Purity of Boron Produced by the Decomposition of Diborane and Subsequent Zone-Melting

Abstract

Boron is prepared from diborane by the van Arkel process under special safety precautions and then purified by zone-refining. No impurities can be detected spectrochemically after the zone-refining. Nevertheless, the purity is controlled with a 2-m grating spectrograph using a DC arc in a nitrogen or argon atmosphere for excitation. Spectrochemical limits of detection are 1 ppm for silicon, and 0. 1–0. 5 ppm for copper, magnesium, calcium, and silver, for example. Neutron activation analysis is also discussed. By this method, the concentrations of tantalum, copper, and arsenic in the zone-refined boron were found to be > 0. 3, > 10–^2–10–³, and > 10–⁴ppm, respectively. No other elements could be identified after activation times up to 120 hr. The main impurity in our boron is carbon (30–50 ppm). Because of the unfavorable distribution coefficient for carbon in boron (approx. 4), removal by zone-refining is very time-consuming. A coulometric, method to determine carbon in boron is discussed.