A review on electrospinning nanofibers in the field of microwave absorption

As important functional materials for absorbing and attenuating incident electromagnetic waves, microwave absorption (MA) materials have found a wide range of applications in civil and military fields. In addition to the study of the compositions, the structural design of the MA materials is also a research hotspot in the field. Among the various structures, the one-dimensional (1D) structure has drawn wide attention because of the unique shape anisotropy and spatial confinement effect. Electrospinning technology has become one of the main ways to prepare continuous 1D micro-nano fibers due to the advantages of many types of spinnable materials, low spinning cost, and high controllability of process parameters. This review involves an introduction and a classification of the research progresses achieved in electrospinning technology concerning MA nanofibers from the perspective of compositions, as well as the list of their minimum reflection loss (RL_min) and effective absorption bandwidth (EAB).     

Porous magnetic carbon nanofibers (P-CNF/Fe) for low-frequency electromagnetic wave absorption synthesized by electrospinning

The recent criteria to evaluate electromagnetic wave absorber include low density, strong absorbency, wide absorption bandwidth and thin absorber thickness, but its performance at low frequencies is always ignored. In this paper, the porous magnetic carbon nanofibers (P-CNF/Fe) for high-efficient electromagnetic wave absorption at low frequencies were fabricated by electrospinning followed by stabilization and carbonization. With the introduction of porous nanostructure, the permittivity of carbon nanofibers was decreased at low frequency and the impedance matching of permittivity and permeability was realized. The electromagnetic absorbing properties were investigated in detail. The minimum reflection coefficient reaches ?44.86?dB at 4.42?GHz, and the widest effective absorption bandwidth (EAB) in the frequency was 3.28 range from 12.96 to 16.24?GHz. Consequently, considering the EM wave absorption performance, P-CNF/Fe synthesized in this work can be a promising candidate in the field of EM wave attenuation.     

Self-cleaning functionalized FeNi/NiFe2O4/NiO/C nanofibers with enhanced microwave absorption performance

Microwave absorbing materials with enhanced microwave absorption performance and self-cleaning function are of great interest for military applications and human health caused by electromagnetic radiation pollution. Herein we report the synthesis of FeNi/NiFe₂O₄/NiO/C nanofibers (NFs) via electrospinning technique using nickel acetylacetonate (Ni(acac)₂), ferric acetylacetonate (Fe(acac)₃), and N, N-dimethylformamide (DMF)/polyacrylonitrile (PAN) solution as precursor. We also show the abilities of the materials to attenuate electromagnetic microwave and their ease of self-cleaning performance. X-ray diffraction patterns and HRTEM images reveal that the materials possess FeNi, NiFe₂O₄, NiO and graphite. HAADF-STEM images show that the magnetic nanoparticles distribute uniformly along the fibers. Contrast experiments had been conducted on different calcination temperatures to elucidate the impedance matching and loss mechanism. Based on the results of the experiment, excellent microwave absorption was exhibited by blending the NiFe₂O₄/NiO/CNFs and FeNi/NiFe₂O₄/NiO/CNFs into paraffin at 50 wt% and 5 wt%, respectively. Moreover, the contact angles (CA) of the as-prepared fiber films calcined at 650, 750 and 950 °C were 143°, 141° and 144°, respectively, indicating that fiber films exhibited excellent hydrophobicity and self-cleaning function. It suggested that the as-obtained NFs had an excellent application prospect in self-cleaning microwave absorbing materials.     

Diameter control of ultrathin zinc oxide nanofibers synthesized by electrospinning

Electrospinning is a versatile technique, which can be used to generate nanofibers from a rich variety of materials. We investigate the variation of a zinc oxide (ZnO)-polyvinylpyrrolidone (PVP) composite structure in morphology by electrospinning from a series of mixture solutions of ZnO sol–gel and PVP. Calcination conditions for the crystallization of ZnO nanofibers and removal of the PVP component from the ZnO-PVP composite nanofibers were also studied. The progression of the ZnO-PVP composite structure from grains to nanofibers was observed, and ZnO-PVP nanofibers as thin as 29.9 ± 0.8 nm on average were successfully fabricated. The size of the resultant ZnO-PVP composite nanofibers was considerably affected by two parameters: the concentrations of zinc acetate and PVP in the precursor solution. The concentration of zinc acetate particularly influenced the diameter distribution of the ZnO-PVP nanofibers. The ZnO-PVP nanofibers could be subsequently converted into ZnO nanofibers of a pure wurtzite phase via calcination in air at 500°C for 2 h.     

Electrochemical properties of polypyrrole/sulfonted SEBS composite nanofibers prepared by electrospinning

The electrochemical behavior of electrospun polypyrrole (PPy)/sulfonated-poly(styrene-ethylene-butylenes-styrene) (S-SEBS) composite nanofibers was investigated, compared to PPy/poly(styrene-ethylene-butylenes-styrene) (SEBS) fibers prepared by a casting method. The electrospun PPy/S-SEBS (E-PSS) fibers were about 300 nm in average diameter, while PPy/SEBS composite (C-PS) prepared by a casting method showed the granular macroporous structure. The effect by both electrospinning and sulfonation results in higher electrochemical capacity due to the increase of doping level, high electrical conductivity, low interfacial resistance, and high reversibility by easy intercalation of Li ion. In addition, sulfonated SEBS induces higher elongation force to jet in the processing of electrospinning due to the role of dopant.     

Characterization on oxidative stabilization of polyacrylonitrile nanofibers prepared by electrospinning

Ultrafine polyacrylonitrile (PAN) fibers, as a precursor of carbon nanofibers, with diameters in the range of 220–760 nm were obtained by electrospinning of PAN solution using N,N-dimethyl formamide (DMF) as solvent. Morphology of the nanofibers for varying concentration and applied voltage was investigated by field emission scanning electron microscopy (FESEM). The thermal properties and structural changes during the oxidative stabilization process were primarily investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) and Raman spectroscopy. The nanofiber diameters increase as the applied voltage is increased and they also increase with an increase in the concentration of the polymer solution. It was also concluded that the electrospun fibers displayed a very sharp exothermic peak at 297.34 °C. A transition temperature observed by FT-IR and Raman was approximately 300 °C, which was closely consistent with the results of DSC and TGA studies. It was also found that oxidative stabilization in air was accompanied by a change in color of nanofibers webs.     

Boosted microwave absorption properties of CoFe2O4 with extraordinary 3D morphologies

Due to their strong magnetic dissipation and low cost, ferrites were one of the first generations of microwave absorbers. However, ferrites also have some drawbacks, such as a low natural resonance frequency (f_r), a lack of dielectric loss, and high density. In order to overcome these drawbacks and improve the microwave dissipation features of ferrites, we successfully prepared CoFe₂O₄ samples with flower-like and crochet ball-like morphologies (named as M1 and M2 samples, respectively). Structural and optical properties were studied by XRD, FTIR, and UV–Vis light absorption. The microwave performance of CoFe₂O₄ was significantly improved with the reflection loss (RL) of M2 of ?40 dB. Furthermore, M1 and M2 samples achieved an ultra-wide effective absorption bandwidth (EAB) of 13 and 12.5 GHz, respectively. It is worth noticing that the EAB of M1 was one of the largest EABs for CoFe₂O₄ that has been reported so far. The excellent microwave dissipation of M1 and M2 samples in the 2–18 GHz frequency range was due to the enhancement of ferrite f_r to the high-frequency range and the introduction of dielectric loss to achieve impedance matching. The flower-like and crochet ball-like morphologies with many pores of M1 and M2 also resolved the high-density issue of CoFe₂O₄. With the relatively good values of RL and EAB combined with low filler loading, thin thickness, and low density, M1 and M2 samples could be expected to be promising microwave absorbers for practical applications.     

Effect of Sr doped the YFeO3 rare earth ortho-ferrite on structure,magnetic properties,and microwave absorption performance

Given the remarkable performances of rare earth multiferroic ortho-ferrites with magnetic optical and dielectric properties, the Y_1-xSr_xFeO₃ (x = 0, 0.05, 0.1, 0.15) perovskite structure microwave absorbing ferrite materials was successfully synthesized by Sr²⁺ ions A-site doping based on sol-gel technology in this paper. The XRD of all samples was refined with FullProf software, which confirmed the formation of the orthogonal perovskite structure (SG: Pnma). The SEM and TEM results display the average particles size of the samples is distributed between 110 and 160 nm. The increase of Sr doping concentration leads to the increase of particles size, which may be related to the growth of preferred orientation and incomplete substitution. The XPS analysis shows that Fe³⁺ was accompanied by the presence of Fe²⁺ with the doping of Sr²⁺ ions and oxygen vacancies increased significantly. The samples change from weak ferromagnetic state to paramagnetic state with the increase of Sr content. The minimum reflection loss (RL) of the Y_0.95Sr_0.05FeO₃ samples at 12.2 GHz reached −30.87 dB with thickness of 2.2 mm, where its effective absorption bandwidth (EAB, RL ≤ −10 dB) reached 2.4 GHz (11.3–13.7 GHz). Moreover, the EAB of the Y_0.85Sr_0.15FeO₃ samples reached 2.64 GHz, and the corresponding range is 9.0–11.6 GHz (X-band).     

Tunable microwave absorption band via rational design of C@TiC nanospheres

To effectively tune material's microwave absorption band, it is necessary to build a special structure and composition. Titanium carbide nanoparticles decorated carbon nanospheres (C@TiC) were prepared by using carbon nanospheres as a initial structure-directing agent, and their absorption band was tuned by rational design of TiC content. With increasing dosage of tetrabutyl titanate (TBT), absorption band of C@TiC dispersed in paraffin (30 wt%) gradually shifted from Ku to S-band, realizing the adjustment of absorption band. For example, when the amount of TBT was 1.0, 1.5 and 2.0 mL, the minimum reflection loss (RL) of C@TiC was about ?49.8, ?52.4 and ?50.3 dB at 17.4, 13.1 and 6.6 GHz, respectively. Excellent performance was attributed to good impedance match and synergistic effect between carbon spheres and TiC nanoparticles. This in-situ phase transition induced nanoheterostructure would provide a way for microwave absorbers with tunable band.     

Tailoring microwave absorption bandwidth of SiCN based composite ceramic fibers by tuning the embedding ratio of Ni3Si

In order to expand the application prospects of SiCN ceramics in the field of microwave (MW) absorption materials, a series of Ni₃Si embedded SiCN ceramic fibers composites (NSF) were prepared by controlling Ni conversion rate through the electrospinning technique and polymer derivation, with the intention of improving the impedance matching degree, enhancing the conductivity and polarization, and further promoting the dielectric loss ability and MW absorption performance of ceramic materials. The microstructure, phase composition, conductivity, MW absorption properties and mechanism of the material were analyzed by a variety of characterization methods. The results show that NSF exhibited high dielectric loss efficiency and desirable effective absorption bandwidth (EAB) when the conversion rate of Ni was 0.5 wt%: The MW of the entire Ku band (12–18 GHz, 6 GHz) could be effectively absorbed by the sample with a thickness of 2.64 mm, and its EAB could cover 6–18 GHz by adjusting its thickness from 1 mm to 5 mm, so its performance is significantly superior to a number of similar SiCN based composite ceramic materials previously reported. To sum up, the NSF prepared in this work exhibits suitable impedance matching degree, good conductivity, obvious polarization effect, excellent dielectric loss ability, and gratifying EAB in MW, and it is expected to become a powerful candidate in the field of broadband MW absorption materials in the future.     

Field-responsive superparamagnetic composite nanofibers by electrospinning

Superparamagnetic polymeric nanofibers were produced via an electrospinning technique from colloidally-stable suspensions of magnetite nanoparticles in polyethylene oxide and polyvinyl alcohol solutions. The magnetite nanoparticles were aligned in columns parallel to the fiber axis direction within the fiber by the electrospinning process. The polymer/magnetite nanofibers exhibited superparamagnetic behavior at room temperature, and deflected in the presence of an applied magnetic field. The mechanical properties of the nanofibers were maintained or improved after incorporating the magnetite nanoparticles.     

Flexible SiC-CNTs hybrid fiber mats for tunable and broadband microwave absorption

Flexible microwave absorbers with high stability are in increasing demand for the applications under harsh conditions. SiC as a functional ceramic material has the feature of high environmental tolerance and adjustable electromagnetic (EM) absorbing properties, making them suitable to be applied for harsh environments. However, the electrical property of SiC requires to be further enhanced to obtain qualified EM absorbing performance. In this work, multiwall carbon nanotubes (CNTs) were introduced to SiC to enhance the electrical properties. Flexible two-dimensional (2D) CNTs loaded SiC fiber mats were prepared as EM absorbers via electrospinning and polymer-derived-ceramic (PDC) methods. The CNTs inside the fibers can form conductive networks and act as reinforcement to ensure high flexibility and enhance the microwave absorption properties of SiC mats. Thus, a reflection loss of ?61 dB and an effective absorption band (EAB) of 2.9 GHz were obtained. More importantly, the EM absorption can be adjusted by tuning the content of CNTs and the EAB can cover the entire X-band by adjusting the material thickness. The work provided a facile strategy to fabricated flexible 2D ceramic mats with high environmental stability and tunable electrical properties, which may shed light on the production of reliable EM absorber for broadband EM absorption applications.     

Hierarchical manganese sillenite Bi12MnO20 microparticles assembled by nanocubes with microwave absorption enhancement

Pure manganese sillenite (Bi₁₂MnO₂₀) microparticles with average sizes of 11.5–16.8 μm were prepared via an oxidation-precipitation method assisted by microwave-heating. Their microstructures are characterized by the shapes of single or multiple intersecting tetrahedrons and the massive nanocubes in ordered arrays on each tetrahedron surface. Such hierarchical Bi₁₂MnO₂₀ microparticles have not been reported so far, while their growth mechanisms may include the formation of active centers, the selective nucleation on active center surfaces and the growth of nanocubes along certain orientations. In electromagnetic determination, noticeable resonances both in permittivity and permeability were revealed beyond relaxations. A cone-like reflection loss peak was obtained with the minimum value of −39.1 dB and a broad effective microwave absorption bandwidth covering 12.7–14.6 GHz. Moreover, the reflection loss peak stagnates at the same frequency regardless of the absorbent thickness variations, suggesting a high performance in microwave attenuation.     

Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites

The hybrid network of Si₃N₄ whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si₃N₄ into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si₃N₄ (C_f-RBSN) composites have been discussed in detail. The C_f reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % C_f-RBSN composite exhibit a minimum reflection loss (RL_min) of ?36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g^?1 for 5.9 mm thickness, while 20 vol % C_f-RBSN composites resulted in RL_min of ?22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si₃N₄ whiskers and C_f.     

Multilayer Ti3C2Tx: From microwave absorption to electromagnetic interference shielding

Ti₃C₂T_x MXene has attracted extensive attention in the field of electromagnetic (EM) protection over recent years. Multilayer Ti₃C₂T_x (M-Ti₃C₂T_x), as an intermediate product of MXene ultra-thin structure, has potential advantages in the field of EM protection. Herein, the M-Ti₃C₂T_x was obtained by HCl/LiF etching Ti₃AlC₂. The microwave absorption (MA) and electromagnetic interference (EMI) shielding performance of Ti₃AlC₂ and M-Ti₃C₂T_x were compared. The mechanism research of MA and EMI shielding indicates that the construction of local conductive network plays a leading role in the EM wave attenuation. The sample with 30% M-Ti₃C₂T_x display RL_min of ?50.26 dB, and corresponding bandwidth of 4.64 GHz at the thickness of 1.7 mm. Especially, the metastructure based on the EM parameters of M-Ti₃C₂T_x/wax exhibits ultra-wide bandwidth (15.54 GHz). Our research will provide a basis for the design of MXene-based EM protection performance.     

Conductive polypyrrole nanofibers via electrospinning: Electrical and morphological properties

Conductive polypyrrole nanofibers with diameters in the range of about 70-300 nm were obtained using electrospinning processes. The conductive nanofibers had well-defined morphology and physical stability. Two methods were employed. Electrospun nanofibers were prepared from a solution mixture of polypyrrole (PPy), and poly(ethylene oxide) (PEO) acted as a carrier in order to improve PPy processability. Both the electrical conductivity and the average diameter of PPy nanofibers can be controlled with the ratio of PPy/PEO content. In addition, pure (without carrier) polypyrrole nanofibers were also able to be formed by electrospinning organic solvent soluble polypyrrole, [(PPy3)⁺ (DEHS)⁻]_x, prepared using the functional doping agent di(2-ethylhexyl) sulfosuccinate sodium salt (NaDEHS) [Jang KS, Lee H, Moon B. Synth Met 2004;143:289-94. [24]]. Electrospun blends of sulfonic acid (SO₃H)-bearing water soluble polypyrrole, [PPy(SO₃H)-DEHS], with PEO acting as a carrier, are also reported. The factors that facilitate the formation of electrical conduction paths through the electrospun nanofiber segments are discussed.     

Preparation of non-woven nanofibers of Bombyx mori silk, Samia cynthia ricini silk and recombinant hybrid silk with electrospinning method

Electrospinning is a good method to obtain nanoscale fibers from polymer solutions. In this paper, we successfully prepared non-woven nanofibers of Bombyx mori and Samia cynthia ricini silk fibroins, and of the recombinant hybrid fiber involving the crystalline domain of B. mori silk and non-crystalline domain of S. c. ricini silk from hexafluoroacetone (HFA) solution using electrospinning method. ¹³C cross polarization/magic angle spinning NMR spectroscopy was used to monitor the structural change of silk fibroins together with the detection of the residual HFA during the process of the fiber formation. Scanning electronic microscope was used to determine the diameters and their distributions of the fibers.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

Upward needleless electrospinning of multiple nanofibers

A new approach to electrospinning of polymer nanofibers is proposed. A two-layer system, with the lower layer being a ferromagnetic suspension and the upper layer a polymer solution, is subject to a normal magnetic field provided by a permanent magnet or a coil. As a result, steady vertical spikes of magnetic suspension perturbed the interlayer interface, as well as the free surface of the uppermost polymer layer. When a normal electric field is applied in addition to the system, the perturbations of the free surface become sites of jetting directed upward. Multiple electrified jets undergo strong stretching by the electric field and bending instability, solvent evaporates and solidified nanofibers deposit on the upper counter-electrode, as in an ordinary electrospinning process. However, the production rate is shown to be higher.     

Structure,magnetic properties and microwave absorption properties of NdFe1-xNixO3

In this paper, a high-purity NdFe_1-xNi_xO₃ perovskite-type material was prepared by a simple sol method. At the same time, adjust the substitution content of nickel to achieve the purpose of adjusting the dielectric properties and magnetic properties. According to the respective instruments, as Ni is substituted into the NdFeO₃, the crystal microstructure will change to a certain extent, and there is a certain causal relationship between the magnetic properties and the bonding. Therefore, by adding a certain amount of nickel, the dielectric properties and magnetic properties can be adjusted to a certain balance point. NdFe_1-xNi_xO₃ material has excellent microwave absorption performance. When x = 0.2, the minimum reflection loss value is ?49.32, and the corresponding impedance matching value is 1, and the effective bandwidth is 2.2 GHz when the thickness is 5.0 mm. The material that adjusts the perovskite structure by Ni element is beneficial to make the microwave absorption peak move from high frequency to low frequency, which has a wider application range and is closer to civil, commercial, military and aerospace.