Reactions and Microstructure Development in Mullite Fibers

Microstructural and compositional changes during heat treatment of sol–gel‐derived mullite fibers with additions of 2 wt% B 2 O 3 , 2 wt% P 2 O 5 , 2 wt% Cr 2 O 3 , and (1 wt% P 2 O 5 + 1 wt% Cr 2 O 3 ) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†‐Al 2 O 3 or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B 2 O 3 and P 2 O 5 increased the temperature of spinel formation and that B 2 O 3 significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B 2 O 3 was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B 2 O 3 had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B 2 O 3 was the most volatile additive.