Sintering mechanism of low-temperature co-fired alumina featuring superior thermal conductivity

Densely sintered alumina is produced with a 5 wt% addition of a CuO-TiO₂-Nb₂O₅-Ag₂O sintering aid under a firing temperature of only 835°C with a prolonged holding time of 96 hours. The sintered material exhibits a thermal conductivity of 20 W/mK, which is significantly greater than that of conventional low-temperature co-fired ceramic (LTCC) materials (~2-7 W/mK). Additionally, suitable dielectric characteristics are observed, such as a relative dielectric constant ε_r of 11.3, quality factor–resonant frequency product Q × f of 4700 GHz, and temperature coefficient of resonant frequency τ_f of −87 ppm/K. The low-temperature densification is revealed to occur mainly in the solid state, before liquid phase formation. Lattice constant measurements, transmission electron microscopy, and energy dispersive spectroscopy reveal an increase in unit cell volume upon densification and the incorporation of Cu²⁺ and Ti⁴⁺ ions into the alumina lattice, which promotes densification. The diffusion speeds of Cu²⁺ and Ti⁴⁺ ions are indirectly affected by Nb and Ag atoms by lowering the additive melting temperature. Therefore, sintering additives with low melting points and elements that incorporate into the lattice of the base material are effective for low-temperature sintering of aluminum-based oxides.