@inproceedings{AltmannSuppeltRuhletal.2024, author = {Altmann, Alexander Anton and Suppelt, Sven and Ruhl, Max and Schaumann, Stephan and Latsch, Bastian and Ben Dali, Omar and Zhukov, Sergey and Flachs, Dennis and Zhang, Xiaoqing and Thielemann, Christiane and Seggern, Heinz von and Kupnik, Mario}, title = {Monolithic Wideband Air-Coupled Ultrasonic Transducer Based on Additively Manufactured Ferroelectrets}, series = {2024 IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)}, booktitle = {2024 IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)}, publisher = {IEEE}, doi = {10.1109/UFFC-JS60046.2024.10793858}, pages = {1 -- 4}, year = {2024}, abstract = {Air-coupled ultrasonic transducers are widely used in non-destructive testing, acoustical sonar systems, and biomedical imaging. These applications require transducers that operate effectively across a broad acoustic frequency spectrum, offer adaptable geometric designs, and increasingly incorporate eco-friendly materials. In this work, we present a monolithic, 3D-printed air-coupled ultrasonic transducer based on ferroelectrets (FEs) and fabricated from biocompatible polylactic acid (PLA). We evaluated the transducer's acoustical performance by measuring the surface velocity of its active area using laser Doppler vibrometry and assessed its robustness during continuous operation over a 19-day period. Additionally, we measured the sound pressure level (SPL) and wideband characteristics in an anechoic chamber across excitation frequencies from 1kHz to 100kHz. At a resonance frequency of 33kHz, our transducer achieved an SPL of 94.3dB and surface velocities up to 37mm/s. The measured bandwidth of 65.2kHz at the -6dB threshold corresponds to a fractional bandwidth of 189\%. The observed exponential decay of the surface velocity, stabilizing at 15\% of its initial amplitude, aligns with the isothermal surface potential decay typically observed in FE films made from PLA. These results demonstrate the effectiveness of the transducer, which features an adaptable backplate for tuning acoustic properties. The low-cost transducer, manufactured from biocompatible PLA, is particularly suited for imaging and biomedical applications furthering green electronics.}, subject = {Ultraschallwandler}, language = {en} } @article{AltmannSuppeltWuestetal.2025, author = {Altmann, Alexander Anton and Suppelt, Sven and W{\"u}st, Philipp and D{\"o}rsam, Jan Helge and Latsch, Bastian and Flachs, Dennis and Blaeser, Andreas and Zhang, Xiaoqing and Thielemann, Christiane and Seggern, Heinz von and Kupnik, Mario}, title = {Self-supporting mid-air 3D printing of single-layer polypropylene structures: Flow rate-dependent analytical modeling and surface characterization}, series = {Additive Manufacturing}, volume = {113}, journal = {Additive Manufacturing}, publisher = {Elsevier BV}, issn = {2214-8604}, doi = {https://doi.org/10.1016/j.addma.2025.105010}, year = {2025}, abstract = {High-precision deposition in material extrusion-based additive manufacturing (MEX-AM), especially in mid-air extrusion, is essential for creating microfluidic channels, integrated sensors, and tissue scaffolds with fine vascular features. However, despite advances in support-free slicing and multi-axis printing, no standardized method exists for fabricating enclosed air cavities with high shape fidelity, particularly in single-layer structures. Reliable deposition at sub-millimeter scales remains challenging, especially when printing low-stiffness polymers such as polypropylene (PP), due to delayed solidification and viscoelastic effects. This study introduces a flow-rate-dependent analytical model to predict extrusion behavior, interfacial bonding, and surface morphology in self-supporting single-layer PP structures. To validate the model, surface roughness, material distribution, and intralayer bonding were quantified using spectral analysis, cross-sectional thickness measurements, and Abbott-Firestone curve evaluation. Results show that reducing the flow rate (relative to 100\% nominal flow through a 0.4 mm nozzle) to 40\% improves surface roughness (Ra) to below 20 m on average across all surfaces, with the lowest Ra of 4.52 ± 0.49 m observed at a 20\% flow rate on the top surfaces enclosing the cavity. The intralayer bonding between adjacent extruded lines increases up to 70\%, improving deposition uniformity. Nozzle-induced smoothing effects enhance surface quality at flow rates of 60\% or lower, while higher flow rates result in increased surface waviness and geometric irregularities. These findings enable the fabrication of enclosed microfluidic channels and functional cavities with well-defined surfaces and reduced mechanical stiffness, suitable for applications involving sensing, controlled deformation, or flexible system integration.}, subject = {Fertigungstechnik}, language = {en} }