Electromagnetic Atteunation And Microwave Absorbing Property Of Cobalt-ferrite Based Absorber

It is widely believed that spinel transition metal oxides show matched impedance matching and attenuation electromagnetic wave ability,thus have been used as high-performance electromagnetic absorber to treat the serious electromagnetic wave interference?EMI?issue.Relying on ferrite absorber,it can achieve a desirable effective frequency bandwidth?f _E?while the absorption layer thickness usually larger than 2.0 mm.The optimal way to reduce the thickness in a commercial region?<2.0 mm?is to boost the dielectric and magnetic loss ability,simultaneously.In this paper,one step-by-step method has been applied to optimize and boost the attenuation ability of ferrite for an improvement of absorption properties in a thick thickness.The main contents are as follows:?1?As we know,conductive loss is the primarily dielectric loss form for ferrite,which is highly relatedto its conductivity.In general,ferrite presenting spinel structure,can be described by a formula of X_x²⁺Y³⁺_1-x[X²⁺_1-xY³⁺_1+x]O₄ which X,Y denoted as the cation ions in tetrahedral or octahedral site. It has been confirmed that different types of cation ions distributing in octahedral site will lead to a changeable conductivity.So,the conductive loss mechanism has been well studied,using Fe_xNi_{1- x}CoFe₂O₄?x=0-1?as an example to reveal the liner relationship among conductivity,conductive loss,effective absorption frequency width.Different from ferrite,the attenuation electromagnetic ability of Co based spinel oxide is only resulted from conductive loss since its nonmagnetic or weak magnetic capability.By various characterization methods,including synchrotron radiation, Mossbauer spectrum and X-ray photoelectron technique,the detailed cation irons types,molars in octahedral site at various x values can be known.In this regard,it can be deduced that the superexchange interaction effect between the adjacent Fe³⁺and Co²⁺plays a positive role on the conductivity which is attributed to their spin states.The results reveal that more Fe³⁺/Co²⁺adjacent cation ion pairs will lead to bigger conductivity values,conductivity loss intensity and effective frequency absorption width.?2?Subsequently,the influences of annealing temperature on the conductivity have been well studied by treating the Fe_0.8Ni_0.2Co₂O₄ at 400-700 ^oC.Then,these Fe_0.8Ni_0.2Co₂O₄ samples were measured by synchrotron radiation.By fitting the K-edge curves of these cation ions,it can be known that the coordination numbers of Fe,Co,Ni ions in these Fe_0.8Ni_0.2Co₂O₄ presents little change.However, average valances of these cation ions after treated at various temperatures are changeable.Thus, the molar numbers of key Fe³⁺and Co²⁺ions are also variation.Finally,it leads to decrease or increase of Fe³⁺/Co²⁺adjacent cation ion pairs.Thus,the conductivity,effective absorption frequency width of samples are changeable.Besides,the effect of anion on the conductivity has been discussed by comparting Fe_0.8Ni_0.2Co₂O₄ and Fe_0.8Ni_0.2Co₂S₄.The results agree well with our proposed that the electron transmission between Fe³⁺and Co²⁺boosts the conductivity and effective frequency absorption width.?3?According to above findings,the Co_xNi_1-xFe₂O₄ ferrite has been developed.when the x fixed at 1.0, the sample shows the biggest conductivity values and effective frequency absorption width.At 1.5 mm,the f_E value is close to 3.9 GHz.To further optimum the f_E value,the optimal sample were then treated at 400-700 ^oC.The f_E value of the sample treated at 500 ^oC is 1.25 times bigger than CoFe₂O₄ witout annealing treatment.?4?At this stage,the optimal ferrte?CoFe₂O₄?achieves desirable conductive loss ability.However, the conductive loss just one form of dielectric loss.Besides,the polarization as another dielectric loss also attributes to the absorption properties.In general,interface polarization is the primarily higher frequency polarization.Design interface and increase the contact area is the effective strategy to boost the interface polarization intensity.So,the core-shell structured CoFe₂O₄@C composite has been developed.Besides,anther control experiments also have been done by loading CoFe₂O₄ quantum dots on carbon sphere,or preparation of film-like ferrite film on graphene.The interface polarization effect can be observed in the loss peak form from the dielectric loss curve. Meanwhile,the peak intensity is stronger with larger contact area.In this part,the Fe value can be increased to 5.9 GHz.?5?In the former section,much progress has been focused on the stronger the dielectric loss ability of ferrite by the synergy effect of conductive loss and polarization.But the magnetic loss ability of ferrite and ferrite composite is weakly.So,in order to continue to boost the attenuation ability,the in-situ growth method has been applied to grow the ferrite on carbonyl iron flakes?CIF?.CIF/ferrites present boosted magnetic loss ability,as compared to ferrite.At the same time,this sample also shows stronger conductive loss and interface polarization intensity.The results revelated that the CIF/CoFe₂O₄ owns the largest absorption properties which the f_E value at 1.5 mm equals to 6.7GHz?...