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研究生:謝承諺
研究生(外文):Cheng-Yan Hsieh
論文名稱:氧化鉍系氧離子導體在還原氣氛下相穩定性及導電性質之研究
論文名稱(外文):Effect of dopant on reduction-resistance of Bi2O3-based solid electrolytes
指導教授:方冠榮
指導教授(外文):Kuan-Zong Fung
學位類別:碩士
校院名稱:國立成功大學
系所名稱:材料科學及工程學系碩博士班
學門:工程學門
學類:材料工程學類
論文種類:學術論文
論文出版年:2003
畢業學年度:91
語文別:中文
論文頁數:87
中文關鍵詞:固態燃料電池電解質氧化鉍還原氣氛
外文關鍵詞:solid oxide fuel cellelectrolytebismuth oxidereducing atmosphere
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固態電解質為具有離子導電特性之導電材料,在實際應用上需具備高導電性及穩定性。目前所知,氧離子導電率最高之氧化物為具有立方氟化鈣結構的高溫δ相的氧化鉍(δ-Bi2O3),但其結構在低於723℃以下之溫度並不穩定,可以藉由稀土氧化物(Er2O3、Y2O3、Gd2O3)的添加來增加立方相的穩定性,然而這些固溶體在還原氣氛下,易被還原成金屬,因此本研究針對固態電解質氧化鉍,添加不同價數的陽離子(Y3+,Nb5+,W6+)探討氧化鉍系固態電解質在還原氣氛下熱處理,添加物對其結構及導電性質的影響。
理論上,要獲得立方晶系氟化鈣結構的Bi2O3系固溶體,可減少氧離子空缺濃度或降低平均陽離子半徑。選擇陽離子半徑較小的Y2O3、Nb2O5、WO3作為添加劑,分別與Bi2O3以固態反應法合成試片。根據實驗結果,Y2O3-Bi2O3,Nb2O5-Bi2O3,WO3-Bi2O3這三種系統,當添加的陽離子和Bi3+的數量比為3:17及1:4時,(Y0.15Bi0.85)2O3,(Y0.2Bi0.8)2O3,以及(Nb0.2Bi0.8)2O3.4為立方晶系氟化鈣結構,而(Nb0.15Bi0.85)2O3.3在800℃燒結後為立方晶系氟化鈣結構,但在750℃熱處理之後,另一具cubic結構的第二相γ-Bi2O3生成,第二相的化學式亦可寫為Bi12.8O19.2,此代表(Nb0.15Bi0.85)2O3.3在低溫為兩cubic共存的情況,而(W0.15Bi0.85)2O3.45為單一的tetragonal結構,(W0.2Bi0.8)2O3.6則是tetragonal相及具orthorhombic相的Bi2WO6兩相共存。WO3-Bi2O3系統無法穩定立方晶系氟化鈣結構主要是由於Bi3+及W6+離子半徑及價數差距太大。此外,試片在5% H2與95% Ar混合的還原氣氛下400℃持溫5小時後,(Y0.15Bi0.85)2O3成為Y2O3和Bi的黑色粉末,(Y0.2Bi0.8)2O3則變黑且表面有Bi生成,其立方晶系氟化鈣結構的晶格常數較還原氣氛熱處理前的晶格常數小,此現象是由於還原氣氛下氧離子空缺濃度增加,以及Bi析出使殘留之立方晶系氟化鈣結構中離子半徑較大的Bi離子所佔比率相對較少所致。至於(Nb0.15Bi0.85)2O3.3,(Nb0.2Bi0.8)2O3.4,(W0.15Bi0.85)2O3.45和(W0.2Bi0.8)2O3.6此4個試片在還原氣氛熱處理後,試片的顏色稍微變黑但無金屬Bi析出,且結構亦無明顯變化,還原氣氛熱處理後的晶格常數同樣有減小的現象,原因為還原氣氛下熱處理氧離子空缺濃度增加所導致。
(Y0.15Bi0.85)2O3與(Y0.2Bi0.8)2O3在還原氣氛下300℃持溫5小時,使用數位電表同時記錄還原氣氛中試片的電阻值,還原熱處理前室溫下試片電阻太大超出數位電表的量測範圍,而還原熱處理後試片電阻減小,顯然有電子傳導的貢獻。(Nb0.15Bi0.85)2O3.3,(Nb0.2Bi0.8)2O3.4,(W0.15Bi0.85)2O3.45和(W0.2Bi0.8)2O3.6分別在400℃、500℃、450℃及500℃有此現象,固溶體中氧空缺濃度及Bi離子含量較少的成分,室溫下可量測到電子導電率的現象,需在較高的溫度熱處理後才發生。此外,在還原氣氛下熱處理時所量測到的導電率皆高於大氣下相同溫度所量到的值,還原氣氛下導電率較高的原因來自於氧離子空缺的增加及伴隨的電子濃度增加。在(W0.2Bi0.8)2O3.6試片的兩側覆蓋[25YSB]0.15[8YSZ]0.85共燒以阻隔電子的傳導,將試片置於還原氣氛下進行導電率的測試,此導電率為(W0.2Bi0.8)2O3.6在還原氣氛下的氧離子導電率,發現此導電率只略高於大氣下的導電率,證明還原氣氛下導電率較大氣下導電率優異的原因主要是由於電子傳導所做的貢獻。由上述的結果可得知,由於Nb2O5-Bi2O3和WO3-Bi2O3系統具有較少的氧離子空缺,因此在還原氣氛下具有較佳的穩定性。
Solid-state electrolytes are materials possessing defects and high ionic conductivity. For practical applications, solid electrolytes require high ionic conductivity and stability. Up to date, the oxygen ionic conductor with highest ionic conductivity is the high temperature cubic Bi2O3 namely, δ-Bi2O3. However, the δ phase is not stable below 723°C, and undergoes a phase transformation to a monoclinic phase. The stability of the cubic phase can be enhanced by the addition of rare earth oxides (such as Er2O3, Y2O3, Gd2O3). But, these solid solutions are easily reduced to metal in a reducing atmosphere. Therefore, the aim of this work was to investigate the effect of aliovalent dopants, Y3+, Nb5+ and W6+ on the crystal structures, conductivities, and microstructures of Bi2O3-based solid electrolytes after exposed to a reducing atmosphere.
Theoretically, in order to stabilize cubic fluorite-structured bismuth oxide based solid solution, it is needed to reduce the average cation radius or the amount of oxygen vacancies. Y2O3, Nb2O5, and WO3 were selected as dopants due to the smaller cation radii than Bi3+. The samples were synthesized by solid state reaction. The ratios of doping cations and Bi3+ were fixed at 3/17 and 1/4 for the Y2O3-Bi2O3, Nb2O5-Bi2O3, and WO3-Bi2O3 systems. Consequently, (Y0.15Bi0.85)2O3, (Y0.2Bi0.8)2O3 and (Nb0.2Bi0.8)2O3.4 exhibited a cubic lattice. The as-sintered (Nb0.15Bi0.85)2O3.3, however, exhibited a metastable single cubic phase. After heating at 750°C for 2.5 hours, Bi12.8O19.2 with γ-Bi2O3-like structure precipitated from the matrix of (Nb0.15Bi0.85)2O3.3. For WO3-Bi2O3 system, (W0.15Bi0.85)2O3.45 consists of a single tetragonal lattice. (W0.2Bi0.8)2O3.6 exhibited a mixture of tetragonal and orthorhombic lattices due to the mismatch in size and valence between Bi and W ions. Additionally, after annealing in the reducing atmosphere(5% H2 and 95% Ar) at 400°C for 5 hours, the bulk of (Y0.15Bi0.85)2O3 was pulverized and so was the surface of (Y0.2Bi0.8)2O3. The XRD analysis indicated that (Y0.15Bi0.85)2O3 was reduced to metallic Bi and Y2O3 while the reduced (Y0.2Bi0.8)2O3 exhibited a mixture of metallic Bi and the cubic solution of Y2O3-Bi2O3. The SEM micrographs of (Y0.15Bi0.85)2O3 and (Y0.2Bi0.8)2O3 show that metallic Bi spheres precipitated with the destruction of oxide surface. The lattice parameters of cubic (Y0.15Bi0.85)2O3 and (Y0.2Bi0.8)2O3 decreased due to the formation of oxygen vacancies after annealing in the reducing atmosphere. The samples of Nb2O5-Bi2O3, and WO3-Bi2O3 systems were slightly blackened but no trace of metallic phase was observed. After annealing in the reducing atmosphere, the lattice parameters of (Nb0.15Bi0.85)2O3.3, (Nb0.2Bi0.8)2O3.4, (W0.15Bi0.85)2O3.45 and (W0.2Bi0.8)2O3.6 also decreased due to the formation of oxygen vacancies.
It is expected that the reduced sample may show a low resistivity at room temperature as a result of the electronic conduction. The reduction of resistivity was observed from the samples of (Y0.15Bi0.85)2O3, (Y0.2Bi0.8)2O3, (Nb0.15Bi0.85)2O3.3, (Nb0.2Bi0.8)2O3.4, (W0.15Bi0.85)2O3.45 and (W0.2Bi0.8)2O3.6 after annealing in the reducing atmosphere at 300°C, 300°C, 400°C, 500°C, 450°C and 500°C respectively. For the samples doped with Nb2O5 and WO3, the resistivity increased again when the samples were cooled to room temperature. This result indicated that Bi2O3 doped with Nb2O5 or WO3 still maintain ionic conduction when annealed under reducing atmosphere at temperature as high as 300℃. It was suggested that Nb2O5 and WO3 effectively enhanced the stability of Bi2O3 under the reducing atmosphere.
The better stability of Nb2O5-Bi2O3, and WO3-Bi2O3 solid solutions can be attributed to the filling of oxygen ions into the vacancies. Consequently, the reduction of Bi ions was suppressed.
中文摘要…………………………………………………………………I
英文摘要………………………………………………………………IV
總目錄…………………………………………………………………VII
圖目錄……………………………………………………………………X
表目錄…………………………………………………………………XIV
第一章緒論………………………………………………………………1
第二章原理及文獻回顧…………………………………………………3
2-1、固態氧化物燃料電池的簡介……………………………………3
2-2、Bi2O3的性質……………………………………………………6
2-2-1、δ-Bi2O3…………………………………………………………8
2-3、Bi2O3-M2O3系統………………………………………………11
2-3-1、Bi2O3-Y2O3.……………………………………………………13
2-4、Bi2O3-M2O5系統…………………………………………………15
2-5、Bi2O3-MO3系統…………………………………………………17
第三章、研究動機與目的………………………………………………20
第四章、實驗步驟及方法………………………………………………22
4-1、試片的合成………………………………………………………23
4-2、X-光繞射分析……………………………………………………23
4-3、SEM顯微結構觀察……………………………………………24
4-4、導電性質測試……………………………………………………24
4-5、還原氣氛下熱處理………………………………………………24
第五章、結果與討論……………………………………………………27
5-1、添加劑對Bi2O3晶體結構的影響………………………………..27
5-1-1、Y2O3的添加對Bi2O3晶體結構的影響…………………………27
5-1-2、Nb2O5的添加對Bi2O3晶體結構的影響………………………30
5-1-3、WO3的添加對Bi2O3晶體結構的影響………………………36
5-2、添加劑對Bi2O3導電率的影響…………………………………45
5-2-1、Y2O3的添加對Bi2O3導電率的影響……………………………45
5-2-2、Nb2O5的添加對Bi2O3導電率的影響…………………………49
5-2-3、WO3的添加對Bi2O3導電率的影響…………………………49
5-3、添加劑對Bi2O3在還原氣氛下晶體結構的影響………………54
5-3-1、Y2O3的添加對Bi2O3在還原氣氛下晶體結構的影響…………54
5-3-2、Nb2O5的添加對Bi2O3在還原氣氛下晶體結構的影響………59
5-3-3、WO3的添加對Bi2O3在還原氣氛下晶體結構的影響…………61
5-4、添加劑對Bi2O3在還原氣氛下導電率的影響…………………66
5-4-1、Y2O3的添加對Bi2O3在還原氣氛下導電率的影響…………66
5-4-2、Nb2O5的添加對Bi2O3在還原氣氛下導電率的影響…………68
5-4-3、WO3的添加對Bi2O3在還原氣氛下導電率的影響……………74
第六章、結論……………………………………………………………82
參考文獻…………………………………………………………………84
致謝……………………………………………………………………87
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