Jiang research group is currently focusing on one topic: mechanical behaviors of heterogeneous soft/hard materials/systems, with emphasis on the multiphysics interactions and specific applications on environmentally driven electronics, energy storage devices, and multi-functional nanomaterials. The work falls under the general area of mechanics of materials, and seeks to develop a clear understanding of the underlying deformation mechanisms by integrating multidisciplinary fields of mechanics, materials, electrical engineering, chemical engineering and chemistry. Specifically, Jiang research group is currently actively engaged in the following topics:
The motivation is to combine materials that have complementary properties to meet the multiple demands in applications. Many rich properties, specifically the “intelligence” of the integrated soft/hard materials are not explored. This research specifically focuses on the heterogeneous gel/hard materials/systems to utilize the environmental sensitivities of gels to drive the deformation of hard materials with applications of environmentally drivable electronics.
The specific research in this area includes:
It is a challenging problem to model the gels when multiply thermodynamics forces (e.g., temperature, electric field), phase transition (e.g., sol-gel), as well as coupled large deformation and mass transfer are involved. The simulations are also challenging because the gel deformation always exhibits time-dependent instabilities.
Our hypothesis is that the intimate interaction among gel and hard materials will enrich their performance. It would be a breakthrough if a common hard material can be triggered by the responsiveness of a gel to make the gel/hard material combination “intelligent”.
The development of the high-performance energy storage devices is extremely important for both fossil-based energy and renewable energy because the generated energy needs to be stored and delivered. This interest has extended from the electrochemistry and materials science societies to the solid mechanics community because there is a serious fracture problem during the electrochemical reactions. For example, in silicon-anode lithium (Li) ion batteries (the one has the highest known theoretical charge capacity), during charge and discharge cycling, Li ions diffuse in and out the Si anode and significant volumetric change (up to 400%) is generated, resulting in pulverization and early capacity fading of the battery cells due to the loss of electrical contact with the current collector.
We are trying to tackle this problem from two complementary directions.
We developed a finite element based numerical method making use of commercially available software package ABAQUS, as a platform at continuum scale to study fully coupled large deformation and mass diffusion problem in Li ion batteries, which is considered to be the basic process in the silicon anode during charging and discharging in lithium ion battery. Through this method, rigorous large deformation, elastic-plasticity of the silicon, various boundary conditions, arbitrary geometry and dimension could be realized. The interaction between anode and the other components of the battery could be studied as an integrated system. It is expected that this framework can be utilized to study many fundamental problems in Li ion batteries, for example the relationship between stress and state of charges, the fracture and damage during lithiation, etc.
We have developed an innovative approach to resolve the stress issues in Si-anode Li-ion batteries by utilizing Si nanostructures as anodes on soft substrates as a means to release the stress induced by Li ion diffusion during charge-discharge cycles, thus realizing high-performance and cyclic stability. The basic concept of this heterogeneous structure is that the diffusion-induced strain in Si anode can deform the soft substrates by generating buckling for the flat Si, and thus the stress in Si is released. Batteries of long cycle life (more than 500 charging/discharging cycles) with exceptionally high discharging capacity (4,137 mAh/g) and better capacity retention (84.6%) have been realized. This stress releasing mechanism suggests a general route for both anode and cathode design, which may facilitate the production of never before seen, high performing lithium ion batteries.
The change of buckling wavelength upon applied strain (e.g., thermal expansion) can be utilized for high sensitivity strain sensing of microelectronic packages and extending to backend films in semiconductor wafers. The buckled thin film grating will be attached to the packages under study, which makes the grating periods change as the local strain changes. Such changes will be recorded with optical system, while the scanning across the sample surface will allow the 2D spatial mapping of the strain change. The mechanism is just the simple reflection.
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