High entropy alloys (HEAs) and medium entropy alloys (MEAs) are new classes of materials, defined as alloys composed of five or more and four or fewer kinds, respectively, of alloying elements with (near-)equiatomic concentrations. In the present article, we reviewed our recent works on ultra-grain refinement of HEAs and MEAs. CoCrFeMnNi HEA and its sub-system MEAs were highly deformed by high-pressure torsion and subsequently annealed under various conditions to obtain fully-recrystallized microstructures with FCC single phase having different mean grain sizes. It was found that ultrafine-grained (UFG) microstructures could be easily obtained by simple thermomechanical processes. Grain size and chemical composition dependence on mechanical properties of the HEA and MEAs were evaluated by tensile tests at room temperature. UFG HEAs and MEAs exhibited characteristic phenomena, such as discontinuous yielding and extra-hardening, similar to other UFG metals. In addition, the UFG HEAs and MEAs showed better strength-ductility balance compared with conventional UFG metals. Friction stresses of HEAs and MEAs were determined from Hall-Petch relationships and found to be much higher than those of pure metals and dilute alloys having FCC structure. Analysis based on theoretical models suggested that the high friction stress reflected atomic-scale heterogeneity in HEAs and MEAs.
Ni-based superalloy samples fabricated by selective laser melting (SLM) in inert gas are known to have lower creep ductility than cast and wrought one. The cause is thought to be due to oxide contamination from the powder surface and the SLM-specific microstructure. In this study, we evaluated the structure of the alloy 718 fabricated by SLM in a vacuum atmosphere that can be expected to suppress oxygen contamination and control the crystal orientation by decreasing the cooling rate. As a result, the microstructure of the samples fabricated by SLM in vacuum exhibits a layered structure, which is composed of a dendrite layer parallel to the building direction and the boundary, and the total thickness corresponds to the stacking thickness. On the other hand, the thickness of the top layer was more than double the additive layer thickness. From these results, the layered structure was formed by a heat-affected layer immediately below the melted part and a dendrite layer formed by melting.
In the last few decades, chalcogenide glasses have received considerable attention as infrared-transmitting materials for infrared systems such as surveillance cameras. In this review, two chalcogenide glass groups are introduced, where germanium sulfide and gallium sulfide are used as the main components. Germanium sulfide glass systems comprise Ge-Ga-Sb-S and Ge-Sb-Sn-S systems, whereas gallium sulfide glass systems comprise Ga-Sb-A-S systems (A = Sn, CsX, (X = Cl, Br, I)). Although these glass systems are free from selenium and arsenic which are commonly used in chalcogenide glasses, they have wide glass-forming composition regions. The glasses are thermally stable against crystallization and can be shaped by molding. Germanium-free gallium sulfide-based glasses are also transparent up to a wavelength of ~13 μm, and their optical window almost covers the atmospheric windows. In addition to the glass-forming regions, the fundamental properties of these glasses including the transmission spectra and refractive indices are presented in this review.
Thin films of metal-organic frameworks (MOFs) have attracted huge interests for various applications (e.g., sensing, catalysts and opto-electronics) due to their large porosity and tunable chemical property. Controlling the position and orientation of MOF crystals on device-scale substrates (over the centimeter scale) is required for the practical use of these sophisticated applications. Cu(OH)2 is a good precursor of the Cu-based MOF thin film due to their high reactivity with organic linkers, which allows for the growth of MOFs under mild conditions. In addition, lattice matching allows for the epitaxial growth of MOFs on Cu(OH)2. This article describes our recent progress on the fabrication of MOF thin films grown on assemblies of nanostructured Cu(OH)2. The approach enables us to control both position and orientation of MOF crystals on a substrate. The present approach for oriented MOF thin film growth is expanded for the oriented MOF-on-MOF thin films where different types of MOF layers possess epitaxial interfaces. A unique anisotropic plasmon resonance is found in the oriented MOF-on-MOF films accommodating Ag nanoparticles. These materials demonstrated here are considered as good candidates for functional porous coatings for solid catalysts, bio-sensors, electrical/optical devises and others.
Photon upconversion is a promising phenomenon that may find application in solar cell, display, and anti-fake printing. Rare-earth-based upconverters are most widely studied, but they always suffer from their weak and narrow absorption at λex = 980 nm. We investigated the plasmon-enhanced upconversion luminescence of CaF2: Yb3+, Er3+ nanocrystals on a plasmonic nanoantenna. We selected titanium nitride (TiN) as the constituent of nanoantenna, considering its better thermal stability compared to the conventional plasmonic metals. Well-defined TiN nanoantennas consisting of the square lattice of TiN nanoparticles were fabricated using a nanoimprint technique. By adjusting the height and period of the nanoantenna, we found that the enhancement factor was strongly influenced by the resonance of the nanoantenna at the pumping wavelength. A maximum of 2.98-fold enhancement was obtained for the upconversion photoluminescence intensity. The mechanism of enhancement was examined using three-dimensional simulation with COMSOL Multiphysics in detail, which was highly consistent with the experimental measurements.
In this review, a dry coating technology to produce composite electrode of an all-solid-state battery is presented. Firstly, from the view point of the dry powder mixing, fundamental concept of the dry coating is reviewed. Secondly, our recent progress in the dry coating technology for cathode active material and sulfide solid electrolyte is presented. The dry coating technology is applied to produce the core-shell structure composite particle composed of cathode active material particle (LiNi1/3Co1/3Mn1/3O2, NCM) and sulfide solid electrolyte (Li3PS4, LPS). We demonstrate that by means of a dry coating process single NCM particle is uniformly coated with LPS without any breakage and attrition of the NCM particle, i.e., NCM@LPS core-shell particle is able to be successfully produced. Finally, performance of the half-cell prepared with NCM@LPS particles is presented. A composite cathode prepared with NCM@LPS particles exhibits much larger contact area between NCM and LPS, resulting in significant improvement of performance of an all-solid-state half-cell.
A 50Li2SO4･50LiPO3 (mol%) glass was prepared by use of a traditional melt-quenching method. The structure and ion-conductivity of the glass were changed by the temperature at which the mother-melt of the glass was melted for the preparation of the glass. The Li2SO4 component was decomposed into Li2O and SO3 gas during the melting and the generated Li2O reacted with PO3- oxoacid-salt unit and produced P2O74- unit in the melt. The conductivity of the glasses was slightly increased with an increase in the temperature at which the melt was obtained for the glass- preparation. The 50Li2SO4･50LiPO3 glass obtained from the melt at 800°C showed the Li+-ion conductivity at 298 K, σ298K = 2.2 x 10-6 Scm-1. An all-solid-state battery was composed with the 50Li2SO4･50LiPO3 glass obtained from the melt at 800°C as solid electrolyte and with the cathode active materials LiNi1/3Mn1/3Co1/3O2. The all-solid-state battery showed good charge-discharge performance at 100°C.
With the aim of creating new lithium-ion conducting oxide glasses for all-solid-state lithium-ion batteries, we tried to prepare the glasses in the compositions (50-x)Li2SO4∙xLi2WO4∙50LiPO3 (mol%) by use of a traditional melt-quenching method. The glass-transition temperatures of the obtained glasses were increased with an increase of the Li2WO4 contents. The bulk glasses showed relatively high lithium-ion conductivities in the range of 10-6 to 10-5 Scm-1 at room temperature. On the other hand, the pellets, which were obtained by pressing the powdered glass samples, showed the conductivities in the range of 10-7 to 10-6 Scm-1 at room temperature. The conductivities of the bulk glasses are about one order higher than those of the pellet samples. An all-solid-state battery was composed with the cathode composites of the cathode active materials LiNi1/3Mn1/3Co1/3O2 and the 25Li2SO4∙25Li2WO4∙50LiPO3 (mol%) glass as solid electrolyte. The all-solid-state battery showed good charge-discharge performance at 100°C.
As a possible candidate of the electrolyte of solid oxide fuel cells (SOFC), ceria substituted YSZ (yttria stabilized zirconia) ceramics have been fabricated. We have already reported that ceria ceramics have improved in strength based on their chemical expansion on reduction annealing. Therefore, we examined the changes in flexural strength and ionic conductivity on reduction annealing. Before annealing both properties decreased with the substitution amount of ceria. However, the reduction annealing treatment improved the strength of the sample, of which degree was proportional to the amount of ceria substituted. Moreover, the 1 mol% ceria-substituted sample had high ionic conductivity and high flexural strength. The sample with that composition sintered under reduction atmosphere improved its mechanical strength by 13% on annealing in air.