Function Of Oxygen Content Research Articles

Thermoelectric figure of merit of oxygen-deficient YBCO perovskites

We have performed a study of temperature dependence of thermal conductivity κ ( T ) , Seebeck coefficient S ( T ) and electrical resistivity ρ ( T ) on oxygen-deficient polycrystalline YBa 2 Cu 3 O 7 - δ ( 0 < δ < 1 ) samples. The measurements of S ( T ) , ρ ( T ) and κ ( T ) were carried out as a function of oxygen content in the temperature range between 77 and 300 K, the influence of oxygen content on the transport properties and the dimensionless figure of merit ( ZT) was studied. As the oxygen content is reducing the Seebeck coefficient and the electrical resistivity show a systematic increase while κ ( T ) decreases. ZT ( T ) exhibits an enhancement reaching values close to 0.3 in the samples with low oxygen levels. This behavior suggests that the charge carrier density decreases while the oxygen level decreases and it opens the possibility to use this kind of materials as active thermoelements, which could work at room temperature and below it.

Resonant inelastic X-ray scattering applied to the electronic structure of strongly correlated systems: The YBCO case

Resonant inelastic soft X-ray scattering is a developing technique well suited to the study of correlation effects in complex materials. We briefly describe its potential for materials science and illustrate its sensitivity with a study of elementary excitations at Cu sites in YBa2Cu3O7−δ as a function of oxygen content. High resolution measurements at the Cu 2p edge reveal structure that, with the help of theoretical models, can be interpreted as due to d–d excitations of the type 3d (x2−y2) to 3d (3z2−r2), Zhang–Rice singlets, and changes in the Cu spin configuration.

Distortion of ReO6 octahedron in the Hg0.82Re0.18Ba2Ca2Cu3O8+d superconductor

Rhenium (Re) LIII edge X-ray absorption spectroscopy was used in order to determine rhenium valence and the local oxygen coordination in Hg0.82Re0.18Ba2Ca2Cu3O8+d polycrystalline samples prepared with three different oxygen content. Thermoelectric power measurements confirmed small increments of charge carrier number as a function of oxygen content. The X-ray absorption near-edge spectroscopy (XANES) analysis showed a valence variation for Re (+6.8, +6.9 and +7.0) in these samples. The extended X-ray absorption fine structure (EXAFS) analysis indicated that the oxygen local order around Re atoms in these Hg0.82Re0.18Ba2Ca2Cu3O8+d samples can be described as a distorted ReO6 octahedron with two different Re–O bound lengths. The valence evaluated by the bound length in the distorted ReO6 octahedron is in agreement with the XANES measurements. Moreover, the distorted ReO6 octahedron and the Cu–Opl angle built a scenario which can justify the high intrinsic term value found in the optimal doped sample under external hydrostatic pressure.

Neutron scattering and gravimetric study ofBI2Sr2CaCu2O8+x high-temperature superconductors

We analysed some of the structural properties ofBI2Sr2CaCu2O8+x superconductors using neutron scattering and intelligent gravimetric analyser instruments from roomtemperature to 750 °C. Both of the experiments showed that the oxygen content of the compound is temperaturedependent and decreases as the temperature increases. Neutron experiment results showedthat the lattice parameters of the compound are a function of oxygen content and theyincrease as the oxygen content decreases.

Influence of Oxygen Content on the Mechanical Properties of Titanium-35Niobium-7Zirconium-5Tantalum Beta Titanium Alloy

Abstract Titanium-35Niobium-7Zirconium-5Tantalum beta titanium alloy (also known as TiOsteum® beta titanium alloy) is a new metallic biomaterial that was designed to have outstanding osseointegratabilty, good mechanical strength, improved ductility, very low elastic modulus, and good hot and cold workability [1]. In this study, mechanical property data are presented from several TiOsteum alloy production lots with ingot oxygen contents ranging from 0.05 to 0.68 %. Additionally, mill annealed mechanical property data collected over the last 10 to 15 years for the more common alpha, alpha-beta, and beta titanium alloys are presented as a function of oxygen content and compared to TiOsteum alloy. The influence of oxygen as an interstitial strengthening element is well established in titanium alloys. This study examines this effect over a broad range of oxygen values, and has significance for the application of the new TiOsteum alloy and other beta titanium alloys being considered for use in medical and surgical devices.

Interplay of doping and structural modulation in superconductingBi2Sr2−xLaxCuO6+δthin films

We have studied the evolution of the structural modulation in epitaxial, $c$-axis-oriented, ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ thin films when varying the La content $x$ and for a given $x$ as a function of oxygen content. A series of thin films with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.8$ has been prepared in situ by rf-magnetron sputtering and characterized by $R(T)$ measurements, Rutherford backscattering spectroscopy, transmission electron microscopy, and x-ray diffraction techniques. The oxygen content of each individual film was varied by thermal annealing across the phase diagram. The evolution of the structural modulation has been thoroughly studied by x-ray diffraction in determining the variation of the amplitude of satellite reflections in special two axes $2\ensuremath{\theta}∕\ensuremath{\theta}\text{\ensuremath{-}}\ensuremath{\theta}$ scans (reciprocal space scans). It is shown that the amplitude of the modulation along the $c$ axis decreases strongly when $x$ increases from 0 to 0.2. It is demonstrated that this variation is essentially governed by La content $x$ and that changing the oxygen content by thermal treatments has a much lower influence, even becoming negligible for $x&gt;0.2$. Such study is important to understand the electronical properties of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ thin films.

Local electronic structure, O–T phase transition and oxygen content in Y 0.8Ca 0.2Ba 2Cu 3O y ( y = 6.84 − 6.32) high- Tc superconductors

Y 0.8Ca 0.2Ba 2Cu 3O y samples with large range of oxygen content, have been systematically studied by means of positron annihilation technology, transport property measurements and X-ray diffraction. The positron lifetime parameters have a strong dependence on oxygen content y, and show evident changes during orthorhombic–tetragonal phase transition ( y = 6.50 ± 0.05). The local electron density n e and vacancy concentration C v are evaluated as a function of oxygen content. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. n e mainly has an effect on high- T c superconductivity, no direct evidence shows that it is related to the O–T phase transition; and C v is mainly caused by oxygen vacancy. The small increase of T c with decreasing y around y ∼ 6.80 is due to the charge carriers over-doping, which is confirmed by the change of n e in this work.

Light-induced modification of a-SiOx II: Laser crystallization

Amorphous hydrogenated silicon suboxides (a-SiOx:H) were deposited by plasma enhanced chemical vapor deposition from the source gases SiH4, H2, and CO2. The band gap of the samples can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at. %. H-effused samples were irradiated by ultraviolet laser pulses with intensities up to 480 mJ/cm2. The structural changes and the crystallization behavior were investigated as a function of oxygen content and laser intensity. A decrease of the melting threshold by a factor of two with increasing oxygen content (0–44 at. %) was observed for the SiOx samples. Above the respective melting thresholds, not only a deterioration of the structural properties but also indications of a segregation of Si crystallites were found. Raman spectroscopy and transmission electron microscopy gave evidence for the existence of Si crystallites up to oxygen contents of 40 at. %. The crystal size reached an optimum for oxygen concentrations between 10 and 30 at. %.

Antiferromagnet-ferromagnet and structural phase transitions inLa0.88MnOxmanganites

The crystal and magnetic structure as well as elastic, magnetic, and electrotransport properties of ${\mathrm{La}}_{0.88}{\mathrm{MnO}}_{x}$ $(2.82l~xl~2.96)$ manganites have been investigated as a function of oxygen content by x-ray, neutron diffraction, Young's modulus, magnetization, and resistivity measurements. The crystal structure of the compounds has been found to be orthorhombic at $xl2.91$ and monoclinic at $xg~2.91.$ The gradual transition from an orbitally ordered state to an orbitally disordered one is observed with increasing temperature or oxygen content. The magnetic properties of the samples have been found to correlate with the type of orbital state. The dynamic orbital correlations favor ferromagnetic state, while A-type antiferromagnetic state is typical for the static Jahn-Teller distortions. The orthorhombic compounds have been found to exhibit a semiconducting behavior, and show a second-order transition into paramagnetic state. The metallic conductivity appears starting from the $x=2.92$ monoclinic compound and, simultaneously, a first-order paramagnet-ferromagnet magnetic phase transition is observed. The properties are discussed in the framework of structurally driven phase separation model.

Magnetic phase diagram of the La0.88MnOx(2.82 ≤ x ≤ 2.96)system

The crystal structure and elastic and magnetic properties of the La0.88MnOx(2.82 ≤ x ≤ 2.96)series as a function of oxygen content have been investigated. Thecrystal structure of the samples has been found to be orthorhombic atx < 2.91 and monoclinicat x ≥ 2.91depending on the oxygen concentration. Both crystalstructure and elastic properties indicate the presence of static(x < 2.87) or dynamic(x ≥ 2.87)Jahn–Teller distortions under the whole range of doping. The strong correlationbetween the type of orbital state and magnetic properties of the samplesis revealed. It is shown that compounds with an antiferromagneticcomponent have features of orbital ordering while ferromagnetic samplesare orbitally disordered. The magnetic phase diagram of the La0.88MnOx(2.82 ≤ x ≤ 2.96)system is proposed on the basis of results obtained.

Effects of oxygen content on the properties of the Hg0.82Re0.18Ba2Ca2Cu3O8+d superconductor

Mercury superconductor samples doped with rhenium, Hg0.82Re0.18Ba2Ca2Cu3O8+d, were produced in a very narrow range of oxygen content (d = 0.05, 0.10 and 0.15). The lattice parameters indicated a slight change of the unitary cell as a function of the oxygen content. Furthermore, the reduction of the volume cell was associated with the slight increment of TC. The measurements of χac versus temperature, under external hydrostatic pressure, have shown sensitive results as a function of distinct oxygen content. In our opinion, this kind of measurement may be used as a tool to determine the oxygen content (underdoped, optimal or overdoped) of samples. For dTC/dP > 1.9 K GPa−1 the sample presents an underdoped oxygen content. One can understand that dTC/dP = (1.9 ± 0.2) K GPa−1 is associated with an intrinsic term value for the optimal oxygen doped sample, Hg0.82Re0.18Ba2Ca2Cu3O8.10.

Effect of oxygen on the optical properties of small silicon pyramidal clusters

Optical absorption and light emission of oxygen-incorporated small silicon $({\mathrm{Si}}_{30}{\mathrm{H}}_{40}{\mathrm{O}}_{i})$ pyramidal clusters as a function of oxygen content were theoretically studied using the self-consistent semiempirical molecular orbital method (modified neglect of diatomic overlap---parametric method 3). In the absolute majority of the cluster configurations with low oxygen content $(i&lt;4)$ the excitations have a strong localized character and result in a significant shift from its equilibrium ground state position of one of the silicon atoms inside the cluster volume. The optical transition energies in those cases range from 2.05 to 2.35 eV. The typical Stokes shift for these structures is of the order of 100 meV. However, for some particular cluster configurations the excitations are localized at silicon sites directly adjacent to embedded oxygen atoms and this results in a considerable reduction of the emission energy down to approximately 1.40--1.60 eV and in an increase of the Stokes shift values to 600--800 meV. The same behavior was traced out for the case where the presence of a silanone $(\mathrm{Si}=\mathrm{O})$ bond at the surface of the ${\mathrm{Si}}_{30}{\mathrm{H}}_{38}\mathrm{O}$ cluster is considered. For intermediate and high oxygen content $(i&gt;4)$ structures, a wide spread of the optical transition energies ranging from 1.60 to 3.00 eV is observed, due to the competition between two opposite tendencies. According to the first one, connected with the increasing of the quantum confinement effects due to oxidation, the optical transition energies tend to increase, whereas the enhanced possibility of involvement of oxygen or oxygen-adjacent silicon atoms in the process tends to decrease the energy transitions.

Fabrication of UV-transparent SixOyNz membranes with a low frequency PECVD reactor

SixOyNz ,S i 3N4 and SiO2 films deposited on silicon in a low frequency PECVD reactor (187.5 kHz) from a SiH4 ,N 2O and/or NH3 gas mixture are always in compressive mechanical stress state after deposition because of ion bombardment of the growing film during deposition. The stress value is, as expected, a decreasing function of oxygen content. It is demonstrated that by reducing drastically the deposition temperature down to 200 8C, stoechiometric silicon nitride films and silicon oxynitride films (0 � O=Si � 1:8) with a low tensile stress can be obtained after high temperature annealing (800 8C). Rutherford backscattering experiments (RBS), elastic recoil detection analyses (ERDA) and infrared spectrometry measurements show that this stress variation is related to the initial composition, to hydrogen desorption from Si‐H, N‐H and O‐H bonds and to cross-linking. By controlling the initial hydrogen concentration with the deposition temperature, the stress value after annealing can be tuned precisely. This simple process was applied to the fabrication of flat dielectric membranes with a thickness in the range of 90‐400 nm by KOH bulk micromachining. # 2002 Elsevier Science B.V. All rights reserved.

Soot and NO formation in methane–oxygen enriched diffusion flames

NO and soot formation were investigated both numerically and experimentally in oxygen-enriched counterflow diffusion flames. Two sets of experiments were conducted. In the first set, the soot volume fraction was measured as a function of oxygen content in the oxidizer jet at constant strain rate (20 s −1). In the second set of experiments, the soot volume fraction was measured as a function of strain rate variation from 10 to 60 s −1 and at constant oxygen content on the oxidizer side. A soot model was developed based on a detailed C 6 gas phase chemistry. The soot and molecular radiation were taken into account. Numerical results were verified against experimental data. The soot volume fraction was predicted with the maximum discrepancy less than 30% for all cases considered. It was found that oxygen variation significantly modified the diffusion flame structure and the flame temperature, resulting in a substantial increase of soot. The temperature increase promotes aromatics production in the fuel pyrolysis zone and changes the relative contributions of the thermal and Fenimore mechanisms into NO formation. As the strain rate increases, the residence time of incipient soot particles in the high temperature zone is reduced and the total amount of soot decreases. High concentration of soot in the flame leads to enhancement of radiant heat exchange: the reduction of temperature due to radiation was found to be between 10 and 50 K. This caused a reduction of peak NO concentrations by 20%–25%. The increase of oxygen content in the oxidizer stream resulted in a reduction of the distance between the plane of the maximum temperature and the stagnation plane.

Influence of oxygen deficiency and Nd/Ba substitution on critical currents in Nd 1+xBa 2−xCu 3O y single crystals

Single crystals of the solid solution Nd 1+xBa 2−xCu 3O y have been grown from BaO-CuO flux in ZrO 2/Y crucibles. Various Nd contents could be achieved by growing in different oxygen partial pressures or by subsequent thermal treatments, which led to a spinodal decomposition. To suppress the influence of oxygen defects, first all crystals have been high-pressure oxidized several times. In a second series, the oxygen content of stoichiometric crystals have been reduced step by step by annealing in 1 bar oxygen at different temperatures. The peak effect of the critical current and the irreversibility field are studied as a function of oxygen content, Nd/Ba ratio, and Nd/Y substitution of the crystals. The influence of these different defects and their combination on the optimization of the critical current density is discussed.

Hall effect in Nd 1.85Ce 0.15CuO y with controlled oxygen content

Nd 1.85Ce 0.15Cu 1.01O y ceramic samples with accurate control of the oxygen content have been prepared to study their electronic properties in comparison with their superconducting behavior. The oxygen content has been varied in the range 3.995 < y < 4.007 by annealing under controlled oxygen partial pressure and subsequent quenching. The samples change from superconducting for the lowest oxygen content (Tc= 20K) to non-superconducting for the highest y value. The normal state Hall effect presents a negative R H coefficient in all the superconducting samples. The Hall effect shows an almost temperature independent behavior in samples with the highest values of T C. They also show no significant change in R H values as a function of oxygen content. It indicates that, in this range of low oxygen concentration, the improvement of superconducting properties by reducing the oxygen content is produced by structural ordering and not by change in the carrier density.

A model of oxygen ordering in YBa 2Cu 3O x. Fragmented-chain structure at 6.5< x

We proposed a two-dimensional Ising model for oxygen ordering in YBa 2Cu 3O x material in which an effective O–O interaction energy with Cu(1) ion in between, besides the occupation of the two oxygen sites, also depends on occupancy of the surrounding oxygen sites. We have shown that the Hamiltonian of the model reduces to that of the anisotropic next-nearest-neighbor Ising (ASYNNNI) model with additional term for three-atom interaction (characterized by the parameter ω). We have shown that, for ω>| V 2|/4 (where V 2<0 denotes copper mediated O–O interaction of the ASYNNNI model), the model includes all three major structural phases, tetragonal, OI, and OII, as ground states at x=0, x=6.5, and x=7.0, respectively, while at off-stoichiometry 6.5< x<7, the model stabilizes the so-called fragmented-chain structure of Zaanen et al. [J. Zaanen, A.T. Paxton, O. Jepsen, O.K. Anderson, Phys. Rev. Lett. 60 (1988) 2685]. We calculated the fractions of 2-fold ( n 2), 3-fold ( n 3) and 4-fold ( n 4) coordinated Cu(1) ions as a function of oxygen content x at constant low temperatures (∝300 K) and obtained characteristic plateaus in n 2( x) and n 4( x), while n 3( x) is very large (∝0.36) for x=6.75. We discussed the impact of the fragmented chain statistics on charge transfer process from Cu(1)–O(1) chains to superconducting CuO 2 sheets and its relevance for occurrence of 60 and 90 K plateaus in T c versus x dependence.

HRTEM and ELNES analysis of polycarbosilane-derived Si–C–O bulk ceramics

A series of analyses by high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) has been carried out to investigate the nanostructure and the chemical bonding of Si–C–O bulk ceramics prepared by pyrolysis of oxygen-controlled polycarbosilane (PCS). HRTEM revealed that the Si–C–O ceramics with higher oxygen content tended to keep an amorphous microstructure. EELS allowed the examination of energy-loss, near-edge-structures (ELNES) that detected clear transition signals of the chemical bonding around Si atoms from Si–C bonding to Si–O bonding with an increase of oxygen content. The microstructure of the PCS-derived Si–C–O bulk ceramics is characterized as a function of oxygen content.

Effect of oxygen content on the magnetic state of La0.5Ca0.5MnO3−γ perovskites

The magnetization and magnetoresistance are studied in La0.5Ca0.5MnO3−γ as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ⩾0.04), from the ferromagnetic to a spin-glass state (γ ⩾0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ⩾0.25). Strongly reduced samples (γ⩾0.25) show a large magnetoresistance despite the absence of Mn3+–Mn4+ pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ⩾0.25) are long-range ordered.

The effect of oxygen reduction on the magnetic properties of REBa 2Cu 3O 6+ x

We report on μSR measurements probing the dynamic magnetic properties of HoBa 2Cu 3O 6+ x and GdBa 2Cu 3O 6+ x powder samples with different oxygen content x. While the longitudinal field relaxation rate in GdBa 2Cu 3O 6+ x does not indicate a change of the dynamics of the Gd magnetic moments as function of oxygen content, there is a strong dependence of the dynamics of the Ho magnetic moments in HoBa 2Cu 3O 6+ x .