Liangke Wu

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.

Improved piezoelectricity of PVDF-HFP/carbon black composite films

We report a substantial improvement of piezoelectricity for poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) copolymer films by introducing carbon black (CB) into the PVDF-HFP to form PVDF-HFP/CB composite films. The optimized output voltage of the composite film at an optimal CB content of 0.5 wt% is found to be 204% of the pristine PVDF-HFP film. Its harvested electrical power density is 464% and 561% of the pristine PVDF-HFP film by using ac and dc circuits, respectively. Through Fourier transform infrared spectroscopy analysis, differential scanning calorimetry analysis, and polarized optical microscopy observations, we clarify the enhancement mechanism of piezoelectricity for the PVDF-HFP/CB composite films. We find that the added CB acts as nucleating agent during the initial formation of crystals, but imposes an insignificant effect on the α–β phase transformation during stretching. We also demonstrate that the addition of optimal CB reduces crystal size yet increases the number of crystals in the composite films. This is beneficial for the formation of elongated, oriented and fibrillar crystalline morphology during stretching and consequently results in a highly efficient poling process. The addition of overdosed CB leads to the formation of undersized crystals, lowered crystallinity, and hence reduced piezoelectric performance of the PVDF-HFP/CB composite films.

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.

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.

Improvement of the piezoelectric properties of PVDF-HFP using AgNWs

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.

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.