View source for Solid

{{short description|State of matter}}
{{Other uses}}
[[File:Insulincrystals.jpg|thumb|300px|[[Single crystal]]line form of solid [[insulin]].]]
{{Continuum mechanics}}
{{Condensed matter physics}}

'''Solid''' is a [[state of matter]] in which [[molecules]] are closely packed and cannot move past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the specific material under consideration.<ref>{{Cite web |title=Young's Modulus of Elasticity – Values for Common Materials |url=https://www.engineeringtoolbox.com/young-modulus-d_417.html |access-date=2025-03-18 |website=www.engineeringtoolbox.com |language=en}}</ref> Solids also always possess the least amount of kinetic energy per atom/molecule relative to other phases<ref name=":0">{{Cite web |date=2025-03-12 |title=Solid {{!}} Definition & Facts {{!}} Britannica |url=https://www.britannica.com/science/solid-state-of-matter |access-date=2025-03-18 |website=www.britannica.com |language=en}}</ref> or, equivalently stated, solids are formed when matter in the liquid / gas phase is cooled below a certain temperature.<ref>{{Citation |last=Cramb |first=A. W. |title=10 - Solidification and steel casting |date=2005-01-01 |work=Fundamentals of Metallurgy |pages=399–452 |editor-last=Seetharaman |editor-first=Seshadri |url=https://www.sciencedirect.com/science/article/abs/pii/B9781855739277500108 |access-date=2025-03-18 |series=Woodhead Publishing Series in Metals and Surface Engineering |publisher=Woodhead Publishing |isbn=978-1-85573-927-7}}</ref> This temperature is called the [[melting point]]<ref>{{Cite web |date=2025-02-19 |title=Melting point {{!}} Definition & Facts {{!}} Britannica |url=https://www.britannica.com/science/melting-point |access-date=2025-03-18 |website=www.britannica.com |language=en}}</ref> of that substance and is an [[intrinsic]]<ref>{{Cite web |title=16.04.04: Properties of Matter: A Forensic Science Approach |url=https://teachersinstitute.yale.edu/curriculum/units/2016/4/16.04.04/2 |access-date=2025-03-18 |website=teachersinstitute.yale.edu}}</ref> property, i.e. independent of how much of the matter there is. All matter in solids can be arranged on a microscopic scale under certain conditions.<ref>{{Cite web |last=updated |first=Mary Bagley last |date=2022-10-20 |title=Matter: Definition & the Five States of Matter |url=https://www.livescience.com/46506-states-of-matter.html |access-date=2025-03-18 |website=livescience.com |language=en}}</ref>

Solids are characterized by [[structural rigidity]] and resistance to applied external forces and pressure.<ref name=":0" /> Unlike liquids, solids do not flow to take on the shape of their container, nor do they expand to fill the entire available volume like a gas.<ref>{{Cite web |date=2014-07-03 |title=7.2: Solids, Liquids, and Gases |url=https://chem.libretexts.org/Courses/Sacramento_City_College/SCC:_Chem_309_-_General_Organic_and_Biochemistry_(Bennett)/Text/07._States_of_Matter_and_the_Gas_Laws/7.2:_Solids_Liquids_and_Gases |access-date=2025-03-18 |website=Chemistry LibreTexts |language=en}}</ref> Much like the other three fundamental phases, solids also [[Thermal expansion|expand when heated]],<ref>{{Cite magazine |last=Allain |first=Rhett |title=Why Do Solids Expand When Heated? |url=https://www.wired.com/2013/08/why-do-solids-expand-when-heated/ |access-date=2025-03-18 |magazine=Wired |language=en-US |issn=1059-1028}}</ref> the thermal energy put into increasing the distance and reducing the [[potential energy]] between atoms. However, solids do this to a much lesser extent.<ref>{{Citation |last=Elert |first=Glenn |title=Thermal Expansion |date=2025 |work=The Physics Hypertextbook |url=https://physics.info/expansion/ |access-date=2025-03-18 |publisher=hypertextbook |language=en}}</ref><ref>{{Cite web |title=Why solids expand on heating |url=https://physics.stackexchange.com/questions/279932/why-solids-expand-on-heating |access-date=2025-03-18 |website=Physics Stack Exchange |language=en}}</ref> When heated to their melting point or [[Sublimation (phase transition)|sublimation point]], solids melt into a liquid or [[Sublimation (phase transition)|sublimate]] directly into a gas, respectively. For solids that directly [[Sublimation (phase transition)|sublimate]] into a gas, the melting point is replaced by the sublimation point.<ref>{{Cite web |date=2019-07-12 |title=3: Phase Changes |url=https://chem.libretexts.org/Courses/Bellarmine_University/BU:_Chem_104_(Christianson)/Phase_1:_The_Phases_of_Matter/3:_Phase_Changes |access-date=2025-03-19 |website=Chemistry LibreTexts |language=en}}</ref> As a rule of thumb, melting will occur if the subjected pressure is higher than the substance's [[triple point]] pressure,<ref name=":6">{{Cite web |date=2013-10-02 |title=Phase Diagrams |url=https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Phase_Transitions/Phase_Diagrams |access-date=2025-03-19 |website=Chemistry LibreTexts |language=en}}</ref> and sublimation will occur otherwise.<ref>{{Cite web |title=Melting versus Sublimation |url=https://www.av8n.com/physics/melt-sublimate.htm |access-date=2025-03-19 |website=www.av8n.com}}</ref> Melting and melting points refer exclusively to transitions between solids and liquids.<ref>{{Cite web |date=2020-02-25 |title=12.5: Melting, Freezing, and Sublimation |url=https://chem.libretexts.org/Courses/Sacramento_City_College/SCC:_CHEM_300_-_Beginning_Chemistry/SCC:_CHEM_300_-_Beginning_Chemistry_(Faculty)/12:_Liquids_Solids_and_Intermolecular_Forces/12.05:_Melting,_Freezing,_and_Sublimation |access-date=2025-03-19 |website=Chemistry LibreTexts |language=en}}</ref> Melting occurs across a great extent of temperatures, ranging from 0.10 K for [[helium-3]] under 30 bars (3 MPa) of pressure,<ref>{{Citation |last=Berglund |first=Peter |title=Deutsch: Phasendiagramm 3He |url=https://commons.wikimedia.org/wiki/File:Phasendiagramm_He3.png |access-date=2025-03-19}}</ref> to around 4,200 K at 1 atm for the composite [[refractory]] material [[hafnium carbonitride]].<ref>{{Cite journal |last1=Cedillos-Barraza |first1=Omar |last2=Manara |first2=Dario |last3=Boboridis |first3=K. |last4=Watkins |first4=Tyson |last5=Grasso |first5=Salvatore |last6=Jayaseelan |first6=Daniel D. |last7=Konings |first7=Rudy J. M. |last8=Reece |first8=Michael J. |last9=Lee |first9=William E. |date=2016-12-01 |title=Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system |journal=Scientific Reports |language=en |volume=6 |issue=1 |pages=37962 |doi=10.1038/srep37962 |pmid=27905481 |pmc=5131352 |bibcode=2016NatSR...637962C |issn=2045-2322}}</ref>

The atoms in a solid are tightly bound to each other in one of two ways: regular [[geometric lattice|geometric lattices]] called [[crystal|crystalline solids]] (e.g. metals, [[ice|water ice]]), or irregular arrangements called [[amorphous solid|amorphous solids]] (e.g. glass, plastic).<ref name=":1">{{Cite web |title=Properties of solids |url=https://www.chem.fsu.edu/chemlab/chm1046course/solids.html |access-date=2025-03-18 |website=www.chem.fsu.edu}}</ref> Molecules and atoms forming crystalline lattices usually organize themselves in a few well-characterized packing structures,<ref name=":1" /> such as body-centered cubic. The adopted structure can and will vary between various pressures and temperatures, as can be seen in [[Phase diagram|phase diagrams]] of the material (e.g. [[Phase diagram#/media/File:Phase diagram of water.svg|that of water]], see left and upper). When the material is composed of a single species of atom/molecule, the phases are designated as [[Allotropy|allotropes]] for atoms (e.g. [[diamond]] / [[graphite]] for [[carbon]]), and as [[Crystal polymorphism|polymorphs]] (e.g. calcite / aragonite for [[calcium carbonate]])<ref>{{Cite journal |last1=Boulos |first1=Ramiz A. |last2=Zhang |first2=Fei |last3=Tjandra |first3=Edwin S. |last4=Martin |first4=Adam D. |last5=Spagnoli |first5=Dino |last6=Raston |first6=Colin L. |date=2014-01-22 |title=Spinning up the polymorphs of calcium carbonate |journal=Scientific Reports |language=en |volume=4 |issue=1 |pages=3616 |doi=10.1038/srep03616 |pmid=24448077 |pmc=3898216 |bibcode=2014NatSR...4.3616B |issn=2045-2322}}</ref> for molecules.

Non-[[Porosity|porous]] solids invariably strongly resist any amount of [[Compression (physics)|compression]] that would otherwise result in a decrease of total [[volume]] regardless of temperature,<ref name=":2">[https://physicspages.com/pdf/Quantum%20mechanics/Degeneracy%20pressure%20in%20a%20solid.pdf DEGENERACY PRESSURE IN A SOLID]</ref> owing to the [[Electron degeneracy pressure|mutual-repulsion]] of neighboring electron clouds among its constituent atoms.<ref name=":2" /><ref name=":3" /> In contrast to solids, gases are [[Ideal gas law|very easily compressed]] as the molecules in a gas are far apart with few intermolecular interactions.<ref>{{Cite web |date=2016-06-27 |title=14.1: Compressibility |url=https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/14:_The_Behavior_of_Gases/14.01:_Compressibility |access-date=2025-03-19 |website=Chemistry LibreTexts |language=en}}</ref> Some solids, especially metallic alloys, can be [[Deformation (physics)|deformed]] or pulled apart with enough force. The degree to which this solid resists deformation in differing directions and axes are quantified by the [[elastic modulus]], [[Ultimate tensile strength|tensile strength]], [[specific strength]], as well as other measurable quantities.<ref>{{Cite web |title=Mechanics of Materials: Strain » Mechanics of Slender Structures {{!}} Boston University |url=https://www.bu.edu/moss/mechanics-of-materials-strain/ |access-date=2025-03-19 |website=www.bu.edu}}</ref>

For the vast majority of substances, the solid phases have the highest [[density]],<ref name=":6" /> moderately higher than that of the liquid phase (if there exists one), and solid blocks of these materials will sink below their liquids.<ref>{{Cite web |last=Communications |first=Grainger Engineering Office of Marketing and |title=Densities of Solids and Liquids |url=https://van.physics.illinois.edu/ask/listing/1588 |access-date=2025-03-19 |website=van.physics.illinois.edu |language=en}}</ref> Exceptions include water ([[Iceberg|icebergs]]), [[gallium]], and [[plutonium]].<ref>{{Cite web |title=FAQ: Water expansion on freezing |url=https://iapws.org/faq1/freeze.html |access-date=2025-03-19 |website=iapws.org}}</ref><ref>{{Cite web |last=staff |date=2017-08-30 |title=Water Expansion When Freezing |url=https://www.sciencefacts.net/why-does-water-expand-when-it-freezes.html |access-date=2025-03-19 |website=Science Facts |language=en-US}}</ref> All naturally occurring elements on the periodic table has a melting point at standard atmospheric pressure, with three exceptions: the [[noble gas]] [[helium]], which remains a liquid even at absolute zero owing to [[zero-point energy]];<ref>{{Cite journal |date=2018-03-16 |title=Solid helium 4 and zero–point energy |url=https://medcraveonline.com/PAIJ/solid-helium-4-and-zerondashpoint-energy.html |journal=Physics & Astronomy International Journal |language=English |volume=2 |issue=2 |doi=10.15406/paij.2018.02.00063 |issn=2576-4543|doi-access=free }}</ref> the [[metalloid]] [[arsenic]], sublimating around 900 K;<ref>{{Cite web |title=Foundry - Lexicon |url=https://www.giessereilexikon.com/en/foundry-lexicon/Encyclopedia/show/arsenic-3648/?cHash=3c2764983cf973eb43529df834069ec0 |access-date=2025-03-19 |website=www.giessereilexikon.com |language=en-EN}}</ref> and the life-forming element carbon, which sublimates around 3,950 K.<ref>{{Cite journal |last=Abrahamson |first=J. |date=1974-04-01 |title=Graphite sublimation temperatures, carbon arcs and crystallite erosion |url=https://www.sciencedirect.com/science/article/abs/pii/0008622374900190 |journal=Carbon |volume=12 |issue=2 |pages=111–141 |doi=10.1016/0008-6223(74)90019-0 |bibcode=1974Carbo..12..111A |issn=0008-6223|url-access=subscription }}</ref>

When applied pressure is released, solids will (very) rapidly re-expand and release the stored energy in the process<ref name=":3">{{Cite web |last=TodayIFoundOut.com |first=Karl Smallwood- |date=2014-01-14 |title=What Happens When Water Freezes in a Box So Strong It Can't Expand? |url=https://gizmodo.com/what-happens-when-water-freezes-in-a-box-so-strong-it-c-1500919062 |access-date=2025-03-18 |website=Gizmodo |language=en-US}}</ref> in a manner somewhat similar to those of gases. An example of this is the (oft-attempted) confinement of freezing water in an inflexible container (of steel, for example).<ref name=":7">{{Cite web |title=Demo 32: Ice Bomb |url=https://www-chem.ucsd.edu/undergraduate/teaching-labs/demos/demo32.html |access-date=2025-03-19 |website=www-chem.ucsd.edu}}</ref> The gradual freezing results in an increase in volume,<ref>{{Cite journal |last1=Powell-Palm |first1=Matthew J. |last2=Rubinsky |first2=Boris |last3=Sun |first3=Wenhao |date=2020-02-21 |title=Freezing water at constant volume and under confinement |url=https://www.nature.com/articles/s42005-020-0303-9 |journal=Communications Physics |language=en |volume=3 |issue=1 |page=39 |doi=10.1038/s42005-020-0303-9 |bibcode=2020CmPhy...3...39P |issn=2399-3650}}</ref> as ice is less dense than water.<ref>{{Cite web |title=Explore - All About Ice - Amazing Expanding Ice |url=https://www.lpi.usra.edu/education/explore/ice/activities/ice_action/expanding_ice/ |access-date=2025-03-19 |website=Lunar and Planetary Institute (LPI) |language=en}}</ref> With no additional volume to expand into, [[Ice|water ice]] subjects the interior to intense pressures, causing the container to explode with great force.<ref name=":7" /><ref>{{Cite web |date=2024-01-24 |title=Risk of Explosion of Plastic Water Bottles When Frozen And Prevention Measures |url=https://www.kimecopak.ca/blogs/news/risk-of-explosion-of-plastic-water-bottles-when-frozen |access-date=2025-03-19 |website=KimEcopak |language=en}}</ref>

Solids' properties on a macroscopic scale can also depend on whether it is contiguous or not. Contiguous (non-aggregate) solids are characterized by [[structural rigidity]] (as in [[Rigid body|rigid bodies]]) and strong resistance to applied forces.<ref name=":0" /> For solids aggregates (e.g. gravel, sand, dust on lunar surface<ref name=":5">{{Cite web |date=2021-06-08 |title=Dust: An Out-of-This World Problem - NASA |url=https://www.nasa.gov/humans-in-space/dust-an-out-of-this-world-problem/ |access-date=2025-03-19 |language=en-US}}</ref>), solid particles can easily slip past one another,<ref name=":4">{{Cite news |last=Munroe |first=Randall |date=2020-11-09 |title=What Makes Sand Soft? |url=https://www.nytimes.com/2020/11/09/science/what-makes-sand-soft.html |access-date=2025-03-18 |work=The New York Times |language=en-US |issn=0362-4331}}</ref> though changes of individual particles ([[quartz]] particles for sand) will still be greatly hindered.<ref>{{Cite web |title=Construction Aggregates 101: What They Are (and Why They Matter) |url=https://www.aem.org/news/construction-aggregates-101-what-they-are-and-why-they-matter |access-date=2025-03-18 |website=www.aem.org |language=en}}</ref> This leads to a perceived softness and ease of compression by operators.<ref>{{Cite web |title=Apollo 11 Image Library |url=https://www.nasa.gov/history/alsj/a11/images11.html |access-date=2025-03-19 |website=www.nasa.gov}}</ref> An illustrating example is the non-firmness of coastal sand<ref name=":4" />and of the lunar regolith.<ref name=":5" />

The branch of [[physics]] that deals with solids is called [[solid-state physics]],<ref>{{Cite web |title=Solid State Physics - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/materials-science/solid-state-physics |access-date=2025-03-18 |website=www.sciencedirect.com}}</ref> and is a major branch of [[condensed matter physics]] (which includes liquids).<ref>{{Cite web |title=Condensed Matter Physics - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/physics-and-astronomy/condensed-matter-physics |access-date=2025-03-18 |website=www.sciencedirect.com}}</ref> [[Materials science]], also one of its numerous branches, is primarily concerned with the way in which a solid's composition and its properties are intertwined.<ref>{{Cite web |date=2025-03-07 |title=Materials science {{!}} Definition, Types, Study, & Facts {{!}} Britannica |url=https://www.britannica.com/technology/materials-science |access-date=2025-03-18 |website=www.britannica.com |language=en}}</ref>

== Microscopic description ==

[[File:Fcc lattice 4.jpg|thumb|Model of closely packed atoms within a crystalline solid]]
The atoms, molecules or ions that make up solids may be arranged in an orderly repeating pattern, or irregularly. Materials whose constituents are arranged in a regular pattern are known as [[crystal]]s. In some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a [[single crystal]]. Solid objects that are large enough to see and handle are rarely composed of a single crystal, but instead are made of a large number of single crystals, known as [[crystallite]]s, whose size can vary from a few nanometers to several meters. Such materials are called [[polycrystal]]line. Almost all common metals, and many [[ceramic]]s, are polycrystalline.

{{multiple image
   | align     = left
   | image1    = Silica.svg
   | alt1      = Atoms of Si and O; each atom has the same number of bonds, but the overall arrangement of the atoms is random.
   | width1    = 200
   | image2    = SiO² Quartz.svg
   | alt2      = Regular hexagonal pattern of Si and O atoms, with a Si atom at each corner and the O atoms at the centre of each side.
   | width2    = 200
   | footer    = Schematic representation of a random-network glassy form (left) and ordered crystalline lattice (right) of identical chemical composition.
}}
In other materials, there is no long-range order in the position of the atoms. These solids are known as [[amorphous solid]]s; examples include [[polystyrene]] and glass.

Whether a solid is crystalline or amorphous depends on the material involved, and the conditions in which it was formed. Solids that are formed by slow cooling will tend to be crystalline, while solids that are frozen rapidly are more likely to be amorphous. Likewise, the specific [[crystal structure]] adopted by a crystalline solid depends on the material involved and on how it was formed.

While many common objects, such as an ice cube or a coin, are chemically identical throughout, many other common materials comprise a number of different substances packed together. For example, a typical [[Rock (geology)|rock]] is an [[Aggregate (geology)|aggregate]] of several different minerals and [[mineraloid]]s, with no specific chemical composition. Wood is a natural organic material consisting primarily of [[cellulose]] fibers embedded in a matrix of organic [[lignin]]. In materials science, [[Composite material|composites]] of more than one constituent material can be designed to have desired properties.

== Classes of solids ==
{{Further|Bonding in solids}}

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 [[ion]]ic [[sodium]] and [[chlorine]], which are held together by [[ionic bond]]s.<ref>{{Cite book |url=https://books.google.com/books?id=_uUlDwAAQBAJ&q=sodium+chloride+made+up+of+ionic+sodium+and+chlorine+ionically+bonded+together&pg=PA39 |title=GENERAL BIOLOGY I: Molecules, Cells and Genes |last=Holley |first=Dennis |date=2017-05-31 |publisher=Dog Ear Publishing |isbn=9781457552748 |language=en}}</ref> In diamond<ref>{{Cite book |url=https://books.google.com/books?id=HI9qBAAAQBAJ&q=in+diamond++atoms+share+electrons+forming+covalent+bonds&pg=PA93 |title=Nanotechnology: Understanding Small Systems, Third Edition |last1=Rogers |first1=Ben |last2=Adams |first2=Jesse |last3=Pennathur |first3=Sumita |date=2014-10-28 |publisher=[[CRC Press]] |isbn=9781482211726 |language=en}}</ref> or silicon, the atoms share [[electron]]s and form [[covalent bond]]s.<ref>{{Cite book |url=https://books.google.com/books?id=on_SBwAAQBAJ&q=in+silicon+atoms+share+electrons+forming+covalent+bonds&pg=PA15 |title=Accidental Injury: Biomechanics and Prevention |last1=Nahum |first1=Alan M. |last2=Melvin |first2=John W. |date=2013-03-09 |publisher=Springer Science & Business Media |isbn=9781475722642 |language=en}}</ref> In metals, electrons are shared in [[metallic bond]]ing.<ref>{{Cite book |url=https://books.google.com/books?id=oEvTt1ZFo_MC&q=in+metal+electrons+shared+through+metallic+bonding&pg=PA60 |title=Materials Science |last1=Narula |first1=G. K. |last2=Narula |first2=K. S. |last3=Gupta |first3=V. K. |date=1989 |publisher=Tata McGraw-Hill Education |isbn=9780074517963 |language=en}}</ref> Some solids, particularly most organic compounds, are held together with [[van der Waals force]]s 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 ===
{{Main|Metal}}

[[File:Chrysler Building detail.jpg|thumb|right|The pinnacle of New York's [[Chrysler Building]], the world's tallest steel-supported brick building, is clad with stainless steel.]]

Metals typically are strong, dense, and good conductors of both [[Electrical conduction|electricity]] and [[Heat conduction|heat]].<ref>{{Cite book |url=https://books.google.com/books?id=0ID973WaVBoC&q=strong%2C+dense%2C+and+good+conductors+of+electricity+and+heat+metals&pg=PA58 |title=Science Foundation |last=Arnold |first=Brian |date=2006-07-01 |publisher=Letts and Lonsdale |isbn=9781843156567 |language=en}}</ref><ref>{{Cite book |last=The Diagram |first=Group authors |url=https://books.google.com/books?id=NH23yrRwbU4C&q=strong%2C+dense%2C+and+good+conductors+of+both+electricity+and+heat+metals&pg=PA78 |title=The Facts on File Chemistry Handbook |date=2009-01-01 |publisher=[[Infobase Publishing]] |isbn=9781438109558 |language=en}}</ref> <!-- There are clearly exceptions to all of these statements (hence the word "typically"), but we don't need to go into them in an introductory article. -->
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 [[alloy]]s.

People have been using metals for a variety of purposes since prehistoric times.
The [[Strength of materials|strength]] and [[:wikt:reliability|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 [[Crust (geology)#Earth's crust|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 [[alloy]]s 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 bond]]ing". In a metal, atoms readily lose their outermost ("valence") [[electron]]s, forming positive [[ion]]s. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud.<ref name="mortimer">{{cite book |author=Mortimer, Charles E. |title=Chemistry: A Conceptual Approach |url=https://archive.org/details/chemistryconcept00mort |url-access=registration |location=New York |publisher=D. Van Nostrad Company |edition=3rd |date=1975 |isbn=0-442-25545-4}}</ref> The large number of [[Free electron model|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 ===
[[File:Different minerals.jpg|thumb|A collection of various minerals.]]
{{Main|Minerals}}

Minerals are naturally occurring solids formed through various geological processes<ref>{{Cite book |url=https://books.google.com/books?id=j9guFwAQZjsC&q=Minerals+are+naturally+occurring+solids+formed+through+various+geological+processes+under+high+pressures.&pg=PA666 |title=Drilling in Extreme Environments: Penetration and Sampling on Earth and other Planets |last1=Bar-Cohen |first1=Yoseph |last2=Zacny |first2=Kris |date=2009-08-04 |publisher=[[John Wiley & Sons]] |isbn=9783527626632 |language=en}}</ref> 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 [[Chemical element#Chemically pure and isotopically pure|pure elements]] and simple [[Salt (chemistry)|salt]]s to very complex [[silicate]]s with thousands of known forms. In contrast, a [[Rock (geology)|rock]] sample is a random aggregate of minerals and/or [[mineraloid]]s, and has no specific chemical composition. The vast majority of the rocks of the [[Earth's crust]] consist of quartz (crystalline SiO<sub>2</sub>), feldspar, mica, [[Chlorite group|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 [[Silicate minerals|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 ===
[[File:Si3N4bearings.jpg|thumb|Si<sub>3</sub>N<sub>4</sub> ceramic bearing parts]]
{{Main|Ceramic engineering}}

Ceramic solids are composed of inorganic compounds, usually [[oxide]]s of chemical elements.<ref>{{Cite web |url=http://autocww.colorado.edu/~toldy3/E64ContentFiles/Construction/Ceramics.html |title=Ceramics |website=autocww.colorado.edu|access-date=|archiveurl = https://web.archive.org/web/20190717132955/http://autocww.colorado.edu/~toldy3/E64ContentFiles/Construction/Ceramics.html|archivedate = 17 July 2019}}</ref> 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 {{convert|1000|to|1600|C|F}}. Exceptions include non-oxide inorganic materials, such as [[nitride]]s, [[boride]]s and [[carbide]]s.

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 [[Ceramics processing|formed]] from fine powders, yielding a fine grained [[polycrystalline]] [[microstructure]] that is filled with [[Scattering|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 (chemistry)|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 K<sub>Ic</sub>) describes the ability of a material with inherent [[Crystallographic defect|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 Engineering|ceramic]] and [[glass-ceramic]] materials that typically exhibit low (and inconsistent) values of K<sub>Ic</sub>.

For an example of applications of ceramics, the extreme hardness of [[Zirconium dioxide|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 vest]]s 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 {{convert|6000|°F|°C|abbr=on}}. 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]] [[heat engine|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===
{{Main|Glass-ceramic}}
[[File:Ceranfeld.jpg|thumb|A high strength glass-ceramic cooktop with negligible [[thermal expansion]].]]

Glass-ceramic materials share many properties with both non-crystalline glasses and [[crystalline]] [[ceramic materials|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 (fluid)|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&nbsp;°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&nbsp;°C) creates hollow, branching rootlike structures called [[fulgurite]] via [[melting|fusion]].

=== Organic solids ===
{{Main|Organic chemistry}}

[[File:PaperAutofluorescence.jpg|thumb|250px|The individual wood pulp fibers in this sample are around 10 [[micrometre|μm]] 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 [[polymer]]s and [[plastic]]s.

==== Wood ====
{{Main|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 ====

[[File:Selfassembly Organic Semiconductor Trixler LMU.jpg|thumb|STM image of self-assembled [[supramolecular]] chains of the [[organic semiconductor]] [[quinacridone]] on [[graphite]].]]

{{Main|Polymer}}

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]]).

[[File:Plastic household items.jpg|thumb|250px|Household items made of various [[kinds of plastic]].]]

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, [[polyester]]s, [[Acrylic resin|acrylics]], [[polyurethane]], and [[polycarbonate]]s, and silicon-based [[silicone]]s. Plastics are generally classified as "commodity", "specialty" and "engineering" plastics.

===Composite materials===
[[File:Stsheat.jpg|thumb|250px|Simulation of the outside of the [[Space Shuttle]] as it heats up to over 1500 °C during re-entry]]
[[File:Kohlenstofffasermatte.jpg|thumb|250px|A cloth of woven [[carbon fiber]] [[Fiber|filament]]s, a common element in [[composite material]]s]]
{{Main|Composite material}}

[[Composite material]]s 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 {{convert|1510|°C|°F|abbr=on}} 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===
[[File:Siliconchip by shapeshifter.png|thumb|Semiconductor chip on crystalline silicon substrate.]]
{{Main|Semiconductors}}
[[Semiconductor]]s 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 cell]]s, [[diode]]s and [[integrated circuit]]s. 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 "[[electron hole|holes]]" in the [[electronic band structure]] of the material. Common semiconductor materials include silicon, [[germanium]] and [[gallium arsenide]].

===Nanomaterials===
{{Main|Nanotechnology}}
[[File:Nano Si 640x480.jpg|thumb|Bulk silicon (left) and silicon nanopowder (right)]]
Many traditional solids exhibit different properties when they shrink to nanometer sizes. For example, [[nanoparticle]]s of usually yellow gold and gray silicon are red in color; gold nanoparticles melt at much lower temperatures (~300&nbsp;°C for 2.5&nbsp;nm size) than the gold slabs (1064&nbsp;°C);<ref>{{Cite journal |last1=Buffat |first1=Ph. |last2=Borel |first2=J.-P. |title=Size effect on the melting temperature of gold particles |journal=Physical Review A |volume=13 |issue=6 |pages=2287 |date=1976 |doi=10.1103/PhysRevA.13.2287 |bibcode=1976PhRvA..13.2287B|url=http://infoscience.epfl.ch/record/100337 }}</ref> and metallic nanowires are much stronger than the corresponding bulk metals.<ref name=pure>{{cite book |url=https://books.google.com/books?id=-Ll6qjWB-RUC&pg=PA164 |pages=164–167 |title=Handbook of materials and techniques for vacuum devices |author=Walter H. Kohl |publisher=Springer |date=1995 |isbn=1-56396-387-6}}</ref><ref>{{cite journal |doi=10.1088/1468-6996/10/4/045004 |title=Inherent tensile strength of molybdenum nanocrystals |date=2009 |last1=Shpak |first1=Anatoly P. |first2=Sergiy O. |first3=Tatjana I |first4=Igor M |journal=Science and Technology of Advanced Materials |volume=10 |page=045004 |last2=Kotrechko |last3=Mazilova |last4=Mikhailovskij |bibcode=2009STAdM..10d5004S |issue=4 |pmc=5090266 |pmid=27877304}}</ref> 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 [[Catalysis|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 cell]]s (SOFC). Lithium, [[Lithium-titanate battery|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|silicon]] [[quantum dot]]s 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===
{{Main|Biomaterials}}
[[File:Woven bone matrix.jpg|thumb|250px|[[Collagen]] [[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==
Physical properties of elements and compounds that provide conclusive evidence of chemical composition include odor, color, volume, density (mass per unit volume), melting point, boiling point, heat capacity, physical form and shape at room temperature (solid, liquid or gas; cubic, trigonal crystals, etc.), hardness, porosity, [[index of refraction]] and many others. This section discusses some physical properties of materials in the solid state.

===Mechanical===
[[File:Torres del Paine, Patagonia (2004).jpg|thumb|300px|[[Granite]] rock formation in the Chilean [[Patagonia]]. Like most [[inorganic]] minerals formed by oxidation in the Earth's atmosphere, granite consists primarily of [[crystalline]] [[silica]] SiO<sub>2</sub> and [[alumina]] Al<sub>2</sub>O<sub>3</sub>.]]

The mechanical properties of materials describe characteristics such as their [[Strength of materials|strength]] and resistance to deformation. For example, steel beams are used in construction because of their high strength, meaning that they neither break nor bend significantly under the applied load.

Mechanical properties include [[Elasticity (physics)|elasticity]], [[Plasticity (physics)|plasticity]], [[tensile strength]], [[compressive strength]], [[shear strength]], [[fracture toughness]], [[ductility]] (low in brittle materials) and [[indentation hardness]]. [[Solid mechanics]] is the study of the behavior of solid matter under external actions such as external forces and temperature changes.

A solid does not exhibit macroscopic flow, as fluids do. Any degree of departure from its original shape is called [[Deformation (engineering)|deformation]]. The proportion of deformation to original size is called strain. If the applied [[stress (mechanics)|stress]] is sufficiently low, almost all solid materials behave in such a way that the strain is directly proportional to the stress ([[Hooke's law]]). The coefficient of the proportion is called the [[modulus of elasticity]] or [[Young's modulus]]. This region of deformation is known as the [[Linear elasticity|linearly elastic]] region. Three models can describe how a solid responds to an applied stress:
* [[Elasticity (physics)|Elasticity]] – When an applied stress is removed, the material returns to its undeformed state.
* [[Viscoelasticity]] – These are materials that behave elastically, but also have [[friction|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.
* [[Plasticity (physics)|Plasticity]] – 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 that is permanent.

Many materials become weaker at high temperatures. Materials that retain their strength at high temperatures, called [[Refractory|refractory materials]], are useful for many purposes. For example, [[glass-ceramic]]s have become extremely useful for countertop cooking, as they exhibit excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000&nbsp;°C.
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.

=== Thermal ===
[[File:1D normal modes (280 kB).gif|thumb|250px|[[Normal modes]] of [[molecular vibration|atomic vibration]] in a [[crystalline]] solid]]

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]].

Thermal properties of solids include [[thermal conductivity]], which is the property of a material that indicates its ability to [[Conduction (heat)|conduct heat]]. Solids also have a [[specific heat capacity]], which is the capacity of a material to store energy in the form of heat (or thermal lattice vibrations).

=== Electrical ===
[[File:Flyingsuperconductor.ogg|thumb|250px|Video of superconducting levitation of [[YBCO]]]]

Electrical properties include both [[electrical resistivity and conductivity]], [[dielectric strength]], [[Permeability (electromagnetism)|electromagnetic permeability]], and [[permittivity]]. Electrical conductors such as metals and alloys are contrasted with electrical insulators such as glasses and ceramics. [[Semiconductors]] 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 [[fast-ion conductor|ionic conductors]].

Many materials also exhibit [[superconductivity]] at low temperatures; they include metallic elements such as tin and aluminium, various metallic alloys, some heavily doped semiconductors, and certain ceramics. The electrical resistivity of most electrical (metallic) conductors generally decreases gradually as the temperature is lowered, but remains finite. In a superconductor, however, 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.

A [[dielectric]], or electrical insulator, is a substance that is highly resistant to the flow of electric current. A dielectric, such as plastic, tends to concentrate an applied electric field within itself, which property is used in capacitors. 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, as well as in electronic filters to differentiate between high-frequency and low-frequency signals.

==== 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 [[electret]]s. For example, the polymer [[polyvinylidene fluoride]] (PVDF) exhibits a piezoelectric response several times larger than the traditional piezoelectric material quartz (crystalline SiO<sub>2</sub>). 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.

=== Optical ===
Materials can [[Transparent materials|transmit]] (e.g. glass) or reflect (e.g. metals) visible light.

Many materials will transmit some wavelengths while blocking others. For example, window glass is transparent to [[Visible spectrum|visible light]], but much less so to most of the frequencies of [[ultraviolet]] light that cause [[sunburn]]. This property is used for frequency-selective optical filters, which can alter the color of incident light.

For some purposes, both the optical and mechanical properties of a material can be of interest. For example, the sensors on an [[infrared homing]] ("heat-seeking") missile must be protected by a cover that is transparent to [[Infrared|infrared radiation]]. The current material of choice for high-speed infrared-guided missile domes is single-crystal [[sapphire]]. The optical transmission of sapphire does not actually extend to cover the entire mid-infrared range (3–5&nbsp;μm), but starts to drop off at wavelengths greater than approximately 4.5&nbsp;μm at room temperature. While the strength of sapphire is better than that of other available mid-range infrared dome materials at room temperature, it weakens above 600&nbsp;°C. A long-standing trade-off exists between optical bandpass and mechanical durability; new materials such as [[transparent ceramics]] or optical nanocomposites may provide improved performance.

Guided lightwave transmission involves the field of fiber optics and the ability of certain glasses to transmit, simultaneously and with low loss of intensity, a range of frequencies (multi-mode optical waveguides) with little interference between them. Optical waveguides are used as components in integrated optical circuits or as the transmission medium in optical communication systems.

====Opto-electronic====
{{Main|Solar cell}}
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 an external 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, handheld calculators, 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, that is not to act as a sole supply but as an additional electricity source.

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. Some solar cells are optimized for light absorption beyond Earth's atmosphere, as well.

==Fields of study==

===Solid-state physics===
{{excerpt|Solid-state physics}}

===Solid-state chemistry===
{{excerpt|Solid-state chemistry}}

===Materials science===
{{excerpt|Materials science}}

==References==
{{Reflist|30em}}

== External links ==
{{Commons category|Solids (matter)}}


{{Table of phase transitions}}
{{Solid objects|state=expanded}}
{{States of matter}}
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