In this study, the mechanical and thermal properties of graphene were systematically investigated using molecular dynamic simulations. The effects of temperature, strain rate and defect on the mechanical properties, including Young’s modulus, fracture strength and fracture strain, were studied. The results indicate that the Young’s modulus, fracture strength and fracture strain of graphene decreased with the increase of temperature, while the fracture strength of graphene along the zigzag direction was more sensitive to the strain rate than that along armchair direction by calculating the strain rate sensitive index. The mechanical properties were significantly reduced with the existence of defect, which was due to more cracks and local stress concentration points. Besides, the thermal conductivity of graphene followed a power law of λ~L0.28, and decreased monotonously with the increase of defect concentration. Compared with the pristine graphene, the thermal conductivity of defective graphene showed a low temperature-dependent behavior since the phonon scattering caused by defect dominated the thermal properties. In addition, the corresponding underlying mechanisms were analyzed by the stress distribution, fracture structure during the deformation and phonon vibration power spectrum.

Owing to its extraordinary physical properties and potential for next generation nanoelectronics, the in-plane graphene/hexagonal boron nitride (Gr/h-BN) heterostructure has been fabricated recently and gained a lot of attention. The defects located at the interface such as vacancies, topological defects are inevitable during the growth process. However, the effects of the defects on the interfacial thermal conductance between the Gr/h-BN interface have not well understood. In this work, the effects of defects on the interfacial thermal conductance across the Gr/h-BN interface have been systematically investigated by using nonequilibrium molecular dynamic simulations. The different types of single-vacancy and Stone–Wales defects were considered. The simulation results showed that the interfacial thermal conductance would decrease linearly with the increase of single-vacancy concentrations and it decreased with the existence of Stone–Wales defects, then reached a platform as concentration increased, the value of which was close to the interfacial thermal conductance of Gr/h-BN with the line defect formed by Stone–Wales defects. The analyses on the phonon vibration power spectra and the stress analysis indicated that the degradation in the in-plane modes accounted for the decrease caused by single-vacancy, while the stress concentration distribution and the ripple appeared near the interface dominated the degradation caused by Stone–Wales defects. Additionally, the effects of system dimensions and temperature on the interfacial thermal conductance were investigated.

In this study, the process-structure-property relationships of thermoformed, woven, carbon fiber (CF)reinforced polyether-ether-ketone (PEEK) composites were investigated, and the associated mechanisms were systematically discussed. Woven CF/PEEK prepregs were prepared using an electrostatic powder coating technique, and composite laminates were fabricated via thermoforming, using four processing temperatures (360, 380, 400, and 420 C) and five holding pressures (1, 3, 5, 7, and 9 MPa), respectively. The macroscopic properties and microstructures of the composite laminates were analyzed using mechanical characterization, thermal characterization, and microscopic observations. The results show that the flexural properties of the woven CF/PEEK composite tended to initially improve and subsequently decline as the processing temperature and holding pressure were increased. They are primarily determined by several complex mechanisms, such as the matrix properties, void defects, and fiber/resin interaction. The interlaminar shear properties of the woven CF/PEEK maintain a similar trend as the temperatures increases from 360 C to 420 C, while it is substantially unchanged as the holding pressure increases from 1 to 9 MPa. They are primarily determined by the mechanisms, such as matrix properties, void defects, and interlayer adhesion. Finally, these mechanisms were systematically discussed. POLYM. COMPOS., 40:3823-3834, 2019.

In this study, the flexural behavior and fracture mechanisms of short carbon fiber reinforced polyether-ether-ketone (SCFR/PEEK) composites at various ambient temperatures were investigated. First, the crystallinity and glass transition temperature (Tg) of PEEK and SCFR/PEEK were analyzed by differential scanning calorimetry analysis and dynamic mechanical analysis tests, respectively. The addition of SCFs increases the Tg but does not change the crystallinity of the PEEK matrix. Then, the three-point flexural tests of PEEK and SCFR/PEEK were performed over the temperature range of 20 to 235 °C, and the temperature-dependencies of the flexural properties of PEEK and SCFR/PEEK were discussed in detail. Finally, the microstructure of SCFR/PEEK was observed using a digital microscope and scanning electron microscope. The results show that the tension crack occurs first, and the crack extends upward leading to the shear crack and compression crack at room temperature. The fracture of SCFR/PEEK is mainly due to the extraction and rupture of SCFs. At high temperatures (above Tg), the tension crack and compression crack both occur, and the strong ductility of the matrix prevents the generation of shear crack. The fracture of SCFR/PEEK is mainly due to the rotation and extraction of SCFs, while the SCFs rupture plays a minor role.

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