Boron is a metalloid of no useful mechanical properties but used as an alloying element in steel. It is the second hardest element after diamond and is an essential plant nutrient. Relatively large quantities of amorphous boron are used as additives in pyrotechnic mixtures, solid rocket propellant fuels, and explosives. High-purity boron (>99.99%) is used in electronics. It is used as a ppm additive for germanium and silicon to make p-type semiconductors. Crystalline high-purity boron is used in thermistors. Boron filaments have been developed as reinforcing material for light-weight, stiff composites for use in commercial and military aircraft recently replaced by graphite filaments. In nuclear technology thin films of boron are used in neutron counters. Boron powder dispersed in polyethylene castings is used for shielding against thermal neutrons. The isotope boron-10 has a large neutron absorption cross section and is used as a control for nuclear reactors, as a shield for nuclear.
