Study On B₂O₃-La₂O₃-MgO-TiO₂ Glass-ceramics Based LTCC Materials

Low Temperature Cofired Ceramics?LTCC?technology has played an important and irreplaceable role of bringing greater multifunctionality,higher performance and sub-miniaturization to electronic elements,components and thereby to the terminal devices with lighter weight and higher reliability.Research and development of new LTCC materials have been drawing much attention.Nowadays,the commercially available LTCC materials are mainly fabricated through two kinds of technical routes:one is out of the pure glass–ceramic systems,such as Ferro A6 from CaO–B₂O₃–SiO₂glass-ceramic,and the other is from the glass/ceramic composites systems,such as the lead borosilicate glass filled with Al₂O₃ of Dupont 951AT.Traditionally,the glasses involved in both cases are mainly from silicate and borosilicate systems.These silicate-based LTCC materials have generally low dielectric constant?less than 10?due to the low dielectric constant of the silicate or borosilicate glasses,which makes the commercially available LTCCs be used only as substrate in most cases and not suitable for the cases where higher dielectric constant is needed.Recently,with the continual rapid progress of novel communication technology and high-density packaging,it is more important and in urgent need to develop new LTCC materials with high dielectric constant??10?.Undoubtedly,one of the most important issues is to seek for suitable non-silicate LTCC glasses or glass–ceramics,which are with dielectric constants higher than traditional silicate or borosilicate-based glasses.In this doctoral dissertation,a non-silicate glass for LTCC was selected from B₂O₃-La₂O₃-MgO-TiO₂?BLMT?system,and the glass structure,the glass-forming region,crystallization process,sintering behavior and dielectric properties of the glass were systematically studied for the first time.Then three ceramics,Ba Ti₄O₉,Li₂ZnTi₃O₈ and Li₂Zn₃Ti₄O₁₂ were synthesized and mixed separately with the BLMT glass to fabricate new LTCCs with high dielectric constant.The crystallization behavior,interreactions between glass and ceramics,sintering mechanism,microstructure and dielectricproperties of the tree composites BLMT/BaTi₄O₉,BLMT/Li₂ZnTi₃O₈ and BLMT/Li₂Zn₃Ti₄O₁₂2 were thoroughly investigated.A series of novel LTCC materials with Q×f values?5000GHz and dielectric constant between from 10 to 25 were obtained.The main work and results of the dissertation are as follows:1)The structure and glass-forming region of the BLMT glass were investigated.The results show that with 42.9mol%of B₂O₃,the BLMT glass-forming region is in the composition range of La₂O₃ 0⁵6.2mol%,MgO 0⁸7.5mol%,TiO₂ 12.5⁶8.7mol%.The network structure of the BLMT glass is composed of[BO₃]and[BO₄]units.The increase of B₂O₃ content will lead to the transition process of[BO₃]to[BO₄].2)The effects of B₂O₃,La₂O₃/MgO,La₂O₃/TiO₂ and MgO/TiO₂ on crystallization behavior of the BLMT glass-ceramics were investigated.It is found that after crystallization heat-treatment,LaBO₃ is found to be the main phase of BLMT glass-ceramics.However,the increase of B₂O₃ content leads to the main phase LaBO₃ change gradually to LaB₃O₆,TiO₂ and MgLaB₅O₁₀.Similarly,with the decrease of MgO/TiO₂in molar ratio,the main phase of the glass-ceramic transforms from LaBO₃ into TiO₂.The main phase precipitated from the glass is always LaBO₃ in the composition range studied with different molar ratio of La₂O₃/MgO and MgO/TiO₂.3)The sintering mechanism of the BLMT glass-ceramics was investigated,and the microwave dielectric properties of the sintered material were characterized.The results indicate that the sintering shrinkage of all samples starts at Tg of the glass,and two different processes are involved during sintering.They are the glass viscous flow sintering densification process that happens at lower temperature and the glass crystallization process at higher temperature.The results of dielectric properties measurement shows that BLMT glass-ceramic possesses positive?_f,?_r=7-15 and Q×f?7000GHz.A LTCC glass composition with excellent dielectric properties and technical characteristics is chosen from BLMT glass system.4)The sintering mechanism,microstructure,phase composition and dielectric properties of the BLMT/BaTi₄O₉ composite were systematically investigated.The results show that full densification of composites can be achieved at 860°C for 20min, nd viscous flow of the BLMT glass dominates the process.The main phase found in all samples is LaBO₃ formed from the crystallization of BLMT glass.BaTi₄O₉ reacts with BLMT glass during sintering,leading to the disappearance of it and the formation of other two new phases BaTi?BO₃?₂ and TiO₂.As the Ba Ti₄O₉ content is increased from 0 to 30wt%,the dielectric constant increases from 8.4 to 20.5,quality factor changes from 9200 to 24000GHz and the temperature coefficient goes from+30 to+145ppm/°C,respectively.Typically,the 30wt%BaTi₄O₉–filled BLMT glass composite sintered at 860°C for 20min displays dielectric properties of?_r=20.5,Q×f=24000GHz and?_f=+145ppm/°C.5)The sintering mechanism,microstructure,phase composition and dielectric properties of BLMT/Li₂ZnTi₃O₈ composites were systematically investigated.The results show that full densification of the composites can be achieved at 900°C for 2h for the composites,and for samples with less than 10wt%BLMT glass the densification can be attributed to non-reactive liquid-phase sintering,while for that with?10wt%glass it is controlled by the co-effects of glass viscous flow sintering and reactive liquid-phase sintering.As the increase of BLMT glass content in the composite,both the main phase LaBO₃ precipitated from BLMT glass and the newly formed rutile increase gradually,while Li₂Zn Ti₃O₈ decrease.With BLMT glass increasing from 2.5 to 80wt%,the dielectric constant demonstrates gradually decrease from 24.6 to 15.3 and the quality factor goes from 49000 to 5800GHz,whereas the temperature coefficient of resonant frequency shows a trend of fall from-12.07 to-14.2ppm/°C?7.5wt%?first then rise to 75ppm/°C.Typically,the composite with 20wt%glass sintered at 900°C exhibits excellent dielectric properties:?_r=22.7,Q×f=19900GHz,and?_f=0.28ppm/°C.6)The sintering mechanism,microstructure,phase composition and dielectric properties of BLMT/Li₂Zn₃Ti₄O₁₂ composites LTCC materials were systematically investigated.The results show that full densification of the composites can be achieved at 900°C for 2h for all composite samples,and for samples with less than 10wt%BLMT glass the densification can be attributed to non-reactive liquid-phase sintering,and for that with?10wt%glass it is controlled by the co-effects of glass viscous flow sintering and reactive liquid-phase sintering.BLMT glass crystallizes during sintering forming aBO₃ as the main crustal phase,and MgLaB₅O₁₀0 and rutile phase are formed too either by crystallization of BLMT glass or by the reaction between BLMT glass and Li₂Zn₃Ti₄O₁₂ ceramic.As BLMT glass is increased,LaBO₃,MgLaB₅O₁₀0 and rutile phases gradually dominates the material and Li₂Zn₃Ti₄O₁₂ phase decreases.With BLMT glass increasing from 2.5 to 80wt%,the dielectric constant demonstrates gradually decrease from 18 to 13 and the quality factor goes from 55000 to 15000GHz,whereas the temperature coefficient of resonant frequency presents a trend of fall from-48 to-55ppm/°C?10wt%?first then rise to+16ppm/°C.Typically,the composite with60wt%glass displays excellent dielectric properties with?_r=14,Q×f=23000GHz,and?_f=-3.56ppm/°C.