Volume 90, Issue 11 - Special Issue: XXXVI. Messtechnisches Symposium des AHMT 2022 / Gastherausgeberin: Ulrike Steinmann

This work studies poisoning by the cyclic siloxane octamethylcyclotetrasiloxane on a commercially available semiconductor gas sensor in TCO (temperature cycled operation). The data is evaluated using the Sauerwald-Baur model and the DSR method (differential surface reduction). The sensitivity towards several gases (volatile organic compounds, hydrogen and carbon monoxide) is evaluated and compared with a sensor in constant temperature operation mode. The physical and chemical processes on the sensitive layer as well as the resulting selectivity towards hydrogen are discussed. A feature is identified that can be derived from the Sauerwald-Baur model (the differential surface oxidation, DSO) and that quantitatively expresses the sensor condition regarding siloxane poisoning. With the help of this feature, a self-compensation of the sensor signal is demonstrated.

Polymers hold great potential for the use in microsensors and organic electronics. They are highly adaptable, easy to process and can contribute new or improved capabilities compared to semiconductors. Direct UV laser lithography also gains increasing attention. Because it avoids expensive photomasks, it is especially attractive where small numbers of specialized microcomponents are needed, like in prototyping. Lithography necessitates materials, which can be shaped by UV radiation. For many microsensor applications, there is the additional requirement of electric conductivity, preferably in the same material. We approached this demand by combining a Novolak and terthiophene doped with copper(II) perchlorate to form an interpenetrating polymer network, which possesses properties of both of its constituents. From this, we manufactured test structures with the UV laser of a micro pattern generator. In previous conference contributions, we showed a first proof of principle. In this publication, we present results of new experiments that demonstrate the characteristics in more detail. We improved our electrical setup to conduct four-terminal measuring. We used it to first verify previous results and investigated the material’s response to alternating currents up to 10 kHz. We then compared the electrical resistivity of differently sized structures for temperatures between 20 and 90 °C and examined long-term stability of their resistance by subjecting samples to temperatures of up to 60 °C for several hours. Additionally, we tested the influence of UV radiation on the resistance. Our samples exhibited good lithographic qualities. Resistivities were around 2 Ω mm and temperature sensitivity up to −407 Ω K −1 . UV radiation induced a partially reversible increase of the electric resistance. The long-term stability of the material was temperature-dependent.

Elastomers with a percolative network of carbon particles are a frequently studied class of materials for applications requiring high elongation and compliant sensors. For novel applications such as soft robots or smart textiles, these have some advantages over traditional strain gauges. However, their functionality is not fully understood. In this work, such materials are investigated as strain sensors in terms of their dynamic behavior, and their current limitations are demonstrated. It becomes clear that such sensors exhibit a non-monotonic behavior under dynamic loads that differs significantly from that observed in quasi-static tests. Two strategies for improving sensor characteristics are derived, modeled, and experimentally tested using the results and an electro-mechanical network model. First, a melt-spinning process that orients the carbon nanotube particles in the process direction creates different degrees of anisotropy. Second, to generate a local negative transverse contraction, an additional auxetic support structure is used. While the resulting anisotropy is insufficient to improve sensor properties, the auxetic structure can significantly improve strain sensitivity.

This work deals with the determination of the uncertainty of measurement data, determined by electrical impedance spectroscopy. Four different types of sand were measured impedimetrically in a measuring cell designed as a plate capacitor in a frequency range from 20 Hz to 1 MHz. The measuring cell was filled ten times with each sand and 20 impedance spectra were recorded for each filling. The uncertainty at each frequency was determined from the measurement data. It was found that the measurement data variance with a given measuring-cell filling was negligibly small. However, it increased by a factor of up to 100 when the measuring cell was repeatedly emptied and re-filled with the same material. We propose a way to estimate a continuous approximation of the uncertainty band of the impedance locus in the complex plane from the discrete uncertainties at each frequency. It uses a Support Vector Machine (SVM) to generate a regression curve using the discrete uncertainties. The result of the regression was used to estimate the uncertainties of an average impedance locus. The said machine learning tool can handle large amounts of data, classes, and influencing variables. In this manner, it can help to identify cause-effect relationships. Furthermore, at the end of this work a possibility to estimate a continuous uncertainty band along the impedance locus curve via SVM regression is shown. This is an extension to the common methodology in literature, where the uncertainty is only determined at selected individual points of the impedance spectrum.

Currently, structural health monitoring (SHM) systems are not in widespread use for monitoring civil structures because of low defect sensitivity and high cross-sensitivities of most SHM techniques available. The potential drop method (PDM), commonly used in material testing, is a possible method for SHM of large metallic civil structures combining high defect sensitivity and high area monitoring capability. The current state of the art lacks experimental evidence of the applicability to large specimens under the demanding operating conditions of SHM. Here, we investigated the PDM as an SHM system experimentally by analyzing the cross-sensitivity to temperature changes and performed a temperature compensation. We present an optimized method for suppressing the unwanted influence of mechanical loads and increasing the defect sensitivity. The temperature influence was separated from the defect-induced impedance change and effectively suppressed by compensation. Thus, cross-sensitivity does not limit PDM in SHM for large metallic civil structures with temperature compensation. PDM is a promising technique for SHM which could facilitate the widespread use of SHM of conductive civil structures.

The increasing demand for micro- and nanofabrication and in parallel the increasing requirements on feature size and resolution is leading to an enormous growth in the field of multi-photon three-dimensional fabrication. To enable new and diverse investigations in this field and to enable high precision for nanofabrication on large areas, a high precision positioning system is combined with an ultra-short pulse laser system. The aim is a modular setup with constant adherence to the Abbe-comparator principle in order to achieve systematic improvements in the area of Direct Laser Writing. For a high-quality identification of the microstructures a measurement tool based on atomic force microscopy is used. To enable the fabrication of continuous micro- and nanostructures on large area, an extremely high positioning precision is used, where no further stitching methods are necessary. Therefore as base of the Direct Laser Writing system the nanopositioning and nanomeasuring machine (NMM-1) is used, which was developed at Technische Universität Ilmenau together with SIOS Meßtechnik GmbH, with a positioning volume of 25 mm × 25 mm × 5 mm and a positioning resolution in the sub-nanometer range. First investigations already confirmed that microfabrication with a Femtosecond Laser and the NMM-1 could be realized and showed the possibility of further developments in the field of Direct Laser Writing. Now the modular structure as a research platform is designed in such a way that the various extensions and measurement setups for large-scale investigations can always be implemented in a metrologically traceable manner. The presented work shows the development of a modular functional setup of an exposure system and NMM-1, which enables micro- and nanofabrication and an improvement in the structure size over large areas.