Alumina/glass composites for low temperature co-fired ceramics (LTCC) and radome applications

A commercial SiO2-Al2O3-CaO based glass at 20 wt% (G-20), 40 wt% (G-40), 50 wt% (G-50), 55 wt% (G-55), and 60 wt% (G-60) was added to Al2O3 in order to determine composite compositions for specific applications such as substrate, low temperature co-fired ceramics (LTCC) and radome. Densification, phase formation and characterization of dielectric, mechanical and thermal properties were studied. The optimum densification temperatures were determined to be 1350°C for the G-20, 1200°C for the G-40, and 850°C for the G-50, G-55 and G-60. The sintering mechanism was liquid phase sintering for the G-20 and G-40, and viscous sintering for the G-50 to G-60. Main crystalline phase was Al2O3 as well as anorthite crystallized after 825°C. Young's modulus decreased from 231 GPa for the G-20 to 105 GPa for the G-60. Flexural strength ranged between 387 MPa for the G-20 and 213 MPa for the G-60. Hardness also decreased with increasing amount of glass; 1058 HV for the G-20 and 679 HV for the G-60. Similarly, fracture toughness was 4.5 MPa/√m for the G-20 and 1.9 MPa/√m for the G-60. Dielectric constant and dielectric loss of the composites (G-20 and G-60) at 5 MHz were 8.76-7.32 and 0.0022-0.0081, respectively. Thermal conductivity decreased with increasing glass content; 1.25 W/m.K for G-40 and 0.98 W/m.K for the G-60 at 75°C. Thermal expansion coefficient decreased with increasing glass such that it was 7.72 ppm/°C for the G-20 and 6.66 ppm/°C for the G-60. Mechanical, thermal and dielectric properties were considered together for possible applications. For substrate applications, all composites were proposed due to low dielectric constant. For LTCC applications, the G-50 to G-60 composites were proposed due to lower sintering temperature, lower dielectric properties together with moderate mechanical and thermal properties. The G-55 composite was successfully fabricated by tape casting and co-fired with silver electrode. For radome applications, the G-20 and G-40 composites were proposed due to lower density, higher sintering temperature, lower Young's modulus and thermal expansion coefficients. Keywords: Glass, glass-ceramic, glass ceramic composite, sintering, liquid phase sintering, viscous sintering, LTCC, substrate, radome.