A divalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include beryl (aquamarine, emerald) and chrysoberyl. The free element is a steel-gray, strong, lightweight brittle alkaline earth metal. It is primarily used as a hardening agent in alloys, notably beryllium copper. Structurally, beryllium's very low density (1.85 times that of water), high melting point (1287 °C), high temperature stability and low coefficient of thermal expansion, make it in many ways an ideal aerospace material, and it has been used in rocket nozzles and is a significant component of planned space telescopes.
Detailed description
Because of its relatively high transparency to X-rays and other ionizing radiation types, beryllium also has a number of uses as filters and windows for radiation and particle physics experiments.
Commercial use of beryllium metal presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium produces a direct corrosive effect to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in susceptible persons.
Because it is not synthesized in stars, beryllium 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.
Characteristics
Physical properties
Beryllium has one of the highest melting points of the light metals. It has exceptional flexural rigidity (Young's modulus 287 GPa). The modulus of elasticity of beryllium is approximately 50% greater than that of steel. The combination of this modulus plus beryllium's relatively low density gives it an unusually fast sound conduction speed at standard conditions (about 12.9 km/s). Other significant properties are the high values for 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 ensure that beryllium demonstrates a unique degree of dimensional stability under conditions of thermal loading.[5]
Chemical properties
Beryllium has the electronic configuration [He] 2s2. Beryllium metal sits above aluminium in the electrochemical series and would be expected to be a reactive metal, however it is passivated by an oxide layer and does not react with air or water even at red heat.[12] Once ignited however beryllium burns brilliantly forming a mixture of beryllium oxide and beryllium nitride.[12] Beryllium dissolves readily in non-oxidizing acids, such as HCl and H2SO4, but not in nitric as this forms the oxide and this behavior is similar to that of aluminium metal. Beryllium, again similarly to aluminium, dissolves in warm alkali to form the beryllate anion, Be(OH)42−, and hydrogen gas.
Occurrence
See also Category: Beryllium minerals The beryllium concentration of the Earth's surface rocks is ca. 4–6 ppm. Beryllium is a constituent of about 100 out of about 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.[5][13][14]
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 very difficult. Although electrolysis of the fused mixture of beryllium fluoride and sodium fluoride was used to isolate beryllium during the nineteenth century, the metal's high melting point makes this process more energy-consuming than the corresponding processes 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.[15]
History
Early analyses of emeralds and beryls yielded always 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. Vauquelin named the new element "glucinum" for the sweet taste of some of its compounds.[22]
Friedrich Wöhler[23] and Antoine Bussy independently isolated beryllium in 1828 by the chemical reaction of metallic potassium with beryllium chloride, as follows:
Applications
It is estimated that most beryllium is used for military applications, so information is not readily available.[27]
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 Be 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.[5]
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.[30][31]
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.
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.[5]
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.[36] They are also found in maintenance and construction materials near magnetic resonance imaging (MRI) machines. In addition to their being very difficult to remove once magnetic tools have become misplaced inside of the MRI machine, the expulsion of any magnetic items as missiles during ordinary operation of the MRI machine is extremely dangerous.[37]
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.[38]
Beryllium is also commonly used as a neutron source in laboratory experiments in which relatively-few neutrons are needed (rather than having to use an entire nuclear reactor). In this, a target of beryllium-9 is bombarded with energetic alpha particles from a radio-isotope. In the nuclear reaction that occurs, beryllium nuclei are transmuted into carbon-12, and one free neutron is emitted, traveling in about the same direction than the alpha particle was heading.
Acoustics
Beryllium's characteristics (low weight and high rigidity) make it useful as a material for high-frequency speaker drivers. Until recently, most beryllium tweeters used an alloy of beryllium and other metals due to beryllium's high cost and difficulty to form. These challenges, coupled with the high performance of beryllium, caused some manufacturers to falsely claim using pure beryllium.[41] Some high-end audio companies manufacture pure beryllium tweeters or speakers using these tweeters. Because beryllium is many times more expensive than titanium, hard to shape due to its brittleness, and toxic if mishandled, these tweeters are limited to high-end home, pro audio, and public address applications.[42][43][44]
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).[45]
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.
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