The Bottom Up Approach is Not Always the Best Processing Method: Dense α-Al2O3/NiAl2O4 Composites



The bottom up approach suggests that atomic scale mixing should permit optimal control of processing for many types of materials in terms of densification rates, final and average grain sizes; and, thereafter, global properties thereby minimizing processing conditions, capital equipment requirements, and energy consumption. The literature indicates that, to date, this axiom has not been tested although numerous researchers have whole-heartedly adopted the concept. Liquid-feed flame spray pyrolysis (LF-FSP) provides atomically mixed NiO·3Al2O3 nanopowders(30–40 nm average particle size, APS) that are a single phase, spinel solid solution. Sintering 56 ± 1 wt% CIPped pellets to 95 ± 1% theoretical density using two different heating schedules produces α-Al2O3/NiAl2O4 composites with grain sizes of 0.9 ± 0.2 µm. For comparative purposes, ball milled Al2O3 are synthesized from both highly atomically-mixed single phase metastable spinel nanopowders and NiAl2O4 30–40 nm APS pellets of the same composition with a submicro­meter length scale mixing and similar green densities are also sintered to 95 ± 2% TD. In both instances, the same microstructures are realized despite the great difference in length scale of mixing. This contrasts greatly with the expectation that the atomically mixed materials would give finer grain sizes at the same densities and with faster sintering times, suggesting that the bottom up approach is not always valid.