Raman spectroscopy at the edges of multilayer graphene

Individual graphene layers in a multilayer graphene sample contribute their own edges. The edge of a graphene layer laid on an n layer graphene (nLG) is a building block for the edges of multilayer graphenes. We found that the D band observed from the edge of the top graphene layer laid on the nLG exhibits an identical line shape to that of disordered (n + 1)LG. Based on the spectral features of the D and 2D bands, we identified two types of alignment configurations at the edges of bilayer and trilayer graphenes, whose edges are well-aligned from their optical images.     

The effect of an oxygen partial pressure gradient on the mechanical behavior of perovskite membrane materials

In application of perovskite as oxygen conducting materials the membrane is operated at elevated temperatures under an oxygen gradient. The effect of the partial pressure difference on the mechanical properties is reported in the current work. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) samples were annealed under an oxygen gradient. The mechanical properties of cross-sections were characterized using indentation testing. Chemical strains for BSCF and LSCF were too small to detect them after cooling to RT by XRD; however, the results suggest that the indentation crack length is affected by chemical strains for LSCF, but not for BSCF. An anisotropy of the indentation crack length and corresponding apparent fracture toughness is related with the interaction of domain switching and residual strain that is probably also associated with the observation that vacuum (10⁻⁵ mbar) annealed LSCF showed surface cracking on heating in air, whereas for BSCF such fracture features were not observed.     

Nano-texture for a wear-resistant and near-frictionless diamond-like carbon

The effect of nano-scale surface texture on wear resistance of diamond-like carbon (DLC) films was studied using a reciprocating ball-on-flat tribometer in dry, humid, and liquid water environments. The nano-scale surface texture was produced by depositing ∼1 μm thick DLC films onto silicon substrates pre-textured with pyramidal wells and polystyrene spheres. The surface roughness of the textured DLC films was about 50 nm in both cases. The friction and wear behavior of the flat and nano-textured DLC films were tested with AISI 440C-grade stainless steel balls at a contact load creating about 360 nm deep Hertzian deformation which is significantly larger than the surface roughness. At this condition, nano-texturing did not affect the friction coefficient, but it significantly reduced the wear of DLC films in dry and humid nitrogen compared to flat DLC. In dry nitrogen, the nano-textured DLC films showed the ultra-low friction without substantial wear of DLC and deposition of thick transfer films onto the counter-surface. The wear reduction appeared to be related to the stress relief in the nano-textured DLC film. In liquid water, surface features on the nano-textured DLC films were diminished due to tribochemical oxidation and material removal at the sliding interface.     

High-conducting Bi4V2−xFexO11-δ ceramics containing Fe2O3 nanocrystals: Structure and properties

The topography, structure, thermal, magnetic, and electrical properties of Bi₄V_2?xFe_xO_11-δ ceramics substituted with x = 0.5 and 0.7 Fe were studied. The microscope analysis showed the presence of iron-rich nanocrystals formed on the Bi-Fe-V-O grains. The X-ray diffraction studies confirmed that grains are built mostly of tetragonal Bi₄V_1.5Fe_0.5O_10.5 phase. Thermal properties analysis showed an order-disorder type γ ? γ? phase transition at a temperature of around 916 K, pronounced in samples doped with x = 0.5 Fe. The magnetic anomaly was observed in ceramics doped with x = 0.7 Fe which was assigned to Morin transition of Fe₂O₃. The conductivity was measured over a wide frequency range from 10 mHz to 1 MHz and at a wide temperature range from 373 to 923 K, using impedance spectroscopy. The D.C. conduction process was due to oxygen vacancies hopping while at low temperatures electron holes hopping is also possible.     

Radiation effects in SiC for nuclear structural applications

Silicon carbide has enjoyed both fundamental study and practical application since the early days of nuclear materials science. In the past decade, with the increased interest in increasing efficiency, solving the real issues of waste disposal, and the constant mission to improve safety of nuclear reactors, silicon carbide has become even more attractive. The purpose of this paper is to discuss recent research that not only strives to understand the remarkable radiation stability of this material, but also the practical application of silicon carbide as waste form and for fission and fusion power applications.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Design and numerical analysis of a planar anode-supported SOFC stack

In the present study, numerical simulations are conducted to examine the flow characteristics and attributes of electrochemical reactions in the stack through three-dimensional analysis using finite volume approach prior to the fabrication of the SOFC stack. The stack flow uniformity index is employed to investigate the flow uniformity whereas in the case of electrochemical modeling, different mathematical models are adopted to predict the characteristics of activation and ohmic overpotentials that occur during electrochemical reactions in the cell. The normalized mass flow rate is found almost same in each cell with flow uniformity index of 0.999. The calculated voltage and power curves under different average current densities are compared with experimental results for the model validation. The changes in the voltage and power of the SOFC stack, current density, temperature, over potential and reactants distributions in relation to varying amounts of reactants flow are also examined. The current density distribution in each cell is observed to vary along the anode flow direction. The temperature difference in each cell is almost same along the flow direction of reactants, and the irreversible resistance showed an opposite trend with a temperature distribution in each cell.     

Crystal plasticity modeling of damage accumulation in dissimilar Mg alloy bi-crystals under high-cycle fatigue

Damage accumulation in Mg AZ31–AZ80 alloy bi-crystals under fatigue loading at room temperature is studied using a modified version of the crystal plasticity finite element model of Abdolvand and Daymond. The model accounts for strain accommodation by both slip and tensile twinning, and is first shown to reasonably describe monotonic single crystal Mg experimental data from the literature. The high cycle fatigue behavior was then investigated in misoriented dissimilar alloy bi-crystals through stress-controlled simulations up to 1000 cycles. Nine different orientation combinations were simulated and the fatigue damage evolution, defined as the cumulative shear strain amplitude, were compared and analyzed. The bi-crystal geometry was used to simulate possible microstructure combinations occurring, for instance within an idealized friction stir weld. Findings suggest that when either of the alloy bi-crystal grains is oriented for basal slip, poor fatigue performance can occur by twinning or slip localization depending upon the neighboring orientation.     

A novel biomedical TiN-embedded TiO2 nanotubes composite coating with remarkable mechanical properties,corrosion, tribocorrosion resistance,and antibacterial activity

Tribocorrosion is a severe problem in dental implants, artificial joints, and other implants, and it will affect the long-term safety of the implants. To improve the deficiencies of titanium alloys, we combined physical vapor deposition technology and anodic oxidation to prepare TiN to embed TiO₂ nanotube composite coatings (NTNTs-TiN). Results show that the hardness of the NTNTs-TiN composite coatings reaches 33.2 ± 0.6 GPa, and the grains of the composite coatings were further refined. The NTNTs-TiN coating has the smallest average coefficient of friction (0.22) during tribocorrosion. The tribocorrosion resistance of NTNTs-TiN coating in SBF is increased by ∼44 and ∼2 times compared with Ti6Al4V alloy and TiN coating, respectively. The capillarity effect of the lower contact angle of NTNTs-TiN can form a continuous water-lubrication film at the interface between the counter-ball and coating and produce a lubrication film composed of Ca, Mg, and P, which reduces the coefficient of friction significantly. The NTNTs/TiN composite coating exhibits the best synergistic effect of wear and corrosion. In addition, the NTNTs-TiN coating also exhibits excellent antimicrobial and corrosion properties, which provides a new solution for the long-term safe use of implants in the human body.