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.

In this article, a strain sensor made from an epoxy-based nanocomposite using a kind of multi-walled carbon nanotubes (MWNTs), i.e., LMWNT-10, was investigated. It was found that the piezoresistivity of this strain sensor is linear and anti-symmetric in tensile and compressive states within a large strain range, which is different from that of the strain sensor using another type of MWNTs, i.e., MWNT-7, in our previous reports. This linear and anti-symmetric piezoresistivity is very suitable for practical sensor applications. In our experiments, both static and dynamic responses of the two sensors were measured. The experimental results indicate that the completely different piezoresistivity characteristics of the two sensors are probably due to the complete different working mechanisms. For the sensor made from LMWNT-10, the present experimental investigations reveal that its working mechanism should be the piezoresistivity of MWNTs due to deformation of MWNTs. On the other hand, for the strain sensor made from MWNT-7, the dominant working mechanism of this sensor is the tunneling effect caused by the distance changes among carbon nanotubes.

We improved the piezoelectric property of poly(vinylidene fluoride) (PVDF) by employing graphene. The reduced graphene oxide (rGO)–PVDF nanocomposites were prepared by a solution casting method and the rGO contents ranged from 0.0 wt% to 0.2 wt%. To induce the piezoelectric β-phase crystal structure, the nanocomposite films were drawn in a ratio of 4–5 and polarized by a step-wise poling method. To evaluate the piezoelectric property, the output voltages of the rGO–PVDF nanocomposite films were measured through extensive experimental vibration tests. The experimental results show that the rGO–PVDF nanocomposite film with 0.05 wt% rGO loading possesses the highest output voltage compared with other loadings, which is around 293% of that of the pure PVDF film. Moreover, it can be found that with the increase of the rGO content from 0 wt% to 0.2 wt%, the output voltage tends to have a peak at 0.05 wt%. The main reason for this phenomenon is that a more β-crystalline phase can be formed at those rGO loadings, as confirmed by XRD and FT-IR spectrum analyses.

In this work, we propose a comprehensive multi-scale three-dimensional (3D) resistor network numerical model to predict the piezoresistivity behavior of a nanocomposite material composed of an insulating polymer matrix and conductive carbon nanotubes (CNTs). This material is expected to be used as highly sensitive resistance-type strain sensors due to its high piezoresistivity defined as the resistance change ratio divided by the mechanical strain. In this multi-scale 3D numerical model, three main working mechanisms, which are well known to induce the piezoresistivity of strain sensors fabricated from nanocomposites, are for the first time considered systematically. They are (a) the change of the internal conductive network formed by the CNTs, (b) the tunneling effect among neighboring CNTs, and (c) the CNTs’ piezoresistivity. Comparisons between the present numerical results and our previous experimental ones were also performed to validate the present numerical model. The influence of the CNTs’ piezoresistivity on the total piezoresistivity of nanocomposite strain sensors is explored in detail and further compared with that of the other two mechanisms. It is found that the first two working mechanisms (i.e., the change of the internal conductive network and the tunneling effect) play a major role on the piezoresistivity of the nanocomposite strain sensors, whereas the contribution from the CNTs’ piezoresistivity is quite small. The present numerical results can provide valuable information for designing highly sensitive resistance-type strain sensors made from various nanocomposites composed of an insulating polymer matrix and conductive nanofillers.

In order to improve the piezoelectric properties of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), we added various amounts of silver nanowires (AgNWs) into PVDF-HFP and N,N-dimethylformamide (DMF) solution to prepare composite films. Stretching and poling were applied to the films to induce the formation of the polar β-phase and reorientation of the dipole moment. The crystal structure of the films was investigated by polarized optical microscopy (POM), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The obtained results showed that the phase transformation mainly occurred in stretching and the reorientation of the dipole moment was attributed to poling. It was also concluded that the AgNWs played a role as nucleate agents in the β-phase formation and the phase transformation. The piezoelectric properties were also evaluated by the output voltage and harvested power density. It was found that the open circuit voltage of 0.1 wt% AgNWs containing films was 52% higher than that of pure PVDF-HFP films. Furthermore, the harvested power density was increased by 159%. Overdose of AgNWs resulted in crystal defects and the lower degree of crystallinity, leading to worse piezoelectric properties.

In this work, the thermal expansion properties of carbon nanotube (CNT)-reinforced nanocomposites with CNT content ranging from 1 to 15 wt% were evaluated using a multi-scale numerical approach, in which the effects of two parameters, i.e., temperature and CNT content, were investigated extensively. For all CNT contents, the obtained results clearly revealed that within a wide low-temperature range (30°C ~ 62°C), thermal contraction is observed, while thermal expansion occurs in a high-temperature range (62°C ~ 120°C). It was found that at any specified CNT content, the thermal expansion properties vary with temperature - as temperature increases, the thermal expansion rate increases linearly. However, at a specified temperature, the absolute value of the thermal expansion rate decreases nonlinearly as the CNT content increases. Moreover, the results provided by the present multi-scale numerical model were in good agreement with those obtained from the corresponding theoretical analyses and experimental measurements in this work, which indicates that this multi-scale numerical approach provides a powerful tool to evaluate the thermal expansion properties of any type of CNT/polymer nanocomposites and therefore promotes the understanding on the thermal behaviors of CNT/polymer nanocomposites for their applications in temperature sensors, nanoelectronics devices, etc.