Dual-Step Sintering of Metal Nanoparticles with Femtosecond Laser Pulses

金属颗粒的选择性激光烧结是一种增材制造工艺，在制造具有复杂几何形状的金属部件方面具有巨大的潜力。利用纳秒和连续波激光来烧结（结合）金属颗粒的当前系统可能遭受高孔隙率和高残余应力。脉冲持续时间小于10^-12 s的飞秒激光具有高关联功率和有限的热影响区，因此可以有效地烧结金属纳米颗粒，以生产具有降低的残余应力的致密部件。也就是说，在使用飞秒激光器有效烧结金属纳米颗粒方面仍然存在挑战，因为高功率脉冲可以在颗粒烧结之前烧蚀（去除）颗粒，从而导致不完整的印刷特征。为了解决这个问题，计划采用双脉冲串策略，其中第一个激光脉冲将颗粒加热到低于发生烧蚀的水平，而第二个脉冲将诱导所需的烧结。成功拓宽飞秒激光加工窗口将有助于制造具有复杂、小尺寸几何形状和高机械/电气/热完整性的金属部件;这种组合为需要精密部件的关键应用行业提供了潜力，例如航空航天。小的相关热影响区意味着该方法也可以对柔性电子产品的制造产生积极影响。此外，该奖项还将为少数民族研究生和本科生提供研究培训，并将通过德克萨斯州奥斯汀地区的教师研究经验（RET）计划制作教学材料进行传播。飞秒激光非热烧蚀广泛用于高精度制造，其中热电子的快速积累可导致原子键的断裂并有助于去除不必要的材料。 这在散装材料的成形中是有益的。然而，在金属纳米颗粒的烧结中，热电子可能导致烧蚀并严重危及所需工艺结果的有效性。为了实现适当的烧结，金属纳米颗粒必须部分熔化，这需要将晶格温度提高到一定阈值以上。然而，热电子效应发生得比电子-声子耦合快得多，因此阻止了晶格温度的期望增加。提出了一种两步烧结策略，该策略将使用双脉冲串来抑制热电子效应，同时仍在广泛的实验条件下促进烧结。为了实现这一点，飞秒激光与贵金属纳米粒子（银，铜，金）的相互作用的机制必须首先被理解。机制将使用超快宽带瞬态吸收光谱，并在飞秒激光烧结实验过程中的瞬态热反射测量。修改后的两个温度模型将被开发来分析实验数据，并提供一个基础，从可行的处理窗口可以被隔离。通过这项研究提供的知识将揭示飞秒激光烧结金属纳米颗粒的动态过程，并弥合理论预测和传统烧结结果之间的长期差距。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Selective laser sintering of metal particles, an additive manufacturing process, has great potential in the manufacture of metal components with complex geometries. Current systems which utilize nanosecond and continuous-wave lasers to sinter (bond) the metal particles can suffer from high porosity and high residual stresses. Femtosecond lasers, with a pulse duration of less than 10^-12 s, have high associated power and a limited heat affected zone, and thus can effectively sinter metal nanoparticles to produce dense parts with reduced residual stresses. That said, challenges remain in the effective sintering of metal nanoparticles with femtosecond lasers as the high-power pulses can ablate (remove) the particles before they are sintered resulting in incomplete printed features. To address this problem, a double-pulse train strategy is planned, whereby the first laser pulse will heat the particles to a level below where ablation occurs, while the second pulse will induce the required sintering. Successfully widening the femtosecond laser processing window will facilitate the manufacture of metal components with complex, small-scale geometries and high mechanical/electrical/thermal integrity; a combination that offers potential for industries requiring precision components for critical applications, i.e. aerospace. The small associated heat affect zone implies the approach could also positively impact the manufacture of flexible electronics. Additionally, this award will provide research training for minority graduate and undergraduate students, and will generate teaching materials for dissemination through Research Experience for Teachers (RET) programs in the Austin, Texas region.Non-thermal ablation by femtosecond lasers is widely used in high-precision manufacturing, where the rapid accumulation of hot electrons can cause the breaking of atomic bonds and facilitate the removal of unnecessary material. This is beneficial in the shaping of bulk materials. However, in the sintering of metal nanoparticles, hot electrons can result in ablation and seriously jeopardize the effectiveness of the desired process outcomes. To achieve proper sintering, metal nanoparticles must be partially melted, which requires increasing the lattice temperature above a certain threshold. Hot electron effects however occur much faster than electron-phonon coupling, hence preventing the desired increase of lattice temperature. A two-step sintering strategy is proposed that will use a double-pulse train to suppress the hot electron effects while still promoting sintering across a broad range of experimental conditions. To achieve this the mechanisms of femtosecond-laser interaction with noble metal nanoparticles (silver, copper, gold) must first be understood. The mechanisms will be investigated using ultrafast broad-band transient absorption spectroscopy, and transient thermoreflectance measurements during femtosecond laser sintering experiments. Modified two-temperature models will be developed to analyze experimental data and to provide a foundation from which viable processing windows can be isolated. Knowledge delivered through this research will reveal the dynamic process of femtosecond-laser sintering of metal nanoparticles and bridge the long-standing gap between theoretical predictions and conventional sintering outcomes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.