Low temperature synthesis of highly pure cordierite materials by spark plasma sintering nano-oxide powders

The development of cordierite ceramics, using traditional materials and conventional methods, remains a key challenge because of the high and narrow sintering temperature range. In this work, single-phase cordierite ceramics were produced by spark plasma sintering nano-oxide powders, at low temperatures. A starting mixture of Al2O3, SiO2, and MgO nano-powders with the composition of stoichiometric cordierite was first sonicated, then, sintered in the temperature range 900-1200 degrees C. The raw powders, sonicated powder mixture, phase transformations, and fracture surface of sintered specimens were characterized by using an x-ray diffractometer and a field emission scanning electron microscope (FE-SEM). The hardness and fracture toughness were measured using the indentation technique. The influence of testing conditions, process parameters, characterization technique, and calculation method on hardness and fracture toughness was investigated. The FE-SEM images, xray maps, and EDS results revealed the homogeneity and stoichiometry of the sonicated powder and sintered samples. Highly pure alpha-cordierite was formed at 1150 degrees C. Samples that were sintered at 1150 and 1200 degrees C had bulk density of 2.58 g/cm(3) (relative density of 100%), and maintained low average grain sizes of 28.68 and 34.95 nm, respectively. With the decrease in the temperature from 1200 to 1150 degrees C, the hardness of cordierite was slightly increased from 8.25 +/- 0.158 to 8.46 +/- 0.188 GPa, the fracture toughness was marginally im proved from 2.2 +/- 0.158 to 2.25 +/- 0.238 MPa mm(1/2), and the critical strain energy release rate was raised from 32.46 to 33.95 J/m(2).