Atmospheric Partial Pressure Research Articles

Selective catalytic oxidation of benzene over Cu/Ti/HZSM-5 under low oxygen pressure for one step synthesis of phenol

The direct hydroxylation of benzene to phenol over a Cu impregnated zeolite catalyst (Cu/HZSM-5) is performed in airflow. Phenol is obtained as a main product in a yield of 2.5%, however, the selectivity of phenol is low (44.0%) due to the formation of by-products such as hydroquinone, p-benzoquinone, CO, and CO2. The use of Cu/HZSM-5 leads to a significantly higher selectivity of phenol (94.0%) under an O2 partial pressure that is lower than the atmospheric O2 partial pressure. It is found that excess O2 in the gas-phase causes the formation of by-products, whereas a decrease of O2 partial pressure prevents the by-products from forming. Moreover, the addition of Ti to Cu/HZSM-5 leads to improved phenol formation activity. Good yield (4.3%) and selectivity (88.0%) of phenol are achieved over Cu/Ti/HZSM-5 under a low O2 partial pressure. The addition of Ti is found to accelerate the formation of Cu+ as an active site. It is therefore speculated that phenol can be formed selectively over Cu/Ti/HZSM-5 under lower O2 partial pressure owing to the increase of Cu+ sites by Ti addition and the prevention of by-product formation by the decreased O2 partial pressure.

An Early Pleistocene atmospheric CO2 record based on pedogenic carbonate from the Chinese loess deposits

The tight coupling between temperature and atmospheric CO2 is shown by ice core records for the past 0.8 million years (Myr). However, the modern atmospheric partial pressure of CO2 (pCO2) has exceeded previous interglacial pCO2 levels over the past 0.8 Myr, suggesting that the earlier part of the Pleistocene and Pliocene might be a better analog of today's radiative forcing of CO2. The early Pleistocene experienced a constant cooling characterized by the intensification of northern hemisphere glaciations. Existing pCO2 records developed from marine organic matter and inorganic precipitates, however, disagree with the trends and absolute values of CO2 over this time interval. Here we present quantitative interglacial pCO2 estimates from ∼2.6–0.9 Myr using the stable carbon isotopic compositions of pedogenic carbonates collected from the Chinese Loess Plateau (CLP). Our pCO2 records provide the first documentation of pCO2 from continental sedimentary deposits over the early Pleistocene. The successive decrease of our pCO2 records is broadly consistent with the increase in deep-sea δO18 and the overall decline of sea surface temperature (SST) at this time, but in contrast with the increasing peak interglacial pCO2 recorded in ice cores for the last 0.8 Myr.

Determining the Phase Stability and Oxygen Nonstoichiometry of Lanthanum Strontium Ferrite Structures By Combining Density Functional Theory and Thermodynamics

Introduction: Mixed Ionic Electronic Conducting (MIEC) materials are used in Solid Oxide Fuel Cells (SOFCs), gas sensors, gas separation membranes, etc. [1] One common MIEC material is La1-x Sr x FeO3-δ (LSF), where x is the fraction of Sr occupying the ABO3 perovskite A-site and δ is the oxygen nonstoichiometry. Doping with mixed multivalent A-site cations or varying the La to Sr concentration ratio can change the oxygen nonstoichiometry, which in turn controls the functionality of LSF in these applications. [2] When x is varied from 1 to 0, LSF changes from cubic SrFeO3-δ (SFO) to rhombohedral La0.5Sr 0.5 FeO3-δ(LSF55) to orthorhombic LaFeO3-δ(LFO). [3] While the ultimate goal for many applications is to computationally identify the La to Sr concentration ratio with the optimum oxygen surface exchange rate, the present study focuses on predicting the La1-x Sr x FeO3-δ (LSF) oxygen nonstoichiometry under SOFC operating conditions using density functional theory (DFT)-based defect thermodynamics calculations. All calculations were performed using spin-polarized DFT with a GGA+U functional. The U parameters on Fe were carefully chosen to capture the Fe oxidation state, the Fe magnetic moment, and the phase stability as xwas varied from 1 to 0. Structure stability: Three LSF concentrations were studied using DFT: SrFeO3, LaFeO3, and La0.5Sr 0.5 FeO3. For each concentration, all three crystalline structures, namely, cubic, rhombohedral, and orthorhombic were compared. [3] After structural minimization, the energetically favorable crystalline structures were a cubic perovskite structure with a = 3.88 Å (experimental 3.86 Å) [4] for SrFeO3, a rhombohedral structure with a=5.51 Å (experimental 5.52 Å) [5] for La0.5Sr 0.5 FeO3, and an orthorhombic structure with a=5.58 Å, b=7.93 Å, c=5.66 Å (experimental a=5.55 Å, b=7.86 Å, c=5.56 Å) [6] for LaFeO3. The Magnetic Moment on Fe: The formal oxidation state on iron is considered to be +4 in SrFeO3, +3 in LaFeO3, and a +3/+4 mixture in LSF that varies as x varies. However in the simulations performed here, a typical Bader charge analysis of the electron density distribution was unable to predict the Fe oxidation state in these structures. Hence, as has been done in experiments, Fe magnetic moment calculations were used to determine the Fe oxidation state. Here, a single U-parameter was used for all LSF structures with different x to compare the magnetic moments of Fe in them. The Fe magnetic moment in all three minimized structures (SrFeO3, LaFeO3, and La0.5Sr 0.5 FeO3) were computed with different Ueff parameters to select a suitable value for Ueff. It was found that Ueff =3 best predicted the Fe magnetic moment as x was varied in LSF. The calculated 0 K magnetic moments for Fe were 3.61 μB for SFO, 3.94 μB for LSF55 and 4.23 μB for LFO. These values are in good agreement with earlier experimental measurements. It is evident from the calculations that for defect free structures, the magnetic moment of Fe linearly varies from 3.61 μB to 4.23 μB as the 3+ to 4+ oxidation state varies as xin LSF changes from 1 to 0. The Oxygen Nonstoichiometry: Since intermediate temperature SOFCs operate at 600-800oC, [6] a thermodynamic correction was added to the DFT+U predicted oxygen formation energy. The vacancy formation energy was calculated as a function of the chemical potential of oxygen, µO (T,p), at SOFC operating condition (equations are provided). In these calculations EO2 DFT was the DFT calculated free energy for oxygen. δG0 O2 was the connection energy between 0 K and room temperature (T0), p0 was the atmospheric partial pressure of oxygen. T was the SOFC operating temperature and p was the operating pressure. Based on the oxygen vacancy formation energy, its concentration was predicted as a function of the chemical potential of O2 as well. Conclusions: Different bulk structures with corresponding oxygen nonstoichiometry were predicted under SOFC operating conditions. The relation between magnetic moment and charge state of Fe had been calculated for LSF, with varying composition of x. How the charge state of Fe controls oxygen nonstoichiometry in LSF was predicted from calculations.

Effect of Variable pCO2 on Ca2+ Removal and Potential Calcification of Cyanobacterial Biofilms —An Experimental Microsensor Study

Two different cyanobacterial biofilms from German karstwater creeks were investigated with respect to their photosynthetic effect on Ca2+ removal and potential CaCO3 precipitation in artificial creek waters of different CO2 partial pressures at a given, constant calcite supersaturation. CO2 partial pressures were adjusted to 350 ppmV, 2200 ppmV and 8700 ppmV respectively, covering the range of Phanerozoic atmospheric CO2 partial pressures inferred from palaeosoils, stomatal indices and model calculations. Microsensor measurements of calcium, pH and oxygen revealed differences in the potential to precipitate CaCO3 between the two model organisms Tychonema-relative strain SAG 2388 and Synechococcus sp. strain SAG 2387. Whereas a strong removal of Ca2+ from the solution was measured at Tychonema-relative biofilm, the Synechococcus sp. biofilm exercised a much lower Ca2+ removal during photosynthesis. Photosynthesis was enhanced in both organisms with increasing CO2 and HCO3−, as indicated by enhanced O2 production, but only for the motile filamentous taxon Tychonema-relative a concomitantly increasing calcium removal was measured. However, model calculations indicate that this short-term Ca2+ binding in the Tychonema-relative is due to complexation to exopolymers or oscillin, with no immediate CaCO3 precipitation. In contrast, Ca2+ and pH measurements at Synechococcus sp. biofilm could be consistent with immediate CaCO3 precipitation at the cells. In both biofilms, pH gradients increase with increasing pCO2 from 350 to 2200 ppmV due to enhanced photosynthesis, but decrease at a pCO2 of 8700 ppmV despite of further enhanced photosynthesis. This observation, regardless whether CO2 or HCO3− is used by the cyanobacteria, is in accordance with hydrochemical modeling demonstrating an increased DIC buffering at high pCO2 conditions. These results indicate that the potential of cyanobacteria to form spatially defined calcification pattern via pH gradients at their cell envelopes ('calcified cyanobacteria') increases at elevated pCO2, while at high pCO2 conditions Ca2+ binding and lowered pH microgradients lead to spatially diffuse calcification without defined cell envelope precipitates.

A new positive relationship between pCO2 and stomatal frequency in Quercus guyavifolia (Fagaceae): a potential proxy for palaeo-CO2 levels.

The inverse relationship between atmospheric CO2 partial pressure (pCO2) and stomatal frequency in many species of plants has been widely used to estimate palaeoatmospheric CO2 (palaeo-CO2) levels; however, the results obtained have been quite variable. This study attempts to find a potential new proxy for palaeo-CO2 levels by analysing stomatal frequency in Quercus guyavifolia (Q. guajavifolia, Fagaceae), an extant dominant species of sclerophyllous forests in the Himalayas with abundant fossil relatives. Stomatal frequency was analysed for extant samples of Q. guyavifolia collected from17 field sites at altitudes ranging between 2493 and 4497 m. Herbarium specimens collected between 1926 and 2011 were also examined. Correlations of pCO2-stomatal frequency were determined using samples from both sources, and these were then applied to Q. preguyavaefolia fossils in order to estimate palaeo-CO2 concentrations for two late-Pliocene floras in south-western China. In contrast to the negative correlations detected for most other species that have been studied, a positive correlation between pCO2 and stomatal frequency was determined in Q. guyavifolia sampled from both extant field collections and historical herbarium specimens. Palaeo-CO2 concentrations were estimated to be approx. 180-240 ppm in the late Pliocene, which is consistent with most other previous estimates. A new positive relationship between pCO2 and stomatal frequency in Q. guyavifolia is presented, which can be applied to the fossils closely related to this species that are widely distributed in the late-Cenozoic strata in order to estimate palaeo-CO2 concentrations. The results show that it is valid to use a positive relationship to estimate palaeo-CO2 concentrations, and the study adds to the variety of stomatal density/index relationships that available for estimating pCO2. The physiological mechanisms underlying this positive response are unclear, however, and require further research.

Photoheterotrophic microalgal hydrogen production using acetate- and butyrate-rich wastewater effluent

Microalgal (Micractinium reisseri YSW05) photo-heterotrophic hydrogen production was investigated with using volatile fatty acids (acetate and butyrate) from dark-fermentation effluent. Dilution ratio (effluent: deionized water) of 1:0 at pH 8.0 supported the maximum hydrogen and biomass production, and the highest acetate and butyrate consumption. Microalgal hydrogen production was higher with continuous light condition compared to alternating dark/light cycle. Hydrogen production correlated with both, biomass production and hydrogenase activity under all experimental conditions. Hydrogenase activity and apparent hydrogen production decreased significantly with increasing headspace oxygen. Interestingly substantial hydrogenase activity was observed at almost atmospheric partial pressure of oxygen (19%), indicating that the enzyme could be oxygen tolerant. The present study demonstrated the application of conventional microbial fermentation effluent for microalgae cultivation and hydrogen production, and showed the presence of oxygen tolerant hydrogenase in M. reisseri which might help to establish a novel strategy for biohydrogen technology in atmospheric condition.

Nickel adsorption on chalk and calcite

Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption studies. The goal was to understand the difference in sorption behavior between synthetic and biogenic calcite. Experiments at atmospheric partial pressure of CO2, in solutions equilibrated with calcite and chalk and pH ranging from 7.7 to 8.8, explored the influence of initial concentration and the amount and type of sorbent on Ni uptake. Adsorption increases with increased surface area and pH. A surface complexation model describes the data well. Stability constants for the Ni surface complex are log KNi=−1.12 on calcite and log KNi=−0.43 and −0.50 on the two chalk samples. The study confirms that synthetic calcite and chalk both take up nickel, but Ni binds more strongly on the biogenic calcite than on inorganically precipitated, synthetic powder, because of the presence of trace amounts of polysaccharides and clay nanoparticles on the chalk surface.

Proteomic and metabolomic responses of Pacific oyster Crassostrea gigas to elevated pCO2 exposure.

The gradually increased atmospheric CO2 partial pressure (pCO2) has thrown the carbonate chemistry off balance and resulted in decreased seawater pH in marine ecosystem, termed ocean acidification (OA). Anthropogenic OA is postulated to affect the physiology of many marine calcifying organisms. However, the susceptibility and metabolic pathways of change in most calcifying animals are still far from being understood. To our knowledge, few studies have focused on the responses induced by pCO2 at both protein and metabolite levels. The pacific oyster C. gigas, widely distributed throughout most of the world's oceans, is a model organism for marine environmental science. In the present study, an integrated metabolomic and proteomic approach was used to elucidate the effects of ocean acidification on Pacific oyster C. gigas, hopefully shedding light on the physiological responses of marine mollusk to the OA stress.

Elevation-dependent responses of tree mast seeding to climate change over 45 years.

We use seed count data from a New Zealand mono-specific mountain beech forest to test for decadal trends in seed production along an elevation gradient in relation to changes in climate. Seedfall was collected (1965 to 2009) from seed trays located on transect lines at fixed elevations along an elevation gradient (1020 to 1370 m). We counted the number of seeds in the catch of each tray, for each year, and determined the number of viable seeds. Climate variables were obtained from a nearby (<2 km) climate station (914-m elevation). Variables were the sum or mean of daily measurements, using periods within each year known to correlate with subsequent interannual variation in seed production. To determine trends in mean seed production, at each elevation, and climate variables, we used generalized least squares (GLS) regression. We demonstrate a trend of increasing total and viable seed production, particularly at higher elevations, which emerged from marked interannual variation. Significant changes in four seasonal climate variables had GLS regression coefficients consistent with predictions of increased seed production. These variables subsumed the effect of year in GLS regressions with a greater influence on seed production with increasing elevation. Regression models enforce a view that the sequence of climate variables was additive in their influence on seed production throughout a reproductive cycle spanning more than 2 years and including three summers. Models with the most support always included summer precipitation as the earliest variable in the sequence followed by summer maximum daily temperatures. We interpret this as reflecting precipitation driven increases in soil nutrient availability enhancing seed production at higher elevations rather than the direct effects of climate, stand development or rising atmospheric CO2 partial pressures. Greater sensitivity of tree seeding at higher elevations to changes in climate reveals how ecosystem responses to climate change will be spatially variable.

Thermodynamic assessment and experimental study of sulphidation of ilmenite and chromite

This study provides an assessment of the high temperature thermochemical reactions of ilmenite and chromite with sulphur and uses the information to analyse the possibility of selective sulphidation of chrome-bearing spinels as a new route for chromite removal from ilmenite concentrates. The work includes both systematic thermodynamic assessment and targeted experimental investigations. Thermodynamic calculation results studying the effect of reactants composition, temperature and different sulphur sources (H2S and elemental S) showed that chromite can be selectively sulphidised at a controlled atmosphere of partial pressure of oxygen (pO2) below 10−10 atm and partial pressure of sulphur (pS2) above 10−6 atm. The addition of carbon with sulphur was found to be useful for the chromite sulphidation reaction. The optimum quantity of sulphur reactant to carbon was found to be 3 : 1 (in mole) for 1 mol of chromite. Sulphidation experiments on a mixture of natural ilmenite and chromite at 1100°C for 5 h using 5%H2S as a sulphur source showed that the ilmenite was preferentially sulphidised first, which was also in a good agreement with the thermodynamic assessment. Sulphidation of a naturally occurring chrome spinel contaminated ilmenite concentrate showed that the degree of weathering also played a role in the sulphidation of the chrome spinel. It was also concluded that H2S is not suitable for selective sulphidation of chrome spinel from the ilmenite concentrate and that tightly controlled pS2 and pO2 conditions are required.

Effect of Atmospheric Carbon Dioxide and Strontium Content on Surface Segregation in Strontium Doped Lanthanum Cobalt Iron Oxide Thin Films

This study investigates surface segregation behavior and phase formation in LaxSr1-xCo0.2Fe0.8O3-δ (LSCF), a commonly used cathode material for solid oxide fuel cells (SOFCs). (100)-oriented LSCF thin films with varying ‘x’ values were deposited on (110)-oriented NdGaO3 (NGO) substrates by pulsed laser deposition (PLD). Compositional changes on the surface of LSCF thin films were measured using total reflection x-ray fluorescence (TXRF) technique in real time at 800°C under different CO2 partial pressures. Ex-situ electronic structure measurements were carried on the post-annealed samples using hard x-ray photoelectron spectroscopy (HAXPES). The thin-films were also characterized in cross-section by high-resolution transmission electron microscopy (HRTEM), and composition profiles in the LSCF matrix adjacent to the surface segregated phases were analyzed using energy-dispersed x-ray spectroscopy (EDS). The thermodynamics and kinetics of the segregation and phase formation phenomena as a function of atmospheric CO2 partial pressure and Sr content is discussed in this presentation.

BaCo0.6Fe0.3Sn0.1O3−δperovskite as a new superior oxygen reduction electrode for intermediate-to-low temperature solid oxide fuel cells

BaCo0.6Fe0.3Sn0.1O3−δ (BCFSn631) is evaluated as an oxygen reduction electrode for intermediate-to-low temperature solid oxide fuel cells (SOFCs). XRD and HR-TEM analysis demonstrate that it is in a simple perovskite phase with cubic lattice symmetry. In situ HT-XRD and ex situ XRD confirm the favorable phase stability of the oxide under a wide range of temperatures and atmospheric oxygen partial pressures. The oxygen nonstoichiometry, electrical conductivity, oxygen reduction activity and electrochemical performance of BCFSn631 are systematically studied through thermogravimetric analysis, electrical conductivity relaxation tests and electrochemical impedance analysis. It has a low thermal expansion coefficient of ∼15.8 × 10−6 K−1 in a temperature range of 25–800 °C and a high oxygen vacancy concentration. Electric conductivity relaxation measurement demonstrates the high oxygen surface exchange and bulk diffusion properties of BCFSn631, comparable to that of the state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrode. Low area specific resistances are obtained for the BCFSn631 electrode in the intermediate temperature range, determined by electrochemical impedance spectroscopy based on symmetrical cell configuration, suggesting its high activity for oxygen reduction reaction. Peak power densities of 1168, 896, 523 and 273 mW cm−2 are obtained from a fuel cell with BCFSn631 electrode at 600, 550, 500 and 450 °C, respectively. In addition, good long-term performance stability is demonstrated. All these results highly promise BCFSn631 as an excellent oxygen reduction electrode for next generation SOFCs.

Multichannel mixed‐conducting hollow fiber membranes for oxygen separation

SignificanceA multichannel mixed‐conducting hollow fiber (MMCHF) membrane, 0.5 wt % Nb2O5‐doped SrCo0.8 Fe0.2O3‐δ (SCFNb), has been successfully prepared by phase inversion and sintering technique. The crystalline structure, morphology, sintering behavior, breaking load, and oxygen permeability of the MMCHF membrane were studied systematically. The MMCHF membrane with porous‐dense asymmetrical microstructure was obtained with the outer diameter of 2.46 mm and inner tetra‐bore diameter of 0.80 mm. The breaking load of the MMCHF membrane was 3–6 times that of conventional single‐channel mixed‐conducting hollow fiber membrane. The MMCHF membrane showed a high oxygen flux which was about two times that of symmetric capillary membrane at similar conditions as well as a good long‐term stability under low oxygen partial pressure atmosphere. This work proposed a new configuration for the mixed‐conducting membranes, combining advantages of multichannel tubular membrane technology and conventional hollow fibers. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1969–1976, 2014

The Potential for Biologically Catalyzed Anaerobic Methane Oxidation on Ancient Mars

This study examines the potential for the biologically mediated anaerobic oxidation of methane (AOM) coupled to sulfate reduction on ancient Mars. Seven distinct fluids representative of putative martian groundwater were used to calculate Gibbs energy values in the presence of dissolved methane under a range of atmospheric CO2 partial pressures. In all scenarios, AOM is exergonic, ranging from -31 to -135 kJ/mol CH4. A reaction transport model was constructed to examine how environmentally relevant parameters such as advection velocity, reactant concentrations, and biomass production rate affect the spatial and temporal dependences of AOM reaction rates. Two geologically supported models for ancient martian AOM are presented: a sulfate-rich groundwater with methane produced from serpentinization by-products, and acid-sulfate fluids with methane from basalt alteration. The simulations presented in this study indicate that AOM could have been a feasible metabolism on ancient Mars, and fossil or isotopic evidence of this metabolic pathway may persist beneath the surface and in surface exposures of eroded ancient terrains.

Sulphide oxidation and carbonate dissolution as a source of CO2 over geological timescales

The observed stability of Earth's climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering, but reconstructions of sea-floor spreading do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth's atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses have been put forward to explain the 'Cenozoic isotope-weathering paradox', and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes, atmospheric CO2 partial pressure and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.

Differential effects of ocean acidification on carbon acquisition in two bloom-forming dinoflagellate species.

Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) with very low CO2 affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO2 partial pressure (pCO2) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom-forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO2 (i.e. 180–1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane-inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO2 range, but we observed a number of species-specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO2-dependent contribution of HCO3− uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO2-independent carbonic anhydrase activity. Comparing both species, a general trade-off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO2.

Roles of an oxygen Frenkel pair in the photoluminescence of Bi3+-doped Y2O3: computational predictions and experimental verifications

The photoluminescence intensity of Bi-doped Y2O3 is increased in a high oxygen partial pressure atmosphere due to the healed oxygen vacancy.

Piezoelectric and dielectric properties of Sn-doped (Na0.5K0.5)NbO3 ceramics processed under low oxygen partial pressure atmosphere

Sn-doped (Na0.5K0.5)NbO3 (Sn-NKN) ceramics fired under various oxygen partial pressure (pO2) conditions have been investigated and discussed in terms of bulk piezoelectric and dielectric properties. X-ray diffraction measurements and Rayleigh analysis indicate that the substitution site of the Sn cations depend on the pO2 atmosphere in the firing process. For pO2 higher than 1.0 × 10−10 atm, Sn cations mainly substitute as Sn4+ at the B-site of perovskite NKN, whereas Sn2+ A-site substitution is favored under a low-pO2 atmosphere. Low-pO2 fired Sn-NKN ceramics exhibit higher relative permittivity, Curie temperature, and piezoelectric coefficient (d33). Sn2+ at A-site acts as a donor and reduces the p-type carrier concentrations that result from an electronic compensation of metal vacancies created through the high volatility of Na and K suboxides. The higher piezoelectricity and resistivity in low-pO2 fired Sn-NKN ceramics make this material suitable for base-metal cofired devices such as Ni-inner-electrode multilayer capacitors and actuators.

WATER LOSS FROM TERRESTRIAL PLANETS WITH CO2-RICH ATMOSPHERES

Water photolysis and hydrogen loss from the upper atmospheres of terrestrial planets is of fundamental importance to climate evolution but remains poorly understood in general. Here we present a range of calculations we performed to study the dependence of water loss rates from terrestrial planets on a range of atmospheric and external parameters. We show that CO2 can only cause significant water loss by increasing surface temperatures over a narrow range of conditions, with cooling of the middle and upper atmosphere acting as a bottleneck on escape in other circumstances. Around G-stars, efficient loss only occurs on planets with intermediate CO2 atmospheric partial pressures (0.1 to 1 bar) that receive a net flux close to the critical runaway greenhouse limit. Because G-star total luminosity increases with time but XUV/UV luminosity decreases, this places strong limits on water loss for planets like Earth. In contrast, for a CO2-rich early Venus, diffusion limits on water loss are only important if clouds caused strong cooling, implying that scenarios where the planet never had surface liquid water are indeed plausible. Around M-stars, water loss is primarily a function of orbital distance, with planets that absorb less flux than ~270 W/m^2 (global mean) unlikely to lose more than one Earth ocean of H2O over their lifetimes unless they lose all their atmospheric N2/CO2 early on. Because of the variability of H2O delivery during accretion, our results suggest that many 'Earth-like' exoplanets in the habitable zone may have ocean-covered surfaces, stable CO2/H2O-rich atmospheres, and high mean surface temperatures.

Oxygen Diffusion and Nonstoichiometry in BiFeO3

Leakage current is a serious problem for the use of ferroelectricity in room-temperature multiferroics BiFeO3, and oxygen nonstoichiometry is considered as one of its principal origins. In order to establish a method to control oxygen content in the compound, we investigated the annealing process of stoichiometric BiFeO3 grains in air and revealed that oxygen diffusion occurs in two steps: (1) the weight of the sample decreases in a short time, which originates from the generation of oxygen deficiency near the surface of the grains; and then (2) it increases gradually and slowly, which originates from oxygen diffusion toward equilibrium in the inner part of the grains, introducing excess oxygen there. Step 1 causes the leakage current, and step 2 tends to cause inhomogeneity of oxygen content as well as the leakage current. Steps 1 and 2 are related to oxygen deficiency and excess oxygen often observed in thin films and bulk crystals, respectively. For the synthesis of homogeneous and highly insulating bulk sample, it is important to avoid these annealing processes, and it is a good way to grow a crystal with stoichiometric oxygen content by the control of atmospheric oxygen partial pressure and taking out its inner part.