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An Aqueous Gelcasting Route to Dense -Si4Al2O2N6–0.5SiO2 Ceramics

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Aqueous particulate slurries containing 45–50 vol%α-Si3N4, α-Al2O3, and Y2O3 powder mixtures were gelcast and sintered for 2–4 h at 1675°–1800°C to obtain dense -Si4Al2O2N6–0.5SiO2 ceramics. For comparison, dense ceramics of the same composition, together with a stoichiometric -Si4Al2O2N6, were also prepared following a conventional dry-powder pressing route. The sintered materials were thoroughly characterized for bulk density, apparent porosity, water-absorption capacity, X-ray diffraction phase, microstructure, hardness, fracture toughness, coefficient of thermal expansion (CTE), and dielectric constant at 16–18 GHz frequency. The characterization results suggest that the sintered properties of -Si4Al2O2N6–0.5SiO2 are little influenced by the processing route, and the in situ generated SiO2 was found to reduce the CTE and dielectric constant of -Si4Al2O2N6 considerably. An aqueous particulate slurry containing a 48 vol%-Si4Al2O2N6–0.5SiO2 precursor mixture was successfully gelcast in an indigenously designed and fabricated aluminum mold to fabricate defect-free crucibles of 500 mL volume. The gelcast -Si4Al2O2N6–0.5SiO2 obtained from a slurry containing 48 vol% solids exhibited a bulk density of 3.13 g/cm3, a -SiAlON phase of ∼90%, a CTE of 3.197 × 10−6 °C−1 (between 30° and 1000°C), a fracture toughness of ∼3.42 MPa·m1/2, a three-point bend strength of ∼199 MPa, and a dielectric constant of ∼6.32 at 17 GHz frequency after sintering for 4 h at 1750°C with 7 wt% Y2O3.
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Document Type: Research Article

Affiliations: Centre for Advanced Ceramics, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad-500 005, AP, India

Publication date: May 1, 2008

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