低功率密度激光合成金刚石的相变机制

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。

Mechanism of Phase Transformation of Diamond Synthesized by Laser with Lower Power Density

In order to prolong the effective actuation duration and improve the transformation ratio of nanodiamonds synthesized by laser irradiation, the process was proposed that the graphite particles suspended in water was irradiated by laser beam with lower power density. The high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) analyse results showed that most of diamond particles with monocrystal structure or multiple twins′ structure were synthesized and the particles size was about 5 nm. When the raw graphite was irradiated by the laser beam with lower power density (106 W·cm-2), the theoretical calculation temperature on the surface of the graphite powders can be up to 5300 K, the mechanism of the phase transformation in the process was that the raw graphite was molten but not generated plasma, then nanodiamond nucleated and grew up from liquid carbon cooled quickly after the laser pulse.