Magnetic and microwave absorbing properties of M-type barium ferrite/graphene oxide composite microwave absorber

In order to improve the absorbing properties of M- type barium ferrite absorbing materials, M-type barium ferrite/graphene oxide composites with different graphene oxide contents were synthesized by the sol-gel autocombustion method. X-ray diffraction （XRD）, a scanning electronic microscopy （ SEM ）, a physical properties measurement system （PPMS-9）, and a vector network analyzer were used to analyze their structure, surface morphology, magnetic and absorbing properties, respectively. The results show that the absorbing band of the composite absorbing material is widened and the absorbing strength is increased compared with the pure M-type barium ferrite. The sample with the content of doped graphene oxide of 3% has the minimum reflectivity at 10 to 18 GHz frequencies. Hence, the doped graphene oxide effectively improves the absorbing properties of M-type barium ferrite.

Preparation and Characterization of M-Type Barium Ferrite Fibers via Aqueous Sol-Gel Process

BaFe12O19 fibers was prepared via an aqueous sol-gel process using Fe（OH）（HCOO）2 synthesized in laboratory and Ba（CH3COO）2 as the original materials and citrate as the chelate. The rheological behaviour of spinnable sol was characterized on rheometer, and the development of gel fibers to barium ferrite fibers was studied by IR, TG and XRD. Morphology observation of the fibers was given on SEM, and the diameter of the obtained fibers was between 5 and 10 um corresponding to different additives. The additives affected the surface tension of the precursor sol which had close relation to the microstructure of fibers. Sucrose and hydroxyethylic cellulose could improve the surface tension while diethanolamine and hexadecylamine reduce that of the decylamine as an additive, well-structured BaFe12O19 precursor sol. And using diethanolamine or hexafibers could be obtained.

The structural and magnetic properties of barium ferrite powders prepared by the sol-gel method

In this paper, M-type hexagonal barium ferrite powders are synthesized using the sol-gel method. A dried precursor heated in air is analyzed in the temperature range from 50 to 1200 ℃ using thermo-gravimetric analysis and differential scanning calorimetry. The effects of the additives and the cacinating temperature on the magnetic properties are investigated, and the results show that single-phase barium ferrite powders can be formed. After heat-treating at 950 ℃ for 4h with 3 wt% additive, the coercivity and saturation magnetization are found to be 440 Oe and 57.9 emu/g, respectively.

Comparison of the Sol-gel Method with the Coprecipitation Technique for Preparation of Hexagonal Barium Ferrite

Hexagonal barium ferrite BaFe12O19 particles were prepared by sol-gel and coprecipitation methods, respectively. The composition of the so-obtained materials was investigated by means of XRD. By the sol-gel method, non-anticipated intermediate crystalline phases, such as γ-Fe2O3, α-Fe2O3, BaCO3, and BaFe2O4 etc., were formed with the delay of the formation of BaFe12O19. The formation of single phase BaFe12O19 required calcination at 850 oC for 4 h. On the other hand, using coprecipitation technique, amorphous hydroxide precursor was directly transferred into BaFe12O19 almost without the formation of intermediate crystalline phases. BaFe12O19 was prepared by calcining at 700 oC for 3 h. The results were confirmed by ESEM and VSM analyses. Based on the already reported results and the observed results in this study, it can be concluded that the coprecipitaion technique is easier to control than the sol-gel method for preparation of BaFe12O19 at a low temperature.

Perpendicularly oriented barium ferrite thin films with low microwave loss,prepared by pulsed laser deposition

Barium ferrite（Ba M） thin films are deposited on platinum coated silicon wafers by pulsed laser deposition（PLD）.The effects of deposition substrate temperature on the microstructure,magnetic and microwave properties of Ba M thin films are investigated in detail.It is found that microstructure,magnetic and microwave properties of Ba M thin film are very sensitive to deposition substrate temperature,and excellent Ba M thin film is obtained when deposition temperature is 910℃ and oxygen pressure is 300 m Torr（1 Torr = 1.3332×102Pa）.X-ray diffraction patterns and atomic force microscopy images show that the best thin film has perpendicular orientation and hexagonal morphology,and the crystallographic alignment degree can be calculated to be 0.94.Hysteresis loops reveal that the squareness ratio（Mr/Ms） is as high as 0.93,the saturated magnetization is 4004 Gs（1 Gs = 104T）,and the anisotropy field is 16.5 kOe（1 Oe = 79.5775 A·m-1）.Ferromagnetic resonance measurements reveal that the gyromagnetic ratio is 2.8 GHz/kOe,and the ferromagnetic resonance linewith is108 Oe at 50 GHz,which means that this thin film has low microwave loss.These properties make the Ba M thin films have potential applications in microwave devices.

Texture and self-biased property of an oriented M-type barium ferrite thick film by tape casting

In this paper, the oriented M-type barium ferrite （BaM） thick films with different thicknesses are prepared by tape casting. It is found that the crystallographic alignment degree （f）, the pore and the squareness ratio （Mr/Ms） are not affected by the thickness of the film. XRD and SEM results show that the thick film has hexagonal morphology with a crystal texture of c-axis grains perpendicular to film plane. The hysteresis curve indicates that the BaM thick film exhibits a self-biased property with a remanent magnetization of 3.30 T, a squareness ratio （Mr/Ms） of 0.81, and a coercivity of 0.40 T. The results show that the BaM thick film has potential for use in self-biasing microwave devices, and also proves that the tape casting technique is capable of fabricating high-quality barium ferrite films, thus providing a unique opportunity to realize the large area production of thick film.

Studies on Progressive Reactions Leading Formations of Different Types of Barium Ferrites

The present investigation is concerned with the reaction of barium and iron nitrates mixtures using three different molar ratios, 1:1 (Ⅰ), 1:2 (Ⅱ) and 2:1 (Ⅲ) at different temperatures as pointed out from the DTA data. The reaction products exhibit 12 compounds namely, Ba(NO3)2, αFe2O3, Fe3O4, BaFeO3, BaFeO2.9, hexagonal BaFeO3-x, tetragonal BaFeO3-x, BaFe2O4, αBaFe2O4, Ba2Fe6O11, Ba5Fe14O26 and BaFe12O19. The formation of these products depend on the molar ratio between the reactants and the reaction temperature. The reaction products were studied by DTA and TG techniques and characterized by X-ray diffraction patterns, magnetic susceptibility data and scanning electron microscopy, SEM.

Synthesis and Properties of Tubular Polyaniline-Barium Ferrite Composite

The polyaniline-barium ferrite composite was synthesized by in situ polymerization of aniline in the presence of BaFe12019 nanoparticles. The structure, morphology, and magnetic properties of samples were characterized by powder X-ray diffraction （XRD）, Fourier transform infrared （FTIR）, scanning electron microscopy （SEM） and vibrating sample magnetometer （VSM）. The testing results showed that the composite exhibited the ferromagnetic and electric behaviors, which benefit for the application of electromagnetic interfence.

Effect of Al-substitution on phase formation and magnetic properties of barium hexaferrite synthesized with sol-gel auto-combustion method

Al-substituted barium ferrite powders were synthesized using the sol-gel auto-combustion method according to the molecular formula BaAlxFe12-xO19 （x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0）. Compared with non-substituted barium ferrite annealing at 1000 ℃, the vibrating sample magnetometer （VSM） measurement manifested that the optimum magnetic properties formation temperature of Al-substituted barium ferrite was 1 100 ℃. The data from X-ray diffractometer （XRD） showed that with increasing x, the lattice constants （a and c） decreased as well as the unit-cell volume Vcell. Magnetic measurement of non-substituted and Al-substituted powders annealed from 900 ℃ to 1 200 ℃ exhibited that the maximum magnetization M （10 kOe）, the remanent magnetization Mr and the coercivity Hc depended strongly on the chemical composition of powder as well as the annealing temperature. When annealing at 1 100 ℃, BaAl0.5Fe11.5O19 of high coercivity Hc （6584 Oe） was produced. Meanwhile, M （10 kOe） and Mr were 42.83 emu/g and 25.65 emu/g, respectively.

Er^(3+)-substituted W-type barium ferrite: preparation and electromagnetic properties

Er3+-substituted W-type barium ferrites Ba1-xErx(Zn0.3Co0.7)2Fe16O27 (x=0.00, 0.05, 0.10, 0.15, 0.20) were synthesized by polymer adsorbent combustion method. Samples were characterized by X-ray diffraction analysis (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and network analyzer to investigate the relationships among Er3+ concentration, crystal structure, surface morphology and electromagnetic properties. All the XRD patterns showed pure phase of W-type barium ferrite when x≤0.15, while the impurity phase of ErFeO3 appeared when x=0.20. The pure W-type barium ferrite showed a hexagonal flake shape. In addition, the microwave electromagnetic properties of samples were analyzed in the frequency range of 2-18 GHz. It was indicated that the electromagnetic properties were significantly improved when Er3+ doping content was 0.10. The reasons were also discussed using electromagnetic theory. The optimized ferrite exhibited excellent microwave absorption performance. The maximum of reflection loss (RL) reached about-27.4 dB and RL was below-10 dB at the frequency range from 8.4 GHz to 18 GHz, when the thickness was 2.6 mm.

Magnetic properties and special morphology of barium ferrite via electrospinning

Barium ferrite micro-/nanofibers with special morphology,nanowires with diameters of 100 nm,nanoribbons with diameters of 1μm,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·m^2·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）.

A neutron diffraction investigation of high valent doped barium ferrite with wideband tunable microwave absorption

The barium ferrite BaTi_(x)Fe_(12−x)O_(19)(x=0.2,0.4,0.6,0.8)(BFTO-x)ceramics doped by Ti4+were synthesized by a modified sol–gel method.The crystal structure and magnetic structure of the samples were determined by neutron diffraction,and confirm that the BFTO-x ceramics were high quality single phase with sheet microstructure.With x increasing from 0.2 to 0.8,the saturation magnetization(M_(s))decreases gradually but the change trend of coercivity(H_(c))is complex under the synergy of the changed grain size and the magnetic crystal anisotropy field.Relying on the high valence of Ti^(4+),double resonance peaks are obtained in the curves of the imaginary part of magnetic conductivity(μ′′)and the resonance peaks could move toward the low frequency with the increase of x,which facilitate the samples perform an excellent wideband modulation microwave absorption property.In the x=0.2 sample,the maximum reflection loss(RL)can reach−44.9 dB at the thickness of only 1.8 mm,and the bandwidth could reach 5.28 GHz at 2 mm when RL is less than−10 dB.All the BFTO-x ceramics show excellent frequency modulation ability varying from 18(x=0.8)to 4 GHz(x=0.4),which covers 81%of the investigated frequency in microwave absorption field.This work not only implements the tunable of electromagnetic parameters but also broadens the application of high-performance microwave absorption devices.

Highly enhanced microwave absorption for carbon nanotube/barium ferrite composite with ultra-low carbon nanotube loading

Barium ferrite(BaFe_(12)O_(19))is considered as potential microwave absorption(MA)material thanks to the large saturation magnetization,high Curie temperature,and excellent chemical stability.The integration of carbon nanotube(CNT)can improve the dielectric loss of BaFe_(12)O_(19)for further enhanced MA perfor-mance,nevertheless,the MA performance is still not desirable because of the poor CNT dispersion in the CNT/BaFe_(12)O_(19)composites,which usually prepared via the ball-milling method,unless high CNT loading was used.Herein,according to the thermal stability of CNT in different atmosphere and the formation mechanism of BaFe_(12)O_(19)from precursor,CNT was introduced in the precursor of BaFe_(12)O_(19)uniformly during auto-ignition process and calcined under different atmosphere.When CNT loading is only 2.0 wt%,the CNT/BaFe_(12)O_(19)composites obtained exhibits a minimum reflection loss(RL_(min))of-43.9 dB and effective bandwidth(with RL<-10 dB)of 3.9 GHz with the thickness of 1.5 mm,which are much supe-rior to-10.2 dB and 2.2 GHz for pure BaFe_(12)O_(19),and-13.6 dB and 2.5 GHz for CNT/BaFe_(12)O_(19)composite prepared by ball-milling method.These results may pave the way to design high-performance BaFe_(12)O_(19)based microwave absorbers.

Microstructure,magnetic properties of hexagonal barium ferrite powder based on calcination temperature and holding time

In this paper,single-phase and fine-grain hexagonal barium ferrite powder was prepared based on the optimal calcination condition.The influence of calcination conditions including temperature and holding time on microstructure and magnetic properties of powder were studied in detail.Firstly,θ-2θ scan X-ray diffraction(XRD) results reveal that it is hard to obtain single phase of powder when the calcination temperature is lower than 850℃.In addition,the calcination time for single phase of barium ferrite powder was reduced with the increase in calcination temperature.Scanning electron microscopy(SEM) images and magnetic hysteresis loops show that the condition of low temperature and long holding time is beneficial for obtaining homogeneous size of grain and excellent magnetic properties.Consequently,hexagonal barium ferrite powder with uniform grain size of~180 nm,high purity and excellent magnetic properties is obtained at optimal calcination condition of 850℃-10.0 h.

Fabrication of Z-type barium ferrite/silica composites with enhanced microwave absorption

The Z-type barium ferrite/silica composites(Z-BCF/SiO2)were fabricated by in situ chemical synthesis method.The composition,structure,morphology and magnetic behavior of the composites were characterized by chemical analysis,IR,XRD,SEM,TEM and VSM.The results indicated that there were some interactions between two components in the Z-BCF/SiO2 composites due to the coating of SiO2 on the Z-BCF particles.The magnetic properties of the Z-BCF/SiO2 composites were evidently less than that of the Z-BCF,owing to the small volume fraction of magnetic components in the samples.Due to the good transmission and loss properties on electromagnetic waves,the composites were better at microwave absorption than the parent component.Therefore,this research laid a foundation for the fabrication of highly efficient microwave absorbing material with enhanced wave impedance matching.

Preparation, Characterizations and Magnetic Properties of Doped Barium Hexaferrites BaFe_(12-2x)Mn_xSn_xO_(19) (x = 0.0-1.0)

A series of doped barium hexaferrites BaFe12-2xMnxSnxO19 （x = 0.0-1.0） particles were prepared by the co-precipitation/molten salt method. The particle size and crystalline of the samples BaFe12-2xMnxSnxO19 decrease with an increase in the doping amount x. When x is less than 0.8, the pure BaFe12-2xMnxSnxO19 particles with hexagonal plate morphology are obtained. The effects of substitution on magnetic properties were evaluated and compared to nomal BaFe12O19. The specific magnetizations （Ms） of doped materials have been significantly improved. Among all these compositions, the BaFe10.4Mn0.8Sn0.8O19 sample has the highest Ms value of 81.8 A？m2？kg-1 at room temperature and its intrinsic coercivity （Hc） is 44.5 kA？m-1. The as-prepared doped barium ferrites exhibit a low temperature coefficient of coercivity close to zero. The coercivity is independent of temperature when x is in the a range 0.5-0.7.

Factors of Impacting the Coprecipitation Process for Synthesizing CoTi-substitued Barium M-type Ferrite Ultrafine Powder

The effect of pH values on synthesizing single-phase CoTi-substituted barium M-type ferrite ultrafine powders,and BaCoTiFe- 10O- 19, was investigated employing corrosion versus pH plot (E-pH plot) for metal element, thermodynamic calculation, and co-dump coprecipitation. The pH values of complete coprecipitation of all Fe 3+, Ti 4+, Co 2+ and Ba 2+ cations are 9-12 and higher than 7.9 on the basis of E-pH plot analysis and thermodynamic calculation, respectively. The minimum pH value necessary to the formation of single-phase BaCoTiFe- 10O- 19 is 8.5 in the light of the co-dump coprecipitation.These results indicate that the coprecipitation process for synthesizing CoTi-substituted barium M-type ferrite ultrafine powders is simultaneously influenced by synergetic coprecipation effect of cations and coordination effect of Cl-anions. The test time of the minimum pH value corresponding to forming a series of single-phase CoTi-substituted barium M-type ferrite ultrafine powders,and BaCo-xTi-xFe- 12-2xO- 19, may be significantly reduced by using the effects of two new factors on the coprecipitation process.

Achieving Ultra-Broad Microwave Absorption Bandwidth Around Millimeter-Wave Atmospheric Window Through an Intentional Manipulation on Multi-Magnetic Resonance Behavior

The utilization of electromagnetic waves is rapidly advancing into the millimeter-wave frequency range,posing increasingly severe challenges in terms of electromagnetic pollution prevention and radar stealth.However,existing millimeter-wave absorbers are still inadequate in addressing these issues due to their monotonous magnetic resonance pattern.In this work,rare-earth La^(3+)and non-magnetic Zr^(4+)ions are simultaneously incorporated into M-type barium ferrite(BaM)to intentionally manipulate the multi-magnetic resonance behavior.By leveraging the contrary impact of La^(3+)and Zr^(4+)ions on magnetocrystalline anisotropy field,the restrictive relationship between intensity and frequency of the multi-magnetic resonance is successfully eliminated.The magnetic resonance peak-differentiating and imitating results confirm that significant multi-magnetic resonance phenomenon emerges around 35 GHz due to the reinforced exchange coupling effect between Fe^(3+)and Fe^(2+)ions.Additionally,Mosbauer spectra analysis,first-principle calculations,and least square fitting collectively identify that additional La^(3+)doping leads to a profound rearrangement of Zr^(4+)occupation and thus makes the portion of polarization/conduction loss increase gradually.As a consequence,the La^(3+)-Zr^(4+)co-doped BaM achieves an ultra-broad bandwidth of 12.5+GHz covering from 27.5 to 40+GHz,which holds remarkable potential for millimeter-wave absorbers around the atmospheric window of 35 GHz.

Enhancement of microwave absorption of nanocomposite BaFe_(12)O_(19)/α-Fe microfibers

Nanocomposite BaFe12019/a-Fe microfibers with diameters of about 1-5 μm are prepared by the organic gel- thermal selective reduction process. The binary phase of BaFe12019 and a-Fe is formed after reduction of the precursor BaFel2019/a-Fe203 microfibers at 350 ℃ for 1 h. These nanocomposite microfibers are fabricated from a-Fe （16-22 nm in diameter） and BaFe12019 particles （36--42 nm in diameter） and basically exhibit a single-phase-like magnetization be- havior, with a high saturation magnetization and coercive force arising from the exchange--coupling interactions of soft a-Fe and hard BaFe12019. The microwave absorption characteristics in a 2-18 GHz frequency range of the nanocomposite BaFe12O19/a-Fe microfibers are mainly influenced by their mass ratio of a-Fe/BaFe12019 and specimen thickness. It is found that the nanocomposite BaFelzO19/a-Fe microfibers with a mass ratio of 1：6 and specimen thickness of 2.5 mm show an optimal reflection loss （RL） of -29.7 dB at 13.5 GHz and the bandwidth with RL exceeding -10 dB covers the whole Ku-band （12.4-18.0 GHz）. This enhancement of microwave absorption can be attributed to the heterostruc- ture of soft, nano, conducting a-Fe particles embedded in hard, nano, semiconducting barium ferrite, which improves the dipolar polarization, interfacial polarization, exchange--coupling interaction, and anisotropic energy in the nanocomposite BaFe12O19/a-Fe microfibers.

Effect of Strontium Substitution on Microstructure and Magnetic Properties of Electrospinning BaFe12O19 Nanofibers

Barium ferrite micro/nano fibers were successfully prepared via the electrostatic spinning by using dimethyl formamide（DMF） as the solvent, and poly vinyl pyrrolidone（PVP） as the spinning auxiliaries. Effects of strontium substitution on the structure, morphology, and magnetic properties were investigated by scanning electron microscope（SEM）, X-ray diffraction analysis（XRD）, and vibration sample magnetometer（VSM）. XRD patterns of the samples confirm that pure barium ferrite fibers form, and the Sr substitution makes the main peaks（110）,（107）, and（114） move to right slightly. Also, the FE-SEM images show that the Sr substituted fibers can keep complete fibrous morphology. Moreover, the VSM results demonstrate that the saturation magnetization can reach 56.7 emu/g when the fibers are calcined at 800℃.