Resarch For The Growth Mechanisms Of Boron In Cu-b Alloys

Boron is the third main group element,which is located in the junction of metal and insulator.Boron has three valence electrons that make it possess the properties of quasi-metals.The electrical conductivity of boron improves with the temperature increasing which makes it has vast application prospects in electronic transmission field.In addition,boron has a large amount of merits,such as high melting point(～2200 ℃),low density(2.340g/cm3),high hardness(Knoop:2160-2900),high calorific value and high young modulus(380-400GPa).Thus,boron is usually as the high temperature reinforced phase and the reinforced phase in metallic matrix.Based on these,the copper melt is selected as the growth solvent of boron in the dissertation,allotropies and nanostructures of boron are studied.Meanwhile,through the regulation and control for the constituent,hypoeutectic(Cu-2wt.%B)and hypereutectic(Cu-5wt.%B)Cu-B alloys are chosen as the major research object.By using different fabrication processes,cooling rate,heat treatment,the morphologies,crystal structures,formation mechanisms of boron allotropies in copper melt are researched.Also,we investigate the stability of variety of boron allotropes under ambient environment.Furthermore,we explore the formation,growth and transformation mechanisms of different boron nanostructures.The main research contents are as follows:(1)The formation mechanisms and stability of α-B and β-B in copper melt at ambient environmentIt’s found that using copper as the solvent instead of platinum,we could synthesize boron.At ambient environment,the coralloid eutectic α-B is a metastable phase which average size is around 2-3μm.The bulk primary β-B is a stable phase which average size is up to 30μm。The growth of coralloid eutectic α-B has three requirements.The formation of constituent supercooling districts and appropriate temperature gradients are the premise conditions.A low eutectic temperature(1013 ℃for Cu-B binary system)and restricted growth space which are provided by copper melt are necessary conditions.Ascribing to the non-equilibrium solidification,primary β-B presents four varieties of typical morphologies:star-shaped,six prismatic,tetrahedral and tip shape of tetrahedral.Cycling-twinning mechanism could be expounded the formation of star-shaped boron.For the other shapes of β-B,those are the result of screw dislocation-driven growth,layer growth and the twin plane re-entrant edge growth work together.(2)The fabrication of boron spheres and its evolution mechanismsIn supercooled copper melt,we put Cu-2wt.%B alloy as the main research object.Through adopting different cooling rate and fabrication processes,boron spheres are synthesis.The formation of boron spheres includes three steps:the aggregation of boron clusters,twin plane re-entrant edge growth and layer growth.Among them,the morphologies of boron will evolve from spheres to petal-like during the growth process of boron spheres.This phenomenon could be expounded by Mullin-Sekerka unstable theory.Meanwhile,the first principle calculation also illustrates that the concentrate of boron and the varied curvature of boron network structures will hinder the formation of closed surface.Thus,those flake network structures which attaching the boron spheres are unable to close forming the petal-like boron particles.(3)The fabrication of boron nanosheets and its chemical environmentThrough grinding the extracted β-B powder,we could fabricate two-dimensional boron nanosheets.The thickness of boron nanosheets is varied,the diameter is around 40nm.By means of XPS,we could analyze the bonding and chemical environment of boron.It’s found that β-B powder includes little B2O3 bonds,B-B bonds take up the majority.After grinding,except for a little B2O3 bonds,B-B bonds have been partly changed into B6O bonds which is an incomplete oxidation state,it means that boron nanosheets have the property of oxidation resistance.