Tuning of electromagnetic wave absorbing properties in Fe-deficient SrFe9.6-xCo1.2Ti1.2O19 hexaferrite-epoxy composites

We report the tunable electromagnetic (EM) wave absorption properties of Fe-deficient SrFe_9.6-xCo_1.2Ti_1.2O₁₉ hexaferrite–epoxy composites. SrFe_9.6-xCo_1.2Ti_1.2O₁₉ hexaferrite powders were prepared via solid-state reaction routes. It was observed that Sr–Ti-rich second phases were formed as x increased, i.e., the Fe content decreased. The ferromagnetic resonance (FMR) frequency of the composites gradually decreased from 8.8 GHz to 4.8 GHz with increasing x, and accordingly, the EM absorption frequency range also gradually changed. The gradual FMR frequency shift was attributed to the compositional shift in the mother phase. It is predicted that the Fe deficiency caused a decrease in the magnetocrystalline anisotropy, and in turn, it shifted the FMR frequency and modified the corresponding EM absorbing properties. All the samples demonstrated a high EM absorption performance with the lowest reflection loss of < −40 dB at the optimized frequency and thickness.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Piezoelectric activities and domain patterns of orthorhombic Ba(Zr,Ti)O3 ceramics

Ba(Zr_xTi_1?x)O₃ (0.025 ≤ x ≤ 0.065) ceramics were prepared by conventional solid-state reaction method. Crystalline structures were analyzed by X-ray diffraction. It was shown that all the Ba(Zr_xTi_1?x)O₃ (0.025 ≤ x ≤ 0.065) ceramics were of orthorhombic phase at room temperature. Piezoelectric activities and domain patterns were investigated and compared with those of BaTiO₃ ceramic. All the Ba(Zr_xTi_1?x)O₃ ceramics showed nearly the same d₃₃ values of about 265 pC/N and the same domain width of about 220 nm. By comparing the grain sizes and domain width of the Ba(Zr_xTi_1?x)O₃ ceramics with those of BaTiO₃ ceramic, it is speculated that the variation of domain width with grain sizes in orthorhombic Ba(Zr_xTi_1?x)O₃ ceramics may be different with that in tetragonal BaTiO₃ ceramic. Besides domain width, the effective inertia mass of domain wall is also considered to be a very important factor that impacts the piezoelectric activities of the Ba(Zr_xTi_1?x)O₃ ceramics.     

Synthesis, characterization and electromagnetic properties of Fe1−xCox alloy flower-like microparticles

Fe_1−xCo_x alloy microparticles with size 3-5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe_1−xCo_x alloy microparticles were measured using a vector network analyzer in the 2-18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8-12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe_1−xCo_x-paraffin (x=0.4, 0.5, and 0.6) composites system. For the Fe_0.5Co_0.5 alloy, the reflection loss (RL) exceeding −20 dB was obtained in the broad frequency range of 5.4-18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe_0.4Co_0.6 sample. The Fe_1−xCo_x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.     

Effect of Y2O3 additive on the microstructure and high-frequency properties of Z-type hexaferrites

In order to improve the high-frequency electromagnetic properties of Z-type hexaferrites such as high cut-off frequency and low dielectric constant, Y₂O₃ was introduced. The influence of Y₂O₃ additive on the phase composition, densification, microstructural and electromagnetic properties of the ceramics with composition of Ba₃(Co_0.4Zn_0.6)₂Y_xFe_24−xO₄₁ (x=0−1) was investigated. The results show that as the amount of Y₂O₃ additive increased, the major phase changed to Z-phase; simultaneously, a small amount of garnet phase appeared. With increasing Y₂O₃ content, the garnet phase separated out on grain boundaries as a secondary phase restraining grain growth. When x was varied from 0 to 1, the dielectric constant decreased and the ferroelectric resonance peak shifted toward a higher frequency. Meanwhile, the initial permeability increased at first, and then decreased with further increasing x, which could be mainly attributed to the change of phase composition and sintering density. With increasing Y₂O₃ content, the minimum reflection point of the samples shifted toward a higher frequency.     

Complex permittivity, permeability and microwave absorbing properties of (Mn2−xZnx)U-type hexaferrite

Complex permittivity, permeability and microwave absorbing properties of a U-type hexaferrite series Ba₄Mn_(2−x)Zn_xFe₃₆O₆₀ (with 0≤x≤2 in step of 0.5) have been examined in the X-band (8.2-12.4 GHz) frequency range. The series have been prepared using conventional solid state reaction route. Microstructural variations with composition have been found with X-ray diffraction (XRD) and scanning electron microgram (SEM). The complex permittivity (ε^?=ε′−jε″) and permeability (μ^?=μ′−jμ″) were measured using vector network analyzer (Agilient Make model PNA E8364B). These parameters were then used for calculating the reflection loss for determination of microwave absorbing properties. Addition of Zn resulted in an increase in reflection loss from −4 dB (or 60 % absorption) in sample with x= 0 to −32 dB (99.92% absorption) in sample with x=1 when the sample thickness was 1.7 mm. Multiple peaks of resonance were obtained in the dielectric and magnetic loss spectra for all samples with x>0. The result indicates that the sample with composition Ba₄MnZnFe₃₆O₆₀, i.e., x=1, can be used effectively for microwave absorption and suppression of electromagnetic interference.     

Dielectric and magnetoelectric properties of BaTiO3–CoMn0.2Fe1.8O4 particulate (0-3) multiferroic composites

Multiferroic properties of (x) CoMn_0.2Fe_1.8O₄–(1-x) BaTiO₃ particulate magnetoelectric (ME) composites with x = 0.1, 0.2, 0.3 M percentage was investigated. The CoMn_0.2Fe_1.8O₄ (CMFO) phase was synthesized by solution combustion route and BaTiO₃ (BT) phase was synthesized by wet chemical method. X-ray diffraction studies revealed the purity of constitute phases; confirmed the manifestation of CMFO and BT within the ME composite structure. The microstructural aspects were observed by using Fe-SEM; revealed the effect of constituent phases on the average grain size of the composites. The temperature dependent dielectric properties for BT exhibited the three anomalies associated to its crystallographic lattice structure change with temperature. Dielectric constant of the composite was found to be decreased with CMFO content. All the composite structures exhibited typical magnetic hysteresis nature at room temperature and showed linear effect on the saturation magnetization of the composite with CMFO content. The ME response was examined at room temperature with an ac magnetic field at 1 kHz, all the composite showed a sharp decreasing behavior of the ME voltage coefficient (α_ME) to an applied dc bias in low field region. The maximum α_ME factor of ～8.51 mV/cm Oe was observed for 10% CMFO–90% BT composition.     

Development of perovskite and phase transition in lead cobalt niobate modified lead zirconate titanate system

Ferroelectric lead zirconate titanate–lead cobalt niobate ceramics with the formula (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃ where x = 0.0–0.5 were fabricated using a high temperature solid-state reaction method. The formation process, the structure and homogeneity of the obtained powders have been investigated by X-ray diffraction method as well as the simultaneous thermal analysis of both differential thermal analysis (DTA) and thermogravimetry analysis (TGA). It was observed that for the binary system (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃, the change in the calcination temperature is approximately linear with respect to the PCoN content in the range x = 0.0–0.5. In addition, X-ray diffraction indicated a phase transformation from a tetragonal to a pseudo-cubic phase when the fraction of PCoN was increased. The dielectric permittivity is remarkably increased by increasing PCoN concentration. The maximum value of remnant polarization P_r (25.3 μC/cm²) was obtained for the 0.5PZT–0.5PCoN ceramic.     

Electrical transport of (1−x)La0.7Ca0.3MnO3+xAl2O3 composites

We report the resistivity (ρ)-temperature (T) patterns in (1-x)La_0,7Ca_0,3MnO₃+xAl₂O₃ composites (0≤x≤0.05) over a temperature regime of 50-300 K. Al₂O₃ addition has increased the resistivity of these composites. The Curie temperature (T_C) is almost independent on the Al₂O₃ content and is about 250 K for all the samples, while the metal-insulator transition temperature (T_MI) decreases with increasing Al₂O₃ content. Based on the phenomenological equation for conductivity under a percolation approach, which is dependent on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental data (ρ−T) from 50 to 300 K and find that the activation barrier increases as Al₂O₃ content increases.     

Synthesis and characterization of MnFe2O4-BiFeO3 multiferroic composites

The mixed spinel-perovskite composites of xMnFe₂O₄-(1-x)BiFeO₃ with x=0, 0.1, 0.2, 0.3 and 0.4 were prepared by solid state reaction method. The structure and grain size were examined by means of X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The XRD results showed that the composites consisted of spinel MnFe₂O₄ and perovskite BiFeO₃ phases after being calcined at the temperature 950 °C for 2 h. The grain size ranged from 0.8 to 1 μm. Magnetization was found to increase with increasing concentration of ferrite content. The variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range. Magnetocapacitance was also observed in the prepared composites, which may be the sign of magnetoelectric coupling in the synthesized composites at room temperature.     

Mixed conductivity and electrocatalytic performance of SrFeO3-δ–SrAl2O4 composite membranes

Oxygen-ionic and electronic transport in dense (SrFe)_1−x(SrAl₂)_xO_z composites, consisting of strontium-deficient Sr(Fe,Al)O_3-δ and SrAl₂O₄ phases, is determined by the properties of perovskite-like solid solution. Increasing the content of SrAl₂O₄, with a total conductivity as low as 5 × 10^− 7 −  10^− 5 S × cm^− 1 at 973–1273 K in air, results in the gradual decrease of the partial conductivities, but also enables the suppression of thermal expansion. Compared to single-phase SrFe_1−xAl_xO_3-δ, (SrFe)_1−x(SrAl₂)_xO_z composites exhibit enhanced thermomechanical properties, while the oxygen permeability of these materials has similar values. The composite membranes exhibit stable performance under air/(H₂–H₂O–N₂) and air/(CH₄–He) gradients at 973–1173 K. The oxidation of dry methane by oxygen permeating through (SrFe)_0.7(SrAl₂)_0.3O_z results in dominant total oxidation, suggesting the necessity to incorporate a reforming catalyst into the ceramic reactors for natural gas conversion.     

Study of magnetic properties and magnetoelectric effect in (x) Ni0.5Zn0.5Fe2O4+(1−x)PZT composites

Magnetoelectric (ME) composites consisting of ferrite phase (x) Ni_0.5Zn_0.5Fe₂O₄+ferroelectric phase (1−x)Pb Zr_0.8Ti_0.2O₃ (Lead Zirconate Titanate—PZT) in which x (mol%) varies between 0 and 1 (0.0≤x≤1.0) was synthesized by double sintering ceramic method. The presence of constituent phases of ferrite, ferroelectric and their composites was confirmed by X-ray diffraction studies. The hysteresis measurement was used to study magnetic properties such as saturation magnetization (M_S) and magnetic moment (μ_B). The existence of single domain (SD) particle in the ferrite phase and mixed (SD+MD) particle in the composites was studied from AC susceptibility measurements. ME voltage coefficient for each mol% of ferrite phase was measured as a function of applied DC magnetic field and at the same time influence of magnetic field on ME response and resistivity of composites was studied. The maximum ME voltage coefficient of 0.84 mV/cm Oe was observed for 15% of ferrite phase and 85% of ferroelectric phase in the composites.     

Preparation and microwave absorption properties of Ce-substituted lithium ferrite

Ce-substituted lithium ferrite, Li_0.5Ce_xFe_2.5−xO₄ (x=0, 0.015 and 0.15), was prepared from metal nitrates and citric acid by the citrate sol-gel method. The thermal decomposition process was investigated by TG-DSC. The phase composition and microstructure of Li_0.5Ce_xFe_2.5−xO₄ was characterized by X-ray powder diffraction analysis (XRD) and a transmission electron microscope (TEM). The complex permittivity and complex permeability and microwave absorption properties of Li_0.5Ce_xFe_2.5−xO₄-paraffin wax composite were measured by the transmission/reflection coaxial line method in the range of 2-18 GHz. It is shown that the substitution of cerium ion had a close effect on the properties of Li_0.5Ce_xFe_2.5−xO₄ ferrites. Also, the present investigation demonstrates that microwave absorbers for applications over 15 GHz, with satisfactory reflection loss, of more than −20 dB for specific frequencies, could be obtained by controlling the substituted Ce element.     

Structural,morphological, magnetic and dielectric characterization of nano-phased antimony doped manganese zinc ferrites

Nano-phased doped Mn–Zn ferrites, viz., Mn_0.5−x/2Zn_0.5−x/2Sb_XFe₂O₄ for x=0 to 0.3 (in steps of 0.05) prepared by hydrothermal method are characterized by X-ray diffraction, Infrared and scanning electron microscopy. XRD and SEM infer the growth of nano-crystalline cubic and hematite (α-Fe₂O₃) phase structures. IR reveals the ferrite phase abundance and metal ion replacement with dopant. Decreasing trend of lattice constant with dopant reflects the preferential replacement of Fe³⁺ions by Sb⁵⁺ion. Doping is found to cause for the decrease (i.e., 46–14 nm) of grain size. An overall trend of decreasing saturation magnetization is observed with doping. Low magnetization is attributed to the diamagnetic nature of dopant, abundance of hematite (α-Fe₂O₃) phase, non-stoichiometry and low temperature (800 °C) sintering conditions. Increasing Yafet–Kittel angle reflects surface spin canting to pronounce lower M_s. Lower coercivity is observed for x≤0.1, while a large H_c results for higher concentrations. High ac resistivity (~10⁶ ohm-cm) and low dielectric loss factor (tan δ~10⁻²–10⁻³) are witnessed. Resistivity is explained on the base of a transformation in the Metal Cation-to-Oxide anion bond configuration and blockade of conductivity path. Retarded hopping (between adjacent B-sites) of carriers across the grain boundaries is addressed. Relatively higher resistivity and low dielectric loss in Sbdoped Mn–Zn ferrite systems pronounce their utility in high frequency applications.     

Complex permittivity, complex permeability and microwave absorption properties of ferrite-paraffin polymer composites

We have investigated the electromagnetic (EM) characteristics of Co_xMn_1−xFe₂O₄ spinel ferrite (where x=0.0, 0.5 and 1.0) nanoparticles (NPs)/paraffin nanocomposite material at 8-20 GHz. Co_xMn_1−xFe₂O₄ NPs have been synthesized by cetyltrimethylammonium assisted hydrothermal route using NaOH. A variation in complex dielectric permittivity and magnetic permeability at room temperature with frequency in the range 8-20 GHz has been studied. Particles showed phase purity and crystallinity in powder X-ray diffraction (XRD) analysis. At the same time, Co_xMn_1−xFe₂O₄ NPs demonstrated a spinel cubic structure from XRD results. A reflection loss of −46.60 dB was found at 10.5 GHz for an absorber thickness of 2 mm. Co_xMn_1−xFe₂O₄ may be attractive candidates for EM wave absorption materials.     

Investigation of magnetic properties of Ni0.2Cu0.2Zn0.6Fe1.96O4–BaTiO3 composites

Composite materials with a ferromagnetic Ni_0.2Cu_0.2Zn₀₆Fe_1.96O₄ phase and a ferroelectric BaTiO₃ phase, xBaTiO₃–(1−x)Ni_0.2Cu_0.2Zn_0.6Fe_1.96O₄, in which x varies from 0 to 1, have been prepared via standard ceramics method with nanosized precursor powders. With the variation of x, typical magnetic hysteresis loops of composites have been observed in the composites at the room temperature. When the content of ferroelectric BaTiO₃ phase increases, the saturation magnetization and initial permeability decrease. Meanwhile, the coercive force tends to increase. Additionally, the cut-off frequency and quality factor of composite materials shift toward higher frequency. The simultaneous presence of nonmagnetic BaTiO₃ in composites pinned the domain wall motion, which lead to the decrease of initial permeability and the increase of cut-off frequency. Microstructure observation shows that the composites possess very fine crystalline grains.     

Influence of zirconium doping on structure,microstructure, dielectric and impedance properties of strontium bismuth niobate ceramics

Efforts have been made in this work to enhance the dielectric properties of SrBi₂Nb₂O₉ (SBN) by partial substitution of Zr⁴⁺ for Nb⁵⁺. Systematic investigations on structure, microstructure, dielectric and impedance properties of the SrBi₂(Nb_2−(4/5)xZr_x)O₉ [where, x = 0, 0.1 and 0.2] ceramic samples were carried out to understand the effect of substitution of Zr⁴⁺ for Nb⁵⁺ in SrBi₂Nb₂O₉. The X-ray diffraction (XRD) investigations indicated that the lattice volume of SrBi₂(Nb_{2− (4/5)x}Zr_x)O₉ with x = 0.1 and 0.2 decreases compared to SBN. The SEM investigations revealed an increase in the size of grains and the change on shape of grains to elongated plate shaped structure with the increase of x (x = 0.1 and 0.2) in SrBi₂(Nb_2−(4/5)xZr_x)O₉. Higher Curie temperature and enhanced peak dielectric constant at the Curie temperature were observed for both the SrBi₂(Nb_2−(4/5)xZr_x)O₉ with x = 0.1 and 0.2 ceramic samples compared to SBN. Among the investigated compositions the higher Curie temperature and enhanced peak dielectric constant at the Curie temperature was observed for SrBi₂(Nb_2−(4/5)xZr_x)O₉ with x = 0.1.     

The role of Ga substitution on magnetic and electromagnetic properties of nano-sized W-type hexagonal ferrites

A series of nano-sized single phase W-type SrGa_xZn₂Fe_16?xO₂₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) hexaferrites prepared by sol gel technique and sintered at 950 °C have been investigated. The thermal decomposition behavior of nitrate–citrate gel of as prepared powder was investigated by means of DTA/TGA analysis. The sintered powders were characterized by FTIR, XRD, SEM, VSM and vector network analyzer (VNA). X-ray diffraction patterns for pure and substituted W-type hexaferrites show the single phase structure with no impurity phase. The lattice parameters (a and c) decrease with the increase of Ga contents (x). The grain size estimated from SEM images is in the range of 139–76 nm which confirms the nanocrystalline nature of the investigated samples. The saturation magnetization (M_s) decreases whereas coercivity (H_c) increases with the increase of Ga contents (x). The values of H_c for all of the investigated samples lie in the range of few hundred oersteds which is one of the necessary conditions for EM materials. The microwave absorption property is enhanced in the frequency (with respect to ?20 dB) from 0.5 to 13 GHz, and the bandwidth reaches 0.899 GHz. The attenuation peak value is ?32 dB at the matching thickness of 3.4 mm.     

Synthesis and characterization of (Bi2O3)1−x−y−z(Gd2O3)x (Sm2O3)y(Eu2O3)z quaternary solid solutions for solid oxide fuel cell

Sm₂O₃, Gd₂O₃, Eu₂O₃ triple-doped Bi₂O₃ based quaternary solid solutions were synthesized as a candidate electrolyte material using the solid-state reaction technique. The structural, thermal and electrical conductivity features of the ceramic samples were examined and compared by using X-ray powder diffraction (XRD), thermal gravimetry/differantial thermal analysis (TG/DTA) and the four-point probe technique (4PPT). The result of XRD measurements indicated that the (Bi₂O₃)_{(1−x−y−z)}(Gd₂O₃)_x(Sm₂O₃)_y(Eu₂O₃)_z (x = 10/y = 10/z = 5, 15, 20 mol % and x = 10/y = 5, 10, 15, 20/z = 10 mol %) samples have a stable face-centered cubic δ-phase and mixed phase crystallographic structure. The phase stability was also checked by the DTA evaluations results. The temperature dependent electrical conductivity measurements showed that the highest electrical conductivity was observed for the sample of the (Bi₂O₃)_0.75(Gd₂O₃)_0.10(Sm₂O₃)_0.05(Eu₂O₃)_0.10 system which has a stable and δ-phase was found as 6.67 × 10⁻³ (Ω cm)⁻¹ at 650 °C. This sample can be used as an electrolyte material in the solid oxide fuel cells (SOFCs) which is possible to operate at intermediate temperature ranges. The activation energy was also calculated at a low temperature range (350–650 °C) and high temperature range (above 650 °C). The values for the samples vary from 0.63 eV to 1.08 eV at low temperature and at high temperature they vary from 0.43 eV to 0.75 eV.     

Enhanced ferroelectric,piezoelectric and dielectric properties of (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 high-temperature piezoelectric composite ceramics

Among Aurivillius layer-structured materials, CaBi₂Nb₂O₉ is a best potential candidate for ultrahigh-temperature applications because of its highest Curie temperature of about 940 °C. In this paper, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ composite ceramics were prepared by conventional solid-state sintering method. The dielectric results show that the introduction of BaZr_0.2Ti_0.8O₃ not only increases the permittivity of the material, but also reduces its dielectric loss. The optimum electrical properties were obtained in the x = 0.01 sample with piezoelectric coefficient (d₃₃) of 15.1 pC/N and high ferroelectric remnant polarization (P_r) of 9.9 μC/cm². Furthermore, the composite samples show good thermal depoling performance, the d₃₃ of the x = 0.01 sample is 13.8 pC/N, which is about 91% of the initial value after depoling at 800 °C. Therefore, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ is one of the candidates for high temperature piezoelectric materials.