Larger Atomic Radius Research Articles

Schematic Interpretation of Anomalies in the Physical Properties of Eu and Yb Among the Lanthanides

Lanthanides are the elements in 6^th period and the 3^rd group of the periodic table. Eu and Yb exhibit some unusual properties compared with the other lanthanides. The author has proposed a diagram to systematically illustrate the properties of the elements, by plotting the Young’s modulus on the ordinate and thermal conductivity on the abscissa. Eu and Yb have much lower Young’s moduli, and are located far from other lanthanides on the diagram. Most lanthanides have hexagonal structures. Eu, however, has a body-centered cubic structure, because it is located on the extension of the curve of alkali metals. Yb has a face-centered cubic (fcc) structure, because it is located on the curve of fcc metals. The positions of Eu and Yb on the diagram are thought to act as a bridge between the lanthanides and other adjacent element groups. As a result, Eu and Yb work as if they were members in the adjacent element groups, which leads to their large atomic radii and their low melting points, etc.

Diffusion of metal ions from a substrate into oxide coatings

Concentrations of metal ions from a substrate were found in coatings adjacent to the substrate by studying the coatings using X-ray photoelectron spectroscopy before and after annealing. Small metal ions easily diffused into the coating from the substrate, whereas larger metal ions had more difficulty doing so because of their large atomic radii. A higher annealing temperature and a lower packing density induced a faster diffusion rate and a higher concentration of metal ions in the coating. Smaller metal ions passed through a SiO2 layer and preferentially accumulated in the Ta2O5 layer due to the migration of oxygen vacancies. These results are relevant for selecting the coating temperature, annealing temperature, and the substrate.

Structural-Ordered Flexible Polypyrrole Films Derived from Vapor Phase Polymerization Process As Ultrahigh-Rate Anodes for Li-Ion and Na-Ion Batteries

Recently, there has been strong market demand for thin, lightweight, flexible and bendable electrodes to make lithium ion batteries (LIBs) or lithium-polymer batteries that are lighter and more flexible than the existing ones, and thus more capable of satisfying for portable soft or bendable electronic devices such as roll-up displays, integrated circuit smart cards and wearable devices for durability in everyday use[1]. In this respect, the concept of all-polymer based batteries and supercapacitors, introduced almost three decades ago[2], has recently gained renewed interest mainly due to long cycle life, low self-discharge rate, endurance to over-discharge, low manufacturing costs, and shape flexibility of conductive polymers. Several important conducting polymers such as polypyrrole (PPy) [3], polyaniline (PAN) [4], and polythiophene (PTh) [5] and their derivatives have been proved to be used not only as the cathode but also as the anode for the secondary battery because they are stable in air and have good electrochemical properties.On the other hand, considering the limited and uneven distribution of lithium minerals, room temperature sodium ion batteries (SIBs) have received growing attention because of their low cost and abundant supply. SIBs and LIBs share the same architecture of battery design. One of the major challenges for SIBs is to discover suitable electrode materials with high and stable sodium storage capabilities. The lithium or sodium storage mechanism of the conductive polymers is reversible n-doped behavior[6]. During discharge process, the cations (Li+ or Na+) stored between the chain layers of polymers and combined with oxygen atom to form oxide[3]. Therefore, the distance between the chain layers is important for the cations insertion and extraction, especially for the sodium ions with larger atomic radius. Among the conducting polymers, PPy have drawn the most attention due to their redox properties, biocompatibility, good electrical conductivity, and chemical stability. However, to the best of our knowledge, very few PPy materials have been reported for Na-ion storage. In this work, we propose PPy thin films with stable chain-layer structure as anodic electrodes for LIBs and SIBs, which synthesized by an easy, inexpensive and scalable vapor phase polymerization (VPP) technique[7]. The ordered pyrrolic chains layer structure of obtained PPy film was characterized by X-ray diffraction (XRD), High-resolution transmission electron microscopy (HR-TEM), Raman spectra and X-ray photoelectron spectra (XPS) examinations. As a consequence, the PPy film demonstrates excellent rate performance and cycling stability as freestanding anodic electrode for both Li-ion and Na-ion storage. In particular, the PPy film anode exhibits an impressive increased reversible capacity of 204.5 mAh g-1 even after 400 cycles at 10 C charge/discharge rate for Li-ion storage. Moreover, a reversible redox capacity of 113.8 mAh g-1 through doping-dedoping reactions of Na-ions into/from the PPy chains is also achieved. The electrochemical Li-ions and Na-ions doping-dedoping behaviors of PPy film was investigated by ex-situ XRD. It was proved that PPy film electrode could be reversibly charged and discharged with lithium and sodium and the ordered pyrrolic chains structure can self-adapt the doped ions. The results may substantially contribute to the progress of the flexible polymer electrode for various ions batteries. Acknowledgements: The authors gratefully acknowledge the support of the National Science Foundation of China (51472161, 51472160, 21403139), the Shanghai Pujiang Program (No. 14PJ1407100), the Key Program for the Fundamental Research of the Science and Technology Commission of Shanghai Municipality (15JC1490800, 12JC1406900) and the International Cooperation Program of the Science and Technology Commission of Shanghai Municipality (14520721700). We acknowledge the support of the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2014048) and the Hujiang Foundation of China (B14006).

Effect of elemental composition on phase formation during milling of multicomponent equiatomic mixtures

Using mechanochemical synthesis through milling of equiatomic multicomponent mixtures of Cr, Fe, Co, Ni, Al, Ti, Mo, and Nb metals in various combinations, we have synthesized powder alloys with different phase compositions: amorphous phase (AP), AP + BCC phase, AP + BCC phase + MO, and FCC + BCC phases. The FCC phase has been shown to be a Ni-based solid solution. The presence of aluminum in a starting mixture helps to stabilize the BCC phase owing to the formation of a disordered B2 phase. Al dissolves in both the BCC and FCC solid solutions, increasing their lattice parameters. In Al-free starting mixtures, Cr is responsible for the formation of the BCC solid solution. The formation of an AP during milling of multicomponent mixtures is favored by the presence of transition metals with a large atomic radius: Ti, Mo, and Nb.

A comparative DFT study on the CO oxidation reaction over Al- and Ge-embedded graphene as efficient metal-free catalysts

In the present study, by means of density functional theory (DFT) calculations, the catalytic oxidation of CO by O2 molecule is investigated over Al- and Ge-embedded graphene. The large atomic radius of these dopant atoms can induce a local surface curvature and modulate the electronic structure properties of the graphene sheet through the charge redistribution. It is found that the adsorption of molecular O2 over Al- or Ge-embedded graphene is stronger than that of CO molecule. The CO oxidation reaction by molecular O2 on Al- and Ge-embedded graphene is comparably studied. The results indicate that a two-step process can occur, namely, CO+O2→CO2+Oads and CO+Oads→CO2. Furthermore, the computed activation energy (Eact) for the first reaction on Ge-doped graphene is lower than that of Al-doped one, and the formation of second CO2 molecule on both surfaces can occur rapidly due to its low energy barrier (0.1eV).

Pd-embedded graphene: An efficient and highly active catalyst for oxidation of CO

Despite numerous efforts performed on the elimination of toxic gases from the air, the oxidation of carbon monoxide (CO) with metal-embedded nanostructures still remains a challenge. The geometry, electronic structure and catalytic properties of Pd-doped graphene (Pd-graphene) are investigated by means of density functional theory (DFT) calculations. The large atomic radius of Pd dopant in graphene can induce the local surface curvature and modulate the electronic structure of the sheet through the charge redistribution effects. Also, Pd-graphene can facilitate the O2 adsorption. Therefore, the catalytic activity of the Pd-graphene for CO oxidation reaction is enhanced. Moreover, the complete CO oxidation reactions on the Pd-graphene include a two-step process of the Langmuir-Hinshelwood (LH) reaction, in which the first step is almost barrier-less (Eact = 0.002 eV) and the second step exhibits an energy barrier of 0.2 eV. The results indicate that the surface activity of graphene-based materials can be drastically improved by introducing the Pd dopants, so Pd-graphene can be a clue for fabricating graphene-based catalysts with high activity toward the oxidation of CO molecule.

Mechanism of Hydrogen Absorption in 1:7 Sm–Co Phase With Higher Fe Content

Hydrogen absorption (HA) capacity of Sm(CobalFe x Cu0.053Zr0.02)7.84 ( $x=0.2$ , 0.3, 0.4, 0.5) as-cast alloys was improved through increasing Fe content and its mechanism was symmetrically analyzed. X-ray diffraction analysis indicates that the main phase of all as-cast alloys was 1:7 phase. Due to lower electronegativity of Fe than that of Co, activation energy of chemical adsorption for 1:7 phase was reduced with increasing Fe content. Due to larger atomic radius of Fe than that of Co, lattice parameters and lattice gaps of 1:7 phases were enlarged with increasing Fe content, and diffusion barrier of H atoms was lowered resulting from increasing lattice parameters and sizes of lattice gaps. These all strengthened the HA capacity of 1:7 phase. Two principles are mentioned to provide theoretical guidance for strengthening HA capacity by adding other elements.

Temperature-Dependent Generalized Planar Fault Energy and Twinnability of Mg Microalloyed with Er, Ho, Dy, Tb, and Gd: First-Principles Study

The generalized planar fault energies, Rice criterion ductility, and twinnability of pure Mg and Mg-RE (RE = Er, Ho, Dy, Tb, and Gd) alloys at different temperature have been investigated using density functional theory. It is shown that all the fault energies and twinnability in the same materials decrease with increasing temperature. However, the ductility has the opposite change trend. On the other hand, alloying rare earth elements will generally decrease the fault energies and increase the ductility and twinnability of Mg at different temperature. It is interesting to note that alloying larger atomic radius will enhance the ductility of Mg more easily and alloying smaller radius will make twinning tendency of Mg more easily. Finally, the electron structure further reveals the underlying mechanisms for the reduction of fault energies with the addition of rare earth elements.

Magnetic Silicon Fullerenes: Experimental Exploration and Theoretical Insight

The present article summarizes progress in research on silicon clusters with encapsulated metal atoms, and specifically focuses on the recent identification of magnetic silicon fullerenes. Considering that C $$_{20}$$ forms the smallest known fullerene, the Si $$_{20}$$ cluster is of particular interest in this context. While the pure hollow Si $$_{20}$$ cage is unstable due to the lack of $$sp^2$$ hybridization, endohedral doping with a range of metal atoms has been considered to be an effective way to stabilize the cage structure. In order to seek out suitable embedded atoms for stabilizing Si $$_{20}$$ , a broad search has been made across elements with relatively large atomic radius. The rare earth elements have been found to be able to stabilize the Si $$_{20}$$ cage in the neutral state by forming R@Si $$_{20}$$ fullerene cages. Among these atoms, Eu@Si $$_{20}$$ has been reported to yield a stable magnetic silicon fullerene. The central europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, based on a stable Eu $$_2$$ Si $$_{30}$$ tube, a magnetic silicon nanotube has been constructed and discussed. These magnetic silicon fullerenes and nanotubes may have potential applications in the fields of spintronics and high-density magnetic storage.

Ab initio investigation of native point defects in Mg24Y5

Eight possible native point defects in Mg24Y5 are studied from the density functional theory (DFT) calculations within GGA approximation. The energetic results show that anti-site defects with lower formation energies are energetically favored over vacancies. Under both Mg-rich and Y-rich conditions, MgY1 defect is dominant due to the lowest defect formation energy, followed by YMg2, MgY2 and YMg1, which reasonably explains the Mg-rich off-stoichiometry of Mg24Y5 alloys. For vacancy defects, the formation energies on the two Mg sub-lattices are smaller than ones on two Y sub-lattices, and VMg1 has the lowest formation energy, while VY2 is most unlikely due to the highest formation energy originating from the large mismatch in atomic size and the strong interaction of Y with surrounding atoms. The defect concentration distribution as a function of temperature and stoichiometry is further obtained from grand canonical statistics. The size factor and electronic structure are further discussed, demonstrating that Mg24Y5 alloy is typical system in which the point defects are affected by strong size effect due to the large atomic size radius and strong electronic factor caused by the intensive interaction of Y with the surrounding atoms.

The influence of HTHP treatment on the OK1 and N3 centers in natural diamond crystals

The structures of nitrogen‐containing paramagnetic centers OK1 and N3 have not been unambiguously determined yet. In this paper, we will present data on the influence of high temperature annealing on the paramagnetic centers OK1 and N3 in diamond. A series of natural colored diamond crystals of cubic habit containing paramagnetic centers OK1 and N3 was studied by electron paramagnetic resonance (EPR), luminescence, and IR spectroscopies. It was found that annealing at temperatures above 2470 K led to the transformation of N3 paramagnetic centers into OK1 paramagnetic centers. Previous work has showed that a temperature in excess of 2400 K are often associated with a local environment change of tetrahedral to octahedral for impurities with large atomic radii. We report a correlation with the intensity of the optical 635.7 nm system and the concentration of the N3 EPR center. However, on stepped annealing 635.7 nm was removed before the N3 EPR center. From the analysis presented, we propose that the N3 EPR center and the 635.7 nm optical system are the manifestation of various defects formed by one impurity atom.

MICROSTRUCTURE AND OXIDATION BEHAVIOR OF ZrSi2-NbSi2 MULTILAYER COATINGS ON AN Nb-Ti-Si-Cr BASE ULTRAHIGH TEMPERATURE ALLOY

The rather poor oxidation resistance of Nb-Si base ultrahigh temperature alloys has seriously limited their practical applications at high temperatures. Niobium disilicide coatings, especially those modified by reactive elements (RE) such as Zr and Y, have been shown to possess good anti-oxidation properties at high temperatures due to the formation of a protective RE-containing SiO2 scale. Halide activated pack cementation (HAPC) is one of the most widely used techniques for preparing protective coatings on Nb-Si base ultrahigh temperature alloys, because compact coatings and metallurgical substrate/coating bonds can be obtained with using this technique. However, only a very limited amount of Zr and Y can be diffused into the coatings by a single co-deposition pack cementation process as a result of their large atomic radii and high melting points. To solve this problem, a method such as magnetron sputtering, which can be used for producing overlay coatings with different composition ratios of coating elements, seems to be feasible. In the present study, ZrSi2-NbSi2 multilayer coatings were prepared on an Nb-Ti-Si-Cr base ultrahigh temperature alloy by first magnetron sputtering 2 μm thick Zr-film, and then Si-Y *国家自然科学基金项目51371145, 51431003和U1453201,高等学校学科创新引智计划项目B080401和凝固技术国家重点实验室 自主研究课题项目116-QP-2014资助 收到初稿日期: 2014-09-09,收到修改稿日期: 2015-04-07 作者简介:李 轩,男, 1982年生,博士生 DOI: 10.11900/0412.1961.2014.00498 第693-700页 pp.693-700

Improvement of thermal stability of amorphous CoFeSiB thin films

The excellent soft magnetic properties of amorphous (a-) CoFeSiB films make it suited for use in the yoke of granular-in-gap sensors, but only if their thermal stability can be improved. To this end, this study investigated the effects of adding small amounts of other metals on the magnetic and structural properties of a-CoFeSiB films. It was found that adding metals with relatively large atomic radii is an effective way to increase thermal stability, with both Ta and Hf showing good thermal stability after annealing at temperatures of 473 to 573 K. Indeed, a -(CoFeSiB)96.2Hf3.8 film was found to maintain its initial coercivity of 0.2 Oe without very little decrease in magnetization after annealing at 623 K. Furthermore, even after annealing at 673 K a -(CoFeSiB)93.0Hf7.0 film still had a relatively low coercivity of approximately 0.5 Oe.

Material design for magnesium alloys with high deformability

Magnesium alloys are very promising structural material, especially because of their low density, but their poor deformability at room temperature is making their commercial application impractical. Some alloying elements have been shown to improve the ductility dramatically, and a search for the yet better alloying elements is now a pressing task. In this study, we investigated the effects of several alloying elements on the mechanical and deformation behaviour of magnesium alloys using both first-principles calculations as well as experiments. The first-principles calculations indicate that the influential key factor for the activation of non-basal slips is the electronegativity and atomic radius. The experimental result proves that the alloys with a similar electronegativity as magnesium and a little larger atomic radius than that of magnesium, such as Ca, Sr and some rare earth elements, show superior ductility due to activation of non-basal dislocation slips. These results propose a promising design principle for the alloys with the improved deformability at room temperature ranges.

The effect of high yttrium solute concentration on the twinning behaviour of magnesium alloys

The deformation behaviour of two single phase binary alloys, Mg–5Y and Mg–10Y, have been examined. In compression, two twin types were observed, the common {101¯2} twin as well as the less common {112¯1} extension twin. It is shown that the {112¯1} twin is much less sensitive to solute concentration than the {101¯2} twin, and it is suggested that the simple atomic shuffle of the {112¯1} twin reduces the solute strengthening imparted by Y additions. The common {101¯2} twin showed significant hardening as a result of alloying with Y. An analysis of solute behaviour has indicated that of the four chemical parameters investigated, i.e. atomic size, shear modulus, electronegativity and solute distribution, it appears to be the larger atomic radius of Y compared to Mg that increases the stress required to activate the {101¯2} twin. It is suggested that the large atomic radius inhibits the atomic shuffling process which accompanies the twinning shear in this twin type.

Li Diffusion in Rare-Earth Metal Doped Battery Materials

One of the key elements determining the performance in Li-ion batteries is the diffusion. All solid-state batteries require fast ionic diffusion in the electrolyte but equally important in the anode and cathode. The electrolyte is required to be electronically non-conducting, the opposite is true for anode and cathode. Various compounds are currently investigated.It is intuitively clear that by widening the Li-ion diffusion channels the activation barrier will be reduced leading to faster diffusion. The lattice parameter can be increased by partially replacing host atoms by impurities with a larger atomic radius. With this strategy in mind, I) the influence of the diffusivity on the lattice parameter is studied employing density functional theory. II) The thermodynamic stability of rare-earth metal atoms as impurities in existing battery materials is calculated in conjunction with the Li diffusion.I) The influence on the lattice parameter is evaluated in two ways. Changing the lattice parameter by introducing impurities or alloying similar materials not only changes the lattice parameter but simultaneously introduces disorder along the Li-ion diffusion pathways. Therefore, changing only the lattice parameter artificially by applying an external strain allows disentangling the contributions from the lattice expansion vs. the disorder effect. At the example of Li diffusion in rare-earth oxides it is shown that the activation energy can be reduced by increasing the lattice parameter in agreement with experiments [1].II) The thermodynamic stability of rare-earth metal atoms in materials suggested as cathode materials like spinel LiMn2O4 and LiCoO2 is calculated. The influence on the Li diffusion due to the introduced lattice distortion and reduced symmetry is discussed. A coarse-grained diffusion model on the basis of the DFT data is introduced containing the relevant changes in the local activation energies due to the presence of the rare-earth metal impurities. The numerical evaluation of the diffusion is carried out by kinetic Monte Carlo simulations. Acknowledgements: This work was supported by the Leibniz-Gemeinschaft under the project No. SAW-2011-PDI-230.[1] S. Tamura, A. Mori and N. Imanaka (2006). Solid State Ionics, 177(26-32), 2727–2730.

Finding Unprecedentedly Low-Thermal-Conductivity Half-Heusler Semiconductors via High-Throughput Materials Modeling

The lattice thermal conductivity ({\kappa}{\omega}) is a key property for many potential applications of compounds. Discovery of materials with very low or high {\kappa}{\omega} remains an experimental challenge due to high costs and time-consuming synthesis procedures. High-throughput computational pre-screening is a valuable approach for significantly reducing the set of candidate compounds. In this article, we introduce efficient methods for reliably estimating the bulk {\kappa}{\omega} for a large number of compounds. The algorithms are based on a combination of machine-learning algorithms, physical insights, and automatic ab-initio calculations. We scanned approximately 79,000 half-Heusler entries in the AFLOWLIB.org database. Among the 450 mechanically stable ordered semiconductors identified, we find that {\kappa}{\omega} spans more than two orders of magnitude- a much larger range than that previously thought. {\kappa}{\omega} is lowest for compounds whose elements in equivalent positions have large atomic radii. We then perform a thorough screening of thermodynamical stability that allows to reduce the list to 77 systems. We can then provide a quantitative estimate of {\kappa}{\omega} for this selected range of systems. Three semiconductors having {\kappa}{\omega} < 5 W /(m K) are proposed for further experimental study.

Effect of substitution of In for Co on magnetostructural coupling and magnetocaloric effect in MnCo1-xInxGe compounds

Effect of replacement of Co by In with larger atomic radius but fewer valence numbers on magnetostructural coupling and magnetocaloric effect is studied in MnCo1-xInxGe compounds. The substitution of Co by a small amount of In (1.5%–2%) can shift martensitic transformation Tstru to lower temperature and make it overlap with Curie temperature TC. As a result, magnetostructural coupling is created and large entropy change (ΔS) takes place. Further increasing In content to x = 0.03 leads to decoupling, but the martensitic transition (Tstru ∼ 249 K) is still close to the magnetic transition (TcA ∼ 269 K). As a result, two close ΔS peaks appear. Mechanism related to different large entropy change in the coupled and decoupled samples are discussed.

Influence of Mo and W on High Temperature Hardness of M&lt;sub&gt;7&lt;/sub&gt;C&lt;sub&gt;3&lt;/sub&gt; Carbide in High Chromium White Cast Iron

The influence of Mo and W addition on the high temperature hardness of M7C3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25mass%Cr. Concentrations of alloying elements in primary M7C3 carbide were measured by EDS. As Mo or W content of the cast iron increases, the concentration of Mo and W in the carbide increased and that of Fe decreased. However, the Cr content was almost the same as about 60mass% in spite of increasing the Mo and W contents. It was found from the XRD results that the lattice constant of M7C3 carbide changed and its attendant volume of a unit cell increased as the Mo or W content was increased. From this point of view, it can be considered that Fe atoms in the M7C3 carbide were substituted by Mo or W which has larger atomic radius than Fe. In all the specimens, the hardness of M7C3 carbides are about 1600HV0.3 at the room temperature, and it decreases gradually with a rise of the test temperature. The decreasing ratio of carbide hardness becomes smaller at high temperature as the Mo or W concentration in M7C3 carbide increases. Thus, the dissolution of Mo or W atom into M7C3 carbide is very effective to maintaining the high temperature hardness of carbide. However, the increment of the hardness became smaller at higher Mo and W contents, so that an excess addition of both elements gave less effect than expected. The fracture toughness of M7C3 carbide at the room temperature was measured using an indentation fracture method, and the values were very similar among the carbides regardless of Mo and W concentration in the M7C3 carbide. [doi:10.2320/matertrans.F-M2014801]

Influence of transition metal on glass formability of Y–TM–B (TM = Fe, Ni) system

Semi-empirical Miedema’s model was combined with the geometric one to determine glass forming ability (GFA) of ternary Y–Fe–B and Y–Ni–B systems. The enthalpies of amorphous phase and solid solution were calculated, as well as the difference between both quantities which reflects the competition during solidification. Moreover, normalized mismatch entropy and glass forming ability parameter were also determined. According to the results, yttrium plays a dominant role due to the large atomic radius and highly negative heat of mixing with the most of alloying elements. The large radii of rare earth elements have been already known to be advantageous in glassy state formation. Enthalpy contour maps for investigated alloys are similar, but significant differences in absolute enthalpy values were observed. Nickel containing system possesses the highest glass forming ability in the wide range of compositions, comparing to Y–Fe–B and Y–Co–B ones.