γ-MnO2/polyaniline composites: Preparation, characterization, and applications in microwave absorption

MnO₂/doped polyaniline (PANI) is prepared by an in situ polymerization method using γ-MnO₂ as the addition agent and hydrochloric acid as the doping agent. Products are characterized by FT-IR, UV-vis, XRD, and TEM. Conductivity, electromagnetic properties, and microwave absorption properties are first discussed on the basis of structural characterization. The as-prepared products of MnO₂/PANI are partially crystalline in nature and spherical in pattern with grain sizes of 50-70 nm. MnO₂ particles are successfully decorated with doped PANI. MnO₂/PANI displays moderate electric conduction, excellent dielectric losses, and microwave absorption capabilities. Compared to pure MnO₂, the dielectric and reflection loss properties of MnO₂/PANI composites exhibit significant improvements, with an effective absorption band at 5 GHz under −10 dB and maximum reflection loss of −21 dB at 13.56 GHz. Pure MnO₂ shows an effective absorption band of 3 GHz under −10 dB and a maximum reflection loss of −14.20 dB at 11.5 GHz. Thus, MnO₂/PANI composites are found to be a promising microwave absorption material.     

Magnetic and electromagnetic absorption properties of FeNi alloy nanoparticles supported by reduced graphene oxide

FeNi alloy nanoparticles (NPs) supported by reduced graphene oxide (RGO) (FeNi/RGO nanocomposites) were successfully synthesized through in‐situ reduction. Large amounts of sphere‐like FeNi NPs are uniformly deposited on the RGO nanosheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the nanocomposites at room temperature. According to the electromagnetic (EM) characteristics, the FeNi/RGO nanocomposites show outstanding EM absorption properties in the 2–18 GHz range, as evidenced by the wide effective absorption bandwidth (up to 3.3 GHz, with reflection loss R_L < –10 dB) and a minimal R_L (–32 dB) at 12.4 GHz with a thickness of 1.5 mm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Beta-manganese dioxide nanorods for sufficient high-temperature electromagnetic interference shielding in X-band

As the development of electronic and communication technology, electromagnetic interference (EMI) shielding and attenuation is an effective strategy to ensure the operation of the electronic devices. Among the materials for high-performance shielding in aerospace industry and related high-temperature working environment, the thermally stable metal oxide semiconductors with narrow band gap are promising candidates. In this work, beta-manganese dioxide (β-MnO₂) nanorods were synthesized by a hydrothermal method. The bulk materials of the β-MnO₂ were fabricated to evaluate the EMI shielding performance in the temperature range of 20–500 °C between 8.2 and 12.4 GHz (X-band). To understand the mechanisms of high-temperature EMI shielding, the contribution of reflection and absorption to EMI shielding was discussed based on temperature-dependent electrical properties and complex permittivity. Highly sufficient shielding effectiveness greater than 20 dB was observed over all the investigated range, suggesting β-MnO₂ nanorods as promising candidates for high-temperature EMI shielding. The results have also established a platform to develop high-temperature EMI shielding materials based on nanoscale semiconductors.     

Adjustable microwave absorption properties of flake shaped (Ni0.5Zn0.5)Fe2O4/Co nanocomposites with stress induced orientation

Flake shaped (Ni_0.5Zn_0.5)Fe₂O₄/Co nanocomposites were successfully fabricated by co-precipitating of Ni-Zn ferrite on the surface of cobalt nanoflakes. The electromagnetic characteristics of the samples were studied at the frequency of 0.1–14 GHz. The results showed that the cobalt nanoflakes in compacted nanocomposites were well orientated, and the nanocomposites were characterized with low optimal reflection loss (RL) of −33.8 dB at 11.5 GHz and broad RL bandwidth for <−20 dB in the frequency range of 7.6–12.1 GHz. At the same time, the position of the absorptive band can be adjusted by changing the mass ratio of ferrite to cobalt in the nanocomposites. It is proposed that the excellent microwave absorption properties are related to the combination of strong shape anisotropy of cobalt nanoflakes and adjustable dielectric loss.     

Double-layer structural design of dielectric ordered mesoporous carbon/paraffin composites for microwave absorption

Microwave absorbing materials composed of ordered mesoporous carbon (OMC) as absorbent and paraffin as matrix were prepared, and their electromagnetic and microwave absorbing properties could be tuned by changing the weight fraction of OMC at 2–18 GHz. The minimum reflection loss (RL) value reached ?9.3 dB at 8.0 GHz and the absorption range with RL lower than ?5 dB was obtained at 5.8–14.4 GHz for a single-layer absorber filled with 1.98 wt.% OMC at 3.0 mm. If a double-layer structure was adopted, the total thickness of the absorber could be reduced below 2.0 mm and the effective absorption range (RL<?10 dB) could be obtained at 8.9–14.3 GHz with a minimal RL of ?28.5 dB at 10.6 GHz. This work demonstrated that dielectric composites could be used as excellent absorbers by adopting reasonable multilayer structures.     

Electromagnetic performance and microwave absorbing property of nanocrystalline Sm2Fe14B compound

A new planar anisotropy Sm₂Fe₁₄B nanocrystal as an electromagnetic absorption material was prepared by melt-spinning method. The electromagnetic and microwave absorbing properties of Sm₂Fe₁₄B nanocrystal/nonmagnetic matrix composite in the frequency range of 0.1–10 GHz were measured and calculated. At the perfect matching point (2.9 GHz), the minimum reflection loss reaches ?42.0 dB at the matching thickness of 3.1 mm. Furthermore, the calculation shows that the normalized input impedance Z _in/Z ₀ equals 1, but the modulus of the ratio between the complex permittivity and permeability |ε/μ| is far away from unity at the perfect matching point. The effective permeability of the composite was simulated using the combination of the Landau–Lifshitz–Gilbert equation and Bruggeman’s effective medium theory; the agreement between the experimental data and the theoretical one demonstrates that the magnetic loss in the composite is mainly caused by natural resonance.     

A superior microwave absorption material: Ni2+-Zr4+ Co-Doped barium ferrite ceramics with large reflection loss and broad bandwidth

Large reflection loss and wide bandwidth are significant targets, determining the microwave absorption ability. However, it is still a challenge to simultaneously satisfy the two conditions. As a multifunctional material, BaFe₁₂O₁₉ possess excellent electromagnetic properties in the microwave frequency band. Due to the natural resonance phenomenon of the material, BaFe₁₂O₁₉ can produce a large magnetic loss which correlates with Fe³⁺ content, and the microwave absorption characteristics of barium ferrite can be modulated by ion doping. As a typical magnetic metal, Ni coupled with high-valence state Zr⁴⁺ doping helps to produce double resonance peaks. In this work, Ni²⁺-Zr⁴⁺ co-doping M-type barium ferrites (BaFe_12-2xNi_xZr_xO₁₉, BNZFO-x, x = 0–0.8) were prepared conveniently by solid-state reaction method. Several necessary measurements to characterize its microwave absorption property have been operated such as morphology, magnetic performance and electromagnetic parameters. The results show that reflection loss and bandwidth can be simply tuned by tailoring Ni²⁺-Zr⁴⁺ content. The reflection loss peak drifts from 18 GHz to 9.76 GHz, which involves a half of the studied frequency range. The maximum reflection loss achieves −60.6 dB and the corresponding bandwidth over −10 dB is 7.68 GHz for BNZFO-0.6 ceramic with only 2.1 mm thickness. Thus, the doping of Ni²⁺-Zr⁴⁺ ion pairs is beneficial to improve the absorbing properties of the material, and the superior microwave absorption property may originate from its inner double natural resonance in micro-scale. The excellent microwave absorption properties suggest that BNZFO-x is a promising candidate applied for designing electromagnetic shielding devices.     

Influence of Cr content on structure and magnetic properties of Fe-Si-Al-Cr powders

As a kind of soft magnetic metallic material, flaky FeSiAl powders have been studied and used widely. Transition metal chromium can improve the magnetic properties of FeSiAl. This article prepared Fe₈₅Si_9.5-xAl_5.5Cr_x (x=0, 2, 4, 6 wt%) alloys powders by adding chromium to replace silicon in alloys. The morphology and microstructure of alloys powders were studied, electromagnetic parameters were measured and microwave absorption properties in the frequency range from 0.5 to 18 GHz were analyzed. With the increase of Cr content, α-Fe (Al, Si) superlattice phases appeared in alloys powders, and then disappeared. Excessive Cr precipitated from the alloys when its content reaches 6 wt%. The minimum reflection loss (-20 dB) among the four powders was 2 wt% Cr content at the frequency of 11.5 GHz. The peaks of reflection loss shifted to the low frequency range with increase in Cr content.     

Dielectric loss and electromagnetic absorption performance study of 1T/2H-MoS2/Mo2S3 nanocomposites with different ratios

High dielectric loss materials have an important application in electromagnetic (EM) absorption fields. In this paper, the ternary nanocomposites: 1T/2H-MoS₂/Mo₂S₃ with heterogeneous interfaces are synthesized by hydrothermal method. XRD, XPS, FTIR, SEM, and TEM measurements are applied to study the structure, morphology, and composition. The frequency spectra of complex permittivity (ε_r-f) are measured in 2–18 GHz by vector network analyzer. The results show that the nanocomposites have higher dielectric loss angle tangents than the reported 2H-MoS₂ absorbers. Based on the ε_r-f spectra, the reflection loss-frequency curves ($RL-f$) are simulated at given thicknesses. An effective absorption bandwidth of 5.2 GHz (12.8–18 GHz) and a $RL$ peak of −29.49 dB are achieved in a thin thickness of 1.62 mm, which are comparable to the reported 2H-MoS₂ absorbers with complex composition, showing that the 1T/2H-MoS₂/Mo₂S₃ nanocomposites have great application potential as an EM wave absorber in the K_u band.     

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.     

Double-layer microwave absorber of nanocrystalline strontium ferrite and iron microfibers

Microwave absorption properties of the nanocrystalline strontium ferrite (SrFe₁₂O₁₉) and iron (α-Fe) microfibers for single-layer and double-layer structures are investigated in a frequency range of 2 GHz-18 GHz. For the single-layer absorbers, the nanocrystalline SrFe₁₂O₁₉ microfibers show some microwave absorptions at 6 GHz-18 GHz, with a minimum reflection loss (RL) value of -11.9 dB at 14.1 GHz for a specimen thickness of 3.0 mm, while for the nanocrystalline α-Fe microfibers, their absorptions largely take place at 15 GHz-18 GHz with the RL value exceeding -10 dB, with a minimum RL value of about -24 dB at 17.5 GHz for a specimen thickness of 0.7 mm. For the double-layer absorber with an absorbing layer of α-Fe microfibers with a thickness of 0.7 mm and matching layer of SrFe₁₂O₁₉ microfibers with a thickness of 1.3 mm, the minimum RL value is about -63 dB at 16.4 GHz and the absorption band width is about 6.7 GHz ranging from 11.3 GHz to 18 GHz with the RL value exceeding -10 dB which covers the whole K_u-band (12.4 GHz-18 GHz) and 27% of X-band (8.2 GHz-12.4 GHz).     

Only Ku-band microwave absorption by Fe3O4/ferrocenyl-CuPc hybrid nanospheres

A novel kind of hybrid nanospheres made of Fe₃O₄ and ferrocenyl-CuPc (FCP) was prepared via effective solvothermal method and performed microwave absorptivity only in Ku-band with minimum reflection loss of −25 dB at 16.0 GHz corresponding to absorbing about 99.7% content of microwave. Scanning electron microscopy images indicated that the nanospheres with uniform particle size distribution have the average diameter of 135 nm. Due to the synergistic reaction between magnetic ferrocenyl-CuPc and Fe₃O₄, the hybrid nanospheres showed novel electromagnetic properties. The real part of complex permittivity of hybrid nanospheres remains stable in the range of 0.5–12.0 GHz and has a large fluctuation at 16.5 GHz. Moreover, the dielectric loss of hybrid nanospheres also appeared a sharp peak at 16.3 GHz with the value of 2.7. The specific gravity of hybrid nanospheres is about 2.08. On the basis of these results, the novel hybrids are believed to have potential applications in the microwave absorbing area in Ku-band.     

Electronic structure and half-metallicity of the new Heusler alloys PtZrTiAl,PdZrTiAl and Pt0.5Pd0.5ZrTiAl

The electronic structure and magnetic properties of the PtZrTiAl, PdZrTiAl and Pt_0.5Pd_0.5ZrTiAl Heusler alloys were investigated using the full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA). For the PtZrTiAl, and PdZrTiAl alloys, the results showed that these Heusler alloys were stable in the Type I structure. The (Pt, Pd)ZrTiAl Heusler alloys are found to exhibit half-metallic ferromagnetism for both the Type I and Type II structure. The total magnetic moments of the PtZrTiAl and PdZrTiAl alloys were obtained to be 3 μ_B per formula unit, which are in agreement with the Slater-Pauling rule m_tot = (Nv ? 18). The half-metalliciy characteristic exists in the relatively wide ranges of 6.06–6.78 Å, and 6.13–6.73 Å for the PtZrTiAl and PdZrTiAl alloys, respectively. To complete the fundamental characteristics of these alloys, Pt_0.5Pd_0.5ZrTiAl is predicted to be a half-metallic ferromagnet with an energy gap of 0.90 eV in the minority spin and a complete spin polarization at the Fermi level. These new Heusler alloys may become ideal candidate material for future spintronic applications.     

Magnetic and microwave absorption properties of BaFe12−x (Mn0.5Cu0.5Zr)x/2O19 synthesized by sol–gel processing

BaFe_12−x (Mn_0.5Cu_0.5Zr)_x/2O₁₉ hexaferrites with x=1, 2 and 3 were prepared by sol–gel process. The ferrite powders possess hexagonal shape and are well separated from one another. The powders of these ferrites were mixed with polyvinylchloride (PVC) plasticizer to be converted into a microwave absorbing composite ferrite with a thickness of 1.8 mm. X-ray diffractometer (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that magnetoplumbite structures for all samples were formed. The sample with higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also the present investigation demonstrates that a microwave absorber using BaFe_12−x(Mn_0.5Cu_0.5Zr)_x/2O₁₉ (x=2 and 3)/PVC with a matching thickness of 1.8 mm can be fabricated for applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.     

Synthesis and studies of trisubstituted biphthalonitrile/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> magnetic hybrid microspheres

New trisubstituted biphthalonitrile/magnetite (TSB/Fe₃O₄) magnetic hybrid microspheres were synthesized from TSB and FeCl₃ · 6H₂O using the method of one-stage thermal temperature crystallization of solvents. The morphology and structure of magnetic hybrid microspheres were inspected using a scanning electron microscope, IR Fourier spectroscopy, and X-ray diffraction. It was found that the grown TSB/Fe₃O₄ magnetic hybrid microspheres represent spherical particles with an average size of ~137 nm and a small size spread. The size and size distribution of magnetic hybrid microspheres can be controlled by a small change in the ratio of TSB and Fe³⁺ ion contents in the microsphere. TSB/Fe₃O₄ hybrid microspheres exhibit a rather high saturation magnetization (58.16 emu g^–1) and new microwave electromagnetic properties, i.e., lower (in comparison with published) dielectric losses at low frequencies; magnetic losses are increased obviously due to an increase in the TSB content. Furthermore, it is detected that magnetic hybrid microspheres absorb microwaves, and strong reflection losses in a wide frequency range are established. The effective reflection loss of–31 dB is obtained in the microwave range from 2 to 16 GHz due to TSB content variations. Wide absorption properties of microwaves along with regular spherical shape and excellent magnetic properties offer wide opportunities for various applications of TSB/Fe₃O₄ magnetic hybrid microspheres as functional materials.     

Magnetic and microwave absorbing properties of magnetite-thermoplastic natural rubber nanocomposites

Magnetic and microwave absorbing properties of thermoplastic natural rubber (TPNR) filled magnetite (Fe₃O₄) nanocomposites were investigated. The TPNR matrix was prepared from polypropylene (PP), natural rubber (NR) and liquid natural rubber (LNR) in the ratio of 70:20:10 with the LNR as the compatibilizer. TPNR-Fe₃O₄ nanocomposites with 4-12 wt% Fe₃O₄ as filler were prepared via a Thermo Haake internal mixer using a melt-blending method. XRD reveals the presence of cubic spinel structure of Fe₃O₄ with the lattice parameter of a=8.395 Å. TEM micrograph shows that the Fe₃O₄ nanoparticles are almost spherical with the size ranging 20-50 nm. The values of saturation magnetization (M_S), remanence (M_R), initial magnetic susceptibility (χ_i) and initial permeability (μ_i) increase, while the coercivity (H_C) decreases with increasing filler content for all compositions. For nanocomposites, the values of the real (ε_r′) and imaginary permittivity (ε_r′′) and imaginary permeability (μ_r′′) increase, while the value of real permeability (μ_r′) decreases as the filler content increases. The absorption or minimum reflection loss (R_L) continuously increases and the dip shifts to a lower frequency region with the increasing of both filler content in nanocomposites and the sample thickness. The R_L is −25.51 dB at 12.65 GHz and the absorbing bandwidth in which the R_L is less than −10 dB is 2.7 GHz when the filler content is 12 wt% at 9 mm sample thickness.     

Microwave characterization of Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> multiferroics at X-band

The microwave characteristics of Pb_1?x Ca _x Fe_0.5Nb_0.5O₃ multiferroics (x = 0.0, 0.4, 0.45, 0.5, 0.55, 0.6), have been investigated as a function of frequency and substitution. The results depict ?13.99 dB reflection loss at 11.65 GHz in composition x = 0.6. Microwave absorption is enhanced with substitution of Ca²⁺ ions and undoped composition 0.0 behaves as electromagnetic shield. The model governing microwave absorption is discussed and different compositions for electromagnetic applications have been suggested.     

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.     

Synthesis,Characterization, and X-Band Permittivity Studies of Polypyrrole–Zirconium Composite

The zirconium dioxide-doped polypyrrole composites with different weight percentage were prepared by in situ polymerization methods. The prepared composites were subjected for characterization with different tools like Fourier transform infrared spectroscopy and scanning electron microscopy. The Fourier transform infrared spectroscopy spectra indicate a characteristic peaks which confirms the formation of composites. Scanning electron microscopy image indicates that the metal oxides are agglomerated with polypyrrole due to that the size of the composite particles increases to 7 µm. The real part of permittivity (?′) at X-band measurements of all samples first decrease and then increase in the higher frequency range. Meanwhile, the values of ?″ for polypyrrole–zirconia composites changes spontaneously due to the scattering of free charges explained by Kramers–Kronig relations. The reflection loss graph shows that the maximum reflection loss (RL) reaches up to ?54.6 dB at 10.8 GHz and the effective absorption bandwidth (RL ≤ ?10 dB) is 6.7 GHz.     

Magnetic and microwave absorbing properties of polyaniline/γ-Fe2O3 nanocomposite

The conducting protonated polyaniline (ES)/γ-Fe₂O₃ nanocomposite with the different γ-Fe₂O₃ content were synthesized by in-situ polymerization. Its morphology, microstructure, DC conductivity and magnetic properties of samples were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), four-wire-technique, and vibrating sample magnetometer (VSM), respectively. The microwave absorbing properties of the nanocomposite powders dispersing in wax coating with the coating thickness of 2 mm were investigated using a vector network analyzers in the frequency range of 7–18 GHz. The pure ES has shown the absorption band with a maximum absorption at approximately 16 GHz and a width (defined as frequency difference between points where the absorption is more than 8 dB) of 3.24 GHz, when 10% γ-Fe₂O₃ by weight is incorporated , the width is broadened to 4.13 GHz and some other absorption bands appear in the range of 7–13 GHz. The parameter dielectric loss tan δ_e (=ε″/ε′) in the 7–18 GHz is found to decrease with increasing γ-Fe₂O₃ contents with 10%, 20%, 30%, respectively, but magnetic loss tan δ_m (=μ″/μ′) increases with increasing γ-Fe₂O₃ contents. The results show that moderate content of γ-Fe₂O₃ nanoparticles embedded in protonated polyaniline matrix may create advanced microwave absorption properties due to simultaneous adjusting of dielectric loss and magnetic loss.