A Systematic Dopant-selection Strategy for Advanced Manufacturing of High Strength Transparent Magnesium Aluminate Spinel

光学透明铝酸镁尖晶石（MgAl2O4， MAS）是应用于航天器和军用车辆窗户等极端环境的候选者之一，传统的硅酸盐玻璃或聚合物不适合在这些环境中使用。目前，加工高强度、高透明度的MAS材料需要较高的成本，这是制约MAS材料应用的主要因素之一。在不损失强度和透明度的情况下实现高密度的有希望的途径是通过添加ppm级杂质来修饰晶界（GB）结构和化学。这些杂质可以改变边界的运动，从而提高材料的强度。这项工作将开发一个系统，用于识别适当的掺杂剂来修改GB原子排列，以提高机械强度和透明度。通过这项工作确定的化学添加剂可以生产具有增强强度和足够透明度的MAS材料。结果。这种方法将为制造透明多晶MAS提供新的途径，适用于大规模制造多晶MAS和其他潜在的陶瓷，以保持国家的全球竞争力和加强国防。这些信息和方法将通过一年一度的显微镜学校进行传播。四种不同类型的稀土元素将被掺杂到多晶和受控双晶MAS材料中，而不是稀土来改变GB强度，从而改变机械性能。掺杂剂的选择基于以下标准：(A)掺杂剂必须是比天然阳离子Mg2+和Al3+更大的阳离子，以实现有效的GB偏析；(B)掺杂剂理想状态为3+态，而不是2+态，以占据两个阳离子位置；(C)掺杂剂的氧化物形式应比Al或Mg具有更高的稳定性（即与氧阴离子的结合更强）。如果这些标准在实验中得到证实，该方法将为设计GB的结构和化学性质提供一种新的策略，从而通过选择掺杂剂来控制GB的强度。采用先进的电子显微镜和精密的显微力学测试对GB的结构和化学进行定量表征，以测量GB的断裂韧性。基于实验结果，我们将建立一个概念，以确定适当的掺杂剂来修改原子排列，以提高机械强度和透明度。本项目建立的概念应适用于其他三元或多组分离子化合物，如钙钛矿SiTiO3和LaMnO3，并适用于其他二元陶瓷材料。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Optically transparent magnesium aluminate spinel (MgAl2O4, MAS) is one of the candidates to use in extreme environments such as spacecraft and military vehicle windows, in which conventional silicate glasses or polymers are not suitable. Currently, the processing of MAS materials with high strength and high transparency requires high costs, which is one of the major limiting factors to prevent the MAS materials from the applications. A promising path for achieving high density without loss of strength and transparency is via modification of grain boundary (GB) structures and chemistry with the addition of ppm-level impurities. These impurities at the GBs can alter the motion of boundaries leading to improved materials strength. The work will develop a system for identifying appropriate dopants to modify GB atomic arrangements for improved mechanical strength and transparency. MAS materials with enhanced strength and sufficient transparency can be produced by chemical additives identified through this work. the results. This approach will provide a new route for manufacturing transparent polycrystalline MAS, applicable for the large-scale manufacturing of polycrystalline MAS and potentially other ceramics needed to maintain the nation’s global competitiveness and enhance national defense. The information and approaches will be disseminated through an annual microscopy school.Four different types of earth-abundant elements will be doped into polycrystalline and controlled bi-crystal MAS materials, instead of rare-earths to alter the GB strength and hence mechanical performance. The dopant selection is based on the following criteria: (A) dopants must be larger cations than the native cations Mg2+ and Al3+ for effective GB segregation, (B) dopants ideally take 3+ state rather than 2+ to occupy both cation sites, and (C) dopants should have higher stability in their oxide forms (i.e. stronger bonding with oxygen anions) than either Al or Mg. If these criteria are proven experimentally, this approach would provide a new strategy for designing GB configuration and chemistry, and hence controlling the GB strength by selecting dopants. The GB structures and chemistry will be quantitatively characterized by advanced electron microscopy together with sophisticated micro-mechanical testing to measure the GB fracture toughness. Based on experimental results, we will establish a concept for identifying appropriate dopants to modify atomic arrangements for improved mechanical strength and transparency. The concept established in this project should be relevant to other ternary or more component ionic compounds such as the perovskites SiTiO3 and LaMnO3 and be applicable to other binary ceramics materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.