Structure, thermal expansion, elastic modulus and dielectric properties of alkali-free boroaluminosilicate glasses with Bi2O3 substitution for CaO

In this paper, alkali-free boroaluminosilicate glasses with low thermal expansion coefficient and low dielectric loss were prepared. The effect of Bi₂O₃ substitution for CaO on structure, thermal expansion, elastic modulus, and dielectric properties of the samples was investigated by Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermal dilatometer, ultrasonic thickness gauge, and impedance analyzer. The results show that the polymerization degree of the glass network structure increases first and then decreases, which reaches the highest with the substitution of 0.5 mol% Bi₂O₃ for CaO. The thermal expansion coefficient and dielectric constant decrease and then increase, while elastic modulus shows the opposite trend, which is attributed to the change in the glass network structure. The atomic mass of Bi much greater than that of Ca causes the obvious increase of glass density. The ionic radius of Bi³⁺ ions is larger than that of Ca²⁺ ions, making it more difficult to migrate through the glass network, reducing dielectric loss. In particular, the sample with 0.5 mol% Bi₂O₃ substitution shows excellent properties, such as low thermal expansion coefficient (3.07 × 10^–6/K), high elastic modulus (80.58 GPa), low dielectric constant (5.52), and dielectric loss (3.15 × 10^–3), which is very suitable for use as chip packaging material.