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

光学透明镁铝尖晶石(MgAl2O4，MAS)是用于航天器和军用车窗等极端环境的候选材料之一，在这些环境中，传统的硅酸盐玻璃或聚合物不适合使用。目前，高强度、高透明度的MAS材料加工成本较高，是制约MAS材料实用化的主要因素之一。在不损失强度和透明度的情况下获得高密度的一个有希望的途径是通过改变晶界(GB)结构和添加ppm级的杂质来进行化学修饰。GB中的这些杂质可以改变边界的运动，从而提高材料的强度。这项工作将开发一种系统，用于识别适当的掺杂剂，以修改GB原子排列，以提高机械强度和透明度。通过本工作确定的化学添加剂可以生产出强度更高、透明度更高的MAS材料。结果。这种方法将为制造透明多晶MAS提供一条新的路线，适用于大规模制造多晶MAS，以及维持国家全球竞争力和加强国防所需的潜在其他陶瓷。这些信息和方法将通过一所年度显微镜学校传播。四种不同类型的富土元素将被掺杂到多晶和受控双晶MAS材料中，而不是稀土，以改变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，才能有效地分离GB；(B)理想情况下，掺杂剂以3+状态占据两个阳离子位置，而不是2+；以及(C)掺杂剂的氧化物形式(即与氧阴离子的结合更强)的稳定性应高于Al或Mg。如果这些准则在实验上得到验证，该方法将为设计GB结构和化学，从而通过选择掺杂剂来控制GB强度提供一种新的策略。用先进的电子显微镜和精密的微机械测试对GB的结构和化学成分进行定量表征，以测量GB的断裂韧性。根据实验结果，我们将建立一个概念，以确定适当的掺杂剂，以改变原子排列，以提高机械强度和透明度。该项目中确立的概念应与其他三元或更多组分的离子化合物相关，如钙钛矿型SiTiO_3和LaMnO_3，并适用于其他二元陶瓷材料。该奖项反映了NSF的法定使命，并已通过使用基金会的智力优势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.