Formation of iron oxides by the oxidation of iron in Fe-MOH-H2O and Fe-MOH-H2O-O2 systems (M = Li,Na, K)

The oxidation behaviours of iron powders, ca. 100 m in diameter, in 5–50 m NaOH, 5–40 m KOH and 5–40 m LiOH solutions at 373–573 K were investigated in the absence and presence of oxygen, where m is molality. The oxidation of iron proceeded noticeably above 423 K to form Fe₃O₄, -Fe₂O₃, -NaFeO₂, -Fe₂O₃, Li _x Fe_3–xO₄ and -LiFeO₂ depending on the reaction conditions. The rate of oxidation in LiOH solutions was much slowerthan those in NaOH and KOH solutions.     

Directional solidification and interface structure of Fe3O4-YIG eutectic

The directional solidification and interface structure of Fe₃O₄-Y₃Fe₅O₁₂ (yttrium iron garnet, YIG) eutectic grown in flowing air were investigated. The microstructure of the eutectic consisted of grains of broken and deformed lamellae. The orientation relationship between Fe₃O₄ and YIG is (0 1 1) Fe₃O₄(5 1 3) YIG, (1 1 1) Fe₃O₄(2 1 1) YIG and (2 1 1)Fe₃O₄(3 1 1) YIG. The two phases of the eutectic showed the same orientation relationship mentioned above, even when the growth rates were changed from 5–90 mm h^–1. High-resolution TEM revealed that a disorder of the garnet structure of YIG occurs along the interface with a width of two or three atomic layers.     

Corrosion of Iron in a Carbonate-Bicarbonate Solution. Part 1. Crystallographic Analysis of Passive Films

Visible passive films are formed on the surface of 1010 steel at 325K in a solution of 1N NaHCO₃ + 1N Na₂CO₃ at anodic potentials up to –320mV_NHE inclusively. For potentials below –400mV, they are dull and contain FeCO₃. For –(390–320)mV, the films are thin blackish bright. Most probably, the diffraction peaks are caused by Fe₃O₄–Fe₂O₃. For –(430–400)mV, the films are dull blackish. Probably, this color is caused by a composite of magnetite and siderite.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Rapid quenching and nitrogenation of Sm2Fe17 alloys

The production of hard magnetic samarium-iron nitrides by rapid quenching of Sm₂Fe₁₇ alloys followed by a nitrogenation treatment is described and analysed. Rapid quenching of Sm₂Fe₁₇ produces almost pure Sm₂Fe₁₇ phase with 40 to 100 nm grain size. At low temperature (T465°C), the nitrogenation treatment under 1 bar N₂-H₂ (5%) mainly produces the definite compound Sm₂Fe₁₇N_3– and follows reasonably well an Arrhenius law, the reaction rate being limited by diffusion through the Sm₂Fe₁₇N_3– reacted layer. At higher temperatures (T465°C), disproportionation reactions simultaneously take place, which transform the metastable nitride into free iron and tiny SmN crystallites (about 10 nm wide). It is concluded that the nitrogenation treatment should be performed at about 400°C in order to reach a sufficient reaction rate for the nitride formation while (possibly) avoiding disproportionation reactions whose by-products (Fe and SmN) are deleterious for permanent magnet applications.     

Microstructural Evolution of Fe2O3 and ZnFe2O4 during Sonochemical Synthesis of Zinc Ferrite

The microstructural evolution of Fe₂O₃ and ZnFe₂O₄ during high-temperature (600–800°C) sonochemical synthesis of zinc ferrite is studied by x-ray diffraction analysis. The results are used to develop a qualitative model for ultrasonically activated solid-state reactions A _s +B _s C _s.     

EPR study of polycrystalline superconductors with YBa2Cu3O7 structure

Electron paramagnetic resonance (EPR) of Gd³⁺, Eu²⁺, and copper ions has been investigated in the high-T_c superconductor with YBa₂Cu₃O_7– structure. It has been established that the system is heterogeneous at 0.150.5 and consists of metallic and dielectric regions. The former arises due to oxygen enrichment while the later due to oxygen deficiency. The integral of exchange interaction between Gd³⁺ localized moments and conduction electrons J_sf=0.016 eV has been determined from the normal state temperature dependence of Gd³⁺ EPR linewidth for metallic regions. T_c depression by gadolinium-localized moments for GdBa₂Cu₃O_7– was estimated to be T_c–2K. Anomalies in linewidth temperature dependence upon transition from the normal to the superconducting state have given information about the value and temperature behavior of the superconductor's energy gap. The model, which gives the opportunity to understand some peculiarities of the EPR signal for YBa₂Cu₃O_7– samples, is proposed in terms of several bottlenecked spinsubsystems: spin-liquid in CuO planes and Cu²⁺-O^– and Cu²⁺-O^2– fragments in CuO chains.     

On the nano-size intergrowth of Sr4Fe6O13±δ and (Sr1?xLax)FeO3?δ phases in the mixed conductor Sr3.6La0.4Fe6Oz

The microstructures of mixed-conducting Sr₄Fe₆O_13±, and Sr_3.6La_0.4Fe₆O _z (nominal composition) have been studied by electron microscopy. The lanthanum free material shows a microstructure containing mainly Sr₄Fe₆O_13±, with some 5 vol% Sr_1–y La _y Fe₁₂O₁₉embedded as micron-size inclusions. The lanthanum-containing material revealed a significantly different microstructure consisting of 20 vol% micron-size Sr_1–y La _y Fe₁₂O₁₉embedded in 40 vol% (Sr_0.85La_0.15)FeO_3–tetragonal perovskite, and 40 vol% of plate-shaped nano-scale intergrowths between (Sr_1–x La _x )FeO_3–and Sr₄Fe₆O_13±phases. Domains with dimensions of 20–50 nm are observed in the tetragonal perovskite when viewed along its fourfold axis. From compositional analysis it is concluded that there is little or no solubility of La in the Sr₄Fe₆O_13±phase. The observed microstructure is important input in explaining the significant effect of La addition on the transport properties of Sr₄Fe₆O_13±materials.     

Formation of oxide phases in the system Fe2O3-NiO

Mixed metal oxides in the system Fe₂O₃-NiO were prepared by coprecipitation of Fe(OH)₃/Ni(OH)₂ and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe₂O₃, NiO and NiFe₂O₄ in samples prepared at 800°C. The oxide phases -Fe₂O₃, NiO, NiFe₂O₄ and a phase with structure similar to NiFe₂O₄ were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe₂O₃-NiO are discussed. Two very strong infrared bands at 553 and 475 cm^–1, a weak intensity infrared band at 383 cm^–1 and two shoulders at 626 and 441 cm^–1 were observed for -Fe₂O₃ prepared at 1100°C. NiFe₂O₄, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm^–1, while NiO showed a broad infrared band at 466 cm^–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.     

Bulk thermal expansion studies of BiCaSrCu2O x and Bi2CaSr2Cu2O x

The compounds BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x were prepared by ceramic techniques and characterized by X-ray powder diffractometry (XRD) and microthermogravimetry (TG) and their bulk thermal expansion measurements were carried out using dilatometry in the temperature range 298T1073 K in air. The results have been analyzed and are compared with those obtained earlier for YBa₂Cu₃O₇. The XRD analysis shows that both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x are single phase in nature, having an orthorhombic symmetry. The TG analysis carried out in oxygen, air, and nitrogen shows negligible weight loss (R~0.1%) on heating to 1073 K, indicating that these two compounds, unlike YBa₂Cu₃O₇, are quite stable. The analysis of bulk thermal expansion data reveals that the average linear thermal expansion coefficient ( ₁) for both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is almost the same ( ₁ 10.5×10^–6 K^–1) and is found to be nearly half of that for YBa₂Cu₃O₇ ( ₁ 18×10^–6 K^–1), suggesting that the interatomic bonding in both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is stronger as compared to YBa₂Cu₃O₇.     

Barium–Strontium and Nickel–Cobalt Solid-State Interdiffusion between Hexagonal W-Ferrites BaM2Fe16O27and SrM2Fe16O27 (M = Ni2+, Co2+)

The Ni, Co, Ba, and Sr profiles in the diffusion zones produced between hexagonal W-ferrites (BaCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/SrNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/BaCo₂Fe₁₆O₂₇, and SrCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇) by annealing at 1520 K were used to evaluate the interdiffusion coefficients of the cations involved by the Boltzmann–Matano method over the whole composition range.     

A sintered magnetoelectric composite material BaTiO3-Ni(Co,Mn) Fe2O4

Magnetoelectric composite materials have been made by sintering a mixture of a piezoelectric and piezomagnetic phase. This paper deals with the preparation and some properties of the combination BaTiO₃-Ni(Co, Mn)Fe₂O₄, and investigates the influence of the cooling rate after sintering, TiO₂ additions, the mole ratio of both phases and the crystallite size of the grains of both phases on some physical properties of the composite material. The maximum value of the conversion factor (E/H)_max found for this material up till now is about 80 mV cm^–1 Oe^–1.     

Glass formation and crystallization of barium ferrite in the Na2O-BaO-Fe2O3-SiO2 system

Amorphous ribbons of approximate composition 0.13Na₂O-0.30BaO-0.30Fe₂O₃-0.27SiO₂ were successfully fabricated using roller-quenching, after melting at 1400C. Devitrification of the amorphous phase by annealing at 710C for 2 h formed a uniform distribution of barium ferrite (BaFe₁₂O₁₉) particles with a mean diameter of 55.88 nm. Other crystalline phases formed on heat treatment were BaFe₂O₄ and Na₂Ba₂Si₂O₇. Devitrified ribbons showed a saturation magnetization (M _S) of 24.41 emu g^–1 and a coercivity (H _C) of 3.06 kOe.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

Stability of Si3N4-Al2O3-ZrO2 composites in oxygen environments

Oxidation of dense Si₃N₄-Al₂O₃-ZrO₂ and Si₃N₄-Al₂O₃ compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<1623 K (H= 100kJ mol^–1)), whereas at T> 1623 K (H = 800kJ mol^–1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si₃N₄-Al₂O₃-ZrO₂ composites compared to other ZrO₂-Si₃N₄ materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.     

Phase Equilibria and Structure of Solid Solutions in the La–Co–Fe–O System at 1100°C

Phase equilibria in the La–Co–Fe–O system are studied at 1100°C in air using samples prepared by the citrate, nitrate, and conventional ceramic routes. The stability regions and structures of solid solutions in the La–Co–Fe–O system are determined by x-ray powder diffraction: LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25, sp. gr. R c; 0.775 y< 1, sp. gr. Pbnm), Co_{1 – y} Fe _y O (0 < y 0.13, NaCl-type structure, sp. gr. Fm3m), and Fe_{3 – x} Co _x O₄ (0.84 x 1.38, sp. gr. Fd3m). The structural parameters of phase-pure solid solutions are determined by the Rietveld method. The composition dependences of lattice parameters are presented for LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25) and Fe_{3 – x} Co _x O₄ (0.84 x 1.38). The 1100°C isotherm of the pseudoternary system La₂O₃–CoO–Fe₂O₃ in air is constructed.     

Preparation of the single phase γ′-(Fe1−x Ni x )4N compounds (0 ≤x ≤ 0.6)

Single phase -Fe₄N type (Fe_1–xNi_x)₄N compounds (0 x 0.6 have been synthesized for the first time by controlled heat treatment of iron-nickel oxalates in a gaseous flow of NH₃ xand H₂. The preparation processes were investigated using differential scanning calorimetry (DSC), X-ray diffraction and Mössbauer spectroscopy. The results confirmed that annealing of oxalates in the NH₃ and H₂ atmosphere included the processes of dehydration, decomposition and reduction, nitrogenation and thermal decomposition of the nitrides. The decomposition and reduction occur simultaneously. The final products depend on the flow rate ratio of NH₃ H₂ and the annealing temperature. The formation conditions for the single phase -Fe₄N type (Fe_1–xNi_x)₄N compounds are related to the nickel concentration, with increasing nickel content, the nitrogenation temperature decreased, in contrast the flow rate ratio of NH₃ H₂ increased.     

Dielectric and Magnetic Properties of Pb(Fe1 /2Nb1 /2)O3–Pb(Fe2 /3W1 /3)O3Solid Solutions

Pb(Fe_1/2Nb_1/2)O₃–Pb(Fe_2/3W_1/3)O₃solid solutions were characterized by dielectric measurements at low frequencies and in the microwave range and magnetic measurements at high frequencies. The observed microwave dispersion was tentatively attributed to the domain mechanism of polarization. The obtained results suggest that the solid solutions studied are potential microwave-absorbing materials.     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

Interaction between a dislocation and an impurity in KCl: Mg2+ single crystals

The interaction between a dislocation and the impurity in KCl: Mg²⁺ (0.035 mol% in the melt) was investigated at 77–178 K with respect to the two models: one is the Fleischer's model and the other the Fleischer's model taking account of the Friedel relation. The latter is termed the F-F. The dependence of strain-rate sensitivity due to the impurities on temperature for the specimen was appropriate to the Fleischer's model than the F-F. Furthermore, the activation enthalpy, H, for the Fleischer's model appeared to be nearly proportional to the temperature in comparison with the F-F. The Friedel relation between effective stress and average length of the dislocation segments is exact for most weak obstacles to dislocation motion. However, above-mentioned results mean that the Friedel relation is not suitable for the interaction between a dislocation and the impurity in the specimen. Then, the value of H(T _c) at the Fleischer's model was found to be 0.61 eV. H(T _c) corresponds to the activation enthalpy for overcoming of the strain field around the impurity by a dislocation at 0 K. In addition, the Gibbs free energy, G ₀, concerning the dislocation motion was determined to be between 0.42 and 0.48 eV on the basis of the following equation ln / = G ₀/(kT_p0)1 – (T/T _c)^{1/2 –1}(T/T _c)^1/2 + ln ₀/where k is the Boltzmann's constant, T the temperature, T _c the critical temperature at which the effective stress due to the impurities is zero, _p0 the effective shear stress without thermal activation, and ₀ the frequency factor.