Theoretical calculation and experiment of microwave electromagnetic property of Ni(C) nanocapsules

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 d B at 12.7 GHz, and below-10 d B, 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.     

Microwave absorption properties of a double-layer absorber based on nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers

The nanocomposite BaFe12O19/α-Fe and nanocrystalline α-Fe microfibers with diameters of 1–5 μm, high aspect ratios and large specific areas are prepared by the citrate gel transformation and reduction process. The nanocomposite BaFe12O19/α-Fe microfibers show some exchange–coupling interactions largely arising from the magnetization hard(BaFe12O19) and soft(α-Fe) nanoparticles. For the microwave absorptions, the double-layer structures consisting of the nanocomposite BaFe12O19/α-Fe and α-Fe microfibers each exhibit a wide band and strong absorption behavior. When the nanocomposite BaFe12O19/α-Fe microfibers are used as a matching layer of 2.3 mm in thickness and α-Fe microfibers as an absorbing layer of 1.2 mm in thickness, the optimal reflection loss(RL) achieves-47 dB at 15.6 GHz, the absorption bandwidth is about 12.7 GHz ranging from 5.3 to 18 GHz, exceeding-20 dB, which covers 72.5% C-band(4.2–8.2 GHz)and whole X-band(8.2–12.4 GHz) and Ku-band(12.4–18 GHz). The enhanced absorption properties of these double-layer absorbers are mainly ascribed to the improvement in impedance matching ability and microwave multi-reflection largely resulting from the dipolar polarization, interfacial polarization, exchange–coupling interaction, and small size effect.     

Fabrication and performance optimization of Mn-Zn ferrite/EP composites as microwave absorbing materials

Magnesium-substituted Mn0.8Zn0.2Fe2O4 ferrite is synthesized by the sol–gel combustion method using citrate acid as the complex agent. The electromagnetic absorbing behaviors of ferrite/polymer coatings fabricated by dispersing Mn–Zn ferrite into epoxy resin （EP） are studied. The microstructure and morphology are characterized by X-ray diffraction and scanning electron microscope. Complex permittivity, complex permeability, and reflection loss of ferrite/EP composite coating are investigated in a low frequency range. It is found that the prepared ferrite particles are traditional cubic spinel ferrite particles with an average size of 200 nm. The results reveal that the electromagnetic microwave absorbing properties are significantly influenced by the weight ratio of ferrite to polymer. The composites with a weight ratio of ferrite/polymer being 3：20 have a maximum reflection loss of –16 dB and wide absorbing band. Thus, the Mn–Zn ferrite is the potential candidate in electromagnetic absorbing application in the low frequency range （10 MHz–1 GHz）.     

Preparation of multi-walled carbon nanotube–Fe composites and their application as light weight and broadband electromagnetic wave absorbers

Multi-walled carbon nanotube （MWCNT）-Fe composites were prepared via the metal organic chemical vapor deposi- tion by depositing iron pentacarbonyl on the surface of MWCNTs. The structural and morphological analyses demonstrated that Fe nanoparticles were deposited on the surface of the MWCNTs. The electromagnetic properties of the MWCNTs were significantly changed, and the absorbing capacity evidently improved after the Fe deposition on the MWCNT surface. A minimum reflection loss of -29.4 dB was observed at 8.39 GHz, and the less than -10 dB bandwidth was about 10.6 GHz, which covered the whole X band （8.2-12.4 GHz） and the whole Ku band （12.4-18 GHz）, indicating that the MWCNT-Fe composites could be used as an effective microwave absorption material.     

Enhanced absorption properties of ordered mesoporous carbon/Co-doped ordered mesoporous carbon double-layer absorbers

Ordered mesoporous carbon (OMC) and metal-doped (M-doped) OMC composites are prepared, and their electromagnetic (EM) parameters are measured. Using the measured EM parameters we calculate the EM wave absorption properties of a double-layer absorber, which is composed of OMC as an absorbing layer and M-doped OMC as the matching layer. The calculated results show that the EM wave absorption performance of OMC/OMC-Co (2.2mm/2.1mm) is improved remarkably. The obtained effective absorption bandwidth is up to 10.3 GHz and the minimum reflection loss reaches 47.6 dB at 14.3 GHz. The enhanced absorption property of OMC/OMC-Co can be attributed to the impedance match between the air and the absorber. Moreover, it can be found that for the absorber with a given matching layer, a larger value of -tanδε (= tan δε absorbing tan δε matching ) can induce better absorption performance, indicating that the difference in impedance between the absorbing layer and the matching layer plays an important role in improving the absorption property of double-layer absorbers.     

Microstructural and electromagnetic properties of MnO2 coated nickel particles with submicron size

Nickel particles with submicron size are prepared by using the solvothermal method. These spheres are then coated with a layer of MnO2 using the soft chemical method. The microstructure is characterized by x-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Energy x-ray dispersive spectrometry and high-resolution images show that the granular composites have a classical core/shell structure with an MnO2 superficial layer,no more than 10 nm in thickness. The hysteresis measurements indicate that these submicron-size Ni composite powders have small remanence and moderate coercivity. The electromagnetic properties of the powders measured by a vector network analyzer in a frequency range of 2-18 GHz are also reported in detail.     

Tunable Dielectric Properties of Carbon Nanotube@Polypyrrole Core-Shell Hybrids by the Shell Thickness for Electromagnetic Wave Absorption

Carbon nanotube@polypyrrole(CNT@PPy) hybrids have been successfully fabricated via a simple in situ chemical oxidation polymerization. The thickness of the PPy shell can be finely controlled in the range of 3.0–6.4 nm. The dielectric loss of core-shell hybrids can be tuned by the shell thickness, resulting in a well-matched characteristic impedance that can enhance electromagnetic wave(EMW) absorption performance. Minimum reflection loss of the hybrid with moderate PPy shell thickness can reach-51.4 dB at 11.8 GHz with a matching thickness of merely 2 mm. Furthermore, the minimum reflection loss values of the hybrid are below-30 d B even at thickness in the range of 1.4–1.9 mm, endowing the possibility of practical application of the hybrids in electromagnetic wave absorption field.     

High microwave absorption performances for single-walled carbon nanotube–epoxy composites with ultra-low loadings

Microwave-absorbing polymeric composites based on single-walled carbon nanotubes(SWNTs) are fabricated via a simple yet versatile method, and these SWNT–epoxy composites exhibit very impressive microwave absorption performances in a range of 2 GHz–18 GHz. For instance, a maximum absorbing value as high as 28 dB can be achieved for each of these SWNT–epoxy composites(1.3-mm thickness) with only 1 wt% loading of SWNTs, and about 4.8 GHz bandwidth,corresponding to a microwave absorption performance higher than 10 dB, is obtained. Furthermore, such low and appropriate loadings of SWNTs also enhance the mechanical strength of the composite. It is suggested that these remarkable results are mainly attributable to the excellent intrinsic properties of SWNTs and their homogeneous dispersion state in the polymer matrix.     

Flexible rGO/Fe_3O_4 NPs/polyurethane film with excellent electromagnetic properties

Large-area and flexible reduced graphene oxide(rGO)/Fe_3O_4 NPs/polyurethane(PU) composite films are fabricated by a facile solution-processable method. The monolayer assembly of Fe_3O_4 nanoparticles with a high particle-stacking density on the graphene oxide(GO) sheets is achieved by mixing two immiscible solutions of Fe_3O_4 nanoparticles in hexane and GO in dimethylformide(DMF) by a mild sonication. The x-ray diffraction and Raman spectrum confirm the reduced process of rGO by a simple thermal treatment. The permittivity value of the composite in a frequency range of 0.1 GHz–18 GHz increases with annealing temperature of GO increasing. For 5-wt% rGO/Fe_3O_4 NPs/PU, the maximum refection loss(RL) of over-35 dB appears at 4.5 GHz when the thickness of film increases to 5 mm. The rGO/Fe_3O_4 NPs/PU film, exhibiting good electromagnetic properties over GHz frequency range, could be a potential candidate as one of microwave absorption materials in flexible electronic devices.     

Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO_2

The effects of TiO_2 on sintering and nonlinear electrical properties of(98.5-x)ZnO–0.5MnO_2–0.5Co_2O_3-0.5Bi_2O_(3–x)TiO_2(x = 0.3,0.5,0.7,0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO_2 doping.The addition of TiO_2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO_2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.     

Absorption Spectra and Refractive Index Changes of an Exciton in a Core/Shell Quantum Dot

The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov–Bohm oscillation in core/shell quantum dots.     

Electromagnetic and Microwave Absorption Properties of Fe Coating on SiC with Metal Organic Chemical Vapor Reaction

SiC-Fe composites are prepared by metal organic chemical vapor deposition （MOCVD） using silicon carbide （SIC＇） and iron pentacarbonyl （Fe（CO）5） as the precursors. The structure and morphology analyses demonstrate that the Fe nanoparticles have been deposited on the surface of the SiC particles. In terms of reflection loss （RL）, the absorbing frequency band （AFB, the value of RL 〈 -10 dB）, and the matching thickness （tin）, SiC-Fe composites show the best performances： minimum RL of - 31.6 dB with tm= 2.0 mm at 15.1 GHz, AFB of 12.9-17.3 OHz, indicating that Fe-doped SiC by MOCVD can significantly improve the electromagnetic properties of SiC and that SiC-Fe composites could be used as an effective microwave absorption materiM.     

Size-dependent thermal stresses in the core–shell nanoparticles

The thermal stress in a magnetic core–shell nanoparticle during a thermal process is an important parameter to be known and controlled in the magnetization process of the core–shell system. In this paper we analyze the stress that appears in a core–shell nanoparticle subjected to a cooling process. The external surface temperature of the system, considered in equilibrium at room temperature, is instantly reduced to a target temperature. The thermal evolution of the system in time and the induced stress are studied using an analytical model based on a time-dependent heat conduction equation and a differential displacement equation in the formalism of elastic displacements. The source of internal stress is the difference in contraction between core and shell materials due to the temperature change. The thermal stress decreases in time and is minimized when the system reaches the thermal equilibrium. The radial and azimuthal stress components depend on system geometry, material properties, and initial and final temperatures. The magnitude of the stress changes the magnetic state of the core–shell system. For some materials, the values of the thermal stresses are larger than their specific elastic limits and the materials begin to deform plastically in the cooling process. The presence of the induced anisotropy due to the plastic deformation modifies the magnetic domain structure and the magnetic behavior of the system.     

Tailoring the optical and magnetic properties of La-BaM hexaferrites by Ni substitution

We investigate the impact of Ni insertion on the structural,optical,and magnetic properties of Ba_0.8La_0.2Fe_12-xNi_xO₁₉hexaferrites(Ni substituted La-BaM hexaferrites).Samples were prepared using the conventional co-precipitation method and sintered at 1000℃for 4 hours to assist the crystallization process.An analysis of the structure of the samples was carried out using an x-ray diffraction(XRD)spectrometer.The M-type hexagonal structure of all the samples was confirmed using XRD spectra.The lattice parameters a and c were found to be in the ranges of 5.8925±0.001 nm–5.8952±0.001 nm and 23.2123±0.001 nm–23.2219±0.001 nm,respectively.The M-type hexagonal nature of the prepared samples was also indicated by the presence of corresponding FT-IR bands and Raman modes in the FT-IR and Raman spectra,respectively.EDX results confirmed the successful synthesis of the samples according to the required stoichiometric ratio.A UV-vis spectrometer was used to record the absorption spectra of the prepared samples in the wavelength range of 200 nm–1100 nm.The optical energy bandgap of the samples was found to be in the range of 1.21 eV–3.39 eV.The M–H loops of the samples were measured at room temperature at an applied magnetic field range of 0 kOe–60 kOe.A high saturation magnetization of 99.92 emu/g was recorded in the sample with x=0 at a microwave operating frequency of 22.2 GHz.This high value of saturation magnetization is due to the substitution of La3+ions at the spin-up(12k,2a,and 2b)sites.The Ni substitution is proven to be a potential candidate for the tuning of the optical and magnetic parameters of M-type hexaferrites.Therefore,we suggest that the prepared samples are suitable for use in magneto-optic applications.     

Structural,magnetic, and dielectric properties of Ni–Zn ferrite and Bi_2O_3 nanocomposites prepared by the sol-gel method

Ni–Zn ferrite and Bi_2O_3 composites were developed by the sol-gel method. The structural, magnetic, and dielectric properties were studied for all the prepared samples. X-ray diffraction(XRD) was performed to study the crystal structure.The results of field emission scanning electron microscopy(FE-SEM) showed that the addition of Bi_2O_3 can increase the grain size of the Ni–Zn ferrite. Magnetic properties were analyzed by a hysteresis loop test and it was found that the saturation magnetization and coercivity decreased with the increase of Bi_2O_3 ratio. In addition, the dielectric properties of the Ni–Zn ferrite were also improved with the addition of Bi_2O_3.     

Electromagnetic wave absorbing properties and hyperfine interactions of Fe–Cu–Nb–Si–B nanocomposites

The Fe–Cu–Nb–Si–B alloy nanocomposite containing two ferromagnetic phases(amorphous phase and nanophase phase) is obtained by properly annealing the as-prepared alloys. High resolution transmission electron microscopy(HRTEM) images show the coexistence of these two phases. It is found that Fe–Si nanograins are surrounded by the retained amorphous ferromagnetic phase. M¨ossbauer spectroscopy measurements show that the nanophase is the D03-type Fe–Si phase, which is employed to find the atomic fractions of resonant57 Fe atoms in these two phases. The microwave permittivity and permeability spectra of Fe–Cu–Nb–Si–B nanocomposite are measured in the frequency range of 0.5 GHz–10 GHz. Large relative microwave permeability values are obtained. The results show that the absorber containing the nanocomposite flakes with a volume fraction of 28.59% exhibits good microwave absorption properties. The reflection loss of the absorber is less than-10 dB in a frequency band of 1.93 GHz–3.20 GHz.     

Enhanced structural and magnetic properties of microwave sintered Li–Ni–Co ferrites prepared by sol–gel method

The properties of lithium ferrites are very sensitive to chemical composition, synthesis method, and sintering techniques. Li–Ni–Co ferrites with compositional formula Li_(0.45-0.5x)Ni_(0.1)Co_xFe_(2.45-0.5x)O_4, where 0.00 ≤ x ≤ 0.1 in steps of 0.02 were prepared by chemical sol–gel method and sintered by microwave sintering technique. The x-ray diffraction patterns confirmed the formation of single phase with spinel structure in all the samples. The structural parameter viz.lattice constant, crystallite size, and x-ray density for these samples were studied and compared with those measured from samples of similar composition prepared by the sol–gel method and sintered by conventional sintering technique. Enhancement in the magnetic properties like Curie temperature, hysteresis parameters was observed by employing sol–gel synthesis combined with microwave sintering. The results obtained and mechanisms involved are discussed in the paper.     

Temperature dependence of line parameters of ~(12)C~(16)O_2 near 2.004 μm studied by tunable diode laser spectroscopy

The absorption spectrum of carbon dioxide at 2.004 μm has been recorded at sample temperatures between 218.0 K and room temperature, by using a high-resolution tunable diode laser absorption spectrometer(TDLAS) combined with a temperature controlled cryogenically cooled absorption cell. The self-, N~(2-), and air-broadening coefficients for nine transitions of ~(12)C~(16)O_2 belonging to the 20012←00001 band in the 4987 cm~(-1)–4998 cm~(-1) region have been measured at different temperatures. From these measurements, we have further determined the temperature dependence exponents of the pressure-broadening coefficients. To the best of our knowledge, the temperature dependence parameters of the collisional broadening coefficients are reported experimentally for the first time for these nine transitions. The measured halfwidth coefficients and the air temperature dependence exponents of these transitions are compared with the available values reported in the literature and HITRAN 2012 database. Agreements and discrepancies are also discussed.     

Low Loss Plastic Terahertz Photonic Band-Gap Fibres

We report a numerical investigation on terahertz wave propagation in plastic photonic band-gap fibres which are characterized by a 19-unit-cell air core and hexagonal Mr holes with rounded corners in cladding. Using the finite element method, the leakage loss and absorption loss are calculated and the transmission properties are analysed. The lowest loss of 0.268 dB/m is obtained. Numerical results show that the fibres could liberate the constraints of background materials beyond the transparency region in terahertz wave band, and efficiently minimize the effect of absorption by background materials, which present great advantage of plastic photonic band-gap fibres in long distance terahertz delivery.     

Enhanced dielectric and optical properties of nanoscale barium hexaferrites for optoelectronics and high frequency application

M-type barium hexaferrites with chemical composition Ba_(1-x)Dy_xFe_(12-y)Cr_yO_(19)(x = 0.0, 0.1, 0.2, and y =0.0, 0.4, 0.5) were synthesized via sol–gel auto-combustion method. The samples were pre-sintered at 400℃ for 3 h and sintered at 950℃ for 5 h. The changes in the structural, dielectric, and optical properties were studied after the substitution of Dy~(3+) and Cr~(3+) ions. X-ray diffraction(XRD) analysis confirms the formation of single phase hexaferrites with the absence of secondary phase. FTIR analysis gives an idea of the formation of hexaferrites with the appearance of two peaks at 438 cm~(-1) and 589 cm~(-1). The field emission scanning electron micrographs(FESEM) show a combination of crystallites with large shapes close to hexagonal platelet-like shape and others with rice or rod-like shapes, whereas EDX and elemental analysis confirm the stoichiometry of prepared samples. The calculated band gap from UV-vis NIR spectroscopy spectra was found to decreases with increase in Dy~(3+)–Cr~(3+)substitution. The dielectric properties were explained on the basis of Maxwell–Wagner model. Enhancement of dielectric constant at higher frequencies was observed in all the samples. Low dielectric loss is also observed in all the samples and Cole–Cole plot shows that grain boundary resistance(Rgb) contribute most to the dielectric properties. The prepared samples exhibit properties that could be useful for optoelectronics and high frequency application.