Translated Abstract
Aerogels are a type of ultralight porous materials. It possesses lots of extreme properties, such as ultra-high specific surface area, ultra-low density, excellent thermal and acoustic resistance and superior elastic recovery, which draws much attention from various industrial and scientific areas.
The superior properties of aerogels result from the microscopic porous structures, and different aerogels have different microscopic porous structures which lead to different mechanical properties. The researches on aerogels mainly focus on the preparation techniques, the modification and enhancement of materials and the applications, while the understanding on the structural properties and the relationship between the structural properties and mechanical properties are still not so clear. In this study, we choose two typical aerogels, i.e. silica aerogel and graphene aerogel, as the research object. We use molecular dynamics simulations, first-principle calculations and phase-field-crystal method to propose the structural model of silica aerogel and graphene aerogel and investigate the relationship between their structural properties and mechanical properties.
In the study of silica aerogel, we propose a two-level structural model with considering the effect of dead-ends to describe the hierarchical structure of silica aerogel, i.e. the effective skeleton model for the 3D structure composed of secondary nanoparticles and the nanoporous silica model inside the secondary particles. At first, we improve the negative ruprturing method which is used to generate atomic porous silica to generate porous silica models with more uniform pores using molecular dynamics. The mechanical properties of these silica models with different densities are investigated. Then we develop a constitutive model for the open-pore porous material with considering the random orientation of its representative volume element. At last, we calculate the mass ratio of the dead-ends in the skeleton of silica aerogels by combining the structural and mechanical properties of these two-level models. This study reveals the existence of the large amount of dead-ends, explains the abnormal exponent in the Young’s modulus- density power-law relation of silica aerogel, and suggests that the enhancement of the connectivity of the skeleton can effectively improve the mechanical properties of silica aerogel.
In the study of the defect effect in graphene, we first use molecular dynamics simulations to investigate how point defects and grain boundaries affect the stiffness and strength of graphene. It shows that the single vacancy, SW defect and 5-7 ring twin grain boundary all slightly weaken the stiffness and strength. In order to utilize the graphene as the corrosion barrier against sulfidation and oxidation for silver surface, experiments are carried out to test the protective effectiveness of graphene by exposing silver sample coated by single-layer graphene in atmosphere with concentrated S. Experiments show that there are silver sulphide particles generated along the grain boundaries of graphene. In order to explain the mechanism how S/O atoms penetration through the grain boundaries of graphene, we use first-principle calculation to investigate the intercalation of sulphur/oxygen atoms into the grain boundaries of graphene. It shows that the single vacancies just facilitate the intercalation of first S/O atom, which would not lead to the intensive corrosion. The intercalated S/O atoms also help created vacancies aside. However, once these newly created vacancies contact and connect each other, it causes larger defects like micro-voids and cracks which at last lead to the catastrophic failure of the grain boundaries and the protection.
In the study of graphene aerogel, we first use phase-field crystal method to build the Schwarz-surface-like graphene (SSG) models. We find that the Young’s modulus of these SSG models with much lower densities is even higher than some metals. Then we propose two SSG-based models to represent the structure of graphene aerogel. We find that the compressive recovery behavior of the graphene aerogel model based on the Schwarz G-like graphene model shows better agreement with experiments.
This study provides a better understanding on the relationship between the structural properties and the mechanical properties of silica aerogel and graphene aerogel, and helps promote the application of aerogel materials.
Translated Keyword
[First-principle calculation, Graphene aerogel, Molecular dynamics simulation, Phase-field-crystal method, Silica aerogel]
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