液相等离子喷涂制备场发射阴极表面纳米金刚石涂层及其形成机理研究

The field emission display (FED) with nano diamond coatings as the cathode emitter offers the advantages of better image quality，less energy consumption and longer service life. Yet cost-effectively large-scale deposition of the diamond coating remains challenging. Furthermore, knowledge about physicochemical behaviors of nano particles in particular nano diamond during thermal spray deposition is insufficiently known. This project aims to fabricate nano diamond coatings by suspension plasma spraying for FED applications. Based on commercially available nano-diamond powders, the diamond slurry which is suitable for thermal spraying is to be developed. Effects of the parameters, e.g. PH value, dispersant and concentration of the suspension, on its performances are to be studied. Optimized deposition will be made for achieving the coatings with high content of nano diamond. The behaviors in terms of changes in structure and morphology of nano-diamond powders in both in-flight and coating formation stages during the suspension plasma spraying is also to be investigated. Microstructure characterization and property evaluation of the diamond coatings will also be conducted. Based on this study, the relationship between processing, microstructure and field emission properties of nano-diamond coatings is to be clarified. This study is to shed light on potential applications of the nano diamond coatings for field emission devices.

纳米金刚石作为阴极发射体的场发射显示器（FED）在图像质量、能耗和使用寿命等方面具有巨大的优势，但低成本、大面积纳米金刚石涂层的制备技术一直存在挑战，同时纳米金刚石在热喷涂过程中的物理化学行为还有待深入探讨。本项目拟利用纳米金刚石粉末，通过研制适用于热喷涂的纳米金刚石浆料，采用液相等离子喷涂技术制备高金刚石含量的纳米结构涂层。研究浆料PH值、分散剂和纳米颗粒浓度等参数对喷涂浆料均匀性和稳定性的影响规律，开展涂层微观结构表征和力学及场发射性能测试，探讨纳米金刚石粉末在等离子焰流中飞行及涂层沉积两个过程中的碳结构变化行为，揭示纳米金刚石涂层结构-性能的相关性，阐明纳米金刚石涂层的形成机理，为液相等离子喷涂制备大尺寸FED阴极表面纳米金刚石涂层提供理论依据与实验支持。

本项目针对在低成本、大面积纳米金刚石涂层的制备技术方面存在的挑战，提出了采用液相等离子喷涂技术制备高金刚石含量的纳米结构涂层，研究纳米金刚石粉末在等离子焰流中飞行及涂层沉积两个过程中的碳结构变化行为，并阐明纳米金刚石涂层的形成机理。.利用NanoSight纳米颗粒追踪分析技术对纳米金刚石颗粒聚集的过程进行了直观、实时的监测，揭示了纳米金刚石颗粒聚集的形成机制。调整浆料的PH值至接近7，同时加入十六烷基三甲基溴化铵作为分散剂，并对浆料进行超声震荡(超声时间30min，功率为99W，温度为30℃)处理，获得具有高分散性和稳定性，且适用于等离子喷涂的液料。通过高速摄像法获得液滴在等离子喷涂过程中的尺寸、温度和速度等数据，与数值模拟分析得出的结果相比较，研究了纳米金刚石液料与等离子焰流的热作用过程，阐明了液滴的尺寸和速度对涂层形成模式的影响规律。通过收集位于等离子焰流中不同空间位置处的单个喷涂粒子并分析其结构、形状、尺寸和分布状态，阐明了液滴在焰流中由液态向扁平粒子单元体转化的过程。通过表征获得的纳米金刚石涂层中颗粒尺寸、分布形态、孔隙率和孔径以及涂层与基体之间结合状态，阐明了涂层的组织结构随制备工艺和热处理条件的演变规律，研究了纳米金刚石涂层微/纳介孔结构在高温下的结构演变规律，阐明热处理工艺对涂层性能的影响。结果表明，经过700℃热处理的纳米金刚石阴极样品的开启电场为8.3V/μm，在20V/μm的电场下场发射电流密度为75μA/cm2。热处理后制得的纳米金刚石涂层样品的场发射性能明显提高，这主要是由于纳米金刚石涂层与金属钛的过渡层之间发生了键合反应而形成Ti-TiC-diamond的网状结构。基于对所制备的场发射阴极开启电场和场发射电流特性的表征，确立了场发射阴极结构的优化设计方法和规律，为液相等离子喷涂制备大尺寸场发射显示器(FED)阴极表面纳米金刚石涂层提供理论依据与实验支持。