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Si3N4-based nanocomposites containing 0–50 wt% TiC0.37N0.63 are directly consolidated at 1700°C by spark plasma sintering, and their reciprocal sliding behavior against a Si3N4 counterbody is investigated under a maximum Hertzian stress of 1.27 GPa in unlubricated conditions. The average grain widths of Si3N4 and TiC0.37N0.63 are about 85 and 90 nm, respectively. The decreasing relative densities of the as-sintered nanocomposites indicate that the nano-TiC0.37N0.63 may introduce pores and reduce the hardness and fracture resistance of the materials. The brittleness index for sliding contacts in all the samples is 25–31, indicating brittle fracture taking place on the wear surface and inducing cavities. When the mean free paths of nano-TiC0.37N0.63 are slightly greater than grain length of Si3N4, the best wear resistance is achieved in Si3N4 containing 20/30 wt% TiC0.37N0.63 due to the process of surface smoothing by triboproducts. Severe wear response can be observed in Si3N4 nanocomposites containing 0, 10, 40, and 50 wt% of TiC0.37N0.63. The wear responses are explained by considering the microstructural parameters (like grain characteristics for both phases and mean free path of nano-TiC0.37N0.63) and contact-induced fracturing behavior, as well as tribochemical reactions.

Poly(butylene succinate) (PBS) is a biodegradable aliphatic polyester and is semicrystalline in nature. It has a wide range of engineering applications owing to its attractive combination of good processability and mechanical properties. PBS nanocomposites with graphene nanosheets were prepared by melt compounding. We report on their structure and their thermal, electrical, and mechanical properties. The thermal stability of the nanocomposite increased by 35°C compared with that of neat PBS. The crystallization and melting behavior of the PBS matrix in the presence of dispersed graphene nanosheets were studied by differential scanning calorimetry and polarised optical microscope. We also observed an interesting phenomenon in the PBS/graphene nanocomposites from our FTIR investigation. β-form of PBS crystals are formed in the composite samples with 5 wt% graphene loading, which otherwise forms in pure PBS only under stress. We observed ∼12% modulus enhancement for 2 wt% PBS/graphene nanocomposites. The electrical conductivity increased to ∼105-fold for 5 wt% PBS/graphene nanocomposites. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Twisted bilayer graphene, in which interlayer interaction plays a critical role in this coupled system, is characterized for its angle-dependent electronic and optical properties. Here, we present a systematic Raman study of single-crystal twisted bilayer graphene grains, with the spectra of each bilayer graphene precisely correlated to its twist angle using combined transmission electron microscopic technique. Van Hove singularities develop as a result of band rehybridization at the crossing Dirac cones of the two layers, giving rise to a critical twist angle that determines the energy separation between the saddle points in the band structure and the resonance Raman spectra accordingly. The 2D mode becomes sensitive to the twist angle, showing the angle-dependent position, peak width, and intensity. Our results interpreted in the framework of angle-dependent double resonance scattering provide an important experimental perspective in understanding the coupled bilayer graphene system. Copyright © 2014 John Wiley & Sons, Ltd.

The effects of microstructure on the mechanical responses and damage evolution of spark-plasma-sintered β-Si3N4-based ceramics has been evaluated through indentation tests. It was found that the nanoceramic and its coarse-grained counterpart exhibit similar elastoplastic behavior in their indentation responses. However, the increased hardness and ratio of elastic work to total work done in the nanoceramic suggest that resistance to plastic deformation is greater than that in the coarser-grained one. The microcracking behavior within the indented materials is also proposed. A finite-element analysis is used to estimate the distribution of compressive and shear stresses within the indented materials. The smaller grain size in bridging ceramic not only enhances the energy dissipation by formation of a higher density of intergranular microcracks along the weak grain boundary phase but also toughens the cracked solid through increasing resistance to plastic sliding and multi-cracks propagation. Prior to formation of the radial cracks, better damage tolerance of nanoceramic is thought to be achieved in this study.

A conductive TiC nanopowder was incorporated into nanosized β-Si3N4-based powder and consolidated by a spark plasma sintering (SPS) technique with a rapid heating process. The influence of the conductive phase on the microstructure development of the Si3N4 matrix was demonstrated. After sintering, the conductive phase transformed into titanium oxycarbonitride. The Si3N4-based composite containing 5 wt% nano-TiCxOyNz shows a larger average grain size and aspect ratio than the monolithic Si3N4-based ceramic. This is possibly because a leakage current hops across the conductive titanium oxycarbonitride grains and causes Joule heating during sintering. The transmission electron microscopy analysis confirmed that dissolution–reprecipitation and coalescence occur. In addition, although the increasing amount of incorporated nano-TiCxOyNz (10 and 20 wt%) decreases the electrical resistivity of the composites, the pinning effect of the titanium-based phase significantly suppresses the grain growth of Si3N4 matrix grains. β-Si3N4-based nanocomposites containing nanosized titanium oxycarbonitride were thus obtained in the present study.

A nanosized β-Si3N4-based powder doped with sintering additives of 6 wt% Al2O3 and 8 wt% Y2O3 were sintered by spark plasma sintering at different heating rates. The in situ shrinkage curves were measured to identify the densification mechanism during the rise in temperature. At slower heating rates (50 and 100°C/min), the nanosized grains are maintained after sintering at 1600°C for 5 min, while anisotropic grain growth is accelerated above 1500°C by applying a rapid heating cycle (200°C/min). In addition to the dynamic Ostwald ripening that occur during the sintering process, the presence of Morie fringes and dislocations is attributed to grain rotation and misfit strain between the subgrains and elongated, large grains. Grain coalescence is found to occur in the rapid heating process.

Pure and 5 wt% of Ag-doped (La 1- x Y x ) 2 Ba 2 CaCu 5 O z superconducting compounds for x = 0-0.5 have been prepared. Analysis of X-ray diffraction patterns shows that the samples are essentially in the single-phase form and they could be refined using the P4/mmm space group in a tetragonal cell. The typical lattice parameters are found to be a = h = 3.879 A, c = 11.646 A for x = 0 and a = h = 3.856 A, c = 11.576 A for x = 0.5 samples. The detailed crystal structure parameters are presented. The average grain size values from scanning electron microscope images are found to be in the order of 1-3 μm. Temperature variations of ac susceptibility and electrical resistivity have been measured. Superconducting transitions with diamagnetic T c ranging from 60 to 75 K have been observed, with the maximum T c for the x = 0.25 sample. Ag-doped samples exhibit improved inter-granular coupling and homogeneous oxygenation.

The polymorphism of poly(vinylidene fluoride) (PVDF) and its nanocomposites was studied by means of solid state nuclear magnetic resonance spectroscopy. 13C cross polarization magic angle spinning (13C CP MAS) NMR spectra were recorded using simultaneous high-power decoupling on both the proton and fluorine channels. Both 1H 13C and 19F 13C CP experiments were conducted, giving identical results apart from intensity variations due to the CP efficiency. Two main resonances for the CF2 and the CH2 groups were observed for both neat PVDF (PVDF-C0) and the nanocomposite containing 2 wt% clay (PVDF-C2) samples. 19F CP MAS spectra were obtained from long proton spin-lock experiments with a shorter contact time. The results showed two strong resonances at −84 and −98 ppm with equal intensities, representing the α-form crystalline structure of PVDF. It was shown that the clay induces the crystallization of PVDF in β-form. Our earlier investigations using thermal analysis and X-ray scattering methods also showed crystal transformation of PVDF in its clay nanocomposites. POLYM. ENG. SCI. 46:1684–1690, 2006. © 2006 Society of Plastics Engineers