Preparation and Improved Grain Conductivity of (Sm[sub 1−x]Y[sub x])[sub 2]Zr[sub 2]O[sub 7] Ceramics

Abstract

͑Sm 1−x Y x ͒ 2 Zr 2 O 7 ͑0 Յ x Յ 0.5͒ ceramics were prepared by a solid-state reaction process at 1973 K for 10 h in air. ͑Sm 1−x Y x ͒ 2 Zr 2 O 7 ͑0 Յ x Յ 0.3͒ ceramics have a single phase of the pyrochlore-type structure, whereas ͑Sm 1−x Y x ͒ 2 Zr 2 O 7 ͑x = 0.4,0.5͒ exhibit a defective fluorite-type structure. The full width at half-maxima in the Raman spectra increase with increasing Y content, which indicates that the degree of structural disorder increases as the yttrium content increases. The electrical conductivity of ͑Sm 1−x Y x ͒ 2 Zr 2 O 7 ceramics was investigated by electrochemical impedance spectroscopy over a frequency range of 1 Hz to 20 MHz in the temperature range of 623-1173 K in air and hydrogen atmospheres. The electrical conductivity of ͑Sm 1−x Y x ͒ 2 Zr 2 O 7 obeys the Arrhenius-type behavior and increases with increasing temperature. The grain conductivity and total conductivity of pyrochlore-type materials are clearly higher than those of defective fluorite-type materials in the intermediate temperature range of 623-1073 K. Both activation energy and pre-exponential factor for grain conductivity increase with increasing Y content. The electrical conductivity measured in the reducing atmosphere of hydrogen shows no significant changes, which proves that the conduction is purely ionic with negligible electronic contribution in the series.Fuel cells are electrochemical energy conversion devices used to generate electricity out of hydrogen. Among various types of fuel cells, solid oxide fuel cells ͑SOFCs͒ are very attractive due to two major advantages: high efficiency of energy conversion and negligible or zero emission of pollutants. 1 Solid electrolytes are the heart and most important component of SOFCs. To a large extent, the operating temperature of SOFCs depends on the nature of electrolytes. Although yttria-stabilized zirconia has been under commercial development, their operating temperatures are too high ͑1073-1273 K͒, which leads to the restriction in material selection and maintenance problems in SOFCs. 2 Therefore, it is essential to reduce the operating temperature to the intermediate temperature of 773-1073 K. Now, the major challenge is to increase the oxide-ion conductivity to allow intermediate-temperature applications. Operation at relatively low temperatures has stimulated many efforts to search for novel electrolytes and electrode materials. 3,4 Oxide-ion conductors of pyrochlore-type structure A 2 B 2 O 7 are potential candidates for intermediate-temperature SOFC applications. 5-11 Rare-earth zirconates have been of considerable interest as potential electrolytes for intermediate-temperature SOFCs due to their high ionic conductivity at relatively low temperatures as well as low cost of raw materials. 6-11 ͑Gd 0.5 Nd 0.5

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

“…At 1023-1173 K, the electrical conductivity of Sm 2 Zr 2 O 7 doped with 2.5-10 mol.% Mg at the Sm site is almost the same as undoped Sm 2 Zr 2 O 7 [18]. For (Sm 1−x Gd x ) 2 Zr 2 O 7 (0 ≤ x ≤ 1.0), (Sm 1−x Y x ) 2 Zr 2 O 7 (0 ≤ x ≤ 0.5) and (Sm 1−x Yb x ) 2 Zr 2 O 7 (0 ≤ x ≤ 1.0) systems, the electrical conductivity of pyrochlore-type compositions is clearly higher than that of defect fluorite-type compositions in the temperature range of 723-1173 K [19][20][21].…”

“…Recently, new A 2 Zr 2 O 7 -type oxides are of considerable scientific interests owing to variable cation radius ratios of r(A 3+ )/r(Zr 4+ ) and oxygen nonstoichiometry [7,[17][18][19][20][21]. The electrical conductivity of the pyrochlore-type Sm 2 Zr 2 O 7 was comparable to those of other oxide-ion conductors in the low-temperature region [17].…”

Electrical Conductivity Improvement of Nd2Ce2O7 Ceramic Co‐doped with Gd2O3 and ZrO2

“…A significant increase in the electrical conductivity is found by suitable substitution of isovalent rareearth cations like Sm or Nd at the Gd site in defect fluorite-type Gd 2 Zr 2 O 7 [13,14]. For (Sm 1-x Yb x ) 2 Zr 2 O 7 (0 ≤ x ≤ 1.0) and (Sm 1-x Y x ) 2 Zr 2 O 7 (0 ≤ x ≤ 0.5) ceramics, the electrical conductivity of pyrochlore-type compositions is obviously higher than that of defect fluorite-type compositions in the temperature range of 723-1,073 K [15,16]. The electrical conductivity of Gd 2 (Sn 1-x Zr x ) 2 O 7 ceramics gradually increases with increasing zirconium content at identical temperature levels [17].…”

Influence of Ce substitution on the order-to-disorder structural transition, thermal expansion and electrical properties in Sm2Zr2−xCexO7 system

“…13 In independent works, Xia et al reported that degree of structural disorder in (Sm 1Àx Y x ) 2 Zr 2 O 7 , (Sm 1Àx Eu x ) 2 Zr 2 O 7 and (Sm 1Àx Mg x ) 2 Zr 2 O 7Àx systems gradually increases with an increase in the substitution of ions, which also inuenced the total ionic conductivity. [14][15][16] The transformation from ordered pyrochlore to a disordered defect uorite structure has been found to be inuential in the ionic conductivities of many other chemical systems like Y 2 Sn 2Àx Zr x O 7 and Nd 2Ày Ho y Zr 2 O 7 also. 17,18 As far as the conducting properties of pyrochlore type crystals are concerned, the 48f oxygen site is reported to play a crucial role in the ion diffusion mechanism in the lattice.…”