錳摻雜鈦酸鉍鈉鉀弛豫型鐵電薄膜之熱處理程序對其能量儲存及電性之影響

Abstract

此研究聚焦於使用溶膠凝膠法製備之無鉛0.8(Bi0.5Na0.5)TiO3-0.2(Bi0.5K0.5)TiO3 鈦酸鉍鈉鉀弛豫型鐵電薄膜之能量儲存特性，並透過研究錳摻雜、熱處理路徑、冷卻速率、氣氛處理四種不同方法提升其表現。

首先，第一部份的實驗目標為改善未摻雜之鈦酸鉍鈉鉀之高漏電流特性，並透過錳摻雜進一步穩定其鐵電表現。研究發現在加入少量的錳摻雜時可以將漏電流控制到最小，進而得到細長的遲滯曲線，展現較好的能量儲存特性。透過多種材料分析方法研究其機制，此漏電抑制主要源於錳摻雜引起之鈣鈦礦結構A位置陽離子之電子分布改變，因而使得3莫耳濃度之錳摻雜展現最佳能量儲存特性，並以此作為後續實驗的材料組成。

第二部份的實驗為研究不同熱處理路徑對能量儲存特性的影響。透過對薄膜進行逐層退火及一次退火兩種熱處理路徑，研究發現相較於逐層退火呈現的純鈣鈦礦結構，一次退火會產生大量的燒綠石二次相，進而形成鈣鈦礦與燒綠石雙相共存的結構。此結構源於高揮發性陽離子於高溫下逸散，並可在稍微降低能量儲存密度的狀況下轉而提升其效率。

除此之外，第三部份的實驗透過使用液態氮淬火、空冷、爐冷三種不同的冷卻方法，研究退火後的冷卻速度對微結構及電性的影響。研究發現較快的降溫速度可提升其極化量，得到較高的能量儲存密度，但會犧牲其效率。由於巨觀分析呈現準正方晶系的結構且無明顯差異，因此進一步透過掃描穿透式電子顯微鏡分析其原子級影像。結果顯示液態氮焠火之薄膜可在極化後產生更高比例之非180度連接之奈米級鐵電域，而空氣冷卻之薄膜則形成短有序長度之立方晶系結構。此外，焠火處理之薄膜也展現較大的晶格扭曲，進而誘發更高的極化量。

最後的實驗為對退火後的薄膜進行氣氛處理，研究此方法對電性及缺陷化學的影響。結果顯示在進行氮氣氣氛處理後，殘存極化量下降及最大極化量上升使得能量儲存表現顯著地提升，推論其源於抑制漏電流以及減少缺陷偶極之影響。在進一步對此研究中的不同熱處理組合施加氣氛處理後，發現摻雜3莫耳濃度錳、逐層退火、液態氮淬火、及氮氣氣氛處理之鈦酸鉍鈉鉀薄膜中得到最佳的能量儲存表現。

本研究分為四大主題—錳摻雜、熱處理路徑、冷卻方法及氣氛處理—並研究其對BNKT-20薄膜之影響。這些方法應能普遍應用於設計與開發高性能介電材料及能量儲存電容器，促進奈米尺度多功能鐵電元件之發展。

In this study, several methods were applied to sol-gel and spin-coating derived lead-free relaxor ferroelectric 0.8(Bi0.5Na0.5)TiO3-0.2(Bi0.5K0.5)TiO3 (abbreviated as BNKT-20) thin films to enhance their energy storage performances. The work was divided into four main sections: Mn doping, annealing routes, cooling methods, and atmosphere treatments.

In the first section, Mn was introduced into the thin film, stabilizing its electrical properties. Structural and chemical analyses showed that Mn was substituted into the lattice, inducing a chemical state change among A-site cations. This modification helped investigate the leakage current mechanism, revealing that 3 mol% Mn-doped BNKT-20 exhibited slim hysteresis curves, beneficial for energy storage applications. This Mn-doped material was used for further investigations.

Secondly, to investigate the effect of different annealing routes, two different methods were applied and compared: annealing layer by layer and annealing once after deposition. The results indicated that thin films annealed once exhibited the coexistence of perovskite BNKT and pyrochlore Bi2Ti2O7 phases due to the high volatility of cations. This phase coexistence led to lower polarization and slimmer hysteresis curves, reducing energy storage density but increasing efficiency.

Furthermore, different cooling methods, including liquid nitrogen quenching, air cooling, and furnace cooling, were applied after the annealing procedure. Electrical property measurements indicated that a faster cooling rate induced larger polarization, enhancing energy storage density but deteriorating efficiency. Although there were no macroscopic differences in the thin films subjected to different cooling methods, their atomic structures were revealed through scanning transmission electron microscopy. Liquid nitrogen quenched thin films exhibited the coexistence of R3c and Pm3 ̅m phases and non-180º domain structure, while air cooled thin films exhibit Pm3 ̅m phase, corresponding to a matrix-like short-range coherence length structure. Additionally, the quenched thin film revealed larger lattice distortion according to the c/a ratio. These factors synergized and contributed to the increased polarization.

The last topic in this study is the effect of atmosphere treatment after annealing. Applying N2 atmosphere treatment resulted in hysteresis curves with smaller remanent polarization and larger maximum polarization by suppressing leakage current and defect dipoles, thereby enhancing energy storage properties. This process was applied to various modified heat treatment combinations in this study, achieving the best energy storage performance in BNKT-20 thin films doped with 3 mol% Mn, annealed layer by layer, and subjected to liquid nitrogen quenching.

This study, composed of four topics—Mn doping, annealing routes, cooling methods, and atmosphere treatments, investigates their effects on BNKT-20 thin films, especially regarding their energy storage properties. These approaches should be generalizable for designing and developing high-performance dielectrics and energy storage capacitors, facilitating the improvement of nanoscale multifunctional ferroelectric devices.