Doaa A Abdel Aziz
Articles written in Bulletin of Materials Science
Volume 34 Issue 1 February 2011 pp 143-148
The influence of SrO (0.0–5.0 wt%) on partial substitution of alpha alumina (corundum) in ceramic composition (95 Al2O3–5B2O3) have been studied by co-precipitated process and their phase composition, microstructure, microchemistry and microwave dielectric properties were studied. Phase composition was revealed by XRD, while microstructure andmicrochemistry were investigated by electron-probe microanalysis (EPMA). The dielectric properties by means of dielectric constant (𝜀r), quality factor (𝑄 × 𝑓) and temperature coefficient of resonant frequency (𝜏f) were measured in the microwave frequency region using a network analyser by the resonance method. The addition of B2O3 and SrO significantly reduced the sintering temperature of alumina ceramic bodies to 1600°C with optimum density (∼ 4 g/cm3) as compared with pure alumina powders recycled from Al dross (3.55 g/cm3 sintered at 1700°C).
The sintered ceramic bodies show different ranges for the dielectric properties. Dielectric constant exists between 24 and 29, quality factor (𝑄 × 𝑓) is ranging between 15,236 and 22,020 GHz with a slight increase than those recorded with commercial alumina (10,000 – 20,000 GHz) and temperature coefficient of resonant frequency (𝜏f) in the –69 to –83 ppm/°C range. The addition of SrO up to 5 wt% leads to a maximum (𝜀r) value (29) due to relatively higher ionic polarizability of Sr2+ than that of the Al3+ and B3+ ions. On the other hand, changing chemical and phase composition with the formation of platelets of Sr-hexaluminate phase (SrAl12O19) results in maximizing value of (𝑄 × 𝑓) up to 22,020 GHz at ≈ 8 GHz and large negative charge of (𝜏f) to –83 ppm/°C.
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