Abstract
In recent years, nanocomposites based on various nano-scale carbon fillers, such as carbon nanotubes (CNTs), are increasingly being thought of as a realistic alternative to conventional smart materials, largely due to their superior electrical properties. Great interest has been generated in building highly sensitive strain sensors with these new nanocomposites. This article reviews the recent significant developments in the field of highly sensitive strain sensors made from CNT/polymer nanocomposites. We focus on the following two topics: electrical conductivity and piezoresistivity of CNT/polymer nanocomposites, and the relationship between them by considering the internal conductive network formed by CNTs, tunneling effect, aspect ratio and piezoresistivity of CNTs themselves, etc. Many recent experimental, theoretical and numerical studies in this field are described in detail to uncover the working mechanisms of this new type of strain sensors and to demonstrate some possible key factors for improving the sensor sensitivity.
Highlights
Various nano-scale carbon fillers of high aspect ratio, such as carbon nanotubes (CNTs) and vapor growth carbon fibers (VGCFs), possess excellent mechanical properties and electrical conductivities.Besides applications of a single CNT in various nanoelectronic applications, such as probes [1] or oscillators [2], CNTs are ideal structural components candidates in various composites and functional composites due to their mechanical reinforcement effects [3,4,5,6,7]
A lot of recent research outcomes in this field have been cited from the aspects of experiments, and numerical modeling. To understand this physical phenomenon more clearly, first we focus on the electrical percolation phenomenon of CNT/polymer nanocomposites, and electrical conductive network formed by CNTs within an insulating matrix
The influences of aspect ratio, aggregation state and curved shapes of CNTs on the percolation threshold and electrical conductivity of nanocomposites are described in detail from many experimental studies, and from a 3D statistical percolation model and a 3D resistor network model proposed by the present authors
Summary
Various nano-scale carbon fillers of high aspect ratio, such as carbon nanotubes (CNTs) and vapor growth carbon fibers (VGCFs), possess excellent mechanical properties and electrical conductivities. A lot of experimental studies have been performed recently, except for [55], there is little literature covering the detailed influences of various factors in fabrication process on the electrical properties of CNT/polymer nanocomposites using in situ polymerization methods For this reason, the present authors prepared the MWNT/epoxy nanocomposites and obtained a low percolation threshold of 0.1 wt.% [63]. This may be explained by easy dispersion of MWNTs in the polymer matrix, and easy formation of a macroscopic conducting network due to small-scale chain-like aggregates of MWNTs, as pointed out in [63]. Besides its application to CNT/polymer nanocomposites, the above formulation [71] has been verified by experimental data of composites with CSFs and nanocomposites with nanofibers
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