Effects of alkaline earth oxides on the densification and microwave properties of low-temperature fired BaO–Al₂O₃–SiO₂ glass–ceramic/Al₂O₃ composites

The effects of alkaline-earth oxides (SrO, CaO, MgO) on the densification and microwave properties of low-temperature fired BaO–Al₂O₃–SiO₂ (BAS) glass–ceramic/Al₂O₃ composites have been investigated. By adding alkaline-earth oxides, the softening and crystallization behaviors of BAS-based glass–ceramics, which determine the densification of composites, are sequentially weakened in the order of BAS, SrO–BaO–Al₂O₃–SiO₂ (SBAS), CaO–BaO–Al₂O₃–SiO₂ (CBAS) and MgO–BaO–Al₂O₃–SiO₂ (MBAS). It is also found in the above BAS-based glass–ceramics that the addition of alkaline-earth oxides plays a more profound role on crystallization behavior than on softening behavior, leading to the expansion of the interval between crystallization and softening temperatures (i.e., densification temperature range) to different degrees depending on the type of oxides added. As a result, the microstructures of oxides-modified glass–ceramic/Al₂O₃ composites (SBAS/Al₂O₃, CBAS/Al₂O₃, MBAS/Al₂O₃) are much denser than those of BAS/Al₂O₃. The properties of such composites are determined by the interplay among density of microstructure, amount and relative ratio of crystalline and residual glass phases related to the crystallization behavior and interfacial reaction. Detailed analyses suggest that CBAS/Al₂O₃ achieves optimal properties among all the four composites. Specifically, for CBAS/Al₂O₃ the dielectric constant, dielectric loss, bending strength and CTE value are 5.81 ± 0.12 (at ~ 10 GHz), (2.04 ± 0.12) × 10⁻³ (at ~ 10 GHz), 155 ± 7 MPa and 6.58 ± 0.11 ppm/°C, respectively. Our findings may provide a new avenue to design LTCC substrate materials as feasible candidates for microwave devices.