【作者】 李和胜；

【导师】 李木森；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 研究发现,向金刚石中掺杂某些元素可以使金刚石获得特殊优异的性能。硼由于具有与碳接近的原子半径,易于进入金刚石晶格,含硼金刚石一直是掺杂金刚石研究的热点。已有的研究发现,含硼金刚石是一种P型半导体材料,甚至还具有超导特性;另外,含硼金刚石还具有明显优于常规金刚石的热稳定性和化学惰性。以含硼金刚石为代表的特种金刚石制备与应用将是二十一世纪人造金刚石行业发展的主要方向之一。含硼金刚石的制备对于丰富人造金刚石的品种,提高其品质,拓展其应用乃至从总体上提升我国人造金刚石行业的技术水平都有十分重要的意义。但是,目前已有的研究大多着眼于含硼金刚石薄膜,对单晶材料少有研究;而且,目前现有的合成含硼金刚石单晶的方法一般条件较为苛刻,生产成本较高,难以在工业化生产条件下获得高品位的含硼金刚石单晶。因此,如何采用较为低廉的原料和较为简便的方法合成优质的含硼金刚石单晶,并进一步对其半导体特性进行研究,便成为含硼金刚石单晶研究深化的紧迫任务。本文在粉末冶金铁基触媒相关研究的基础上,向触媒原材料中添加合理的硼源材料,制备含硼粉末冶金铁基触媒。使用制备出的触媒匹配人造金刚石专用石墨组成Fe-Ni-C-B反应体系,在高温高压下合成含硼金刚石单晶。通过对含硼触媒的成分、高温高压合成工艺和提纯工艺的优化设计,系统研究了Fe-Ni-C-B系高温高压合成含硼金刚石单晶的工艺。通过金刚石晶体结构和性能的系统表征,研究了硼对金刚石晶体结构和性能的影响。通过对高温高压下含硼金刚石单晶在Fe-Ni-C-B系中的碳源供给、形成机制以及生长机制的讨论,系统研究了含硼金刚石单晶的高温高压合成机理。本文以含硼金刚石单晶合成工艺为主线,从触媒制备、合成工艺、结构与性能表征和合成机理等几个主要方面,系统开展了含硼金刚石单晶的实验分析和理论研究工作。本文从铁基触媒原材料优选及制备工艺优化入手,为粉末冶金方法制备含硼触媒奠定了工艺基础。通过对触媒原材料质量(主要是氧含量)的严格控制,优化金属粉末配比和添加石墨粉,改进了铁基触媒的成分构成;提出了粉末轧制-烧结-冲制新的制备工艺,提高了触媒的质量和贵重金属镍的利用率。从硼源材料优选,硼源合理添加量的选择以及触媒成分多元合金化三个方面对含硼粉末冶金铁基触媒的成分进行了优化设计。首先优选出六方氮化硼作为硼源材料,并对其适宜的添加量进行了探讨。试验证明,硼源添加量应为a-2a,过量添加会影响金刚石的品位。进而以铜为例,证明在触媒成分中添加有益元素的多元合金化可以明显提高金刚石的品位。从优选石墨、改进合成压块组装结构和设计新的合成工艺三个方面对高温高压合成金刚石工艺进行了优化设计。优选出G4D石墨作为合成含硼金刚石单晶的碳源材料;借鉴粉末工艺对合成压块的组装结构进行了改进,提高了腔体内压力、温度的稳定性;以保证金刚石的优晶生长为目的设计了压力功率动态匹配合成工艺,并通过设备改造和压力标定及温度测量完成了这一新工艺设计;同时,确定出含硼金刚石单晶在Fe-Ni-C-B系中的优晶生长区:P=5.5-5.7GPa,T=1400-1500℃。利用铁基触媒及其包覆膜具有铁磁性和脆性大的特点,设计了一套单纯依靠机械方法提纯金刚石的新工艺。经试验验证,新工艺既可以有效提纯金刚石,且无污染,方法简单,机械化程度高,具有重要的推广应用价值。应用现代分析测试技术对含硼金刚石单晶的晶体结构和主要性能进行了系统表征。试验结果表明,合成的金刚石单晶受硼的影响,表面比较粗糙,{111}面较发达。Raman特征峰的偏移提供了硼进入金刚石晶格的间接证据;而红外吸收光谱则直接探测到了含硼金刚石晶体内部的B-C键。采用第一原理的模拟计算表明,硼在金刚石晶格中易于以置换原子的形式存在。由于硼原子对晶体表面碳原子的取代,有效地阻止或延缓了金刚石的氧化,使得含硼金刚石单晶具有明显优于常规金刚石的热稳定性:表面起始氧化温度提高了约170℃,氧化过程的表观活化能约为常规金刚石的3.5倍。试验结果和理论分析进一步证明,金属碳化物才是金刚石生长的直接碳源,且触媒熔体中金属碳化物的充分形成直接影响金刚石的碳源供给,并进而影响金刚石的合成效果。依据金属包覆膜物相结构系统表征的结果,借鉴Fe-Ni-C系中金刚石的合成机理,讨论了含硼金刚石在Fe-Ni-C-B系中的形成机制。试验发现,硼是以金属-碳-硼化合物的形式溶入金属包覆膜内,经金属中间相的催化反应而析出活性硼原子(团),再向金刚石扩散,其扩散的路径、形式与碳相同。含硼金刚石单晶的形成依赖于金属-碳-硼化合物在包覆膜内层的分解。本文还依据对金刚石单晶/金属包覆膜界面微观结构表征的结果,借鉴经典的晶体生长理论,讨论了含硼金刚石单晶在Fe-Ni-C-B系中的生长机制。研究结果表明,含硼金刚石单晶在Fe-Ni-C-B系中是以层状方式长大的。这种层状生长的台阶来源前期以二维晶核为主,后期则以位错为主。自金属包覆膜中脱溶析出的层片状碳-硼原子团扩散到达金刚石单晶表面,在生长台阶前端被吸附,长成含硼金刚石单晶的一部分。随着台阶的不断扩展,新的台阶在刚长成的晶面上继续形成,含硼金刚石单晶则以层状堆叠的方式逐步生长。更多还原

【Abstract】 It has been found that the properties of diamond can be significantly enhanced by doping some elements into diamond lattice. Boron atoms, whose atomic radius is close to that of carbon, can enter into diamond lattice relatively easily and nowadays, boron-doped diamond has become a hot spot due to the related advantages in practice. For instance, boron-doped diamond is semi-conductive or even super-conductive and the thermal stability and chemical inertia of boron-doped diamond are much better than those of the normal one. In addition, boron-doped diamond can be manufactured by many kinds of methods. The manufacture and application of special diamond, which is represented by the boron-doped one, will be the important innovative orientation in 21st century. The study on manufacture of boron-doped diamond is significant for the technical progress of the whole industry in China for it will enrich the species of synthetic diamond, upgrade its properties and extend its application fields.However, at present, most studies focus on boron-doped aggregate crystals and poly- crystalline films and only a few on single crystals. Furthermore, almost all the methods for synthetic boron-doped diamond single crystal require strict conditions and involve high cost; therefore, it is hard to obtain high quality boron-doped diamond single crystal under existing industrial conditions.In this light, it is very urgent to obtain synthetic high-quality boron-doped diamond single crystal with cheaper raw materials and simpler method.This paper mainly focuses on synthesis of boron-doped diamond single crystal in Fe-Ni-C-B system under high-pressure and high-temperature (HPHT). The Fe-Ni-C-B reactive system is composed of boron-doped iron-based catalyst alloy and graphite. The boron-doped iron-based catalyst alloy was made by powder metallurgy method by adding boron source into the raw materials. The synthesis technology of boron-doped diamond was promoted by redesigning the composition of the catalyst, improving synthesis process and developing new purification technique. At the same time, the synthesis mechanism of boron-doped diamond under HPHT, consisting of the carbon source and mechanism for diamond growth as well as the formation route, was also studied systemically. This paper is organized based on the whole preparation process of boron-doped diamond including catalyst manufacture, HPHT synthesis and purification, characterization of structure and properties of diamond as well as the synthesis mechanism.As the first step of diamond synthesis, the composition of iron-based catalyst was enhanced by modifying the ratio of pure iron powder to nickel and introducing graphite powder. The chemical composition of raw materials was strictly controlled, the oxygen content, especially. A new kind of formation method for catalyst preparation was developed from powder rolling to sintering, then to punching. This method is helpful to increase the product yield and utilization efficiency of nickel.The composition of boron-doped catalyst was designed by optimizing the boron source and its addition level. The multi-component alloying of catalyst by adding some elements such as copper was discussed as one part of composition design. On the basis of the experimental results, hexagonal boron nitride (hBN) is considered as a feasible boron source. In addition, the addition quantity of hBN should be controlled in the range of a-2a because excessive addition will damage the quality of diamond crystal. Moreover, the added copper is helpful to control the crystal growth rate and enhance the quality of diamond.G4D type graphite disc is admitted as the suitable carbon source for boron-doped diamond synthesis. The structure of assembly bulk was improved by adopting the means of powder catalyst mode to enhance the stability of temperature and pressure in synthesis cell. A new pressure and temperature dynamic match technology was proposed aiming to control the pressure and temperature within the range for high-quality diamond growth. Then the new synthesis technology is realized with the modified high-pressure apparatus. In addition, the real pressure in synthesis cell was calibrated and the temperature in synthesis cell was measured by thermocouple. Due to the experimental work mentioned above, the growth region for high-quality boron-doped diamond single crystal in Fe-Ni-C-B system was definite: P=5.5-5.7GPa,T=1400-1500℃.A new diamond purification process was created by fully utilizing the ferromagnetism and high brittleness of iron-based catalyst and metallic film. The diamond single crystals can be purified completely with mechanical separation methods. Furthermore, the new process is more valuable in practice with no pollution, lower cost and simpler operation.The shape of diamond crystals is cube-octahedral with rough surface and developed {111} faces due to the presence of boron. The shift of Raman scattering peak suggests boron atoms entering into diamond lattice. The B-C covalent bond in diamond is directly found in IR absorption spectra. The boron atoms are inclined to substitute the carbon atoms in diamond lattice on basis of the calculation results with the first principle. In addition, due to the substitution of boron to carbon on the crystal surface, the thermal stability of boron-doped diamond is more excellent than that of the normal one. The surface initial oxidation temperature of the boron-doped diamond is advanced by 170℃, and the apparent activation energy of oxidation process of boron-doped diamond is 3.5 times as much as that of the normal one.The experimental results of this paper further prove that the direct carbon source for diamond growth in Fe-Ni-C-B system under HPHT is metallic carbides, not graphite. The yield and quality of synthetic diamond crystals are determined by the full formation of metallic carbide in catalyst melt, i.e. the solution degree of graphite in it.The diffusion form and route of boron are the same to those of carbon in the process of diamond formation. The boron-doped metallic carbides diffuse through the metallic film surrounding diamond and arrive on the film/diamond interface, where active boron and carbon atoms will precipitate from the carbides and enter into diamond. In general, the formation of boron-doped diamond depends on the decomposition of carbides on the film/diamond interface.Plenty of trace of diamond layer growth was found via modern materials testing technique. The origin of growth steps for diamond layer growth is two-dimensional nucleation at the initial stage and more growth steps originate from the dislocations that result from the mismatch between boron and carbon atom. The layer boron-carbon atom groups, namely the growth unit, which arrive on the surface growing diamond, will be absorbed on the side of the steps and become one part of diamond crystal.更多还原