Solid-State-Activated Sintering of ZnAl 2 O 4 Ceramics Containing Cu 3 Nb 2 O 8 with Superior Dielectric and Thermal Properties

Low-temperature co-fired ceramics (LTCCs) are dielectric materials that can be co-fired with Ag or Cu; however, conventional LTCC materials are mostly poorly thermally conductive, which is problematic and requires improvement. We focused on ZnAl O (gahnite) as a base material. With its high thermal conductivity (~59 W·m ·K reported for 0.83ZnAl O -0.17TiO ), ZnAl O is potentially more thermally conductive than Al O (alumina); however, it sinters densely at a moderate temperature (~1500 °C). The addition of only 4 wt.% of Cu Nb O significantly lowered the sintering temperature of ZnAl O to 910 °C, which is lower than the melting point of silver (961 °C). The sample fired at 960 °C for 384 h exhibited a relative permittivity ( ) of 9.2, a quality factor by resonant frequency ( × ) value of 105,000 GHz, and a temperature coefficient of the resonant frequency ( ) of -56 ppm·K . The sample exhibited a thermal conductivity of 10.1 W·m ·K , which exceeds that of conventional LTCCs (~2-7 W·m ·K ); hence, it is a superior LTCC candidate. In addition, a mixed powder of the Cu Nb O additive and ZnAl O has a melting temperature that is not significantly different from that (~970 °C) of the pristine Cu Nb O additive. The sample appears to densify in the solid state through a solid-state-activated sintering mechanism.