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为了延长激光法合成纳米金刚石的有效作用时间从而提高合成效率,提出采用功率密度低、脉宽长的毫秒脉冲激光照射循环水介质中石墨颗粒合成纳米金刚石的新工艺。高分辨透射电镜(HRTEM)及X射线衍射(XRD)分析结果表明,产物中含大量具有球形单晶体结构或五重孪晶结构的金刚石颗粒(平均颗粒尺寸约为5 nm)。通过对纳米金刚石微观组织结构的分析以及理论计算测算出低功率密度(106W.cm-2)毫秒脉冲激光照射时石墨颗粒表面可达到最高温度5300 K,认为该功率密度毫秒脉冲激光照射石墨颗粒时,不能产生碳等离子体,只能使石墨颗粒熔融,得到液态碳。因此生成纳米金刚石的相变机制为:激光脉冲开始时,石墨颗粒吸收激光能量快速升温并达到熔融状态,激光脉冲过后,碳液滴迅速冷却,金刚石形核并长大得到纳米晶。
In order to prolong the effective time of laser synthesized nanodiamonds and improve the synthesis efficiency, a new process of nanocrystalline diamonds synthesized by graphite particles in circulating aqueous media by using low power density and pulse width millisecond pulsed lasers was proposed. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) results show that the product contains a large number of spherical particles with spherical single crystal structure or quintic twin structure (average particle size of about 5 nm). By analyzing the microstructure of nanodiamonds and calculating theoretically, the highest temperature of graphite particles can reach 5300 K on the surface of pulsed laser with low power density (106W · cm-2), and it is considered that when the power density microsecond pulsed laser irradiates graphite particles , Can not produce carbon plasma, graphite particles can only be melted to obtain liquid carbon. Therefore, the phase transformation mechanism of nano-diamond is as follows: At the beginning of the laser pulse, the graphite particles absorb the laser energy and rapidly reach the melting state. After the laser pulse, the carbon droplets rapidly cool and the diamond nucleates and grows to obtain nanocrystals.