Wavy Graphene-Like Network Forming during Pyrolysis of Polyacrylonitrile into Carbon Fiber.

Carbon fiber (CF) obtained by pyrolysis of polyacrylonitrile (PAN-CF) surpasses metals in properties suitable for diverse applications such as aircraft manufacture and power turbine blades. PAN-CF obtained by pyrolysis at 1200-1400 °C shows a remarkably high tensile strength of 7 GPa, much higher than pitch-based CF (pb-CF) consisting of piles of pure graphene networks. However, little information has been available on the atomistic structure of PAN-CF and on how it forms during pyrolysis. We pyrolyzed an acrylonitrile 9-mer in a carbon nanotube, monitored the course of the reaction using atomic-resolution electron microscopy and Raman spectroscopy, and found that this oligomer forms a thermally reactive wavy graphene-like network (WGN) at 1200-1400 °C during slow graphitization taking place between 900 and 1800 °C. Ptychographic microscopic analysis indicated that such material consists of 5-, 6-, and larger-membered rings; hence, it is not flat but wavy. The experimental data suggest that, during PAN-CF manufacturing, many layers of WGN hierarchically pile up to form a chemically and physically interdigitated noncrystalline phase that resists fracture and increases the tensile strength─the properties expected for high-entropy materials. pb-CF using nearly pure carbon starting material, on the other hand, forms a crystalline graphene network and is brittle.

Immense Reduction in Interfacial Resistance between Sulfide Electrolyte and Positive Electrode.

Low interfacial resistance between the solid sulfide electrolyte and the electrode is critical for developing all-solid-state Li batteries; however, the origin of interfacial resistance has not been quantitatively reported in the literature. This study reports the resistance values across the interface between an amorphous Li3PS4 solid electrolyte and a LiCoO2(001) epitaxial thin film electrode in a thin-film Li battery model. High interfacial resistance is observed, which is attributed to the spontaneous formation of an interfacial layer between the solid electrolyte and the positive electrode upon contact. That is, the interfacial resistance originates from an interphase mixed layer instead of a space charge layer. The introduction of a 10 nm thick Li3PO4 buffer layer between the solid electrolyte and positive electrode layers suppresses the formation of the interphase mixed layer, thereby leading to a 2800-fold decrease in the interfacial resistance. These results provide insight into reducing the interfacial resistance of all-solid-state Li batteries with sulfide electrolytes by utilizing buffer layers.

Lone-Pair-Induced Intra- and Interlayer Polarizations in Sillén-Aurivillius Layered Perovskite Bi4NbO8Br.

Sillén-Aurivillius layered perovskite oxyhalides Bi4MO8X (M = Nb, Ta; X = Cl, Br) are of great interest because of their potential as lead-free ferroelectrics in addition to their function as visible-light-responsive photocatalysts. In this work, we revisited the crystal structure of Bi4NbO8Br (space group: P21cn), revealing that the intralayer polarization is not based on the reported NbO6 octahedral tilting but is derived from the stereochemically active Bi3+ lone pair electrons (LPEs) and the octahedral off-centering of Nb5+ cations. The revised structure (space group: Ic) has additional interlayer polarizations (calculated: 0.6 µC/cm2), in agreement with recent experiments on Bi4NbO8Br. The occurrence of polarization due to stereochemically active LPEs and Nb-site off-centering is similar to Aurivillius-type ferroelectrics (e.g., Bi2WO6), with comparable spontaneous polarizations in the in-plane direction (calculated: 43.5 µC/cm2). This, together with the out-of-plane polarization, indicates that Sillén-Aurivillius compounds have great potential as ferroelectric materials.

Cross-sectional transmission electron microscopy observation of sub-nano-sized molybdenum carbide crystals in carbon nanotubes.

Cross-sectional observation of molybdenum carbide nanocrystals inside carbon nanotubes was successfully conducted in this study. The nanocrystals were generated by irradiating as-synthesized encapsulated molybdenum oxide crystals with an intense electron beam; the most probable composition of the crystals was determined to be α-MoC1-x by electron diffraction, high-resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. Thinning processes using a focused ion beam and an Ar-ion mill enabled cross-sectional observations along the tube axis. As a result, it became clear that the molybdenum carbide crystals show translational symmetry with a parallelogram configuration having preferred {111} facets when observed from the [110] direction, despite the sub-nanometer order of the crystals.

Nanodomain structures with hierarchical inhomogeneities in PMN-PT.

The nanometric domain configuration of (1 - x) Pb(Mg(1/3)Nb(2/3))O(3-x)PbTiO(3) [(1 - x)PMN-xPT] single crystals in the monoclinic phase around a morphotropic phase boundary (MPB) has been examined thoroughly by means of transmission electron microscopy (TEM). Domain structures with hierarchically inhomogeneous configuration were found in the monoclinic phase near the MPB region around x ~ 0.32, which are characterized as nanoscaled lamella-type domain structures with ~10 nm width inside macroscopic-sized banded domains with 100 to 200 nm width. To elucidate formation processes of the domain structures with hierarchically inhomogeneous configuration, an in situ TEM observation of changes of the domain structures in the temperature window between 298K and 500K was carried out. It is revealed that these nanoscaled lamella-type domain structures with ~10 nm width appear inside the banded domains as a result of the tetragonal structure and are inherent to the monoclinic phase.