O-rich Phases Research Articles

Understanding lithium, sodium, and potassium storage mechanisms in silicon oxycarbide

The use of silicon oxycarbides (SiOCs) as anode materials in lithium-ion batteries and sodium-ion batteries has risen considerably in recent years. However, the amorphous and complex structures of SiOCs that contains C-rich and O-rich SiOC phases make it difficult to clarify Li+- and Na+-ion storage mechanisms experimentally. This study uncovers the Li+-, Na+-, and K+-ion storage mechanisms in both the O-rich SiO1.5C0.5 and C-rich SiO0.5C1.5 structures using the density functional theory. The ions inserted at the initial discharge process fill the microvoids in the SiOCs. A further ion insertion causes Si–O and Si–C bond cleavage, and thus results in the formation of a large-size free volume, which is favorable for subsequent ion insertion. The reasons for the high Li+-ion storage capacity as compared to Na+-ion are less severe volume expansion, more favorable formation of Li-rich Si compounds and Li–Si alloys. The theoretical K+-ion storage capacities in the O-rich SiO1.5C0.5 and C-rich SiO0.5C1.5 phases are much lower (335 and 186 mAh g−1, respectively) than those of Li+-ion (519 and 681 mAh g−1, respectively) and Na+-ion storages (335 and 681 mAh g−1, respectively). The huge structural instability caused by the repulsive interaction between the K+ ions results in the low storage capacity.

Effect of input powder attributes on optimized processing and as-built tensile properties in laser powder bed fusion of AlSi10Mg alloy

A systematic material characterization approach is presented to demonstrate the critical impact of input powder attributes on the optimized parameters and the as-built properties in the Laser Powder Bed Fusion (LPBF) of AlSi10Mg alloy. Two batches of powders (conventional and specialty) with widely different attributes, including particles’ morphology, size distribution, internal oxidation, surface chemistry and roughness, were separately investigated for the optimized LPBF parameters. It is showed that the specialty powder with a spherical morphology and a larger mean particle size exhibits a higher flowability, as well as a higher apparent density. The latter, together with a smoother surface topography in the specialty powder particles (thus a lower laser absorptivity), necessitates a higher volumetric energy density (VED) requirement for a complete fusion during the LPBF. The as-built microstructures which resulted from both powders show a similar evolution of total porosity content, melt-pool boundaries and solidification structure, both qualitatively and quantitatively, which is consistent with their comparable YS values. The specialty powder, however, yields a higher UTS and %El in the as-built state, which is believed to be due to a scarce presence of O-rich particles (which otherwise preside primarily within the intercellular regions, as is the case for the conventional powder). The emergence of such O-rich particles in the as-built microstructure could be traced back to the powder source (i.e., in the form of internal O-rich phases and a thick oxide surface layer in the conventional powder). In other words, an O-lean powder translates into an O-lean printed part. One-time recycling of the specialty powder is shown to have a negligible effect on its morphological characteristics, and thus on the optimized set of parameters, consistently yielding as-built YS values similar to those of the original powder. The as-built UTS and %El values resulting from the recycled powder, however, are notably lower than those from the original powder, which are suggested to be due to the presence of contaminants and/or oxide inclusions resulting from the LPBF processing itself and/or during the recycling procedure (e.g., sieving). We believe the correlations established here between the input powder attributes and the LPBF performance of AlSi10Mg alloy can be applicable to many metallic systems.

New strategy for increasing sodium-ion uptake in silicon oxycarbides

Silicon oxycarbides (SiOCs) are considered promising sodium-ion battery anode materials. However, the total and low-voltage plateau capacities of SiOCs are low (below 200 and 95 mAh g−1, respectively), which hinders the manufacturing of high-density electrodes. Herein a new strategy for improving the electrochemical performance of SiOCs was proposed by considering the Na+ ion storage mechanisms in SiOCs. High-capacity SiOCs were synthesized by increasing the amount of C- and O-rich SiOxCy phases and reducing the amount of inactive SiO2, SiC, and Cfree phases. The compositions of SiOCs were controlled by adjusting the synthesis conditions (e.g., sweep gas and pyrolysis time) and C content of the SiOC precursors. The use of N2 sweep gas instead of H2/Ar suppressed the formation of SiC and Cfree. In addition, the amount of inactive SiO2 phase in SiOCs was further suppressed using a C-rich precursor. Consequently, the synthesized SiOC, which was enriched with C- and O-rich SiOxCy phases, delivered a high reversible capacity of 234 mAh g−1 at a current density of 25 mA g−1. In constant-current/constant voltage mode, the reversible capacity was further increased to 299 mAh g−1, which was close to the theoretical maximum capacity of 315 mAh g−1. After 140 cycles, the reversible capacity was stabilized to 160 mAh g−1. The initial capacity loss during long-term cycling, which was mostly caused by the progressive decrease in the low-voltage plateau capacities, was attributed to the increase in cell polarization.

Effects of oxygen variation on the improved structural stability, electronic and optical properties of ZnTeO compounds

Crystalline ZnTeO thin films are promising materials for next generation photovoltaics. However, their structural stability and optical nonlinearity potential in bulk form have not been reported. Here, structural, electronic and optical properties of ZnTeO composites have been thoroughly studied using genetic algorithm and density functional theory (DFT). Energetically, mechanically and dynamically stable O-rich phases, namely Zn2Te2O6 and ZnTeO4, were obtained. Ground-state properties such as lattice constants and simulated XRD were analyzed and compared to the experimental literature wherever possible. With a G0W0 corrected band gap, these semiconducting phases display several desirable features, namely, Jahn–Teller distorted cations, hardness and shear anisotropy-induced optical nonlinearity that increase monotonically as O concentration elevates. Such trends appear to be consistent with that seen in the experimental study of ZnTeO thin film. It is observed that Zn-d, Te-p and O-p states have immense influence towards the electronic properties of these structures. Both phases exhibit steep elevation of absorption throughout the ultraviolet (UV) range, hitting peak value of ~5.0 × 105 cm−1. Of particular interest, the non-centrosymmetric ZnTeO4 has second harmonic generation coefficients (9.84 pm V−1 and 2.33 pm V−1 at static limit) greater than borates crystal and large birefringence that exceeds 0.08 in deep UV region, thus highlighting its potential pedigree as new optical materials in UV range.

Theoretical evidence for unexpected O-rich phases at corners of MgO surfaces

Realistic oxide materials are often semiconductors, in particular at elevated temperatures, and their surfaces contain undercoordinated atoms at structural defects such as steps and corners. Using hybrid density-functional theory and ab initio atomistic thermodynamics, we investigate the interplay of bond-making, bond-breaking, and charge-carrier trapping at the corner defects at the (100) surface of a $p$-doped MgO in thermodynamic equilibrium with an ${\mathrm{O}}_{2}$ atmosphere. We show that by manipulating the coordination of surface atoms, one can drastically change and even reverse the order of stability of reduced versus oxidized surface sites.

The puzzle of the CNO isotope ratios in asymptotic giant branch carbon stars

Previous determinations of the oxygen isotopic ratios in AGB carbon stars were at odds with the existing theoretical predictions. We aim to redetermine the oxygen ratios in these stars using new spectral analysis tools and further develop discussions on the carbon and nitrogen isotopic ratios in order to elucidate this problem. Oxygen isotopic ratios were derived from spectra in the K-band in a sample of galactic AGB carbon stars of different spectral types and near solar metallicity. Synthetic spectra calculated in LTE with spherical carbon-rich atmosphere models and updated molecular line lists were used. The CNO isotope ratios derived in a homogeneous way, were compared with theoretical predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code assuming extra mixing both during the RGB and AGB phases. For most of the stars the 16O/17O/18O ratios derived are in good agreement with theoretical predictions confirming that, for AGB stars, are established using the values reached after the FDU according to the initial stellar mass. This fact, as far as the oxygen isotopic ratios are concerned, leaves little space for the operation of any extra mixing mechanism during the AGB phase. Nevertheless, for a few stars with large 16O/17O/18O, the operation of such a mechanism might be required, although their observed 12C/13C and 14N/15N ratios would be difficult to reconcile within this scenario. Furthermore, J-type stars tend to have lower 16O/17O ratios than the normal carbon stars, as already indicated in previous studies. Excluding these peculiar stars, AGB carbon stars occupy the same region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram, showing little spread. This reinforces the idea that these grains were probably formed in low-mass stars during the previous O-rich phases.

Friction and Wear Properties of αAlB<sub>12</sub>-NiAl Cermet Prepared by Spark Plasma Sintering

In this study, αAlB12-20vol% NiAl cermet disk specimens were prepared by spark plasma sintering, and their microstructure, Knoop hardness, fracture toughness, and friction and wear properties were investigated. The αAlB12-20vol% NiAl disk specimens were obtained by spark plasma sintering blended αAlB12 and NiAl powder at 1573 K for 600 seconds. No reaction product phases were observed between the αAlB12 and NiAl phases. The αAlB12-20vol% NiAl disk specimens exhibited friction coefficients lower than 0.2 and specific wear rates as low as 1.3 × 10-6 mm3/Nm when sliding against Si3N4 ball specimens in water. O-rich phases were observed on the worn surfaces of the NiAl and αAlB12-20vol% NiAl disk specimens after sliding against Si3N4 ball specimens in water. The Knoop hardness of the disk specimens was as high as 10 GPa and the fracture toughness was as high as 7 MPa m1/2.

A high-performance liquid chromatography assay with a triazole-bonded column for evaluation of d-amino acid oxidase activity.

Elution profiles of kynurenic acid (KYNA) and 7-chlorokynurenic acid (Cl-KYNA) were examined by high-performance liquid chromatography (HPLC) using a triazole-bonded stationary phase column (Cosmosil® HILIC) under isocratic elution of a mobile phase consisting of CH3 CN-aqueous 10 mm ammonium formate between pH 3.0 and 6.0. The capacity factors of KYNA and Cl-KYNA varied with both the CH3 CN content and the pH of the mobile phase. The elution order of KYNA and Cl-KYNA was reversed between the CH3 CN- and H2 O-rich mobile phases, suggesting that hydrophilic interactions and anion-exchange interactions caused retention of KYNA and Cl-KYNA in the CH3 CN- and H2 O-rich mobile phases, respectively. The present HPLC method using a triazole-bonded column and fluorescence detection (excitation 250 nm, emission 398 nm) was applied to monitor in vitro production of KYNA from d-kynurenine (d-KYN) by d-amino acid oxidase (DAO) using Cl-KYNA as an internal standard. A single KYNA peak was clearly observed after enzymatic reaction of d-KYN with DAO. Production of KYNA from d-KYN was suppressed by the addition of commercial DAO inhibitors. The present HPLC method can be used to evaluate DAO activity and DAO inhibitory effects in candidate drugs for the treatment of schizophrenia.

Sintering and characterization of ZrN and (Dy,Zr)N as surrogate materials for fast reactor nitride fuel

Pellets of inert matrix material ZrN, and surrogate nitride fuel material (Dy0.4Zr0.6)N, are fabricated for the purpose of investigating the origin and the effect of carbon and oxygen impurity concentrations. Oxygen concentrations of up to 1.2wt% are deliberately introduced into the materials with two separate methods. The achievable pellet densities of these materials, as a function of O content, sintering temperature and dimensional powder properties are determined. O dissolved into (Dy,Zr)N increases the achievable densities to a larger extent than if dissolved into ZrN. The segregation of O-rich phases in ZrN indicates a low O solubility in the material. Oxygen pick-up during the fabrication of the product as well as its exposure to air is demonstrated. The quality of the materials is monitored by the systematic analysis of O, N and C contents throughout the fabrication and sintering processes, supported by XRD and SEM analyses.

Comprehensive Insights into the Structural and Chemical Changes in Mixed-Anion FeOF Electrodes by Using Operando PDF and NMR Spectroscopy

In-depth analysis of operando X-ray pair distribution function (PDF) data is combined with Li NMR spectroscopy to gain comprehensive insights into the electrochemical reaction mechanism of high-performance iron oxyfluoride electrodes. While the full discharge capacity could be recovered upon charge, implying reversibility of the electrochemical reaction, the atomic structure of the electrode formed after cycling (discharge-charge) differs from the pristine uncycled electrode material. Instead, the "active" electrode that forms upon cycling is a nanocomposite of an amorphous rutile phase and a nanoscale rock salt phase. Bond valence sum analysis, based on the precise structural parameters (bond lengths and coordination number) extracted from the in situ PDF data, suggests that anion partitioning occurs during the electrochemical reaction, with the rutile phase being F-rich and the rock salt phase being O-rich. The F- and O-rich phases react sequentially; Fe in a F-rich environment reacts preferentially during both discharge and charge.

Oxygen-Induced Transformations of an FeO(111) Film on Pt(111): A Combined DFT and STM Study

The structural stability of an FeO(111) film supported on Pt(111) was studied by density functional theory (DFT) as a function of oxygen pressure. The results showed formation of O-rich phases at elevated O-2 pressures and revealed a site specificity of the oxidation process within the coincidence (Moire) structure between FeO(111) and Pt(111), ultimately resulting in an ordered pattern of O-Fe-O trilayer islands, as observed by scanning tunneling microscopy (STM). In addition, high resolution STM images revealed a (root 3 x root 3)R30 degrees superstructure of the FeO2 islands with respect to pristine FeO(111). This structure is rationalized by DFT in terms of strong relaxations within the Fe sublayer and can be considered as an intermediate state of the FeO(111) transformation into an Fe2O3(0001) film.

Secondary phases and interfaces in a nitrogen-atmosphere sintered Al alloy: Transmission electron microscopy evidence for the formation of AlN during liquid phase sintering

A comprehensive transmission electron microscopy study has been made of the secondary phases and their interfaces with the matrix in an alloy of Al–2Mg–2Si–0.25Cu sintered in nitrogen. AlN was detected both at the Al–Mg2Si interface and inside Mg2Si grains as strings of nanocrystallites. Mg2Si did not exist at the sintering temperature; it was the solidified residue of the sintering liquid. The observation confirms the formation of AlN during liquid phase sintering of aluminium alloys in nitrogen. The likely pore filling processes are discussed in the light of the distribution of the AlN nanocrystallites. Two Al-, Si- and O-rich secondary phases were also identified, suggesting that, in addition to Mg, Si may have also played a role in disrupting the Al2O3 film that enveloped each Al powder particle. These findings improve the fundamental basis for understanding the sintering of aluminium alloys in nitrogen and the role of Si.

Alloy surface segregation in reactive environments: First-principles atomistic thermodynamics study ofAg3Pd(111)in oxygen atmospheres

We present a first-principles atomistic thermodynamics framework to describe the structure, composition, and segregation profile of an alloy surface in contact with a (reactive) environment. The method is illustrated with the application to a ${\mathrm{Ag}}_{3}\mathrm{Pd}(111)$ surface in an oxygen atmosphere, and we analyze trends in segregation, adsorption, and surface free energies. We observe a wide range of oxygen adsorption energies on the various alloy surface configurations, including binding that is stronger than on a $\mathrm{Pd}(111)$ surface and weaker than that on a $\mathrm{Ag}(111)$ surface. This and the consideration of even small amounts of nonstoichiometries in the ordered bulk alloy are found to be crucial to accurately model the Pd surface segregation occurring in increasingly O-rich gas phases.

Phase diagram for theNi∕Al2O3interface and relationships to adhesion

First-principles calculations conducted over a broad range of atomic configurations have been used to determine the phase diagram and work of separation for $\mathrm{Ni}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interfaces. Seven interfacial phases have been identified. The results reveal that the strongest (O-rich) phases derive their strength from ionic $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{O}$ bonds across the interface, reminiscent of $\mathrm{NiO}$. The Al-rich phases are also strong, exhibiting a mix of ${\mathrm{Ni}}_{3}\mathrm{Al}$-like and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-like interfacial bonds. The stoichiometric interfaces are the weakest since they are formed from the ground-state ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ surface.

Microstructure of Explosively Compacted Nd-Fe-B Magnets

The microstructure of explosively compacted Nd-Fe-B alloys has been investigated by means of transmission electron microscopy and X-ray diffraction. It is shown that there are three kinds of phases, i.e. the matrix phase Nd 2 Fe 1 4 B, O-rich phases and Nd-rich phases in explosively compacted Nd-Fe-B magnets. The matrix Nd 2 Fe 1 4 B phase is tetragonal, with lattice parameters of a = 0.88 nm and c = 1.22 nm. The O-rich phase was present at the two-grain boundary and three-grain junctions, with the fcc structure, and the lattice parameter is a = 0.559 nm. The O-rich phase belongs to the compound Nd-Fe-O, and the contents of O, Nd and Fe are 45-60 at%, 20-40 at% and 10-21 at%, respectively. It is also shown that a large number of block-shaped Nd-rich phases are in a two-grain boundary or three-grain junctions or Nd 2 Fe 1 4 B phase, with the hcp structure and the lattice parameters of a = 0.395 nm and c = 0.628 nm. The Nd-rich phases belong to compound Nd-O, and the contents of Nd and O are 80-85 at% and 10-15 at%, respectively. Also a few dislocations in the boundary phase have been observed.

Effect of grain alignment and processing temperature on critical currents in YBa2Cu3O7-δ sintered compacts

Magnetically aligned YBa2Cu3O7-δ ceramics show resistivities approaching that of single crystals and improved transport critical currents in a magnetic field. Reduced microcracking and increased transport along the ab-plane are believed responsible for the improved performance over nonaligned ceramics. Aligned and nonaligned samples were prepared in parallel using a range of sintering and annealing temperatures. A rapid rise in density for samples sintered above 900°C in oxygen is accompanied by rapid grain growth, improved alignment, a drop in the room temperature resistivity, and an increase in the critical current. The presence of low melting point (i.e., Ba–Cu–O-rich) phases at grain boundaries is believed responsible for the rapid densification. However, the presence of this phase does not appear to be the most important factor limiting Jc. A high temperature oxygen anneal at 900°C improved performance as compared to anneals at 500°C, possibly due to the removal of carbon. For aligned samples sintered at 980°C, critical currents are over 200 A/cm2 at 77 K, ¼ tesla, and room temperature resistivities are below 350 μΩ-cm.

The role of fluids in the development of a granulite facies transition zone in S India

The Bodinayakkanur area of Tamil Nadu lies in the transition zone (P = 5 kbar, T = 700°C) from amphibolite to granulite facies metamorphism, in the Archaean high grade terrain of S India. Within the zone 3 closely associated rock units occur: (1) Precursor grey gneiss. (2) Pink migmatitic and metasomatic gneiss, (3) Green charnockitic granulites. Field observation shows that the complex inter-relations between these 3 rock types can be related to influx of fabric- and layer-guided CO 2 - and H 2 O + K 2 O-rich fluid phases, derived from a deep-seated source.