Fabrication and characterization of compositionally-engineered glass/ceramic/nano-filler composites for LTCC and radome applications

It was aimed to optimize mechanical, thermal and dielectric properties of (SiO2-Al2O3-CaO based) glass/ceramic/(and nanofiller (hexagonal boron nitride, aluminium nitride) composites for functional and structural applications. A base alumina/55 weight percent glass composition was first selected and then engineered to improve overall physical properties. For this purpose, mullite as the replacement for alumina as well as addition of nano fillers (e.g., platelet-shaped nano hexagonal boron nitride particles or nano aluminium nitride powders) were primarily considered. Densification, phase formation, microstructure evolution, mechanical properties (hardness, flexural strength, elastic constant), dielectric properties (dielectric constant and loss) and thermal properties (thermal conductivity, thermal expansion coefficient) were investigated as a function of glass content (10 to 60 weight percent), nano filler content (1 to 10 weight percent) and crystalline phases (mullite or alumina). Densification mechanism was liquid phase sintering for the 10-30 weight percent glass and viscous sintering for the 50-60 weight percent glass compositions. Nano fillers aluminium nitride and hexagonal boron nitride neither chemically reacted with other phases nor decomposed due to sintering temperatures less than 950 C. Closed porosity was found to be a critical parameter for the nano filler-added composites. The overall results were evaluated to comply with some potential structural and functional applications such as low temperature co-fired ceramics (e.g., mullite/55 weight percent glass/10 weight percent hBN; bulk density=2.3 g/cm3 after sintering at 900 °C, dielectric constant (loss) = 5.13 (0.003) at 5 MHz, thermal conductivity = 1.91 W/m·K at 25 °C, thermal expansion coefficient= 4.89 ppm/°C, flexural strength=71 MPa and elastic modulus=71 GPa) and radome (mullite/20 weight percent glass; high sintering temperature of 1450°C and highest thermal shock resistance parameter of 162°C together with dielectric constant (loss)=7.12 (0.0025) at 5 MHz (and dielectric constant=5.9 at 4–10 GHz), thermal conductivity=3.72 W/m·K, thermal expansion coefficient=4.27 ppm/°C, flexural strength=180 MPa, elastic modulus=189 GPa). These results are comparable with respect to the commercial products. Finally, low-temperature co-fired ceramic and radome prototypes were successfully fabricated by tape casting and gel casting, respectively. Keywords: Low-temperature co-fired ceramics, radome material, dielectric properties, thermal properties, mechanical properties.