Study on Microwave Dielectric Property of Carbon Black and Short Carbon Fibers

Carbon black and carbon fibers of different lengths were introduced in different matrices at different ratios to explore their microwave dielectric properties under 8.2 GHz-12.4 GHz. It is found that the actual dielectric constants of the samples containing carbon black are in a two-order function of the contents of carbon black （ε＇, ε＂=Av2＋Bv＋C） and the complex dielectric constants show an obvious frequency response. Of the added fibers of different lengths, the 4 mm-long one could well disperse in the matrices having not only good frequency response, but also larger real parts, imaginary parts and loss values. The imaginary parts and the loss values （tanδ）of the samples with 4 mm-long carbon fibers added increase linearly with the contents of fiber increasing. So it is practicable to adjust the dielectric parameters of the material in a wide range by changing the added amount of carbon black, and the carbon fiber or altering the lengths of the carbon fiber added.

Preparation and Microwave Absorption Property of Sr_2FeMoO_6 Powder

Double perovskite oxide Sr_2FeMoO_6 powder is prepared by a solid state reaction method.The microwave absorption properties of Sr_2FeMoO_6 and paraffin wax composites are studied in the frequency range of 2—18GHz at room temperature.The optimum absorption-36.7dB is achieved at 17.7GHz with a matching thickness of 5.0mm,which indicates that Sr_2FeMoO_6/paraffin composites can be potential microwave absorbers in a relatively high frequency range.The excellent microwave absorption properties are attributed to the good electromagnetic match between dielectric loss and magnetic loss.The dielectric loss is considered to be caused by orientation polarization and interfacial polarization,while the magnetic loss is caused by natural resonance in the low frequency range,eddy current loss as well as exchange resonance in the high frequency range.

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 microwave hexaferrites by a sol-gel process

s Ba_4Zn_zCo_(2-z)Fe_(36)O_(60) hexaferrites with Z = 2.0, 1.5, 1.0, 0.5, and0.0 were prepared by a citrate Sol-gel process. The complex dielectric constant and complexpermeability of hexaferrite-paraffin wax composites were measured in the range from 0.1 to 6.0 GHz.Measured results showed that both the complex dielectric constant and the dielectric loss exhibitedno significant change as the measuring frequency increasing except for the sample with Z = 0.0. Thereal part of permeability decreased with increasing frequency for all samples in the whole measuringfrequency range, and the natural resonance phenomena were observed in all μ″ spectrum but Co_2U.The results also indicate that the content of zinc closely affects the microwave properties ofBa_4Zn_zCo_(2-z)Fe_(36)O_(60) ferrites, and the greater the zinc content, the lower the naturalresonance frequency is.

STUDY ON PREPARATION, COMPLEX PERMITTIVITY,PERMEABILITY AND MICROWAVE PROPERTIES OF BaZnzCo(2-z)Fe16O27/SiO2 MICROCRYSTALLINE GLASS CERAMICS

BaZnzCo2-zFe16O27/SiO2 microcrystalline glass ceramics with Z=0.0, 0.5, 0.8 and 1.1 were prepared at temperature 1200℃ for 5h by citrate sol-gel process. The complex dielectric constant and complex permeability of BaZnzCo2-zFe16 O27/SiO2-paraffin wax composites had been measured. Both the complex dielectric constant and dielectric loss exhibit almost no variation with frequency in 0.1-6.0GHz. The real part of permeability decreases with increasing frequency for all samples, and the imaginary part of permeability exhibits a clear resonance peak at 1.44GHz for Co2 W/SiO2, 1.32GHz for Zn0.5Co1.5 W/SiO2, 1.20GHz for Zn0.8Co1.2W/SiO2, and 1.08GHz for Zn1.1 Co0.9 W/SiO2. It is also showed that the content of zinc has a close effect on the magnetic properties of BaZnzCo2-zFe16O27/SiO2 microcrystalline glass ceramics, the greater the zinc content, the higher the values of imaginary part of permeability and magnetic loss.

Microwave absorption properties of FeSiBNbCu glass-covered amorphous wires

Coaxially dielectric samples consisting of different packing ratios of glass-covered Fe73.5Si13.5B9Nb3Cu1 amorphous wires embedded in a paraffin wax matrix were fabricated, and the influence of short-wire packing ratio (3%-9% in mass fraction) and thickness (1-7 mm) on the microwave absorption properties was systematically investigated in microwave frequency range of 2-18 GHz. X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and scalar network analyzer (SNA) were used for characterizing microstructure and evaluating microwave absorption properties. Experimental results show the significant frequency (6-18 GHz) dependence of the complex relative permeability and permittivity. The reflection loss (RL) with different thickness and short-wire packing ratio reveals that the composite sample containing 7% exhibits better microwave absorption behavior with its minimum value of RL reaching-34 dB in thickness of 3 mm at 14 GHz. Therefore, it is significantly useful to develop microwire-dielectric materials with much wider absorption band for microwave absorption applications.

Electromagnetic and microwave absorbing properties of FeCoB powder composites

The electromagnetic and microwave absorbing properties of FeCoB powder composites prepared by sin- gle-roller melt-spinning and mechanical milling processes were investigated in this paper. The result indicates that the flake-like powders are obtained. As milling time increases, the flake-like powder particles tend to agglomerate, causing the flake-like powders decrease gradually. The milling time plays an important role in the electromagnetic parameters which relates to the shape and size of the powder particles. The calculation shows that the sample milled for 6 h could achieve an optimal reflection loss of -11.5 dB at 5.8 GHz, with mass fraction of 83 % and a matching thickness of 1.8 mm. The result also indicates that the microwave absorbing properties of the FeCoB powder composites are adjustable by changing their thickness, and can be applied as a thinner microwave absorbing material in the range of 2-8 GHz.

Manufacture of Zn-Co Substituted Y-Type Barium Hexagonal Ferrites by Citrate Precursor Route and Their Microwave Properties

Ba2ZnxCo2-xFe12O22 hexaferrites with x = 0.0, 0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by citrate sol-gel process. The complex dielectric constant and complex permeability of hexaferrites were studied as functions of the measuring frequency, composition and sintering temperature in the range 100 MHz similar to 6 GHz. The natural resonance phenomenon is observed in mu' spectrum for all Y-type hexaferrites, which resonance frequency is dependent on zinc content and firing temperature. The natural resonance frequencies Of Co2Y hexaferrite were calculated, which is in agreement with the measuring results. The reflection loss of those ferrites is measuring frequency as well as absorber thickness dependent, and the higher the frequency and the thickness are, the higher the reflection loss is.

Microstructure and microwave electromagnetic properties of Dy^(3+)-doped W-type hexaferrites

Ba 1 x Dy x Co 2 Fe 16 O 27 （x = 0.00, 0.05, 0.10, 0.15, and 0.20） was prepared by the solid-state method. The phase structure was studied using powder X-ray diffraction （XRD）, the electromagnetic properties were measured, and the reflection loss of Dy 3＋ -doped ferrite material was calculated using electromagnetic parameters by the transmission line theory. All XRD patterns showed the single phase of the magnetoplumbite barium ferrite without other intermediate phase when x ≤ 0.15. The values of ε ′ and ε ″ increased slightly with Dy 3＋ ions doping. The values of μ″ and μ′ were improved with Dy 3＋ doping, exhibiting excellent microwave magnetic performance. The reasons have also been discussed using the electromagnetic theory. Dy substitution could increase microwave-absorbing performance and broaden frequency band （reflection loss （RL） -10 dB）, and the absorbing peak shifted to high-frequency position. When x = 0.2, ferrite layer exhibited the most excellent microwave-absorbing performance at a thin matching thickness of 1.5 mm. The peak value of RL was around -15 dB, and the frequency band （RL -10 dB） was about 7 GHz （from 8 to 15 GHz）.

Complex permittivity, permeability, and microwave absorption of barium ferrite by citrate sol-gel process

BaFe12O19 hexaferrites were prepared by citrate sol-gel process. The formation temperature was much lower than that of the conventional ceramic method. The complex dielectric constant and complex permeability of hexaferrite-paraffin wax composites had been measured by the transmission/reflection coaxial line method in the range from 200 MHz to 6 GHz. The measurement results showed that both the complex dielectric constant and dielectric loss exhibited no significant variation with the measuring frequency increase. It also showed that the real part of permeability decreased as the frequency increasing for all samples, the resonance phenomena due to domain wall resonance was observed in ?' spectra, the calcining temperature strongly affects the microwave properties of BaFe12O19 ferrites, the higher the calcining temperature, the lower the resonance frequency.

Low-temperature sintering and microwave dielectric properties of Li_2MgTi_3O_8 ceramics doped with BaCu(B_2O_5)

The influences of BaCu（B2O5） （BCB） addition on sintering, microstructure and microwave dielectric properties of Li2MgTi308 ceramics were investigated using X-ray diffractometry, scanning electron microscopy and microwave dielectric measurements. The experimental results show that a small amount of BaCu（B2O5） addition can effectively reduce the sintering temperature to 900℃, and induce only a limited degradation of the microwave dielectric properties. Typically, the best microwave dielectric properties of er24.5, Q×f =24 622 GHz, rf=4.2×10-6℃ -1 are obtained for 1.0% BCB-doped Li2MgTi3O8 ceramics sintered at 900℃ for 3 h. The BCB-doped Li2MgTi3O8 ceramics can be compatible with Ag electrode, which may be a strong candidate for low temperature co-fired ceramics applications.

Synthesis of flowerlike nickel particles and their microwave absorbing properties

Two kinds of nickel particles with flower-like struc~＇es assembled with a number of nano-flakes were synthesized and the relationship of their morphology and microwave absorbing properties was studied. The electromagnetic parameters of these flower-like Ni were measured with vector network analyzer at 2-18 GHz frequency and the reflection losses （RL） with different sample thicknesses were calculated. The results indicate that the flower-like nickel-wax composites with the sample thickness less than 2 mm show excellent absorbing ability. This result is expected to play a guiding role in the preparation of the highly efficient absorber.

Crystal microstructure, infrared absorption, and microwave electromagnetic properties of (La_(1-x)Dy_x)_(2/3)Sr_(1/3)MnO_3

The manganite perovskite polycrystal samples of （La1-xDyx）2/3Sr1/3MnO3 （x = 0, 0.1, 0.2, 0.35, and 0.5） doped with Dy were prepared by solid state reaction in atmosphere to measure their X-ray diffraction （XRD） patterns, scanning electric microscope （SEM） images, infrared absorption spectra, and microwave electromagnetic properties. The displacement of the XRD peaks of the samples was found, and the 2θ increases from 0.05o to 0.5o. The grains of undoped La2/3Sr1/3MnO3 not only have the greatest size, but also the most regular shape. The size of the grains decreases as the Dy doping content increases from 0 to 0.5. The infrared absorption spectra of all samples were measured at room temperature. An absorption peak corresponding to the stretching vibration mode of Mn-O bonds appears within the range of 591-629 cm-1. The absorption peak shifts from a higher frequency to a lower one with the decrease of the average ionic radius of A-site. The frequency de- pendence of microwave-absorbing properties, imaginary components of the complex magnetic permeability μ＂ and dielectric permeability ε＂ for all samples was measured at room temperature from 8 to 13 GHz. The results show that the loss of microwave absorption can be attrib- uted to both the magnetic and electric losses. The increase of Dy content not only enhances the microwave absorption but also causes the displacement of the absorption peaks.

Microstructure and microwave dielectric properties of ZnO-B_2O_3 added (Ca_(0.254)Li_(0.19)Sm_(0.14))TiO_3 ceramics

The effects of ZnO-B2O3 （ZB2） on the sintering behavior and microwave dielectric properties of （Ca0.254Li0.19Sm0.14）TiO3 ceramics were investigated. The densities of the specimens reached the maximum value by adding 3 wt.% ZB2 and then decreased. The sintering temperature of the specimens was lowered from 1300 to 1100℃ without degradation of the microwave dielectric properties. The （Ca0.254Li0.19Sm0.14）TiO3＋ 3 wt.% ZB2 sintered at 1100℃ for 3 h showed good microwave dielectric properties, εr= 108.2, Qf= 6545 GHz, and rf= 6.5 ppm/℃, respectively, indicating that ZB2 was an effective sintering aid to improve the densification and microwave dielectric properties of （Ca0.254Li0.19Sm0.14）TiO3 ceramics.

Microwave Absorbing Performance of RE-Fe Based Alloys

REI3Fes4Cr3（RE=Ce, Pr, Tb, Er） and Pr13_XFes4Cr3Ti（x=0, 2, 4, 6） alloy powders were prepared by are smelting method and high energy ball milling technique. The phase structure and the morphology of the alloy powders were investigated by X-ray diffraction （XRD） and scanning electron microscopy （SEM）, and their microwave absorbing properties were determined by a vector network analyzer. The results show that the alloys with light rare earths （Ce, Pr） have good low frequency absorbing property and those with heavy rare earths （Tb, Er） exhibit an improved high frequency absorbing property. The minimum refleetivity at the absorbing peak frequency of RE,FeuCr3（RE=Ce, Pr, Tb, Er） are -9.49 dB at 5.76 GHz, -22.38 dB at 7.92 GHz, -18.52 dB at 11.68 GHz and -17.59 dB at 10.24 GHz, respectively. The absorbing bandwidth under -10 dB of the Pr13FesaCr3 powder was widened from 1.91 GI-Iz to 3.89 GHz by adding 2% Ti, but the reflectivity of the alloy was increased from -22.38 dB to -14.91 riB.

Effect of Rare Earth Element on Microwave Absorption Properties of Nano-Lithium Ferrite

Superfine powders of nano-lithium ferrite doped with different kinds and amount of rare earth element were prepared by sol-gel method. Their photograph was taken by transmission electron microscopy. From it, we can see most of particles are less than 100 nm and average diameter of superfine powders is 50 nm. Then their microwave absorption properties are measured by power ratio method. The result indicates that microwave absorption properties of nano-lithium ferrite doped with rare earth element change obviously. Different kinds and amount of rare earth element make different influences. Magnetic hysteresis loop is surveyed by vibrating sample magnetometer. We find that there are some relationships between magnetism and microwave absorption properties.

Influence of Er^(3+) Substitution on Microwave Magnetic Performance of BaFe_(12)O_(19)

Barium hexaferrites doped with Er3+, Ba1-xErxFe12O19, x=0.0, 0.01, 0.02, 0.04, 0.06, were prepared by the conventional ceramic technology. The structure and electromagnetic properties of the calcined samples were studied using powder X-ray diffraction (XRD) and network analyzer (Agilent 8722ET). All the XRD patterns showed the single phase of the magnetoplumbite barium ferrite without other intermediate phase when x is below 0.02. The lattice parameters of ferrites doped with Er3+ decreased, indicating that the substitution of Er3+ occurs on Ba2+ basis site and results in a contract of the crystal cell. The microwave electromagnetic properties of the samples have been studied at the frequency range from 2 to 18 GHz. It was shown that ε′ increased slightly, and the maximum of ε″ appeared at low frequency position with Er3+ doping. μ″ and μ′ were improved significantly when x=0.01, and the maxima are 2 and 1.7, respectively. The reasons were also discussed using electromagnetic theory.

Mossbauer spectroscopy studies on the particle size distribution effect of Fe–B–P amorphous alloy on the microwave absorption properties

An Fe-based nanocrystalline alloy powder is important for application in microwave absorption,and the particle size has a critical impact on the electromagnetic microwave parameters.Therefore,it is necessary to study further the effects of the particle size on such parameters and improve the microwave absorption performance of Febased nanocrystalline powers.In this study,Fe-B-P particles were prepared through a synthetic approach consisting of an aqueous chemical reduction and a ball milling treatment.We investigated the effects of ball milling on the microstructure and electromagnetic properties of Fe-B-P particles.The experimental results indicate that the Fe-B-P particles synthesized through an aqueous chemical reduction are amorphous spheres.Fe-B-P particles with an original particle size of 200-1200 nm can be milled into an irregular shape with the size reduced to\500 nm after 0.5 h of ball milling,and subsequently,the particles become smaller with increases in the milling time,with traces of Fe2O3 generated on the particle surface.The results of the Mo¨ssbauer spectra show that a portion of the small particles demonstrate a superparamagnetic property.The volume proportions of the superparamagnetic component increase from 13.1 to 15.8%as the treatment time increases.We measured the permittivity and permeability spectra of Fe-B-P particles within the frequency range of 2-18 GHz.The reflection loss(RL)is-10 dB for an absorber thickness of 1.7-5.0 mm.The RL is-20 dB for an absorber thickness of 1.9-2.7 mm.The microwave absorption properties of samples with the same thickness are improved with an increase in the treatment time and are shifted to a higher frequency,which will broaden the bandwidth of the absorption as well.

Changes in sulfur form during coal desulfurization with microwave:Effect on coal properties

The changes in sulfur form in coal were analyzed by sulfur K-XANES(K-edge X-ray absorption near edge structures) spectra before and after the coal microwave desulfurization in a Na OH solution. After the desulfurization, the pyritic sulfur content of coal decreased significantly from 53.6% to 39.2%, while the sulfate sulfur content increased from 17.3% to 34.6%. Only a small amount of thiophene sulfur(20.1–16.1%) was removed. Some sulfur-containing components were oxidized to sulfate sulfur. Under the optimum conditions, the ash content decreased, while the volatile content increased. The calorific value of coal slightly decreased with a slight variation in the amplitudes. The overall structure of coal did not change significantly based on Fourier transform infrared(FTIR) spectral analyses. Thus, the desulfurization of coal with microwave irradiation in a Na OH solution did not significantly change the properties of coal.

Influence of CaxSr1-x(0≤x≤1)Substitution for Zn on Microwave Dielectric Properties of Li2ZnTi3O8 Ceramic as Temperature Stable Materials

A temperature stable Li2Zn0.95(SrxCa1-x)0.05Ti3O8(0≤x≤1)ceramics were fabricated using a conventional solid-state route sintered at 1100℃for 4 h.The XRD results indicate that the main phase Li2ZnTi3O8 and secondary phase including SrxCa1-xTiO3(0≤x≤1)solid solution and TiO2 co-exist in composite and form a stable composite system when the(CaxSr1-x)(0≤x≤1)substitutes for Zn of Li2ZnTi3O8 ceramic.As x is increased from 0 to 1,the relative permittivity(εr)increases from 26.65 to 27.12,and the quality factor(Q×f)increases from 63300 to 66600 GHz.With the increased of x,the temperature coefficient of resonant frequency(τf)increases from 0.27 to 8.23 ppm/℃,and then decreases to 3.51 ppm/℃.On the whole,the Li2Zn0.95(SrxCa1-x)0.05Ti3O8(0≤x≤1)ceramics show excellent comprehensive properties of middleεr=25-27,higher Q×f≥60000 GHz andτf≤±8.5 ppm/℃.