微結構對鈦酸鋇介電及鐵電性質的影響

Abstract

因為鈦酸鋇獨特的介電及鐵電性質，而被廣泛使用於電熱調節器，壓電的偵測裝置，陶瓷介電材料，和非揮發性記憶體等。本研究使用五種不同的燒結條件燒結出不同的微結構藉以探討微結構對鈦酸鋇介電及鐵電性質的影響。 當燒結溫度大於1330℃時，因為液相的生成，Ti-excess BaTiO3會產生異常的晶粒成長。本實驗藉由影像分析探討晶粒分布和燒結條件的關係。當燒結溫度到達1335℃時，異常晶粒成長開始產生，形成同時具有大晶粒和小晶粒的bimodal microstructure。而當燒結溫度到達1350℃時，這種bimodal microstructure消失，取而代之的是100%的大晶粒的微結構。 實驗中發現，鈦酸鋇的介電常數隨著晶粒減小和密度增加而增加，而絕緣電阻率則和密度較為相關，其隨著密度的增加而增加。損失因子和晶粒大小與密度息息相關，因為晶粒大小會影響domain wall 移動進而影響損失因子，而孔洞中的水氣也會對其造成影響。實驗中發現，若試片密度小於88%，則完全沒有鐵電的性質且有大的漏電流產生。而大晶粒的試片有較高的remanent polarization且展現軟性的鐵電材料特質。 實驗中發現，小晶粒的試片對交流電場展現較佳的抗性和較好的可靠度，因為小晶粒的介電強度和破壞強度皆較大晶粒試片為佳。我們也發現在交流電場作用之下，有銀遷移的產生。可能的原因是交流電場作用導致裂縫的產生而提供了銀一個較佳的傳導途徑，進而導致銀遷移。

Due to the unique dielectric and ferroelectric properties, barium titanate has been widely used as positive temperature coefficient thermistors, piezoelectrics, capacitors, and dynamic memory etc. In the present study, five sintering conditions are applied to produce the specimens with various microstructures. The effects of microstructures such as grain size and density on the dielectric and ferroelectric properties are then investigated. Abnormal grain growth is formed in the Ti-excess BaTiO3 due to the formation of liquid phase at > 1330℃. By applying image analysis, the relationships between grain size distribution and sintering conditions are established. As the sintering temperature reaches 1335℃, abnormal grain growth is occurred; a bimodal microstructure of both fine grains and coarse grains is formed. As the sintering temperature is raised to 1350℃, the bimodal microstructure disappeared, a 100% coarse grains microstructure formed instead. The dielectric constant increases with decreasing grain size and increasing density. The insulation resistivity is dependent of the density, showing a increase with increase of density. The dissipation factor is dependent of both the grain size and the density due to the domain wall movement and the absorbed water in the pores. The specimen with density＜88% shows no ferroelectric properties and a large leakage current. The coarse-grained specimen shows a higher value of remanent polarization and behaves as a soft ferroelectric material. The fine-grained specimen shows a better resistance under cyclic electric field and exhibits a better reliability. It is due to the dielectric strength and fracture strength of fine-grained specimen is higher. We also found that silver migration is taken place when a cyclic electric field is applied. It may due to that the Ag migrate through the fatigue cracks and flaws produced by the cyclic electric field.