High-performance Microwave Absorption Research Articles

Porous carbon/Fe composites from waste fabric for high-efficiency electromagnetic wave absorption

• Waste fabrics are up-recycled into high-performance microwave absorbers. • Tannic acid-Fe 2+ coating catalyzes the polymer fiber into graphitized carbon fiber. • The recycling method is simple and environmentally friendly. • The minimum RL of −50.5 dB at 1.7 mm can be attained. Nowadays, a large amount of waste fabrics has brought huge environmental and resource problems, while the traditional recycling routes are downcycling with low efficiency and complex separation processes. Herein, we demonstrate a green and facile route to up-recycle waste fabrics (cotton-polyester blending or cotton fabrics) into a high-performance microwave absorber. In this design, an assistant coating was deposited onto the fiber surface of the waste fabric through the assembly of tannic acid-Fe 2+ (TA-Fe 2+ ), which effectively catalyzed the polymer fiber into highly graphitized porous carbon fiber at high temperatures. Meanwhile, the Fe nanoparticles from the reduction of Fe 2+ ions were evenly embedded on the surface of porous carbon fibers. Consequently, the resultant porous carbon/Fe composites with hierarchical microstructures displayed excellent microwave absorption performances. Typically, the microwave absorber from waste polyester-cotton blended fabric could reach a minimum reflection loss ( RL ) of -50.5 dB at a thin coating thickness of 1.7 mm, and the minimum RL from waste cotton fabric absorber reached -47.1 dB at a thickness of 2.5 mm. This work provides a new idea to convert waste fabrics into high value-added microwave absorbing materials in a simple and environmentally friendly way, which will help reuse resources and protect the environment.

Microstructure optimization of core@shell structured MSe2/FeSe2@MoSe2 (M = Co, Ni) flower-like multicomponent nanocomposites towards high-efficiency microwave absorption

• Core@shell structured M Se 2 /FeSe 2 @MoSe 2 ( M =Co, Ni) flower-like MCNCs were selectively synthesize through a simple route. • The obtained core@shell structured MCNCs with the different MoSe 2 contents and flower-like morphologies could be tuned. • The obtained M Se 2 /FeSe 2 @MoSe 2 ( M =Co, Ni) flower-like MCNCs displayed very outstanding comprehensive MAPs. • Our findings provided a new and effective pathway to develop the strong wideband MAs. In this work, we put forward a scheme to exquisitely design and selectively synthesize the core@shell structured M Se 2 /FeSe 2 @MoSe 2 ( M = Co, Ni) flower-like multicomponent nanocomposites (MCNCs) through a simple two-step hydrothermal reaction on the surfaces of M Fe 2 O 4 nanospheres with the certain amounts of Mo and Se sources. With increasing the amounts of Mo and Se sources, the obtained core@shell structured M Se 2 /FeSe 2 @MoSe 2 ( M = Co, Ni) MCNCs with the enhanced content of MoSe 2 and improved flower-like geometry morphology could be produced on a large scale. The obtained results revealed that the as-prepared samples displayed improved comprehensive microwave absorption properties (CMAPs) with the increased amounts of Mo and Se sources. The as-prepared CoSe 2 /FeSe 2 @MoSe 2 and NiSe 2 /FeSe 2 @MoSe 2 MCNCs with the well-defined flower-like morphology could simultaneously present the outstanding CMAPs in terms of strong absorption capability, wide absorption bandwidth, and thin matching thicknesses, which mainly originated from the conduction loss and flower-like geometry morphology. Therefore, the findings not only develop the very desirable candidates for high-performance microwave absorption materials but also pave a new way for optimizing the CMAPs through tailoring morphology engineering.

Tunable high-performance microwave absorption of cobalt nanoparticles wrapped in N-self-doped carbon nanofibers at ultralow filler loadings

Manufacturing lightweight, ultralow filler loading, broad bandwidth and highly efficient microwave absorbers is still a challenge. Herein, we have synthesized Co nanoparticles decorated N-self-doped carbon nanofibers (Co@NCNFs) composite by electrospinning followed by a two-step heat treatment and investigated its electromagnetic (EM) and microwave absorption performances under different filler contents (from 2.5 to 7.5 wt%) in paraffin wax matrix. It is found that the Co@NCNFs composite has strong ferromagnetism and moderate graphitization degree as well as abundant polar groups and defects, and Co nanoparticles encapsulated by graphitic carbon layers with sizes around 45–115 nm are uniformly dispersed in the NCNFs. With increasing the filler loading, the EM attenuation ability is gradually strengthened while the impedance matching level is just the opposite. The EM absorbing properties of the composites are more superior within 5.0 wt% Co@NCNFs loading owing to the synergy of improved impedance matching and satisfactory EM dissipation ability. Under an ultralow filler loading of 2.5 wt%, the optimal reflection loss (RL) can reach as strong as −49.3 dB at 17.83 GHz with a thin thickness of 1.61 mm and the maximum effective absorption bandwidth (EAB, RL<−10.0 dB) is as wide as 5.44 GHz (12.56–16.99 GHz) at 1.92 mm. When the filler loading is only 5.0 wt%, a minimum RL value of −52.3 dB is achieved at 6.36 GHz with the thickness of 3.03 mm and the maximum EAB is 4.64 GHz (13.36–18.00 GHz) at 1.42 mm. Compared to other Co/C composite absorbers, the as-prepared Co@NCNFs composite with well-designed constitute and structure demonstrates the better comprehensive absorption performance at a much lower filler loading. Therefore, the present Co@NCNFs may be more promising for practical applications as lightweight and high-performance EM wave absorbers especially in the medium and high frequency range.

Enhanced broadband microwave absorption of Fe/C core-shell nanofibers in X and Ku bands

Fe/C core-shell nanofibers were prepared via a coaxial electrospinning technique integrated with high-temperature carbonization, in which the Fe(NO3)3-containing PAN solution and Fe(NO3)3 constituted the shell and core structures, respectively. The core-shell structure was perfectly maintained after carbonization, and Fe nanoparticles were uniformly distributed within the carbon nanofibers. The effects of the microscopic morphology and crystallinity of the carbon components on the electromagnetic properties, matching characteristics, and electromagnetic wave absorption properties of the Fe/C nanofibers were investigated by adjusting the carbonization temperature. As-prepared Fe/C-800 °C core-shell nanofibers mixed with 10 wt% of paraffin showed a minimal reflection loss (RLmin) of −20.6 dB under 11.52 GHz at 2.5 mm of thickness and lower than −20 dB under 2.3–5.0 mm of thickness. The efficient absorption bandwidth (EAB) was 6.24 GHz in the range of 11.76–18 GHz at 2.1 mm, covering the whole Ku band. The absorption bandwidth can cover the whole X and Ku bands by varying the sample thickness. The efficient absorption bandwidth was 3.84 GHz in the range of 8.08–11.92 GHz at 2.9 mm, covering the X band. The effective complementarities between the magnetic loss of Fe nanoparticles, the dielectric loss of PAN-based carbon, and the core-shell structure promoted impedance matching and multiple interface polarization, endowing the produced composites with prominent absorption characteristics of microwaves. Therefore, the prepared Fe/C core-shell nanofibers are an ideal lightweight and broadband electromagnetic wave (EMW)-absorbing material that exhibits high-performance microwave absorption.

Two-Dimensional Basalt/Ni Microflakes with Uniform and Compact Nanolayers for Optimized Microwave Absorption Performance.

It has been accepted that the uniform distribution of magnetic metal particles is beneficial to microwave absorption, while why the homogeneous magnetic particles on the dielectric substrate improve the electromagnetic loss is still unclear. Herein, metal Ni nanoparticles, two-dimensional (2D) basalt/scattered Ni, and basalt/uniform Ni microflakes are obtained through a pretreatment and electroless deposition process. In comparison to Ni nanoparticles and basalt/scattered Ni, the basalt/Ni microflakes with largely uniform and compact Ni nanolayers on basalt, breaking the percolation limit, are favorable for enhanced electromagnetic attenuation. The Ni nanolayers are convenient for construction of a microscale conductive net and migration of an electron. The 2D heterostructures constructed by basalt substrates and decorated Ni layers boost multiple scattering absorption and promote interfacial polarization. Meanwhile, exposed Ni does not inhibit magnetic resonance, enabling strong magnetic coupling. Consequently, the basalt/Ni microflakes with uniform Ni nanolayers demonstrate better microwave absorption with a minimum reflection loss of -30 dB and an effective absorption bandwidth of 3 GHz at 1 mm. This work shows that the uniform and compact magnetic metal nanolayers are effective in improving the dielectric loss and magnetic loss simultaneously to achieve the high-performance microwave absorption.

Rational design of carbon-rich silicon oxycarbide nanospheres for high-performance microwave absorbers

Polymer-derived-ceramics (PDCs) are considered as a promising candidate for excellent microwave absorption under harsh environments due to their controllable microstructure at the molecular level. Herein, a series of silicon oxycarbide (SiOC) nanospheres are successfully fabricated through a PDC method and then pyrolyzed under argon atmosphere at different temperatures and the evolutions of microstructure and their microwave absorption properties are investigated simultaneously. It is found that the microwave absorption properties mainly depend on carbon content. With a relative carbon content of 38.13%, the carbon-rich SiOC nanospheres-1 pyrolyzed at 1400 °C possess excellent microwave absorption properties with a minimum reflection loss (RL) value of −56.28 dB at 6.8 GHz with a thickness of 2.93 mm. Electromagnetic analysis demonstrates that proper impedance matching, intrinsic attenuation ability, and the multiple reflection are responsible for the excellent RL. This work will provide a novel approach to obtain the PDCs with excellent microwave absorption properties.

Metal-organic framework (MOF) derived porous NiCo2O4 and ZnCo2O4 spinels with high microwave absorption performance for environmentally friendly EMI shielding applications

Sheet-like NiCo2O4 and ZnCo2O4 spinels were prepared by metal-organic framework (MOF) as sacrificial template to derive into porous microwave absorber materials. The NiCo and ZnCo MOFs obtained by using 2-methylimidazole linker were decomposed to the sheet-like NiCo2O4 and ZnCo2O4 powders, respectively, with the similar specific surface area of 75 m2/g and the pore volume of 0.31 cm3/g. However, the NiCo2O4 and ZnCo2O4 absorbers had the distinct microwave absorption performance on account of their various physicochemical properties caused by the nature of Ni and Zn elements. From the practical viewpoint, the MOF-derived NiCo2O4 sample had the broader effective absorption bandwidth (>3 GHz) at smaller thicknesses (<3 mm) in the Ku and X bands, whereas a significant fraction of C2, C1, and S bands could be absorbed by the MOF-derived ZnCo2O4 sample.

Controllable synthesis of FeSe2/rGO porous composites towards an excellent electromagnetic wave absorption with broadened bandwidth

Due to the escalating demand for electronic dependability and defense security, there has been a surge in research into broadband and lightweight microwave absorbers. Porous composites that are lightweight and plentiful in interfaces have the potential to be high-performance absorbers due to their ability to attenuate waves in a balanced manner and match impedance. “Using a solvothermal technique we generated FeSe2/rGO composites with a porous topology. By varying the weight of rGO, the electromagnetic properties of FeSe2/rGO composites may be finely tuned. Impedance matching and attenuation capability are both improved as a direct result of the porous structure and the appropriate electromagnetic parameters. FeSe2/rGO composites benefit from the tunable composition, porous structure, and strong synergistic effect between FeSe2 and rGO sheets and display outstanding microwave absorption performance with an ultrabroad bandwidth approaching 5.2 GHz with a thin thickness of 1.6 mm which covers 75% of the studied frequency range. At the same thickness, a significant reflection loss of −43.7 dB is attained. This work not only enables the tuning of electromagnetic parameters but also expands the use of high-performance microwave absorption devices. Remarkable microwave absorption ability, of the porous composites FeSe2/rGO can be utilized as a high-performance microwave absorber.”

Ultralight N-doped platanus acerifolia biomass carbon microtubes/RGO composite aerogel with enhanced mechanical properties and high-performance microwave absorption

Biomass-derived carbon material, an ideal lightweight and sustainable electromagnetic wave (EMW) absorption material, has adjustable dielectric properties, which is one of its greatest advantages. However, how to comprehensively control its composite dielectric constant to achieve strong microwave absorption (MA), wide effective absorption bandwidth (EAB), light mass, and thin thickness is still challenging. In this study, the N-doped biomass carbon microtubes/RGO composite aerogel (N-BCMT/RGO) with a three-dimensional multistage pore network structure was prepared by using the waste Platanus acerifolia tree fruit-derived biomass microtubes as the base, through heteroatom doping and composite conductive material. Among them, the doping of nitrogen atoms introduces numerous defects and promotes the formation of a heterogeneous interface, which plays an important role in regulating conduction and polarization loss. Secondly, the introduction of conductive material effectively promotes the hopping and migration of electrons, which further enhances the conduction loss. Moreover, the close binding of BCMT and RGO causes N-BCMT/RGO aerogel to exhibit enhanced mechanical properties. The special multistage hole structure allows N-BCMT/RGO to exhibit good impedance matching and ultra-low density (0.0121 g/cm3), which also exhibits an ultra-wide EAB of 8.36 GHz and a minimum reflection loss (RLmin) of −55.45 dB at an ultra-low filler loading of 2 wt%. This opens up an avenue for the realization of novel microwave absorbers with light weight and high performance, and the proposed comprehensive control strategy for the composite permittivity of biomass-carbon materials provides new ideas for activating the MA behavior of metal-free carbon-based absorbers.

CoNi-MoS2 composites with high microwave absorption performance

• The use of CoNi alloy to improve the microwave absorption properties of MoS2. • The CoNi-MoS2 has a reflection loss of −60.93 dB with the thickness of 4.93 mm. • This work provides a strategy for the synthesis of efficient microwave absorbers. Composites consisting of magnetic CoNi particles and flower MoS 2 (CoNi-MoS 2 ) have been prepared, by using a hydrothermal reaction method, for microwave absorption. When the CoNi-MoS 2 molar ratio is 1:1, optimal absorption performance is achieved, with reflection loss of −60.93 dB at 5.68 GHz, while the thickness is 4.93 mm. The outstanding microwave absorption performance is attributed to the synergy of the magnetic and dielectric losses, which greatly improves the impedance matching and increases the decay constants of the materials.

Fe@Fe3C Core-Shell Nanoparticles Embedded in Polyvinyl Alcohol-Derived Porous Carbon toward Light and High-Performance Microwave Absorption

High-performance microwave absorbents play a key role in information technology field. In this work, excellent microwave absorption property was achieved via embedded Fe@Fe3C core–shell nanoparticles in polyvinyl alcohol-derived porous carbon matrix. The results indicate that the microwave absorption performance will be greatly enhanced due to the interface between the nanoparticles and matrix, as well as the core–shell and porous structure, increase the energy consumption capacity via dipole polarization, interface loss and the multiple reflection/scattering. The maximum reflection loss (RLmax) reaches −56.88 dB at 11.3 GHz with the thickness of 2.11 mm, and the effective absorption bandwidth (EAB, RL<−10 dB) can get up to 5.2 GHz (from 12.9 to 18 GHz) with the thickness of 1.64 mm. Moreover, due to the low density of the porous carbon matrix, the total weight of the absorbents can be greatly induced, which provides an efficient way for lightweight and high-performance microwave absorption materials design.

Enhanced heat conductance and microwave absorption of 2D laminated Ti3C2Tx MXene microflakes via steering surface, defects, and interlayer spacing

In this paper, 2D laminated Ti3C2Tx MXene was successfully synthesized via etching the Al layer on Ti3AlC2 precursors in a one-step hydrothermal reaction. Controlling the etching time (t) handily adjusted the defects, surface groups, interlayer spacing, and surface oxide layer of laminated Ti3C2Tx MXene. With the prolonged etching time, the electrical conductivity and heat conductance changed in an inverted U-type. Moreover, the permittivity and low-frequency attenuation ability of Ti3C2Tx were enhanced, but the impedance matching decreased. The laminated Ti3C2Tx MXene harvest a maximal effective bandwidth of 6.32 GHz (RL ≤ −10 dB, 1.4 mm in thickness) with a maximal absorption of − 33.37 dB at t = 72 h and the optimal heat conductance of 2.746 W/(m·K) at t = 12 h. The outstanding comprehensive properties are owing to the synergistic effects of interface, defect/dipole, orientation, and crossing polarizations, caused by 2D laminated structure with appropriate layer spacing, surface functional groups, and defects. Consequent upon this fact, our strategies offer some significant insights for designing high-performance microwave absorption and heat conductance materials.

Halloysite nanotube-based nest-like composite microspheres with enhanced microwave absorption ability

Exploration of 3D nest-like materials has been demonstrated to be an effective strategy to afford high-performance microwave absorption ability. In this work, a facile approach was developed to construct nest-like microspheres via a microgel-assisted method, in which the natural product halloysite nanotube (HNT) was employed as the starting material. The nest-like micromorphology was demonstrated, and the particle size can be well controlled. The obtained HNTs-based nest-like microsphere-anchored Ag nanoparticles exhibit desirable microwave absorption ability. The optimal reflection loss is −23.34 dB at 7.03 GHz with a thickness of 3.5 mm and an effective absorption bandwidth of 2.54 GHz (5.89–8.43 GHz). The result provided a facile approach to construct 3D nest-like microspheres starting from 1D nanomaterials, and other potential applications should be desirable.

Rational Design of Yolk Core-Shell Structure MnO-Co@C Nanospheres for High-Performance Microwave Absorption

MnO-Co@C nanospheres were fabricated by in situ polymerizing and high-temperature carbonizing processes. This unique synthesis method does not require any template or reducing gas. The synthesized multicore-shell structure has a shell of about 500 nm and multiple nuclei of several tens of nanometers. Subsequently, extensive experiments were conducted to adjust the material composition of the nanospheres by adjusting the amount of resorcinol and formaldehyde. The results showed that the obtained material performed best when resorcinol and formaldehyde were added to 0.2 g MnCo2O4 at 0.3 g and 0.42 mL, respectively. The efficient absorption bandwidth (EAB) value reaches 3.3 GHz when the absorber thickness is 3 mm. The reflection loss (RL) is up to −23.8 dB when the frequency is at 8.6 GHz. The unique yolk core-shell structure gives the material a heterogeneous interface, and the enhanced interfacial polarization loss causes the enhanced dielectric loss. The carbon layer with microporosity also causes conduction loss and multiple reflections. The composite structure formed by metallic Co, MnO, and carbon has better impedance matching and improved microwave absorption capability.

Architecture inspired structure engineering toward carbon nanotube hybrid for microwave absorption promotion

SummaryConstructing sophisticated 3D structure has been shown to be fruitful in developing carbon nanotubes (CNTs) microwave absorbers (MAs). However, issues with the unclear electromagnetic (EM) responding synergy of CNTs toward substrate and the limited dissipation property caused by the large dense CNTs networks throughout the reported models still need to be resolved. Inspired by the creeper-window-room-structured architecture, an analogous conformal nanostructure of amorphous carbon/CNTs (N-AC/CNTs) hybrid is constructed through an in situ autocatalytic planting approach. By this model, not only the inheritance of frequency dependence characteristic but the co-inheritance of lossy behavior and impedance matching is demonstrated. Moreover, by virtue of the unique structure, a synergistic reinforcing dielectric loss from conductive loss and dielectric polarization was introduced. Therefore, N-AC/CNTs-750 shows impressive EM performance. This work hereby unveils the synergy of EM response from CNTs toward substrate, and provides a pioneering insight into developing architecture-inspired structure engineering to construct high-performance MAs.

Recent progress of polyaniline-based composites in the field of microwave absorption

Electromagnetic waves pollution as one of the major pollutions, has seriously endangered the human health and the normal use of electronic equipments. The development of high-performance microwave absorption (MA) materials containing wide absorption bandwidth, strong absorption capacity and light weight has become imminent. Due to excellent electrical conductivity, corrosion resistance, and unique doping mechanism, polyaniline (PANI) is expected to be a highly efficient microwave absorber. However, the lack of magnetic loss and multi-interface polarization of single-component PANI makes it difficult to meet practical applications. Therefore, compounding PANI with other lossy materials is used to improve its MA performance. This paper reviews the research background and the mechanism of PANI-based MA materials. Subsequently, the research progress of PANI/carbon composites, PANI/ferrite composites, PANI/natural composites and multicomponent PANI-based composites is introduced. The challenges and the prospects for the development of PANI-based composites in MA are presented, and the directions and the priorities for future research are pointed out.

Fe/Fe3O4/biomass carbon derived from agaric to achieve high-performance microwave absorption

Recently, biomass carbon materials as electromagnetic wave-absorbing materials have received widespread attention due to their easy availability, low cost, and environmental friendliness. However, for the vast majority of microwave absorbing materials that possess only dielectric losses, it is difficult for them to achieve satisfactory microwave absorbing performance. This paper proposes a new strategy to prepare a magnetic biomass composite material with tunable high-performance microwave absorption using agaric as the biomass carbon precursor and ferric nitrate as the magnetic precursor. When the carbonization temperature is 800 °C, the Fe/Fe3O4/biomass carbon (Fe/Fe3O4/BC) composite exhibits excellent microwave absorption performance in the X-band. At a thickness of 2.06 mm and a frequency of 9.63 GHz, its minimum reflection loss reaches −30.41 dB, and the effective absorption bandwidth is 2.45 GHz. In the carbonization process, ferric nitrate is transformed into magnetic Fe3O4 and Fe nanoparticles. The magnetic losses generated by Fe/Fe3O4 and the dielectric losses of biomass carbon, interfacial polarization, dipole polarization and multiple reflection synergies improve the impedance matching characteristics of the composite and improve the microwave absorption performance. Therefore, the prepared Fe/Fe3O4/BC composite material has exhibits great potential as a high-efficiency microwave absorber.

Tailorable microwave absorption properties of macro-porous core@shell structured SiC@Ti3SiC2 via molten salt shielded synthesis (MS3) method in air

High-performance microwave absorption and lightweight design have evolved into the foremost critical considerations in the practical application of modern microwave absorbing materials. In view of this, macro-porous core-shell structured SiC@Ti3SiC2 were prepared through a facile molten salt shielded synthesis method in air. Distinctly, the minimum reflection loss (RL) value of SiC@Ti3SiC2 could reach − 68.59 dB at 9.76 GHz with the bandwidth of RL< −10 dB of 5.6 GHz when the absorber thickness is 2.3 mm. The synergistic mechanism of the multiple scattering, conductive loss, and significant polarization effects of macro-porous (average 53 nm) core-shell structured SiC compounded with nano-layered Ti3SiC2 (∼60 nm) is reviewed for the considerable improvement of electromagnetic wave absorption properties. This work provides a practical reference for the design and synthesis of high-performance Ti3SiC2-based microwave absorbers.

Controllable Synthesis of Hierarchical Dendrite-Like Iron-Based Hybrids with Boosting Microwave Absorption Performance

Fabricating high-performance microwave absorption materials with excellent attenuation property covering low frequency ranges are urgently needed for applications in our daily life. Herein, we put forward the controllable synthesis of hierarchical dendrite-like iron-carbon hybrids (Fe-C based hybrids) through three steps, including the hydrothermal method, chemical vapor deposition strategy and hydrogen reduction using K3[Fe(CN)6] and C2H2 as Fe and C source, respectively. Due to the existence of interface polarization effect, natural resonance, and exchange resonance generated from Fe-C based hybrids, the microwave absorption property in low frequency bands of the as-fabricated Fe-C based hybrids are growing as the filler loading increases. The Fe-C based hybrids achieved the minimum reflection loss value (RLmin) at 2.95 GHz as low as -31.73 dB at 4.5 mm, and the effective absorption bandwidth can reach up to 9.19 GHz in the range of thickness from 1.5 to 5.0 mm, covering the entire L, S, C, and X-band frequency bands. The novel structure design provides a facile and meaningful strategy for fabricating two-dimensional hierarchical dendrite-like Fe-C based hybrids as high-performance microwave absorbers.

PVP-assisted hydrothermal synthesis of rod-like NiCo2O4 powders as high-performance microwave absorbers

Rod-like NiCo2O4 powders were synthesized by hydrothermal method in the presence of polyvinylpyrrolidone (PVP) surfactant for absorption the microwaves (1–18 GHz). The structural, microstructural, magnetic, and electromagnetic properties were analyzed by various characterization techniques. By adding the PVP surfactant, the wood-like texture morphology was transformed to the rod-like morphology composed of spherical nanoparticles (∼15 nm). The single-phase NiCo2O4 obtained by PVP agent had the higher crystallinity and oxygen vacancies in compared with the wood-like texture NiCo2O4 morphology. The specific surface area and pore volume decreased from 83 to 65 m2/g and from 0.13 to 0.05 cm3/g by using PVP agent, respectively. The rod-like NiCo2O4 powders exhibited the wider effective absorption band of ∼4.9 GHz in the Ku (12–18 GHz) band at a matching thickness of 1.4 mm in comparison with 3.9 GHz for the wood-like NiCo2O4 powders at the matching thickness of 1.2 mm. The better microwave absorption performance of the rod-like NiCo2O4 powders was attributed to their good impedance matching caused by their lower permittivity and higher permeability.