Study On Microstructure, Performance And Relative Theory Of Boron-doped Diamond

In this paper,boron carbide acted as boron source was doped into Fe-Ni-C-B system catalyst made with powder metallurgy method.Boron doped diamond was synthesized with graphite and catalyst under the condition of high temperature and high pressure(HTHP).The effect of boron quantity, morphology of iron based catalyst and growth of crystal were systemically studied by means of diamond shape measurement system(Diashape),scanning electron microscopy(SEM),transmission electron microscopy(TEM),Raman spectroscopy(Raman),X-ray diffraction(XRD)and field emission scanning electron microscope(FESEM).And correlative theory of diamond synthesis was studied.After rolling and heat treatment in 1150℃at inertia atmosphere,the performance of catalyst made of iron,nickel,graphite and boron carbide were improved,and the catalysts were fit to forging to catalyst pieces which suitable for diamond synthesis.The diffusibility of carbon was strong,and it was evenly distributed.The diffusibility of iron and nickel was low,and they were easily formed black group of rich iron and white group of rich nickel.All the groups were mixed evenly.Most powders were maintained their original shape. There were lots of holes among powders,and all of thera formed loose network structure.Molten metal reacted with graphite to produce a mass of primary cementite at the process of diamond synthesis under HTHP.Most of the austenite was batten shape,and it was the main carbon source in the nucleation and growth of diamond.The thick primary cementite put the nucleation and growth of diamond at a disadvantage,and the thin primary cementite advantaged the nucleation and growth of diamond.Therefore,the growth of diamond had a direct relation with the quantity,shape and distribution of primary cementite which provide carbon needed by diamond when it grew. Boron is one of the main factors in the amount,morphology and distribution of primary cementite in catalyst.With adding different content of boron carbide to iron-based catalyst,the shapes of primary cementite were different,and it was directly influenced the nucleation and growth of boron doped diamond.The morphology of boron doped diamond crystal effect its static compressive strength.The granularity of 1#,2#,and 3# three different diamond mainly concentrated between the 50/60 and 40/50.The degree of ellipse and roundness in sample 1 was higher,and there were little defects in it. Therefore,the static compressive strength was higher.And sample 2 and 3 relatively took second place.Boron has effect of the growth of the diamond crystal plane.When the content of boron carbide in catalyst improved from 0.1 wt%to 0.2 wt%,boron could restrain the growth of(111)plane in diamond,and promote the growth of (220)and(311)plane.And when the content of boron carbide in catalyst improved from 0.2 wt%to 0.3 wt%,the effect of boron on crystal plane growth is reverse.The analysis with XRD and Raman spectroscopy was as same as the results observed with field emission electron microscopy.The content of boron could influence the antioxidation of the diamond. The antioxidation of boron doped diamond was tgenerally higher than that of conventional diamond.With the increase of boron content,the initial oxidation temperature increased.Because the boron atoms in the surface of diamond was bonded with three carbon atoms,they were formed a boron skin structure,and it slow down the oxidation rate of diamond.In the nitrogen atmosphere,the carbon atoms in the diamond did not contact with oxygen in the air,so the oxidation of the carbon atoms did not take place.There were little difference of antioxidation between the boron doped diamond and conventional diamond. Because the impurities adsorbed in the surface of diamond were ablated in the high temperature,there was slight fluctuation in difference thermal analysis (DTA)curves of the diamond.The basic unit of diamond was tetrahedron.The defects in diamond surface were the coupling track of carbon tetrahedron among the crystal planes and its adjacent planes.The figure that carbon tetrahedron intersected with {111} was triangle,and the figure that carbon tetrahedron intersected with {100} was a line.The plane of the pit which shape like the pyramid was formed with the intersection of {100} and {110}.With the formation of the pit, the area of interface between the diamond and melt catalyst was improved,and the rate that diamond nucleus absorbed carbon atoms was increase.While,the carbon atoms were easily bonded to the corner of the bit,so the growth of diamond was improved.The hexagonal steps and triangular pit on diamond (111)plane were the diagrams that carbon tetrahedron intersected with {111}.The steps were easily formed in the diamond after the carbon atoms born two-dimensional nucleation in the {111} plane.There were twists in the step, and that the carbon atoms were easily bonded to the twist of the step promote the growth of the step resulting in a layered crystal growth.Because the angle of the both sides of the serrated step in diamond {111} plane was cute,the plane grew fast did not annexed by that grew slow.The concurrence of both plane result in maintaining twist in step,and making diamond grew fast.The growth interface of diamond was located in a zone of component cooling.The solute was discharged from the flange of growth interface along the moving direction of interface and the side of flange.The solute was rich in the clearance of the flange,and the pole structure was formed in the diamond.When calculating the valence electron density of a crystal plane in complex crystal,it should be consider of the valence electron of atoms near the crystal plane.The concept of valid valence electron density was put forward.The calculation based on EET theory and analysis result show that,the valid valence electron density of the(111)plane is higher than that of the(110) plane.As the higher outside stress were applied on diamond,the weak bond between the(111)planes is easily broken up and diamond was cleaved along (111)plane.The conclusion based on the valid valence electron density was in good agreement with the result observed in the experiment.Therefore,the cleavage surface of diamond could be determined with the calculation of the valid valence electron density,and analyzing cleavage fracture in complex crystal with it was feasible.