Study of Rapid Heat Transfer Phenomena of Nano-second

在水中进行了激光脉冲加热金属表面的实验和理论研究，以了解纳米秒级的热和流体流动现象的物理性质。在以汞为测试表面的实验中，发现激光照射刚结束时，由于水和汞的相变产生激波，下一阶段在毫秒内形成半球形汽泡，最后阶段在几秒内，汞表面发生变形。而在空气中加热的情况下，汞的等离子体在受到激波照射后的几微秒内就会出现，而汞表面不会发生变形。利用硅固体表面在水中进行的实验中，由于水在水中以声速运动时发生相变而产生强烈的激波。相变可以理解为当硅的表面温度达到水的均匀成核温度时发生。在空气中加热硅表面时，会观察到激波，但这主要是由于空气膨胀引起的。为了解释这些实验结果，基于以下事实，提出了一个简单的激波产生模型：(1)传热由通常的抛物线热传导控制（非傅立叶效应和量子效应不显著）；(2)液体对流可以忽略，但水的可压缩性变得重要；(3)金属表面达到水的均匀成核温度时发生相变，产生激波。该模型很好地解释了冲击波强度与激光辐照功率的关系。

The experimental and theoretical study of laser pulse heating of metal surfaces was conducted in water to make clear the physics of heat and fluid flow phenomena in nano-second order. In the experiment employing mercury as the test surface, it was found that the shock wave due to phase change of water and mercury is generated just after laser irradiation and, at the next stage of milli-seconds, a semi-spherical vapor bubble is formed and , in the last stage of a few seconds, the deformation of mercury surface is followed. In contrast, in case of heating in air, plasma of mercury appears in several micro-seconds after the irradiation accompanied by shock wave without deformation of mercury surface. In the experiment employing the silicon solid surface in water, a strong shock wave appears due to the phase change of water which travels in water with the speed of sound velocity. The phase change may be understand to occur when the surface temperature of silicon reaches the homogeneous nucleation temperature of water. In the case of heating of silicon surface in air, the shock wave is observed but this is mainly due to the expansion of air. In order to explain those experimental results, simple model of shock wave generation is proposed basing on the facts that (1)heat transfer is governed by usual parabolic heat conduction (non-Fourier and quantum effects are not significant) ; (2) liquid convection can be neglected but compressibility of water becomes important ; (3) phase change takes place when the metal surface reaches the homogeneous nucleation temperature of water, which yields the shock wave. This model explain well the dependence of shock wave strength on laser irradiation power.