The Study On Particles Growth And Millimeter Wave Properties For Hexagonal Ferrite

M-type barium hexaferrite exhibits relatively strong uniaxial anisotropy,high natural resonance frequency and high saturation magnetization etc,which ensures that it has a great application potential in the millimeter wave band?26.5-40GHz?.In this study,M-type barium hexaferrite is prepared through the hydrothermal method,the reaction process is controlled by the oxidant,and a detailed study has been carried out.The effects of oxidant and alkali concentration on the reaction process,the change of the products morphology are also studied.According to the composition,morphology,millimeter wave magnetic properties of the products,some conclusions are obtained:1.When the reaction process is under the control of weak oxidant KNO₃,the reaction of anoxic nitrate reduction coupled with iron oxidation can only be carried out under the hydrothermal system of high temperature and pressure.Through adjusting the oxidizing ability of the solution system,the pure M-type barium hexaferrite with typical hexagonal plate morphology,uniform size and perfect crystallization can be prepared in a variety of alkali concentrations.However,when the hydrothermal reaction is carried out under the action of strong oxidant H2O2,the oxidation reaction occurs in the preparation of the precursor solution and exhibits very drastic,which can lead to impurity ?-Fe₂O₃ in the final product and have the damaged hexagonal structure with many defects in the grain boundary.The product can be purified through the pickling process.2.For the hydrothermal synthesis reaction with iron ion as the raw material,there is no oxidation process in the whole reaction leading to the appearance of different reaction mechanism.With the increase of alkali concentration,the morphology of the products becomes increasingly thin.In the ultra high alkali concentration,the prepared particles are oriented in the plane as the form of "flexible paper" after the filtration process.In a word,the product morphology can be controlled by changing the reaction process3.According to the oxidation ability,we have summarized the reaction process of synthesizing barium ferrite with ferrous ion as raw material that is carried out in multiple steps.First,the intermediate product of ?-Fe₂O₃ is prepared during the oxidation process.Then,the ?-Fe₂O₃ particles convert to M-type barium hexaferrite through the further reaction.However,when the oxidation ability of the solution is insufficient,Fe3O4 will be synthesized and cannot be further converted.On the other hand,if the oxidation ability of the solution was too strong,the oxidation process will be carried out with a fast reaction rate that a part of ?-Fe₂O₃ particles will grow up rapidly after the nucleation process nd cannot participate in the further reaction for synthesizing M-type barium hexaferrite particles.4.The resonant frequency of the M-type barium hexaferrite with typical hexagonal plate morphology is about 40 GHz or higher,through combining a part of impurity phase Fe3O4 and ?-Fe₂O₃,the resonance frequency of the products can be reduced to the millimeter wave band.The reflection loss of the synthesized composites can be-19.7dB and have the 7.6 GHz available bandwidth when the absortion thickness is 2mm,which enables the application of millimeter wave absorbtion.For the reaction under the effect of strong oxidant H2O2,the hexagonal structure of the products is severely damaged,which leads to the decrease of anisotropy field.Then,the resonance frequency of the products is reduced to 35-38 GHz.However,for the very thin particles prepared by the addition of ferric ions,the shape anisotropy will increase the anisotropy field will be enhanced.Then,the resonance frequency of the products will increase and be out of the millimeter wave band range.In summary,the shift of resonance frequency can be realized by changing the morphology and component of the products.