Study On Influences Of Ionic Doping And Performances Improvement For High Dielectric Constant Gyromagnetic Ferrites

With the rapid development of the communication technology,the microwave device is more and more important nowadays,and it has attracted much attention and been widely used in many filed.As an important category,passive devices such as circulators,isolators,and phase shifters take the advantage of the tensor permeability characteristics of gyromagnetic ferrite materials to realize functions such as signal circulation and isolation,so as to composite the microwave communication system.The electromagnetic wave propagates through the ferrite material,and its wavelength is inversely proportional to the square root of the ferrite dielectric constant.The miniaturization of such devices is an important for the miniaturization of the circuit system.The yttrium iron garnet（YIG）has attracted much attention due to its excellent magnetoelectric properties,especially the ultra-low loss.The dielectric constant of this kind of material is usually about 15.The size of passive devices will be greatly reduced if the dielectric constant could be increased with excellent magnetoelectric properties.At the same time,this kind of microwave device is usually fabricated one by one,leading to a low consistency,which greatly affects the production efficiency.The emergence of the low temperature co-fired ceramic technology（LTCC）provides an effective solution for the miniaturization,planar integration and batch fabrication of microwave devices.However,since YIG ferrite materials are generally sintered at 1450°C,the matching co-firing between YIG and silver electrode by the LTCC technology（below 961°C）cannot be achieved.In addition,the low temperature sintering of YIG ferrite leads to defects in its microstructure,increased pores,and sharp deterioration of performance.Therefore,it is important to decrease the sintering temperature of YIG ferrite with excellent magnetoelectric properties.In order to increase the dielectric constant of YIG ferrite and reduce its sintering temperature,we used a large number of Bi³⁺ions to replace the ion in YIG,and optimized the sintering temperature through doping.The material synthesized in this study has a high dielectric constant.Besides,the performance and practical value of the prepared materials are verified through the simulation and fabrication of the circulator with double Y-junction strapline.Firstly,the Bi:YIG ferrite was co-substituted with Ca²⁺-Zr⁴⁺ions,and its magnetoelectric properties at low temperature sintering were explored.The samples sintered at 950℃have garnet phase,and appropriate amount of Zr⁴⁺ion substitution improved the grain growth and densification,and promoted the accumulation of charges at the grain boundary.Besides,it increased the interface polarization and dielectric constant,which is higher than 30 at 1MHz-0.5GHz frequency range.For the x=0.3 sample,it has excellent magnetic properties,that is 1926.68Gs for saturation magnetization,3.93Oe for coercivity,and 446Oe for ferromagnetic resonance linewidth.Secondly,based on the co-substitution of Ca²⁺-Zr⁴⁺ions,the effect of iron-deficient formulation on the crystal structure,microscopic morphology,density,dielectric properties,magnetic energy properties,and gyromagnetic properties of YIG ferrite was studied.It shows that an appropriate amount of iron deficiency can help to improve the density of materials and reduce electromagnetic losses.The performance of YIG ferrite has been improved as x=0.2:M_s=27.78emu/g,H_c=3.80Oe,4πM_s=2039.51Gs,andΔH=326Oe.In order to furtherly reduce the loss,the combined substitution of Ca²⁺-Sn⁴⁺ions are used to improve the performance of the above YIG ferrite.For the x=0.2 sample,the densest one was obtained at 952°C,and the magnetoelectric properties have also been greatly improved.The dielectric properties have no obvious variation,and the dielectric constant remains high.The saturation magnetization was 1843.34Gs,the coercivity was2.51Oe,and the ferromagnetic resonance linewidth was 190Oe.Finally,according to the low-temperature sintered YIG ferrite material discussed above,a strapline circulator with double Y-junction was designed by the HFSS three-dimensional simulation software.The working frequency of designed circulator is 8GHz-10GHz.The standing wave ratio VSWR is less than 1.35,and the return loss and isolation are more than 20d B.Besides,the transmission loss is smaller than 1d B.According to the designed circulator,the real object has been fabricated.The feasibility of the designed circulator has been verified,and this type of device can be manufactured on a large scale using LTCC technology.