鈦酸鉍鈉基薄膜於形態相邊界區之鐵電性與結構精算研究

鐵電材料的壓電性質可被應用於致動器、能量擷取器、感應器等元件。隨著國際環保意識的提高，無鉛鐵電陶瓷的開發已是重要議題，然而目前對鈦酸鉍鈉基[(Bi0.5Na0.5)TiO3 (BNT)-based]無鉛鐵電薄薄膜的研究報導卻相當有限。原因在於當此材料被製成薄膜後會顯現極大的漏電流造成鐵電性質下降，因此在鐵電元件的應用領域上有相當的困難。本研究通過溶膠-凝膠法(sol-gel)，分別以兩種不同的溶膠製程途徑沉積 Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT-18)薄膜於Pt/Ti/SiO2/Si(100)基板上。兩種製程主要的差異為選擇溶劑的不同以及乾燥控制化學添加劑(Drying Control Chemical Addi-tives; DCCA)的添加與否。第一種溶膠系統使用乙二醇單甲醚作為溶劑，優點為溶膠所需的老化時間短，但卻有難以控制其水解縮合反應速率使溶膠品質不夠穩定的問題，導致BNKT-18薄膜的再現性太低。第二種溶膠系統是使用異丙醇與去離子水作為溶劑，並添加了乙醯丙酮作為螯合劑、甲醯胺作為乾燥控制化學添加劑。這樣的溶膠製備過程不僅解決了水解縮合反應速率不穩定的問題，更改善了BNKT-18薄膜的微結構與緻密度。更重要的是，此溶膠系統有利於製備出更精準化學計量比的薄膜。低掠角X光繞射與電滯曲線分析的結果顯示，以第二種溶膠系統製備之BNKT-18薄膜具單一鈣鈦礦結構，且漏電流小、鐵電性質顯著。本文的另一項研究主題為，使用第二種溶膠系統製備BNT-based薄膜，探討(1-x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3 (BNKT-100x)、(1-y)(Bi0.5Na0.5)TiO3-yBaTiO3 (BNBT-100y)二元系統薄膜之形態相邊界區(Morphotropic Phase Boundary; MPB)之組成範圍與鐵電性質。本研究使用Rietveld refinement方法對BNT-based薄膜之GIXRD圖譜進行定量相分析，結果表明BNKT-100x薄膜之MPB組成範圍位於0.14 ≤ x ≤ 0.18，為rhombohedral、cubic與tetragonal三相共存，電滯曲線量測的結果顯示MPB組成時具有較佳的鐵電表現；然而，當BNBT-100y組成位於MPB附近時，由於其組成相主要為調製結構(modulated structure)，而非單純的rhombohedral或tetragonal對稱，因此大幅增加了其結構精算的難度，而電滯曲線量測的結果顯示於y = 0.06時具有最好的鐵電表現。

關鍵字

鈦酸鉍鈉；無鉛鐵電材料；溶膠-凝膠法；螯合劑；乾燥控制化學添加劑；形態相邊界區；結構精算

並列摘要

The piezoelectricity of ferroelectric materials is the basis for the operation of electro-mechanical devices such as actuators, energy harvesters, and pressure sensors. With in-creasing awareness of environmental protection, the development of lead-free ferroe-lectric materials has been an ongoing effort and attracted great attention. In this study, the sol-gel processing parameters and crystalline phases of (Bi0.5Na0.5)TiO3 (BNT)-based lead-free ferroelectric thin films were investigated. First, Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT-18) thin films were selected to exam two different sol preparation routes. The main differences between the two were the choice of solvent and the addition of polymeric agents. The first sol route was prepared using 2-methoxyethanol as the solvent. It required a relatively short aging time; however, the sol was unstable due to the difficulty in controlling the rate of hydrolysis and conden-sation reaction, which led to the low reproducibility of the BNKT-18 thin films. The second sol route used isopropyl alcohol and DI water to form the solvent mix and added acetylacetone as a chelating agent and formamide as a drying control chemical additive (DCCA). Such a mixture not only solved the problem of unstable hydrolysis and condensation reaction but also improved the microstructure and density of the BNKT-18 thin films. The second sol route allowed more precise control of the stoi-chiometry of the thin films. Results from the grazing incident X-ray diffraction (GIXRD) and ferroelectric polarization measurements showed that the BNKT-18 thin films prepared by the second sol route exhibited a single-phase perovskite structure and low leakage currents. Next, the morphotropic phase boundaries (MPB) of two BNT-based thin films prepared by the second sol route: (1-x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3 (abbreviated as BNKT-100x) and (1-y)(Bi0.5Na0.5)TiO3-yBaTiO3 (abbreviated as BNBT-100y) were de-fined. Quantitative phase analyses of the BNKT-100x and BNBT-100y thin films were performed using the Rietveld refinement method to analyze the GIXRD patterns. The results indicated that the MPB of the BNKT-100x thin films existed in the composition range of 0.14 ≤ x ≤ 0.18 with the coexistence of rhombohedral, cubic, and tetragonal phases. The MPB compositions exhibited a softer ferroelectric hysteresis with larger remanent polarization values. In contrast, the MPB of the BNBT-100y thin films was difficult to define by the Rietveld refinement method since the films exhibited modu-lated structures rather than a simple rhombohedral or tetragonal symmetry. The BNBT-6 thin film exhibited the best ferroelectric performance among the BNBT-100y thin films studied.

並列關鍵字

(Bi0.5Na0.5)TiO3 ； Lead-free ferroelectrics ； Sol-gel ； Morphotropic phase boundary ； Rietveld refinement

參考文獻

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