Equilibrium Oxygen Partial Pressure Research Articles

Experimental Studies on the Sulfide Capacities of CaO-SiO2-CrOx Slags

To understand the desulfurization process during the refining of Cr-containing steel grades, this work was initiated to study the reactions between Cr-sulfur and chromium-containing slags. The sulfide capacities of CaO-SiO2-CrOx pseudo-ternary slags were measured using the traditional gas-slag equilibration technique between 1823 K and 1923 K. Sixteen different slag compositions were examined, and two different equilibrium oxygen partial pressures were used to understand the impact of the varying valence of Cr on the sulfide capacities. The results showed that log10 Cs varied linearly with the reciprocal T, and the slope was higher than the corresponding value reported for the binary CaO-SiO2 of corresponding composition. It was difficult to isolate the relative effects of the bi- and trivalent Cr in the slags because the Cr2+/Cr3+ ratio was influenced by the basicity of the slag. By using the equation developed by these authors earlier that related Cr2+/Cr3+ with basicity, oxygen partial pressure, and temperature, it was possible to obtain an approximate trend of the CrO effect on the sulfide capacities; viz. the sulfide shows a decreasing trend as Cr2+ replaces Ca2+ in the slag. With a continued increase of Cr2+ content, indications of the occurrence of a minimum point were observed; beyond which the sulfide capacities showed a slight increasing trend. The latter was attributed, based on slag-structure analysis by Gaskell et al., to the increasing extent of the polymerization reaction releasing oxygen ions for sulfide reactions.

Oxygen partial pressure dependence of surface tension and its temperature coefficient for metallic melts: a discussion from the viewpoint of solubility and adsorption of oxygen

Surface tension of molten Si, Ag, and Fe–18Cr–8Ni alloy (Type-304 stainless steel) were measured as a function of the temperature and oxygen partial pressure of an ambient atmosphere by an oscillating drop method using electromagnetic levitation, which assures measurement over a wide range of temperatures below and above the melting point and of oxygen partial pressure. For molten silicon, the lower limit of measured temperature was determined by equilibrium oxygen partial pressure for SiO2 formation; as oxygen partial pressure increases, undercooled conditions necessary for taking measurements become smaller. This is attributed to the low solubility of oxygen in molten silicon. For Ag, however, surface tension can be measured in a wide range of temperature and oxygen partial pressures due to the high solubility of oxygen. A boomerang-shaped behavior of surface tension was observed for Ag in an atmosphere with high oxygen partial pressure; surface tension has a maximum value when plotted against temperature. This boomerang-shaped behavior is attributed to desorption of oxygen at high temperature; surface tension increases with increasing temperature. The boomerang-shaped behavior was also observed for Fe–18Cr–8Ni alloy.

Effects of dopants and oxygen non-stoichiometry on thermodynamic and magnetic properties of micro- and nanostructured perovskite type manganites

The aim of the paper is to evidence the effect of the composition and oxygen stoichiometry on the thermodynamic, spin dynamics and magnetic properties of some micro- and nanostructured substituted manganites La1–xAxMn1–yByO3–δ (A=Ca; B=Co, Fe; x=0·33; y=0, 0·02 and 0·04). The partial molar thermodynamic properties of oxygen dissolution in the perovskite phase as well the equilibrium partial pressures of oxygen have been obtained using solid electrolyte electrochemical cells [electromotive force measurement (EMF)] method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. Using electron paramagnetic resonance spectroscopy, the spin dynamics has been investigated. Particular attention was devoted to the discussion concerning the influence of doping on polaron activation energy Ea and exchange coupling integral J between Mn spins. The results were correlated with the variation of both saturation magnetisation Ms and Curie temperature Tc. The data were discussed in relation to significant changes in the overall concentration of defects. New features related to the modifications in properties connected with the nanocrystalline state were evidenced.

Influence of Sn-substitution on temperature dependence and magnetic disaccommodation of manganese–zinc ferrites

In this paper, the effects of Sn-substitution on temperature dependence and magnetic disaccommodation of manganese–zinc ferrites were investigated. Toroidal cores were prepared by the conventional ceramic process and sintered at 1360 °C for 4 h in atmosphere controlled by using the equation for equilibrium oxygen partial pressure. The experimental results show that the substitution of Sn 4+ in manganese–zinc ferrites can influence the thermal stability and disaccommodation remarkably. Secondly, the temperature dependence of the initial permeability μ i and disaccommodation of Sn-substitution manganese–zinc ferrites have an internal relationship. The experimental results are explained and compared with those of Ti-substitution manganese–zinc ferrite.

Development of Agile Titania Sensors Via High‐Temperature Reductive Etching Process (HiTREP©): I. Structural Reorganization

High selectivity, enhanced sensitivity, short response times and long shelf‐life are much sought‐after features in solid‐state chemical sensors for the detection and metering of gas(es) of interest. Because the sensing mechanism of semiconducting oxides is invariably surface dominated, benign microscopic features are desirable to realize a useful sensing material. In principle, such morphological features could be incorporated in a number of semiconducting oxides by employing a technique based on thermodynamic consideration of the metal/metal oxide coexistence. By dynamically modulating the equilibrium oxygen partial pressure across the metal/metal oxide proximity line, renewed formulation and growth of an oxide surface on an atomic/submolecular level with exotic morphological features under conditions of oxygen “deprivation” or “enrichment” has been achieved practically in a number of potential ceramic sensor systems. In the case of oxides that are not amenable to such classical oxygen partial pressure modulation, a novel high‐temperature reductive etching process (HiTREP©) could be exploited to recreate the smart nanofeatures to impart the desired accentuation effect. This surface modification method was applied to a new commercially available aqueous plasma electro‐deposited (PED) titania thick film, and the microscopic results of this strategy are presented.

Influence of Composition and Particle Size on Spin Dynamics and Thermodynamic Properties of Magnetoresistive Perovskites

The thermodynamic behavior and spin dynamics of the colossal magnetoresistive (CMR) perovskites of general formula La(1-x)(A)xMn(1-y)(B)yO3 (where A is an alkaline earth, and B = Al, In) have been studied in order to evidence the effect of composition and the influence of nanocrystallinity on the thermodynamic and magnetic characteristics. By using electron paramagnetic resonance (EPR) spectroscopy, the behavior of the exchange coupling integral (J) between Mn spins and the polaron activation energy (Ea) have been investigated. The thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained by using solid electrolyte electrochemical cells method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with measurements of the electromotive force (EMF). The results were correlated with the average Mn valence values as determined by redox titration. The properties of the rare-earth manganites are strongly affected by the A- and B-site substitution and by the oxygen nonstoichiometry. New features related to the modifications in properties connected with the nanocrystalline state were evidenced. The correlation existing between the magnetic and thermodynamic characteristics were discussed in relation to significant changes in the overall concentration of defects.

Standard molar Gibbs energy of formation of Pb 5Bi 8O 17 and PbBi 12O 19 and phase diagram of the Pb–Bi–O system

Equilibrium phase fields of the ternary Pb–Bi–O system were established by long-term equilibration in the temperature range 660–840 K. Using these results, the partial phase diagram of the ternary system has been constructed. The standard molar Gibbs energy of formation of the ternary oxides, 〈Pb 5Bi 8O 17〉 and 〈PbBi 12O 19〉, was determined by measuring the equilibrium oxygen partial pressures over the relevant phase fields by static manometry in conjunction with a solid oxide electrolyte-based emf cell. Further, the standard molar Gibbs energy of formation of 〈Pb 3O 4〉 was also measured by the manometric method. The oxide that coexists with the Pb–Bi eutectic alloy (LBE) has been experimentally confirmed as [β-PbO] ss. Using the thermochemical data measured, the compositions of Pb–Bi alloys that coexist with [β-PbO] ss and ternary oxides were derived and reported.

化合物半導体からの希少金属リサイクルに関するインジウム-アンチモン-酸素系の熱力学的研究

Phase equilibria and the activities of the components in indium-antimony-oxygen ternary system have been investigated at temperature of 1173 K to discuss the recovery of indium and antimony from used InSb compound semiconductors. Using the activities of indium and antimony in the indium-antimony binary system reported in literature, the equilibrium partial pressures of oxygen were calculated by applying the Gibbs-Duhem equations to the phase relation of metal-oxide equilibrium. The vapor pressures of indium, antimony and antimony oxide, Sb4O6, were then calculated. It was found that the vapor pressure of antimony was much larger than the values of indium and antimony oxide except in the vicinity of pure antimony composition, indicating that antimony might preferentially vaporize in remelting of used InSb compound semiconductors for recycling. This suggests that change in InSb composition may occur. However, the equilibrium partial pressure of oxygen calculated indicates that the indium oxide is stable compared to antimony oxide, Sb2O3. Then the melting experiment of InSb compound was conducted under the condition of simulated air atmosphere. The indium oxide film formed was observed on the metal surface and it prevented vaporization of antimony. Consequently, the recycling of InSb compound semiconductor can be expected by remelting in an oxidation atmosphere like air without a change in InSb composition.

Attempt to study Marangoni flow of low-Pr-number fluids using a liquid bridge of silver

In order to experimentally investigate the Marangoni flow of low-Prandtl-number fluids in a liquid bridge geometry under the condition of small Marangoni numbers close to the critical Marangoni numbers Ma c1 and Ma c2, the formation of a liquid bridge of silver was attempted. The available temperature difference between the upper and lower rods to obtain a small Marangoni number, such as Ma = 50, was calculated for a 5 mm high liquid bridge for several molten metals. For molten silver, the possible temperature difference was estimated to be 16 K, whereas, for molten silicon, this was 0.38 K, which is unrealistic for the purposes of experiments. For silver, a free surface can be obtained in the wide range of oxygen partial pressures, whereas, for molten silicon, the available oxygen partial pressure range is very small; equilibrium oxygen partial pressure for SiO 2 formation is as low as 1.1 × 10 −14 Pa. A liquid bridge of molten silver was successfully prepared and temperature oscillation was observed; the estimated Marangoni number was 160 and oscillation frequency was 0.26 Hz.

Point defect chemistry of YBa 2Cu 3O 6.5+ δ

A novel mathematical approach for defect concentration calculations proposed by Poulsen [F.W. Poulsen, Solid State Ionics, 129 (2000) 145] for oxides with rather small deviations from stoichiometry also applies to perovskite-type oxides with large ranges of oxygen nonstoichiometry. Point defect chemistry calculations have been performed for YBa 2 Cu 3 O 6.5 + δ for which experimental equilibrium oxygen partial pressure isotherms are well known. The model applied in this work describes experimental results very well in the p O 2 range of 1 0 − 5 to 10 bar and for temperatures between 450 and 800 ° C.

Influence of quencher on microstructure and magnetic properties of manganese-zinc ferrites

The effects of quencher after calcination on the microstructure and magnetic properties of manganese-zinc ferrites were investigated by measuring the magnetic properties, electrical resistivity and density. The powder of Mn 0.68Zn 0.25Fe 2.07O 4 composition was prepared by adopting the conventional ceramic technique. Toroidal cores were sintered at 1350 °C for 4 h in atmosphere controlled by using the equation for equilibrium oxygen partial pressure. The fracture surface micrographs of samples were observed by scanning electron microscope. The results show that the inner stress of calcined powder increases, abnormal grains of ferrite grow up, initial permeability goes down and power losses of ferrite rise with the increase in quenching temperature, and the microstructure and magnetic properties of manganese-zinc ferrites can be improved with the gradual cooling of calcined powder to room temperature (25 °C).

Solubility of Ni in BaTiO3 during co-firing

Nickel is now frequently used as internal electrodes for multilayer ceramic capacitors (MLCC). In order to evaluate the effect of the presence of metallic Ni on the properties of BaTiO3-based dielectrics, nano-sized Ni particles are mixed thoroughly with BaTiO3 powder and subsequently sintered at 1,330 °C in nitrogen. The oxygen partial pressure in the nitrogen is slightly higher than the equilibrium oxygen partial pressure for the Ni→NiO reaction. Under such a condition, an upper limit of 5,400 ± 540 ppm for the solubility of Ni in BaTiO3 is detected. The Ni solute acts as an acceptor to BaTiO3, and its presence increases the reduction resistance of BaTiO3 significantly.

An EPR characterization of vanadium in CaO and Na2O based Al2O3–SiO2 glasses

The relationship between glass composition and local structure was examined by electron paramagnetic resonance (EPR) analysis of vanadium(IV) in CaO and Na2O based Al2O3–SiO2 glasses. The characteristic EPR spectrum of vanadium(IV) was affected with change in glass composition, equilibrium oxygen partial pressures and melting temperature. From the available previous reports and current study, it seems that a close relationship exists between octahedral and tetrahedral vanadium, which depends upon temperature. The results are important to understanding of vanadium behavior as a candidate in Al–V–Si BMGs and their nano-composites.

Feasibility study of the new rutile extraction process from natural ilmenite ore based on the oxidation reaction

Phase relations in the Fe2O3-FeTiO3-TiO2 system were investigated by equilibrating synthetic samples in evacuated sealed quartz tubes at a temperature of 1373 K. The equilibrium partial pressure of oxygen was measured by the electromotive force (EMF) method in the temperature range of 1273 to 1373 K. The phase diagram and oxygen partial pressure diagram in the titanium-iron-oxygen ternary system were then constructed at 1373 K. Rutile extraction from natural ilmenite ore was discussed from the thermodynamic viewpoint. It is found that rutile can be produced from common natural ilmenite ores not only by the reduction as the conventional titanium-rich slag process but also by an oxidation. Then, the oxidation experiment was conducted in air using Australian ilmenite ore to obtain rutile as one of the coexistent phases. Magnetic separation and leaching experiments for synthesized pseudobrookite and reagent rutile were conducted to confirm the possibility of separation of rutile from pseudobrookite. A new rutile extraction process was then proposed.

Defect Structure and Electrical Conductivity of the Ruddlesden−Popper Phases Sr3FeMO6+δ (M = Co, Ni)

In this work, we report a systematic study of the oxygen nonstoichiometry, high-temperature thermodynamics, and transport properties of the perovskite-related mixed conductor Sr3FeMO6+δ (M = Co, Ni). Thermogravimetry was used to determine the oxygen content change (6+δ) as a function of temperature (T) and equilibrium oxygen partial pressure (pO2) within the range 1 × 10-5 atm < pO2 < 1 atm and 400 °C ≤ T ≤ 1000 °C. From the experimental values of the oxygen chemical potential (μO2), we determined both the partial molar enthalpy (hO2) and the partial molar entropy (sO2) for the composition range 5.9 < 6+δ < 6.6. Thermodynamic data were fitted using a straightforward defect model. The model was deduced from the mass action law assuming oxygen-vacancy formation and involving different fractions of localized and delocalized charge carriers in iron sites and the metal transition M3d−O2p band, respectively. Electrical resistivity measurements as a function of pO2 at constant temperature were performed in the 650 °C ≤ T ≤ 1000 °C temperature range for Sr3FeCoO6+δ and Sr3FeNiO6+δ compounds. The activation energy values for the electrical-transport process at constant oxygen content values were obtained from the combination of electrical conductivity and thermogravimetry data. The electrical conductivity data are discussed within the frame of large polaron behavior in agreement with the thermodynamic data.

The Effect of Gas Flow Rate on the Oxide Scale Morphology of a 10%Cr-Ferritic Steels in Ar-H&lt;sub&gt;2&lt;/sub&gt;O and Ar-H&lt;sub&gt;2&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O Mixtures

The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.

Fine-tuning of ceramic-based chemical sensors via novel microstructural modification: Part II: Low level CO sensing by molybdenum oxide, MoO 3

Employing a buffer gas mixture of CO and CO 2, the equilibrium oxygen partial pressure was varied across the Mo/MoO 3 proximity line by varying the ratio of concentration of the two gases, as described in an earlier communication. This led to the reduction of MoO 3 in one case and oxidation of the reduced phase (Mo or MoO 2) in the other with a concomitant variation in the microstructural features of the resulting species. The formation and growth of the new oxide surface under conditions of oxygen potential modulation has been found to impart enhanced sensing characteristics to the MoO 3-based chemical sensors for low levels of carbon monoxide (14–100 ppm CO) in the ambient.

Thermodynamic properties and spin dynamics of some micro and nanostructured magnetoresistive lanthanum manganites

The spin dynamics and thermodynamic properties of micro- and nanostructured lanthanum manganites La 0.67Ca 0.33Mn 1− x Me x O 3 (Me = Al; x = 0–0.05) have been studied in order to evidence new features related to the effect of different compositional variables on the thermodynamic and magnetic behaviour. By electron paramagnetic resonance (EPR) spectroscopy, the influence of the grain size and the Mn substitution effects on the spin dynamics were investigated. A solid state electrochemical method was used to obtain the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen. The influence of the oxygen stoichiometry change on the thermodynamic properties has been investigated. The results were correlated with the average Mn valence values as determined by redox titration. In this paper an alternative method is proposed to estimate the contribution made by oxygen vacancies in balancing the local charge in CMR manganites.

Effects of calcining and sintering parameters on the magnetic properties of high-permeability MnZn ferrites

For asymmetric digital subscriber line (ADSL) technology applications, new MnZn ferrites are required with not only high initial permeability, but also with low hysteresis material constant /spl eta//sub B/. The effects of calcining and sintering temperatures and oxygen equilibrium partial pressure on the magnetic properties of MnZn ferrites are studied in this work. It was found that raising the calcining temperature required a rise in the sintering temperature to simultaneously achieve maximum permeability and the lowest /spl eta//sub B/ value. Both grain size and porosity had an effect on the initial permeability of the samples. Porosity had an even larger effect on the /spl eta//sub B/ value than the initial permeability. Oxygen equilibrium partial pressure also had an important effect on the initial permeability and /spl eta//sub B/ value. In our testing range, both the initial permeability and the /spl eta//sub B/ value decreased with increasing value of A (oxygen parameter). In order to optimize synthesis conditions for the high initial permeability and low /spl eta//sub B/ value, it was best to calcine the sample at 950/spl deg/C and sinter it at 1370/spl deg/C, with A=7.55.

Thermodynamic and phase relation study of the Ni–Ge–O system in the solid state

Abstract In order to clarify the phase relation in the Ni–Ge–O system and to obtain the activity of germanium in solid Ni–Ge alloys, phase equilibrium experiments and EMF measurements with a zirconia-based solid electrolyte are performed from 950 to 1100 K. The results are as follows: 1) Ni-rich alloys with less than 24.0 at.% Ge cannot coexist with GeO2 (tetragonal). 2) In solid Ni–Ge alloys with 24.0–44.7 at.% Ge, the activity of Ge changes sharply in the Ni5Ge3 phase region and the temperature dependence of the activity is very small. 3) Equilibrium oxygen partial pressures are obtained in the whole composition range in the ternary Ni–Ge–O system at 1050 K.