Mechanical and thermal properties of light weight boron‐mullite Al₅BO₉

Abstract

Al5BO9 is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al5BO9 have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al5BO9 prepared by spark plasma sintering of solid‐state reaction synthesized Al5BO9 powders. The bulk (B), shear (G), and Young's (E) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness (Hv) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m1/2, which are close to those of mullite. Al5BO9 has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie αa = (4.40 ± 0.21) × 10−6 K−1, αb = (7.11 ± 0.18) × 10−6 K−1, αc = (6.70 ± 0.29) × 10−6 K−1 from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower αa is resulted from the edge‐shared AlO6 octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10−6 K−1. The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al5BO9 a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles.

Preparation and properties of porous Al5BO9 for high‐temperature wave‐transparent and thermal insulating applications

“…27 Experimentally, phase pure Al 5 BO 9 powders were synthesized by solid-state reaction, 30,31 molten salt, 24 sol-gel methods, 32 and bulk Al 5 BO 9 was successfully sintered through spark plasma sintering (SPS). 26 Al 5 BO 9 has been proven to have comparable flexural strength (277 ± 35 MPa), compressive strength (814 ± 75 MPa), and fracture toughness (2.4 ± .3 MPa) to mullite. In addition, low Pugh's ratio G/B (.574) indicates its intrinsic damage tolerance.…”

“…15,22 Boron-mullite Al 5 BO 9 possesses a unique combination of wide band gap, low density, low thermal conductivity and high strength, which is prospective as ceramic wafer seal structure and porous high-temperature wavetransparent material of hypersonic vehicles. [23][24][25][26][27][28][29] Previous work of Zhou and Xiang 25 predicted that Al 5 BO 9 had a low theoretical density of 2.94 g/cm 3 , and its theoretical thermal conductivity declined with temperature following κ = 2404.78/T. The absorption spectrum study showed that Al 5 BO 9 could transmit electromagnetic waves, making it potential as a wave-transparent material.…”

“…Besides, Al 5 BO 9 has the lowest specific thermal conductivity S (14.48 × 10 6 W g m −4 K −1 ) among the current candidate materials for ceramic wafer structure applications. 26 Although porous Al 5 BO 9 is a promising lightweight insulating and electromagnetic wave-transmitting material with potential applications in aerospace industry, the preparation method for porous Al 5 BO 9 is not readily available yet, which hinders its widespread applications. Previous works have demonstrated that porous oxide and nonoxide ceramics can be manufactured by the in situ reaction and partial sintering method with controllable density and porosity, which is a simple and pore-forming agentfree method.…”

“…The absorption spectrum study showed that Al 5 BO 9 could transmit electromagnetic waves, making it potential as a wave‐transparent material 27 . Experimentally, phase pure Al 5 BO 9 powders were synthesized by solid‐state reaction, 30,31 molten salt, 24 sol–gel methods, 32 and bulk Al 5 BO 9 was successfully sintered through spark plasma sintering (SPS) 26 . Al 5 BO 9 has been proven to have comparable flexural strength (277 ± 35 MPa), compressive strength (814 ± 75 MPa), and fracture toughness (2.4 ± .3 MPa) to mullite.…”

“…The average linear coefficient of thermal expansion (CTE) of Al 5 BO 9 is (6.05 ± .06) × 10 −6 K −1 , and the experimental thermal conductivity declines with temperature following κ = 1336.39/ T + 1.97. Besides, Al 5 BO 9 has the lowest specific thermal conductivity S (14.48 × 10 6 W g m −4 K −1 ) among the current candidate materials for ceramic wafer structure applications 26 …”