CuO-TiO2-Nb2O5 was utilized as additive (5 wt%) to obtain densified alumina using the firing temperature of 885 °C for 96 h. Densification started at ~835 °C, which was lower than the melting point of the additive (~967 °C). The melting temperature of the powder mixture consisting of the additive and alumina was measured, but no significant changes were observed compared with the melting temperature of the additive. Furthermore, the lattice constants measurements of the alumina sample obtained after heat treatment at 885 °C revealed an increase in unit cell volume, which suggested the incorporation of Cu and Ti components into alumina according to the TEM-EDS analysis. In addition, the timing of the increase in alumina lattice constant and that of the rapid increase in the sintered density of the additive-containing alumina coincided. These results indicated that the densification of the sample occurred in solid state (solid-state-activated sintering). The sample fired at 935 °C for 6 h exhibited the thermal conductivity of 22 W/mK, which was higher than that of conventional low-temperature co-fired ceramic materials (~2-7 W/mK), the relative dielectric constant (εr) of 10.2, the quality factor multiplied by the resonant frequency (Q × f) value of 47000 GHz, and the temperature coefficient of resonant frequency (τf) of-50 ppm/K.
Recent studies by the authors have been intended to develop sintering additives that can make alumina sintered at low temperature with a small quantity. In the case of use of CuO-TiO 2-Nb 2 O 5-Ag 2 O type additives, high-density alumina was obtained by firing at a temperature of 900℃ or below, even though additives content was only 5mass%. In this study, co-firing of above materials with Ag-electrodes at 900℃ or below was examined in order to clarify the relationship between chemical compositions and their reactivity. The results showed that reactivity was highly affected by the Ag 2 O content in the sintering additives. When the content of Ag 2 O was higher, any disappearance of Ag-electrodes was not found and electric properties were almost the same as those of conventional LTCC materials. Finally, these results indicated the obtained materials had possibility to be applied to LTCC modules having high 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 εr of 11.3, quality factor–resonant frequency product Q × f of 4700 GHz, and temperature coefficient of resonant frequency τf 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.
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