AI 中文总结
本研究开发移动边界扩散模型，数值模拟发现固态反应存在三种动力学 regime，亚抛物线 regime 的特性由扩散系数比和晶粒生长动力学决定，有限时间拟合可得到表观指数。
AI 中文摘要
在固态扩散偶中，常观察到反应层的非抛物线生长，但微观结构演化与由此产生的动力学之间的关系仍未完全明晰。该效应通常归因于晶粒生长，晶粒生长会逐步降低快速晶界扩散的贡献，导致多晶产物层的有效扩散系数在反应过程中发生演化。本研究开发了一种移动边界扩散模型，用于描述产物层生长，同时考虑持续形成的产物的局部晶粒生长历史，以及体扩散和晶界扩散。数值模拟表明，反应可经历三种不同的动力学 regime：由晶界扩散主导的初始近似抛物线 regime、与有效扩散系数强空间变化相关的瞬态亚抛物线 regime，以及后续由体扩散主导的近似抛物线 regime。亚抛物线 regime 的幅度和持续时间取决于晶界与体扩散的相对扩散系数，以及晶粒生长的动力学。因此，瞬时生长指数会连续演化，并不代表唯一的动力学常数。不过，对有限实验时间间隔内的模拟层厚数据进行拟合，可得到明确的表观指数，表明瞬态过程如何表现为遵循单一幂律。
英文摘要
Non-parabolic growth of reaction layers is frequently observed in solid-state diffusion couples, but the relationship between microstructural evolution and the resulting kinetics remains incompletely understood. This effect is commonly attributed to grain growth, which can progressively reduce the contribution of fast grain-boundary diffusion, causing the effective diffusivity of a polycrystalline product layer to evolve during reaction. This work develops a moving-boundary diffusion model to describe product-layer growth while accounting for the local grain-growth history of the continuously formed product, as well as both bulk and grain-boundary diffusion. Numerical simulations show that the reaction can pass through three distinct kinetic regimes: an initial approximately parabolic regime dominated by grain-boundary diffusion, a transient sub-parabolic regime associated with strong spatial variation of the effective diffusivity, and a subsequent approximately parabolic regime dominated by bulk diffusion. The magnitude and duration of the sub-parabolic regime depend on the relative grain-boundary and bulk diffusivities and on the kinetics of grain growth. The instantaneous growth exponent therefore evolves continuously and does not represent a unique kinetic constant. Nevertheless, fitting simulated layer-thickness data over finite experimental time intervals produces well-defined apparent exponents, demonstrating how a transient process can appear to obey a single power law.
