Enhanced absorption properties of ordered mesoporous carbon/Co-doped ordered mesoporous carbon double-layer absorbers

Ordered mesoporous carbon (OMC) and metal-doped (M-doped) OMC composites are prepared, and their electromagnetic (EM) parameters are measured. Using the measured EM parameters we calculate the EM wave absorption properties of a double-layer absorber, which is composed of OMC as an absorbing layer and M-doped OMC as the matching layer. The calculated results show that the EM wave absorption performance of OMC/OMC-Co (2.2mm/2.1mm) is improved remarkably. The obtained effective absorption bandwidth is up to 10.3 GHz and the minimum reflection loss reaches 47.6 dB at 14.3 GHz. The enhanced absorption property of OMC/OMC-Co can be attributed to the impedance match between the air and the absorber. Moreover, it can be found that for the absorber with a given matching layer, a larger value of -tanδε (= tan δε absorbing tan δε matching ) can induce better absorption performance, indicating that the difference in impedance between the absorbing layer and the matching layer plays an important role in improving the absorption property of double-layer absorbers.     

Preparation and microwave absorption properties of electroless Co-Ni-P coated strontium ferrite powder

A new type of Co-Ni-P coated strontium ferrite nanocomposite was prepared with electroless plating enhanced by ultrasonic wave at room temperature. The plating process was studied carefully. The morphology, crystal structure and microwave absorption properties of the Co-Ni-P coated powder were studied with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX) and vector network analyzer. The results show that the strontium ferrite powder was successfully coated with Co-Ni-P alloy and possesses excellent microwave absorption properties. The maximum microwave loss of the composite powder reaches −44.12 dB. The bandwidth with the loss above −10 dB exceeds 13.8 GHz.     

Electromagnetic absorption properties of flowerlike cobalt composites at microwave frequencies

In this work,we report the electromagnetic absorption(EMA) properties of composites consisting of micrometersized cobalt with flowerlike architecture synthesized by a facile hydrothermal reduction method.Compared with the conventional spherical Co-paraffin composites,the flowerlike Co-paraffin composites are favorable with respect to EMA performance in the low frequency region,ascribing interfacial polarization loss and Ohmic loss to the improvement in the impedance match.     

Structural and magnetic properties of nanocrystalline stannic substituted cobalt ferrite

The structural and magnetic properties of the spinel ferrite system Co_1+xFe_2−2xSn_xO₄ (x=0.0–1.0) have been studied. Samples in the series were prepared by the ceramic technique. The structural and microstructural evolutions of the nanophase have been studied using X-ray powder diffraction and the Rietveld method. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is partially an inverse spinel. Far infrared absorption spectra show two significant absorption bands, around 600 cm⁻¹ and 425 cm⁻¹, which are respectively attributed to tetrahedral (A) and octahedral [B] vibrations of the spinel. Scanning Electron Microscopy (SEM) was used to study surface morphology. SEM images reveal particles in the nanosize range. The transmission electronic microscope (TEM) reveals that the grains are spherical in shape. TEM analysis confirmed the X-ray results. The magnetic properties of the prepared samples were characterized by using a vibrating sample magnetometer.     

Remanence of the interparticle interactions and its influence on the microwave absorption in Co-ferrite

Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperatures: 400, 450, 500, 550 and 600 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17–26 nm range as function of the annealing temperature. The Mössbauer spectra at room temperature showed the presence of a partial inverse spinel structure. Saturation magnetization and the coercive field are strongly dependent on the annealing temperature and they can be associated with variations of the nanoparticles size. Microwave power absorption (MPA) (dP/dH) measurements were carried out as a function of DC field (H_DC) in asymmetric sweeps in the 0 kOeH_DC9 kOe range, at X-band (9.4 GHz), for all annealing temperatures. The large hysteresis in the MPA is due to interparticle interaction associated with its demagnetizing-like nature.     

Fabrication and performance optimization of Mn-Zn ferrite/EP composites as microwave absorbing materials

Magnesium-substituted Mn0.8Zn0.2Fe2O4 ferrite is synthesized by the sol–gel combustion method using citrate acid as the complex agent. The electromagnetic absorbing behaviors of ferrite/polymer coatings fabricated by dispersing Mn–Zn ferrite into epoxy resin （EP） are studied. The microstructure and morphology are characterized by X-ray diffraction and scanning electron microscope. Complex permittivity, complex permeability, and reflection loss of ferrite/EP composite coating are investigated in a low frequency range. It is found that the prepared ferrite particles are traditional cubic spinel ferrite particles with an average size of 200 nm. The results reveal that the electromagnetic microwave absorbing properties are significantly influenced by the weight ratio of ferrite to polymer. The composites with a weight ratio of ferrite/polymer being 3：20 have a maximum reflection loss of –16 dB and wide absorbing band. Thus, the Mn–Zn ferrite is the potential candidate in electromagnetic absorbing application in the low frequency range （10 MHz–1 GHz）.     

Microwave absorption properties of Ce-substituted M-type barium ferrite

Ce-substituted barium ferrite with chemical composition BaCe_0.05Fe_11.95O₁₉ has been prepared by the citrate sol-gel method. The phase composition of BaCe_0.05Fe_11.95O₁₉ was characterized by X-ray powder diffraction analysis (XRD). The complex permittivity and complex permeability, microwave absorption properties of the resulting powder were measured by the transmission/reflection coaxial line method in the range of 8-13 GHz. The results show that the resulting powder has a minimum reflection loss value of - 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm.     

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

The complex permittivity (ε′–jε″), complex permeability (μ′–jμ″) and microwave absorption properties of ferrite–polymer composites prepared with different ferrite ratios of 50%, 60%, 70% and 80% in polyurethane (PU) matrix have been investigated in X-band (8.2–12.4 GHz) frequency range. The M-type hexaferrite composition BaCo⁺²_0.9Fe⁺²_0.05Si⁺⁴_0.95Fe⁺³_10.1O₁₉ was prepared by solid-state reaction technique, whereas commercial PU was used to prepare the composites. At higher GHz frequencies, ferrite's permeabilities are drastically reduced, however, the forced conversion of Fe⁺³ to Fe⁺² ions that involves electron hopping, could have increased the dielectric losses in the chosen composition. We have measured complex permittivity and permeability using a vector network analyzer (HP/Agilent model PNA E8364B) and software module 85071. All the parameters ε′, ε″, μ′ and μ″ are found to increase with increased ferrite contents. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 80% ferrite content has shown a minimum reflection loss of −24.5 dB (>99% power absorption) at 12 GHz with the −20 dB bandwidth over the extended frequency range of 11–13 GHz for an absorber thickness of 1.6 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology).     

Poly(sodium-p-styrenesulfonate) assisted microwave synthesis of ordered mesoporous carbon supported Pd nanoparticles for formic acid electro-oxidation

Pd nanoparticles highly dispersed onto the surface of ordered mesoporous carbons (OMCs) were synthesized successfully by poly(sodium-p-styrenesulfonate) (PSS) assisted microwave synthesis. Here, PSS served as a bifunctional molecule both for solubilizing and dispersing OMCs into aqueous solution and for jointing Pd²⁺ to facilitate the subsequent uniform formation of Pd nanoparticles on their surfaces. The effects of PSS on structural and electrochemical properties of Pd/OMCs were investigated. It was found that the addition of PSS facilitated Pd nanoparticles to disperse on the carbon surface. Electrochemical properties showed that Pd catalysts prepared with addition of PSS displayed better electrochemical activity and stability for formic acid electro-oxidation than those without PSS.     

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.     

Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding

Surface cladding of Magnesium ferrite (MgFe₂O₄) was found to be helpful for effectively decreasing the permittivity of Fe flakes. So the electromagnetic impedance matching was improved, resulting in a good microwave absorption. Careful characterizations showed that the giant decreases of permittivity were ascribed to the high resistivity of surface due to the MgFe₂O₄ cladding. A high frequency microwave absorption property with thin absorber thickness was obtained for the Fe flakes with 10 wt% MgFe₂O₄.     

稀土掺杂对钴铁氧体电子结构和磁性能影响的理论研究

尖晶石型铁氧体是十分重要的磁性材料之一, 具有独特的物理性质、化学特性、磁学特性和电子特性. 其中尖晶石型钴铁氧体具有较好的电磁性质而被广泛应用. 本文基于密度泛函理论(DFT) 的第一性原理平面波赝势法, 结合广义梯度近似(GGA+U), 研究了CoRE_0.125Fe_1.875O₄ (RE = Nd, Eu, Gd)体系的电子结构和磁性能. 结果表明随着稀土元素从Nd到Gd掺杂体系晶胞的晶格常数呈递减趋势. 磁性能依赖于稀土离子(RE³⁺)4f轨道未配对的电子数, 掺杂Eu和Gd能够提高钴铁氧体体系的磁矩, 主要因为它们3+价态离子具有较多未配对的4f电子, 因而对磁性能的影响较大. 然而Nd 的掺杂对体系磁性能的影响很小, 这是由于它的离子半径较大, 导致晶格发生畸变.     

Composition and magnetic properties of cobalt ferrite nano-particles prepared by the co-precipitation method

Cobalt ferrite nano-particles were prepared using the co-precipitation method followed by annealing treatment. The formation of nano-particles with different composition, microstructure and sizes were confirmed by X-ray diffraction, Raman, thermogravimetric-differential thermal analysis and transmission electron microscope. The magnetic hysteresis loops measured at room temperature revealed smaller effective magnetic anisotropy constant, coercivity and remanence ratio for the samples prepared by adding the NaOH solutions into the mixed solutions of Co²⁺ and Fe³⁺ ions due to the formation of Co³⁺ ions. A small saturation magnetization and an enhanced coercivity were observed for the nano-particles prepared by adding the mixed solutions of Co²⁺ and Fe³⁺ ions into the NaOH solutions, which was related to the formation of outer layers with poor crystallization on the surfaces of the cobalt ferrite nano-crystals. Furthermore, the existence of these outer layers induced the oxidation of Co²⁺ ions in cobalt ferrite nano-crystals at 200 and 300 °C, and led to a large change on the composition and magnetic properties.     

Stability and textural properties of cobalt incorporated MCM-48 mesoporous molecular sieve

Transition metal cobalt incorporated MCM-48 mesoporous molecular sieves (CoMCM-48) with different Co contents were synthesized hydrothermally at 120 °C for 24 h by directly adding fluoride ions to the initial gel. The resulting materials were characterized by means of XRD, TEM, FT-IR, UV-vis, TPR and N₂ physical adsorption, respectively. The effect of various factors, such as the Si/Co molar ratio, calcination temperature and hydrothermal treatment time, on the crystalline structure and textural properties of CoMCM-48 was investigated in detail. The results show that the CoMCM-48 mesoporous materials with high specific surface area were successfully synthesized. A small amount substitution of Co for Si in MCM-48 did not significantly change the textural properties while the higher cobalt incorporated leads to decrease of the surface area and deterioration of structural regularity. Furthermore, the resulting CoMCM-48 still retained the cubic mesoporous framework even after calcination at 800 °C for 4 h or hydrothermal treatment at 100 °C for 24 h.     

Influence of vacuum sintering on microstructure and magnetic properties of magnetostrictive cobalt ferrite

Differences in the microstructure and magnetic properties of highly magnetostrictive cobalt ferrite resulting from the effects of different vacuum sintering temperatures and times have been investigated. A vacuum environment was chosen to allow direct comparison of results with air-sintered samples which are more often reported in the literature. It was found that vacuum sintering resulted in the development of a solid solution second phase with composition Co_1−xFe_xO₄ (x∼0.33). There was a decrease in magnetostriction as a result of the formation of the second phase. Furthermore, differences in sintering temperatures were found to have a greater effect on the magnetostriction than differences in sintering times. It was found that the first order cubic anisotropy coefficient initially increased with both sintering temperature and time, before peaking and decreasing to its lowest measured value. The lowest anisotropy was therefore achieved with samples sintered at higher temperatures and longer times.     

Low field microwave absorption and magnetization process in CoFeNi electroplated wires

Ferromagnetic resonance （FMR）, Ferromagnetic antirresonance （FMAR） and low field magnetoimpedance （MI） are the characteristic features of high frequency losses in applied fields. While some results on FMR and FMAR in CoFeNi electroplated wires were reported earlier, here we present microwave absorption in CuBe wires electroplated by 1 μm FeCoNi magnetic layer at very low fields. These data are comparatively analysed together with longitudinal hysteresis loops in order to reveal the correlation between power absorption and magnetization processes. Microwave studies are made by using the cavity perturbation method at 9.65 GHz for a DC field parallel to the sample axis, and with microwave magnetic field hrf parallel or perpendicular to the wire axis. Two peaks have been observed in all samples, one is due to FMR, and the other is, at very low fields, related to MI. The MI peaks represent minima in power absorption. By comparing with the hysteresis loop we remark the close correspondence between the MI phenomena in the axial mode and the concomitant magnetization process.     

Effect of heat treatment on the magnetic and magnetoelastic properties of cobalt ferrite

The influence of different heat treatments on the magnetic and magnetoelastic properties of highly magnetostrictive CoFe₂O₄ has been investigated. The first order cubic anisotropy coefficient, coercive field, magnetostriction and high strain sensitivity were observed to decrease as the heat treatment temperature increased. The saturation magnetization of the samples on the other hand increased with increase in heat treatment temperature. These changes were not accompanied by any observable changes in crystal structure or composition and are indicative of migration of Co²⁺ from the octahedral sites (B-sites) to the tetrahedral sites (A-sites) and Fe³⁺ from the A-sites to the B-sites of the spinel structure. Different distributions of the cations at the two distinct lattice sites can strongly affect the magnetic and magnetoelastic properties of these materials.     

Synthesis of cobalt ferrite nano-particles and their magnetic characterization

Cobalt ferrite nano-particles (CoFe₂O₄) were synthesized by the co-precipitation method with ammonium hydroxide as an alkaline solution. The reactions were carried out at different temperatures between 20 and 80 °C. The nano-particles have been investigated by magnetic measurements, X-ray powder diffraction and transmission electron microscopy. The average crystallite size of the synthesized samples was between 11 and 45 nm, which was found to be dependent on both pH value of the reaction and annealing temperatures. However, lattice parameters, interplane spacing and grain size were controlled by varying the annealing temperature. Magnetic characterization of the nano-samples were carried out using a vibrating sample magnetometer at room temperature. The saturation magnetization was computed and found to lie between 5 and 67 emu/g depending on the particle size of the studied sample. The coercivity was found to exhibit non-monotonic behavior with the particle size. Such behavior can be accounted for by the combination between surface anisotropy and thermal energies. The ratio of remanence magnetization to saturation magnetization was found to exhibit almost linear dependence on the particle size.     

Electromagnetic wave absorption properties of Ba(CoTi)xFe12-2xO19@BiFeO3 in hundreds of megahertz band

The high-performance electromagnetic (EM) wave absorption material Ba(CoTi)$_{x}$Fe$_{12-2x}$O$_{19}$@BiFeO$_{3}$ was prepared by solid-state reaction, and its EM wave absorption properties were deeply studied. The results revealed that Ba(CoTi)$_{x}$Fe$_{12-2x}$O$_{19}$@BiFeO$_{3}$ could obtain excellent absorption properties in hundreds of megahertz by adjusting the Co$^{2+}$-Ti$^{4+}$ content. The best comprehensive property was obtained for $x=1.2$, where the optimal reflection loss ($RL$) value reaches $-30.42$ dB at about 600 MHz with thickness of 3.5 mm, and the corresponding effective absorption band covers the frequency range of 437 MHz-1 GHz. Moreover, the EM wave absorption mechanism was studied based on the simulation methods. The simulated results showed that the excellent EM wave absorption properties of Ba(CoTi)$_{x}$Fe$_{12-2x}$O$_{19}$@BiFeO$_{3}$ mainly originated from the internal loss caused by natural resonance, and the interface cancelation further improved the absorption properties and resulted in $RL$ peaks.