Influence of processing route and SiO₂ on sintering ability, CTE, and dielectric constant of β-Si₄Al₂O₂N₆

Abstract

Dense β-Si₄Al₂O₂N₆ and β-Si₄Al₂O₂N₆-0.5SiO₂ ceramics were obtained from α-Si₃N₄, α-Al₂O₃, AlN, and Y₂O₃ upon sintering green bodies consolidated by aqueous gel casting. For comparison purposes, a β-Si₄Al₂O₂N₆ was also prepared by the conventional dry-powder processing route. In the case of gel-cast β-Si₄Al₂O₂N₆, the as-purchased AlN powder was treated with H₃PO₄ and Al(H₂PO₄)₃ prior to use along with α-Si₃N₄, α-Al₂O₃, and Y₂O₃. The gel-cast β-Si₄Al₂O₂N₆ exhibited superior hardness (1423 ± 6 Hv), fracture toughness (3.95 ± 0.3 MPa·m^1/2), and coefficient of thermal expansion (CTE) (3.798 × 10⁻⁶/°C between 30 and 1000 °C) in comparison to the ceramic consolidated by conventional dry pressing, which exhibited only 1317 ± 5 Hv, 3.30 ± 0.2 MPa·m^1/2, and 3.532 × 10⁻⁶/°C between 30 and 700 °C. The in situ-generated ~9 wt% SiO₂ has considerably reduced the dielectric constant and CTE of β-Si₄Al₂O₂N₆ from 7.30 to 6.32 and from 3.798 × 10⁻⁶/°C to 3.519 × 10⁻⁶/°C, respectively. The loss tangent property of the investigated materials was little influenced by the variation of chemical composition and processing route.