Yaolu Liu

Piezoresistive Strain Sensors Made from Carbon Nanotubes Based Polymer Nanocomposites

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.

A carbon nanotube/polymer strain sensor with linear and anti-symmetric piezoresistivity

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.

Evaluation of piezoelectric property of reduced graphene oxide (rGO)–poly(vinylidene fluoride) nanocomposites

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.

Characterization of damage size in metallic plates using Lamb waves

By employing the Lamb waves for monitoring the structural integrity, a theory was proposed to approximately evaluate the damage size based on the reflection intensity information from the damage. First, to relate the wave reflection intensity to the size and shape of the damage, this theory was constructed for an arbitrary elliptical damage, which can be changed into a crack-like or a circular damage. Then, this theory was verified by employing the numerical results predicted by a powerful numerical approach for through-thickness elliptical holes in aluminum plates. Furthermore, experimental verification was also performed for through-thickness circular holes in aluminum plates. By assuming a damage of a circular shape, this theory was further extended to approximately predict the damage size using a coarse sensor/actuator network after identifying the damage location. To validate the proposed method, a four PZT transducer network was used to experimentally identify the locations and sizes of a circular hole and an elliptical hole in aluminum square plates.

Damage evaluation based on a wave energy flow map using multiple PZT sensors.

A new wave energy flow (WEF) map concept was proposed in this work. Based on it, an improved technique incorporating the laser scanning method and Bettis reciprocal theorem was developed to evaluate the shape and size of damage as well as to realize visualization of wave propagation. In this technique, a simple signal processing algorithm was proposed to construct the WEF map when waves propagate through an inspection region, and multiple lead zirconate titanate (PZT) sensors were employed to improve inspection reliability. Various damages in aluminum and carbon fiber reinforced plastic laminated plates were experimentally and numerically evaluated to validate this technique. The results show that it can effectively evaluate the shape and size of damage from wave field variations around the damage in the WEF map.

Multi-scale numerical simulations of thermal expansion properties of CNT-reinforced nanocomposites

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.

Enhancement of PVDF’s piezoelectricity by VGCF and MWNT

Multi-walled carbon nanotube (MWNT) and vapor grown carbon fiber (VGCF) were blended into poly (vinylidene fluoride) (PVDF) to enhance the piezoelectricity of the neat polymer. The PVDF composite films were prepared by solution casting method, stretched uniaxially and poled in silicon oil. The nanofiller contents range from 0.05 to 0.3 wt.%. Open circuit output voltage and energy harvesting tests indicate that both the PVDF/MWNT and PVDF/VGCF composite films approached the maximum output at the nanofiller content of 0.05 wt.%. Compared to the neat PVDF films, the maximum increasing rates of open circuit voltage and harvested power density are 24% and 47% for the PVDF/MWNT films and 15% and 78% for the PVDF/VGCF films, respectively. X-ray diffraction analysis showed an increase in content of the β phase in the PVDF composites; thus, the piezoelectric properties, which are dependent on β phase content, were enhanced. Stretching of the films leads to the transformation of PVDF from α phase to β phase form. Moreover, the addition of nanofillers, such as MWNT and VGCF, improves this transformation since the nanofillers provide a phase transformation nucleation function.

Improved piezoelectric properties of poly(vinylidene fluoride) nanocomposites containing multi-walled carbon nanotubes

We improved the piezoelectric properties of poly(vinylidene fluoride) (PVDF) by employing multi-walled carbon nanotubes (MWCNTs) as nanofillers. The MWCNT/PVDF nanocomposite was prepared by the solution casting method with MWCNT content ranging from 0.0 to 0.3?wt%. To induce the piezoelectric ?-phase crystal structure, the nanocomposite films were drawn to 400%?500% elongation and polarized with a step-wise poling method. To evaluate the piezoelectric properties, the output voltages of the nanocomposite films were measured through extensive experimental vibration tests. The experimental results show that the nanocomposite film with 0.05?wt% MWCNT loading possesses the highest output voltage, around two times higher than that of pure PVDF film, as compared to the other loadings. The main reason for this phenomenon is that more ?-crystalline phase can be formed at this MWCNT loading, as confirmed by x-ray diffraction and Fourier transform infrared spectroscopy spectral analysis and polarized optical microscopy observations.

Optimal Excitation Frequency of Lamb Waves for Delamination Detection in CFRP Laminates

To improve the reliability of delamination detection techniques based on Lamb waves for carbon fibre reinforced plastics (CFRP) composite laminates, the influence of excitation frequency of Lamb waves on the intensity of reflected waves from a delamination in cross-ply CFRP delaminated beams was investigated. The A0 Lamb wave mode was chosen although the conclusion in this work is also applicable to S0 mode. A Chebyshev pseudospectral Mindlin plate element proposed by authors was employed to simulate the A 0 mode propagation and the interaction of the A0 mode with the delamination. Numerical results show that the sensitivity of A0 mode to the delamination at different frequencies is remarkably different. Each delamination with different lengths or different locations along the through-thickness direction of the laminates generally has at least one or more optimal excitation frequencies which can generate the strongest reflected wave signals from the delamination. The reason for this phenomenon was also investigated by analyzing the natural vibration of the local upper- and lower-delaminated regions. The optimal excitation frequencies were identified to be close to the resonant frequencies of the local delaminated regions with pure flexural vibration modes similar to the deformation pattern of A0 mode.

The interfacial mechanical properties of functionalized graphene–polymer nanocomposites

The interfacial mechanical properties between graphene (GR) and a polymer matrix play a key role in load transfer capability for GR/polymer nanocomposites. Grafting of polymer molecular chains on GR can improve the dispersion of the GR in a polymer matrix and change the interfacial mechanical properties between the GR and the polymer matrix. In this work, we investigated the interfacial mechanical properties between GR functionalized with polymer molecular chains and a polyethylene (PE) matrix using molecular dynamics simulations. The influences of grafting density and chain length on the interfacial mechanical properties were analyzed. The results show that grafting of short PE molecular chains on GR can significantly improve the interfacial shear strength and interfacial Mode-II fracture toughness in functionalized GR/PE nanocomposites.