Dual-Step Sintering of Metal Nanoparticles with Femtosecond Laser Pulses

金属颗粒选择性激光烧结是一种添加剂制造工艺，在制造复杂几何形状的金属零件方面具有很大的潜力。目前使用纳秒和连续波激光来烧结(粘结)金属颗粒的系统会受到高孔隙率和高残余应力的影响。S脉冲宽度小于10^-12的飞秒激光具有高相关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，以降低残余应力来生产致密零件。也就是说，在用飞秒激光有效地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被烧结之前烧蚀(移除)颗粒，从而导致不完整的印刷特征。为了解决这个问题，计划了一个双脉冲序列策略，根据该策略，第一个激光脉冲将把颗粒加热到发生烧蚀的水平以下，而第二个脉冲将诱导所需的烧结。成功地拓宽飞秒激光加工窗口将有助于制造具有复杂、小规模几何形状和高机械/电气/热完整性的金属部件；这一组合为关键应用(如航空航天)需要精密部件的行业提供了潜力。较小的相关热影响区域意味着，这种方法也可能对柔性电子产品的制造产生积极影响。此外，该奖项将为少数族裔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的教师研究经验(RET)计划生成教材供分发。飞秒激光的非热烧蚀广泛用于高精度制造，在高精度制造中，热电子的快速积累可能会导致原子键断裂，并有助于去除不必要的材料。这对散装材料的成型是有利的。然而，在金属纳米颗粒的烧结过程中，热电子会导致烧蚀，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的阈值以上。然而，热电子效应的发生速度比电子-声子耦合快得多，因此阻止了晶格温度的预期增加。提出了一种两步烧结策略，该策略将使用双脉冲序列来抑制热电子效应，同时在广泛的实验条件下仍能促进烧结。要实现这一点，必须首先了解飞秒激光与贵金属纳米颗粒(银、铜、金)相互作用的机制。在飞秒激光烧结实验中，将使用超快宽带瞬时吸收光谱和瞬时热反射测量来研究其机理。将开发改进的双温度模型来分析实验数据，并提供一个可以隔离可行的处理窗口的基础。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智力优势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.