The research objective of this grant is to elucidate the sub-nanometer structural and chemical characteristics of transparent magnesium aluminate spinel grain boundaries that are responsible for intergranular fracture when LiF is used as a sintering aid. It has long been known that partially-disordered intergranular films (IGFs) at the 1-2 nm scale can have a profound effect on the behavior of ceramic materials, and it is now becoming evident that newly-revealed thinner layers (complexions) can have similar effects on macroscopic behavior. Although conventional High-Resolution TEM studies have not identified residual LiF in fully-processed liquid phase sintered spinel, light elements like Li and F are not detectable by this technique in the quantities typical of many boundary complexions so there is high likelihood that sub-nanometer layers of boundary LiF are, indeed, behind the poor boundary strength. The proposed program will test this proposition in a highly coordinated set of experiments correlating the mechanical properties of doped grain boundaries with their associated grain boundary chemistry and structure. This will be accomplished through the marriage of cutting-edge aberration-corrected Scanning Transmission Electron Microscopy (ac STEM) and newly-developed microscale fracture testing techniques that are suitable for evaluating the local fracture properties of boundaries.
The results of this program will inform the development of transparent, fracture-resistant spinel materials for spacecraft windows, sensor domes, armor, and protective goggles and face shields. Although the study of interface complexions is in its early days, the long-term impact of the proposed work is through the contribution of a key element (the link between boundary fracture behavior and complexion) to a developing scientific framework for documenting, understanding and exploiting complexions in ceramic materials. There is great potential to achieve unique combinations of properties in many ceramic (and even metallic) systems by tailoring grain boundary structure and chemistry according to the complexion framework. The development of micro-scale fracture techniques that can be performed using commercially-available instruments will present, for the first time, the opportunity for a large number of people and industries to be directly involved in carrying out quantitative fracture measurements with very small volumes of material. Impact on human development will be primarily through the education and training of the graduate students and undergraduates who will be involved with the project, who will have access to the latest microscopy and microtesting technologies. Impact on the local community will be achieved through continuing participation in outreach activities including ?NanoDays? at the Da Vinci Discovery Center of Science and Technology in Allentown, PA.
The overarching research objective of this grant is to lay the groundwork for understanding the effects of adding small quantities of certain elements and compounds to transparent magnesium aluminate spinel ceramic material. Certain elements are known to segregate to the grain boundaries of spinel, substantially altering the development of the microstructure during processing. This, in turn, can have a strong effect on the mechanical behavior of the final material. During the performance of this work a number of new techniques have been developed that will be of use to future research and development activities. In the long run, the results of this program will inform the development of transparent, fracture-resistant spinel materials for spacecraft windows, sensor domes, armor, and protective goggles and face shields. Intellectual Merit Key outcomes of this program include (1) demonstration that micrometer-scale cantilever bend tests can accurately measure the fracture behavior of ceramic materials including, but not limited to, spinel; (2) proof that adding very small amounts of ytterbium, a 'rare earth element', to spinel can strengthen the grain boundaries; (3) proof that the rare earth elements, despite being chemically similar to one another, can have drastically different grain boundary segregation behavior and therefore during processing they influence microstructure evolution in different ways. For example, small amounts of europium, another rare earth element, segregates in spinel grain boundaries in a significantly different fashion as compared to ytterbium and creates very different microstructures despite being present in a concentration of less than 500 ppm. Understanding these phenomena brings us one step closer to being able to design and fabricate spinel components with optimal mechanical and optical properties at reasonable cost. Broader Impact The results of this study have been disseminated to the scientific community through 5 graduate student presentations, 2 presentations by the Co-PI and PI, and 1 journal publication. Three additional journal publications are in draft form. The project has fully or partially supported the training of 4 graduate students in the areas of ceramic processing, in-situ nanomechanical testing, and aberration corrected analytical electron microscopy. Two of the graduate students are female. One student has completed his PhD degree and now works for a U.S. company. The other graduate students are expanding their skills in other NSF-funded research projects. Micromechanical testing has received considerable attention of the past two decades, but most of the emphasis has been on thermal-mechanical behavior of semiconductor materials and on metal plasticity. The present research program is the first to apply emerging commercially-available tools and new techniques to the analysis of ceramic grain boundaries. The development of micro-scale fracture techniques that can be performed using these instruments present, for the first time, the opportunity for a large number of people and industries to be directly involved in carrying out quantitative fracture measurements with very small volumes of material. In fracture mechanics the role of grain boundaries in mechanical failure of materials, especially ceramic materials, has been well established. However, characterization of individual grain boundaries and relating it to the grain boundary segregation behavior has never been attempted before in a systematic fashion. The fundamental knowledge about mechanical characteristics of individual grain boundaries will pave the path for developing materials and structures with specific mechanical properties by means of grain boundary engineering. Faculty and graduate students supported by this grant, as well as undergraduate volunteers, participated in three K-12 outreach activities (CHOICES, NanoDays, and MatCamp). The Lehigh University CHOICES program is an engineering-themed outreach event that exclusively targets middle school aged girls. NanoDays is a larger outreach event that reached over 6000 residents of the Lehigh Valley during the past three years. It is focused on introducing nanotechnology tools and concepts to the general public, and to children in grades K-8 in particular. MatCamp is partially sponsored by ASM International. Each year it engages 16 high school students in hands-on activities to introduce them to the concepts of materials science and engineering.