Double-layer electromagnetic wave absorber based on barium titanate/carbon nanotube nanocomposites

The electromagnetic wave absorption properties of double-layer barium titanate/carbon nanotube (BTO/CNT) nanocomposites were evaluated. The BTO/CNT nanomaterials were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy. The reflection loss (R.L.) of the samples was calculated based on the measured complex permittivity and permeability. The minimum R.L. of single-layer BTO/CNT 30 wt% nanocomposites sample with a thickness of 1.1 mm reached ~−30.3 dB (over 99.9% absorption) at 13.8 GHz, and the bandwidth of the reflection loss less than −10 dB (over 90% absorption) was 1.5 GHz. The double-layer composites consist of BTO/CNT 30 wt% (absorption layer) with thickness of 1.0 mm and BTO 30 wt% (matching layer) with thickness of 0.3 mm showed a minimum R.L. of ~−63.7 dB (over 99.9999% absorption) at 13.7 GHz, and the bandwidth of the reflection loss less than −10 dB was 1.7 GHz. Wider response bandwidth, >1.7 GHz also can be achieved with different designs of double-layer absorbers. The R.L. significantly improved and wider response bandwidth can be obtained with double-layer composites. The capability to modulate the absorption and bandwidth of these samples to suit various applications in different frequency bands indicates that these nanocomposites could be an excellent electromagnetic wave absorber.     

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.     

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).     

Flexible,hydrophobic SiC ceramic nanofibers used as high frequency electromagnetic wave absorbers

In this paper, flexible hydrophobic SiC ceramic nanofibers have been successfully fabricated via electrospinning and subsequent high temperature heat treatment. The synthesized SiC ceramic nanofibers show excellent flexibility without any breakage even under a bending angle of 142.6°, and high hydrophobicity with a water contact angle of 149.05°. The SiC nanofibers exhibit excellent electromagnetic (EM) wave absorption properties with an effective absorption bandwidth (reflection loss (RL) <−10 dB, 90% EM wave absorbed) of 4–18 GHz. The maximum reflection loss of SiC ceramic nanofibers reaches −19.4 dB at 5.84 GHz. In addition, the nanofibers are environmentally stable in 2 mol/L NaOH solution for 2 h and high temperature of 500 °C in air atmosphere. The excellent EM wave absorption performance, flexibility, hydrophobic properties, corrosion resistant properties in alkali environment and high temperature stability make SiC ceramic nanofibers to be a potential candidate for EM wave absorption used in harsh environment.     

Controllable synthesis of porous CxNy nanofibers with enhanced electromagnetic wave absorption property

Porous C_xN_y nanofibers are controllably synthesized by a simple two-step method. The prepared samples possess uniform micropores and a chemical composition of C_0.73 N_0.27 with a surface area of 329 m² g⁻¹. The obtained C_xN_y nanofibers exhibit remarkable electromagnetic (EM) wave absorption properties when compared with conventional one-dimensional carbon materials. The minimum reflection loss (RL) reaches −36 dB at 2.7 GHz when the ratio of the C_xN_y absorbent added in paraffin matrix is only 1:3. The bandwidth of the RL below −10 dB covers 7.7 GHz (8.1–15.8 GHz) at the sample thickness of 2.5 mm. A possible EM wave loss mechanism was proposed in detail. The multiple reflection and dielectric loss could govern the excellent EM absorption leading the product to a probable application in stealth materials.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

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.     

α-MnO2 nanorods-polyaniline (PANI) nanocomposites synthesized by polymer coating and grafting approaches for screening EMI pollution

In this work, α-MnO₂ nanorods-polyaniline nanocomposites were synthesized using polymer coating and grafting approaches. The synthesized nanocomposites were characterized by XRD, FESEM, EDAX, TEM, TG-DTA and FT-IR techniques. The Electromagnetic properties of prepared samples were measured using vector network analyzer in the 8–18 GHz (X and Ku-Band) frequency region. The α-MnO₂-NH₂-PANI nanocomposite synthesized by grafting approach showed better electrical conductivity, excellent dielectric loss with superior microwave absorption ability. In comparison with pure MnO₂, the microwave absorption characteristics of α-MnO₂-NH₂-PANI nanocomposite display considerable improvements, with an effective absorption band at 10.8 GHz and 14.5 GHz under ?10 dB and minimum reflection loss (RL) of ?30.79 dB at 14.5 GHz. The α-MnO₂-NH₂-PANI sample also showed considerable shielding effectiveness (SE) i.e. ?20.85 dB in the 8–18 GHz frequency region. The observed value of RL and SE surpasses the required value for being utilized at a commercial level. These results are surely helpful to explore the microwave absorption study of different combinations of organic/inorganic nanocomposite materials particularly for shielding and microwave absorption applications.     

Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles

Ceramics-polymer nanocomposites consisting of core-shell structured BaTiO₃@Al₂O₃ (BT@Al₂O₃) nanoparticles as the filler and poly(vinylidene fluoride) (PVDF) as the polymer matrix were fabricated by solution casting. At the same volume fraction, the BT@Al₂O₃/PVDF nanocomposites, with larger dielectric constant and higher energy density, outperformed the BT/PVDF nanocomposites. The 2.5 vol% BT@Al₂O₃/PVDF nanocomposites at 360 MV/m had a double more energy density than pure PVDF at 400 MV/m (6.19 vs. 2.30 J/cm³), and a remarkably 42% lower remnant polarization than the 2.5 vol% BT/PVDF nanocomposites (0.99 vs. 1.69 μC/cm² at 300 MV/m). Such significant enhancement was closely related to the surface modification by Al₂O₃, which improved the insulation of BT nanoparticles and reduced the contrast of dielectric constant between the filler and the PVDF matrix.     

Enhanced radar and infrared compatible stealth properties in hierarchical SnO2@ZnO nanostructures

Hierarchical SnO₂@ZnO nanostructures are successfully synthesized in a large scale by using a simple hydrothermal method. The SnO₂ nanowires epitaxially grow on the non-polarized plane of ZnO nanorods with a six-fold symmetry. The radar wave absorbing and infrared emissivity properties of hierarchical SnO₂@ZnO nanostructures are studied. Such hybrid hierarchical SnO₂@ZnO nanostructures show enhanced radar and infrared compatible stealth properties than ZnO or SnO₂. The minimum reflection loss (RL) is −23.51 dB at 9.2 GHz with a bandwidth (RL<−10 dB) of 3.5 GHz and the average infrared emissivity in middle-infrared band and far-infrared band are around 0.65 and 0.89, respectively.     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

Polymer-assistant ceramic nanocomposite materials for advanced fuel cell technologies

In this study,nanocomposites of LaCePr-oxide (LCP) and Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) with different contents of polyvinylidene fluoride (PVDF) were prepared and applied to solid oxide fuel cells. The composite materials were characterized by X-ray diffraction analysis (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electrochemical impedance spectrum (EIS). The effect of PVDF concentration on the conductivity and performance of the fuel cells was investigated. It was found that PVDF plays a template role of pore forming in the nanocomposites, and the changed microstructure by as-formed pores greatly influences the electrochemical property of the nanocomposites. The cell with 3 wt% PVDF heat-treated at 210 °C achieved the highest power density of 982 mW cm⁻² at 520 °C, which enhanced performance by more than 57% than when no heat-treatment was implemented. It is 66% higher than the cell with no PVDF and no heat-treatment. Pores formed by PVDF after heat-treatment enlarged the triple phase boundary (TPB), which results in improved fuel cell performance.     

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.     

Fast discharge and high energy density of nanocomposite capacitors using Ba0.6Sr0.4TiO3 nanofibers

Nanocomposites combining high breakdown strength (BDS) polymer and high dielectric permittivity ceramic fillers have shown great potential for pulsed power application. Here a new composite material based on surface-functionalized Ba_0.6Sr_0.4TiO₃ nanofibers/poly(vinylidene fluoride) (BST NF/PVDF) has been prepared by solution casting. The nanocomposites containing 2.5 vol% isopropyl dioleic(dioctylphosphate) titanate (NDZ 101)-functionalized BST NF (N-h-BST NF) have large energy density of 6.95 J cm⁻³ at 380 MV m⁻¹, which is 1.85 times larger than that of the pure PVDF at the same electric field. Also, the discharge speed of the nanocomposites containing 7.5 vol% N-h-BST NF is approximately 0.11 μs. The good properties, together with the large energy density and fast discharge speed, make this material a promising candidate for pulsed power capacitor.     

Controlled synthesis and electromagnetic wave absorption properties of core-shell Fe3O4@SiO2 nanospheres decorated graphene

Fe₃O₄/reduced graphene oxide (RGO) nanocomposite was synthesized by a simple hydrothermal method and then SiO₂ coated onto Fe₃O₄ by a modified Stӧber method. The transmission electron microscopy and field emission scanning electron microscopy characterization indicate that masses of Fe₃O₄@SiO₂ core-shell structure nanospheres attached to the RGO sheets, and that the thicknesses of SiO₂ shells are about 20–40 nm. The X-ray diffractograms and Raman spectra illustrate that the synthesized samples consist of highly crystallized cubic Fe₃O₄, amorphous SiO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis loops reveal the ferromagnetic behavior of the samples at room temperature. In addition, the Fe₃O₄@SiO₂/RGO paraffin composite exhibit excellent electromagnetic wave absorption properties at room temperature in the frequency range of 2–18 GHz, which are attributed to the effective complementarities between the dielectric loss and magnetic loss. For Fe₃O₄@SiO₂/RGO-1 and Fe₃O₄@SiO₂/RGO-2 nanocomposite, the minimum reflection loss can reach −26.4 dB and −16.3 dB with the thickness of 1.5 mm, respectively. The effective absorption bandwidth of the samples can reach more than 10.0 GHz with the thickness in the range of 1.5–3.0 mm. It is demonstrated that such nanocomposite could be used as a promising candidate in electromagnetic wave absorption area.     

Control of dispersion frequency of BaTiO3-based ceramics applicable to thin absorber for millimeter electromagnetic wave

Elements such as B, Li and Na were doped to barium titanate, BaTiO₃ in order to control dielectric dispersion. Addition of 3 mol% Li₂O lowered the dispersion at frequency of 0.53 MHz, while addition of 3 mol% B₂O₃ or Na₂O did not affect dispersion frequency. BaTiO₃ doped with 0.3 mol% Li₂O showed dielectric dispersion at around 2.5 GHz. An electromagnetic (EM) wave absorber using the doped BaTiO₃ plate was tried to produce for millimeter frequency range. A matching layer of 0.5 mm thick ceramic plate with relative permittivity 21 was attached to it to suppress reflection of incident EM wave due to the discontinuity at the boundary between the BaTiO₃ and air. The obtained EM wave absorber had reflectivity of −45 dB at 31 GHz and −25 dB at 95 GHz, respectively.     

Microwave absorption study of carbon nano tubes dispersed hard/soft ferrite nanocomposite

Nickel substituted strontium hexaferrite, SrNi₂Fe₁₀O₁₉·(SrFe₁₂O₁₉/NiFe₂O₄) nanoparticles have been synthesized by low combustion method by citrate precursors using sol to gel (S–G) followed by gel to nano crystalline (G–N) conversion. The resulting ‘as-synthesized’ powder is heat treated (HT) at 800 and 1000 °C for 4 h in nitrogen atmosphere. The hysteresis loops show an increase in saturation magnetization from 27.443 to 63.706 emu/g with increasing HT temperatures. The multiwalled carbon nano tubes (CNTs) were synthesized by thermal decomposition of acetylene gas over iron-catalyst deposited on silicon wafer in the temperature range of 750–800 °C. A microwave absorbing medium is prepared by adding CNTs in the nickel substituted strontium hexaferrite nanoparticles. Addition of certain mass of CNTs improves the microwave absorption properties and wave band of SrFe₁₂O₁₉/NiFe₂O₄ absorbent. When 10 wt% CNTs is mixed with SrFe₁₂O₁₉/NiFe₂O₄ nanoparticles to fabricate a composite with 2 mm thickness, the maximum reflection loss reaches to ?36.817 dB at 9.292 GHz and ?10 dB bandwidth reaches 3.27 GHz.     

Dielectric and microwave absorption properties of polymer derived SiCN ceramics annealed in N2 atmosphere

Porous SiCN ceramics were successfully fabricated by pyrolysis of a kind of polysilazane. The effects of annealing temperature on the microstructure evolution, direct-current electrical conductivity, dielectric properties, and microwave absorption properties of SiCN in the frequency range 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, SiC, Si₃N₄ and free carbon nanodomains are gradually formed in the SiCN. Both the SiC and free carbon nanodomains lead to the increases of the complex relative permittivity and loss tangent of SiCN. With the increase of the annealing temperature, the average real permittivity, imaginary permittivity and loss tangent increase from 4.4, 0.2 and 0.05 to 13.8, 6.3 and 0.46, respectively. The minimum reflection coefficient and the frequency bandwidth below −10 dB for SiCN annealed at 1500 °C are −53 dB and 3.02 GHz, indicating good microwave absorption properties.     

Influence of oxygen plasma treatment parameters on poly(vinylidene fluoride) electrospun fiber mats wettability

Electrospun poly(vinylidene fluoride) (PVDF) fiber mats find applications in an increasing number of areas, such as battery separators, filtration and detection membranes, due to their excellent properties. However, there are limitations due to the hydrophobic nature and low surface energy of PVDF. In this work, oxygen plasma treatment has been applied in order to modify the surface wettability of PVDF fiber mats and superhydrophilic PVDF electrospun membranes have been obtained. Further, plasma treatment does not significantly influences fiber average size (∼400 ± 200 nm), morphology, electroactive β-phase content (∼80–85%) or the degree of crystallinity (X_c of 42 ± 2%), allowing to maintain the excellent physical–chemical characteristics of PVDF. Plasma treatment mainly induces surface chemistry modifications, such as the introduction of oxygen and release of fluorine atoms that significantly changes polymer membrane wettability by a reduction of the contact angle of the polymer fibers and an overall decrease of the surface tension of the membranes.     

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.