Etude de la cinetique d'oxydation de magnetites finement divisees. I - Influence de la substitution par l'aluminium

The kinetics of finely divided, (Fe²⁺Fe³⁺_2?xAl³⁺_x)O₄ type (0 ? x ? 2), substitution magnetites oxidation into metastable phase γ(Fe³⁺_1?yAl³⁺_y)₂O₃ (x = 3y) has been accounted for by means of the law of diffusion, under variable working conditions, of the cationic vacancies generated at solid-gas interface with a diffusion coefficient that decreases when the stoichiometric difference is high. The crystallites size and the trivalent substituant percentage increases, in both cases, slow down the reaction rate.     

Study of the oxidation kinetics of finely-divided magnetites IV. Influence of zinc substitution

The oxidation kinetics of zinc substituted magnetites (Fe²⁺_1?xZn²⁺_xFe³⁺₂)O^2?₄ (O < x < 1) into the γ defect phase of type x(Zn²⁺Fe³⁺₂O^2?₄), $\text{3}\text{2}\text{(1?}\text{x}\text{)}\text{Fe}{}^{\mathrm{3+}}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion with a chemical diffusion coefficient depending on vacancy concentration. For compounds rich in zinc the diffusion coefficient is constant and the rate law written as v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{4}$ is in agreement with a partial association of defects. For low substitution extents the diffusion coefficient varies with the transformation extent α and exponent n from the pressure law is equal to $\text{1}\text{2}$, which agrees with a total association of defects.     

Study of the oxidation kinetics of finely-divided magnetites III — Influence of chromium and aluminum surstitution

The oxidation kinetics of aluminum and chromium-substituted magnetites (Fe²⁺Cr³⁺_xAl³⁺_2?x)O^2?₄ (0< x < 2) into the γ defect phase of type $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{)}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion under variable working conditions for samples prepared at 700°C and whose size is less than 400 Å. When particle size is caused to increase by annealing the kinetic curves are sigmoidal but only for specimens with high chromium substitution. For compounds rich in chromium observations of morphology show facetting during oxidation but no fracturing was observed.     

Structural and Magnetic Studies on Chromium substituted Ni-Zn Nano Ferrite Synthesized by Citrate Gel Auto Combustion Method

Nano crystalline Ni_0.5Zn_0.5Cr_xFe_2?xO₄ particles with x varying from 0.00 to 0.25 in steps of 0.05 have been synthesized through citrate gel autocombustion method. When Ni_0.5Zn_0.5Fe₂O₄ nano particles were annealed at 1000 °C, the crystallite size increased while the lattice constant decreased slightly. For, the Cr³⁺ substituted samples annealed at 1000 °C, the variation in lattice constant, bondlengths, M_e-M_e distances and other structural parameters have been attributed to the dissimilarity in the ionic radius of the displaced (Fe³⁺) ion and the substituted (Cr³⁺) ion. Thermal studies indicated the autocombustion process which is an exothermic reaction between the nitrates salt solutions and the citric acid took place at about a temperature of 400 °C for Ni_0.5Zn_0.5Fe₂O₄. The M-H loops for all samples indicated a soft ferrite nature for all samples. The non-saturated hysteresis loop and high coercivity for the as prepared Ni_0.5Zn_0.5Fe₂O₄ nano particles has been attributed to the core-shell structure of the fine particles. When annealed at 1000 °C the saturation magnetization of Ni_0.5Zn_0.5Fe₂O₄ nano particles increased and attained the bulk value (70emu/gm). The specific saturation magnetization has been observed to decrease with increasing Cr³⁺ substitution and is ascribed to the reduction in the predominant A-B exchange interaction mechanism. By considering the site preferences cations a suitable distribution of the cations among the A & B-sites has been proposed for Ni_0.5Zn_0.5Cr_xFe_2?xO₄ nano particles annealed at 1000 °C and has been verified using the X-ray diffraction line intensity calculations. The FT-IR spectra of the annealed ferrite powders showed two significant absorption bands in the wave numbers around 400 cm^?1 & 580 cm^?1 and an additional shoulder at 360cm^?1. The position and width of the bands have been observed to vary with Cr³⁺ substitution. The results of IR spectra are in support of the proposed cation distribution.     

Rate law versus oxygen pressure in spinel oxidation to γ lacunar spinels

In the controlled oxidation of spinels (Fe²⁺Fe³⁺_2?xM³⁺_x)O^2?₄ (M³⁺=Al³⁺, Cr³⁺ ; 0 < x < 2) and (Fe²⁺Al³⁺_2?xCr³⁺_x)O^2?₄ to the metastable phases γ(Fe³⁺_1?yM³⁺_y)₂O^2?₃ and $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{O}{}^{\mathrm{2?}}{}_{\mathrm{3?}}\text{(0 <}\text{y}\text{<}\text{2}\text{3}\text{)}$, over the temperature range 200–450°C, the rate law is usually v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{2}$. This law is well interpreted if we consider a total association vacancy-positive charge. However, for low oxidation extents a (a < 0.3) the law written as is in better agreement with a partial association of defects. For a total dissociation of defects exponent n of the pressure law is always smaller.     

Effect of Cr doping on the phase structure,surface appearance and magnetic property of BiFeO<Subscript>3</Subscript> thin films prepared via sol–gel technology

Multiferroic BiFeO₃ (BFO) and BiFe_1?xCr_xO₃ (BF_1?xC_xO, x?=?0.05, 0.1, 0.15 and 0.2) thin films were successfully synthesized on silicon (111) substrates via sol–gel technology. The effect of Cr³⁺ ion doping on the phase structure, surface morphology, valence states for Fe element and magnetic property was investigated. The introduction for simulation images of ionic space arrangement was to better comprehend the substitution site and superexchange interaction between the Fe³⁺ (Cr³⁺) and O^2? ions. The phase structure of Cr-doping thin films transition from rhombohedral to orthorhombic was confirmed by the X-ray diffraction (XRD) and Raman measurements, and the obtained results also demonstrated that the Cr³⁺ ions successfully located in Fe²⁺ and Fe³⁺ ions sites of BFO lattice system. The Field Emission Scanning Electron Microscopy (FESEM) patterns clearly exhibited that the grains sizes were remarkably decreased by Cr³⁺ ions doping, and the surfaces textures got glossier and smoother judging from the Atomic Force Microscope (AFM) images. The dense surface structure can restrict the O^2? ions escaping from the lattices system, which is beneficial for the release of magnetic property due to superexchange interaction of improvement. It was found that the saturation magnetization (Ms) was significantly linearly increased accompanying the adding of Cr-doping due to destroying of spatial modulation helical structure and enhancing of superexchange interaction. Moreover, the Hall-effect results firstly revealed that the carrier concentration and mobility rate played significant roles in magnetoelectric effect behaviors.     

Microwave properties of the ceramic compositions M2+Ti4+ 1−x Me4+O3

The systems MgZr _x TiO₃, Mg_1?y Ca _y Ti_1?x Sn _x O₃, where Ti⁴⁺ is partially substituted by Zr⁴⁺ or Sn⁴⁺, and a series of compositions Mg (Ca_1.5Ta_0.2) _x Ti_1?x O₃, where Ti⁴⁺ is partially substituted by the four-valent cation (Ca_1.5Ta _0.2)⁴⁺, have been synthesized. For all compositions it was found that the permittivity ε_r and quality factor Q decreased monotonically with increasing substitution factor x. The measurements were carried out within the frequency range 9–11 GHz. At lower x, the quality factor had high values, so that such compositions could be used as microwave resonators with controllable parameters (for instance, for satellite television). The relationships between Q and x which have been obtained, together with the changes in density, are interpreted.     

New rutile solid solutions,Ti1−4xLixM3xO2: M = Nb,Ta, Sb

Three extensive new rutile solid solution series have been prepared in which Ti⁴⁺ is replaced by a combination of Li⁺ and a pentavalent cation: Nb⁵⁺, Ta⁵⁺, Sb⁵⁺. The formulae are Ti_1?4xLi_xM_3xO₂: 0 < x ? 0.15, M = Ta; 0 < x ? 0.17, M = Nb; 0 < x ? 0.12, M = Sb. The solid solutions were characterised by X-ray powder diffraction and density measurements. In addition to the rutile solid solutions, LiNb₃O₈ forms a limited range of solid solutions, Li_1?yNb_3?3yTi_4yO₈: 0 < y ? 0.06.     

Etude des solutions solides entre le sesquioxyde de fer cubique γ-Fe2O3 et le ferrite de zinc ZnFe2O4

The authors have prepared metastable solid solutions between cubic iron sesquioxide γ-Fe₂O₃ and zinc ferrite ZnFe₂O₄. They give the main characteristics of these defect spinels. Fe³⁺ ions and vacancies are replaced by Zn²⁺ ions on tetrahedral sites. The lattice parameter and the thermal stability increase with the substitution rate. The magnetic behavior is comparable to that observed for solid solutions M_1?yZn_yFe₂O₄.     

Study the structure stability of NiFe2−x Cr x O4 (x = 0, 0.08) during H2/CO2 cycle reaction

H₂/CO₂ cycle reaction activities of spinel structure NiFe_2−x Cr _x O₄ (x = 0, 0.08) prepared by co-precipitation were determined. The results showed that pure NiFe₂O₄ had almost lost its CO₂ decomposition activities after 15 cycles, while Cr³⁺ doped NiFe₂O₄ still had about 40% of the initial reaction activity value after 50 reaction cycles. The magnetic properties of samples annealed at 350 °C indicated that M _s and M _r decreased from 32.49 to 26.04 emu/g and 9.39 to 7.31 emu/g, respectively, but H _c increased from 230 to 1800 Oe with the increasing of Cr³⁺content. XRD Rietveld analysis showed that there appeared 23.15% Fe _y Ni_1−y (0<y < 1) and no Fe₃C in pure NiFe₂O₄ system after the first H₂/CO₂ cycle reaction. With the increasing of cycle times, the phase abundance of NiFe₂O₄ decreased rapidly. At the same time, Fe₃C appeared and its content increased fast. After 15 cycles, the phase abundance of NiFe₂O₄ is less than 5%wt, but those of Fe _y Ni_1−y (0 < y < 1) and Fe₃C enhanced to 48.15 %wt and 46.92 %wt, respectively. However, the cycle reaction life of NiFe_2−x Cr _x O₄ (x = 0.08) was much longer than NiFe₂O₄. The spinel structure stability was improved dramatically because of the existing of Cr³⁺ in the cell of NiFe₂O₄. After 50 cycles, the phase abundance of NiFe₂O₄ still had 20 %wt.     

Phase Composition of Heat-Treatment Products in the ZrO(OH)2–Y(OH)3–FeOOH System

The phase composition of the heat-treatment products in the ZrO(OH)₂–Y(OH)₃–FeOOH system is determined as a function of the precipitation procedure and calcination temperature (620–1570 K) for the compositions 0.97ZrO₂· xY₂O₃· yFe₂O₃(x+ y= 0.03; x= 0, 0.01, 0.015, 0.02, 0.03) and (1 – x– y)ZrO₂· xY₂O₃· yFe₂O₃(x= y= 0.02, 0.025, 0.03, 0.04). At a given ZrO₂: stabilizer ratio, partial substitution of Fe³⁺for Y³⁺increases the degree of ZrO₂stabilization and retards the low-temperature degradation of the material.     

Effects of double-doping on physical properties of V2O3-based alloys

The electrical resistivity of several types of (V_1?x?yM_xM′_y)₂O₃ alloys have been investigated, where M and M′ represent Al, Ga, and 3d transition metal elements. It was found that effects previously obtained by incorporation of M = Al, Cr, may be counteracted by use of M′ = Ti or Ga. On the basis of this information 3-dimensional phase diagrams have been sketched out; several sets of critical points have been encountered. The physical characteristics of (Ti_1?yCr_y)₂O₃ have also been determined; the electrical properties of the alloy y = 0.4 and 0.6 are remarkably similar to those of (V_1?xCr_x)₂O₃ with x = 0.02, 0.03.     

Ti-Ca-Al-doped YCrO3 pigments: XRD and UV-vis investigation

Compounds based on the YCrO₃ perovskite structure, doped with Al, Ti and Ca, were studied by XRD and DRS, aiming at assessing their potential as green ceramic pigments. Nine samples were synthesized by the ceramic route according to the Y_1−yCa_yCr_1−x−yAl_xTi_yO₃ stoichiometry, where 0 < x < 0.5 and 0 < y < 0.2. Doping affected the phase composition (minor amounts of Y₂O₃ and Y-Ti oxides were found) and induced a series of structural rearrangements in perovskite without symmetry changes, involving unit cell parameters, metal-oxygen distances and distortion of cation sites. Different trends were observed varying x and y. Optical properties were influenced by changes in perovskite crystal structure and stoichiometry. In particular, the oxidation of Cr³⁺ to Cr⁴⁺ had a deleterious effect on its green colour, turning to gray-brown. The occurrence of Cr⁴⁺ in the perovskite lattice was required to balance the replacement of Y³⁺ by Ca²⁺ ions not fully compensated by the designed coupled substitution of Ti⁴⁺ for Cr³⁺.     

Le systeme V1−xFexO2: Proprietes structurales et magnetiques

The system V_1?xFe_xO₂ (x ? 0.020) was investigated by X-ray diffraction, DTA, magnetic susceptibility measurements and Mössbauer spectroscopy. The phase diagram and the magnetic properties are similar to those of the V_1?xCr_xO₂ and V_1?xAl_xO₂ systems previously studied. In oxygen deficient materials appears a Magnéli type shear phase, the Fe³⁺ ions occupying the shear planes.     

Magnetic and crystallographic properties of Fe1−x(Cu0.5In0.5)xCr2S4 spinels

Fe_1?x(Cu_0.5In_0.5)_xCr₂S₄ spinel powders with 0 ≤ x ≤ 1 were prepared. Their lattice constant (a_o) increases linearly with x from a_o = 0.9995 nm for x = 0 to 1.0065 nm for x = 1. Cu⁺, In³⁺, and Fe²⁺ ions occupy tetrahedral A sites of the spinel lattice and Cr³⁺ ions the octahedral B sites. Spinels with 0 ≤ x ≤ 0.82 are ferrimagnets with Curie temperatures decreasing from 171 K for x = 0 to 116 K for x = 0.82. Spinels with 0.82< x≤ 1 are antiferromagnets with Néel temperatures between 31 K and 36 K. The magnetic moment of Fe_0.18Cu_0.41In_0.41Cr₂S₄ spinels was determined by susceptibility measurements to be 5.85 $\text{μ}{}_{\mathrm{<mtext>B</mtext>}}\text{molecule}$, which is equal to the calculated spin-only magnetic moment.     

Effect of combined doping (y<Superscript>3 +</Superscript> + fe<Superscript>3 +</Superscript>) on structural features of nanodispersed zirconium oxide

Fully stabilized zirconium dioxide is widely used. One of the basic requirements to this material is the thermal stability of the structure. The most effective stabilizer for zirconium oxide is yttrium oxide. However, the structure of Y-ZrO₂ degraded at low temperature. Partial substitution of Fe^{3 +} for Y³⁺ decreases both the crystallization and sintering temperature of zirconia ceramic. The aim of present work is the investigation of structural peculiarities of zirconium oxide stabilized by combined dopant depending on chemical composition, synthesis conditions and heat treatment. The polymorphic composition of a ZrO₂-based materials has been determined in series of samples that correspond to the formula [1−(x+y)]ZrO₂⋅xY₂O₃⋅yFe₂O₃ in the temperature range 620–1570 K. It has been found that at the same molar ratio ZrO₂ : doping oxides, the degree of ZrO₂ stabilization increases, and the low-temperature degradation process is retarded by the partial substitution of Fe^{3 +} for Y³⁺. Nonequivalent sites of Fe^{3 +} ions have been identified: two with octahedral coordination for CPH and three with octa-, penta- and tetrahedral coordination for SPH. The possibility of cluster distribution of Fe³⁺ ions and the dependence of the number of vacancies on synthesis conditions have been shown.     

Electrical Resistivity Behavior and VRH Transport Mechanism in Semiconducting La0.6Sr0.4Mn1−2x Fe x Cr x O3 (0.10≤x≤0.25) Manganites

The transport properties and conduction mechanism in La_0.6Sr_0.4Mn_1?2x Fe _x Cr _x O₃ (0≤x≤0.3) have been investigated. The undoped samples show metal–semiconductor transition with a peak of resistivity at a temperature T _P , whereas for all doped compounds the semiconducting behavior persists in the whole temperature range. The insertion of Cr³⁺ and Fe³⁺ ions leads to the increase of resistivity because the simultaneous substitution of Fe³⁺ and Cr³⁺ for Mn³⁺ reduces the number of available hopping sites for the Mn e _g↑ electron and suppresses the double-exchange mechanism. It was found that the transport mechanism for substituted samples is dominated by the variable range hopping of small polarons between localized states in a model where the various parameters estimated from Mott’s relation obey the variable range hopping (VRH) mechanism.     

Mössbauer resonance studies in the CaTiO3Ca2Fe2O5 system

A Mössbauer resonance study of the CaTiO₃-Ca₂Fe₂O₅ system - i.e. the Ca₂Fe_2xTi_2?2xO_6?x solid solution (0?x?1) - shows a continuous evolution of the local iron environment over the entire composition range. For 0.50?x?1 the investigation confirms a previous structural model of alterning layers of octahedra and tetrahedra. For lower concentrations of oxygen vacancies, evidence is given of Fe³⁺ ions randomly distributed in octahedral and even in tetrahedral sites. Titanium atoms apparently have only octahedral surroundings. A long range order is detected when $\text{x}\text{? 0.50}$.     

Synthesis of Fe–Co alloy and cobalt–magnetite composites doped with Nd3+ by using iron disproportionation

Iron–cobalt alloy and cobalt–magnetite composites doped with Nd³⁺ (Co _x Fe_1?x /Co _y Fe_1?y Nd _z Fe_2?z O₄) in which the Fe alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe(OH)₂ and the reduction of Co(II) by Fe⁰ in a concentrated KOH solution. Powder X-ray diffraction, scanning electron microscope and vibrating sample magnetometer were employed to characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe(II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by Nd³⁺ ions on structure, magnetic properties of the composites were investigated. From the perspective of thermodynamics, we explain the postulated mechanism of the disproportionation reaction.     

Effect of Cr substitution on microwave absorption of BaFe12O19

The chromium substituted barium hexaferrites were prepared by self-propagating combustion method. The crystalline structure, complex permittivity, complex permeability, and hyperfine parameters of BaFe_12−xCr_xO₁₉ (x varies from 0.2 to 1.0 in a step of 0.2) were measured with X-ray diffraction, vector network analyzer and Mössbauer spectroscopy. At 850 °C, only a part of Cr³⁺ ions are permitted to enter the lattice of barium ferrite. With further calcination at 1000 °C, all Cr³⁺ ions enter the lattice. After substitution, the complex permittivity is increased. The Cr³⁺ ions substitute for the Fe³⁺ ions on the 2a site, and simultaneously lead to a quadrupole splitting on the 4f₁ site. These changes decrease the anisotropy field, which are accordant with the spectra of μ″. In the crystalline cells of the substituted barium ferrites, some Fe²⁺ ions are formed. It results in a bigger dielectric loss.