Henning Heuer

This study investigates the capability of high-frequency eddy-current (HFEC) testing for the characterization of carbon fiber-reinforced polymer (CFRP) laminates layup by analyzing the angular response of a transmitter-receiver (T-R) eddy-current probe. Three-dimensional finite-element simulations were performed to analyze the distribution of induced currents and the angular resistance response in laminates with different fiber orientations and stacking sequences. The simulations demonstrate that the angular positions of the response peaks identify the fiber orientations, whereas their relative amplitudes provide quantitative information on the ply stacking sequence. The numerical predictions were validated experimentally using an HFEC imaging system. Both numerical and experimental results show that the measured HFEC response is governed by the combined effects of probe orientation, fiber orientation, and scan direction, demonstrating the potential of the proposed approach for non-destructive characterization of CFRP laminate layup.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) provide a path to mass-produced cost-efficient transducers for clinical imaging and wearables. However, their inherent nonlinearities, especially at high excitation voltages, can degrade performance and image quality. This requires fast non-contact characterization of individual CMUT transducers directly on a wafer. This work demonstrates the first application of the Synchronized Swept-Sine Technique (SST) in Laser-Doppler Vibrometry (LDV) for comprehensive characterization of CMUTs on individual cell level. In the SST, the extraction of both linear and nonlinear harmonic transfer functions from a single measurement is possible by excitation with an exponential chirp and application of an inverse filter. Using a scanning LDV system, we characterized individual cells in a 64-cell CMUT array (50 μm diameter) across a 1...16 MHz frequency range in both air and oil. The method reveals quantitative spectral data, including resonance at 6.4 MHz and second-harmonic distortion at –11.8 dB relative to the fundamental for the investigated CMUT. Results demonstrate significant cell-to-cell variations in harmonic transfer functions and reveal nonlinear behavior even in non-excited cells due to inter-cell acoustic coupling. These findings establish SST-LDV as a powerful complementary technique to electrical impedance measurements. The method provides spatially resolved mechanical insights that are crucial for the development of CMUTs, model validation, and potentially, quality control in high-throughput MEMS manufacturing.

Emerging ultrasound imaging technologies such as wearables and miniaturized invasive devices require exceptional piezoelectric performance alongside flexibility, small form factors, biocompatibility and scalable production capabilities. To address this, we investigate the embedding of high-performance lead magnesium niobate-lead titanate (PMN-PT) transducers in a flexible and biocompatible Liquid Crystal Polymer (LCP) substrate with integrated conductor tracks. This approach aims to improve mechanical reliability and enable industrial-scale production of compact ultrasound devices. The piezoelectric elements were contacted during the embedding process, involving exposure to heat and pressure, requiring a subsequent repolarization process to restore the piezoelectric behavior. Performance was validated against encapsulated wire-bonded reference samples through electrical impedance measurements and acoustic pitch-catch experiments using a hydrophone in a water tank, with different sound incidence angles. After repoling, the embedded single crystal transducer demonstrated expected piezoelectric behavior in impedance data and achieved a peak negative output pressure amplitude of 4.70 kPa (measured at 10 mm distance in water), compared to 4.45 kPa for the encapsulated wire-bonded reference sample, representing a difference of ≈ 5 %. Directivity patterns showed minimal differences in emission angle, with resonance frequencies varying between prototypes due to the inherent characteristics of different manufacturing approaches in the acoustic stack. This embedding approach shows promising results for miniaturized applications, including external ultrasound wearables and patches, as well as invasive devices such as intravascular and implantable ultrasound systems.

State‐of‐the‐art methods to monitor the degree of wetting of lithium‐ion batteries during production, which are applicable at an industrial scale, are not capable of determining the spatial electrolyte distribution in the porous network of electrodes and separator. Ultrasound can bridge this gap as it has previously been proven to be a suitable method to monitor or evaluate the infiltration of porous structures, for example, epoxy resin in carbon fiber composite. Herein, the acoustic properties of different battery cell materials are evaluated over a wide frequency range in both the dry and wet states, for the first time. Furthermore, a collection of input parameters, from the experimental setup and the evaluation routine to the acoustic parameters of the battery materials themselves, are provided. These parameters are needed for the adaptation of ultrasonic nondestructive testing to the monitoring of the wetting process of a lithium‐ion battery. Most importantly, this work demonstrates that a large change in the sound velocity (up to 100%) is observed when the porous structure of a lithium‐ion battery is filled with electrolyte and thus can serve as an appropriate evaluation figure of merit.

Driven by the electrification and the increasing use of security-relevant electronics thermomechanical fatigue of solder joints gains in importance. This research investigates the geometric attributes of solder joints in a high detailed way based on two-pole resistors. Using a 3D profilometer the shape of the solder meniscus, the standoff height, the offset and the tilt angle of the component are analyzed. A void calculation based on radioscopic measurements and the results of solder paste inspection supplement the comprehensive analysis of the solder joint characteristics. These studies show correlations between the initial attributes of solder joints themselves and the thermomechanical fatigue process due to temperature cycling. The initial geometric attributes of the components are also compared with the values of the same components after thermomechanical loading. The level of damage was measured by using shear tests and a failure probability according to Weibull was determined.

The suppression of unnecessary radio-electronic noise and the protection of electronic devices from electromagnetic interference by the use of pliable highly microwave radiation absorbing composite materials based on polymers or rubbers filled with conductive and magnetic fillers have been proposed. Since the working frequency bands of electronic devices and systems are rapidly expanding up to the millimeter wave range, the capabilities of absorbing and shielding composites should be evaluated for increasing operating frequency. The point is that the absorption capacity of conductive and magnetic fillers essentially decreases as the frequency increases. Therefore, this paper is devoted to the absorbing capabilities of composites filled with high-loss dielectric fillers, in which absorption significantly increases as frequency rises, and it is possible to achieve the maximum frequency selective of absorption due to electromagnetic and electromechanical resonances.