Effects of mannitol on the synthesis of ultra-fine ZrB₂ powders

ZrB₂ particles were synthesized by a sol–gel method using zirconium oxychloride, boric acid, and mannitol as starting materials. Mannitol helps to form the Zr–O–B in the sol–gel system to improve the stability of the precursor. Mannitol acts both as a modifier, as well as carbon source. High purity of ZrB₂ powder can be obtained with a molar ratio of B/Zr = 2.3, C/Zr = 6.6 after carbothermal reduction reaction at a low temperature of 1350 °C for 1 h. The yield rate of the modified ZrB₂ precursor can be increased to 53.4%. Additionally, the ultra fine ZrB₂ powder exhibits rod-like and well-crystallized particles morphology with a particle size of ~200–300 nm. For comparison purpose, sucrose or phenolic resin substituted for mannitol, acting as carbon source were carried out as contrast experiments. It revealed that mannitol helps to form chelate complex with H₃BO₃, which promotes the stability of the precursor, increases the yield of ZrB₂, and reduce the preparation temperature of the ZrB₂ powders.

(a) SEM image of sample 3 heated at 1350 °C for 1 h. (b) XRD patterns of sample 3 heated at different temperatures for 1 h from 1000 to 1450 °C.