Study On Preparation Of Boron Nanosheets And Their Reinforcement On Copper Matrix Composite

Two-dimensional ultrathin boron nanosheets have a wide range of applications in the fields of energy storage,catalysis and cancer diagnosis and treatment,but the unique bonding structure of boron,"three-center two-electron" bond,makes it more likely to form a bulk structure rather than a sheet structure.Synthesis of boron nanosheets by means of solid phase diffusion has been achieved in copper medium,solving the problem that boron is not easy to form sheets due to its special bond structure.The crystal structures and growth mechanisms of boron nanosheets are also discussed.On this basis,in this paper,using Cu-B alloys as raw materials,boron nanosheets have been introduced into copper-based composites as one of the reinforcing phases by powder metallurgy method successfully.A copper matrix composite reinforced by dispersed boron was prepared,and the balance of mechanical properties and neutron absorption properties was achieved.This paper has important theoretical significance and research value for the preparation,growth and application of boron nanosheets.The main research contents are as follows:(1)Synthesis of boron nanosheets in copper mediumBoron has three electrons in its outermost shell,and it tends to form bulks rather than sheets due to its unique electron bonding.In this paper,boron nanosheets are prepared in copper medium and solid phase diffusion is the method.There are three processes during preparation:mechanical exfoliation,recombination and extraction.Boron nanosheets with a literal size ranging from a few nanometers to several hundred nanometers and a thickness of less than 6 nanometers can be obtained.Through the chemical environment analysis of boron by XPS,it can be concluded that B-B bonds are dominant,implying that boron nanosheets are hardly oxidized.Through the analysis of Raman spectra and the observation of high resolution transmission microscope,it can be observed that the boron nanosheets have two crystal structures:?-Rhombohedra boron and a-Rhombohedra boron,as well as two basic morphologies:hexagonal and rhombic sheets.Rhombic ?-Rhombohedra boron has twin growth mechanism.(2)The preparation and properties of copper matrix composite reinforeced by dispersed boronIn this paper,a copper matrix composite reinforced by dispersed boron,including boron nanosheets,has been prepared by powder metallurgy.The processes are as follows:Cu-B alloys with a mass fraction of 2wt.%B are atomized into Cu-B powders and milled,and then the powders are precompacted under a hydraulic pressure and sintered in a sintering furnace without pressure.The hardness and elastic modulus of the composite are tested by nano indentation.The hardness of the composite reaches 3.04GPa and the elastic modulus is about 107GPa,indicating that it possesses good mechanical properties.The neutron absorption properties of the composite are tested and the relationship between neutron absorption cross section and neutron energy is obtained.The effects of boron content,abundance of 10B and morphology of second phase on the neutron absorption property of the material are discussed.(3)The interfacial relationship between the copper matrix and the second phase in the compositeThe mechanical properties of conventional second phase reinforced metal matrix composites are greatly affected by the poor interfacial bonding between matrix and reinforcements.In view of this,the interfaces between copper matrix and reinforcements are studied in this paper.The content of elemental B reaches 98%or more in the second phase,while a small amount of elemental Al,Cu and O are dissolved.There are three crystal structures of boron,?-Rhombohedra boron,a-Rhombohedra boron and ?-Tetragonal boron.Through high resolution transmission electron microscopy,it is found that the all of three structures of boron have good interfacial bonds with copper matrix.In addition,some high density twins are observed in both copper and boron,and a small number of lattice distortion regions with certain width form at phase boundary due to the reduction of interface energy,which are also important reasons for the high hardness of the composite.