Mixed Alkali Effect for Mitigation of Sodium-Accelerated Corrosion of Silicon Nitride Ceramics

0102340 Du本项目旨在研究Kyocera的SN 282（涡轮级Si 3 N4陶瓷）和化学气相沉积Si 3 N4在含有单一碱（钠、铯）和混合碱（钠和铯）元素的氧气环境中的腐蚀行为中的混合碱效应（MAE）。 旨在揭示MAE在Si 3 N4陶瓷高温缓蚀方面的潜力，建立对MAE的认识。 将回答的问题包括：（1）与单一碱金属（Na或Cs）的情况相比，Na和Cs在Si 3 N4上的氧化物层中的结合是否会相互延迟，如果是这样，定量相关性是什么？(2)在混合碱和单一碱条件下，腐蚀动力学（速率和活化能）和氧化物特性（成分、相、形态和结构）将如何比较？ (3)腐蚀中的MAE是否是碱比的函数（例如，[Na]/[Cs]）在氧化物和环境中的作用？(4)在某些碱性成分下是否存在腐蚀最小值？(5)MAE将如何影响腐蚀机制？SN 282中的添加剂和杂质阳离子对（1）-（5）有什么影响？ 该项目的目标将通过评估Si 3 N4在O2-NaNO 3，O2-CsNO 3和O2-NaNO 3-CsNO 3气体混合物中的腐蚀动力学来实现，这些气体混合物在很宽的温度范围内具有选定的成分，并通过使用各种分析工具来表征氧化层。硅基陶瓷对钠盐热腐蚀的敏感性仍然是开发先进燃气涡轮机和其他结构应用中陶瓷优势的严重障碍。 迫切需要寻找减轻腐蚀问题的方法，以实现其预期的应用。 本计画旨在探讨硅酸盐玻璃中的混合碱效应（MAE），以减缓氮化硅陶瓷在含钠环境中的腐蚀。 MAE是指当添加不同的碱金属氧化物时，玻璃的许多性质发生显著的非线性变化。 该建议的前提基于两个已知的事实。 首先，氮化硅的钠加速腐蚀是由于钠掺入导致氧化层溶解。 其次，MAE降低了碱活性，并将硅酸盐中的碱扩散率降低了2至6个数量级。 该项目代表了已知的第一次尝试使用MAE概念来解决硅形成陶瓷的钠加速腐蚀问题。 这将产生丰富的动力学和分析信息和见解MAE在高温腐蚀的氮化硅。 所产生的知识将为设计和用户群体提供实际开发MAE的科学依据，用于先进涡轮机发动机和其他要求苛刻的应用中的二氧化硅成型陶瓷的腐蚀减缓和耐久性改进。

0102340DuThis project is proposed to study the mixed alkali effect (MAE) in the corrosion behavior of Kyocera's SN282, a turbine-grade Si3N4 ceramic, and chemically vapor deposited Si3N4 in oxygen environments containing single alkali (sodium, cesium) and mixed alkali (sodium and cesium) elements. It aims to reveal the potential and establish the understanding of MAE in high-temperature corrosion retardation of Si3N4 ceramics. Questions that will be answered include: (1) Will the incorporation of Na and Cs in the oxide layer on Si3N4 be mutually retarded compared to a single alkali (Na or Cs) situation, and if so, what is the quantitative correlation? (2) How will the corrosion kinetics (rate and activation energy) and oxide characteristics (composition, phase, morphology, and structure) compare under mixed alkali and single alkali conditions? (3) Is MAE in corrosion a function of alkali ratio (e.g., [Na]/[Cs]) in the oxide and in the environment? (4) Do corrosion minima exist at some alkali composition(s)? (5) How will MAE affect the corrosion mechanism? And (6) What are the effects, if any, of the additive and impurity cations in SN282 on (1)-(5)? The objectives of the project will be achieved by evaluating the corrosion kinetics of Si3N4 in O2-NaNO3, O2-CsNO3, and O2-NaNO3-CsNO3 gas mixtures of selected compositions over a broad temperature range and by characterizing the oxide layers using various analytical tools. %%%The susceptibility of silica-forming ceramics to hot corrosion by sodium salts remains a severe roadblock to the exploitation of ceramic benefits in advanced gas turbine engines and other structural applications. A critical need exists to search for means to mitigate the corrosion problem in order for the realization of their projected applications. This project aims to explore the mixed alkali effect (MAE) in silicate glasses for corrosion retardation of silicon nitride ceramics in sodium-containing environments. MAE refers to the pronounced, non-linear changes in many properties of a glass when a dissimilar alkali oxide is added. The premise of the proposal rests on two known facts. First, sodium-accelerated corrosion of silicon nitride results from the dissolution of the oxidation layer through sodium incorporation. Second, MAE lowers the alkali activity and reduces alkali diffusivity in silicates by 2 to 6 orders of magnitude. This project represents the first known attempt to use the MAE concept to address sodium-accelerated corrosion of silica-forming ceramics. It will yield rich kinetic and analytical information on and insights into MAE in high-temperature corrosion of Si3N4. The knowledge generated will provide the design and user communities with the scientific basis for practical exploitation of MAE for corrosion retardation and durability improvement of silica-forming ceramics for advanced turbine engines and other demanding applications where they would otherwise be unsuitable.