Iron dispersed carbon composites formed from iron-polyvinylalcohol complexes

Iron-polyvinylalcohol (Fe-PVA) complexes have been pyrolysed at the temperatures up to 1000 K, and the iron-carbon composites formed have been characterized. The yield of carbon was much higher for the complexes than for PVA alone. The degree of carbon graphitization and the chemical form of iron species were dependent on the pyrolysis temperature. About 30 wt% fine particles of Fe₃O₄ or -Fe were dispersed in the matrix of amorphous carbon at 800 or 900 K, respectively. At 1000 K, -Fe was partly transformed to Fe₃C, and the agglomeration of -Fe was not so significant. At this temperature the carbon was graphitized, which resulted in a lowering of the surface area of the composite. It is suggested that the graphitization proceeds through the mechanism involving the formation and subsequent decomposition of Fe₃C. Thus, the use of Fe-PVA complexes achieves a high yield of carbon and a high dispersion of a large amount of iron species throughout the carbon matrix.     

Stability region of Eu<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>O<Subscript>3+δ</Subscript> solid solutions in air

The homogeneity range of EuMnO₃ has been determined using x-ray diffraction analysis of single-and mixed-phase Eu_2?x Mn_xO_3+δ samples (0.90≤x≤1.20, Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Eu-Mn-O system in air. The dependences of unit-cell parameters on x and synthesis temperature are presented for the samples synthesized at 1100 and 1400°C and for EuMnO₃, respectively. The solubility of europium oxide in EuMnO₃ is tentatively attributed to structural defects, and that of managanese oxides is interpreted in terms of structural defects, oxygen nonstoichiometry of europium manganite, the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺, and partial substitution of the resulting Mn²⁺ for Eu³⁺ on the cuboctahedral site of the perovskite-like structure. To check these assumptions, systematic studies of the oxygen nonstoichiometry and structure of Eu_2?x Mn_xO_3+δ solid solutions synthesized at different temperatures are needed.     

Cationic surface segregation in doped LaMnO<Subscript>3</Subscript>

The surface cation chemistry in (La, A)MnO₃ (A = Ca, Sr and Ba) is investigated using first-principles thermodynamics. We find that, all three dopants tend to segregate to the surface over a wide range of T– \( p_{{{\text{O}}_{2} }} \) conditions and the tendency for segregation increases with the increase in the dopant cationic size. Moving toward the low oxygen pressure, dopants prefer to remain in the surface regions accompanied by the appropriate number of charge compensating oxygen vacancies. The situation when dopants remain in the bulk regions tends to occur close to the thermodynamic conditions that also favor the decomposition of LaMnO₃. The present work serves as an important step toward understanding of factors governing the cationic surface segregation in doped LaMnO₃ and opens a pathway to study other important chemical environments (such as water- and CO₂-containing air) which are crucially given the fact that the ‘real-world’ air enhances cationic segregation.     

Effect of Al content on the microstructure and mechanical behavior of two-phase FeNiMnAl alloys

The effects of varying the Al content in the range of 11–15 at.% on the microstructure and room-temperature mechanical properties of lamellar-structured FeNiMnAl alloys with compositions close to Fe₃₀Ni₂₀Mn₃₅Al₁₅ have been studied, and the temperature dependence of the yield strength of one composition, i.e., Fe₃₆Ni₁₈Mn₃₃Al₁₃, has been investigated. All alloys consisted of B2 and fcc phases. Decreasing the Al content initially (13 and 14 at.% Al) led to marked increases in both the fcc phase fraction and fcc lamellar spacing, λ, but, on further reducing the Al content (11 and 12 at.% Al), the lamellar structure was no longer present. The elongation to fracture of the FeNiMnAl alloys increased with the decreasing Al concentration from 6.5 % at 15 at.% Al to 31.1 % at 11 at.% Al with a concomitant decrease in the yield stress from 820 to 255 MPa. For the lamellar-structured alloys, the yield stress, σ _y, obeyed a Hall–Petch-type relationship with λ, i.e., $ \sigma_{y } = \sigma^{\prime}_{0} + k^{\prime}\lambda^{ - 1} $ , where $ \sigma^{\prime}_{0} $ is the lattice resistance, and k′ is a constant. The compressive yield stress of Fe₃₆Ni₁₈Mn₃₃Al₁₃ was found to be independent of temperature up to 700 K, after which it decreased dramatically because of the softening of the B2 phase. All alloys showed ductile fracture modes.     

A study on age hardening in cast Fe-Mn-Al-Si-C alloys

Phase transformations in a group of cast and solution treated (at 1100° C) Fe-Mn-Al-Si-C alloys during isothermal ageing have been investigated as a function of (a) ageing temperature (500 to 800° C), (b) aluminium (5 to 10%) and carbon (0.27 to 0.9%) contents for fixed manganese ( 30%) and silicon ( 1.5%) levels. Irrespective of the ageing temperature and carbon content, 5% Al alloys do not undergo appreciable ageing. Ageing tendency increases with increase in aluminium and carbon contents. Hardening is caused by the precipitation of Al(Fe, Mn)C _x phase inside the austenite grain and Mn₁₂Si₇Al₅ phase at the grain boundaries. The sequence of precipitation of the phases depends on the ageing temperature and composition. Cold working by rolling, after homogenization at 1100° C, accelerates ageing.     

Structural and optical study of GaMnAs/GaAs

GaMnAs/GaAs was obtained with mass-analyzed low energy dual ion beam depostion technique with Mn ion energy of 1000 eV and a dose of 1.5 × 10¹⁸ Mn⁺/cm² at the substrate temperature of 400°C and was annealed at 840°C. X-ray diffraction spectra showed that Ga_5.2Mn, Ga₅Mn₈, -Mn and Mn₃Ga were obtained in the as-grown sample. After annealing Mn₃Ga and -Mn disappeared, Ga₅Mn₈ tended to disappear, Ga_5.2Mn crystallized better and new phase of Mn₂As was generated. The photoluminescence spectra of the as-grown sample showed that the 1.5042 eV GaAs exciton peak, 1.4875 eV peak involving a carbon acceptor and a broad band near 1.35 eV. After annealing at 840°C, the 1.5042 eV peak and 1.4875 eV shifted to 1.5065 and 1.4894 eV, respectively, and the photoluminescence intensity of the 1.35 eV band increased greatly.     

Reactivity of oxygen-deficient Mn(II)-bearing ferrites (Mn x Fe3-x O4-δ, O⩽x⩽1, δ>0) toward CO2 decomposition to carbon

The reduction of CO₂ to carbon was studied in oxygen-deficient Mn(II)-bearing ferrites (Mn _x Fe_3-x O_4-, Ox1, >0) at 300 °C. They were prepared by reducing Mn(II)-bearing ferrites with H₂ gas at 300°C. The oxygen-deficient Mn(II)-bearing ferrites showed a single phase with a spinel structure having an oxygen deficiency. The decomposition reaction of CO₂ to carbon was accompanied by oxidation of the oxygen-deficient Mn(II)-bearing ferrites. The decomposition rate slowed when the Mn(II) content in the Mn(II)-bearing ferrites increased. A Mössbauer study of the phase changes of the solid samples during the H₂ reduction and CO₂ decomposition indicated the following. Increases in the Mn(II) content lowered the electron conductivity of the Mn(II)-bearing ferrites. Increases in the oxygen deficiency, , contributed to an increase in electron conductivity and suggested that electron conduction due to the electron hopping determines the reductivity of CO₂ to carbon by the donation of an electron at adsorption sites.     

Metal-insulator transition in epitaxial films of LaMnO3 manganites grown by magnetron sputtering

We have studied thin films of LaMnO₃ manganite grown by RF magnetron sputtering at high pressure on crystalline substrates with cubic symmetry. It is established that these films exhibit a metal-insulator transition, whereas LaMnO₃ grown on orthorhombic substrates remains in a dielectric state. The parameters of the metal-insulator transition have been studied as dependent on the level and symmetry of mechanical stresses that arise during the epitaxial growth of LaMnO₃ films on various substrates. The resistance of LaMnO₃ films grown on SrTiO₃ substrates has been studied as a function of the film thickness. It is found that the presence of excess oxygen due to substitution in the cation system can significantly influence the Mn⁴⁺/Mn³⁺ ion ratio in the film and thus lead to the appearance of the metal-insulator transition.     

An investigation of shelf-life of strontium doped LaMnO3 materials

The shelf-life of Sr doped LaMnO₃ (LSM) materials of varying stoichiometric compositions prepared by wet chemical synthesis was investigated under storage conditions of low and high humidity at ambient temperature for 350 days. It has been found that the adhesion, chemical stability and stability of the electrode performance of LSM materials are critically dependent on the A-site stoichiometry (i.e., (La+Sr)/Mn ratio) of the LSM materials and much less dependent on storage conditions (humidity level and storage time) over the period investigated. In this respect, LSM materials with stoichiometric and sub-stoichiometric compositions where (La+Sr)/Mn1 are stable and LSM with A-site sub-stoichiometry ((La+Sr)/Mn0.9) is effective in achieving good adherence, high chemical stability and high stability of electrode performance.     

Residual stresses after thermomechanical treatment involving nonuniform plastic deformation

Summary It was shown that nonuniform plastic deformation during TMT may lead to the appearance of high residual stresses of the first kind.For equal geometrical parameters of deformation (i. e., for equal deformation gradients and degrees of deformation), the magnitude of residual stresses is determined primarily by the mechanical properties of the phases deformed, (in the present case G, G₁, and _S) and, consequently, substantially depends on the deformation temperature t_d. Residual stresses of considerable magnitude, which must be allowed for in calculating the load-carrying capacity of parts subjected to TMT, are produced at t_d below the recrystallization temperature. Consequently, TMT which involves plastic deformation at 400°–550° C may produce residual stresses comparable to those produced by cold working.Practically no residual stresses are produced by HTMT; this is because structural steels have a very low resistance to deformation at temperatures above .No substantial differences were observed between the residual stresses produced by deformation of supercooled austenite and products of non-martensite decomposition, when the treatment was carried out at the same t_d.Quenching after deformation prevents stress relaxation and so increases the level of residual stresses which, however, do not reach the level of stress that would be produced by corresponding cold deformation.It was shown on the example of TMT with deformation in torsion that residual stresses after this treatment may serve as an additional source of directional strengthening. Particularly promising with respect to increasing the fatigue strength of machine parts should be surface TMT, which produces residual compressive stresses in the surface layers.     

Nanocrystalline LaMnO3 preparation and kinetics of crystallization process

Precursor of nanocrystalline LaMnO₃ was synthesized by solid-state reaction at low heat using La(NO₃)₃·6H₂O, MnSO₄·H₂O, and Na₂CO₃·10H₂O as raw materials. XRD analysis showed that precursor was a mixture containing orthorhombic La₂(CO₃)₃·8H₂O and rhombohedral MnCO₃. When the precursor was calcined at 800 ^°C for 2 h, pure phase LaMnO₃ with rhombohedral structure was obtained. Magnetic characterization indicated that rhombohedral LaMnO₃ behaved weak magnetic properties. The thermal process of the precursor experienced four steps, which involved the dehydration of crystallization water at first, and then decomposition of manganese carbonate into MnO₂, and decomposition of La₂(CO₃)₃ and MnO₂ together into La₂O₂CO₃ and Mn₂O₃, and lastly reaction of monoclinic La₂O₂CO₃ with Mn₂O₃ and formation of rhombohedral LaMnO₃. Based on the Kissinger equation, the value of the activation energy associated with the formation of rhombohedral LaMnO₃ was determined to be 260 kJ mol^?1. The value of the Avrami exponent, n, was equal to 1.68, which suggested that crystallization process of LaMnO₃ was the random nucleation and growth of nuclei reaction.     

Effect of high-pressure high-temperature processing on the phase composition of Mn<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1) alloys

High-pressure high-temperature processing (p = 7 GPa, T = 2300 K) has been shown to alter the phase composition of Mn_2?x Cr _x Sb alloys, resulting in a two-phase state: Mn₃Sb (sp. gr. Pm3m, no. 221) and a hexagonal phase of variable composition (sp. gr. P6₃/mmc, no. 194). With increasing chromium concentration, the content of the hexagonal phase increases, while that of the cubic phase drops. In addition, the occupancy of the bipyramidal interstitial sites in the hexagonal phase increases, reaching a maximum in the single-phase alloy MnCrSb (Ni₂In structure).     

Microstructural characteristics and carbon content of Al2O3 films as a function of deposition parameters

Microstructural characteristics and carbon content in aluminum oxide thin films deposited on silicon substrates are reported as a function of deposition parameters such as substrate temperature and molar concentration of the spraying solution. The films were deposited using the spray pyrolysis technique from a spraying solution of aluminum acetylacetonate (Al(acac)₃), dissolved in N,N-dimethylformamide (N,N-DMF), at temperatures in the range from 500 to 650 °C. Water mist was added during the deposition process and no further post deposition thermal treatments were given to these films. The films have a stoichiometry close to that expected for Al₂O₃, although, residual carbon from the deposition process was found to be present in these films. The surface roughness of the films was less than 20 , with deposition rates up to 540 /min, low chemical etch rates and activation energies around 27.6 kJ/mol were also determined. High resolution transmission electron microscopy observation of these films show the presence of a tiny 5Al₂O₃ : H₂O crystalline phase embedded in a dense amorphous matrix. It was found that the average crystallite diameter of this phase depended on the deposition temperature as well as on the molar concentration of the spraying solution.     

Magnetic properties of LaInO<Subscript>3</Subscript>-based perovskite-structure photoluminescent materials doped with Nd<Superscript>3+</Superscript>, Cr<Superscript>3+</Superscript>, and Mn<Superscript>3+</Superscript> ions

Single-phase ceramic samples of La_1–xNd_xInO₃ (0.007 ≤ x ≤ 0.05), LaIn_0.99M_0.01O₃, and La_0.95Nd_0.05In_0.995M_0.005O₃ (M = Cr³⁺ and Mn³⁺) solid solutions have been prepared by solid-state reactions, and their crystal structure, magnetic field dependences of their specific magnetization at 5 and 300 K, and temperature dependences of their molar magnetic susceptibility have been studied. It has been shown that the 300-K specific magnetization of the La_1–xNd_xInO₃ (x = 0.02, 0.05), La_0.95Nd_0.05In_0.995M_0.005O₃ (M = Cr³⁺ and Mn³⁺), and LaIn_0.99Mn_0.01O₃ solid solutions increases linearly with increasing magnetic field strength up to 14 T and that the magnitude of the 300-K specific magnetization of the La_0.993Nd_0.007InO₃ and LaIn_0.99Cr_0.01O₃ solid solutions increases linearly, but they have diamagnetic magnetization. At a temperature of 5 K, the magnetization of all the indates studied here increases nonlinearly with increasing magnetic field strength, gradually approaching magnetic saturation, without, however, reaching it in a magnetic field of 14 T. In the temperature range where the Curie–Weiss law is obeyed (5–30 K), the effective magnetic moments obtained for the Nd³⁺ ion (\({\mu _{effN{d^{3 + }}}}\)) in the La_1–xNd_xInO₃ solid solutions with x = 0.007, 0.02, and 0.05 are 2.95μ_B, 3.09μ_B, and 2.75μ_B, respectively, which is well below the theoretical value \({\mu _{effN{d^{3 + }}}}\)= 3.62μ_B. The effective magnetic moments of the Cr³⁺ and Mn³⁺ ions in the LaIn_0.99Cr_0.01O₃ and LaIn_0.99Mn_0.01O₃ solid solutions are 3.87μ^B and 5.11μ^B, respectively, and differ only slightly from the theoretical values \({\mu _{effC{r^{3 + }}}}\)= 3.87μ^B and \({\mu _{effM{n^{3 + }}}}\)= 4.9μ^B.     

The decomposition of the solid solution state in the temperature range 20 to 200° C in an Al-Zn-Mg alloy

The decomposition of the supersaturated solid solution of an Al-3.2wt% Zn-2.2 wt% Mg alloy has been investigated in the temperature of 20 to 200° C, by small-angle X-ray scattering, electrical resistivity and mechanical measurements. On the basis of the results obtained, three subsequent stages of the decomposition process can be distinguished. Between 20 and 70° the basic process is the nucleation and growth of G.P. zones, the volume fraction of which increases logarithmically with time. A transition stage is observed between 80 and 100° in which the volume fraction increases linearly with time. Above 90° C, the growth kinetics of the volume fraction shows a definite incubation period at the beginning of ageing, while the yield stress increases monotonically. In the temperature range 100 to 160° C, the formation of the phase takes place. Below 100° a linear connection between the yield stress and (fR)^1/2 is found from which the specific surface energy to cut a G.P. zone is calculated to be ₀= 0.21 Nm m^–2.     

Spherical particles and their surface properties

Spherical particles of transition metal oxides (vanadium oxide, manganese oxide and iron oxide) were formed by the use of O₂-H₂ flame fusion and the effect of varying raw material on the product particles was investigated. Particles ranging in size from 50 to 0.01 m were produced simultaneously by explosive decomposition of raw material which is composed of higher valent metal ions, and melting to give V₂O₅ V₂O₄ + O₂; Mn₂O₃ 2MnO + O₂; and 3Fe₂O₃ 2Fe₃O₄ + O₂. During cooling from the molten state particles were reoxidized completely or partially (in the surface layer) into V₂O₅, Mn₃O₄ and Fe₂O₃, respectively, depending upon the particle size. A cavity was observed electron microscopically in the smaller V₂O₅ particles, while fine single crystal (exactly polyhedral but almost spherical) particles of Mn₃O₄ and Fe₃O₄ were obtained. The isoelectric points of spherical Mn₃O₄, Fe₃O₄ and Fe₂O₃ particles were determined to be 4.0, 5.1 and 5.1, respectively.     

Effects of Magnetization on Hole Localization and MnO6 Octahedra Disorder in Hole-Doped Lanthanum Manganese Perovskites

We review the distortions of the MnO₆ octahedra reduced by magnetization in hole-doped lanthanum manganese perovskites. The systems we consider include the colossal magnetoresistance (CMR) samples La_1–x Ca _x MnO₃ (x=0.21, 0.25, 0.30), La_0.67Ba_0.33MnO₃, and a poorer quality La_0.67Pb_0.33MnO₃ sample. We also report preliminary work on three samples of oxygen-doped LaMnO₃₊ and a lanthanum-deficient La_0.9MnO₃ sample. We find the same exponential relationship between the removal of the distortion and the sample magnetization in the Ba- and Pb-doped CMR samples as was found previously for the Ca-doped samples. The MnO₆ distortion in the oxygen-doped materials is found to slightly reduce below the magnetic transition, although much less so than in the CMR samples. Above T _c, the antiferromagnetic LaMnO_3.006 sample shows a softer temperature dependence of the Mn–O bond length distribution broadening. Surprisingly, even this sample shows deviations from thermal (Debye) behavior near T _N, possibly due to FM coupling within MnO planes.     

Homogeneity Range of ScMnO<Subscript>3</Subscript> in Air

Sc_cMn_{2 – c} O₃ (0.9 c 1.1) samples were prepared by reacting Sc₂O₃ and Mn₂O₃ in air between 800 and 1350°C and were characterized by x-ray diffraction. The results were used to determine the homogeneity range of ScMnO₃. The Mn- and Sc-rich phase boundaries of ScMnO₃ are at c 0.975 (800–1250°C) and c 1.025 (800–1200°C), respectively. At higher temperatures, the homogeneity range narrows down. At 1305 ± 5°C, stoichiometric ScMnO₃ dissociates to form Sc₂O₃- and Mn₃O₄-based solid solutions.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 608–610.Original Russian Text Copyright © 2005 by Golikov, Balakirev, Titova, Fedorova, Antonov.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the microstructure and electrical properties of LaMnO<Subscript>3</Subscript>-based NTC thermistors

A study to develop a new system of negative temperature coefficient thermistors for wide temperature range, A series of Mn-based perovskite-structured ceramics of composition (LaMn_1?x Al _x O₃)_0.9(Al₂O₃)_0.1 has been synthesized by conventional solid state reaction at 1350?°C. The X-ray diffraction patterns showed that for all the samples, the substitution of manganese by aluminum up to x?=?0.1 preserved the rhombohedral perovskite LaMnO₃-like phase. For x?=?0.2, apart from the LaMnO₃-like structure, a second perovskite phase based on the cubic LaAlO₃ structure was formed. For x?=?0.3 and 0.4, the phase present was LaAlO₃ -type structure. The grain sizes of the sintered body detected by scanning electron microscope were decreased with increasing Al₂O₃ content. The resistivity increases with increasing the Al content. The obtained values of ρ _25?°C and B _25/50 and E _a are in the range of 10–13103 Ω cm, 1813–2794 K, 0.156–0.241 eV, respectively. The resistance variation (ΔR/R) was <0.241% and the minimum value (0.0483%) was obtained for aging at 125?°C at 500 h. The aim of this work was explored new composite ceramics materials, which could be used as potential candidates for wide temperature range from ?100 to 500?°C thermistors applications.     

Effect of Air Annealing on the Structural and Magnetic Properties of LaMnO3 Perovskite Produced by Reactive Spark Plasma Sintering Route

Perovskite-type lanthanum manganite (LaMnO₃) was synthesized from Mn₂O₃ and La₂O₃ precursor powders using the Spark Plasma Sintering technique (SPS). The in situ reaction proceeds in a few minutes at 1000 °C under 50 MPa with a very high heating rate of 100 °C/min. The Rietveld refinement of the X-ray powder diffraction shows that LaMnO₃ sample crystallizes in a perovskite structure of O′-type orthorhombic symmetry with Pbnm space group. After annealing at 1000 °C in air for 10 hrs, the symmetry changes to rhombohedral with R-3c space group. For the as prepared sample, two critical temperature points in the temperature dependence of the ZFC and FC magnetization occurring at 125 K and 145 K are observed. They are suggested to be related to A-type antiferromagnetic Néel temperature and ferromagnetic short range order, respectively. The ferromagnetic component in the annealed sample increases indicating the expected variation in the concentration of Mn³⁺ and Mn⁴⁺ ions, which was attributed to the oxygen stoichiometry effects.