Nuclear magnetic resonance microimaging investigation of membrane electrode assembly of fuel cells: morphology and solvent dynamics

The structure and solvent (water, methanol, etc.) dynamics of a number of fuel membrane electrode assembly (MEA) samples are studied with nuclear magnetic resonance microimaging with spatial resolution of tens of micrometers. The micrometer-scale inhomogeneity of the samples is observed and confirmed with various weighting methods. In particular, diffusion coefficients at different positions in MEA are clearly differentiated. Furthermore, chemical shift selection imaging enables one to investigate the spatial distribution and dynamics of individual chemical groups. These types of information offer us insights into the working principle of fuel cell and pave the way to in situ studies of operating fuel cells.     

6×6 matrix formalism of optical elements for modeling and analyzing 3D optical systems

Although conventional 2×2 ray matrices, i.e. ABCD matrices, provide a convenient means of obtaining initial estimates of the performance of an optical system during the early stages of the design process, they are suitable only for optical systems with the axisymmetric property. Accordingly, this study utilizes the 6×6 matrix formalism, which was proposed by our group, for optical boundary surfaces to develop a new approach for modeling and analyzing 3D optical systems comprising multiple lenses and/or mirrors and clarifying many of the system’s basic properties, e.g. the effective focal length, primary aberration and cardinal points, and so forth. To reduce the complexity of the modeling process, general matrix formalisms of sub-systems in a 3D optical system are presented. The validity of the proposed approach is evaluated by modeling and analyzing a simple 3D optical system and comparing the results with those obtained from the skew ray-tracing computer program and commercial optical design software. The results confirm that the proposed methodology provides a convenient means of obtaining initial insights into a variety of 3D optical systems with non-coplanar axes.     

Nanoindentation of vertical ZnO nanowires

We report the experimental observations of buckling instabilities of vertical well-aligned single-crystal ZnO nanowires prepared on ZnO:Ga/glass templates. It was found that critical buckling load and buckling energy of the ZnO nanowires were 215 μN and 3.69×10⁻¹¹ J, respectively. It was also found that Young's modulus of the ZnO nanowires were 232 and 454 GPa while critical buckling strains were 0.35% and 0.18% when fixed–fixed column mode and fixed–pinned column mode were used, respectively.     

Simple expressions for the maximum omnidirectional bandgap of bilayer photonic crystals

We propose three dimensionless approximate expressions to predict the thickness filling factor, gap center, and gap width of the maximum omnidirectional gap (MODG) for various refractive indices in one-dimensional photonic crystals. These expressions are simple and do not include trigonometric or inverse trigonometric functions. It is easy to obtain the MODG from given refractive indices but also to estimate the refractive indices from the MODG based on the results.     

Rheological aspect on electrospinning of polyamide 6 solutions

Polyamide 6 (PA6) solutions in formic acid (FA) and deionized water cosolvent may behave as polyelectrolyte or neutral solutions depending on the cosolvent composition. In this study, both polyelectrolyte and neutral PA6 solutions were prepared for electrospinning, and their spinnability was correlated with their rheological properties. In addition, the effects of PA6 average molecular weight and carbon nanocapsule (CNC) nanoparticle addition on solution rheology and electrospinnability were investigated. Microstructure and thermal properties of the as-spun fibers were identified by wide-angle X-ray diffraction, polarized Fourier infrared spectroscopy, and differential scanning calorimetry (DSC). Due to the chain expansion, polyelectrolyte solutions with 99 vol.% FA solvent possess much lower entanglement concentration (?_e, ∼1 wt.%) than neutral solutions (∼7 wt.%) prepared by 90 and 85 vol.% FA solvent. Compared with the neutral solution, the polyelectrolyte solution is more advantageous because a lower concentration is sufficient to obtain bead-free PA6 fibers. However, at a concentrated regime of 15 wt.% solution, the obtained fibers exhibit a larger diameter due to the higher entanglement density. For the crystalline structure, the content and orientation of α-form crystals are higher in the PA6 fibers obtained from the polyelectrolyte than from the neutral solution. When PA6 with a lower molecular weight is used, a higher concentration is required to develop the entangled chains to produce bead-free fibers. Homogeneous PA6 solutions filled with CNCs exhibit more elastic behavior than unfilled solutions due to the presence of the CNC–CNC network, aside from the entangled network of PA6 chains. Electrospinning of the CNC-filled solutions yields PA6 fibers with CNC aggregates protruding from the fiber surface. The inclusion of CNC in the PA6/FA solution produces fibers possessing enhanced α-form crystals with reduced orientation. In all cases, DSC heating traces of the as-spun fibers identify a high melting temperature (HMT) phase of PA6. The amount of HMT phase decreases, provided that more water or CNCs are added into the PA6/FA solution for electrospinning.     

Large negative differential resistance in graphene nanoribbon superlattices

A graphene nanoribbon superlattice with a large negative differential resistance (NDR) is proposed. Our results show that the peak-to-valley ratio (PVR) of the graphene superlattices can reach 21 at room temperature with bias voltages between 90–220 mV, which is quite large compared with the one of traditional graphene-based devices. It is found that the NDR is strongly influenced by the thicknesses of the potential barrier. Therefore, the NDR effect can be optimized by designing a proper barrier thickness. The large NDR effect can be attributed to the splitting of the gap in transmission spectrum (segment of Wannier–Stark ladder) with larger thicknesses of barrier when the applied voltage increases.     

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as ThII Synthons

Thorium redox chemistry is extremely scarce due to the high stability of Th^IV. Here we report two unique examples of thorium arenide complexes prepared by reduction of a Th^IV-siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(O^tBu)₃)₃Th(η⁶-C₁₀H₈)] ( 1 ) and the inverse-sandwich complex [K(OSi(O^tBu)₃)₃Th]₂(μ-η⁶,η⁶-C₁₀H₈)] ( 2 ). The electrons stored in these complexes allow the reduction of a broad range of substrates (N₂O, AdN₃, CO₂, HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN₂ to yield the unexpected Th^IV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.