触媒微观结构与cBN单晶合成机理的相关性研究

Cubic boron nitride (cBN) single crystal can be synthesized from hexagonal boron nitride (hBN) with catalysts at high pressure and high temperature (HPHT), and the catalyzed or synthesized process of boron and nitride atoms must be achieved by catalysts. In reality, the correlative synthesis mechanism study of cBN has no perfect theoretical support at home and abroad, so the quality of large single crystals lay behind that of the advanced western companies. High quality cBN crystals can only be obtained on limited ranges of HPHT, these ranges were presented as V-shape in P-T equilibrium diagram, and in these ranges catalyst melt has the structure of short-range order. However, the catalyst structure obtained by "quenching" is closely related to its state at HPHT, so it can provide certain effective evidences for the study of cBN single crystal synthesis mechanism under critical conditions. .On the base of preparatory works, the in-situ microstructures and electronic structures of interface between catalyst and cBN crystal, which obtained by HPHT synthesis and then "quenching", were mainly studies by focus iron beam cutting technology, high resolution transmission electron microscopy, electron energy loss spectroscopy, and so on. These works are aim to find the experimental supports for the transformation process of cBN from hBN. On these bases, the first principle, the empirical electron theory of solids and molecules (EET), and the improved Thomas-Fermi-Dirac theory by Cheng (TFDC) are used to provide theoretical supports for the transformation process of cBN. By these theoretical studies, the electron density and energy in different crystal planes, including hBN, cBN and catalyst, can be obtained, and by these means the interactions of different phases can be analyzed. In the end, base on the experimental and theoretical supports, the technical experiments with novel catalysts and different technical parameters at HPHT are carried out to study the relations between the catalyst microstructures and the quality of cBN, and these works are beneficial to the control of catalyst structures at HPHT. The purpose of the plan is to make clear the synthesis mechanism of cBN single crystal, and lay a theoretical and experimental foundation for the synthesis of large particle and high quality cBN single crystals.

有触媒的静态高温高压合成cBN单晶，只有通过触媒才能实现对B、N原子的催化或合成，然而目前对其作用机理尚不明确，致使我国大单晶质量落后于国际同行。优质cBN单晶只生长在较窄的高温高压"V"形区内，此区域内触媒熔体存在短程有序结构，而快冷后的触媒结构与触媒熔体存在相关性。本申请在前期研究基础上，主要运用聚焦离子束切割、高分辨电镜以及电子能量损失谱等手段，获得合成后快冷的cBN/触媒界面原位结构和界面层电子结构信息，寻找cBN转变的实验依据；利用第一性原理、EET和TFDC理论，获得cBN、hBN、触媒结构各晶面的电子密度和晶面能，以此分析各相之间的相互作用，寻找cBN转变的理论依据；最后根据实验和理论依据选择新型触媒剂和工艺进行合成实验，研究触媒结构与cBN单晶质量的关系，以实现对高温高压触媒结构的控制。通过本研究，明确cBN单晶合成机理，为合成出大颗粒、高品质cBN单晶奠定理论与实验基础。

采用静态高温高压触媒法合成cBN单晶，只有通过触媒才能实现对B、N原子的催化或合成。然而，目前对其作用机理尚不明确，致使对大颗粒cBN单晶合成工艺缺乏理论指导。优质的cBN单晶只生长在较窄的高温高压“V”形区内，在此区域触媒熔体存在着短程有序结构，而快冷后的触媒结构与触媒熔体存在相关性。本项目以合成实验、结构表征、理论计算为研究思路，围绕触媒结构与cBN单晶合成机理的相关性开展研究，取得了以下重要成果。. 1）通过研究触媒种类、粒度、籽晶等以及高温高压合成工艺参数，优化了触媒体系和合成工艺。采用Li3N-hBN体系+籽晶和一次升温、慢超压工艺，批量合成出了≥50目的大颗粒cBN单晶，其产量占比为56.8%，而且单晶晶型好、强度高（各粒度强度均优于国家标准）。此部分为合成优质大颗粒cBN单晶提供了重要的实验依据。. 2）利用现代检测手段，分层对cBN单晶周围触媒的物相结构组成、精细结构（包括电子结构）进行了系统表征，发现：在高温高压下cBN单晶是逐步转变的，其转变和生长均来自hBN的直接相变，在此过程中Li3BN2相起催化作用。此部分为揭示cBN合成机理提供了直接证据。. 3）利用固体与分子经验电子理论、晶面能理论以及高温高压热力学理论，研究了cBN、hBN和Li3BN2在高温高压下的相互作用，多种计算结果基本具有一致性，并且与表征结果相吻合。此部分基本明确了静态高温高压触媒法的cBN单晶合成机理。. 4）在研究合成后的触媒组织结构和形貌与cBN单晶合成效果的相关性以及明确cBN合成机理的基础上，提出了利于合成后触媒层中Li3BN2物相和管状组织形成和均匀分布的思路，以实现对高温高压触媒结构的控制。此部分初步实现了对高温高压触媒结构的控制。. 上述研究基本明确了静态高温高压触媒法合成cBN单晶机理，为合成大颗粒、高品质cBN单晶奠定了理论与实验基础。. 通过本项目研究，已发表SCI和EI收录论文各7篇，授权中国国家发明专利3项。