Formation of TiB2–Al2O3 and NbB2–Al2O3 composites by combustion synthesis involving thermite reactions

Abstract

Preparation of TiB 2 –Al 2 O 3 and NbB 2 –Al 2 O 3 in situ composites with a broad range of phase composition was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions of different types. Thermite mixtures of Al–TiO 2 and Al–TiO 2 –B 2 O 3 were incorporated with the Ti–B combustion system to produce the composites of TiB 2 –Al 2 O 3 , within which the increase of the thermite mixture for a higher content of Al 2 O 3 decreased the reaction temperature and combustion wave velocity. This implies that the thermite reaction of Al with TiO 2 reduces the exothermicity of the overall SHS process. In the synthesis of NbB 2 –Al 2 O 3 composite, two thermite mixtures of Al–Nb 2 O 5 and Al–Nb 2 O 5 –B 2 O 3 were added to the Nb–B combustion system and both of which were found to increase the combustion temperature and propagation rate of the flame front. This is due to the highly exothermic nature of the thermite reaction between Al and Nb 2 O 5 . For both kinds of composites, it was found that adoption of B 2 O 3 as one of the thermite reagents improved the product formation effectively. The XRD analysis shows that the final products composed of no more than TiB 2 and Al 2 O 3 are obtained from the powder compacts containing the thermite mixture of Al–TiO 2 –B 2 O 3 . On formation of the NbB 2 –Al 2 O 3 composite, NbB 2 is identified as the major boride phase in the products involving the thermite reactions of Al–Nb 2 O 5 –B 2 O 3 , while Nb 3 B 4 dominates in the case of using Al and Nb 2 O 5 as the thermite reagents.