Magnetic properties of Co3O4 polycrystal powder

The results of investigations of magnetic properties of Co₃O₄ polycrystals in powder morphology with average crystallite size of 10 µm are presented. The temperature dependence of the magnetic susceptibility, measured in a wide temperature interval (1.5≤T≤400 K) using SQUID magnetometer, as well as electron spin resonance (ESR) spectra measured at X-band and Q-band frequency ranges have been studied. Antiferromagnetism with a Néel temperature T_N at about 39 K was observed from the analysis of evolution of molar magnetic susceptibility and ESR intensity as a function of temperature. ESR parameters namely g-factor, intensity (I_ESR), g-value, and linewidth ΔH for powder Co₃O₄ have been obtained. Some deviations from the expected values of some magnetic properties were determined and discussed.     

Methyl ethyl ketone combustion over La-transition metal (Cr, Co, Ni, Mn) perovskites

A series of LaBO₃ (B = Cr, Co, Ni, Mn) and La_0.9K_0.1MnO_3+δ perovskites have been prepared and tested as catalysts in the combustion of methyl ethyl ketone (MEK) at two concentration levels in air. Complete MEK conversion can be achieved for the most concentrated stream (1250 ppmv, WHSV = 425 h⁻¹) at temperatures between 270 °C (manganite) and 345 °C (chromite). Activity is governed by the nature of the cation in position B and related to reducibility, being comparable for manganite activity with that of the much more expensive Pt-supported catalysts. Doping with K of lanthanum manganite produces an increase in surface area, as well as the formation of non-stoichiometric oxygen and a greater proportion of Mn⁴⁺ on the surface. All these factors may have a role in increasing its activity for catalytic combustion. Catalytic results suggest a marked influence of MEK concentration on the combustion rate. MEK oxidation to CO₂ goes through acetaldehyde as intermediate product; methyl vinyl ketone and diacetyl (2,3-butanedione) were also formed, albeit in very low amounts. Nevertheless, acetaldehyde yield is zero at complete conversion, so the combustion of MEK can be carried out over these perovskite systems with 100% selectivity for CO₂.     

Multiferroic properties of La/Er/Mn/Co multi-doped BiFeO3 thin films

Bi_0.9-xLa_xEr_0.1Fe_0.96Co_0.02Mn_0.02O₃ (BLa_xEFMCO) thin films were prepared by sol-gel method. The grain size, grain boundary resistance, oxygen vacancies and the amount of Fe²⁺ of the films were reduced by multi-ion doping to reduce the built-in electric field of the films. An applied voltage was adopted to regulate the effects of the directional alignment of the oxygen vacancies, defects, and defect pairs on the ferroelectric domains at the grain boundaries to control the ferroelectric polarization of the films. Meanwhile, the capacitance peak also reveals the effects of the ferroelectric domains switching, the migration of oxygen vacancies, and the directional alignment of defect pairs on the ferroelectric properties. In addition, the remnant polarization value of the BLa_0.01EFMCO thin film reaches 152?μC/cm², the squareness of the hysteresis loop (R_sq) is calculated to be 1.03, and the maximum switching current is 1.50?mA. The typical butterfly curves under positive and negative electric fields indicate the films with the enhanced ferroelectric properties. Moreover, the BLa_0.01EFMCO thin film exhibits the enhanced ferromagnetic properties, and its saturation magnetization (M_s) is 2.32 emu/cm³. Therefore, the ferroelectric properties of the BFO film can be enhanced by the multi-ion doped BFO film to reduce the grain boundary resistance (Rgb), the interface Schottky barrier formed by the asymmetric electrode material at the top and bottom of the film, and the built-in electric field formed by the film internal defect or defect pairs.     

Formation and properties of high entropy oxides in Co-Cr-Fe-Mg-Mn-Ni-O system: Novel (Cr,Fe,Mg,Mn,Ni)3O4 and (Co,Cr,Fe,Mg,Mn)3O4 high entropy spinels

Possibility of formation of quinary and senary equimolar high entropy oxides from the Co-Cr-Fe-Mg-Mn-Ni-O system is presented. Different proposed compositions are synthesized using the solid-state reaction route at high temperatures (900−1100 °C) and quenched to room temperature. Phase composition of the samples is studied, showing tendency toward formation of two main phases: rock salt-structured Fm-3 m and spinel-structured Fd-3 m. It is documented that the annealing temperature has a profound effect on stability of both structures, and at 1100 °C usually the highest content of Fm-3 m phase is usually observed. Three different oxides, namely, (Co,Cr,Fe,Mn,Ni)₃O₄, (Co,Cr,Fe,Mg,Mn)₃O₄ and (Cr,Fe,Mg,Mn,Ni)₃O₄ are obtained as single-phase materials, which structure can be described as the high entropy Fd-3 m spinel one. The latter two compounds have not been previously reported in the literature. Activated character of the electrical conductivity dependence on temperature is observed, with relatively high total conductivity at high temperatures and corresponding high absolute values of Seebeck coefficient.     

Mechanical and physical properties of LiNi0.33Mn0.33Co0.33O2 (NMC)

Lithium Nickel Manganese Cobalt Oxide (NMC) is one of the most common oxide cathode materials for Li-ion batteries. NMC is also under consideration for use in all solid-state batteries. However, differences in the coefficients of thermal expansion (CTE) between NMC and the solid electrolyte during composite electrode fabrication and differential expansion and contraction during electrochemical cycling will cause stresses possibly resulting in electrode fracture and battery capacity fade. As a consequence, we hot-pressed phase-pure polycrystalline NMC with controlled density and accurately measured the mechanical (elastic modulus, shear modulus, Poisson’s ratio and nanoindentation hardness) and physical properties (CTE and thermal conductivity). We believe that this is the first report of the mechanical and physical properties of commercially available NMC and these experimental data are important to predict or increase the cycle life of NMC as a cathode material for state-of-the-art Li-ion and advanced solid-state batteries.     

Physicochemical properties of rock salt-type ordered Sr2MMoO6 (M = Mg,Mn, Fe,Co, Ni) double perovskites

B-site rock salt-type ordered Sr₂MMoO₆ (M = Mg, Mn, Fe, Co, Ni) oxides have been systematically investigated in terms of their crystal structure, oxygen non-stoichiometry, transport properties, thermal expansion and chemical stability. Structural evolution on temperature of the studied materials was interpreted on a basis of octahedra rotation, with I4/m → Fm-3m phase transition. In the case of P2₁/n → I4/m transformation recorded for Sr₂MnMoO₆, a region of coexistence of two phases was observed. The Fe-containing compound was found to possess very high electrical conductivity, around 1000 S cm⁻¹ in 5 vol.% H₂ in Ar, and small negative Seebeck coefficient. Studies show that Mn- and Fe-containing compounds are stable only in reducing conditions, Co- and Ni-containing ones are stable only in oxidizing conditions, while Sr₂MgMoO₆ remains stable in a wide range of oxygen partial pressures. Recorded power density of a button-type SOFC with Sr₂FeMoO₆-based anode was 0.32 W cm⁻² at 900 °C.     

Magnetic and electrical properties of La0.7Ca0.3Mn0.95Co0.05O3 epitaxial layers by pulsed laser deposition

Epitaxial La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (LCMCO) thin films were prepared on (1 0 0) LaAlO₃ single-crystal substrates by pulsed laser deposition (PLD). We have been studied using X-ray diffraction (XRD), electrical transport magneto-transport and dc magnetization. XRD pattern reflects that all films have c-axis epitaxial growth on LaAlO₃ substrates. The decrease in out-of-plane cell parameter specifies a progressive relaxation of in the plane compressive strain as the thickness film is increases. From the dc magnetization measurements, it is observed that ferromagnetic to paramagnetic transition temperature increases with increase in the film thickness. Magneto-resistance and temperature coefficient of resistance increases with thickness of film and have maximum value near its metal to insulator transition temperature.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr,Co, Fe,Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Microwave dielectric characteristics of Li2(Mg0.94M0.06)Ti3O8 (M=Zn,Co, and Mn) ceramics

Li₂(Mg_0.94M_0.06)Ti₃O₈ (M=Zn, Co, and Mn) ceramics were synthesized by the conventional solid-state reaction route. The effect of M (Zn, Co, and Mn) substitution on the structure, microstructure and microwave dielectric properties of Li₂(Mg_0.94M_0.06)Ti₃O₈ has been investigated. The XRD patterns of sintered samples revealed the single-phase formation with spinel structure. With the increase in ionic radius of M, the Qf value decrease is attributed to the decrease of packing fraction and grain size. The Li₂(Mg_0.94Zn_0.06)Ti₃O₈ ceramic sintered at 1075 °C for 4 h showed the best microwave dielectric properties with a dielectric constant of 27.1, a Qf value of 44 800 GHz, and a temperature coefficient of resonant frequency of (+)1.9 ppm/°C.     

Co from partial reduction of La(Co,Fe)O3 perovskites for Fischer–Tropsch synthesis

Small Co clusters (d<10 nm) supported over mixed La–Co–Fe perovskites were successfully synthesized. These catalysts were active for Fischer–Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO₃ is maintained for the mixed perovskite when x>0.5, the orthorhombic phase of LaFeO₃ is found for x<0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co–Fe mixed perovskites (x≥0.5) cannot be used as precursors for Fischer–Tropsch catalysts: their partial reduction only consists in a complete reduction of Co³⁺ into Co²⁺.

The partial reduction of orthorhombic perovskites (x<0.5) leads to active Fischer–Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600 °C, a maximum of 8.6 wt.% of Co⁰ can be extracted from LaCo_0.40Fe_0.60O₃ (compared to only 2 wt.% after calcination at 750 °C). The catalyst is then composed of Co⁰ particles of 10 nm on a stable deficient perovskite LaCo_0.05³⁺Fe_0.60³⁺O_2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 °C, 40% of which was composed of the C₂–C₄ fraction.     

Investigation of Structural,Morphological and Magnetic Properties of MFe2O4 (M = Co,Ni, Zn,Cu, Mn) Obtained by Thermal Decomposition

The structural, morphological and magnetic properties of MFe₂O₄ (M = Co, Ni, Zn, Cu, Mn) type ferrites produced by thermal decomposition at 700 and 1000 °C were studied. The thermal analysis revealed that the ferrites are formed at up to 350 °C. After heat treatment at 1000 °C, single-phase ferrite nanoparticles were attained, while after heat treatment at 700 °C, the CoFe₂O₄ was accompanied by Co₃O₄ and the MnFe₂O₄ by α-Fe₂O₃. The particle size of the spherical shape in the nanoscale region was confirmed by transmission electron microscopy. The specific surface area below 0.5 m²/g suggested a non–porous structure with particle agglomeration that limits nitrogen absorption. By heat treatment at 1000 °C, superparamagnetic CoFe₂O₄ nanoparticles and paramagnetic NiFe₂O₄, MnFe₂O₄, CuFe₂O₄ and ZnFe₂O₄ nanoparticles were obtained.     

Crystal structure,magnetic properties,and magnetocaloric effect in B-site disordered RE2CrMnO6 (RE = Ho and Er) perovskites

In this work, polycrystalline oxides viz., Ho₂CrMnO₆ (HCMO) and Er₂CrMnO₆ (ECMO), were prepared using the sol-gel process, and their crystal structure, magnetic properties, and magnetocaloric effects (MCEs) were studied. X-ray refinement results demonstrate that both oxides exhibited the B-site disordered perovskite type structure (Pbnm space group). A ferromagnetic-paramagnetic phase transition as well as large reversible MCEs was also observed at Curie temperatures (T_C) of ~6.1 and ~5.2 K for HCMO and ECMO, respectively. For a magnetic field change (ΔH) of 0–7 T, the maximum magnetic entropy change (-ΔS_M), temperature-averaged entropy change (TEC 3), and relative cooling power (RCP) were estimated to be 11.03 J/kgK, 11.02 J/kgK, and 322.7 J/kg for HCMO, and 12.94 J/kgK, 12.80 J/kgK and 277.5 J/kg for ECMO.     

Structural,magnetic, electrical and electrochemical properties of SrCoO2.5, Sr9Co2Mn5O21 and SrMnO3 compounds

SrCo_1−xMn_xO_3−d (0 ≤ x ≤ 1) were synthesized by conventional solid state reaction method. We found that Mn substitution hinders the decomposition of carbonate at 900 °C and increases the synthesis temperature of SrCo_1−xMn_xO_3−d to 1200 °C. Three different main phases emerge as the Mn content is increased; SrCoO_2.5, Sr₉Co₂Mn₅O₂₁ and SrMnO_3. Structural properties of the samples were investigated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Valance states of the transition metal (TM) ions were estimated based on lattice parameters and ionic radii of TM ions. High temperature resistivity measurements show hysteresis upon heating and cooling and is attributed to oxygen release without any endothermic/exothermic activity. The spin configurations of Co and Mn ions in SrCoO_2.5, Sr₉Co₂Mn₅O₂₁ and SrMnO₃ were estimated by Curie-Weiss fitting of χ-T curves. Electrochemical measurements show two anodic/cathodic peaks indicating two main types of oxygen sites in the unit cell.     

Hexavalent (Me - W/Mo)-modified (Ba,Ca)TiO3-Bi(Mg,Me)O3 perovskites for high-temperature dielectrics

We report on the synthesis of complex lead-free perovskite-type (1−x)(Ba_0.8Ca_0.2)TiO₃–xBi(Mg_0.75W_0.25)O₃ (BCT-xBMW) and (1−x)(Ba_0.8Ca_0.2)TiO₃-xBi(Mg_0.75Mo_0.25)O₃ (BCT-xBMM) solid solutions via conventional solid-state reaction route. The sintering temperature was adjusted as a function of composition x to obtain dense samples (relative densities over 95%) at the same time minimizing bismuth evaporation. X-ray diffraction analysis shows the formation of single-phase perovskites for 0 ≤ x ≤ 0.10 in the BCT-xBMW series and increasing concentrations of impurity phases for x ≥ 0.15 and for x ≥ 0.05 in BCT-xBMM. A transition from a tetragonal to pseudo-cubic perovskite structure is observed in BCT-xBMW and BCT-xBMM at x = 0.05. The dielectric response has been characterized between −60°C and 300°C for BCT-xBMW, and between 30°C and 300°C for BCT-xBMM using impedance spectroscopy, showing a transition from ferroelectric to relaxor-like behavior at x ≥ 0.05. Additional polarization and Raman spectroscopy measurements reveal the occurrence of highly disordered systems. Analysis of the Raman spectra indicates structural phase changes and lattice modifications caused by chemical substitution. For the composition 0.8Ba_0.8Ca_0.2TiO₃-0.2Bi(Mg_0.75W_0.25)O₃, a temperature-stable permittivity of about 600 (±15% between −60°C and 300°C) and small losses of tanδ < 0.02 for T ≤ 230°C at 1 kHz are observed_, making it a suitable dielectric material for high-temperature capacitors.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr, Co, Fe, Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Electrochemical surface properties of Co3O4 electrodes

The electrochemical behaviour of Co₃O₄ layers deposited by thermal decomposition of Co(NO₃)₂ at 200–500°C on titanium supports with and without an interlayer of RuO₂ has been studied by cyclic voltammetry, chronopotentiometry and potential step experiments in alkaline solutions. Such variables as the calcination temperature, the solution pH, the potential sweep rate and the oxide loading have been investigated in detail to determine their influence on voltammetric peaks and voltammetric charge. Insight has been gained into the relevance of the latter to surface area determination and to proton diffusion into the oxide layer. The role of the support-active layer interface and especially that of the RuO₂ interlayer has been scrutinized. The importance of surface studies for the understanding of the electrocatalytic behaviour of Co₃O₄ electrodes has been analysed.Presented at the 37th Meeting of the International Society of Electrochemistry, Vilnius, USSR, 25–29 August 1986.     

Rapid microwave-assisted synthesis and magnetic properties of high-entropy spinel (Cr0.2Mn0.2Fe0.2Co0.2-xNi0.2Znx)3O4 nanoparticles

High-entropy oxide (HEO) has recently become popular because of its unique multifunctional performance. In this study, we developed a novel microwave-assisted method for the production of HEO nanoparticles with the composition (Cr_0.2Fe_0.2Mn_0.2Co_0.2-xNi_0.2Zn_x)₃O₄ (x = 0, 0.05, 0.1, and 0.2). The results revealed that all metallic elements were uniformly distributed throughout the single-phase cubic spinel structure of the HEO nanoparticles. The particle size distributions of four fabricated samples ranged from 10 to 50 nm. Because of its numerous advantages such as the ultrafast and low-temperature fabrication of nanoscale and high-purity products at a relatively low cost, the suggested methodology is an excellent synthesis method. The original HEO spinel (x = 0) achieved saturated magnetization (M_s) and coercivity (H_c) values of 24.3 emu/g and 160 Oe, respectively, at room temperature. Zinc substitution in the HEO composition indicated that M_s and H_c decreased with increasing zinc concentration owing to its non-magnetic nature.