Optimization of the phase composition and structure of LaB6 modified ZrB2-SiC multiphase powders synthesized by sol-gel and boro/carbothermal reduction method

Abstract

In this study, ultrafine ZrB₂-SiC-LaB₆ multiphase powders were synthesized using a combination of sol-gel and boro/carbothermal reduction methods. The microstructure of multiphase powders was investigated for the effects of heat-treatment temperature, water and carbon source content. The results showed that the optimum conditions for the preparation of uniformly dispersed ZrB₂-SiC-LaB₆ multiphase powders with a diameter of about 300 nm were as follows: the heat-treatment temperature was 1500 °C, $n_{H_{2}O}$: n_Glu= 30:1, and n_Glu: (n_Zr+n_Si+n_B+n_La+n_Glu)= 0.24:1. The multiphase grains consisted of eutectic structures formed between the ZrB₂ phase and either the LaB₆ phase or the LaBO₃ phase. Further, the addition of 2.5 wt% graphite led to a large number of rod-like grains in eutectic structures, which was attributed to the high concentration of C or CO atmosphere coupled with the direction of the stable hot gas flow. The formation of multiphase grains is due to the presence of unstable hexagonal crystal system La₂O₃ in the liquid phase, which forms a eutectic structure with ZrB₂ grains of the same crystal system and reacts with B₂O₃ to form the more stable LaBO₃ or LaB₆ phase. This study provides a theoretical foundation and experimental basis for the application of ZrB₂-SiC-LaB₆ multiphase powders.