High-temperature nanoarchitected materials with unique thermo-mechanical properties
Final Report Abstract
During this DFG Research Fellowship, at the University of California, Irvine, the applicant, Dr. Jens Bauer and his collaborators have, with a number of high impact publications, substantially contributed to the advancement of architected materials and two-photon polymerization (TPP) additive manufacturing, one of the key technologies to synthesize nanoarchitected materials, as well as microfluidic, biomedical, and micro-optical devices, and much more. Aiming to enhance the thermo-mechanical properties of nanoarchitected materials, three different TPP materials have been studied and optimized regarding mechanical strength, stiffness and toughness, as well as thermal conductivity and temperature stability. TPP technology is rapidly progressing, however, fabrication is still largely empirical, hindered by the lack of systematic data on material properties, and limited knowledge on their dependence on the process parameters. This project experimentally established these correlations for the acrylate-based resin IP-Dip (one of the most common TPP materials), over a large range of process parameters and length scales ranging from nanometers to centimeters. A threshold-based model was shown to accurately predict the mechanical properties depending on the process parameters, laying the foundation for a universal quantitative predictability of the mechanical properties of TPP-derived structures. While TPP is limited to mechanically weak polymers, pyrolysis is an effective route to transform polymeric templates into ceramics. Today, the most relevant materials for nanoarchitected structures, which can be synthesized via TPP, is pyrolytic glassy carbon. During pyrolysis, the resin is thermally decomposed in vacuum or inert atmosphere at around 1000°C, forming glassy carbon, with excellent chemical resistance, thermal stability, low density, and high strength. Similarly to TPP polymers themselves, TPP-derived ceramics were not well understood, though. We showed that the mechanical properties of pyrolytic glassy carbon, derived from TPP-printed IP-Dip, can, as the polymer itself, be tailored via TPP-printing parameter selection. The glassy carbon thereby inherits the process parameter-dependency from the polymer, as the ratio of sp²-bonded to disordered carbon, which determines the mechanical properties, depends on the degree of cross-links in the pre-pyrolysis polymer. Based on thermal conductivity measurements with individual nanobars at 300-800K TPP glassy carbon was found an excellent insulator. With 2 W/mK, specimens outperformed most monolithic solids and were up to 75% less conductive than bulk glassy carbon. Thus, showing that nanoscale glassy carbon is a well-suited for thermo-mechanical nanoarchitected metamaterials. Ceramics would be the ideal engineering materials, if their brittleness could be overcome. However, ductility in ceramics rapidly disappears when samples reach micrometer dimensions. Despite the shown advantages, TPP-glassy carbon is no exception. Brittle behavior with size- and geometry-sensitive properties, as well as shrinkage up to 90% upon pyrolysis complicate application. Overcoming these limitations, we showed the manufacturing of ultra-strong yet ductile SiOC ceramics via TPP of a pre-ceramic siloxane resin and subsequent pyrolysis. We presented high quality SiOC architectures with feature down to ~200 nm and shrinkages upon pyrolysis of only ~30%. This SiOC formulation is the strongest, stiffest and most tough two-photon polymerizable material reported to date. We found strengths of 7 GPa, yet ductile deformation behavior with strains up to 25%. Remarkably, these properties were very consistent across the entire range of examined specimens, up to diameters of 20 µm. Our findings for the first time demonstrate accessibility of ductility and ultra-high strength far beyond the nanoscale, as well as straightforward printability via TPP, with significant implications on the design and fabrication of micro- and meso- scale engineering systems. In the past, nanoarchitected materials have successfully utilized the ultra-high strength of nanoscale constituent materials; however, topologies were largely limited to inefficient beam-geometries in terms of stiffness and strength and toughness. Within this project novel shell- and plate-based nanoarchitectures were developed and shown to overcome the limitations of beam-based designs. We made shell-nanoarchitectures, whose topology was generated by extracting the phase-interface of a numericallysimulated spinodally decomposed solid, combining ultra-high tougness and exceptional strength and stiffness at low weight. We found non-catastrophic deformation up to 80% strain, and energy absorption up to one order of magnitude higher than for other nano-, micro- and macro-architectures and solids, and state-of-the-art impact protection structures. At the same time, strength and stiffness were on par with the most advanced, yet brittle beam-nanolattices. Finite element simulations showed, optimized shell thickness-to-curvature-radius ratios suppress catastrophic failure by impeding propagation of critically oriented cracks. In contrast to most architected materials, spinodal architectures may be easily manufacturable on an industrial scale and could become the next generation of superior cellular materials for structural applications. Furthermore, we demonstrated the first plate-nanolattices which are the only materials to experimentally achieve the theoretical stiffness and strength limits of an isotropic porous solid. Demonstrating specific strengths surpassing those of bulk diamond and performance improvements up to 639%, over the best beam-nanolattices, we provided the groundwork for establishing plate- and shell-architecture as another superior design principals for architected materials.
Publications
- Adv. Mater. 2017, 29, 1701850
J. Bauer, L. R. Meza, T. A. Schaedler, R. Schwaiger, X. Zheng, L. Valdevit,
(See online at https://doi.org/10.1002/adma.201701850) - Adv. Mater. Technol. 2019, 1900146
J. Bauer, A. Guell Izard, Y. Zhang, T. Baldacchini, L. Valdevit
(See online at https://doi.org/10.1002/admt.201900146) - Matter 2019
J. Bauer, C. Crook, A. Guell Izard, Z. C. Eckel, N. Ruvalcaba, T. A. Schaedler, L. Valdevit
(See online at https://doi.org/10.1016/j.matt.2019.09.009) - Small 2019, 1903834
A. Guell Izard, J. Bauer, C. Crook, V. Turlo, L. Valdevit
(See online at https://doi.org/10.1002/smll.201903834)
