Research On The Synthesis And Optimized Properties Of Titanium Oxides Based High Permittivity And Lithuim Oxides Based Low Permittivity Microwave Dielectric Ceramics

To meet the specific requriment of fifth-generation?5G?communication and applications,microwave integrated circuit has the trend of pursuing extremely high frequency and broad band-width.In the field of microwave ceramic devices,there are requirements of miniaturization,high thermo-stability,ultra-low dielectric loss,etc.The research of a new generation of multifunctional microwave dielectric ceramics is mainly focusing on the following two directions,?1?exploring new systems with dielectric constant??_r?greater than 100;?2?forging novel materials with ultra-high quality factor?Q×f?higher than 100,000 GHz.This study is based on three typical dielectric ceramics,such as the ABO₃-type perovskites with ultra-high dielectric constant(Na_1/2Sm_1/2TiO₃?NST???_r>100?and Ca_0.35Nd_0.35Li_0.25TiO₃?CLNT???_r>130?ceramics)and NaCl-type rock-salts with ultra-high Q value?Li₂Mg₃BO₆?B=Ti,Sn?,?_r?20 and Q×f?100,000 GHz?.We are capable of realizing the control of its microstructure and dielectric properties,obtaining the regulation mechanism of crystal structure and dielectric properties,and synthesizing a series of high dielectric constant??_r>100?ceramics and temperature-stabilized ceramics with ultra-high Q values??_r?20,Q×f?100,000 GHz,?_f⁰ ppm/??.Herein,the main research results are as summarized as follows:?1?The structure model of NST is determined to be an orthorhombic perovskite in Pnma space group.The crystal structure of NST has certain amounts of vacancies at A-site and oxygen vacancies.In order to suppress the intrinsic loss caused by free electrons and Ti reduction,we adopt a one-time synthesis process to introduce acceptor Cr³⁺into the ceramic to supress free electrons.The XPS analysis suggests that the Cr³⁺effectively inhibits the generation of Ti³⁺.The CeO₂ is introduced into the NST ceramic to surpress the intrinsic loss caused by phonon scattering of vacancies.As Ce⁴⁺enters the A-site in a proper concentration,the Q×f value is improved due to suppression of the vacancies or replacement of(Na_1/2Sm_1/2)²⁺;but excessive amount of Ce⁴⁺will enter the B-site to deteriorate dielectric properties.The?_f value is highly dependent on the B-site bond valence,and the introduction of Cr³⁺and Ce⁴⁺is benefitial for enhancing the B-site bond valence,and the?_f can be improved to some extent.With the purpose of tuning the?_r and?_f in NST ceramics,we utilize Nd/Sn and Nd/Zr to co-substitute for ions at A/B sites.We first focus on improving the?_r through substitution of Nd³⁺for Sm³⁺in NST ceramics,increasing the?_r from 100.0 to 110.0.Next,Sn and Zr are used to substitute for Ti ions,respectively.Within the solid solution range,the B site substitution decreases t?t<0.9596,the increasing tilting in oxygen octahedron?,and?_f can be effectively adjusted from 206ppm/?to 0 ppm/?,but when it is beyond the solid solution range,the appearance of the Nd₂Ti₂O₇ phase can seriously deteriorate the dielectric properties.In particular,for the case of Nd/Zr cosubstituiton,Na_0.5Nd_0.2Sm_0.3Ti_0.98Zr_0.02O₃has the excellent microwave dielectric properties:?_r=107,Q×f=9,600 GHz and?_f=190.2 ppm/?.?2?For the CLNT based high dielectric constant??_r>130?ceramics,we first replace Ti⁴⁺with Al³⁺in Ca_0.35Nd_0.35Li_0.25Ti_1-x-x Al_xO₃?CLNTA_x?system without using isostatic pressing,giving rise to improvement of Q×f value and simplification of synthesis.The crystal model of CLNTA_x is determined to be an orthorhombic perovskite with Pnma group.In the region of x?0.03,the intrinsic loss can be reduced due to the improvement of atomic packing fraction and the increrase in relative density of ceramics.But with an increasing Al content,the experimental dielectric polarizability(?_obs)and thoeritical dielectric polarizability(?_theo)decreases while the??=?_obs-?_theo gradually increases;as we know,the smaller B-site bond valence indicates a weaker bond strength bewteen oxygen and cations at B-site,resulting in an enhanced phonon scattering at B site.Therefore,both of the?_r and?_f are deteriorated because of the decreasing bond valence.In particular,Ca_0.35Nd_0.35Li_0.25Ti_0.97Al_0.03O₃?CLNTA3?ceramics has the good properties of?_r=128.0,Q×f=4,329 GHz and?_f=74.8 ppm/?.In order to manipulate the?_f value of CLNT ceramics,we use the composite ions((Zn_1/3Ta_2/3)⁴⁺and(Mg_1/3Ta_2/3)⁴⁺)to replace with Ti at B-sites.In the study of CLNT?ZT?x,the Raman spectroscopy shows that the A_g mode is very sensitive to the short-range order?SRO?effect for 0?x?0.08.The different charges and ionic radius between Zn²⁺and Ta⁵⁺provide the driving force to form the SRO structure,and also increase the intensity of the A_g mode.With an increasing(Zn_1/3Ta_2/3)⁴⁺content,the degree of SRO and oxygen octahedral distortion become stronger.The?_f is greatly influenced by the increase in the tilting of the[TiO6]octahedron and the secondary phase,which can be tuned from 67.9 ppm/?to 10.3 ppm/?.In the study of CLNT?MT?x ceramics,most of results are similar to that of CLNT?ZT?x ceramics.But it is interesting to find that intensity of the satellite peaks??200?and?0 02??of?1 2 1?gradually increases,indicating that the increase of(Mg_1/3Ta_2/3)⁴⁺content contributes to the growth of{100}and{001}planes.Typically,for x=0.08,the CLNT?MT?0.08 ceramics show excellent microwave dielectric properties of?_r=121.5,Q×f=3,865 GHz,?_f=+22.1 ppm/?,which has a relatively low?_f value.In order to synthesize a low temperature sintered ceramic with high dielectric constant,we adopt BaCu?B₂O₅?as a sintering aid to study the low-temperature firing mechanism for CLNTA3.Typically,the CLNTA3 with 5 wt.%BCB2 has the excellent microwave dielectric properties of?_r=110,Q×f=2,979 GHz and?_f=41.2 ppm/?.?3?For the Li₂Mg₃BO₆?B=Ti,Sn?ceramics with ultra-high Q value,we study the mechanism of manipulating dense sintering,phase evolution,crystal structure and dielectric properties.We first exploited a controlled sintering method of lithium-rich protective atmosphere to realize the dense sintering of Li₂Mg₃SnO₆ ceramics.The novel sinering method can effectively suppress the pores or cure the microcracks in ceramics.In the study of phase evolution in Li_2+xMg₃SnO₆,the Li₂SnO₃ can reaction with MgO,and generates the cubic Li₂Mg₃SnO₆ solid solution.However,as is exposed tohigh temperature air,Li₂Mg₃SnO₆ is decomposed into Mg₂SnO₄,MgO and Li₂O,and the resultant product of MgO will continue to react with Li₂Mg₃SnO₆ to form a Li₂Mg₃SnO₆-like phase.Based on the reaction between Li₂SnO₃ and MgO,we have derived a series of Li₂SnO₃-MgO solid solution,which has the chemical formulas of Li_2/3?1-x?Sn_1/3?1-x?Mg_x O and Li₂Mg_y SnO_3+y?x=0⁴/7,y=0⁴,x=y/?3+y??.The controlled sintering method of protective atmosphere is popularized to synthesize Li_2/3?1-x?Sn_1/3?1-x?Mg_xO ceramics,and it turns out that the novel method is suitable for the dense sintering of LSMxO ceramics.In the range of 0?x?4/7,the analysis of TEM shows that Li₂Mg₃SnO₆?x=1/2?has a short-range ordering structure?superlattice?while there are no such observations in other concentrations,and the superlattice structure contributes to the ultra-high Q value of Li₂Mg₃SnO₆?x=1/2?.The dielectric properties of LSMxO ceramics is highly dependent on phase composition,dielectric polarizability,bond valence,full width at half maximum of A_g vibration mode,packing fraction,average grain size and distortion of oxygen octahedral.Finally,the LSMxO ceramics offers the great freedom in tuning dielectric properties and obtaining materials with excellent perfromance,including the Li₂SnO₃/Li₂Mg₃SnO₆ multiphase ceramics with properties of?_r=15.43,Q×f=80902 GHz and?_f=+5.61 ppm/?and ultra-high Q Li₂Mg₃SnO₆?x=1/2?ceramic with properties of?_r=12.7,Q×f=168330 GHz and?_f=-27.4 ppm/???4?In the study of Li₂Mg₃TiO₆ ceramics,we first get the mechanism of manipulating phase transition and dielectric properties through substitution of Mg for Ca in Li₂Mg3-_xCa_xTiO₆?x=0⁰.18,LMCxT?ceramics.In the phase evolution,MgO can react with Li₂TiO₃ to generate a Li₂Mg₃TiO₆ solid solution with face-centered cubic rock salt structure in Fm-3m space group.In the Li₂Mg3-_xCa_xTiO₆ system,Ca actually cannot be incorporated into the lattcie of Li₂Mg₃TiO₆ due to its large radii,but it results in the formation of CaTiO₃ phase in Li₂Mg3-_xCa_x TiO₆ systems which forms a stable Li₂Mg₃TiO₆/CaTiO₃ system.As a result,we synthesize a system with two phases by one synthesizing process,which shows the excellent properties of?_r=17.8,Q×f=102246GHz and?_f=-0.7 ppm/?.We also find that though there is a Mg deficiency in Li₂Mg3-_xTiO₆,the samples still present Li₂Mg₃TiO₆-like phase,which requires further investigation of phase evlution of Li₂Mg3-_x TiO₆.Based on the evolutionary relationship between Li₂TiO₃,MgO,Li₂Mg_3-xTiO₆ and Li₂Mg₃TiO₆,we derive a series of Li₂TiO₃-MgO solid solution with chemical formulas of Li_2/3?1-x?Ti_1/3?1-x?Mg_xO and Li₂Mg_y TiO_3+y?x=0⁴/7,y=0⁴,x=y/?3+y??.The Li_2/3?1-x?Ti_1/3?1-x?Mg_x O ceramics can be well-sintered,which confirms the reliaiblity of the controlled sintering method of protective atmosphere.In the range of 0⁴/7,TEM analysis shows that Li₂Mg₃TiO₆?x=1/2?has a short-range ordering structure?superlattice?whereas there are no such observations in other concentrations,and the superlattice structure contributes to the ultra-high Q value of Li₂Mg₃TiO₆?x=1/2?.The dielectric performance of LTMxO ceramics is highly dependent on phase composition,dielectric polarizability,FWHM of Ag vibration mode and unit cell volume.For x=1/7,the Li₂Mg_0.5TiO_3.5 solid solution shows a typical temperature-stable property of?_r=21.5,Q×f=82,495 GHz and?_f=-5.3 ppm/?;for x=1/2,Li₂Mg₃TiO₆ceramicshas an ultra-high Q×f value of 148,713 GHz.