In this research project, a new class of materials with controlled transversely modulated heterophase nanostructures (TMNS) will be developed. The research will integrate theory, modeling, experimental characterization, and design of TMNS with controlled scale and morphology. The basic idea of this research effort is to design TMNS by exploiting epitaxial self-assembling of constituent phases on a crystalline substrate. Formation of such self-assembled nanostructures requires establishing epitaxial relations between each phase and the substrate. These epitaxial relations lead to self-organization of constituent phases and formation of 3D heteroepitaxial nanostructures with coherent or semi-coherent interfaces. By selecting different substrates or substrate orientations and changing the thickness of the nanostructured layer, it is possible to control morphology of the self-assembled nanostructures on a scale that is difficult to obtain with other techniques. Because of the nanoscale of the component phases, dislocation-mediated mechanisms are suppressed resulting in significant elastic strain. Therefore, controlling this stress becomes a new mechanism for manipulating film properties, similar to semiconductor heterostructures. The goal of this research is to develop experimentally verified theoretical principles and computational tools to design materials with modulated nanostructures using epitaxial control. The ability to control morphology, scale, and stress state will be demonstrated. Self-assembled modulated structures on substrates will be formed as a result of either: (a) solid-solid phase transformation (polymorphic, martensitic, or eutectoid), or (b) eutectic crystallization from an amorphous or liquid phase. As a consequence of this research, new principles of design will be developed for thin film materials consisting of controlled heterophase nanostructures for tailoring of interfaces at the nanoscale, as well as the associated processing, characterization, and modeling techniques necessary to realize TMNS.
NON-TECHNICAL SUMMARY
Nanostructured materials are important for a wide spectrum of structural and functional applications, such as sensors, actuators, magnetic recording media, wear resistant coatings, high temperature or corrosion resistant structural materials, and thermoelectric devices. This research will provide an entirely new principle for designing materials with controlled heterophase nanostructures that will lead to materials that are stronger, better at sensing, and more durable, as well as new materials that would not otherwise be possible such as multilayered composite structures whose properties can be actively tuned through self-assembly of the nanostructures. Broader impacts of this research include a coupled theoretical and experimental approach to research and education that ensures broad access to the knowledge needed to enhance the interest and skills of future engineers and researchers using sputtering techniques, nanoindentation, and computational materials science.
The goal of this research is to develop experimentally verified theoretical principles and computational tools to design materials with modulated nanostructures using epitaxial control. We will demonstrate the ability to control morphology, scale, and stress state. These principles can be used to develop a new class of materials: self-assembled nanocomposites. To achieve the goal of the proposed research the following tasks will be accomplished: (1) Theory and modeling of epitaxially-controlled phase transformations; (2) Processes for forming TMNS; and (3) Characterization of TMNS films. Self-assembled modulated structures on substrates will be formed as a result of either: (a) solid-solid phase transformation (polymorphic, martensitic, or eutectoid), or (b) eutectic crystallization from an amorphous or liquid phase. Key findings of this research project are summarized as follows: 1. Determination of Processing Parameters for Epitaxial Control of TMNS at a Ag-Si Interface This work established a processing technique to fabricate TMNS at the interface between Ag foil and Si wafer. Crystal orientation of the Si was found to determine Ag nanostructure morphology while annealing duration determined structure size and periodicity. Annealing in an oxygen rich environment (air) was discovered to be a critical parameter to prevent bulk diffusion between the layers and enable modulated nanostructure growth. Annealing specimens in vacuum resulted in bulk diffusion and eutectic solidification, establishing the need for oxide layer growth to retard diffusion and enable structure growth. EBSD analysis revealed the epitaxial relation between the single crystal Ag nanostructures and the surrounding Si. 2. Formation of Transversely Modulated Nanostructures in Epitaxial Pd Thin Films via Hydrogenation Transversely modulated nanoscale surface structures were fabricated in epitaxial Pd film via high temperature and high pressure gas phase hydrogenation. These structures are oriented similarly to coherent hydride formations seen in, suggesting their presence is due to coherent β phase hydride formation. A thickness effect was observed, where 50 nm films displayed ordered plates with a much smaller aspect ratio than those formed in a 100 nm film. An edge-gradient effect was observed in some specimens, suggesting that phase fraction of the ordered nanostructures may be dependent on internal stress of the film. In other words, the stress along the edges of the film may be relaxed during heating due to differences in thermal coefficients and interfacial defect formation, resulting in varying equilibrium nanostructure concentration. 3. Development of a Thermodynamic Model for Predicting the Control or Elimination of Hysteresis due to Metal-hydride Transformations in Epitaxial Thin Films A thermodynamic approach has been formulated that predicts elimination of intrinsic hysteresis during isomorphic phase transformation in metal-hydride epitaxial thin films via substrate constraint. This is due to the addition of an elastic energy term in the total free energy which represents elastic interaction between the film and substrate due to film-substrate misfit and epitaxial misfit of the phases. This term makes the free energy of the two-phase state a nonconvex function, making the chemical potential an increasing linear function in this range. Intellectual contributions were made in three areas: (1) Development of principles of design for thin film materials with controlled heterophase nanostructures for tailoring of interfaces at the nanoscale, (2) Development of a new self-consistent, experimentally-verified model for phase transformations under epitaxial control, (3) Development of new techniques for processing TMNS metallic thin films, (4) Development of new methods for characterizing TMNS to provide fundamental insight into the relationship between the cooperative behavior of the nanostructure and the mechanical properties of metallic thin films From the new method of for materials design developed here, there are three directions of possible practical applications of TMNS: (1) modification of physical properties, (2) tailoring of interfacial properties, and (3) formation of non-equilibrium structures that are not possible in bulk materials or uniform epilayers. These will impact the development of new structural and functional multiphase nanomaterials for a wide spectrum of applications, such as sensors, actuators, magnetic recording media, wear resistant coatings, high temperature or corrosion resistant structural materials, and thermoelectric devices. The creation of such self-assembled modulated structures in epitaxial films can lead to a new class of macroscopic composite materials consisting of alternating passive and transformable layers with self-assembling nanostructures. Broader impacts were also made in the following educational and societal areas: (1) strengthening the practical knowledge and experience of students who will serve as future researchers in the design of nanostructured materials with a new course, "Materials by Design", (2) a coupled theoretical and experimental approach to research and education that ensures broad access to the knowledge needed to enhance the interest and skills of future engineers and researchers using sputtering techniques, nanoindentation, and computational materials science, and (3) enhanced diversity within the mechanics and materials community through the participation of underrepresented minorities in the proposed research efforts.
