鐵電材料在MPB的高介電/壓電性能探討

Abstract

鐵電材料為常見智能材料之一，鐵電材料之壓電性質被廣泛運用在感測器與致動器。而鐵電材料在接近MPB時出現的高壓電與介電性能更是近十年來熱門的研究題材。而過去的文獻中指出鐵電材料在接近MPB時是以多階層狀微結構存在著(Jin et al. 2003; Wang 2006; Schonau et al. 2007; Wang 2007)。因此有別與之前文獻以能量最小化求得單一鐵電疇接近MPB時的壓電與介電性能，本篇論文以總自由能最小化加上層狀結構之諧和條件求解層狀結構之等效壓電與介電性能，並與微觀力學所計算出之等效係數作比較。 本篇論文發現多階層狀等效壓電介電係數並不等於個別鐵電疇之體積分率相加，並且以總自由能最小化求得之等效係數與微觀力學所計算出之等效係數相比較，在外加場極小時兩者會求出相同的係數，亦即材料在基態之切線模數。而在接近MPB時，層狀結構在巨觀極化方向的縱向壓電性能與介電性能皆大幅上升，且高於單一鐵電疇在極化方向之壓電與介電性能。

Ferroelectric materials have been widely recognized as one of important smart materials due to their excellent dielectric and piezoelectric behavior, and are therefore often used in actuator and sensor applications. It has been widely known that the abnormally large dielectric and piezoelectric behavior can be observed by forming MPB (morphotropic phase boundary) due to chemical alloying in certain conventional lead-based ferroelectrics. The explanations vary due to different observations of experiment. One of the important observations is the appearance of fine scales of laminar domain patterns as the composition approaches MPB (Wang et al. 2003; Wang 2006; Schonau et al. 2007; Wang 2007). As a result, the convention models based on single domain state for explaining the enhancement of piezoelectricity are questionable. Here, we propose a model based on energy minimizing laminated domain pattern for studying the effective dielectric and piezoelectric properties at composition near MPB. It is found that the effective coefficients are not equivalent to the simple volume average of the distinct domain states. The results are consistent to those base on micromechanics calculations when the applied field is small. In addition, the effective dielectric and longitude piezoelectric coefficients at the poling direction are much larger than those calculated based on the single domain state.