Phase transition & oxygen ion conductivity of (R,A)Zr2O7.

• Main Authors: Huan-Shuo Chang, 張桓碩
• Other Authors: Huey-Chuen I. Kao.
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/25882080931261373849

碩士 === 淡江大學 === 化學學系碩士班 === 102 === 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, 2.88*10-3 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.