Preparation Of Ions Doped Barium Ferrite And Its Microwave Absorption Properties In Millimeter Frequency Range

With the development of modern communication technology,our daily life becomes more convenient.However,it also causes a serious issue called electromagnetic interference(EMI).Moreover,the requirements of anti-radar coating for military equipment get severer due to the improvement of radar system.To solve the EMI problem and meet the demand of military stealth,extensive work has been done to obtain microwave absorbing materials(MAMs)with strong absorption,broad band,light weight,tunable absorption frequency a:nd thin matching thickness.Overall,it is important to study absorbing materials for civil use and military use.Nowadays,the main fire control radars work around the millimeter wave atmospheric window of 35 GHz.The absorption materials aiming at the corresponding frequency range are hence highly needed.M-type barium ferrite BaFe12O19(BFO)has been one of the most potential microwave absorbers because of its large magnetic 1oss from natural resonance.The natural resonance frequency of barium ferrite is-45 GHz,which can be adjusted to lower frequency range by non-1agnetic or weak magnetic ions sLbstituting for Fe3+ ions for decreasing the magnetocrystalline anisotropy.Consequently,the effective absorption frequency range of barium ferrite can be tuned to?35 GHz.However,the singleiresonance and low dielectric loss of the ferrites lead to narrow absorption band and thick matching thickness,restricting their application.To solve above issues,ions and ion combinations with higher valence and larger radii are seltcted to substitute for Fe3+ ions in barium ferrite.On one hand,it will result in part of the Fe3+ ions transforming to Fe2+ ions to keep charge balance.Hence,the exchange coupling between Fe3+ and Fe2+ ions will probably be formed,contributing new Lande factors(g)larger than that of Fe3+(2.00).The existence of multiple g values will lead to multiple magnetic resonance peaks and thus multiple reflection loss peaks,broadening the bandwidth.On the other hand,the ions with larger radii substituting for Fe3+ will cause lattice expansion in barium ferrite.Some oxygen vacancies might be induced to weaken the expansion,increasing the conductivity and thus the permittivity.It will eventually reduce the matching thickness according to quarter-wavelength theory.In this work,Nb5+ doped barium ferrites(BNFO)were synthesized by sol-gel method.Strong RL-54 dB,broad BW 11.9+ GHz and thin dmp1 0.86 mm in the sample with x=0.6 sintered at 1350? for 3 h around the millimeter-wave atmospheric window of 35 GHz.Furthermore,the influence of Nb5+ ions doping on the formation process of the BaFe12O19 phase is also analyzed in this work.The influence mechanism of Nb5+ concentration and heat treatment time for the formation temperature and morphology of barium ferrite is investigated.The connections between material conposition,heat treatment conditions and the EM parameters are established.The controls of the EM parameters and the order of quarter-wave matching thickness on the absorption properties are also successfully revealed by comb ining the impedance matching principle,quarter-wave theory and attenuation mechanism.Zr4+ doped barium frrites powders(BFZJ)were also synthesized by sol-gel method.The substitution sites of Zr4+ ions for Fe3+ ions are analyzed.The effects of the substitution sites on Fe2+ and oxygIn vacancy concentrations aId thus on EM parameters and absorption properties of the barium ferrites are researched.The results show that the Zr4+ ions substitute for Fe3+ ions at 4f1 site below x=0.1 initially and at 2b site above x=0.1 afterwards.Fe2+ ions and oxygen vacancies are generated in the ferrite with Zr4+ ions at 4f1 site.The formed oxygen vacancies are consumed and Fe2+ ions keep stable with Zr4+ ions at 2b site.Ultimately,BW of?10 GHz,RL of?-40 dB and dm of?0.8 mm are obtained around millimeter-wave atmospheric window of 35 GHz in the BFZO with x=0.2.Actually,the substitution amount of Fe3+ ions is limited in the samples with only Zr4+ or Nb5+ ions doping,which restricts the application frequency range of the ferrites with broad bandwidth and thin matching thickness to be around 24?40 GHz.Nb5+-Ni2+ co-doped barium ferrites BaNbxNixFe12-2xO19(x=0?8)were synthesized to broaden the application frequency range.The co-doping ofNb5+ and Ni2+ ions lowers the average valence difference between the doping ions and Fe3+ions,which will increase the substitution amount of Fe3+ ions and thus broaden the application frequency range.The relationship between the substitution amounts of Nb5+ and Ni2+ ions for Fe3+ ions in barium ferrites and the adding amounts of Nb5?and Ni2+ ions were analyzed.The results show that broad BW of 4.5+-9.3 GHz,strong RL of-17.4?-31.8 dB and wide tunable frequency range(<18 GHz->40 GHz)are achieved at thin dm of 0.85?1.45 mm in BaNbxNixFe12-2xO19 simultaneously.On the other hand,considering that the larger Zr4+ ions substituting for Fe3+ will lead to lattice expansion while the smaller Ti4+ ions substituting for Fe3+ ions will lead to lattice contraction The opposite effects to the lattice will cancel out each other and hence more Fe3+ ions probably be replaced,broadening the application frequency range.Hence,Zr4+-Ti4+ co-doped barium ferrites were synthesized by sol-gel method.The effects of Zr4+-Ti4+ content and sintering temperature on the phase structure,morphology,EM parameters and absorption properties of the barium ferrite are discussed.The results show that single-phased barium ferrite phase can be achieved in the samples with x=0?0.4 sintered at 1300??1400? for 3 h.The grain size increases with sintering temperature increasing while decreases initially and increases afterwards with doping amount increasing.The multi-resonance phenomenon enhances with doping content and heat temperature improving,whilst the permittivity enhance with temperature improving and increase firstly and then decrease gradually with Zr4--Ti4+ content increasing.Finally,RL of?50 dB,of?12 GHz andd dm of?1 mm can be attained in the samples with x?0.2 sintered at 1400?.Besides,the effective absorption frequency range can be adjusted rapidy from 40+ GHz to 18-GHz.