A modified sintering method to prepare phase pure AlN ceramics: Structural and dielectric studies for microwave applications

Abstract

Aluminium nitride (AlN) requires a very high temperature >1900 °C in a specific environment (in the presence of nitrogen) to be densified. Many researchers have used different techniques for sintering: microwave sintering, spark plasma sintering, and conventional sintering. Microwave sintering is dangerous for operating, spark plasma sintering is expensive and conventional sintering is economical but time-consuming and difficult to obtain pure AlN phase. Preparing pure AlN ceramics is challenging because of the high sintering temperature. Since oxidation will occur at high temperatures while sintering, this study deals with the preparation of pure AlN ceramics and their dielectric and structural properties. In this study, AlN ceramics are prepared by conventional sintering at low-temperature 1700 °C for 30h by adopting the powder bed method in the presence of nitrogen. This powder bed consists of 70% AlN powder and 30% C (carbon) powder. These sintered AlN ceramics achieved maximum relative density ∼95% with wurtzite structure of space group P63mc, space group number 186, confirmed from XRD refinement. It is significant to note that the powder bed eliminated the formation of Al2O3. The improvement in the density at the lower sintering temperatures is attributed to smaller initial particle sizes. The Raman spectra of AlN ceramics exhibits six phonon modes of wurtzite structure. The average grain size of sintered AlN ceramic is 1.45 μ m. The temperature-dependent Raman spectrum of AlN ceramics exhibits structure stability at various temperatures with stable wurtzite structure phonon modes. These sintered AlN ceramics exhibited good microwave dielectric properties as dielectric permittivity (εr = 8.83) and dissipation factor (tanδ = 2.686 × 10−4). In this study, the AlN ceramic exhibits the best dielectric properties in the broad frequency range 1 MHz–100 MHz at a different temperature from −140 °C to 200 °C. The obtained properties of AlN ceramics are essential and appropriate for RF- window and electronic applications.