Development of highly sensitive piezoresistive sensor exploiting PDMS-infiltrated 3D graphene sponge

This study deals with the possible applications of three-dimensional foam-like graphene structures. Starting from a Ni porous structure, the deposition of graphene is performed by Chemical Vapour Deposition (CVD) process on commercial Ni foam scaffolds. Then, in order to guarantee mechanical support for the successive steps, Poly-Methyl MethAcrylate (PMMA) is infiltrated and, successively, the Ni is etched. In this way, a hollow free-standing graphene foam with pores filled with PMMA is formed. Subsequently, by infiltrating a polymer into a hollow graphene structure, it is possible to obtain a three-dimensional (3D) graphene foam (GF)/polydimethylsiloxane (PDMS) composite. At the end of the process, PMMA is removed in an acetone solution. The excellent electrical and mechanical proprieties derived from the GF/PDMS composite, combined with the structure of GFs, lead to the flexible and stretchable conductors with unique potential applications in wearable sensing and communication devices. Indeed, the prepared 3D composites can be bent, stretched and twisted without breaking the structure. The main advantages of these materials are low mass density, large surface area and high electrical conductivity, the latter coming from the interconnected graphene structure forming the GFs that allows fast charge transport. Conversely, the main problems encountered in these GF/PDMS composites are the weak interfacial interaction between graphene and the polymer matrix, the non-uniform distribution of the graphene sheets within the polymer and the possible low quality and/or high inter-sheet junction contact resistance. This technique guarantees many degrees of freedom: for example, by tuning the porosity of the Ni scaffold, the network and pore structures of GFs can be easily tailored. Moreover, by performing the CVD processes, it is possible to obtain different thicknesses changing the deposition time and the precursor concentration. Indeed, a higher methane (CH4) concentration leads to an increase in the number of graphene layers and this results in a difference in the specific surface area, density and electrical conductivity of GFs. During this work, morphological, compositional and structural characterizations have been carried out by means of SEM, EDX, XRD, and Raman Spectroscopy. Electrical characterizations can be performed focusing on different aspects. Due to the already mentioned properties, indeed, the reaction of the graphene foam to mechanical stresses is studied by means of different tests such as bending, compression, pressure and the dependence of the GFs on temperature. These new technologies have particular challenges to overcome, such as the difficulty of dispersing graphene into polymer matrices due to the large aspect ratio and strong interaction, which can cause a detachment of the graphene sheets from matrices. Secondly, severe deterioration during the stretching must be avoided. In particular, for considerable strain, the conductivity gradually decreases due to possible cracking of the conductive networks. The PDMS substrate allows to improve the strain distribution and, in this way, the structure possesses a highly deformable geometry.