Abstract
摘要:現有的文獻中,鮮少有研究對於鐵電單晶晶體方位與外加電場或力電場方向之相對耦合角度,以及此耦合角度對於單晶體應變表現進行探討。因此本計劃將針對此主題在實驗與模擬上進行深入研究。鐵電單晶材料在電場極化作用下,因鐵電域(ferroelectric domain)之高度規則性排列,可獲得極佳之鐵電與壓電性質。在設計鐵電致動器的領域上,如何從鐵電單晶體的極化旋轉(polarization switching)過程中擷取巨幅之電致應變(electrostrain)乃為重要的研究目標之一。在鐵電單晶體內,極化旋轉可造成的應變總量依據於兩個重要因素:一、單晶方位與外加力電場方向間的耦合效應。二、在極化旋轉過程中,電偶極轉動時所依循的「變體」(variant)路徑。在同一角度之極化旋轉下,可因不同的「晶面-外加場」方向耦合與不同的偶極轉動路徑,進而造成極為相異之應變總量。本計畫在實驗上,將晶體方位{100}、{110}、與{111}之鈦酸鋇(BaTiO3)鐵電單晶體,在單軸力電耦合施加下,可量測到極化與應變遲滯表現。藉著改變外加有效場方向與單晶晶面方位的搭配組合,得以探討因電偶極轉動路徑不同所造成有效應變量之差異。故可利用不同晶體方位之BaTiO3單晶,搭配一個固定單軸方向之力電場來完成實驗目標。在模擬計算方面,研究中將採用「多階層狀鐵電域」(multirank laminating domains)模型,來模擬BaTiO3單晶體在不同「晶面-外加場」方向耦合下之極化與應變遲滯演變。藉由將其與實驗與理論數值之比較,深入探討模型參數之設定值與有效性。在單晶模型中,不同電偶極轉動路徑間的比例關係亦會整併成校正因子,用以調整在宏觀遲滯模擬上之精準度。藉由在BaTiO3單晶體上的研究基礎,本計畫亦將利用正方晶系之「織構」(textured)多晶無鉛陶瓷(Bi0.5Na0.5)TiO3-BaTiO3(簡寫為BNBT),製備出新式高應變非線性鐵電致動器雛型。與單晶鐵電材料相較之下,織構多晶鐵電陶瓷的優勢是:在製程上相對容易與便宜,更重要的是其晶粒具備優選成長方向,而此特徵使其具備類單晶之鐵電與壓電表現。在本計劃中,織構BNBT陶瓷將採用「反應模板晶粒成長法」(RTGG)製備。量測與模擬不同優選結晶方位之織構BNBT陶瓷之極化與應變遲滯表現(i.e.,利用單晶模型搭配統計均勻法)。BNBT鐵電致動器雛型則採用多層堆疊與外殼施壓設計,以驗證機械偏壓對於電致應變量提升上之協助,也藉由致動器的表現指標來檢視模擬計算之可靠度。
Abstract: Among all ferroelectric materials, single crystal ferroelectrics have received great attentions recently due to their ability to provide a high degree of polarization domain alignment with electrical poling, which could in turn give rise to superb piezoelectric characteristics. The achievement of large electrostrain from the single crystals is therefore an imperative goal for the designers and manufacturers of ferroelectric actuators. The strain magnitude produced by the polarization switching within a single crystal ferroelectric depends on two important factors: (1) the coupling between the direction of external loading and the crystallographic orientation of the single crystal, and (2) the "rotation route" adopted by the polarization to complete its switching. It is essential that a polarization switching action goes through variants at different angles relative to the loading direction in order to produce a noticeable strain. Currently in the literature there is a significant lack of studies both experimentally and theoretically on the directional coupling between the crystallographic orientation of a ferroelectric single crystal and the external electrical or electromechanical loading direction. This insufficiency shall be addressed in this study – the switching characteristics of tetragonal barium titanate (BaTiO3) single crystals of crystallographic orientations {100}, {110} and {111} under uniaxial electromechanical loading will be investigated. This will be achieved through examining their polarization and strain hystereses under loading. The coupling between the direction of external loading and the crystallographic plane of the single crystal will be varied to study the effectiveness of straining induced by different polarization rotation routes. This will be achieved by loading the BaTiO3 crystals of different orientations at an identical fixed-directional loading setup. A model framework based on energy minimization criteria and multirank laminating domains will be used to compose parameters which govern the evolutions of polarization and strain for the BaTiO3 single crystals at different crystal-load directional couplings. By calculating the differences between the measured, theoretical and simulated hystereses, the values and validity of the modeling parameters can be adjusted so that the proportions of different polarization rotation routes can be incorporated into the single crystal model as a correction factor for the simulated macroscopic strains and polarizations on the hysteresis path. The knowledge obtained from the studies of BaTiO3 single crystals will then serve as a guideline to design high-strain non-linear ferroelectric actuators made of tetragonal-based "textured" polycrystalline ceramics (Bi0.5Na0.5)TiO3-BaTiO3 (abbreviated as BNBT). Comparing to the single crystal ferroelectrics, the textured ferroelectric ceramics are considerably cheaper in terms of processing cost, and most importantly, they still possess large piezoelectric coefficients due to the preferential grain growth, making them exhibit single crystal-like characteristics. In this study, textured BNBT ceramics will be prepared via reactive-templated grain growth (RTGG) with Bi4Ti3O12 microcrystalline templates. The polarization and strain hystereses of the textured BNBT ceramics of different preferred crystallographic orientations under uniaxial electromechanical loading will be measured and then simulated using the developed single crystal model in conjunction with a statistical homogenization scheme. High-strain non-linear ferroelectric actuators made of the textured BNBT ceramics will be prototyped. The developed BNBT actuators will be of a multilayer stack design with a compressive bolt-clamping mechanism to test the concept of large strain actuation induced by bias stress-assisted domain switchings. The performance indices of the BNBT actuators will be used to validate the modeling works.
Keyword(s)
鐵電單晶
力電耦合
方向耦合
偶極轉動
變體路徑
行為模型
織構鐵電陶瓷
致動元件雛型
Ferroelectricity
Single crystals
Electromechanical loading
Directional coupling
Switching route
Constitutive modeling
Textured ferroelectrics
Actuator prototyping