Study On Low-temperture Sintering,Structure And Properties Of Lithium-based Microwave Dielectric Ceramics

In this paper,the Li₂ZnTi₃O₈（LZT）ceramics with good microwave dielectric properties were prepared by the solid-phase sintering method.The performance of the LZT ceramics were optimized by means of non-stoichiometry,adding sintering additives and doping substitution methods.The connection between the preparation process,phase composition,microstructure,and microwave dielectric properties of the LZT ceramics have been explored.The main research work on this thesis is listed as follows:（1）The best sintering system was determined by studying the sintering system of the LZT ceramics.The experimental results showed that only a single LZT phase formed at different temperatures and different heating rates.When the samples are heated at5℃/min and sintered at 1075℃for 4 h,the highest relative density can be obtained,and the samples have the best microwave dielectric properties ofε_r=24.7,Q×f=49,527 GHz（@7.434 GHz）,τ_f=-12.31 ppm/°C.（2）In order to solve the problem of Li volatilization in the sintering process of the LZT ceramics,the L_2+xZT（x=0.02,0.04,0.06,0.08）ceramics were prepared by a non-stoichiometric method.It is reported that L_2+xZT ceramics formed a single LZT phase,and an appropriate amount of Li can promote grain growth,reduced defects such as pores,and promoted the densification of ceramics.In general,the L_2.06ZT ceramics sintered at1075℃for 4 h showed microwave dielectric properties ofε_r=25.64,Q×f=58,186 GHz,τ_f=-13.53 ppm/°C.（3）In order to verify whether LZT ceramics can meet the application requirements of LTCC technology,the LZT-x wt.%Li F（x=1,2,3,4）ceramics were prepared.The relationship between dielectric properties,sintering properties and microstructure of LZT-x wt.%Li F（x=1,2,3,4）ceramics were explored.The experimental results showed that Li F can successfully introduce a liquid phase sintering mechanism during the sintering process,and reduce the sintering temperature to 950℃of the LZT ceramics.The LZT-x wt.%Li F had no second phase generated during sintering,and it had good chemical compatibility with Ag.The optimal microwave dielectric properties ofε_r=24.4,Q×f=48,065 GHz,τ_f=-15.86 ppm/°C were yielded in LZT ceramics with 2 wt.%Li F at950℃,which makes them suitable for the application of LTCC technology.（4）In order to make the frequency range of the LZT ceramics applications larger and make it more competitive in the field of microwave dielectric ceramics,the LZT_3-xCe_x（x=0.01,0.02,0.03,0.04）ceramics were prepared.The test results indicated that LZT and Ce O₂ phases coexist in the sintering process.Due to the existence of Ce O₂ phase,the dielectric constant andτ_f value of LZT_3-xCe_x ceramics is reduced to a certain extent with LZT ceramics,but Ce O₂ can promote sintering compaction,improve the microstructure,and increase the Q×f value.When sintered at 1050℃,the LZT_2.97Ce_0.03 samples dielectric properties ofε_r=24.54,Q×f=54,618 GHz,τ_f=-14.89 ppm/°C.（5）In order to improve the dielectric properties of the LZT ceramics,the LZT-x wt.%Nb₂O₅（x=0,1,2,3,4）ceramics were prepared and the influence of Nb₂O₅ on dielectric properties of LZT ceramics has been investigated.The test results illustrated that Nb₂O₅can promote the sintering and grain growth of the LZT ceramics.In addition,Nb₂O₅ can reduce the number of oxygen vacancies,inhibit the reduction process of Ti⁴⁺to Ti³⁺to a certain extent,and can significantly improve the quality factors and increase the density of the LZT ceramics.Raman spectroscopy further explained the relationship between crystal structure and dielectric properties.The LZT-3 wt.%Nb₂O₅ ceramics sintered at1075℃had the most excellent microwave dielectric properties,i.e.,ε_r=26.52,Q×f=66,304 GHz,τ_f=-14.2 ppm/°C.To sum up,the LZT ceramics have excellent microwave dielectric properties,which can meet the technical requirements of LTCC by adding Li F,and can have the most excellent microwave dielectric properties by adding Nb₂O₅.As a microwave dielectric ceramic system with potential application value,the LZT can be used as one of the candidate materials in the field of microwave communication.