Abstract
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 ZnAl2O4 (gahnite) as a base material. With its high thermal conductivity (~59 W·m−1·K−1 reported for 0.83ZnAl2O4–0.17TiO2), ZnAl2O4 is potentially more thermally conductive than Al2O3 (alumina); however, it sinters densely at a moderate temperature (~1500 °C). The addition of only 4 wt.% of Cu3Nb2O8 significantly lowered the sintering temperature of ZnAl2O4 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 (εr) of 9.2, a quality factor by resonant frequency (Q × f) value of 105,000 GHz, and a temperature coefficient of the resonant frequency (τf) 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 Cu3Nb2O8 additive and ZnAl2O4 has a melting temperature that is not significantly different from that (~970 °C) of the pristine Cu3Nb2O8 additive. The sample appears to densify in the solid state through a solid-state-activated sintering mechanism.
Highlights
Low-temperature co-fired ceramics (LTCCs) are dielectric materials that can be co-fired with Ag or Cu, which are metals that exhibit low-resistance conduction at temperatures below their melting points (961 and 1084 ◦ C, respectively) [1,2]
The addition of a small amount of the calcined Cu3 Nb2 O8 sintering aid to ZnAl2 O4, a highly thermally conductive dielectric material, successfully enabled ZnAl2 O4 sintering at temperatures below the melting point of silver (961 ◦ C), resulting in better dielectric properties than those obtained by the addition of the non-calcined CuO–Nb2 O5 sintering aid
The sample fired at 960 ◦ C exhibited more suitable dielectric characteristics, such as a relative permittivity εr of 9.2, a Q × f of 105,000 GHz, and a temperature coefficient of resonant frequency τ f of −56 ppm·K−1 when the holding time was prolonged from 2 h to 384 h
Summary
Low-temperature co-fired ceramics (LTCCs) are dielectric materials that can be co-fired with Ag or Cu, which are metals that exhibit low-resistance conduction at temperatures below their melting points (961 and 1084 ◦ C, respectively) [1,2]. Aluminum-based oxide ceramics, such as Al2 O3 (alumina), are relatively highly thermally conductive; a large amount (approximately 50% or more of the total) of a poorly thermally conductive low-softening-point glass needs to be added to achieve low-temperature sintering when used as the base material. The majority of these conventional LTCC materials are poorly thermally conductive (approximately 2–7 W·m−1 ·K−1 ), which is a shortcoming [4,5]. The heat-generation densities of semiconductor-based electronic components, such as light-emitting diodes (LEDs), mounted on LTCC multilayer devices have recently been reported to be increasing [6,7]; highly thermally conductive LTCC materials are in demand. We previously developed sintering additives for alumina using highly thermally conductive (~30 W·m−1 ·K−1 )
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