René Streubel

Continuous multigram nanoparticle synthesis by high-power, high-repetition-rate ultrafast laser ablation in liquids

Utilizing a novel laser system consisting of a 500 W, 10 MHz, 3 ps laser source which is fully synchronized with a polygon scanner reaching scanning speeds up to 500 m/s, we explore the possibilities to increase the productivity of nanoparticle synthesis by laser ablation in liquids. By exploiting the high scanning speed, laser-induced cavitation bubbles are spatially bypassed at high repetition rates and continuous multigram ablation rates up to 4 g/h are demonstrated for platinum, gold, silver, aluminum, copper, and titanium. Furthermore, the applicable, ablation-effective repetition rate is increased by two orders of magnitude. The ultrafast ablation mechanisms are investigated for different laser fluences, repetition rates, interpulse distances, and ablation times, while the resulting trends are successfully described by validating a model developed for ultrafast laser ablation in air to hold in liquids as well.

Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids

The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h(-1) for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s(-1). This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL.

Pulsed laser ablation of a continuously-fed wire in liquid flow for high-yield production of silver nanoparticles

Using wires of defined diameters instead of a planar target for pulsed laser ablation in liquid results in significant increase of ablation efficiency and nanoparticle productivity up to a factor of 15. We identified several competitive phenomena based on thermal conductivity, reflectivity and cavitation bubble shape that affect the ablation efficiency when the geometry of the target is changed. On the basis of the obtained results, this work represents an intriguing starting point for further developments related to the up-scaling of pulsed laser ablation in liquid environments at the industrial level.

Depositing laser-generated nanoparticles on powders for additive manufacturing of oxide dispersed strengthened alloy parts via laser metal deposition

We present a novel route for the adsorption of pulsed laser-dispersed nanoparticles onto metal powders in aqueous solution without using any binders or surfactants. By electrostatic interaction, we deposit Y2O3 nanoparticles onto iron–chromium based powders and obtain a high dispersion of nano-sized particles on the metallic powders. Within the additively manufactured component, we show that the particle spacing of the oxide inclusion can be adjusted by the initial mass fraction of the adsorbed Y2O3 particles on the micropowder. Thus, our procedure constitutes a robust route for additive manufacturing of oxide dispersion-strengthened alloys via oxide nanoparticles supported on steel micropowders.

High-productivity synthesis of ligand-free nanoparticles by laser ablation in liquids

In typical wet chemical synthesis methods for nanoparticles (such as precipitation and sol-gel techniques) organic adsorbates are used, or marks of the residual reagents are left. Unfortunately, both these factors cause the deactivation of nanoparticle surfaces. That is, the chemisorbed or physisorbed ligands weaken the catalytic activity, interfere with surface-sensitive spectroscopy, and hinder surface-assisted mass spectrometry. In contrast, the physico-chemical synthesis method of pulsed laser ablation in liquid (PLAL) produces ligand-free, highly pure nanoparticles. In recent years, this technique has thus emerged as a way to produce a variety of nanoparticles with unique properties. Research in this field has progressed rapidly with regard to the design of innovative composites1–3 and their application in laser-assisted material processing techniques (e.g., rapid prototyping4 and laser lithography5). In comparison to ligand-coated nanoparticles, the laser-generated nanoparticles exhibit higher conjugation efficiency,6 higher grafting density,7 high electroaffinity toward charged biomolecules,8 and novel preparation routes for heterogeneous catalysts.9 With the current laser-based synthesis methods, however, it is difficult to achieve the large amounts of nanoparticles required for industrial applications. In PLAL methods, a laser pulse interacts with a target material and ignites a plasma, which in turn leads to the formation of a cavitation bubble (of vapor gas). Nanoparticles form inside this bubble10 and then disperse into the liquid after its collapse. In addition, the cavitation bubble causes the scattering of consecutive laser pulses if they arrive before the bubble collapses (which usually takes about 200 microseconds). The lifetime, as well as the dimensions (diameter of up to 1mm), of the cavitation Figure 1. (a) Schematic illustration of the novel pulsed laser ablation in liquid (PLAL) setup used to produce nanoparticles. The apparatus consists of a high-power laser system coupled with a polygon scanning system (which can reach ultrasonic speeds) in a specially designed liquid flow reactor. vfocus: Scanning speed of the focus. (b) Schematic diagram of the flow-through reactor, showing how the laser pulses spatially bypass the cavitation bubbles with interpulse distances of up to 400 m.

Pilot-scale synthesis of catalysis-relevant nanoparticles by high-power ultrafast laser ablation in liquids (Conference Presentation)

Pulsed Laser Ablation in Liquids is an innovative method, which is used to obtain colloidal solutions of nanoparticles that show unique properties and are not achievable by conventional synthesis methods. However, this method lacks of key parameters and scaling factors as well as a correlation between these factors and the occurring operating costs. During the laser driven synthesis cavitation bubbles filled with nanoparticles are formed. These cavitation bubbles along with already dispersed nanoparticles in the solution are the two major factors that limit the energy that can be coupled into the target material by shielding subsequent laser pulses. While the latter shielding effect can be avoided by suitable fluid handling avoiding the former is more difficult due to the lifetime (~100μs) and the size (~100μm) of cavitation bubbles which depend on the laser energy and pulse duration. In this work we present a strategy to scale up the process by enhancing the productivity of the synthesis. This approach utilizes a MHz-repetition rate laser system consisting of a 500W ps-laser source and a laser scanner that reaches a scanning speed of up to 500m/s. This unique system enables spatial bypassing the cavitation bubble and thereby applying most of the laser energy to the target. By using this system productivities of up to 5 gram per hour are demonstrated in a continuous process.