Strong Microwave Absorption Research Articles

Composites with Koch fractal activated carbon fiber felt screens for strong microwave absorption

Abstract This work presents a strong microwave absorber with Koch fractal activated carbon fiber (ACF) felt screens. The effects of arrangements (spacing d) and structure parameters (iteration number n, initial side-length L) of Koch fractal units on the microwave absorption (MA) properties were investigated. It was observed that the optimal spacing d was obtained at 20 mm, and the reflection loss (RL) decreased in the low frequency-range from 6 to 9 GHz, but increased in the high-frequency range from 16 to 18 GHz apparently with increasing n and L. The strongest MA could reach – 62.8 dB. The relationship between ACF area ratio and RL was described.

Microwave-Induced Interfacial Nanobubbles

A new method for generating nanobubbles via microwave irradiation was verified and quantified. AFM measurement showed that nanobubbles with diameters ranging from 200 to 600 nm were generated at a water-HOPG surface by applying microwave radiation to aqueous solutions with 9.0-30.0 mg/L dissolved oxygen. Graphite displays strong microwave absorption and transmits high thermal energy to the surface. Because of the high dielectric constant (20 °C, 80 F/m) and dielectric loss factor, water molecules have a strong ability to absorb microwave radiation. The thermal and nonthermal effects of microwave radiation made contributions to decreasing the gas solubility, thus facilitating nanobubble nucleation. The yield of nanobubbles increased about 10-fold when the irradiation time increased from 60 to 120 s at 200 W of microwave radiation. The nanobubble density increased from 0.8 to 15 μm-2 by improving the working power from 200 to 600 W. An apparent improvement in nanobubbles yield was obtained between 300 and 400 W, and the resulting temperature was 34-52 °C. When the initial dissolved oxygen increased from 11.3 to 30.0 mg/L, the density of nanobubbles increased from 1.2 to 13 μm-2. The generation of nanobubbles could be well controlled by adjusting the gas concentration, microwave power, or irradiation time. The method may be valuable in preparing surface nanobubbles quickly and conveniently for various applications, such as catalysis, hypoxia/anoxia remediation, and templates for preparing nanoscale materials.

The origin of the strong microwave absorption in black TiO2

In this study, the mechanism of the strong microwave absorption in black TiO2 nanoparticles has been investigated both experimentally and theoretically. In experiment, the amorphous TiO2 nanoparticles/paraffin wax composites show the reflection loss (RL) of −4.0 dB, which is much smaller compared with the RL of −49.0 dB in those core/shell structure ones. Theoretically, the calculation illustrates that the accumulated charge of 1013 cm−3 at the core/shell interface results in the plasmon resonance with the incident microwave frequency at 9.3 GHz and 27.0 GHz. The microwave absorption enhancement of the black TiO2 nanoparticles is proposed to originate from the synergy mechanism between their crystalline-cores and amorphous-shells, rather than the defects and impurities in amorphous phase.

Nanoscale polygonal carbon: a unique low-loading filler for effective microwave absorption

In this work, silica template was utilized into the phenol resin to fabricate a novel nanoscale polygonal carbon (NPC) toward effective microwave absorption fillers. The as-prepared NPC with polygonal features presents substantial improvement in the complex permittivity of the wax-based composites. The porous structure coupled with electrical conduction and polygonal characteristic enables the NPC to form an effective electrical loss network via the formation of unique 3D pathways. Therefore, the composite embedded with NPC delivers strong microwave absorption up to −38 dB at a low filler loading of 3 wt%, with a much broadened effective absorption bandwidth in the range of 7–13 GHz. In addition, the fundamental mechanisms of the complex permittivity and broadened effective absorption bandwidth have been intensively discussed, indicating more advantages in comparison with some graphene- or carbon nanotube-based fillers. The results suggest that the exclusive performance induced by the NPC promises great potential in the achievement of advanced microwave absorption composites with low-loading fillers.

Microwave Absorbing Properties of MnAlFe Magnetic Powder

MnAlFe alloy was prepared by the combined use of smelting, high-energy ball milling and tempering heat treatment process. The phase composition and microwave absorbing properties of MnAlFe alloy were investigated by X-ray diffractometer(XRD) and vector network analyzer. The results show that the powders consist of Al8Mn5 single phase with different Fe content; the dielectric loss and the magnetic loss increases with the addition of Fe, the resonant frequency and absorption peak of ε″ and μ″ moves towardslower frequency region simultaneously. The (Al8Mn5)99Fe exhibits preferably comprehensive microwave absorbing properties in the range of 6-18 GHz. The value of the minimum reflectivity and absorption peak frequency with the coating thickness of 1.5 mm, are -23.1 dB, 15.04 GHz respectively. Owing to the wide applications of electromagnetic waves in the range of 1–4 GHz for mobile phone, local area network, radar systems and so on over the past few decades, the serious problems of the electromagnetic interference and electromagnetic compatibility have become increasingly prominent in recent years. Microwave absorbing materials, which possess special performance for absorbing electromagnetic wave, have been applied in both civil and military fields and play an important role in camouflage technology. In military field, stealth technology to reduce and eliminate the electromagnetic radiation and electromagnetic interference resulting from the broad applications of various kinds of electronic and microwave instruments, stealth technology plays an important strategic role in the wars in future[1-2]. The microwave absorbing materials can be used to reduce radar cross section for military use. For this reason, the microwave absorbing material is vitally important for both civil and military fields[3-7]. In view of this utilization, excellent microwave absorbing materials should exhibit strong microwave absorption properties over a wide frequency range and need to be thin and light in weight. It has an important meaning to the application of magnetic microwave absorbing materials. MnAl alloy can be used as microwave absorbent. It has properties such as light in weight, good thermal stability and corrosion resistance[8-11]. But as permanent magnets, MnAl has shortcomings such as their difficulties of fabrication and high costs that prevent them from being used as permanent magnets. A great value would be brought to either theory or practice through proper treatments as further improvement in properties. In this paper, we will discuss the effects of changing relative content of Fe on absorption properties of Al8Mn5 alloy.

Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules**Project supported by the Science and Technology Program of Guangdong Province, China (Grant Nos. 2014B010106005, 2013B051000077, and 2015A050502047) and the Science and Technology Program of Guangzhou City, China (Grant No. 201508030018).

With the combination of the dielectric loss of the carbon layer with the magnetic loss of the ferromagnetic metal core, carbon-coated nickel (Ni(C)) nanoparticles are expected to be the promising microwave absorbers. Microwave electromagnetic parameters and reflection loss in a frequency range of 2 GHz–18 GHz for paraffin-Ni(C) composites are investigated. The values of relative complex permittivity and permeability, the dielectric and magnetic loss tangent of paraffin-Ni(C) composites are measured, respectively, when the weight ratios of Ni(C) nanoparticles are equal to 10 wt%, 40 wt%, 50 wt%, 70 wt%, and 80 wt% in paraffin-Ni(C) composites. The results reveal that Ni(C) nanoparticles exhibit a peak of magnetic loss at about 13 GHz, suggesting that magnetic loss and a natural resonance could be found at that frequency. Based on the measured complex permittivity and permeability, the reflection losses of paraffin-Ni(C) composites with different weight ratios of Ni(C) nanoparticles and coating thickness values are simulated according to the transmission line theory. An excellent microwave absorption is obtained. To be proved by the experimental results, the reflection loss of composite with a coating thickness of 2 mm is measured by the Arch method. The results indicate that the maximum reflection loss reaches −26.73 dB at 12.7 GHz, and below −10 dB, the bandwidth is about 4 GHz. The fact that the measured absorption position is consistent with the calculated results suggests that a good electromagnetic match and a strong microwave absorption can be established in Ni(C) nanoparticles. The excellent Ni(C) microwave absorber is prepared by choosing an optimum layer number and the weight ratio of Ni(C) nanoparticles in paraffin-Ni(C) composites.

Lanthanum and Neodymium Doped Barium Ferrite-TiO₂/MCNTs/poly(3-methyl thiophene) Composites with Nest Structures: Preparation, Characterization and Electromagnetic Microwave Absorption Properties.

We report herein the synthesis of a novel nest structured electromagnetic composite through in-situ chemical polymerization of 3-methyl thiophene (3MT) in the presence of the BaFe11.92(LaNd)0.04O19-TiO2 (BFTO) nanoparticles and MCNTs. As an absorbing material, the BFTO/MCNTs/P3MT/wax composites were prepared at various loadings of BFTO/MCNTs/P3MT (0.2:0.10:1.0 ~ 0.2:0.30:1.0), and they exhibited strong microwave absorption properties in the range of 1.0–18 GHz. When the loading of BFTO/MCNTs/P3MT is 0.2:0.30:1.0, the composite has a strongest absorbing peak at 11.04 GHz, and achieves a maximum absorbing value of −21.56 dB. The absorbing peak position moves to higher frequencies with the increase of MCNTs content. The mechanism for microwave absorption of these composites has been explained in detail.

CoNi@SiO2 @TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption.

The synthesis of CoNi@SiO2 @TiO2 core-shell and CoNi@Air@TiO2 yolk-shell microspheres is reported for the first time. Owing to the magnetic-dielectric synergistic effect, the obtained CoNi@SiO2 @TiO2 microspheres exhibit outstanding microwave absorption performance with a maximum reflection loss of -58.2 dB and wide bandwidth of 8.1 GHz (8.0-16.1 GHz, < -10 dB).

Tailor-Made Distribution of Nanoparticles in Blend Structure toward Outstanding Electromagnetic Interference Shielding.

Engineering blend structure with tailor-made distribution of nanoparticles is the prime requisite to obtain materials with extraordinary properties. Herein, a unique strategy of distributing nanoparticles in different phases of a blend structure has resulted in >99% blocking of incoming electromagnetic (EM) radiation. This is accomplished by designing a ternary polymer blend structure using polycarbonate (PC), poly(vinylidene fluoride) (PVDF), and poly(methyl methacrylate) (PMMA) to simultaneously improve the structural, electrical, and electromagnetic interference shielding (EMI). The blend structure was made conducting by preferentially localizing the multi-wall nanotubes (MWNTs) in the PVDF phase. By taking advantage of "π-π stacking" MWNTs was noncovalently modified with an imidazolium based ionic liquid (IL). Interestingly, the enhanced dispersion of IL-MWNTs in PVDF improved the electrical conductivity of the blends significantly. While one key requisite to attenuate EM radiation (i.e., electrical conductivity) was achieved using MWNTs, the magnetic properties of the blend structure was tuned by introducing barium ferrite (BaFe) nanoparticles, which can interact with the incoming EM radiation. By suitably modifying the surface of BaFe nanoparticles, we can tailor their localization under the macroscopic processing condition. The precise localization of BaFe nanoparticles in the PC phase, due to nucleophilic substitution reaction, and the MWNTs in the PVDF phase not only improved the conductivity but also facilitated in absorption of the incoming microwave radiation due to synergetic effect from MWNT and BaFe. The shielding effectiveness (SE) was measured in X and Ku band, and an enhanced SE of -37 dB was noted at 18 GHz frequency. PMMA, which acted as an interfacial modifier in PC/PVDF blends further, resulting in a significant enhancement in the mechanical properties besides retaining high SE. This study opens a new avenue in designing mechanically strong microwave absorbers with a suitable combination of materials.

Influences of doping Cr/Fe/Ta on the performance of Ni/CeO2 catalyst under microwave irradiation in dry reforming of CH4

The structure of Ni/CeO2 catalyst with doping of Cr, Fe and Ta was investigated with XRD, N2 physisorption, XPS and HRTEM and the catalytic activity of the catalysts under microwave irradiation in dry reforming of methane was tested in a microwave reactor. The results show that the introduction of Cr and Ta to Ni/CeO2 can enhance the interaction between Ni and the support/promoter and inhibit the enlargement of NiO particles during the synthesis. The CH4 conversions in dry reforming on the catalysts follow the order: Ni/CeO2<2Fe–Ni<2Ta–Ni<2Cr–Ni. The superior performance of 2Ta–Ni and 2Cr–Ni may be attributed to the locally-heated Ni particles caused by the strong microwave absorption of the in-situ grown graphene attached on them under microwave irradiation.

Preparation of reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O4 nanocomposites and their excellent microwave absorption properties

A novel reduced graphene oxide/Ni0.4Zn0.4Co0.2Fe2O4 nanocomposite was firstly prepared by a facile and efficient mechanical mixing method with good safety, high controllability and excellent quantifiability. The microstructure and morphology were examined by FTIR, XRD, TG, SEM and EDS. The measured electromagnetic parameters indicated that the rGO/NZCF-15 composite exhibited excellent microwave absorption properties and extraordinary broad absorption bandwidths due to both dielectric loss and magnetic loss. The minimum reflection loss of the rGO/NZCF-15 composite was −38.7dB at 15.2GHz for a layer of only 1.9mm thickness, and the bandwidth corresponding to RL less than −10dB can reach 6.2GHz (11.8–18.0GHz). This novel composite with such strong microwave absorption could be used as a new candidate for selective frequency shielding and lightweight EM wave absorbing material.

Microwave absorption by hydrogen plasma in carbon nanotubes films with layered dielectric structure

A layered structure model is proposed for microwave dielectric properties of nonhomogeneous hydrogen plasma in carbon nanotubes (CNTs) film. Using the transfer matrix method for solving electromagnetic wave propagation equation, the microwave attenuation of the film is calculated in the range of 0–30 GHz under different conditions. It is found theoretically that with the increase of hydrogen plasma nonhomogeneity, the frequency bandwidth of strong microwave absorption by the film increases markedly. The application of a moderate static magnetic field can effectively improve microwave attenuation properties of hydrogen plasma in CNTs. The numerical results are in good agreement with the available experimental data.

Strong microwave absorption of hydrogenated wide bandgap semiconductor nanoparticles.

Electromagnetic interactions in the microelectronvolt (μeV) or microwave region have numerous important applications in both civil and military fields, such as electronic communications, signal protection, and antireflective coatings on airplanes against microwave detection. Traditionally, nonmagnetic wide-bandgap metal oxide semiconductors lack these μeV electronic transitions and applications. Here, we demonstrate that these metal oxides can be fabricated as good microwave absorbers using a 2D electron gas plasma resonance at the disorder/order interface generated by a hydrogenation process. Using ZnO and TiO2 nanoparticles as examples, we show that large absorption with reflection loss values as large as -49.0 dB (99.99999%) is obtained in the microwave region. The frequency of absorption can be tuned with the particle size and hydrogenation condition. These results may pave the way for new applications for wide bandgap semiconductors, especially in the μeV regime.

CoxNi100-x nanoparticles encapsulated by curved graphite layers: controlled in situ metal-catalytic preparation and broadband microwave absorption.

We report a one-step approach for preparing dispersive CoxNi100-x nanoparticles completely encapsulated by curved graphite layers. The nanoparticles were prepared by evaporating Co-Ni alloys and the shell of graphite layers was formed by in situ metal-catalytic growth on the surface of nanoparticles whose layer number was controlled by tuning the Co content of the alloys. By modulating the composition of the magnetic core and the layer number of the shell, the magnetic and dielectric properties of these core/shell structures are simultaneously optimized and their permeability and permittivity were improved to obtain the enhanced electromagnetic match. As a result, the bandwidth of reflection loss (RL) exceeding -20 dB (99% absorption) of the nanocapsules is 9.6 GHz for S1, 12.8 GHz for S2, 13.5 GHz for S3 and 14.2 GHz for S4. The optimal RL value reaches -53 dB at 13.2 GHz for an absorber thickness of 2.55 mm. An optimized impedance match by controlling the growth of the core and shell is responsible for this extraordinary microwave absorption.

Partially crystallized TiO2 for microwave absorption

Amorphous TiO2 synthesized via Ti anodization is partially crystallized and displays strong microwave absorption ability. Interfacial electric field forms at the boundary of crystalline–amorphous phases due to the uneven oxygen-vacancy distributions in the two phases, amplifying the microwave absorption.

Synthesis, magnetic resonance and microwave absorption properties of cobalt nanospheres

By using a simple and low-cost liquid reduction method, we have synthesized cobalt nanospheres on a large scale. The materials were characterized, and the result showed that as-prepared products were cobalt nanospheres, assembled by nanosheets in parallel, with a size of around 100 nm. The electromagnetic behaviors of the cobalt nanospheres, including permittivity () and permeability (), were also investigated as a function of frequency in the microwave frequency range of 2–18 GHz. The permittivity presented multiple dielectric resonance peaks whilst the permeability displayed dual obvious magnetic resonance peaks, manifestly different from other cobalt particles in comparison. The calculated reflection loss (RL) indicated there were two strong microwave absorption peaks over the microwave frequency range of 2–18 GHz, which was in accord with the magnetic resonance peaks. The result revealed that the magnetic loss contributed even more than dielectric loss to the microwave absorption for the cobalt nanospheres.

Effects of carbon nanotube dispersion methods on the radar absorbing properties of MWCNT/epoxy nanocomposites

Radar absorbing materials (RAMs) for practical applications are expected not only to have strong microwave absorption and a wide absorption bandwidth, but also to be lightweight, to have a fine thickness and acceptable structural performance, as well as being cost-effective. Although the dispersion of carbon-nanofillers in polymer matrices is a key factor determining the microwave absorbing properties of the composites, there have few studies on these effects. To our knowledge, to date, the realization of pristine multi-walled carbon nanotube (MWCNT)/polymer composites as RAMs in industrial production has been restricted, due to high CNT contents or large composite thicknesses. Thus, in this work, two MWCNT dispersion processing methods, a solution process with surfactant-aid and a ball-milling dispersion, were investigated to fabricate pristine MWCNT/epoxy nanocomposites. The effects of the different dispersion processes, CNT loading, and composite thickness on CNT dispersion in the matrix, were observed by TEM, and the electrical conductivity and X-band absorbing performance of the composites were assessed. The use of an ionic surfactant to aid the dispersion of CNTs in solution resulted in the best RAMs, with a good compromise among effective X-band absorption, small composite thickness, and very low CNT content. The ball-milling method also resulted in materials with a low CNT content and microwave absorbing performance acceptable for industrial applications. Moreover, it offers a very simple and efficient route suitable for low-cost, mass production of RAMs. The results showed that by facile approaches of dispersing pristine commercial MWCNTs in an epoxy resin matrix, composites of only 2–3 mm thickness and as little as 0.25–0.5 wt% CNT loading could be obtained, with a relatively wide X-band operating bandwidth and maximum absorptions exceeding 18–25 dB.

Silver nanoparticles supported on CeO2-SBA-15 by microwave irradiation possess metal-support interactions and enhanced catalytic activity.

Metal-support interactions (MSIs) and particle size play important roles in catalytic reactions. For the first time, silver nanoparticles supported on CeO2-SBA-15 supports are reported that possess tunable particle size and MSIs, as prepared by microwave (MW) irradiation, owing to strong charge polarization of CeO2 clusters (i.e., MW absorption). Characterizations, including TEM, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure, were carried out to disclose the influence of CeO2 contents on the Ag particle size, MSI effect between Ag nanoparticles and CeO2-SBA-15 supports, and the strong MW absorption of CeO2 clusters that contribute to the MSIs during Ag deposition. The Ag particle sizes were controllably tuned from 1.9 to 3.9 nm by changing the loading amounts of CeO2 from 0.5 to 2.0 wt%. The Ag nanoparticle size was predominantly responsible for the high turnover frequency (TOF) of 0.41 min(-1) in ammonia borane dehydrogenation, whereas both particle size and MSIs contributed to the high TOF of 555 min(-1) in 4-nitrophenol reduction for Ag/0.5CeO2-SBA-15, which were twice as large as those of Ag/SBA-15 without CeO2 and Ag/CeO2-SBA-15 prepared by conventional oil-bath heating.

SERS active Ag encapsulated Fe@SiO2 nanorods in electromagnetic wave absorption and crystal violet detection

The present work is focused on the preparation of Fe nanorods by the chemical reduction of FeCl3 (aq) using NaBH4 in the presence of glycerol as template followed by annealing of the product at 500°C in the presence of H2 gas flow. Subsequently, its surface has been modified by silica followed by silver nanoparticles to form silica coated Fe (Fe@SiO2) and Ag encapsulated Fe@SiO2 nanostructure employing the Stöber method and silver mirror reaction respectively. XRD pattern of the products confirmed the formation of bcc phase of iron and fcc phase of silver, though silica remained amorphous. FESEM images established the growth of iron nanorods from the annealed product and also formation of silica and silver coating on its surface. The appearance of the characteristics bands in FTIR confirmed the presence of SiO2 on the Fe surface. Magnetic measurements at room temperature indicated the ferromagnetic behavior of as prepared iron nanorods, Fe@SiO2 and silver encapsulated Fe@SiO2 nanostructures. All the samples exhibited strong microwave absorption property in the high frequency range (10GHz), though it is superior for Ag encapsulated Fe@SiO2 (−14.7dB) compared with Fe@SiO2 (−9.7dB) nanostructures of the same thickness. The synthesized Ag encapsulated Fe@SiO2 nanostructure also exhibited the SERS phenomena, which is useful in the detection of the carcinogenic dye crystal violet (CV) upto the concentration of 10−10M. All these findings clearly demonstrate that the Ag encapsulated Fe@SiO2 nanostructure could efficiently be used in the environmental remediation.

Electromagnetic and microwave absorbing properties of RGO@hematite core–shell nanostructure/PVDF composites

Abstract RGO@hematite nanohybrids were fabricated by in situ growth method under mild wet-chemical conditions. A series of characterization results indicate that the as-prepared hematite nanoparticles with relatively uniform sizes are embedded in RGO layers to form unique core–shell nanostructures. As an absorbing material, the RGO@hematite/polyvinylidene fluoride (PVDF) composites were prepared at various loadings of RGO@hematite (0–15 wt%), and they exhibited strong microwave absorption in the microwave range of 2.0–18.0 GHz. For example, RGO@hematite/PVDF composite with 5 wt% loading has a strong absorbing peak at 5.76 GHz and achieves a maximum absorbing value of −43.97 dB. The absorbing peak position moves to lower frequencies with the increase of RGO@hematite loading. The mechanism for the enhanced wave absorbing properties was explained in detail.