Sensors and Actuators A 289 (2019) 157–164 Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna Aligned carbon nanotube based sensors for strain sensing applications A. Santos a,∗ , L. Amorim a , J.P. Nunes a , L.A. Rocha b , A.F. Silva b , J.C. Viana a a IPC/i3N - Institute for Polymers and Composites, University of Minho, Guimarães, Portugal b CMEMS – Center for MicroElectroMehanical Systems, University of Minho, Guimarães, Portugal a r t i c l e i n f o Article history: Received 10 September 2018 Received in revised form 21 February 2019 Accepted 21 February 2019 Available online 25 February 2019 Keywords: CNT Sensor Strain monitoring Anisotropy a b s t r a c t This paper presents an aligned carbon nanotube (CNT)-based strain sensor. Vertical aligned carbon nano- tubes (VA-CNT), synthesized by chemical vapour deposition (CVD), were knocked down onto polymeric films, in order to obtain a thin 10 × 10 × 0.05 mm CNT patch. Different polymeric substrates, ADEXepoxy, polyethylene terephthalate (PET) and polyimide (PI) were used. The samples’ morphology before and after the knock down process, specifically their alignment, was observed by scanning electron microscopy (SEM). The good quality of the synthesized VA-CNT was assessed by Raman spectroscopy. Furthermore, transmission electron microscopy (TEM) analysis was carried out to determine the average wall num- ber and diameters (inner and outer) of the VA-CNT. A MATLAB software with an adapted Van der Pauw method for anisotropic conductors was developed to determine the electric properties of the obtained samples, which were strained in the transverse (X) and parallel (Y) directions with respect to the CNT alignment. The electric anisotropy, defined as electric resistance ratio between obtained measurements along the X (R xx ) and Y (R yy ) -axes, decreases with deformation increment when the sample was strained in the Y-direction, while it increases when strained in the X-direction. Moreover, the obtained Gauge factor values showed a much sensitive response to deformation, i.e., approximately 47% increase in GF values, when the samples are strained transversely to CNT alignment. These results showed that the piezoresistive CNT/polymeric based sensor produced is suitable for strain sensing applications. © 2019 Elsevier B.V. All rights reserved. 1. Introduction In the last decades, carbon nanotubes (CNT) have been studied for several applications, namely aerospace, aeronautic, or micro- electronics, among others. CNT structural, electric, mechanical and electromechanical properties made them suitable for strain sensors [1]. Moreover, the anisotropic electric properties, due to the CNT alignment, can be an advantage for sensing in different directions or with different direction sensitivity. CNT have been used for piezoresistive polymer-based sensors by adopting three main approaches: (a) as conductive matrix fillers in polymer nanocomposites as thin films [2,3]; (b) as thin bucky paper films [4,5]; and (c) as aligned forests [6,7]. However, the use of CNT as strain sensors rise a few manu- facturing issues that become drawbacks on their widely adoption. Main manufacturing steps of CNT-polymer nanocomposite include ∗ Corresponding author. E-mail address: b7525@dep.uminho.pt (A. Santos). mechanical mixing and deposition methods, such as filtration, coat- ing and dip casting, among others, which present difficulties in the uniformity of CNT dispersion, in the avoidance of agglomer- ates and in the process of CNT alignment [8]. CNT bucky paper sensors can be produced by dispersion and deposition methods, such as vacuum filtration (the most common method), drop cast- ing, or hot-press compression, among others, which can also result in a non homogeneous CNT distribution [4,5,9]. Vertical-aligned carbon nanotubes (VA-CNT) forests impregnation also shows full impregnation deficiency and uncertainty in maintaining the CNT alignment. The most common method for VA-CNT growth is via chemi- cal vapour deposition (CVD), which shows high process variability and several decisive factors for the CNT electric properties, such as growth time, temperature, catalytic type and thickness, gas mixture and additional delamination step (hydrogen flow after the growth). Some authors studied the effects of CNT length [10] that can be con- trolled by CVD growth time, and of the interplay of CNT structure and density [11], which can be controlled by the catalytic thick- ness, on the VA-CNT anisotropic electric properties. Higher growth time results in longer CNT and lower electric resistance [10]. Sup- port/catalytic interaction and adequate hydrogen pre-treatment time (c.a. 2–5 min) affects CNT structure [12], with higher catalytic thickness resulting in two opposite effects: the electric conductivity https://doi.org/10.1016/j.sna.2019.02.026 0924-4247/© 2019 Elsevier B.V. All rights reserved.