Influence of R²⁺ (R = Mg, Ca, Sr) partial substitution for Ba²⁺ on structures and the thermal properties of BaO–Al₂O₃–SiO₂–B₂O₃ LTCC materials

Abstract

A combination of dielectric and thermal properties is the prime factor for developing glass–ceramics for practical aspects in low-temperature co-fired ceramic technology. In this work, we report the low melting-point compositions of BaO–Al₂O₃–SiO₂–B₂O₃ and BaAl₂Si₂O₈ ceramics at 900 °C. The phase evolutions, microstructure, dielectric, and thermal expansion properties corresponding to the partial replacement of Ba²⁺ with alkaline earth metal ions R²⁺, i.e., Sr²⁺, Ca²⁺, and Mg²⁺ were investigated. The results indicated that, because the lattice distortion of hexacelsian structure was caused by solid solutions of substitution ions to BaAl₂Si₂O₈ ceramics, the phase transition of hexacelsian can be gradually inhabited with the decrease of the substitution ion radius. Notably, the partial substitution of Mg²⁺ in the Ba²⁺ can completely inhibit the phase transition and promote the formation of orthorhombic celsian. By the substitution of 40 mol% of Ba²⁺ to Mg²⁺, physical properties, i.e., density of 2.22 g⋅cm⁻³, ε_r of 4.08 at 12 GHz, and tanδ of 2.7 × 10⁻³, are obtained. Importantly, the coefficient of thermal expansion (CTE) of 4.02 × 10⁻⁶/°C close to that of monocrystalline silicon is also successfully achieved. The new BaAl₂Si₂O₈ ceramics with low CTE and reliable thermal stability demonstrated the potential for application in chip packaging.