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(2)O(4) (gahnite) as a base material. With its high thermal conductivity (~59 W·m(−1)·K(−1) reported for 0.83ZnAl(2)O(4)–0.17TiO(2)), ZnAl(2)O(4) is potentially more thermally conductive than Al(2)O(3) (alumina); however, it sinters densely at a moderate temperature (~1500 °C). The addition of only 4 wt.% of Cu(3)Nb(2)O(8) significantly lowered the sintering temperature of ZnAl(2)O(4) 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 (ε(r)) of 9.2, a quality factor by resonant frequency (Q × f) value of 105,000 GHz, and a temperature coefficient of the resonant frequency (τ(f)) of −56 ppm·K(−1). The sample exhibited a thermal conductivity of 10.1 W·m(−1)·K(−1), which exceeds that of conventional LTCCs (~2–7 W·m(−1)·K(−1)); hence, it is a superior LTCC candidate. In addition, a mixed powder of the Cu(3)Nb(2)O(8) additive and ZnAl(2)O(4) has a melting temperature that is not significantly different from that (~970 °C) of the pristine Cu(3)Nb(2)O(8) additive. The sample appears to densify in the solid state through a solid-state-activated sintering mechanism.