Study On Growth Mechanism Of Boron Crystal And Boron Spheres In Cu-B Alloys

Boron has excellent physical and chemical properties,and it has broad application prospects in aerospace,electronic transmission,nuclear energy,medicine and other fields.Nowadays,the preparation processes of boron are still immature,and it is difficult to realize large-scale preparation of high-purity boron,this result in high prices of boron and limit its further application.In this work,the process of preparing boron by copper melt method was optimized,and the effect of aluminum on the growth process of primary boron in Cu-B alloy was studied.The single crystal morphology of primary boron in Cu-B alloy was analyzed by adjusting the cooling rate of Cu-B alloy,and on this basis,the formation process of twin boron was discussed.Using magnesium as an alloying element realizes the spheroidization of eutectic boron in Cu-B alloy.Spheroidized Cu-B alloy was extracted,boron spheres with different morphologies were obtained,namely,sub-micron solid boron spheres,hollow boron spheres,and spherical coating structures.The formation processes of boron spheres were analyzed on atomic scale,and the precipitation process of boron nanospheres was analyzed by in-situ observation.The main research results of this paper are as follows:(1)Research on the growth mode of boron crystalsIn this thesis,a series of Cu-B alloys were prepared by adjusting the cooling rate,and the crystallographic appearances of primary boron in the alloys were studied.The exposed faces of primary boron were {101} and {001} crystal faces.With the change of the position of {001} crystal faces and the increase in size,the morphology of {101}crystal faces changed from parallelograms to pentagons,triangles and trapezoids in turn,and the crystallographic appearances of primary boron also changeed.In the process of crystal growth,boron crystals whose {101} crystal faces are parallelogram,five deformation and triangle can form pentagonal twins to reduce the overall surface energy.However,boron crystals with trapezoidal {101} crystal faces reduce the surface energy by forming ordinary twins,polysynthetic twin,parallel grouping and pentagonal dodecahedrons,and the formed ordinary twins can form large-sized boron sheets through twin groove growth.(2)Relevant research on the preparation of elemental boron using the melt method that used copper as a medium.In the traditional copper melt method to prepare elemental boron,the copper matrix in Cu-B alloy is removed by nitric acid,which makes raw copper unrecyclable.In this thesis,electrolysis was used to remove the copper matrix in Cu-B alloy to realize the recycling of copper.Using scanning electron microscope,X-ray diffractometer and raman spectroscopy to study the effect of aluminum on the structure of primary boron in Cu-B alloys,it was found that excessive aluminum can induce the transformation of?-B to ?-B or AlB12 and form a core-shell structure or layered structure.This transformation was beneficial to the growth of primary boron,and as a result,the content of primary boron in Cu-B alloy increased and the content of eutectic boron decreased.(3)Study on the spheroidization of eutectic boron and the growth process of boron spheresOn the basis of rapid solidification,the spheroidization of eutectic boron in Cu-B alloy was successfully achieved by using 0.2 wt.%magnesium.Sub-micron boron spheres are formed in the initial stage of the eutectic reaction and have curved lattice stripes.In order to maintain this curved morphology,crystal defects such as stacking faults,lattice distortions,twins and edge dislocations were formed in the boron spheres.Hollow boron spheres and cladding structures were formed at the end of the eutectic reaction.The hollow boron sphere possessed polycrystalline structure and the lattice stripes on the surface were not bent,but there were twin structures at the junction of adjacent crystals;the cladding structures were combination of hollow boron spheres and amorphous boron spheres.Amorphous boron spheres were formed by solidification below the glass transition temperature.The formation process of boron nanospheres was the crystallization process of amorphous boron.The crystallized boron nanospheres possessed ?-B structure,and as the crystallization process progresses,the boron nanospheres gradually broke away from the amorphous matrix.