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本工作构建了基于巨势(Grand potential)的氮化铀烧结相场模型，模型中考虑了物质扩散和烧结颗粒运动，该模型可以扩大界面宽度以增大模拟体系的空间尺度。首先，对所构建模型进行了验证分析，界面平衡后相场变量呈现出对称分布，颗粒的刚体运动机制可以显著促进致密化过程。然后，模拟了不同温度下的双颗粒烧结过程，结果表明烧结颈的增长过程符合幂函数关系，幂指数n的大小为7.14，表明该过程中的主要传质机制为表面扩散。随着烧结温度的升高，烧结颈增长速率加快，晶界内部的空位最大偏析量增加。最后，研究了不同温度下的多颗粒烧结过程，烧结颈之间接触重叠形成复杂的晶界结构，内部孔隙由不规则形状向圆弧形转化。致密化过程中孔隙先是以空位的形式富集在晶界处，再沿晶界传输至外部气相或体积更大的孔隙中。平均孔径先缓慢增加后保持稳定。随着烧结温度由1723 K增加至1873 K，致密化程度不断加深。Due to its high thermal conductivity and uranium density, uranium nitride (UN) has great application prospects in various nuclear facilities. However, sintering is an important step during the preparation of UN fuel, and the properties of UN pellets in reactor are significantly affected by sintering parameters. Therefore, using numerical simulation techniques to investigate the sintering mechanism of UN fuel is of great significance. In this work, a phase-field model for the sintering of UN based on the grand potential is established, which simultaneously incorporates the rigid body motion of particles and the mass diffusion. This model enables the expansion of the interface width, thereby increasing the spatial scale of the simulation system. Firstly, a validation analysis of the constructed model is conducted. The phase-field variables exhibit a symmetric distribution at the locally equilibrated interface. The rigid body motion of particles significantly promotes the densification process. Subsequently, the sintering process of two particles is simulated at different temperatures. The results show that the growth of the sintering neck follows a power function relationship with the power exponent n of 7.14, indicating that the dominant mass transfer mechanism is surface diffusion. As the sintering temperature increases, the sintering neck growth accelerates, and the maximum concentration of vacancies within the grain boundary increases. Finally, the multi-particle sintering is investigated at different temperatures. The contact and overlap between sintering necks form a complex grain boundary structure, and the internal pores transform from irregular to circular shapes. During the densification, vacancies originating from pores segregate to grain boundaries and then diffuse to the external gas phase or larger pores. The average pore size initially increases slowly and then remains stable. As the sintering temperature increases from 1723 K to 1873 K, the degree of densification progressively improves.
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Keywords:
- Phase-field simulation /
- Uranium nitride /
- Sintering /
- Grand potential /
- Diffusion
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