Low-temperature co-fired ceramic (LTCC) technology becomes a cornerstone in the fabrication of high-frequency, high-density microelectronic devices. However, a critical challenge lies in a thermal expansion mismatch between conventional LTCC substrates (typically <7 × 10^-6 ℃^-1) and printed circuit boards (PCBs, 12-16 × 10^-6 ℃^-1), leading to interfacial delamination and compromised reliability. To address this issue, developing LTCC materials with high and linear thermal expansion coefficients is essential. Among some potential candidates, BaO-B₂O₃-SiO₂ glass-ceramics stand out due to their tunable properties and relatively high thermal expansion. Nevertheless, achieving dense sintering at < 900 ℃, while maintaining a high thermal expansion coefficient (>11 × 10^-6 ℃^-1), remains a key obstacle. This study was to investigate the synergistic effects of incorporating CaO and SrO into BaO-B₂O₃-SiO₂ glass-ceramics on the sintering behavior, crystallization characteristics, and overall performance for LTCC applications.

Four glass compositions were designed based on the BaO-B₂O₃-SiO₂ system at different additions of CaO and SrO (in mole fraction), i.e., BBS1 (baseline), BBS2 (with 10% CaO), BBS3 (with 10% SrO), and BBS4 (with 5% CaO and 5% SrO). Raw materials including BaCO₃, CaCO₃, SrCO₃, H₃BO₃, and SiO₂ were melted at 1400 ℃ for 2 h, quenched, and ground into powders with a consistent particle size (D₅₀ ≈ 5 μm). The powders were pressed into pellets and bars, then sintered at 800 ℃ for 20 min. The sintering behavior was analyzed by high-temperature microscopy and dilatometry. The glass transition (T_g) and crystallization (T_c) temperatures were determined by differential scanning calorimetry (DSC). The phase identification and crystallinity were evaluated by X-ray diffraction (XRD), and the microstructural features were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The thermal expansion, density, dielectric properties, flexural strength, and hardness were measured to assess the performance of the sintered glass-ceramics.

The introduction of CaO and SrO significantly lowers the onset (T_fs) and maximum shrinkage (T_ms) temperatures, with SrO having a more pronounced effect. For instance, T_fs decreases from 720 ℃ (BBS1) to 654 ℃ (BBS3), and T_ms decreases from 794 ℃ to 761 ℃. This indicates enhanced sintering kinetics due to the larger ionic radius and higher polarizability of Sr²⁺, which facilitates atomic mobility and reduces activation energy. The results of DSC reveal that the addition of CaO and SrO also decreases T_g and T_c, with SrO again exhibiting a greater influence. The ΔT (T_c-T_g) value, which reflects the processing window for sintering, is increased in SrO-containing samples, indicating an improved sinter-ability.

The XRD patterns show that the baseline sample (BBS1) primarily crystallizes into Ba₂Si₄O₁₀, while CaO addition (BBS2) promotes the formation of Ba₅(Si₈O₂₁), and SrO addition (BBS3) and co-addition (BBS4) favor forming Ba₂Si₃O₈. The crystallinity increases due to CaO and SrO incorporation, reaching a maximum of 77.76% in BBS2, indicating that CaO is more effective in promoting crystallization due to its smaller ionic radius and higher field strength, which enhances network disruption and nucleation. The Raman spectra confirm a structural transformation from [BO₃] to [BO₄] units, indicating a more connected and dense glass network.

The microstructural analysiss reveals that the baseline sample has a porous structure with dispersed spherical crystals. In contrast, CaO and SrO additions lead to denser microstructures with interlocked crystal morphologies. Particularly, BBS4 exhibits a uniform distribution of plate-like Ba₂Si₃O₈ crystals and reduced porosity, contributing to its superior mechanical properties.

Thermal expansion coefficients (30-300 ℃) range from 11.52 × 10^-6 ℃^-1 to 12.43 × 10^-6 ℃^-1, with BBS4 showing a balanced value of 11.94 × 10^-6 ℃^-1. The slight decrease compared to the baseline is attributed to the increased [BO₄] content and denser structure. The dielectric constants at 1 MHz vary between 6.51 and 7.82, with BBS3 showing the maximum value due to its high-polarizability Sr²⁺ and Ba₂Si₃O₈ phase. The flexural strength and hardness are significantly improved in CaO- and SrO-modified samples, with BBS4 achieving the maximum values (i.e., 124.36 MPa and 953.88 HV). These enhancements are attributed to increased crystallinity, reduced porosity, and optimized crystal interlocking.

This study demonstrated that the co-incorporation of CaO and SrO into BaO-B₂O₃-SiO₂ lass-ceramics effectively modulated their sintering and crystallization behaviors, enabling dense sintering at 800 ℃. SrO played a dominant role in lowering sintering temperatures, while CaO was more effective in promoting crystallization. The synergistic effect of both oxides led to a glass-ceramic with a high thermal expansion coefficient (i.e., 11.94 × 10^-6 ℃^-1), excellent mechanical strength (i.e., 124.36 MPa), and favorable dielectric properties (i.e., 8.71 at 1 MHz), making it highly suitable for LTCC applications. The findings could provide a promising pathway for developing high-performance, low-temperature sintering glass-ceramics with tailored thermal and mechanical properties.