Giant Piezoresistive Variation of Metal Particles Dispersed in PDMS Matrix
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Giant Piezoresistive Variation of Metal Particles Dispersed in PDMS Matrix Stefano Stassi1, 2, Giancarlo Canavese1, Mariangela Lombardi1, Andrea Guerriero1,3 and Candido Fabrizio Pirri1,3 1
Centre for Space Human Robotics, IIT-Italian Institute of Technology, C.so Trento 21, 10129 Torino, Italy 2
Dept. of Physics, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
3
Dept. of Materials Science and Chemical Engineering, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy ABSTRACT An Investigation of the piezoresistive response of a metal-polymer composite based on nickel conductive filler in a polydimethylsiloxane (PDMS) insulating matrix for tactile sensor application is presented in this paper. Lacking a mechanical deformation, the prepared composites show no electric conductivity, even though the metal particle content is well above the expected percolation threshold. In contrast, when subjected to uniaxial compression, the electric resistance is strongly reduced. A variation of up to nine orders of magnitude was registered. The thickness of the insulating layer between particles decreases when the sample composite is compressed. Therefore, the electric conduction which is related to a tunneling phenomena, increases exponentially. This behavior is further enhanced by the presence of very sharp nanometric spikes on the particles surface which act as field enhancement factors. In the presented work, the piezoresistive behavior of the composite, the stability in time of the resistance value and the response to several cycles of compression and decompression are evaluated on samples with different physical parameters like nickel content, PDMS copolymer/curing agent ratio and thickness. INTRODUCTION Electrical conductive polymers have been studied for 40 years and they continue to stimulate both scientific studies and experimental applications. Two ways can be followed to obtain conductive polymers: producing a polymer that is intrinsically conducting, or adding a conductive filler to an electrically insulating polymeric matrix. The conductive mechanisms of filled polymeric composites can be divided into two main families. In the former, well known as pressure conductive rubber, the randomly distributed electrical conductive particles are in physical contact with each other when the samples are mechanically loaded [1-3]. Furthermore, it has been demonstrated that changing the strain, the variation of the electrical conduction of the sample can be attributed to the change in the conducting particles contacts which increase when the samples are mechanically deformed [2]. In literature, different percolation models have been proposed to describe the variation in resistivity as a function of filler concentration [4,5]. The percolation models describe the conduction with the presence of electrical paths between two filler particles, but they generally fail below the percolation threshold. Hybrid piezoresistive polymers of the second group (known as quantum tunnelling composite) diff
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