Analyzing the mechanical behavior of supercrystals
Cong Yan defended his PhD thesis at the Department of Mechanical engineering on July 2nd.
The structures of natural materials are often replicated in human technology to improve performance, efficiency and sustainability. That鈥檚 because natural materials can have very useful qualities such as exceptional stiffness, hardness and fracture toughness. These attributes arise from their intricate hierarchical structures. A promising strategy to replicate such architectures is through a bottom-up approach in which nanoparticles surface-functionalized with organic ligands can self-assemble into long-range ordered structures, known as supercrystals, driven by interparticle interactions. Cong Yan focuses in his PhD research on understanding the mechanical behavior of these supercrystals.
uses nanoindention, with particular focus on their flow behavior and time-dependent deformation for his research. Meanwhile, ligands crosslinking is demonstrated to strengthen interparticle interactions and its influence on these mechanical behaviors is explored. This research is crucial to be able to develop robust, bulk hierarchical structures.
Elastic modulus and hardness ratio
Knowledge about the flow behavior of materials is essential to figure out how they behave under external loading and it鈥檚 also crucial to be able to do predictive studies. Cong Yan started his research with standard nanoindentation measurements to investigate the elastic modulus and hardness of supercrystals. He found that the ratio of these two qualities of supercrystals correlates with their potential in dissipating the energy. Crosslinked supercrystals typically show a higher ratio and tend to behave more brittlely. Consequently, this ratio serves as a useful indicator to assess the flow behavior of supercrystals.
Stress-strain curves
A more accurate assessment of flow behavior relies on their stress-strain curves. To address the challenge of non-uniform stress fields under the indent, an innovative approach based on the expanding cavity model is developed to extract the flow curve from spherical nanoindentation. Its validity is further confirmed through finite element analysis. The proposed protocol is applicable well beyond supercrystals, and it offers broad applicability for characterizing small-scale components, as well as for localized analyses in multi-phase or polycrystalline materials, thereby extending the capabilities of nanoindentation technique.
Time-dependent behavior
The time-dependent behavior of supercrystals is another critical consideration, particularly ensuring the dimensional stability during a long-term service. It is first examined in terms of creep: deformation continues under the constant load holding. The underlying mechanism is proposed as the compaction of organic ligands facilitating the movement of nanoparticles, as implied by the activation volume. Subsequently, a free volume-based model is proposed to predict the creep behavior of supercrystals. This model emphasizes the role of free volume in impacting the relaxation of organic ligands and it offers a practical, simulation-free approach for implementation - without requiring finite element analysis.
Another aspect of time-dependent behavior explored is the response of supercrystals to oscillatory loading, in other words fatigue. Supercrystals without ligands crosslinking exhibit a longer fatigue life, attributed to a greater potential in dissipating energy under the oscillatory loading. The difference in dissipating energy is primarily governed by the nature of functionalizing organic ligands, specifically, whether they are crosslinked or not.
Understanding supercrystals
With this research, Cong Yan addresses a critical knowledge gap in understanding of mechanical behaviors of supercrystals and he expands the applicability of nanoindentation technique towards more complex material behaviors. The organic ligands are revealed to play a vital role in controlling the mechanical behavior of supercrystals, offering opportunities to tune their structural performance. The mechanical properties of supercrystals can thus be tailored and used for the design of robust, bulk hierarchical materials with customized properties for a broad spectrum of applications.
Title of PhD thesis: . Supervisors: Assistant Prof. Diletta Guintini and Prof. Marc Geers.