Satoshi Atobe

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.

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.

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.

Pull-out simulations of a capped carbon nanotube in carbon nanotube-reinforced nanocomposites

Systematic atomic simulations based on molecular mechanics were conducted to investigate the pull-out behavior of a capped carbon nanotube (CNT) in CNT-reinforced nanocomposites. Two common cases were studied: the pull-out of a complete CNT from a polymer matrix in a CNT/polymer nanocomposite and the pull-out of the broken outer walls of a CNT from the intact inner walls (i.e., the sword-in-sheath mode) in a CNT/alumina nanocomposite. By analyzing the obtained relationship between the energy increment (i.e., the difference in the potential energy between two consecutive pull-out steps) and the pull-out displacement, a set of simple empirical formulas based on the nanotube diameter was developed to predict the corresponding pull-out force. The predictions from these formulas are quite consistent with the experimental results. Moreover, the much higher pull-out force for a capped CNT than that of the corresponding open-ended CNT implies a significant contribution from the CNT cap to the interfacial properties of the CNT-reinforced nanocomposites. This finding provides a valuable insight for designing nanocomposites with desirable mechanical properties.

Ultrasensitive strain sensors of multiwalled carbon nanotube/epoxy nanocomposite using dielectric loss tangent

In this work, the dielectric loss tangent (tan δ) of a series of strain sensors, fabricated from an epoxy nanocomposite with multi-wall carbon nanotube (MWCNT) content varying at 1 wt. % – 5 wt. %, was characterized experimentally. The effects of four parameters including frequency, strain of nanocomposite, MWCNT content, and loading voltage were investigated extensively. Moreover, an alternative current gauge factor KAC was developed. The largest value of KAC was found to be 256 for the nanocomposite strain sensor with 1 wt. % MWCNT content at 0.6% tensile strain, which indicates the ultra-sensitivity of the present strain sensor.

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.

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.

Impact damage monitoring of FRP pressure vessels based on impact force identification

In this paper, a method for monitoring impact damage of FRP pressure vessels is discussed. To estimate the location and extent of the impact damage, strains are measured at several locations on the surface of the structure, and impact force identification is performed using those data so as to obtain information on the impact which caused the damage. The location and force history of the impact are identified using an identification method based on experimental transfer matrices which relate the impact force to the corresponding strain responses. Here, experimental transfer matrices are determined from the measured data acquired by impact tests that do not wreak impact damages. The location of the impact damage is predicted from the identified impact location. As to the extent of the impact damage, the peak value and the duration of the identified force history are used to estimate the damage initiation and the damage size, respectively. In order to examine the validity of the method, monitoring of impact damages induced by drop-weight impact tests is demonstrated.

Investigation of interfacial mechanical properties of graphene-polymer nanocomposites

Abstract A series of graphene (GR) pull-out simulations based on molecular dynamics (MD) were carried out to investigate the interfacial mechanical properties between GR and a polymer matrix (polyethylene: PE). The effects of pull-out velocity, number of vacancy defect in GR and temperature on the interfacial mechanical properties of a GR/PE nanocomposite system were explored. The obtained results showed that the pull-out velocity and the temperature have significant influences on the interfacial mechanical properties for a perfect GR. Moderate vacancy defects in GR can effectively enhance the interfacial mechanical properties in GR-based polymer nanocomposites.