Preparation And Properties Of Porous Boron Nitride And Antioxidation Coatings On Carbon Fibers Surface

Carbon fibers,new fibers materials with high strengh,high modulus and low density,possess excellent physical and chemical properties.Therefore,carbon fiber reinforced composites have been widely used in aerospace industry,various engineering applications,leisure,entertainment and other fields.But,the oxidation resistance property of carbon fibers is relatively poor.The weight loss will occur above 450 ? in the air and as the temperature rises,the weight loss rate will increase obviously,which can lead to mechanical properties of carbon fibers reinforced composites sharp decline.Therefore,the improvement of oxidation resistance at high temperature is very important for carbon fibers application.Recently,preparation coatings on carbon fibers surface to improve oxidation resistance has attracted scholars' extensive attention at home and abroad.Boron nitride(BN)possesses many excellent properties,such as excellent antioxidant property,good thermal conductivity and so on.Meanwhile,the coefficient of thermal expansion is similar to that of carbon fibers.Accordingly,BN has become the preferred coatings material to improve the oxidation resistance of carbon fibers.In addition,the porous BN materials have large specific surface area and strong adsorption ability,which can be used as catalyst carrier,hydrogen storage as well as gas adsorption and separation.In this paper,the BN coatings are prepared on carbon fibers surface by precursor pyrolysis.Firstly,the effects caused by the precursor ratio and decomposition temperature on the BN samples structure are investigated.It has been found that the samples have the best crystal structure when the precursor decomposes at the ratio of 1:3.The crystalline structure of the samples varies significantly with temperature.The samples obtained at lower temperature is mainly amorphous and with temperature rising,the crystalline structure is gradually changed from amorphous structure to the two-dimensional ordered t-BN sructure,and then to the three-dimensional ordered h-BN structure.The formation mechanism that the precursor decomposes to form BN is also explored.The t-BN coatings are prepared on carbon fibers surface by the dipping method and the influence of dipping times on the coatings quality is studied.It has been found that the continuous and uniform coatings with the thickness about 100nm are formed on the surface after three dipping times and combine well with the substrate.The initial oxidation temperature of the coated carbon fibers is raised about 80 0C.Furthermore,the h-BN coatings are prepared on the carbon fibers surface by the precursor powder immersion reaction method.The coatings with 500nm thickness are uniform and bonded closely with the substrate,which enhances the initial oxidation temperature more than 150 C.The preparation of BN coatings cause less damage for mechanical properties of carbon fibers.Meanwhile,the resistivity is significantly increased,which is beneficial to improve its electrical insulation.The C-doped SiO2 antioxidation coatings are prepared by the sol-gel method to improve the oxidation resistance of carbon fibers.The coatings structure varying with temperature and the covered area on carbon fibers surface changing with the immersion times are systematically analysed.It has been found that the C-doped SiO2 coatings obtained by heat treatment at 900 C have good crystallinity.The coatings formed after three times immersion in sol are compact and uniform,and completely coate with carbon fibers substrate.There are a few residual particles and no cracks appears on the surface.The thickness of coatings is about 200nm,which the initial oxidation temperature raises about 110?.The oxidation resistance of carbon fibers improves significantly.But,the mechanical properties of the coated carbon fibers are damaged greatly and only 80%of the initial tensile strength is retained.The porous BN samples are prepared by precursor pyrolysis at high temperature.Firstly,it is proved that the prepared samples are BN material by using the multiple test and characterization techniques.The changes in the structure and morphology of the samples with the precursor ratio and heat treatment temperature are investigated.Meanwhile,the pore structure and crystallization degree of the samples are observed.The adsorption property,specific surface area,porosity and pore size distribution of the samples are analysed.The following conclusions are obtained:(1)The porous BN materials are successfully prepared by the precursor pyrolysis at high temperature.(2)The precursor ratio has little effect but temperature has significant influence on the structure and morphology of the samples.The structure of the samples obtained at lower temperature is mainly amorphous and the morphology is spherical and granular with serious agglomeration.With the pyrolysis temperature rising,the samples gradually changes amorphous state to hexagonal layered structure,the morphology gradually changes from granular and spherical to flake.(3)The element composition of the samples also changes with temperature.In the BN-1000 sample,the sum of B and N elements is about 60%and there are many C and O impurity elements.Nevertheless,in the BN-1500 sample,the sum of B and N elements is more than 93%with little C and O element content.The pure porous BN material are obtained.(4)The adsorption property and pore structure of the samples vary significantly with temperature.At lower temperature,the specific surface area and total pore volume are extremely small.As the pyrolysis temperature rises,the specific surface area and total pore volume are greatly increased and reach the maximum at 1500?.In the BN-1500 sample,the specific surface area and the total pore volume reach 1138.4m2·g-1 is 0.7287cm·g-1 respectively.The pore structure of samples are mainly micropores and the pore size distributes uniformly.(5)The thermal stability are compared between the prepared the porous BN samples and commercial BN powder.The porous BN samples have relatively poor thermal stability due to the existence of a large number of pores.Above 800? the partial BN molecule will be oxidized and transformed into B2O3,which leads to an increase in weight.