Study And Application On Garnet Ferrite For L-Band Circulator

The microwave ferrite circulator plays a role of signal isolation,matching and gain enhancement between microwave system receiver and transmitter（T/R module） as well as in the circuit,and is an indispensable key device in the electronic system.Compared with other high-frequency band devices,L-band ferrite circulator has low frequency,long wavelength,ability to pass obstacles and low propagation attenuation.So it is widely applied in long distance transmissions such as weather forecasting,radio broadcasting and satellite communication.The performance of L-band microwave circulator mainly depends on the ferrite material applied to it.Therefore,this paper first studied the garnet ferrite material for the L-band circulator.According to the performance requirements of the device on the 4πM_s,T_c,ΔH,temperature stability and dielectric loss of materials.The traditional oxide ceramic process was used combined with theoretical analysis to determine the basic formula of the material as Y_2.7-yCa_0.3-zGd_yAl_xFe_4.68-x-zZr_0.3V_zMn_0.02O₁₂.The effects of Al³⁺,Gd³⁺ and V⁵⁺substitutions,and process parameters such as sintering temperature,Bi₂O₃ additive and iron deficiency during preparation of garnet ferrite materials were investigated experimentally.In order to reduce the 4πM_s of the material,the effect of Al³⁺ substitution on the properties of the material was investigated.The results show that:（1）Proper amount of Al³⁺ substitution has no significant effect on the crystal phase composition of the sample.The samples in each group are pure garnet phases.（2）As the substitution amount of Al³⁺ in the formulation increases,the density decreases,the porosity increases,the shrinkage rate decreases,and the sufficient degree of sintering decreases.This is because the required energy to form Y₃Al₅O₁₂ is higher than that to form Y₃Fe₅O₁₂,thus the more Al³⁺ is substituted,the greater the sintering activation energy is required for the material,and the more difficult the sintering is,thus the sintering activation energy Q of the sample increases.（3）With the increase of Al³⁺ substitution,the 4πM_s,T_c andΔH of the material are significantly reduced.When the substitution amount is 1.2,the performance of the material almost meets the requirements of the L-band circulator.In order to improve the temperature stability of the material,the Gd³⁺ substitution was carried out.The results show that:（1）There is no significant difference in the crystal phase composition of the Gd³⁺ substituted samples in each group,all of which were pure garnet phases.Gd³⁺ substitution has no obvious effect on the T_c of the sample,but can change the sample offset point T_d,thereby improving the temperature stability of the material.（2）As the Gd³⁺ substitution increases,the sample T_d gradually increases.The temperature stability in the range of-55℃⁺85℃ first decreases and then increases,the value reached the minimum 3.60‰/℃ when the substitution amount is 0.4.（3）Gd³⁺ substitution increases the density and the ΔH of the sample,but has little effect on the porosity and shrinkage.In order to improve the microstructure of the sample and reduce the ΔH of the material,the V⁵⁺ substitution was carried out.The results show that proper amount of V⁵⁺ substitution promoted the sintering of the material.With the increase of V⁵⁺ substitution,the porosity P of the sample decreased,the shrinkageηincreased,the microstructure improved,and the 4πM_s,T_c andΔH of sample decreased.The results of the sintering temperature experiment show that as the sintering temperature of the sample increases,the average grain size of the sample and 4πM_s increase first and then decrease.The P andΔH decrease first and then increase,and reach a minimum at 1320℃.With the increase of doping amount of Bi₂O₃ additive,the apparent density d_b of the sample increases first and then decreases,P decreases first and then increases.When the doping amount is 0.1wt%,the microstructure of the sample is the best.At the same time,4πM_s is the largest andΔH is the smallest.A small amount of iron deficiency has no effect on the crystal phase composition of the sample.As the iron deficiency of the material increases,the dielectric loss tanδεof the sample decreases.Finallyfortheobtainedsample,thematerialformulationis Y2.3Ca0.25Gd0.4Al1.2Fe3.43Zr0.3V0.05Mn0.02O12,the sintering temperature is 1320℃,the doping amount of Bi₂O₃ and BaTiO₃ is 0.1wt%respectively.The magnetic properties of the sample are:4πM_s=305Gs（normal temperature）,T_c=115℃,α4πMs=3.60‰/℃（-55℃to+85℃）.The microwave properties of the sample at 9.25GHz and normal temperature are:ΔH=38 Oe,tanδε=1.77×10^-4 andε_r=12.Based on the garnet ferrite material prepared above,the simulation design of ferrite microstrip circulator was carried out with HFSS software.First,the center junction radius of the circulator and the thickness of the ferrite substrate are calculated according to the circulating condition.The microstrip line size is calculated according to the impedance matching theory to obtain the initial model and size parameters of the circulator.The initial circulator model has the center junction radius of approximately26 mm,the ferrite substrate thickness of 6 mm,the large Y junction length of 24 mm,and width of 15 mm.The HFSS software was then used to simulate and optimize the initial model and structure of the circulator.After optimization,the substrate volume is66×66×3.6 mm³.The simulation results show that in the range of 0.81 GHz⁰.98 GHz,the return loss|S11| and the isolation|S31| are both bigger than 20 dB,the insertion loss is|S21| is less than 0.3 dB,the standing wave ratio VSWR is less than 1.5,and has a good circulate effect.