High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.     

Flower-like microparticles and novel superparamagnetic properties of new binary Co1/2Fe1/2(H2PO4)2·2H2O obtained by a rapid solid state route at ambient temperature

A new binary Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O was synthesized by a simple, rapid and cost-effective method using CoCO₃-Fe(c)-H₃PO₄ system at ambient temperature. Thermal treatment of the obtained Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O at 600 °C yielded as a binary cobalt iron cyclotetraphosphate CoFeP₄O₁₂. The FTIR and XRD results of the synthesized Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and its final decomposed product CoFeP₄O₁₂ indicate the monoclinic phases with space group P2₁/n and C2/c, respectively. The particle morphologies of both binary metal compounds appear the flower-like microparticle shapes. Room temperature magnetization results show novel superparamagnetic behaviors of the Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and its final decomposed product CoFeP₄O₁₂, having no hysteresis loops in the range of ±10,000 Oe with the specific magnetization values of 0.045 and 12.502 emu/g at 10 kOe, respectively. The dominant physical properties of the obtained binary metal compounds (Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and CoFeP₄O₁₂) are compared with the single compounds (M(H₂PO₄)₂·2H₂O and M₂P₄O₁₂; where M = Co, Fe), indicating the presence of Co ions in substitution position of Fe ions.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Study of Ln0.6Sr0.4Co0.8Mn0.2O3-δ (Ln=La, Gd, Sm or Nd) as the cathode materials for intermediate temperature SOFC

Perovskite type oxides Ln_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ (Ln=La, Gd, Sm, or Nd) have been prepared by the solid state reaction of corresponding oxides. The crystal parameters of the compositions were determined by XRD powder diffraction, which revealed that all the compositions have orthorhombic structure. The reaction test of all samples with Ce_0.8Gd_0.2O_1.9 was carried out at 1200 °C for 48 h, and no reaction product was detected by XRD. The change in mass of La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ as a function of temperature was determined by thermogravimetric analysis (TGA). The electrical conductivity of the sintered samples were measured as a function of temperature from 200 to 1000 °C. The highest conductivity of about 1400 S cm⁻¹ was found in La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ. The cathodic polarization of these oxides electrodes deposited on Ce_0.8Gd_0.2O_1.9 tablet was studied at 500-800 °C in air.     

Synthesis and characterization of the colossal magnetoresistance manganite La1.2(Sr1.4Ca0.4)Mn2O7 by citrate gel

Single-phase La_1.2(Sr_1.4Ca_0.4)Mn₂O₇ has been synthesized from the aqueous solution of metal nitrates and citric acid by the sol-gel technique. Small particle size with high homogeneity of the powder was obtained. The valence of Mn is determined to be 3.45±0.05 by chemical titration. The MR ratio [ρ−ρ(H)]/ρ(H), is 115% (102 K, 1.5 T) for the composition prepared by the citrate route, and is nearly three times larger than that of the sample prepared by solid state reaction.     

Preparation, characterization and microwave absorption properties of polyaniline/Co0.5Zn0.5Fe2O4 nanocomposite

The polyaniline (PAni)/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was prepared by an in situ polymerization in an aqueous solution. The products were characterized by Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-vis) spectrometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The average particle size of the PAni/Co_0.5Zn_0.5Fe₂O₄ was estimated to be about 70 nm by TEM. The reflection loss (dB) of the nanocomposite was measured at different microwave frequencies in X-band (8.2-12.4 GHz), U-band (12.4-18 GHz) and K-band (18-26.5 GHz) by radar cross-section (RCS) method according to the national standard GJB-2038-94. The results showed the reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was higher than that of the PAni. The maximum reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was about −39.9 dB at 22.4 GHz with a bandwidth of 5 GHz (full frequency width at about a half of the peak response). In conclusion, this sample is a good microwave shielding and absorbing materials at higher frequency.     

Dielectric properties of (1−x)(Mg0.95Co0.05)TiO3-xCaTiO3 ceramic system at microwave frequency

The microwave dielectric properties and the microstructures of the (1−x)MgTiO₃-xCaTiO₃ ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275°C, the 0.95(Mg_0.95Co_0.05)TiO₃-0.05CaTiO₃ ceramics possesses excellent microwave dielectric properties: a dielectric constant ε_r of 20.3, a Q×f value of 107 000 ( at 7 GHz) and a τ_f value of −22.8 ppm/°C. By appropriately adjusting the x value in the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramic system, zero τ_f value can be achieved. With x=0.07, a dielectric constant ε_γ of 21.6, a Q×f value of 92 000 (at 7 GHz) and a τ_f value of −1.8 ppm/°C was obtained for 0.93(Mg_0.95Co_0.05)TiO₃-0.07CaTiO₃ ceramics sintered at 1275°C for 4 h.     

Assessment of the optimum degree of Sr3Fe2MoO9 electron-doping through oxygen removal: An X-ray powder diffraction and Fe Mössbauer spectroscopy study

We describe the preparation and structural characterization by X-ray powder diffraction (XRPD) and Mössbauer spectroscopy of three electron-doped perovskites Sr₃Fe₂MoO_9−δ with Fe/Mo = 2 obtained from Sr₃Fe₂MoO₉. The compounds were synthesized by topotactic reduction with H₂/N₂ (5/95) at 600, 700 and 800 °C. Above 800 °C the Fe/Mo ratio changes from Fe/Mo = 2-1 < Fe/Mo < 2. The structural refinements of the XRPD data for the reduced perovskites were carried out by the Rietveld profile analysis method. The crystal structure of these phases is cubic, space group , with cationic disorder at the two different B sites that can be populated in variable proportions by the Fe atoms. The Mössbauer spectra allowed determining the evolution of the different species formed after the treatments at different temperatures and confirm that Fe ions in the samples reduced at 600, 700 and 800 °C are only in the high-spin Fe³⁺ electronic state.     

Preparation of cobalt-zinc ferrite (Co0.8Zn0.2Fe2O4) nanopowder via combustion method and investigation of its magnetic properties

Cobalt-zinc ferrite (Co_0.8Zn_0.2Fe₂O₄) was prepared by combustion method, using cobalt, zinc and iron nitrates. The crystallinity of the as-burnt powder was developed by annealing at 700 °C. Crystalline phase was investigated by XRD. Using Williamson-Hall method, the average crystallite sizes for nanoparticles were determined to be about 27 nm before and 37 nm after annealing, and residual stresses for annealed particles were omitted. The morphology of the annealed sample was investigated by TEM and the mean particle size was determined to be about 30 nm. The final stoichiometry of the sample after annealing showed good agreement with the initial stoichiometry using atomic absorption spectrometry. Magnetic properties of the annealed sample such as saturation magnetization, remanence magnetization, and coercivity measured at room temperature were 70 emu/g, 14 emu/g, and 270 Oe, respectively. The Curie temperature of the sample was determined to be 350 °C using AC-susceptibility technique.     

Crystal and magnetic study of the disordered perovskites Ca(Mn0.5Sb0.5)O3 and Ca(Fe0.5Sb0.5)O3

We have investigated the double perovskites Ca₂MSbO₆ (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M_0.5Sb_0.5)O₃. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO₆ octahedra of Ca(Mn_0.5Sb_0.5)O₃, in contrast with the ordered double perovskite Sr₂MnSbO₆. Ca(Fe_0.5Sb_0.5)O₃ behaves as an antiferromagnet with an ordered magnetic moment for Fe³⁺ of 1.53(4)μ_B and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn_0.5Sb_0.5)O₃ cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

The magnetoelectric perovskite Sr3Fe2TeO9: An insight from magnetic measurements and neutron powder diffraction

A study of the crystallographic and magnetic structures of the double perovskite Sr₃Fe₂TeO₉ has been carried out on a polycrystalline sample using neutron powder diffraction (NPD) data between 10 and 650 K. An analysis of the NPD patterns at room temperature has shown that this compound crystallises in the tetragonal space group I4/m with a = 5.5614(7) Å and c = 7.867(1) Å and has a partially ordered arrangement of Fe and Te at the B-sites. The compound undergoes an I4/m → Fm-3m improper ferroelectric phase transition near 460 K. A low-temperature ferrimagnetic ordering (below T_N = 260 K) has been followed from the magnetisation measurements and sequential NPD data analysis. In good agreement with magnetic measurements the ferrimagnetic structure with very weak magnetisation is defined by the propagation vector k = (0, 0, 0). In addition to the obtained experimental results on magnetic and electric properties some aspects of magnetoelectricity in this perovskite are also discussed and compared with those of another quaternary oxide Sr₃Fe₂B⁶⁺O₉.     

Static magnetic properties of Co and Ru substituted Ba-Sr ferrite

M-type hexagonal ferrite powders, Ba_0.5Sr_0.5Co_xRu_xFe_(12−2x)O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) have been synthesized by conventional ceramic method. Magnetic properties have been investigated as a function of substitution of Co and Ru ions at applied external field of 10 kOe. XRD and SEM revealed hexagonal structure for these ferrites. The Co and Ru ions substitution cause increase in saturation magnetization and rapid decrease in magnetocrystalline anisotropy at lower substitution. The magnetic parameters variation has been explained by taking into account preferential site occupancy of sublattice sites by substituted ions. Curie temperature decreases with substitution due to weakening of superexchange interaction. The obtained hysteresis parameters suggest that the proposed materials cannot be used for recording applications.     

Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3

The system of (1 − y)(Mg_0.6Zn_0.4)_1−xCo_xTiO₃-yCaTiO₃ was investigated to optimize its microwave dielectric properties by adopting appropriate contents of Co and Ca and by controlling sintering conditions. The effect of Co substitution was to enhance densification and Qf value, while the addition of CaTiO₃ resulted in increases of dielectric constant and TCF. As an optimal compositions, 0.93(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃-0.07CaTiO₃ successfully demonstrated a dielectric constant of 23.04, a Qf of 79,460 GHz and a TCF value of +1.4 ppm/°C after firing at a relatively lower sintering temperature of 1200 °C. The increase of sintering temperature beyond 1200 °C tended to degrade overall microwave dielectric properties presumably due to Zn volatilization as evidenced by the presence of a Zn-deficient phase (MgTi₂O₅) at 1400 °C. An attempt to establish the correlation between microstructure characteristics and dielectric properties was made in this dielectric system where the extensive range of firing temperature up to 1400 °C was evaluated.     

Synthesis and characterization of K2NiF4-type CaLnCoO4 (Ln = Sm and Gd)

K₂NiF₄-type CaLnCoO₄ (Ln = Sm and Gd) has been synthesized at 1173 or 1223 K in air using citric acid (CA) and ethylene glycol (EG). CaLnCoO₄ (Ln = Sm and Gd) has an orthorhombic structure with the space group Bmab. The average particle sizes are approximately 300 nm for CaSmCoO₄ and approximately 170 nm for CaGdCoO₄, respectively. The global instability index (GII) indicates that the crystal structure of CaGdCoO₄ is more stable than that of CaSmCoO₄. CaLnCoO₄ (Ln = Sm and Gd) is a p-type semiconductor and shows paramagnetic behavior above 5 K. The 1/χ-T curve of CaSmCoO₄ deviates from the Curie-Weiss law, whereas the 1/χ-T curve of CaGdCoO₄ follows the Curie-Weiss law in the temperature range of 5 ≤ T ≤ 300 K. From the values of the observed effective magnetic moment (μ_eff) of CaLnCoO₄ (Ln = Sm and Gd), it is considered that the spin state of the Co³⁺ ion is low.     

Improved thermoelectric properties in directionally grown Bi2Sr2Co1.8Oy ceramics by Pb for Bi substitution

Thermoelectric performances of misfit cobaltites can be controlled by grain orientation and/or by cation substitution. Both processes have been simultaneously performed by directional solidification at 0.03 m/h, using the laser floating zone technique, of Bi_2−xPb_xSr₂Co_1.8O_y (with x = 0.0, 0.2, 0.4 and 0.6). The microstructure has shown two different main phases as a function of Pb content, a Co-poor phase for low Pb content (0.0 and 0.2) and a Co-rich one for higher Pb substitution. These microstructural changes are reflected on the thermoelectric properties leading to an important decrease on the resistivity and increase of thermopower for samples with 0.4Pb substitution. Both improvements lead to power factor values higher than usual in textured misfit cobaltites.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.