Why is boron a good refractory material


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boron is a chemical element in the periodic table of the elements with the symbol B and the atomic number 5. The trivalent, rare semimetal occurs enriched in some minable deposits.

Boron exists in several modifications. Amorphous boron is a brown powder. Several allotropic modifications of crystalline boron are known.


Boron compounds (from Persian buraH about arabic Buraq and (Greek βοραχου or lat. borax "Borsaures Natron", "Borax") have been known for thousands of years. In ancient Egypt, the mineral sodium bicarbonate was used for mummification, which, in addition to other compounds, also contains borates. Borax glass has been used in the Chinese Empire since the 4th century. Boron compounds were used to make glass in ancient Rome.

It was not until 1808 that Joseph Louis Gay-Lussac and Jacques Thenard produced boron by means of reduction with potassium and, at the same time, independently of this, later Sir Humphry Davy by electrolysis of boric acid. In 1824 Jöns Jacob Berzelius recognized the elementary character of the material. The American chemist W. Weintraub succeeded in preparing pure crystallized boron in 1909.

Name, symbol, atomic numberBoron, B, 5
Group, period, block13, 2, p
CAS number7440-42-8
Mass fraction of the earth's envelope16 ppm
Physical statefirmly
Crystal structurerhombohedral
density2.460 g / cm3
Mohs hardness9,3
Melting point2349 K (2076 ° C)
boiling point4200 K (3927 ° C)
Molar volume4,39 · 10-6 m3/ mol
Heat of evaporation507 kJ / mol
Heat of fusion50 kJ / mol
Speed ​​of sound16,200 m / s at 293.15 K.
Specific heat capacity1260 J / (kg K)
Electric conductivity1,0 · 10-4 A / (V m)
Thermal conductivity27 W / (m K)
Oxidation states3
Oxides (basicity)B.2O3 (slightly angry)
Electronegativity2.04 (Pauling scale)


Boron occurs naturally only in oxygen-containing compounds. Large deposits are located in Turkey, the USA (Mojave Desert) and Argentina. Staßfurt potash salts contain small amounts of associated boracite.

The largest borate mines are located in Turkey (approx. 72% of the world’s occurrence) in the vicinity of Boron (Kramer deposit) in California and Kırka. The minerals borax, kernite and colemanite are mined.

Extraction and presentation

Amorphous boron is produced by the reduction of boron trioxide, B2O3, made with magnesium powder.

Crystalline boron is obtained through

  • Heating amorphous boron to over 1400 ° C
  • Reduction of boron trichloride by hydrogen on a hot tungsten wire
  • thermal decomposition of the hydride diborane
  • Melt-flow electrolysis of boric acid


The most thermodynamically stable form is the β-rhombohedral modification. It has a complicated structure with 105 boron atoms per unit cell. The simplest allotropic modification is the α-rhombohedral form of boron. It contains 12 boron atoms per unit cell.

α-tetragonal boron, the crystalline form of boron shown first, contains 50 boron atoms in the unit cell (according to the formula (B.12)4B.2), but can also be used, for example, as inclusion compound B, depending on the production conditions50C.2 or B.50N2 are present. In α-tetragonal boron free from foreign atoms, a single boron atom always connects four B.12-Icosahedron with each other. Each icosahedron has connections to two individual boron atoms and ten other icosahedra.

The dominant structural unit in the modifications of boron is the B.12-Icosahedron. The metallic boron is black, very hard and a poor conductor at room temperature. It does not occur in nature.

Researchers at the ETH in Zurich produced an ionic crystal from extremely pure boron. To do this, the material had to be exposed to a pressure of up to 30 gigapascals and a temperature of 1500 degrees Celsius.


Because of the high ionization energy, boron does not contain any B.3+-Cations known. The complicated structures in many boron compounds and their properties show that the description of the bonding relationships as covalent, metallic or ionic is very simplistic and must be replaced by a molecular orbital (MO) approach.

Boron is permeable to infrared light. At room temperature it shows a low electrical conductivity, which increases sharply at higher temperatures.

Boron has the highest tensile strength of all known elements and the second highest hardness, only surpassed by the carbon modification diamond. Boron modifications are physically and chemically similar to hard ceramics such as silicon carbide or tungsten carbide.

The ability of boron to form stable spatial networks via covalent bonds is a further indication of the chemical similarity of boron to its period neighbors carbon and silicon.

Boron is inert up to 400 ° C, at higher temperatures it becomes a strong reducing agent. At temperatures above 700 ° C it burns in air to form boron trioxide B.2O3. Boron is not attacked by boiling hydrochloric or hydrofluoric acid. Even oxidizing, hot, concentrated sulfuric acid only attacks boron at temperatures above 200 ° C, hot concentrated phosphoric acid only at temperatures above 600 ° C.

If you solve B2O3 in water, the very weak boric acid is formed. Their volatile esters, most clearly trimethyl borate, give flames a strong green color.

An important research discipline of today's inorganic chemistry is that of the compounds of boron with hydrogen (boranes), as well as with hydrogen and nitrogen, which are surprisingly similar to hydrocarbons (isoelectronic), e.g. borazole B.3N3H6 ("Inorganic Benzene"). A number of organic boron compounds are also known, for example boronic acids.


The economically most important compound is borax (sodium tetraborate decahydrate, Na2B.4O7 · 10 H.2O) for the production of insulating materials and bleaching materials (perborates). Other applications: (selection)

Elemental boron

  • Additive for rocket fuels
  • Boron nitrate mixtures as detonators for airbags
  • Crystalline boron and boron fibers for applications with extremely high strength and rigidity: components for helicopter rotors, tennis racks, golf clubs and fishing rods. Because of the low radar echo also in the stealth fighter-bomber F-117 and Northrop B-2
  • Fireworks and light ammunition (because of the intense green flame)
  • p-doping in silicon
  • thermochemical surface hardening, boriding
  • Nuclear applications of 10B: Control rods in nuclear reactors, detection of neutrons, neutron shielding (radiation protective clothing and walls; in steels for storage vessels for nuclear fuels) via the nuclear reaction10B (n, α)7Li due to the very high cross-section for neutrons with a gamma-ray-free reaction product. In addition, a certain, variable amount of boric acid is added to the cooling water of pressurized water reactors. This is used to control the chain reaction.
  • Neutron source in reactors 11B (α, n)14N

Boron compounds

  • Boron-silicate fibers for thermal insulation
  • optical fiber
  • Manufacture of refractory borosilicate glasses (Pyrex, Duran)
  • Neodymium-iron-boron compounds for the production of strongest magnets. They are used for magnetic resonance tomographs, micromotors and hard drives (positioning of the read / write heads), permanent magnet rotors (e.g. stepper and servo motors), linear motors for positioning axes.
  • Brake and clutch linings
  • Armor, bulletproof vests
  • Disinfecting aviation fuel
  • Rhenium diboride (ReB2) is a solid harder than diamond


Boron is possibly an essential trace element that has an influence on bone metabolism and brain function, among other things. Doses over 100 mg / day can cause symptoms of intoxication. Plants are sometimes very sensitive to boron, so that certain sensitive plants (willows, fruit trees, artichokes) have a tendency to boron chlorosis at concentrations of more than 1 mg / l boron (clinical picture characterized by the increased formation of brown spots) and can eventually die off.

safety instructions

Elemental boron in small doses is not toxic. However, some boron compounds such as boranes (boron hydrogen compounds) are highly toxic and must be handled with great care. Boron trioxide, boric acid and borates were classified as toxic to reproduction with the 30th ATP in the EU from summer 2009.


In analytical chemistry, boron can be quantitatively detected using the curcumin method in the form of the red-colored complex rosocyanine. For this purpose, a sample of the boron-containing material is digested by oxidation. The boric acid formed by the digestion can then be determined colorimetrically.

Based on an article in Wikipedia.de

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Date of the last change: Jena, the: 21.11. 2018