| Summary: | 博士 === 國立成功大學 === 材料科學與工程學研究所 === 85 === Strontium barium niobate (SBN) ceramic is a good electro-optic material and has been widely used. Though the properties of the single- crystal SBN have been intensively studied, there are still some restrictions in their applications because of the small size, low mechanical strength, and high cost. Hence, it has been intrigued to develop the strontium barium niobate ceramic. However, it was reported that highly dense SBN was difficult to fabricate when pressureless sintering was used. The purposes of this study were to evaluate the reaction kinetics and sintering behavior and the effect of atmospheres on reaction- and normal-sintering.
The solid state reaction of SrNb2O6 (SN) and BaNb2O6 (BN) to form SrxBa1-xNb2O6 (SBN) at different temperatures and heating rates was investigated. The reaction kinetics were analyzed by X-ray diffraction for quenched samples and the internal standard method was used to quantify the extent of the reaction. A non-isothermal kinetic empirical model was employed to evaluate the activation energy and rate constant of reaction for forming SBN with different ratios of Sr/Ba. It was found that the kinetic form would change above and below a transition at about α≒60%， which might be due to the change of the frequency factor. As the heating rate and the ratio of Sr/Ba are increased, the fully reacted temperatures of SBN increase. It is suggested that SBN with a prepared ratio might form directly rather than a variety of SrxBa1-xNb2O6 during reaction. The reaction mechanism was suggested and discussed. The higher the molar ratio of Sr/Ba, the higher the activation energy for forming SBN.
The sintering behavior and microstructural evolution of reaction-and normal-sintering have been investigated and compared. Hg-penetration was used to evaluate the pore size distribution and SEM was employed to observe the microstructural development during sintering. In comparing the densification behavior of the reaction sintering and normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became higher after reaction. It was also found that the chemical reaction occurred prior to densification but a high-density and uniform microstructure obtained for the reaction- sintered sample was pronouncedly enhanced when the reaction was almost completed. A higher density (>98%Dth), uniform and fine-grained (3~4μm) microstructure of SBN could be achieved by using the reaction sintering.
The sintering behavior of SrxBa1-xNb2O6 (x=0.5, 0.6, and 0.7) powders with similar particle size was studied. The modified statistical theory of sintering was used to evaluate the activation energy. It was found that the densification rate increased and the evaluated activation energy of SBN decreased with the increase of the molar ratio of Sr/Ba. It was observed that in sintering SBN50, below 1300℃, the densification rate decreased with the increase of the oxygen partial pressure, and above 1300℃, the oxygen partial pressure became insignificant influence on the densification. The relationship of sintering behavior and structure of SBN was discussed and the Nb5+ ion was suggested as the rate-limiting species in sintering SBN. The final relative density of the sample sintered in O2 could reach 98.7%Dth.
Finally, based on the results of this study, it was found that the density of the reaction-sintered sample sintered at 1300℃ for 0.2~0.5h in O2 could have high density ~99%Dth and a uniform and fine microstructure. Moreover, if those samples were further heat-treated at appropriated conditions, their density could reach ~99.5%Dth and they would become translucent.
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