| Summary: | 碩士 === 國立成功大學 === 資源工程學系碩博士班 === 92 === Abstract
Cordierite has low dielectric constant, low thermal expansion coefficient, high resistivity, high conductivity and good mechanical strength; it is mainly used in the fabrication of catalytic converter honeycomb supports used in automobiles, self-cleaning ovens and industrial heat exchangers for turbines. Besides, cordierite is expected to have great potential as a substrate material for integrated circuit board replacing alumina, which has a relatively low dielectric constant (~5) at the high frequency region.
Cordierite is a ternary compound of the MgO-Al2O3-SiO2 system, with the stoichiometric composition of 2MgO．2Al2O3．5SiO2. A sol-gel process was utilized to prepare low-temperature sinterable ceramics with a stoichiometric cordierite composition, in which Bi2O3 additives were incorporated to control the crystallization process and lowering the sintering temperature
In this study, ultrafine powders were precipitated by titrating NH4OH into the aqueous solution of MgCl2, AlCl3, TEOS and alcohol. The suitable pH value would be 9 and the above for the reaction to be completed. By calcining the powders, α-cordierite precursor was obtained.
Comparing the ceramic bulks prepared by low temperature calcinations (500℃) and high temperature calcinations (1400℃), the previous sample has denser structure, which means that low temperature calcinations has a better influence in sintering behavior.
The highest quality factors found in this study were 10,000 ~ 20,000 in 1 MHz, and 404 ~ 620 at high frequency region (18 MHz), both with dielectric constant of ~5, which considered as excellent dielectric properties.
Ceramic bulks sintered at 870 ~ 970℃ for 8 hours with 15 ~ 20 wt% Bi2O3 additives have thermal expansion coefficients of 17 ~ 20 x 10-7 /℃, which match the sili con’s (30 x 10-7 /℃) very well.
The resistivity coefficient of the ceramic bulks without Bi2O3 additives were lower compared to those with Bi2O3 additive. As the amount of Bi2O3 additives increased, the resistivity coefficient increased and became stable. The best resistivity coefficient found in this study was 4 x 1011 Ωcm.
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