Phase Equilibria in the Iron Oxide-Cobalt Oxide-Phosphorus Oxide System

Solid-state equilibria at 1000°C were determined in the Fe₂O₃-FePO₄-Co₃(PO₄)₂-CoO area of the Fe-Co-P-O system in air. Two new ternary compounds were observed: CoFe(PO₄)O, an oxyphosphate which melts incongruently at 1130°C, and Co₃Fe₄(PO₄)₆, an orthophosphate which melts incongruently at 1080°C. The ramifications of the liquidus behavior for the formation of rapidly solidified cobalt ferrite from cobalt ferrite-phosphate melts are discussed.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Magneto-Dielectric Properties of Mg–Cu–Co Ferrite Ceramics: I. Densification Behavior and Microstructure Development

The densification, grain growth, and microstructure development of Mg–Cu–Co ferrite ceramics (MgFe_1.98O₄, Mg_{1− x} Cu _x Fe_1.98O₄, with x =0.10–0.30 and Mg_{0.90− x} Co _x Cu_0.10Fe_1.98O₄, with x =0.05–0.20) were studied. The primary objective was to develop magneto-dielectric materials for miniaturization of high frequency and very-high frequency antennas. It was found that magnesium ferrite (MgFe_1.98O₄) is a promising magneto-dielectric material. However, due to its poor densification, it could not be fully sintered at a temperature below 1200°C. High-temperature sintering resulted in undesirable electrical and dielectric properties, due to the formation of Fe²⁺ ions. The poor densification and slow grain growth rate of MgFe_1.98O₄ can be considerably improved by incorporating Cu, due to the occurrence of liquid-phase sintering at a high temperature. A critical concentration of Cu was observed for Mg_{1− x} Cu _x Fe_1.98O₄, above which both densification and grain growth were maximized or saturated. The presence of Co did not have a significant influence on the densification and grain growth of the Mg-based ferrite ceramics.     

Synthesis and Magneto-mechanical Properties of Ce-TZP/La M-Type Hexaferrite Composite

An in situ composite composed of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) and La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was synthesized from a powder mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO. The dense Ce-TZP dispersed with platelike La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ crystals as a second phase were formed after sintering from 1250° to 1350°C. The saturation magnetization of the in situ composite Ce-TZP/La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was proportional to the mass fraction of the hexaferrite second phase in Ce-TZP. The coercivity of the composite with a 20 mass% of second phase decreased from 9.14 to 2.52 kOe (from 728 to 201 kA/m) after the pulverization of the composite. The susceptibility (χ) increased by 15%–25% under uniaxial stress on the composite. The change of the susceptibility (Δχ/χ) value increased with decreasing the mass fraction of the second phase in the composite. The Δχ was found to increase linearly with applied stress and abruptly change on cracking, which is expected for the application in fracture sensing of the composite.     

Effect of Particle Size and Concentration on Microwave-Absorbing Properties of CuxCo2−xY (x=0, 1) Hexaferrite Composites

Co₂Y-type hexaferrites (Ba₂Cu _x Co_{2− x} Fe₁₂O₂₂, x =0,1) have been fabricated by the conventional ceramic process. The complex permittivity and permeability of Co₂Y hexaferrite composites have been measured using a vector network analyzer in the frequency range of 2–18 GHz. The results show that Co₂Y hexaferrite composites have a high permeability and a low permittivity. This can be adjusted by a CuO additive and particle sizes. The reflection loss (RL) values of Co₂Y hexaferrite composites backed with a conducting plate indicate that the composites possess good microwave-absorbing properties. The minimum RL value of the composites was below −30 dB. The microwave-absorbing materials with 10 GHz (8 GHz) bandwidth for attenuation of 5 dB (10 dB) can be achieved.     

Preparation and Characterization of Nanocrystalline Misch-Metal-Substituted Yttrium Iron Garnet Powder by the Sol–Gel Combustion Process

Nanocrystalline Y_{3− x} MM _x Fe₅O₁₂ powders (MM denotes Misch-metal, x =0.0, 0.25, 0.5, 0.75, and 1.0) were synthesized by a sol–gel combustion method. Magnetic properties and crystalline structures were investigated using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope. The XRD patterns showed that the single-phase garnet of Y_{3− x} MM _x Fe₅O₁₂ was formed at x values ≤1.0. The saturation magnetization of powders increased with decreasing MM content and reached the maximum value at Y₃Fe₅O₁₂. The crystallite size of powders calcined at 800°C for 3 h was in the range of 38–53 nm.     

Effect of La Doping on the Phase Conversion, Microstructure Change, and Electrical Properties of Bi2Fe4O9 Ceramics

Undoped and La-doped Bi₂Fe₄O₉ ceramics were synthesized using a soft chemical method. It is observed that in calcining La-doped Bi₂Fe₄O₉, Bi(La)FeO₃ phase rather than Bi_{2− x} La _x Fe₄O₉ gradually increases with increasing La doping content. The phase conversion from mullite-type structure of Bi₂Fe₄O₉ to rhombohedrally distorted perovskite one of Bi(La)FeO₃ with increasing La doping content indicates that La doping can stabilize the structure of BiFeO₃. This is further evidenced that Bi₂Fe₄O₉ can be directly converted to Bi(La)FeO₃ by heating the mixtures of nominal composition of Bi₂Fe₄O₉/ x La₂O₃. Furthermore, the microstructure changes and the room temperature hysteresis loops and leakage current for Bi_{2− x} La _x Fe₄O₉ with x =0 and 0.02 were characterized.     

Reaction Condition for the Synthesis of Ultrafine Particles of the High-Vacancy-Content Zn(II)- Bearing Ferrites from Iron(III) Tartrate Solution

Reaction conditions for the synthesis of ultraflne particles of ferrite solid Solutions between ZnFe₂O₄ and γ-Fe₂O₃ rep-resented by x(ZnFe₂O₄)- y(Fe₃O₄)- z(γ-Fe₂O₃), where x + y + z = 1 (0.09 x 0.73, y 0.08,0.22 z 0.88) (high-vacancy-content Zn(II)- bearing ferrites), were studied using the strongly alkaline Solutions of Fe(III) tartrate con-taining Zn(II) ions at 100°C. The crystal growth of the ferrite particles was enhanced with the Zn(II) ions in the reaction solution. The number of nuclei of the ferrite was dependent on the Fe(III) or the tartrate concentration, which influenced the ultraflne particle size of the ferrite. The average particle size of the high-vacancy-content Zn(II)- bearing ferrites ranged from 7 to 38 nm. The room-temperature Mössbauer effect of the high-vacancy-content Zn(II)- bearing ferrite (x = 0.34–0.37, y 0.02, z = 0.62–0.65) revealed the superparamagnetic nature for ultrafine particle sizes less than 20 nm. The ferrite was transferred from superparamagnetic to ferrimagnetic with an increase in the particle size (>20 nm) at room temperature.     

Mössbauer Spectroscopy Studies on Zn-Bearing Ferrite

Ion distribution in Zn-bearing ferrite Zn _x Fe_{3− x} O₄ was examined using Mössbauer spectra at room temperature. Ion distribution models I and II have been proposed for the samples prepared by a solid-state reaction and from an aqueous Zn(II)–Fe(II) hydroxides suspension, respectively.
(I) (II)     

Preparation of Y-type Barium Hexaferrite by the Glass–Ceramic Method

Y-type Ba-hexaferrites (Ba₂Zn₂Fe₁₂O₂₂), which have been identified as electromagnetic wave absorbers, were prepared by the glass–ceramic method. Glasses with the composition 0.1ZnO·0.9( x B₂O₃· y BaO·(1− x − y )Fe₂O₃) were prepared, and the precipitation of Y-type Ba-hexaferrite from the glass matrix was investigated. Y-type Ba-hexaferrite were precipitated by heating glass specimens with roughly the composition 0.1ZnO·0.9(0.2B₂O₃·0.5BaO·0.3Fe₂O₃) at temperatures above about 1073 K. The lower temperature limit at which single-phase Y-type powder was obtained after leaching with a dilute HCl solution was about 1093 K. The low-temperature formation of Y-type Ba-hexaferrite was linked to the generation of a liquid phase. The shape of Y-type crystals depended strongly on the heating temperature and changed from a plate-like hexagon to a complex polyhedron with increasing heating temperature.     

Defect Structure and Electrical Properties of La1−ySryFe1−xAlxO3−δ

La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO₃–SrFeO_3−δ–LaAlO₃ system were investigated by X-ray powder diffraction analysis. The defect structure of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe_{1− x} Al_xO₃ was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ promotes the formation of anion vacancies and Fe⁴⁺ that lead to higher conductivity.     

Synthesis of Pure and Manganese-, Nickel-, and Zinc-Doped Ferrite Particles in Water-in-Oil Microemulsions

In recent years, the materials research focuses toward synthesis of finer and finer microstructural features. The unique properties of nanosized particles outweigh their higher production costs. Precipitation in microemulsion is one technique, which promises to produce small particles of controlled size and morphology at reasonable cost. The present study demonstrates the synthesis of nanocrystalline α-Fe₂O₃(hematite), Mn_0.5Zn_0.5Fe₂O_4, and Ni_0.5Zn_0.5Fe₂O₄ particles in a reverse micellar microemulsion system [water–iso-octane–AOT (sodium di-2ethylhexylsulfosuccinate)]. The synthesis of α-Fe₂O₃ is performed to obtain baseline data for the synthesis of Mn_0.5Zn_0.5Fe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ in the microemulsion system. Nanosized, spherical α-Fe₂O₃, Ni-Zn ferrite, and Mn-Zn ferrite particles (20–80 nm) with very narrow particle size distribution are synthesized. Crystallization of the particles is obtained at temperatures as low as 300°C.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Structural Study on PbO–SiO2 Glasses by X-Ray and Neutron Diffraction and 29Si MAS NMR Measurements

Structure of x PbO–(100− x )SiO₂ ( x =25–89) glasses has been investigated by means of the X-ray and neutron diffraction and ²⁹Si MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb–O correlation was observed at 0.23 nm, indicating that the PbO₃ trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb–Pb correlation at ∼0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb₂O₄ unit, which consists of the edge-shared PbO₃ trigonal pyramids. These results strongly imply that the Pb₂O₄ units participate in the glass network constructed by SiO₄ tetrahedra even at low PbO content. Differing from other lead-containing glass systems, these structural characteristics of Pb ions in the PbO–SiO₂ glass system are responsible for the extremely wide glass-forming region.     

Hydrothermal Preparation of Ultrafine Ferrites and Their Sintering

Ultrafine and nearly spherical ferrites such as NiFe₂O₄, ZnFe₂O₄, Ni_xZn_1−xFe₂O₄, Mn_0.5Zn_0.5Fe₂O₄, and CoFe₂O₄ were prepared under mild hydrothermal conditions by precipitating from metal nitrates with aqueous ammonia. These hydrothermal ferrite powders were shown to sinter to almost theoretical density at 1000°C without any sintering aids.     

Microstructure of (Fe0.2Co0.8)0.8(Fe2.38Co0.62O4): A Magnetic Oxidation Resistant Composite Formed by Coprecipitation

The metal–ferrite composite (Fe_0.2Co_0.8)_0.8(Fe_2.38Co_0.62O₄) has been studied by X-ray diffractometry measurements and high-resolution transmission electron microscopy. Spinel ferrite occurs in highly crystalline domains 100–150 nm in size, and the iron–cobalt alloy occurs in smaller and less-crystalline domains (10–20 nm). Both phases are heterogeneous in composition. The metal is embedded in the spinel phase, located near the edges, and overlaid by a poorly crystallized layer or misshapen regions containing small spinel crystals and amorphous phases. By annealing under vacuum up to 800°C, the misshapen regions disappear and the size of the metallic regions increases. The concentration of iron in the metallic regions decreases and their structure changes to face-centered cubic, while the spinel becomes enriched in iron.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

High-Temperature Oxidation of a Cu–Ni Based Cermet: Kinetic and Microstructural Study

The oxidation of a cermet composed of nickel ferrite (Ni _x Fe_{3− x} O₄), nickel oxide (Ni _y Fe_{1− y} O), and Ni–Cu alloy (Cu _z Ni_{1− z} ) (with x =0.8, y =0.84, and z =0.44) has been studied by thermogravimetry, at 960°C, under controlled oxygen partial pressure. The rate of oxidation increases with the oxygen partial pressure up to 5.1 × 10³ Pa, then it becomes independent of the oxygen pressure. This is due to the presence of the two copper oxides, CuO and Cu₂O, above this oxygen pressure. Scanning electron microscopy observation of oxidized samples has shown that the oxidation mechanism involves both external and internal oxidation.     

Nonisothermal Crystallization Kinetics of BixY3−xFe5O12 (0.25≤x≤1.00) Prepared from Coprecipitation Process

The nonisothermal crystallization kinetic of Bi _x Y_{3− x} Fe₅O₁₂ (0.25≤ x ≤1.00) powders prepared by coprecipitation process has been investigated. The activation energy of crystallization was calculated by differential scanning calorimetry at different heating rates. The activation energies of crystallization of Bi _x Y_{3− x} Fe₅O₁₂ system are 492, 447, 377, and 353 kJ/mol and the Avrami constant n are 3.49, 2.25, 2.48, and 2.33 for x =0.25, 0.50, 0.75, and 1.00, respectively. The Avrami exponent values (1< n <3) were consistent with surface and internal crystallizations occurring simultaneously for 0.50≤ x ≤1.00, the value ( n >3) for the Avrami exponent was consistent with bulk crystallization domination in Bi _x Y_{3− x} Fe₅O₁₂ system. The results reveal that increasing the substitution amount of bismuth for yttrium would significantly decrease activation energy in Bi _x Y_{3− x} Fe₅O₁₂ system.