Novel Synthesis of Single Crystal Entropy-Stabilized Ceramics: Identifying Critical Reaction and Transport Parameters

Non-Technical Abstract

The ability to produce functional ceramics having tailored geometries has been elusive given the complexity of the underlying processes. The proposed work investigates a novel approach to fabricate such ceramics via solid-state synthesis, a process that involves a chemical reaction of mobile atoms to create a desired product phase. The focus of these studies is two-phase systems that react to form a novel product-phase, namely a so-called entropy-stabilized ceramic (ESC) that is stable at high temperatures.

Two new prototypes of substantial technological relevance are of particular interest here. First, the CuO-CuAl2O4-CuAlO2 system provides an opportunity for the design of new functional electronic devices such as transparent transistors and junction diodes, and CuAlO2 also has potential as a sensor material and for oxygen storage material applications.

Second, the In-Sn-O system may be exploited to fabricate single-crystal thermoelectrics (e.g., In4Sn3O12), which are devices that turn temperature differences into voltages. This combined experimental and modeling program seeks to describe quantitatively the creation and propagation of the desired product phase for each system so that it may be readily fabricated, with the following overarching aims: 1) elucidate the reaction mechanism resulting in the product phase (i.e., how does the reaction occur?); 2) understand the importance of elastic fields on the reaction (i.e., how does the material deform during a reaction?); 3) characterize the diffusion kinetics along internal interfaces (i.e., how do atoms migrate near phase boundaries?); and 4) design patterned single-crystal phases with unique microstructures (i.e., geometries) for specific applications.

In short, the intellectual merit of the proposed work is that it will identify the critical processes and reaction parameters in the solid-state reaction to form an ESC. Some broader impacts of this program include the education of the next generation of computational materials scientists using tools developed here and the use of interactive demonstrations developed here to encourage under-represented minorities in existing programs (e.g., CHOICES for middle-school girls) to pursue STEM opportunities.

Technical Abstract

The ability to exploit solid-state reactions to produce functional ceramics having tailored microstructures has been elusive given the complex interplay among various kinetic processes and the resulting pattern of an evolving product phase. While stochastic reaction-diffusion simulations provide important insight on the kinetics and microstructural evolution, the details of the underlying reaction mechanisms and transport pathways are not well understood.

The proposed work, supported by the Ceramics program in the Division of Materials Research, seeks to elucidate these mechanisms by considering several prototypical systems including Co-Ti-O, CuO-CuAl2O4-CuAlO2 and Sn-In-O. These systems were chosen given their technological importance; for example, as a sensor and an oxygen storage material (e.g., CuAlO2) and as a single-crystal thermoelectric (e.g., In4Sn3O12).

While for the first system it is known that it proceeds from a duplex two-phase structure to a single crystal of CoTi2O5, a so-called entropy stabilized ceramic (ESC), the important role of misfit strain in dictating transformation kinetics and morphological development remains unclear and will be examined here. The proposed program will also investigate the mechanisms and kinetics of transport along interphase boundaries (IBs) – processes that have received very little attention in the ceramics community.

In particular, advanced electron microscope techniques will be employed to characterize both the composition and orientation of phases at the reaction front, and the detailed structure of the IBs. In tandem, numerical simulations will be used to assess the impact of induced transformation stresses as well as boundaries with different degrees of free volume.

Finally, to develop greater intuition about underlying kinetic processes, a Whipple-like solution will also be formulated to study reaction and diffusion at a prototypical IB. In short, the intellectual merit of the proposed work is that it will identify the critical processes and reaction parameters in the solid-state reaction to form an ESC. The broader impacts of this work are designed to foster interest in STEM field subjects, particularly in under-represented groups.

This will be accomplished, for example, via the CHOICES (Charting Horizons and Opportunities In Careers in Engineering and Science) program, which is dedicated to encouraging middle school girls to consider careers in science and engineering, and a data science boot camp that will provide students with the basic background skills to apply machine learning techniques in materials science and engineering.

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.