(R,A)Zr2O7之相變化與氧離子導電度研究

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Title:	(R,A)Zr2O7之相變化與氧離子導電度研究
Other Titles:	Phase transition & oxygen ion conductivity of (R,A)Zr2O7
Authors:	張桓碩;Chang, Huan-Shuo
Contributors:	淡江大學化學學系碩士班高惠春;Kao, Huey-Chuen I.
Keywords:	離子導體;焦綠石;螢石;固態電解質;ionic conductivity;pyrochlore;fluorite;solid state electrolyte
Date:	2014
Issue Date:	2015-05-01 16:11:09 (UTC+8)
Abstract:	本研究的樣品的樣品共有 (Nd2-xErx)Zr2O7 (x = 0 – 1.2)、(Sm2-yDyy)Zr2O7 (x = 0 – 1.2)、(Gd2-zZnz)Zr2O7-0.5z (z = 0 – 0.6) 三系列，皆以檸檬酸鹽前驅物法製備，前驅物法製備，空氣中熱處理空氣中熱處理空氣中熱處理的溫度為的溫度為的溫度為 1600 1600 1600 1600 1600 oC 10 h C 10 hC 10 hC 10 hC 10 h，相對緻密度皆相對緻密度皆相對緻密度皆有 94% 以上。 Nd、Sm和Gd系列離子半徑比 (rA/rZr) 範圍分別為1.452 – 1.541、1.455 – 1.498和1.33 – 1.44，A是指Zr之外的陽離子。離子半徑比小於1.463為螢石相，大於為焦綠石相，等於1.463則需從X-光繞射圖譜判斷。樣品在螢石相轉為焦綠石相時，導電度明顯躍升，Nd和Sm 系列取代量分別為0.4和0.2時，有最高 σgrain rain 導電度，在700 oC 下分別為分別為 1.55(3)10-3 和4.7(1)10-3 S•cm-1。Sm2Zr2O7離子半徑比 (rA/rB) 比 Nd2Zr2O7 靠近相轉的邊界，透過取代，得到的最高導電度也較高。A2Zr2O7 焦綠石相的樣品隨著 A 位置的平均離子半徑比增加，8b O(3) 位置上的氧離子佔有率下降，48f O(1) 位置的 x-座標增加。A–O(1) 鍵長與48f O(1) 位置的 x-座標有關，x-座標偏離特殊位置0.037時，焦綠石相的樣品會有最高的導電度， A–O(1) 鍵長為2.56 Å 時，與 R2Zr2O7 系列中最高導電度的 Eu2Zr2O7 之 A–O(1) 為 2.55 Å 接近。推測以上的結構因子是決定導電度的要素。利用 Zn2+ 取代 Gd3+，增加 (Gd2-zZnz)Zr2O7-0.5z (z = 0 – 0.6) 系列空缺，使得氧離子在結構內更容易傳導。取代量為0.2時導電度最高，為 2.88* 10-3 S•cm-1，與焦綠石相的結果相當。增加氧空缺也是增加導電度的方法之一。以上樣品的活化能隨離子半徑比增加，a-軸變長，導致活化能下降。螢石相晶粒的活化能約在0.90 – 1.23 eV之間；焦綠石相約在0.7 – 1.02 eV 之間。螢石相樣品因為氧原子皆在特殊位置，且佔有率一致，比較不利於氧離子傳導。焦綠石相則因為 O(1) x-座標的位移和 O(3) 位置上的氧較少，使得氧離子在晶格內比較容易傳導，所以後者的活化能低於前者。總之，焦綠石的導電度優於螢石相，不過，增加螢石相氧空缺，也有助於提升其導電度。 In this study, (Nd2-xErx)Zr2O7 (x = 0 – 1.2), (Sm2-yDyy)Zr2O7 (y = 0 – 1.2) and (Gd2-zZnz)Zr2O7-0.5z (z = 0 – 0.6) were prepared by a citric acid precursor method and sintered at 1600oC 10 h in air. All of them have relative densities higher than 94 %. Ionic radius ratio (rA/rZr) of the Nd, Sm and Gd series are in the range of 1.452 – 1.541, 1.455 – 1.498 and 1.33 – 1.44, respectively, where A is the cations other than Zr. With a ratio smaller than 1.463, fluorite phase is obtained. On the other hand, it is pyrochlore. For the ratio equals to 1.463, X-ray diffraction peaks were examined to determine the crystalline phase. When the phase transformed from fluorite to pyrochlore, a small jump was found in the grain conductivity increased rapidly. At 700oC, (Nd1.6Er0.4)Zr2O7 and (Sm1.8Dy0.2)Zr2O7 have the highest values, which are 1.55(3)* 10-3 S•cm-1 and 4.7(1)10-3 S•cm-1, respectively. Because the rA/rZr of the Sm2Zr2O7 is closer to the phase transition boundary than that of the (Nd2-xErx)Zr2O7, through the substitution, Sm series has a higher conductivity than its counterpart. In the pyrochlore structure, increasing the cation size at the A-site, average rA/rZr increases and the occupancy factor of the 8b O(3)-site decreases, as well as the x-fractional coordinate of the 48f O(1)-site and the A–O(1) bond length increases. When the x-fractional coordinate of the O(1)-site shift 0.037 and A–O(1) length reaches to 2.56 Å, which is close to the Eu–O(1) 2.56Å bond in Eu2Zr2O7, optimal conductivity was observed. These structural factors are important in determining the conductivity among the zirconates mentioned in this report. Replacing part of the Gd3+ by Zn2+, in the (Gd2-zZnz)Zr2O7-0.5z (z = 0 – 0.6), oxygen vacancy was created and conductivity was increased. (Gd1.8Zn0.2)Zr2O6.9 has the highest conductivity, 2.8810-3 S•cm-1 at 700C, which is comparable to the pyrochlore samples. Increasing oxgen vacancies is another way to increase the conductivity. Grain activation energy (Ea) found for the fluorite samples is in the range of 0.90 – 1.23 eV and for the pyrochlore phase is 0.70 – 1.02 eV. In the former, all of the oxygen atoms are evenly distributed at the specific sites, however for the latter, there is an O(3)-site with lesser oxygen atom occupancy, which leads to the shift of the x-coordinate of the O(1)-site and causes an increase of the A-O(1) bond length. Therefore, pyrochlore phase has a smaller Ea than fluorite.
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