Influence Of Cations Doping And Sintering Atmosphere On Microstructure And Electrical Properties Of A₂Zr₂O₇ Ceramic

The rare-earth zirconate compounds including((Nd0.7Yb0.3)1–xSmx)2Zr2O7,((Sm0.5Gd0.5)1–xYbx)2Zr2O7,((Sm0.5Gd0.5)1–xSrx)2Zr2O7–δ,((Sm0.5Gd0.5)1–xCax)2Zr2O7–δ and Sm Gd(Zr1–xNbx)2O7+δ were synthesized by solid state reaction method with air or vacuum atmosphere. The compounds were analyzed by Raman spectroscopy, X-ray diffraction, scanning electron microscopy and electrochemical impedance spectroscopy. So we can get informations about microstructure, electrical conductivity and thermal expansion coefficient.The oxygen vacancy concentrations in A2Zr2O7 ceramic doped with different cations and sintered in different atmospheres were calculated. With increasing the oxygen partial pressure, the concentration of oxygen vacancies gradually decreases. A high concentration of oxygen vacancies can be obtained by doping bivalent cations at A site due to defect reactions. However, rare-earth zirconate ceramic doping with pentavalent elements at Zr site is not suitable for sintering in a low oxygen partial pressure.The crystal structure of A2Zr2O7 ceramic is mainly controlled by r(A3+)/r(Zr4+). The Sm Gd Zr2O7 ceramic has a pyrochlore structure. The degree of ordering decreases with the doping of Yb. When Yb content is 15 mol.%, the ceramic exhibits a defect fluorite structure. The rare-earth zirconates have a pyrochlore structure with doping the cations of Sr, Ca and Nb. However, the solubilities of Sr and Ca in rare-earth zirconate are not high enough since Sr and Ca cations tend to form a perovskite structure of Sr Zr O3 and Ca Zr O3. Sr and Ca cations can act as sintering aid to promote the densification process may have the effect of sintering aids.((Sm0.5Gd0.5)1–xYbx)2Zr2O7 ceramics sintered in vacuum have a similar structure to those sintered in air. The((Nd0.7Yb0.3)1–xSmx)2Zr2O7 sintered in vacuum exhibits an ordered pyrochlore structure.Both increasing the concentration of mobile oxygen vacancies caused by disorder and enhancing the activation energy of oxygen vacancies movement can influence the electrical conductivity. However, these two factors are competing.((Sm0.5Gd0.5)1–xYbx)2Zr2O7 ceramic exhibits a defect fluorite with doping of Yb. The electrical conductivity is the highest at x=0.05. Therefore, it can be deduced that a certain degree of ordering can increase the electrical conductivity. In((Sm0.5Gd0.5)1–xCax)2Zr2O7 ceramics, the electrical conductivity at x=0.025 is the highest in this series. The defect reaction after doping pentavalent cations may decrease in the number of oxygen vacancies in Sm Gd(Zr1–xNbx)2O7+δ ceramics and promote the occurrence of proton conductivity, which is beneficial to the improvement of electrical conductivity. Sm Gd(Zr0.95Nb0.05)2O7+δ ceramic has the highest electrical conductivity of 1.41×10–2Scm–1 at 1173 K among all the rare-earth zirconates. For((Sm0.5Gd0.5)1–xYbx)2Zr2O7 and((Nd0.7Yb0.3)1–xSmx)2Zr2O7 ceramics sintered in vacuum, the oxygen vacancies increase greatly, which promotes the enhancement of electrical conductivity. The average linear expansion coefficients of((Nd0.7Yb0.3)1–xSmx)2Zr2O7,((Sm0.5Gd0.5)1–xYbx)2Zr2O7 and Sm Gd(Zr1–xNbx)2O7+δ increase slightly as compared with 8YSZ, and((Nd0.7Yb0.3)1–xSmx)2Zr2O7 ceramics sintered in air have the highest average linear expansion coefficients.