Low-temperature Sintering Mechanism, Dielectric, and Thermal Properties of CuO-TiO₂-Nb₂O₅-containing Al₂O₃ Ceramics Featuring Superior Thermal Conductivity

Abstract

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.

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

“…There is no phase diagram for CuO‐TiO 2 ‐Nb 2 O 5 ‐Ag 2 O (additive “A”) or CuO‐TiO 2 ‐Nb 2 O 5 (additive “B”); hence, accurate eutectic temperatures are not known. Thus, T m was also measured for the peripheral compositions; however, T m did not drop below 868 and 967°C, respectively 15,16,18 . Therefore, the eutectic temperatures were equivalent to T m .…”

“…A T / T m ratio above 1.00 corresponds to liquid phase formation; below 1.00, densification occurs solely by solid‐state mechanisms. The T m of additives “B” and “C” were 967°C 16 and 1008°C, respectively. Additive “D” (Ag 2 O) reduces to Ag at approximately 200°C; therefore, the value of T m for additive “D” was set to 961°C, which is the melting temperature of Ag.…”

“…Thus, T m was also measured for the peripheral compositions; however, T m did not drop below 868 and 967°C, respectively. 15,16,18 Therefore, the eutectic temperatures were equivalent to T m . For additive "C" (CuO-TiO 2 ), Rubia et al 27 reported the eutectic temperature of CuO-TiO 2 as 1000°C at a Cu:Ti molar ratio of 75:25.…”

“…However, none of these binary additives yield satisfactory sintering performance at temperatures below 1000°C. Nevertheless, dense alumina has been produced at sintering temperatures of 925‐1000°C with the use of binary or ternary sintering additives such as 5‐10 wt% CuO‐Nb 2 O 5 or CuO‐TiO 2 ‐Nb 2 O 5 15‐17 . Particularly good sintering performance was obtained with the CuO‐TiO 2 ‐NbO 2.5 ternary sintering additive (molar ratio 4:1:4), because a Cu‐Ti‐Nb‐O‐type complex oxide (presumed to be Cu 4 TiNb 4 O x ) was generated almost entirely in a single phase, promoting alumina sintering 15 .…”

“…Nevertheless, dense alumina has been produced at sintering temperatures of 925-1000°C with the use of binary or ternary sintering additives such as 5-10 wt% CuO-Nb 2 O 5 or CuO-TiO 2 -Nb 2 O 5 . [15][16][17] Particularly good sintering performance was obtained with the CuO-TiO 2 -NbO 2.5 ternary sintering additive (molar ratio 4:1:4), because a Cu-Ti-Nb-O-type complex oxide (presumed to be Cu 4 TiNb 4 O x ) was generated almost entirely in a single phase, promoting alumina sintering. 15 Furthermore, quaternary sintering additives such as CuO-TiO 2 -Nb 2 O 5 -Ag 2 O [18][19][20] can reduce the sintering temperature to 900°C or below, 19,20 enabling co-firing with Ag electrodes.…”