Effect of electropulsing on the precipitation of NbC x N1−x from austenite phase

The precipitation of NbC _x N_1?x significantly affects the high-temperature mechanical properties of Nb microalloyed steel. The control of the precipitation behaviour of NbC _x N_1?x is the key, especially the nucleation and precipitation from Nb solid solution. We explore to use electropulsing to promote the precipitation of NbC _x N_1?x through thermodynamic model, chemical phase analysis, XRD and TEM. The modelling research show that electropulsing intensity significantly promotes the nucleation of NbC _x N_1?x in austenite phase. The higher the electropulsing temperature is, the better the promotion effect is. The chemical phase analysis and XRD investigation results show that electropulsing can increase the amount of precipitation of NbC _x N_1?x by 50% at 950°C compared to the precipitation without electropulsing.     

Strongly Coupled Carbon Nanosheets/Molybdenum Carbide Nanocluster Hollow Nanospheres for High‐Performance Aprotic Li–O2 Battery

A highly efficient oxygen electrode is indispensable for achieving high‐performance aprotic lithium–O₂ batteries. Herein, it is demonstrated that strongly coupled carbon nanosheets/molybdenum carbide (α‐MoC_1?x) nanocluster hierarchical hybrid hollow spheres (denoted as MoC_1?x/HSC) can work well as cathode for boosting the performance of lithium–O₂ batteries. The important feature of MoC_1?x/HSC is that the α‐MoC_1?x nanoclusters, uniformly incorporated into carbon nanosheets, can not only effectively prevent the nanoclusters from agglomeration, but also help enhance the interaction between the nanoclusters and the conductive substrate during the charge and discharge process. As a consequence, the MoC_1?x/HSC shows significantly improved electrocatalytic performance in an aprotic Li–O₂ battery with greatly reduced charge and discharge overpotentials and long cycle stability. The ex situ scanning electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy studies reveal that the mechanism for the high‐performance Li–O₂ battery using MoC_1?x/HSC as cathode is that the incorporated molybdenum carbide nanoclusters can make oxygen reduction on their surfaces easy, and finally form amorphous film‐like Li‐deficient Li₂O₂ with the ability to decompose at a low potential. To the best of knowledge, the MoC_1?x/HSC of this paper is among the best cathode materials for lithium–O₂ batteries reported to date.     

Selective,low-temperature synthesis of niobium carbide and a mixed (niobium/tungsten) carbide from metal oxide-polyacrylonitrile composites by carbothermal reduction

Composites of polyacrylonitrile (PAN) with the layered oxides (C₆H₁₃NH₃)Nb₃O₈, (C₈H₁₇NH₃)Nb₃O₈ and -(C₆H₁₃NH₃)NbWO₆ undergo carbothermal reduction in an argon atmosphere at 1000 °C to give the cubic carbides NbC_x and (Nb, W)C_x, respectively. Reduction of the Nb₃O₈/PAN composites to NbC_x proceeds via the formation of tetragonal NbO₂, with no other intermediates being detected. Formation of NbC_x begins at 800 °C but is not complete until 1000 °C. The resultant carbide appears in a highly porous form in admixture with approximately 50% wt/wt amorphous carbon. The carbide content, x, of cubic NbC_x increases with heating time (at 1000 °C) as expected. Values of x ranging from 0.69–0.95 have been observed. The cubic mixed carbide, (Nb, W) C_x, is formed similarly from the -NbWO₆ system via an alkylammonium form in the presence of PAN, although progressive separation into cubic NbC_x and hexagonal WC_x occurs at temperatures above 1000 °C. The -NbWO₆ system does not form a well-defined alkylammonium salt; instead a mixture of -HNbWO₆ with PAN gives rise to a very poorly crystalline (Nb, W) carbide on reduction. In all cases, both a layered oxide and PAN are necessary to form the pure carbides at 1000 °C. The oxide/PAN composites appear to be intimate physical mixtures rather than ordered layered nanocomposites.     

N2O decomposition over (Mg6−xAx) MnO8 (A = Li, Al)

Two different murdochite-type mixed oxides, (Mg_6–x Li _x )MnO₈ (x = 0, 0.1, 0.2 and 0.3) and (Mg_6–x Al _x )MnO₈ (x = 0, 0.2, 0.4, 0.6) were examined for the catalytic decomposition of N₂O in order to make clear the effects of mixed valencies of pairing manganese ions and oxygen vacancies. The valence of manganese ions and the amount of surface oxygen vacancies have been examined with X-ray photoelectron spectroscopy (XPS). (Mg_6–x Li _x )MnO₈ had mixed valence manganese ions and oxygen vacancies on the surface after the substitution. The substituted (Mg_6–x Al _x )MnO₈ had a mixed valence state but oxygen vacancies decresed with x and excess oxygen over stoichiometry was observed at x = 0.4 and 0.6. The reaction rate of N₂O decomposition increased after substitution with lithium but hardly increased after the substitution with aluminum in (Mg_6–x A _x )MnO₈. We assumed that the presence of oxygen vacancies on the surface along with pairing altervalent manganese ions affected strongly to enhance the reactivity of N₂O decomposition.     

Electrical resistivities in single crystals of TiC x and VC x

TiC _x and VC _x have wide non-stoichiometric composition ranges which come only from the carbon vacancies. High purity single crystals with controlled compositions were prepared, and their electrical resistivities at 4.2 and 298 K were measured as a function of composition. In the case of TiC _x whose carbon vacancies are disordered in the lattice, the dependence of the residual resistivity on the composition was interpreted' by applying Nordheim's rule to the vacancy scattering and by considering the change in carrier density due to the introduction of vacancies. In addition, the difference in the resistivity between 4.2 and 298 K was discussed. In the case of VC _x whose carbon vacancies are ordered, the dependence of the resistivity on the composition ranges of the ordered phases was examined in detail.     

Early stage donor-vacancy clusters in germanium

There is considerable experimental evidence that vacancies in Ge dominate several solid state reactions that range from self-diffusivity to metal and dopant transport. It is therefore vital that we fully understand how vacancies interact with other point defects in Ge. Here we have a look at the properties of small donor-vacancy (Sb _n V _m with m,n ≤ 2) complexes in Ge by ab-initio density functional modeling. Particular attention has been payed to binding energies and to the electronic activity of the complexes. We found that all aggregates may contribute to the n→ p type conversion that is typically observed under prolonged MeV irradiation conditions. In general, Sb _n V _m defects are double acceptors. It is also suggested that spontaneous formation of Sb₃V complexes may limit the activation level of donors introduced by ion implantation.     

Preparation and Structure of AgGa1 – x In x Se2 Single Crystals

AgGa_{1 – x} In _x Se₂ single crystals grown by the Bridgman–Stockbarger method, using presynthesized charges with x = 0.27, 0.335, and 0.375 are studied by x-ray diffraction (phase analysis, determination of lattice parameters, structure refinement of powdered crystals by the Rietveld profile analysis method, and single-crystal structural analysis). The results suggest that the crystals may contain Se and Ag vacancies. The polarity of the crystals is found to depend on their composition.     

Interaction energy parameters in hypostoichiometric mono-carbides,TiCx and NbCx,evaluated by statistical thermodynamics

Abstract

Interaction energy E(C-C) between nearest neighbour C atoms and E(C-M) between a C atom and M atom in hypostoichiometric monocarbide MC_x (M = Ti or Nb) were evaluated from the available a(C)-T-x relationships by statistical thermodynamic analysis (a(C): carbon activity, T: temperature, x: C/M atom ratio). Estimated E(C-C) values were positive (i.e. the C-C interaction is repulsive) for both TiC_x and NbC_x and E(C-C) varied with T showing a trend of weakening repulsion with rising T. On the other hand, the C-M interaction appeared to be strongly attractive.     

Room-Temperature Ferromagnetism in Co-doped ZnO Prepared by Microemulsion

Zn_1?x Co _x O (x=0, 0.005, 0.01, 0.02, 0.03, 0.06, 0.12) powders have been synthesized by the microemulsion method. XRD patterns confirm the pure wurtzite structure without any impurity phase. TEM images illustrate that Zn_1?x Co _x O particles are formed by aggregation of small crystallites. The magnetization measurements show clear room-temperature ferromagnetism in Zn_1?x Co _x O (x=0, 0.005, 0.01), and the saturated ferromagnetic magnetization increased with increasing Co concentration to 0.01. However, the ferromagnetic magnetization decreased drastically with further increasing Co concentration (x>0.01). PL spectra clearly show an increasing concentration of oxygen vacancies with increasing Co concentration to 0.01, and strong suppression of oxygen vacancies with further increasing Co concentration in Zn_1?x Co _x O. We conclude that the high concentration of oxygen vacancies is the key origin for the observed ferromagnetism in Co-doped ZnO.     

Vacancy ordering and lithium insertion in III2VI3 nanowires

Superlattice structures resulting from vacancy ordering have been observed in many materials. Here we report vacancy ordering behavior in III₂VI₃ nanowires. The formation of layer-like structural vacancies has been achieved during the synthesis of In₂Se₃ nanowires through a vapor-transport route. Doping In₂Se₃ nanowires with small amounts of Ga during synthesis can completely change the structural vacancy ordering from a layer-like to a screw-like pattern for (In _x Ga_1−x )₂Se₃ nanowires. Lithium atoms can fill in the layer-like structural vacancies of In₂Se₃ nanowires and generate new types of vacancy and lithium atom ordering superlattices. The screw-patterned vacancies of (In _x Ga_1−x )₂Se₃ nanowires show reversible lithium insertion. Our results contribute to the understanding of structure property correlations of III₂VI₃ materials used in lithium ion storage, photovoltaics, and phase change memory.     

Transformation studies of M- to W-type hexaferrite in BaFe12−2x Co3x O19

Samples of general formula BaFe_12–2x Co_3xO₁₉ (x=0.1, 0.3, 0.5) have been synthesized by ceramic as well as citrate gel routes. The X-ray diffraction patterns indicate that M-type phase forms for x=0.1, while W-type phase is favourable for x=0.3 and 0.5. The results for x=0.3 are particularly interesting. While the X-ray diffraction data show evidence of a monophasic W-type hexaferrite, the magnetic, thermal and microstructural investigations indicate a secondary phase presumably at grain boundaries. Transformation of M- to W-type hexaferrite has been explained in terms of different possibilities such as nucleation of secondary phases, formation of vacancies and different site occupation of ions in the structure.     

Solubility of lanthanum in strontium titanate in oxygen-rich atmospheres

Perovskite (ABO₃)-type lanthanum substituted strontium titanate ceramics (lanthanum content x), which had been sintered in pure oxygen at 1400 °C, were investigated from x = 0 up to x = 0.6 by light optical and scanning electron microscopic means in conjunction with X-ray analysis, X-ray diffraction, and pyconometry in order to determine the mechanism being responsible for the compensation of the electronic excess charge resulting from the "donor" lanthanum. A pure strontium vacancy compensation mechanism was observed for lanthanum contents up to x = 0.3. Above x = 0.4 titanium vacancies occur additionally but their concentration remains negligible compared to the predominating strontium vacancies. No indication of a solubility limit of lanthanum at x = 0.4, as stated in former works was observed. At x = 0.5 and 0.6 the lattice structure was found to be slightly distorted, tetragonally and orthorhombically, respectively. The lattice parameter obeys Vegard’s law up to the end member La_2/3□_1/3TiO₃ (□: vacant site). These results were completely confirmed by pycnometry data. This revised version was published online in November 2006 with corrections to the Cover Date.     

Explanation of Co Magnetic Moment Enhancement in (Co,Cu)-Doped ZnO by First-Principle Calculations

By ab-initio calculations on Zn_0.95?x Co_0.05Cu _x O, we study the variations of magnetic moments versus Cu concentration. The electronic structure is calculated by using the Korringa?CKohn?CRostoker (KKR) method combined with coherent potential approximation (CPA). We show that the total magnetic moment and magnetic moment of Co increase with increasing Cu content. From a density of state (DOS) analysis, we propose an explanation of the enhancement of the Co magnetic moment versus Cu concentration.     

Electronic Structure and Magnetic Properties of Doped GaAs (Zinc Blende) with Double Impurities and Defects

Using ab-initio calculations based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of a system based on GaAs material. These properties have been studied, with different point defects in GaAs, which are Gallium interstitials (Ga_i), Gallium antisites (Ga_As), Gallium vacancies (V_Ga), Arsenic interstitials (As_i) and Arsenic antisites (As_Ga), Arsenic vacancies (V_As), and in-doped or codoped system Ga_1?x Co _x As and Ga_1?y?z Co _y Fe _z As, respectively. This work presents detailed information about total and local density of states at different concentrations of these defects and doped elements; the stability of the ferromagnetic state compared with the spin-glass state has been discussed. The result of electronic properties shows that codoped GaAs material with Fe and Co become ferromagnetic, also doped GaAs with Co or Fe with a defect (hole in Ga) is stable in the ferromagnetic state. The Curie temperature is estimated from the total energy difference between the Disorder Local Moment (DLM) and the (FM) state by using a mapping on the Heisenberg model in mean field approximation.     

The evolution of structure and properties in GdMn(1−x)TixO3 ceramics

In this work, the effects of Ti doping on the microstructure, dielectric, and magnetic properties of GdMn_(1?x)Ti_xO₃ (x?=?0.00–0.15) ceramic samples synthesized using a solid-state reaction were investigated. All the experimental samples formed a single-phase structure, and no structural transformation occurred within the experimental doping range; however, Ti doping caused lattice shrinkage. Ti doping reduced the grain size, and the microstructure of the synthesized samples appeared more compact in scanning electron microscopy images. The lattice distortion of GdMn_(1?x)Ti_xO₃ caused by Ti substitution at the Mn sites resulted in changes in the Raman vibration modes. X-ray photoelectron spectroscopy results showed that the valence state transition of the Ti and Mn ions occurred and the concentration of Ti⁴⁺, Mn³⁺ ions and oxygen vacancies changed due to the charge compensation induced by Ti doping. Ti doping had a significant influence on the size and concentration of cation vacancies in the GdMn_(1?x)Ti_xO₃ samples. Appropriate Ti doping was shown to reduce the dielectric loss, improve the frequency stability of the dielectric constant, and significantly affect the long-range ordering of Gd³⁺ magnetic moments and clearly reduce magnetization.

     

The nonstoichiometry and physical properties of the Nd1−x Ca1+x FeO4−y system

The solid solutions of the Nd_1–x Ca_1+x FeO_4–y system for compositions ofx=0.000,0.125, 0.250,0.375, and 0.500 are prepared by drip pyrolysis. XRD analysis shows all the solid solutions are tetragonal I_4/mmm. The Fe⁴⁺ ratio to the total Fe ions or value has a maximum for the compositionx=0.375. From the X-ray powder diffraction analysis and the Mössbauer spectroscopy, the distortion and symmetry change of oxygen octahedra of Fe ions are observed. The structural change of oxygen octahedra of Fe ions strongly affects the physical properties. The solid solution whenx=0.000 shows a weak ferromagnetic behaviour due to the spin canting of the distorted octahedra. The other solid solutions withx=0.125, 0.250, 0.375, and 0.500 show a paramagnetic behaviour over room temperature. The decrease of the magnetic transition temperature is due to the distortion of oxygen octahedra of Fe ions and the existence of the Fe⁴⁺ ion. The formation site of oxygen vacancies plays an important role in the conductivity of the Nd_1–x Ca_1+x FeO_4–y system. Although the oxygen vacancies in [Nd, Ca]-O layer have little effect on conductivity, the oxygen vacancies in the FeO₂ plane of the perovskite layer act as electron trapping sites and thus increase the activation energy.     

Stability of the nitrogen-deficient Ti2AlNx MAX phase in Ar2+-irradiated (Ti,Al)N/Ti2AlNx multilayers

The effects of 100 keV Ar²⁺ ion irradiation on the structure and stability of multilayered dc sputtered and annealed thin films of (Ti,Al)N/Ti₂AlN _x have been investigated with X-ray diffraction and transmission electron microscopy. It is shown that the multilayer structure is preserved as well as the crystalline structures of both components. This demonstrates a high tolerance of the Ti₂AlN _x MAX phase in multilayers to damage induced by nuclear elastic interactions. It is suggested that damage recovery may originate from native nitrogen vacancies and from the interfaces that act as sinks for point defects, like Frenkel pairs, created during the irradiation.     

ZrC Segregation to the Surface of Dilute Solid Solutions of Zirconium Carbide in Niobium Carbide

Precipitation of crystalline ZrC in the surface region of dilute ZrC–NbC solid solutions (<1.0 mol % ZrC) was revealed by electron microscopy, x-ray microanalysis, laser mass spectrometry, and x-ray diffraction. The ZrC precipitation was shown to be a consequence of the diffusional decomposition of the dilute Zr_{1 – x} Nb _x C solid solutions. The boundaries of the solid-miscibility gap in the ZrC _y –NbC _y"system were calculated. The segregation energy of zirconium carbide was estimated at –33.3 kJ/mol.     

Oxygen Vacancy‐Engineered PEGylated MoO3−x Nanoparticles with Superior Sulfite Oxidase Mimetic Activity for Vitamin B1 Detection

Sulfite oxidase (SuO_x) is a molybdenum‐dependent enzyme that catalyzes the oxidation of sulfite to sulfate to maintain the intracellular levels of sulfite at an appropriate low level. The deficiency of SuO_x would cause severe neurological damage and infant diseases, which makes SuO_x of tremendous biomedical importance. Herein, a SuO_x mimic nanozyme of PEGylated (polyethylene glycol)‐MoO_3?x nanoparticles (P‐MoO_3?x NPs) with abundant oxygen vacancies created by vacancy‐engineering is reported. Their level of SuO_x‐like activity is 12 times higher than that of bulk‐MoO₃. It is also established that the superior increased enzyme mimetic activity is due to the introduction of the oxygen vacancies acting as catalytic hotspots, which allows better sulfite capture ability. It is found that vitamin B1 (VB1) inhibits the SuO_x mimic activity of P‐MoO_3?x NPs through the irreversible cleavage by sulfite and the electrostatic interaction with P‐MoO_3?x NPs. A colorimetric platform is developed for the detection of VB1 with high sensitivity (the low detection limit is 0.46 µg mL^?1) and good selectivity. These findings pave the way for further investigating the nanozyme which possess intrinsic SuO_x mimicing activity and is thus a promising candidate for biomedical detection.     

{111} twinning structure and interfacial energy in nonstoichiometric TiCx with ordered carbon vacancies

Here we report the experimental and theoretical investigations of the {111} twinning structures in nonstoichiometric TiC_0.7 with ordered carbon vacancies. Owing to the ordering of carbon vacancies, a cubic Ti₂C-type ordered phase is formed in nonstoichiometric TiC_x (0.53 ≤ x ≤ 0.81). Via the TEM measurements, the {111}-specific twinning structure with incoherent boundary has been observed in ordered TiC_0.7. The first principles calculations indicate that the presence of ordered carbon vacancies leads to reduction in the energy of {111}_Ti type twinning interface, thus favoring the formation and stabilization of {111}_Ti-specific twinning structure in the ordered TiC_x.