Equilibrium Oxygen Content Research Articles

Cathode water management towards improved performance of protonic ceramic fuel cells

Over protonic ceramic fuel cells (PCFCs) cathode, both oxygen reduction reaction (ORR) and hydration reaction happen. To maximize the performance, the cathode requires sufficient proton conductivity for facilitating water formation and large number of active sites and high intrinsic activity for catalyzing ORR. Herein, we demonstrate hydration reaction and ORR over PCFC cathode can be optimally balanced through controlling the air flow rate, which adjusts the equilibrium water and oxygen content in the surrounding atmosphere of the cathode. Then the suitable proton uptake can enhance the proton conductivity of the cathode without negatively affecting ORR. The different characteristics of various cathode materials suggest their different optimal operation conditions. Specifically, the optimal performance of SrTi0.1Fe0.9O3-δ, Ba0.5Sr0.5Co0.8Fe0.2O3-δ and BaCo0.4Fe0.4Zr0.1Y0.1O3-δ cathodes in PCFCs at 600 °C is reached at different water content in the air atmosphere of 472, 592 and 735 mW cm−2, respectively, showing increased peak power density (PPD) of 13%, 10% and 6% of the corresponding cells as compared to that operating near static air atmosphere. Our study reveals the important environmental demands of PCFC cathode during the operation and provides useful guidance for the further performance optimization.

Investigation of deoxidation of high-silicon austenitic sulfuric acid-resistant stainless steel

The oxygen content in high-silicon austenitic sulfuric acid-resistant stainless steels is one of the most detrimental parameters to their corrosion resistance. Based on the ion-molecular coexistence theory (IMCT), a thermodynamic model of the slag-steel reaction of austenitic stainless steel containing 5.0 wt% Si with CaF2-CaO-Al2O3-MgO-SiO2 pentabasic slag was developed to investigate the deoxidation reaction and the oxygen control mechanism of the steel. The model was validated through experiments proposed in this study. The results of the slag-steel reaction indicated that the equilibrium oxygen content was determined by the greater of two factors: w[O]%, Si obtained from the [Si]-[O] equilibrium reaction controlled by the activity of SiO2 and w[O]%, Al obtained from the [Al]-[O] equilibrium reaction controlled by the activity of Al2O3. The system temperature and the basicity of slag are the most crucial among the multiple variables affecting the equilibrium oxygen content compared with Al2O3 and CaF2 in slag. However, achieving an ultra-low oxygen steel, both a basicity of slag greater than two and a low activity of Al2O3 in slag should be maintained. The total oxygen content in steel can reach a minimum value of 3.4 ppm when the slag composition encompasses w(CaF2)% = 29.38, w(CaO)% = 44.07, w(SiO2)% = 14.69, w(MgO)% = 9.89, w(Al2O3)% = 1.96. The high basicity of slag reduces the total oxygen content of stainless steel, whereas the influence of redox reactions between Si and Al results in a higher Al content in steel and the formation of more inclusions during solidification. Thus, the optimal Al2O3 content is less than 4% and the optimal basicity is 2.4 during the refining process.

Deoxidation and inclusion evolution of FeNi-based expansion alloy smelted by slagging method

ABSTRACT The composition of CaO-SiO2-MgO-Al2O3-CaF2 slag was designed by thermodynamic calculation of [Al]-[O] equilibrium of FeNi-based expansion alloy. On this basis, the deoxidation and inclusion evolution in the process of slagging and smelting FeNi-based expansion alloy in the medium frequency induction furnace were investigated. The results show that the oxygen content in the alloy melt is mainly controlled by the [Fe]-[O] equilibrium, and the change trend of the equilibrium oxygen content calculated by [Fe]-[O] equilibrium is consistent with the actual oxygen content. The total oxygen content of the final ingot is 45 ppm. After Al deoxidation in the melting process, the inclusions in the alloy are transformed into composite oxide inclusions rich in Al2O3 and MgO. After final deoxidation with Si-Ca alloy, the contents of Al2O3 and MgO in the inclusions decreased significantly, while the CaO content increased. And there are mainly CaO-SiO2-MgO-Al2O3 composite oxide inclusions in the ingot.

Determination of real-time oxygen transfer rate based on an electrochemical method

The interfacial oxygen transfer rate is one of the main factors to control the composition of alloys. The commonly employed method of studying the interfacial oxygen transfer rate is the chemical composition analysis; however, it is difficult to be studied in situ. Here, a new method of measuring the oxygen transfer rate at the gas–slag and slag–metal interfaces was reported based on electrochemical analyses. The interfacial oxygen transfer rate in the smelting process of Inconel 718 superalloy was investigated at 1723, 1773, 1823, and 1873 K. The experimental results show that the electrochemical method can measure the real-time oxygen content; hence, this method is promising in controlling the oxygen content in alloys. As the temperature increased, both the equilibrium oxygen content and the rate of oxygen absorption increased significantly, and the increase was the most obvious when the temperature was 1873 K. The possible reason is that the increase in temperature weakens the mass transfer resistance of the electric double layer at the interface, thus accelerating the oxygen transfer rate.

Defect formation peculiarities and redox properties of novel oxygen carrier material LaCu0.5Ti0.5O3±δ at elevated temperatures

The disordered perovskite-like phase LaCu0.5Ti0.5O3±δ (LCTO) with Pnma space group is synthesized by solid-state reaction route. Coulometric and thermogravimetric measurements in 300–1223 K temperature and 10−16 – 0.21 atm oxygen partial pressure ranges reveal narrow equilibrium oxygen content interval for the studied compound. Moreover, LCTO is shown to possess both over- and non-stoichiometry depending on external conditions. The complete reduction of LaCu0.5Ti0.5O3±δ results in a formation of La2O3, La2Ti2O7–x oxides and metallic copper. Defect chemistry modeling and DFT calculations evidence oxygen vacancies formed in LaCu0.5Ti0.5O3±δ at reduction can be considered mobile species. A decomposition mechanism for studied cuprate is proposed based on experimental results.

Influence of refining process and utilization of different slags on inclusions, titanium yield and total oxygen content of Ti-stabilized 321 stainless steel

Ti-stabilized 321 stainless steel was prepared using an electric arc furnace, argon oxygen decarburization (AOD) furnace, ladle furnace (LF), and continuous casting processes. In addition, the effect of refining process and utilization of different slags on the evolution of inclusions, titanium yield, and oxygen content was systematically investigated by experimental and thermodynamic analysis. The results reveal that the total oxygen content (TO) and inclusion density decreased during the refining process. The spherical CaO–SiO2–Al2O3–MgO inclusions existed in the 321 stainless steel after the AOD process. Moreover, prior to the Ti addition, the spherical CaO–Al2O3–MgO–SiO2 inclusions were observed during LF refining process. However, Ti addition resulted in multilayer CaO–Al2O3–MgO–TiOx inclusions. Two different samples were prepared by conventional CaO–Al2O3-based slag (Heat-1) and TiO2-rich CaO–Al2O3-based slag (Heat-2). The statistical analysis revealed that the density of inclusions and the TiOx content in CaO–Al2O3–MgO–TiOx inclusions found in Heat-2 sample are much lower than those in the Heat-1 sample. Furthermore, the TO content and Ti yield during the LF refining process were controlled by using TiO2-rich calcium aluminate synthetic slag. These results were consistent with the ion–molecule coexistence theory and FactSage™7.2 software calculations. When TiO2-rich CaO–Al2O3-based slag was used, the TiO2 activity of the slag increased, and the equilibrium oxygen content significantly decreased from the AOD to LF processes. Therefore, the higher TiO2 activity of slag and lower equilibrium oxygen content suppressed the undesirable reactions between Ti and O.

Peculiar Properties of Electrochemically Oxidized SmBaCo2−xMnxO5+δ (x = 0; 0.5 and 1) A-Site Ordered Perovskites

Fully-stoichiometric SmBaCo2-xMnxO6 oxides (x = 0, 0.5, 1) were obtained through the electrochemical oxidation method performed in 1 M KOH solution from starting materials having close to equilibrium oxygen content. Cycling voltammetry scans allow us to recognize the voltage range (0.3–0.55 V vs. Hg/HgO electrode) for which electrochemical oxidation occurs with high efficiency. In a similarly performed galvanostatic experiment, the value of the stabilized voltage recorded during the oxidation increased with higher Mn content, which seems to relate to the electronic structure of the compounds. Results of the iodometric titration and thermogravimetric analysis prove that the proposed technique allows for an increase in the oxygen content in SmBaCo2-xMnxO5+δ materials to values close to 6 (δ ≈ 1). While the expected significant enhancement of the total conductivity was observed for the oxidized samples, surprisingly, their crystal structure only underwent slight modification. This can be interpreted as due to the unique nature of the oxygen intercalation process at room temperature.

Thermodynamic properties of zirconium-oxygen solid solution and its deoxidation in calcium chloride molten salt

Thermodynamic properties of zirconium-oxygen (Zr–O) solid solution and its deoxidation by calcium in CaCl2 molten salt were studied using a chemical equilibration technique. In the temperature range from 1173 to 1373 K, the equilibrium oxygen contents in Zr were measured experimentally under coexistence of Ca and CaO in CaCl2 molten salt. The oxygen content in Zr was reduced from about 800 ppm to a minimum of less than 100 ppm. Then aCaO sensor — niobium-oxygen (Nb–O) solid solution was employed to equilibrate with Zr–O solid solution to determine the oxygen activity coefficient in Zr–O solid solution. Zr and Nb specimens were submerged in Ca-saturated CaCl2 molten salt containing different amounts of CaO. CaCl2 melt was used for a homogeneous reaction and composition. The experimental results shown that Henry's law held and the activity coefficient of oxygen was constant within the concentration range of this study. Based on the result, the standard formation Gibbs free energy of Zr–O solid solution was obtained as a function of temperature:1/2O2 (g) = O (in Zr)ΔGZr−Oθ=−543577+64.3T(1173−1373K)The p-t-x relationship of the deoxidation system was obtained simultaneously, which make the deoxidation limit and variation trend of oxygen content in Zr–O solid solution predictable and controllable under different conditions. Oxygen distribution coefficient between Zr–O solid solution and Nb–O solid solution was found in agreement with the theoretical value and independent of oxygen potential of the system.

The interplay of oxygen non-stoichiometry and defects in Ga-doped layered cobaltites

The formation of solid solutions PrBaCo2–xGaxO6–δ, where х = 0, 0.05, 0.10, 0.15 and 0.20, with tetragonal structure is observed at incorporation of gallium in the double perovskite cobaltite PrBaCo2O6–δ. The magnetic susceptibility data evidence a transition at 140–160 K from ferromagnetic to paramagnetic phase characterized the presence of low-spin Co4+ with Co3+ cations adopting all possible spin states. The temperature increase is accompanied with a gradual decrease in the amount of low-spin Co3+ cations in the paramagnetic phase. The variations of equilibrium oxygen content in doped cobaltites with changes of oxygen pressure and temperature are used for determination of oxygen chemical potential, oxygen partial thermodynamic functions and parameters that govern defect formation reactions in PrBaCo2–xGaxO6–δ. The consistency of the thermodynamic analysis is confirmed by favorable coincidence of the experimental and calculated plots for the partial thermodynamic functions. Arguments are presented showing that oxygen defects in the cobaltites can be considered as weakly interacting even at high concentration. The gallium doping is found to improve densification of ceramic samples, electron hole conductivity and to simultaneously increase the population of O2 vacancies important for ion transport.

Research on the oxygen content in 304L stainless steel during VOD-LF-CC process based on IMCT

The total oxygen in 304L stainless steel produced by the VOD-LF-CC process is investigated in this research. The equilibrium oxygen content in the molten steel during the smelting process is calculated based on the ion-molecule coexistence theory. The results show that it is feasible to use the developed thermodynamic model for calculating the equilibrium oxygen content in CaO–SiO2–Al2O3–MgO–FeO–Cr2O3 slags equilibrated with 304L molten steel. The variation law for total oxygen is in good agreement with the equilibrium oxygen content. The oxygen content of the oxide-inclusion decreases with the number density of inclusions, which decrease at different stages in the process. The content of FeO and Cr2O3 in the slag has no obvious effect on the equilibrium oxygen content when the basicity of the slag remains constant. Combining the proposed method with industrial practice, the optimum VOD process parameters for obtaining a low equilibrium oxygen content are as follows: 1600–1650 °C of the temperature after the VOD stage, 0.4–0.5% of the Si content after reduction, and 2.2–2.5 of the slag basicity.

Advanced electrochemical properties of Pr0.9Y0.1BaCo1.8Ni0.2O6–δ – Ce0.8Sm0.2O1.9 composite as cathode material for IT–SOFCs

The co-doped double perovskite-type cobaltite Pr0.9Y0.1BaCo1.8Ni0.2O6–δ (PYBCN) is synthesized via organo-metallic precursors. The concentrations of electronic and ionic defects in PYBCN at variations of gaseous environment are obtained from the analysis of the experimental isothermal plots for equilibrium oxygen content (6 – δ) vs. oxygen partial pressure pO2 in the ambient gaseous phase. It is argued that nickel doping is beneficial for enhanced formation of vacancies in O2– positions controlling oxygen ion transport in the cobaltite. The cobaltite based dual-phase composite with 30 wt% addition of samarium doped ceria Ce0.8Sm0.2O1.9 (SDC) is found to have considerably reduced thermal expansion compared to the parent cobaltite. The impedance measurements show that the area specific resistance (ASR) of the composite decreases to 0.11 Ω cm2 at 973 K with the activation energy 1.2 eV. Further increase of the temperature is accompanied by the decline of the activation energy to 0.9 eV, and ASR attains 0.045 Ω cm2 at 1073 K. The observed small ASRs are attributed to a large concentration of O2–vacancies in the cobaltite component of the composite. The cathodic overvoltage for oxygen reduction that does not exceed 40 mV at 1073 K and current density 1000 mA cm−2, moderate thermal expansion, good conductivity and high electro-chemical activity distinguish the developed composite as one of the promising cathode materials for intermediate temperature solid oxide fuel cell applications.

Improved structural stability, electron transport and defect formation in PrBaCo2–xAlxO6–δ

The double perovskite-like solid solutions PrBaCo2–xAlxO6–δ are obtained via combustion of glycerol-nitrate organo-metallic precursors. The aluminum doping occurs favorable for mitigation of thermal expansion and stabilization of the tetragonal structure in a wide range of temperature changes. The variations of equilibrium oxygen content in PrBaCo1.9Al0.1O6–δ are measured with a coulometric titration technique, and analyzed in terms of defect chemistry. It is shown that aluminum incorporation is accompanied by formation of rigid AlO6 octahedra in the crystal structure and enhanced disproportionation of Co3+ cations. The developed defect model is successfully applied in order to explain the data for conductivity and thermopower. The structural stability, moderate thermal expansion and high conductivity represent an advantageous properties combination for the using of PrBaCo1.9Al0.1O6–δ cobaltite in various high-temperature solid state electrochemical devices.

Improved stability and defect structure of yttrium doped cobaltite PrBaCo2O6–δ

The experimental data for equilibrium oxygen content in double perovskite cobaltite Pr0.9Y0.1BaCo2O6–δ show that the incorporation of yttrium in place of praseodymium results in stabilization of the tetragonal structure in wide limits of oxygen pressure and temperature. The thermodynamic model calculations of defect formation reactions demonstrate that the doping is accompanied with enhanced electron and oxygen ion disordering. The observed properties combination is promising for the using of the yttrium modified cobaltite as a cathode in intermediate temperature SOFCs and a ceramic material for oxygen separation and storage.

Oxygen thermodynamics and defect chemistry of nonstoichiometric manganites Ca0.6−уSr0.4LaуMnO3−δ

The chemical potential of oxygen ΔμO(δ,y,T) relative to the standard state is calculated from experimental data for equilibrium oxygen content in Ca0.6–уSr0.4LaуMnO3–δ at different values of temperature and oxygen partial pressure in the gaseous phase. The respective values of partial molar enthalpy ΔH¯O(δ,y) and entropy ΔS¯O(δ,y) are derived from the linear plots for ΔμO(δ,y,T) vs. temperature. Based on the ideal solution approximation, the equations are obtained that show interrelation of ΔH¯O(δ,y) and ΔS¯O(δ,y) with thermodynamic parameters of defect formation reactions and concentration of defects. It is shown that thermal excitation of Mn4+ cations considerably impacts equilibration of defect species in the crystal structure of Ca0.6–уSr0.4LaуMnO3–δ.

Defect formation in double perovskites PrBaCo2−xCuxO5 +δ at elevated temperatures

The variations of equilibrium oxygen content with oxygen pressure and temperature were measured in copper doped double perovskite cobaltite PrBaCo2−xCuxO5+δ. The results of thermodynamic analysis show that oxygen pressure decrease and temperature increase both favor the appearance of oxygen vacancies in O2 structural positions. It is suggested that oxygen migration over O3 positions in double perovskite cobaltites occurs via mediation of O2 positions. Copper doping is shown to facilitate formation of oxygen vacancies in O2 positions, which is expected to be advantageous for enhanced oxygen ion transport.

Structural phase transitions in YBaCo4O7 + x cobaltate upon variations in oxygen content, according to X-ray diffraction data obtained using synchrotron radiation

According to thermogravimetric data, the equilibrium oxygen content in layered YBaCo4O7 + x cobaltate varies stepwise from x ≈ 1.4 to x ≈ 0 when temperature is varied in the range of 300–400°C. Detailed ex situ and in situ structural investigations using synchrotron radiation show that variation in the oxygen content in YBaCo4O7 + x is accompanied by substantial reordering of the crystal structure and has the character of a first-order phase transition between two rigorously determined phases that differ by the parameters and symmetry of the crystal lattice as well as by the oxygen content. In addition to uniquely high oxygen liberation upon heating, dx/dT, great mechanical stresses that lead to the material cracking and complicate its practical use are a consequence of the character of stepwise transitions in YBaCo4O7 + x.

Effect of Al2O3 Refractories on Oxygen Content of Molten Fe^|^ndash;Cr Alloy

Thermodynamic information on the equilibrium between dissolved Cr and O in steel melt is the basic knowledge to control Cr content in the stainless steel. The case of using alumina refractories is considered in the present work. Molten Fe–Cr alloy equilibrated with Cr2O3–Al2O3 solid solution when Cr content of the alloy was high and equilibrated with FetO·(Cr, Al)2O3 solid solution when Cr content of the alloy was low. The equilibrium oxygen contents in Fe–Cr alloy at given Cr contents using Al2O3 crucible in the present work were lower than those using Cr2O3 crucible reported previously. It was found that oxide in equilibrium with Fe–Cr alloy will control and affect the O content when Al content is low.

Thermodynamic stability, structural and electrical characterization of mixed ionic and electronic conductor La2Mo2O8.96

Thermogravimetric analysis (TGA) technique in controlled oxygen partial pressure (pO(2)) atmospheres has been used to obtain equilibrium oxygen content data as a function of pO(2) on the La(2)Mo(2)O(9-δ) system resulting from the partial reduction of fast oxide-ion conductor La(2)Mo(2)O(9) (LM). Thermodynamic conditions for stabilization of crystalline La(7)Mo(7)O(30) and amorphous La(2)Mo(2)O(7-y) at 718 °C have been determined and discussed. At 608 °C, the compound reported for the first time La(2)Mo(2)O(8.96) (LM896) has been found. The crystalline form and transition temperature in LM896 have been identified by X-ray diffraction at room temperature (XRD) and at controlled temperature. Conductivity curves obtained by electrochemical impedance spectroscopy (EIS) as a function of temperature for both LM and LM896 have been compared. The results indicate that LM896 is a mixed ionic and electronic conductor (MIEC).

On Physico‐Chemical and Technical Limits in Clean Steel Production

AbstractGreat progresses in steel cleanliness have been attained during the last decades. In the measures of oxygen the lowest levels are approaching 5 ppm in Otot i.e. close to the thermodynamic limit of Al deoxidation. As the thermodynamics of the reaction system is firmly anchored to the steel chemistry and thus to the properties of steel and the final product, it is useful to examine the thermodynamic constraints for selected steels in the refining and casting processes. In most steels Al‐O equilibrium determines the limit of deoxidation. Calculations by applying thermodynamic software showed that in selected “common” steels the equilibrium oxygen content varied from below 5 ppm up to 30 ppm or higher depending on the aluminium content, interaction effect of other alloying elements and temperature.For lower oxygen content there are several possibilities. The first one is to use stronger deoxidizing additions like Ca, Zr, Ce etc. However, they can be problematic as substitutes as they produce inclusions which influence steel properties. The second way is to intensify the deoxidizing power of certain elements e.g. Si by bringing the steel in intimate contact with a proper slag with low SiO2 activity in ladle treatment with violent stirring. In the calculation example it was possible to decrease oxygen content from 30 ppm to the level below 10 ppm in C/Mn/Si steel at very low Al level. The third potential but unused process is vacuum deoxidation especially for medium and high carbon steels. Equilibrium oxygen contents below 1 ppm are thermodynamically easily attainable. But in practice the vacuum treatment should be designed to intensify the carbon‐oxygen reaction on the top surface of the stirred steel liquid. Also the eventual reactions with refractory materials should be suppressed.

Oxygen nonstoichiometry and exchange kinetics of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ

The oxygen nonstoichiometry of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF5582) was investigated by thermogravimetry resulting in values of 2.40 < (3 − δ ) <2.57 for the equilibrium oxygen content in the range 600 ≤ T/°C ≤ 900 and 1E− 4 < pO 2/bar < 0.4. The oxygen exchange kinetics was studied by electrical conductivity relaxation as a function of temperature in the range 550 ≤ T/°C ≤ 725 with chemical diffusion coefficients 1E− 6 < D chem/cm 2 s − 1 < 3E− 5 and surface exchange coefficients 2E− 4 < k chem/cm s − 1 < 3E− 3. The activation energies of the kinetic parameters amount to E a( D chem) = 86 ± 8 kJ mol − 1 and E a( k chem) = 64 ± 12 kJ mol − 1 . Self-diffusion and surface exchange coefficients, as well as ionic conductivities are estimated.