Cobaltite Spinel Research Articles

Iron doped manganese cobaltite spinel coatings produced by electrophoretic co-deposition on interconnects for solid oxide cells: Microstructural and electrical characterization

We report a systematic microstructural and electrical characterization of iron doped Mn–Co spinel coatings processed by electrophoretic co-deposition of Mn1.5Co1.5O4 and Fe2O3 powders on Crofer 22 APU and AISI 441 steel substrates. Iron addition to Mn–Co spinel coating leads to a reduction of the area specific resistance on both substrates, after 3200 h at 750 °C. The Fe doped Mn–Co coating both leads to a thinner oxide scale and reduces the sub scale oxidation for the Crofer 22 APU substrate. Fe doped Mn–Co on AISI 441 shows both a thicker oxide scale and low area specific resistance values, likely due to a doping effect of the oxide scale by minor alloying elements. The different mechanisms by which iron doping of Mn–Co spinels can influence elemental interdiffusion at the steel-oxide scale-coating interfaces and relative contributions to the overall area specific resistance are evaluated by means of advanced electron microscopy. The promising results are further confirmed in a cell test, where the Fe doped MnCo coated interconnect does not induce any degradation of the oxygen electrode, proving its efficiency.

A general hierarchical flower-shaped cobalt oxide spinel template: facile method, morphology control, and enhanced saturation magnetization

A robust and general method for producing cobaltite spinel nanostructures containing a broad array of additional elements is presented.

Nano spinel cobaltites and their catalytic and electrochemical properties: facile synthesis of metal (Co, Ni, and Zn) and mixed metal (Co–Ni and Co–Zn) complexes of Schiff bases prepared from α-ketoglutaric acid and ethyl carbazate

Nanocobaltites synthesized from solid solution precursors showed strong photocatalytic activity and enhanced electrochemical properties.

Simple and facile synthesis of pure nanocrystalline spinel cobaltite by thermal decomposition of solid solution precursors

Water assisted cobalt quinolinate complex with general formula [Co(Hqui)2(H2O)2], is used as the solid solution precursors for the synthesis of the spinel cobaltite at low temperature by convenient method (i.e., thermal decomposition). The cobalt precursor was characterized by spectroscopic, thermal and powder X-ray diffraction studies. The crystal and molecular structure of cobalt quinolinate complex is crystallized in monoclinic crystal system with P21/n space group. Generally, the quinolinate anion acts as a tridentate ligand to create polymeric structure, but in this complex, the anion attains bidentate (O, N) mode to generate monomeric structure and it is rare. The parent quinolinic acid (H2qui; λem. = 383 nm) and its complex displays a strong fluorescence emission at 405 nm when excitation at 330 nm. The compound exhibits endo-followed by exothermic decomposition to produce cobalt oxides around 450 °C by simultaneous TG–DTA method. The cobaltite (Co3O4) derived from the cobalt quinolinate solid solution precursor are obtained by heating the complex at 450, 550 and 650 °C in a silica crucible for 3 h in muffle furnace. The high homogeneity of spinel cobaltite nanoparticles were characterized by infrared, UV–Visible, powder X-ray diffraction, scanning electron microscope coupled with EDX analysis and transmission electron microscope studies. The characterization shows that the spinel cobaltite oxide is highly homogenous and nano crystalline nature (~ 70 nm).

Investigations on the enhanced photocatalytic activity of (Ag, La) substituted nickel cobaltite spinels

This work reports the synthesis of NiCo2O4 (NCO), Ni0.95Ag0.05Co2O4 (NACO), and Ni0.95Ag0.05Co1.95La0.05O4 (NACO) by a simple sol-gel route and the evaluation of their photocatalytic activity towards the organic dye Evans blue. The synthesized photocatalysts were characterized by analytical techniques such as XRD, FTIR, SEM, particle analyzer and UV–Vis spectroscopy. The XRD patterns proved that the prepared samples crystallized in the cubic spinel structure. The related structural parameters were also evaluated. The SEM micrographs revealed that all the samples possessed agglomerated uneven structure. The particle size was found to decrease with the addition of dopants. The photocatalytic efficiency was validated using UV–vis spectroscopy, which proved the relatively narrow bandgap of the spinels. The photocatalytic activity of the undoped and doped samples against the Evans blue dye under visible light irradiation has been systematically studied. The superior photocatalytic activity was witnessed for (Ag, La) co-doped NiCo2O4. Conclusively, a possible mechanism for the photo-degradation of the dye was established. Hence rare earth doped cobaltites serve as a potential candidate for resolving the crucial environmental pollution.

Nickel doped cobaltite spinel as a solar selective absorber coating for efficient photothermal conversion with a low thermal radiative loss at high operating temperatures

Concentrated Solar Thermal (CST) system efficiency depends mainly on the operating temperature. One of the key components in the CST system is the receiver tube to attain higher temperatures, where the challenge remains on developing a solar absorber coating with high photothermal conversion and thermal stability at high temperatures. Nickel doped cobaltite (NixCo3-xO4; 0 ≤ x ≥ 1) thin film has been developed with spinel oxide phase on top of stainless steel substrates using a simple wet-chemical dip coating method as a base absorber layer (absorptance, α = 0.92; emittance, ε = 0.14). On top of the base absorber layer, coating integrated with silica (SiO2) as an optical enhancement layer (OPEL) has been deposited to make the coating more selective (α = 0.94 and ε = 0.13). A thorough investigation has been done to characterize the samples through UV–Vis–NIR spectrophotometer, Fourier transform infra-red spectroscopy, X-ray diffraction, transmission electron microscopy and thermogravimetric analyzer for the optical and physiochemical properties. Besides, the optimized spinel coating exhibits a low radiative loss with a minimum thermal emissivity of 0.07 at 100 °C and 0.18 at 500 °C, which indicates that the nickel cobaltite spinels are a very good candidate for high temperature solar selective applications.

Cation-Substitution-Tuned Oxygen Electrocatalyst of Spinel Cobaltite MCo2O4 (M = Fe, Co, and Ni) Hexagonal Nanoplates for Rechargeable Zn-Air Batteries

The spinel cobalt oxide (Co3O4) nanoplate exposed with hexagonal {111} facets is demonstrated to be a highly active catalyst, while the effect of cation substitution on the oxygen electrocatalysis is still unclear. Herein, the electrocatalytic activity of cation-substituted spinel cobaltite MCo2O4 (M = Fe, Co, and Ni) nanoplates with the {111} facets is investigated systematically by experiments and theoretical calculations. For both oxygen reduction and evolution reactions, Ni-substituted Co3O4 hexagonal nanoplates show the best activity. It is mainly attributed to the increased surface energy per unit area and the enhanced oxygen species absorption ability, which are also evidenced by density functional theory calculations. Moreover, the three kinds of MCo2O4 nanoplates are applied in Zn-air batteries and the corresponding electrochemical performance is tested. Among the three batteries, NiCo2O4 hexagonal nanoplates also enable the highest peak power density of 110.3 mW cm−2 and the most stable discharge-charge voltage profiles for 50 cycles, indicating that NiCo2O4 nanoplates are the promising catalyst for further Zn-air battery applications. Besides, this work illustrates that the substitution of Co by Ni or Fe can remarkably change the electronic structural states, thus tuning the electrochemical properties of the hexagonal Co3O4 nanoplates.

Role of dilution on the electronic structure and magnetic ordering of spinel cobaltites

By means of the first-principles density functional theory ($\mathrm{DFT}+U$) calculations and experiments, we investigate the role of dilution on the structural, magnetic, electronic, and optical properties of the antiferromagnetic (AFM) spinel ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ having N\'eel temperature $({T}_{\text{N}})\ensuremath{\sim}30$ K. As the octahedral cobalt site of spinel lattice is diluted with Ge, Al, Ti, Ru, and Sn cations, we observe a substantial increase in the size of the unit cell as well as destruction of the long-range magnetic ordering with a spin-orbit compensation effect. The ferrimagnetic ordering in diluted inverse spinels such as ${\mathrm{Co}}_{2}\mathrm{\ensuremath{\Sigma}}{\mathrm{O}}_{4}$ ($\mathrm{\ensuremath{\Sigma}}$ = Ti and Sn) emerges due to the difference in the magnetic moments of two sublattices A (3.87 ${\ensuremath{\mu}}_{\text{B}}$) and B (4.16 ${\ensuremath{\mu}}_{\text{B}}$ for ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ and 5.19 ${\ensuremath{\mu}}_{\text{B}}$ for ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$). Experiments and DFT calculations indicate antiferromagnetic configuration for ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$, ${\mathrm{Co}}_{2}{\mathrm{AlO}}_{4}$ (${T}_{\text{N}}\ensuremath{\sim}4.8$ K) spinels with an equal and opposite moment of $\ensuremath{\sim}2.60$ ${\ensuremath{\mu}}_{\text{B}}$ in tetrahedral sites of divalent Co ions and negligible contribution from trivalent B-site Co due to the complete filling of ${t}_{2g}$ levels having a giant crystal field of $\ensuremath{\sim}2.5$ and 1.8 eV, respectively. However, in ${\mathrm{Co}}_{2}{\mathrm{GeO}}_{4}$ (${T}_{\text{N}}\ensuremath{\sim}20.4$ K) case AFM behavior originates due to the opposite spins at octahedral sites of divalent Co ions. The remaining spinels ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ (${T}_{\text{N}}\ensuremath{\sim}47.8$ K), ${\mathrm{Co}}_{2}{\mathrm{RuO}}_{4}$ (${T}_{\text{N}}\ensuremath{\sim}16$ K), and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ (${T}_{\text{N}}\ensuremath{\sim}41$ K) are more favorable to ferrimagnetic structure as evident from our magnetization measurements with a different temperature dependence of magnetic moments A(T) and B(T) at tetrahedral A and octahedral B sites, respectively. The variation in the energy band gap (${E}_{g}=1.68\ensuremath{\rightarrow}3.28$ eV for ${\mathrm{Co}}_{2}{\mathrm{RuO}}_{4}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{Co}}_{2}{\mathrm{GeO}}_{4}$) obtained from $\mathrm{DFT}+U$ calculations are in good agreement with our experimental results (${E}_{g}=1.52\ensuremath{\rightarrow}3.16$ eV) obtained from the diffusive reflectance spectroscopy. The extent of exchange splitting ${\mathrm{\ensuremath{\Delta}}}_{\text{EX}}^{{e}_{g}}$ of tetrahedral ${\mathrm{Co}}^{2+}$ varies between 1.8 and 1.3 eV for ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{AlO}}_{4}$, respectively. However, ${\mathrm{\ensuremath{\Delta}}}_{\text{EX}}^{{t}_{2g}}$ exhibits a decreasing trend ($5.2\ensuremath{\rightarrow}3.6$ eV for ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}\ensuremath{\rightarrow}{\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$) with increasing the lattice parameter, except for cobalt-orthogermanate ${\mathrm{Co}}_{2}{\mathrm{GeO}}_{4}$.

Elaboration, characterization and first principle studies of MnCo2O4 nanomaterials prepared from non-standard raw materials

Moroccan non-standard materials are used for the first time to synthesize nanoparticles of Manganese Cobaltite Spinel Oxide (MnCo2O4) at low temperature by the molten salt technique. The products have been characterized to determine their chemical compositions, phases and morphologies. The x-ray Diffraction (XRD) patterns show the formation of a single-phase MnCo2O4 nanoparticles at three different temperatures with a particle size between 29 and 41 nm. Fourier Transform-Infrared spectra (FTIR) analysis confirms the formation of spinel MnCo2O4 without any bands of parasitic phases or impurities, and gives, in good agreement with the results of Thermo-Gravimetric Analysis (TGA), ample information leading to the deduction of the cationic distribution of the products. Moreover, the majority of the elaborated nanoparticles have a polyhedral shape. First principal calculation results show that the inverse spinel structure with a ferrimagnetic ordering is the most stable configuration for MnCo2O4 which agrees with the experimental results.

Sol-gel synthesis, characterization, and supercapacitor applications of MCo2O4 (M = Ni, Mn, Cu, Zn) cobaltite spinels

High performance MCo2O4spinels (M = Ni, Mn, Cu, Zn) were synthesized by the sol gel method (citrate) and their capacitive behavior was investigated in alkaline electrolyte. Their structural, morphological, functional groups and textural properties were characterized by TG/DSC, XRD, SEM, FTIR, EDS and BET. The capacitive properties of spinel MCo2O4 samples were thoroughly investigated in 1 M KOH aqueous electrolyte using cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results revealed high stability of the samples and excellent electrochemical reversibility, and exhibited specific capacity depending on the nature of the transition metal ion M. A high specific capacitance of 285 F g−1 was measured for CuCo2O4 and a low capacitance of 158 F g−1 for ZnCo2O4.In addition, MCo2O4 spinels displayed good stability during long-term cycles with a cycling efficiency which exceeds75% after 1000 cycles. The obtained results classified MCo2O4 cobaltite spinels as most promising materials for their application in super capacitors.

Correction: Effect of cation substitution on the pseudocapacitive performance of spinel cobaltite MCo2O4 (M = Mn, Ni, Cu, and Co)

Correction for ‘Effect of cation substitution on the pseudocapacitive performance of spinel cobaltite MCo2O4 (M = Mn, Ni, Cu, and Co)’ by Shude Liu et al., J. Mater. Chem. A, 2018, DOI: 10.1039/c8ta00540k.

Studies on spinel cobaltites, MCo2O4 (M = Mn, Zn, Fe, Ni and Co) and their functional properties

Optimization of electrodes for charge storage with appropriate processing conditions places significant challenges in the developments for high performance charge storage devices. In this article, metal cobaltite spinels of formula MCo2O4 (where M = Mn, Zn, Fe, Ni and Co) are synthesized by oxalate decomposition method followed by calcination at three typical temperatures, viz. 350, 550, and 750°C and examined their performance variation when used as anodes in lithium ion batteries. Phase and structure of the materials are studied by powder x-ray diffraction (XRD) technique. Single phase MnCo2O4,ZnCo2O4 and Co3O4 are obtained for all different temperatures 350°C, 550°C and 750°C; whereas FeCo2O4 and NiCo2O4 contained their constituent binary phases even after repeated calcination. Morphologies of the materials are studied via scanning electron microscopy (SEM): needle-shaped particles of MnCo2O4 and ZnCo2O4, submicron sized particles of FeCo2O4 and agglomerated submicron particle of NiCo2O4 are observed. Galvanostatic cycling has been conducted in the voltage range 0.005–3.0V vs. Li at a current density of 60mAg−1 up to 50 cycles to study their Li storage capabilities. Highest observed charge capacities are: MnCo2O4 – 365mAhg−1 (750°C); ZnCo2O4 – 516mAhg−1 (550°C); FeCo2O4 – 480mAhg−1 (550°C); NiCo2O4 – 384mAhg−1 (750°C); and Co3O4 – 675mAhg−1 (350°C). The Co3O4 showed the highest reversible capacity of 675mAhg−1; the NiO present in NiCo2O4 acts as a buffer layer that results in improved cycling stability; the ZnCo2O4 with long needle-like shows good cycling stability.

Magnetic Properties of Spinel Cobaltite A Co<sub>2</sub>O<sub>4 </sub>(A=Zn and Li)

We synthesized polycrystals of cobaltite spinels LiCo2O4, ZnCo2O4, and their mixed crystal (Li1-xZnx)Co2O4, and performed the magnetic susceptibility and specific heat measurements. LiCo2O4 with the Weiss temperature ΘW ~ -114 K exhibits an antiferromagnetic transition at TN ~ 30 K. On the other hand, ZnCo2O4 with the Weiss temperature ΘW ~ -90 K exhibits the absence of magnetic phase transition down to low temperature (2 K), indicating the presence of strong frustration. Taking into account the absence of magnetic phase transition in the orbital-degenerate ZnCo2O4, it is suggested that the antiferromagnetic transition in the charge-orbital-degenerate LiCo2O4 is driven by the charge degree of freedom. Furthermore, the magnetic properties of (Li1-xZnx)Co2O4 suggest that the antiferromagnetic transition in LiCo2O4 is sensitively suppressed by diluting the charge degeneracy.

Nanostructured MnCo2O4 synthesized via co-precipitation method for SOFC interconnect application

Nanostructured manganese cobaltite spinel oxide (MnCo2O4) was successfully synthesized by co-precipitation method. X-ray diffraction (XRD) results revealed that the molar ratio of OH−/NO3− = 1.5 in the precipitation stage is more appropriate for achieving MnCo2O4 phase. The results of thermal analysis (TGA/DTA) along with the XRD results revealed that the MnCo2O4 phase remains stable up to 1050 °C. Morphological studies show that by increasing the calcination temperature from 350 to 550 and 1000 °C, morphology of the particles varies from plate-like to quasi spherical and polyhedral shape, respectively. Density and porosity measurement of cold pressed and sintered samples showed that by employing un-calcined powder, a dense sample with 98% of the theoretical density could be obtained at the sintering temperature of 1000 °C. Selected synthesized powders were coated on the AISI 430 ferritic stainless steel coupons and the results demonstrated that using un-calcined powder, leads to the formation of a highly dense and adhesive coating layer due to the very high tendency of un-calcined powder for densification.

Porous Spinel Cobaltite with Core-Shell Architectures As High-Performance Catalysts for Rechargeable Lithium Air Batteries

Nowadays, the lithium-oxygen battery has captured world-wide attention recently because of its extremely high theoretical energy density. A typical nonaqueous Li-O2 battery consists of a lithium−metal anode, organic electrolyte, and a porous air cathode exposed to gaseous O2 during cell operation. The air electrode is crucial to improve the electrochemical performance for rechargeable nonaqueous lithium–air batteries. Among the components of the air electrode, the catalyst is important in that it can enhance the charge reaction by reducing the voltage required to dissociate the reaction products (such as Li2O2) into lithium metal and oxygen. The electrocatalyst is currently being investigated with special attention being paid to solve the sluggish kinetics related to the main electrochemical reactions, as well as the instability of the discharge products. Recently, a variety of electrocatalysts, including noble metals, transition metal oxides and carbon-based materials, have been explored in Li-O2 cells with a high reversible capacity and a lower charge potential for oxygen evolution reaction (OER) than bare carbon. A problem for the cathode lies in that the solid discharge products are insoluble and thus precipitate in the pores of cathode, which gradually block the catalytic sites as well as the diffusion pathways of electrolyte and oxygen, especially for the inner part next to the separator, and eventually degrade the performance of Li-O2 batteries. Considering this point, how to improve the pore utilization and promote the O2 transport in the electrode plays an important role in determining the battery performance. Therefore, it is challenging and highly desirable to develop optimum electrocatalysts to efficiently catalyze Li-O2 reactions while simultaneously facilitate rapid oxygen and electrolyte diffusion. In this work, we present a facile solvothermal synthesis of 3D hierarchical spinel core-shell microspheres with a porous structure and applied as efficient 3D electrocatalysts in nonaqueous Li-O2 batteries. Such unique structure can improve the availability of the catalytic sites and facilitate the diffusion of electrons and reactants. Due to the synergistic effect of high catalytic activity of the NiCo2O4 and unique porous core-shell, the 3D hiercarchical NiCo2O4 catalyst shows superior specific capacity, rate capability, and cycle stability in Li-O2 batteries. Furthermore, other spinel oxides (e.g. MnCo2O4 and ZnCo2O4) are also have been synthesized through similar procedures, suggesting the generality and feasibility of this facile strategy. The superior catalytic performance of NiCo2O4 was further examined in Li-O2 batteries. It is found that the Li–O2 battery with the NiCo2O4 electrode exhibits rather stable specific capacities above 7000 mAh g-1 carbon for five cycles and can keep specific capacities to be 6100 mAh g-1 carbon after 10 cycles at a current density of 100 mA g-1 carbon. Such performance can make NiCo2O4 core-shell microspheres as one of the best mixed metal oxides catalysts in Li-O2 batteries. Remarkably, it is found that our strategy could be extended to fabricate other spinel catalyst electrodes, such as MnCo2O4 and ZnCo2O4, suggesting the generality and feasibility of this facile strategy. In summary, we have demonstrated controlled synthesis of hierarchical spinel core-shell microspheres by a facile solvothermal method. The spinel microspheres constructed by porous nanoplates can act as high-performance catalysts in Li-O2 batteries. By taking advantage of the superior electrocatalytic activity and porous unique features, the as-prepared catalyst electrode exhibited low overpotentials, high rate capacity as well as excellent long-term cyclability. Significantly, this work offers exciting possibilities for the development of new functional materials in high-density storage devices Acknowledgements The authors are thankful for the support by funding from the Singapore National Research Foundation (NRF-CRP10-2012-06).

Microstructural evolution in manganese cobaltite films grown on Crofer 22 APU substrates by pulsed laser deposition

Abstract The use of pulsed laser deposition to produce high-quality manganese cobaltite spinel coatings on Crofer 22 APU substrates has been investigated, and the microstructures of the coatings have been studied using a combination of X-ray diffraction and electron microscopy techniques. It is shown that deposition from ceramic Mn1.5Co1.5O4 targets at a chamber pressure of 2.8 × 10− 4 mTorr N2 with the substrate pre-heated to 400 °C resulted in smooth, dense polycrystalline coatings. These coatings contain a mixture of rock-salt (Mn,Co)O and FCC Co phases due to reduction of the target material in the plume during deposition. Post-deposition annealing at atmospheric pressure in laboratory air at 800 °C led to re-oxidation of the coatings to give spinel phases with a thin chromia layer at the interface with the substrate. An annealing time of 0.5 h resulted in a coating with a fine uniform mixture of tetragonal and cubic spinel phases. The presence of pores at the location of the prior Co grains and at the original coating surface was used to infer the mechanisms by which the re-oxidation occurs. An annealing time of 14 h led to phase segregation in the coatings with more Co-rich cubic spinel at the surface and at the coating/substrate interface, and more Mn-rich tetragonal spinel in the center of the coating. There is Cr in the cubic phase only, indicating that the phase segregation occurs due to outward diffusion of Cr from the substrate through the chromia layer. There are local variations in the porosity and surface morphology of the coating, which appear to be correlated with the orientation of the underlying substrate grains. These observations indicate that control of grain structure in the substrate may be necessary to promote microstructural stability in such coatings.

Observation of large low‐field magnetoresistance in spinel cobaltite: A new half‐metal

Low‐field magnetoresistance is an effective and energy‐saving way to use half‐metallic materials in magnetic reading heads and magnetic random access memory. Common spin‐polarized materials with low field magnetoresistance effect are perovskite‐type manganese, cobalt, and molybdenum oxides. In this study, we report a new type of spinel cobaltite materials, self‐assembled nanocrystalline NiCo2O4, which shows large low field magnetoresistance as large as –19.1% at 0.5 T and –50% at 9 T (2 K). The large low field magnetoresistance is attributed to the fast magnetization rotation of the core nanocrystals. The surface spin‐glass is responsible for the observed weak saturation of magnetoresistance under high fields. Our calculation demonstrates that the half‐metallicity of NiCo2O4 comes from the hopping eg electrons within the tetrahedral Co‐atoms and the octahedral Ni‐atoms. The discovery of large low‐field magnetoresistance in simple spinel oxide NiCo2O4, a non‐perovskite oxide, leads to an extended family of low‐field magnetoresistance materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

ALCHEMI studies of site occupancies in Cr-, Ni-, and Fe-substituted manganese cobaltite spinels

The effects of Cr, Ni, and Fe substitution into manganese cobaltite (MCO) spinels are of great interest due to the roles that the diffusion of these cations play in reaction layer development during high temperature exposure of MCO-coated alloys. Here we report a study on a series of model Cr-, Ni-, and Fe-substituted MCO spinel ceramics produced by consolidation of combustion-synthesized oxide powders. The cation site occupancies in these samples have been studied by X-ray spectrometry-based Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI) experiments in the transmission electron microscope, with the data being analyzed using the ordering tie-line approach. In Cr-substituted samples, the Cr ions lie on the octahedral B sites and the Co ions reside on the tetrahedral A sites with Mn occupying the remaining sites. In Ni-substituted samples, all of the Ni ions occupy the B sites and the Co and Mn ions tend to lie on A and B sites, respectively. In contrast to the Cr-substituted samples, there is some mixing of the Co and Mn ions on the two types of sites at lower Ni contents. In Fe-substituted samples with lower Fe contents, all of the Mn ions occupy the B sites, roughly equal proportions of Fe ions occupy the A and B sites, and Co ions fill the remaining sites. With increasing Fe content, the degree of order decreases which ultimately results in the High Fe sample exhibiting no channeling evidence for preferred site occupation. These ALCHEMI data could provide a useful insight into the role of cation sub-lattice site preference in the formation of reaction layers in MCO-coated stainless steels and superalloys.

Pure and Ni-substituted Co3O4 spinel catalysts for direct N2O decomposition

A series of NixCo1-xCo2O4 (0 ≤ x ≤ 1) spinel catalysts were prepared by the co-precipitation method and used for direct N2O decomposition. The decomposition pathway of the parent precipitates was characterized by thermal analysis. The catalysts were calcined at 500 °C for 3 h and characterized by powder X-ray diffraction, Fourier transform infrared, and N2 adsorption-desorption. Nickel cobaltite spinel was formed in the solid state reaction between NiO and Co3O4. The N2O decomposition measurement revealed significant increase in the activity of Co3O4 spinel oxide catalyst with the partial replacement of Co2+ by Ni2+. The activity of this series of catalysts was controlled by the degree of Co2+ substitution by Ni2+, spinel crystallite size, catalyst surface area, presence of residual K+, and calcination temperature.

Flash-sintering of MnCo2O4 and its relation to phase stability

Abstract The flash-sintering behavior of manganese cobaltite spinel (MnCo 2 O 4 ) is analyzed in the present work. It is shown that the MnCo 2 O 4 is flash-sintered at 120–150 °C under 15.0–17.5 V cm −1 , which is substantially lower than the conventional-sintering temperatures of 1080 °C and more. We have also demonstrated that the flash-sintering is a transient phenomenon, where the power dissipation rises quickly at first and then results to Joule heating. The extent of sintering is confirmed through SEM, where a dense and pore - free morphology is observed for the well-sintered samples. The growth of secondary phase as a function of sintering temperature in both conventional and flash processes is monitored by XRD. Consistent changes in I – V curves observed at 200–700 °C, suggest that the rapid increase of the conductivity during flash-effect follows the hopping mechanism of usual conductivity phenomenon. On the basis of correlation between the conductivity, phase-stability and microstructure, a mechanism for flash-sintering has been proposed.