【作者】 刘占国；

【导师】 周玉； 欧阳家虎；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 本文以ZrOCl2·8H2O和稀土氧化物为原材料,采用化学共沉淀-煅烧法成功制备了稀土锆酸盐(Sm1–xGdx)2Zr2O7、(Gd1–xYbx)2Zr2O7、(Sm1–xYbx)2Zr2O7和(Nd1–xYbx)2Zr2O7(0≤x≤1.0)粉体;经冷等静压成型和无压烧结后得到致密的陶瓷块体材料。采用X射线衍射、拉曼光谱、扫描电镜、透射电镜和能谱等手段研究了稀土锆酸盐材料的微观组织结构及其转变机理;采用激光热导仪、高温热膨胀仪和交流阻抗谱等手段测试了稀土锆酸盐材料的热扩散系数、热膨胀系数和电导率。(Sm1–xGdx)2Zr2O7、(Gd1–xYbx)2Zr2O7和(Sm1–xYbx)2Zr2O7稀土锆酸盐为完全固溶体,其晶体结构主要与A2Zr2O7中阳离子半径比值(r(A3+)/r(Zr4+))有关。当阳离子半径比值r(A3+)/r(Zr4+)<1.46时,为无序的缺陷型萤石结构;当阳离子半径比值r(A3+)/r(Zr4+)>1.46时,为有序的烧绿石结构。对于Gd2Zr2O7来说,阳离子半径比值r(Gd3+)/r(Zr4+)=1.46,处于烧绿石结构与缺陷型萤石结构的相边界处;本文中Gd2Zr2O7的烧结温度高于其有序无序转变温度(1803K),所以Gd2Zr2O7为无序的缺陷型萤石结构。在(Nd1–xYbx)2Zr2O7固溶体中,由于稀土阳离子Nd3+与Yb3+半径相差较大,没有形成完全固溶体。(Nd1–xYbx)2Zr2O7 (0≤x≤0.25)为单相烧绿石结构,(Nd1–xYbx)2Zr2O7(0.45≤x≤1.00)为单相缺陷型萤石结构,而(Nd1–xYbx)2Zr2O7(0.30≤x≤0.40)为烧绿石结构与缺陷型萤石结构共存。稀土锆酸盐(Ln1–xYbx)2Zr2O7(Ln=Gd, Sm, Nd)在室温到1673K范围内的热导率在1.35–1.96W·m–1·K–1之间,且(Ln1–xYbx)2Zr2O7固溶体的热导率随着温度的升高而逐渐减小,在1073–1273K附近达到最小值,继续升高温度有略微增大的趋势;在相同温度条件下,(Ln1–xYbx)2Zr2O7固溶体在A位两种稀土元素(Ln3+与Yb3+)等摩尔(x=0.5)时具有最低的热导率,这是由于当x=0.5时其有效声子平均自由程最小。在本文的稀土锆酸盐材料中,Sm2Zr2O7的热膨胀系数最大,Yb2Zr2O7的热膨胀系数最小,主要与晶体结构和氧空位分布有关。(Ln1–xYbx)2Zr2O7固溶体的平均热膨胀系数在373–1673K范围内处于10.52–11.78×10–6K–1之间,并且随着体系中Yb含量的增加逐渐减小。稀土锆酸盐(Sm1–xGdx)2Zr2O7和(Ln1–xYbx)2Zr2O7(Ln=Gd, Sm, Nd)的晶粒电导率与晶体结构有直接关系。当A2Zr2O7中阳离子半径比值r(A3+)/r(Zr4+)在1.48附近时,晶粒电导率达到最大值;并且在A位两种稀土阳离子半径相差最小的材料(Sm0.5Gd0.5)2Zr2O7中,晶粒电导率达到了本文中所有研究体系的最大值(2.69×10–2S·cm–1, 1173K)。在缺陷型萤石结构的稀土锆酸盐中,晶粒电导率随着阳离子半径比值的增大逐渐增大,这与晶胞自由体积逐渐增加有关;而在烧绿石结构稀土锆酸盐中,晶粒电导率随着阳离子半径比值的增大逐渐降低,这主要是由结构有序化程度增加导致的48f位置氧空位数量减少引起的。Gd2Zr2O7与V2O5在973K时的反应产物为ZrV2O7和GdVO4;而在1023–1273K之间的反应产物为GdVO4和m-ZrO2,反应产物的不同主要取决于ZrV2O7的热稳定性。在973–1273K之间,Gd2Zr2O7与等摩尔的V2O5和Na2SO4的混合物发生反应,生成GdVO4和m-ZrO2。稀土锆酸盐材料在高温下与氧化铝会发生化学反应。随着氧化铝含量的增加,在反应产物中先后出现了钙钛矿结构的NdAlO3和少量六方β-Al2O3结构的NdAl11O18。ZrO2–NdO1.5–AlO1.5陶瓷的热导率在室温到1673K范围内处在1.50–3.22W·m–1·K–1之间;在相同温度条件下,ZrO2–AlO1.5–NdO1.5陶瓷的热导率随着添加氧化铝含量的增加逐渐增大。在1523K时,ZrO2–AlO1.5–NdO1.5陶瓷的热膨胀系数在10.45–11.42×10–6K–1之间,与目前使用的热障涂层陶瓷层材料6–8wt.%Y2O3–ZrO2和稀土锆酸盐材料的热膨胀系数相近。更多还原

【Abstract】 (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.更多还原