Study On The High-performance M-type Barium Hexaferrite And Its Applications On Millimeter-wave Circulator

With the rapid progress of modern space-based radar technology,especially the application and promotion of millimeter-wave technology,high-frequency,miniaturization and low loss have become the development orientation of microwave devices in the future.The microwave circulators developed based on the spinel and garnet ferrites requires an external permanent magnet to provide indispensable biasing fields,which makes it difficult for the entire systems to achieve integration and miniaturization.On the contrary,M-type barium hexaferrites possess high anistropy,which can realize the self-biasing of the circulator at zero field,as well as it can improve the operating frequency of the circulator to enable it work in the millimeter-wave frequency band.Therefore,the developments of high-performance M-type barium hexaferrites are of great significance for the miniaturization and high-frequency development of microwave circulators.This article will focus on the application requirements of self-biased circulators,utilizing solid-state sintering method to prepare M-type barium hexaferrite.In terms of materials,based on first-principles calculation,cation distribution and exchange interaction of barium hexaferrites were investigated,and a comprehensive exploration of relationship between ion substitution,doping modification,and process optimization,and magnetic properties were also performed.On the device side,Optimization of simulation design and implementation of the self-biased circulator,to which a self-developed and high-performance M-type barium hexaferrite was applied,were fulfilled.Firstly,La-,La-Co and La-Cu substituted M-type barium hexaferrites were synthesized by solid-state sintering method,and the influences from each component of composition on the properties of M-type barium hexaferrites were investigated.The results indicate that:（1）La-substitution can increase the anisotropy of M-type barium hexaferrite,but has an inhibitory effect on the grain growth.The first-principles calculation of the exchange integrals of the system manifests that La-substitution can reduce the exchange interaction between the 4f₁-4f₂、4f₁-12k、4f₂-12k、2f₁-2f₁、2f₂-2f₂ and 4k-8k sublattices,while 2a-4f₁、2a-12k、2b-4f₁ and 2b-12k sublattices will be enhanced.When the exchange correlation potential U_eff=6.7 eV,the Curie temperature T_c calculated using the random phase approximation theory is reasonable;（2）The Rietveld structural refinement combined with Raman and XPS analysis confirms that cobalt ions with large anisotropic constant enter the M-type barium hexaferrite lattice in a trivalent form,and preferentially occupies the 2a,4f_l,and 12k sites.Therefore,the appropriate amount of La-Co substitution can increase the remanence 4πM_r and the anisotropy field H_a of the M-type barium hexaferrite,but the density d of the sintered hexaferrites decrease otherwise.Calculated by the Néel ferromagnetic molecular field theory,the molecular field coefficients between the 2b-4f₂、12k-4f₁、2a-4f₁、12k-4f₂ and 2b-12k sublattices are reduced,so that the Curie temperature T_c is decreased;（3）First-principles calculations are carried out in combination with Raman and Rietveld structural refinement.It is validated that Cu²⁺occupies the 2b and 4f₂ sites,and the occupied ratio is about 1:2.Meanwhile,the appropriate amount of La-Cu substitution can significantly increase the remanence 4πM_r,anisotropy field H_a,and density d of the M-type barium hexaferrite,while reduce the porosity of the ferrite inside p.Secondly,on the basis of the assured Ba_0.8La_0.2Fe_11.8Cu_0.2O₁₉ composition,the microstructure and properties of M-type barium hexaferrite doped with Bi₂O₃ and Bi₂O₃+CuO were investigated,and the sintering mechanism of Bi₂O₃+CuO doped hexaferrites were also discussed.The results show that:（1）Low melting point Bi₂O₃ can form a liquid phase at the grain boundary and promote the solid-phase reaction.Therefore,the approariate amount of Bi₂O₃ can reduce the porosity of the Ba_0.8La_0.2Fe_11.8Cu_0.2O₁₉ hexaferrites,increase the density d of the sintered sample and the remanence4πM_r;（2）In the combination of Bi₂O₃ and CuO,Bi₂O₃ and CuO at the grain boundary can generate Bi₂Cu O₄ at 700°C,the melting point of Bi₂CuO₄ and Bi₂O₃ coexistence phase is lower than that of Bi₂O₃,Therefore,proper doping of Bi₂O₃+CuO can promote the uniformity and densification of hexaferrite particles,and further increase the density d and remanence 4πM_r of sintered hexaferrites.Then,the effects of preparation process（calcination and secondary ball milling）on the properties of M-type barium hexaferrite were investigated,and a high-performance M-type barium hexaferrite was fabricated by magnetic-field press.The results suggest that:（1）The calcination temperature will affect the sintering activity of the hexaferrites,and the appropriate calcination temperature can regulate the growth rate of the grain,make its growth densified and uniform,and then increase the density d and remanence 4πM_r of the sintered hexaferrites;（2）selecting the appropriate second ball milling duration can improve the microstructure and properties of the hexaferrites,nevertheless,second ball milling for a long time can bring some impurities,and then deterioration of properties;（3）adopting Ba_0.8La_0.2Fe_11.8Cu_0.2O₁₉9 as the composition,Bi₂O₃+CuO as sintering aid,and calcined at 1150°C,the M-type barium hexaferrite prepared by magnetic field press has a high c-axis orientation when the second ball milling duration is 14 h.The remanence4πM_r and the linewidthΔH of the ferromagnetic resonance of the hexaferrite are better than those reported literatures on the bulk M-type barium hexaferrites.Finally,based on the developed high-performance M-type barium hexaferrites,the structural size of the self-biased circulator is optimized through HFSS simulation software,and the circulator is fabricated by the collaboration of the semiconductor lithography process and magnetron sputtering technology.The results illustrate that:（1）The thickness h of the ferrite substrate and the radius R of the central conductor can regulate the coupling between the ferrite and the electromagnetic wave as well as the port impedance matching of the circulator,and the decrease of the radius R of the central conductor can reduce the resonant frequency of the"+"mode and the"-"mode,and thus the center frequency f₀ of the circulator shifts to a low frequency;（2）Aλ_g/4 wavelength converter is deployed as a large Y junction whose size alteration of the large Y junction mainly affects the return loss of the circulator|S₁₁|and isolation|S₁₂|,and the center frequency f₀ of the circulator shifts to a low frequency along with its enhanced length L_Y or diminished width W_y;（3）The application of double Y junction can increase the bandwidth of the circulator,and unlike the length L_Y of the small Y junction,its width W_y has little impact on the center frequency f₀,while reducing the size of the small Y junction can reduce the insertion loss|S₂₁|,increase the return loss|S₁₁|and the isolation|S₁₂|;（4）The self-biased circulator is smaller,and in frequency range of 29.5 GHz to 29.9 GHz,its physical return loss|S₁₁|and isolation|S₁₂|are both greater than 15 dB with simultaneous insertion loss|S₂₁|<2.8 dB,self-biased circulator perform a good circulating function.