Electrodeposition of zirconium diboride from oxides dissolved in molten cryolite

Abstract

Zirconium diboride has been deposited from B₂O₃ and ZrO₂ dissolved in molten Na₃AlF₆ using nickel as the cathode material. The mechanism of the deposition and the morphology of the deposit were shown to be strongly dependent upon melt composition and current density. A series of nickel boride diffusion compounds was also identified by electron microprobe analysis. The compounds Ni₃B, Ni₂B, Ni₄B₃ and NiB were formed quite readily at lower current densities. The ZrB₂ deposit was mainly scaly and/or dendritic and was usually non-adherent. Three competing reaction sequences were found to be in operation:

• 1.

  (i) boron deposition and nickel boride formation by diffusion,

• 2.

  (ii) boron deposition and subsequent chemical reaction with ZrO₂ to form ZrB₂, and

• 3.

  (iii) codeposition of boron and zirconium to form ZrB₂.