Solid Solution Shells Research Articles

Specific Hydrogen Spillover Pathways Generated on Graphene Oxide Enabling the Formation of Non-Equilibrium Alloy Nanoparticles.

The phenomenon of hydrogen spillover is investigated as a means of realizing a hydrogen-based society for over half a century. Herein, a graphene oxide having a precisely tuned architecture via calcination in air to introduce ether groups onto basal planes along with carbon defects is reported. This material provides specific pathways for the spillover of atomic hydrogen and has practical applications with regard to the synthesis of non-equilibrium solid-solution alloy nanoparticles. A combination of experimental work and simulations confirmed that the presence of ether groups associated with carbon defects facilitated hydrogen spillover within the basal planes of this graphene oxide. This enhanced hydrogen spillover ability, in turn, enables the simultaneous reduction of Ru3+ and Ni2+ ions to form RuNi alloy nanoparticles under hydrogen reduction conditions. Energy dispersive X-ray and X-ray absorption near edge structure simulations establish that this strategy forms unique alloy nanoparticles each comprising a Ru core with a RuNi solid-solution shell having a hexagonal close-packed structure. These non-equilibrium RuNi alloy nanoparticles exhibit greater catalytic activity than monometallic Ru nanoparticles during the hydrolysis of ammonia borane.

Microstructures and mechanical properties of reactive spark plasma-sintered ZrB2–SiC–MoSi2 composites

Ternary ZrB2–SiC–MoSi2 composites with different nominal volume fractions of SiC (10–25 vol%) and MoSi2 (5–20 vol%) were sintered by reactive spark plasma sintering (RSPS) at 1650 °C and 35 MPa from the starting powders of ZrB2, Si, Mo and activated charcoal. The densification behaviour, microstructures and mechanical properties were comprehensively investigated. SiC clusters and MoSi2 were formed by in-situ reactions and the improved sintering efficiency was attributed to the mass transfer by liquid Si. The formation of a core–shell (Zr, Mo)B2 solid solution was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). With the addition of the nominal MoSi2 content up to 15 vol%, the molar ratio of MoB2 in the solid solution shell increased owing to the increase of Mo; the densification and mechanical properties decreased owing to the softness of MoSi2, the cracks, and the large size of SiC clusters. The relative density, Vickers hardness, and fracture toughness of ZrB2–SiC–MoSi2 composites were significantly better than those of bulk ZrB2 owing to the combined effect of phase enhancement and solid solution strengthening.

Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB2–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB2 and TaSi2) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB2–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB2 core and a (Zr, Ta)B2 solid solution shell. The additives helped refine the ZrB2 grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB2 grains and the semi-coherent ZrB2/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB2 or TaC hardened the ZrB2–20SiC ceramics, whereas the addition of Ta or TaSi2 reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB2- and TaSi2-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi2-included composites to ∼4.72 MPa⸱m1/2.

New technique for producing items from TiC-(Ni-alloy) cermet for severe application conditions

A new technique for producing complex-shaped items from wear-resistant TiC-(Ni-alloy) cermets is described. Cermets are produced by infiltration of TiC-preforms with Ni-alloy, which combines the advantages of powder metallurgy and casting. A special vacuum induction furnace is designed to effectively implement the new technique, which enhances the production capacity and quality of a cermet item against other known technologies. The structure of new cermets is characterized by a uniform distribution of TiC grains in the Ni matrix. During infiltration, a considerable part of tungsten and some part of molybdenum migrate in TiC grains forming solid solution shells on them. The shell ensures the connection between TiC and Ni-alloy and high physical and mechanical properties of the cermet. The physical and mechanical properties of the TiC-(Ni-alloy) cermet at temperatures up to 1100 °С and the results of practical use of items from cermets in production are presented. The shop tests showed the increase of the durability of a cermet ball valve by more than 6 times as compared with conventional wedge valves.

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part I: Processing and microstructure

This study systematically correlates processing with quantitative microstructural information over an extended compositional range for ZrB2-MoSi2 ceramics, with MoSi2 contents ranging from 5 to 70 vol% and diboride starting particle sizes ranging from 3 to 12 μm. Fifteen different ceramics were hot pressed between 1750 and 1925 °C. Plastic deformation of MoSi2 contributed to initial densification, but some of the MoSi2 decomposed during the later stages of hot pressing. Finer diboride particles required lower temperatures to densify (1750 to 1850 °C) compared to coarser diboride particles (1900 °C). Increasing MoSi2 content led to a decrease in sintering temperature. As MoSi2 content increased, ZrB2 grain size decreased and MoSi2 cluster size increased. Starting powders with lower impurity contents and isothermal vacuum holds contributed to lower oxide impurity contents in the final ceramics. A diboride core-shell microstructure involving (Zr1-x,Mox)B2 solid solutions formed in all compositions with Mo contents in the solid solution shells ranging from 3 to 6 at%.This work identified specific relationships between starting composition, processing conditions and final microstructure, showing how microstructure and properties could be tailored by processing. The outcomes of this extensive study will serve as guidelines for the design of other structural ceramics that have to attain determinate thermo-mechanical properties for targeted applications.

Densification behavior of ZrB2-MoSi2 ceramics: The formation and evolution of core-shell solid solution structures

The role of solid solution shells in the densification behavior of ZrB2-MoSi2 ceramics was analyzed for varying ZrB2 starting particle sizes and MoSi2 contents. The formation of core-shell structures in hot-pressed ceramics in the ZrB2-MoSi2 system was confirmed by SEM, TEM and X-ray diffraction analysis. Microstructure analysis established that each ZrB2 core and its (Zr,Mo)B2 solid solution shell were isostructural with no detectable crystallographic misfit. Two single phase (Zr1-XMoX)B2 solid solutions were synthesized by reactive hot pressing to confirm that ZrB2-MoB2 solid solutions obey a quasi-linear Vegard's law. Additional (Si-free) ZrB2-Mo compositions, ZrB2/Mo/ZrB2 sandwich structures, or ZrB2-MoSi2 ceramics (quenched to room temperature) were hot pressed to study a transient liquid phase and its interdependency on ZrB2 powder purity, connectivity of pathways for mass transfer, wettability, or solubility as a function of applied pressure. The solid solution shells were the result of mass transfer by competing solid state mechanisms of surface and grain boundary diffusion during sintering that may have been boosted by a fugitive Si-based transient liquid phase. Mo was incorporated in diffusion-deposited diboride material at particle-particle necks. The volume fraction of solid solution shells and their Mo contents were affected by processing temperatures. In addition, plastic deformation of MoSi2 filled some closed porosity to aid the ceramics in achieving near full density.

High temperature deformation and microstructural evolution of core-shell structured titanium alloy

The hot deformation behavior and microstructural evolution of a core-shell (soft coarse-grained Ti cores and hard Ti-N solid solution shells) structured titanium alloy were investigated under 800–950 °C at a strain rate range of 0.001–1 s−1. The core-shell structured titanium alloys exhibit a higher flow stress (195 MPa deformed at 800 °C/0.1 s−1) compared with that of pure Ti (48 MPa deformed at 800 °C/0.1 s−1), suggesting that core-shell structure promotes a strengthening effect of titanium alloys. The nitrogen solution strengthened hard shells squash along compression direction playing a roll of skeleton and the soft cores accommodate the deformation, leading to inhomogeneous deformation. Dynamic recrystallization takes place mainly adjacent to the shells due to stress and strain concentration. The improvement compression stress of core-shell structured titanium alloys is attributed to the nitrogen-induced solution strengthening and core-shell structure itself, together with the inhabited plastic deformation and dynamic recrystallization by core-shell structure.

Elevated tensile properties of Ti-O alloy with a novel core-shell structure

Oxidized Ti powder was consolidated successfully by using spark plasma sintering into a fully dense bulk core-shell (CS) structured Ti alloy with isolated soft coarse-grained Ti cores and hard Ti-O solid solution shells. The temperature effect on the mechanical responses of CS Ti was assessed experimentally through tensile testing during 500–700°C at a constant strain rate of 0.0005s−1 and compared with those for its annealed counterpart and a pure Ti compact. All samples exhibit a decreased strength with the temperature. CS Ti exhibits a significantly enhanced tensile strength at all the testing temperatures compared with CP Ti. Contrary to the substantial brittleness of the annealed sample at 500°C and 600°C, CS Ti presents a much enhanced ductility at elevated temperatures, being 12% at 500°C and 77% at 600°C. At 700°C, all the samples show comparable elongation. The superior mechanical performances of CS Ti at elevated temperatures can be attributed to its novel heterogeneous microstructure along with high O content.

Fabrication of Ta2O5 Dispersion-Strengthened Mo-Si-B Alloy by Powder Metallurgical Method

In this study, we investigate the effect of oxide dispersion strengthening on mechanical properties by dispersion of nano-sized Ta2O5 particles in Mo-Si-B alloy. A Mo-Si-B core-shell powder consisting of two intermetallic compounds of Mo5SiB2 and Mo3Si as the core and nano-sized Mo solid solution surrounding intermetallic compounds was fabricated by chemical vapor transport. And Mo-Si-B core-shell powder with uniformly dispersed nano-sized Ta2O5 particles on the surface of a Mo solid solution shell was produced by a wet blending process with TaCl5 solution and heat treatment. Then, pressureless sintering was performed at 1400°C for 3 h under a H2 atmosphere. The hardness and fracture toughness of the Ta2O5-dispersed Mo-Si-B alloy were measured using Vickers hardness and 3-point bending tests, respectively. The Vickers hardness and fracture toughness of the fabricated Mo-Si-B-Ta2O5 alloy were more improved than that of the Mo-Si-B alloy fabricated using core-shell powder with no addition of Ta2O5 particles (Mo-Si-B alloy: 353 Hv, 13.5 MPa·√m, Mo-Si-B-Ta2O5 alloy: 509 Hv, 15.1 MPa·√m).

Core-shell structured titanium-nitrogen alloys with high strength, high thermal stability and good plasticity

Multifunctional materials with more than two good properties are widely required in modern industries. However, some properties are often trade-off with each other by single microstructural designation. For example, nanostructured materials have high strength, but low ductility and thermal stability. Here by means of spark plasma sintering (SPS) of nitrided Ti particles, we synthesized bulk core-shell structured Ti alloys with isolated soft coarse-grained Ti cores and hard Ti-N solid solution shells. The core-shell Ti alloys exhibit a high yield strength (~1.4 GPa) comparable to that of nanostructured states and high thermal stability (over 1100 °C, 0.71 of melting temperature), contributed by the hard Ti-N shells, as well as a good plasticity (fracture plasticity of 12%) due to the soft Ti cores. Our results demonstrate that this core-shell structure offers a design pathway towards an advanced material with enhancing strength-plasticity-thermal stability synergy.

Combined effects of WC and SiC on densification and thermo-mechanical stability of ZrB2 ceramics

ZrB2-based mixtures containing WC and SiC in various amounts were hot-pressed at 1930°C, achieving final relative densities above 94%. The diboride matrix of the sintered ceramics was constituted by (Zr,W)B2 solid solution shells grown onto original ZrB2 grains. WC phase reacted during sintering leaving W-monoboride and mixed Zr,W-carbide as secondary phases. Room temperature flexure strength ranged from 540 to 630MPa, but performances of major significance were determined at 1500°C in air, with values exceeding 700MPa. SiC was vital to improve the oxidation resistance at 1500°C compared to SiC-free ZrB2-based ceramic. W, encased in the shell, retarded the inward diffusion of oxygen compared to the pure ZrB2.

Facile synthesis and photocatalytic properties of ZnO core/ZnS-CdS solid solution shell nanorods grown vertically on reductive graphene oxide.

In the present study, ZnS-CdS solid solution sensitized ZnO nanorods were anchored on graphene sheets by combining a hydrothermal process and ion exchange technique, and the significant influence of CdS content in the shell on photo absorption and photocatalytic performance were investigated. Electron microscopic images reveal that the as-prepared nanocomposites display a sandwich-like 3D structure, consisting of ZnO nanorods with a ZnxCd1-xS or CdS shell grown vertically on both sides of the graphene sheets. UV/Vis DRS shows that the solid solution sensitized nanocomposites have enhanced visible light absorption and also exhibited a red-shift of the band-edge as compared to RGO/ZnO and RGO/ZnO@ZnS. Fluorescence emission spectra indicate that the deposition of CdS on the shell with an appropriate CdS/ZnS ratio and the incorporation of graphene causes improved charge separation. The photocatalytic experiments demonstrate that the RGO/ZnO@ZnxCd1-xS nanocomposites possess much higher photocatalytic activity for H2 evolution than the RGO/ZnO nanorods and RGO/ZnO@ZnS core/shell nanorods. Under the irradiation of a 300 W Xe lamp, the highest photocatalytic hydrogen production rate of 1865 μmol h(-1) g(-1) is observed over the RGO/ZnO@Zn0.6Zn0.4S sample, which is about 2.1 and 1.4 times more active than RGO/ZnO and RGO/ZnO@ZnS, respectively. Under the irradiation of visible light (>420 nm), the RGO/ZnO and RGO/ZnO@ZnS nanorods are barely active, whereas RGO/ZnO@Zn0.6Zn0.4S displays a hydrogen production rate of 160 μmol h(-1) g(-1). The highly improved performance of the composites can be ascribed to the increased light absorption and efficient charge separation.

Potassium-Doped Ni–MgO–ZrO2 Catalysts for Dry Reforming of Methane to Synthesis Gas

Ni/K–MgO–ZrO2 catalysts for dry reforming of methane, with a range of Mg/Zr ratios and each containing about 10 wt% Ni, were prepared via Ni nitrate impregnation on MgO–ZrO2 supports synthesized by co-precipitation using K2CO3. It was found that a proportion of the potassium of the precipitant remained in the samples and improved the stability of the catalysts in the reaction. It was also shown that reduction of the catalysts at 1,023 K without calcination in air is necessary for stable and high activity; calcination in air at 1,073 K gives a deterioration of the catalytic properties, leading to rapid deactivation during the reaction. The order of the CH4 conversions of the reduced catalysts after 14 h on stream was as follows: Ni/K–Mg5Zr2 ~ Ni/K–Mg ≥ Ni/K–Mg2Zr5 ≫ Ni/K–Zr. A catalyst with 0.95 wt% K on MgO–ZrO2 with a Mg:Zr mole ratio of 5:2 showed the best resistance to deactivation. Experiments in a microbalance system showed that there was only negligible coke deposition on the surface of this sample. This behaviour was attributed to the presence of Ni nanoparticles with a diameter of less than 10 nm located on a MgO/NiO solid solution shell doped by K ions; this in turn covers a core of tetragonal ZrO2 and/or a MgO/ZrO2 solid solution. This conclusion was supported by EDS/TEM, XPS, XRD and H2 chemisorption measurements.

Investigation of the Precipitation in Mg-Zr Alloys

The Mg-Zr alloys with the Zr contents of 0.2%, 0.4%, 0.6% and 0.8% were prepared using induction melting, metal mold centrifugal casting process. The microstructures of the alloys were analyzed by scanning electron microscope, energy spectrum analysis, transmission electron microscope and their mechanical properties were also measured with tensile test. The results show that there are many micro- and nano-sized Zr-rich or Zr particles dispersed in Mg-Zr alloys. The nano-sized Zr-rich particle consists of a rectangle shaped Zr core and its surrounding Mg (Zr) solid solution shell with round exterior; a single Zr particle appears a coherent lattice relationship with matrix.

Improved Electrochemical Performances of Surface-Modified Spinel LiMn2 O 4 for Long Cycle Life Lithium-Ion Batteries

The surface of spinel was modified with inorganic salt by a sol-gel method to improve its electrochemical performance at elevated temperatures. Compared with pristine spinel surface-modification improved the cycling stability of the material at elevated temperatures. The capacity retention of -modified spinel was more than 94% after 500 cycles at room temperature and more than 90% after 200 cycles at 55°C at 1C charge-discharge rate. Thermogravimetry/differential scanning calorimetry studies showed that the formation process of on was different from that of pure Powder X-ray diffraction indicated that spinel was the only detected phase in -modified X-ray photoelectron spectroscopy showed that the Al content in the spinel decreased with the detection depth beneath the particle surface of the sample. Introduction of Al and Li into changed the electronic structures of its O and Mn atoms on the particle surface, but the electronic structure of Al remained unchanged. Therefore a surface solid solution of was formed on The thickness of the surface solid solution was estimated to be about 12 nm. The improved electrochemical performance of surface-modified spinel was attributed to the protection effect of the surface solid solution shell while the improved stability of crystalline structure was assigned to the entrance of and into the spinel crystalline structure. © 2003 The Electrochemical Society. All rights reserved.