Effect Of Stacking Sequence On Thermal And Mechanical Properties Of Al-B-O Ceramics

Boron mullite ceramics(Al₅BO₉,Al₁₈B₄O₃₃,Al₄B₂O₉)and compounds without mullite crystal structure(Al₄B₆O₁₅ and Al BO₃)in Al-B-O system generally have superior properties such as low density,wide band gap and high melting point,which are promising for the applications including thermal protectioin,wave transparent and thermal sealing in hypersonic vehicles.However,the current research is mainly concentrated on the theoretical calculation of Al₅BO₉ and synthesis methods of Al₁₈B₄O₃₃ and Al₄B₂O₉,lacking systematic theoretical research and exploration of mechanical and thermal properties.Based on the above research background,the main works of this paper are as follows:?1?Theoretical study of Al-B-O system materials.The structure,electrical structure,mechanical and thermal properties in Al-B-O system are systematically investigated using first principle calculations.The effects of stacking structure and mechanical properties on thermal conductivity are also discussed.The results show that Al₅BO₉ is a light-weight wave transparent material with wide band gap,low thermal conductivity and good damage resistance.The minimum thermal conductivity is estimated to be 1.42 W/?m·K?.Al BO₃ has a structure similar to Al₂O₃and a thermal conductivity of 25.3 W/?m·K?at room temperature?RT?.Al₃BO₆has mechanical properties between Al₅BO₉ and Al BO₃,while Al₄B₆O₁₅ has a primary octahedral backbone topology with a wide band gap of 7.6 e V.The minimum thermal conductivity of Al₄B₆O₁₅is estimated to be 1.71 W/?m·K?.?2?Synthesis and characterization of Al₅BO₉powders.Al₂O₃ and B₂O₃ are used as raw materials.A solid-phase reaction method is used to calcine the raw material mixture with a molar ratio of 5:1.06 under atmospheric pressure and air atmosphere at 1200°C for 2 h to obtain phase pure Al₅BO₉ powders.As reaction temperature increases,the product changes from low temperature phase Al₄B₂O₉ to high temperature phase Al₅BO₉.The crystal grains of Al₅BO₉ are rod-shaped with uniform size and intact shape.?3?Mechanical and thermal properties of bulk Al₅BO₉prepared by spark plasma sintering of Al₅BO₉powders.Al₅BO₉has bulk?B?,shear?G?and Young's?E?moduli consistent with theoretical values.The flexural strength,compressive strength and fracture toughness of Al₅BO₉are close to those of mullite.The low Pugh's ratio?G/B=0.574?indicates its intrinsic damage tolerance.Research on thermal properties thermal conductivity and low specific thermal conductivity(14.48×10⁶ W·g·m^-4·K^-1).The low thermal conductivity and low density endow Al₅BO₉a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles.?4?Preparation and characterization of porous Al₅BO₉ceramics.Porous Al₅BO₉ceramic is successfully prepared without addition of pore-forming agent by partial sintering.The porosity and density of porous Al₅BO₉can be controlled by adjusting the green density.Further investigations reveal that porous Al₅BO₉has a uniform pore structure with narrow pore size distribution.Moreover,compressive strength,thermal diffusivity,thermal conductivity and dielectric constant of porous Al₅BO₉ are studied.The results show that porous Al₅BO₉ has high compressive strength?211±18 MPa?,low thermal conductivity,and good thermal stability.Porous Al₅BO₉ has a low significantly with temperature and frequency.High compressive strength,low thermal conductivity,and good dielectric properties suggest porous Al₅BO₉ ceramic as a candidate material for high temperature wave transmission.