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Study On Microstructure And Physical Properties Of A2Zr2O7 -type Rare-earth Zirconates

Posted on:2010-09-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z G LiuFull Text:PDF
GTID:1101360332957801Subject:Materials science
Abstract/Summary:PDF Full Text Request
(Sm1–xGdx)2Zr2O7, (Gd1–xYbx)2Zr2O7, (Sm1–xYbx)2Zr2O7 and (Nd1–xYbx)2Zr2O7 (0≤x≤1.0) powders were synthesized by the chemical-coprecipitation and calcination method using ZrOCl2·8H2O and rare-earth oxides as starting materials. The synthesized powders were compacted by cold isostatic pressing, and were then pressureless-sintered to prepare dense ceramic bulk materials. The microstructure of rare-earth zirconate solid solutions were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The thermal diffusivity, thermal expansion coefficient and electrical conductivity of rare-earth zirconate solid solutions were investigated by laser flash method, push-rod dilatometer and AC impedance spectroscopy.(Sm1–xGdx)2Zr2O7, (Gd1–xYbx)2Zr2O7 and (Sm1–xYbx)2Zr2O7 zirconates are complete solid solutions, whose crystal structures are mainly governed by the cation radius ratio of r(A3+)/r(Zr4+) in A2Zr2O7 system. The rare-earth zirconates exhibit a defect fluorite-type structure for r(A3+)/r(Zr4+)<1.46, and a pyrochlore-type structure for r(A3+)/r(Zr4+)>1.46. However, the cation radius ratio of Gd2Zr2O7, r(Gd3+)/r(Zr4+)=1.46, resides just at the phase boundary between pyrochlore- and defect fluorite-type structures. As the sintered temperature used in this study is higher than the order–disorder transition temperature, Gd2Zr2O7 exhibits only a defect fluorite structure. (Nd1–xYbx)2Zr2O7 are not complete solid solutions owing to the large difference in ionic radius of Nd3+ and Yb3+. The crystal structures of (Nd1–xYbx)2Zr2O7 have been found to be pyrochlores for 0≤x≤0.25, defect fluorites for 0.45≤x≤1.00 and a mixture of these for 0.30≤x≤0.40.The thermal conductivities of (Ln1–xYbx)2Zr2O7(Ln=Gd, Sm, Nd) solid solutions are located within the range of 1.35 to 1.96W·m–1·K–1 from room temperature to 1673K. The thermal conductivities of (Ln1–xYbx)2Zr2O7 first gradually decrease with increasing temperature, and then increase slightly above 1073–1273K due to the increased radiation contribution. At identical temperature levels, (Ln0.5Yb0.5)2Zr2O7 solid solutions have the lowest thermal conductivity due to the reduced cation mean free path at the compositional combination of equal molar Yb3+ and Ln3+ cations. Among all the rare-earth zirconates in this study, Sm2Zr2O7 exhibits the highest thermal expansion coefficient, however, Yb2Zr2O7 has the lowest thermal expansion, which depends mainly upon crystal structure and distribution of oxygen vacancy. The average thermal expansion coefficients of (Ln1–xYbx)2Zr2O7 solid solutions are within the range of 10.52–11.78×10–6K–1 from 373 to 1673K, and gradually decrease with increasing Yb content.The grain conductivities of (Sm1–xGdx)2Zr2O7 and (Ln1–xYbx)2Zr2O7(Ln=Gd, Sm, Nd) solid solutions are closely related to their own crystal structures. The grain conductivity of rare-earth zirconates has a maximum when the cation radius ratio of r(A3+)/r(Zr4+) is close to 1.48 in A2Zr2O7 system. (Sm0.5Gd0.5)2Zr2O7 solid solution with the smallest difference in cation radius at the A sites exhibits a highest grain conductivity of 2.69×10–2S·cm–1 at 1173K. For rare-earth zirconates with a defect fluorite structure, the grain conductivity increases gradually with increasing the cation radius ratio, which is related to increasing unit cell free volume. However, for rare-earth zirconates with a pyrochlore structure, the grain conductivity decreases gradually with increasing the cation radius ratio due to the decrease of oxygen vacancies at 48f sites caused by the increase of structural ordering degree.Gd2Zr2O7 reacts with V2O5 and forms ZrV2O7 and GdVO4 at 973K. However, in the temperature range of 1023–1273K, Gd2Zr2O7 reacts with V2O5 and finally forms GdVO4 and m-ZrO2, which could be explained based on the thermal instability of ZrV2O7. Gd2Zr2O7 reacts with a mixture of V2O5 and Na2SO4 to form GdVO4 and m-ZrO2 in the temperature range of 973–1273K. Rare-earth zirconates can react with Al2O3 at elevated temperatures. In ZrO2–NdO1.5–AlO1.5 ceramics, perovskite-like NdAlO3 and small amounts of hexagonalβ-Al2O3-type NdAl11O18 are also found with increasing Al2O3 content, besides the ZrO2-Nd2O3 solid solutions. The thermal conductivities of ZrO2–NdO1.5–AlO1.5 ceramics are located within the range of 1.50 to 3.22W·m–1·K–1 from room temperature to 1673K, and increase gradually with the increase of alumina concent in raw materials under identical temperature conditions. The thermal expansion coefficients of ZrO2–NdO1.5–AlO1.5 ceramics are within the range of 10.45 to 11.42×10–6 K–1 at 1523K, which are of the same order of magnitude as currently used 6–8wt.% yttria stabilized zirconia and rare-earth zirconates for thermal barrier coating applications.
Keywords/Search Tags:rare-earth zirconates, microstructure, thermal conductivity, thermal expansion, electrical conductivity, hot corrosion
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