Inorganic
Main article: Inorganic chemistry
Major branches of inorganic groups include minerals in the form of metal oxide compounds 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 and 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.
Organic
Main article: Organic chemistry
Organic chemistry 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, and the halogens (fluorine, chlorine, bromine, iodine). They may also contain the elements phosphorus or sulfur. 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 of course 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.
Monomers can have various chemical substituents, or functonal groups, which can affect the chemical properties of organic compounds, such as solubility and chemical reactivity, as well as the physical properties, 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 which is classified as 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.
Sol-gel chemistry
The sol-gel process is a wet-chemical technique for the fabrication of materials (typically a metal oxide) starting from a chemical solution that reacts to produce nanosized colloidal particles (or sol). Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid. The result is a system composed of solid particles (size ranging from 1 nm to 1 micron) dispersed in a solvent. A drying process serves to remove the liquid phase from the gel, yielding a micro-porous amorphous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties, such as (visco)elasticity, and stuctural integrity.
The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize fine powders. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology.
With the viscosity of a sol adjusted into a proper range, both optical quality glass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors and thermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed by precipitation (e.g. dental & biomedical applications).
Note: The lightest known solids are aerogels. The lightest aerogel produced has a density of 1.9 mg/cm3 or 1.9 kg/m3 (1/530 the density of water).
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.
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.
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:
Thermal
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.
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.
Superconductivity occurs in many materials, including metals like tin and aluminium, various metallic alloys, some heavily-doped semiconductors, and certain metal oxide ceramics components which have been checmically doped. The electrical resistivity of most electrical (metallic) conductors generally decreases gradually as the temperature is lowered. However, in copper and silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of copper shows some resistance. In a superconductor however, despite these imperfections, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source.
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.
Optical
Main article: Transparent materials
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.
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.
Mechanical
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Main article: Fracture mechanics
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.
Thermo-mechanical
Main article: Refractories
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.
"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.
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.
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.
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