Tension Deformation Behavior And Microscopic Mechanism Of Amorphous Silica

Diatoms are a kind of single-celled photosynthetic plankton living in the waterenvironment.The frustuleswhich are mainly composed of amorphous SiO2areunique anddiverse, andhave scales from nanometers to micrometers and multi-level complexporous structures. The structures exhibit excellent mechanical, optical and transportproperties, which are increasing people’s interest and attention.Amorphous SiO2is a sort of brittle material which exists in the nature abundantly.In the state of bulk phase, once the initial crack propagation starts, amorphous SiO2iseasy to lose efficacy, and absorbs little energy in its fracture process. However, recentexperiments and calculations found that in micro-and nano-scale, the diatom shell wallcomposed of amorphous SiO2showed good mechanical properties though the structureof multi-holes arranged in existence. Meanwhile, it has high stiffness and fracturetoughness, whose value is even in the same range as widely-used engineering materials.In this paper, by usinglarge-scale molecular dynamics simulations based on thefirst principles reactive force field ReaxFF, we simulated thedeformation-fractureprocess of bulk SiO2structure and amorphous SiO2perforated foil structures.We studiedthe influence of microstructure and dimensions on the mechanical properties ofmaterials, revealing the microscopic deformation mechanism of structures. The maincontents show as follows:①The preparation method of the modelby the process ofmelting-quenchingwasdetermined， and the sample of amorphous SiO2wasprepared.Meanwhile, we determined simulation method of the tensile deformation-fracture process.②Molecular dynamics simulations of thedeformation-fracture processofamorphous SiO2blockstructurewith different length dimensions were carried out andstudying the effect of size on the mechanical properties of amorphous SiO2blockstructure. The stress-strainbehaviors show the small model exhibits a better ultimatestrength, ductility and toughness than the large model; Analysis of the microstructureevolution of the system reveal the generation and expansion of voids plays an importantrole in the fracture behavior of the model.The greaterirreversible change of structureleads to the small-scale modeltransits from brittleness to ductility, and makingthe small-scale model shows better ultimate stress, toughness, and a larger range of plasticdeformation than the large-scale model.③Molecular dynamics simulations were carried out to study the uniaxialtensiledeformation-fracturebehavior of amorphous SiO2perforated thin sheetstructures,and studying the effect of structures length and the diameter of hole on themechanical properties of amorphous SiO2perforated thin sheet structures. Our studyshows that the plastic range, toughness, maximum stress and failure strain increase withdecreasing diameter of hole for the same length dimensions of foil structure. However,when the diameter to length ratio is the same，smaller-size model shows bettermechanical properties than large-size model. Through analyzing atomic configurationand the equivalent Von Mises stress, the failure of small-size model displays localizednecking first, and then voids growing, gathering and extending in the necking regionuntil failure occurs, while failure of large-size model exhibits brittle characteristics, thenmodels appear cracks and disconnect quickly after that.