【作者】 宿庆财；

【导师】 李木森；

【作者基本信息】 山东大学 ， 材料学， 2008， 博士

【摘要】 本文以碳化硼为硼源,利用粉末冶金法制备了Fe-Ni-C-B系触媒片,以石墨为碳源,在高温高压条件下合成了几种含硼量不同的金刚石单晶。利用Diashape金刚石形貌分析仪、扫描电镜(SEM)、透射电镜(TEM)、拉曼光谱(Raman)、X射线衍射仪(XRD)和场发射扫描电镜(FESEM)等系统地研究了含硼量对铁基触媒组织形态和金刚石形貌与微观结构的影响,并对相关理论进行了研究。用铁粉、镍粉、碳粉和碳化硼粉末轧制而成的铁基触媒片在惰性气氛条件下1150℃烧结处理后,明显改善了片状铁基触媒的机械性能,适合于冲压成合成金刚石用的触媒片。碳元素的扩散性强,在触媒中的分布比较均匀。而铁、镍元素的扩散性较差,在触媒中易于形成白色的富镍团簇和浅黑色的富铁团簇,这些偏聚的团簇较为均匀地混合在一起。触媒中粉末颗粒大部分保持原有的形态,颗粒间还有一些黑色的孔洞,形成网络状疏松结构。在高温高压合成金刚石的过程中,熔融触媒与石墨相互反应,转变生成大量的初生渗碳体。触媒中的初生渗碳体多呈板条状,是金刚石形核和长大的主要碳源。较宽的初生渗碳体板条对金刚石形核和长大不利,较细的初生渗碳体对金刚石形核和长大则有利。因此,金刚石的生长与提供碳源的初生渗碳体数量、形状和分布均匀性直接相关。硼是影响触媒中初生渗碳体的数量、形状和分布的主要原因之一。在铁基触媒中加入的碳化硼含量不同,触媒中的初生渗碳体组织形态也不相同,从而直接影响金刚石的形核和长大。含硼金刚石的晶体形态影响了其静态抗压强度。1#、2#、3#三种不同含硼量的金刚石粒度主要集中在50/60和40/50之间。1#金刚石具有较好椭圆度和圆度,晶形较有规则,晶体内缺陷较少,结构较完整,具有较高的静态抗压强度。2#、3#两种金刚石相对次之。硼对金刚石晶体的生长习性也有影响。当触媒中的碳化硼含量由0.1wt%增加到0.2wt%时,硼可以抑制金刚石(111)面的生长,而促进(220)和(311)面的生长。当触媒中的碳化硼含量由0.2wt%增加到0.3wt%时,硼对晶面生长的作用却相反。XRD和Raman光谱分析的结果与场发射电镜观察的现象是一致的。硼含量对金刚石的抗氧化性能有较大的影响。含硼金刚石的抗氧化性能一般高于常规金刚石,随着硼含量的增加,起始氧化温度增高。由于硼原子在金刚石表面与三个碳原子成键后,形成硼皮结构,减缓了金刚石的氧化速率。在氮气保护气氛下,由于金刚石中的碳原子不能与空气中的氧原子接触而产生氧化,因此含硼金刚石与常规金刚石的抗氧化性能差别不大。而金刚石的差热温度曲线略有波动,这是由于金刚石表面吸附的杂质在高温下烧蚀所致。金刚石的基本生长单元是四面体,金刚石表面的部分缺陷是碳四面体在晶体各个面族上与相邻晶面相交的轨迹。碳四面体在{100}面显露的是直线,在{111}面上显露的是三角形。{100}和{110}面的相交,在金刚石晶体的{100}面上形成四棱锥形的凹坑,因而使得金刚石与熔融触媒的接触面积增大,金刚石晶核吸收碳原子的速度增加。同时,四棱锥凹坑的相邻侧面的夹角处更容易吸附碳原子,进一步促进了金刚石生长。在金刚石(111)面上的六边形台阶和三角形凹坑是碳四面体在该晶面上显露的形貌特征。碳原子在{111}面上二维形核后易于出现台阶,台阶的存在必然伴随扭折。由于台阶扭折处容易吸附碳原子,促进台阶在{111}面上不断运动,导致晶体呈层状生长。金刚石{111}晶面上存在锯齿状台阶,其两侧晶面的夹角为锐角,使得这两个晶面不会因一个晶面生长速度过快而被生长速度较慢的晶面所吞噬。当两个晶面同时存在时,可以保持台阶的扭折状态,从而提高了金刚石的生长速度。金刚石生长界面处于狭窄的成分过冷区,由于生长界面上的凸缘可以沿界面推移方向和凸缘两侧排出溶质,导致凸缘间隙处的溶质富集,使得金刚石内部出现胞状结构。计算复杂晶体中某一晶面的价电子密度时应考虑邻近晶面的原子对该晶面的影响,据此提出了有效价电子密度的概念。基于EET理论的计算结果表明,(111)面的有效价电子密度大于(110)面,当金刚石受外力作用时,(111)面间较弱的键相对更容易断裂,因而导致沿(111)面产生解理。这与实验观察结果基本吻合。因此,根据有效价电子密度可以判定金刚石的解理面,进而分析复杂晶体的解理断裂。更多还原

【Abstract】 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.更多还原