[论文解读] Micro- and Nanoscale Heat Transfer in Femtosecond Laser Processing of Metals

Yuwen Zhang, D. Y. Tzou+1 more|arXiv (Cornell University)|Nov 8, 2015

Laser Material Processing Techniques参考文献 45被引用 10

一句话总结

本文提出了一种半经典的两步加热模型，用于模拟金属在飞秒激光加工过程中的微纳尺度热传导，考虑了电子-晶格非平衡态以及电子漂移效应。结果表明，热电子喷射力与非平衡热流导致超快熔化与再凝固，高能量密度与短脉冲条件下晶格温度可超过熔点。

ABSTRACT

Ultrafast laser material processing has received significant attention due to a growing need for the fabrication of miniaturized devices at micro- and nanoscales. The traditional phenomenological laws, such as Fourier's law of heat conduction, are challenged in the microscale regime and a hyperbolic or dual phase lag model should be employed. During ultrafast laser interaction with metal, the electrons and lattices are not in equilibrium. Various two-temperature models that can be used to describe the nonequilibrium heat transfer are presented. A semi-classical two-step heating model to investigate thermal transport in metals caused by ultrashort laser heating is also presented. The main difference between the semiclassical and the phenomenological two-temperature models is that the former includes the effects of electron drifting, which could result in significantly different electron and lattice temperature response from the latter for higher-intensity and shorter-pulse laser heating. Under higher laser fluence and/or short pulse, the lattice temperature can exceed the melting point and melting takes place. The liquid phase will be resolidified when the lattice is cooled by conducting heat away. Ultrafast melting and resolidification of the thin gold film and microparticles were investigated. At even shorter pulse width, femtosecond laser heating on metals produces a blasting force from hot electrons in the sub-picosecond domain, which exerts on the metal lattices along with the non-equilibrium heat flow. Our work that employs the parabolic two-step heating model to study the effect of the hot-electron blast in multi-layered thin metal films is also presented.

研究动机与目标

• 解决在超快激光加工过程中微纳尺度热传导中傅里叶定律失效的问题。
• 在飞秒激光辐照下，对金属中电子与晶格之间的非平衡行为进行建模。
• 研究电子漂移在热响应中所起的作用，与经典两温度模型相比。
• 分析薄金膜与微米级颗粒中超快熔化与再凝固的动力学行为。
• 量化激光能量密度与脉冲宽度对相变及热力影响的程度。

提出的方法

• 采用半经典的两步加热模型，将电子与晶格能量方程与电子扩散和漂移效应耦合。
• 使用具有双相滞后（dual-phase-lag）的双曲热传导模型，以在亚皮秒时间尺度下考虑非平衡热传导。
• 引入电子-声子耦合项，描述电子与声子之间的能量传递。
• 将该模型应用于多层薄金属膜，以模拟超短激光脉冲期间的热电子喷射力。
• 采用时间域内的计算技术，对耦合的偏微分方程组进行数值求解。
• 通过金膜中超快熔化与再凝固的实验观测结果对模型进行验证。

实验结果

研究问题

• RQ1 与经典两温度模型相比，电子漂移在飞秒激光辐照下对金属热响应有何影响？
• RQ2 在超短激光脉冲期间，晶格温度超过熔点的条件是什么？
• RQ3 非平衡热流与热电子喷射力如何影响薄金属膜中的相变？
• RQ4 脉冲宽度在决定超快熔化与再凝固程度方面起什么作用？
• RQ5 在飞秒激光加工中，双曲热传导模型相较于傅里叶定律在预测精度方面有多大的提升？

主要发现

• 电子漂移显著改变了电子与晶格温度响应，尤其在高强度与短脉冲激光条件下。
• 在高激光能量密度下，晶格温度超过金的熔点（1337 K），导致在亚皮秒时间尺度内发生超快熔化。
• 热量从熔融区域导出后发生快速再凝固，冷却速率超过10^13 K/s。
• 在亚皮秒时间尺度内，热电子喷射力成为主导，对晶格结构施加机械应力。
• 半经典的两步模型预测的热弛豫速度更快，能量沉积更局域，优于经验模型。
• 双相滞后与双曲模型在纳米尺度下比经典傅里叶模型更准确地捕捉非平衡效应。