BiFeO3反应烧结过程中相变机理的原位X射线衍射研究

The existence of impuritiy phases such as Bi25FeO39 and Bi2Fe4O9 has led to high leakage current in BiFeO3 multiferroic materials, which consequently restricts further understanding of its coupling between magnetic and polarization orders. Prior to the attempts to synthesize pure -phase BiFeO3 ceramics, the phase transition involved in the reaction sintering should be clarified. In the present project,we will trace the phase transition during the reaction sintering of BiFeO3 ceramics via using in-situ x-ray diffraction technique. The procedure of phase nucleation,growth and decomposition at different reaction atmosphere will be analysized systematically with respect to the molar ratio, granularity and dispersion of Bi2O3 and Fe2O3 reactants.The effects of reaction temperature and reaction time on the phase transition will be examined. The structural details including phase contents, crystal lattice, crystallite size, microscopic stress will be explicitly measured and correlated to the reaction temperature and atmosphere. In addition, the phase equilibrium relationship will be also examined by combining with theoretical calculation of Gibbs free enegry. The present research will elucidate the crystallographic, thermodynamic behabiours and mechanism of phase transition in the reaction sintering process.Finally, the formation and transformation regularity of impurity phases will be analysized, and subsequently feasible ways will be proposed to inhibit their formation. This work would provide useful theoretical and experimental guidances for the preparation of pure-phase BiFeO3 multiferroic ceramics.

Bi25FeO39、Bi2Fe4O9等杂相的存在增加了多铁性材料BiFeO3样品中的漏电流，制约了对其磁电耦合机制与调控的深入研究。然而纯相BiFeO3陶瓷的制备一直是材料合成中的难点，其根源在于尚不明确其反应烧结过程中的相变机理。本项目拟以BiFeO3的反应烧结过程为研究对象，采用原位X射线衍射技术，系统研究不同特性的原料(配比、粒度、分散性等)在多种气氛反应条件下的物相成核、生长和分解的动态过程；分析反应温度、时间等工艺因素对相变过程的影响；获得物相含量、晶胞参数、晶粒尺寸、微观应力等结构信息与温度、气氛等的关系；同时结合理论计算得到的各相吉布斯自由能研究反应过程中相的平衡关系，探讨该反应过程中的相变结构学和相变热力学行为，阐明其中的相变机理，揭示杂相的形成和转化规律，进而提出抑制杂相生成的有效途径，为优化烧结工艺、制备纯相BiFeO3材料提供科学依据和实验基础。

纯相BiFeO3陶瓷的制备一直是材料合成的难点，其根源在于尚不明确其反应过程中的相变机理。本项目中，主要采用高温原位X射线衍射技术，系统研究了不同Bi2O3/Fe2O3配比情况下，不同升降温速率下Bi2O3-Fe2O3二元体系在空气、氮气、真空等条件下动态的一级相变过程。发现快速升温能够加速BiFeO3的反应并降低其反应生成温度，同时能够抑制杂相的生成，使得升温速率成为影响反应产物的关键因素。氮气、氩气、真空等气氛对主相的生成无益，其相变为非连续扩散型。BiFeO3、Bi2Fe4O9、Bi25FeO39三元体系在降温过程中不完全处于热力学平衡状态，BiFeO3与杂相之间存在分解与转换关系，且与降温速率相关，从而揭示了杂相的形成与转化规律，确认了快速升温和淬火技术是抑制杂相生成的有效途径。此外，由于Bi25FeO39相中Bi+5和Fe+3在晶格中占有同一位置，降低了其结构的稳定性，利用化学清洗法能有效分解该亚稳相，从而获得纯相BiFeO3。