Effects of Additives on the Densification and Microwave Dielectric Properties of Binary CaO–B2O3–SiO2 Glass

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Glass systems, consisting of binary CaO–B2O3–SiO2 (CBS) glass (referred to as glass AB) and various amounts of Li2CO3, TiO2, and Al2O3 additives, were prepared in this study. The effects of the additives on the densification and microwave dielectric properties of binary CBS glass were investigated. Binary glass AB was not densified until the sintering temperature reached 875 °C; however, with the addition of 1 wt % Li2CO3 (referred to as glass L1), the glass achieved a maximum density at 825 °C. The resultant glass-ceramics contained a major phase quartz, SiO2, and a minor phase, CaB2O4, and had a dielectric constant ($\varepsilon_{\text{r}}$) varying from 4.10 to 4.23 and a minimum $\tan\delta$ of 0.0015, as frequency increased from 4.3 to 18.6 GHz. For glass L1 with further addition of the ceramic filler Al2O3 (up to 30 wt %), the maximum densification was reached at 775 °C and the formation of CaB2O3 and SiO2 crystallites was suppressed. Both $\varepsilon_{\text{r}}$ and $\tan\delta$ increased with the Al2O3 content of the glass-ceramics. Further addition of the crystallization agent TiO2 (up to 5 wt %) to glass L1 slightly reduced the sintering temperature to 800 °C. The X-ray diffraction (XRD) intensities of the quartz SiO2 and CaB2O4 crystallites as well as the $\varepsilon_{\text{r}}$ and $\tan\delta$ values of the resultant glass-ceramic increased with the amount of TiO2 added.
2010-02-25