Tactile sensor devices exploiting the tunnelling conduction in piezoresistive composites

The thesis reports on the preparation of three piezoresistive composites using different metal particles as filler in a silicone (PDMS) matrix. The results obtained from the functional characterizations performed under compressive and tensile stresses are well supported by the theoretical models and showed that the conduction mechanism in the metal-polymer composites is based on a quantum tunnelling effect. The phenomenon is further enhanced by the sharp tip morphology of the metal particles used. In particular when using spiky nickel particles, the composites undergo a variation of resistance up to nine orders of magnitude under an applied pressure. The possibility to obtain a huge variation in resistance upon a small deformation of the samples makes these composites a well performing functional material for sensor applications. Moreover the simplicity of the synthesis process, the low cost of the materials and the mechanical flexibility favor their choice among the possible sensing materials for tactile sensors. Piezoresistive composites were subsequently implemented in two different sensor architectures. The first measures the resistance variation of a 8x8 array of sensing element and reproduces the pressure distribution on a 3D graphic software. The second exploits both the resistance and capacitance variation of the tunnelling conductive material with an extremely low power quasi-digital frequency converter methods. Thanks to this measuring methods, the sensor was able to resolve 1 gr of applied load.