Structuring electrodes of lithium-ion batteries with liquid electrolyte leads to an increase in the performance of the battery cell, as shown in several studies. The effective diffusion of the lithium ions can be significantly increased by introducing microchannels. This leads on the one hand to higher charge and discharge rates, and on the other hand to an increase in the number of charge cycles and thus to a longer lifetime. Even though there are already many publications on the laser structuring of electrodes, the fundamental interaction mechanisms have not yet been fully developed without doubt. Also, the lack of progress productivity is currently preventing industrial application. For this reason, the influence of pulse-bursts on the laser structuring process of the active material of electrodes was investigated within the scope of this work. The influence of the variation of the number of pulse bursts, the repetition rate as well as the peak fluence on the ablation rate and the ablation efficiency was investigated. The results were evaluated by an automated analysis procedure to allow a systematic and statistical assessment of 110 induced features per parameter set in a time-saving manner. It was shown that the efficiency of the individual pulses could be increased by up to 75% when machining NMC. Due to the pulse bursts, a five times greater increase in the ablation rate could be achieved on the anode side. In addition, cross-sections of the introduced pores were examined by scanning electron microscopy combined with EDS analysis to identify a possible thermal influence of the laser material processing.
In recent years, various studies have described the potential of Direct Laser Interference Patterning (DLIP) technology for technical surfaces in industrial applications. The focus of the studies is currently on the variability of the structural patterns (also in combination with Direct Laser Writing processes), the increase of the aspect-ratio and the upscaling to high processing speeds as well as large area structuring using mainly laser-scanner systems. Common are spatial periods of a few microns. However, for some DLIP applications regarding the structuring of tool steel the focus is different. Namely it can be beneficial to achieve a variable spatial period while maintaining a well-defined structure. Examples are molding or embossing processes where the structured tool steel is used as a template. Again, the spatial periods ranges in the magnitude of micrometers or below. This study describes the development of an automated system technology for a high-precision DLIP setup without using a laser-scanner-system. Two coherent beams from a beam source with ultrashort laser pulses of 10 ps and a wavelength of 532 nm are superimposed on a metallic workpiece to ablate line-like periodic patterns by interference. The characteristic spatial period created in the process is dependent on the incident angle between the two beams. It can be varied in the micro- and sub-micrometer range by implementing axes to automate the beam guidance and subsequently controlling the incident angle. The setup is calibrated and fine-tuned in a process where the spatial periods structured at different incident angles are validated using an Atomic Force Microscope (AFM). Further automation is achieved by developing a user interface for intuitive control. Finally, various spatial periods are structured onto tool steel for industrial applications in the field of the automotive industry.
The technology of 3D printing is conquering the world and awakens the interest of many users in the most varying of applications. New formulation approaches for photo-sensitive thiol-ene resins in combination with various printing technologies, like stereolithography (SLA), projection based printing/digital light processing (DLP) or two-photon polymerization (TPP) are presented. Thiol-ene polymerizations are known for its fast and quantitative reaction and to form highly homogeneous polymer networks. As the resins are locally and temporally photo-curable the polymerization type is very promising for 3D-printing. By using suitable wavelengths, photoinitiator-free fabrication is feasible for single- and two photon induced polymerization. In this paper divinyl ethers of polyethylene glycols in combination with star-shaped tetrathiols were used to design a simple test-system for photo-curable thiol-ene resins. In order to control and improve curing depth and lateral resolution in 3D-polymerization processes, either additives in chemical formulation or process parameters can be changed. The achieved curing depth and resolution limits depend on the applied fabrication method. While two-/multiphoton induced lithography offers the possibility of micron- to sub-micron resolution it lacks in built-up speed. Hence single-photon polymerization is a fast alternative with optimization potential in sub-10-micron resolution. Absorber- and initiator free compositions were developed in order to avoid aging, yellowing and toxicity of resulting products. They can be cured with UV-laser radiation below 300 nm. The development at Fraunhofer ILT is focusing on new applications in the field of medical products and implants, technical products with respect to mechanical properties or optical properties of 3D-printed objects. Recent process results with model system (polyethylene glycol divinylether/ Pentaerithrytol tetrakis (3-mercaptopropionat), Raman measurements of polymer conversion and surface modifications using bifunctional crosslinkers are presented with advantages, drawbacks and a general outlook.
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