Experimental and Theoretical Study on Piezoresistive Behavior of Compressible Melamine Sponge Modified by Carbon-based Fillers

High-performance compression sensors have been playing an increasingly important role in human motion detection, health monitoring and human-machine interfaces over recent years. However, it remains a great challenge to develop theoretical models providing practical guidance to the sensor design. Herein, carbon black (CB), carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were respectively incorporated into porous melamine sponges by a facile approach of dip-coating to fabricate compression sensors. Uniaxial compression-resistance tests show that the compressibility, stability and piezoresistive sensitivity of sensors could be tailored by the filler type and concentration. A model considering the number of conductive pathways (NCP) is given to describe the relationship between the resistance change and applied compression, showing extremely good agreement with the experimental data. Also, the correlation between the equivalent filler volume fraction and conductivity is described by the other two models proposed by McLachlan and Kirkpatrick, revealing the electrical percolation thresholds (empty set(c)) for the conductive systems under compression. Among the three fillers, CB particles endowed the composite with the best piezoresistive sensitivity but the largest empty set(c) due to its small size and aspect ratio. A combination of experimental study and theoretical model opens up a way of further understanding the piezoresistive sensing behavior as well as optimizing the electrical property and piezoresistivity of compressive conductive polymer composite.