Microstructure and microwave absorption properties of Fe3O4/dextran/SnO2 multilayer microspheres

Fe₃O₄/dextran/SnO₂ multilayer microspheres have been successfully designed and synthesized by solvothermal and hydrothermal reactions. Dextran worked as a linker between Fe₃O₄ core and SnO₂ shell. It can not only prevent the oxidation of Fe₃O₄ but also be carbonize to another absorber carbon black. The as-synthesized microspheres were about 320 nm in size and well-defined in shape. The maximum reflection loss of Fe₃O₄/dextran/SnO₂ microspheres and paraffin wax composites could reach 20.26 dB at 4.72 GHz, and the bandwidth with a reflection loss less than ? 10 dB was 4.86 GHz with 4 mm in thickness. The excellent microwave absorption properties of the composites were attributed to the special multilayer structures of Fe₃O₄/dextran/SnO₂ microspheres and the effective complementarity between dielectric loss and magnetic loss.     

Fe3O4/carbon composite nanofiber absorber with enhanced microwave absorption performance

Fe₃O₄/carbon composite nanofibers were prepared by electrospinning polyacrylonitrile (PAN)/acetyl acetone iron (AAI)/dimethyl formamide (DMF) solution, followed by stabilization and carbonization. SEM and TEM observations reveal that the fibers are lengthy and uniform, and are loaded with well-distributed Fe₃O₄ nanoparticles, which are evidenced by XRD. Electrical and magnetic properties of the samples were studied to show the effect of enhancement of electrical conductivity and magnetic hysteresis performance. Finally, the permittivity and permeability parameters were measured by a vector network analyzer, and the reflectivity loss was calculated based on Transmission Line Theory. Results show that Fe₃O₄/C composite nanofibers exhibit enhanced properties of microwave absorption as compared to those of pure carbon nanofibers by: decreasing reflectivity loss values; widening absorption width and improving performance in low frequency (2–5 GHz) absorption. Absorption properties can be tuned by changing AAI content, carbonization temperature, composite fiber/paraffin ratio and coating thickness. It is shown that with coating thickness of 5 mm and fiber/paraffin ratio of 5 wt.%, the bandwidth for reflection loss under ?5 dB can reach a maximum of 12–13 GHz in the range of 2–18 GHz, accompanying with a minimum reflection loss of ?40 to ?45 dB, and preferred low frequency band absorption can also be obtained. The mechanisms for the enhanced absorption performance were briefly discussed. It is supposed that this kind of composite material is promising for resolving the problems of weak absorption in the low frequency range and narrow bandwidth absorption.     

Double-layer microwave absorber based on nanocrystalline Zn0.5Ni0.5Fe2O4/α-Fe microfibers

The microwave absorption properties of the nanocrystalline NiZn ferrite (Zn_0.5Ni_0.5Fe₂O₄) and iron (α-Fe) microfibers with single-layer and double-layer structures were investigated in the frequency range of 2–18 GHz. The double-layer absorbers have much better microwave absorption properties than the single-layer absorbers, and the microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing layer and matching layer. With the absorbing layer thickness 0.7 mm of α-Fe microfibers–wax composite and the matching layer thickness 1.5 mm of Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite, the minimum reflection loss (RL) reaches about −71 dB at 16.2 GHz and the absorption band width is about 9.2 GHz ranging from 8.8 to 18 GHz with the RL value exceeding −10 dB. While, when the absorbing layer is the Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite with thickness 1.8 mm and the matching layer is the α-Fe microfibers–wax composite with thickness 0.2 mm, the RL value achieves the minimum about −73 dB at 13.8 GHz and the absorption band width is about 10.2 GHz ranging from 7.8 to 18 GHz with the RL value exceeding −10 dB, which covers the whole X-band (8.2–12.4 GHz) and K_u-band (12.4–18 GHz).     

Properties of Co0.5Ni0.5Fe2O4/carbon nanotubes/polyimide nanocomposites for microwave absorption

High-temperature microwave absorbing materials are of great interest due to their ability to withstand high temperatures. Multi-walled carbon nanotubes (MWNTs) were surface modified by Ar plasma and Co_0.5Ni_0.5Fe₂O₄ nanoparticles were doped onto the surface of the MWNTs by a chemical co-precipitation method. Co_0.5Ni_0.5Fe₂O₄/MWNTs powders were then added to polyimide to prepare nanocomposites for microwave absorption. After plasma modification, the surface of the MWNTs produced carboxyl groups, which are beneficial for interfacial bonding between the MWNTs and PI. The glass transition temperature of the nanocomposites was 261 °C and their thermostability was preserved up to 500 °C. The maximum reflection loss (RL) value of nanocomposites containing 0.75 wt% modified MWNTs was ?24.37 dB and the frequency range where the RL value was less than ?10 dB was 5.1 GHz from 7.8 to 12.9 GHz.     

Capture and release of genomic DNA by PEI modified Fe3O4/Au nanoparticles

Polyethylenimine (PEI) modified Fe₃O₄/Au nanoparticles were synthesized in aqueous solution and characterized by photo correlation spectroscopy (PCS) and vibrating sample magnetometer (VSM). The so-obtained Fe₃O₄/Au-PEI nanoparticles were capable of efficient electrostatic capture of DNA. The maximum amount of genomic DNA captured on 1.0 mg Fe₃O₄/Au-PEI nanoparticles was 90 μg. The DNA release behavior was studied and the DNA recovery from Fe₃O₄/Au-PEI nanoparticles approached 100% under optimal conditions. DNA extraction from mammalian cells using Fe₃O₄/Au-PEI nanoparticles was successfully performed. Up to approximately 43.1 μg of high-purity (OD₂₆₀/OD₂₈₀ ratio = 1.81) genomic DNA was extracted from 10 mg of liver tissue. The results indicated that the prepared Fe₃O₄/Au-PEI nanoparticles could be successfully used for DNA capture and release.     

Graphene-enhanced microwave absorption properties of Fe3O4/SiO2 nanorods

Fe₃O₄/SiO₂/graphene composite composed of Fe₃O₄/SiO₂ core–shell nanorods and graphene nanosheets were synthesized by a facile wet chemical method. Structure and morphology studies reveal that the Fe₃O₄/SiO₂ nanorods with porous structure and large aspect ratio are densely wrapped by the graphene nanosheets. By changing the graphene content, the electromagnetic properties of the Fe₃O₄/SiO₂/graphene composite can be well tuned. When the weight ratio of Fe₃O₄/SiO₂ to graphene reaches an appropriate value, excellent microwave absorption performance is achieved due to the large electromagnetic losses and good impedance matching. The Fe₃O₄/SiO₂/graphene composite with graphene content of 5 wt.% shows the minimum reflection loss of −27.1 dB at 12.2 GHz when the coating layer thickness is only 1.5 mm.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Electromagnetic and microwave absorption performance of some transition metal doped La0.7Sr0.3Mn1−xTMxO3±δ (TM = Fe,Co or Ni)

In this study the transition metal doped La_0.7Sr_0.3Mn_1?xTM_xO_3±δ (TM = Fe, Co or Ni, x = 0, 0.2) powders were fabricated by the conventional solid state reaction method. The compositions, morphologies and crystal structures were characterized using different method. The influences of the incorporation of TM into La_0.7Sr_0.3MnO_3±δ on the complex permittivity, complex permeability and microwave absorption performance were investigated in the range of 5.85–18 GHz. It is found that the electromagnetic loss has been enhanced after TM doping. And the microwave absorption properties have been significantly improved. In present study La_0.7Sr_0.3Mn_0.8Fe_0.2O_3±δ had the best microwave absorption properties. The maximum reflection loss was 27.67 dB at 10.97 GHz, and the absorbing bandwidth above 6 dB was 6.80 GHz with 2 mm thickness.     

Efficient purification of His-tagged protein by superparamagnetic Fe3O4/Au–ANTA–Co2 + nanoparticles

Superparamagnetic Fe₃O₄/Au nanoparticles were synthesized and surface modified with mercaptopropionic acid (MPA), followed by conjugating Nα,Nα-Bis(carboxymethyl)-l-lysine hydrate (ANTA) and subsequently chelating Co^2 +. The resulting Fe₃O₄/Au–ANTA–Co^2 + nanoparticles have an average size of 210 nm in aqueous solution, and a magnetization of 36 emu/g, endowing the magnetic nanoparticles with excellent magnetic responsivity and dispersity. The Co^2 + ions in the magnetic nanoparticle shell provide docking site for histidine, and the Fe₃O₄/Au–ANTA–Co^2 + nanoparticles exhibit excellent performance in binding of a His-tagged protein with a binding capacity of 74 μg/mg. The magnetic nanoparticles show highly selective purification of the His-tagged protein from Escherichia coli lysate. Therefore, the obtained Fe₃O₄/Au–ANTA–Co^2 + nanoparticles exhibited excellent performance in the direct separation of His-tagged protein from cell lysate.     

Enhancement of electric conductivity by incorporation of Ag into core/shell structure of Fe3O4/Ag/PPy NPs

Core–shell structure of Fe₃O₄/Ag/polypyrrole (PPy) nano-particles (NPs) was prepared by a facile method through the redox reaction of silver nitrate and pyrrole in the presence of polyvinyl pyrrolidone (PVP) as protection agent. The presence of PVP has an effect on the morphology and structure of the Fe₃O₄/Ag/PPy NPs. Rod-shaped Fe₃O₄/Ag/PPy NPs were obtained with the increase of the concentration of PVP to 0.125 mM, in which the wire-like core was formed by the aggregation of Fe₃O₄ and Ag nanoparticles. At the same time, the PPy shell of the NPs became more uniform and thicker with the increase of the concentrations of PVP and AgNO₃ in the solution. The electric conductivity of the NPs can be enhanced to 335 ± 8 S/cm by incorporation of the Ag into the NPs. At the same time, the thickness of the PPy shell affects the electric conductivity of the samples. All the NPs in the present work exhibit superparamagnetic behavior.     

Electromagnetic and microwave-absorbing properties of magnetite decorated multiwalled carbon nanotubes prepared with poly(N-vinyl-2-pyrrolidone)

The magnetite (Fe₃O₄) decorated multiwalled carbon nanotubes (MWNTs) hybrids were prepared by an in situ chemical precipitation method using poly(N-vinyl-2-pyrrolidone) (PVP) as dispersant. The structure and morphology of hybrids are characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and transmission electron-microscopy (TEM). The TEM investigation shows that the Fe₃O₄/MWNTs hybrids exhibit less entangled structure and many more Fe₃O₄ particles are attached homogeneously on the surface of MWNTs, which indicated that PVP can indeed help MWNTs to disperse in isolated form. The electromagnetic and absorbing properties were investigated in a frequency of 2–18 GHz. The results show that the Fe₃O₄/MWNTs hybrids exhibit a superparamagnetic behavior and possess a saturation magnetization of 22.9 emu/g. The maximum reflection loss is ?35.8 dB at 8.56 GHz, and the bandwidth below ?10 dB is more than 2.32 GHz. More importantly, a new reflection loss peak occurs at the frequency of 14.6 GHz, which indicates that the Fe₃O₄/MWNTs hybrids have better absorption properties in the high-frequency.     

Preparation,characterization and application of Fe3O4/ZnO core/shell magnetic nanoparticles

Fe₃O₄ magnetic nanoparticles (MNPs) were synthesized by a co-precipitation method. The phase purity was confirmed by X-ray powder diffraction (XRD) analysis. The crystal size was found to be 10 nm from transmission electron microscopy (TEM). It is evidenced that the surface of Fe₃O₄ MNPs was modified by sodium citrate. The Fe₃O₄/ZnO core/shell MNPs were obtained by coating the MNPs with direct precipitation using zinc acetate and ammonium carbonate. The precursor was firstly dried and then calcined at 350 °C. The antioxidation tests indicated that the core/shell MNPs give better antioxidation than that of the Fe₃O₄ MNPs. The photocatalytic degradation of methyl orange revealed that the core/shell MNPs have higher photocatalytic activity than that of the ZnO nanoparticles. Separation of the core/shell MNPs from the aqueous suspension using a magnet provides an easy way to recycle the core/shell MNPs. After four-time recycling, the photocatalytic degradation percentage of the core/shell MNPs is about 70%.     

Microwave-polyol controlled synthesis and magnetic properties of monodisperse CoxNi1−xFe2O4/MWCNT nanocomposites

Monodisperse Co_xNi_1−xFe₂O₄ nanoparticles (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) with controllable composition attached on the multi-walled carbon nanotubes (MWCNTs) were prepared by microwave-polyol method. The composition of Co_xNi_1−xFe₂O₄ nanoparticles can be controlled through adjusting the atomic ratios of cobalt and nickel nitrate in the mixed solution. The influence of the microwave power and microwave irradiation time on the monodispersion of nanoparticles was also investigated. The results show quasi-spherical Co_xNi_1−xFe₂O₄ nanoparticles with the face-centered cubic structure and average crystallite size (6 nm) are uniformly dispersed on MWCNTs. The saturation magnetization of Co_xNi_1−xFe₂O₄/MWCNT nanocomposites increases gradually from 12.90 to 20.03 emu/g with increasing Co²⁺ concentration. The coercivity is almost zero at room temperature, which indicates the superparamagnetic behavior.     

Structural,magnetic and microwave absorption characteristics of BaCoxMnxTi2xFe12 − 4xO19

The effect of Mn⁺²Co⁺²Ti⁺⁴ substitution on microwave absorption has been studied for BaCo_xMn_xTi_2xFe_12 ? 4xO₁₉ ferrite–acrylic resin composites, where x varies from 0.3 to 0.5 in steps of 0.1, in frequency range from 12 to 20 GHz. X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer, and vector network analyzer were used to analyze the structures, electromagnetic and microwave absorption properties. The results showed that, the magnetoplumbite structures for all samples have been formed. Based on microwave measurement on reflectivity, BaCo_xMn_xTi_2xFe_12 ? 4xO₁₉ may be a good candidate for electromagnetic compatibility and other practical applications at high frequency.     

New dielectric materials of xSrTiO3–(1 − x)Ca(Mg1/3Nb2/3)O3 ceramic system at microwave frequency

The crystal structures and the microwave dielectric properties of the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ perovskite ceramic system have been investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. SrTiO₃ has dielectric properties of dielectric constant ε_r ∼ 205, Q × f value ∼ 4200 GHz and a large positive τ_f value ∼ 1700 ppm/°C. Ca(Mg_1/3Nb_2/3)O₃ possesses high dielectric constant (ε_r ∼ 28), high quality factor (Q × f value ∼ 58,000 at 7 GHz) and negative τ_f value (− 48 ppm/°C). As the x value varies from 0.2 to 0.8, the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ system has the dielectric properties as follows: 40 < ε_r < 123, 4600 < Q × f < 33,400 and − 23 < τ_f < 600. A new microwave dielectric material, 0.3SrTiO₃–0.7Ca(Mg_1/3Nb_2/3)O₃, applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ε_r ∼ 46, a Q × f value ∼ 29,300 GHz (at 6.8 GHz) and a τ_f value ∼− 2 ppm/°C. A near-zero τ_f value can be achieved by adjusting the x value of xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ ceramics.     

Synthesis and characterization of silica core/nano-ferrite shell particles

Core/shell particles were synthesized by assembling oppositely charged ferrite (Fe₃O₄ or NiFe₂O₄) nanoparticles on the surface of monodispersed silica core particles (having size ～0.4 μm) prepared by hydrolysis and condensation of tetraethylortosilicate. Optimal conditions for synthesis of silica core/nano-Fe₃O₄ shell particles were found at pH ～ 5.4. The obtained particles have superparamagnetic behavior above a blocking temperature of ≈25 K, which make them very attractive for a broad range of biomedical and bioengineering applications. Incorporation of nickel into ferrite structure could not be achieved at lower pH value, so functionalization of core particles was required. Incorporation of nickel into ferrite structure was successful at pH above 7, however at higher pH the formation rate of nickel–ferrite particles becomes very fast and the self-aggregation dominates the competing formation of the nickel–ferrite shell. Because of that the self-aggregation was prevented by surface modification of nickel–ferrite nanoparticles with citric acid before their deposition on the functionalized silica core and homogenous and continuous NiFe₂O₄ shell was finally obtained.     

Magnetically-sensitive shape memory polyurethane composites crosslinked with multi-walled carbon nanotubes

Magnetically-sensitive polyurethane composites, which were crosslinked with multi-walled carbon nanotubes (MWCNTs) and were filled with Fe₃O₄ nanoparticles, were synthesized via in situ polymerization method. MWCNTs pretreated with nitric acid were used as crosslinking agents. Because of the crosslinking of MWCNTs with polyurethane prepolymer, the properties of the composites with a high content of Fe₃O₄ nanoparticles, especially the mechanical properties, were significantly improved. The composites showed excellent shape memory properties in both 45 °C hot water and an alternating magnetic field (f = 45 kHz, H = 29.7 kA m⁻¹). The shape recovery time was less than one minute and the shape recovery rate was over 95% in the alternating magnetic field.     

Preparation of chitosan/magnetite composite beads and their application for removal of Pb(II) and Ni(II) from aqueous solution

A simple and effective process has been proposed to prepare chitosan/magnetite nanocomposite beads with saturation magnetization value as high as uncoated Fe₃O₄ nanoparticles (ca. 54 emu/g). The reason was that the coating chitosan layer was so thin that it did not affect magnetic properties of these composite beads. Especially, chitosan on the surface of the magnetic Fe₃O₄ nanoparticles is available for coordinating with heavy metal ions, making those ions removed with the assistance of external magnets. Maximum adsorption capacities for Pb(II) and Ni(II), occurred at pH 6 and under room temperature were as high as 63.33 and 52.55 mg/g respectively, according to Langmuir isotherm model. These results permitted to conclude that chitosan/magnetite nanocomposite beads could serve as a promising adsorbent not only for Pb(II) and Ni(II) (pH = 4–6) but also for other heavy metal ions in wastewater treatment technology.     

Preparation of Fe3O4–chitosan nanoparticles used for hyperthermia

The Fe₃O₄–chitosan nanoparticles with core-shell structure have been prepared by crosslinking method. Oleic acid modified Fe₃O₄ nanoparticles were firstly prepared by co-precipitation then chitosan was added to coat on the surface of the Fe₃O₄ nanoparticles by physical absorption. The Fe₃O₄–chitosan nanoparticles were obtained by crosslinking the amino groups on the chitosan using glutaraldehyde. Transmission electron microscopy showed that the Fe₃O₄–chitosan nanoparticles were quasi-spherical with a mean diameter of 10.5 nm. X-ray diffraction pattern and X-ray photoelectron spectra indicated that the magnetic nanoparticles were pure Fe₃O₄ with a cubic inverse spinel structure. The modification using chitosan did not result in a phase change. The binding of chitosan to the Fe₃O₄ nanoparticles was also demonstrated by the measurement of fourier transform infrared spectra and thermogravimetric analysis. Magnetic measurement revealed that the saturation magnetization of the composite nanoparticles was 30.7 emu/g and the nanoparticles were superparamagnetic at room temperature. Furthermore, the inductive heating property of the composite nanoparticles in an alternating current magnetic field was investigated and the results indicated that the heating effect was significant. The Fe₃O₄–chitosan nanoparticles prepared have great potential in hyperthermia.     

Microwave dielectric properties of xNd(Zn1/2Ti1/2)O3–(1 − x)CaTiO3 ceramics

The microwave dielectric properties of xNd(Zn_1/2Ti_1/2)O₃–(1 − x)CaTiO₃ have been investigated. The system has been prepared by a conventional solid state ceramic route. Nd(Zn_1/2Ti_1/2)O₃ (NZT) possesses a dielectric constant (ε_r) of 32, a high quality factor (Q × f) of 170,000 GHz and a temperature coefficient of resonant frequency (τ_f) of − 42 ppm/°C. In order to produce a temperature-stable material, the addition of CaTiO₃ leads to a near-zero temperature variation of resonant frequency. In general, the microwave quality factor (Q × f) decreased as x increased and the temperature coefficient of resonant frequency (τ_f) was approximately linearly proportional to permittivity. The dielectric constant decreases from 77 to 32 as x varies from 0.2 to 1.0. The dielectric constant (ε_r) of 45, Q × f value of 56,000 (at 6 GHz) and temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C were obtained for 0.5Nd(Zn_1/2Ti_1/2)O₃–0.5CaTiO₃ ceramics sintered at 1300 °C for 4 h. As the content of x increases, the highest Q × f value of 136,200 GHz for x = 0.8 is achieved at the sintering temperature 1300 °C.