Predictions on the Phase Constitution of SmCo7-XMx Alloys by Data Mining.

Based on a home-built Sm-Co-based alloys database, this work proposes a support vector machine model to study the concurrent effects of element doping and microstructure scale on the phase constitution of SmCo7-based alloys. The results indicated that the doping element's melting point and electronegativity difference with Co are the key features that affect the stability of the 1:7 H phase. High-throughput predictions on the phase constitution of SmCo7-based alloys with various characteristics were achieved. It was found that doping elements with electronegativity differences with Co that are smaller than 0.05 can significantly enhance 1:7 H phase stability in a broad range of grain sizes. When the electronegativity difference increases to 0.4, the phase stability becomes more dependent on the melting point of the doping element, the doping concentration, and the mean grain size of the alloy. The present data-driven method and the proposed rule for 1:7 H phase stabilization were confirmed by experiments. This work provides a quantitative strategy for composition design and tailoring grain size to achieve high stability of the 1:7 H phase in Sm-Co-based permanent magnets. The present method is applicable for evaluating the phase stability of a wide range of metastable alloys.

Exciton Self-Trapping Dynamics in 1D Perovskite Single Crystals: Effect of Quantum Tunnelling.

We present experimental and theoretical investigations of the photophysics in the one-dimensional (1D) hybrid organic-inorganic perovskite (HOIP) white-light emitter, [DMEDA]PbBr4. It is found that the broadband-emission nature of the 1D perovskite is similar to the case of two-dimensional (2D) HOIP materials, exciton self-trapping (ST) is the dominant mechanism. By comprehensive spectroscopic investigations, we observed direct evidence of exciton crossing the energy barrier separating free and ST states through quantum tunnelling. Moreover, we consider the lattice shrinking mechanisms at low temperatures and interpret the ST exciton formation process using a configuration coordinate diagram. We propose that the energy barrier separating free and ST excitons is temperature-dependent, and consequently, the manner of excitons crossing it is highly dependent on the exciting energy and temperature. For excitons located at the bottom of the free excitonic states, the quantum tunnelling is the dominant channel to the ST states.

Femtosecond visualization of oxygen vacancies in metal oxides.

Oxygen vacancies often determine the electronic structure of metal oxides, but existing techniques cannot distinguish the oxygen-vacancy sites in the crystal structure. We report here that time-resolved optical spectroscopy can solve this challenge and determine the spatial locations of oxygen vacancies. Using tungsten oxides as examples, we identified the true oxygen-vacancy sites in WO2.9 and WO2.72, typical derivatives of WO3 and determined their fingerprint optoelectronic features. We find that a metastable band with a three-stage evolution dynamics of the excited states is present in WO2.9 but is absent in WO2.72. By comparison with model bandstructure calculations, this enables determination of the most closely neighbored oxygen-vacancy pairs in the crystal structure of WO2.72, for which two oxygen vacancies are ortho-positioned to a single W atom as a sole configuration among all O─W bonds. These findings verify the existence of preference rules of oxygen vacancies in metal oxides.

How non-ferromagnetic Mn enhances the magnetization of SmCo7 based alloys.

This work was focused on the effects of Mn doping on the phase stability and magnetic performance of SmCo7 based alloys. Particularly, the role of Mn in the improvement of the magnetization of the SmCo7 matrix, as well as its mechanisms, was examined in detail. The metastable SmCo7 single phase was well stabilized by the appropriate content of Mn doping and nanostructuring of the alloy. It was discovered that the non-ferromagnetic element Mn can enhance magnetization effectively. By tailoring the Mn content and nanostructuring, the prepared SmCo7-xMnx alloy achieved good comprehensive magnetic properties. The mechanisms for the magnetization enhancement by Mn and the coupled effect of Mn doping and nanostructuring on the magnetic properties were proposed based on the characterization of magnetic structures and the model calculations of magnetic moments.

Distinguishing contributions of ceramic matrix and binder metal to the plasticity of nanocrystalline cermets.

Using the typical WC-Co cemented carbide as an example, the interactions of dislocations within the ceramic matrix and the binder metal, as well as the possible cooperation and competition between the matrix and binder during deformation of the nanocrystalline cermets, were studied by molecular dynamics simulations. It was found that at the same level of strain, the dislocations in Co have more complex configurations in the cermet with higher Co content. With loading, the ratio between mobile and sessile dislocations in Co becomes stable earlier in the high-Co cermet. The strain threshold for the nucleation of dislocations in WC increases with Co content. At the later stage of deformation, the growth rate of WC dislocation density increases more rapidly in the cermet with lower Co content, which exhibits an opposite tendency compared with Co dislocation density. The relative contribution of Co and WC to the plasticity of the cermet varies in the deformation process. With a low Co content, the density of WC dislocations becomes higher than that of Co dislocations at larger strains, indicating that WC may contribute more than Co to the plasticity of the nanocrystalline cermet at the final deformation stage. The findings in the present work will be applicable to a large variety of ceramic-metal composite materials.

Transient Electronic Depletion and Lattice Expansion Induced Ultrafast Bandedge Plasmons.

Strong optical excitation of plasmonic nanostructures may induce simultaneous interband and intraband electronic transitions. However, interaction mechanisms between interband, intraband, and plasmon-band processes have not been thoroughly understood. In particular, optical-heating-induced lattice expansion, which definitely leads to shift of the Fermi level, has not been taken into account in plasmonic studies. Here, it is shown that plasmonic bandedge shift is responsible for the optical modulation on the boundary between plasmonic electron oscillation and interband transitions via investigations on gold nanofilms and nanoparticles. Strong optical excitation induces transient depletion of the conduction band just below the Fermi level through intraband transitions, while the subsequent lattice heating induces transient thermal expansion and hence lowers the Fermi level. Both effects reduce the threshold for interband transitions and therefore push the plasmonic bandedge to the red. These discoveries introduce a first correlation between plasmonic response and optical excitation induced thermal expansion of lattices. The revealed Fermi-level adjustment mechanism allows alignment of electronic levels at the metal-semiconductor interfaces, which applies to all conductive materials and renders reliable physics for the design of plasmonic or optoelectronic devices.

Hierarchical nanostructured W-Cu composite with outstanding hardness and wear resistance.

Hierarchical nanostructured W-Cu composite with an average W size below 200 nm and nanocrystalline structure inside the W phase was obtained by refining the inner structure of the initial ultrafine powders combined with high-pressure spark plasma sintering. It revealed that an atomic scale combination can be formed at both the W grain boundaries and W/Cu interfaces. Accordingly, the nanostructured W-Cu composite exhibits twofold hardness and greatly improved wear resistance with satisfactory electrical conductivity, as compared to those of their fine grain structured counterparts. The upgraded wear resistance is attributed to the restricted micro-plowing and the mechanically mixed layer, induced by a refined microstructure, intrinsic high hardness, and the composition modulation on the wear surface.

Evaluation of interfacial stability and strength of cermets based on work function.

A new method based on work function to analyze the interfacial stability and strength of ceramic-metal composites was proposed in this work. The interfacial work function gradient and interfacial elastic modulus were evaluated experimentally using WC-Co and TiC-Co as the examples. It found that a stable and strongly bonded interface had a gradually changing interfacial work function, while a weak interface exhibited a steep work function changing across the interface. The spatial resolution of the experimental analysis could be down to 10 nm with a high work function sensitivity. First-principles calculations were conducted to analyze the electronic configurations across the interfaces. They revealed the potential distribution across the interfaces in the sub-nano scale. They demonstrated that the interface with a smaller interfacial work function gradient had smaller interface energy and stronger interfacial bonds, and thus the interface was more stable and stronger. The calculation disclosed the mechanism of the experimental observations of the interfacial work function. Both the experimental and theoretical studies confirmed that the interfacial work function gradient could be a measure of the interactions across the interfaces. The effectiveness of the established model was demonstrated by analyzing the stability of thin films at WC/Co interfaces. This study provides a new method to evaluate the interfacial stability and bonding strength for cermets.

Solute segregation and thermal stability of nanocrystalline solid solution systems.

A model coupling first principles and thermodynamics was developed to describe the thermal stability of a nanograin structure in solid solution alloys. The thermodynamic functions of solute segregation and conditions for thermal stabilization were demonstrated for both strongly and weakly solute-segregating systems. The dependence of segregation behavior on the grain size, solute concentration and temperature was quantified, where the parameters to control destabilization of the nanograin structure at a given temperature were predicted. For the first time it was found that there exists a transformation from the single-extreme to dual-extreme rule of the total Gibbs free energies of the solid solution systems with the decrease of solute concentration or increase of temperature. The model calculations were confirmed quantitatively by the experimental results, and a nanocrystalline W-10 at%Sc solid solution with a highly stable grain structure in a broad range from room temperature to 1600 K was prepared. The universal mechanism disclosed in this study will facilitate the design of nanocrystalline alloys with high thermal stability through matching of the doping element and the initial grain size.

How hard metal becomes soft: crystallographic analysis on the mechanical behavior of ultra-coarse cemented carbide.

Investigation into the temperature dependence of the mechanical behavior of ultra-coarse grained cemented carbide materials is highly demanded due to its service conditions of concurrent applied stress and high temperature. In the present study, based on the designed experiments and microstructural characterization combined with crystallographic analysis, the evolution of slip systems, motion and interaction of dislocations with temperature are quantified for the WC hard phase. Mechanisms are proposed for the formation of the sessile dislocations in the main slip systems at the room temperature and the glissile dislocations in the new slip systems activated at high temperatures. Furthermore, the correlation of the plastic strain and fracture toughness with the temperature-dependent slip activation, dislocation reaction and transformation is explained quantitatively. Enlightened by the present findings, potential approach to enhance the high-temperature strength of ultra-coarse cemented carbides based on WC strengthening was suggested.

Rough Structure of Electrodeposition as a Template for an Ultrarobust Self-Cleaning Surface.

Superhydrophobic surfaces with self-cleaning properties have been developed based on roughness on the micro- and nanometer scales and low-energy surfaces. However, such surfaces are fragile and stop functioning when exposed to oil. Addressing these challenges, here we show an ultrarobust self-cleaning surface fabricated by a process of metal electrodeposition of a rough structure that is subsequently coated with fluorinated metal-oxide nanoparticles. Scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction were employed to characterize the surfaces. The micro- and nanoscale roughness jointly with the low surface energy imparted by the fluorinated nanoparticles yielded surfaces with water contact angle of 164.1° and a sliding angle of 3.2°. Most interestingly, the surface exhibits fascinating mechanical stability after finger-wipe, knife-scratch, sand abrasion, and sandpaper abrasion tests. It is found that the surface with superamphiphobic properties has excellent repellency toward common corrosive liquids and low-surface-energy substances. Amazingly, the surface exhibited excellent self-cleaning ability and remained intact even after its top layer was exposed to 50 abrasion cycles with sandpaper and oil contamination. It is believed that this simple, unique, and practical method can provide new approaches for effectively solving the stability issue of superhydrophobic surfaces and could extend to a variety of metallic materials.

The thermal stability of the nanograin structure in a weak solute segregation system.

A hybrid model that combines first principles calculations and thermodynamic evaluation was developed to describe the thermal stability of a nanocrystalline solid solution with weak segregation. The dependence of the solute segregation behavior on the electronic structure, solute concentration, grain size and temperature was demonstrated, using the nanocrystalline Cu-Zn system as an example. The modeling results show that the segregation energy changes with the solute concentration in a form of nonmonotonic function. The change in the total Gibbs free energy indicates that at a constant solute concentration and a given temperature, a nanocrystalline structure can remain stable when the initial grain size is controlled in a critical range. In experiments, dense nanocrystalline Cu-Zn alloy bulk was prepared, and a series of annealing experiments were performed to examine the thermal stability of the nanograins. The experimental measurements confirmed the model predictions that with a certain solute concentration, a state of steady nanograin growth can be achieved at high temperatures when the initial grain size is controlled in a critical range. The present work proposes that in weak solute segregation systems, the nanograin structure can be kept thermally stable by adjusting the solute concentration and initial grain size.

Safety of inactivated monovalent pandemic (H1N1) 2009 vaccination during pregnancy: a population-based study in Taiwan.

BACKGROUND: Pregnant women were prioritized for H1N1 vaccination during the 2009-2010 pandemic. Safety concerns exist with vaccinating pregnant women, particularly in their first trimesters. METHODS: We linked computerized data on H1N1 vaccination, National Health Insurance, and Taiwan Birth Registry and identified events of spontaneous abortions (SABs) and all singleton births that occurred/delivered during November 1, 2009-September 30, 2010. The observation period for each case of SAB (6-19 weeks gestation) was divided into period at risk (1-28 days after vaccination) and control periods (the remaining person-days until SAB). The self-controlled case series method for truncated observational periods assessed the incidence rate ratio (IRR) of SAB during the 1-28 days compared with those in the control period. The case-control design matched each case of adverse fetal outcomes to up to 10 controls on fetal sex and year/month of pregnancy onset, and calculated matched odds ratio (OR) on H1N1 vaccination at <14 or ≥14 weeks gestation. RESULTS: Sixty-five women with SAB had received H1N1 vaccination at 6-19 weeks gestation. The IRR of SAB for the risk period 1-28 days was 1.03 (95% confidence interval [CI] 0.55-1.93). Among the 147,294 live births and 1354 stillbirths, maternal H1N1 vaccine receipt at <14 weeks gestation was associated with significantly reduced likelihood of small for gestational age (SGA) birth (OR 0.72, 95% CI 0.61-0.84) and birth defect (OR 0.46, 95% CI 0.22-1.00), whereas receipt at ≥14 weeks gestation was associated with significantly reduced likelihood of stillbirth (OR 0.63, 95% CI 0.46-0.86), prematurity (OR 0.90, 95% CI 0.83-0.97), low birth weight (OR 0.81, 95% CI 0.74-0.88), and SGA birth (OR 0.90, 95% CI 0.84-0.97). CONCLUSIONS: H1N1 vaccination during pregnancy did not increase risk of SAB or adverse fetal outcomes.