Norbert Ackerl

Last Name

Ackerl

First Name

Norbert

ORCID

0000-0001-5882-3243

Show all metadata

Search Results

Publications 1 - 10 of 30

Laser-Microstructured Copper Reveals Selectivity Patterns in the Electrocatalytic Reduction of CO2
Veenstra, Florentine L.P.; Ackerl, Norbert; Martín, Antonio J.; et al. (2020)
Chem
The strategy of engineering the local chemical environment to direct selectivity in the electroreduction of CO2 toward value-added products is only qualitatively understood. The unfeasibility of local concentration measurements and the limited applicability of simulations to practical systems hinder more precise guidelines. Herein, we quantify the impact of the (electro)chemical environment on the selectivity pattern using microstructured Cu electrodes prepared by ultrashort pulse laser ablation. We created regularly distributed micro-probes and assessed their product distributions at distinct overpotentials. The regular geometry enabled the accurate simulation of the local pH and CO2 concentration. Selectivity maps useful for mechanistic and applied studies emerged. They reveal clear patterns for C1-C3 products suggesting novel insights such as the presence of two reaction mechanisms for propanol. The effect on the selectivity pattern of operation parameters such as enhanced mass transport and electrolyte composition was also predicted by the maps.
CAM solution for quasi-tangential laser ablation of complex 3D workpieces
Ackerl, Norbert; Gysel, Johannes; Warhanek, Maximilian; et al. (2019)
Lasers in Manufacturing 2019. Online Proceedings
A fast forward computation of the laser ablation paths and hatches for complex 3D geometries enabled by a computer-aided manufacturing (CAM) toolbox is presented. Specifically, the laser paths for quasi-tangential irradiance condition on a rotary specimen are calculated and the machine code for a laser manufacturing system with up to seven synchronously controllable axes generated. In order to set the parameters for the path determination, an empirical parameter study points to the material specific removal rate in accordance with pulse and line overlap. Therefore, a manifold use of this tool is possible for all laser sources from continuous wave to ultra-short pulses. This leads to a unique flexible way, hith-erto not available for fast prototyping applications. Following, laser manufactured specimens with complex contour shapes reveal the high potential of this approach. High-precision laser manufactured dental implants and diamond-grinding tools with tolerances in the micrometer range serve as demonstrators.
Geometry assessment of ultra-short pulsed laser drilled micro-holes
Putzer, Matthias; Ackerl, Norbert; Wegener, Konrad (2021)
The International Journal of Advanced Manufacturing Technology
Ultra-short pulsed laser ablation enables a defined generation of micro-holes. A parameter study on the ablation characteristics of copper clearly reveals a benefit for green wavelength with lower threshold fluence, simultaneously increasing the Rayleigh length. The use of a circular drilling method allows a defined manufacturing of micro boreholes and micro through-holes with 35 mu m diameter of up to 165 mu m and 300 mu m length. Introducing high-resolution micro-computed X-ray tomography studying the micro-hole evolution and adjacent geometrical transformations reveals micrometer resolution and high usability. The conical geometry evolving up to an aspect ratio of 5:1 fits well to established models known for percussion drilling. However, increasing the number of pulses leads to non-conical geometry evolution, and this resulting geometry is studied for the first time. Henceforth, the exact geometrical evolution from conical to cylindrical shape upon laser drilling can be resolved revealing the impact of multiple reflections at the generated steep flanks.
Laser Surface Functionalization from Fundamentals to Application
Ackerl, Norbert (2020)
In the last decade stable ultra-short pulsed (USP) laser systems have become more widely available and are especially useful to ablate materials in a defined manner. Using small pulse energies mitigates heat input to the specimen useful for removing heat-sensitive materials. The exact ablation mechanism is still controversially discussed and as well the formation of sub-wavelength ripples. Utilizing laser radiation for microstructuring and cutting can potentially bridge the manufacturing feature-size gap of conventional techniques and clean room technology enabling innovative applications. However, there is a need in developing novel machine tool concepts and processing strategies with respect to a certain use considering this force-free process. In order to tailor the functionality governed by topography and chemistry, the ablation effects have to be unraveled exploiting the use of USP laser machining routines. Light-matter interaction mechanisms leading to ablation and evolution of self-assembled micro- and nanostructures have to be controlled transferring these unique patterns to application. This thesis presents experimental configurations developed to address the challenges just described. The test-beds consist of a combination using mechanical and optical axes encompassing a USP laser system, modifying optics and a beam delivery. Combined processes with radial and quasi-tangential irradiation condition are established for a fast production, where the controller speed is identified being the bottleneck for further acceleration. The impact of laser machining to the specimens is discussed based on microscopy and spectroscopy data using photons and electrons. Fundamental studies on single- and multi-pulse ablation reveals precursor ripple structures as origin of cone-like-protrusions (CLP) observed at steel samples. Stop-motion imaging combined with micro-structural assessments points to an evolution of topology from laser-induced periodic surface structures (LIPSS) - supra-wavelength ripples - to CLP after more than one laser pass. This evolution depends on the polarization state, fluence, total energy, and the grain orientation of the specimen. Moreover, the LIPSS spatial periodicity is tuned by wavelength being the major influence and the orientation controlled by wave plates. These findings enable a defined laser micro-machining of a wide range of materials from metals to dielectrics with the processing strategy being the key to success. Micrometer features are manufactured with high-precision paving the way for a rational surface design leading to a tailored function. The work proposes laser machining as a viable technology for many applications and especially for heat-sensitive and ultra-hard materials to introduce a distinct function. New laser marking strategies facilitate a coloration proven with oxide layers on steel and titanium substrates using interference and additionally LIPSS as diffraction gratings. Hierarchical structures on copper allow to control the wettability enabling passive droplet movement, mixing, and drop-wise condensation. Innovative hatched cutting and drilling strategies allow a production of high-precision hard shadow masks for lithography processing and chemical micro-reactors pointing to a selective production of hydrocarbons in electrocatalysis. Defined ablation of ceramics makes the USP laser machining of dental implants possible introducing customized biointerfaces to alter the osseointegration. However, quasi-tangential USP processing induces a sub-surface phase transition of the alumina-toughened zirconia being the origin of crack initiation. Using a customized scanhead-free setup enables the use of high average power for laser conditioning of super-abrasive grinding tools. This highlights the use of USP laser machining reaching a sub-micrometer precision on millimeter long contours with persistent diamond phase. Taking advantage of the selectivity between dissimilar materials with corresponding threshold fluence allows a laser sharpening to generate protrusions in favor of high-speed and precision grinding processes.
Laser marking and coloration of Ti-6Al-4V with ultrashort pulses
Ackerl, Norbert; Gugger, Pascal; Wegener, Konrad (2020)
Journal of Laser Applications
© 2020 Author(s). A routine for color marking using oxide layers and laser-induced periodic surface structures is presented. Titanium and alloys thereof are marked with pixelated graphics at a high resolution with tempering colors. A computational approach for the laser path calculation enables a fast-forward marking of complex designs. The color map attained from a laser parameter studies enables vivid coloration. The minimal color pixel size is given by the optical setup and laser wavelength at near-infrared and green radiation to the focal spot size. A pixel size of 35 and 20 μ m was reached within this study, and no cross talk and distinctness between adjacent colors were observed. The oxide layer growth is sensitive on the applied laser strategy and parameter setting; however, a set of stable coloration conditions is conceived. Hitherto, fine color nuances in blue unravel the unique potential of this approach by oxidizing the substrate leading to a defined oxide layer thickness at a high repetition rate below the threshold fluence. Additionally, ultrashort pulsed laser pulses below 10 ps enable the generation of laser-induced periodic surface structures. In the low spatial frequency regime, these structures are correlated with the polarization direction of the laser light. Afterward, diffraction gratings with rotated spatial periodicity are manufactured using a half-wave plate. This allows forgery-proof marking strategies, where both mechanisms could be superimposed to increase the information density and complicating counterfeit product labeling.
Experimental and theoretical investigation of ultrashort pulsed laser ablation of diamond
Boerner, Paul; Hajri, Melik; Ackerl, Norbert; et al. (2019)
Journal of Laser Applications
Ultra-Short Pulsed Laser Marking and Coloration of Metals with Segmented Pixel Parameter Transformation
Ackerl, Norbert; Gugger, Pascal; Warhanek, Maximilian; et al. (2020)
Journal of Laser Micro / Nanoengineering
Toward application of hierarchical structures by ultrashort pulsed laser ablation
Ackerl, Norbert; Boerner, Paul; Wegener, Konrad (2019)
Journal of Laser Applications
Model structures of molten salt-promoted MgO to probe the mechanism of MgCO3 formation during CO2 capture at a solid-liquid interface
Hansen Bork, Alexander; Ackerl, Norbert; Reuteler, Joakim; et al. (2022)
Journal of Materials Chemistry A
MgO is a promising solid oxide-based sorbent to capture anthropogenic CO2 emissions due to its high theoretical gravimetric CO2 uptake and its abundance. When MgO is coated with alkali metal salts such as LiNO3, NaNO3, KNO3, or their mixtures, the kinetics of the CO2 uptake reaction is significantly faster resulting in a 15 times higher CO2 uptake compared to bare MgO. However, the underlying mechanism that leads to this dramatic increase in the carbonation rate is still unclear. This study aims to determine the most favourable location for the nucleation and growth of MgCO3 and more specifically, whether the carbonation occurs preferentially at the buried interface, the triple phase boundary (TPB), and/or inside the molten salt of the NaNO3-MgO system. For this purpose, a model system consisting of a MgO single crystal that is structured by ultra-short pulse laser ablation and coated with NaNO3 as the promoter is used. To identify the location of nucleation and growth of MgCO3, micro X-ray computed tomography, scanning electron microscopy, Raman microspectroscopy and optical profilometry were applied. We found that MgCO3 forms at the NaNO3/MgO interface and not inside the melt. Moreover, there was no preferential nucleation of MgCO3 at the TPB when compared to the buried interface. Furthermore, it is found that there is no observable CO2 diffusion limitation in the nucleation step. However, it was observed that CO2 diffusion limits MgCO3 crystal growth, i.e. the growth rate of MgCO3 is approximately an order of magnitude faster in shallow grooves compared to that in deep grooves.
Efficiency Study on Single Pulse, Burst Mode and Multi Pulse Ultra-Short Pulsed Ablation of Pure Copper
Ackerl, Norbert; Wegener, Konrad (2019)
2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Publications 1 - 10 of 30

Norbert Ackerl

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results