Influence of manganese substitution into the A-site of perovskite type Ca1−xMnxTiO3 ceramic

Ca_1−xMn_xTiO₃ (x = 0–1.0) perovskite ceramics were prepared by conventional solid state reaction. XRD was used to confirm the microcrystalline nature of the Ca_1−xMn_xTiO₃ crystals. For the x = 0 composition, the XRD patterns were those of a single orthorhombic perovskite while for x = 0.2–0.8, the XRD spectra were those of two orthorhombic perovskite phases: CaTiO₃ and MnTiO₃. For x = 1, XRD pattern was that of the MnTiO₃ phase only. The morphology and particle size of the grains of the different composition were observed using SEM. The size of the particles increased from 0.2 μm to 2–3 μm as x increased from 0 to 0.6. The room temperature dielectric constant at the frequency of 110 kHz for the x = 0.2 and x = 1.0 ceramics were ∼3.41 × 10⁴ and ∼4.99 × 10³, respectively. The ESR linewidth of samples increased with increasing manganese content due to the formation of magnetic cluster. Our ESR studies indicate that the manganese ions are in the Mn⁴⁺ state.     

Nanocrystalline/nanosized manganese substituted nickel ferrites – Ni1−xMnxFe2O4 obtained by ceramic-mechanical milling route

Manganese substituted nickel ferrites, Ni_1−xMn_xFe₂O₄ (x=0, 0.3, 0.5 and 0.7) have been obtained by a combined method, heat treatment and subsequent mechanical milling. The samples were characterised by X-ray diffraction, differential scanning calorimetry and magnetic measurements. The increase of the Mn²⁺ cations amount into the spinel structure leads to a significant expansion of the cubic spinel structure lattice parameter. The crystallite size decreases with increasing milling time up to 120 min, more rapidly for the nickel–manganese ferrites with a large amount of Mn²⁺ cations (x=0.7). After only 15 min of milling the mean crystallites size is less than 25 nm for all synthesised ferrites. The Néel temperature decreases by increasing Mn²⁺ cation amount from 585 °C for x=0 up to 380 °C for x=0.7. The magnetisation of the ferrite increases by introducing more manganese cations into the spinel structure. The magnetisation of the milled samples decreases by increasing milling time for each ratio among Ni and Mn cations and tends to be difficult to saturate, a behaviour assigned to the spin canted effect.     

State of manganese atoms in the Nd1+x Sr2−x Mn2O7−δ solid solution

The oxidation state of manganese in the Nd_2?x Sr_1+x Mn₂O_7?δ solid solution was determined by X-ray photoelectron spectroscopy and by calculating the oxygen nonstoichiometry based on the gravimetric data. As a result of the heterovalent replacement of Nd³⁺ with Sr²⁺, the change in the oxidation state of manganese occurs in different ways, i.e., it increases at x > 0 and decreases at x < 0. In the latter case, some oxygen ions acquire the oxidation state of ?1. The samples slowly cooled under oxidative conditions possess a significant positive oxygen non-stoichiometry, which tends to decrease after Nd³⁺ is replaced with Sr²⁺. An excess of oxygen stabilizes the crystal structure of Nd_1+x Sr_2?x Mn₂O_7?δ.     

Characterization of structure and electrochemical properties of lithium manganese oxides for lithium secondary batteries hydrothermally synthesized from δ-KxMnO2

Lithium manganese oxides have attracted much attention as cathode materials for lithium secondary batteries in view of their high capacity and low toxicity. In this study, layered manganese oxide (δ-K_xMnO₂) has been synthesized by thermal decomposition of KMnO₄, and four lithium manganese oxide phases have been synthesized for the first time by mild hydrothermal reactions of this material with different lithium compounds. The lithium manganese oxides were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission (ICPE) spectroscopy, and chemical redox titration. The four materials obtained are rock salt structure Li₂MnO₃, hollandite (BaMn₈O₁₆) structure α-MnO₂, spinel structure LiMn₂O₄, and birnessite structure Li_xMnO₂. Their electrochemical properties used as cathode material for secondary lithium batteries have been investigated. Of the four lithium manganese oxides, birnessite structure Li_xMnO₂ demonstrated the most stable cycling behavior with high Coulombic efficiency. Its reversible capacity reaches 155 mAh g⁻¹, indicating that it is a viable cathode material for lithium secondary batteries.     

A microbial fuel cell using manganese oxide oxygen reduction catalysts

Microbial fuel cells (MFCs) are a potential method for enhanced water and waste treatment, which offer the additional benefit of energy generation. Manganese oxide was prepared by a simple chemical oxidation using potassium permanganate. Carbon-supported manganese oxide nanoparticles were successfully characterised as cathode materials for MFCs. The manganese oxide particles when used in a two-chamber MFC, using inoculum from an anaerobically digested sewage sludge, were found to exhibit similar oxygen reduction performance to that in separate electrochemical tests. MFC tests were conducted in a simple two chamber cell using aqueous air-saturated catholytes separated from the anode chamber by a Nafion membrane. MFC peak power densities were ca. 161 mW m^?2 for MnO _x /C compared to 193 mW m^?2 for a benchmark Pt/C, in neutral solution at room temperature. The catalyst materials demonstrated good stability in the 7.0–10.0 pH range. Theoretical (IR free) peak power densities were 937 mW m^?2 for MnO _x /C compared with 1037 mW m^?2 for Pt/C in the same experimental conditions: showing the MFCs performances can easily be improved by using more favourable conditions (more conductive electrolyte, improved cathode catalyst etc.). Our studies indicated that the use of our low cost MnO _x /C catalysts is of potential interest for the future application of MFC systems.     

High Catalytic Activity in CO Oxidation over MnO x Nanocrystals

Manganese oxides of various stoichiometry were prepared via Mn-oxalate precipitation followed by thermal decomposition in the presence of oxygen. A non-stoichiometric manganese oxide, MnO _x (x = 1.61…1.67) was obtained by annealing at 633 K and demonstrated superior CO oxidation activity, i.e. full CO conversion at room temperature and below. The activity gradually decreased with time-on-stream of the reactants but could be easily recovered by heating at 633 K in the presence of oxygen. CO oxidation over MnO _x in the absence of oxygen proved to be possible with reduced rates and demonstrated a Mars—van Krevelen—type mechanism to be in operation. A TEM structural analysis showed the MnO _x phase to form microrods with large aspect ratio which broke up into nanocrystalline manganese oxide (MnO _x ) particles with diameters below 3 nm and a BET specific surface area of 525 m²/g. Annealing at 798 K rather than 633 K produced well crystalline Mn₂O₃ which showed lower CO oxidation activity, i.e. 100% CO conversion at 335 K. The catalytic performance in CO oxidation of various Mn-oxides either studied in this work or elsewhere was compared on the basis of specific reaction rates.     

Comportement electrochimique des oxydes mixtes MexCo1 − x Mn2O4 (Me = Ni,Cu; 0,75 ⩾ x ⩾ 0,25)

It is demonstrated through the electrochemical reduction of cobalt manganese spinels that it is possible to increase the cathodic reactivity by replacement of cobalt ions by nickel and copper cations. The reduction reaction occurs on active sites formed by Mn⁴⁺ ions associated, in octahedral sites, with Mn³⁺ ions, for the Ni_xCo_{1 ? x} Mn₂O₄ compounds. For the copper manganites oxides, Cu_xCo_{1 ? x}Mn₂O₄, the electrochemical reaction is likely to occur by the redox on solid state between Mn³⁺ and Cu²⁺ cations.     

Gleichgewichtselektrodepotentiale von MeO2Hx(yH2O)-elektroden—I. Thermodynamik

Non-stoichiometric oxides of the general formula MeO₂H_x (yH₂O) in contact with aqueous electrolytes are discussed with regard to different equilibria between solid and liquid phase. Corresponding relations for the equilibrium electrode potential ε are derived.If the equilibria with respect to H⁺ and Me²⁺ ions and H₂O) are established at the same time, the value of ε is determined if two of the three variables pH, aα_Me²⁺ (activity of the Me²⁺ ions in the electrolyte) and x are given. As one further result of the thermodynamic treatment relations between differential changes of ε and of the composition of the solution are obtained. Discussing the pH dependence of the electrode potential it is shown that (∂ε/∂pH)aα_Me²⁺ increases with increasing value of x.This of particular importance for oxides with large stoichiometric width, eg manganese dioxide.Finally it is shown that the ratio between the activities of hydrogen and metal in the oxide is given by the corresponding ratio of hydrogen ions and metal ions in the solution.     

Magnetic properties of Cu1+xMn2−xO4 and Ni1+xMn2−xO4 solid solutions

Magnetic properties of two spinel oxides solid solutions, Cu_1+xMn_2−xO₄ and Ni_1+xMn_2−xO₄, are reported. These series are characterized by two magnetic transitions: the upper one, of ferrimagnetic type, occurs at about 85 K (for copper-based) and at 105–110 K (for nickel-based spinels), independently of the x-content; the lower transition may be related to a Néel-type collinear ordering and takes place at 30 and 45 K, respectively. Application of moderate fields (H > 250 Oe) make both transitions to merge into one broad maximum in the magnetization, which takes place at lower temperature when applying larger fields. Magnetization cycles with temperature (ZFC/FC) or field (loops) allowed us to well characterize the ordered state. The effective moment follows the expected behavior when manganese ions are being substituted by ions of lower magnetic moment (Ni²⁺ and Cu²⁺).     

Enhanced room temperature coefficient of resistivity (RT-TCR) and broad metal-insulator transition temperature (TMI) of La0.67Ca0.33-xAgxMnO3 polycrystalline ceramics

Polycrystalline La_0.67Ca_0.33-xAg_xMnO₃ (LCAMO, x = 0, 0.06, 0.15, 0.18, and 0.24) ceramics were fabricated by conventional sol-gel route at relatively low sintering temperature of 1100 °C for 12 h. Effects of silver content (x) on crystal structure, grain size, resistivity and magnetic properties of as-prepared LCAMO specimens were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), elemental mapping and energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and standard four-probe method (ρ-T). The data from XPS, XRD and EDS revealed that silver existed as Ag⁺ ions in the lattice matrix position of LCAMO ceramics. Broad metal-insulator transition temperature (T_MI) values ranging from 267.0 K (x = 0) to 302.6 K (x = 0.24) were obtained with LCAMO specimens prepared with variable Ag⁺ added contents. Peak temperature coefficient of resistivity (TCR) enhanced from 4.1% K⁻¹ at 263.2 K (for x = 0) to 10.9% K⁻¹ at 278.5 K (for x = 0.18), and reached 7.5% K⁻¹ at room-temperature (295.9 K) for x = 0.24. Meanwhile, magnetoresistance (MR) of materials reached 17.7% at room-temperature (299.2 K) for x = 0.24. Overall, these findings demonstrated that Ag doping was beneficial for improving electrical and magnetic properties of LCAMO materials. In summary, LCAMO ceramics achieved RT-TCR and MR at optimal Ag stoichiometric ratio, promising for applications in infrared bolometers or magnetic sensors.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Preparation and structures of trinuclear manganese(II) complexes with N-2-pyridiylmethylidene-2-hydroxy-5-substituted-phenylamine

The trinuclear manganese(II) complexes, [Mn^II₃(OAc)₄(pap)₂(H₂O)₂] and [Mn^II₃(OAc)₄(5-Cl-pap)₂(MeOH)₂] were prepared by the reaction of tridentate Schiff base ligands X-papH (X=H, Cl), [N-2-pyridiylmethylidene-2-hydroxy-5-substituted-phenylamine], and Mn^II(OAc)₂·2H₂O. In the molecular structures of these complexes, two terminal manganese ions are coordinated with one oxygen and two nitrogen atoms of X-pap, two oxygen atoms of OAc⁻ and a solvent molecule, to form a distorted octahedral structure where the central manganese ion resides on a center of symmetry and is surrounded by an O₆ donor set of four oxygen atoms from four bridging OAc⁻ and two phenolic oxygen atoms of two X-pap ligands.     

Influence of Ti4+ substitution for Ta5+ on the crystal structure,Raman spectra,and microwave dielectric properties of Ba3Ta4-4xTi4+5xO21 ceramics

This paper systematically investigated the influence of Ti⁴⁺ substitution for Ta⁵⁺ on the phase composition and microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1, 0.2, and 0.3) ceramics with hexagonal tungsten bronze-like structures. X-ray diffraction and Rietveld refinement results indicated that single-phase Ba₃Ta₄Ti₄O₂₁ could be obtained only with the x values of 0.1 and 0.2, and a secondary phase was detected at an x value of 0.3. The valence state of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.2) ceramics was analyzed through X-ray photoelectron spectroscopy. Increasing Ti⁴⁺/Ta⁵⁺ ratios could reduce sintering temperature and improve the microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ solid solutions. However, the dielectric properties, particularly the quality factor, of Ba₃Ta_4-4xTi_4+5xO₂₁ ceramics deteriorated severely as a result of oxygen vacancy defects caused by the transition of the valence state from Ti⁴⁺ to Ti³⁺ when x = 0.2 and the coexistence of the secondary phase when x = 0.3. Infrared reflectivity spectroscopy was performed to explore the intrinsic dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics. The measured and extrapolated microwave dielectric properties of Ba₃Ta_4-4xTi_4+5xO₂₁ (x = 0.1) ceramics sintered at 1240 °C for 6 h were ε_r ~ 46.5, Q × f = 13,900 GHz, τ_f ~ +49.4 ppm/°C, and ε_r ~ 44, Q × f = 34,850 GHz.     

(La,Sr)(Mn,Me)O3 manganites doped with d metals: Study of charge compensation mechanisms by crystallographic and magnetic characterizations

A comparison between theoretically calculated unit cell volume and interatomic distances in the system La_0.7Sr_0.3Mn_1−xMe_xO_3+δ (where Me = Cu, Fe, Cr, Ti) and the experimental data obtained by the full-profile Rietveld X-ray analysis as well as an analysis of magnetic properties allowed us to suggest possible mechanisms of charge compensation occurring when d metals substitute for manganese. It has been shown that in the case when copper, iron, chromium and titanium ions substitute for manganese ions in the system La_0.7Sr_0.3Mn_1−xMe_xO₃ charge compensation is described by the model 2Mn³⁺ → Mn⁴⁺ + Cu²⁺, Mn³⁺ → Fe³⁺, Mn³⁺ → Cr³⁺ and Mn⁴⁺ → Ti⁴⁺, respectively. In the latter case, a decrease in oxygen nonstoichiometry occurs with increasing x.     

Comparison of multi-valent manganese oxides (Mn4+, Mn3+, and Mn2+) doping in BiFeO3-BaTiO3 piezoelectric ceramics

Different manganese oxides-doping effects were compared in piezoceramic BiFeO₃-BaTiO₃ system. 0.67Bi_1.05(Fe_0.99Mn^x_0.01)O₃-0.33BaTiO₃ (valence state x = 4+, 3+, and 2+) ceramics were prepared via a solid-state reaction process followed by furnace-cooling (FC) or water-quenching (WQ) process. For the FC ceramics, the direct piezoelectric sensor coefficient (d₃₃) was almost independent of valence state of doped Mn, while d₃₃ depended on the fraction of Fe³⁺/Fe²⁺ in WQ ceramics. The d₃₃ value was highest for the donor Mn⁴⁺-doped ceramic, among the FC ceramics, with 175 pC/N. However, acceptor-doping with Mn²⁺ prevented the transition of Fe ion valence state from 3+ to 2+ in the WQ ceramics, the Mn²⁺-doped WQ ceramic showed the largest d₃₃ of 313 pC/N and converse piezoelectric actuator coefficient, d₃₃^* of 352 pm/V, with high Curie phase transition temperature (482 °C).     

Synthesis and electromagnetic properties of one-dimensional La3+-Doped mullite based on first-principles simulation

One-dimensional La³⁺-doped mullite of diameter 20–30 nm and length 2–3 μm was prepared by a molten salt method. The electrical properties of La_(x)Al_(6-x)Si₂O₁₃ (x = 0, 0.5%, 1.0%, 2.0%) can be effectively adjusted by the La³⁺ doping content. The electrical conductivity increased from 1.18 × 10⁻⁸ (x = 0) to 2.09 × 10⁻⁶ S cm⁻¹ (x = 2.0%). The real part of permittivity was improved from 2.5 (x = 0) to 3.2 (x = 2.0%) in the frequency range 2–18 GHz. The presence of La³⁺ increased the dielectric loss types and improved the characteristic impedance matching, leading to excellent electromagnetic wave absorption with a minimum reflection loss of −18 dB. First-principles calculations revealed that La doping can decrease the band gap of mullite from 3.677 to 3.541 eV and induce the formation of local dipole moments, which can ultimately improve both the electrical conductivity and the dielectric polarization.     

Enhanced energy storage performances of CaTiO3-based ceramic through A-site Sm3+ doping and A-site vacancy

Ceramic-based capacitors for energy storage devices require simultaneously high energy density and efficiency. Achieving high electric breakdown field based on linear dielectrics is crucial. Here, A-site Sm³⁺ doped perovskite Ca_1-1.5xSm_x□_0.5xTiO₃ ceramics with introduced A-site vacancies (V_A) were prepared. All Ca_1-1.5xSm_x□_0.5xTiO₃ ceramics crystallize in an orthorhombic structure, with lattice constants a, b, and c linearly decreased. As a result of Sm³⁺ dopants and V_A, the grain size decreased while the ceramics’ density was improved. The permittivity decreases from 176 (x = 0) to 135 (x = 0.1), but tanδ is effectively constrained (～10^?4). What’s more, the dielectric breakdown strength is significantly improved from 429 kV/cm (x = 0) to 547 kV/cm (x = 0.1) with dielectric linearity is maintained. The optimum energy storage density of 2 J/cm³ (x = 0.02) with ultrahigh energy efficiency of over 93.7 % is achieved, which are superior to many existing linear dielectrics and relaxor ferroelectrics. This work confirms the energy-storage enhancement through chemical modifications and microstructural engineering.     

Co-optimization of Na and K doping for improved room-temperature TCR of La0.7(Na0.3-xKx)MnO3 polycrystalline ceramics

Polycrystalline La_0.7(Na_0.3-xK_x)MnO₃ (LNKMO, x = 0.10, 0.15, 0.20, 0.25, and 0.26) ceramics were successfully compounded by adopting conventional sol-gel technology. The physical properties of as-prepared specimens were closely related to their morphology and internal structure, which were characterized and analyzed via X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Results confirmed that La⁺ ions located at A-sites in crystal lattice were partially substituted by doped Na⁺ and K⁺ ions, which resulted in rotation and distortion of MnO₆ octahedron. Lattice distortion was primary factor behind double exchange (DE) mechanism and Jahn-Teller (JT) effects. In addition, Na and K dopants altered relative amount of Mn³⁺ and Mn⁴⁺ ions, causing intensity variation in DE effect. These changes contributed to a decline in resistivity and an increase in peak resistance temperature (T_k) with increasing K doping level. Meanwhile, optimal temperature coefficient of resistance (TCR) value of LNKMO ceramics reached 8.48% K^?1 at 292.14 K when x = 0.25. This work reveals the mechanism of Na and K co-doping to optimize electrical transport properties of LNKMO manganese oxides and provides excellent material for the fabrication of uncooled infrared bolometers.     

Multiplicity of photoluminescence in Raman spectroscopy and defect chemistry of (Ba1−xRx)(Ti1−xHox)O3 (R = La,Pr, Nd,Sm) dielectric ceramics

(Ba_1?xR_x)(Ti_1?xHo_x)O₃ (R = La, Pr, Nd, Sm; x ≥ 0.04) (BRTH) ceramics were prepared using a mixed oxides method. The solubility limits in BRTH with R = La, Pr, Nd, Sm were determined by XRD to be x = 0.11, 0.12, 0.06, and 0.14, respectively. The ionic radius of R at Ti-site plays a decisive role in the solubility limit in BRTH. Only BRTH with R = La satisfied Vegard's law. The multiplicity of photoluminescence (PL) signals of Nd³⁺/Ho³⁺ and Sm³⁺/Ho³⁺ in Raman scattering under 532-nm excitation laser and the high-permittivity abnormality for the denser BRTH with R = Sm and at x = 0.07 were reported. The PL provided the evidence of a small number of Ho³⁺ at Ba-site in BRTH and it was determined that the number of Ba-site Ho³⁺ ions increased from 0.05 at% at R = La to 0.19 at% at R = Sm with increasing atomic number of light rare earth. BRTH exhibited a much broadened dielectric-temperature characteristics, marked by ×5 T, ×6 T, ×7 T, and ×8 S dielectric specifications for BRTH with R = La, Pr, Nd, Sm and at x = 0.06, respectively, and they exhibited lower dielectric loss (tan δ < 0.015) at room temperature. The dielectric-peak temperature (T_m) of BRTH decreased linearly at a rate of less than ?21 °C/%(R/Ho). The defect chemistry, solubility limit, lower dielectric loss, and dielectric abnormality are discussed.     

Novel synthesis method for quaternary Cd(Cu,Zn)Se thin films and its characterizations

Nowadays, the search for novel compounds by chemical synthesis is in trend. Herein, we report the deposition of Cd_1-x-yZn_xCu_ySe (0.025 ≤ x = y ≤ 0.15) films by facile, industry-oriented chemical synthesis. The Cd_1-x-yZn_xCu_ySe thin films were deposited at the optimized growth conditions (temperature = 70 ± 0.1 °C, pH = 10.3 ± 0.1, substrate rotation speed = 70 ± 2 rpm and time = 100 min). As-synthesized thin films were characterized for physical, chemical, topographical and electrical attributes. The study of vibrational modes in Cd_1-x-yZn_xCu_ySe thin films was done with the help of Raman spectroscopy. Improvement in surface topography with the integration of Cu²⁺ and Zn²⁺ into the CdSe lattice has been noticed by the atomic force microscopy (AFM). The electrochemical impedance spectroscopy revealed lower values of R_s and R_ct for x = y = 0.05 composition. Chemical deposition of Cd_1-x-yZn_xCu_ySe thin films may offer an excellent way to fabricate quaternary chalcogenide-based absorber materials for solar cells.