The nano-graphite sheet/alumina composites were prepared in situ by a facile impregnation-reduction process. The microstructure of the composites was analyzed by X-ray diffraction (XRD), and the final phase composition after reduction is Al2O3, metal Fe and graphite crystal. Scanning electron microscopy (SEM) images show that the particle size of Fe is about 20 nm, and the lamellae thickness of the graphite is about 30 nm. Then, the dielectric properties and conductive mechanism of the composites were investigated experimentally in the frequency range of 0.01–1.00 GHz by impedance analyzer. The results show that the real part of permittivity of composites increases with Fe3+ concentration, which is due to the increase in interfacial polarization between Fe and Al2O3 and the three-dimensional network of lamellar graphite formation. Therefore, tunable microtopography and electrical parameters of nano-graphite sheet/alumina composites can be realized by changing Fe3+ concentration.

Barium ferrite micro-/nanofibers with special morphology, nanowires with diameters of 100 nm, nanoribbons with diameters of 1 μm, and nanotubes with outer diameter of about 300 nm while inner diameter of 100 nm were successfully prepared via electrospinning using different solvents (dimethyl formamide (DMF), solutions of deionized water and ethyl alcohol, and solutions of deionized water and acetic acid, respectively). The barium ferrite micro-/nanofibers were characterized by scanning electron microscope (SEM), X-ray diffraction analysis (XRD), and vibration sample magnetometer (VSM). The results demonstrate that the pure BaFe12O19 ferrite phase is successfully formed. And the SEM results show excellent morphologies. The magnetic hysteresis loops demonstrate that their magnetic properties are quite different with different morphologies. The specific saturation magnetization is approximately the same (46.12–49.17 A·m2·kg−1), but the coercivity of the BaFe12O19 increases from wires (190.08 kA·m−1), ribbons (224.16 kA·m−1) to tubes (258.88 kA·m−1).

Cermets are widely applied as attenuating materials due to high electromagnetic loss and better mechanical properties. In this paper, multiphase cermets composed of iron, iron oxide and alumina were successfully prepared by a two-step in situ synthesis process, which includes pressureless sintering and a selective reduction in hydrogen atmosphere. The phase composition and microstructure of cermets were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. It is shown that nanosized cuboid Fe particles and octahedral Fe3O4 particles are distributed in alumina matrix. The permittivity and permeability of composites were tested with radio frequency impedance analyzer (0.01–1.00 GHz). The results show that permittivity presents obvious frequency dispersion. Furthermore, dielectric constants of multiphase cermets get enlarged due to the enhancement of interfacial polarization. On the other hand, there is a magnetic loss peak in permeability spectra, which indicates typical relaxation behavior. It is possible to achieve better electromagnetic attenuation property by adjusting process parameters.

The carbon/silicon nitride (C/Si3N4) composites consisting of amorphous carbon dispersed in porous Si3N4 matrix were herein prepared by facile impregnation-carbonization process at low temperature. The microstructures and electrical properties of C/Si3N4 composites were investigated in detail. A percolation phenomenon and an insulator–conductor transition appeared in the composites with the increase of carbon content. The formation of continuous conducting carbon network led to the plasma-like negative permittivity behavior in the composites above the percolation threshold (between 8.1 and 10.9 vol%), and the frequency dispersions of negative permittivity can be fitted well by Drude model. Carbon materials can be regarded as a good candidate for realizing negative permittivity, and the preparation of C/Si3N4 composites by the facile impregnation-carbonization approach offers the important possibility of tuning the negative permittivity.

•Epsilon-negative composite that has very high dielectric loss (tan δ~100@10 MHz).•Plasma oscillation of delocalized electrons is contribute to negative permittivity.•The negative permittivity behavior can be fitted well by Drude model.•Conductivity spectra below percolation threshold conformed to Jonscher's power law.Loss materials have great potential applications in electromagnetic wave absorption and attenuation. In this paper, we report an epsilon-negative composite that has very high dielectric loss (tan δ~100@10 MHz). This percolative polymer composite is filled with spherical micron-sized copper particles. Below the percolation threshold, conductivity spectra showed obvious frequency dispersion and conformed to the Jonscher's power law, indicating hopping conduction. When composites reached up to percolated state, the conduction mechanism changed to electron-like conduction. It is suggested that the electron transfer between copper particles leads to a very high loss of the composites.

Metamaterials are artificial periodic structures with negative permittivity and permeability. Several interesting properties can be obtained in metamaterials, such as negative index behavior, which can be used for building perfect lenses, cloaking, antennas, etc. As the metamaterial’s properties are determined by its structure, the key challenge is to reduce the fabrication cost of the periodic structure on the micrometer or nanometer scale for realistic applications. In this paper, we experimentally demonstrate a new one-step method for the fabrication of a large-area infrared metamaterial at extremely low cost. A metallic mesh is used as a shadow mask during the pulsed laser deposition (PLD) process to fabricate a FeNi/SiC/FeNi multilayer sandwich structure on Si substrate (cm2 level). The sample shows a strong absorption peak in the infrared frequency range, and the absorption intensity changes with the sample’s geometry.

Recently, the negative index materials (NIMs), also called double negative materials (DNMs) are receiving an increasing attention due to their many unique electromagnetic properties and attractive applications prospect. DNMs are now well established in “artificial” metamaterials, but it remains a challenge from the perspective of “natural” materials, especially in single phase materials. Herein, the La1−xSrxMnO3 (LSMO, x = 0.1, 0.2, 0.3, 0.4, 0.5) ceramics with different Sr mole ratio (x) were prepared by combining the sol–gel process with auto-combustion, followed by cold-pressing and sintering. The dielectric properties of LSMO were studied at frequency ranges from 104 Hz to 108 Hz. The negative permittivity was realized, and the permittivity and ac conductivity were drastically tuned by the Sr-doping effect. Experimental results and theoretical simulation results indicate that the negative permittivity is attributed to the Lorentz resonance (x = 0.1) and the plasma oscillation (x > 0.1), respectively. It is further found that the doping elements tune the permittivity by adjusting the concentration of the effective electrons, which is further supported by the frequency dependences of the conductivity (σ′ac–f).

Copper/yttrium iron garnet (Cu/Y3Fe5O12) cermets with tailored microstructures were prepared via a facile wet chemical process. The radio-frequency permeability and permittivity spectra of the cermets were investigated in detail. A percolation phenomenon appears with increasing copper contents, and an ultra-low percolation threshold (<5.47 vol%) is obtained. For the cermets below percolation threshold, the frequency dispersion of permittivity is featured with a poly-dispersive Debye type relaxation process. For the cermets above but still near percolation threshold, plasma-like negative permittivity featured with fano-like resonances was obtained. The Cu/Y3Fe5O12 cermet is a potential candidate for electromagnetic cloaking, microwave attenuation and telecommunications, etc.

Negative parameter materials (NPMs) with negative permittivity and/or negative permeability have attracted increasing attention in recent years. In this work, the tunable negative electromagnetic parameters of copper/yttrium iron garnet (Cu/YIG) composites, which were prepared by an in situ synthesis process, were investigated in a radio frequency regime. When they reached the percolated state, Fano-like resonances are observed and the permittivity changes from negative to positive. In addition, the combined contributions of the magnetic resonance of ferrimagnetic YIG particles and the diamagnetic response of the current loop bring about negative permeability in high frequency. Furthermore, the negative permittivity and permeability could be controllable by an external magnetic field. Hopefully, it is indicated that the in situ synthesis process offers a facile and versatile approach to fabricate NPMs.

Negative permittivity is one of the most important properties in the realization of double negative medium or negative index materials. In this paper, tunable negative permittivity in the radio frequency range has been obtained in composites with Fe78Si9B13 amorphous alloy dispersed in an epoxy matrix. The microstructure and dielectric properties of Fe78Si9B13/epoxy composites are investigated in detail. The results indicate that when the Fe78Si9B13 content is beyond the percolation threshold, the plasma oscillation of delocalized electrons in interconnected Fe78Si9B13 leads to negative permittivity. By controlling the effective concentration of free electrons, the negative permittivity of the Fe78Si9B13/epoxy composites could be easily adjusted. Additionally, the frequency dispersion behaviors of the conductivity conform to the Jonscher's power law below percolation threshold, demonstrating that the conductive mechanism is hopping conduction. The realization of tunable negative permittivity in Fe78Si9B13/epoxy composites gives a new and high-efficient way toward double negative materials.

Iron–alumina composites consisting of different loadings of iron particles dispersed in an alumina matrix were prepared via a facile impregnation–calcination process. The frequency dispersions of conductivity and permittivity were investigated in detail. An ultra low percolation threshold of 2.3 vol%, which is much lower than that of dense metal–ceramic composites, was obtained. Meanwhile, a significant enhancement of permittivity ε′ (from ∼7.5 to ∼800) was achieved when the iron content increases from 0 to 4.2 vol% at 10 MHz. The ultra low percolation threshold can be explained by the fact that the porous microstructure of the composites will facilitate the formation of a layer of two dimensional conductive networks on the pore wall of porous alumina. And the significant enhancement of permittivity should be attributed to the interfacial polarization phenomenon that takes place at the iron–alumina interfaces. This paper demonstrates that the loading of a conductive component into a porous matrix is an effective way to fabricate composites with simultaneously high permittivity and ultra-low percolation threshold. Hopefully, various porous metal–ceramic composites with tailored dielectric properties could be fabricated using the impregnation–calcination process.

For double negative materials (DNMs), these are usually observed in artificial metamaterials or some metal/ceramic composites. Here, an interesting negative permittivity behavior of single-phase perovskite La1−xSrxMnO3 (LSMO) is found. The LSMO bulks with an x value of 0.2 or 0.5 were prepared by a sol–gel/sintering process. From microstructure characterization, the bulks are nearly in single phase and the major phase belongs to the perovskite structure; meanwhile, the bulks are of high density. Furthermore, the experimental data of the plasma-like negative permittivity are fitted well by a lossy Drude model, suggesting a plasma frequency of 3.85 GHz (x = 0.5). The complex permeability of LSMO presents a negative susceptibility (μ′ < 1) at a frequency range of 0.2–1 GHz (x = 0.2) and 0.6–1 GHz (x = 0.5). These results have important implications for the realization of double negative properties in single-phase LSMO as a promising candidate for DNMs.

In this paper, the microstructure and dielectric properties of ion-doped La0.7Sr0.3MnO3 are investigated in detail. The polycrystalline ceramics of La0.7Sr0.3Mn1−yMyO3 (M = Fe, Ni, Cu; y = 0.3 or 0.5) are prepared by the sol–gel/sintering method. The symmetry of the crystal structure is improved and the dielectric properties obtained in the frequency range from 80 MHz to 1 GHz can be tuned by the doping of Fe, Ni, and Cu ions. With regard to the dielectric properties in lossy ceramics, the decrease of real permittivity ε′r for the doping of Fe and Ni will be effective for impedance-matching; the enhanced dielectric losses in the Ni and Cu doping samples will promote stronger absorption. Thus, the improved dielectric properties make ion-doped La0.7Sr0.3MnO3 promising candidates for lossy ceramics in microwave electronics. In addition, the experimental results of permittivity were checked by K–K relations, and the frequency dispersion behaviors of the conductivity within a certain frequency range accord with the Jonscher's power law in the form of σ′ac(f) ∝ (2πf)n, demonstrating that the conductive mechanism is hopping conduction.

In this paper, we report a new method to fabricate luminescent and macroporous Eu3+ doped Y2O3 (Y2O3:Eu3+)-coated silica monoliths. Macroporous silica monoliths were firstly prepared through freeze drying emulsions containing silica nanoparticles (silica NPs) and calcination. Then Y2O3:Eu3+ layers were loaded onto the macroporous silica monoliths by the wet impregnation method. The Y2O3:Eu3+-coated silica monoliths were characterized by field emission scanning electron microscopy, X-ray diffraction and mercury intrusion porosimetry. The silica NPs content, emulsion composition and dip times played important roles on morphologies of the Y2O3:Eu3+-coated silica monoliths. It was found that the silica monoliths still remained good macroporous structures after coating the Y2O3:Eu3+ layers. The loading amount of Y2O3:Eu3+ was also controlled by the dip times. Finally, luminescence properties of the Y2O3:Eu3+-coated silica monoliths were examined and they gave a strong emission peak of 612 nm attributed to the 5D0 → 7F2 transition of Eu3+. This method was simple, convenient and provided a novel route for fabrication of luminescent and macroporous silica monoliths.

Ag/Y3Fe5O12 random composites of silver particles randomly hosted in porous Y3Fe5O12 were prepared using a facile impregnation–calcination process. With the increase of silver content, the silver particles interconnected with each other near the percolation threshold, leading to the formation of electrical conductive silver networks. The plasma oscillation of delocalized electrons in interconnected silver particles leads to the negative permittivity. The negative permittivity behaviour was analyzed using the Drude model and equivalent circuit models. Meanwhile, the diamagnetic response of silver networks combined with the magnetic resonance of Y3Fe5O12 results in the negative permeability. That is to say, simultaneous negative permittivity and negative permeability were realized in the Ag/Y3Fe5O12 random composites. Hopefully, tunable double negative property could be realized by adjusting the microstructure and composition of the composites. External DC magnetic field could also be applied to control the electromagnetic properties of the composites.

We use a selective reduction reaction to fabricate a new kind of dual composite, which has a “composite-within-a-composite” structure. Based on the different reduction behavior of Fe2O3, an Fe-rich structure (Fe–Fe3O4–Al2O3) is formed in an Al-rich structure (FeAl2O4–Al2O3). The Fe-rich structure can bring down the concentration of free electrons to reduce the energy loss, without losing its metallic behaviour at the same time. Near the percolation threshold, a conductor–insulator transition appears and the dual composites show totally different electromagnetic responses, which can be well explained by effective medium theory. By controlling the parameters of the selective reduction process, we can get a tunable negative permittivity, which implies the ability to continuously change their electromagnetic properties.

FeWO4 (FWO) nanocrystals were prepared at 180 °C by a simple hydrothermal method, and carbon-coated FWO (FWO/C) was obtained at 550 °C using pyrrole as a carbon source. The FWO/C obtained from the product hydrothermally treated for 5 h exhibits reversible capacities of 771.6, 743.8, 670.6, 532.6, 342.2, and 184.0 mAh g–1 at the current densities of 100, 200, 400, 800, 1600, and 3200 mA g–1, respectively, whereas that from the product treated for 0.5 h achieves a reversible capacity of 205.9 mAh g–1 after cycling 200 times at a current density of 800 mA g–1. The excellent electrochemical performance of the FWO/C results from the combination of the nanocrystals with good electron transport performance and the nitrogen-doped carbon coating.Keywords: cyclic performance; hydrothermal treatment; impedance; N-doping; rate capability; tungstate;

Fe–Mn–O composite oxides with various Fe/Mn molar ratios were prepared by a simple coprecipitation method followed by calcining at 600 °C, and carbon-coated oxides were obtained by pyrolyzing pyrrole at 550 °C. The cycling and rate performance of the oxides as anode materials are greatly associated with the Fe/Mn molar ratio. The carbon-coated oxides with a molar ratio of 2:1 exhibit a stable reversible capacity of 651.8 mA h g–1 at a current density of 100 mA g–1 after 90 cycles, and the capacities of 567.7, 501.3, 390.7, and 203.8 mA h g–1 at varied densities of 200, 400, 800, and 1600 mA g–1, respectively. The electrochemical performance is superior to that of single Fe3O4 or MnO prepared under the same conditions. The enhanced performance could be ascribed to the smaller particle size of Fe–Mn–O than the individuals, the mutual segregation of heterogeneous oxides of Fe3O4 and MnO during delithiation, and heterogeneous elements of Fe and Mn during lithiation.Keywords: anode material; composite oxide; coprecipitation; cycling performance; electrochemical performance; rate capability;

The aim of this paper is to synthesize α-(Fe, Al)2O3 solid solutions from precursors prepared by the sol–gel method. The powders were characterized by X-ray diffraction, particle size analysis and TGA-DSC. The results indicated that the precursor prepared by sol–gel method will translate into α-(Fe, Al)2O3 solid solutions after calcined at 1300 °C for 3 h, with the average grain size of 14 μm. And Fe2O3 reduced the temperature of the γ-Al2O to α-Al2O3 phase transition from 1,300 to 850 °C.

The embedding of metal nanoparticles into an insulating ceramic matrix can provide encapsulation and prevent their oxidation and agglomeration. A nanoembedment powder with the Fe nanoparticles embedded into Al2O3 matrix is prepared by high-energy ball milling. Starting from the highly exothermic reactant mixture of magnetite and aluminum, Fe nanoparticles were in situ formed in Al2O3 matrix by mechanochemical reaction. It is found that a post-reaction milling significantly narrows the size distribution of Fe nanoparticles. The mechanism for the two-stage milling process was proposed. The microwave permeability of nanoembedments exhibited a multiresonance behavior, which was the evidence for monodispersed Fe nanoparticles. The Fe@Al2O3 nanoembedments are potential candidates as microwave absorber and left-handed materials.Graphical abstractA nanoembedment powder with the Fe nanoparticles embedded into Al2O3 matrix is prepared by high-energy ball milling. It is found that extensive milling after mechanochemical reaction significantly narrows the size distribution of Fe nanoparticles, which is evidenced by a multiresonance behavior in the microwave permeability spectra. And the mechanism for the two-stage milling process is proposed.Highlights► Fe nanoparticles were added to Al2O3 by high-energy ball milling. ► Post-reaction milling distinctly narrows the size distribution of Fe nanoparticles. ► Monodispersed Fe nanoparticles were evidenced by multiresonance behavior. ►The mechanism for the two-stage milling process is proposed.

The iron nanoparticles embedded in alumina powder (Fe@Al2O3 nanoembedments) were prepared by a mechanochemical process. Their potential for microwave absorber is evaluated. The insulating embedment provides encapsulation and prevents oxidation and agglomeration of metal nanoparticles. The hysteresis loop of such nanoembedments at room temperature exhibits a superparamagnetic behavior, while the microwave permeability spectra show a multiresonance behavior. The maximum reflection loss can reach −21.4 dB at a higher frequency of ∼13.3 GHz as the thickness is 1.4 mm. The coexistence of superparamagnetic loss and multiresonant loss is responsible for microwave absorption. The Fe@Al2O3 nanoembedments may become attractive candidates for a new type of microwave absorber.Highlights► We prepared Fe@Al2O3 nanocomposites by mechanochemical process. ► The nano−sized metal particle embedded uniformly in the Al2O3 matrix. ► The microwave properties were measured by Vector Network Analyzer. ► The maximum reflection loss can reach −21.4 dB at a higher frequency of ∼13.3 GHz.

Fe/Al2O3 composite consisting of iron particles dispersed in a porous alumina host has been prepared by wet impregnation and subsequent heat treatment in hydrogen. Thermogravimetric and differential thermal analysis was used to study the thermal behavior of Fe(NO3)3 loaded in porous alumina. H2-temperature programmed reduction was adopted to analyze the reduction behavior of α-Fe2O3 loaded into porous alumina. The morphology and particle size of the magnetic particles were evaluated by scanning electron microscope, while the phase identification and structural analysis of the samples were examined by X-ray diffraction technique. The magnetic properties of the nanocomposite were investigated by means of vibrating sample magnetometer and 57Fe Mössbauer spectrometry. The Mössbauer spectra indicated that all of the α-Fe2O3 changed into α-Fe during the reduction process; and, microscopic observations revealed that iron particles with an average diameter of ~200 nm were dispersed homogeneously on the pore walls of the porous alumina.

Mechanical alloying (MA) has been utilized to synthesize many equilibrium and/or nonequilibrium phases. The structural evolution of iron aluminides mechanically alloyed from a powder mixture of iron and aluminum has been studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It is found that, during mechanical milling under argon atmosphere, aluminum dissolves gradually into the bcc lattice of α-Fe, resulting in the formation of bcc-Fe3Al solid solutions, locally with B2-ordered structure in some region. TEM investigation indicates that NNAPBs are formed during MA. Nanocrystalline microstructure is achieved in powders after 25-h milling, with the grain size about 78 nm.

In situ process has been utilized to fabricate TiAl/Al2O3 composites, with thermodynamically compatible matrices and reinforcements being obtained, through displacement reaction between TiO2 and Al. However, the reaction involves intermediate steps between the starting and final materials, affecting the final phases and consequently the properties. In this paper, a model-free Kissinger-type method is applied to the non-isothermal differential scanning calorimetry (DSC) data to evaluate the reaction kinetics. The results reveal that the reaction between TiO2 and Al is driven by two competing modes: two-stage liquid combustion, and one-stage solid combustion. Following solid combustion mode, the metastable fcc-TiAl, confirmed by TEM investigation, is formed as precursor for the final ordered γ-TiAl.

For double negative materials (DNMs), these are usually observed in artificial metamaterials or some metal/ceramic composites. Here, an interesting negative permittivity behavior of single-phase perovskite La1−xSrxMnO3 (LSMO) is found. The LSMO bulks with an x value of 0.2 or 0.5 were prepared by a sol–gel/sintering process. From microstructure characterization, the bulks are nearly in single phase and the major phase belongs to the perovskite structure; meanwhile, the bulks are of high density. Furthermore, the experimental data of the plasma-like negative permittivity are fitted well by a lossy Drude model, suggesting a plasma frequency of 3.85 GHz (x = 0.5). The complex permeability of LSMO presents a negative susceptibility (μ′ < 1) at a frequency range of 0.2–1 GHz (x = 0.2) and 0.6–1 GHz (x = 0.5). These results have important implications for the realization of double negative properties in single-phase LSMO as a promising candidate for DNMs.

We use a selective reduction reaction to fabricate a new kind of dual composite, which has a “composite-within-a-composite” structure. Based on the different reduction behavior of Fe2O3, an Fe-rich structure (Fe–Fe3O4–Al2O3) is formed in an Al-rich structure (FeAl2O4–Al2O3). The Fe-rich structure can bring down the concentration of free electrons to reduce the energy loss, without losing its metallic behaviour at the same time. Near the percolation threshold, a conductor–insulator transition appears and the dual composites show totally different electromagnetic responses, which can be well explained by effective medium theory. By controlling the parameters of the selective reduction process, we can get a tunable negative permittivity, which implies the ability to continuously change their electromagnetic properties.

Iron–alumina composites consisting of different loadings of iron particles dispersed in an alumina matrix were prepared via a facile impregnation–calcination process. The frequency dispersions of conductivity and permittivity were investigated in detail. An ultra low percolation threshold of 2.3 vol%, which is much lower than that of dense metal–ceramic composites, was obtained. Meanwhile, a significant enhancement of permittivity ε′ (from ∼7.5 to ∼800) was achieved when the iron content increases from 0 to 4.2 vol% at 10 MHz. The ultra low percolation threshold can be explained by the fact that the porous microstructure of the composites will facilitate the formation of a layer of two dimensional conductive networks on the pore wall of porous alumina. And the significant enhancement of permittivity should be attributed to the interfacial polarization phenomenon that takes place at the iron–alumina interfaces. This paper demonstrates that the loading of a conductive component into a porous matrix is an effective way to fabricate composites with simultaneously high permittivity and ultra-low percolation threshold. Hopefully, various porous metal–ceramic composites with tailored dielectric properties could be fabricated using the impregnation–calcination process.

Ag/Y3Fe5O12 random composites of silver particles randomly hosted in porous Y3Fe5O12 were prepared using a facile impregnation–calcination process. With the increase of silver content, the silver particles interconnected with each other near the percolation threshold, leading to the formation of electrical conductive silver networks. The plasma oscillation of delocalized electrons in interconnected silver particles leads to the negative permittivity. The negative permittivity behaviour was analyzed using the Drude model and equivalent circuit models. Meanwhile, the diamagnetic response of silver networks combined with the magnetic resonance of Y3Fe5O12 results in the negative permeability. That is to say, simultaneous negative permittivity and negative permeability were realized in the Ag/Y3Fe5O12 random composites. Hopefully, tunable double negative property could be realized by adjusting the microstructure and composition of the composites. External DC magnetic field could also be applied to control the electromagnetic properties of the composites.