Gel casting of mullite for radome applications

GEL CASTING OF MULLITE FOR RADOME APPLICATIONS ABSTRACT Gel casting is a colloidal processing route to produce large and 3D complex products. It was aimed to investigate gel casting of mullite ceramics because this method has not been studied to produce dense mullite ceramics in literature. Mullite was chosen due to its excellent creep and thermal shock resistance at high temperatures, and low dielectric properties suitable for electronic and radome applications. Gel casting parameters such as monomer types, their ratios, effect of initiator and catalyzer were studied. Densification behavior, dielectric properties, mechanical properties and thermal properties were characterized in detail for radome applications. Initial mullite powder had 5-10% glass phase and 1 % corundum. Ball milling for 48 h decreased mean particle size from 3-5 m to 0.28 m. 3.4 wt% nano TiO2 (e.g., 21 nm) was added to form a liquid phase to aid densification. Gelation of the slurries was completed in 3 to 5 minutes at 25°C for the samples with catalyzer and in 35 to 65 minutes at 70°C for the samples without catalyzer. All samples reached to the relative densities of 97 to 98.2 % after sintering at 1540°C for 2 h. Backscattered SEM images revealed anisotropically grown mullite grains, glassy phase located as islands between the grains, hexagonally-shaped Al2O3 grains, white-color grains rich in ZrO2 and TiO2, and pores with sharp corners. All samples had hardness values between 1077 HV to 1089 HV, and fracture toughness values between 2.4 to 2.5 MPa. √m. The Weibull modulus was calculated as 12.33. Thermal conductivity increased and ranged from 1.5 W/m.K at 50°C to 3.75 W/m.K at 550°C due to the presence of a glassy phase. Thermal expansion coefficient was 5.26 ppm/°C between 30°C and 800°C. The dielectric constant was nearly 7.5 up to 10 GHz and loss tangent was 0.0031 at 5 MHz. Mullite is a suitable candidate for radome applications because it has a higher thermal shock resistance parameter (142°C) than Al2O3 and has advantages in terms of sintering in air as opposed to Si3N4 and BN ceramics. A large block (240x250x35 mm3) was successfully casted, and then easily machined to the desired shapes. Keywords: Mullite, Gel casting, Liquid phase sintering, Mechanical properties, Thermal Properties, Dielectric Properties, Radome