Controlled Oxygen Partial Pressure Research Articles

Role of mixed oxidant and controlled oxygen partial pressure heat treatment to regulate the dielectric properties and resistivity of SrTiO3-based CP ceramics

Sr0.99La0.01TiO3 (SLT10) colossal permittivity (CP) ceramic materials were synthesized using solid phase method and secondary heat treated using mixed oxidant 35%Bi2O3–10%Al2O3–20%MgO–25%CuO–10%SiO2 oxidant. The heat treatment atmospheres were chosen as O2, air, and Ar to correspond to different oxygen partial pressures. The dielectric properties of the Sr0.99La0.01TiO3-Percolated (SLT10-P) ceramics were investigated, and the contribution of the oxidant to the resistivity enhancement was verified by complex impedance spectroscopy and DC resistivity tests. Excellent dielectric properties were obtained for SLT10-P Air ceramic sample (16543 for dielectric permittivity and 0.016 for loss tangent), and the resistivity was increased from 2.13 × 108 Ω cm (SLT10) to 1.23 × 1011 Ω cm (SLT10-P Air). The resistance degradation mechanism of SLT10-P ceramics and oxygen vacancy migration in the process of resistance degradation were studied by using resistance monitoring device and thermally stimulated depolarization currents (TSDC) test. The introduction of oxidant optimizes the temperature stability and resistivity of SLT10 ceramics, but it will lead to the decrease of dielectric permittivity. The atmosphere of heat treatment changed the concentration of oxygen vacancy in ceramics. When SLT10-P O2 was treated in O2 atmosphere, the concentration of oxygen vacancy in ceramics decreased, which led to the decrease of dielectric permittivity and the increase of grain boundary resistance. For SLT10-P Ar ceramic, more oxygen vacancies and free electrons are induced by low oxygen partial pressure heat treatment, which enhances the dipole effect of electron pinning defects and leads to colossal dielectric permittivity, but high concentration of oxygen vacancies will affect the resistance decline. In SLT10-P Air ceramic, the coexistence of colossal permittivity and low loss tangent and high resistance is realized.

Synthesis of Fe3Al@Al2O3 flakes showing improved microwave absorption performance after servicing at high temperatures

Most magnetic loss-type absorbers exhibit very poor microwave absorption performance at high temperatures. Herein, we developed a thermal treatment and controlled oxygen partial pressure process to design a Fe3Al@Al2O3 core-shell structure by inducing the in situ oxidation of Al elements in the Fe3Al particles. Owing to the in situ growth of the dense and thin Al2O3 layer on the Fe3Al surface, the oxidation issue at high temeratures can be effectively alleviated. Compared to the Fe3Al counterpart without a Al2O3 shell that tends to be oxidized at high temperatures, the Fe3Al@Al2O3 absorber keeps good microwave attenuation properties and impedance matching ratios even at a temperature of 400 ℃, with the minimum reflection loss of −52.4 dB at 5.8 GHz. Moreover, the effective absorption bandwidth is 4.9 GHz. The new findings demonstrated in this work provides a possible approach for the exploration of efficient microwave absorption materials used at high temperatures.

Periodic oxygen-defective ZrO2−x films with room-temperature ferromagnetism produced by precisely controlling oxidation of metallic Zr foil

Ferromagnetism can be induced by introducing oxygen vacancies into metallic oxides. The oxygen deficiencies introduced into ZrO2 samples thus far have been present in unquantifiable concentrations and randomly scattered near the surface. We successfully produced room-temperature-ferromagnetic 9 μm-thick ZrO2−x films via a stepwise oxidation process consisting of annealing metallic Zr foil in air and under a controlled oxygen partial pressure. The ZrO2−x films are composed of a periodic structure with a spacing of 3–4 nm; this structure originated from the presence and absence of oxygen deficiencies. The unique structure in the ZrO2−x phase consisting of the periodic arrangements of oxygen deficiencies can be produced by the stepwise oxidation using Zr metal. Our proposed process should open the door for new applications of defect-driven ferromagnetic films of undoped magnetic oxides with a periodic oxygen-defective structure.

Low temperature synthesis of VO2 and hysteresis free VOx thin films with high temperature coefficient of resistance for bolometer applications

Vanadium dioxide (VO2) is an insulator to metal phase transition material, which finds many electronic and optical applications due to its reversible switching while VOx-based bolometers are widely used in commercial uncooled infrared detectors due to their high-temperature coefficient of resistance and low resistivity. Atmospheric pressure thermal oxidation (APTO) of Vanadium (V) is a simple process used to synthesize VO2 films without the need for precise control of oxygen partial pressure unlike other synthesis methods. In this work, we report the effect of reducing the V oxidation temperature to 250–450 °C during APTO and synthesize films with different oxidation durations. For an optimized oxidation duration at each oxidation temperature, VO2 films are obtained with more than two orders of resistance switching. This optimum oxidation duration is found to decrease exponentially with increase in oxidation temperature with an activation energy of 1.05 eV. The phase transition temperature of these VO2 films is found to decrease monotonically with decrease in oxidation temperature. We also observe that partially oxidized (oxidation time less than optimized oxidation duration to form VO2) VOx films show broad resistance switching from room temperature up to 70 °C instead of steep switching at the phase transition temperature. The partially oxidized film shows a temperature coefficient of resistance (TCR) as high as ∼10%/K, which is 3–4 times greater than TCR of commercial VOx thin films used in bolometers while showing similar resistivity. The resistivity of these films also exhibits almost zero hysteresis with temperature, an important requirement for bolometer applications. This study will widen the usefulness of the APTO process for fabricating VO2-based devices and high TCR VOx films for bolometer applications. The demonstrated low temperature process will especially be suitable to integrate VO2/VOx on substrates which can sustain only low temperatures such as some of the flexible substrates.

Phase Equilibrium studies in the system “FeO”–SiO2–Al2O3–CaO–MgO at 1200 °C and Po2 10-8atm

Iron oxides and silica are the major components of copper smelting slag. The oxides of aluminum, calcium and magnesium are also present in the slag that is introduced through copper concentrate, flux and refractories. Liquidus temperatures of the copper smelting slags are usually controlled by Fe/SiO2. The concentrations of Al2O3, CaO and MgO, and FeO/Fe2O3 in the slag can also affect the liquidus temperatures where FeO/Fe2O3 is a function of oxygen partial pressure. High temperature equilibration under controlled oxygen partial pressure followed by quenching and electron probe microanalysis were used to determine the compositions of the liquid and solid phases at 1200 °C and Po2 10-8 atm. The experimental results are presented in the forms of pseudo-ternary sections “FeO”-CaO-SiO2 at fixed 2, 4 and 6 wt pct MgO, and 2 + 2, 4 + 4 and 6 + 6 wt pct MgO + Al2O3. Spinel and tridymite are the major primary phases in the composition range investigated. In addition, CaSiO3, pyroxene, olivine, and melilite are also present. The isotherms in the spinel and tridymite primary phase fields move towards higher SiO2 concentration directions with increasing CaO, Al2O3, and MgO concentrations. The experimentally determined results are compared with the FactSage calculations.

Ruddlesden‐Popper‐type perovskite Sr3Fe2O7−δ for enhanced thermochemical energy storage

AbstractPerovskite has been considered a promising thermochemical energy storage material. Such materials can perform redox reactions reversibly under the control of oxygen partial pressure over a wide range of temperatures. Layered perovskites have been poorly studied as energy storage material, although their oxygen species exhibit good oxidation activity. In this work, Ruddlesden‐Popper‐type quasi‐2D perovskite Sr3Fe2O7‐δ and 3D perovskite SrFeO3‐δ were prepared for the testing of thermochemical energy storage properties. It was shown that the degree of reduction reaction for Sr3Fe2O7‐δ was much greater than that of SrFeO3‐δ, with change of non‐stoichiometry up to 0.79. The combined effect of thermodynamic parameters for samples on heat storage behavior was studied by Van't Hoff method. The reduction entropy of Sr3Fe2O7‐δ is much higher than that of SrFeO3‐δ, which explains the large promotion in the reaction degree of SrFeO3‐δ. The total reduction enthalpy of Sr3Fe2O7‐δ is about 2.8 times that of SrFeO3‐δ, with both reduction enthalpy and reaction entropy affecting the heat storage capacity. Sr3Fe2O7‐δ also has an attractive spectral absorption of 96.92% in the range of 300–2500 nm, which makes it advantageous in volumetric solar collector. Overall, Sr3Fe2O7‐δ offers improved performance in terms of thermochemical energy storage compared to SrFeO3‐δ.image

Crystal structure and oxygen nonstoichiometry of ErBa2Fe3O8

Ten samples across the range of oxygen nonstoichometry of the ErBa2FeIII3O8 triple perovskite were obtained by high-temperature equilibration and quenching from flowing gas of controlled oxygen partial pressure. The crystal structure accommodating additional oxygen upon oxidation, and oxygen vacancies in the central coordination octahedron upon reduction, was studied by synchrotron X-ray powder diffraction. Close to the homogeneity limit upon reduction, the tetragonal P4/mmm structure distorts orthorhombic upon ordering of vacancies. The structural compromises across the nonstoichiometry range are discussed in terms of bond-valence sums at the atoms.

Study on the Increase of Co Concentration and the Purification of La–Co Cosubstituted M-Type Sr Ferrite by Oxygen Partial Pressure Control

Study on the Increase of Co Concentration and the Purification of La–Co Cosubstituted M-Type Sr Ferrite by Oxygen Partial Pressure Control

Effect of Oxygen on Water Vapor Oxidation of Fe by Oxygen Partial Pressure Control Using Gas Sensor

An oxygen pump sensor was installed in front of an oxidation furnace to supply and discharge oxygen to the Ar–H2O atmosphere and accurately control the oxygen partial pressure in the atmosphere. Furthermore, the oxidation rate was set after the oxidation furnace. It was measured by an oxygen sensor and a hydrogen sensor. Pure iron was used as the sample. The purpose of this study was to clarify the oxidation behavior of water vapor and oxygen. As a result, it was clarified that the amount of hydrogen generated by the oxidation of water vapor and Fe decreases when oxygen is supplied. In addition, when the oxygen supply amount increased, oxidation by the water vapor did not occur. On the other hand, it was clarified that when oxygen in the atmosphere is discharged, oxidation by oxygen is suppressed and only oxidation by the water vapor occurs. In the cross-sectional microstructure after oxidation, Fe3O4 was formed in the outer layer in the environment where the oxygen was discharged, but by supplying oxygen, the oxide layer became thicker and Fe2O3 was formed in the outer layer. The mass gain increased as the oxygen was supplied. Considering the ratio of oxidation by water vapor and oxidation by oxygen, it was clarified by this method that the mass gain is greater when the oxidation by oxygen is dominant.

Synchrotron-based techniques for characterizing STCH water-splitting materials

Understanding the role of oxygen vacancy–induced atomic and electronic structural changes to complex metal oxides during water-splitting processes is paramount to advancing the field of solar thermochemical hydrogen production (STCH). The formulation and confirmation of a mechanism for these types of chemical reactions necessitate a multifaceted experimental approach, featuring advanced structural characterization methods. Synchrotron X-ray techniques are essential to the rapidly advancing field of STCH in part due to properties such as high brilliance, high coherence, and variable energy that provide sensitivity, resolution, and rapid data acquisition times required for the characterization of complex metal oxides during water-splitting cycles. X-ray diffraction (XRD) is commonly used for determining the structures and phase purity of new materials synthesized by solid-state techniques and monitoring the structural integrity of oxides during water-splitting processes (e.g., oxygen vacancy–induced lattice expansion). X-ray absorption spectroscopy (XAS) is an element-specific technique and is sensitive to local atomic and electronic changes encountered around metal coordination centers during redox. While in operando measurements are desirable, the experimental conditions required for such measurements (high temperatures, controlled oxygen partial pressures, and H2O) practically necessitate in situ measurements that do not meet all operating conditions or ex situ measurements. Here, we highlight the application of synchrotron X-ray scattering and spectroscopic techniques using both in situ and ex situ measurements, emphasizing the advantages and limitations of each method as they relate to water-splitting processes. The best practices are discussed for preparing quenched states of reduction and performing synchrotron measurements, which focus on XRD and XAS at soft (e.g., oxygen K-edge, transition metal L-edges, and lanthanide M-edges) and hard (e.g., transition metal K-edges and lanthanide L-edges) X-ray energies. The X-ray absorption spectra of these complex oxides are a convolution of multiple contributions with accurate interpretation being contingent on computational methods. The state-of-the-art methods are discussed that enable peak positions and intensities to be related to material electronic and structural properties. Through careful experimental design, these studies can elucidate complex structure–property relationships as they pertain to nonstoichiometric water splitting. A survey of modern approaches for the evaluation of water-splitting materials at synchrotron sources under various experimental conditions is provided, and available software for data analysis is discussed.

Study on the Increase of Co Concentration and the Purification of La-Co Cosubstituted M-type Sr Ferrite by Oxygen Partial Pressure Control

Study on the Increase of Co Concentration and the Purification of La-Co Cosubstituted M-type Sr Ferrite by Oxygen Partial Pressure Control

Effect of asymmetric power distribution in bipolar reactive sputtering on the optoelectronic and microstructure properties of titanium dioxide for solar water splitting

Titanium dioxide is one of the most commonly used materials for solar water splitting, because of its excellent corrosion resistance. However, its optoelectronic properties are not sufficient to achieve high solar to hydrogen conversion. It is known that the choice of deposition method has a great influence on the optoelectronic and structural properties of titanium dioxide.Here we combine two different sputtering configurations and vary the process parameters to obtain thin films of black TiOx. Bipolar magnetron sputtering from two metallic titanium targets is used in combination with an oxygen partial pressure control via a closed-loop residual gas monitoring system. The power distribution between the two targets in the bipolar sputtering mode is thereby varied from sample to sample, while the oxygen partial pressure remains constant. This causes one target to shift into the oxidic regime, while the other target shifts into the metallic regime. Because the metallic target has a higher sputter yield the process leads to off-stochiometric samples. These samples exhibit a broad-band absorption and an increased photoelectrochemical activity (up to five times) compared to samples without the modification of the power distribution. The results are discussed based on the optoelectronic and microstructural analysis of the thin films.

Spatter oxidation during laser powder bed fusion of Alloy 718: Dependence on oxygen content in the process atmosphere

In laser powder bed fusion ( L -PBF), powder degradation is mainly driven by the accumulation of highly oxidized spatter particles in the powder bed. Although the amount of spattering can be controlled by the melt pool stability, spatter formation is an unavoidable characteristic of PBF processes. Oxidized spatter risks defect formation in the printed components. However, the factors influencing the level of spatter oxidation during L -PBF processing are not yet fully understood. Herein, the residual oxygen in the process atmosphere was reduced from the traditionally applied 1000–20 ppm using an oxygen partial pressure control system to process Alloy 718 powder. Spatter particles accumulated on the gas inlet were further analyzed to reveal the effect of the oxygen content in the process atmosphere on the spatter oxidation by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Increasing the residual oxygen in the process atmosphere increased surface coverage by oxide phases rich in Al and Cr. The XPS analysis confirmed that the surface of Alloy 718 spatter particles were covered with Al- and Cr-based oxides, whose thickness increased with the oxygen content in the process atmosphere. The bulk oxygen content in the spatter powder showed the same trend with approximately thrice the oxygen content in spatters generated at 1000 ppm O 2 (608 ppm O in the sample) compared to spatters generated with oxygen at 20 ppm (206 ppm O in the sample). Thermodynamic simulations demonstrate a transition from thick Al- and Cr-based mixed corundum and spinel-type oxides to Al-based corundum oxide with decreasing oxygen partial pressure, consistent with the XPS findings. • The spatter oxidation decreased with the residual oxygen content in the L -PBF atmosphere. • Significant changes in the surface oxide chemistry of spatter particles occurred as the residual oxygen varied. • The thickness of oxide phases decreased with the residual oxygen content. • The surface oxide was significantly enriched with Al and Cr as the oxygen level in the L -PBF atmosphere increased. • Thermodynamic calculations confirm preferential Al oxidation at lower oxygen levels compared to Cr.

Defect chemistry and oxygen nonstoichiometry of CeBaFe2O5.5

While lanthanoids Ln ​= ​Gd to Pr form Ba–Ln ordered double-cell iron perovskites with a wide equilibrium nonstoichiometry from LnBaFe2O5 to over O5.5, the Ce variant of this study no longer forms the CeBaFe2O5 composition. Point-defect equilibria behind oxygen nonstoichiometry of the CeIIIBaFeIII2O5.5 that forms are studied by quenching from flowing gas of controlled oxygen partial pressures pO2 and temperatures. Their thermodynamic parameters are evaluated on the defect model of pure undoped CeIIIBaFeIII2O5.5 with an electronic-defect pair e′ and h•, anion-Frenkel pair Oi′′ and vO••, and redox behavior at equilibrium with oxygen in the reaction atmospheres to fit the experimentally determined nonstoichiometry.

Oxygen nonstoichiometry in LnBaFe2O5+w across Ln = Nd, Sm, Gd

In double-cell perovskites LnBaFe2O5+w, only lanthanoids Ln ​= ​Pr to Gd support wide equilibrium nonstoichiometry w from 0 to >0.5. This study investigates point-defect equilibria behind the variation in w along Ln ​= ​Nd, Sm, Gd by quenching the oxides from flowing gas of controlled oxygen partial pressures and temperatures. The experimental data of w as a function of pO2 and T are least-squares fit via standard enthalpies and entropies of the defect-equilibrium reactions of formation for electronic-defect pair e′ and h• and for anion-Frenkel pair Oi′′ and vO••, and of the Fe2+ oxidation. NdBaFe2O5+w is closest to behaving as a pure undoped oxide at 1000 ​°C, where it has a single plateau in the w versus pO2 plot at the w ​= ​0.5 level. A second plateau, at w ​= ​0, emerges at lower temperatures. The smaller Sm and Gd increase the size difference versus Ba, and their defect equilibria show two plateaus even at 1000 ​°C. The temperature-dependent defect equilibria across this series are therefore approximated by a formal Ln3+ for Ba2+ donor-doping model, and the results are presented in Brouwer diagrams of defect concentrations. The decrease of the anion-Frenkel defect-equilibrium constant KaF due to decreasing ionic size from Nd3+ towards Gd3+ is magnified upon decrease in temperature that feeds the entropy of the anion-Frenkel disorder; at 400 ​°C, Ln ​= ​Nd has KaF ​= ​10−5.4, Sm 10−15.0 and Gd 10−24.3.

Infrared furnace for in situ neutron single-crystal diffraction studies in controlled gas atmospheres at high temperatures

To understand oxygen diffusion mechanisms in non-stoichiometric oxides, the possibility to explore structural changes as a function of the oxygen partial pressure with temperature and related oxygen bulk stoichiometry is mandatory. This article reports on the realization of a high-temperature furnace, suitable for single-crystal neutron diffraction, working continuously at temperatures of up to 1000°C at different and adjustable partial gas pressures of up to 2 bar (1 bar = 100 kPa). This allows exploration of the phase diagrams of non-stoichiometric oxides under in situ conditions and controlled oxygen partial pressure. As a pilot study, the structural changes of Pr2NiO4+δ were explored at room temperature (δ ≃ 0.24) and at 900°C under 1 bar P(O2) (δ ≃ 0.13) as well as under secondary vacuum (approximately 10−5 mbar) conditions yielding a δ close to zero. The strong anharmonic displacements of the apical oxygen atoms along the [110] shallow diffusion pathway, which were previously observed at room temperature and 400°C, become more isotropic at 900°C. The study shows that the anisotropic oxygen displacements, here related to lattice instabilities, play a major role in understanding oxygen diffusion pathways and related activation energies at moderate temperatures. This also shows the importance of the availability of reaction cells for single-crystal neutron diffraction to explore the phase diagram and associated structural changes of non-stoichiometric oxygen ion conductors and respective diffusion mechanisms.

Integrated experimental liquidus and modelling studies of the ternary AgO0.5-FeO1.5-SiO2 system in equilibrium with metallic Ag

Phase equilibria of the AgO 0.5 -“FeO 1.5 ”-SiO 2 system was investigated in the range of 1563–1913 K (1290–1640 °C) for oxide liquid (slag) in equilibrium with the gas with controlled oxygen partial pressure, Ag metal or Ag-Au alloy, and solid oxide phases: (a) tridymite, cristobalite (SiO 2 ); (b) hematite (Fe 2 O 3 ), spinel (Fe 3 O 4 ) and wüstite (FeO 1+x ). High-temperature equilibration on silica, iridium or iron substrates, followed by quenching and direct measurement of the Ag, Au, Fe and Si concentrations in the liquid and solid phases with the electron probe X-ray microanalysis (EPMA) or laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) were used for accurate characterization of the system. Present data were used to develop the thermodynamic models for all phases in this system using FactSage computer package. The present study is a part of the investigation of the thermochemistry of complex multicomponent pyrometallurgical systems. • As a part of the recycling process, the copper and lead smelters require a reliable thermodynamic database for successful recovery of the precious metals such as Ag. • This article demonstrates the high temperature phase equilibria of the AgO 0.5 -“FeO 1.5 ″-SiO 2 ternary system. • This system is a part of the complex multicomponent pyrometallurgical systems (PbO-ZnO-Cu 2 O-FeO-Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 -MgO-S-As, Sn, Sb, Bi, Ag, Au, Ni, Cr) for the copper and lead smelting and recycling industries. • The data obtained in this study were used for developing FactSage thermodynamic database.

Oxygen vacancy induced phase and conductivity transition of epitaxial BaTiO3−δ films directly grown on Ge (001) without surface passivation

The heterogeneous epitaxial system of BaTiO3/Ge (BTO/Ge) is of great interest for both fundamental research and device applications, thanks to its quasi-lattice-matching feature and the integration of functional oxides on semiconductors. Currently, the heteroepitaxial growth of crystalline BTO films on Ge includes the utilization of ultrahigh vacuum tools and complex surface passivation pre-treatment as well as careful control of oxygen partial pressure during the growth. Meanwhile, oxygen vacancies in oxides strongly impact their structural and electrical properties. Here, we report a facile method to directly grow single crystalline BTO films on Ge using pulsed laser deposition. The strict control of oxygen partial pressure ensures a sharp interface with an atom-to-atom registry and also leads to the oxygen-deficient characteristics of BTO. The epitaxial relationship of BTO and Ge is [110] BTO (001)//[100] Ge (001). Detailed crystallographic studies on BTO films with different thicknesses show that, for the films with a thickness less than 20 nm, BTO shows a mixture of tetragonal and cubic phases due to the oxygen vacancies and the strain from the Ge substrate and the cubic phase eventually dominates as the film thickness increases. Such oxygen-deficient BTO films reveal conducting characteristics rather than dielectric properties. The oxygen vacancies can be partly “cured” after a low temperature annealing process. These results not only demonstrate the possibility to directly grow single crystalline oxides on semiconductors without surface passivation but also highlight the importance of oxygen vacancies and lattice strain on the crystallographic and electrical properties of BTO films.

Influence of Oxygen Vacancies on the Structure of BiVO4

We study oxygen vacancies in the tetragonal scheelite phase of bismuth vanadate and identify stable oxygen-deficient structures. Upon subjecting these to variable-cell optimization, we find that oxygen vacancies give rise to significant structural distortions, the degree of which exhibits a vacancy concentration dependence. Furthermore, we show that these distortions give rise to splitting of powder X-ray diffraction peaks, yielding patterns similar to that of the monoclinic scheelite phase, and that these effects are also present at finite temperatures. Our results highlight the need for characterization methods beyond X-ray diffraction for identifying the phase of synthesized bismuth vanadate samples and the importance of oxygen partial pressure control during synthesis.

Formation of Protective Alumina on Ni3Al By Oxygen Partial Pressure Control Using Oxygen Pump Sensor

Introduction The oxidation of Ni aluminide with a low Al concentration produces NiAl2O4 and Al2O3 on the surface. It has been reported that the formation of NiO during the early stage of oxidation leads to the formation of NiAl2O4, resulting in poor-oxidation resistance. In this study, an oxygen pump-sensor was installed in front of the oxidation furnace to accurately control the oxygen partial pressure in the atmosphere. During the initial stage of oxidation, the partial pressure was reduced to a level at which NiO was not generated, and the partial pressure of oxygen was rapidly increased to the temperature at which α-Al2O3 was formed. Oxygen Partial Pressure Control and Oxidation Kinetics Assessment Fig.1 shows the oxidation test equipment used in this study. An oxygen sensor and oxygen pump sensor were installed at the latter stage of the electric furnace, and the oxygen partial pressure and hydrogen oxidation current (hydrogen concentration) of the discharged gas atmosphere were measured. Yttria-stabilized zirconia tubes (8 mol%Y2O3-ZrO2) were used for the oxygen sensor and oxygen pump sensor.For the oxygen sensor, the electromotive force was measured and substituted into the Nernst equation (Eq. 1) to obtain the oxygen partial pressure. E=(RT/4F)ln(P O2(ref)/P O2(mea)) (1)where, R: gas constant, T: temperature, F: Faraday constant, E: measured electromotive force, : oxygen partial pressure in the reference gas (atmosphere 0.21 atm) and : measured oxygen partial pressure.The oxygen pump sensor consists of a sensor part and a pump part. The electromotive force is measured by the sensor, and the electromotive force is set so that the initial oxygen partial pressure is obtained. Since it is an oxide ion conductor, when a current is applied to the pump section, oxygen is supplied from the inside wall of the tube in the pump section via reactions (2) (inside wall) and(3) (outside wall).2O2-→O2+4e- (2)O2+4e-→2O2- (3)As a result, the oxygen partial pressure of the pump section is maintained at the initial oxygen partial pressure.Furthermore, by applying a negative current at the pump section, oxygen in the tube can be discharged by reaction (2) on the outside and reaction (3) on the inside. The experiment was performed by controlling the oxygen partial pressure of the atmosphere in this manner.The calculation of the oxidation rate using the oxygen pump sensor was performed according to Faraday's law (Eq. 4). J=I/4F (4)where, F: Faraday constant, I: applied current, and J: amount of oxygen supplied to the measurement system by the oxygen pump. The amount of hydrogen was calculated from the supplied amount of oxygen J, and the oxidation rate was calculated. Results Fig.2 shows the results of a oxidation test in which the oxygen partial pressure P O2 of the Ni-25at.%Al alloy was changed from 10-19 atm to 10-6 atm at 1050 °C. In the figure, change in oxygen partial pressure, the oxidation current, and the oxidation rate are shown in (a), (b), (c), respectively. The P O2 change (a) shows that the initial P O2 is 10-19 atm. Thereafter, when the temperature reached 1050 ° C, the P O2 was increased to 10-6 atm. It can be seen from the change in the current value (b) that the current abruptly increased when the P O2 was changed. In other words, it can be seen that oxidation rapidly progressed when the P O2 was changed, which agrees with the oxidation rate result(c).The oxidation rate rapidly increased when P O2 was changed. then showed a low oxidation rate at 1250 °C.Fig.3 shows the cross-sectional microstructure and the corresponding element map by EPMA after the oxidation experiment when the P O2 was changed from 10-19 atm to 10-6 atm at 1050 °C. During the initial stage of the oxidation, the P O2 was lowered, and a thin Al2O3 film was formed on the surface by increasing the P O2 at 1050 °C. When the P O2 was changed, only Al2O3 was formed. Therefore, at the beginning of oxidation, the P O2 is reduced to avoid formation of Ni oxide. It was clarified that when the P O2 was increased at 1050 ° C at which the a-Al2O3 was formed, only Al2O3 with a high oxidation resistance was formed. The oxide formed in this manner is expected to have high oxidation resistance. Conclusion The accurate control of oxygen partial pressure in the furnace is successfully conducted by applying the oxygen sensor and oxygen pump sensor. Using the furnace, we the direct formation of Al2O3 on Ni3Al was confirmed.AcknowledgementsPart of this study was supported by the NIMS Joint Research Hub Program. Figure 1