稀土铕离子掺杂对拟薄水铝石晶体结构和相变的影响

Effects of Eu^(3+) Ion Dopant on Crystal Structure and Phase Transformation of Pseudoboehmite

研究了Eu3+离子对拟薄水铝石高温相变及微结构的影响。XRD及热分析结果表明,拟薄水铝石在560,882及1224℃分别经历-γAlOOH→-γAl2O3,γ-Al2O3→-θAl2O3和-θAl2O3→-αAl2O3相变。Eu3+掺杂后-γAlOOH→-γAl2O3相变温度不变,-γAl2O3→-θAl2O3和-θAl2O3→-αAl2O3相变温度分别提高到1054和1237℃。晶胞体积增大及衍射峰2θ值减小说明Eu3+离子以部分离子取代的形式进入-γAl2O3及-θAl2O3晶格。Eu3+离子进入晶格抑制了γ-Al2O3及-θAl2O3晶粒生长速率。-θAl2O3→-αAl2O3相变过程中Eu3+离子随Al3+一起迁移,并与Al3+离子及O2-离子形成了一个新生化合物相EuAl12O19。分析认为,该化合物在-αAl2O3晶界间存在,阻碍了-θAl2O3→-αAl2O3相转变过程中离子扩散速率,使-θAl2O3→-αAl2O3相变活化能由822.0提高到1063.15 kJ.mol-1。Eu3+离子掺杂使-θAl2O3→-αAl2O3相变过程由形核控制转变为表面扩散控制。

The effect of Eu^3＋ ions on the phase transformation and microstructure of pseudoboehmite xerogel was studied. The samples with out and with Eu^3＋ ions were sintered at different temperatures. DSC and XRD results indicate that the pseuoboehmite xerogel experiences phase transformations of γ-A1OOH→γ-Al2O3, γ-Al2O3→θ-Al2O3 and θ-Al2O3 → α-Al2O3 at 560, 882 and 1224 ℃, respectively. Upon adding 2%（mole fraction） Eu^3＋ to the sample, the phase transformation temperatures of γ-Al2O3→θ-Al2O3 and θ-Al2O3→α-Al2O3 increase to 1124 and 1237 ℃, respectively. The calculated lattice constants results show a significant expansion in the γ-Al2O3 phase, relative to the virgin sample. A decrease in 20 values in θ-Al2O3 results from doping effect with Eu^3＋ ions. It indicates that a partial substitution of the Eu^3＋ ions on Al^3＋ sites in γ-Al2O3 and θ-Al2O3 phases is occurred. The crystal growth of γ-Al2O3 and θ-Al2O3 phases is hindered by the substitution Eu^3＋ ions. A newly formed compound EuAl12O19 at higher temperature is detected and this possibly attributes to the increase of the active energy of θ-Al2O3→α-Al2O3 phase transformarion. Upon adding 2% （mole fraction） Eu^3＋ to the sample, the limited step of diffusion of Eu^3＋ turns from nucleation to surface control during θ-Al2O3→α- Al2O3 phase transformation.