Oxidation Mechanism of SiC Coated Layer in Air Atmosphere at 1400℃
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摘要: 在工业规模大内径喷射流化床内通过化学气相沉积法制备包覆燃料颗粒SiC包覆层,进行反应堆空气进入(AI)模拟工况1400℃氛围下3.5 h高温氧化实验,并采用高分辨扫描电镜(SEM)以及能谱分析(EDS)对SiC材料氧化前后表面及断面进行表征分析。结果表明,开始时SiC包覆层微米级的SiC细小晶体结构非常清晰,在高温时SiC细小晶体结构逐渐被氧化形成玻璃状光滑的Si氧化物,该玻璃状氧化物会阻止氧的扩散渗入,保护SiC包覆层以避免进一步氧化。由于SiC和Si的氧化物的膨胀系数和弹性模量不同,两者间存在层错应力,且Si的氧化物晶体化会导致氧化层体积变化,从而逐渐形成斑点状裂纹。随温度升高,氧又渗入裂纹的界面继续氧化,并且把形成的氧化层逐层剥离掉。SiC包覆层断面的分析表明,氧化层只有1μm左右,说明氧气的扩散和氧化过程非常缓慢。断面SEM分析发现,Si的氧化层和SiC包覆层之间有气孔存在并呈线性分布。EDS分析表明,剥离氧化层后的包覆层Si和C原子比不变,应具有同样的阻挡裂变气体的功能。Abstract: The SiC coating layer of HTGR fuel particles is the most critical layer in hindering the release of fission products.In this paper,SiC layer was prepared in a spout fluidized bed by chemical vapor deposi-tion on an industrial scale.High temperature oxidation experiments were carried out at different temperatures at 1400℃.The microstructure and composition were studied using different characterization methods,in-cluding EDS and SEM analysis.High-resolution SEM results showed that the oxidation started from the sur-face and gradually formed a punctate distribution of the oxide.The small crystal structure(μm) of the SiC surface was very clear at the beginning and this crystal structure was oxidized at high temperatures,and disappeared gradually to form oxides of Si on the SiC surface.There was a stacking fault stress due to the different thermal expansion coefficient and elastic modulus of SiC and Si oxides,leading to the gradual formation of cracks around the punctates,and then with the increasing temperature,the crack further increased the oxidation rate of SiC,finally the formed Si oxides layer fell off from the particles.It can be seen clearly that the layer was peeling when the temperature was increased to 1400℃.But the oxidation layer was very thin(~1μm),so the remain SiC coated layer has the same function of hindering fission products in the TRISO particles.EDS results can also validate these conclusions.
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Key words:
- Nuclear fuel particles /
- SiC coated layer /
- Oxidation behavior /
- Microstructure analysis
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