作者:Stefano Modena、Gian Domenico Soraru、Yigal Blum、Rishi Raj
DOI:10.1111/j.1551-2916.2005.00043.x
日期:2005.2
The oxidation kinetics of non‐oxides depend on the inward diffusion of oxygen from the environment through the passivating silica overgrowth, the outward diffusion of the effluent species, e.g., CO, produced by the oxidation reaction at the interface, and the chemical driving forces for diffusion. An analysis that combines these factors into a unified theory is presented. The analysis is applied to experiments on the oxidation of polymer‐derived amorphous silicon oxycarbide (SiCO) ceramics containing different amounts of carbon. The comparison between theory and experiment suggests that the activity of the so‐called “free carbon” in SiCO is likely to be less than unity, which explains why the oxidation of SiCO is passive in nature. Further, the analysis provides quantitative answers to the following questions: (a) How is the effective diffusivity for the parabolic rate constant related to the composition of the substrate, the inward diffusivity of oxygen, and the outward diffusivity of CO? (b) How does the rate constant depend on the activity of carbon in the substrate and on the activity of carbon in the environment? (c) How is the pressure of CO generated at the interface related to the carbon activity and the diffusion coefficients? The analysis points towards the need for systematic experiments in controlled O2/CO2 environments for a more complete understanding of the oxidation kinetics of carbon‐based ceramics.
非氧化物的氧化动力学依赖于氧气从环境通过钝化二氧化硅层向内的扩散、氧化反应界面产生的流出物(如CO)向外的扩散,以及扩散的化学驱动力。本文提出了一种将这些因素结合到一个统一理论中进行分析的方法。该分析应用于不同含碳量的聚硅氧烷(SiCO)陶瓷氧化实验。理论与实验的比较表明,SiCO中所谓的“自由碳”的活性很可能小于1,这解释了为什么SiCO的氧化具有本质性的被动性。进一步,该分析对以下问题提供了定量答案:(a) 与抛物线速率常数相关的有效扩散率与基底的成分、氧气的内扩散率以及CO的外扩散率如何相关?(b) 速率常数如何取决于基底中碳的活性和环境中碳的活性?(c) 界面处生成的CO的压力如何与碳的活性和扩散系数相关?该分析指出,为了更全面地理解碳基陶瓷的氧化动力学,需要在可控的O2/CO2环境中进行系统实验。