| Compared with 4G communication system,5G network has the characteristics of high frequency,high signal speed,and large bandwidth.Especially the development of5G large-scale multiple-input multiple-output(Large Scale MIMO)antenna has spawned its underlying devices(such as filters and resonators)requirements of miniaturization,light weight,high integration,high frequency,ultra-low insertion loss,and high temperature stability,etc.Therefore,microwave dielectric ceramic materials with large-scale applications in 5G systems need to have an ultra-high Q×f value(Q×f>100,000GHz)to reduce the insertion loss of passive devices or improve the signal gain of active devices,a low dielectric permittivity(?r<20)ensures its high signal speed applications in high-frequency bands,a near-zero temperature coefficient of the resonant frequency(?f?0 ppm/°C)value provides frequency temperature stability,and a low density(?<3.4g/cm3)enabling it to be used in large scale integrated circuit or phased array antenna.This dissertation aims to investigate the underlying relationship between the phase composition,degree of structural order,chemical bond characteristics,crystal growth status,phonon vibration mode,etc.,and microwave dielectric properties of the multi-component microwave dielectric ceramics with the rock salt structure,including the Li3Mg2NbO6-based systems,the systems within the Li2TiO3-Li3Nb O4-Mg O pseudo ternary phase diagram and Li4MgWO6-based systems with the low sintering temperature,providing relevant experimental and theoretical model guidance for the development of microwave dielectric ceramics for 5G systems with low dielectric permittivity,ultra-low loss,and high temperature stability.The main contents of this dissertation include:(1)To improve the dielectric loss(Q×f=79,643 GHz)and optimize the temperature coefficient of the resonant frequency(?f=-27.2 ppm/°C)of Li3Mg2NbO6 with ordered rock-salt structure,in this dissertation,the Ti and Sn were used as non-equivalent ions to substitute on the complex sites of Mg and Nb of the Li3Mg2NbO6.These substitution forms satisfy the overall charge conservation of the system,and the general chemical formula is Li3Mg2-x/3MxNb1-2x/3O6(M=Ti and Sn).The results indicated that the substitution of Ti and Sn could enable the systems to form multi-component solid solutions in a wide composition range,and the substitution led to the continuous orthorhombic-cubic phase evolution and the ordered-disordered structural transformation in the solid solution composition;the substitution of non-equivalent ions of Ti and Sn on the Li3Mg2NbO6 ordered rock-salt superstructure can be selective and will preferentially substitute Nb-Mg-Nb ion clusters.Further TEM analysis of the Sn-substituted samples revealed that such substitution patterns lead to the formation of the reconstructed superlattices and coherent phase interfaces between the orthorhombic and cubic lattices.The transformation of part of the ordered structure to the disorder after a small amount of substitution can release part of the internal stress of the ordered arrangement.Because there is continuity between the transformation of the two structures(the formation of coherent phase interfaces),the stress increased by the lattice distortion was limited,thus the total stress of the system decreases and the loss decreases accordingly.(2)Compared to the systems composed of unary or binary rock-salt structured compounds,the Li2Ti O3-Li3Nb O4-Mg O pseudo-ternary phase diagram has higher degrees of freedom to obtain the composition of compounds with excellent performance.First,the series of Li5Mg ABO8(A=Ti,Sn;B=Nb,Ta)ceramics were designed and prepared based on the phase diagram method.All samples show a single-phase disordered cubic rock-salt structure,which indirectly indicated that there were more multi-component rock-salt solid solutions in the pseudo-ternary phase system.Secondly,a Li3+xMg2–2xNb1-xTi2xO6(0?x?1)multi-component system,where the Li3Mg2Nb O6 and Li2Ti O3 compounds were employed as end members,was designed by phase diagram design method.It was found that the system formed a solid solution in the whole composition range,and the orthorhombic-cubic-monoclinic phase evolution accompanied by the order-disorder-order transition occurred with the increase of substitution amount.By analyzing the calculated configuration entropy results of different phases,it was proved that the disordered structure is more conducive to the stability of the strained structure than the ordered structure,which explained the driving force of the spontaneous phase evolution.TEM test results showed that the orthorhombic-cubic phase evolution was a continuous transition,and a coherent phase interface was formed between the two phases;besides,a series of reconstructed superlattices were formed near the coherent phase interface based on both the orthorhombic and cubic lattices.The formation of the reconstructed superlattice originated from the selective substitution of cations and the interfacial stress of the phase boundary.The compositions with ultra-high Q×f values in the system all appeared in the transition stage of the phase evolution.Further tests by terahertz time-domain spectroscopy proved that the samples in the phase evolution stage,ordered structure because of the lesser phonon anharmonic vibration.The x=0.9 sample has the optimal comprehensive performance of:?r=20.4,Q×f=90,300 GHz,?f=2.9ppm/°C.(3)Using the commonly applied experimental trial-and-error method to study the performance of each composition in the ternary phase diagram is a gigantic project that is time consuming.Therefore,the machine learning algorithm,including the Gauss process regression model and efficient global optimization selector,was used to conduct an accelerated search of extremely high Q×f value in the(1-x)Li3Mg Nb O5-x Li2Ti O3system of the Li2Ti O3-Li3Nb O4-Mg O pseudo-ternary phase diagram.The composition with an extremely high Q×f value was successfully searched after three iterations by the designed algorithm,which greatly saved the time and cost compared with the experimental trial-and-error method.The cubic-monoclinic phase evolution in the Li2Ti O3-rich end of the(1-x)Li3Mg Nb O5-x Li2Ti O3 system and the observed cubic-monoclinic coherent phase interface and reconstructed superlattice based on the monoclinic structure near the phase interface of the x=0.85 sample further proved that this type of continuous phase transition structure is deeply related to low dielectric loss.The x=0.85 sample has the optimal properties as follows:?r=18.6,Q×f=104,300 GHz,?f=-3.5 ppm/°C,and it also has a low bulk density of?=3.12g/cm3,which shows a huge application potential in millimeter-wave devices.(4)New compounds with rock-salt structures suitable for the LTCC application were investigated.For the first time,the orthorhombic Li4Mg WO6 compound was synthesized by the solid-state reaction method,it was proved that Li4Mg WO6 has an ordered salt rock superstructure,and it was found that it could be sintered and well densified at 950?.The optimal comprehensive properties were as follows:?r=15.06,Q×f=28,300 GHz,and?f=0.9 ppm/?.The sintering properties of Li4Mg WO6 were further improved by using Li F as the sintering aid.It was found that Li F has completely entered into the Li4Mg WO6lattice and solid solutions were formed in all compositional ranges,and the addition of excess Li F caused the phase evolution of the system from an ordered orthorhombic structure to a disordered cubic structure.When 0.3 mol of Li F was added,the optimal sintering temperature of the system was reduced from 950?to 840?,and the loss did not deteriorate.Li4Mg WO6 based ceramics have successfully expanded the application prospect of rock salt structured compounds in the LTCC field. |
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