This grant provides funding for a nanoparticle sintering study across multi-scales through quantifying three-dimensional (3D) structural evolution of different nanoparticle packings and connecting the microstructural characteristics with macroscopic shrinkage. The primary issues to be addressed are particle agglomeration, excessive grain growth, complex nano-/micro-structural evolution, and non-uniform shrinkage. Sintering is a high temperature process whereby particles, in this case nanoparticles (with diameters less than 100 micrometers), are consolidated into solid form. The process involves multi-scale events that range from atomic diffusion to macroscopic shrinkage and creates stable nanostructures for long term use. Experiments will be carried out to create homogeneous and agglomerated titanium dioxide and zirconium dioxide nanoparticle packing structures across all scales. The resulting nanostructure will be quantitatively described and correlated with grain growth and macroscopic shrinkage through nanostructural electron tomography and focused ion beam 3D rendering.
If successful, the results of this research will provide understanding to the key issues in nanoparticle sintering. These include the effects of different nanoparticle packings, excessive grain growth, complex nano-/micro-structural evolution, and non-uniform shrinkage. The team will develop theories that link nano-/micro-structural evolution and macroscopic densification throughout the entire sintering process. The integrated understanding across multi-scales will provide comprehensive sintering knowledge and enable the field to challenge the heretofore-accepted sintering theories and perspectives. With proper adjustment, the methodology and breakthrough can also be applied to conventional micron-sized particle sintering which has numerous applications. New theory-guided sintering processes resulting from this program will also allow for more energy efficient sintering practices. The research methodology has wide-ranging significance in multi-scale device integration while improving process reliability and manufacturability.
Intellectual merit: Our work provides fundamental understanding to the colloidal suspension processing and micro-scale feature array templating using nanoparticles. It demonstrates the unexplored potentials of patterning to create different feature shapes and hierarchically sized nanopatterns. Micron-sized feature arrays with feature sizes ranging from 250 nm to 1.5 μm are obtained. This technique is general and applicable to most nanoparticle suspensions, and will open numerous opportunities for micron-sized device fabrication. These new findings demonstrate that well-defined nanoparticle packing structures at different length scales, very desirably from nano-, to micro-, and to macro-scales, can be created. Due to the essential application of co-dispersion on nanostructure formation, we have explored preparing metal oxide nanoparticle and polymer composite for meeting the requirement of stamping. To achieve uniform-dispersed and high-loading hybrid solutions, surface modification, functionalization, and heat treatment are introduced to disperse nanoparticles. Solutions with different solid loading and nanoparticle volume fraction are also studied. By measuring the viscosity, the flowability of hybrid solutions has been evaluated. In our work, 3D microstructure reconstruction has been used to quantitatively evaluate pore evolution during sintering. Pore shape, distribution, and connectivity are displayed in 3D based on actual microstructure data and show drastic differences from the conventional description of pores. Pore volume fraction and pore number density are extracted from the 3D microstructure reconstruction and compared. Pore size distribution frequency with sintering time is obtained. New tortuosity concept is provided in order to properly represent pore shape changes during sintering. The microstructural evolution of nanoparticle materials after different sintering conditions is quantitatively presented. Focused ion beam (FIB) is used as a critical tool for sintering microstructural evolution understanding. 3D images are constructed with Amira software based on a series of 2D images obtained from focused ion beam (FIB) cutting of samples with an interval of 10 nm. Pore connectivity, pore numbers, grain-pore interfacial areas, and pore tortuosity of the sintered samples are calculated by IDL software using the 3D images. Our new tortuosity definition describes the actual sample microstructural changes while the other two in the literature fail to do so. FIB cutting and 3D microstructure reconstruction offer many new aspects of sintering microstructure that have not been available before. We also studied focused ion beam (FIB) patterning guidance effect on the nanopore development during the subsequent anodization on uneven surfaces. Different shape, uneven features are first created on aluminum surfaces. After that, hexagonally arranged nanoconcaves with different inter-concave distances are patterned across the surfaces. The pore development in different anodization electrolytes as well as with different interpore distances and feature surface curvatures are studied. The results show that FIB guided anodization can produce ordered nanopore arrays on various uneven surfaces. In the nanopore depth direction, pores grow perpendicularly to the tangential direction of the surface, which leads to pore splitting or termination based on the surface curvature. Broader impacts: The nanoparticles studied, TiO2, ZnO, and ZrO2, represent exciting possibilities in energy production, sensing, electronic, structural, and optical applications with the potentials of providing unprecedented material performance. The nanoparticle-based patterning and synergistic multi-scale sintering studies will offer new capabilities of effective manipulation of complex, device-level architectures for Coulomb blockade, metal-insulator transition, and super-paramagnetism. The research methodology has wide-ranging significance in multi-scale device integration while improving process reliability and producibility. The drastically improved ability of optimizing specific sintering processes means more energy efficient high temperature practices and thus more energy saving. This program represents transformative research because we have established a lithographic patterning process, a special grain growth control technique for porous material sintering, and a 3D method of quantifying the sintering microstructures in search of an effective approach to produce large surface area materials with complicated nanopatterns and nanostructures. These patterns are expected to deliver numerous direct device fabrication capabilities with high controllability. This program offers graduate study and research opportunities. In addition, the program offers research experience for undergraduate engineering students, especially female undergraduate and minority students. We were able to attract four female undergraduate students to work on this NSF project. In addition, we reached out to female engineering freshmen and high school students for nanomaterials education and female engineering student recruitment, education, and retention. Our educational theme is solidly based on nano-education. We have actively participated in Women’s Preview Weekend, Hypatia female engineering community building, C-Tech2, Blast, and Imagination summer camp. Our educational theme is solidly based on nano-education. For off-campus outreach, we have actively participated in the Summer Academy of Virginia State University in order to interact with minority institutions. These efforts will not only improve female and minority students’ understanding and knowledge base about nanotechnology and nanomaterials but more importantly will show them the career potentials in engineering. The activities will contribute to a new generation of professionals prepared to move our society in ‘nano’.
