| The application of electromagnetic wave in the military and its products, such as electromagnetic interference and electromagnetic pollution, make absorbing material as the hot pot of various countries’research. The discovering for absorbing material with high strength and broad bandwidth is very significant in both military and civil field. M-type barium ferrite with larger magneto-crystalline anisotropy field, high saturation magnetization, big coercive force, high resistivity and good chemical stability, has the greatest potential in high frequency microwave applications.The electromagnetic waves with the frequency of26.5-40GHz have been widely applied in military radar detection, due to its all-weather characteristic of centimeter wave and high resolution imaging characteristic of millimeter wave simultaneously. At high frequency, ferrite mainly absorbs electromagnetic wave by natural resonance. The frequency of natural resonance of barium ferrite which is higher than40GHz can be red shifted to26.5-40GHz hopefully by ion doping. To keep charge balance, the substitution from cations with higher valent to Fe3+, will result in the change from Fe3+to Fe2+. Superexchange coupling effect existing in between Fe3+and Fe2+the precession frequency of whose coupling magnetic moment is higher than that of Fe3+, can lead to the double resonance mechanism and then the double absorbing peak. The aim of absorbing in broad band can be reached by the superposition of double absorbing peak.In this paper, barium ferrite doped by Ti BaFe12-xTixO19(BFTO) was prepared by citrate sol-gel at different annealing temperature. The effect from annealing temperature and the content of Ti to crystallization, electromagnetic properties and then broadband absorption characteristics was studied. The conclusion is presented as follow:Being doped with Ti, the additional phase α-Fe2O3is quite easy to form in the BFTO powders. This process will generate iron vacancies in the barium ferrite, leading to a suppression on the transformation of Fe3+into Fe2+. Therefore, an elevated sintering temperature is needed to remove the α-Fe2O3phase from the system. Single-phased BFTO can be obtained at1100℃、1200℃and1300℃However, excessive introduction of Ti will induce the formation of Ba6Fe45Ti17O106phase, which is not favorable for the comprehensive functionality of materials. Thus, the combinational effect of sintering temperature and Ti content should both be considered. The anisotropy field in the BFTO powders can be significantly reduced when Ti ions are introduced into the2b lattice positions in barium ferrite. The magnetic loss of BFTO between26.5-40GHz are mainly originated from natural resonance. The doping of Ti ions can reduce the anisotropy field and result in a decreased resonance frequency of the material. Meanwhile, The introduction of Ti impels the Fe3+ions to transform into Fe2+ions, and this process generates superexchange coupling resonance structure between Fe3+and Fe2+in the BFTO samples. The particular coupling effect contributes a Lande factor which is different with that of Fe3+. Consequently, dual resonance magnetic loss mechanism is expected in the material. The superposition of such dual resonance loss peaks results in a broader magnetic loss range. The reflection loss absorbing peaks in the BFTO powders are induced by natural resonance, and the dual absorbing peaks are derived from the dual resonance structure. The superposition of such absorbing peaks makes the broadening of frequency bandwidth possible to realize more absorbing of microwaves. When the samples were sintered1100℃at for3h, a maximum frequency bandwidth of11.77GHz was observed at x=0.6. As to the sample sintered at1200℃for3h, a maximum frequency bandwidth of11.73GHz was obtained at x=0.7.In this paper, a method of prolonging the calcination time and elevating the sintering temperature was adopted to effectively introduce oxygen vacancies into the BFTO system. Such a process renders more Fe3+to transform into Fe2+, thus the superexchange coupling resonance effect is enhanced between Fe3+and Fe2+, further broadening the absoring bandwidth of the BFTO powders. The frequency bandwidth for x=0.6sintered at1200℃for7.5h attains11.82GHz, while the sample sintered at1300℃for3h with x=0.8could achieve12.23GHz.The BFTO ceramics were also prepared for the purpose of miniaturization. The results revealed that Ti doping could reduce the calcination temperature of barium ferrite. This fact is favorable for the BFTO material to be used in integration of electronic devices because it can maintain a relatively high remnant magnetization together with its soft magnetic properties. |
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