Dual-Step Sintering of Metal Nanoparticles with Femtosecond Laser Pulses

University Of Texas At Austin, Austin TX

Investigators

Abstract

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.