MnO3 Powder Research Articles

The effects of annealing treatment on the morphology and microwave absorption properties of α-MnO2

Chemically synthesized α-MnO2 powder was annealed at different temperatures with (NH4)2S2O8 as the oxidant and MnSO4 as the manganese source. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and vector network analysis. Results indicated that the structures and morphologies of the products did not change remarkably, mainly consisting of α-MnO2 with a sea urchin-like shape, when the annealing temperature was below 500 °C. However, it was observed that the powder changed from α-MnO2 to Mn2O3 at 500 °C and 600 °C. The scanning electron microscopy images showed that the morphologies of the resulted Mn2O3 powder were microspheres in various sizes. Microwave absorption properties of the as-synthesized samples were also discussed, which showed that the dielectric loss tangent, magnetic loss tangent and reflection loss could be improved significantly by increasing the annealing temperature appropriately (<500 °C).

High surface area nanocrystalline hausmannite synthesized by a solvent-free route

Nanocrystalline high surface area Mn3O4 powder was obtained at low temperature by a solvent-free route. The precursor was a mixture of manganese (II) acetate, 3,6,9-trioxadecanoic acid (TODA) and ammonium acetate that were intimately mixed by grounding in an agate mortar. Nanocrystalline Mn3O4 was obtained by thermal treatment at 120°C. Powder X-ray diffraction, selected area electron diffraction, high resolution transmission electron microscopy, and Fourier transformed infrared characterization confirmed the formation of the hausmannite phase. The as-prepared mesoporous material has high specific surface area (120m2g−1). The performances of tape casted Mn3O4 nanopowder electrodes were investigated as anode material for lithium ion batteries. High capacity values were achieved at diverse C rates. Capacity fading was found to be dependent on the upper cut off voltage, the presence of a plateau at 2.25V vs. Li+/Li being detrimental for long term cyclability.

Effects of sintering temperatures on the infrared emissivity of La0.7Sr0.3MnO3

La0.7Sr0.3MnO3 powders were fabricated by the traditional solid state reaction at different temperatures. The crystal structure, infrared transmittance spectra, resistivity and infrared normal emissivity ɛN in the range of 8–14μm were examined using different methods. Effects of sintering temperatures on the infrared emissivity were investigated. It is found that the emissivity of La0.7Sr0.3MnO3 powders decreased first and increased then with the sintering temperature's increase from 1273K to 1523K, and the lowest infrared emissivity was obtained when calcined at 1473K. The trend of the variation of the infrared normal emissivity with sintering temperature's increase was opposite to that of crystallite size and similar to that of resistivity.

Control of nonstoichiometric defects in manganese oxides by self-propagating high-temperature synthesis

Nonstoichiometric compounds of Mn1−δO were self-propagating high-temperature synthesized by ignition of thoroughly mixed powders of Mn, NaClO4, and MnO without additional external heating. The mixing ratio was systematically varied to control δ. Single phase Mn1−δO products containing nonstoichiometric defects were obtained for 0<δ≤0.025. With increase in δ, Mn3O4 content of the products is increased. Double phases of Mn1−δO and Mn3O4 were obtained at 0.025<δ≤0.150. The lattice parameter, a, of the single phase Mn1−δO was well explained by the linear equation: a=−0.0667δ+4.448.

Preparation of Ca0.8Sm0.2MnO3 powders and effects of calcination temperature on structure and electrical property

The Ca0.8Sm0.2MnO3 (CSM) powders were prepared through sol–gel auto-combustion technology. The preparation temperature was determined by TG-DTA, FTIR, and XRD analyses, which indicated that the synthetic temperature of CSM powders was 900°C. Effects of calcination temperatures higher than 900°Con the structure and electrical property of CSM powders were also investigated. The crystallography data calculated from XRD spectra illustrated that the cell parameters which related to the resistivity of CSM powders were affected by calcination temperatures. SEM and EDAX analyses demonstrated that the higher calcination temperature led to the larger size of grains. The suitable crystallite size was helpful to decreasing the resistivity, and the CSM powders with the lowest resistivity of 0.2920Ω•m were obtained by calcining the powders at 1000°C.

Magnetic and magnetocaloric properties of monovalent substituted La0.5Ca0.45A0.05MnO3 (A = Na, Ag, K) perovskite manganites

The effects of monovalent doping in the A-site on the structural, magnetic and magnetocaloric properties in La0.5Ca0.45A0.05MnO3 (A = Na, Ag, K) powder perovskite manganites have been investigated. Our compounds have been synthesized using the conventional solid state reaction at high temperature. Rietveled refinements of X-ray diffraction patterns at room temperature show that all our powder samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. Magnetization measurements versus temperature in a magnetic applied field of 0.05T indicate that our investigated samples display a paramagnetic- ferromagnetic transition with decreasing temperature. Moreover, in La0.5 Ca0.45Ag0.05MnO3 compound, a second transition toward an antiferromagnetic state is still present around 150K. The Curie temperatures Tc are found to be 270K, 238K and 240K for A = Na, Ag and K respectively. From magnetization measurements versus magnetic applied field we have determined the magnetic entropy change as well as the relative cooling powerRCP for all our samples. The maximum magnetic entropy change , in a magnetic field change of 5T is found to be 1.02, 2.43 and 1.85Jkg/K for A = Na, Ag and K respectively. The La0.5Ca0.45Ag0.05MnO3 sample exhibits the largest RCP value of 217.32 J/kg under 5T.

MnO Powder as Anode Material for Lithium Ion Battery

MnO powder was synthesized by a facile pyrolysis method using Mn(Ac)2•4H2O powder as the starting material at 800°C for 3h under nitrogen atmosphere. The morphology and structure character of MnO powder was studied by Field-emission scanning electron microscopy (SEM) and X-ray Diffraction. The electrochemical performance of the product was evaluated by galvanostatic charge/discharge experiment. The results shows that MnO powder can deliver a capacity of 604mAh/g at current density of 0.1C, and a capacity retention of 88% is acquired after 150 cycles at 1C rate.

Highly Efficient Hydrogen Generation from H2O/H2O2/MnO2System

The photocatalytic hydrogen generation from H2O/H2O2/MnO2 system and H2O2 concentration dependence of the efficiency were studied for the first time. The MnO2 powder was synthesized by an aqueous reaction followed by low temperature calcination. The powder appeared very stable in water. The absorption edge of the MnO2 powder was ∼1248 nm, corresponding to a band gap of ∼0.68eV. The experiment results of the hydrogen generation revealed that the efficiency was very high and increased as increase in H2O2 concentration. A mechanism for the hydrogen generation was mainly discussed.

Giant Seebeck coefficient thermoelectric device of MnO2 powder

We report a giant Seebeck coefficient (S) thermoelectric (TE) material—MnO2 powder—and the design of a practical MnO2 TE generator to light up a regular light emitting diode (LED). The S of MnO2 powder was measured to be higher than 20 000 μV K−1, which is about 100 times higher than the state-of-the-art of Bi2Te3, one of the best TE materials. The giant S is very possibly closely related to the surface density of the electronic state (DOS), the Fermi energy level. The very high S and therefore high power factor is technologically important for transient TE cooling.

Structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3(x = 0.0, 0.1 and 0.2)

We investigate the effect of Erbium doping on the structural, magnetic and magnetocaloric properties of Pr0.6-xErxSr0.4MnO3 (x = 0.0, 0.1 and 0.2) powder samples. Our polycrystalline samples were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted orthorhombic system with Pnma space group. All our synthesized samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. The magnetic transition temperature TC is found to decrease continuously with increasing Er concentration. It was found that the maximum value of the magnetic entropy change for Pr0.4Er0.2Sr0.4MnO3 sample reached max M

Microstructural, magnetic and optical characterizations of nanocrystalline Zn1−xMnxO dilute magnetic semiconductors synthesized by mechanical alloying

Nanocrystalline Mn doped Zn1−xMnxO (x=0.02, 0.05, 0.07, 0.10) dilute magnetic semiconductors (DMSs) have been synthesized by mechanical alloying the stoichiometric mixtures of ZnO and MnO powders for different durations in a high energy planetary ball mill at room temperature under air medium. In the course of milling, the peak broadening of ZnO reflections increases continuously and MnO reflections disappear completely after 2h of milling. Microstructure characterization of ball milled samples employing the Rietveld structure refinement using X-ray diffraction data reveals that the Zn2+ ions are replaced by Mn2+ ions and lattice parameters of ZnO matrix increase slowly due to this substitution. Particle size of these DMS decreases continuously with increasing Mn concentrations. XPS measurement indicates that the doped Mn ions possess both Mn2+ and Mn4+ valance states. Magnetic characterization of DMS samples shows room temperature ferromagnetism (RTF), which confirms the inclusion of Mn2+ in ZnO lattice and the RTF can be explained in the light of RKKY exchange interaction. Optical characterization of these DMS by UV–vis spectroscopy clearly reveals a red-shift and consequently the optical band-gap decreases with increasing Mn concentrations.

Facile synthesis of monodisperse MnO nanoparticles from bulk MnO

A simple method is presented for the one-pot synthesis of monodisperse MnO nanoparticles (NPs) using bulk MnO as the starting material. In the proposed approach, bulk MnO powder is dissolved in hot oleic acid to form manganese oleate (Mn (RCOO ) 2) and a decomposition process is then performed to obtain MnO NPs. The proposed method is environmentally friendly in that the only by-product is water, which can be easily distilled out. Conceptually, the synthesis method can be viewed as a controlled dissolution recrystallization process. It is shown that the size of the NPs can be controlled over the range of 6–32 nm via an appropriate specification of the precursor-to-oleic acid ratio and the reaction temperature (by the choice of an appropriate high b.p organic solvent). Finally, the size-dependent magnetic properties of three different monodisperse MnO NPs (7.7 nm, 19.4 nm and 31.7 nm) are studied. It is shown that the monodisperse MnO NPs exhibit a weak ferromagnetic behavior due to uncompensated surface spins at low temperatures.

Electrocatalytic Activity of Non-Stoichiometric Perovskites toward Oxygen Reduction Reaction in Alkaline Electrolytes

Perovskite LaxCa0.4MnO3 (x = 0.4, 0.5, 0.6) powder was prepared through a sol-gel method and characterized by X-ray diffraction (XRD), a gas adsorption technique (BET) and transmission electron microscopy (TEM). The electrocatalytic properties of LaxCa0.4MnO3/C composites towards the oxygen reduction reaction (ORR) were studied using rotating ring-disk electrode (RRDE) techniques and Koutecky-Levich theory in both 1 M and 6 M KOH electrolytes. The results show better ORR activities for non-stoichiometric perovskites than that for stoichiometric La0.6Ca0.4MnO3. The overall electron transfer numbers for these LaxCa0.4MnO3 composites are in the range of 3.2-3.7, and with decreasing values of x the electron transfer number increases and accordingly H2O2 production decreases. These results suggest that the existence of a Mn reduction/oxidation pair in Ca-doped nonstoichiometric perovskites could activate the ORR reaction sites, resulting in improved catalytic activity.

Thermoelectric properties of Ca0.8Dy0.2MnO3 synthesized by solution combustion process

High-quality Ca0.8Dy0.2MnO3 nano-powders were synthesized by the solution combustion process. The size of the synthesized Ca0.8Dy0.2MnO3 powders was approximately 23 nm. The green pellets were sintered at 1150-1300°C at a step size of 50°C. Sintered Ca0.8Dy0.2MnO3 bodies crystallized in the perovskite structure with an orthorhombic symmetry. The sintering temperature did not affect the Seebeck coefficient, but significantly affected the electrical conductivity. The electrical conductivity of Ca0.8Dy0.2MnO3 increased with increasing temperature, indicating a semiconducting behavior. The absolute value of the Seebeck coefficient gradually increased with an increase in temperature. The highest power factor (3.7 × 10-5 Wm-1 K-2 at 800°C) was obtained for Ca0.8Dy0.2MnO3 sintered at 1,250°C. In this study, we investigated the microstructure and thermoelectric properties of Ca0.8Dy0.2MnO3, depending on sintering temperature.

Facile In-Situ Synthesis of Manganese Dioxide Nanosheets on Cellulose Fibers and their Application in Oxidative Decomposition of Formaldehyde

In situ synthesis of manganese dioxide was carried out under ambient conditions on porous cellulose fibers as support. MnO2/cellulose composites with different MnO2 loadings were obtained by tuning the concentration of aqueous KMnO4. Physical and chemical properties of the MnO2/cellulose composites were characterized by SEM, TEM, and XRD. When the concentration of aqueous KMnO4 falls in the range of 6.33–12.6 mM, the obtained MnO2 shows a nanosheet morphology and is uniformly coated on the cellulose fiber. The catalytic activities of MnO2/cellulose composites were investigated for the oxidative decomposition of HCHO. The nanostructured MnO2/cellulose composites show excellent catalytic performance for the oxidative decomposition of HCHO. Among them, 8.86 wt % MnO2/cellulose has the highest catalytic activity. The HCHO conversion per milligram of MnO2 of 8.86 wt % MnO2/cellulose is about 9–17 times as high as that of birnessite MnO2 powder prepared by a hydrothermal method. The catalytic activity was found to be dependent not only on the content of MnO2, but also on the adsorption of HCHO on cellulose fibers as well. These two factors are supposed to give rise to the highest catalytic activity for 8.86 wt % MnO2/cellulose. The high efficiency combined with the easy applicability makes the MnO2/cellulose composites more promising for practical applications as compared to MnO2 powders.

Precession electron diffraction of Mn2O3and PbMnO2.75: solving structures where X-rays fail

The structure solutions of the two phases Mn(2)O(3) and PbMnO(2.75) by precession electron diffraction (PED) are presented. In the powder samples used these structures could not be solved by X-ray diffraction because Mn(2)O(3) is a minority phase in an MnO(2) powder and the complex structure of PbMnO(2.75) leads to severe peak overlap in the powder diffraction pattern. The influence of different parameters on the structure solution is studied, i.e. the number of reflections measured, precession angle, resolution limit and Lorentz-type correction. It is shown that the number of reflections is the most important parameter for successful structure solution from PED data.

ChemInform Abstract: Large Magnetic Entropy Change Near Room Temperature in La0.7(Ca0.27Ag0.03)MnO3 Perovskite.

AbstractLa0.7 (Ca1‐xAgx)0.3MnO3 (x = 0, 0.1, 0.2, 0.7, 1) powders are prepared by solid state reaction of La2O3, CaCO3, Ag2CO3, and MnO2 (1350 °C, 24 h) and characterized by powder XRD and magnetization measurements.

Nanoparticles: Well Shaped Mn3O4 Nano‐octahedra with Anomalous Magnetic Behavior and Enhanced Photodecomposition Properties (Small 4/2011)

AbstractVery uniform and well shaped Mn3O4 nano‐octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape‐directing agent. The nano‐octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano‐octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn3O4 nano‐octahedra exhibit anomalous magnetic properties: the Mn3O4 nano‐octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn3O4 nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn3O4 nano‐octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn3O4 nano‐octahedra is much superior to that of commercial Mn3O4 powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn3O4 nano‐octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn3O4 nano‐octahedra.

Synthesis, structural and magnetic properties of nanostructured Ca0.9Gd0.1MnO3 obtained by modified glycine nitrate procedure (MGNP)

Ca0.9Gd0.1MnO3 nanopowders with perovskite type crystal structure were synthesized by modified glycine nitrate procedure. Nanopowders were prepared by combining glycine with metal nitrates and/or metal acetates in their appropriate stoichiometric ratios. Modification of the procedure was performed by partial replacement of nitrates by acetates, in order to control the burn-up reaction. Obtained Ca0.9Gd0.1MnO3 powders were calcinated in the temperature interval from 850°C to 950°C for 10min. Properties such as phase evolution, lattice parameters, chemical composition and magnetic properties were monitored by DTA, X-ray diffraction, SEM/EDS and magnetic measurements. Magnetic measurements performed at the sample with the smallest crystallite size showed that a 10% of Gd3+ substituted Ca2+ ions changes antiferromagnetic properties of CaMnO3 by the introduction of ferromagnetic interaction due to a double exchange between Mn3+ and Mn4+ ions. Presence of competing interactions and their randomness lead to a formation of a spin glass state below Neel temperature TN=110K. From the high temperature magnetic susceptibility measurements effective magnetic moment of manganese ions is determined which lies between the values for Mn3+ and Mn4+ ions.

Well Shaped Mn3O4 Nano‐octahedra with Anomalous Magnetic Behavior and Enhanced Photodecomposition Properties

Very uniform and well shaped Mn₃O₄ nano-octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape-directing agent. The nano-octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano-octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn₃O₄ nano-octahedra exhibit anomalous magnetic properties: the Mn₃O₄ nano-octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn₃O₄ nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn₃O₄ nano-octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn₃O₄ nano-octahedra is much superior to that of commercial Mn₃O₄ powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn₃O₄ nano-octahedra.