Synthesis of boron nitride from ferroboron Part 1 – Nitriding

Abstract

Thermodynamic calculations and nitriding experiments have been carried out to convert ferroboron precursor to boron nitride in a mixture of NH₃ with N₂ gases under ambient pressure. The results show that complete nitridation can be achieved at 673 K with 20 vol.-%NH₃ in the gas mixture. The nitrided product consists of amorphous boron nitride, iron, and metastable iron nitrides. The measured weight gain follows a two stage, near parabolic kinetics accompanied by volume expansion, which causes spalling of the outer nitrided layer and facilitates further nitriding. An alternative reaction route using 100 vol.-%N₂ is also possible, but a higher reaction temperature (≥∼1700 K) is needed. The product from the high temperature route contains partially crystallised (hexagonal) boron nitride, iron, and virtually no metastable iron nitride.