Increase Of Mn4 Research Articles

Bi3+ and Mn4+ co-doped La2MgGeO6 blue-red tunable emission phosphors based on energy transfer for agricultural applications

Nowadays, the requirements for plant lighting for plant growth are becoming stronger. Under excitation of ultraviolet light 200–350 nm, Bi3+ and Mn4+ doped La2MgGeO6 phosphors can emit blue and red, respectively. By co-doping, blue-red emission can be achieved, which can be used in agricultural production. Based on energy transfer from Bi3+ to Mn4+, the luminescence intensity of Mn4+ can be doubled. The transfer efficiency of Bi3+ to Mn4+ increases linearly with the increase of Mn4+ concentration. The blue/red intensity ratio can be adjusted to meet the different needs of different plants by changing the doping concentrations of Bi3+ and Mn4+. Our research indicates that Bi3+ and Mn4+ co-activate blue-red phosphors have potential applications in agricultural production.

Deep red emitting Mg2La2SnO7:Mn4+ phosphor: Synthesis and luminescence properties

Mg2La2SnO7:Mn4+ phosphor is synthesized by the solid-state reaction method in air. Monitored at 706 nm, excitation spectrum of Mg2La2SnO7:Mn4+ phosphor including peaks at ∼ 300, 333, 395, and 516 nm can be observed in the range of 220–600 nm. With excitation at 333 nm, emission band of Mg2La2SnO7:Mn4+ phosphor is shown in the range of 640–780 nm owing to the 2E → 4A2 transitions of Mn4+ ion. The optimal Mn4+ ion concentration is ∼ 0.3 mol% in Mg2La2SnO7:Mn4+ phosphor. The lifetime decreases from 1.23 to 0.83 ms with the increase of Mn4+ concentration from 0.1 to 1.2 mol%. There is only one luminescence center (Mn4+) in Mg2La2SnO7:Mn4+ phosphor, which is confirmed by the luminescence properties, decay curves, and time-resolved emission spectra. Mg2La2SnO7:Mn4+ phosphor has good thermal stability. Luminous mechanism is explained by energy level diagram of Mn4+ ion and the thermal quenching phenomenon is analyzed by the configuration coordinate diagram of Mn4+ ion.

Fabrication and properties of high B value [Mn1.56Co0.96Ni0.48O4]1−x[SrMnO3]x (0 ≤ x ≤ 0.5) spinel–perovskite composite NTC films

The [Mn1.56Co0.96Ni0.48O4]1−x[SrMnO3]x ((MCN)1−x(SM)x, 0 ≤ x ≤ 0.5) ceramic thin composite films were grown on SiO2/Si (100) substrates by the chemical solution deposition (CSD) method. The results of X-ray diffraction showed that the (MCN)1−x(SM)x (0 < x ≤ 0.5) films were a resemblance of the spinel–perovskite composite structure. The field emission scanning electron microscope analyses presented a considerable improvement in relative density with the increasing of the SM contents. In particular, the thermal constant of (MCN)1−x(SM)x ceramic film increased from 3289 to 5390 K with the increasing of the SM contents (x) from 0 to 0.5. This feature was attributed to the increase in the grain boundary resistance and the increase in Mn3+/Mn4+ pairs as the SM contents increased. From the spectroscopic ellipsometer spectroscopy results, it was found that the (MCN)0.5(SM)0.5 film had the biggest refractive index n value of 2.36 at 1.5 eV and a strong absorption peak at 4.0 eV. The sensitivity of MCN spinel ceramic film was enhanced by the compounding of SM. It was a significant disposition in the study of negative temperature coefficient thermistors.

Influence of silver addition on microstructures and transport properties of La0.67Ca0.33MnO3:Ag x composites

In order to obtain high temperature coefficient of resistance (TCR) value of La0.67Ca0.33MnO3:Ag x (LCMO:Ag x ) composites, samples with different Ag contents (x = 0, 0.1, 0.2, 0.25, 0.3, and 0.5) were prepared by sol–gel method. X-ray diffraction analyses indicated that all samples had orthorhombic perovskite structures. As x increased, lattice parameters (a, b, c) and cell volumes underwent slight expansions. Interestingly, the addition of Ag dramatically affected TCR and magneto-resistance (MR) values. Elevated TCR value up to 53.46%·K−1 at 277 K was observed for LCMO:Ag x composites with added Ag at the composition of x = 0.1. Meanwhile, MR value at 263 K reached 71% at the magnetic field of 1 T for samples with Ag composition of x = 0.25. The increase in Mn4+/Mn3+ ratio and improvement in crystallization caused by added Ag was found responsible for the elevated values of TCR, MR, and Tp. These findings may have practical use in high-performance magneto-resistive manganites.

Improvement of electrical properties of single-phase film thermistors by a Ni0.75Mn2.25O4/LaMnO3 bilayer structure

The LaMnO3 film, Ni0.75Mn2.25O4/LaMnO3 supported bilayers and Ni0.75Mn2.25O4 film thermistors have been successfully fabricated through pulsed laser deposition (PLD) method. XRD analysis reveals that no chemical reaction occurs between LaMnO3 and Ni0.75Mn2.25O4 during deposition in the coexistence of bilayer materials. However, the resistivity at room-temperature of thin film with a Ni0.75Mn2.25O4/LaMnO3 bilayer is 19.6 Ω cm. It is much lower than the resistivity of the LaMnO3 (580 Ω cm) and Ni0.75Mn2.25O4 single phase films (94.2 Ω cm). Such feature is attributed to the existence of the parallel equivalent circuit in bilayer, the decrease in the grain boundary area and the increase in Mn3+/Mn4+ ratio. In addition, the electrical measurement shows that the thermal constant of the bilayer samples are almost the same as that of single layer Ni0.75Mn2.25O4 film. This study suggests that a bilayer concept is proposed to adjust the electrical properties of NTC thermistors. We believe the above results are very instructive for the applications and further research of Mn-based NTC oxides.

Manganese Oxide/Poly(3,4-ethylenedioxythiophene) Hybrid Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

Manganese oxide/poly(3,4-ethylene-dioxythiophene) (MnOx/ PEDOT) nanostructured hybrid thin films were prepared using a simple anodic electrodeposition process from aqueous solution, and then tested for oxygen reduction reaction (ORR) activity in alkaline electrolyte using rotating disk electrode and rotating ring disk electrode methods. MnOx/PEDOT provided improvements over MnOx-only and PEDOT-only control films, with > 0.2 V decrease in onset and half-wave overpotentials, and > 1.5 times increase in terminal current density. The MnOx/PEDOT film exhibited only a slightly lower n value (n = 3.86-3.92) than the 20% Pt/C benchmark electrocatalyst (n = 3.98) across all potentials. MnOx/PEDOT also displayed a more positive half-wave potential and superior electrocatalytic selectivity for the ORR upon methanol exposure than 20% Pt/C. The high activity and synergism of MnOx/PEDOT towards the ORR is attributed to effective intermixing/dispersion of the two materials, intimate substrate contact with improved charge transfer processes attained by co-electrodepositing MnOx with PEDOT and due to the increase in Mn3+ content at the surface of the oxide.

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72µm to 0.55µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2K for x=0.0–90K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255K for x=0.0–125K for x=0.25. Lowering of irreversible temperature (Tirr) from 250K for x=0.0–135.4K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni2+ charge state of Ni ions and increase in Mn4+/Mn3+ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.

Relationships between Mn3+ Content, Structural Ordering, Phase Transformation, and Kinetic Properties in LiNixMn2–xO4 Cathode Materials

Micrometer-sized LiNixMn2–xO4 (0.3 ≤ x ≤ 0.5) single crystals with (111) surface facets were synthesized and characterized by 6Li magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and electrochemical studies. All three techniques were sensitive to cation disorder and the corroborated results showed that structural ordering improves with x. The transition from the ordered to the disordered spinel was triggered by an increase in Mn3+ content, which was accomplished either by a change in chemical composition or postsynthesis thermal treatment. Disordering led to increased solid solution behavior, reduced two-phase transformation domains, and improved transport properties during Li extraction and insertion. Further increasing Mn3+ content in already disordered structure extends the solid solution domain and eliminates the presence of phase II; however, this has limited effect on rate capability. The study demonstrates the dominant role of structural ordering in morpholo...

Role of Cu-Ion Doping in Cu-α-MnO2 Nanowire Electrocatalysts for the Oxygen Reduction Reaction

The role of Cu-ion doping in α-MnO2 electrocatalysts for the oxygen reduction reaction in alkaline electrolyte was investigated. Cu-doped α-MnO2 nanowires (Cu-α-MnO2) were prepared with varying amounts (up to ∼3%) of Cu2+ using a hydrothermal method. The electrocatalytic data indicate that Cu-α-MnO2 nanowires have up to 74% higher terminal current densities, 2.5 times enhanced kinetic rate constants, and 66% lower charge transfer resistances that trend with Cu content, exceeding values attained by α-MnO2 alone. The observed improvement in catalytic behavior correlates with an increase in Mn3+ content at the surface of the Cu-α-MnO2 nanowires. The Mn3+/Mn4+ couple is the mediator for the rate-limiting redox-driven O2/OH– exchange. O2 adsorbs via an axial site (the eg orbital on the Mn3+ d4 ion) at the surface or at edge defects of the nanowire, and the increase in covalent nature of the nanowire with Cu-ion doping leads to stabilization of O2 adsorbates and faster rates of reduction. A smaller crystallite ...

Microstructural characterization and electrical conductivity of CuxMn3−xO4 (0.9≤x≤1.3) spinels produced by optimized glycine–nitrate combustion and mechanical milling processes

In this research, the electrical conductivity of CuxMn3−xO4 (0.9≤x≤1.3) spinel powders prepared by an optimized glycine–nitrate process (GNP) followed by high-energy mechanical milling was studied. The samples were characterized by X-ray diffraction (XRD), thermal gravimetric and differential thermal analysis (TG/DTA), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), 4-probe DC and Archimedes methods. Different glycine/nitrate (G/N) ratios of 0.56, 1.11 and 1.67 were employed, from which the 1.11 (stoichiometric ratio) resulted in the formation of desired spinel phase. The as-prepared powder was calcined at 500°C for 2h under Ar atmosphere (referred as sample A). To evaluate the effect of grain size on electrical conductivity, sample A was milled for 1h (sample B) and 3h (sample C) and pelletized under a pressure of 50MPa. The results showed that sample A had a very low relative density after cold pressing (~40%) leading to a significantly lower conductivity (14.4Scm−1 at 750°C) with an activation energy of 0.76eV as compared to the samples B (29Scm−1 at 750°C) and C (23Scm−1 at 750°C), with the activation energies of 0.67 and 0.72, respectively. Sample C (average grain size of ~10nm), with larger fraction of grain boundaries, exhibited lower conductivity with respect to sample B (average grain size of ~22nm) due to the higher level of charge carrier scattering.The optimized GNP-mechanical milling process was then applied on CuxMn3−xO4 (0.9≤x≤1.3) spinels to probe the effect of copper content on electrical conductivity. In this case, all stoichiometries were sintered at 1250°C for 2h. The results showed that the conductivity increases from 52 to 140Scm−1 by increasing the copper content in the range of 0.9≤x≤1.3 at 750°C. This arises from a gradual increase in Mn3+–Mn4+ octahedrally located ion pairs.

Magnetic Mn substituted cobalt zinc ferrite systems: Structural, electrical and magnetic properties and their role in photo-catalytic degradation of methyl orange azo dye

The present work focuses on the effect of replacement of Fe3+ ions by the Mn3+ ions in cobalt zinc ferrites (Co0.6Zn0.4MnxFe2−xO4 (0.2, 0.4, 0.6, 0.8 and 1.0) on the structural, magnetic, electrical and catalytic properties. Powder X-ray diffraction studies confirmed that all the samples possessed cubic spinel structure with Fd-3m space groups. The saturation magnetization was found to decrease with increase in Mn3+ ions concentration. The drift mobility of all compositions was found to decrease with increase in temperature which could be attributed to the semiconductor nature of nanoferrites. The photo-catalytic activity of all the nanoferrites was evaluated by the degradation of methyl orange dye and it was observed that the degradation of methyl orange dye was enhanced with increase in Mn3+ ions concentration from 0.2 to 1.0. This might be due to the octahedral site preference and higher redox potential of manganese ion as compared those of iron.

Fabrication and properties of Mn1.56Co0.96Ni0.48O4 free-standing ultrathin chips

Mn1.56Co0.96Ni0.48O4 (MCN) free-standing ultrathin chips are successfully fabricated by using screen printing. The structure, electrical and IR absorption properties have been investigated as a function of the sintering temperature. The X-ray diffraction, X-ray photoelectron spectroscopy and field emission scanning electron microscope analyses show remarkable improvements in crystallinity, stoichiometry and relative density for MCN chips. From the electrical experimental results, it is found that the resistivity of the chip samples sharply decreases from 2660 to 21.9Ωcm. Such feature is attributed to the decrease in the grain boundary resistance and the increase in Mn3+/Mn4+ ratio with increasing sintering temperature. Furthermore, Fourier transform infrared spectra show that the absorption bands ν1 (around 1160cm−1) at 1100°C almost disappears while the others still remain. It is ascribed to the increase in the density of samples. The intensities of symmetry bands ν2 (850cm−1) are also found to be consistent with an increase in ratio of Mn3+ to Mn4+ ions with increasing sintering temperature.

Effect of CuMn2O4 spinel in Cu–Mn oxide catalysts on selective catalytic reduction of NOx with NH3 at low temperature

Using the impregnation method, a series of Cu–Mn oxide catalysts were prepared and investigated for the selective catalytic reduction (SCR) of NOx with NH3 at temperatures ranging from 353 K to 453 K. The 0.05Cu–MnOx/TiO2 catalyst shows the highest activity and yields nearly 100% NOx conversion at 453 K using GHSV = 40 000 h−1, while the 0.20Cu–MnOx/TiO2 catalyst exhibits a certain level of potassium tolerance. In addition, the catalysts show favorable stability and water resistance. According to the XRD, EDS and SCR performance results, the existence of a new crystallized CuMn2O4 spinel phase is the dominant parameter for outstanding SCR activity between 413 K and 453 K. TPR, XPS and in situ DRIFT experiments indicate that CuMn2O4 is responsible for low reduction temperature, strong interaction between manganese oxides and copper oxides, high Mn3+ content and numerous acid sites on the surface. Compared with MnOx/TiO2 catalysts, Cu–Mn oxide catalysts could reduce the poisoning effect of potassium, illustrating that the CuMn2O4 phase may play a significant role in K-tolerance. Meanwhile, based on a certain level of potassium tolerance in CuMn2O4, an oxidation mechanism for NO is proposed due to the increase in Mn3+ and the special structure of a spinel oxide.

Mn3+ substituted Co–Cd ferrites, CoCd0.4MnxFe1.6−xO4 (0.1 ⩽ x ⩽ 0.6): Cation distribution, structural, magnetic and electrical properties

The substitution of Mn3+ ions in cobalt–cadmium ferrites (synthesized via sol–gel auto-combustion method) is found to remarkably influence their structural, magnetic and electrical properties. The structural evolutions of the nanophase, investigated using powder X-ray diffraction (XRD) studies reveal the formation of single-phased cubic spinel structures, with Fd-3m space group. As the Mn3+ concentration is increased, the values of the structural parameters like lattice constant, crystallite size, X-ray density, bulk density and porosity undergo significant changes. The value of lattice constant is found to be ∼8.40Å and the average crystallite size is in the range of 41–45nm. The porosity in the entire ferrite samples is low (4.0–10.2%). The magnetic properties, like saturation magnetization and coercivity, show a decreasing trend with increase in Mn3+ concentration. Using the values of saturation magnetization, the cation distribution has been proposed. The proposed cation distribution is further correlated with the electrical properties. It is observed that the D.C. resistivity increases with Mn3+ concentration, suggesting occupancy of Mn3+ ions in the octahedral B-sites which leads to the dilution of Fe2+–Fe3+ conduction. The activation energy calculated from the D.C. resistivity vs temperature curves is found to be ∼0.50eV.

Sol–gel preparation of efficient red phosphor Mg2TiO4:Mn4+ and XAFS investigation on the substitution of Mn4+ for Ti4+

An efficient red phosphor based on the substitution of Mn4+ for Ti4+ in the lattice of Mg2TiO4 was prepared through a sol–gel route. The phosphor was characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and UV-vis spectroscopy and the luminescent properties of the samples with varying Mn4+ concentrations were investigated. The photoluminescence of the sol–gel prepared Mg2TiO4:Mn4+ phosphor was compared with the corresponding sample prepared via solid-state reaction and the commercial CaAlSiN3:Eu2+. X-ray absorption fine structure (XAFS) investigation reveals the oxidation state and coordination environment of the Mn ion in the crystal structure of Mg2TiO4 and indicates the substitution of Mn4+ for Ti-sites. Moreover, a decrease in lattice parameter was observed with the increase of Mn4+ content in Mg2TiO4, further confirming the replacement of Ti4+ ions by smaller Mn4+ ions in the host lattice.

Catalytic N2O decomposition on Pr0.8Ba0.2MnO3 type perovskite catalyst for industrial emission control

Ba substituted PrMnO3 type perovskite catalysts (Pr1−xBaxMnO3 with x=0.1–0.4) have been studied for N2O decomposition reaction. These catalysts were prepared by a combination of co-precipitation and impregnation methods. They are characterized in detail by means of XRD, BET-SA, SEM, EDX, O2-TPD, H2-TPR and XPS analysis. The catalytic activity of Ba substituted PrMnO3 catalyst was observed to be relatively better than the bare catalyst, thereby showing the promotional effect of barium, and Pr0.8Ba0.2MnO3 with 20mol% substitution was found to be the optimized composition. The Pr0.8Ba0.2MnO3 catalyst composition was also prepared in supported form using ceramic honeycomb by following in situ co-precipitation method and tested for N2O decomposition reaction under simulated feed conditions. Supported catalyst shows 92% conversion of N2O at 550°C with a maximum of 0.0984mmol of N2O decomposed per gram of the catalyst, per unit time in the presence of NO and O2, which was higher than that obtained for unsupported catalyst. O2-TPD studies inferred that Ba incorporation results in increase of Mn4+/Mn3+ ratio of PrMnO3 catalyst, thereby confirming the substitution of Ba in perovskite structure. TPR studies also provided the clear evidence to this effect. This improved redox property of Ba substituted perovskite catalyst was correlated to its enhanced catalytic activity for N2O decomposition.

Evolution of structural and magnetic properties with varying oxygen content in low-bandwidth manganite Pr0.9Ca0.1MnO3 thin films

The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr1−xCaxMnO3 (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn4+ ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn4+ ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn3+ ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.

Effect of Mn4+ doping on the microstructure and electrical property of BiFeO3 ceramic

Multiferroic BiFe1-xMnxO3 (x= 0, 0.05, 0.10, 0.15, 0.20) (represented as BF1-xMxO) ceramics are prepared by the conventional solid state reaction technique. The effects of Mn4+ doping on density, phase structure, morphology, dielectric and ferroelectric properties are investigated. The X-ray diffraction patterns of the samples indicate that the typical perovskite phase structure of BiFeO3 is formed, and a phase transition starts near x= 0.05, i.e., the phase structure is distorted from rhombohedral to orthorhombic by Mn4+ doping. The dielectric susceptibility of the sample is significantly increased and the dielectric loss is slightly increased with the increase of Mn4+ content. The dielectric constant r of the BiFe0.85Mn0.15O3 ceramic at 10 kHz is as high as 1065, 22 times larger than that for pure BiFeO3. It is suggested by hysteresis loop measurements that the ferroelectric property of the BF1-xMxO ceramics is improved and the remanent polarization is increased by Mn4+ doping. This is probably because Mn4+ is more stable than Fe3+, and the B-site doping with higher valent Mn4+ could reduce the volatilization of Bi3+ and suppress the valence fluctuation of Fe3+, thereby reducing the concentration of oxygen vacancies and the leakage current in the ceramic.

Preparation of La-Mn-O Perovskite Catalyst by Microwave Irradiation Method and its Application to Methane Combustion

La-Mn-O perovskite catalyst was successfully prepared by microwave irradiation processing using only several minutes for heat treatment, and characterized by XRD, SEM, XPS, H2-TPR and O2-TPD. The catalyst by microwave irradiation processing with 6 min, mainly non-stoichiometric excess oxygen LaMnO3.15 phase, shows excellent catalytic performance. Compared with the sample prepared by conventional method, it possesses relative higher surface area, the increase in Mn4+ and oxygen vacancy content and the enhancement in the mobility of lattice oxygen which play an important part in the methane combustion.

Thermal hysteresis of permeability and transport properties of Mn substituted Mg–Cu–Zn ferrites

Mn substituted Mg–Cu–Zn ferrites of composition Mg0.35Cu0.20Zn0.45O(Fe2−xMnx O3)0.97 have been prepared by the standard double sintering ceramic technique. X-ray diffraction patterns of the samples showed single phase cubic spinel structure without any detectable impurity phases. The lattice constant is found to increase linearly with increase in Mn3+ ion concentration obeying Vegard's law. The initial permeability (μi) of the Mg–Cu–Zn ferrites exhibits thermal hysteresis when the temperature is cycled from above the Curie temperature TC to below. The sharp decrease of μi at T = TC indicates that the samples have high homogeneity according to Globus et al. The Curie temperature TC of the studied ferrite system was determined from the μi–T curves where the Hopkinson type of effect at the TC has been observed with the manifestation of a sharp fall in permeability. The Curie temperature TC is found to increase with increasing Mn content. Dc electrical resistivity increases significantly with the increase in Mn content. The ac resistivity (ρac) and dielectric constant (ε′) of the samples are found to decrease with increase in frequency, exhibiting normal ferrimagnetic behaviour. Dielectric relaxation peaks were observed for the frequency dependence of dielectric loss tangent curves. ε′ increases as the temperature increases, which is the normal dielectric behaviour of the magnetic semiconductor ferrite. The observed variation of electrical and dielectric properties are explained on the basis of Fe2+/Fe3+ ionic concentration as well as the electronic hopping frequency between Fe2+ and Fe3+ ions in the present samples.