高温原位拉曼光谱研究BiFeO3陶瓷反应烧结相变过程

多铁性材料BiFeO3(BFO)由于具有潜在的磁电耦合效应而备受关注,但纯相陶瓷的制备始终是一个难点,部分原因在于对其反应烧结相变规律的认识尚不充分。高温原位拉曼光谱技术(HT-Raman)是表征复杂的固体相变及反应的有力手段。首次利用HT-Raman,研究了不同配比(1∶1, 1.03∶1和1.05∶1)的Bi2O3-Fe2O3在不同升降温速率(10和100℃·min-1)下的反应烧结相变过程,以及降温时反应产物的收缩效应。结果表明:Bi2O3-Fe2O3反应烧结生成BiFeO3的过程中,会产生中间过渡相Bi2Fe4O9和Bi25FeO39∶Bi2O3-Fe2O3配比为1.03∶1、升降温速率较快时,产物中杂相含量最少,可见Bi过量及较快的升降温速率能有效抑制杂相的生成。降温过程中,发现BFO的A1-1峰位随着温度降低发生蓝移,且二者呈良好的线性关系,这说明降温过程中BFO仅因温度变化产生晶格收缩,并没有结构相变。此外,还利用二维X射线衍射(2D-XRD)及背散射电子衍射(EBSD),表征了烧结产物的相组成及形貌。XRD结果也显示Bi过量时杂相含量较少,与拉曼结果一致。结合2D-XRD和EBSD的结果可知, Bi过量时烧结产物晶粒尺寸较大且均一,可见快速升降温有利于晶粒的成核与生长。研究结果可帮助进一步认清反应烧结规律,并指导纯相BiFeO3基陶瓷的制备。

In-situ Raman Spectroscopy Study on the Phase Transformation of BiFeO3 Ceramics in the Reaction Sintering Process

The multiferric BiFeO3(BFO) has attracted much attention due to its magnetoelectric coupling effects. However, the phase transformation involved in the reaction sintering is still not clarified, which consequently restricts the synthesis of pure-phase BiFeO3 ceramics. In-situ High Temperature Raman Spectroscopy(in-situ HT-Raman)is a powerful means to characterize complex solid phase transitions and reactions. In this paper, the phase transformation in the reaction sintering of BiFeO3 ceramics with different molar ratio(1∶1, 1.03∶1 or 1.05∶1) of Bi2O3-Fe2O3 and heating/cooling rates(10, 100 ℃·min-1) was studied by In-situ HT-Raman for the first time. We also studied the thermal contraction of the sintering product in cooling process. Results showed that two intermediate transition phases Bi2Fe4O9 and Bi25FeO39 will be produced in the reaction sintering process of BFO ceramics. The content of impurity phases mainly depends on the molar ratio of Bi2O3-Fe2O3 and heating/cooling rates, and 1.03∶1 with higher rate is optimum. There is a good linear relation ship between BFO A1-1 peak position and temperature in the cooling process, which indicates that the temperature reduction only brings about the lattice contraction of BFO ceramic and no structural change. Moreover, the phase composition and morphologies of sintering products were measured by Two Dimensional X-ray Diffraction(2 D-XRD) and Electron Backscattered Diffraction(EBSD). XRD results showed that the content of impurity phases is relatively low when Bi is excessive, which is consistent with Raman results. Combined 2 D-XRD and EBSD results, rapid heating/cooling rate is beneficial to the grains nucleation and growth, on account of the larger and more uniform grains when Bi exceeds 3%. This work provides useful experimental guidance for the preparation of pure-phase BiFeO3 ceramics by further elucidating its mechanism of phase transformation in the reaction sintering.