The forces between the atoms in a solid can take a variety of forms. For example, a crystal of
sodium chloride (common salt) is made up of
ionic
sodium and
chlorine, which are held together by
ionic bonds. In diamond or silicon, the atoms share
electrons and form
covalent bonds. In metals, electrons are shared in
metallic bonding. Some solids, particularly most organic compounds, are held together with
van der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding.
Metals , the world's tallest steel-supported brick building, is clad with stainless steel. Metals typically are strong, dense, and good conductors of both
electricity and
heat. The bulk of the elements in the
periodic table, those to the left of a diagonal line drawn from
boron to
polonium, are metals. Mixtures of two or more elements in which the major component is a metal are known as
alloys. People have been using metals for a variety of purposes since prehistoric times. The
strength and
reliability of metals has led to their widespread use in construction of buildings and other structures, as well as in most vehicles, many appliances and tools, pipes, road signs and railroad tracks. Iron and aluminium are the two most commonly used structural metals. They are also the most abundant metals in the
Earth's crust. Iron is most commonly used in the form of an alloy, steel, which contains up to 2.1%
carbon, making it much harder than pure iron. Because metals are good conductors of electricity, they are valuable in electrical appliances and for carrying an
electric current over long distances with little energy loss or dissipation. Thus, electrical power grids rely on metal cables to distribute electricity. Home electrical systems, for example, are wired with copper for its good conducting properties and easy machinability. The high
thermal conductivity of most metals also makes them useful for stovetop cooking utensils. 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. Metallic solids are held together by a high density of shared, delocalized electrons, known as "
metallic bonding". In a metal, atoms readily lose their outermost ("valence")
electrons, forming positive
ions. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud. The large number of
free electrons gives metals their high values of electrical and thermal conductivity. The free electrons also prevent transmission of visible light, making metals opaque, shiny and
lustrous. More advanced models of metal properties consider the effect of the positive ions cores on the delocalised electrons. As most metals have crystalline structure, those ions are usually arranged into a periodic lattice. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the
nearly free electron model.
Minerals Minerals are naturally occurring solids formed through various geological processes under high pressures. To be classified as a true mineral, a substance must have a
crystal structure with uniform physical properties throughout. Minerals range in composition from
pure elements and simple
salts to very complex
silicates with thousands of known forms. In contrast, a
rock sample is a random aggregate of minerals and/or
mineraloids, and has no specific chemical composition. The vast majority of the rocks of the
Earth's crust consist of quartz (crystalline SiO2), feldspar, mica,
chlorite,
kaolin, calcite,
epidote,
olivine,
augite,
hornblende,
magnetite,
hematite,
limonite and a few other minerals. Some minerals, like
quartz,
mica or
feldspar are common, while others have been found in only a few locations worldwide. The largest group of minerals by far is the
silicates (most rocks are ≥95% silicates), which are composed largely of
silicon and
oxygen, with the addition of ions of aluminium,
magnesium, iron,
calcium and other metals.
Ceramics Ceramic solids are composed of inorganic compounds, usually
oxides of chemical elements. They are chemically inert, and often are capable of withstanding chemical erosion that occurs in an acidic or caustic environment. Ceramics generally can withstand high temperatures ranging from . Exceptions include non-oxide inorganic materials, such as
nitrides,
borides and
carbides. 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. Most ceramic materials, such as alumina and its compounds, are
formed from fine powders, yielding a fine grained
polycrystalline microstructure that is filled with
light-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. Advanced ceramics are also used in the medicine, electrical and electronics industries.
Ceramic engineering is the science and technology of creating solid-state ceramic materials, parts and devices. This is done either by the action of heat, or, at lower temperatures, using
precipitation reactions from 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. Mechanically speaking, ceramic materials are brittle, hard, strong in compression and 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 KIc) 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 that typically exhibit low (and inconsistent) values of KIc. For an example of applications of ceramics, the extreme hardness of
zirconia is utilized in the manufacture of knife blades, as well as other industrial cutting tools. Ceramics such as
alumina,
boron carbide and
silicon carbide have been used in
bulletproof vests to repel large-caliber rifle fire.
Silicon nitride parts are used in ceramic ball bearings, where their high hardness makes them wear resistant. In general, ceramics are also chemically resistant and can be used in wet environments where steel bearings would be susceptible to oxidation (or rust). As another example of ceramic applications, in the early 1980s,
Toyota researched production of an
adiabatic ceramic engine with an
operating temperature of over . Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as
waste heat in order to prevent a meltdown of the metallic parts. Work is also being done in developing ceramic parts for
gas turbine engines. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel. Such engines are not in production, however, because the manufacturing of ceramic parts in the sufficient precision and durability is difficult and costly. Processing methods often result in a wide distribution of microscopic flaws that frequently play a detrimental role in the sintering process, resulting in the proliferation of cracks, and ultimate mechanical failure.
Glass ceramics . Glass-ceramic materials share many properties with both non-crystalline glasses and
crystalline ceramics. They are formed as a glass, and then partially crystallized by heat treatment, producing both
amorphous and
crystalline phases so that crystalline grains are embedded within a non-crystalline intergranular phase. Glass-ceramics are used to make cookware (originally known by the brand name
CorningWare) and stovetops that have high resistance to
thermal shock and extremely low
permeability to liquids. The negative
coefficient of thermal expansion of the crystalline ceramic phase can be balanced with the positive coefficient of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net coefficient of thermal expansion close to zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C. Glass ceramics may also occur naturally when
lightning strikes the crystalline (e.g. quartz) grains found in most beach
sand. In this case, the extreme and immediate heat of the lightning (~2500 °C) creates hollow, branching rootlike structures called
fulgurite via
fusion.
Organic solids in diameter. Organic chemistry studies 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 such as
nitrogen,
oxygen and the halogens:
fluorine,
chlorine,
bromine and
iodine. Some organic compounds may also contain the elements
phosphorus or
sulfur. Examples of organic solids include wood,
paraffin wax,
naphthalene and a wide variety of
polymers and
plastics.
Wood Wood is a natural organic material consisting primarily of
cellulose fibers embedded in a matrix of
lignin. Regarding mechanical properties, the fibers are strong in tension, and the lignin matrix resists compression. Thus wood has been an important construction material since humans began building shelters and using boats. Wood to be used for construction work is commonly known as
lumber or
timber. In construction, wood is not only a structural material, but is also used to form the mould for concrete. Wood-based materials are also extensively used for packaging (e.g. cardboard) and paper, which are both created from the refined pulp. The chemical pulping processes use a combination of high temperature and alkaline (kraft) or acidic (sulfite) chemicals to break the chemical bonds of the lignin before burning it out.
Polymers chains of the
organic semiconductor quinacridone on
graphite. 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 functional 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 provided the tensile strength for natural fibers and ropes, and by the early 19th century natural rubber was in widespread use. Polymers are the raw materials (the resins) used to make what are commonly called 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 that have been around, and that are in current widespread use, include carbon-based
polyethylene,
polypropylene,
polyvinyl chloride,
polystyrene, nylons,
polyesters,
acrylics,
polyurethane, and
polycarbonates, and silicon-based
silicones. Plastics are generally classified as "commodity", "specialty" and "engineering" plastics.
Composite materials as it heats up to over 1500 °C during re-entry
filaments, a common element in
composite materials
Composite materials contain two or more macroscopic phases, one of which is often ceramic. For example, a continuous matrix, and a dispersed phase of ceramic particles or fibers. Applications of composite materials range from structural elements such as steel-reinforced concrete, to the thermally insulative tiles that play a key and integral role in NASA's
Space Shuttle thermal protection system, which is used to protect the surface of the shuttle from the heat of re-entry into the Earth's atmosphere. One example is
Reinforced Carbon-Carbon (RCC), the light gray material that withstands reentry temperatures up to and protects the nose cap and leading edges of Space Shuttle's wings. RCC is a
laminated composite material made from
graphite rayon cloth and impregnated with a
phenolic resin. After curing at high temperature in an autoclave, the laminate is pyrolized to convert the resin to carbon, impregnated with
furfural alcohol in a vacuum chamber, and cured/pyrolized to convert the furfural alcohol to carbon. In order to provide oxidation resistance for reuse capability, the outer layers of the RCC are converted to silicon carbide. Domestic examples of composites 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. Thus, the matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials provides the designer with the choice of an optimum combination.
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 into electrical energy. In a metallic conductor, current is carried by the flow of electrons, but in semiconductors, current can be carried either by electrons or by the positively charged "
holes" in the
electronic band structure of the material. Common semiconductor materials include silicon,
germanium and
gallium arsenide.
Nanomaterials Many traditional solids exhibit different properties when they shrink to nanometer sizes. For example,
nanoparticles of usually yellow gold and gray silicon are red in color; gold nanoparticles melt at much lower temperatures (~300 °C for 2.5 nm size) than the gold slabs (1064 °C); and metallic nanowires are much stronger than the corresponding bulk metals. The high surface area of nanoparticles makes them extremely attractive for certain applications in the field of energy. For example, platinum metals may provide improvements as 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 are being applied in lithium-ion batteries. 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. Here again, surface area of the nanoparticles (and thin films) plays a critical role in maximizing the amount of absorbed radiation.
Biomaterials fibers of woven
bone Many natural (or biological) materials are complex composites with remarkable mechanical properties. 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. Thus,
self-assembly is emerging as a new strategy in the chemical synthesis of high performance biomaterials. ==Physical properties==