静电纺制备Fe3O4/PEK-C纳米复合膜及其吸波性能

静电纺丝技术是一种新颖、高效且简单的制备连续纳米纤维的方法,纳米复合纤维膜的优异特点赋予了纳米吸波剂新的吸波通道。本文采用静电纺丝工艺制备Fe3O4/PEK-C纳米复合纤维膜,利用SEM和TGA表征纳米复合纤维膜的微观形貌和热稳定性,用矢量网络分析仪测试样品在8.2~12.4 GHz的电磁参数与吸波性能。结果表明,Fe3O4/PEK-C纳米复合纤维膜呈现出超细纤维彼此交织构成的立体网络结构,其热稳定性、复介电常数和复磁导率均随着Fe3O4含量的增加而增加,介电损耗和磁损耗得到加强。当纳米复合纤维膜的厚度为1.8 mm时,其反射损耗在整个测试波段均处于-5 dB以下,-10 dB以下有效吸收频宽为2 GHz,频率在8.6 GHz处吸收强度达到最大值-15.4 dB。预期可作为隐身复合材料的吸波功能层。     

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.     

Synthesis and characterisation of the double perovskite Ba2(Zn0.5Ti0.5X)O6 (X = Nb, Ta) ceramics

Ba₂(Zn_0.5Ti_0.5X)O₆ compounds from the general ABO₃ perovskite family were synthesized by the classical solid-state route for X = Nb and Ta with various A/B ratios (1.005, 1 and 0.995). After the calcination step at 1100 °C, both compounds (X = Nb and Ta) contain mainly the cubic disordered ‘Ba₂(Zn_0.5Ti_0.5X)O₆’ phase but traces of BaTiO₃ and secondary phases are often detectable. Nevertheless, after the sintering stage at higher temperature (from 1300 to 1500 °C) and for all A/B ratios investigated, Ti enters into the cubic perovskite structure, resulting in the formation of a unique ‘Ba₂(Zn_0.5Ti_0.5X)O₆’ phase. Attractive dielectric properties have been measured on the tantalum-based compound for A/B = 0.995 (Q ∼2000 at 7.4 GHz and ? = 39.6) as well as on the niobium-based phase for A/B = 1.005 (Q ∼2200 at 6.1 GHz and ? = 54.8). All these characteristics were confirmed at 1 MHz and a linear dependence of the permittivity versus temperature from −60 to 180 °C has also been evidenced for both formulations. Sinterability, dielectric properties and microstructure of such compounds are discussed with respect to the stoichiometry.     

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.     

Combustion synthesis of La0.7Sr0.3Co0.5Fe0.5O3 (LSCF) porous materials for application as cathode in IT-SOFC

La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) porous materials have attracted a substantial interest for application as cathode in solid oxide fuel cells of intermediate temperature (IT-SOFC). This work investigates the effect of different propellants (urea, glycine, citric acid and sucrose) in the preparation of LSCF powders by the combustion method and also the influence of the sintering temperature on the porosity and electrical conductivity. TGA profiles of the as-prepared samples showed a lower weight loss for the sample prepared with glycine, associated with the higher combustion temperature. XRD patterns presented characteristic reflections of LSFC perovskite and a small formation of secondary phases, with nanometric crystallite sizes (9-20 nm). SEM analysis revealed the loose and porous structure of the powder materials. Densification studies were carried within 950-1100 °C, showing that porosity decreased with increasing sintering temperature. Electrical conductivity was measured in the temperature range 300-800 °C and correlated with the sintering temperature.     

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl_0.5Fe_0.5O_2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10⁻³ to 5×10⁻³. The analysis of structural parameters showed that the ionic transport in the CaAl_0.5Fe_0.5O_2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl_0.5Fe_0.5O_2.5+δ lattice. The average thermal expansion coefficients of CaAl_0.5Fe_0.5O_2.5+δ ceramics in air are 16.7×10⁻⁶ and 12.6×10⁻⁶ K⁻¹ at 370-850 and 930-1300 K, respectively.     

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.     

Synthesis and electromagnetic properties of microwave absorbing material: Cox(Cu0.5Zn0.5)1−xFe2O4 (0 < x < 1) nanoparticles

Spinel ferrite Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ over a compositional range 0 < x < 1 was prepared using a simple hydrothermal method. Particle sizes could be varied from 14 to 25 nm by changing the x value. X-ray diffraction results confirmed that all the as-prepared nanoparticles revealed typical spinel structure and transmission electron microscopy images showed that the particle size of the samples increased with increasing x value. The magnetic properties of the as-prepared Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ nanoparticles have been systematically examined. The maximum saturation magnetization existed at the highest Co content (x = 1). The electromagnetic properties of all the samples have been measured by an Agilent network analyzer and the results showed that Co_0.1(Cu_0.5Zn_0.5)_0.9Fe₂O₄ possessed the best microwave absorbing properties.     

Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

Double-layer microwave absorber based on nanocrystalline Zn0.5Ni0.5Fe2O4/α-Fe microfibers

The microwave absorption properties of the nanocrystalline NiZn ferrite (Zn_0.5Ni_0.5Fe₂O₄) and iron (α-Fe) microfibers with single-layer and double-layer structures were investigated in the frequency range of 2–18 GHz. The double-layer absorbers have much better microwave absorption properties than the single-layer absorbers, and the microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing layer and matching layer. With the absorbing layer thickness 0.7 mm of α-Fe microfibers–wax composite and the matching layer thickness 1.5 mm of Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite, the minimum reflection loss (RL) reaches about −71 dB at 16.2 GHz and the absorption band width is about 9.2 GHz ranging from 8.8 to 18 GHz with the RL value exceeding −10 dB. While, when the absorbing layer is the Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite with thickness 1.8 mm and the matching layer is the α-Fe microfibers–wax composite with thickness 0.2 mm, the RL value achieves the minimum about −73 dB at 13.8 GHz and the absorption band width is about 10.2 GHz ranging from 7.8 to 18 GHz with the RL value exceeding −10 dB, which covers the whole X-band (8.2–12.4 GHz) and K_u-band (12.4–18 GHz).     

Low temperature synthesis of Fe3O4 micro-spheres and its microwave absorption properties

Fe₃O₄ micro-spheres were synthesized by a simple chemical method at 90 °C. It was demonstrated that the final size and morphology of Fe₃O₄ was significantly affected by the additive NaF. The as-synthesized products were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and vector network analysis. The complex permittivity and permeability of paraffin wax and Fe₃O₄ with different Fe₃O₄ volume fraction were measured to increase linearly with increasing volume fraction of Fe₃O₄. When the matching thickness is 4 mm, the calculated reflection loss reaches a maximum value of −45.2 dB at 4.67 GHz with 40% volume fraction of Fe₃O₄.     

Sol-gel auto-combustion synthesis of Ni0.5Zn0.5Fe2O4/(SiO2)x (x = 10, 20, 30 wt.%) nanocomposites and their characterizations

A nitrate-citrate-silica gel was prepared from metallic nitrates, citric acid and tetraethoxysilane (TEOS) by sol-gel process, and it was further used to synthesize Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites by auto-combustion. The obtained Ni_0.5Zn_0.5Fe₂O₄/(SiO₂)_x (x = 10, 20, 30 wt.%) samples were characterized by IR, ²⁹Si CP/MAS NMR, XRD, TEM, EPR and impedance analyzer measurements. Particle size of these composites was calculated from Scherrer's formula, and that decreased with increasing SiO₂ content. The content of TEOS in the starting solution affects the interaction between NiZn ferrite and silica, and then determines the particle size, dielectric properties and the EPR properties (ΔH_PP, g factor, N_S and T₂) of the as-synthesized powder.     

Thermal, dielectric and microwave absorption properties of polyaniline-CoFe2O4 nanocomposites

The present paper deals with the synthesis of conducting ferromagnetic polyaniline-CoFe₂O₄ (PC) nanocomposites via one-step chemical oxidative polymerization of aniline in the presence of CoFe₂O₄ nanoparticles (30-40 nm). These nanocomposites of PC have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and vibrating sample magnetometer (VSM). Extended thermal analysis has revealed that the activation energy of these nanocomposites varies from 75.3 to 84.3 kJ/mol as compared to the activation energy of 50.3 kJ/mol for polyaniline-DBSA. In addition, dielectric and microwave absorption properties of the nanocomposites have been measured in the frequency range of 12.4-18 GHz (Ku-band) which demonstrate that more than 99% attenuation of microwaves (SE_A = 21.5 dB) has been achieved using these nanocomposites. Systematic investigations reveal that the CoFe₂O₄ nanoparticles in the polyaniline matrix have phenomenal effect in determining the microwave absorption properties of the nanocomposites.     

Structural, magnetic and dielectric properties of Bi5−xLaxTi3Co0.5Fe0.5O15 ceramics

The Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4) ceramics were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The cell volume of the composites increases monotonously with the increase of La content, which indicates that La ions have been incorporated into the lattice of Bi₅Ti₃Co_0.5Fe_0.5O₁₅. The magnetic measurements show that La doping on Bi sites has enhanced the magnetization of Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4). Both the dielectric constants and loss tangent of all the samples decrease on increasing frequency and then become almost constant at room temperature. The La doped Bi₅Ti₃Co_0.5Fe_0.5O₁₅ samples exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization and lower losses at room temperature.     

Complex permeability and microwave absorbing properties of planar anisotropy carbonyl-iron/Ni0.5Zn0.5Fe2O4 composite in quasimicrowave band

Planar anisotropy carbonyl-iron (PACI)/Ni_0.5Zn_0.5Fe₂O₄ composite as absorbent filler in quasimicrowave band has been synthesized via ball-milling technique and solvothermal method. The effective permeability of the composite was measured and calculated. The result indicates that the magnetic loss in the composite is mainly caused by the natural resonance. Compared with the uncoated PACI particles, the permittivity of the composite decreased dramatically, and hence a dramatic enhancement of reflection loss (RL) was obtained in quasimicrowave band. This result indicates that our PACI/ferrite composite can be used as potential microwave absorbers in quasimicrowave band for its novel microwave properties.     

Synthesis and microwave dielectric properties of Sr3Zn1−xMgxNb2O9 phases

Oxides of the type, Sr₃Zn_1−xMg_xNb₂O₉ (0≤x≤1) have been obtained by the ceramic method. These oxides crystallize in the hexagonal cell corresponding to ordered triple perovskites. Sintered disks show nearly frequency-independent dielectric constant for all the compositions. Compositions sintered at 1425°C yield dielectric constant of 20-22 at ∼6 GHz, with quality factor ranging from 1300 to 1500. Sr₃Zn_0.5Mg_0.5Nb₂O₉ shows a very low temperature coefficient of resonant frequency (τ_f) of +4 ppm/°C.     

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.     

Electromagnetic wave absorption properties of mesoporous Fe3O4/C nanocomposites

In this work, the mesoporous Fe₃O₄/C nanocomposites with a yolk-shell structure (Fe₃O₄@void@C) were prepared by a silica-assisted strategy, and their microstructure, magnetic properties, and microwave absorption were studied in detail. The BET surface area and the total pore volume of Fe₃O₄@void@C are 171.5 m² g⁻¹ and 0.19 cm³ g⁻¹, respectively. The composites show a saturation magnetization of 35.4 emu/g and reduced hysteresis loss at room temperature. The Fe₃O₄@void@C nanocomposites exhibit the obvious complementarities between complex permittivity and permeability. A minimum reflection loss value of −18.1 dB was obtained for the absorber thickness reaching 2.0 mm, and even the Fe₃O₄@void@C nanocomposites possess a lower reflection loss of-10 dB under 2 GHz bandwidth. We believe that the investigations on the Fe₃O₄@void@C nanocomposite open up a route to develop a new type of composite, which is considered as a promising candidate for microwave absorber.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.