Densely packed glass structure caused by seven-coordinated Zr in high elastic modulus Al2O3–SiO2–ZrO2 glasses

Al 2 O 3 –SiO 2 –ZrO 2 ternary glasses were fabricated using a levitation technique. The addition of ZrO 2 to the Al 2 O 3 –SiO 2 glasses strongly affected their thermal, mechanical, and structural properties. Several compositions were partially vitrified at the laser-melted area without levitation although their melting required temperatures higher than 2000°C. With increasing ZrO 2 content, the elastic moduli linearly increased, and the 50Al 2 O 3 –20SiO 2 –30ZrO 2 glass exhibited a Young′s modulus of 166 GPa and Vickers hardness H V of 11 GPa. Conversely, crack resistance significantly decreased with the addition of ZrO 2 . Density measurements, Zr L 2,3 -edge and K -edge X-ray absorption fine structure analyses, and 27 Al and 29 Si magic-angle spinning nuclear magnetic resonance spectroscopy were performed to investigate the local structure around Zr, Al, and Si in the glasses. Zr formed distorted ZrO 7 as in monoclinic ZrO 2 , which has been rarely found in conventional oxide glasses. The highly oxygen-coordinated Al atoms such as AlO 5 and AlO 6 , were the main components in the glasses rather than AlO 4 . The majority of Si atoms form SiO 4 with four bridging oxygen (Q 4 ). Among the four bridging oxygens, the number of oxygens connected to Al or Zr clearly increased with decreasing SiO 2 content. The high packing density of the ternary glasses that resulted in high elastic moduli originated from the highly oxygen-coordinated Zr and Al and their close bonding with SiO 4 without generating nonbridging oxygens.