简介概要

Preparation and microwave absorption properties of Ni（Co/Zn/Cu）Fe₂O₄/SiC@SiO₂ composites

来源期刊：Rare Metals2019年第1期

论文作者：Peng Wang Ping-An Liu Sheng Ye

文章页码：59 - 63

摘要：Ni（Co/Zn/Cu）Fe₂O₄/SiC@SiO₂, a microwave absorber, was prepared by the sol-gel method. The phase structure and the morphology of the microwave absorbers were characterized by X-Ray Diffraction（XRD） and scanning electron microscopy（SEM）, respectively. Laser sizer（LS） and X-ray photoelectron spectroscopy（XPS） analysis show the core-shell structure of SiC@SiO₂. Coaxial method was used to measure the microwave absorption properties of the prepared composites in the frequency range of 2-18 GHz. When 70 wt% SiC is wrapped by 30 wt% SiO₂,and 50 wt% NiFe₂O₄ is added into 50 wt% SiC@SiO₂, the as-prepared powders are found to have advanced microwave absorption properties with a minimum reflection loss（RL） of -32.26 dB at about 6.08 GHz, and the available bandwidth is approximately 2.1 GHz when the RL is below -10 dB.

稀有金属(英文版) 2019,38(01),59-63

Preparation and microwave absorption properties of Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂ composites

Peng Wang Ping-An Liu Sheng Ye

School of Materials Science and Engineering, South China University of Technology

作者简介：*Ping-An Liu e-mail:paliu@scut.edu.cn;

收稿日期：21 April 2015

基金：financially supported by the Military Project of the Ministry of National Defense of China (No. JPPT-125-2-168);

Preparation and microwave absorption properties of Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂ composites

Peng Wang Ping-An Liu Sheng Ye

School of Materials Science and Engineering, South China University of Technology

Abstract：

Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂, a microwave absorber, was prepared by the sol-gel method. The phase structure and the morphology of the microwave absorbers were characterized by X-Ray Diffraction(XRD) and scanning electron microscopy(SEM), respectively. Laser sizer(LS) and X-ray photoelectron spectroscopy(XPS) analysis show the core-shell structure of SiC@SiO₂. Coaxial method was used to measure the microwave absorption properties of the prepared composites in the frequency range of 2-18 GHz. When 70 wt% SiC is wrapped by 30 wt% SiO₂,and 50 wt% NiFe₂O₄ is added into 50 wt% SiC@SiO₂, the as-prepared powders are found to have advanced microwave absorption properties with a minimum reflection loss(RL) of -32.26 dB at about 6.08 GHz, and the available bandwidth is approximately 2.1 GHz when the RL is below -10 dB.

Keyword：

Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂; Microwave absorption property; Sol-gel method; Core-shell structure;

Received： 21 April 2015

1 Introduction

As one of the main electronic countermeasures to make aircrafts,missiles and other military hardware less detectable,considerable attention has been paid to the stealth technology [ 1, 2] .Silicon carbide (SiC) has been considered as a promising candidate for microwave absorbing material because of its high breakdown field strength,high saturated carrier drift velocity and relatively good microwave absorbing property.However,SiC does not have good microwave absorbing property due to its high relative complex permittivity compared with that of air,which leads to the impedance mismatch between the materials and the air [ 3, 4] .Nowadays,the focus is shifted to SiC composites [ 5, 6, 7] ,surface modification of SiC [ 4, 8] and the special structures of SiC [ 9, 10] ,just like the interpenetrating structure composed of an microwave transparent phase and a microwave absorber [ 11] .On the one hand,ferrite is widely used as microwave absorbing materials because of their features related to low cost,design flexibility and microwave properties.However,the application of ferrite is limited [ 12, 13, 14, 15, 16] .Limited work was reported on the synthesis of the composites about SiC and spinel ferrite,which might meet the wide requirement for microwave absorbing applications.The aim of this article is partly to develop new radar absorbing structures (RASs) to minimize the reflected wave [ 17, 18] ,especially the core-shell structure.Then,the dielectric loss of SiC and the hysteresis loss of spinel ferrite were applied to the composites prepared by a sol-gel method.

2 Experimental

2.1 Sample preparation

Mixtures were obtained by mixing the commercially available silicon carbide (SiC,1.5 g,mean particle size of about8μm) with deionized water and ethanol,and analyticalgrade ethyl silicate (C₈H₂₀O₄Si,5.6 ml) was subsequently added into the solution,whose pH was adjusted to about 2.08by hydrochloric acid.Then,the ethanol was evaporated at60C.After that the dry 70wt%SiC@30wt%SiO₂ powders were synthesized through heating the dry gel at 400℃for2 h,and the 70wt%SiC@30wt%SiO₂ was selected because it exhibited the most excellent microwave absorbing property among other composites with different weight ratios obtained by experiments.

Stoichiometric amou nts of ferric nitrate (Fe(NO₃)3.9H₂O,98.5%) and nickel nitrate (Ni(NO₃)2.6H₂O,99.0%) were weighed and dissolved in deionized water.The solution and the as-prepared SiC@SiO₂ with different weight ratios were fully mixed.After stirring for 10 min,analytical-grade citric acid (C₆H₈O₇·H₂O) was blended into the solution to make a colloidal suspension.Ammonia solution was added to adjust the pH to about 7.05.Fin ally,the raw materials,cobalt nitrate(Co(NO₃)₂·6H₂O,99%),copper nitrate (Cu(NO₃)₂·3H₂O,99%) or zinc nitrate (Zn(NO₃)2·6H₂O,99%) were used,respectively,to obtain a series of absorbers after heating at800℃for 2 h with a heating rate of 15℃.min^-1 under a Ar atmosphere of 0.5 MPa.

2.2 Measurements of properties

The crystalline phase of the samples was observed by X-ray diffractometer (XRD,XPert PRO,PANalytical Corporation,Netherlands) using Cu Kαradiation (λ=0.15418 nm) as the source,and the scanned angles were ranged from 5°to 90°.The surface morphologies,particle size and structure of the absorbers were measured by scanning electron microscopy (SEM,Nova NanoSEM 430,FEI,America),Laser sizer (LS,BT-9300S,China) and X-ray photoelectron spectroscopy (XPS,PHI X-tool,Al Kα,15 kV,Japan),respectively.The electromagnetic parameters of microwave absorbers were investigated with coaxial method using a vector network analyzer(AV3629D,the Forty-First Research Institute,China),and the frequency range was 2-18 GHz.For the samples to be tested,they were prepared by mixing the powders with the paraffin(weight ratio of 1:4).In this study,the reflection loss (RL) of a sample was calculated according to the following equations [ 19] to predict the microwave absorption properties of samples:

where z_in is the normalized input impedance of microwave absorption layer,j is the imaginary unit,c is the velocity of light,.f is frequency.μ_r is relative complex permeability,ε_ris relative complex permittivity,and d (d=3.0 mm) is the thickness of an absorber.

3 Results and discussion

3.1 Phase composition and structures of SiC@SiO2

XRD patterns of the raw SiC and the as-prepared composites are revealed in Fig.1.The presence of strong peaks from 2θ=30°to 80°illustrates that the main phase among this composite is in the form ofα-SiC and their crystal system is hexagonal.The appearance of a broad peak around 2θ=23°stands for the SiO₂ phase,and it is amorphous.

Fig.1 XRD patterns of synthesized SiC@SiO₂ powders and raw SiC powders

3.2 Characteristics of SiC@SiO2

Figure 2 shows the morphological characteristics of the raw SiC powders and the prepared 70wt%SiC@30wt%-SiO₂ composite.It can be seen that the raw SiC particles are angular distinctly (Fig.2a).However,the synthesized SiC@SiO₂ powders do not have sharp edges and corners;thus,obviously SiC powders have their surfaces coated to form new structures,which are confirmed by the size distributions in Figs.3 and 4.As presented in Fig.3a,b,the mean particle diameter of the raw SiC powders and the SiO₂ powders prepared via sol-gel method is 8.589 and11.94μm,respectively.However,the average particle diameter of the manufactured 70wt%SiC@30wt%SiO₂composites is 21.28μm (Fig.3c).Therefore,the surface of the SiC must be covered with a new kind of material.

High-resolution XPS spectra of sample deposited on carbon tape were recorded to prove the core-shell structure of prepared SiC@SiO₂.The peaks at 102.7,287.8 and531.7 eV in the survey scan are attributed to the characteristic doublets of Si,C and O elements,respectively(Fig.4a).Three points are chosen on the sample,and after a quantitative analysis of XPS spectra (Fig.4a-c),the surface contents of Si,C and O are listed in Table 1.Apparently,the O content on SiC@SiO₂ is much higher than that on SiC@SiO₂ in 2000 nm depth,and C content increases sharply from the surface to depth in 2000 nm(Table 1).This result is applicable to the values obtained at Points 1,2 and 3,which further proves that SiC particles covered by SiO₂ are core-shell structure.This conclusion is consistent with the data in Figs.2 and 3.

Fig.2 SEM images of a SiC and b 70wt%SiC@30%wt%SiO₂ composite

Fig.3 Size difference (diff) distribution of a raw SiC,b SiO₂ made by sol-gel method and c 70wt%SiC@30%wt%SiO₂ composite

Fig.4 XPS spectra of a surface,b C 1s and c O 1s of 70wt%SiC@30%wt%SiO₂ composite

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Table 1 Si,C,O contents of Points 1-3 in Fig.4 on surface and in2000 nm depth (at%)

3.3 Analysis of Ni(Co/Zn/Cu)Fe2O4/SiC@SiO2composites

XRD patterns of Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂ composites are illustrated in Fig.5.Characteristic peaks from20=33°to 75°in the spectra indicate the successful synthesis of NiFe₂O₄ and the existence of SiC,in accordance with the powder diffraction card database JCPDS54-964 and JCPDS1-74-1302.The diffraction peaks at 26.6°,25.1°and 31.7°represent SiO₂ and by-product Fe₂SiO₄(JCPDS 1-71-1667),respectively.However,Fe₂SiO₄ peaks disappear in XRD pattern when ferrite content is becoming fewer,and SiO₂ is hard to be detected because of its slight addition.

Fig.5 XRD patterns of synthesized NiFe₂O₄/SiC@SiO₂ powders with different ferrite contents

The dielectric loss (tan -,whereε'is the real partof complex permittivity andε"is the imaginary part ofcomplex permittivity) [ 20] ,magnetic loss (tan ,whereμ'is the real part of complex permeability andμ"is the imaginary part of complex permeability) [ 21] and reflectivity affect the absorbing property of a material.They represent the microwave attenuation property and the impedance match characteristics of materials,respectively.The higher the tanδ(tanδ_m and tanδ_e) is,the better the microwave attenuation property is.Figure 6 shows the variation of tanσas a function of ferrite contents.As shown in Fig.6a,b,the magnetic loss increases with the increase of amount of ferrite.The absorbing mechanism of ferrite is mainly magnetic loss,including domain wall resonance,natural resonance and hysteresis loss [ 22] ,while that of SiC@SiO₂ is mainly dielectric loss.Furthermore,magnetic loss of the NiFe₂O₄/SiC@SiO₂ arises with the frequency increasing from 2 to 18 GHz except for NiFe₂O₄/SiC@-SiO₂,while the dielectric loss is totally opposite (Fig.6a,b).So dielectric loss plays the most important role in the absorbing property of the synthesized powders at low frequencies,and magnetic loss is the main cause at high frequencies.Because of the difference in complex permittivity between SiC@SiO₂ and NiFe₂O₄ ferrite,interface scattering may be generated,thus leading to more microwave attenuation.

Fig.7 Reflectivity of raw SiC and synthesized NiFe₂O₄/SiC@SiO₂powders with various ferrite contents

Curves of the NiFe₂O₄/SiC@SiO₂ with different ferrite contents are shown in Fig.7 (d=3 mm).Compared with the pure SiC and the inpidual SiC@SiO₂,the sample with50 wt%NiFe₂O₄ exhibits the most excellent microwave absorption properties.The minimum reflectivity reaches around-32.26 dB at about 6.08 GHz when 50 wt%NiFe₂O₄ is contained in the composites,and the effective absorption frequency ranges from 5.3 to 7.4 GHz(RL<-10 dB).Microwave absorption peaks shift toward the low frequency regions as the content of the ferrite increases from 0 wt%to 100 wt%.

Figure 8 reveals the RL of each sample as a function of frequency.It is obviously observed that as the tape of elements doped in the ferrite changes,the RL value of samples alters in lower frequency range of 5-11 GHz.The maximum RL value of NiFe₂O₄/SiC@SiO₂ is about-32.26 dB at 6.08 GHz,and CuFe₂O₄/SiC@SiO₂ and ZnFe₂O₄/SiC@SiO₂ have the minimum RL values of-4.3 dB at 8.7 GHz and-12.9 dB at 8.5 GHz,respectively.It indicates that different kinds of ferrite can shift the matching frequency and alter the minimum RL.NiFe₂O₄/SiC@SiO₂ powders have a better absorbing property than others,whereas the optimal content of CoFe₂O₄,CuFe₂O₄ and ZnFe₂O₄ will be studied in a further research.

Fig.6 Hysteresis losses a and dielectric losses b of synthesized NiFe₂O₄/SiC@SiO₂ with various ferrite contents

Fig.8 Reflectivity of synthesized composites with various element types doped in ferrite (50 wt%SiC@SiO₂)

4 Conclusion

The effect of weight ratio between spinel ferrite and SiC@SiO₂ was studied,and the microwave absorption properties of the Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂ composites with different element types doped in the ferrite were investigated.Core-shell structural 70wt%SiC@30wt%-SiO₂ particles are obtained through a sol-gel process,and their mean particle diameter is 21.28μm.Factors determining the variation of the reflectivity of the NiFe₂O₄/SiC@SiO₂ composites were investigated.As expected,among all the Ni(Co/Zn/Cu)Fe₂O₄/SiC@SiO₂ composites,NiFe₂O₄/SiC@SiO₂ with 50 wt%NiFe₂O₄ exhibits the most excellent microwave absorption properties in the low frequency region because of the synergistic effect between sol phases,and the minimum RL value of the NiFe₂O₄/SiC@SiO₂ is-32.26 dB at 6.08 GHz with the thickness of3 mm,which is better than those of the pure SiC and the70wt%SiC@30wt%SiO₂.