Low Temperature Sintering and Dielectric Properties of Ca-B-Si-O Glass/Ca_xMg_1-xTiO₃ LTCC Materials with Medium Dielectric Constant

Glass/ceramic system LTCC materials have excellent properties, such as feasible design and three-dimensional packaging. Functional ceramic films prepared by using tape casting can achieve the functionalization and chip type packaging of microelectronic chips. Borosilicate glass and alumina ceramic materials could be combined to form glass/ceramic LTCC materials with promising sintering performance. In this study, Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ materials were developed with low sintering temperature through regulating the composition and process. Thermal properties, microstructure, low-temperature sintering performance and dielectric properties of the Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ materials with medium dielectric constant were studied for the first time, in order to for high-frequency applications.

The Ca-B-Si-O glass batches were prepared by using mixing method, followed by melting at 1400–1500 ℃ to produce borosilicate glass. Then, the Ca-B-Si-O glass powder with mean diameter of 3.27 μm was obtained through ball milling. The LTCC composite materials were based on 40–55 wt.% Ca-B-Si-O glass and Ca_xMg_1-xTiO₃. The Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ material with high properties was obtained by using low-temperature sintering at 800–925 ℃, combined with adjustment in composition, structure and firing process. The particle distribution profiles of the Ca-B-Si-O glass and Ca_xMg_1-xTiO₃ powder were measured using NSKC-1 Photo Sizer tester. The DSC curves of glass/Ca_xMg_1-xTiO₃ were recorded using STA449C thermal analyzer. Bulk density of the Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ sintered bodies was measured using Archimedes’ method. Phase composition of the Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ sintered bodies was examined using D8 FOCUS X-ray diffractometer. Cross-sectional structures of the Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ sintered bodies were characterized using Zeiss Sigma HD scanning electron microscopy. Dielectric constant and dielectric loss were measured using KEYSIGHT E5080B network analyzer. Coefficient of thermal expansion(CTE)of the sample was measured using DIL 402EP thermal expansion meter over 25–500 ℃ at 5 ℃·min⁻¹. Three-point bending strength was measured using CMT6208 electronic universal tensile testing machine.

Softening temperature of the Ca-B-Si-O glass is 693 ℃. With increasing value of x, the softening temperature was gradually increased, while the Tf value of the glass/CaTiO₃ reached 713 ℃. Moreover, the shape of the crystallization heat-releasing peak of Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ material varied with the different composite materials. With increasing firing temperature, the bulk density first increased, and then decreased due to the overburning. The transition temperature of different samples is not consistent. The high-temperature molten Ca-B-Si-O glass was homogeneous and totally amorphous. Quartz and wollastonite were crystalized from the Ca-B-Si-O glass at 875 ℃, while CaTiO(SiO₄) and CaTiSiO₅ crystals were formed in the glass/Ca_0.5Mg_0.5TiO₃ and glass/CaTiO₃ precipitated at 875 ℃, respectively. Ca-B-Si-O glass can be sintered at 750 ℃, with a small number of closed pores. As the sintering temperature was increased to 750 ℃, liquid phase was formed, which led to rapid densification. After sintering at 875 ℃, relatively smooth cross-section was observed, with a small number of closed pores. The glass/CaTiO₃ samples had a large number of pores after sintering at 925 ℃, due to the overburning, thus leading to rapid decrease in density. The maximum dielectric constants were 16.71 and 25.18, for the glass/Ca_0.5Mg_0.5TiO₃ and glass/CaTiO₃, respectively. The Ca-B-Si-O glass/Ca_xMg_1-xTiO₃ could be sintered at 875 ℃. The glass/CaTiO₃ materials with 50 wt.% CaTiO₃ showed the highest dielectric properties, with density ρ of 3.17 g·cm⁻³, dielectric constant ε_r of 25.18 at 7 GHz, dielectric loss tanδ of 0.0009 at 7 GHz, bending strength of 165 MPa and the thermal expansion coefficient of 7.17×10-6 ℃⁻¹.

The composition of Ca_xMg_1-xTiO₃ has different effects on densification, thermal and dielectric properties of Ca-B-Si-O glass/Ca_xMg_1-xTiO₃. With appropriate Ca_xMg_1-xTiO₃ content, quartz crystals were formed in the Ca-B-Si-O glass, which is beneficial for promoting the sintering density and dielectric properties of glass/ceramic materials. The glass/Ca_xMg_1-xTiO₃ materials with excellent dielectric properties can be obtained through adjusting the value of x (x=0.5–1.0 mole fraction). Especially, the Ca-B-Si-O glass/CaTiO₃ sample sintered at 875 ℃ exhibited full densification, with a bulk density of 3.17 g·cm^-3, a dielectric constant of 25.18 at 7 GHz and a dielectric loss of 0.0009, making it potential candidate for low-temperature co-fired ceramics.