Mechanical, thermal, and dielectric properties of glass mullite composites for low-temperature cofired ceramic and radome applications

SiO2-Al2O3-CaO-based glass (10-60 wt%)/mullite composites were investigated for the LTCC and radome applications. The optimum densification temperatures decreased from 1550 degrees C (10 wt% glass) to 1400 degrees C (30 wt% glass) by means of liquid-phase sintering, and to 850 degrees C-825 degrees C (50-60 wt% glass) by means of viscous phase sintering. XRD analysis showed that mullite was the main phase as well as in situ crystallized anorthite after 825 degrees C. The composite with 20 wt% glass was a suitable candidate for the radome applications (bulk density = 2.86 g/cm(3) after sintering at 1450 degrees C, dielectric constant (loss) = 7.12 (0.0025) at 5 MHz, thermal expansion coefficient = 4.27 ppm/degrees C between 25 degrees C and 800 degrees C, thermal shock resistance parameter = 162 degrees C), and the composite with 50wt% glass was a suitable candidate for the low-temperature cofired ceramic applications (bulk density = 2.64 g/cm(3) after sintering at 850 degrees C, dielectric constant (loss) = 6.79 (0.0043) at 5 MHz, thermal conductivity = 2.11 W/m center dot K at 25 degrees C, and thermal expansion coefficient = 3.93 ppm/degrees C between 25 degrees C and 300 degrees C).