Research On Low-temperature Sintered La₂O₃-CaO-B₂O₃-SiO₂/Al₂O₃ Glass Ceramics And Fabrication Of RF Devices

With the rapid development of 5G/6G technology,which drives electronic components towards miniaturization,high integration and chip-based development,low temperature co-fired microwave dielectric ceramics（LTCC）have become the research focus.In this dissertation,a composite comprising of La₂O₃-CaO-B₂O₃-based（LCB）glass and Al₂O₃ceramic was utilized to produce glass/ceramics,which exhibited excellent comprehensive properties by regulating the glass composition and glass content.Furthermore,two RF devices,namely low-pass filter and Luneburg lens were successfully fabricated based on this material,which fulfill the engineering application.The specific findings are presented as follows:Firstly,the effects of glass formers B₂O₃ and SiO₂/B₂O₃ on the network structure and precrystallisation properties of La₂O₃-CaO-B₂O₃-based glass were investigated.It was found that the network structure of both LCB glass and La₂O₃-CaO-B₂O₃-SiO₂（LCBS）glass consist of[BO₃]triangle and[BO₄]tetrahedron.The network structure of LCB glass is strengthened by the transformation from[BO₃]triangle to[BO₄]tetrahedron with the increase of B₂O₃content.When the content of B₂O₃ exceeds 70 mol%,boron anomaly occurs and the glass network structure is loosened.In LCBS glass,with the increase of SiO₂/B₂O₃ content,the transformation of[BO₃]triangle to[BO₄]tetrahedron will occur in the glass network structure,enhancing the network structure.The crystallization phase of LCB glass and LCBS glass are composed of La BO₃,La（BO₂）₃ and Ca B₂O₄.Secondly,the sintering mechanism,and the influence of LCBS glass composition and content on the structure and properties of LCBS/Al₂O₃ glass/ceramics were investigated.The results show that the glass/ceramics interface is well wettable during the sintering process and the sintering mechanism is a partially reactive liquid phase sintering.Low temperature co-fired ceramics with low dielectric constants（4～8）,low dielectric losses（0.0022～0.0040）,high thermal conductivity（1.5～3.0 W/m·K）,high flexural strengths（102～154 MPa）,and suitable thermal expansion coefficients（6.13～6.63 ppm/°C）could be obtained by modulating the composition of the material.40（6La₂O₃-24CaO-55B₂O₃-15SiO₂）/60Al₂O₃（G40）sample has excellent performance,with dense sintering at 850℃,ε_r=6.87（13 GHz）,tanδ=0.0022,and could be co-fired with Ag powder,no chemical reaction,which shows good chemical compatibility.Subsequently,using G40 as the primary material,an investigation was carried out to examine the effects of dispersant,plasticizer/binder（R）,and solid content on the flow casting performance of the ceramic slurry,with the aim of obtaining a ceramic slurry suitable for application in LTCC technology.A Chebyshev low-pass filter with a cut-off frequency of 1GHz was designed and fabricated.It was found that the optimum ratio of the cast slurry was26 wt%solids,68 wt%ethanol/butanone,2 wt%castor oil,1.75 wt%polyvinyl butyral,（PEG-400+butylbenzyl phthalate）/polyvinyl butyral=1,and 0.5 wt%n-butanol,which yielded a viscosity of 0.586 Pa·s and a shear thinning behaviour,rendering it suitable for the casting process.Sintered samples at 850℃showed excellent performance:ε_r=7.22（1 MHz）,tanδ=0.0012,flexural strength of 241 MPa,a thermal expansion coefficient of 6.67 ppm/℃,and demonstrated no diffusion phenomenon with Ag electrodes.Furthermore,the prepared filter possessed a cut-off frequency of 1 GHz and an insertion loss of 0.92 d B,which indicates great potential for market application.Finally,using G40 as the primary material,the high solid content ceramic paste suitable for SLA（Stereolithography Apparatus,abbreviated as SLA）3D printing technolog was investigated.A Ku-band Luneburg Lens was designed and prepared.It was found that as the solid content increased from 60 wt%to 80 wt%,the viscosity of the paste increased and the curing depth decreased.The maximum printable solid content was 75 wt%,yielding a curing depth of 0.24 mm and a viscosity of 7.33 Pa·s.After the sintering process,the sintered sample’s dielectric constant increased from 5.22 to 6.64,while the dielectric loss increased from 0.0024to 0.0035,andτ_franged between-29.35 ppm/℃and-46.82 ppm/℃.The sample with 65wt%solids had the best comprehensive performance,with a density of 3.0135 g/cm³,ε_r=5.56（13 GHz）,tanδ=0.0024,τ_f=-32.76 ppm/℃.The Luneburg Lens sintered at 850℃achieves±15°beam scanning in the Ku band,and a gain enhancement over 5 d B to the feed source.The technical route of low-temperature sintered ceramic medium combined with SLA photocuring 3D printing process provides an economical and effective approach to developing microwave/RF devices with low cost and complex structure and promotes their integration in engineering application.