Preparation And Properties Of High Temperature Resistant Al（H₂PO₄）₃ Composite Films

High-temperature absorbing materials have always been rare in the sphere of absorbing materials,and the current research on high-temperature absorbing materials is limited to the lack of high-temperature absorbing substrates.Currently available absorbing substrates are rarely used at high temperatures,and absorbing substrates such as Al₂O₃,which can be used at high temperatures,are difficult to form and to obtain.Phosphate adhesives are widely used in the aerospace industry because of heir low curing temperature,short production cycle,excellent mechanical properties,heat resistance and dielectric properties,high bond strength,high temperature structural stability and environmental friendliness.In this thesis,the properties of aluminum dihydrogen phosphate matrix were studied by using the prepared aluminum dihydrogen phosphate as the adhesive matrix,and the bonding mechanism of the matrix was discussed.When Al（H₂PO₄）₃ is cured,an amorphous glassy mosaic structure is formed which is bonded by a reinforced PO₄^3-tetrahedron and a bridged hydrogen bond.When the Al（H₂PO₄）₃ matrix is heated,an amorphous glassy state and a crystalline hydrate are formed first,and then the crystallization water is removed and the phosphide molecules are dehydrated and condensed to form aluminum metaphosphate.Finally,the aluminum metaphosphate is polymerized into a cyclic metaphosphoric acid and a crystal transformation occurs.The adhesive and composite film were prepared by adding SiO₂ and SiC as curing agents respectively in the matrix,and the effects of the heat treatment temperature of the adhesive and the amount of curing agent on the film were studied.After the addition of the curing agent,no new phase was formed in the system during the heat treatment of the adhesive.The mechanical properties of the two films are better at temperatures below 400℃.At 230℃,the Vickers hardness of the films in both systems is over 200MPa.However,the mechanical properties of the films were unsatisfactory above 600℃.The Vickers hardness of Al（H₂PO₄）₃/SiO₂ film was 11.61MPa at 600℃,and the hardness of Al（H₂PO₄）₃/SiC film was 16.38MPa.It was found that Al（H₂PO₄）₃/SiC film has better mechanical properties when the temperature and curing agent content are the same,probably because the thermal expansion coefficients of SiC and metaphosphate are closer at high temperature.In order to obtain films with a better performance at high temperature,a composite film was prepared by heat-treated Poly hydrogen methyl silicone oil(PHMS₃₅₀),which is the first time to be introduced into a phosphate system to prepare a heat resistant composite material,and the preparation process of the film is much easier compared with the first two.It is found that the hardness of Al（H₂PO₄）₃/PHMS₃₅₀ film is as high as77.33MPa at 800℃,and the mechanical properties are obviously better than the first two films.This is mainly because the Poly hydrogen methyl silicone oil undergoes ceramization at high temperature to form hydrated silica,which serves to strengthen the internal structure of the bonded film and the toughening of the nanoparticles.This paper also discussed the electromagnetic properties of the three films and compares them.It is found that all three films have a certain dielectric loss capability,and do not have magnetic loss capability,and the dielectric loss capability is nearly the same.