Boron is used in many technology areas as a result of certain properties such as its high enthalpy of oxide formation, low molecular weight and good chemical stability. Large amounts of amorphous boron are used as an additive in pyrotechnic mixtures. In chemical synthesis, boron is used as a starting material for producing borides and also as a flux material during soldering.
Since January 2001, a discovery by Prof. Akimitsu (Nature, Vol. 410, No. 6824 (2001), 63-64) has created a furor: Prof. Akimitsu found that the well-known chemical compound magnesium diboride (MgB2) has superconductive properties at temperatures below 40 Kelvin. In contrast to so-called cuprate-based superconductors, magnesium diboride exhibits advantageous properties for use as a superconductor in wires and other applications (such as in sintered bodies). Magnesium diboride is usually produced by the reaction of finely divided boron and magnesium powders with one another.
As a result of the method of wire manufacture (inclusion of the magnesium diboride or of a mixture of elemental boron and magnesium in a metal sheath and subsequent drawing and, if appropriate, subsequent heat treatment to achieve a chemical reaction between boron and magnesium to provide magnesium diboride in the case of a mixture of magnesium and boron being used (in situ process) to obtain a metal wire with a magnesium diboride core), various requirements are placed on the magnesium diboride which have hitherto not been achieved. Besides a high fraction of amorphous boron, a high purity, for example, a low content of oxygen, nitrogen, anionic impurities such as chloride or fluoride, but also customary metallic impurities such as alkali metal ions and alkaline earth metal ions and also other metal ions, are required. A relatively small particle size is likewise required, as is the absence of oversized individual particles since these individual particles can lead to the tearing of the wire upon drawing, and impurities can result in a lower current bearing capacity. Oversized individual particles (“oversize”) also prevent the complete chemical reaction of the boron with magnesium to give magnesium diboride during the processes that form the basis of the wire manufacture. The chemical reactivity of the boron is furthermore reduced by coating the surface with boron oxide and borates, which is reflected in a longer reaction time and the required higher reaction temperatures. This is a disadvantage, particularly in the case of the in situ process for superconductive wire manufacture.