Radical diffusion engineering: tailored nanocomposite materials for piezoresistive inkjet printed strain measurement

We present a comprehensive study related to UV-curable nanocomposite (NC) materials, based on acrylic matrix containing Ag nanoparticles (NPs) formed by in situ reduction and co-formulated with titania NPs. Addition of titania produces a diffusion limited aggregation of in situ formed Ag NPs during photocuring due to radical propagation, allowing to obtain electromechanical percolation at very low solid content. Keeping low the solid content is important, considering the cost of raw materials. Compared to NCs based on spherical fillers, where percolation is reached at very high solid contents (around 70%), by radical engineering we could approach it by adding 5 to 30% of Ag precursor (Ag content 2 to 10%). These NCs are characterized by a low viscosity at room temperature, allowing full processability by means of inkjet printing (IjP), as well as good electrical properties after curing, ranging from metallic to dissipative, in their annealed state. We present morphological, chemo-physical and electrical characterisation, as well as outstanding piezoresistive properties of these materials in the thin film state and after direct patterning by means of IjP. The goal was to realize low cost printed strain-gages featuring improved characteristics when compared to available commercial products. We obtain diffusion-engineered unstructured materials featuring gauge factors (GF) as high as 13.4, corresponding to a seven-fold increase with respect to commercial metallic alloys. Measurements performed on structured NC IjP strain gauges produce GF up to 220, corresponding to a hundred-fold increase in comparison with commercial devices.