Search

Im Gegensatz zu herkömmlichen Dehnungsmessstreifen können Carbon nanotube (CNT)-basierte Komposite zusätzlich eine ausgeprägte Druck-abhängigkeit des Widerstandes aufweisen. Deshalb können Drucksensoren aus CNT-Nanokomposite ohne den Einsatz von Verformungskörpern wie z. B. Biegebalken aufgebaut werden. Die möglichen Anwendungsgebiete für diese direkt messenden Sensoren wurden in der vorliegenden Arbeit bei drei industriellen Anwendungen wie z. B. bei Robotergreifarmen gezeigt. Die Zielstellung dieser Arbeit ist die Entwicklung und Charakterisierung eines neuartigen Sensors aus CNT-Nanokomposite. Unter Verwendung von Multi-walled carbon nanotube (MWCNT)-Epoxidharz und interdigitalen Elektroden soll der Sensor auf wenigen Quadratzentimetern Drücke im Megapascal-Bereich und somit Kräfte im Kilonewton-Bereich messen können. Durch die Auswahl geeigneter Werkstoffe und die Modellierung mit der Finite Element Methode wurde der Sensorentwurf durchgeführt sowie der Messbereich abgeschätzt. Die Herstellung der MWCNT-Epoxidharz-Dispersion erfolgte durch mechanische Mischverfahren. Anschließend wurden aus der Dispersion druckempfindliche Schichten mit der Schablonendrucktechnik hergestellt. Dabei wurden die Herstellungs-parameter und besonders der Füllstoffgehalt der MWCNTs variiert, um deren Einflüsse auf das mechanische, thermische und elektrische Verhalten zu untersuchen. Die Charakterisierung der mechanischen Kenngrößen erfolgte mit Zugversuchen und dynamisch-mechanischer Analyse. In den Untersuchungen zeigen die MWCNT-Komposite eine signifikante Steigerung der Zugfestigkeit und eine Erhöhung der Glasübergangstemperatur gegenüber reinem Epoxidharz. Die Abhängigkeiten der Druckempfindlichkeit und der Temperaturempfindlichkeit vom Füllstoffgehalt wurden untersucht. Eine besonders hohe Druckempfindlichkeit, aber auch Temperaturempfind-lichkeit wurde bei Proben mit geringem Füllstoffgehalt (1 wt% und 1,25 wt%) festgestellt. Es ist also wichtig, die richtige Materialkombination für diese Art Sensor zu finden. Die realisierten Sensoren liefern zuverlässige Antwortsignale bei wiederholten Belastungen bis zu einer Belastung von 20 MPa (entspricht 2 kN). Zusätzlich wurde der Temperatureinfluss in einem Bereich von −20 °C bis 50 °C durch eine Wheatstonesche Brückenschaltung kompensiert. Die vorliegende Arbeit zeigt, dass eine zuverlässige Druckmessung mit einer Temperaturmessabweichung von 0,214 MPa/10 K gewährleistet werden kann. In contrast to conventional metallic strain gauges, carbon nanotube (CNT) composites have an additional pressure sensitivity. Therefore, deformation elements such as bending beam is not needed by using pressure sensors, which are based on CNT nanocomposite. The possible areas of application for these pressure direct measured sensors were showed in three industrial application such as robot gripper. The focus of this work is the development and characterization of a new sensor manufactured from CNT nanocomposite. By using multi-walled carbon nanotube (MWCNT) epoxy and interdigital electrodes the sensor, which has a dimension of few square centimetre, should measure a pressure in mega Pascal range and hence a force in kilo newton range. By the selection of suitable materials and the modelling using finite element method, the sensor design as well as the measurement range were carried out. The MWCNT epoxy dispersion is manufactured by using a mechanical mixing process. Subsequent, the dispersion is used to fabricate pressure sensitive layers by stencil printing methods. Thereby, the fabrication parameters and especially the filler content of the MWCNTs were varied for the mechanical, thermal and electrical investigation. The characterization of the mechanical characteristic values were carried out by using tensile test and dynamic mechanical analysis. The results show a significant increasing of the tensile strength and glass transition temperature in comparison to neat epoxy. Additionally, the influence of the filler content to the pressure and thermal sensitivity were investigated. A highly pressure sensitivity but also a highly thermal sensitivity are obtained for samples with lower filler contents (1 wt% and 1.25 wt%). Therefore, a suitable material combination has to be chosen. The fabricated sensors show reliable response signals by repeated excitations up to 20 MPa (meets to 2 KN). Moreover, the temperature influence ranged from -20 °C to 50 °C was compensated with a Wheatstone bridge. This work demonstrate a direct pressure sensitive sensor with reliable response signals by a thermal deviation of 0.214 MPa/10K.

The present work is focussed on the optimisation of a process for the generation of epoxy/layered silicate nanocomposites with improved barrier properties. To enhance the effectivity of the layered silicate particles it is necessary to seperate the single layers (exfoliation) and consequently to enlarge the interface between the components. The process of exfoliation depends on thermodynamic and kinetic actions that can be cost- and energy consuming. By adding liquid epoxy components and organic modified layered silicates corresponding nanocpomposites are formed. Therefor a sufficient compatibility of epoxy-components and layered silicate is advantegeous. Hyper branched polyalkoxysiloxane (PAOS) is a SiO2 precursor molecule with emerging properties. It is distinguished by a low viscosity, good miscibility with many organic solvents and various modifiability. Due to this properties PAOS can act as an intermediate for the compatibility between the Bisphenol-F-epoxy matrix and organically modified layered silicates used in this work. PAOS acts as exfoliation agent as well and contributes to improved barrier properties of the composite. To determine the layer distances different analytical approaches were used. To obtain basic information regarding intercalation or exfoliation and consequently changes in the distance of the single layers X-ray diffraction measurements(XRD)are very sufficient. For quantification of an exfoliation TEM-measurements were carried out and evaluated by using a semiquantitative method. Furthermore a model for determining the degree of exfoliation from 1H-NMR-solid state measurements of the longitudinal relaxation time (TH1) was developed. By modification of PAOS, applying cocondensation- or transesterification reactions, it was possible to adapt PAOS functional groups to the organic modification of layered silicates. This adjustment in compatibility resulted in increased layer distances and an improved distribution of the single layers in the epoxy matrix. The resulting nanocomsite exhibited an improved water vapor barrier of 22.5% compared to the unfilled epoxy resin. This nanocomposite contained 2 wt.-% (7.5 vol.-%) of layered silicate. Within this work a maximum filling content of 5 wt.-% (18.5 vol.-%) could be reached. When higher filling contents were used a perfect wetting of the silicate layers by PAOS and epoxy resin was not longer given. Possibly this degree of filling is not sufficient to reach the target of a super-barrier layer. By using and combining suitable PAOS-and layered silicate modifications improved barrier properties of epoxy/layered silicate/PAOS nanocomposites were obtained. Another system with different modification showed improved mechanical properties. A nanocomposite system combining improved mechanical and barrier properties was not generated within the framework of this thesis. Additionally the reduction of water vapor permeability of 22.5% is not sufficient for a super-barrier layer. One approach for a further improvement of the barrier properties can be an increase of the filling factor in the nanocomposite. A suitable solvent, as shown in some parts of this thesis, could be helpful as well as a controlled sedimentation of layered silicate particles.