Boride
In borides there are chemical compounds which the element boron included and in which the reaction partner a lower generally electronegativity has. Arsenic boride , for example, would be an exception . Counterexamples are boron nitride and boron carbide , which, in purely formal terms, are not counted among the borides.
Most borides are compounds with metals , many of which have ceramic properties and are therefore classified as non-oxide ceramics . For example, the borides of titanium and zirconium have a much higher electrical and thermal conductivity than the corresponding metals . As will be explained later, this phenomenon is explained by its structure.
Manufacturing
There are various ways of producing borides on an industrial scale. The three most common are education by:
- the union of the elements or reaction of the hydride with boron
- the electrolysis of molten salts
- the reduction of the corresponding metal oxides with carbon and boron carbide
Bond conditions in the borides
The illustration of the bonding relationships in borides is a more complex topic. The bond models change with increasing boron content, as briefly named below (from low to high content):
- Borides with isolated boron atoms, so-called low-boron borides (e.g. M 4 B, M 3 B, M 2 B, M 5 B 2 , M 5 B 3 , M 7 B 3 )
- Borides with single and double chains of boron atoms
- Borides with two-dimensional networks (MB 2 , M 2 B 5 )
- Borides with three-dimensional networks (MB 4 , MB 6 , MB 12 )
The third group contains some of the best conductive, hardest and highest melting types such as titanium boride (TiB 2 ). This type is built up by alternating layers of densely packed metal atoms and hexagonal boron networks as shown in the following figure. The good conductivity mentioned above is explained by the layer structure.
use
The use of borides is usually limited to special applications, as there are often cheaper compounds with similar material properties. However, the borides of the lanthanides, which are suitable as excellent electron emitters, are important. Because of their hardness, their chemical and thermal resistance, the borides are materials with the highest demands and are used, for example, in high-temperature furnaces, turbine blades and rocket nozzles. Probably the most important boride is titanium boride (TiB 2 ), which is characterized by its high hardness, the high melting point at over 3200 ° C and its electrical conductivity and is used under appropriate extreme conditions.
In 2001 it was discovered that magnesium diboride MgB 2 is a superconductor with a critical temperature of T C = 39 K. This material has all the prerequisites (good processability, high critical current density, high critical magnetic field) to replace triniob tin Nb 3 Sn, which has been used commercially as a low-temperature superconductor (e.g. in MRT devices) in the next few years. The critical temperature can be increased by a few Kelvin by doping with carbon.
Lanthanum hexaboride (LaB 6 ) has an extremely low electron work function . Lanthanum hexaboride ceramics are used in plasma technology and in some electron microscopes as an electron source .
Examples
Lanthanum hexaboride , magnesium diboride , plutonium boride
literature
- R. Telle: Boride - a new generation of hard materials? , Chemistry in our time, 22nd year 1988, No. 3, pp. 93-99, ISSN 0009-2851
Individual evidence
- ↑ Encyclopedia of Physics: work function
- ↑ sindlhauser.de: PeroLan - Cathodes Lanthanum Hexaboride (LaB6) - Cathodes for plasma technology ( Memento from March 17, 2011 in the Internet Archive )