Nanostructured functional Ti-Ag electrodes for large deformation sensor applications

This paper reports on the development of thin film-based stretchable electrodes, suitable for different types of sensors. Columnar Ti–Ag thin films with a Ag content of 8 at.% were prepared by D.C. magnetron sputtering on carbon nanotube/poly(vinylidene fluoride) CNT/PVDF piezoresistive composites. GLancing Angle Deposition, GLAD, technique was used to change the typical normal columnar growth microstructure obtained by conventional sputtering, into different growing architectures, such as inclined columns and zigzag profiles, in order to tune mechanical and electrical responses of the materials. Three different incident angles of the particle flux, α = 40°, 60° and 80°, were used to deposit Ti–Ag thin films with the different architectures. Upon uniaxial stretching of the prepared zigzag thin films, the resistance of the thin film starts increasing smoothly for strains up to 3%. Above 10% strain, a sharp increase of the electrical resistance is observed due to film mechanical failure and therefore interruption of the electrical conductivity pathways. Furthermore, the influence of the thin film architecture was also studied with respect to the performance of piezoresistive sensors based on carbon nanotube/poly(vinylidene fluoride), CNT/PVDF, composites in which Ti–Ag coated films served as electrodes for signal acquisition. Electromechanical tests were thus performed in composites with CNT contents close to the percolation threshold, where the electromechanical response, characterized by the Gauge Factor, is the largest. The stability of the piezoresistive response was analyzed for the various architectures of the GLAD sputtered Ti–Ag electrodes, including incident angle and number of zigzag periods. It is shown that thin film architecture has a pronounced influence in the overall sensor response, where the best results are obtained for piezoresistive polymer composites coated with Ti–Ag films, produced with intermediate (α = 60°) incident angles.This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2014 and the project Matepro - Optimizing Materials and Processes", ref. NORTE-07-0124-FEDER-000037", co-funded by the "Programa Operacional Regional do Norte" (ON.2 - O Novo Norte), under the "Quadro de Referencia Estrategico Nacional" (QREN), through the "Fundo Europeu de Desenvolvimento Regional" (FEDER). The authors also thank FCT for financial support under project PTDC/CTM-NAN/112574/2009. AF thanks the FCT for grant SFRH/BD/69796/2010