Boride

At borides are chemical compounds that contain the element boron, and in which the reactants usually has a lower electronegativity. An exception would, for example, represent the Arsenborid. Counter-examples are boron nitride and boron carbide, which are purely formal not counted among the borides.

Most borides are compounds of metals, many of which have ceramic properties and thus be counted as non-oxide ceramics. For example, the borides of titanium and zirconium in relation to the corresponding metals have a much higher electrical and thermal conductivity. This phenomenon can be explained, as explained later in more detail, by their structure.

Production

There are various ways to produce borides in an industrial scale. The three most common are the formation 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

Bonding in borides

The illustration of the bonding in borides is a more complex issue. The binding models are changing with increasing boron content as briefly below named ( from low to high level ):

The third group contains some of the best current-conducting, hardest and highest-melting types such as titanium boride ( TiB2 ). Built this type is represented by alternating layers of densely packed - metal atoms and hexagonal Bornetzwerke as in the figure below. Due to the layer structure of the above-mentioned good conductivities explain.

Use

The use of the borides is limited usually to special applications, since there is often less expensive than compounds with similar material properties. Significantly, however, the borides of the lanthanides, which are suitable as excellent electron emitter. Because of their hardness, chemical and thermal resistance are the borides materials with the highest standards and are used for example in high temperature furnaces, turbine blades and rocket nozzles. The most significant boride is titanium boride ( TiB2 ), which is characterized by its high hardness, high melting point of about 3200 ° C and its electrical conductivity and is used under appropriate extreme conditions.

In 2001 it was discovered that magnesium diboride MgB2 a superconductor with a critical temperature of TC = 39 K is. This material possesses all the prerequisites ( good processability, high critical current density, high critical magnetic field ) used to fill the existing commercially available as low-temperature superconductors (eg in MRI machines ) Triniobzinn Nb3Sn to replace in the next few years. By doping with carbon, the critical temperature can be increased by a few degrees Kelvin.

Lanthanum hexaboride ( LaB6 ) has an extremely low electron work function. Ceramics of lanthanum found in the plasma technology and in some electron as an electron source use.

Examples

Lanthanum, magnesium diboride, Plutoniumboride