Fabrication of transparent mullite ceramic by spark plasma sintering from powders synthesized via sol–gel process combined with pulse current heating

• Nanocrystal mullite powders are synthesized by pulse current heating.
• Dense and fine grained mullite ceramics are fabricated at low temperature.
• Infrared transparency and mechanical properties of mullite ceramics are investigated.

Mullite nanopowders were synthesized by combining the advantages of the sol–gel process with the rapid synthesis provided by pulse current heating. The mullite ceramic with an infrared transmittance of 83–88% in the wavelength range from 2.5 to 4 μm with a fine grain size of 200 nm was obtained by spark plasma sintering at 1350 °C. Due to the high relative density and the small grain size, the hardness and toughness values of the sample reached 17.82 GPa and 3.6 MPa m1/2, respectively. In contrast, when the mullite powders synthesized in a muffle furnace, an intermediate phase occurred so that the powder synthesis required high crystallization temperatures and resulted in agglomerated particles. Thus, the mullite ceramics required high temperatures for densification. As a result, the optical and mechanical properties of the ceramics were poor due to the low relative density and the elongated grain growth.

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