Synthesis and reaction sintering of YB4 ceramics

Abstract Within this work, the preparation of yttrium tetraboride (YB 4 ) in the form of powder as well as bulk material was investigated. Powders were synthesized via four different reaction methods, including direct synthesis from elemental powders, reduction of yttrium oxide with boron, boron carbide reduction, and combined boron carbide/carbothermal reduction at 1500 °C, 1700 °C and 1900 °C. Pure YB 4 powder was successfully synthesized using the combined boron carbide/carbothermal reduction method. Secondary phases, especially Y 2 O 3 , YB 2 or YBO 3 , were found in powders prepared using the other three methods. Bulk material was prepared using direct synthesis from elements by reactive hot-pressing. Influence of temperature and boron content on densification and phase evolution of samples was studied. In situ reaction sintering was performed using conventional hot-pressing at temperatures from 1100 °C to 1800 °C in vacuum. The amount of boron varied from the stoichiometric content to 5 and 10 wt% excess (with respect to the reaction from elemental powders). Stoichiometric reactions led primarily to the formation of YB 2 and YB 4 and several secondary phases such as Y 2 O 3 , YBO 3 and Y 16.86 B 8 O 38 . YB 4 as a main phase was formed only at elevated temperatures (1700 °C and 1800 °C) but certain content of impurities were still present. Excess of B resulted in the formation of YB 4 as a primary phase in all prepared samples with a small content of YBO 3 and/or Y 16.86 B 8 O 38 . Moreover, SEM analysis revealed the presence of unreacted boron.