Al5BO9: 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 Al5BO9 were investigated using a combination of first‐principles calculations, Debye model, Clack model, and Slack's equation. The results show that Al5BO9 has a wide band gap of 6.4 eV. The bonding in BO3 triangle and AlO4 tetrahedron is covalent‐ionic, whereas that in AlO6 octahedra and AlO5 bipyramids is ionic‐covalent. The maximum (245 GPa) and minimum Young's modulus (161 GPa) are along  and  direction, respectively. Based on the low shear modulus (78 GPa) and low Pugh's ratio (G/B = 0.497), Al5BO9 is predicted as a damage‐tolerant ceramic. The most likely slip systems are (100) and (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.
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