Development of hard/soft ferrite nanocomposite for enhanced microwave absorption

Nickel and zinc substituted strontium hexaferrite, SrFe₁₁Zn_0.5Ni_0.5O₁₉ (SrFe₁₂O₁₉/NiFe₂O₄/ZnFe₂O₄) nanoparticles having super paramagnetic nature are synthesized by co-precipitation of chloride salts using 7.5 M sodium hydroxide solution. The resulting precursors are heat treated (HT) at 900 and 1200 °C for 4 h in nitrogen atmosphere. During heat treatment, transformation proceeds as a constant rate of nucleation and three dimensional growth with an activation energy of 176.79 kJ/mol. The hysteresis loops show an increase in saturation magnetization from 1.042 to 59.789 emu/g with increasing HT temperatures. The ‘as-synthesized’ particles with spherical and needle shapes have size in the range of 20–25 nm. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate and pyramidal shapes with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties are estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches −29.62 dB (99% power attenuation) at 10.21 GHz which suits its application in RADAR absorbing materials.     

Fast synthesis of SrFe12O19 hexaferrite in a single-mode microwave cavity

The M-type SrFe₁₂O₁₉ hexaferrite has been synthesized by a microwave solid state reaction process – a fast heating process – in a home-made 2.45 GHz single-mode microwave cavity. Starting from SrCO₃ and Fe₂O₃ mixtures (1:6 ratio), cold pressed samples have been heated up in the microwave electric field without needing any susceptor, demonstrating the good coupling of the precursors with this microwave mode in our experimental setup.After optimization of the experimental conditions, the properties of the obtained ceramics, including structure, microstructure and magnetic properties, are compared with those of ceramics synthesized by conventional solid state reaction. With that microwave process, it is found that SrFe₁₂O₁₉ ceramics prepared in less than 30 min exhibit magnetic properties similar to those of the same compound produced by a conventional process. This highlights the potentialities of the technique to synthesize hexaferrite ceramics.     

Synthesis of hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance

A ternary functional composite NiFe₂O₄@MnO₂@graphene was synthesized successfully via a facile method. The phase constitution, microstructures, morphologies and chemical compositions of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). It was observed that the NiFe₂O₄ nanoparticles were coated by hierarchically MnO₂ shells and distributed on the surface of graphene. Investigations of EM wave absorption indicated that NiFe₂O₄@MnO₂@ graphene composite has the strongest reflection loss of −47.4 dB at 7.4 GHz at the matching thickness of 3 mm, compared to NiFe₂O₄ and NiFe₂O₄@MnO₂, and its maximum absorption bandwidth (<−10 dB) is 4.3 GHz (from 5.1 to 9.4 GHz). The enhanced microwave absorption performance can be attributed to the hierarchical structure of MnO₂, void space between MnO₂ and graphene, and better impedance matching of ternary composite. The above results indicate that the novel hierarchical NiFe₂O₄@MnO₂@graphene composite, with intense absorption and wide absorption bandwidth, would be a promising absorber with less EM wave interference.     

Development of novel magnetic-dielectric ceramics for enhancement of reflection loss in X band

The purpose of this research was to develop BaFe_9.5Al_1.5CrO₁₉-xCaCu₃Ti₄O₁₂ nanocomposites (x=10%, 20%, 30%, 40%, 50%) and investigate their structural and magnetic features. The substituted barium hexaferrite (BaFe_9.5Al_1.5CrO₁₉) nanoparticles and calcium copper titanate (CaCu₃Ti₄O₁₂) particles were synthesized by the auto-combustion sol-gel method. The structural, chemical composition and morphology of CaCu₃Ti₄O₁₂ (CCTO) and the nanocomposites were investigated by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The magnetic and microwave properties of nanocomposites were also investigated by vibrating sample magnetometer and vector network analyzer, respectively. The results confirmed that 1100 °C is the optimum synthesis temperature for CCTO, the mean particles size of the CCTO particles changing from 220 nm (at 850 °C) to 2.18 µm (1250 °C). The SEM micrograph revealed that in all of the BaM-xCCTO nanocomposites (x=10%, 20%, 30%, 40%, 50%), the CCTO dielectric particles were attached to the substituted barium hexaferrite nanoparticles, indicating the effectiveness of the adopted synthesis method. Due to the presence of a dielectric phase in the nanocomposites the saturation magnetization decreases from 22 emu/g to 12 emu/g. The coercive field was a slightly larger than substituted barium hexaferrite and increased from 5.558 kOe for substituted barium hexaferrite to 5.813 kOe for BaM-50CCTO due to hindered motion of the domain walls by the dielectric phase and also to the collective behavior of agglomerated barium ferrite nanoparticles. The BaM-30CCTO nanocomposite shows the highest value of reflection loss compared to other nanocomposites. The reflection dip frequency of BaM-30CCTO nanocomposite was −48.85 dB at 10.93 GHz.     

Improved magnetic and electromagnetic absorption properties of xSrFe12O19/(1−x)NiFe2O4 composites

xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites (0 ≤ x ≤ 1.0) were synthesized by using a conventional solid-state synthetic route. The results show that magnetic hysteresis loops of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are similar to those of individual component ferrites, except for the 0.1SrFe₁₂O₁₉/0.9NiFe₂O₄ and 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄, suggesting that the hard/soft magnetic phases are well exchange-coupled. The saturation magnetization, coercivity, and remanent magnetization of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are increased with increasing content of SrFe₁₂O₁₉, with maximal values of 42.1 Am² kg⁻¹, 78.7 kA m⁻¹, 17.2 Am² kg⁻¹, respectively, as the content x is about 0.5. They are higher than those of the individual components, implying that interface coupling is present in the magnetic composites. The coercivity and remanent magnetization of the composites are increased initially with increasing sintering temperature and then show a downward tendency. For the component SrFe₁₂O₁₉ and NiFe₂O₄, the minimum reflection losses are −12.5 dB and −18.3 dB at match thicknesses of 2.5 mm and 2 mm, respectively. Compared with those of the component SrFe₁₂O₁₉ and NiFe₂O₄, the microwave absorption performances of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are improved remarkably, especially for the samples of x = 0.3 and x = 0.9. The minimum reflection losses values of the 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄ composite are −31.6 dB (12.7 GHz) and −20.2 dB (13 GHz), while those of the 0.9SrFe₁₂O₁₉/0.1NiFe₂O₄ composites are −23.7 dB (16.3 GHz) and −33.5 dB (15.8 GHz), as the matching thicknesses are 2.5 mm and 2 mm, respectively. Therefore, the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites could be used as potential microwave absorption materials.     

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber composites

Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe₃O₄) nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe₃O₄–CNTs–HPCFs composites were investigated in the frequency range of 2–18 GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15 dB covers a wide frequency range from 10.2 to 18 GHz with the thickness of 1.5–3.0 mm, and the minimum reflection loss is −50.9 dB at 14.03 GHz with a 2.5 mm thick sample layer. Microwave absorbing mechanism of the Fe₃O₄–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe₃O₄ and CNTs–HPCFs.     

Magnetic and microwave properties of BaFe12O19 substituted with magnetic,non-magnetic and dielectric ions

Barium hexaferrite particles were synthesized with conventional solid state reaction route. 1% boron (B₂O₃) was added to the initial mixture of oxides to inhibit crystal growth at lower temperatures. Magnetic (Mn²⁺, Co²⁺, Ni²⁺and Cu²⁺), non-magnetic (Zn²⁺) and dielectric (Ti⁴⁺) ions were replaced by one Fe³⁺ ion of barium hexaferrite to shift the ferromagnetic resonance frequency to low frequencies and to increase the magnetic and dielectric losses. The structural and morphological characterization of samples was done by X-ray powder diffractometer and scanning electron microscopy. Magnetic and microwave properties were determined by vibrating sample magnetometer and vector network analyzer, respectively. The maximum saturation magnetization and the highest reflection losses of −34 dB at 10 GHz, with absoption bandwidth of 1.6 GHz at −20 dB, were observed in Cu²⁺–Ti⁴⁺ and Zn²⁺–Ti⁴⁺ substituted samples. The mechanism of microwave energy dissipation is due to the impedance matching at matching thickness. It was also observed that as the sample thickness increases, the resonance frequency decreases exponentially.     

Effect of La-Ni substitution on structural,magnetic and microwave absorption properties of barium ferrite

In this paper, La-Ni substituted barium ferrite nanoparticles were prepared by a co-precipitation method. The morphology, structure, magnetic and microwave absorption properties of samples were accomplished by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analysis. From the results evaluation, it can be seen that the magnetoplumbite structure for all of the samples have been formed and the average crystallite sizes of Ba_1−xLa_xFe_12−xNi_xO₁₉ nanoparticles within in the range of 50.9–65.5 nm. Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ exhibits a remarkable reflection loss of −13.5 dB at 13.05 GHz with a matching thickness of 1.5 mm. The reflection loss results indicate that Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ nanoparticles may be used as a potential for thin microwave absorbers.     

Controlled synthesis of Fe3O4@SnO2/RGO nanocomposite for microwave absorption enhancement

A ternary nanocomposite of Fe₃O₄@SnO₂/reduced graphene oxide (RGO) with different contents of SnO₂ nanoparticles was synthesized by a simple and efficient three-step method. The transmission electron microscopy and field emission scanning electron microscopy characterization display that plenty of Fe₃O₄@SnO₂ core–shell structure nanoparticles are well distributed on the surface of RGO sheets. The X-ray diffractograms show that the products consist of highly crystallized cubic Fe₃O₄, tetragonal SnO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the products at room temperature. The microwave absorption properties of paraffin containing 50 wt% products were investigated at room temperature in the frequency range of 2–18 GHz by a vector network analyzer. The electromagnetic data show that the maximum reflection loss is −45.5 dB and −29.5 dB for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. Meanwhile, the reflection loss less than −10 dB is up to 14.4 GHz and 13.8 GHz for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. It is believed that such nanocomposite could be used as promising microwave absorbers.     

Fabrication of microwave absorbing CoFe2O4 coatings with robust superhydrophobicity on natural wood surfaces

A superhydrophobic wood surface with microwave absorption property was prepared based on the formation of CoFe₂O₄ nanoparticles and subsequent hydrophobization using fluorinated alkylsilane (FAS). Meanwhile, sticky epoxy resin was worked as a caking agent by adhering abundant of CoFe₂O₄ nanoparticles to wood surface. The as-prepared superhydrophobic coatings on wood maintain stable superhydrophobicity after suffering a significant abrasion. Moreover, the complex permeability and permittivity of the coated wood composites were measured in the frequency range of 2–18 GHz by vector network analysis. The microwave absorption properties were elucidated by the traditional coaxial line method. The results show that the as-prepared wood composites have excellent microwave absorption properties at the frequency of 16 GHz, and the minimum reflection loss can reach −12.3 dB. The approach presented may provide further routes for designing outdoor wood wave absorbers with a specified absorption frequency.     

Study of strontium ferrites substituted by lanthanum on the structural and magnetic properties

M-type strontium ferrites, Sr_0.8La_0.2Fe₁₂O₁₉ have been synthesized by conventional ceramic process. The effects of lanthanum addition and sintering temperature on microstructures and magnetic properties of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ samples were investigated. Microstructural analysis of the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ specimens, sintered at different temperatures revealed that average grain sizes of SrFe₁₂O₁₉ ferrites were larger than that of Sr_0.8La_0.2Fe₁₂O₁₉ ferrite and increased with increasing sintering temperature. The X-ray diffraction (XRD) results confirmed the strontium hexagonal ferrite phase of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ compounds. A maximum coercivity value of 4850 Oe and maximum saturation magnetization value of 102 emu/g were obtained for the SrFe₁₂O₁₉ ferrite sintered at 1150 °C and for the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ ferrites sintered at 1300 °C, respectively. The remanence (Mr) of Sr_0.8La_0.2Fe₁₂O₁₉ sample sintered at 1200 °C possesses the maximum value of 60 emu/g.     

Fabrication of carbon encapsulated Co3O4 nanoparticles embedded in porous graphitic carbon nanosheets for microwave absorber

Carbon-encapsulated Co₃O₄ nanoparticles homogeneously embedded 2D (two-dimensional) porous graphitic carbon (PGC) nanosheets were prepared by a facile and scalable synthesis method. With assistance of sodium chloride, the Co₃O₄ nanoparticles (10–20 nm) with magnetic loss were well encapsulated by onion-like carbon shells homogeneously embedded porous graphitic carbon nanosheets (thickness of less than 50 nm) with dielectric loss. In the architecture, the well impedance matching for microwave absorption can be obtained by the synergetic effect between Co₃O₄ nanoparticles and encapsulated porous carbon nanosheets. The minimum reflection loss value of −32.3 dB was observed at 11.4 GHz with a matching thickness of 2.3 mm for 2D Co₃O₄@C@PGC nanosheets. The 2D Co₃O₄@C@PGC nanosheets can be used as a kind of candidate for microwave absorbing materials.     

High electromagnetic wave absorbing performance of activated hollow carbon fibers decorated with CNTs and Ni nanoparticles

Activated hollow carbon fibers (ACHFs) decorated with carbon nanotubes (CNTs) and nickel nanoparticles (CNTs–Ni–ACHFs) were prepared by thermal reduction and chemical vapor deposition technique. Microwave reflection loss, permittivity and permeability of CNTs–Ni–ACHFs composites as novel electromagnetic wave absorbents were studied in the frequency range of 2–18 GHz. It was demonstrated that CNTs–Ni–ACHFs absorbents possessed the best microwave absorbing performances whose minimum reflection loss was −43.457 dB at 13.10 GHz with a thickness of 2.0 mm, which is much better than those of Ni–ACHFs and ACHFs samples. The dielectric polarizations and magnetic loss derived from the effect of the porous structures, Ni nanoparticles, and defects in the CNTs–Ni–ACHFs composites are playing an important role for the excellent microwave absorbing performances.     

Coupling CoFe2O4 and SnS2 nanoparticles with reduced graphene oxide as a high-performance electromagnetic wave absorber

The reduced graphene oxide (RGO)/CoFe₂O₄/SnS₂ composites have been successfully synthesized by two-step hydrothermal processes. TEM results show that CoFe₂O₄ and SnS₂ nanoparticles with both diameters about 5–10 nm are well dispersed on the surface of graphene. Compared with RGO/CoFe₂O₄ composites, the as-prepared RGO/CoFe₂O₄/SnS₂ composites exhibit excellent electromagnetic (EM) wave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum reflection loss of RGO/CoFe₂O₄/SnS₂ composites is −54.4 dB at 16.5 GHz with thickness of only 1.6 mm and the absorption bandwidth with the reflection loss below −10 dB is up to 12.0 GHz (from 6.0 to 18.0 GHz) with a thickness in the range of 1.5–4.0 mm. And especially, they cover the whole X band (8.0–12.0 GHz), which could be used for military radar and direct broadcast satellite (DBS).     

Cation distribution and magnetic analysis of wideband microwave absorptive CoxNi1−xFe2O4 ferrites

Co_xNi_1−xFe₂O₄ ferrites (x=0, 0.2, 0.4, 0.4, 0.6, 0.8 and 1) were prepared by a sol-gel auto-combustion method. The samples were structurally characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), and Fourier transform infrared spectroscopy (FTIR). The XRD patterns confirmed single phase formation of spinel structure. Cation distribution estimated from XRD data suggested the mixed spinel structure of ferrite. The EDX analysis was in good agreement with the nominal composition. The results of FTIR analysis indicated that the functional groups of Co-Ni spinel ferrite were formed during the combustion process. According to FE-SEM micrographs, by addition of cobalt ion the average particle size of substituted nickel ferrite was gradually became smaller from 450 nm to 280 nm. Magnetic measurement using vibrating sample magnetometer (VSM) showed an increase in saturation magnetization and coercivity by Co²⁺ substitution in nickel ferrite. For Co_0.8Ni_0.2Fe₂O₄ sample, M_s and H_c reaches as high as 93 emu/g and 420 Oe, respectively. The reflection loss properties of the nanocomposites were investigated in the frequency range of 8–12 GHz, using vector network analyzer (VNA). Cobalt substitution could enhance reflection loss of NiFe₂O₄ ferrite. The maximum reflection loss value of the Co²⁺ substituted Ni ferrite was ~ −26 dB (i.e. over 99% absorption) at 9.7 GHz with bandwidth of 4 GHz (RL<– 10 dB) through the entire frequency range of X-band.     

Preparation of Fe3O4 particles from copper/iron ore cinder and their microwave absorption properties

In this study, magnetic Fe₃O₄ particles were prepared from copper/iron ore cider by precipitation oxidization method. The yield of Fe was 82.6 at%. XRD, TEM, SEM, EDS and microwave network analyzer were used to characterize the particles. The results showed that Fe₃O₄ particles were well crystallized and possessed an octahedral morphology, and the crystal size was about 200 nm; the sample with 70 wt% Fe₃O₄ exhibited the optimal absorbing ability, the minimum reflection loss was ?42.7 dB at 14.08 GHz and the bandwidth less than ?10 dB was about 4.2 GHz when the sample thickness was 1.9 mm. It was clearly demonstrated that the Fe₃O₄ particles prepared from copper/iron ore cider could be used as an effective microwave absorbing material.     

Magnetic reduced graphene oxide nanocomposite as an effective electromagnetic wave absorber and its absorbing mechanism

A magnetic reduced graphene oxide (MRGO) composite consisting of graphene oxide and Fe₃O₄ particles in the range of 5–20 nm has been prepared by the one-pot hydrothermal process. RGO nanosheets provide flexible substrates for nanoparticle decoration, while Fe₃O₄ nanoparticles can also effectively prevent nanosheets to restack each other. Compared with previously literature, the synthesized RGO-Fe₃O₄ composite exhibits excellent electromagnetic wave absorption. The minimum reflection loss (R_L) value of −49.05 dB has been observed at 14.16 GHz with a thickness of 2.08 mm. The absorption bandwidth (R_L<−10 dB) corresponding to the minimum R_L is 4.60 GHz. The electromagnetic wave absorption properties of the RGO-Fe₃O₄ composite have been interpreted through the quarter-wavelength matching model.     

Tunability and microwave dielectric properties of BaO–SrO–Nd2O3–TiO2 ceramics

Ferroelectrics such as barium–strontium titanates (BSTO) and Mg-doped BSTO are well known as promising candidates for the application in microwave tunable devices including phase shifters, filters, and others operating at microwave frequencies. In this study new bulk ceramics based on BaTiO₃ (BTO)–SrTiO₃ (STO) with addition of BaNd₂Ti₄O₁₂ (BNT) solid solution was investigated. The phase correlations, size, and nature of boundaries between phases were studied using scanning electron microscopy (SEM). The effect of compositional change on the unit cell parameters of the perovskite phase, microwave dielectric properties, and tunability under DC field had been studied. The materials with dielectric constant ∼320–700, Qf = 1950–3000 GHz at 3.5 GHz and tunability ∼8.0–31.7% at E = 1 V/μm were achieved.     

Hollow polyaniline/Fe3O4 microsphere composites: Preparation,characterization, and applications in microwave absorption

Hollow polyaniline/Fe₃O₄ microsphere composites with electromagnetic properties were successfully prepared by decorating the surface of hollow polyaniline/sulfonated polystyrene microspheres with various amounts of Fe₃O₄ magnetic nanoparticles using sulfonated polystyrene (SPS) as hard templates and then removing the templates with tetrahydrofuran (THF). The synthesized hollow microsphere composites were characterized by FT-IR, UV/Vis spectrophotometry, SEM, XRD, elemental analysis, TGA, and measurement of their magnetic parameters. Experimental results indicated that the microspheres were well-defined in size (1.50–1.80 μm) and shape, and that they were superparamagnetic with maximum saturation magnetization values of 3.88 emu/g with a 12.37 wt% content of Fe₃O₄ magnetic nanoparticles. Measurements of the electromagnetic parameters of the samples showed that the maximum bandwidth was 8.0 GHz over ?10 dB of reflection loss in the 2–18 GHz range when the Fe₃O₄ content in the hollow polyaniline/Fe₃O₄ microsphere composites was 7.33 wt%.     

Synthesis of multi-walled carbon nanotube/doped barium hexaferrite nanocomposites: An investigation of structural,magnetic and microwave absorption properties

Nanocomposite BaCu_xMg_xZr_2xFe_12−4xO₁₉/MWCNTs with different substitutions (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized using a two stage chemical route. The reported structural, magnetic and microwave absorption properties of samples were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and vector network analysis (VNA). Results showed that the doped hexaferrite nanoparticles and BaCu_xMg_xZr_2xFe_12−4xO₁₉/MWCNTs nanocomposites were synthesized successfully in all samples. The SEM and TEM micrographs proved that the MWCNTs structure was not destroyed after the acid treatment process while the nanoparticles could be absorbed on the surface of MWCNTs as a layer. The evaluation of VSM analysis also proved the maximum magnetization increase at first, and then decreased on the further increase of the dopant content. The values of coercivity varied in the range of 4689–251 (G). The values of real and imaginary parts of the permittivity of the BaM samples (x=0.0–0.5) with MWCNT was much higher than those of the BaM samples without MWCNT. It was also found that the highly doped sample (x=0.5)/MWCNT had a minimum reflection loss value of –23.1 dB with a suitable bandwidth about 6 GHz at a matching thickness of 2.1 mm. In addition, the reflection loss proved to be dependent on the absorber thickness; while with increasing the thickness of absorbers, the resonance frequencies shifted to a lower regime.