釔離子摻雜對鈦酸鋇之微結構,電性與光解關係之研究

Abstract

本論文主要在探討近化學計量比(鋇/鈦原子比例 = 0.999)的鈦酸鋇摻雜不同含量釔離子，在空氣中，經過1460 ℃持溫90分鐘的燒結，對於顯微結構，電性與光解關係之關聯性。在鈦酸鋇摻雜釔離子含量從0 mol%增加到2.0 mol%，透過顯微結構的觀察，我們可以知道微結構的演化，是先由液相幫助燒結，而在燒結的過程中逐步形成高緻密的微結構(即摻雜0.2 mol%釔離子)。在鈦酸鋇中摻雜高含量的釔離子(摻雜2.0 mol%釔離子)後，微結構形成具有熔融狀的晶粒及次微米孔洞的多孔結構。在微結構演化過程中摻雜1.0 mol%釔離子的鈦酸鋇扮演中間過渡階段，其微結構的特徵為具有異常大的孔洞以及表面形態異常粗糙的晶粒。在導電性方面，當摻雜釔離子從0.02 mol%增加0.2 mol%時，電子載子濃度與導電率皆大幅增加。而 n 型半導體的載子濃度增加，不僅增加了半導體特性，透過 c/a 比例的連續減少，也證實在此摻雜成份範圍，釔離子是取代鋇離子的位置。在0.2 mol%釔離子的鈦酸鋇中，透過具有豐富陽離子和缺乏氧的析出物在晶界形成，使此成份不但具有較高的電子濃度，而且還有良好的傳導路徑，透過EPMA分析結果指出釔離子在晶界的濃度較晶粒高，因此晶界相提供穩定的自由電子補償機制，而且晶界亦提供了傳導電子路徑，因此在摻雜0.2 mol%釔離子的鈦酸鋇具有最佳的導電率。當鈦酸鋇中釔離子摻雜含量從0.2 mol%增加到1.0 mol%時，n 型半導體特性被降低，不過由於微結構的缺陷提供了電子傳導的路徑，因此1.0 mol%的成份中有較高的電子遷移率。由於摻雜2.0 mol%釔離子鈦酸鋇具有多孔的微結構，因此亦具備高的電子遷移率。鈦酸鋇中摻雜釔離子含量從0-2.0 mol%，主要的電荷補償機制被從蕭特基缺陷轉變為自由電子補償最後變成氧空位/電洞補償。透過霍爾效應量測結果可知，摻雜釔離子含量增加，從 n 型半導體特性變成 p 型半導體特性，支援電荷補償機制的變化。透過X光繞射儀結果結合、顯微術、質譜儀和霍爾效應數據，發現摻雜1.0 mol%釔離子為一個臨界濃度，在此濃度下，摻雜的釔離子分別取代不同的位置，而產生不同的電荷補償機制和半導體特性。DSC分析結果指出摻雜不同含量釔離子的鈦酸鋇在130 ℃的相轉變溫度(Tc)隨著摻雜釔離子含量增加呈現先上升後下降的現象；在摻雜0-0.1 mol%釔離子的鈦酸鋇中，Tc 會隨著釔離子摻雜含量增加而上升，這是因為材料內部的蕭特基缺陷濃度降低所造成的。在摻雜0.2 mol%釔離子的鈦酸鋇中，因為低熔點第二相的生成而導致 Tc 下降。在摻雜1.0-2.0 mol%釔離子的鈦酸鋇中，則是因為氧空缺濃度增加與低熔點第二相含量增加，而造成 Tc 下降。在結合電子自旋共振光譜、陰極射線激發光譜、與光解反應分析結果可得知，具有 與淺層施體或受體能階的鈦酸鋇成份(例如，摻雜0.1 mol%釔離子)，在可見光下具有較佳之光觸媒與光電流反應；在紫外光下，則是以具有 且高缺陷濃度的鈦酸鋇成份(例如，摻雜0.2 mol%釔離子)有較佳之光觸媒與光電流表現。

Intricate connections among the microstructural effect, semiconducting tendency and charge compensation behavior of yttrium (Y3+) dopants in near-stoichiometric barium titanate (BaTiO3; Ba/Ti atomic ratio = 0.999) ceramics sintered at 1460 ◦C in air are examined. It is found that with increasing Y3+ doping from 0 to 2.0 mol%, the microstructure of BaTiO3 evolves from a liquid-phase-assisted dense-sintered microstructure to a highly porous microstructure characterized by connected pores and loose lattices of fused submicrometre grains. When Y3+ doping is increased progressively from 0.02 to 0.2 mol%, the (negative) majority carrier concentration and conductivity are increased substantially by 4-8 orders of magnitude. This increase in n-type semiconductor characteristics is contributed not only by the increasing substitution of Y3+ for Ba2+ in host BaTiO3, but also by the formation of yttrium-rich and/or oxygen-deficient precipitates at the grain boundaries. The grain boundary phases would therefore stabilize the mechanism of free electron compensation and enable the transportation of electrons through the grain boundaries. Through a combined interpretation of the characterization data, Y3+ doping at 1.0 mol% is found to be the critical doping amount separating different site-occupying behaviors of Y3+ in the BaTiO3 cation sites, which eventually lead to different charge compensation mechanisms and semiconductor properties. As the amount of Y3+ doping is increased from 0 to 0.1 mol%, the Curie temperature for the cubic-to-tetragonal phase transition (Tc) increases due to the reduction in Schottky defects. At the Y3+ doping of 0.2 mol%, Tc starts to decrease due to the formation of low-temperature second phases. Finally, when Y3+ doping is increased to 1.0-2.0 mol%, Tc decreases further due to the increase in oxygen vacancy concentration and the formation of low-temperature second phases. Experimental data from the electron paramagnetic resonance, cathodoluminescsnce and photocatalytic characterizations show that 0.1 mol% Y-doped BaTiO3, which possesses polarons and shallow donor or acceptor energy levels, is suitable for photocatalytic applications under visible light irradiation; while 0.2 mol% Y-doped BaTiO3, which has a higher overall defect concentration, is suitable for photocatalytic applications under UV irradiation.