Session: 17-15-01: Society-Wide Micro/Nano Poster Forum

Paper Number: 99553

99553 - Dual-Step Sintering of Cu Nanoparticles With Femtosecond Laser 

Selective laser sintering with femtosecond (fs) lasers and metal nanoparticles (NPs) can achieve high precision and dense sub-micron features with lower porosity, low oxidation and reduced residual stress, due to the high peak power and highly localized heat affected zone. Successful sintering of metal NPs with fs laser is challenging due to the ablation caused by hot electron effects, such as blast and Coulomb explosion. Electron firstly absorb the laser energy and becomes a hot electron gas with a temperature of several thousand Kelvins. These hot electrons can ablate nanoparticles within a couple of picoseconds, way before the energy is passed to the lattice, and hence hinder the sintering process. The ablation can occur through three paths: (i) the hot electron blast, (ii) the Coulomb explosion, and (iii) the Coulomb repulsion. When a laser is illuminated onto a metal NP, electrons are drawn out and flowing over the surface of the NP due to a thermionic emission. At the same time, inside NPs are positively charged since the electrons are moved to the surface. The density gradient of charged particles in the NP can generate high internal pressure and hence blasting force. Simultaneously, an internal electric field is built up in the NP, which can be strong enough to break the NP into small parts, the so-called Coulomb explosion. Moreover, there is a repulsive force among the positively charged NPs, and this force results in the Coulomb repulsion. In this study, we proposed a dual-step sintering strategy that deposits the laser energy into metal NPs via multiple steps to lower the electron temperature significantly and while still maintaining a high enough lattice temperature for sintering in order to avoid the ablation. To demonstrate this concept, we utilized a pair of fs laser pulses with a time delay at picosecond scale to sinter Cu NPs. The delay time is chosen to be comparable with the electron-phonon coupling time so that the highest electron temperature would not be too high to cause ablation, while the final lattice temperature is still enough for sintering. When the delay time is slightly longer than the electron-phonon coupling time of Cu NPs, it is shown that the ablation area is drastically reduced and the power window for successful sintering can be extended by about 2 times. At the same time, the heat affected zone can be reduced by 66% (area). Our results showed that comparing with sintering with single femtosecond laser pulse train, with this dual-step sintering strategy: (i) the ablation could be suppressed significantly and the sintering window could be extended by twice; (ii) the heat affected zone in the glass substrate can be reduced by more than half; (iii) there is clear improvement on surface smoothness. This new strategy can be adopted for all the SLS processes with fs laser and unlock the power of fs-SLS for future applications.

Presenting Author: Janghan Park The University of Texas at Austin

Presenting Author Biography: Janghan Park is a PhD student at The University of Texas at Austin majoring in Mechanical Engineering. He received his B.S. degree from University of New Mexico. His research interests are selective laser sintering and light-matter interactions using optics and lasers.

Authors:

Janghan Park The University of Texas at Austin
Yaguo Wang The University of Texas at Austin