymers (plastics) and those naturally occurring as biopolymers.

The key feature that distinguishes polymers from other molecules is the repetition of many identical, similar, or complementary molecular subunits in these chains. These subunits, the monomers, are small molecules of low to moderate molecular weight, and are linked to each other during polymerization.

Instead of being identical, similar monomers can have various chemical substituents. Thus, functional groups can affect the chemical properties of monomers, such as solubility and chemical reactivity. In addition, functional groups can affect the physical properties of monomers, such as hardness, density, mechanical or tensile strength, abrasion resistance, heat resistance, transparency, color, etc.). In proteins, these differences give the polymer the ability to adopt a biologically active conformation in preference to others (see self-assembly).

People have been using natural organic polymers for centuries in the form of waxes and shellac - a thermoplastic polymer. A plant polymer named cellulose provides the tensile strength for natural fibers and ropes, and by the early 19th century natural rubber was in widespread use.

Eventually, inventors learned to improve the properties of natural polymers. Natural rubber was sensitive to temperature, becoming sticky and smelly in hot weather and brittle in cold weather. In 1834, Friedrich Ludersdorf of Germany and Nathaniel Hayward of the U.S., independently discovered that adding sulfur to raw rubber helped prevent the material from becoming sticky. This process is called vulcanization.

In 1907, the invention and refinement of a synthetic polymer commonly known as Bakelite signaled the dawning of the age of plastics. Bakelite is a brand named for a material based on a thermosetting phenol formaldehyde resin called polyoxybenzylmethylenglycolanhydride. First discovered in 1872 by Leo Baekeland, it is formed by the reaction under heat and pressure of phenol and formaldehyde, generally with a wood flour filler. It was the first plastic made from synthetic polymers. It was used for its nonconductive and heat-resistant properties in radio and telephone casings as well as electrical insulators. Since the invention of the first artificial polymer, the family has quickly grown with the invention of others.

Common synthetic organic polymers are polyethylene, polypropylene, nylon, polytetrafluoroethylene (PTFE, Teflon or Gore-Tex), rayon (or cellophane, cellulose based fiber), polystyrene (or styrofoam), polyurethane (epoxy), polyesters, polymethylmethacrylate (PMMA, acrylic glass or Plexiglas), polyvinylchloride (PVC), polyisobutylene (butyl rubber), Tupperware, Formica plastic laminate, and high-strength Kevlar. Also important are both natural and synthetic rubber as well as the polymerized butadiene, a component of synthetic rubber.

Most of these examples are generic terms, and many varieties of each of these may exist, with their physical characteristics finely tuned for a specific use. Changing the conditions of polymerization alters the chemical composition of the product by modifying the chain length (degree of polymerization), branching, or tacticity. With a single monomer as a start, the product is a homopolymer. Further, secondary components may be added to create a heteropolymer (copolymer), and the degree of clustering of the different components can also be controlled. Physical characteristics (hardness, density, mechanical or tensile strength, abrasion resistance, heat resistance, transparency, color, etc.) will depend on the final composition.

Plastics can be classified in many ways but most commonly by their polymer backbone (polyvinyl chloride, polyethylene, acrylic, silicone, urethane, etc.). Other classifications include thermoplastic vs. thermoset, elastomer, engineering plastic, addition or condensation, and glass transition temperature or T_g. Many plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which a liquid transforms into a solid) and one or more glass transitions (temperatures at which liquid transforms into glass).

[edit] Nanotechnology

Modern chemical synthesis has reached the point where it is possible to prepare small molecules to an infinite variety of structure, purpose and function. These methods are used today to produce a wide variety of useful chemical compounds such as pharmaceuticals or commercial polymers. This raises the question of extending this kind of control to the next length and size scale, seeking methods to assemble these single molecules into supramolecular assemblies consisting of many molecules arranged in a carefully controlled manner.

These approaches use the concepts of molecular self-assembly and/or supramolecular chemistry to automatically arrange themselves into some useful conformation through a bottom-up approach. The concept of molecular recognition is especially important. Molecules can be designed so that a specific conformation or arrangement is favored due to various intermolecular forces. The base pairing rules for nucleic acids (e.g. DNA double helix) are a direct result of this, as is the specificity of an enzyme being targeted to a single substrate, or the specific folding of the protein itself. Thus, two or more components can be designed to be complementary and mutually attractive so that they make a more complex and useful whole.

Such molecular ("bottom-up") approaches should be able to produce devices in parallel and much cheaper than the traditional, macroscopic "top-down" methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases. Most useful structures require complex and thermodynamically unlikely arrangements of atoms. Nevertheless, there are many examples of self-assembly based on molecular recognition in biology besides nucleic acids. The challenge for nanotechnology is whether these principles can be used to synthesize novel biomaterials in addition to natural ones.

[edit] Applications

Nanotechnology is playing an increasing role in solving the world energy crisis. Because of their high surface area, platinum metals may be ideal candidates for automotive fuel catalysts, as well as proton exchange membrane (PEM) fuel cells. Also, ceramic oxides (or cermets) of lanthanum, cerium, manganese and nickel are now being developed as solid oxide fuel cells (SOFC).

Lithium, lithium titanate and tantalum nanoparticles will likely be found in the next generation of lithium ion batteries for powering up all-electric vehicles. Silicon nanoparticles have been shown to dramatically expand the storage capacity of lithium ion batteries during the expansion/contraction cycle. Silicon nanowires cycle without significant degradation and present the potential for use in batteries with greatly expanded storage times.

Silicon nanoparticles are also being used in new forms of solar energy cells. Thin film deposition of silicon quantum dots on the polycrystalline silicon substrate of a photovoltaic (solar) cell increases voltage output as much as 60% by fluorescing the incoming light prior to capture. Again, surface area of the nanoparticles (and thin films) plays a critical role in maximizing the amount of absorbed radiation.

[edit] Physical properties

Physical properties of elements and compounds which provide conclusive evidence of chemical composition include odor, color, volume, density (mass / volume), melting point, boiling point, heat capacity, physical form at room temperature (solid, liquid or gas), hardness, porosity, and index of refraction. Physical properties, which constitute the emerging study of the science of materials in the solid state, include the following:

[edit] Electronic

Electrical properties include conductivity, resistance, impedance and capacitance. Electrical conductors such as metals and alloys are contrasted with electrical insulators such as glasses and ceramics. Semiconductors (e.g. Si, GaAs) behave somewhere in between. Whereas conductivity in metals is caused by electrons, both electrons and holes contribute to current in semiconductors. Alternatively, ions support electric current in ionic conductors.

[edit] Optical

Materials can transmit (glass) or reflect visible light (metals). Frequency selective optical filters can be used to alter or enhance the brightness and contrast of a digital image. Guided lightwave transmission involves the emerging field of fiber optics and the ability of certain glassy compositions as medium of transmission for a range of frequencies simultaneously (multi-mode optical waveguides) with little or no interference between competing waveforms. This resonant mode of energy and data transmission via electromagnetic wave propagation, though low powered, is virtually lossless.

Optical waveguides are used as components in integrated optical circuits (e.g. light-emitting diodes LEDs) or as the transmission medium in optical communication systems. Also of value is the sensitivity of materials to radiation in the thermal infrared portion of the electromagnetic spectrum. This heat-seeking ability is responsible for such diverse optical phenomena as night vision and infrared luminescence.

[edit] Photovoltaics

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Fundamentally, the device needs to fulfill only two functions: photo-generation of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity (simply put, carrying electrons off through a metal contact into a wire or other circuit). This conversion is called the photoelectric effect, and the field of research related to solar cells is known as photovoltaics.

Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, and consumer systems, such as handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.

All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. The materials used in solar cells tend to have the property of preferentially absorbing the wavelengths of solar light that reach the earth surface. However, some solar cells are optimized for light absorption beyond Earth's atmosphere as well.

Silicon remains the only material that is well-researched in both bulk and thin-film configurations. Crystalline silicon was the material used in the earliest successful photovoltaic devices, and is still the most widely used photovoltaic material.

[edit] Dielectric

A dielectric, or electrical insulator, is a substance that is highly resistant to the flow of electric current. A dielectric tends to concentrate an applied electric field within itself. The use of many plastics as dielectrics in capacitors presents several advantages. A capacitor is an electrical device that can store energy in the electric field between a pair of closely spaced conductors (called 'plates'). When voltage is applied to the capacitor, electric charges of equal magnitude, but opposite polarity, build up on each plate. Capacitors are used in electrical circuits as energy-storage devices. They are also used in electronic filters to differentiate between high-frequency and low-frequency signals.

[edit] Mechanical

Mechanical properties are important in structural and building materials as well as textile fabrics. They characterize the strength of materials and include elasticity / plasticity, tensile strength, compressive strength, shear strength, fracture toughness, ductility (low in brittle materials), and indentation hardness.

A solid does not exhibit macroscopic flow, as fluids do. Any degree of departure from its original shape is called deformation. The proportion of deformation to original size is called strain. If the applied stress is sufficiently low (or the imposed strain is small enough), almost all solid materials behave in such a way that the strain is directly proportional to the stress. The coefficient of the proportion is called the modulus of elasticity or Young's modulus. This region of deformation is known as the linearly elastic region. Three models can describe how a solid responds to an applied stress:

• Elastically – When an applied stress is removed, the material returns to its undeformed state. Linearly elastic materials, those that deform proportionally to the applied load, can be described by the linear elasticity equations such as Hooke's law.

• Viscoelastically – These are materials that behave elastically, but also have damping. When the applied stress is removed, work has to be done against the damping effects and is converted to heat within the material. This results in a hysteresis loop in the stress–strain curve. This implies that the mechanical response has a time-dependence.

• Plastically – Materials that behave elastically generally do so when the applied stress is less than a yield value. When the stress is greater than the yield stress, the material behaves plastically and does not return to its previous state. That is, irreversible plastic deformation (or viscous flow) occurs after yield which is permanent.

[edit] Thermo-mechanical

Thermo-mechanical properties such as thermal conductivity focus on the mechanical stability of a material at elevated temperatures. Also important is the specific heat capacity of a material to store energy in the form of heat (or thermal vibrations). In the aerospace industry, high performance materials used in the design of aircraft and/or spacecraft exteriors must have a high resistance to thermal shock. Thus, synthetic fibers spun out of organic polymers and polymer/ ceramic /metal composite materials and fiber-reinforced polymers are now being designed with this purpose in mind.

A high strength glass-ceramic cooktop with negligible thermal expansion.

"Glass ceramics" include a mix of lithium and aluminosilicates which yields an array of materials with interesting thermomechanical properties. The most commercially important of these have the distinction of being impervious to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking. The negative thermal expansion coefficient (TEC) of the crystalline ceramic phase can be balanced with the positive TEC of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net TEC near zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.

[edit] Electro-mechanical

Piezoelectricity is the ability of crystals to generate a voltage in response to an applied mechanical stress. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. Polymer materials like rubber, wool, hair, wood fiber, and silk often behave as electrets. For example, the polymer polyvinylidene fluoride (PVDF) exhibits a piezoelectric response several times larger than the traditional piezoelectri material quartz (crystalline SiO₂). The deformation (~0.1%) lends itself to useful technical applications such as high voltage sources, loudspeakers, lasers, as well as chemical, biological, and acousto-optic sensors and/or transducers.

[edit] Thermo-electrical

Chalcogenide glasses are formed from the elements in group VI of the periodic table, particularly sulfur (S), selenium (Se) and tellurium (Te), which react with more electro-positive elements, such as antimony (Sb), silver (Ag) and germanium (Ge). These are extremely versatile compounds, in that they can be crystalline or amorphous, metallic or semiconducting, and conductors of ions or electrons. Already important in optical storage discs and fibers, they are now being proposed as the basis for solid-state memory technologies. Moreover, chalcogenide glass materials form the basis of CD and DVD technologies.

A compact disc (CD) utilizing chalcogenide glasses for solid-state memory technology.

Electrical switching in chalcogenide semiconductors emerged in the 1960s, when the amorphous chalcogenide Te₄₈As₃₀Si₁₂Ge₁₀ was found to exhibit sharp, reversible transitions in electrical resistance above a threshold voltage. The switching mechanism would appear initiated by fast purely electronic processes. If current is allowed to persist in the non-crystalline material, it heats up and changes to crystalline form. This is equivalent to information being written on it. A crystalline region may be melted by exposure to a brief, intense pulse of heat. Subsequent rapid cooling then sends the melted region back through the glass transition. Conversely, a lower-intensity heat pulse of longer duration will crystallize an amorphous region.

Attempts to induce the glassy–crystal transformation of chalcogenides by electrical means form the basis of phase-change random-access memory (PC-RAM). Examples of such phase change materials are GeSbTe and AgInSbTe. In optical discs, the phase change layer is usually sandwiched between dielectric layers of ZnS-SiO₂. Other less common such materials are InSe, SbSe, SbTe, InSbSe, InSbTe, GeSbSe, GeSbTeSe, and AgInSbSeTe.

Diffusion of both electrons and ions participate in electromigration — widely studied as a degradation mechanism of the electrical conductors used in modern integrated circuits. Thus, a unified approach to the study of chalcogenides, assessing the collective roles of atoms, ions and electrons, may prove essential for both device performance and reliability.^{[7][8][9][10]}

[edit] See also

[edit] References

[edit] Further reading

• Davidge, R.W., Mechanical Behavior of Ceramics, Cambridge Solid State Science Series, Eds. Clarke, D.R., et al. (1979)

• Lawn, B.R., Fracture of Brittle Solids, Cambridge Solid State Science Series, 2nd Edn., Eds. Clarke, D.R., et al. (1993)

• Green, D., An Introduction to the Mechanical Properties of Ceramics, Cambridge Solid State Science Series, Eds. Clarke, D.R., et al.(1998)

[edit] External links

• UC Berkeley Chemistry lecture on solids

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