Al 5 BO 9 : A Wide Band Gap, Damage‐Tolerant, and Thermal Insulating Lightweight Material for High‐Temperature Applications

The electronic structure, chemical bonding, mechanical, and thermal properties of a B‐containing mullite Al 5 BO 9 were investigated using a combination of first‐principles calculations, Debye model, Clack model, and Slack's equation. The results show that Al 5 BO 9 has a wide band gap of 6.4 eV. The bonding in BO 3 triangle and AlO 4 tetrahedron is covalent‐ionic, whereas that in AlO 6 octahedra and AlO 5 bipyramids is ionic‐covalent. The maximum (245 GPa) and minimum Young's modulus (161 GPa) are along [100] and [010] direction, respectively. Based on the low shear modulus (78 GPa) and low Pugh's ratio ( G/B = 0.497), Al 5 BO 9 is predicted as a damage‐tolerant ceramic. The most likely slip systems are [010](100) and [100](010). The high Debye temperature (956 K) indicates good high‐temperature stability. The minimum thermal conductivity is estimated to be 1.59 W·(m·K) −1 , and the thermal conductivity decreases with temperature as 2071.9/T.