Oxygen-poor Condition Research Articles

Energetic, Optical, and Electronic Properties of Intrinsic Electron-Trapping Defects in YAlO3: A Hybrid DFT Study

The formation energies of cation antisite defects (YAl and AlY), oxygen vacancies (VO), and nearest-neighbor defect complexes (YAl–AlY and YAl–VO) in various charge states in the YAlO3 crystal are calculated using density functional theory (DFT) with a modified PBE0 hybrid functional containing 32% Hartree–Fock (HF) exchange. It is found that the formation of YAl is more energetically favorable than AlY under oxygen-poor condition, consistent with the fact that the latter was not observed in experiments. On the basis of calculated optical transition energies associated with the excitons trapped at YAl, VO, and YAl–VO, the two emission bands observed under excitonic excitation at low temperature are identified. Electronic properties of YAl–VO complexes in the neutral and singly negative charge states are finally investigated. It shows that the extra electron added into the negative charge state is mainly localized at 4d orbitals of YAl with a two-component feature of its density distribution extending axia...

Experimental Study on Catalytic Combustion Parameters of Gasoline Vapor in Oxygen-Poor Condition

This paper studied gasoline vapor combustion catalyzed by Pd/Al2O3 in oxygen-poor condition. By adjusting the gasoline vapor and oxygen, the inlet temperature and flow rate, collecting data of temperature difference between outlet and inlet as well as the change of gas mixture, the study analyzed the various factors in the catalytic combustion process, and optimized the process control parameters. The results indicated that catalytic combustion was more efficient at the ignition temperature of 274 °C, burned 50% of gasoline vapor in oxygen-poor condition (O2 fraction was 12%).

Selection rule of preferred doping site for n-type oxides

Using first-principles calculations and analysis, we show that to create shallow n-type dopants in oxides, anion site doping is preferred for more covalent oxides such as SnO2 and cation site doping is preferred for more ionic oxides such as ZnO. This is because for more ionic oxides, the conduction band minimum (CBM) state actually contains a considerable amount of O 3s orbitals, thus anion site doping can cause large perturbation on the CBM and consequently produces deeper donor levels. We also show that whether it is cation site doping or anion site doping, the oxygen-poor condition should always be used.

Oxygen vacancy in LiTiPO 5 and LiTi 2(PO 4) 3: A first-principles study

The structures of LiTiPO 5 and LiTi 2(PO 4) 3, as well as the possibility of oxygen vacancies formation in the systems are studied by first-principles calculations. It is found that oxygen vacancies can be formed in LiTiPO 5 and LiTi 2(PO 4) 3 under oxygen poor condition. The formation of oxygen vacancies introduce a defect band within their band gaps, which is expected to improve the electronic conductivity of LiTiPO 5 and LiTi 2(PO 4) 3 significantly. Meanwhile, a great concentration of oxygen vacancies may increase the discharge voltage of LiTiPO 5 and LiTi 2(PO 4) 3.

FIRST-PRINCIPLES INVESTIGATION OF STRUCTURAL AND ELECTRONIC PROPERTIES OF THE RECONSTRUCTED ZnO$(000\bar 1)$ and $(0001) (\sqrt{3}\times\sqrt{3})-R30^\circ$ SURFACES

The surface O or Zn vacancy of ZnO is extremely important, so in this paper we have studied the structural and electronic properties of the ZnO [Formula: see text] O -polar surface with O vacancy and [Formula: see text] Zn -polar surface with Zn vacancy comparatively using the first-principles calculations. For the former case, the topmost oxygen atoms relax outward and the angle between Zn – O – Zn bonds in the topmost double-layer decreases to 105.01° with respect to the unrelaxed surface. However, for the latter case, the topmost zinc atoms relax inward and the angle between O – Zn – O bonds in the topmost double-layer increases to 117.61° with respect to the unrelaxed surface. Both the two reconstructed surfaces become semiconducting property in contrast to the metallic property of the unreconstructed surface. The calculated surface vacancy formation energy [Formula: see text] indicates that the surface with O vacancy is slightly more easier to be formed compared to the surface with Zn vacancy. By employing the technique of ab initio atomistic thermodynamics, we calculated the surface Gibbs free energy to determine the lowest-energy structure, indicating that the surface with O vacancy is more stable than the surface with Zn vacancy under the oxygen poor condition.

Modulation of carrier density in ZnO nanowires without impurity doping

ZnO nanowire based field effect transistor devices show the distinct performance depending on whether comparably oxygen-rich or oxygen-poor conditions were used for nanowire growth. Higher on-state current flows through the ZnO nanowire channel grown under oxygen-poor condition. A possible origin of this characteristic is discussed based on a photoluminescence analysis of the nanowire samples. The observed effect should be taken into account for ZnO nanowire based devices and applications.

The origin of p-type conduction in (P, N) codoped ZnO

P monodoped and (P, N) codoped ZnO are investigated by the first-principles calculations. It is found that the substitutional P defect at O site (PO) and interstitial P (Pi) contribute little to the p-type conductivity of ZnO samples under equilibrium condition. Zinc vacancies (VZn) and PZn-2VZn complex are demonstrated to be shallow acceptors with ionization energies around 100 meV, but they are easily compensated by PZn defect. Fortunately, PZn–4NO complexes may have lower formation energy than that of PZn under Zn-rich condition by proper choices of P and N sources. In addition, the neutral PZn–3NO passive defects may form an impurity band right above the valence-band maximum of ZnO as in earlier reported (Ga,N) or (Zr,N) doped ZnO. This significantly reduces the acceptor level of PZn–4NO complexes and helps improving the p-type conductivity in ZnO. It is suggested that a better (P, N) codoped p-type ZnO could be obtained under oxygen-poor condition.

Vitreoscilla hemoglobin aids respiration under hypoxic conditions in its native host

When Vitreoscilla were grown in medium containing 60mM sodium nitrite under both normal and limited aeration conditions, the levels of Vitreoscilla hemoglobin (VHb) were decreased by greater than 90%, while the levels of the terminal respiratory oxidase, cytochrome bo, were increased 350% under normal aeration and 7-23% under limited aeration. Cytochrome function, as measured by both NADH and ubiquinol oxidases for cells grown under both conditions, increased in parallel (by 150-222% and 8-56%, respectively, for the two activities). Nitrite in the medium inhibited Vitreoscilla growth at both normal and limited aeration. The inhibition of VHb at 60mM nitrite decreased whole cell respiration to the greatest degree in stationary phase for growth in limited aeration conditions, which was the most oxygen poor condition tested. These results are consistent with the originally proposed role for VHb, as an aid to respiration under hypoxic conditions.

Study on anomalous n-type conduction of P-doped ZnO using P2O5 dopant source

The unexpected n-type conduction observed in P-doped ZnO thin films fabricated from rf magnetron sputtering, was studied systematically through a combined approach of experiment and computer modeling. The carrier stability was predicted from first-principles density functional theory and chemical thermodynamic calculations. It demonstrated that, under oxygen-poor growth condition and low temperature, the stable doping defect PO−1 may have negative effect on n-type conduction and, under oxygen-poor growth condition and high temperature, the stable doping defect may contribute significantly to the n-type conduction. Furthermore, under oxygen-rich growth condition, the stable doping defect PZn1 may help to maintain the n-type conduction at high oxygen partial pressures. Our model predictions are in good agreement with experimental observations in anomalous conduction of P2O5-doped ZnO thin films and provide scientific explanation. This research not only revealed increased fundamental understanding on electronic behaviors but also provided a fabrication strategy for P-doped n-type ZnO.

Multicenter study on the clinical value of fetal pulse oximetry: I. Methodologic evaluation

Objective: Our purpose was to evaluate the feasibility of intrapartum fetal pulse oximetry, the distribution of fetal oxygen saturation values, and the relationship with the neonatal outcome in a population with an abnormal fetal heart rate. Study Design: A prospective multicenter observational study was performed from June 1994 to November 1995. Fetal oxygen saturation was continuously recorded with use of a Nellcor N-400 fetal pulse oximeter in case of an abnormal fetal heart rate during labor. Simultaneous readings of fetal oxygen saturation and fetal blood analysis were obtained at inclusion and before birth. Feasibility, adverse effects, distribution of fetal oxygen saturation values, and relationship with neonatal outcome were assessed. Results: One hundred seventy-four patients were included. From 172 attempted sensor placements, the procedure was impossible in three cases and fetal oxygen saturation values were obtained in 164 cases (95.3%). Physicians considered sensor placement an easier task than an attempt at fetal blood analysis (easy in 87.5% vs 78.9% for fetal blood analysis, p = 0.03). The mean reliable signal time (±SD) was 64.7% ± 32% during the first stage. There were no serious adverse effects in the study population. The mean fetal oxygen saturation during the first stage of labor was 42.2% ± 8.0% (10th to 90th percentile range 30% to 53%). Fetal oxygen saturation was significantly correlated with scalp pH ( r = 0.29, p = 0.01) but not with neonatal umbilical artery pH or gas values. There was a significant association between low fetal oxygen saturation (<30%) and poor neonatal condition. Conclusion: The feasibility of fetal pulse oximetry is satisfactory in clinical practice. It is easy to use and provides a fair rate of recorded values, even in a population with suspicion of fetal distress. A low fetal oxygen saturation is significantly associated with an abnormal neonatal outcome.

Temperature dependence of thin film metal/MoO 3/metal capacitors

Thin film capacitors of MoO 3 prepared in a vacuum of the order of 10 −6 Torr are found to exhibit maxima in their capacitance versus temperature characteristics and a monotonic rise in the conductance versus temperature characteristics. These results are compared with those previously reported for oxygen-rich MoO 3 capacitors. In the case of the oxygen-rich samples the experimental results were analysed using an equivalent circuit consisting of impedances representing the metal-insulator interfaces and the interior bulk of the metal/insulator/metal capacitors. Using the same model, but taking into account the oxygen-poor condition of the present films, the experimental results are shown to be consistent with the theoretical results.