Negative Thermal Expansion Occurs in a Bauxite-Based Aluminum Borate Ceramic Whisker with a Nanofiber Microstructure

Whisker-like microstructured ceramic was synthesized using bauxite [Al(OH)₃] raw material and boric acid (H₃BO₃). Stoichiometric (Al₁₈B₄O₃₃) and non-stoichiometric compositions were prepared through a ceramic route or a solid reaction method, respectively. The suggested compositional design resulted in two crystalline phases: aluminum borate (Al₂₀B₄O₃₆) and α-corundum (Al₂O₃). An increase in the B₂O₃/Al₂O₃ ratio led to the formation of the aluminum borate phase rather than the α-corundum phase; however, in general, an increase in the ratio corresponds to an increase in the α-corundum phase. The studied microstructure was characterized as interlocking wire-like and fiber-like crystals that form whisker-like textures. The densities of the sintered samples were between 3.2340 and 2.7474 g/cc, and in general, density decreased as the aluminum borate phase increased. The coefficient of thermal expansion ranged from − 3.73 to 3.79 × 10⁻⁷/°C (20 °C to 500 °C), which demonstrates the outstanding physical properties of the ceramic.