O-rich Atmospheres Research Articles

Impact of evolution of structural defects on the ferromagnetic and electrical properties of laser deposited Indium-doped SnO2 thin films

Impact of non-magnetic cationic-substitution on the evolution of structural defects and correlated ferromagnetic and electrical properties are investigated in series of pulsed laser deposited Sn1-xInxO2 (0.0 ≤ x ≤ 0.12) thin films. Beyond the nominal doping concentration of 2 at.% (i.e. x > 0.02), Indium (In)-doped SnO2 film switches to exhibit from n-type to p-type electrical conductivity and simultaneously the magnetization (MS) as well as the Curie temperature (TC) of the films increase significantly. Estimated values of ‘MS’ and ‘TC’ are found to achieve as large as 15.21 emu/cm3 and 540 K respectively when In-doping concentration approaches towards x = 0.08 and afterwards tend to decrease abruptly. Various spectroscopic techniques including Positron Annihilation Lifetime Spectroscopy (PALS) have detected the existence of Sn vacancy (VSn) defects within Sn1-xInxO2 films arise as the effect of In-substitution at Sn site (InSn) under O-rich atmosphere. The estimated positron lifetimes and the increase of line-shape S-parameter confirm the rise of VSn defects which serve as the major source of magnetic moments in non-magnetic host SnO2. Besides, InSn defects introduce excess holes within SnO2 lattice and thereby the magnetic spin-spin RKKY interaction between near-by VSn defects are mediated ferromagetically through the localized holes. For x > 0.08, stabilization of various donor-type defects such as Sn interstitial (Sni), indium interstitial (Ini) actually compensates the acceptors that leads to reduce the effective hole density consequently diminishing the strength of ferromagnetism within SnO2. Hence, tuning of such ferromagnetic and semiconducting properties through non-magnetic cationic substitution in transparent conducting oxides can be very promising in the field of next-generation spintronics.

Evolution of lamellar architecture and microstructure during redox cycling of Fe-Co and Fe-Cu foams

The effects of alloying Fe with 25 at% Co or 30 at% Cu are studied in freeze-cast lamellar foams subjected to redox cycling under H 2 - and H 2 O-rich atmospheres at 800 ºC, relevant to metal-air batteries. In unalloyed Fe foams, redox cycling causes irreversible Kirkendall porosity growth within lamellae, leading to fracture and buckling in the lamellar architecture which in turn leads to foam densification after a few cycles. By contrast, Fe-25Co lamellae develop, upon oxidation, a pure Co core and a Fe 3 O 4 shell which decreases buckling and Kirkendall pore growth, thus slowing sintering and densification of the lamellar foams. After Fe 3 O 4 reduction, the Fe-rich shells and Co-rich cores of the lamellae re-homogenize by diffusion to the original single-phase Fe-25Co alloy, achieving a reversible microstructure upon a full redox cycle. After 10 cycles, average channel porosity (between lamellae) undergoes only a small decrease (from 62 % to 46 %), with minimal Kirkendall pore coarsening in the lamellae, consistent with strong sintering resistance in the Fe-25Co foams. The Fe-30Cu foams also display lamellae with Cu core and a Fe 3 O 4 shell structure after oxidation, since Cu, like Co, does not oxidize under steam. However, the lack of solubility of Cu in Fe prevents re-homogenization after Fe 3 O 4 reduction, so the resulting Cu-core / Fe-shell lamellae undergo severe sintering and densification upon subsequent redox cycling, with channel porosity reducing from 61 % to 9 % after just 5 cycles. For both systems, operando x-ray diffraction during redox cycling reveals that Cu, unlike Co, doubles the Fe oxidation rate, as compared to pure Fe foams. • Fe-25Co and Fe-30Cu metallic foams are created with high porosity and lamellar architecture. • Operando X-ray diffraction is used to track H 2 /H 2 O redox reactions in these foams at 850 °C. • Fe-25Co foams show excellent stability over 10 full redox cycles at 850 °C. • Fe-30Cu foams sinter during cycling, but Cu accelerates redox kinetics.

Structure and role of carbon-related defects in yttrium aluminum garnet

This paper presents ab-initio calculations of composition and energy structure of carbon-related defects in carbon-doped yttrium aluminum garnet, Y3Al5O12. Defect formation energies are considered for isolated point defects and their combinations in case of O-poor atmosphere, which is inherent to crystal growth under Ar + CO gas conditions, and O-rich atmosphere used at post-growth annealing. The point CAl and complex carbon-related defects, such as Ci-VAl, and CAl-VO with lower formation energies should play a role of electron acceptors that compete with oxygen vacancies for electron capture. These results are discussed in conjunction with specific properties of carbon-doped garnets, in particular, no F-type center formation in annealed crystals, even under high irradiation doses.

Catalytic CO Oxidation on Nanocatalysts

CO oxidation, as one of the simplest catalytic reactions, has been widely studied in heterogeneous catalysis. Since CO can be easily adsorbed on active metals like Au, Pt, Pd, Rh, and Ru, many researchers have attempted to observe surface phenomena, including adsorption, surface restructuring, and oxidation kinetics on diverse metal surfaces ranging from single crystals to nanoparticles (NPs). The rapid advances in nanochemistry have further stimulated studies of catalytic reactions, and in-depth understanding of CO oxidation is possible based on analyses of how the size, shape, and composition affect the activity and determine the most effective active site of metal NPs. Recent research has demonstrated that the CO oxidation activity varies with the size of metallic NPs. The oxidation state of the NPs varies as a function of the size, and the surface oxide layers formed on NPs have been found to be important in enhancing or suppressing CO oxidation. Surface segregation of bimetallic NPs also influences the CO oxidation rate. The NP surfaces undergo significant adsorbate-induced structural changes, as confirmed by various in situ characterization techniques under catalytically relevant CO oxidation conditions. Based on the knowledge of support-induced catalytic properties, various metal-support interactions have been investigated for enhancing CO oxidation. By changing the reaction environments to either CO- or O-rich atmospheres, the synergetic effect at the interfaces of NPs and support oxides have been largely clarified. Versatile nanostructures with confined shells of small metal NPs have also been designed as core@shell-type catalysts. Using in situ characterization techniques combined with well-defined NPs, it is possible to study CO oxidation on the molecular level in order to gain vital mechanistic insights under catalytic working conditions.

Redox variations in the inner solar system with new constraints from vanadium XANES in spinels

![][1] Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known ![Formula][2] . We obtain values, in order of increasing ![Formula][3] , from IW-3 for lodranites and acapulcoites, to diogenites, brachinites (near IW), ALH 84001, terrestrial basalt, hornblende-bearing R chondrite LAP 04840 (IW+1.6), and finally ranging up to IW+3.1 for CK chondrites (where the ![Formula][4] of a sample relative to the ![Formula][5] of the IW buffer at specific T ). To place the significance of these new measurements into context we then review the range of oxygen fugacities recorded in major achondrite groups, chondritic and primitive materials, and planetary materials. This range extends from IW-8 to IW+2. Several chondrite groups associated with aqueous alteration exhibit values that are slightly higher than this range, suggesting that water and oxidation may be linked. The range in planetary materials is even wider than that defined by meteorite groups. Earth and Mars exhibit values higher than IW+2, due to a critical role played by pressure. Pressure allows dissolution of volatiles into magmas, which can later cause oxidation or reduction during fractionation, cooling, and degassing. Fluid mobility, either in the sub-arc mantle and crust, or in regions of metasomatism, can generate values >IW+2, again suggesting an important link between water and oxidation. At the very least, Earth exhibits a higher range of oxidation than other planets and astromaterials due to the presence of an O-rich atmosphere, liquid water, and hydrated interior. New analytical techniques and sample suites will revolutionize our understanding of oxygen fugacity variation in the inner solar system, and the origin of our solar system in general. [1]: /embed/graphic-1.gif [2]: /embed/mml-math-1.gif [3]: /embed/mml-math-2.gif [4]: /embed/mml-math-3.gif [5]: /embed/mml-math-4.gif

TRANSMISSION SPECTROSCOPY OF THE HOT JUPITER WASP-12b FROM 0.7 TO 5 μm

Since the first report of a potentially non-solar carbon-to-oxygen ratio (C/O) in its dayside atmosphere, the highly irradiated exoplanet WASP-12b has been under intense scrutiny and the subject of many follow-up observations. Additionally, the recent discovery of stellar binary companions ~1" from WASP-12 has obfuscated interpretation of the observational data. Here we present new ground-based multi-object transmission-spectroscopy observations of WASP-12b that we acquired over two consecutive nights in the red optical with Gemini-N/GMOS. After correcting for the influence of WASP-12's stellar companions, we find that these data rule out a cloud-free, H2 atmosphere with no additional opacity sources. We detect features in the transmission spectrum that may be attributed to metal oxides (such as TiO and VO) for an O-rich atmosphere or to metal hydrides (such as TiH) for a C-rich atmosphere. We also reanalyzed NIR transit-spectroscopy observations of WASP-12b from HST/WFC3 and broadband transit photometry from Warm Spitzer. We attribute the broad spectral features in the WFC3 data to either H2O or CH4 and HCN for an O-rich or C-rich atmosphere, respectively. The Spitzer data suggest shallower transit depths than the models predict at infrared wavelengths, albeit at low statistical significance. A multi-instrument, broad-wavelength analysis of WASP-12b suggests that the transmission spectrum is well approximated by a simple Rayleigh scattering model with a planet terminator temperature of 1870 +/- 130 K. We conclude that additional high-precision data and isolated spectroscopic measurements of the companion stars are required to place definitive constraints on the composition of WASP-12b's atmosphere.

The effects of sintering atmosphere on microstructures and electrical properties of lead-free (Bi0.5Na0.5)TiO3-based ceramics

0.93(Bi0.5Na0.5)TiO3–0.07BaTiO3 (BNT–7BT) piezoelectric ceramics sintered in O2 and N2 have been investigated to understand the effects of sintering atmosphere in defects, microstructures, and dielectric properties. The BNT–7BT ceramics sintered in O2 and N2 exhibit a major pseudo-cubic structure with slight distortion from the cubic cell. The specimen sintered in N2 atmosphere shows larger grain sizes than the specimen sintered in O2. The X-ray photoelectron spectroscopy (XPS), SEM–EDS, and TEM–EDS suggest that specimen sintered in N2 atmosphere exhibits more ion vacancies. Leakage current measurements suggest more oxygen vacancies for specimen sintered in N2. A higher remnant polarization and lower coercive field were observed for specimen sintered in O2. Strain vs. E field (S–E) loops display higher E-field induced strain up to 0.256% and higher dielectric permittivity for specimen sintered in O2. This work suggests that sintering in the O-rich atmosphere can reduce defects and enhances ferroelectric, dielectric, and piezoelectric properties.

Identification of oxygen and zinc vacancy optical signals in ZnO

Photoluminescence spectroscopy has been used to study single crystalline ZnO samples systematically annealed in inert, Zn-rich and O-rich atmospheres. A striking correlation is observed between the choice of annealing ambient and the position of the deep band emission (DBE) often detected in ZnO. In particular, annealing in O2 results in a DBE at 2.35±0.05eV, whereas annealing in the presence of metallic Zn results in DBE at 2.53±0.05eV. The authors attribute the former band to zinc vacancy (VZn) related defects and the latter to oxygen vacancy (VO) related defects. Additional confirmation for the VO and VZn peak identification comes from the observation that the effect is reversible when O- and Zn-rich annealing conditions are switched. After annealing in the presence of ZnO powder, there is no indication for the VZn- or VO-related bands, but the authors observe a low intensity yellow luminescence band peaking at 2.17eV, probably related to Li, a common impurity in hydrothermally grown ZnO.