Low Temperature Solution Synthesis and Microwave Absorption Properties of Multiwalled Carbon Nanotubes/Fe3O4 Composites

Multiwalled carbon nanotubes (MWCNTs)/Fe₃O₄ nanocomposites were synthesized via a simple low temperature solution method. The phase structures and morphologies of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the Fe₃O₄ spheres of about 150 nm were linked with MWCNTs. The microwave absorption properties of the MWCNTs/Fe₃O₄ nanocomposites were measured by vector network analysis (VNA). A wide region of microwave absorption was achieved due to dual magnetic and dielectric losses. When the matching thickness is 2 mm, the reflection loss (RL) of the sample exceeding ?10 dB was obtained at the frequency range of 9.9–12.4 GHz, with an optimal RL of ?29.8 dB at 11.04 GHz. A possible mechanism of the improved microwave absorption properties of the composites was discussed.     

Enhanced electromagnetic wave absorption properties of polyaniline-coated Fe3O4/reduced graphene oxide nanocomposites

Fe₃O₄-reduced graphene oxide-polyaniline (Fe₃O₄–RGO–PANI) ternary electromagnetic wave absorbing materials were prepared by in situ polymerization of aniline monomer on the surface of Fe₃O₄–RGO nanocomposites. The morphology, structure and other physical properties of the nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, vibration sample magnetism, etc. The electromagnetic wave absorbing properties of composite materials were measured by using a vector network analyzer. The PANI–Fe₃O₄–RGO nanocomposites demonstrated that the maximum reflection loss was ?36.5 dB at 7.4 GHz with a thickness of 4.5 mm and the absorption bandwidth with the reflection loss below ?10 dB was up to 12.0 GHz with a thickness in the range of 2.5–5.0 mm, suggesting that the microwave absorption properties and the absorption bandwidth were greatly enhanced by coating with polyaniline (PANI). The strong absorption characteristics of PANI–Fe₃O₄–RGO ternary composites indicated their potential application as the electromagnetic wave absorbing material.     

Preparation of core-shell structured hollow glass microspheres/BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/Ag composites with excellent microwave absorbing properties

Hollow glass microspheres/barium ferrite (HGM/BaFe₁₂O₁₉) was first prepared via co-precipitation reaction, which was then performed to fabricate the HGM/BaFe₁₂O₁₉/Ag composites by chemical plating method. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM) were utilized to characterize the structures, morphologies and properties of the resultant composites. Results showed that a homogeneous and complete BaFe₁₂O₁₉ shell was coated on the surface of the HGM, and HGM/BaFe₁₂O₁₉ composites were also fully covered with Ag particles. The conductivity of the HGM/BaFe₁₂O₁₉/Ag composites was 1.24?×?10² S/cm, whereas the saturation magnetizations of the composites was reduced to 12.76 emu/g. The microwave absorption properties of the HGMs/BaFe₁₂O₁₉/Ag composites were significantly improved compared with those of HGMs/BaFe₁₂O₁₉ composites and BaFe₁₂O₁₉ particles. The reflection loss (R) showed that the bandwidth of reflection loss of HGM/BaFe₁₂O₁₉/Ag less than ?10 dB (90% absorption) was 2.1 GHz (from 10.3 to 12.4 GHz), herein, the minimum loss value was ?19.7 dB at 12.4 GHz.     

A comparative study of Fe3O4/polyaniline composites with octahedral and microspherical inorganic kernels

Studies have been carried out on the microwave absorption properties of the title composites. The dielectric loss of each material correlated well with its microwave absorption properties. However, these properties are not affected significantly by whether the kernel is octahedral or hollow spherical. Samples with variable amounts of polyaniline were synthesized and it was found that the maximum reflection loss occurs at around 50 % of polyaniline for both octahedral and micro-spherical Fe₃O₄/polyaniline composites showing that the amount of interface between Fe₃O₄ and polyaniline plays a key part in enhancing the absorbance although other effects are also important. Hollow spherical Fe₃O₄ was synthesized using ammonium acetate. However, it was noted that when sodium acetate was used, fewer cracks appeared in the material. The presence of cracks is evidence for the hollow nature of spherical Fe₃O₄. A clad growth mechanism is proposed for the formation of the composite material.     

Preparation and microwave absorbing properties of hollow glass microspheres/Fe3O4/Ag composites with core–shell structure

The low-density, conductive and magnetic hollow glass microspheres (HGM)/Fe₃O₄/Ag composites have been successfully synthesized via co-precipitation and chemical plating method. The morphology, composition, microstructure, magnetic and microwave absorbing properties of the composites were investigated based on the analyses of the results using scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, vibrating sample magnetometer and vector network analyzer. The results showed that the HGM/Fe₃O₄ composites were successfully prepared, and the coating layers on the surface of HGM are compact and continuous. Moreover, the final composites were completely covered with Ag nanoparticles. With the addition of Ag nanoparticles, the saturation magnetization of the HGM/Fe₃O₄ composites reduces from 32.08 to 14.77 emu/g, whereas its conductivity increases to 0.48 S/cm. The reflection loss (R) of HGM/Fe₃O₄/Ag composites is lower than ?10 dB at 8.2–8.7, 9.6–10.8 and 11.4–11.9 GHz, and the minimum loss value is ?19.1 dB at 9.9 GHz.     

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

Synthesis and microwave absorption property of ternary composites: polyaniline/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Ba<Subscript>3</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>24</Subscript>O<Subscript>41</Subscript>

Polyaniline (PANI)/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite was prepared by an in-situ polymerization method. The phase structure, morphology and magnetic properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite were characterized by XRD, FT-IR, SEM, TEM, and VSM, respectively. The microwave absorption properties of the composite were investigated by using a vector network analyzer in the 2–18 GHz frequency range. The results show that the maximum reflection loss value of the PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite reaches ?30.5 dB at 10.5 GHz with a thickness of 3 mm and the bandwidth of reflection loss below ?10 dB reaches up to 1.2 GHz. The excellent microwave absorption properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite due to the enhanced impedance match between dielectric loss and magnetic loss.     

Effect of magnetite bridged carbon nanotube/graphene networks on the properties of polyarylene ether nitrile

We report the fabrication and properties of polyarylene ether nitrile (PEN) nanocomposite with 3D carbon nanotubes/graphene sheets network (Fe₃O₄-CNT/GS) bridged by magnetite. The Fe₃O₄-CNT/GS is firstly fabricated by one-step solvothermal method after the synthesis of phthalonitrile functionalized CNT (CNT-CN) and GO (GO-CNT). Fe₃O₄-CNT/GS is characterized by XPS and XRD, while the 3D frame of it is confirmed by SEM observation. Then, the obtained Fe₃O₄-CNT/GS is introduced into phthalonitrile end-capped polyarylene ether nitrile (PEN-Ph) to prepare the composites by solution-mixing assembly and solution casting method. Finally, the obtained PEN based nanocomposites are further treated at 320 °C to improve the properties of the composites. To study the effect of Fe₃O₄-CNT/GS on the PEN-Ph, the micro-morphologies, mechanical, thermal and dielectric properties of the obtained (Fe₃O₄-CNT/GS)/PEN nanocomposites films are investigated. Besides, the influence of the Fe₃O₄-CNT/GS content and the heat-treatment on the properties of the PEN composites are also investigated. The results show that Fe₃O₄-CNT/GS can improve the dielectric properties and maintain good mechanical properties of PEN composite simultaneously.     

Low-temperature spin spray deposited ferrite/piezoelectric thin film magnetoelectric heterostructures with strong magnetoelectric coupling

We report low-temperature spin spray deposited Fe₃O₄/ZnO thin film microwave magnetic/piezoelectric magnetoelectric heterostructures. A voltage induced effective ferromagnetic resonance field of 14 Oe was realized in Fe₃O₄/ZnO magnetoelectric (ME) heterostructures. Compared with most thin film magnetoelectric heterostructures prepared by high temperature (>600 °C) deposition methods, for example, pulsed laser deposition, molecular beam epitaxy, or sputtering, Fe₃O₄/ZnO ME heterostructures have much lower deposition temperature (<100 °C) at a much lower cost and less energy dissipation, which can be readily integrated in different integrated circuits.     

Synthesis and microwave absorption enhancement of yolk–shell Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@C microspheres

Rational design on the microstructure of microwave-absorbing materials is paving the way for upgrading their performances in electromagnetic pollution prevention. In this study, a Fe₃O₄/C composite with unique yolk–shell microstructure (YS-Fe₃O₄@C) is successfully fabricated by a silica-assisted route. It is found that carbon shells in this composite can make up the shortages of Fe₃O₄ microspheres in dielectric loss ability, while they may more or less attenuate the intrinsically magnetic loss of Fe₃O₄ microspheres. The microwave absorption properties of YS-Fe₃O₄@C are evaluated in the frequency range of 2.0–18.0 GHz in terms of the measured complex permittivity and complex permeability. The results demonstrate that YS-Fe₃O₄@C can exhibit much better performance than bare Fe₃O₄ microspheres and individual carbon materials, as well as core–shell Fe₃O₄/C composite (CS-Fe₃O₄@C), where strong reflection loss and wide response bandwidth can be achieved simultaneously. With an absorber thickness of 2.0 mm, the maximum reflection loss is ?73.1 dB at 14.6 GHz and a bandwidth over ?10.0 dB is in the range of 12.3–18.0 GHz. It can be proved that the unique yolk–shell microstructure is helpful to reinforce the dielectric loss ability and create an optimized matching of characteristic impedance in the composite.     

Electromagnetic and microwave absorbing properties of magnetite nanoparticles decorated carbon nanotubes/polyaniline multiphase heterostructures

Magnetite nanoparticles decorated CNTs/PANI multiphase heterostructures were prepared by polymerization of aniline monomer and an additional process of the coprecipitation of Fe²⁺ and Fe³⁺. Scanning electron microscopy and transmission electron microscopy observation indicated that the monodispersed magnetite nanoparticles were uniformly decorated on the surface of CNTs/PANI. The formation of magnetite nanoparticles on CNTs/PANI was mainly through a preferentially position-selective precipitation process. More interestingly, a portion of Fe₃O₄ nanoparticles was found to form core–shell structures with PANI. The effects of different additional amounts of NH₂Fe(SO₄)₂·6H₂O reactant on the magnetic properties and microwave absorbing performances of CNTs/PANI/Fe₃O₄ heterostructures were investigated. The CNTs/PANI/Fe₃O₄ multiphase heterostructures were proved to be superparamagnetic. The microwave absorption measurement showed that the CNTs/PANI/Fe₃O₄ samples under 1.5 g of NH₂Fe(SO₄)₂·6H₂O condition exhibited much more effective absorption performance. These results suggested the novel CNTs/PANI/Fe₃O₄ multiphase heterostructures with PANI as the second phase may be potential candidate for microwave absorption systems.     

The effect of polyethylenimine on the microwave absorbing properties of a hybrid microwave absorber of Fe3O4/MWNTs

The hybrid microwave absorber of Fe₃O₄/multi-walled carbon nanotubes (MWNTs)modified with polyethylenimine (PEI) polymers was fabricated by chemical co-precipitation. The structure and morphology of hybrids are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron-microscopy (TEM). The effect of PEI on the distribution of Fe₃O₄ nanoparticles and the microwave absorbing properties of hybrid microwave absorber of Fe₃O₄/MWNTs were investigated. The TEM results show that Fe₃O₄ nanoparticles are attached homogeneously on MWNTs, which indicates that the adding of PEI is effective to control the distribution of Fe₃O₄ nanoparticles on the surface of MWNTs. The microwave absorbing properties results show that the maximum reflection loss (RL) of PEI modified Fe₃O₄/MWNTs hybrids is improved significantly, which is ?30.69 dB at 7.24 GHz and ?10 dB bandwidth is 1.84 GHz. However, the RL of the Fe₃O₄/MWNTs without PEI is ?21.96 dB at 7.02 GHz and ?10 dB bandwidth is 1.2 GHz.     

Template-induced covalent assembly of hybrid particles for the facile fabrication of magnetic Fe3O4–polymer hybrid hollow microspheres

Magnetic Fe₃O₄–poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) hybrid hollow microspheres, in which Fe₃O₄ nanoparticles were firmly incorporated in the cross-linked PZS shell, have been fabricated through the formation of core/shell PS/Fe₃O₄–PZS composites based on template-induced covalent assembly method, followed by core removal in a tetrahydrofuran solution. The morphology, composition, thermal property, and magnetic property of the magnetic hollow microspheres were characterized by scanning electron microscope, transmission electron microscope, Fourier transform infrared spectra, energy dispersive X-ray spectroscopy, X-ray diffraction, thermogravimetric analysis, and vibrating sample magnetometer respectively. Results indicated that the typical hollow microspheres had about 950 nm of inner diameter and about 210 nm of shell thickness, 440 °C of initial decomposition temperature under nitrogen atmosphere, and 13.3 emu g^?1 of magnetization saturation. Furthermore, the shell thickness of magnetic hollow microspheres could be easily controlled by tuning the mass ratio of PS to comonomers. As-fabricated hybrid hollow microspheres exhibit great potential as good catalyst supports.     

Self-reducing coal-derived carbon/Ni3Fe magnetic composites with frequency-dependent microwave absorption performance

Coal-derived carbon/Ni₃Fe magnetic composites with frequency dependent microwave absorption performance were prepared at low temperatures (750–850 °C) using coal as the carbon source. The Ni₃Fe alloy was successfully formed due to the carbothermal reaction and reducing gas. SEM images indicate the surface becomes rougher and the number of interlayer of the composites increases with increasing reaction temperature. Consistently, high degree of graphitization of the coal-derived carbon was confirmed by using Raman spectroscopy. Specifically, coal-derived carbon/Ni₃Fe magnetic composites exhibit frequency-dependent microwave absorption characteristics at 2–18 GHz, that is, as the reaction temperature rises from 750 °C to 850 °C, the minimum reflection loss gradually shifts to low frequencies. Among them, CC/Ni₃Fe_(8 0 0)-0.4 exhibits a minimum reflection loss of ?60.76 dB at 16.64 GHz, while the thickness is only 1.28 mm. Such a clean strategy provides experience for the environmental application of coal and microwave absorption. Meanwhile, a lightweight, stable and efficient microwave absorber has been developed.     

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.     

Effect of sintering temperatures on properties of BaO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glass/silica composites for CBGA package

BaO–B₂O₃–SiO₂–Al₂O₃ (BBSA) glass/silica composites synthesized by solid-state reaction method were developed for CBGA packages, and the effects of sintering temperature (900–950 °C) on the phase transformation, microstructure, thermal, mechanical and electrical properties were investigated. XRD results show that the major phases quartz and cristobalite, and the minor phase BaSi₂O₅ are detected in BBSA composites. Furthermore, it was found that the quartz phase transforms to cristobalite phase at 930–940 °C. The formation of cristobalite phase with higher coefficient of thermal expansion (CTE) led to the increase of CTE value of BBSA composites. However, excessive cristobalite phase content would degrade the mechanical properties and the linearity of thermal expansion of the ceramics. BBSA composites sintered at 920 °C exhibited excellent properties: low dielectric constant and loss (ε_r = 6.2, tanδ = 10^?4 at 1 MHz), high bending strength (179 MPa), high CTE (12.19 ppm/°C) as well as superior linearity of the thermal expansion.     

The superior electromagnetic properties of carbonyl-iron/Fe91.2Si3.1P2.9Sb2.8 composites powder and impedance match mechanism

Carbonyl-iron and Fe_91.2Si_3.1P_2.9Sb_2.8 powder used as dual-fillers for thin microwave absorbers were firstly prepared by a simple mechanical mixture technique. The patterns and magnetic properties of carbonyl-iron and Fe_91.2Si_3.1P_2.9Sb_2.8 were characterized by scanning electron microscope and vibrating sample magnetometer The electromagnetic parameters were measured in the 2–7 GHz range by a HP8720B vector network analyzer. In comparison with carbonyl-iron and Fe_91.2Si_3.1P_2.9Sb_2.8 powder, the carbonyl-iron/Fe_91.2Si_3.1P_2.9Sb_2.8 composites powder exhibited excellent microwave absorption properties in the 2–7 GHz frequency range. The reflection loss was found to <?20 dB in the 2–7 GHz range for thickness of 2–5.3 mm, and the minimum reflection loss of ?37 dB was observed at 5.2 GHz with a matching thickness of 2.5 mm. The excellent microwave absorption properties were firstly explained by using quantitatively coefficient of electromagnetic matching. In addition, a strong natural resonance was found in the carbonyl-iron/Fe_91.2Si_3.1P_2.9Sb_2.8 composites powder as an important reason bringing about the excellent microwave absorption.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

Influence of hydrothermal treatment on the microstructure and oxidation resistance of a Zn4B2O7·H2O (4ZnO·B2O3·H2O) coating for C/C composites

Antioxidant modification for C/C composites by in situ hydrothermal synthesise at 140 °C of a 4ZnO·B₂O₃·H₂O crystallite coating has been successfully achieved. The influence of hydrothermal time on the phase composition, microstructure of the as-prepared Zn₄B₂O₇·H₂O (4ZnO·B₂O₃·H₂O), and its antioxidant modification for C/C composites were investigated. Samples were characterised by XRD, SEM, isothermal oxidation test and TG-DSC. Results show that, 4ZnO·B₂O₃·H₂O crystalline coating is achieved on the surface of C/C composites after the hydrothermal treatment at 140 °C for time in the range of 2–12 h. A smooth and crack-free 4ZnO·B₂O₃·H₂O layer can be obtained when the hydrothermal time reaches 8 h. Isothermal oxidation test demonstrates that the oxidation resistance of C/C composites is improved. The as-modified composites exhibit only 1.52 g·cm^?2 weight loss after oxidation at 600 °C for 15 h, while the non-modified one shows a 6.57 g·cm^?2 weight loss after only 10 h oxidation. For the uncoated C/C composite the oxidation rate is approximately linear with time (non-protective oxidation), thus at 15 h exposure one can estimate the mass loss to be 6.57 g·cm^?2 after 10 h for direct comparison with the coated samples.     

Effect of Iron Particle Size and Volume Fraction on the Magnetic Properties of Fe/Silicate Glass Soft Magnetic Composites

Fe/Silicate glass soft magnetic composites (SMC) were fabricated by powder metallurgy with 1000 MPa pressure at room temperature, and then annealed at 750 °C for 90 min. A continuous and discontinuous layer of iron oxygen compounds FeO and Fe₃O₄ exit at the interface. Very fine crystalline phases Na₁₂Ca₃Fe₂(Si₆O₁₈)₂ were formed in silicate glass. Crystallite formation introducing the CTE mismatch between Fe and silicate glass results in thermal stress. Therefore, the magnetic properties decrease. The particle size and volume fraction of iron powder are larger, DC magnetic properties are better, but the core loss is greater. For the Fe/ silicate glass composite, the hysteresis loss plays a leading role in iron.