2022, Schultes, Günter, Cerino, Mario, Lellig, Angela, Koch, Marcus

The family of sputter deposited granular metal-based carbon-containing sensor films is known for their high sensitivity transforming force-dependent strain into electrical resistance change. Among them nickel–carbon thin films possess a gauge factor of up to 30, compared to only 2 for traditional sensor films of metal alloys. This high sensitivity is based on disordered interparticle tunneling through barriers of graphite-like carbon walls between metal–carbon particles of columnar shape. Force and pressure sensors would benefit a lot from the elevated piezoresistivity. A disadvantage, however, is a disturbing temporal creep and drift of the resistance under load and temperature. This contribution shows how to stabilize such sensor films. A significant stabilization is achieved by partially replacing nickel with chromium, albeit at the expense of sensitivity. The more chromium used in these NixCr1−x-C layers, the higher the optimum annealing temperature can be selected and the better the electrical stabilization. A good compromise while maintaining sensitivities well above the standard of 2 is identified for films with x=0.5 to 0.9, stabilized by optimized temperature treatments. The stabilizing effect of chromium is revealed by transmission electron microscopy with elemental analysis. The post-annealing drives segregation processes in the layer material. While the interior of the layer is depleted of chromium and carbon, boundary layers are formed. Chromium is enriched near the surface boundary, oxidized in air and forms chromium-rich oxide sub-layers, which are chemically very stable and protect against further reactions and corrosion. As a result, creep and drift errors are greatly reduced, so that the optimized sensor coatings are now suitable for widespread use.

2016, Schmälzlin, Elmar, Moralejo, Benito, Bodenmüller, Daniel, Darvin, Maxim E., Thiede, Gisela, Roth, Martin M.

Step-by-step, time-consuming scanning of the sample is still the state-of-the-art in imaging Raman spectroscopy. Even for a few 100 image points the measurement time may add up to minutes or hours. A radical decrease in measurement time can be achieved by applying multiplex spectrographs coupled to imaging fiber bundles that are successfully used in astronomy. For optimal use of the scarce and expensive observation time at astronomical observatories, special high-performance spectrograph systems were developed. They are designed for recording thousands of spatially resolved spectra of a two-dimensional image field within one single exposure. Transferring this technology to imaging Raman spectroscopy allows a considerably faster acquisition of chemical maps. Currently, an imaging field of up to 1 cm2 can be investigated. For porcine skin the required measurement time is less than 1 min. For this reason, this technique is of particular interest for medical diagnostics, e.g., the identification of potentially cancerous abnormalities of skin tissue.

2017, Schröder, Kerstin, Rothhardt, Manfred, Ecke, Wolfgang, Richter, Uwe, Sonntag, André, Bartelt, Hartmut

Fibre optic sensors are excellent tools to use for monitoring high-voltage current collectors. Because of their small cross section and electrical neutrality, they are easily integrated into the current collector strip and are well specialized for detection of high-speed load events. The conventional contact force measurement with four force sensors below the collector strips can also be simplified by using fibre optic force and acceleration sensors.

2018, Schwebke, Silvan, Werner, Ulf, Schultes, Günter

Granular and columnar nickel-carbon composites may exhibit large strain sensitivity, which makes them an interesting sensor material. Based on experimental results and morphological characterization of the material, we develop a model of the electron transport in the film and use it to explain its piezoresistive effect. First we describe a model for the electron transport from particle to particle. The model is then applied in Monte Carlo simulations of the resistance and strain properties of the disordered films that give a first explanation of film properties. The simulations give insights into the origin of the transverse sensitivity and show the influence of various parameters such as particle separation and geometric disorder. An important influence towards larger strain sensitivity is local strain enhancement due to different elastic moduli of metal particles and carbon matrix.

2018, Schultes, Günter, Schmid-Engel, Hanna, Schwebke, Silvan, Werner, Ulf

We have produced granular films based on carbon and different transition metals by means of plasma deposition processes. Some of the films possess an increased strain sensitivity compared to metallic films. They respond to strain almost linearly with gauge factors of up to 30 if strained longitudinally, while in the transverse direction about half of the effect is still measured. In addition, the film's thermal coefficient of resistance is adjustable by the metal concentration. The influence of metal concentration was investigated for the elements Ni, Pd, Fe, Pt, W, and Cr, while the elements Co, Au, Ag, Al, Ti, and Cu were studied briefly. Only Ni and Pd have a pronounced strain sensitivity at 55- €±- €5- €at.- €% (atomic percent) of metal, among which Ni–C is far more stable. Two phases are identified by transmission electron microscopy and X-ray diffraction: metal-containing nanocolumns densely packed in a surrounding carbon phase. We differentiate three groups of metals, due to their respective affinity to carbon. It turns out that only nickel has the capability to bond and form a stable and closed encapsulation of GLC around each nanoparticle. In this structure, the electron transport is in part accomplished by tunneling processes across the basal planes of the graphitic encapsulation. Hence, we hold these tunneling processes responsible for the increased gauge factors of Ni–C composites. The other elements are unable to form graphitic encapsulations and thus do not exhibit elevated gauge factors.

2016, Windisch, M., Eichhorn, K.-J., Lienig, J., Gerlach, G., Schulze, L.

The volume production of industrial hydrogel sensors lacks a quality-assuring manufacturing technique for thin polymer films with reproducible properties. Overcoming this problem requires a paradigm change from the current recipe-driven manufacturing process to a specification-driven one. This requires techniques to measure quality-determining hydrogel film properties as well as tools and methods for the control and optimization of the manufacturing process. In this paper we present an approach that comprehensively addresses these issues. The influence of process parameters on the hydrogel film properties and the resulting sensor characteristics have been assessed by means of batch manufacturing tests and the application of several measurement techniques. Based on these investigations, we present novel methods and a tool for the optimization of the cross-linking process step, with the latter being crucial for the sensor sensitivity. Our approach is applicable to various sensor designs with different hydrogels. It has been successfully tested with a sensor solution for surface technology based on PVA/PAA hydrogel as sensing layer and a piezoelectric thickness shear resonator as transducer. Finally, unresolved issues regarding the measurement of hydrogel film parameters are outlined for future research.