High Vacancy Content Research Articles

Crystal facet engineering of K-OMS-2 for active site identification in catalytic ozone decomposition

Manganese oxide catalysts (MnOx) are expected to be utilized to catalytic ozone (O3) decomposition to curb indoor O3 pollution. However, the accumulation of oxygen-containing species (*OO) on the active site (oxygen vacancy, Ov) of MnOx limits the practical application. Herein, α-MnO2 with exposed {100} (Mn-100), {110} (Mn-110), or {310} (Mn-310) facet was synthesized, respectively. Mn-110 has preferred performance and stability, maintaining 99.9 % O3 conversion after 8 h under 35 % RH. Experimental and theoretical results indicated that the Mn-100 facet owns lower Ov content and *OO adsorption energy, exhibiting the highest specific area normalized activity. Mn-310 has a higher vacancy content and enhanced *OO adsorption energy, which resulted in a rapid accumulation of *OO species. Mn-110 has a moderate Ov content and *OO adsorption and thus exhibits optimal activity. This study hopes to point out a new optimization strategy for MnOx catalysts.

Influence of Vacancies in Manganese Hexacyanoferrate Cathode for Organic Na-Ion Batteries: A Structural Perspective.

Manganese hexacyanoferrates (MnHCF) are promising positive electrode materials for non-aqueous batteries, including Na-ion batteries, due to their large specific capacity (>130 mAh g-1 ), high discharge potential and sustainability. Typically, the electrochemical reaction of MnHCF associates with phase and structural changes, due to the Jahn-Teller (JT) distortion of Mn sites upon the charge process. To understand the effect of the MnHCF structure on its electrochemical performance, two MnHCF materials with different vacancies content are investigated herein. The electrochemical results show that the sample with lower vacancy content (4 %) exhibits relatively higher capacity retention of 99.1 % and 92.6 % at 2nd and 10th cycles, respectively, with respect to 97.4 % and 79.3 % in sample with higher vacancy content (11 %). Ex-situ X-ray absorption spectroscopy (XAS) and ex situ X-ray diffraction (XRD) characterization results show that a weaker cooperative JT-distortion effect and relatively smaller crystal structure modification occurred for the material with lower vacancies, which explains the better electrochemical performance in cycled electrodes.

Structural response of phyllomanganates to wet aging and aqueous Mn(II)

Naturally occurring Mn(IV/III) oxides are often formed through microbial Mn(II) oxidation, resulting in reactive phyllomanganates with varying Mn(IV), Mn(III), and vacancy contents. Residual aqueous Mn(II) may adsorb in the interlayer of phyllomanganates above vacancies in their octahedral sheets. The potential for interlayer Mn(II)-layer Mn(IV) comproportionation reactions and subsequent formation of structural Mn(III) suggests that aqueous Mn(II) may cause phyllomanganate structural changes that alters mineral reactivity or trace metal scavenging. Here we examine the effects of aging phyllomanganates with varying initial vacancy and Mn(III) content in the presence and absence of dissolved Mn(II) at pH 4 and 7. Three phyllomanganates were studied: two exhibiting turbostratic layer stacking (δ-MnO2 with high vacancy content and hexagonal birnessite with both vacancies and Mn(III) substitutions) and one with rotationally ordered layer stacking (triclinic birnessite containing predominantly Mn(III) substitutions). Structural analyses suggest that during aging at pH 4, Mn(II) adsorbs above vacancies and promotes the formation of phyllomanganates with rotationally ordered sheets and mixed symmetries arranged into supercells, while structural Mn(III) undergoes disproportionation. These structural changes at pH 4 correlate with reduced Mn(II) uptake onto triclinic and hexagonal birnessite after 25days relative to 48h of reaction, indicating that phyllomanganate reactivity decreases upon aging with Mn(II), or that recrystallization processes involving Mn(II) uptake occur over 25days. At pH 7, Mn(II) adsorbs and causes limited structural effects, primarily increasing sheet stacking in δ-MnO2. These results show that aging-induced structural changes in phyllomanganates are affected by aqueous Mn(II), pH, and initial solid-phase Mn(III) content. Such restructuring likely alters manganese oxide reactions with other constituents in environmental and geologic systems, particularly trace metals and redox-active compounds.

The role of processing route on the microstructure of 14YWT nanostructured ferritic alloy

Nanostructured ferritic alloys have outstanding high temperature creep properties and enhanced tolerance to radiation damage over conventional ferritic alloys. To achieve these properties, NFAs are fabricated by mechanical alloying of metallic and yttria powders. Atom probe tomography has demonstrated that milling times of at least 40 h are required to produce a uniform distribution of solutes in the flakes. After milling and hot extrusion, the microstructure consists of α-Fe, high number densities of Ti–Y–O-vacancy-enriched nanoclusters, and coarse Y2Ti2O7 and Ti(O,C,N) precipitates on the grain boundaries. In contrast, the as-cast condition consists of α-Fe with 50–100 μm irregularly-shaped Y2Ti2O7 pyrochlore precipitates with smaller embedded precipitates with the Y3Al5O12 (yttrium–aluminum garnet) crystal structure indicating that this traditional processing route is not a viable approach to achieve the desired microstructure. The nano-hardnesses were also substantially different, i.e., 4 and 8 GPa for the as-cast and as-extruded conditions, respectively. These variances can be explained by the microstructural differences and the effects of the high vacancy content introduced by mechanical alloying, and the strong binding energy of vacancies with O, Ti, and Y atoms that retard diffusion.

Synthesis, Thermodynamic Stability and Diffusion Mechanism of Al5Fe2-Based Coatings

Aluminide coating of steels enables more efficient power generation through higher operating temperatures. Low-temperature (T < 660 °C) pack cementation aluminide coatings form an Al5Fe2 phase which allows for the development of a large Al flux, but the mechanism is not clear. The coating structures and resultant oxides were examined in both austenitic and ferritic steels at 1,000 and 800 °C to evaluate the high temperature oxidation behavior in air. To understand the relatively fast Al diffusion, the stability of the Al5Fe2 phase and the defect structure have been examined by a cluster expansion method with density functional theory calculations. The Al5Fe2 phase has a low site occupancy and a high vacancy content that promotes rapid kinetics. The high vacancy concentration in the Al5Fe2 phase can be traced to the interaction between Al and vacancies along the [001] chains. The analysis offers useful guidance to enable an effective control of low temperature aluminizing.

Kinetic Monte Carlo simulations of nanocrystalline film deposition

A full diffusion kinetic Monte Carlo algorithm is used to model nanocrystalline film deposition, and study the mechanisms of grain nucleation and microstructure formation in such films. The major finding of this work is that new grain nucleation occurs predominantly on surface peaks. Consequently, development of a nanocrystalline structure is promoted by a growth surface with nanoscale roughness, on which new grains can nucleate and grow separately from one another. The grain minor dimension (in the plane of the film) is primarily dictated by surface peak spacing, which in turn is reduced at low temperatures and high deposition rates. The grain major dimension (in the growth direction) is related to the probability of nucleating new grains on top of pre-existing ones, with finer grains being formed at low temperatures and low grain boundary energies. Because vacancies kinetically pin grain boundaries, high vacancy content, which is obtained at high deposition rate, also favors nanograins. Consistent with empirical observations common in the experimental literature, it is found that as grains shrink, they transition from elongated to equiaxed.

Phase-field simulation of sintering and related phenomena – A vacancy diffusion approach

A phase-field model of sintering and related phenomena in a two-phase system and in a multi-phase system is presented. We consider diffusion of vacancies as the atomic mechanism for redistribution of material and we will use the familiar model of thermal vacancies in a crystal as our energy formulation. The solid material will thus be characterized by a low vacancy content and the surroundings by a very high vacancy content and a very low content of atoms. The surface of the solid body will be characterized by a continuous variation in vacancy content. The temporal development of particles during solid state sintering with effects such as wetting is shown in various simulations.

Structural and magnetic study of self- doped La1−x−yCaxØyMno3

Abstract A detailed study of the combined effect of divalent ion and A-site vacancy self-doping in ferromagnetic La 1− x − y Ca x O y MnO 3 manganite system is presented. It is found that the system can accommodate high vacancy content, with a strong lattice distortion for y>0.12. Near the stoichiometric maximum T c ( x =0.33, y =0), vacancies do not degrade T c , and the magnetoresistance can even be enhanced.

Compression of seven vacancy-ordered phases ofScxSto 50 GPa

The ${\mathrm{Sc}}_{x}\mathrm{S}$ system, $(0.8&lt;x&lt;~1.0)$ has a lattice based on the cubic rocksalt structure with vacancies only in the metal sublattice. Our compression experiments reveal that the zero-pressure isothermal bulk modulus increases with increasing vacancy content from about 100 GPa to about 150 GPa. This is due to enhanced Sc $3d\ensuremath{-}\mathrm{S}$ $2p$ orbital interaction and resulting stronger Sc-S bonds in the samples with high vacancy content. The fourth-order Birch-Murnaghan equations of state obtained for each composition converge above 40 GPa, and samples with a larger Sc deficiency decompressed from 50 GPa to below 20 GPa show a slight increase in volume. We propose the formation of ${\mathrm{S}}_{2}^{\mathrm{\ensuremath{-}}2}$ dumbbell pairs (similar to those in transition-metal disulfides) from the dimerization of ${\mathrm{S}}^{\mathrm{\ensuremath{-}}2}$ ions above 20 GPa in the compositions with more vacancies as an explanation of these observations.

Magnetic properties of natural goethite-III. Magnetic behaviour and properties of minerals originating from goethite dehydration during thermal demagnetization

The magnetic behaviour during stepwise demagnetization of a set of artificial samples, containing well-defined natural goethite from five localities (chemical composition, crystallite size and grain size are known) is reported. Differential Scanning Calorimetry measurements indicated that the goethites converted to haematite between 260° and 360°C. In this temperature range a small decrease in initial susceptibility and an additional remanence decay leading toward an extra bending point in the thermal decay curve are observed. Because titanomagnetite or pyrrhotite are absent in the samples, this extra bending point is tentatively interpreted as being due to recrystallization of the minor haematite already present in the original goethite concentrates, facilitated by water made available through the goethite/haematite conversion. At much higher temperatures, at some 600°C, a self-reversal of the remanence is observed in most specimens. The haematite derived from the goethites between 260° and 360°C appeared to be prone to further alteration at successive higher demagnetization temperatures. Chemical alterations in the magnetic mineral suite, starting at temperatures from 380°C upward—depending on the grain size of the original goethite—were inferred from an increase in initial susceptibility. The susceptibility behaviour showed an appreciable variation, due to the creation of varying trace amounts of magnetite, which appeared to be already present after the 400°C step. After the final 685°C demagnetization step the magnetic mineralogy was usually dominated by magnetite. The observed variation is thought to be the result of local within-specimen differences in availability of the amount and possibly also the composition of the vapour phase. The vapour phase is due to chemical reactions between the matrix constituents: waterglass and calcite, leading to a CO2/H2O vapour phase. Reducing capacity, necessary for the magnetite formation, is envisaged to be created by decomposition of trace amounts of organic matter at relatively low temperatures, up to some 400°C. At higher temperatures (over 550°C) decomposition of traces of ferrous iron bearing clayminerals presumably donates the ferrous iron. Differences in the magnetite/haematite ratio as well as in the properties of both minerals were assessed in some detail with more or less routine rockmagnetic methods: acquisition of the isothermal remanent magnetization at room temperature (in fields up to 11.2MA m-1 or 14 T), AF demagnetization and low-temperature cycling of the saturation remanence. This was done after the 400°C step to study the magnetic properties of the newly formed haematite and after the 685°C step to evaluate the properties of the magnetite and more evolved haematite. After the 400°C step the haematite appeared to be very fine-crystalline and magnetically extremely hard. Its saturation remanence is considerably lower than that of well-crystalline haematite. After the 685°C step its hardness has decreased and its saturation remanence slightly increased. Chemical differences between the goethites do not show up as distinct haematite properties, only Mn-bearing goethite yields a softer haematite type. The magnetite formed in all goethite samples seems to have more or less identical properties depending on its thermal growth history. It has a maximum blocking temperature of some 560°C, which does not point to a high vacancy content. High values for the magnetite/haematite saturation remanence ratios indicate magnetite grain sizes from very close to the superparamagnetic threshold size to approximately 1µm, i.e. the goethite grains have been converted to intimately intergrown haematite/magnetite aggregates.

Thermoanalytical studies on cation distribution in submicronic titanomagnetites

Non-isothermal and isothermal TG studies on the cation distribution in submicronic titanomagnetites; Fe3−xTixO4 with 0<x<0.82 are reported. For x<0.61, these titanomagnetites could be oxidized at temperatures below 500° to titanium-substituted magnetites which have a higher vacancy content than that of γ-Fe2O3. The DTA results show that the temperature of oxidation of Fe2+ to Fe3+ ions is increased, whereas the temperature of phase change is decreased with increasing titanium substitution. The kinetics of oxidation is found to be governed by the law of diffusion for variable working conditions and for different extents of the vacancies created at the solid-gas interface. The diffusion of iron(II) ions located at octahedral sites proceeds with a lower activation energy than that of iron(II) ions located at tetrahedral sites. Diffusion rates were discerned to be composition-dependent and the variations can be identified with the distribution of the iron ions in sublattices.