Etching of diamond octahedrons at high temperatures and pressure with controlled oxygen partial pressure

Etching of a natural diamond octahedron was carried out at temperatures of 800 to 1400° C and at pressures of 15 and 40 kbar under controlled oxygen partial pressure in the range 10⁻¹⁷ to 10⁴ atm by use of oxygen buffers. Well-defined etch pits of equiangular triangule outline were formed. When the results were plotted based on logP _{O 2} versus 1/T, reversal of the pit orientation clearly occurred on a boundary curve expressed by an equation, logP _{O 2}=−9.0×10⁴/T+63, whereP _{O 2} (atm) andT (K) are oxygen partial pressure and temperature, respectively. Etch pits with the same orientation as an octahedral face were produced in a low temperature and highP _{O 2} region, and those with the opposite orientation, i.e. the same as for natural “trigon”, were produced in the other region.     

MOF-Derived Mn2O3 Nanocage with Oxygen Vacancies as Efficient Cathode Catalysts for Li–O2 Batteries

Designing efficient and cost-effective electrocatalysts is the primary imperative for addressing the pivotal concerns confronting lithium–oxygen batteries (LOBs). The microstructure of the catalyst is one of the key factors that influence the catalytic performance. This study proceeds to the advantage of metal-organic frameworks (MOFs) derivatives by annealing manganese 1,2,3-triazolate (MET-2) at different temperatures to optimize Mn₂O₃ crystals for special microstructures. It is found that at 350 °C annealing temperature, the derived Mn₂O₃ nanocage maintains the structure of MOF, the inherited high porosity and large specific surface area provide more channels for Li⁺ and O₂ diffusion, beside the oxygen vacancies on the surface of Mn₂O₃ nanocages enhance the electrocatalytic activity. With the synergy of unique structure and rich oxygen vacancies, the Mn₂O₃ nanocage exhibits ultrahigh discharge capacity (21 070.6 mAh g⁻¹ at 500 mA g⁻¹) and excellent cycling stability (180 cycles at the limited capacity of 600 mAh g⁻¹ with a current of 500 mA g⁻¹). This study demonstrates that the Mn₂O₃ nanocage structure containing oxygen vacancies can significantly enhance catalytic performance for LOBs, which provide a simple method for structurally designed transition metal oxide electrocatalysts.     

Numerical simulation of the effect of atomic oxygen impurity on the formation of population inversion during axisymmetric mixing of iodine and excited oxygen flows

We have performed a parametric analysis of the influence of the percentage fraction of atomic oxygen in the O₂:O₂(¹Δ):O flow on the formation of population inversion for a laser transition in iodine atoms and the temperature regime at an elevated pressure of P = 5 Torr in a system with axisymmetric injection of a I₂: He mixture to the flow of oxygen excited by an electric-discharge singlet oxygen generator.     

The Interaction of Zirconia with Oxygen and Its Defect Structure

The mass action law is used to describe the reversible exchange between the crystal lattice of ZrO₂ and the ambient medium, and the special features of positioning of interstitial particles in the lattice are taken into account. The ways of equilibrium interaction between the ZrO_2−x lattice with the ambient medium (types of defects that arise) at different partial pressures of oxygen p(O₂) and temperatures T are determined, along with the form of the correlation x = f[p(O₂)], the variation of its composition, the number of charge carriers, mass, volume, and density. The obtained results are compared with the available experimental data. It is demonstrated that, at low temperatures, the lattice defects in pure ZrO₂ are as follows: at high values of p(O₂) - oxygen vacancies in the anion sublattice and singly charged anions of oxygen in the interstice, and at low values of p(O₂) - vacancies in the anion sublattice and electrons localized on the adjacent cations. At high temperatures, the lattice defects are as follows: at high values of p(O₂) - singly charged anions of oxygen and quadruply charged cations of zirconium at the interstice, and at low values of p(O₂) - only neutral zirconium atoms at the interstice.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 4, 2005, pp. 556–567.Original Russian Text Copyright © 2005 by E. P. Pakhomov.     

Dielectric properties of 3d transition metal substituted BaTiO3 ceramics containing the hexagonal phase formation

The role of transition metals of the 3d series from V (Z = 23) to Zn (Z = 30) is investigated in modifying the crystallographic phase contents, microstructure and the dielectric properties of BaTiO₃ ceramics containing ≤10 at% substituents. All the transition metals brought about the phase conversion to hexagonal BaTiO₃ and the hexagonality is found to depend on Ba/Ti ratio as well as the processing conditions including the sintering temperature and the post sinter annealing. The ε_r-T characteristics are modified with increasing hexagonality by way of the tremendous decrease in dielectric constant with broad and diminished ε_max for the mixed phase ceramics giving way to flat ε_r-T curves for totally hexagonal specimens. Doping with >1% Zn²⁺ or ≥5% Mg²⁺ also render the ceramics completely hexagonal indicating that the crystal field effects of the 3d orbital electrons are not the cause for the conversion to hexagonal phase. Electron paramagnetic resonance (EPR) spectra of Mn-doped ceramics reveal the prevalence of defect complexes involving oxygen vacancies and different valence states of Mn occupying the Ti-sites within the corner-sharing as well as face-sharing octahedra present in hexagonal BaTiO₃. EPR results also indicate defect complexes involving electron localization at Ti-sites and oxygen vacancies around the face-sharing octahedra. On annealing the ceramics in lower oxygen partial pressures at elevated temperatures leads to the reversion to corner shared (Ti³⁺–V_O) defect complex accompanied by the conversion to cubic/tetragonal phase. The alterations in the oxygen vacancy-metal impurity defect complexes and the modifications in the oxygen close packing are the cause for the prevalence of hexagonal BaTiO₃ at room temperature.     

Effects of Ho2O3 addition on defects of BaTiO3

Effects of Ho₂O₃ addition on defects of BaTiO₃ ceramic have been studied in terms of electrical conductivity at 1200 °C as a function of oxygen partial pressure (P_O2°) and oxygen vacancy concentration. The substitution of Ho³⁺ for the Ti site in Ba(Ti_1−xHo_x)O_3−0.5x resulted in a significant shift of conductivity minimum toward lower oxygen pressures and showed an acceptor-doped behavior. The solubility limit of Ho on Ti sites was confirmed less than 3.0 mol% by measuring the electrical conductivity and the lattice constant. Oxygen vacancy concentrations were calculated from the positions of P_O2° in the conductivity minima and were in good agreement with theoretically estimated values within the solubility limit. The Curie point moved to lower temperatures with increasing the oxygen vacancy concentration and Ho contents.     

Vacancies as a constitutive element for the design of nanocluster-strengthened ferritic steels

The existence of nanoclusters that are thermodynamically stable at elevated temperatures is truly intriguing because of its scientific implications and potential applications. Highly stable nanoclusters have been observed by atom probe tomography in iron-based alloys at temperatures close to 1400°C (0.92T _m) that appear to defy the stability constraints of artificially created nanostructured materials. The ∼4-nm-diameter Ti-, Y- and Oenrichednanoclusters are identified in the new form of a highly defective material state with vacancies as the critical alloying component and with (Ti + Y):O ratio different from the stable TiO₂ and Y₂Ti₂O₇ oxides. Vacancies play an indispensable role in enhancing the oxygen solubility and increasing the oxygen binding energy in the presence of Ti and Y, resulting in the stabilization of coherent nanoclusters. Atom probe tomography characterizations and theoretical predictions indicate that vacancies can be exploited for the first time as a nanoscale constituent to design materials with far superior high temperature properties.     

Observations of p-type semiconductivity in titanium dioxide at room temperature

Titanium dioxide (TiO₂) is widely considered to be a strongly n-type semiconductor due to its tendency for oxygen deficiency. In this work, however, room temperature p-type semiconductivity has been observed in rutile TiO₂, as determined using surface photovoltage spectroscopy and cyclic voltammetry subsequent to controlled processing at elevated temperatures. In particular, room temperature p-type semiconductivity has been imposed via strong oxidation [p(O₂) = 75 kPa] at elevated temperatures (1273 K), followed by rapid cooling in the same gas phase. It is reasoned that under such conditions, the observed p-type behaviour is due to the formation of titanium vacancies (acceptor-type ionic defects) at the TiO₂ surface. It is also concluded that the extremely slow diffusion kinetics of these defects towards the bulk is what limits the p-type behaviour to the surface and near surface regions. Nevertheless, this could be overcome by applying appropriately lengthy annealing times. The reported observation of p-type semiconductivity in TiO₂ is expected to have far reaching consequences for this intensely researched material.     

Electrical conductivity of fluorite phases in the system Y2O3-Ta2O5-MgO

Electrical conductivity of fluorite phases in the system Y₂O₃-Ta₂O₅-MgO has been studied. Y_0.8Ta_0.2O_1.7 fluorite phase, having 15 mol% oxygen ion vacancies, showed the maximum electrical conductivity. Electrical conductivity of Y_0.8Ta_0.2O_1.7 fluorite phase increased linearly with P^1/2 _HO due to the contribution of protonic conduction in water vapour. The electrical conductivity of the fluorite phase increased more remarkably at lower temperature, 700°C, than at higher temperatures, 800°C and 900°C. The obtained results imply that the protonic conduction greatly contributes to the electrical conductivity of the fluorite phases in the system Y₂O₃-Ta₂O₅-MgO in water vapour.     

Oxygen gas-sensing behaviour of V2O5–SnO–TeO2 glass

The d.c. conductivity, σ, and the oxygen gas-sensing behaviour of V₂O₅–SnO–TeO₂ glass prepared by press-quenching were studied in argon and oxygen gas atmospheres at temperatures ranging from 303–473 K. The glass of 50V₂O₅·20SnO·30TeO₂ (mol %) was n-type semiconducting. The high-temperature conductivity was lower in oxygen and higher in argon than that in air. This was explained by the V⁴⁺ ions in the glass being oxidized by oxygen which had diffused into the glass, resulting in an increase in V⁵⁺ with time. The experimental relationship between σ and oxygen partial pressure, P _O2, agreed quantitatively with the theoretical relation σ ∝ P _O2 ^-1/4 . Changes in conductivity by switching the atmospheres between oxygen and argon gases were found to be reproducible. From the data of these dynamic changes, the oxygen gas sensitivity, S, at 473 K was obtained to be 1.3 in oxygen atmosphere. The dynamic changes could be quantitatively explained by an oxygen diffusion model. Throughout these discussions, the present tellurite glass was found to possess a potential applicability as an oxygen gas sensor. This revised version was published online in November 2006 with corrections to the Cover Date.     

The mechanical and structural properties of Y1Ba2Cu3O7−x superconductors

Mechanical losses spectroscopy of two superconducting Y₁Ba₂Cu₃O_7–x samples (T _c=93 K) measured at 5, 10, 20 and 30 MHz revealed four peaks. As the frequency of measurement was increased, the peaks shifted to higher temperatures. The relaxation process was found to have an activation enthalpy of 0.188, 0.092, 0.209 and 0.314 eV with an attempt frequency,f _o, equal to 2.46 × 10¹⁵, 5.95 × 10¹⁰, 1.64 × 10¹⁴, and 7.82 × 10¹³ Hz for P₁, P₂, P₃ and P₄ peaks, respectively. The mechanism responsible for these relaxation peaks is discussed. The infrared spectra of some of these compounds provide evidence for oxygen rearrangement on going from tetragonal to orthorhombic phase. X-ray diffraction analysis shows that these compounds correspond to a single phase which is an orthorhombic perovskite structure.     

Investigation of strontium-doped La(Cr,Mn)O3 for solid oxide fuel cells

The (La, Sr) (Cr, Mn)O₃ system was investigated in an effort to develop an interconnect and cathode materials for solid oxide fuel cells. Sintering studies were done in air at temperatures below 1500°C. Significant improvements in densification were observed with substitution of 50 mol% Mn for chromium and a density of 95% theoretical was achieved with the substitution of 70 mol% Mn for chromium in the La(Cr, Mn)O₃ system. Electrical conductivity (d.c.) measurements were made as a function of temperature and oxygen activity. At 1000°C and 1 atm oxygen, the electrical conductivity ranged from 2.2–20 S cm^–1 for LaCr_0.8Mn_0.4O₃ and La_0.9Sr_0.1Cr_0.2Mn_0.8O₃, respectively. All of the compositions showed similar dependence of electrical conductivity on the oxygen activity. Dependence was small at high oxygen activities; as the oxygen activity decreased, a break in electrical conductivity at 10^–12 atm and 1000°C was observed, and then the electrical conductivity decreased asP _O2 ^1/4 . Sintering and electrical conductivity studies indicate that La_0.9Sr_0.1Cr_0.2Mn_0.8O₃ appears to be a candidate for solid oxide fuel cell applications.     

Electrical conductivity in Ta2O5

The defect structure of undoped polycrystalline Ta₂O₅ was investigated by determining the temperature [850–1050°C] and oxygen partial pressure [10⁰–10^?19 atm.] dependence of the electrical conductivity. The data were found to be proportional to the $\text{～?}\text{1}\text{4}\text{th}$ power of the oxygen partial pressure for the oxygen pressure range <10^?8 atm. and independent of the oxygen partial pressure for P_O2 > 10^?6 atm. The enthalpy of formation of doubly ionized oxygen vacancies plus two electrons is estimated to be 118.31 Kcal/mole [5.13 eV]. The observed conductivity data are explained on the basis of the presence of unknown acceptor impurities in the undoped samples.     

Slower Removing Ligands of Metal Organic Frameworks Enables Higher Electrocatalytic Performance of Derived Nanomaterials

The widely used route of high‐temperature pyrolysis for transformation of Prussian blue analogs (PBAs) to functional nanomaterials leads to the fast removal of CN^? ligands, and thus the formation of large metal aggregates and the loss of porous structures inside PBAs. Here, a controllable pyrolysis route at low temperature is reported for retaining the confined effect of CN^? ligands to metal cations during the whole pyrolysis process, thereby preparing high‐surface‐area cubes comprising disordered bimetallic oxides (i.e., Co₃O₄ and Fe₂O₃) nanoparticles. The disordered structure of Co₃O₄ enables the exposure of abundant oxygen vacancies. Notably, for the first time, it is found that the in situ generated CoOOH during the oxygen evolution reaction (OER) can inherit the oxygen vacancies of pristine Co₃O₄ (i.e., before OER), and such CoOOH with abundant oxygen vacancies adsorbs two ^?OH in the following Co³⁺ to Co⁴⁺ for markedly promoting OER. However, during the similar step, the ordered Co₃O₄ with less oxygen vacancies only involves one ^?OH, resulting in the additional overpotentials for adsorbing ^?OH. Consequently, with high surface area and disordered Co₃O₄, the as‐synthesized electrocatalysts have a low potential of 237 mV at 10 mA cm^?2, surpassing most of reported electrocatalysts.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.     

Photoluminescence and low-voltage cathodoluminescence characteristics of blue phosphor, ZnGa2O4:M (M = Na, Ag)

Photoluminescence and low-voltage cathodoluminescence characteristics of ZnGa₂O₄ phosphor doped with monovalent ions has been studied. Monovalent ions such as Na⁺ and Ag⁺ are incorporated into ZnGa₂O₄ lattices in order to increase the concentration of oxygen vacancies in the spinel lattice. By doping low concentrations of monovalent ions (Na⁺, Ag⁺) into ZnGa₂O₄, the self-activated blue luminescence originated from oxygen vacancies is enhanced. Also, the blue luminescence intensity is enhanced more along with a good color purity by annealing ZnGa₂O₄:Na⁺ in a reducing atmosphere, which is due to increasing the concentration of oxygen vacancies even more. The luminescence band at the UV region (λ_max=360 nm) does not become the major luminescence band by introducing Na⁺ ion into the ZnGa₂O₄ lattice, while the UV luminescence band becomes the major one by annealing the undoped ZnGa₂O₄ in a reducing atmosphere.     

Preparation of YVO4 powder from the Y2O3 + V2O5 + H2O system by a hydrolysed colloid reaction (HCR) technique

Prior to the formation of YVO₄ in the Y₂O₃ + V₂O₅ + H₂O system, two intermediate, partially hydrophobic, complex colloidal mixtures with metastable characteristics can be produced at room temperature and atmospheric pressure. The ball-milled system, having both hydrophobic and hydrophilic species, transforms into the stable yttrium orthovanadate phase due to intensive hydrolysis. At room temperature an orange mixture (possessing dispersed Y₂O₃ and 4Y₂O_3–P (OH) _p ^p+ ·2VO ₃ ^– , Y₂O_3–p (OH) _p ^p+ ·6VO ₃ ^– ·xH₂O-like heteroaggregations) formed by 20 h mixing at pH ca. 4.0 transforms slowly, another red-brown heavily flocculated colloidal mixture (with dispersed Y₂O₃ and Y₂O_3–p (OH) _p ^p+ ·V₁₀O ₂₈ ^6– ·yH₂O-like aggregation) formed by 70 h mixing at pH ca. 4.5 transforms rapidly into YVO₄ in water. During additional mixing of highly diluted red-brown mixtures this transformation can be completed at room temperature. At elevated temperatures (50–95 °C) the orange mixture precipitates into a red-brown decavanadate-type precipitatium which subsequently can also rapidly hydrolyse into an orthovanadate phase in the diluted aqueous systems. Both vanadium excess meta-and decavanadate-type aggregations exhibit amorphous character by X-ray diffraction.The semi-hydrophobic colloidal structure can modify the dissociation mechanism, which prevents the system from returning to the starting oxides, and gives a new HCR technique for YVO₄ preparation with a simple hydrolysis process at low temperatures and atmospheric pressure.     

Catalytic-free growth of ZnGa2O4 nanowires on amorphous carbon layers

Zinc gallate (ZnGa₂O₄) nanowires were directly grown on the amorphous carbon-coated silicon substrates using a facile chemical vapor deposition method without any metal catalysts. The growth mechanism can be attributed to a self-organization vapor-liquid-solid (VLS) process. The amorphous carbon layer plays an important role in the nucleation and growth process of the ZnGa₂O₄ nanowires. The photoluminescence (PL) of the nanowires shows a broad, strong green emission band centered at 532 nm and a weak UV emission band at 381 nm, which can be attributed to a large amount of ionized oxygen vacancies and the combination of Ga³⁺ ions with free electrons in coordinated oxygen vacancies, respectively.     

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si₃N₄-Y₂O₃ systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si₃N₄-Y₂O₃-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm⁻² in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm⁻² at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.     

Elastic properties of partially-stabilized HfO2 compositions

The effect of metastable tetragonal HfO₂ on the elastic properties of partially stabilized HfO₂ doped with Er₂O₃, Y₂O₃ or Eu₂O₃ was studied using a sonic resonance technique. The elastic moduli were monitored at elevated temperatures to follow the effect of the tetragonal-monoclinic phase transformation. Elastic moduli were also determined as a function of porosity and found to follow linear relations. The non-linear modulus versus temperature relations from room temperature to 500° C are explained in terms of the oxygen vacancies present in the fluorite type phase.