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

Lehigh University, Bethlehem PA

Investigators

Abstract

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.