Improvement of the mode II interface fracture toughness of glass fiber reinforced plastics/aluminum laminates through vapor grown carbon fiber interleaves.

The effects of acid treatment, vapor grown carbon fiber (VGCF) interlayer and the angle, i.e., 0° and 90°, between the rolling stripes of an aluminum (Al) plate and the fiber direction of glass fiber reinforced plastics (GFRP) on the mode II interlaminar mechanical properties of GFRP/Al laminates were investigated. The experimental results of an end notched flexure test demonstrate that the acid treatment and the proper addition of VGCF can effectively improve the critical load and mode II fracture toughness of GFRP/Al laminates. The specimens with acid treatment and 10 g m-2 VGCF addition possess the highest mode II fracture toughness, i.e., 269% and 385% increases in the 0° and 90° specimens, respectively compared to those corresponding pristine ones. Due to the induced anisotropy by the rolling stripes on the aluminum plate, the 90° specimens possess 15.3%-73.6% higher mode II fracture toughness compared to the 0° specimens. The improvement mechanisms were explored by the observation of crack propagation path and fracture surface with optical, laser scanning and scanning electron microscopies. Moreover, finite element analyses were carried out based on the cohesive zone model to verify the experimental fracture toughness and to predict the interface shear strength between the aluminum plates and GFRP laminates.

An efficient algorithm embedded in an ultrasonic visualization technique for damage inspection using the AE sensor excitation method.

To improve the reliability of a Lamb wave visualization technique and to obtain more information about structural damages (e.g., size and shape), we put forward a new signal processing algorithm to identify damage more clearly in an inspection region. Since the kinetic energy of material particles in a damaged area would suddenly change when ultrasonic waves encounter the damage, the new algorithm embedded in the wave visualization technique is aimed at monitoring the kinetic energy variations of all points in an inspection region to construct a damage diagnostic image. To validate the new algorithm, three kinds of surface damages on the center of aluminum plates, including two non-penetrative slits with different depths and a circular dent, were experimentally inspected. From the experimental results, it can be found that the new algorithm can remarkably enhance the quality of the diagnostic image, especially for some minor defects.

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 Betti's 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.

Temperature-dependent piezoresistivity in an MWCNT/epoxy nanocomposite temperature sensor with ultrahigh performance.

A temperature sensor was fabricated from a polymer nanocomposite with multi-walled carbon nanotube (MWCNT) as nanofiller (i.e., MWCNT/epoxy). The electrical resistance and temperature coefficient of resistance (TCR) of the temperature sensor were characterized experimentally. The effects of temperature (within the range 333-373 K) and MWCNT content (within the range 1-5 wt%) were investigated thoroughly. It was found that the resistance increases with increasing temperature and decreasing MWCNT content. However, the resistance change ratio related to the TCR increases with increasing temperature and MWCNT content. The highest value of TCR (0.021 K(-1)), which was observed in the case of 5 wt% MWCNT, is much higher than those of traditional metals and MWCNT-based temperature sensors. Moreover, the corresponding numerical simulation-conducted to explain the above temperature-dependent piezoresistivity of the nanocomposite temperature sensor-indicated the key role of a temperature-dependent tunneling effect.

Preparation of Novel Graphene/Silicone Rubber Nanocomposite Dielectric Foams.

In order to obtain high dielectric silicone rubber (SR)-based nanocomposites, graphene (Gr) was added by ultrasonication and mechanical mixing for the preparation of a microporous structure. It was discovered that the Gr content and the expansion rate had a great impact on the cellular structure. Based on the effects of the Gr content and the expansion rate on the dielectric property, hybrid materials were prepared and better properties appeared, as expected. For all samples, the dielectric constant increased with the Gr content until 3 wt% and then decreased. When the Gr content was 3 wt% and the expansion rate was 2, the dielectric constant reached 18.14 (1 kHz), which was 55% higher than that of the non-expansion sample (11.74) and several times that of the pure sample (3~6). Meanwhile, the dielectric loss was less than 0.01. This work proposed a method for producing high dielectric materials with important applications in the field of capacitors, sensors, and micro-resistors.

Boosting Output Performance of Triboelectric Nanogenerator via Mutual Coupling Effects Enabled Photon-Carriers and Plasmon.

Boosting the output performance of triboelectric nanogenerators via some unique methods is always a meaningful way to speed up their commercialization. However, the available approach to boost performance is mainly restricted to one physics effect based and the basic research of boosting performance via mutual coupling effects is little research. Herein, a new strategy is creatively proposed based on charge traps from mutual coupling effects, generated from g-C3 N4 /MXene-Au composites, to further promote the output performance of triboelectric nanogenerator. It is found that photon-generated carriers coupling surface plasmon effect enables composites filled into tribo-material with visible light is an excellent value in boosting performance. The charge traps from mutual coupling effects for boosting performance are analyzed theoretically and verified by experiments. The output power of boosting-triboelectric nanogenerator (TENG) achieves a sixfold enhancement (20 mW) of normal TENG with polydimethylsiloxane (PDMS) in ambient conditions. This work provides a profound understanding of the working mechanism of mutual coupling effects boosting the performance of TENG and an effective way for promoting TENG output.

Synergistic Delamination Toughening of Glass Fiber-Aluminum Laminates by Surface Treatment and Graphene Oxide Interleaf.

The synergistic effects of surface treatment and interleaf on the interlaminar mechanical properties of glass fiber-aluminum laminates were studied. Aluminum sheets were treated with alkaline etching. Meanwhile, a graphene oxide (GO) interleaf was introduced between the aluminum sheet and the glass fiber-reinforced epoxy composite. Double cantilever beam and end-notched flexure tests were employed to evaluate the interlaminar fracture toughness of the glass fiber-aluminum laminates. The obtained results show that the toughening efficiency of the interleaf is dependent on the aluminum surface characteristics as well as the GO loading. Further comparison reveals that the highest mode-I and mode-II fracture toughnesses are obtained in the specimens with alkali etching treatment and addition of GO interleaf with 0.5 wt% of GO loading, which are 510% and 381% higher in comparison to the plain specimen. Fracture surfaces were observed to further uncover the reinforcement mechanisms.

Enhancement of thermal energy transport across the graphene/h-BN heterostructure interface.

Enhancing thermal energy transport is critical for the applications of 2-dimensional materials. Here, we explored the methods of enhancing the interfacial thermal energy transport across the graphene (GR)/hexagonal boron nitride (h-BN) heterostructure interface, and revealed the enhancement mechanisms of interfacial thermal energy transport by applying non-equilibrium molecular dynamics (NEMD) simulations. The computational results indicated that both doping and interface topography optimization could effectively improve the interfacial thermal conductance (ITC) of the GR/h-BN heterostructure. In particular, the enhancement of the zigzag interface topography led to a much better result than the other methods. Doping and interface topography optimization increased the overlap of the phonon density of states (PDOS). Temperature had a negligible effect on the ITC of the GR/h-BN heterostructure when the temperature exceeded 600 K.

A double-helix-structured triboelectric nanogenerator enhanced with positive charge traps for self-powered temperature sensing and smart-home control systems.

Triboelectric nanogenerators (TENGs) have been in spotlight for their excellent capability to drive miniature electronics. Herein, we report a sophisticated double-helix-structured triboelectric nanogenerator (DHS-TENG) enhanced with positive charge traps for self-powered temperature sensing and smart-home control system. The DHS-TENG increases the charge density on the contact surfaces by taking advantage of the ferroelectric characteristics of polyvinylidene fluoride (PVDF). In addition, the flexible double-helix-structure endows DHS-TENG with excellent elastic property as it has no external supporting materials. The reported DHS-TENG, with the dimensions of 3 cm × 3 cm × 5 cm and a light weight of 10 g, can deliver a peak output power of 9.03 mW under a loading resistance of 4 MΩ. It also delivers an enhanced output performance of 460 V, 140 µA and 400 nC under a constant contact force of 40 N. Furthermore, the DHS-TENG is capable of powering 120 green LEDs and enabling a temperature sensor to work properly. In particular, the DHS-TENG demonstrates the capability of successful remote data transmission for application in smart-home control systems within 10 meters.

Tomographic reconstruction of damage images in hollow cylinders using Lamb waves.

Lamb wave tomography (LWT) is a potential and efficient technique for non-destructive tomographic reconstruction of damage images in structural components or materials. A two-stage inverse algorithm proposed by the authors for quickly reconstructing the damage images was applied to hollow cylinders. An aluminum hollow cylinder with an internal surface pit and a Carbon Fiber Reinforced Plastic (CFRP) laminated hollow cylinder with an artificial internal surface damage were used to validate the proposed method. The results show that the present method is capable of successfully reconstructing the images of the above damages in a larger inspection area with much less experimental data compared to some conventional ultrasonic tomography techniques.

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.

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 ß-phase crystal structure, the nanocomposite films were drawn in a ratio of 4­5 and polarized by a step-wise poling method. To evaluate the piezoelectric property, the output voltages of the rGO­PVDF nanocomposite films were measured through extensive experimental vibration tests. The experimental results show that the rGO­PVDF 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 ß-crystalline phase can be formed at those rGO loadings, as confirmed by XRD and FT-IR spectrum analyses.