Basic Research On Millimeter Wave Green Gyromagnetic Tapes And LTCF Phase Shifters

With the rapid development of electronic information technology,the microwave ferrite devices have been widely used for radar system,artificial intelligence,satellite communications,and other advanced technologies.In particular,ferrite phase shifter has become an important component of phased array radar system.In order to meet the development of high frequency,miniaturization and integration of ferrite devices and systems,low temperature co-fired ferrite（LTCF）technology has become a key passive integration technology.As excellent materials for fabricating millimeter wave ferrite phase shifters,Li series ferrites,have some problems such as small grain size and low densification at low sintering temperature（900 ℃）,which lead to deterioration of gyromagnetic and dielectric properties of the Li series ferrites.Therefore,the preparation of gyromagnet ferrite materials with low sintering temperature and excellent properties are key to realize millimeter wave LTCF phase shifter.This dissertation focused on studying the low temperature sintering of gyromagnetic Li series ferrites applied to millimeter wave LTCF phase shifter,exploring the influence of low softening temperature glass doping,low melting point composite oxides doping,nanocrystalline implantation and process parameter optimization on microstructures,gyromagnetic properties and dielectric properties of ferrites,preparing the green gyromagnetic tapes and ferrite substrates with excellent properties,and designing and fabricating a LTCF phase shifter applied to millimeter wave.Firstly,the influences of V2O5-ZnO-B2O3（VZB）,Bi2O3-ZnO-B2O3（BZB）and Li2CO3-B2O3-SiO2（LBS）glasses on the performance of low temperature sintered LiZnTiMn ferrites were systematically studied.These studies found that,（1）Due to low softening temperature（398 ℃）of VZB glass,the glass could be converted into liquid phase in the sintering process,which could effectively promote the grain growth and densification.Also,gyromagnet properties and dielectric properties of the samples were significantly improved.When sintered at 900 ℃,the samples with 0.5 wt.%VZB glass could not only enhance Ms and Br/Bs,but also reduce?H and tanδε.（2）BZB glass could form liquid phase and cover the surface of grains during the sintering process,which enhanced the mass transfer rate and promoted grain growth of solid phase through capillary force and viscous flow.In addition,SEM images indicated that the microstructures of samples were very sensitive to the content of BZB glass.In a word,adding a small amount of BZB glass（0.3 wt.%）could obtain a uniform microstructure ferrite.Finally,when sintering temperature is 900 ℃,the sample had excellent gyromagnetic properties and dielectric properties:Ms=73.71 emu/g,Br/Bs=0.89,?H=161 Oe,tanδε=3.13×10^-4.（3）The LBS glass could form liquid phase and dissolve in LiZnTiMn ferrites.The proper doping amount of LBS glass could promote uniform densification of grain growth,improve the Ms and reduce?H of samples.Secondly,the effects of low-melting-point composite oxides（V2O5-CuO,Bi2O3-CuO,Bi2O3-Li2CO3）on sintering characteristics,gyromagnetic properties and dielectric properties of LiZnTiMn ferrites were studied.The results showed that,（1）V2O5-CuO additive could form CuV2O6-V2O5 mixture at 600 ℃,which had a low melting point（644 ℃）.The grain growth and densification of the material could be enhanced and compact microstructure could be obtained through the liquid phase sintering mechanism.When additive amount x=0.5 wt.%,the sample presented a unique microstructure of small grains located at the junction of large grains,which exhibits excellent gyromagnetic properties and dielectric properties.（2）Bi2O3-CuO mixture had significant effects on sintering characteristics and microstructure of LiZnTiMn ferrites.As the content of Bi2O3 increased,the grain size of the sample gradually increased and the number of pores gradually decreased.When x=0.21 wt.%,the sample sintered at 900 ℃ had the maximum of Br/Bs（0.91）,the minimum of?H（155 Oe）and tanδε(2.92×10^-4).When the content of Bi₂O₃ in the mixture was too high,the grains abnormally grew（grain size was more than 20μm）and the microstructure uniformity deteriorated,resulting in a rapid decrease in the Br/Bs and an increase in the?H.（3）When an appropriate proportion of Bi2O3-Li2CO3 mixture was doped,the LiZnTiMn ferrites with low sintering temperature（900 ℃）remained excellent gyromagnetic properties.For example,when 0.3 wt.%Bi2O3 and 0.2 wt.%Li2CO3 were added,the saturation flux density increased to 341 mT,the remanence ratio improved to 0.9,and the ferromagnetic resonance line width decreased to 162 Oe.Then,the effects of nanocrystalline placement,pre-sintering temperature and holding time on properties of LiZn ferrites under low sintering temperature were studied.The researches showed that,（1）LiZnTi nanocrystals（26 nm）prepared by self-propagating sol-gel method had good activity,which could be well integrated with LiZnTi pre-sintering powder.With the increase of nanocrystalline content,the grain size of the samples gradually decreased.When 5 wt.%nanocrystalline was added,SEM images showed that the microstructure of sample had been effectively regulated and improved,and the grains are uniform and compact.The sample sintered at 900 ℃ had excellent gyromagnetic properties and dielectric properties:Bs=364 mT,?H=145Oe,tanδε=2.45×10^-4.（2）Proper pre-sintering temperature could adjust the activity of the pre-sintering powder,prevent the secondary growth and discontinuous growth of grains at final sintering process,and improve the gyromagnetic properties.（3）The increase of holding time at final sintering process could promote grain growth and reduce the number of pores.However,it was easy to reduce the uniformity of the grains size due to too long insulation time,resulting in a rapidly reduction of the remanence ratio.Finally,with respect to device design and fabrication,we studied of LTCF process and prepared the green gyromagnetic tapes with excellent performance.And the ferrite substrates,which could be used in LTCF phase shifter,were obtained through adjustment of sintering process.In the aspect of devices,the millimeter waves LTCF phase shifters with microstrip and stripline structure were designed and simulated by using ANSYS electronics 17.0.The millimeter waves LTCF microstrip phase shifter was realized by using the self-developed green gyromagnetic tapes.Test results showed that,the insertion loss was greater than-4 dB,the return loss was less than-10 dB in the frequency range of 30.7³1.7 GHz.And at the frequency of 31.2 GHz,the insertion loss was-3.22 dB,the return loss was-12.57 dB,the phase shift was 243.7°.