Investigation On Ni₂U Barium Ferrite And The Application In Circulator

For microwave ferrite circulators,traditional spinel or garnet ferrite circulators require an additional magnetic bias field at work,which is not conducive to the miniaturization and lightweight development of circulators.But the textured hexagonal barium ferrite can accompolish the self-biased ciculator due to its high remanence ratio（M_r/M_s）and magnetocrystalline anisotropy（H_k）,which is of great significance for the miniaturization and lightweight development of circulators.Ni₂U barium ferrite has appropriate saturation magnetization（4πM_s）,high magnetocrystalline anisotropy and remanence,and shows the potential for self-biased circulator in Ku band.In this paper,Ni₂U barium ferrites were prepared by an oxide ceramic process.The influences of preparation technology,additive and ion substitution on the material crystalline phase,microstructure and static magnetic were analyzed in detail.Basing on the Ni₂U barium ferrite material,the structure design and simulation of a self-biased ferrite circulator applied in Ku band were carried out.Firstly,the preparation technology including calcined temperature,sintering temperature and secondary milling time were investigated.The research results show that:（1）The suitable caicined temperature reduces the impure phase,improve the uniformity of microstructure,increase the density,4πM_s and M_r/M_s,and decrease the coercivity（H_c）.（2）The increase in sintering temperature accelerates the reaction rate,enhances the densification,4πM_s and M_r/M_s,and reduces H_c accordingly.When the sintering temperature increases furtherly,the Z-type phase forms,the sample is overburnt,and the magnetic properties get worse.（3）Using a wet molding process,the orientation degree（F）,4πM_r,M_r/M_s and anisotropy field（H_a）increase for sintered samples as the second milling time increases.When the second milling time is 12 h,F and M_r/M_s reach to 88%and 0.9,respectively,and H_a is12.5 kOe.When the second milling time increases furtherly,the orientation degree and magnetic properties get worse.Secondly,the researches on Bi₂O₃ and CuO additives,Co²⁺and Zn²⁺ions substitution were carried out.The results show that:（1）The appropriate amount of Bi₂O₃ gives rise to the sufficient solid phase reaction,so increases the 4πM_s and 4πM_r,and decrease H_c,but excessive Bi₂O₃ results in BiFe O₃ as the secondary phase,and the static magnetic properties deteriorate accordingly.（2）CuO additive improves the d of the sintered samples,increases 4πM_s and 4πM_r and decreases H_c.With 1 wt%Bi₂O₃ and 2 wt%CuO,the density reaches 5.13 g/cm³,M_r/M_s is 0.48,and H_c is 550 Oe for the sintered sample.（3）Zn²⁺substitution increases 4πM_s,and significantly decreases the 4πM_r and H_c.Meanwhile,the crystalline anisotropy constant（K₁）and H_k decrease,and the sample shows a tendency towards soft magnetism.When the substitution amount ofZn²⁺is 0.8,K₁ and H_k decrease to 0.9×10⁵ erg/cm³ and 1.8kOe,respectively.（4）Co²⁺substitution increases 4πM_s and 4πM_r,and changes the anisotropic type of the sintered sample from c-axis to plane.Thus,H_k and H_c decrease.The results also show that when the substitution amount Co²⁺is 0.8,the sample has the optimum static magnetic properties.Based on the formula of Ba₄Ni_1.2Co_0.8Fe₃₆O₆₀,the textured ferrite was prepared by magnetic forming process.For the sintered sample,4πM_s is 3155 Gs,F is 90%,M_r/M_s is 0.92,H_c is 427 Oe,and K_a and H_a are 1.9×10⁵erg/cm³ and 8.3 kOe,respectively.Finally,based on the textured Ba₄Ni_1.2Co_0.8Fe₃₆O₆₀0 ferrite,the structure design and simulation of a Ku-band self-biased ferrite circulator were carried out using Ansoft HFSS software.The optimized results show that,at 14.5¹5.5 GHz,the return loss(S₁₁)and isolation degree(S₁₂)are both higher than 20 dB,the insertion loss(S₂₁)is lower than 1 dB,the standing wave ratio（VSWR）is lower than 1.5,and the bandwidth is 1.0GHz.