A Medley of Potpourri

iso	NA	half-life	DM	DE (MeV)	DP
⁵⁴Fe	5.8%	>3.1×10²² y	(β⁺β⁺)	0.6800	⁵⁴Cr
⁵⁵Fe	syn	2.73 y	ε	0.231	⁵⁵Mn
⁵⁶Fe	91.72%	⁵⁶Fe is stable with 30 neutrons
⁵⁷Fe	2.2%	⁵⁷Fe is stable with 31 neutrons
⁵⁸Fe	0.28%	⁵⁸Fe is stable with 32 neutrons
⁵⁹Fe	syn	44.503 d	β⁻	1.565	⁵⁹Co
⁶⁰Fe	syn	2.6×10⁶ y	β⁻	3.978	⁶⁰Co

Decay modes in parentheses are predicted, but have not yet been observed

Iron is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal in the first transition series.^[4] It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Its abundance in rocky planets like Earth is due to its abundant production by fusion in high-mass stars, where the production of nickel-56 (which decays to the most common isotope of iron) is the last nuclear fusion reaction that is exothermic. Consequently, radioactive nickel is the last element to be produced before the violent collapse of a supernova scatters precursor radionuclide of iron into space.

Like other group 8 elements, iron exists in a wide range of oxidation states, −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen and water. Fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike many other metals which form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.

Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is soft (softer than aluminium)^{[clarification needed]}, but is unobtainable by smelting. The material is significantly hardened and strengthened by impurities, in particular carbon, from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, which has a high carbon content. Further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and low carbon iron alloys along with other metals (alloy steels) are by far the most common metals in industrial use, due to their great range of desirable properties and the widespread abundance of iron-bearing rock.

Iron chemical compounds have many uses. Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the halogens and the chalcogens. Among its organometallic compounds is ferrocene, the first sandwich compound discovered.

Iron plays an important role in biology, forming complexes with molecular oxygen in hemoglobin and myoglobin; these two compounds are common oxygen transport proteins in vertebrates. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.

Characteristics

Mechanical properties

Characteristic values of tensile strength (TS) and Brinell hardness (BH) of different forms of iron.^[5]^[6]
Material	TS (MPa)	BH (Brinell)
Iron whiskers	11000
Ausformed (hardened) steel	2930	850–1200
Martensitic steel	2070	600
Bainitic steel	1380	400
Pearlitic steel	1200	350
Cold-worked iron	690	200
Small-grain iron	340	100
Carbon-containing iron	140	40
Pure, single-crystal iron	10	3

The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the Brinell test, Rockwell test and the Vickers hardness test. The data on iron is so consistent that it is often used to calibrate measurements or to compare tests.^[6]^[7] However, the mechanical properties of iron are significantly affected by the sample's purity: pure research-purpose single crystals of iron are actually softer than aluminium,^[5] and the purest industrially produced iron (99.99%) has a hardness of 20–30 Brinell.^[8] An increase in the carbon content of the iron will initially cause a significant corresponding increase in the iron's hardness and tensile strength. Maximum hardness of 65 R_c is achieved with a 0.6% carbon content, although this produces a metal with a low tensile strength.^[9]

Molar volume vs. pressure for α iron at room temperature

Because of its significance for planetary cores, the physical properties of iron at high pressures and temperatures have also been studied extensively. The form of iron that is stable under standard conditions can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form, as described in the next section.

Phase diagram and allotropes

Iron represents an example of allotropy in a metal. There are at least four allotropic forms of iron, known as α, γ, δ, and ε; at very high pressures, some controversial experimental evidence exists for a phase β stable at very high pressures and temperatures.^[10]

Low-pressure phase diagram of pure iron

As molten iron cools it crystallizes at 1538 °C into its δ allotrope, which has a body-centered cubic (bcc) crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic (fcc) crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope, or ferrite. Finally, at 770 °C (the Curie point, T_c) iron becomes magnetic. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in "domain structure", where each domain contains iron atoms with a particular electronic spin. In unmagnetized iron, all the electronic spins of the atoms within one domain are in the same direction, however, the neighboring domains point in various other directions and thus over all they cancel each other out. As a result, the iron is unmagnetized. In magnetized iron, the electronic spins of all the domains are aligned, so that the magnetic effects of neighboring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about 10 micrometres across.^[11] At pressures above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into a hexagonal close-packed (hcp) structure, which is also known as ε-iron; the higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure. The β-phase, if it exists, would appear at pressures of at least 50 GPa and temperatures of at least 1500 K; it has been thought to have an orthorhombic or a double hcp structure.^[10]

Iron is of greatest importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties, and an understanding of the properties of the allotropes of iron is key to the manufacture of good quality steels.

α-iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).^[12]

Above 912 °C and up to 1400 °C α-iron undergoes a phase transition from bcc to the fcc configuration of γ-iron, also called austenite. This is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.^[11]

The high-pressure phases of iron are important as endmember models for the solid parts of planetary cores. The inner core of the Earth is generally assumed to consist essentially of an iron-nickel alloy with ε (or β) structure.

The melting point of iron is experimentally well defined for pressures up to approximately 50 GPa. For higher pressures, different studies placed the γ-ε-liquid triple point at pressures differing by tens of gigapascals and yielded differences of more than 1000 K for the melting point. Generally speaking, molecular dynamics computer simulations of iron melting and shock wave experiments suggest higher melting points and a much steeper slope of the melting curve than static experiments carried out in diamond anvil cells.^[13]

Isotopes

Naturally occurring iron consists of four stable isotopes: 5.845% of ⁵⁴Fe, 91.754% of ⁵⁶Fe, 2.119% of ⁵⁷Fe and 0.282% of ⁵⁸Fe. Of these stable isotopes, only ⁵⁷Fe has a nuclear spin (−¹⁄₂). The nuclide ⁵⁴Fe is predicted to undergo double beta decay, but this process had never been observed experimentally for these nuclei, and only the lower limit on the half-life was established: t_1/2>3.1×10²² years.
⁶⁰Fe is an extinct radionuclide of long half-life (2.6 million years).^[14] It is not found on Earth, but its ultimate decay product is the stable nuclide nickel-60.

Much of the past work on measuring the isotopic composition of Fe has focused on determining ⁶⁰Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.^[15]

The most abundant iron isotope ⁵⁶Fe is of particular interest to nuclear scientists as it represents the most common endpoint of nucleosynthesis. It is often cited, falsely, as the isotope of highest binding energy, a distinction which actually belongs to nickel-62.^[16] Since ⁵⁶Ni is easily produced from lighter nuclei in the alpha process in nuclear reactions in supernovae (see silicon burning process), nickel-56 (14 alpha particles) is the endpoint of fusion chains inside extremely massive stars, since addition of another alpha particle would result in zinc-60, which requires a great deal more energy. This nickel-56, which has a half-life of about 6 days, is therefore made in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the supernova remnant gas cloud, first to radioactive cobalt-56, and then stable iron-56. This last nuclide is therefore common in the universe, relative to other stable metals of approximately the same atomic weight.

In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of ⁶⁰Ni, the daughter product of ⁶⁰Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of ⁶⁰Fe at the time of formation of the Solar System. Possibly the energy released by the decay of ⁶⁰Fe contributed, together with the energy released by decay of the radionuclide ²⁶Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni present in extraterrestrial material may also provide further insight into the origin of the Solar System and its early history.^[17]

Nuclei of iron atoms have some of the highest binding energies per nucleon, surpassed only by the nickel isotope ⁶²Ni. This is formed by nuclear fusion in stars. Although a further tiny energy gain could be extracted by synthesizing ⁶²Ni, conditions in stars are unsuitable for this process to be favored. Elemental distribution on Earth greatly favors iron over nickel, and also presumably in supernova element production.^[18]

Iron-56 is the heaviest stable isotope produced by the alpha process in stellar nucleosynthesis; elements heavier than iron and nickel require a supernova for their formation. Iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth.

Nucleosynthesis

Iron is created by extremely large, extremely hot (over 2.5 billion kelvin) stars through the silicon burning process. It is the heaviest stable element to be produced in this manner. The process starts with the second largest stable nucleus created by silicon burning, which is calcium. One stable nucleus of calcium fuses with one helium nucleus, creating unstable titanium. Before the titanium decays, it can fuse with another helium nucleus, creating unstable chromium. Before the chromium decays, it can fuse with another helium nucleus, creating unstable iron. Before the iron decays, it can fuse with another helium nucleus, creating unstable nickel-56. Any further fusion of nickel-56 consumes energy instead of producing energy, so after the production of nickel-56, the star does not produce the energy necessary to keep the core from collapsing. Eventually, the nickel-56 decays to unstable cobalt-56, which in turn decays to stable iron-56. When the core of the star collapses, it creates a supernova. Supernovas also create additional forms of stable iron via the r-process.

Occurrence

Planetary occurrence

Iron meteorites, similar in composition to the Earth's inner- and outer core

Iron is the sixth most abundant element in the Universe, and the most common refractory element.^[19] It is formed as the final exothermic stage of stellar nucleosynthesis, by silicon fusion in massive stars.

Metallic or native iron is rarely found on the surface of the Earth because it tends to oxidize, but its oxides are pervasive and represent the primary ores. While it makes up about 5% of the Earth's crust, both the Earth's inner and outer core are believed to consist largely of an iron-nickel alloy constituting 35% of the mass of the Earth as a whole. Iron is consequently the most abundant element on Earth, but only the fourth most abundant element in the Earth's crust.^[20]^[21] Most of the iron in the crust is found combined with oxygen as iron oxide minerals such as hematite (Fe₂O₃) and magnetite (Fe₃O₄). Large deposits of iron are found in banded iron formations. These geological formations are a type of rock consisting of repeated thin layers of iron oxides alternating with bands of iron-poor shale and chert. The banded iron formations were laid down in the time between 3,700 million years ago and 1,800 million years ago^[22]^[23]

About 1 in 20 meteorites consist of the unique iron-nickel minerals taenite (35–80% iron) and kamacite (90–95% iron). Although rare, iron meteorites are the main form of natural metallic iron on the Earth's surface.^[24]

The red color of the surface of Mars is derived from an iron oxide-rich regolith. This has been proven by Mössbauer spectroscopy.^[25]

Stocks in use in society

According to the International Resource Panel's Metal Stocks in Society report, the global stock of iron in use in society is 2200 kg per capita. Much of this is in more-developed countries (7000–14000 kg per capita) rather than less-developed countries (2000 kg per capita).

Chemistry and compounds

Iron forms compounds mainly in the +2 and +3 oxidation states. Traditionally, iron(II) compounds are called ferrous, and iron(III) compounds ferric. Iron also occurs in higher oxidation states, an example being the purple potassium ferrate (K₂FeO₄) which contains iron in its +6 oxidation state. Iron(IV) is a common intermediate in many biochemical oxidation reactions.^[26]^[27] Numerous organometallic compounds contain formal oxidation states of +1, 0, −1, or even −2. The oxidation states and other bonding properties are often assessed using the technique of Mössbauer spectroscopy.^[28] There are also many mixed valence compounds that contain both iron(II) and iron(III) centers, such as magnetite and Prussian blue (Fe₄(Fe[CN]₆)₃).^[27] The latter is used as the traditional "blue" in blueprints.^[29]

Hydrated iron(III) chloride, also known as ferric chloride

The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO₄·7H₂O) and iron(III) chloride (FeCl₃). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt ((NH₄)₂Fe(SO₄)₂·6H₂O). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.^[27]

Unlike many other metals, iron does not form amalgams with mercury. As a result, mercury is traded in standardized 76 pound flasks (34 kg) made of iron.^[30]

Binary compounds

Iron reacts with oxygen in the air to form various oxide and hydroxide compounds; the most common are iron(II,III) oxide (Fe₃O₄), and iron(III) oxide (Fe₂O₃). Iron(II) oxide also exists, though it is unstable at room temperature. These oxides are the principal ores for the production of iron (see bloomery and blast furnace). They are also used in the production of ferrites, useful magnetic storage media in computers, and pigments. The best known sulfide is iron pyrite (FeS₂), also known as fool's gold owing to its golden luster.^[27]

The binary ferrous and ferric halides are well known, with the exception of ferric iodide. The ferrous halides typically arise from treating iron metal with the corresponding binary halogen acid to give the corresponding hydrated salts.^[27]

Fe + 2 HX → FeX₂ + H₂

Iron reacts with fluorine, chlorine, and bromine to give the corresponding ferric halides, ferric chloride being the most common:

2 Fe + 3 X₂ → 2 FeX₃ (X = F, Cl, Br)

Coordination and organometallic compounds

Prussian blue

Several cyanide complexes are known. The most famous example is Prussian blue, (Fe₄(Fe[CN]₆)₃). Potassium ferricyanide and potassium ferrocyanide are also known; the formation of Prussian blue upon reaction with iron(II) and iron(III) respectively forms the basis of a "wet" chemical test.^[27]
Prussian blue is also used as an antidote for thallium and radioactive caesium poisoning.^[31]^[32]
Prussian blue can be used in laundry bluing to correct the yellowish tint left by ferrous salts in water.

Ferrocene

Several carbonyl compounds of iron are known. The premier iron(0) compound is iron pentacarbonyl, Fe(CO)₅, which is used to produce carbonyl iron powder, a highly reactive form of metallic iron. Thermolysis of iron pentacarbonyl gives the trinuclear cluster, triiron dodecacarbonyl. Collman's reagent, disodium tetracarbonylferrate, is a useful reagent for organic chemistry; it contains iron in the −2 oxidation state. Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1 oxidation state.^[33]

Ferrocene is an extremely stable complex. The first sandwich compound, it contains an iron(II) center with two cyclopentadienyl ligands bonded through all ten carbon atoms. This arrangement was a shocking novelty when it was first discovered,^[34] but the discovery of ferrocene has led to a new branch of organometallic chemistry. Ferrocene itself can be used as the backbone of a ligand, e.g. dppf. Ferrocene can itself be oxidized to the ferrocenium cation (Fc⁺); the ferrocene/ferrocenium couple is often used as a reference in electrochemistry.^[35]

History

Wrought iron

The symbol for Mars has been used since antiquity to represent iron.

An pillar, slightly fluted, with some ornamentation at its top. It is black, slightly weathered to a dark brown near the base. It is around 7 meters (23 feet) tall. It stands upon a raised circular base of stone, and is surrounded by a short, square fence.

The Delhi iron pillar is an example of the iron extraction and processing methodologies of early India. The iron pillar at Delhi has withstood corrosion for the last 1600 years.

Iron objects of great age are much rarer than objects made of gold or silver due to the ease of corrosion of iron.^[36] Beads made from meteoric iron in 3500 BCE or earlier were found in Gerzah, Egypt by G. A. Wainwright.^[37] The beads contain 7.5% nickel, which is a signature of meteoric origin since iron found in the Earth's crust has very little to no nickel content. Meteoric iron was highly regarded due to its origin in the heavens and was often used to forge weapons and tools or whole specimens placed in churches.^[37] Items that were likely made of iron by Egyptians date from 2500 to 3000 BCE.^[36] Iron had a distinct advantage over bronze in warfare implements. It was much harder and more durable than bronze, although susceptible to rust. However, this is contested. Hittitologist Trevor Bryce argues that before advanced iron-working techniques were developed in India, meteoritic iron weapons used by early Mesopotamian armies had a tendency to shatter in combat, due to their high carbon content.^[38]

The first iron production started in the Middle Bronze Age but it took several centuries before iron displaced bronze. Samples of smelted iron from Asmar, Mesopotamia and Tall Chagar Bazaar in northern Syria were made sometime between 2700 and 3000 BCE.^[39] The Hittites appear to be the first to understand the production of iron from its ores and regard it highly in their society. They began to smelt iron between 1500 and 1200 BCE and the practice spread to the rest of the Near East after their empire fell in 1180 BCE.^[39] The subsequent period is called the Iron Age. Iron smelting, and thus the Iron Age, reached Europe two hundred years later and arrived in Zimbabwe, Africa by the 8th century.^[39] In China, iron only appears circa 700–500 BCE.^[40] Iron smelting may have been introduced into China through Central Asia.^[41] The earliest evidence of the use of a blast furnace in China dates to the 1st century AD,^[42] and cupola furnaces were used as early as the Warring States period (403–221 BCE).^[43] Usage of the blast and cupola furnace remained widespread during the Song and Tang Dynasties.^[44]

Artifacts of smelted iron are found in India dating from 1800 to 1200 BCE,^[45] and in the Levant from about 1500 BCE (suggesting smelting in Anatolia or the Caucasus).^[46]^[47]

The Book of Genesis, fourth chapter, verse 22 contains the first mention of iron in the Old Testament of the Bible; "Tubal-cain, an instructor of every artificer in brass and iron."^[36] Other verses allude to iron mining (Job 28:2), iron used as a stylus (Job 19:24), furnace (Deuteronomy 4:20), chariots (Joshua 17:16), nails (I Chron. 22:3), saws and axes (II Sam. 12:31), and cooking utensils (Ezekiel 4:3).^[48] The metal is also mentioned in the New Testament, for example in Acts chapter 12 verse 10, "[Peter passed through] the iron gate that leadeth unto the city" of Antioch.^[49]

Iron working was introduced to Greece in the late 11th century BCE.^[50] The spread of ironworking in Central and Western Europe is associated with Celtic expansion. According to Pliny the Elder, iron use was common in the Roman era.^[37] The annual iron output of the Roman Empire is estimated at 84,750 t,^[51] while the similarly populous Han China produced around 5,000 t.^[52]

During the Industrial Revolution in Britain, Henry Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative production systems. In 1783 he patented the puddling process for refining iron ore. It was later improved by others, including Joseph Hall.

Cast iron

Cast iron was first produced in China during 5th century BCE,^[53] but was hardly in Europe until the medieval period.^[54]^[55] The earliest cast iron artifacts were discovered by archaeologists in what is now modern Luhe County, Jiangsu in China. Cast iron was used in ancient China for warfare, agriculture, and architecture.^[56] During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.

Coalbrookdale by Night, 1801. Blast furnaces light the iron making town of Coalbrookdale.

Medieval blast furnaces were about 10 feet (3.0 m) tall and made of fireproof brick; forced air was usually provided by hand-operated bellows.^[55] Modern blast furnaces have grown much bigger.

In 1709, Abraham Darby I established a coke-fired blast furnace to produce cast iron. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution. Toward the end of the 18th century, cast iron began to replace wrought iron for certain purposes, because it was cheaper. Carbon content in iron wasn't implicated as the reason for the differences in properties of wrought iron, cast iron, and steel until the 18th century.^[39]

Since iron was becoming cheaper and more plentiful, it also became a major structural material following the building of the innovative first iron bridge in 1778.

Steel

Steel (with smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity by using a bloomery. Blacksmiths in Luristan in western Iran were making good steel by 1000 BCE.^[39] Then improved versions, Wootz steel by India and Damascus steel were developed around 300 BCE and 500 CE respectively. These methods were specialized, and so steel did not become a major commodity until the 1850s.^[57]
New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This made steel much more economical, thereby leading to wrought iron no longer being produced.^[58]

Foundations of modern chemistry

Antoine Lavoisier used the reaction of water steam with metallic iron inside an incandescent iron tube to produce hydrogen in his experiments leading to the demonstration of the mass conservation.
Anaerobic oxidation of iron at high temperature can be schematically represented by the following reactions:

Fe + H₂O → FeO + H₂

2 Fe + 3 H₂O → Fe₂O₃ + 3 H₂

3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂

Production of metallic iron

Industrial routes

The production of iron or steel is a process consisting of two main stages, unless the desired product is cast iron. In the first stage pig iron is produced in a blast furnace. Alternatively, it may be directly reduced. The second stage, pig iron is converted to wrought iron or steel.

The fining process of smelting iron ore to make wrought iron from pig iron, with the right illustration displaying men working a blast furnace, from the Tiangong Kaiwu encyclopedia, published in 1637 by Song Yingxing.

How iron was extracted in the 19th century

For a few limited purposes like electromagnet cores, pure iron is produced by electrolysis of a ferrous sulfate solution

Blast furnace processing

Industrial iron production starts with iron ores, principally hematite, which has a nominal formula Fe₂O₃, and magnetite, with the formula Fe₃O₄. These ores are reduced to the metal in a carbothermic reaction, i.e. by treatment with carbon. The conversion is typically conducted in in a blast furnace at temperatures of about 2000 °C. Carbon is provided in the form of coke. The process also contains a flux such as limestone, which is used to remove silicaceous minerals in the ore, which would otherwise clog the furnace. The coke and limestone are fed into the top of the furnace, while a massive blast of heated air, about 4 tons per ton of iron,^[55] is forced into the furnace at the bottom.In the furnace, the coke reacts with oxygen in the air blast to produce carbon monoxide:

2 C + O₂ → 2 CO

The carbon monoxide reduces the iron ore (in the chemical equation below, hematite) to molten iron, becoming carbon dioxide in the process:

Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂

Some iron in the high-temperature lower region of the furnace reacts directly with the coke:

2 Fe₂O₃ + 3 C → 4 Fe + 3 CO₂

The flux present to melt impurities in the ore is principally limestone (calcium carbonate) and dolomite (calcium-magnesium carbonate). Other specialized fluxes are used depending on the details of the ore. In the heat of the furnace the limestone flux decomposes to calcium oxide (also known as quicklime):

CaCO₃ → CaO + CO₂

Then calcium oxide combines with silicon dioxide to form a liquid slag.

CaO + SiO₂ → CaSiO₃

The slag melts in the heat of the furnace. In the bottom of the furnace, the molten slag floats on top of the denser molten iron, and apertures in the side of the furnace are opened to run off the iron and the slag separately. The iron, once cooled, is called pig iron, while the slag can be used as a material in road construction or to improve mineral-poor soils for agriculture^[55]

This heap of iron ore pellets will be used in steel production.

Direct iron reduction

Owing to environmental concerns, alternative methods of processing iron have been developed. "Direct iron reduction" reduces iron ore to a powder called "sponge" iron or "direct" iron that is suitable for steelmaking.^[55] Two main reactions comprise the direct reduction process:

Natural gas is partially oxidized (with heat and a catalyst):

2 CH₄ + O₂ → 2 CO + 4 H₂

These gases are then treated with iron ore in a furnace, producing solid sponge iron:

Fe₂O₃ + CO + 2 H₂ → 2 Fe + CO₂ + 2 H₂O

Silica is removed by adding a limestone flux as described above.

Further processes

Iron-carbon phase diagram, various stable solid solution forms

Pig iron is not pure iron, but has 4–5% carbon dissolved in it with small amounts of other impurities like sulfur, magnesium, phosphorus and manganese. As the carbon is the major impurity, the iron (pig iron) becomes brittle and hard. This form of iron, also known as cast iron, is used to cast articles in foundries such as stoves, pipes, radiators, lamp-posts and rails.

Alternatively pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron). Various processes have been used for this, including finery forges, puddling furnaces, Bessemer converters, open hearth furnaces, basic oxygen furnaces, and electric arc furnaces. In all cases, the objective is to oxidize some or all of the carbon, together with other impurities. On the other hand, other metals may be added to make alloy steels.

Annealing involves the heating of a piece of steel to 700–800 °C for several hours and then gradual cooling. It makes the steel softer and more workable.

Laboratory methods

Metallic iron is generally produced in the laboratory by two methods. One route is electrolysis of ferrous chloride onto an iron cathode. The second method involves reduction of iron oxides with hydrogen gas at about 500 °C.^[59]

Applications

Iron powder

Metallurgical

Iron production 2009 (million tonnes)^[60]
Country	Iron ore	Pig iron	Direct iron	Steel
China	1,114.9	549.4		573.6
Australia	393.9	4.4		5.2
Brazil	305.0	25.1	0.011	26.5
Japan		66.9		87.5
India	257.4	38.2	23.4	63.5
Russia	92.1	43.9	4.7	60.0
Ukraine	65.8	25.7		29.9
South Korea	0.1	27.3		48.6
Germany	0.4	20.1	0.38	32.7
World	1,594.9	914.0	64.5	1,232.4

Iron is the most widely used of all the metals, accounting for 95% of worldwide metal production.^{[citation needed]} Its low cost and high strength make it indispensable in engineering applications such as the construction of machinery and machine tools, automobiles, the hulls of large ships, and structural components for buildings. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.

Commercially available iron is classified based on purity and the abundance of additives. Pig iron has 3.5–4.5% carbon^[61] and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of manganese. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.

"White" cast irons contain their carbon in the form of cementite, or iron-carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron-carbide, a very pale, silvery, shiny material, hence the appellation.

In gray iron the carbon exists as separate, fine flakes of graphite, and also renders the material brittle due to the sharp edged flakes of graphite that produce stress concentration sites within the material. A newer variant of gray iron, referred to as ductile iron is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.

Wrought iron contains less than 0.25% carbon but large amounts of slag that give it a fibrous characteristic.^[61] It is a tough, malleable product, but not as fusible as pig iron. If honed to an edge, it loses it quickly. Wrought iron is characterized by the presence of fine fibers of slag entrapped within the metal. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel for traditional "wrought iron" products and blacksmithing.

Mild steel corrodes more readily than wrought iron, but is cheaper and more widely available. Carbon steel contains 2.0% carbon or less,^[62] with small amounts of manganese, sulfur, phosphorus, and silicon. Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. Their alloy content raises their cost, and so they are usually only employed for specialist uses. One common alloy steel, though, is stainless steel. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.

A graph of attenuation coefficient vs. energy between 1 meV and 100 keV for several photon scattering mechanisms.

Photon mass attenuation coefficient for iron.

Apart from traditional applications, iron is also used for protection from ionizing radiation. Although it is lighter than another traditional protection material, lead, it is much stronger mechanically. The attenuation of radiation as a function of energy is shown in the graph.

The main disadvantage of iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way. Painting, galvanization, passivation, plastic coating and bluing are all used to protect iron from rust by excluding water and oxygen or by cathodic protection.

Iron compounds

Although its metallurgical role is dominant in terms of amounts, iron compounds are pervasive in industry as well being used in many niche uses. Iron catalysts are traditionally used in the Haber-Bosch Process for the production of ammonia and the Fischer-Tropsch process for conversion of carbon monoxide to hydrocarbons for fuels and lubricants.^[63] Powdered iron in an acidic solvent was used in the Bechamp reduction the reduction of nitrobenzene to aniline.^[64]

Iron(III) chloride finds use in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an etchant for copper in the manufacture of printed circuit boards.^[65] It can also be dissolved in alcohol to form tincture of iron. The other halides tend to be limited to laboratory uses.

Iron(II) sulfate is used as a precursor to other iron compounds. It is also used to reduce chromate in cement. It is used to fortify foods and treat iron deficiency anemia. These are its main uses. Iron(III) sulfate is used in settling minute sewage particles in tank water. Iron(II) chloride is used as a reducing flocculating agent, in the formation of iron complexes and magnetic iron oxides, and as a reducing agent in organic synthesis.

Biological role

Iron is abundant in biology.^[66] Iron-proteins are found in all living organisms, ranging from the evolutionarily primitive archaea to humans. The color of blood is due to the hemoglobin, an iron-containing protein. As illustrated by hemoglobin, iron is often bound to cofactors, e.g. in hemes.
The iron-sulfur clusters are pervasive and include nitrogenase, the enzymes responsible for biological nitrogen fixation. Influential theories of evolution have invoked a role for iron sulfides in the iron-sulfur world theory.

Structure of Heme b, in the protein additional ligand(s) would be attached to Fe.

Iron is a necessary trace element found in nearly all living organisms. Iron-containing enzymes and proteins, often containing heme prosthetic groups, participate in many biological oxidations and in transport. Examples of proteins found in higher organisms include hemoglobin, cytochrome (see high-valent iron), and catalase.^[67]

Bioinorganic compounds

The most commonly known and studied "bioinorganic" compounds of iron (i.e., iron compounds used in biology) are the heme proteins: examples are hemoglobin, myoglobin, and cytochrome P450.
These compounds can transport gases, build enzymes, and be used in transferring electrons. Metalloproteins are a group of proteins with metal ion cofactors. Some examples of iron metalloproteins are ferritin and rubredoxin. Many enzymes vital to life contain iron, such as catalase, lipoxygenases, and IRE-BP.

Health and diet

Iron is pervasive, but particularly rich sources of dietary iron include red meat, lentils, beans, poultry, fish, leaf vegetables, watercress, tofu, chickpeas, black-eyed peas, blackstrap molasses, fortified bread, and fortified breakfast cereals. Iron in low amounts is found in molasses, teff, and farina. Iron in meat (heme iron) is more easily absorbed than iron in vegetables.^[68] Although some studies suggest that heme/hemoglobin from red meat has effects which may increase the likelihood of colorectal cancer,^[69]^[70] there is still some controversy,^[71] and even a few studies suggesting that there is not enough evidence to support such claims.^[72]
Iron provided by dietary supplements is often found as iron(II) fumarate, although iron sulfate is cheaper and is absorbed equally well. Elemental iron, or reduced iron, despite being absorbed at only one third to two thirds the efficiency (relative to iron sulfate),^[73] is often added to foods such as breakfast cereals or enriched wheat flour. Iron is most available to the body when chelated to amino acids^[74] and is also available for use as a common iron supplement. Often the amino acid chosen for this purpose is the cheapest and most common amino acid, glycine, leading to "iron glycinate" supplements.^[75] The Recommended Dietary Allowance (RDA) for iron varies considerably based on age, gender, and source of dietary iron (heme-based iron has higher bioavailability).^[76] Infants may require iron supplements if they are bottle-fed cow's milk.^[77] Blood donors and pregnant women are at special risk of low iron levels and are often advised to supplement their iron intake.^[78]

Uptake and storage

Iron acquisition poses a problem for aerobic organisms, because ferric iron is poorly soluble near neutral pH. Thus, bacteria have evolved high-affinity sequestering agents called siderophores.^[79]^[80]^[81]

After uptake, in cells, iron storage is carefully regulated; "free" iron ions do not exist as such. A major component of this regulation is the protein transferrin, which binds iron ions absorbed from the duodenum and carries it in the blood to cells.^[82] In animals, plants, and fungi, iron is often the metal ion incorporated into the heme complex. Heme is an essential component of cytochrome proteins, which mediate redox reactions, and of oxygen carrier proteins such as hemoglobin, myoglobin, and leghemoglobin.

Inorganic iron contributes to redox reactions in the iron-sulfur clusters of many enzymes, such as nitrogenase (involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. Non-heme iron proteins include the enzymes methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to deoxyribose; DNA biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrates) and purple acid phosphatase (hydrolysis of phosphate esters).

Iron distribution is heavily regulated in mammals, partly because iron ions have a high potential for biological toxicity.^[83]

Regulation of uptake

Iron uptake is tightly regulated by the human body, which has no regulated physiological means of excreting iron. Only small amounts of iron are lost daily due to mucosal and skin epithelial cell sloughing, so control of iron levels is mostly by regulating uptake.^[84] Regulation of iron uptake is impaired in some people as a result of a genetic defect that maps to the HLA-H gene region on chromosome 6. In these people, excessive iron intake can result in iron overload disorders, such as hemochromatosis. Many people have a genetic susceptibility to iron overload without realizing it or being aware of a family history of the problem. For this reason, it is advised that people do not take iron supplements unless they suffer from iron deficiency and have consulted a doctor.

Hemochromatosis is estimated to cause disease in between 0.3 and 0.8% of Caucasians.^[85]MRI finds that iron accumulates in the hippocampus of the brains of those with Alzheimer's disease and in the substantia nigra of those with Parkinson disease.^[86]

Bioremediation

Iron-eating bacteria live in the hulls of sunken ships such as the Titanic.^[87] The acidophile bacteria Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus spp., Acidianus brierleyi and Sulfobacillus thermosulfidooxidans can oxidize ferrous iron enzymically.^[88] A sample of the fungus Aspergillus niger was found growing from gold mining solution, and was found to contain cyano metal complexes such as gold, silver, copper iron and zinc. The fungus also plays a role in the solubilization of heavy metal sulfides.^[89]

Permeable reactive barriers

Zerovalent iron is the main reactive material for permeable reactive barriers.^[90]

Toxicity

NFPA 704 "fire diamond"
1 0 1
Fire diamond for powdered iron metal

Main article: Iron poisoning

Large amounts of ingested iron can cause excessive levels of iron in the blood. High blood levels of free ferrous iron react with peroxides to produce free radicals, which are highly reactive and can damage DNA, proteins, lipids, and other cellular components. Thus, iron toxicity occurs when there is free iron in the cell, which generally occurs when iron levels exceed the capacity of transferrin to bind the iron. Damage to the cells of the gastrointestinal tract can also prevent them from regulating iron absorption leading to further increases in blood levels. Iron typically damages cells in the heart, liver and elsewhere, which can cause significant adverse effects, including coma, metabolic acidosis, shock, liver failure, coagulopathy, adult respiratory distress syndrome, long-term organ damage, and even death.^[91] Humans experience iron toxicity above 20 milligrams of iron for every kilogram of mass, and 60 milligrams per kilogram is considered a lethal dose.^[92] Overconsumption of iron, often the result of children eating large quantities of ferrous sulfate tablets intended for adult consumption, is one of the most common toxicological causes of death in children under six.^[92] The Dietary Reference Intake (DRI) lists the Tolerable Upper Intake Level (UL) for adults as 45 mg/day. For children under fourteen years old the UL is 40 mg/day.

The medical management of iron toxicity is complicated, and can include use of a specific chelating agent called deferoxamine to bind and expel excess iron from the body.^[91]^[93]

Potential applications of graphene

From Wikipedia, the free encyclopedia

Potential graphene applications include lightweight, thin, flexible, yet durable display screens, electric circuits, and solar cells, as well as various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.^[1]

In 2008, graphene produced by exfoliation was one of the most expensive materials on Earth, with a sample the area of a cross section of a human hair costing more than $1,000 as of April 2008 (about $100,000,000/cm²).^[2] Since then, exfoliation procedures have been scaled up, and now companies sell graphene in large quantities.^[3] The price of epitaxial graphene on Silicon carbide is dominated by the substrate price, which was approximately $100/cm² as of 2009.

Hong and his team in South Korea pioneered the synthesis of large-scale graphene films using chemical vapour deposition (CVD) on thin nickel layers, which triggered research on practical applications,^[4] with wafer sizes up to 30 inches (760 mm) reported.^[5]

In 2013, the European Union made a €1 billion grant to be used for research into potential graphene applications.^[6] In 2013 the Graphene Flagship consortium formed, including Chalmers University of Technology and seven other European universities and research centers, along with Nokia.^[7]

Medicine

Tissue engineering

Graphene has been investigated for tissue engineering. It has been used as a reinforcing agent to improve the mechanical properties of biodegradable polymeric nanocomposites for engineering bone tissue applications.^[8] Dispersion of low weight % of graphene (~0.02 wt.%) increased in compressive and flexural mechanical properties of polymeric nanocomposites.

Contrast agents/bioimaging

Functionalized and surfactant dispersed graphene solutions have been designed as blood pool MRI contrast agents.^[9] Additionally, iodine and manganese incorporating graphene nanoparticles have served as multimodal MRI-CT contrast agents.^[10] Graphene micro- and nano-particles have served as contrast agents for photoacoustic and thermoacoustic tomography.^[11] Graphene has also been reported to be efficiently taken up cancerous cells thereby enabling the design of drug delivery agents for cancer therapy.^[12] Graphene nanoparticles of various morphologies are non-toxic at low concentrations and do not alter stem cell differentiation suggesting that they may be safe to use for biomedical applications.^[13]

Polymerase chain reaction

Graphene is reported to have enhanced PCR by increasing the yield of DNA product.^[14] Experiments revealed that graphene's thermal conductivity could be the main factor behind this result. Graphene yields DNA product equivalent to positive control with up to 65% reduction in PCR cycles.^{[citation needed]}

Devices

Graphene's modifiable chemistry, large surface area, atomic thickness and molecularly gatable structure make antibody-functionalized graphene sheets excellent candidates for mammalian and microbial detection and diagnosis devices.^[15] Graphene is so thin water has near-perfect wetting transparency which is an important property particularly in developing bio-sensor applications.^[16] This means that a sensors coated in graphene have as much contact with an aqueous system as an uncoated sensor, while it remains protected mechanically from its environment.

Energy of the electrons with wavenumber k in graphene, calculated in the Tight Binding-approximation. The unoccupied (occupied) states, colored in blue–red (yellow–green), touch each other without energy gap exactly at the above-mentioned six k-vectors.

Integration of graphene (thickness of 0.34 nm) layers as nanoelectrodes into a nanopore^[17] can potentially solve a bottleneck for nanopore-based single-molecule DNA sequencing.

On November 20, 2013 the Bill & Melinda Gates Foundation awarded $100,000 'to develop new elastic composite materials for condoms containing nanomaterials like graphene'.^[18]

In 2014, graphene-based, transparent (across infrared to ultraviolet frequencies), flexible, implantable medical sensor microarrays were announced that allow the viewing of brain tissue hidden by implants. Optical transparency was >90%. Applications demonstrated include optogenetic activation of focal cortical areas, in vivo imaging of cortical vasculature via fluorescence microscopy and 3D optical coherence tomography.^[19]^[20]

Drug delivery[edit]

Researchers in Monash University discovered that the sheet of graphene oxide can be transformed into liquid crystal droplets spontaneously – like a polymer - simply by placing the material in a solution and manipulating the pH. The graphene droplets change their structure at the presence of an external magnetic field. This finding opens the door for potential use of carrying drug in the graphene droplets and drug release upon reaching the targeted tissue when the droplets change shape under the magnetic field. Another possible application is in disease detection if graphene is found to change shape at the presence of certain disease markers such as toxins.^[21]^[22]

A graphene ‘flying carpet’ was demonstrated to deliver two anti-cancer drugs sequentially to the lung tumor cells (A549 cell) in a mouse model. Doxorubicin (DOX) is embedded onto the graphene sheet, while the molecules of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) are linked to the nanostructure via short peptide chains. Injected intravenously, the graphene strips with the drug playload preferentially concentrate to the cancer cells due to common blood vessel leakage around the tumor. Receptors on the cancer cell membrane bind TRAIL and cell surface enzymes clip the peptide thus release the drug onto the cell surface. Without the bulky TRAIL, the graphene strips with the embedded DOX are swallowed into the cells. The intracellular acidic environment promotes DOX’s release from graphene. TRAIL on the cell surface triggers the apoptosis while DOX attacks the nucleus. These two drugs work synergistically and were found to be more effective than either drug alone.^[23]^[24]

Testing

In 2014 a graphene based blood glucose testing product was announced.^[25]^[26]

Electronics

For integrated circuits, graphene has a high carrier mobility, as well as low noise, allowing it to be used as the channel in a field-effect transistor. Single sheets of graphene are hard to produce and even harder to make on an appropriate substrate.^[27]

In 2008, the smallest transistor so far, one atom thick, 10 atoms wide was made of graphene.^[28] IBM announced in December 2008 that they had fabricated and characterized graphene transistors operating at GHz frequencies.^[29] In May 2009, an n-type transistor was announced meaning that both n and p-type graphene transistors had been created.^[30]^[31] A functional graphene integrated circuit was demonstrated – a complementary inverter consisting of one p- and one n-type graphene transistor.^[32] However, this inverter suffered from a very low voltage gain.

According to a January 2010 report,^[33] graphene was epitaxially grown on SiC in a quantity and with quality suitable for mass production of integrated circuits. At high temperatures, the quantum Hall effect could be measured in these samples. IBM built 'processors' using 100 GHz transistors on 2-inch (51 mm) graphene sheets.^[34]

In June 2011, IBM researchers announced that they had succeeded in creating the first graphene-based integrated circuit, a broadband radio mixer.^[35] The circuit handled frequencies up to 10 GHz. Its performance was unaffected by temperatures up to 127 °C.

In June 2013 an 8 transistor 1.28 GHz ring oscillator circuit was described.^[36]

Transistors

Graphene exhibits a pronounced response to perpendicular external electric fields, potentially forming field-effect transistors (FET). A 2004 paper documented FETs with an on-off ratio of ~30 at room temperature.^{[citation needed]} A 2006 paper announced an all-graphene planar FET with side gates.^[37] Their devices showed changes of 2% at cryogenic temperatures. The first top-gated FET (on–off ratio of <2 2007.="" class="reference" demonstrated="" id="cite_ref-38" in="" sup="" was="">[38]

Graphene nanoribbons may prove generally capable of replacing silicon as a semiconductor.^[39]
US patent 7015142 for graphene-based electronics was issued in 2006. In 2008, researchers at MIT Lincoln Lab produced hundreds of transistors on a single chip^[40] and in 2009, very high frequency transistors were produced at Hughes Research Laboratories.^[41]

A 2008 paper demonstrated a switching effect based on a reversible chemical modification of the graphene layer that gives an on–off ratio of greater than six orders of magnitude. These reversible switches could potentially be employed in nonvolatile memories.^[42]

In 2009, researchers demonstrated four different types of logic gates, each composed of a single graphene transistor.^[43]

Practical uses for these circuits are limited by the very small voltage gain they exhibit. Typically, the amplitude of the output signal is about 40 times less than that of the input signal. Moreover, none of these circuits operated at frequencies higher than 25 kHz.

In the same year, tight-binding numerical simulations^[44] demonstrated that the band-gap induced in graphene bilayer field effect transistors is not sufficiently large for high-performance transistors for digital applications, but can be sufficient for ultra-low voltage applications, when exploiting a tunnel-FET architecture.^[45]

In February 2010, researchers announced transistors with an on/off rate of 100 gigahertz, far exceeding the rates of previous attempts, and exceeding the speed of silicon transistors with an equal gate length. The 240 nm devices were made with conventional silicon-manufacturing equipment.^[46]^[47]^[48]

In November 2011, researchers used 3d printing (additive manufacturing) as a method for fabricating graphene devices.^[49]

In 2013, researchers demonstrated graphene's high mobility in a detector that allows broad band frequency selectivity ranging from the THz to IR region (0.76–33 THz)^[50] A separate group created a terahertz-speed transistor with bistable characteristics, which means that the device can spontaneously switch between two electronic states. The device consists of two layers of graphene separated by an insulating layer of boron nitride a few atomic layers thick. Electrons move through this barrier by quantum tunneling. These new transistors exhibit “negative differential conductance,” whereby the same electrical current flows at two different applied voltages.^[51]

Graphene does not have an energy band-gap, which presents a hurdle for its applications in digital logic gates. The efforts to induce a band-gap in graphene via quantum confinement or surface functionalization have not resulted in a breakthrough. The negative differential resistance experimentally observed in graphene field-effect transistors of "conventional" design allows for construction of viable non-Boolean computational architectures with the gap-less graphene. The negative differential resistance — observed under certain biasing schemes — is an intrinsic property of graphene resulting from its symmetric band structure. The results present a conceptual change in graphene research and indicate an alternative route for graphene's applications in information processing.^[52]

In 2013 researchers reported the creation of transistors printed on flexible plastic that operate at 25 gigahertz, sufficient for communications circuits and that can be fabricated at scale. The researchers first fabricate the non-graphene-containing structures—the electrodes and gates—on plastic sheets. Separately, they grow large graphene sheets on metal, then peel it off and transfer it to the plastic. Finally, they top the sheet with a waterproof layer. The devices work after being soaked in water, and are flexible enough to be folded.^[53]

Trilayer graphene

An electric field can change trilayer graphene's crystal structure, transforming its behavior from metal-like to semiconductor-like. A sharp metal scanning tunneling microscopy tip was able to move the domain border between the upper and lower graphene configurations. One side of the material behaves as a metal, while the other side behaves as a semiconductor. Trilayer graphene can be stacked in either Bernal or rhombohedral configurations, which can exist in a single flake. The two domains are separated by a precise boundary at which the middle layer is strained to accommodate the transition from one stacking pattern to the other.^[54]

Silicon transistors function as either p-type or n-type semiconductors, whereas graphene could operate as both. This lowers costs and is more versatile. The technique provides the basis for a field-effect transistor. Scalable manufacturing techiques have yet to be developed.^[54]

In trilayer graphene, the two stacking configurations exhibit very different electronic properties. The region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon–carbon bond distance. The free-energy difference between the two stacking configurations scales quadratically with electric field, favoring rhombohedral stacking as the electric field increases.^[54]

This ability to control the stacking order opens the way to new devices that combine structural and electrical properties.^[54]^[55]

Graphene-based transistors could be much thinner than modern silicon devices, allowing faster and smaller configurations.^{[citation needed]}

Transparent conducting electrode

Graphene's high electrical conductivity and high optical transparency make it a candidate for transparent conducting electrodes, required for such applications as touchscreens, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes. In particular, graphene's mechanical strength and flexibility are advantageous compared to indium tin oxide, which is brittle. Graphene films may be deposited from solution over large areas.^[56]^[57]

Large-area, continuous, transparent and highly conducting few-layered graphene ﬁlms were produced by chemical vapor deposition and used as anodes for application in photovoltaic devices. A power conversion efficiency (PCE) up to 1.71% was demonstrated, which is 55.2% of the PCE of a control device based on indium tin oxide.^[58]

Organic light-emitting diodes (OLEDs) with graphene anodes have been demonstrated.^[59] The electronic and optical performance of graphene-based devices are similar to devices made with indium tin oxide.

A carbon-based device called a light-emitting electrochemical cell (LEC) was demonstrated with chemically-derived graphene as the cathode and the conductive polymer PEDOT as the anode.^[60] Unlike its predecessors, this device contains only carbon-based electrodes, with no metal.^{[citation needed]}

In 2014 a prototype graphene-based flexible display was demonstrated.^[61]

Frequency multiplier

In 2009, researchers built experimental graphene frequency multipliers that take an incoming signal of a certain frequency and output a signal at a multiple of that frequency.^[62]

Optoelectronics

Graphene strongly interacts with photons, with the potential for direct band-gap creation. This is promising for optoelectronic and nanophotonic devices. Light interaction arises due to the Van Hove singularity. Graphene displays different time scales in response to photon interaction, ranging from femtoseconds (ultra-fast) to picoseconds. Potential uses include transparent films, touch screens and light emitters or as a plasmonic device that confines light and alters wavelengths.^[63]

Hall effect sensors

Due to extremely high electron mobility, graphene may be used for production of highly sensitive Hall effect sensors.^[64] Potential application of such sensors is connected with DC current transformers for special applications.^{[citation needed]}

Quantum dots

Graphene quantum dots (GQDs) keep all dimensions less than 10 nm. Their size and edge crystallography govern their electrical, magnetic, optical and chemical properties. GQDs can be produced via graphite nanotomy^[65] or via bottom-up, solution-based routes (Diels-Alder, cyclotrimerization and/or cyclodehydrogenation reactions).^[66] GQDs with controlled structure can be incorporated into applications in electronics, optoelectronics and electromagnetics. Quantum confinement can be created by changing the width of graphene nanoribbons (GNRs) at selected points along the ribbon.^[28]^[67] It is studied as a catalyst for fuel cells.^[68]

Light processing

Optical modulator

When the Fermi level of graphene is tuned, its optical absorption can be changed. In 2011, researchers reported the first graphene-based optical modulator. Operating at 1.2 GHz without a temperature controller, this modulator has a broad bandwidth (from 1.3 to 1.6 μm) and small footprint (~25 μm²).^[69]

Infrared light detection

Graphene, reacts to the infrared spectrum at room temperature, albeit with sensitivity 100 to 1000 times too low for practical applications. However, two graphene layers separated by an insulator allowed an electric field produced by holes left by photo-freed electrons in one layer to affect a current running through the other layer. The process produces little heat, making it suitable for use in night-vision optics. The sandwich is thin enough to be integrated in handheld devices, eyeglass-mounted computers and even contact lenses.^[70]

Energy

Generation

Ethanol distillation

Graphene oxide membranes allow water vapor to pass through, but are impermeable to other liquids and gases.^[71] This phenomenon has been used for further distilling of vodka to higher alcohol concentrations, in a room-temperature laboratory, without the application of heat or vacuum as used in traditional distillation methods.^[72] Further development and commercialization of such membranes could revolutionize the economics of biofuel production and the alcoholic beverage industry.^{[citation needed]}

Solar cells

Graphene has a unique combination of high electrical conductivity and optical transparency, which make it a candidate for use in solar cells. A single sheet of graphene is a zero-bandgap semiconductor whose charge carriers are delocalized over large areas, implying that carrier scattering does not occur. Because this material only absorbs 2.6% of green light and 2.3% of red light,^[73] it is a candidate for applications requiring a transparent conductor. Graphene can be assembled into a film electrode with low roughness. However, graphene films produced via solution processing contain lattice defects and grain boundaries that act as recombination centers and decrease the material's electrical conductivity.
Thus, these films must be made thicker than one atomic layer to obtain useful sheet resistances. This added resistance can be combatted by incorporating conductive filler materials, such as a silica matrix. Reduced graphene film's electrical conductivity can be improved by attaching large aromatic molecules such as pyrene-1-sulfonic acid sodium salt (PyS) and the disodium salt of 3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid (PDI). These molecules, under high temperatures, facilitate better π-conjugation of the graphene basal plane. Graphene films have high transparency in the visible and near-infrared regions and are chemically and thermally stable.^[74]

For graphene to be used in commercial solar cells, large-scale production is required. However, no scalable process for producing graphene is available, including the peeling of pyrolytic graphene or thermal decomposition of silicon carbide.^[74]

Graphene's high charge mobilities recommend it for use as a charge collector and transporter in photovoltaics (PV). Using graphene as a photoactive material requires its bandgap to be 1.4–1.9 eV. In 2010, single cell efficiencies of nanostructured graphene-based PVs of over 12% were achieved. According to P. Mukhopadhyay and R. K. Gupta organic photovoltaics could be "devices in which semiconducting graphene is used as the photoactive material and metallic graphene is used as the conductive electrodes".^[74]

In 2010, Xinming Li and Hongwei Zhu from Tsinghua University first reported graphene-silicon heterojunction solar cell, where graphene served as a transparent electrode and introduced a built-in electric field near the interface between the graphene and n-type silicon to help collect photo-generated carriers. More studies promote this new type of photovoltaic device.^[75] For example, in 2012 researchers from the University of Florida reported efficiency of 8.6% for a prototype cell consisting of a wafer of silicon coated with a layer of graphene doped with trifluoromethanesulfonyl-amide (TFSA). Furthermore, Xinming Li found chemical doping could improve the graphene characteristics and significantly enhance the efficiency of graphene-silicon solar cell to 9.6% in 2013.^[76]

In 2013 another team claimed to have reached 15.6% percent using a combination of titanium oxide and graphene as a charge collector and perovskite as a sunlight absorber. The device is manufacturable at temperatures under 150 °C (302 °F) using solution-based deposition. This lowers production costs and offers the potential using flexible plastics.^[77]

Large scale production of highly transparent graphene films by chemical vapor deposition was achieved in 2008. In this process, ultra-thin graphene sheets are created by first depositing carbon atoms in the form of graphene films on a nickel plate from methane gas. A protective layer of thermoplastic is laid over the graphene layer and the nickel underneath is dissolved in an acid bath. The final step is to attach the plastic-protected graphene to a flexible polymer sheet, which can then be incorporated into an OPV cell. Graphene/polymer sheets range in size up to 150 square centimeters and can be used to create dense arrays of flexible OPV cells. It may eventually be possible to run printing presses covering extensive areas with inexpensive solar cells, much like newspaper presses print newspapers (roll-to-roll).^[78]

Silicon generates only one current-driving electron for each photon it absorbs, while graphene can produce multiple electrons. Solar cells made with graphene could offer 60% conversion efficiency – double the widely accepted maximum efficiency of silicon cells.^[79]

Fuel cells

Appropriately perforated graphene (and hexagonal boron nitride hBN) can allow protons to pass through it, offering the potential for using graphene monolayers as a barrier that blocks hydrogen atoms but not protons/ionized hydrogen (hydrogen atoms with their electrons stripped off). They could even be used to extract hydrogen gas out of the atmosphere that could power electric generators with ambient air.^[80]

The membranes are more effective at elevated temperatures and when covered with catalytic nanoparticles such as platinum.^[80]

Graphene could solve a major problem for fuel cells: fuel crossover that reduces efficiency and durability.^[80]

At room temperature, proton conductivity with monolayer hBN, outperforms graphene, with resistivity to proton flow of about 10 Ω cm² and a low activation energy of about 0.3 electronvolts. At higher temperatures, graphene outperforms with resistivity estimated to fall below 10⁻³ Ω cm² above 250 degrees Celsius.^[81]

Storage

Supercapacitor

Due to graphene's high surface area to mass ratio, one potential application is in the conductive plates of supercapacitors.^[82]

In February 2013 researchers announced a novel technique to produce graphene supercapacitors based on the DVD burner reduction approach.^[83]

In 2014 a supercapacitor was announced that was claimed to achieve energy density comparable to current lithium-ion batteries.^[25]^[26]

In 2015 the technique was adapted to produce stacked, 3-D supercapacitors. Laser-induced graphene was produced on both sides of a polymer sheet. The sections were then stacked, separated by solid electrolytes, making multiple microsupercapacitors. The stacked configuration substantially increased the energy density of the result. In testing, the researchers charged and discharged the devices for thousands of cycles with almost no loss of capacitance.^[84]

Electrode for Li-ion batteries

Stable Li-ion cycling has recently been demonstrated in bi- and few layer graphene films grown on nickel substrates,^[85] while single layer graphene films have been demonstrated as a protective layer against corrosion in battery components such as the battery case.^[86] This creates possibilities for flexible electrodes for microscale Li-ion batteries where the anode acts as the active material as well as the current collector.^[87]

There are also silicon-graphene anode Li-ion batteries.^[88]

Hydrogen storage

Hydrogenation-assisted graphene origami (HAGO) was used to cause approximately square graphene sheets to fold into a cage can store hydrogen at 9.5 percent by weight. The U.S. Department of Energy had set a goal of 7.5 percent hydrogen by 2020. An electric field causes the box to open and close.^[89]

Rechargeable battery

Researchers at Northwestern University built a lithium-ion battery made of graphene and silicon, which was claimed to last over a week on a single charge and only took 15 minutes to charge.^[90]

Sensors

Molecular adsorbtion

Theoretically graphene makes an excellent sensor due to its 2D structure. The fact that its entire volume is exposed to its surrounding environment makes it very efficient to detect adsorbed molecules. However, similar to carbon nanotubes, graphene has no dangling bonds on its surface.
Gaseous molecules cannot be readily adsorbed onto graphene surfaces, so intrinsically graphene is insensitive.^[91] The sensitivity of graphene chemical gas sensors can be dramatically enhanced by functionalization, for example, coating the film with a thin layer of certain polymers. The thin polymer layer acts like a concentrator that absorbs gaseous molecules. The molecule absorption introduces a local change in electrical resistance of graphene sensors. While this effect occurs in other materials, graphene is superior due to its high electrical conductivity (even when few carriers are present) and low noise, which makes this change in resistance detectable.^[92]

Piezoelectric effect

Density functional theory simulations predict that depositing certain adatoms on graphene can render it piezoelectrically responsive to an electric field applied in the out-of-plane direction. This type of locally engineered piezoelectricity is similar in magnitude to that of bulk piezoelectric materials and makes graphene a candidate for control and sensing in nanoscale devices.^[93]

Body motion

Rubber bands infused with graphene("G-bands") can be used as inexpensive body sensors. The bands remain pliable and can be used as a sensor to measure breathing, heart rate, or movement. Lightweight sensor suits for vulnerable patients could make it possible to remotely monitor subtle movement. These sensors display 10-fold increases in resistance and work at strains exceeding 800%. Gauge factors of up to 35 were observed. Such sensors can function at vibration frequencies of at least 160 Hz. At 60 Hz, strains of at least 6% at strain rates exceeding 6000%/s can be monitored.^[94]

Environmental

Contaminent removal

Graphene oxide is non-toxic and biodegradable. Its surface is covered with epoxy, hydroxyl, and carboxyl groups that interact with cations and anions. It is soluble in water and forms stable colloid suspensions in other liquids because it is amphiphilic (able to mix with water or oil). Dispersed in liquids it shows excellent sorption capacities. It can remove copper, cobalt, cadmium, arsenate and organic solvents.^[95]

In 2013 it was shown to be able to remove radioactive nuclides from water, including radioactive isotopes of actinides (elements with atomic numbers 89 to 103, including thorium, uranium, neptunium, plutonium and americium) and lanthanides (the ‘rare earths’ with atomic numbers 57 to 71, including europium).^[95]

Even at concentrations < 0.1 g/L, radionuclide sorption proceeds rapidly. At pH between 4 and 8, graphene oxide removes over 90% of nuclides, including uranium and europium. At pH >7, more than 70% of strontium and technicium are removed with up to 20% of neptunium.^[95]

Water filtration

Research suggests that graphene filters could outperform other techniques of desalination by a significant margin.^[96]

Other

Plasmonics and metamaterials

Graphene accommodates a plasmonic surface mode, observed recently via near field infrared optical microscopy techniques.^[97]^[98] and infrared spectroscopy ^[99] Potential applications are in the terahertz to midinfrared frequencies,^[100] such as terahertz and midinfrared light modulators, passive terahertz filters, midinfrared photodetectors and biosensors.

Lubricant

Scientists discovered using graphene as a lubricant works better than traditionally used graphite. A one atom thick layer of graphene in between a steel ball and steel disc lasted for 6,500 cycles. Conventional lubricants lasted 1,000 cycles.^[101]

Radio wave absorption

Stacked graphene layers on a quartz substrate increased the absorption of millimeter (radio) waves by 90 per cent over 125 – 165 GHz bandwidth, extensible to microwave and low-terahertz frequencies, while remaining transparent to visible light. For example, graphene could be used as a coating for buildings or windows to block radio waves. Absorption is a result of mutually coupled Fabry–Perot resonators represented by each graphene-quartz substrate. A repeated transfer-and-etch process was used to control surface resistivity.^[102]^[103]

Redox

Graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process that modifies electronic and optical properties. Oxidation and reduction are related to resistive switching.^[104]

Nanoantennas

A graphene-based plasmonic nano-antenna (GPN) can operate efficiently at millimeter radio wavelengths. The wavelength of surface plasmon polaritons for a given frequency is several hundred times smaller than the wavelength of freely propagating electromagnetic waves of the same frequency. These speed and size differences enable efficient graphene-based antennas to be far smaller than conventional alternatives. The latter operate at frequencies 100-1000 times larger than GPNs, producing .01-.001 as many photons.^[105]

An electromagnetic (EM) wave directed vertically onto a graphene surface excites the graphene into oscillations that interact with those in the dielectric on which the graphene is mounted, thereby forming surface plasmon polaritons (SPP). When the antenna becomes resonant (an integral number of SPP wavelengths fit into the physical dimensions of the graphene), the SPP/EM coupling increases greatly, efficiently transferring energy between the two.^[105]

A phased array antenna 100 µm in diameter could produce 300 GHz beams only a few degrees in diameter, instead of the 180 degree radiation from tsa conventional metal antenna of that size. Potential uses include smart dust, low-power terabit wireless networks^[105] and photonics.^[106]

A nanoscale gold rod antenna captured and transformed EM energy into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable. The plasmon wavefronts can be directly controlled by adjusting antenna geometry. The waves were focused (by curving the antenna) and refracted (by a prism-shaped graphene bilayer because the conductivity in the two-atom-thick prism is larger than in the surrounding one-atom-thick layer.)^[106]

Sound transducers

Graphene provides relatively good frequency response, suggesting uses in audio speakers. Its light weight may make it suitable for microphones as well.^[107]

Waterproof coating

Graphene could potentially usher in a new generation of waterproof devices whose chassis may not need to be sealed like today's devices.^[90]^{[dubious – discuss]}

Coolant additive

Graphene's high thermal conductivity suggests that it could be used as an additive in coolants. Preliminary research work showed that 5% graphene by volume can enhance the thermal conductivity of a base fluid by 86%.^[108] Another application due to graphene's enhanced thermal conductivity was found in PCR.^[14]

Reference material

Graphene's properties suggest it as a reference material for characterizing electroconductive and transparent materials. One layer of graphene absorbs 2.3% of red light.^[109]

This property was used to define the conductivity of transparency that combines sheet resistance and transparency. This parameter was used to compare materials without the use of two independent parameters.^[110]

Thermal management

In 2011, researchers reported that a three-dimensional, vertically aligned, functionalized multilayer graphene architecture can be an approach for graphene-based thermal interfacial materials (TIMs) with superior thermal conductivity and ultra-low interfacial thermal resistance between graphene and metal.^[111]

Graphene-metal composites can be utilized in thermal interface materials.^[112]

Adding a layer of graphene to each side of a copper film increased the metal's heat-conducting properties up to 24%. This suggests the possibility of using them for semiconductor interconnects in computer chips. The improvement is the result of changes in copper’s nano- and microstructure, not from graphene’s independent action as an additional heat conducting channel. High temperature chemical vapor deposition stimulates grain size growth in copper films. The larger grain sizes improve heat conduction. The heat conduction improvement was more pronounced in thinner copper films, which is useful as copper interconnects shrink.^[113]

Structural material

Graphene's strength, stiffness and lightness suggested it for use with carbon fiber. Graphene has been used as a reinforcing agent to improve the mechanical properties of biodegradable polymeric nanocomposites for engineering bone tissue.^[114]

Catalyst

In 2014, researchers at The University of Western Australia discovered nano sized fragments of graphene can speed up the rate of chemical reactions.^[115]

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