Removal of carbon impurities from boride powders: ZrB2, NbB2, CrB2, and LaB6

Boride powders of ZrB2, NbB2, CrB2, and LaB6 generally contain 0.1-0.5 wt.% of carbon. To remove the carbon impurities, the borides were mixed with the corresponding metal oxides and boron powders and heated in a vacuum at 1600 oC to eliminate carbon as carbon monoxide gas. That is, ZrB2 (C) + x(ZrO2 + 4B) → (1 + x)ZrB2 + 2xBO↑ (+CO↑) in the case of ZrB2. This process decreased the carbon content by more than one order of magnitude to less than 0.01 wt.% (<100 ppm), which was low enough for use in feed rods for floating-zone crystal growth