Beryllium

iso	NA	half-life	DM	DE (MeV)	DP
⁷Be	trace	53.12 d	ε	0.862	⁷Li
⁷Be	trace	53.12 d	γ	0.477	-
⁸Be	trace	7×10⁻¹⁷s	α		⁴He
⁹Be	100%	⁹Be is stable with 5 neutrons
¹⁰Be	trace	1.36×10⁶ y	β⁻	0.556	¹⁰B

Beryllium ( /bəˈrɪliəm/ bə-ril-ee-əm) is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl (aquamarine, emerald) and chrysoberyl. As a free element it is a steel-gray, strong, lightweight and brittle alkaline earth metal.

Beryllium is used primarily as a hardening agent in alloys, notably beryllium copper. In structural applications, high flexural rigidity, thermal stability, thermal conductivity and low density (1.85 times that of water) make beryllium a quality aerospace material for high-speed aircraft, missiles, space vehicles and communication satellites. Because of its low density and atomic mass, beryllium is relatively transparent to X-rays and other forms of ionizing radiation; therefore, it is the most common window material for X-ray equipment and in particle physics experiments. The high thermal conductivity of beryllium and beryllium oxide have led to their use in heat transport and heat sinking applications.

The commercial use of beryllium metal presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium is corrosive to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in some people. Because any Beryllium synthesized in stars is short-lived, it is a relatively rare element in both the Earth and the universe. The element is not known to be necessary or useful for either plant or animal life.^[3]

Characteristics

Physical properties

Beryllium has exceptional flexural rigidity (Young's modulus 287 GPa) and a reasonably high melting point. The modulus of elasticity of beryllium is approximately 50% greater than that of steel. The combination of this modulus and a relatively low density results in an unusually fast sound conduction speed in beryllium – about 12.9 km/s at ambient conditions. Other significant properties are high specific heat (1925 J·kg⁻¹·K⁻¹) and thermal conductivity (216 W·m⁻¹·K⁻¹), which make beryllium the metal with the best heat dissipation characteristics per unit weight. In combination with the relatively low coefficient of linear thermal expansion (11.4×10⁻⁶ K⁻¹), these characteristics result in a unique stability under conditions of thermal loading.^[4]

Nuclear properties

Beryllium has a large scattering cross section for high-energy neutrons, about 6 barns for energies above ~0.01 eV. Therefore, it effectively slows the neutrons to the thermal energy range of below 0.03 eV, where the total cross section is at least an order of magnitude lower – exact value strongly depends on the purity and size of the crystallites in the material. The predominant beryllium isotope ⁹Be also undergoes a (n,2n) neutron reaction to ⁸Be, which then instantaneously breaks into two alpha particles; that is, beryllium is a neutron multiplier, releasing more neutrons than it absorbs. This nuclear reaction is:^[5]

9
4Be + n → 2(4
2He) + 2n

As a metal, beryllium is transparent to most wavelengths of X-rays and gamma rays, making it useful for the output windows of X-ray tubes and other such apparatus. It is also a good source for the relatively-small numbers of free neutrons in the laboratory which are liberated when beryllium nuclei are struck by energetic alpha particles^[4] producing the nuclear reaction

9
4Be + 4
2He → 12
6C + n , where 4
2He is an alpha particle and 12
6C is a carbon-12 nucleus.^[5]

Isotopes and nucleosynthesis

Main articles: Isotopes of beryllium and beryllium-10

Both stable and unstable isotopes of beryllium are created in stars, but these do not last long. It is believed that most of the stable beryllium in the universe was created when cosmic rays induced fission in heavier elements found in interstellar gas and dust.^[6]

Beryllium contains only one stable isotope, ⁹Be, and therefore is a monoisotopic element. Cosmogenic ¹⁰Be is produced in the atmosphere of the Earth by the cosmic ray spallation of oxygen and nitrogen. Cosmogenic ¹⁰Be accumulates at the soil surface, where its relatively long half-life (1.36 million years) permits a long residence time before decaying to boron-10. Thus, ¹⁰Be and its daughter products are used to examine natural soil erosion, soil formation and the development of lateritic soils, and as a proxy for measurement of the variations in solar activity and the age of ice cores.^[7]

The production of ¹⁰Be is inversely proportional to the solar activity, because the increased solar wind during periods of high solar magnetic activity in turn decreases the flux of galactic cosmic rays that reach the Earth. Nuclear explosions also form ¹⁰Be by the reaction of fast neutrons with ¹³C in the carbon dioxide in air. This is one of the indicators of past activity at nuclear weapon test sites.^[8]

The isotope ⁷Be (half-life 53 days) is also cosmogenic, and shows an atmospheric abundance linked to sunspots much like ¹⁰Be. ⁸Be has a very short half-life of about 7×10⁻¹⁷ s that contributes to its significant cosmological role, as elements heavier than beryllium could not have been produced by nuclear fusion in the Big Bang.^[9] This is due to the lack of sufficient time during the Big Bang's nucleosynthesis phase to produce carbon by the fusion of ⁴He nuclei and the very low concentrations of available beryllium-8. The British astronomer Sir Fred Hoyle first showed that the energy levels of ⁸Be and ¹²C allow carbon production by the so-called triple-alpha process in helium-fueled stars where more nucleosynthesis time is available, thus making creation of carbon-based life possible from the gas and dust ejected by supernovas (see also Big Bang nucleosynthesis).^[10]

The innermost electrons of beryllium may contribute to chemical bonding. Therefore, when ⁷Be decays by electron capture, it does so by taking electrons from atomic orbitals that may participate in bonding. This makes its decay rate dependent to a measurable degree upon its electron configuration – a rare occurrence in nuclear decay.^[11]

The shortest-lived known isotope of beryllium is ¹³Be which decays through neutron emission. It has a half-life of 2.7 × 10⁻²¹ s. ⁶Be is also very short-lived with a half-life of 5.0 × 10⁻²¹ s.^[12] The exotic isotopes ¹¹Be and ¹⁴Be are known to exhibit a nuclear halo.^[13] This phenomenon can be understood as the nuclei of ¹¹Be and ¹⁴Be have, respectively, 1 and 4 neutrons orbiting substantially outside the classical Fermi 'waterdrop' model of the nucleus.

Occurrence

Beryllium is scarce in the universe. With a concentration of 1 part per billion (ppb) by weight, it is much less abundant than all elements preceding niobium, with the exception of boron, which has a similar abundance.^[14] Beryllium is similarly rare in the Sun with a concentration of 0.1 ppb by weight, similar to that of rhenium.^[15]

The beryllium concentration in the Earth's surface rocks is ca. 4–6 ppm by atoms. Beryllium is a major constituent of about 100 out of some 4000 known minerals, the most important of which are bertrandite (Be₄Si₂O₇(OH)₂), beryl (Al₂Be₃Si₆O₁₈), chrysoberyl (Al₂BeO₄) and phenakite (Be₂SiO₄). Precious forms of beryl are aquamarine, bixbite and emerald.^[4]^[16]^[17] In sea water, beryllium is exceedingly rare, more so than even scandium, comprising only 0.0006 ppb by weight.^[18] In stream water, however, beryllium is more abundant with 0.1 ppb by weight.^[19]

Production

Because of its high affinity for oxygen at elevated temperatures, and its ability to reduce water when its oxide film is removed, the extraction of beryllium from its compounds is a difficult process. Electrolysis of a mixture of beryllium fluoride and sodium fluoride was used to isolate beryllium during the 19th century. The metal's high melting point makes this process more energy-consuming than corresponding processes used for the alkali metals. Early in the 20th century, the production of beryllium by the thermal decomposition of beryllium iodide was investigated following the success of a similar process for the production of zirconium, but this process proved to be uneconomical for volume production.^[20]

Pure beryllium metal did not become readily available until 1957, even though it had been used as an alloying metal to harden and toughen copper much earlier. Beryllium could be produced by reducing beryllium compounds such as beryllium chloride with metallic potassium or sodium. Currently most beryllium is produced by reducing beryllium fluoride with purified magnesium. The price on the American market for vacuum-cast beryllium ingots was about $338 per pound ($745 per kilogram) in 2001.^[21] The chemical equation for the key reaction is as follows:

BeF₂ + Mg → MgF₂ + Be

The United States, China and Kazakhstan are the only three countries involved in the industrial scale extraction of beryllium.^[22] In the US, Brush Wellman Inc. is the main producer of beryllium and beryllium products.^[23] This company smelts its beryllium ore, which contains the mineral bertrandite, and which comes mostly from the company-owned Spor Mountain deposit in the State of Utah. The smelting and other refining of the beryllium is carried out at a factory 10 miles north of Delta, Utah,^[24] a location chosen for its remoteness and proximity to the Intermountain Power Project.^[25] Between 1998 and 2008, the world's production of beryllium had decreased from 343 to about 200 tonnes, of which 176 tonnes (88%) came from the United States.^[26]^[27]

Chemical properties

Beryllium metal is located above aluminium in the electrochemical series and therefore is expected to show significant chemical activity; however, it is passivated by an oxide layer and does not react with air or water even at red heat.^[28] Once ignited, beryllium burns brilliantly forming a mixture of beryllium oxide and beryllium nitride.^[28] Beryllium dissolves readily in non-oxidizing acids, such as HCl and diluted H₂SO₄, but not in nitric acid as this forms the oxide. This behavior is similar to that of aluminium metal. Beryllium, again similarly to aluminium, dissolves in warm alkali solutions to form the beryllate anion, Be(OH)₄²⁻, and hydrogen gas.

The beryllium atom has the electronic configuration [He] 2s². In beryllium compounds the two electrons are lost and beryllium is in the +2 oxidation state; the only evidence of lower valence of beryllium is in the solubility of the metal in BeCl₂.^[29] The small atomic radius leads to significant covalent character in beryllium's bonding.^[28] Beryllium is 4-fold coordinated in virtually all of its derivatives, e.g. [Be(H₂O)₄]²⁺ and tetrahaloberyllates, BeX₄²⁻. This characteristic is employed in analytical techniques using EDTA as a ligand. EDTA preferentially forms octahedral complexes – thus absorbing other cations such as Al³⁺ which might interfere – for example, in the solvent extraction of a complex formed between Be²⁺ and acetylacetone.^[30]

Solutions of beryllium salts, e.g. beryllium sulfate and beryllium nitrate, are acidic because of hydrolysis of the [Be(H₂O)₄]²⁺ ion.

[Be(H₂O)₄]²⁺ + H₂O [Be(H₂O)₃(OH)]⁺ + H₃O⁺

Other products of hydrolysis include the trimeric ion [Be₃(OH)₃(H₂O)₆]³⁺. Beryllium hydroxide, Be(OH)₂, is insoluble even in acidic solutions with pH less than 6, that is at biological pH. It is amphoteric and dissolves in strongly alkaline solutions.

Beryllium forms binary compounds with many non-metals. Anhydrous halides are known for F, Cl, Br and I. BeF₂ has a silica-like structure with corner-shared BeF₄ tetrahedra. BeCl₂ and BeBr₂ have chain structures with edge-shared tetrahedra. All beryllium halides have a linear monomeric molecular structure in the gas phase.^[28]

Beryllium oxide, BeO, is a white refractory solid, which has the wurtzite crystal structure and a thermal conductivity as high as in some metals. BeO is amphoteric. Salts of beryllium can be produced by treating Be(OH)₂ with acid.^[28] Beryllium sulfide, selenide and telluride are known, all having the zincblende structure.^[29]

Beryllium nitride, Be₃N₂ is a high-melting-point compound which is readily hydrolyzed. Beryllium azide, BeN₆ is known and beryllium phosphide, Be₃P₂ has a similar structure to Be₃N₂. Basic beryllium nitrate and basic beryllium acetate have similar tetrahedral structures with four beryllium atoms coordinated to a central oxide ion.^[29] A number of beryllium borides are known, such as Be₅B, Be₄B, Be₂B, BeB₂, BeB₆ and BeB₁₂. Beryllium carbide, Be₂C, is a refractory brick-red compound that reacts with water to give methane.^[29] No beryllium silicide has been identified.^[28]

History

Early analyses of emeralds and beryls always yielded similar elements, leading to the fallacious conclusion that both substances are aluminium silicates. René Just Haüy discovered that both crystals show strong similarities, and he asked the chemist Louis-Nicolas Vauquelin for a chemical analysis. Vauquelin was able to separate the aluminium from the beryllium by dissolving the aluminium hydroxide in an additional alkali.^[31] Vauquelin named the new element "glucinum" for the sweet taste of some of its compounds.^[32]

Friedrich Wöhler^[33] and Antoine Bussy^[34] independently isolated beryllium in 1828 by the chemical reaction of metallic potassium with beryllium chloride, as follows:

BeCl₂ + 2 K → 2 KCl + Be

The potassium itself had been produced by the electrolysis of its compounds, a newly discovered process. This chemical method yielded only small grains of beryllium from which no ingot of metal could be cast or hammered. The direct electrolysis of a molten mixture of beryllium fluoride and sodium fluoride by Paul Lebeau in 1898 resulted in the first pure samples of beryllium.^[32] It took until World War I (1914–18) before significant amounts of beryllium were produced, but large-scale production was not started until early 1930s. It saw a rapid increase during World War II, due to the rising demand for hard beryllium-copper alloys and phosphors for fluorescent lights. In the first years, most fluorescent lamps used zinc orthosilicate with varying content of beryllium to emit greenish light. Small additions of magnesium tungstate improved the blue part of the spectrum yielding acceptable white. After it was discovered that beryllium was toxic, halophosphate-based phosphors took over.^[35]

Etymology

Early usage of the word beryllium can be traced to many languages, including Latin Beryllus; French Béry; Greek βήρυλλος, bērullos, beryl; Prakrit veruliya (वॆरुलिय‌); Pāli veḷuriya (वेलुरिय), veḷiru (भेलिरु) or viḷar (भिलर्) – "to become pale," in reference to the pale semiprecious gemstone beryl. The original source is probably the Sanskrit word वैडूर्य vaidurya-, which is of Dravidian origin and could be derived from the name of the modern city of Belur.^[36] For about 160 years, beryllium was also known as glucinum or glucinium (with the accompanying chemical symbol "Gl",^[37]), the name coming from the Greek word for sweet: γλυκυς, due to the sweet taste of beryllium salts.

Applications

It is estimated that most beryllium is used for military applications, so information is not readily available.^[38]

Radiation windows

Because of its low atomic number and very low absorption for X-rays, the oldest and still one of the most important applications of beryllium is in radiation windows for X-ray tubes. Extreme demands are placed on purity and cleanliness of beryllium to avoid artifacts in the X-ray images. Thin beryllium foils are used as radiation windows for X-ray detectors, and the extremely low absorption minimizes the heating effects caused by high intensity, low energy X-rays typical of synchrotron radiation. Vacuum-tight windows and beam-tubes for radiation experiments on synchrotrons are manufactured exclusively from beryllium. In scientific setups for various X-ray emission studies (e.g., energy-dispersive X-ray spectroscopy) the sample holder is usually made of beryllium because its emitted X-rays have much lower energies (~100 eV) than X-rays from most studied materials.^[4]

Low atomic number also makes beryllium relatively transparent to energetic particles. Therefore it is used to build the beam pipe around the collision region in particle physics setups, such as all four main detector experiments at the Large Hadron Collider (ALICE, ATLAS, CMS, LHCb),^[39] the Tevatron and the SLAC. The low density of beryllium allows collision products to reach the surrounding detectors without significant interaction, its stiffness allows a powerful vacuum to be produced within the pipe to minimize interaction with gases, its thermal stability allows it to function correctly at temperatures of only a few degrees above absolute zero, and its diamagnetic nature keeps it from interfering with the complex multipole magnet systems used to steer and focus the particle beams.^[40]

Mechanical applications

Because of its stiffness, light weight and dimensional stability over a wide temperature range, beryllium metal is used for lightweight structural components in the defense and aerospace industries in high-speed aircraft, missiles, space vehicles and communication satellites. Several liquid-fuel rockets use nozzles of pure beryllium.^[41]^[42] A small number of bicycle frames were built with beryllium, at "astonishing" prices.^[43]

Beryllium is used as an alloying agent in the production of beryllium copper, which contains up to 2.5% beryllium. Beryllium-copper alloys are used in many applications because of their combination of high electrical and thermal conductivity, high strength and hardness, nonmagnetic properties, along with good corrosion and fatigue resistance. These applications include the making of spot welding electrodes, springs, non-sparking tools and electrical contacts.

The excellent elastic rigidity of beryllium has led to its extensive use in precision instrumentation, e.g. in gyroscope inertial guidance systems and in support structures for optical systems.^[4] Beryllium-copper alloys were also applied as a hardening agent in Jason pistols, which were used to strip paint from the hulls of ships.^[44]

An earlier major application of beryllium was in brakes for military aircraft because of its hardness, high melting point and exceptional heat dissipation. Environmental considerations have led to substitution by other materials.^[4]

To reduce cost, beryllium may be fabricated with large amounts of aluminium, resulting in AlBeMet (trade name). This blend is cheaper, while still retaining many desirable properties.

Mirrors

Beryllium mirrors are of particular interest. Large-area mirrors, frequently with a honeycomb support structure, are used, for example, in meteorological satellites where low weight and long-term dimensional stability are critical. Smaller beryllium mirrors are used in optical guidance systems and in fire-control systems, e.g. in the German-made Leopard 1 and Leopard 2 main battle tanks. In these systems, very rapid movement of the mirror is required which again dictates low mass and high rigidity. Usually the beryllium mirror is coated with hard electroless nickel plating which can be more easily polished to a finer optical finish than beryllium. In some applications, though, the beryllium blank is polished without any coating. This is particularly applicable to cryogenic operation where thermal expansion mismatch can cause the coating to buckle.^[4]

The James Webb Space Telescope^[45] will have 18 hexagonal beryllium sections for its mirrors. Because JWST will face a temperature of 33 K, the mirror is made of beryllium, capable of handling extreme cold better than glass. Beryllium contracts and deforms less than glass – and remains more uniform – in such temperatures.^[46] For the same reason, the optics of the Spitzer Space Telescope are entirely built of beryllium metal.^[47]

Magnetic applications

Beryllium is non-magnetic. Therefore, tools fabricated out of beryllium are used by naval or military explosive ordnance disposal teams for work on or near naval mines, since these mines commonly have magnetic fuzes.^[48] They are also found in maintenance and construction materials near magnetic resonance imaging (MRI) machines because of the high magnetic fields generated by them.^[49] In the fields of radio communications and powerful (usually military) radars, hand tools made of beryllium are used to tune the highly magnetic klystrons, magnetrons, traveling wave tubes, etc., that are used for generating high levels of microwave power in the transmitters.^[50]

Nuclear applications

Thin plates or foils of beryllium are sometimes used in nuclear weapon designs as the very outer layer of the plutonium pits in the primary stages of thermonuclear bombs, placed to surround the fissile material. These layers of beryllium are good "pushers" for the implosion of the plutonium-239, and they are also good neutron reflectors, just as they are in beryllium-moderated nuclear reactors.^[51]

Beryllium is also commonly used as a neutron source in laboratory experiments in which relatively few neutrons are needed (rather than having to use a nuclear reactor). For this purpose, a target of beryllium-9 is bombarded with energetic alpha particles from a radio-isotope such as polonium-210, radium-226, plutonium-239, or americium-241. In the nuclear reaction that occurs, a beryllium nucleus is transmuted into carbon-12, and one free neutron is emitted, traveling in about the same direction as the alpha particle was heading. Such neutron sources, named "urchin" neutron initiators, were used some in early atomic bombs.^[51]

Beryllium is also used at the Joint European Torus nuclear-fusion research laboratory, and it will be used in the more advanced ITER to condition the components which face the plasma.^[52] Beryllium has also been proposed as a cladding material for nuclear fuel rods, owing to its good combination of mechanical, chemical and nuclear properties.^[4] Beryllium fluoride is one of the constituent salts of the eutectic salt mixture FLiBe, which is used as a solvent, moderator and coolant in many hypothetical molten salt reactor designs.^[53]

Acoustics

Low weight and high rigidity of beryllium make it useful as a material for high-frequency speaker drivers. Because beryllium is expensive (many times more than titanium), hard to shape due to its brittleness, and toxic if mishandled, beryllium tweeters are limited to high-end home, pro audio and public address applications.^[54]^[55]^[56] More often, beryllium is alloyed with other metals, which is sometimes not disclosed for marketing purposes.^[57]

Electronic

Beryllium is a p-type dopant in III-V compound semiconductors. It is widely used in materials such as GaAs, AlGaAs, InGaAs and InAlAs grown by molecular beam epitaxy (MBE).^[58] Cross-rolled beryllium sheet is an excellent structural support for printed circuit boards in surface-mount technology. In critical electronic applications, beryllium is both a structural support and heat sink. The application also requires a coefficient of thermal expansion that is well matched to the alumina and polyimide-glass substrates. The beryllium-beryllium oxide composite "E-Materials" have been specially designed for these electronic applications and have the additional advantage that the thermal expansion coefficient can be tailored to match diverse substrate materials.^[4]

Beryllium oxide is useful for many applications that require the combined properties of an electrical insulator and an excellent heat conductor, with high strength and hardness, and a very high melting point. Beryllium oxide is frequently used as an insulator base plate in high-power transistors in radio frequency transmitters for telecommunications. Beryllium oxide is also being studied for use in increasing the thermal conductivity of uranium dioxide nuclear fuel pellets.^[59] Beryllium compounds were used in fluorescent lighting tubes, but this use was discontinued because of the disease berylliosis which developed in the workers who were making the tubes.^[60]

Complexes

Beryllium(II) readily forms complexes with strong donating ligands such as phosphine oxides and arsine oxides. There have been extensive studies of these complexes which show the stability of the O-Be bond.

Toxicity

Main article: Beryllium poisoning

The toxicity of beryllium depends upon the duration, intensity and frequency of exposure (features of dose), as well as the form of beryllium and the route of exposure (e.g., inhalation, dermal, ingestion). According to the International Agency for Research on Cancer (IARC), beryllium and beryllium compounds are Category 1 carcinogens; they are carcinogenic to animals (including humans).^[61] Chronic berylliosis is a pulmonary and systemic granulomatous disease caused by exposure to beryllium. Acute beryllium disease in the form of chemical pneumonitis was first reported in Europe in 1933 and in the United States in 1943. Cases of chronic berylliosis were first described in 1946 among workers in plants manufacturing fluorescent lamps in Massachusetts. Chronic berylliosis resembles sarcoidosis in many respects, and the differential diagnosis is often difficult. It killed some early workers in nuclear weapons design, such as Herbert L. Anderson.^[62]

Early researchers tasted beryllium and its various compounds for sweetness in order to verify its presence. Modern diagnostic equipment no longer necessitates this highly risky procedure and no attempt should be made to ingest this highly toxic substance. Beryllium and its compounds should be handled with great care and special precautions must be taken when carrying out any activity which could result in the release of beryllium dust (lung cancer is a possible result of prolonged exposure to beryllium laden dust). Although the use of beryllium compounds in fluorescent lighting tubes was discontinued in 1949, potential for exposure to beryllium exists in the nuclear and aerospace industries and in the refining of beryllium metal and melting of beryllium-containing alloys, the manufacturing of electronic devices, and the handling of other beryllium-containing material.^[63]

A successful test for beryllium in air and on surfaces has been recently developed and published as an international voluntary consensus standard ASTM D7202. The procedure uses dilute ammonium bifluoride for dissolution and fluorescence detection with beryllium bound to sulfonated hydroxybenzoquinoline, allowing up to 100 times more sensitive detection than the recommended limit for beryllium concentration in the workplace. Fluorescence increases with increasing beryllium concentration. The new procedure has been successfully tested on a variety of surfaces and is effective for the dissolution and ultratrace detection of refractory beryllium oxide and siliceous beryllium (ASTM D7458).^[64]^[65]