Piezoresistive behavior and multi-directional strain sensing ability of carbon nanotube-graphene nanoplatelet hybrid sheets

Free-standing carbon nanomaterial hybrid sheets, consisting of multi-walled carbon nanotubes (MWCNTs), exfoliated graphite nanoplatelets (xGnPs) and nanographene platelets (NGPs) of different lengths and lateral dimensions, have been prepared using various material combinations and compositions. When subjected to tensile strains, the carbon nanomaterial sheets showed piezoresistive behavior, characterized by a change in electrical resistance with applied strain. Simultaneous measurement of resistance changes among multiple electrodes placed on the periphery of the hybrid sheets showed the dependence of resistance changes on strain direction, which potentially allows multi-directional strain sensing. Various combinations of MWCNT length, xGnP size and MWCNT-to-xGnP/NGP ratio result in different specific surface areas and nanoparticle interactions, which serve as critical factors for controlling the sensitivity of hybrid sheets. The smaller the nanoplatelet size and the higher the content as compared to MWCNT, the higher the sensitivity. Buckypapers, which are free-standing sheets composed of CNTs, are used as the control materials, and the unique characteristics of hybrid sheets are discussed.