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Polymer electrolyte fuel cells (PEFCs) are drawing attention as energy conversion devices for next generations because of their highly efficient, environmentally benign, and portable features. In the last five decades, three distinguishable innovations were achieved in terms of proton conductive membranes and electrodes: introduction of perfluorinated membranes into PEFCs, adoption of ionomers for electrodes, and increased toughness of membranes by reinforced membranes. The efficiency, cost, and durability achieved from the past three innovations are still not enough to replace competing technologies such as combustion engines. In this review, the authors would elucidate the three different methods based on nanotechnology to overcome the limits: nanoporous carbon-supported catalysts, nanocomposite membranes, and nanostructured membrane electrode assemblies, which will bring the fourth innovation to PEFCs. With the innovation, PEFCs will fulfill the goals of being clean-energy conversion devices in the major applications of stationary, portable, and vehicle markets.
In our previous paper, the expanding cavity model (ECM) and Lamé solution were used to obtain an analytical expression for the scale ratio between hardness (H) to reduced modulus (Er) and unloading work (Wu) to total work (Wt) of indentation for elastic-perfectly plastic materials. In this paper, the more general work-hardening (linear and power-law) materials are studied. Our previous conclusions that this ratio depends mainly on the conical angle of indenter, holds not only for elastic perfectly-plastic materials, but also for work-hardening materials. These results were also verified by numerical simulations.
The effect of Pd concentration on the soldering reaction between Ni and Sn–xPd alloys (x = 0–0.5 wt%) was investigated in this study. When the Pd concentration was low (x ≤ 0.05 wt%), the predominant reaction product was a layer of Ni3Sn4. In contrast, an additional (Pd,Ni)Sn4 layer deposited over the Ni3Sn4 in the case of above 0.2 wt%. This microstructure evolution significantly weakened the strength of the interface, deteriorating the reliability of solder joints. A Pd–Ni–Sn isotherm simulated by the CALPHAD method was used to rationalize the above transition in the reaction product(s).
The influence of high magnetic field on nitriding behavior was investigated in a mixture of NH3 and H2. It was found that high magnetic field could shift the equilibrium of nitriding reaction; this proved that the critical nitrogen potential to form γ′-Fe4N and ε-Fe3N phase was evidently enhanced compared with conventional nitriding. This research provides a new approach for a selective nitriding process.
The parallel nano-scanning calorimeter (PnSC) is a silicon-based micromachined device for calorimetric measurement of nanoscale materials in a high-throughput methodology. The device contains an array of nanocalorimeters. Each nanocalorimeter consists of a silicon nitride membrane and a tungsten heating element that also serves as a temperature gauge. The small mass of the individual nanocalorimeters enables measurements on samples as small as a few hundred nanograms at heating rates up to 104 K/s. The sensitivity of the device is demonstrated through the analysis of the melting transformation of a 25-nm indium film. To demonstrate the combinatorial capabilities, the device is used to analyze a Ni–Ti–Zr sample library. The as-deposited amorphous samples are crystallized by local heating in a process that lasts just tens of milliseconds. The martensite–austenite transformation in the Ni–Ti–Zr shape memory alloy system is analyzed and the dependence of transformation temperature and specific heat on composition is revealed.
Near-stoichiometric (NS) (Mg:)Er:LiNbO3 crystals were grown from melts containing 0.0/0.5, 0.5/0.5, and 1.0/0.5 mol%/mol% MgO/Er2O3. Crystal composition and optical properties studies show that the Li2O contents in these crystals increase all by ∼1 mol% relative to the congruent point. The 1.0 mol% MgO-doped NS crystal is just near optical damage threshold and withstands a 488 nm light intensity >0.74 MW/cm2 without optical damage. Unpolarized absorption spectra of these NS crystals were measured, and the Er3+ absorption cross-section spectra were determined. The Er3+ spectroscopic properties were studied by Judd–Ofelt theory. The results show that as the crystal composition approaches the stoichiometry, the Er3+ spectroscopic properties change definitely. The Er3+ ion in the NS crystal has smaller absorption cross section and hence weaker oscillator strength, lower emission rate, and longer radiative lifetime. Nevertheless, the radiative quantum efficiency is retained. In addition, the MgO codoping has less effect on the Er3+ spectroscopic properties.
Emission properties of Er3+ in Ga2S3–GeS2–Sb2S3 glasses at the mid-infrared region were investigated from the viewpoint of their dependence on the concentration of the active ion and the glass composition. In the Judd–Ofelt analysis, no variation in omega parameters were observed when GeS2 was replaced by Ga2S3, while Ω2 increased as Sb2S3 was replaced by Ga2S3. This is due to the structural similarity and difference between the glass network units, GaS4 and GeS4 tetrahedra, and SbS3 pyramid. Clear mid-infrared emissions were observed at 2750 and 4300 nm assigned to the 4I11/2 → 4I13/2 and 4I9/2 → 4I11/2 transitions, respectively. The lifetime of the initial level of the 4.3 μm emission, 4I9/2, rapidly decreased with the Er3+ concentration because of the cross relaxation of this level, which can take place even at considerably low Er3+ concentration. The cross-relaxation processes were suppressed by the increase in the content of Ga2S3 because the solubility of Er3+ ions in the glasses increases with the Ga2S3 content.
SrY2O4:Eu3+ phosphors were synthesized by both the solid-state reaction method and the sol-gel method, and their photoluminescence in vacuum ultraviolet (VUV) and the ultraviolet (UV) region were evaluated. The excitation spectra of SrY2O4:Eu3+ phosphors prepared by solid-state reaction show another excitation band centered at 324 nm except for the charge-transfer bands (CTB) of Eu3+ when monitored at 610 nm, and a blue emission band around 406 nm is observed when excited at 324 nm, which could be associated with defects. Both the excitation and emission bands mentioned above disappear when the samples were prepared by the sol-gel method. SrY1.98O4:0.02Eu3+ phosphors synthesized by the sol-gel method exhibit a higher emission intensity under 147 nm excitation compared with solid-state reaction technology. The main reason could be that the samples prepared by the solution-based route have more regular and uniform morphologies.
Hierarchically structured zinc oxide was prepared from zinc acetylacetonate by a microwave-assisted process. The zinc oxide formed nanoparticles that are packed in substructured spherical agglomerates with a diameter of 0.5 μm. Nitrogen adsorption, x-ray diffraction, and dilatometry were used to investigate the densification. Ion beam method was applied to prepare cross sections and enable microstructural analysis. Three regimes of microstructural evolution were identified on different scales during sintering. In the first regime, nanoparticles changed morphology and densification occurred only in the interiors of the agglomerates. In the second regime, agglomerates became hollow and built necks. Simultaneously, densification set in on the macroscopic scale. A drastic homogenization of the microstructure was observed that marked the beginning of the third regime, where densification and grain growth occurred.
Resistance degradation of Ca-doped BaTiO3 ceramics was investigated. A series of coarse and fine-grained (Ba1–xCax)TiO3 with only Ba site incorporation ranging x from 0 to 0.1, and Ba(Ti1–yCay)O3 ceramics with only Ti site incorporation ranging y from 0 to 0.015, were prepared with similar grain sizes. The increase of x did not cause any distinct difference in degradation, whereas an increase in y caused a significant resistance degradation in both coarse and fine-grained specimens. The variation of ionic transference number (tion) as evaluated by the Warburg impedance was negligible with increase in x, but significantly increased with the increase in y. These results demonstrate that the decrease of lattice parameters and lattice shrinkage by the Ba site incorporation of Ca has little influence on the resistance degradation, and that the oxygen vacancy concentration generated by the Ti site incorporation of acceptor Ca is a very important factor that governs resistance degradation.
Lead barium niobate (PBN or PbxBa1–xNb2O6) is a promising tungsten bronze ceramic system that has a morphotropic phase boundary between the orthorhombic and tetragonal phases at x ≈ 0.63, where the spontaneous polarization (Ps ≈ 60–70 μC/cm2) and other ferroelectric properties are known to be higher. However, even textured PBN60 ceramics have low Ps (∼23.9 μC/cm2) and piezoelectric charge coefficient (d33 ≈ 236 pC/N) as compared to the single crystal counterparts. The aim of this study is to control powder processing, green body formation, and sintering conditions to enhance both densification and electrical properties. Therefore, samples were prepared by tape casting methods using single phase PBN60 and reactive mixture of PbNb2O6 and BaNb2O6 powders. Three wt% excess PbO was found to be necessary for densification. Our results showed that undoped PBN60 ceramics reached Ps = 33 μC/cm2, d33 = 305 pC/N, and had a Tc = 340–350 °C. These results are much higher than the reported values in the literature, which can be attributed to the careful ceramic processing such as tape casting (e.g., homogenous green structure), annealing (e.g., control of excess grain boundary phase), and liquid phase sintering (e.g., higher densification).
To study the crystallization kinetics of β-Si3N4 in Si–B–C–N polymer-derived ceramics, the amorphous ceramics with composition SiC1.6N1.0B0.4 were synthesized and then isothermally annealed at 1700, 1775 and 1850 °C. The integrated intensities of β-Si3N4 x-ray diffraction (XRD) patterns were used to examine the course of crystallization. The average size of the Si3N4 nanocrystallites was analyzed by means of the XRD measurements and energy-filtering transmission electron microscopy. It was realized that the nanocrystallite dimensions change insignificantly within the time period of crystallization; however, they depend significantly on the temperature. Subsequently, the kinetics of the β-Si3N4 crystallization was analyzed. Consequently, large activation energy in the range of 11.5 eV was estimated. Moreover, continuous nucleation and diffusion-controlled growth have been concluded as the main mechanisms of the crystallization process. Further analysis points at the crucial role of the nucleation rate in the crystallization kinetics of β-Si3N4.
Ti/AlTiN/Ti-diamondlike carbon (DLC) composite coatings were deposited by mid-frequency magnetron sputtering and Hall ion source-assisted deposition on high-speed steel W18Cr4V substrates. The coating microstructure and mechanical properties, including hardness, elastic modulus, coefficient of friction, and wear properties were investigated by scanning electron microscopy, Raman spectroscopy, scratch and ball-on-disk friction tests, respectively. Fairly smooth composite coating with strong interfacial adhesion and good mechanical properties was produced. The substrate bias increases sp3 bonds contents in the DLC layer, thus coating hardness increased from 14 to 24 GPa and elastic modulus from 190 to 230 GPa with the increased substrate bias. Adhesion of interfaces between Ti-DLC and AlTiN layer, AlTiN and the steel substrate decreased with the substrate bias. The coefficient of friction is between 0.10 and 0.15, except when the substrate bias is 500 V, it is 0.2. Composite coating wear resistance increased with the substrate bias.
After sputter deposition of Sn (layer thickness of 350 nm) on Cu substrates and during subsequent aging at room temperature, Cu and Sn reacted to form the intermetallic phase Cu6Sn5 in the Sn layer at the Cu/Sn interface, which led within a few hours of aging to the development of a compressive stress parallel to the Cu/Sn interface in the Sn layer. One day after aging at room-temperature whisker formation occurred on the surface of the Sn layer. It was shown that whisker growth is associated with long-range Sn diffusion parallel to the Cu/Sn interface. Sn layers of the same thickness sputter deposited on pure Si substrates exhibited throughout the same aging time at room temperature a tensile stress parallel to the Cu/Sn interface (no intermetallic phase formation took place) and whisker formation did not occur. The interrelationship of intermetallic compound formation, stress development, and whisker growth is discussed.
The formation of Zn whiskers threatens the reliable operation of electronic equipment with an electrical shorting hazard. As with tin whiskers (much more intensively researched than Zn whiskers), the mechanism of formation is still not clear. This work investigated the Zn whisker growth mechanism for an electroplated Zn coating above a carbon steel substrate from a raised floor tile. Iron–zinc (Fe–Zn) intermetallic and Zn oxides were identified by x-ray diffraction analysis (XRD) and electron probe microanalysis (EPMA). Fe–Zn intermetallic compounds formed on the surface of the Zn layer in addition to the interface between the Zn coating and the steel substrate. Zn oxides formed primarily on the surface of the Zn coating. Fe–Zn intermetallic compounds and Zn oxide formation can be the source of a residual stress that promotes Zn diffusion to the surface of electroplated Zn coating, resulting in the formation of Zn whiskers.
In this study, Ti-based metallic glass matrix composites with high plasticity have been developed by controlling characteristic and volume fraction of primary phase embedded in the glass matrix. By careful alloy design procedure, the compositions of β/glass phases, which are in metastable equilibrium have been properly selected, therefore the mechanical properties can be tailored by selecting the alloy compositions between the composition of β and glass phases. The relation between the compressive yield strength and volume fraction of β phase is well described using the rule of mixtures.
Mg-based metallic glass interpenetrating phase composites (IPCs) containing 30–70 vol% titanium was fabricated in this study. The effects of reinforced phase volume fraction and interspace on the mechanical properties were investigated systematically. With increasing the volume fraction of titanium, the fracture strength and strain increased up to 1860 MPa and 44%, respectively. The results showed that the critical volume fraction (around 40%) of Ti metal should be required for significantly improving plasticity of IPC. Decreasing the interspace of the titanium phase could lead to enhancement of yield and fracture strength. The deformation behavior and strengthening mechanisms were discussed in detail.
Because of the lack of universal contact models for nonlinear strain problems, indentation analysis on rubberlike materials is confined to small deformation in which Hertz's solution is applied. Recognizing that deep indentation may provide more material information, in this paper we propose a nonlinear elastic model for large spherical indentation of rubberlike materials based on the higher-order approximation of spherical function and Sneddon's solution. The effect of limiting network stretch is studied on the initial elastic modulus for lightly cross-linked rubbers. With the comparisons of the finite-element simulation and the experimental result, the proposed model is verified to predict the large indentation of rubberlike materials over the indentation depth of 0.8 times the indenter radius.
Hardness of glass is known to be related to the resistance to permanent deformation. However, the mechanism of permanent deformation of glass under a sharp diamond indenter is not clear yet. One of the deformation modes of oxide glass at room temperature is permanent densification. In this study, the indentation-induced densification of soda-lime glass under diamond indenters was evaluated from the volume recovery of indentation imprint by thermal annealing. The volume change of the indentation imprint by annealing corresponds to the densified volume under the indenter. Using some kinds of diamond indenters, which have different inclined face angles, the ratios of densified volume to the total “lost” volume under the indenters were determined. With an increase in the inclined face angle, the densification contribution decreased and the shear-flow contribution increased. This indenter-shape dependence of densification in glass is discussed in terms of the stress dependence of the deformation mechanisms in glass.
Conical indentation methods to determine residual stress are proposed by examining the finite element solutions based on the incremental plasticity theory. We first note that hardness depends on the magnitude and sign of residual stress and material properties and can change by up to 20% over a specific range of elastic tensile and compressive residual stress, although some prior indentation studies reported that hardness is hardly affected by residual stress. By analyzing the characteristics of conical indentation, we then select some normalized indentation parameters, which are free from the effect of indenter tip rounding. Adopting dimensional analysis, we present practical conical indentation methods for the evaluation of elastic/plastic equi- and nonequi-biaxial residual stresses. The validity of developed approaches is confirmed by applying them to the experimental evaluation of four-point bending stress.