6533b835fe1ef96bd129e910

RESEARCH PRODUCT

Reaction Sequence in the Preparation of NiAl2O4 Spinel-Mullite Composites by Sol-Gel

subject

description

NiAl 2 O 4 spinel-mullite composites were prepared by simultaneous replacement of Al by Ti and/or Ni in 3:2 stoichiometric mullite. Specimens having nominal compositions 3(Al 2-2x Ni x Ti x O 3 ).2SiO 2 (x = 0, 0.025, 0.05, 0.2) and 3(Al 2-x M x O 3) .2SiO 2 (M = Ni 2+ or Ti 4+ and x = 0.05) were synthesized by sol-gel techniques, which provide homogeneous gels in the SiO 2 -Al 2 O 3 system. Gel structures investigated by infrared (IR) spectroscopy revealed the formation of Al-O-Si bonds in dried gels. The reaction sequence of gel-derived glasses, previously obtained by preheating gels at 750°C for 3 h, was evaluated by X-ray powder diffraction (XRD) and ultraviolet-visible (UV-vis) spectroscopy. All samples crystallized at around 1000°C from an amorphous state, but unexpectedly the first crystalline phase was Al-Si spinel in all aluminum-substituted specimens ; i.e., a change in the sequence of reaction with respect to the 3:2 stoichiometric mullite was produced. NiAl 2 O 4 spinel was almost simultaneously detected. Two processes of mullite crystallization were observed. The temperature of formation of mullite was the lowest for the higher substituted sample. The microstructure of the final NiAl 2 O 4 spinel-mullite composites found in all Ni-containing samples after annealing for 96 h was examined by scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX), which revealed the presence of small particles of NiAl 2 O 4 spinel dispersed in a mullite matrix. For annealed compositions with the larger Al replacement, i.e., when x = 0.2, a small amount of very small Al 2 TiO 5 particles was also detected by XRD and transmission electron microscopy (TEM).