Co-rich Oxide Research Articles

Surface Reconstruction on Metal Nitride during Photo-oxidation.

The efficient conversion and storage of solar energy for chemical fuel production presents a challenge in sustainable energy technologies. Metal nitrides (MNs) possess unique structures that make them multi-functional catalysts for water splitting. However, the thermodynamic instability of MNs often results in the formation of surface oxide layers and ambiguous reaction mechanisms. Herein, we present on the photo-induced reconstruction of a Mo-rich@Co-rich bi-layer on ternary cobalt-molybdenum nitride (Co3 Mo3 N) surfaces, resulting in improved effectiveness for solar water splitting. During a photo-oxidation process, the uniform initial surface oxide layer is reconstructed into an amorphous Co-rich oxide surface layer and a subsurface Mo-N layer. The Co-rich outer layer provides active sites for photocatalytic oxygen evolution reaction (POER), while the Mo-rich sublayer promotes charge transfer and enhances the oxidation resistance of Co3 Mo3 N. Additionally, the surface reconstruction yields a shortened Co-Mo bond length, weakening the adsorption of hydrogen and resulting in improved performance for both photocatalytic hydrogen evolution reaction (PHER) and POER. This work provides insight into the surface structure-to-activity relationships of MNs in solar energy conversion, and is expected to have significant implications for the design of metal nitride-based catalysts in sustainable energy technologies.

Cognition on oxidation behavior of Ni-based superalloy GH4742 when exposed to water vapor

The oxidation behavior of Ni-based superalloy GH4742 exposed to water vapor was investigated at 900–1100 °C for 50 h in Ar+20%H2O, with Ar+20%O2 included for comparison. The presence of H2O is adverse to formation of continuous Cr2O3 oxide layer but contributes to generation of big voids. Based on this, Ni2+ and Co2+ are more easily to diffuse towards the outermost oxide layer (OOL), forming non-protective Ni- and Co-rich oxides with high proportion. This leads to a higher weight gain in Ar+20%H2O than that in Ar+20%O2. At elevated temperatures, the spallation of oxide scale occurs while the consumption of Cr is reduced in Ar+20%H2O, and the contents of Ni- and Co-rich oxides at this moment are reduced in the OOL. The oxidation mechanism was also discussed.

Microstructural and oxidation effects on fatigue crack initiation mechanisms in a turbine disc alloy

Effects of microstructure and oxidation on fatigue crack initiation and early propagation processes were investigated in RR1000 turbine disc alloy with different γ′ distributions and carbide distributions on the grain boundary. Fatigue tests were carried out under three-point bending and trapezoidal waveform loading (with a 90 s dwell) at 650 °C in air. The failure mode in both γ′ variants is clearly characterised by intergranular features. A number of fatigue cracks are seen to initiate at grain boundaries with bulged Co-rich oxides at the surface and/or interfaces between carbides and grain boundaries, resulting from oxidation damage assisted by applied loading. Reduced lifetime is closely linked to significant intergranular crack initiation and frequent consequent crack coalescence events, which results in enhanced fatigue crack growth (FCG) rates. The extent of intergranular features and enhanced FCG are more marked where more continuous carbides exist at the grain boundary.

Sulfur poisoning behavior of La1-xSrxCo1-yFeyO3-δ thin films with different compositions

Sulfur in the air stream is one of the primary contaminants affecting the stability of cathodes in solid oxide fuel cells (SOFCs). In this work, sulfur poisoning of La1-xSrxCo1-yFeyO3-δ (LSCF) thin films with different compositions was investigated. The composition of the cathode was systematically controlled by preparing La0.75Sr0.42Co0.15Fe0.68O3-δ (A-site excess LSCF) and La0.60Sr0.39Co0.20Fe0.81O3-δ (stoichiometric LSCF) thin films on Gd-doped ceria (GDC) substrates by pulsed laser deposition. The LSCF films were then heat-treated at 800 °C for 100 h in synthetic air with a trace amount of SO2 (ppb level). As an informative complement to the microstructural and compositional analyses, secondary ion mass spectrometry was also performed to effectively determine the extent of sulfur accumulations from LSCF cathode surface to GDC. The results showed that sulfur poisoning behavior of LSCF cathodes depends strongly on composition. The A-site excess LSCF films exhibited significant SrSO4 formation with severe porosity, whereas, only an isolated SrSO4 formation was observed in the stoichiometric LSCF films; instead, the partial poisoning by sulfur minimizes excessive porosity while enhancing Co-rich oxide precipitation. Electrochemical impedance spectroscopy was carried out to further elucidate the influence of cathode composition on cell stability. Thermodynamic considerations have been made to clarify the plausible chemical reaction mechanism between LSCF of different compositions with SO2.

The role of Dy doping on oxidation behavior of Co-40Mn/Co coating for solid oxide fuel cell metal interconnects

Co-38Mn-2Dy and Co-40Mn alloy coatings are prepared by using high-energy micro-arc alloying process (HEMAA) and a Co transition layer is firstly obtained on 304 SS. Microstructures and compositions of Co-Mn(Dy)/Co coatings after preparation and oxidation are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Coatings are the splash appearance and fine-grained structure with a metallurgical bonding to substrate. The addition of Dy in Co-Mn coating can effectively improve the oxide scale adhesion and reducing the high temperature oxidation rate of coating. The outward diffusion of Fe and Cr from substrate is remarkably suppressed by the external oxide layer for Co-38Mn-2Dy/Co coating. And Dy-rich oxides, Mn-rich oxides and oxide pegs are found in oxide layer after oxidation for 100 h, respectively. Co-rich oxides favor to form on the top scale because of high oxygen partial pressure. The value of area specific resistance (ASR) is measured to be low for Co-38Mn-2Dy/Co coating at 800 °C.

Influence of orientation-dependent grain boundary oxidation on fatigue cracking behaviour in an advanced Ni-based superalloy

Fatigue tests have been conducted on an advanced disc Ni-based superalloy [low solvus, high refractory (LSHR) alloy] at 650 °C in air under three-point bend loading to investigate the role of orientation-dependent grain boundary (GB) oxidation in crack initiation and early propagation. It is found that crack initiation occurs mainly from bulged GB oxides, and cracks then predominantly propagate along the oxidised grain boundaries. These bulged oxides are extremely enriched in Co and preferentially form at the boundaries between high and low Schmid factor grains which are inclined normal to the applied tensile stress direction. Meanwhile, relatively flat/thin Ni/Ti/Al-rich oxide complexes also form at other grain boundaries, but they appear to be much less detrimental in fatigue crack initiation and propagation compared with the bulged GB Co-rich oxide complexes.

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725°C under three-point loading. Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650°C to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10−2–10−3Pa. Cracks mainly initiate from grain boundaries or γ/γ′ interfaces due to the formation and subsequent cracking of Cr-rich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature.

Acid-Delithiated Li1-x(NiyCo1-y)1+xO2 as Insertion Electrodes in Lithium Batteries

Monophase Li1-x(NiyCo1-y)1+xO2 oxides (0 ≤ y ≤ 0.4 and 0.8 ≤ y ≤ 0.9) with a layered structure (R3̄m space group) have been prepared by interaction of (NiyCo1-y)3O4 (0 ≤ y ≤ 0.4) and NiyCo1-yO (0.8 ≤ y ≤ 0.9) with LiOH at low temperature (450°c). As in the case of high-temperature analogues, cobalt substitution for nickel stabilizes the Ni3+ ions and improves the two-dimensionality of the crystal lattice. Acid treatment of LT and HT oxides leads to a partial dissolution of the solid and to removal of lithium and "impurity" cobalt and/or nickel from LiO2 layers of the trigonal framework. For cobalt-rich oxides, the ion extraction is accompanied by partial Li+/H+ exchange, especially for LT samples. Acid-delithiated oxides are studied as cathode materials in lithium batteries. The resulting cells show a high value of the initial voltage (4 V), which allows a direct discharge of the cell without previous charge. For LT samples, the discharge and charge processes occur at lower voltage (especially for Co-rich oxides) and the reversibility of the cell is lower as compared to that of HT analogues. The better reversibility of the cell in the 4.0-3.2 V range is found for HT Ni-rich oxide. The composition of the acid-treated oxides can be recovered without an enhancement polarization of the cell. XRD analysis and IR spectroscopy show good reversibility of lithium insertion and extraction reactions by chemical and electrochemical procedures.

Oxidation of four palladium-rich ceramic fusing alloys

Oxidation of four ceramic fusing alloys with varying amounts of palladium (35–79 wt%) and with different oxidizing elements (In, Sn, Ga, Cu, Co) was studied by scanning electron microscopy. X-ray diffraction and X-ray photoelectron spectroscopy. The alloy containing cobalt and gallium oxidized mainly externally forming a rather continuous Co-rich oxide layer on the surface and a Ga-rich layer underneath it. The other alloys were oxidized internally. The oxidation behaviour of Pd was found to be strongly dependent on the other oxidizing constituents.